The Role of Novel T Cell Costimulatory Pathways in Autoimmunity and Transplantation : Journal of the American Society of Nephrology

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The Role of Novel T Cell Costimulatory Pathways in Autoimmunity and Transplantation

Yamada, Akira; Salama, Alan D.; Sayegh, Mohamed H.

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Journal of the American Society of Nephrology 13(2):p 559-575, February 2002. | DOI: 10.1681/ASN.V132559
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The CD28-B7 and CD154-CD40 pathways have been described as the critical costimulatory pathways for T cell activation. Blockade of these pathways has been reported to regulate both autoimmune and alloimmune responses in experimental models and in human disease. However, studies have indicated that inhibition of these pathways is insufficient to reproducibly induce long-lasting immunologic tolerance in experimental autoimmunity and transplantation models. This suggests that host immune reactivity toward the autoantigens or graft may persist despite optimal blockade of these pathways. These findings may be explained by the presence of immune mechanisms that are known to be relatively resistant to CD28-B7 and/or CD154-CD40 blockade, such as those involving CD8+ T cells (in some transplant models), primed or memory T cells, and natural killer (NK) cells (in autoimmunity and transplantation). Alternatively, other costimulatory pathways may provide the necessary second signals for complete T cell activation. These two possibilities are of course not mutually exclusive. The recent discovery of new members of the CD28-B7 family, inducible costimulator (ICOS), its ligand, B7RP-1, as well as programmed death–1 (PD-1) and its ligands, PD-L1 and PD-L2, have therefore been of major interest. Furthermore, recent data have demonstrated that other molecules belonging to the tumor necrosis factor (TNF) superfamily and their receptors (TNF-R), including 4–1BB, CD30, CD134 (OX40), and CD27, and their respective ligands, 4–1BBL, CD30L, CD134L, and CD70, also act as efficient costimulatory molecules for T cells. The important role that these newly discovered pathways play in regulation of T cell responses in both autoimmunity and transplantation is only now becoming apparent. In some cases, these pathways may be subdominant (or redundant) and exert potent effects on T cell reactivity only in the absence of or after suboptimal costimulation through CD28-B7 and CD154-CD40. However, in other cases these pathways can play a pivotal role in T cell activation or differentiation that may be dependent on the particular stage of the ongoing immune response. Finally, there are important yet complex interactions between these novel T cell costimulatory pathways and both the CD28-B7 and CD154-CD40 pathways, which determine the outcome of a particular immune response in vivo. In this review, we summarize the biology of these pathways, highlight their roles, their hierarchy of dominance and interactions, and finally promote ideas regarding their therapeutic manipulation for the treatment of autoimmune diseases and as immunotherapy in transplantation.

T Cell Activation

T cells require two collaborative but distinct signals for full activation (1,2) (Figure 1). The first signal (signal one) is provided by the engagement of the T cell receptor (TCR) with its specific peptide antigen, bound to the MHC molecules on the surface of antigen-presenting cells (APC). The second costimulatory signal (signal two) is provided by engagement of T cell surface receptors with their specific ligands on APC (Figure 1). Signaling through the TCR alone without signal two can lead to a state of T cell unresponsiveness that is termed anergy or to apoptosis. Importantly, not all costimulatory molecules provide a “positive” signal; some provide “negative” signals that result in physiologic termination of immune responses (3) (Figure 1). The balance between positive and negative T cell costimulatory signals plays a critical role in protecting the organism against invading foreign antigens and preventing the development of autoimmunity.

Figure 1. :
(A) Postive signaling pathways. T cell activation requires two signals. Signal one, the ligation of the T cell receptor with its antigen, which is presented on the surface of MHC molecules on antigen-presenting cells (APC), and signal two, the ligation of costimulatory molecules on T cells with their respective ligands on APC. Through a series of secondary signals, the T cell subsequently undergoes proliferation, cytokine production, and further differentiation into its effector state. (B) Negative signaling pathways. Some costimulatory signals can also lead to negative T cell signaling, resulting in cellular anergy, loss of proliferative capacity, and reduction of cytokine production. These pathways may also be involved in the generation of regulatory cells.

The Conventional T Cell Costimulatory Pathways

The CD28/CTLA4-B7 Pathway

The CD28-B7 T cell costimulatory pathway is one of the best characterized and is critical for T cell activation (47) (Figure 2). CD28, present on T cells, has two known ligands, B7–1 (CD80) and B7–2 (CD86), both of which are expressed primarily on activated APC, such as dendritic cells, macrophages, and B cells. When activated, T cells upregulate CTLA-4, a molecule that is structurally similar to CD28 that also binds both B7–1 and B7–2. Interaction of CD28 with B7–1 and B7–2 provides a positive signal, which results in full T cell activation, including cytokine production, clonal expansion, enhanced T cell survival, and provision of B cell help (8). CTLA-4 has a higher affinity for B7–1 than B7–2, and functions to provide a “negative” signal resulting in physiologic termination of T cell responses (911) (Figure 1). The importance of CTLA-4 as a negative regulatory costimulatory molecule for T cells is highlighted by the observation that CTLA-4–deficient mice develop a fatal lymphoproliferative disorder with multiorgan autoimmune disease (12,13). Furthermore, recent evidence suggests that the CTLA-4 negative signaling pathway may be required for induction of acquired tolerance in vivo (1416). Indeed, it has been hypothesized that CTLA-4 may function as a master switch for peripheral T cell tolerance (17). Therefore, strategies that promote CTLA4-mediated negative signaling could be very useful therapeutically in T cell–mediated diseases. Ligation of CD28 by B7–1 or B7–2 can be blocked by anti-B7–1 or anti-B7–2 monoclonal antibodies, respectively, or by CTLA4Ig, a recombinant fusion protein containing the extracellular domain of CTLA-4 fused to an Ig heavy chain tail. CTLA4Ig binds to both B7–1 and B7–2 with higher affinity than does CD28, and thus acts as a competitive inhibitor of CD28 binding to B7–1/B7–2, resulting in blockade of CD28-B7 costimulation.

Figure 2. :
The CD28-B7 family of costimulatory molecules. Both CD28 and CTLA4 contain a motif (MYPPPY) that is necessary for binding to B7–1 and B7–2. Other members of the family lack this motif and are therefore prevented from binding these ligands. The net effect on cellular function after stimulation through these pathways is dependent on the temporal expression patterns of these molecules during T cell activation and the combination of positive and negative signals delivered.

Blockade of the B7 Pathway in Transplantation.

We and others (1822) have shown that CD28-B7 T cell costimulatory blockade prevents acute allograft rejection and induces donor-specific tolerance in several animal models, although this is not a universal finding in all models or strain combinations. In addition, CD28-B7 blockade prevents development (2325) and interrupts progression (26,27) of chronic allograft rejection in minor antigen-mismatched transplant models. However, B7 blockade is less effective in preventing chronic vasculopathy in fully allogeneic transplant models, in which chronic administration of CTLA4Ig or co-administration of donor antigen with CTLA4Ig is required to attenuate development of chronic rejection (28,29). Interestingly, while blockade of both B7–1 and B7–2 are necessary to prevent allograft rejection and promote long-term engraftment in acute rejection models (30), selective inhibition of signaling through B7–1 is sufficient for prevention of chronic rejection (27). We have recently reported this in a rat cardiac transplant model, where selective B7–1 blockade was ineffective in preventing acute graft loss but prevented progression of chronic allograft vasculopathy (27). These data are consistent with those of Furukawa et al. (31), which demonstrate that allograft vasculopathy is significantly attenuated in B7–1 knockout and B7–1/B7–2 double knockout but not B7–2 knockout mice compared with wild type littermates.

Creation of CD28 and B7–1/B7–2 deficient animals has helped shed light into the functions of the CD28-B7 T cell costimulatory pathways in allograft rejection. It is interesting that although B7–1/B7–2–double deficient recipients fail to reject vascularized allografts (32,33), CD28-deficient animals have been reported to reject allografts with some delay (34,35). It appears that both CD8+ T cells (35) and NK cells (36) play important roles in CD28-independent allograft rejection. This is a clinically relevant observation, because it may explain the mechanisms of resistance to CD28-B7 blockade in some allograft models (37). Whether these CD8+ T cells are dependent on one or more of the new T cell costimulatory pathways for activation remains to be determined (see below).

Blockade of B7 Pathway in Autoimmunity.

Inhibition of the CD28-B7 pathway has also been shown to be effective in the prevention and treatment of established diverse autoimmune diseases in both experimental animal models and patients. In experimental autoimmune glomerulonephritis (EAG), an animal model of human anti-glomerular basement membrane (GBM) disease, there was significant attenuation of clinical disease, anti-GBM autoantibody production, and renal mononuclear cell infiltration in animals treated with CTLA4Ig (38). Furthermore, selective blockade of B7–1 by a mutant form of CTLA4Ig produced similar disease regulation, demonstrating that B7–1–mediated signaling is central to autoreactive T cell activation in this model. Differential effects of signaling by B7–1 or B7–2 have also been demonstrated in other autoimmune models, including lupus nephritis in the MRL-lpr/lpr mice, experimental autoimmune encephalomyelitis (EAE), and diabetes in susceptible nonobese diabetic (NOD) mice. Combined blockade of B7–1 and B7–2 in MRL mice attenuated lupus-like renal disease and was associated with suppressed autoantibody production. However, deficiency or inhibition of B7–1 or B7–2 alone resulted in similar levels of pathogenic autoantibodies. Only in the animals lacking B7–2 was there diminished renal Ig deposition and attenuated pathology (39). The B7–1–deficient animals developed more severe nephritis despite similar autoantibody levels, further demonstrating the lack of correlation between antibody titer and disease (40). In EAE, treatment of animals with anti-B7–1 antibody prevents the development of disease, whereas anti-B7–2 antibody exacerbates it (41), although this is not a universal observation in all models (42,43). In the NOD mice, anti-B7–2 treatment suppresses diabetes, but anti-B7–1 antibody alone or in combination with anti-B7–2 antibody accelerates disease. Furthermore, only early treatment with anti-B7–2 prevents the development of diabetes, but it interestingly has no effect on the inflammatory insulitis (44). B7 costimulation signaling through CD28 is also implicated in the development of collagen-induced arthritis, autoimmune thyroiditis, autoimmune uveitis, and myasthenia gravis (39,40,4548). However, CD28-B7 blockade may not completely abrogate disease, but rather diminishes severity and alters T cell and antibody phenotypes. In experimental myasthenia, for example, CD28 deficiency renders animals less susceptible to disease, but only deficiency of CD154 (see below) confers complete disease resistance (45). Furthermore, although CD28 deficiency protects animals from EAE, disease can be induced after second immunization with antigen, suggesting that alternative pathways can be used for full T cell activation (49).

B7 blockade by CTLA4Ig has been studied in patients in a phase I trial as treatment for severe psoriasis vulgaris (50,51) and in phase II trials for therapy of rheumatoid arthritis. CTLA4Ig is currently undergoing trials in other patient groups, including those with multiple sclerosis and lupus nephritis and in renal transplant recipients (8,52). There are currently more preparations of CTLA4Ig that are being tested clinically. In addition, there are several preparations of humanized anti-B7–1 and anti-B7–2 monoclonal antibodies. Importantly, the experimental animal data showing distinct functions of B7–1 and B7–2 in regulating the autoimmune response in various disease models underscores the need to design tailor-made therapeutic strategies in humans with various autoimmune diseases.

The CD154-CD40 Pathway

There has recently been much interest in studying the role of CD154 and its ligand CD40 in the process of allograft rejection and in the regulation of autoimmune disease (8,53). CD154 is expressed on activated T cells, and CD40 is expressed on APC, including B lymphocytes. CD154-CD40 interaction provides a bidirectional signal for T and B cell activation, thus underlying its importance in T cell–B cell collaboration. CD40 signaling of B cells is critical for Ig switching, and the absence of CD154 characterizes the hyper IgM X-lined syndrome (54). It has been questioned, however, whether CD154 acts directly to transduce a costimulatory signal to the T cell, or indirectly, as ligation of CD40 on APC is a strong inducer of B7 expression (55,56).

CD154-CD40 Blockade in Transplantation.

Larsen et al. (57) have shown that blocking this pathway with an antibody to CD154 is efficient in preventing acute graft rejection in a mouse cardiac allograft model. Our group (58) reported similar results and demonstrated downregulation of B7–1 expression in cardiac allografts of animals treated with anti-CD154. In our study (58) and in a study by Parker et al. (59) using islet transplantation, coadministration of donor cells synergizes with CD154 blockade to prolong graft survival and induce donor-specific tolerance. In addition, this strategy resulted in the prevention of chronic rejection (60), although others have found contradictory data. CD154 blockade alone was found not to prevent the development of chronic rejection (61,62), and Shimizu et al. (63) recently showed that CD154-deficient animals develop chronic allograft vasculopathy despite long-term allograft survival. In these cases, it has been suggested that CD154 blockade–resistant CD8+ T cells (61), perhaps through one or more of the new pathways, may play a role in the pathogenesis of chronic allograft rejection.

A number of studies have demonstrated synergy between B7 and CD154 blockade with or without donor antigen. Larsen et al. (62) reported that simultaneous inhibition of these two pathways led to prolongation of murine skin allograft survival and prevented the development of chronic cardiac allograft vasculopathy. Wekerle et al. (64,65) reported that combined B7 and CD154 blockade may substitute for T cell depletion and irradiation (when high-dose donor bone marrow was used), in the induction of mixed allogeneic chimerism and deletional tolerance in a mouse skin transplant model. Similar observations were reported by Larsen’s group (66), which used CD154 blockade and donor bone marrow.

Preclinical studies indicating the efficacy of CTLA4Ig and a humanized anti-CD154 monoclonal antibody in primate renal (67,68) and islet (6971) transplantation models have also been reported. Both these agents have been shown to prolong graft survival, but there are no data to indicate that by themselves they reproducibly induce donor-specific tolerance in primates (72). However, when anti-CD154 monoclonal antibody was used as part of a strategy to induce mixed allogeneic chimerism in a renal transplant model (73), the primates did develop donor-specific tolerance. However, some recipients developed thromboembolic complications that responded to anticoagulation with heparin. Such a complication was also observed in some humans entered in the phase I-II renal transplant trial with the humanized anti-CD154 (Biogen Inc., Cambridge, MA) monoclonal antibody, resulting in premature termination of the trial. The exact mechanisms underlying these complications and the plans for future development of this agent in transplantation remain unclear.

Of interest is the interaction between conventional immunosuppressive drugs and costimulatory pathway blockade. Although some drug regimens (containing calcineurin inhibitors) may be detrimental to the effects of T cell costimulatory blockade (57,68,74), others (such as rapamycin) may be beneficial (75). The working hypothesis is that calcineurin inhibitors may inhibit, while rapamycin promotes, activation-induced T cell death (AICD), a mechanism that is required for induction of tolerance by CD154 and B7 blockade (75,76). Calcineurin inhibitors also inhibit expression of CTLA4 (77), which may be necessary for induction of tolerance by T cell costimulatory blockade (30). However, we have recently shown that while rapamycin is indeed synergistic with CD154 blockade, calcineurin inhibitors do not universally impair long-term graft survival in all models (78,79). In our model, late introduction of calcineurin inhibitors to animals treated with CD154 blockade, led to the development of chronic allograft vasculopathy, indicating that this type of strategy may not be clinically desirable in humans (78). These collective observations demonstrate that the interactions between T cell costimulatory blockade and immunosuppressive drugs are complex but extremely important to understand so as to develop clinically relevant strategies to translate into humans.

CD154-CD40 in Autoimmunity.

In numerous autoimmune diseases, blockade of the CD154-CD40 pathway has been shown to abrogate or suppress disease. This is especially true of diseases in which B cell activation is of fundamental importance, such as systemic lupus erythematosus (SLE) and myasthenia gravis (MG), because the CD154-CD40 pathway is critical in T cell–B cell interaction and activation. For example, in models of SLE, disease may be retarded by a brief treatment course with anti-CD154 antibody (80). In experimental MG, blockade of the CD40 pathway alone renders the animals completely resistant to disease induction (45). Other autoimmune diseases can also be modulated by blockade of this pathway. Using models of spontaneous diabetes in rodents, recurrence of autoimmunity (in transplanted isografts) was diminished after treatment with anti-CD154 antibodies, although the efficacy was greater in rats than mice (81,82). This is consistent with previous observations indicating that CD154 blockade protected NOD mice from developing diabetes when therapy is initiated early but that therapy was ineffective for established disease (83).

In EAE, deficiency of CD40 within the central nervous system is sufficient to diminish the intensity and duration of disease, despite the demonstration of adequate T cell activation within the peripheral immune system (84). Although CD154-CD40 blockade alone is highly efficacious in autoimmune disease, as is found in certain transplantation models, there is synergy with blockade of the B7-CD28 pathway. For example, in a model of SLE, CD154-CD40 blockade alone retards disease, but when combined with CTLA4Ig therapy, renal disease may be completely prevented and survival significantly improved (80).

Humanized anti-CD154 antibodies are currently undergoing phase I-II testing in autoimmune diseases, including lupus nephritis, although at least one preparation (Biogen Inc.) has been associated with thromboembolic complications, and those trials have been terminated prematurely (see above). Other preparations (IDEC Pharmaceuticals, San Diego, CA) have not been reported to cause similar complications and are currently under investigation.

The Novel Costimulatory Pathways

Several novel T cell costimulatory pathways have recently been described (85). The ICOS–B7RP-1 and PD-1–PD-L pathways are related to the CD28-B7 family (Figure 2). Furthermore, several new members of the TNF–TNF-R superfamily, of which CD514-CD40 is the prototype, have also been found to be efficient costimualtory molecules (Figure 3). The expression patterns and functions of these pathways are complex and as yet not clearly defined in experimental systems including autoimmunity and transplantation. Few data are available regarding the in vivo expression of the molecules involved and their roles in human disease. In addition, their potential interactions with the CD28/CTLA4-B7 and CD154-CD40 pathways remain incompletely understood. Their roles are being investigated by using a combination of monoclonal antibodies, fusion proteins, and novel gene knockout animals. We have begun to understand how these molecules are regulated during immune responses and what effects they exert. Finally, data are emerging on the interactions between these novel pathways and conventional immunosuppressive agents, which will be important in the planning of future treatment strategies in both transplantation and autoimmunity.

Figure 3. :
The TNF–TNF-R superfamily of molecules. A number of ligand receptor pairs from this superfamily can act as efficient costimulatory molecules. Through their interactions, both T and B cell activation may occur and result in a variety of cell effector functions.

Novel CD28/CTLA4-B7 Family Pathways (Figure 2)

The ICOS–B7RP-1 Pathway.

The newly discovered CD28 homologue, ICOS, is a T cell costimulatory molecule first reported on activated human T cells (86,87). Human ICOS shares 24% identity (and 39% similarity) with human CD28 and 17% identity (and 39% similarity) with human CTLA-4 (88). The MYPPPY motif, which is required for the binding of CD28 and CTLA-4 to B7 ligands (89), is not conserved in ICOS; instead, it is replaced by a FDPPPF motif. Thus, ICOS does not bind B7–1 or B7–2. Similarly, the L-ICOS ligand, B7RP-1 (which has also been named L-COS, B7h, B7H-2, GL-50) (87,9093) binds ICOS but not CD28 or CTLA-4.

In a similar manner to CD28, signaling through ICOS can result in enhanced T cell proliferation and cytokine production, induce T cell upregulation of CD154, and stimulate T cells to provide help for Ig production by B cells (86). However, ICOS has several properties that are distinct from CD28 and thus make it particularly intriguing. Whereas CD28 is constitutively expressed on T cells, ICOS is induced after TCR engagement and is thus expressed only on activated T cells and resting memory T cells (87), suggesting an important role in providing costimulatory signals to activated T cells (94). This is of some importance because it is known that unlike antigen-inexperienced (nai[Combining Diaeresis]ve) T cells, which require CD28 signaling for proliferation and cytokine production, optimal activation and differentiation of recently activated T cells or memory cells can occur independently of CD28 costimulation (85,95). Expression of ICOS is enhanced by CD28 costimulation, and ICOS upregulation is markedly reduced in the absence of B7–1 and B7–2, suggesting that some of the functions ascribed to CD28 may be due in part to ICOS signaling (96). B7RP-1 expression is still incompletely understood. Early data suggests that it may be constitutively expressed at low levels on antigen presenting cells and certain parenchymal cells (such as renal tubular epithelial cells, prostate epithelial cells and brain tissue) and appears to be upregulated in inflammatory states (97,98). Whereas interferon-γ (IFN-γ) stimulation upregulates both B7RP-1 and B7–1/B7–2 on dendritic cells (DC), TNF-α and lipopolysaccharide (LPS) have differential effects, downregulating B7RP-1 and upregulating B7–1/B7–2 (95). What role this pattern of parenchymal expression plays in regulation of immune responses in normal and diseased tissue remains to be determined.

The functional effect of ICOS ligation was demonstrated by using a signaling anti-ICOS monoclonal antibody, which resulted in enhanced T cell proliferation and production of several cytokines (interleukin-4 [IL-4], IL-5, IL-10, IFN-γ, TNF-α, and GM-CSF) (86). ICOS may have a critical role in regulating Th2 cell differentiation. The inducible expression of ICOS and its preferential induction of IL-4 and IL-10 suggest that ICOS may amplify and regulate T helper cell differentiation. Coyle et al. (94) have reported that ICOS is an important costimulatory receptor for both recently activated T cells and for Th2 but not Th1 effector cells. Inhibition of ICOS may be effective in suppressing the function of recently activated T helper cells, inhibiting the secretion of both IL-4 and IFN-γ. However, under circumstances where strong immune deviation occurs, the contribution of ICOS to T cell activation may be restricted to Th2 helper cells. Indeed, ICOS-Ig administration suppressed Th2 cell–mediated airway hyperreactivity in the absence of suppressive effects on Th1-mediated alterations in airway functions (94).

ICOS costimulation is involved in both alloimmune responses and those to nominal antigens, because ICOS–B7RP-1 blockade with ICOS-Ig fusion protein suppressed proliferation of T cell responding to allogeneic DC as well as to tetanus toxoid in vitro (90). In vivo studies have suggested complex interactions between ICOS and the CD28-B7 and CD154-CD40 pathways. Inhibition of ICOS in CD28-deficient mice further reduced Th1/Th2 polarization in murine viral and parasitic infection models (99). Blocking of ICOS alone had a limited but significant capacity to downregulate T helper cell subset development. In contrast, cytotoxic T lymphocyte (CTL) responses remained unaffected by blocking ICOS. Taken together, these data suggest that ICOS can regulate both CD28-dependent and CD28-independent CD4+ subset responses but not CD8-mediated CTL responses in vivo (99).

ICOS-deficient mice exhibit profound deficits in Ig isotype class switching and germinal center formation. Class switching can be restored in ICOS-deficient mice by CD40 stimulation, demonstrating critical interactions between the ICOS–B7RP-1 and the CD154-CD40 pathways (100). Differentiated ICOS-deficient cells are able to produce IFN-γ and IL-10 but fail to express IL-4 upon restimulation. Furthermore, significantly higher numbers of CD4+ ICOS-deficient T cells retain the nai[Combining Diaeresis]ve phenotype (CD62Lhigh) after cellular activation. ICOS-deficient T cells do not proliferate in response to immunogens (such as keyhole-limpet hemocyanin) administered in alum, but they do if the antigen is coadministered with complete Freund’s adjuvant (CFA), suggesting that strong inflammatory responses induced by the CFA can bypass the requirement for ICOS. ICOS is not required for Th2 differentiation, but rather regulates IL-4 and IL-13 production by effector cells. In EAE, ICOS-deficient mice developed greatly enhanced disease compared with wild type mice (101). This may reflect impaired production of the regulatory Th2 cytokines IL-4, IL-13, and/or IL-10.

Collectively, the above data demonstrate that ICOS stimulation is important in T cell activation and differentiation, and in T cell–B cell interactions. In addition, there are complex, yet important, interactions between the ICOS–B7RP-1 pathway and the CD28-B7 and CD154-CD40 pathways. Indeed, Ozkaynak et al. (97) recently demonstrated that the blockade of ICOS–B7RP-1 pathway effectively inhibited the development of chronic rejection in association with CD154-CD40 pathway blockade, using a murine cardiac transplant model. Furthermore, ICOS blockade prevented acute rejection and, with concurrent donor-specific transfusion or cyclosporine, induced long-term graft survival.

The contribution of ICOS to T cell–mediated immune responses and the functional consequences of ICOS inhibition may be critically influenced by both the nature of the immune response and the timing of intervention with ICOS blockade strategies. For example, the effect ICOS blockade had on the development of EAE was in part dependent on the disease stage (induction or effector stage) when it was administered. Treatment with anti-ICOS antibody during antigen priming (days 1 to 10) resulted in worsening of disease, increased IFN-γ production, increased chemokine expression, greater T cell proliferation, and reduced IgG1 antibody levels, all consistent with a greater Th1 response. Delayed treatment (days 9 to 20) produced the opposite effect, with significantly attenuated disease, decreased IFN-γ production, and reduced chemokine expression and cellular infiltration into the target organ (102).

Current investigations are actively aimed at exploring the functions and mechanisms of ICOS–B7RP-1 interactions in various transplantation and autoimmune models.

PD-1 and its Ligands, PD-L1 and PD-L2.

The newest member of the CD28 superfamily to be described is PD-1. Like CD28, ICOS, and CTLA4, it is a transmembrane protein of the Ig superfamily, and like CTLA4 it possesses only a single V-like domain and an immunoreceptor tyrosine–based inhibitory motif (ITIM) within its cytoplasmic tail (Figure 2). It shares 23% homology with CTLA4, but it lacks the MYPPPY motif required for B7–1 and B7–2 binding. PD-1 receptor is found on activated T and B cells as well as myeloid cells such as macrophages. It binds two known ligands, PD-L1 and PD-L2, found on professional APC, such as DC and monocytes, but also found constitutively on certain parenchymal cells (in the heart, lung, and kidney) as well as on a subpopulation of T and B cells (103,104). In an analgous manner to CTLA4, engagement of PD-1 by its ligands results in a negative regulatory effect, with inhibition of downstream cellular signaling events, diminished cellular proliferation, and cytokine production. However, some of these effects are dependent on antigen dose. For example, at high antigen concentrations, cytokine production but not cell proliferation is diminished (104). Furthermore, PD-1 deficiency (similar to CTLA4) results in autoimmune phenomena, including splenomegaly, B cell expansion with increased serum immunoglobulins, lupus-like glomerulonephritis, arthritis, and autoimmune cardiomyopathy (105). The exact phenotype varies dependent on the background strain in which the knockouts are generated. PD-1 ligation is sufficient to downregulate suboptimal CD28-mediated signaling (103). Thus, after T cell activation both CTLA4 and PD-1 are upregulated and serve to contain the T cell response. Both molecules therefore appear to play important roles in the maintenance of tolerance. Although PD-1 deficiency leads to some modulation of thymic selection (106), it appears to play a more prominent role in peripheral rather than central tolerance. The possible expression of PD-L on parenchymal cells (which can upregulate class II MHC and present antigens to T cells, but do not express B7) suggests that PD-1–PD-L signaling may to some extent underlie the tolerogenic capacity of these nonprofessional APC. By way of contrast, early reports suggest that B7RP-1 (the ICOS ligand) is upregulated on parenchymal cells, such as renal tubular epithelial cells, after activation (for example, by IFN-γ in vitro) (98). The net effect of signaling through these different pathways on T cells present in inflamed tissues will therefore be complex, and the balance may dictate the final outcome of the immune response.

The TNF–TNF-R Pathways (Figure 3)

The CD154-CD40 interaction is the prototypic pathway of the TNF–TNF-R superfamily of molecules, representing one of a series of receptor-ligand interactions that are important in T cell activation and T cell–B cell interactions. These pathways have the capacity to both provide direct T cell costimulation and interact with other costimulatory pathways such as CD28-B7 and ICOS–B7RP-1. The TNF–TNF-R superfamily contains a number of member pairs, including 4–1BB-4–1BBL, CD30-CD30L, CD134-CD134L, and CD70-CD27. Each of these molecular receptor-ligand interactions has been reported to have unique costimulatory functions. These will be discussed individually. Although other newly described members of the family, such as LIGHT-HVEM (107), also exist, there are as yet limited data on their role in autoimmunity and transplantation and they will not be considered further in this review.

The 4-1BB–4-1BBL Pathway.

4-1BB (CD137, ILA), a member of the TNF-R family, exists as both a 30-kD monomer and a 55-kD homodimer (108). 4–1BBL is a member of the TNF family and exists as a disulfide-linked homodimer (109). 4–1BB is primarily expressed on activated CD4+ and CD8+ T cells (108) as well as on activated NK cells (110). 4–1BB expression peaks 2 to 3 d after cell activation (108,111). 4–1BBL is expressed on mature DC (112) and on activated B cells and macrophages (113). However, due to the initial low levels of expression of 4–1BBL after activation (85) it has been suggested that this pathway would not play a major role in the initiation of the immune response. Furthermore, anti–4-1BB monoclonal antibody has a greater effect on previously activated T cells than on resting T cells, preventing the cells from undergoing AICD (111). Like CD95 (Fas), another TNF-R member, 4–1BB is involved in induction of lymphocyte apoptosis. Although 4–1BB induces expression of CD95 on resting primary T and B cells, induction of apoptosis by 4–1BB is independent of CD95 because anti-CD95 antibody fragments do not block 4–1BB-induced apoptosis (114).

Stimulation of 4–1BB induces higher levels of CD8+ than CD4+ T cell proliferation (115) and appears to be critical for CD8+ T cell survival (116). Furthermore, 4-1BBL–deficient mice have an impaired ability to generate CTL responses to influenza virus (117). However, 4–1BB participates in promoting IL-2 production by resting CD4+ T cells, confirming that 4–1BBL can also play a role in antigen-specific CD4+ T cell responses (118). Moreover, anti–4-1BB monoclonal antibody can induce helper T cell anergy and effectively block T cell–dependent B cell responses (119).

After repeated stimulation, human CD4+ T cells proliferate negligibly in response to anti-CD3 and anti-CD28 monoclonal antibodies (mAb) but show enhanced responses to combined anti-CD3, anti-CD28, and anti–4-1BB mAb (120). These data suggests that 4–1BB plays a later role in the immune response than CD28 and that 4–1BB functions to perpetuate the immune response after CD28 downmodulation. Whereas anti–4-1BB mAb preferentially stimulate CD8+ T cell proliferation, CD28 ligation exerts a more significant proliferative effect on CD4+ cells (115,121). The 4–1BB may, however, be subdominant, because response of nai[Combining Diaeresis]ve CD8+ T cells is dependent on 4–1BB only when CD28 molecules are absent (85). Furthermore, whereas CD28 plays a role in initial T cell expansion, 4-1BB–4-1BBL exerts its effects by sustaining established CD4+ and CD8+ T cell responses and enhancing cell division and T cell effector function (122). 4-1BB–4-1BBL interaction contributes to the development of an allogeneic Th2 response by CD4+CD28 T cells (123). However, treatment of activated human T cells with an intact CD28 pathway using anti–4-1BB promotes a Th1 response (120). Therefore, the T cell response and phenotype that results is dependent on not only the ligation of 4–1BB but on the status of the CD28-B7 pathway. Because 4–1BB may function during the later stages of an immune response, possibly to sustain T cell activation after CD28 downregulation, it may be of importance in conditions of chronic immune stimulation.

Treatment with a stimulating anti–4-1BB monoclonal antibody leads to accelerated allograft rejection in both murine heart and skin transplantation models (115). CD28 or 4-1BBL–deficient mice reject both MHC and minor antigen-incompatible skin grafts without delay, and CD28/4–1BB double-deficient mice experience prolonged graft survival for both mismatches (although all of the grafts were eventually rejected within 30 d) (117). Tan et al. (124) reported that both 4–1BB and 4–1BBL transcripts were expressed in rejecting grafts using a murine cardiac transplant model. They also demonstrated that 4–1BB promotes CD8+ T cell proliferation by both enhancing signals through the IL-2 receptor and by other IL-2–independent mechanisms.

Interestingly, transfection of either B7–1 or B7–2 into certain lymphoma cell lines does not render the lines immunogenic, but the additional transfection of 4–1BBL results in a tumor that is highly immunogenic and can confer long-lasting protection against subsequent challenge with parental tumor in vivo (125). Furthermore, the 4-1BBL–expressing tumors were capable of priming CTL responses against 4-1BBL–transfected as well as parental tumors in the absence of CD28, although cytokine production was lower, resulting in a weaker CTL recall response and reduced ability to survive challenge with parental tumor (126).

In primary mixed lymphocyte reactions, a significant reduction in the response was observed when either 4–1BBFc or CTLA4Ig was added to the cultures, although CTLA4Ig had the greater effect (127). In other strain combinations, 4-1BB–alkaline phosphatase conjugate (4-1BB–AP) and CTLA4Ig added individually profoundly blocked proliferation of alloreactive T cells, and the combination of the two completely abrogated the response (128). Furthermore, measurement of CTL responses demonstrated that CD28+ T cells killing allogeneic target cells were only moderately inhibited by blocking of 4-1BB–4-1BBL interaction, whereas CD28-deficient T cell killing of the allogeneic target was completely blocked by inclusion of 4-1BB–AP in the cultures (117). Thus, 4–1BBL and CD28 may play redundant roles in allogeneic CTL responses. The functions of the 41BB-4–1BBL in costimulation of CD8+ T cells makes it an attractive target for investigation in models where conventional T cell costimulatory blockade of B7 and CD154 are not optimally effective (37).

The CD30-CD30L Pathway.

CD30 was originally described as a marker of Reed-Sternberg cells in Hodgkin lymphoma (129). CD30 is expressed by activated but not by resting B or T cells (129133). It has been proposed that CD30 is preferentially expressed on Th2 cells (130), although this is not universally accepted (134). T cell expression of CD30 is dependent on the presence of CD28 costimulatory signals or exogenous IL-4 during primary T cell activation (131). CD30L is a transmembrane protein of the TNF family that is expressed by T and B lymphocytes, macrophages, and a variety of hematopoietic cells and tumors (135,136). Using activated splenocytes, it was demonstrated that CD30L is expressed primarily on CD4+ T cells, with peak expression at days 1 and 2, whereas CD30 is expressed primarily on CD8+ T cells, with peak expression on days 4 and 5 (130). The CD30L has been reported to act both as a costimulator for the proliferation of T cells and as a mediator of cytotoxicity through induction of apoptosis (135,136). Mice deficient in CD30 showed a mild impairment in thymic negative selection, and activation-induced death of thymocytes after CD3 crosslinking is impaired both in vivo and in vitro (137).

Although function of CD30-CD30L interaction is largely unknown, in vitro studies have shown that it has effects on both cell activation and cell death (130,131,133,135,138,139). Gruss et al. (135) demonstrated that CD30-CD30L interaction enhance or reduce proliferation of many different CD30+ human lymphoma cell lines. Using lymph node cells, Gilfillan et al. (131) showed that CD30 signaling has a costimulatory effect during a secondary stimulation with anti-CD3. In addition to this role in cell proliferation, CD30 signaling also regulates T cells by inducing apoptosis. Lee et al. (139) reported that in combination with signals transduced by the TCR, CD30 signaling induces Fas-independent cell death in T cell hybridomas. Moreover, Telford et al. (133) showed that CD30-regulated, Fas-independent apoptosis occurred in murine CD8+ T cells after cessation of TCR signals. Several reports have shown that CD30-CD30L interaction induces immune deviation to Th2. Stimulation of CD30 by plate-bound agonistic anti-CD30 directly signaled for IL-5 but not IFN-γ production by CD30+ CTL lines (130). In addition, costimulation of peripheral blood mononuclear cells with agonistic anti-CD30 antibody resulted in preferential development of antigen-specific T cell lines and clones showing a Th2-like profile of cytokine secretion. Furthermore, blockade in bulk culture of CD30-CD30L interaction shifted the development of antigen-specific T cells toward Th1-like phenotype (140). These observations suggest that CD30 triggering of activated Th cells by CD30L-expressing APC may represent an important costimulatory signaling for the development of Th2-type responses. However, contrary to the in vitro observation, in vivo blockade of CD30L could not abrogate murine experimental leishmaniasis, a Th2-mediated disease (141).

CD30 signaling limits the proliferative potential of autoreactive CD8+ effector T cells and protects the body against autoimmune diabetes mediated by CD8+ T cells in mice (142). Furthermore, transplantation of CD30-deficient mice, both MHC class I and class II disparate skin or heart grafts, were rejected faster than control animals (143). This could be due to impaired apoptosis of alloreactive T cells or due to an imbalance of the alloimmune response toward a Th1 phenotype. Further studies are required to explore the effects and mechanisms of CD30-CD30L blockade in experimental models of autoimmunity and transplantation.

The CD134-CD134 Pathway.

CD134 (OX40) was originally identified as a cell surface antigen on activated rat CD4+ T cells. The human, rat, and mouse CD134 genes were subsequently cloned and shown to belong to the TNF-R superfamily (144146). CD134 ligand (OX40L) is a type II membrane protein with limited homology to TNF and has been shown to bind to and costimulate CD134+ T cells in vitro (147,148). When CD134 is engaged by anti-CD134 monoclonal antibody or CD134L it generates a costimulatory signal that can be as potent as CD28 (149). Engagement promotes effector and memory-effector T cell functions by upregulating IL-2 production and increasing the life span of effector T cells.

Expression of CD134 is restricted to activated T cells in humans and rodents (145,150). Expression of CD134L has been documented on activated murine B cells (145,151), human dendritic cells (152), human vascular endothelial cells (153), and HTLV-1-transformed T cells (154). Both in vitro and in vivo activation of nai[Combining Diaeresis]ve T cells results in transient expression of CD134 with a peak at 24 to 48 h and down regulation by 96 to 120 h (155). Although CD134 expression is augmented by CD28, it can occur independently (156).

The CD134-CD134L pathway appears to be particularly important for regulating the extent of CD4+ T cell expansion in the primary T cell response and thus the ability of T cells to persist as a population over time (157). CD134-deficient T cells secrete IL-2 and proliferate normally during the initial period of activation, but cannot be sustained during the latter phases of the primary response and exhibit decreased survival over time. Mice lacking CD134 generate lower frequencies of antigen-specific CD4+ T cells late in the primary response and lower frequencies of surviving memory cells as compared to wild type animals. Furthermore, CD134 and CD134L-deficient mice demonstrate not only impaired T cell proliferation but also diminished Th1 and Th2 cytokine production (155,158161). Therefore CD134-CD134L interactions help regulate primary T cell expansion and T cell memory (157). This function may be particularly relevant for chronic autoimmune diseases and development of chronic rejection where prolonged antigen exposure occurs.

Early studies suggested that CD134-CD134L interactions were also necessary for B cell activation and humoral immunity (151,162). However, studies using CD134- and CD134L-deficient mice have demonstrated that CD134-CD134L interactions are not essential (or are redundant) for germinal center formation and antibody responses to antigens or infectious agents (158161). However, transgenic expression of CD134 on dendritic cells (163) does lead to increased numbers of CXCR-5 CD4+ T cells in B cell follicles (156), which may provide augmented T cell help for B cell function. Furthermore, CD134-deficient mice have been reported to be severely impaired in their ability to generate a Th2 response in response to allergen-induced airway disease (164). These mice also exhibit diminished lung inflammation and significantly attenuated airway hyperreactivity (164). CD134-CD134L is also important in other Th2 CD4+ T cell responses including infections with leishmaniasis (141,165).

CD134-CD134L interactions are critical in autoimmune responses with evidence that signaling through CD134 can break peripheral T cell tolerance (166). Targeting this pathway diminishes disease in EAE (167,168) and in a model of inflammatory bowel disease (169). CD134-Ig administration to mice with colitis ameliorated disease was associated with reduced tissue T cell infiltrates as well as diminished TNF-α, IL-1, IL-12, and IFN-γ production (169). Numerous groups have investigated the effect of CD134 pathway blockade in murine EAE. Administration of anti-CD134L antibody effectively ameliorated EAE in both actively induced and adoptively transferred models (170). Interestingly, anti-CD134L monoclonal antibody treatment did not inhibit the development of pathogenic T cells, their proliferative responses or IFN-γ production as evidenced by restimulation of draining lymph node cells with antigen, and these cells effectively transferred EAE to nai[Combining Diaeresis]ve mice. However, flow cytometric analysis showed that the anti-CD134L antibody treatment inhibited the accumulation of CD134-expressing CD4+ T cells in the spinal cord and the migration of adoptively transferred CD4+ T cells. Interestingly, immunohistochemical analysis revealed prominent CD134L staining on endothelial cells in the inflamed spinal cord. What role this may play in maintenance of the immune response and cell migration remains to be defined. Recently Chitnis et al. (49) from our group demonstrated that anti-CD134L monoclonal antibody therapy protected animals from EAE in CD28-deficient but not wild type mice. Furthermore, using CD134-deficient mice, Ndhlovu et al. (171) reported that abortive T cell priming greatly reduced the clinical manifestations of actively induced EAE associated with a reduction in IFN-γ, IL-2, and IL-6 production.

Although there is CD28-independent costimulation of T cells by CD134L (172), there appears to be synergy between the CD28-B7 and CD134-CD134L pathways. Studies using fibroblast transfectants expressing B7–1 and/or CD134L demonstrated that together CD134L and B7–1 enhance T cell proliferation and cytokine production, especially IL-2 production (155). It is possible that while CD28-B7 costimulation regulates early events, driving cell cycle progression and initial T cell expansion, the CD134-CD134L interaction promotes a more sustained cytokine and proliferative response. This would lead to less cell death and higher frequencies of antigen-specific T cells. Therefore, blockade of this pathway may enhance the ability of B7 and/or CD154 blockade to promote deletional tolerance and may thus prove to be therapeutic importance, especially in stringent transplant models that are relatively resistant to B7 or CD154 blockade alone. In contrast, CD134-transgenic mice develop more severe EAE after a delayed onset, and both CD134-transgenic/CD28-deficient and CD134-transgenic/CD40-deficient mice fail to develop EAE, demonstrating the necessity of these molecules (171).

The CD27-CD70 Pathway.

CD27, another TNF-R superfamily member has been implicated in T cell activation, T cell development, and T cell–dependent antibody production by B cells (173,174). Its ligand, CD70, is a type II transmembrane glycoprotein belonging to the TNF family. It is found on medullary thymic epithelium and is rapidly induced on both T and B cells after cellular activation. CD70 expression on B cells is enhanced by CD40 signaling and is downregulated by IL-4 (175).

Murine CD70 transfectants exhibit a potent costimulatory activity for anti-CD3–stimulated T cell proliferation, which is inhibited by anti-CD70 far more efficiently than murine CD27-Ig (176). Using knockout animals Hendriks et al. (177) suggested that CD27 makes essential contributions to mature CD4+ and CD8+ T cell functions: CD27-supported antigen-specific expansion (but not effector cell maturation) of nai[Combining Diaeresis]ve T cells independent of the cell cycle-promoting activities of CD28 and IL-2. Primary CD4+ and CD8+ T cell responses to influenza virus were impaired in CD27-deficient mice. Effects of CD27-deficiency were most profound on T cell memory, reflected by delayed response kinetics and reduction in number of CD8+ virus-specific T cell to levels seen in primary responses. Furthermore, in the mixed lymphocyte culture using wild type mice, CD27-CD70 interaction induced the generation of cytotoxic T cells (178).

Two reports indicate that CD70 and CD134L on activated B cells could provide CD28-independent costimulatory signals to T cells (172,176). Moreover, CD27-CD70 interactions complement CD40 ligation on B cells, playing a key role in T-dependent B cell responses, and being responsible for plasma cell differentiation (179). Furthermore, CD27-mediated activation may be involved in the NK-cell–mediated innate immunity against virus-infected or transformed cells expressing CD70 (174).

Nakajima et al. (180) reported that treatment of SJL mice with the anti-CD70 monoclonal antibody prevented EAE. The therapeutic effect was not due to the inhibition of T cell priming and antibody production by B cells or immune deviation, although TNF-α production was suppressed. Two separate groups (181,182) recently reported that coexpression of CD70 and B7–1 on tumor cells enhances antitumor immune responses, and this observation could be applicable for prevention of graft rejection. The findings that the CD27-CD70 pathway is important for CD8+ T cell and NK cell functions as well as memory T cell generation and its interaction with CD154-CD40 (179) suggest a key therapeutic target for prevention of alloantibody-mediated chronic allograft vasculopathy and perhaps induction of tolerance in stringent transplant models. Preliminary work from our group indicates that CD27-CD70 blockade might be particularly effective in promoting long-term allograft survival in CD28-deficient animals in which both CD8+ T cells and NK cells play a key role (35).


Recent advances in our knowledge of T cell activation have suggested that inhibiting T cell costimulatory pathways may be an effective way to promote antigen-specific tolerance of transplants and to prevent or treat autoimmune diseases. Blockade of the B7 and CD154 pathways has already shown great promise in certain rodent and primate transplant models and to a much more limited extent in certain human diseases. However, blockade of these conventional T cell costimulatory pathways may not be sufficient to induce tolerance in more stringent transplant models or to inhibit the primed or memory T cell response in autoimmune diseases. Therefore, understanding the functions and mechanisms of other T cell costimulatory pathways in various immune responses may allow for a more efficacious blockade of T cell responses and provide hope of achieving reproducible, robust tolerance in humans (183). On the basis of the known biology of the pathways we have highlighted and the effects seen after their inhibition, it seems likely that one or more of them may prove to be promising therapeutically (Table 1), possibly with combined B7 and/or CD154 blockade. Further experimental studies will be needed to understand which pathways are critical for particular disease states (especially in patients where data are lacking), at what time points, and how these pathways interact with conventional immunosuppressants as well as with interruption of other costimulatory pathways. Our challenge now is to further precisely define the functions of these pathways and the way they interact during autoimmune disease and after transplantation so that the full potential for therapeutic manipulation can be realized.

Table 1:
Potential therapeutic uses of novel T costimulatory pathways blockadea
1. Bretscher P, Cohn M: A theory of self-nonself discrimination. Science 169: 1042–1049, 1970
2. Janeway CA, Jr, Bottomly K: Signals and signs for lymphocyte responses. Cell 76: 275–285, 1994
3. Van Parijs L, Abbas AK: Homeostasis and self-tolerance in the immune system: Turning lymphocytes off. Science 280: 243–248, 1998
4. Linsley PS, Ledbetter JA: The role of the CD28 receptor during T cell responses to antigen. Annu Rev Immunol 11: 191–212, 1993
5. June CH, Bluestone JA, Nadler LM, Thompson CB: The B7 and CD28 receptor families. Immunol Today 15: 321–331, 1994
    6. Thompson CB: Distinct roles for the costimulatory ligands B7–1 and B7–2 in T helper cell differentiation. Cell 81: 979–982, 1995
      7. Bluestone JA: New perspectives of CD28-B7-mediated T cell costimulation. Immunity 2: 555–559, 1995
      8. Sayegh MH, Turka LA: The role of T-cell costimulatory activation pathways in transplant rejection. N Engl J Med 338: 1813–1821, 1998
      9. Linsley PS, Brady W, Urnes M, Grosmaire LS, Damle NK, Ledbetter JA: CTLA-4 is a second receptor for the B cell activation antigen B7. J Exp Med 174: 561–569, 1991
      10. Walunas TL, Lenschow DJ, Bakker CY, Linsley PS, Freeman GJ, Green JM, Thompson CB, Bluestone JA: CTLA-4 can function as a negative regulator of T cell activation. Immunity 1: 405–413, 1994
        11. Walunas TL, Bakker CY, Bluestone JA: CTLA-4 ligation blocks CD28-dependent T cell activation. J Exp Med 183: 2541–2550, 1996
        12. Tivol EA, Borriello F, Schweitzer AN, Lynch WP, Bluestone JA, Sharpe AH: Loss of CTLA-4 leads to massive lymphoproliferation and fatal multiorgan tissue destruction, revealing a critical negative regualtory role of CTLA-4. Immunity 3: 541–547, 1995
        13. Waterhouse P, Penninger JM, Timms E, Wakeham A, Shahinian A, Lee KP, Thompson CB, Griesser H, Mak TW: Lymphoproliferative disorders with early lethality in mice deficient in CTLA4. Science 270: 985–988, 1995
        14. Perez V, Parijs LV, Biuckians A, Zheng X, Strom T, Abbas A: Induction of peripheral T cell tolerance in vivo required CTLA-4 engagement. immunity 6: 411–417, 1997
        15. Greenwald RJ, Boussiotis VA, Lorsbach RB, Abbas AK, Sharpe AH: CTLA-4 regulates induction of anergy in vivo. Immunity 14: 145–155, 2001
          16. Issazadeh S, Zhang M, Sayegh MH, Khoury SJ: Acquired thymic tolerance: Role of CTLA4 in the initiation and maintenance of tolerance in a clinically relevant autoimmune disease model. J Immunol 162: 761–765, 1999
          17. Bluestone JA: Is CTLA-4 a master switch for peripheral T cell tolerance? J Immunol 158: 1989–1993, 1997
          18. Turka LA, Linsley PS, Lin H, Brady W, Leiden JM, Wei RQ, Gibson ML, Zheng XG, Myrdal S, Gordon D: T-cell activation by the CD28 ligand B7 is required for cardiac allograft rejection in vivo. Proc Natl Acad Sci USA 89: 11102–11105, 1992
          19. Lin H, Bolling SF, Linsley PS, Wei RQ, Gordon D, Thompson CB, Turka LA: Long-term acceptance of major histocompatibility complex mismatched cardiac allografts induced by CTLA4Ig plus donor-specific transfusion. J Exp Med 178: 1801–1806, 1993
            20. Pearson TC, Alexander DZ, Winn KJ, Linsley PS, Lowry RP, Larsen CP: Transplantation tolerance induced by CTLA4-Ig. Transplantation 57: 1701–1706, 1994
              21. Pearson T, Alexander D, Hendrix R, Elwood E PSL, Winn K, Larsen C: CTLA4-Ig plus bone marrow induces long-term allograft survival and donor specific unresponsiveness in the murine model. Evidence for hematopoietic chimerism. Transplantation 61: 997–1004, 1995
                22. Sayegh MH, Akalin E, Hancock WW, Russell ME, Carpenter CB, Linsley PS, Turka LA: CD28-B7 blockade after alloantigenic challenge in vivo inhibits Th1 cytokines but spares Th2. J Exp Med 181: 1869–1874, 1995
                23. Russell ME, Hancock WW, Akalin E, Wallace AF, Glysing-Jensen T, Willett TA, Sayegh MH: Chronic cardiac rejection in the LEW to F344 rat model. Blockade of CD 28:B 7 costimulation by CTLA4Ig modulates T cell and macrophage activation and attenuates arteriosclerosis. J Clin Invest 97: 833–838, 1996
                24. Azuma H, Chandraker A, Nadeau K, Hancock WW, Carpenter CB, Tilney NL, Sayegh MH: Blockade of T-cell costimulation prevents development of experimental chronic renal allograft rejection. Proc Natl Acad Sci USA 93: 12439–12444, 1996
                  25. Chandraker A, Russell ME, Glysing-Jensen T, Willett TA, Sayegh MH: T-cell costimulatory blockade in experimental chronic cardiac allograft rejection: Effects of cyclosporine and donor antigen. Transplantation 63: 1053–1058, 1997
                  26. Chandraker A, Azuma H, Nadeau K, Carpenter CB, Tilney NL, Hancock WW, Sayegh MH: Late blockade of T cell costimulation interrupts progression of experimental chronic allograft rejection. J Clin Invest 101: 2309–2318, 1998
                  27. Kim KS, Denton MD, Chandraker A, Knoflach A, Milord R, Waaga AM, Turka LA, Russell ME, Peach R, Sayegh MH: CD28-B7-mediated T cell costimulation in chronic cardiac allograft rejection: Differential role of B7–1 in initiation versus progression of graft arteriosclerosis. Am J Pathol 158: 977–986, 2001
                  28. Glysing-Jensen T, Raisanen-Sokolowski A, Sayegh MH, Russell ME: Chronic blockade of CD28-B7-mediated T-cell costimulation by CTLA4Ig reduces intimal thickening in MHC class I and II incompatible mouse heart allografts. Transplantation 64: 1641–1645, 1997
                  29. Sayegh M, Zheng X-G, Magee C, Hancock W, Turka L: Donor antigen is necessary for the prevention of chronic rejection in CTLA4Ig-treated murine cardiac allografts. Transplantation 64: 1646–1650, 1997
                  30. Judge TA, Wu Z, Zheng XG, Sharpe AH, Sayegh MH, Turka LA: The role of CD80, CD86, and CTLA 4 in alloimmune responses and the induction of long-term allograft survival. J Immunol 162, 1947–1951, 1999
                  31. Furukawa Y, Mandelbrot DA, Libby P, Sharpe AH, Mitchell RN: Association of B7–1 co-stimulation with the development of graft arterial disease. Studies using mice lacking B7-1, B7–2, or B7–1/B7–2. Am J Pathol 157: 473–484, 2000
                  32. Mandelbrot DA, Furukawa Y, McAdam AJ, Alexander SI, Libby P, Mitchell RN, Sharpe AH: Expression of B7 molecules in recipient, not donor, mice determines the survival of cardiac allografts. J Immunol 163: 3753–3757, 1999
                  33. Szot GL, Zhou P, Sharpe AH, He G, Kim O, Newell KA, Bluestone JA, Thistlethwaite JR, Jr: Absence of host B7 expression is sufficient for long-term murine vascularized heart allograft survival. Transplantation 69: 904–909, 2000
                  34. Lin H, Rathmell JC, Gray GS, Thompson CB, Leiden JM, Alegre ML: Cytotoxic T lymphocyte antigen 4 (CTLA4) blockade accelerates the acute rejection of cardiac allografts in CD28-deficient mice: CTLA4 can function independently of CD28. J Exp Med 188, 199–204, 1998
                  35. Yamada A, Kishimoto K, Dong VM, Sho M, Anosova NG, Benichou G, Mandelbrot DM, Sharpe AH, Turka LA, Auchincloss HJ, Sayegh MH: CD28 independent costimulation of T cells in alloimmune responses. J Immunol 167: 140–146, 2001
                  36. Maier S, Tertilt C, Chambron N, Gerauer K, Huser N, Heidecke CD, Pfeffer K: Inhibition of natural killer cells results in acceptance of cardiac allografts in CD28−/− mice. Nat Med 7: 557–562, 2001
                  37. Trambley J, Bingaman AW, Lin A, Elwood ET, Waitze SY, Ha J, Durham MM, Corbascio M, Cowan SR, Pearson TC, Larsen CP: Asialo GM1(+) CD8(+) T cells play a critical role in costimulation blockade-resistant allograft rejection. J Clin Invest 104: 1715–1722, 1999
                  38. Reynolds J, Tam FW, Chandraker A, Smith J, Karkar AM, Cross J, Peach R, Sayegh MH, Pusey CD: CD28-B7 blockade prevents the development of experimental autoimmune glomerulonephritis. J Clin Invest 105: 643–651, 2000
                  39. Liang B, Kashgarian MJ, Sharpe AH, Mamula MJ: Autoantibody responses and pathology regulated by B7–1 and B7–2 costimulation in MRL/lpr lupus. J Immunol 165: 3436–3443, 2000
                  40. Liang B, Gee RJ, Kashgarian MJ, Sharpe AH, Mamula MJ: B7 costimulation in the development of lupus: Autoimmunity arises either in the absence of B7.1/B7.2 or in the presence of anti-b7.1/B7.2 blocking antibodies. J Immunol 163: 2322–2329, 1999
                  41. Kuchroo VK, Das MP, Brown JA, Ranger AM, Zamvil SS, Sobel RA, Weiner HL, Nabavi N, Glimcher LH: B7–1 and B7–2 costimulatory molecules activate differentially the Th1/Th2 developmental pathways: Application to autoimmune disease therapy. Cell 80: 707–718, 1995
                  42. Gallon L, Chandraker A, Issazadeh S, Peach R, Linsley PS, Turka LA, Sayegh MH, Khoury SJ: Differential effects of B7–1 blockade in the rat experimental autoimmune encephalomyelitis model. J Immunol 159: 4212–4216, 1997
                  43. Schaub M, Issazadeh S, Stadlbauer TH, Peach R, Sayegh MH, Khoury SJ: Costimulatory signal blockade in murine relapsing experimental autoimmune encephalomyelitis. J Neuroimmunol 96: 158–166, 1999
                  44. Lenschow DJ, Ho SC, Sattar H, Rhee L, Gray G, Nabavi N, Herold KC, Bluestone JA: Differential effects of anti-B7–1 and anti-B7–2 monoclonal antibody treatment on the development of diabetes in the nonobese diabetic mouse. J Exp Med 181: 1145–1155, 1995
                  45. Shi FD, He B, Li H, Matusevicius D, Link H, Ljunggren HG: Differential requirements for CD28 and CD40 ligand in the induction of experimental autoimmune myasthenia gravis. Eur J Immunol 28: 3587–3593, 1998
                  46. Tada Y, Nagasawa K, Ho A, Morito F, Ushiyama O, Suzuki N, Ohta H, Mak TW: CD28-deficient mice are highly resistant to collagen-induced arthritis. J Immunol 162: 203–208, 1999
                    47. Peterson KE, Sharp GC, Tang H, Braley-Mullen H: B7.2 has opposing roles during the activation versus effector stages of experimental autoimmune thyroiditis. J Immunol 162: 1859–1867, 1999
                      48. Shao H, Woon MD, Nakamura S, Sohn JH, Morton PA, Bora NS, Kaplan HJ: Requirement of B7-mediated costimulation in the induction of experimental autoimmune anterior uveitis. Invest Ophthalmol Vis Sci 42: 2016–2021, 2001
                      49. Chitnis T, Najafian N, Abdallah KA, Dong V, Yagita H, Sayegh MH, Khoury SJ: CD28-independent induction of experimental autoimmune encephalomyelitis. J Clin Invest 107: 575–583, 2001
                      50. Abrams JR, Lebwohl MG, Guzzo CA, Jegasothy BV, Goldfarb MT, Goffe BS, Menter A, Lowe NJ, Krueger G, Brown MJ, Weiner RS, Birkhofer MJ, Warner GL, Berry KK, Linsley PS, Krueger JG, Ochs HD, Kelley SL, Kang S: CTLA4Ig-mediated blockade of T-cell costimulation in patients with psoriasis vulgaris. J Clin Invest 103: 1243–1252, 1999
                      51. Abrams JR, Kelley SL, Hayes E, Kikuchi T, Brown MJ, Kang S, Lebwohl MG, Guzzo CA, Jegasothy BV, Linsley PS, Krueger JG: Blockade of T lymphocyte costimulation with cytotoxic T lymphocyte- associated antigen 4-immunoglobulin (CTLA4Ig) reverses the cellular pathology of psoriatic plaques, including the activation of keratinocytes, dendritic cells, and endothelial cells. J Exp Med 192: 681–694, 2000
                      52. Sayegh MH: Finally, CTLA4Ig graduates to the clinic. J Clin Invest 103: 1223–1225, 1999
                      53. Noelle RJ: CD40 and its ligand in host defense. Immunity 4: 415–419, 1996
                      54. Jain A, Atkinson TP, Lipsky PE, Slater JE, Nelson DL, Strober W: Defects of T-cell effector function and post-thymic maturation in X- linked hyper-IgM syndrome. J Clin Invest 103: 1151–1158, 1999
                      55. Klaus SJ, Pinchuk LM, Ochs HD, Law CL, Fanslow WC, Armitage RJ, Clark EA: Costimulation through CD28 enhances T cell-dependent B cell activation via CD40-CD40L interaction. J Immunol 152: 5643–5652, 1994
                      56. Ranheim EA, Kipps TJ: Activated T cells induce expression of B7/BB1 on normal or leukemic B cells through a CD40-dependent signal. J Exp Med 177: 925–935, 1993
                      57. Larsen CP, Alexander DZ, Hollenbaugh D, Elwood ET, Ritchie SC, Aruffo A, Hendrix R, Pearson TC: CD40-gp39 interactions play a critical role during allograft rejection. Suppression of allograft rejection by blockade of the CD40-gp39 pathway. Transplantation 61: 4–9, 1996
                      58. Hancock WW, Sayegh MH, Zheng XG, Peach R, Linsley PS, Turka LA: Costimulatory function and expression of CD40 ligand. CD80 and CD86 in vascularized murine cardiac allograft rejection. Proc Natl Acad Sci USA 93: 13967–13972, 1996
                      59. Parker DC, Greiner DL, Phillips NE, Appel MC, Steele AW, Durie FH, Noelle RJ, Mordes JP, Rossini AA: Survival of mouse pancreatic islet allografts in recipients treated with allogeneic small lymphocytes and antibody to CD40 ligand. Proc Natl Acad Sci USA 92: 9560–9564, 1995
                      60. Hancock WW, Buelow R, Sayegh MH, Turka LA: Antibody-induced transplant arteriosclerosis is prevented by graft expression of anti-oxidant and anti-apoptotic genes [In Process Citation]. Nat Med 4: 1392–1396, 1998
                      61. Ensminger SM, Witzke O, Spriewald BM, Morrison K, Morris PJ, Rose ML, Wood KJ: CD8+ T cells contribute to the development of transplant arteriosclerosis despite CD154 blockade. Transplantation 69: 2609–2612, 2000
                      62. Larsen CP, Elwood ET, Alexander DZ, Ritchie SC, Hendrix R, Tucker-Burden C, Cho HR, Aruffo A, Hollenbaugh D, Linsley PS, Winn KJ, Pearson TC: Long-term acceptance of skin and cardiac allografts after blocking CD40 and CD28 pathways. Nature 381: 434–438, 1996
                      63. Shimizu K, Schonbeck U, Mach F, Libby P, Mitchell RN: Host CD40 ligand deficiency induces long-term allograft survival and donor-specific tolerance in mouse cardiac transplantation but does not prevent graft arteriosclerosis. J Immunol 165: 3506–3518, 2000
                      64. Wekerle T, Sayegh MH, Hill J, Zhao Y, Chandraker A, Swenson KG, Zhao G, Sykes M: Extrathymic T cell deletion and allogeneic stem cell engraftment induced with costimulatory blockade is followed by central T cell tolerance. J Exp Med 187: 2037–2044, 1998
                      65. Wekerle T, Kurtz J, Ito H, Ronquillo JV, Dong V, Zhao G, Shaffer J, Sayegh MH, Sykes M: Allogeneic bone marrow transplantation with co-stimulatory blockade induces macrochimerism and tolerance without cytoreductive host treatment. Nat Med 6: 464–469, 2000
                      66. Durham MM, Bingaman AW, Adams AB, Ha J, Waitze SY, Pearson TC, Larsen CP: Cutting edge: administration of anti-CD40 ligand and donor bone marrow leads to hemopoietic chimerism and donor-specific tolerance without cytoreductive conditioning. J Immunol 165: 1–4, 2000
                      67. Kirk AD, Harlan DM, Armstrong NN, Davis TA, Dong Y, Gray GS, Hong X, Thomas D, Fechner JH, Jr, Knechtle SJ: CTLA4-Ig and anti-CD40 ligand prevent renal allograft rejection in primates. Proc Natl Acad Sci USA 94: 8789–8794, 1997
                      68. Kirk AD, Burkly LC, Batty DS, Baumgartner RE, Berning JD, Buchanan K, Fechner JH, Jr, Germond RL, Kampen RL, Patterson NB, Swanson SJ, Tadaki DK, TenHoor CN, White L, Knechtle SJ, Harlan DM: Treatment with humanized monoclonal antibody against CD154 prevents acute renal allograft rejection in nonhuman primates. Nat Med 5: 686–693, 1999
                      69. Levisetti MG, Padrid PA, Szot GL, Mittal N, Meehan SM, Wardrip CL, Gray GS, Bruce DS, Thistlethwaite JR, Jr, Bluestone JA: Immunosuppressive effects of human CTLA4Ig in a non-human primate model of allogeneic pancreatic islet transplantation. J Immunol 159: 5187–5191, 1997
                      70. Kenyon NS, Chatzipetrou M, Masetti M, Ranuncoli A, Oliveira M, Wagner JL, Kirk AD, Harlan DM, Burkly LC, Ricordi C: Long-term survival and function of intrahepatic islet allografts in rhesus monkeys treated with humanized anti-CD154 [In Process Citation]. Proc Natl Acad Sci USA 96: 8132–8137, 1999
                        71. Kenyon NS, Fernandez LA, Lehmann R, Masetti M, Ranuncoli A, Chatzipetrou M, Iaria G, Han D, Wagner JL, Ruiz P, Berho M, Inverardi L, Alejandro R, Mintz DH, Kirk AD, Harlan DM, Burkly LC, Ricordi C: Long-term survival and function of intrahepatic islet allografts in baboons treated with humanized anti-CD154. Diabetes 48: 1473–1481, 1999
                        72. Yamada A, Sayegh MH: CD154-CD40 pathway in transplantation. Transplantation 73: 51–55, 2002
                        73. Kawai T, Andrews D, Colvin RB, Sachs DH, Cosimi AB: Thromboembolic complications after treatment with monoclonal antibody against CD40 ligand. Nat Med 6: 114, 2000
                        74. Smiley ST, Csizmadia V, Gao W, Turka LA, Hancock WW: Differential effects of cyclosporine A, methylprednisolone, mycophenolate, and rapamycin on CD154 induction and requirement for NFkappaB: Implications for tolerance induction. Transplantation 70: 415–419, 2000
                        75. Li Y, Li XC, Zheng XX, Wells AD, Turka LA, Strom TB: Blocking both signal 1 and signal 2 of T-cell activation prevents apoptosis of alloreactive T cells and induction of peripheral allograft tolerance. Nat Med 5: 1298–1302, 1999
                        76. Li XC, Strom TB, Turka LA, Wells AD: T cell death and transplantation tolerance. Immunity 14: 407–416, 2001
                        77. Fecteau S, Basadonna GP, Freitas A, Ariyan C, Sayegh MH, Rothstein DM: CTLA-4 up-regulation plays a role in tolerance mediated by CD45. Nat Immunol 2: 58–63, 2001
                        78. Sho M, Najafian N, Salama AD, Yamada A, Sayegh MH: New insights into the interaction between costimulation blockade and conventional immunosuppression in vivo. Am J Transplantation 1: 142A, 2001
                        79. Yuan X, Dong VM, Coito AJ, Wagga AM, Lenhard M, Chandraker A, Benjamin CD, SAyegh MH: A novel CD154 monoclonal antibody in acute and chronic rat vascularised cardiac allograft rejection. Am J Transplant 1: 244A, 2001
                        80. Daikh DI, Finck BK, Linsley PS, Hollenbaugh D, Wofsy D: Long-term inhibition of murine lupus by brief simultaneous blockade of the B7/CD28 and CD40/gp39 costimulation pathways. J Immunol 159: 3104–3108, 1997
                        81. Kover KL, Geng Z, Hess DM, Benjamin CD, Moore WV: Anti-CD154 (CD40L) prevents recurrence of diabetes in islet isografts in the DR-BB rat. Diabetes 49: 1666–1670, 2000
                        82. Molano RD, Berney T, Li H, Cattan P, Pileggi A, Vizzardelli C, Kenyon NS, Ricordi C, Burkly LC, Inverardi L: Prolonged islet graft survival in NOD mice by blockade of the CD40- CD154 pathway of T-cell costimulation. Diabetes 50: 270–276, 2001
                        83. Balasa B, Krahl T, Patstone G, Lee J, Tisch R, McDevitt HO, Sarvetnick N: CD40 ligand-CD40 interactions are necessary for the initiation of insulitis and diabetes in nonobese diabetic mice. J Immunol 159: 4620–4627, 1997
                        84. Becher B, Durell BG, Miga AV, Hickey WF, Noelle RJ: The clinical course of experimental autoimmune encephalomyelitis and inflammation is controlled by the expression of CD40 within the central nervous system. J Exp Med 193: 967–974, 2001
                        85. Watts TH, DeBenedette MA: T cell co-stimulatory molecules other than CD28. Curr Opin Immunol 11: 286–293, 1999
                        86. Hutloff A, Dittrich AM, Beier KC, Eljaschewitsch B, Kraft R, Anagnostopoulos I, Kroczek RA: ICOS is an inducible T-cell co-stimulator structurally and functionally related to CD28. Nature 397: 263–266, 1999
                        87. Yoshinaga SK, Whoriskey JS, Khare SD, Sarmiento U, Guo J, Horan T, Shih G, Zhang M, Coccia MA, Kohno T, Tafuri-Bladt A, Brankow D, Campbell P, Chang D, Chiu L, Dai T, Duncan G, Elliott GS, Hui A, McCabe SM, Scully S, Shahinian A, Shaklee CL, Van G, Mak TW: T-cell co-stimulation through B7RP-1 and ICOS. Nature 402: 827–832, 1999
                        88. Brodie D, Collins AV, Iaboni A, Fennelly JA, Sparks LM, Xu XN, van der Merwe PA, Davis SJ: LICOS, a primordial costimulatory ligand? Curr Biol 10: 333–336, 2000
                        89. Peach RJ, Bajorath J, Brady W, Leytze G, Greene J, Naemura J, Linsley PS: Complementarity determining region 1 (CDR1)- and CDR3-analogous regions in CTLA-4 and CD28 determine the binding to B7–1. J Exp Med 180: 2049–2058, 1994
                        90. Aicher A, Hayden-Ledbetter M, Brady WA, Pezzutto A, Richter G, Magaletti D, Buckwalter S, Ledbetter JA, Clark EA: Characterization of human inducible costimulator ligand expression and function. J Immunol 164: 4689–4696, 2000
                        91. Ling V, Wu PW, Finnerty HF, Bean KM, Spaulding V, Fouser LA, Leonard JP, Hunter SE, Zollner R, Thomas JL, Miyashiro JS, Jacobs KA, Collins M: Cutting edge: Identification of GL50, a novel B7-like protein that functionally binds to ICOS receptor. J Immunol 164: 1653–1657, 2000
                          92. Swallow MM, Wallin JJ, Sha WC: B7h, a novel costimulatory homolog of B7.1 and B7.2, is induced by TNFalpha. Immunity 11: 423–432, 1999
                            93. Mages HW, Hutloff A, Heuck C, Buchner K, Himmelbauer H, Oliveri F, Kroczek RA: Molecular cloning and characterization of murine ICOS and identification of B7h as ICOS ligand. Eur J Immunol 30: 1040–1047, 2000
                            94. Coyle AJ, Lehar S, Lloyd C, Tian J, Delaney T, Manning S, Nguyen T, Burwell T, Schneider H, Gonzalo JA, Gosselin M, Owen LR, Rudd CE, Gutierrez-Ramos JC: The CD28-related molecule ICOS is required for effective T cell–dependent immune responses. Immunity 13: 95–105, 2000
                            95. Coyle AJ, Gutierrez-Ramos JC: The expanding B7 superfamily: Increasing complexity in costimulatory signals regulating T cell function. Nat Immunol 2: 203–209, 2001
                            96. McAdam AJ, Chang TT, Lumelsky AE, Greenfield EA, Boussiotis VA, Duke-Cohan JS, Chernova T, Malenkovich N, Jabs C, Kuchroo VK, Ling V, Collins M, Sharpe AH, Freeman GJ: Mouse inducible costimulatory molecule (ICOS) expression is enhanced by CD28 costimulation and regulates differentiation of CD4+ T cells. J Immunol 165: 5035–5040, 2000
                            97. Ozkaynak E, Gao W, Shemmeri N, Wang C, Gutierrez-Ramos JC, Amaral J, Qin S, Rottman JB, Coyle AJ, Hancock WW: Importance of ICOS-B7RP-1 costimulation in acute and chronic allograft rejection. Nat Immunol 2: 591–596, 2001
                            98. Wahl P, Bilic G, Neuweiler J, Yoshinaga SK, Wuthrich RP: B7RP-1 a novel renal tubular epithelial antigen with costimulatory function. J Am Soc Nephrol 12: 643A, 2001
                            99. Kopf M, Coyle AJ, Schmitz N, Barner M, Oxenius A, Gallimore A, Gutierrez-Ramos JC, Bachmann MF: Inducible costimulator protein (ICOS) controls T helper cell subset polarization after virus and parasite infection. J Exp Med 192: 53–61, 2000
                            100. McAdam AJ, Greenwald RJ, Levin MA, Chernova T, Malenkovich N, Ling V, Freeman GJ, Sharpe AH: ICOS is critical for CD40-mediated antibody class switching. Nature 409: 102–105, 2001
                            101. Dong C, Juedes AE, Temann UA, Shresta S, Allison JP, Ruddle NH, Flavell RA: ICOS co-stimulatory receptor is essential for T-cell activation and function. Nature 409: 97–101, 2001
                            102. Rottman JB, Smith T, Tonra JR, Ganley K, Bloom T, Silva R, Pierce B, Gutierrez-Ramos JC, Ozkaynak E, Coyle AJ: The costimulatory molecule ICOS plays an important role in the immunopathogenesis of EAE. Nat Immunol 2: 605–611, 2001
                            103. Freeman GJ, Long AJ, Iwai Y, Bourque K, Chernova T, Nishimura H, Fitz LJ, Malenkovich N, Okazaki T, Byrne MC, Horton HF, Fouser L, Carter L, Ling V, Bowman MR, Carreno BM, Collins M, Wood CR, Honjo T: Engagement of the PD-1 immunoinhibitory receptor by a novel B7 family member leads to negative regulation of lymphocyte activation. J Exp Med 192: 1027–1034, 2000
                            104. Latchman Y, Wood CR, Chernova T, Chaudhary D, Borde M, Chernova I, Iwai Y, Long AJ, Brown JA, Nunes R, Greenfield EA, Bourque K, Boussiotis VA, Carter LL, Carreno BM, Malenkovich N, Nishimura H, Okazaki T, Honjo T, Sharpe AH, Freeman GJ: PD-L2 is a second ligand for PD-I and inhibits T cell activation. Nat Immunol 2: 261–268, 2001
                            105. Nishimura H, Nose M, Hiai H, Minato N, Honjo T: Development of lupus-like autoimmune diseases by disruption of the PD-1 gene encoding an ITIM motif-carrying immunoreceptor. Immunity 11: 141–151, 1999
                            106. Nishimura H, Honjo T, Minato N: Facilitation of beta selection and modification of positive selection in the thymus of PD-1-deficient mice. J Exp Med 191: 891–898, 2000
                            107. Pakala SV, Ilic A, Chen L, Sarvetnick N: TNF-alpha receptor 1 (p55) on islets is necessary for the expression of LIGHT on diabetogenic T cells. Clin Immunol 100, 198–207, 2001
                            108. Pollok KE, Kim YJ, Zhou Z, Hurtado J, Kim KK, Pickard RT, Kwon BS: Inducible T cell antigen 4–1BB. Analysis of expression and function. J Immunol 150: 771–781, 1993
                            109. Goodwin RG, Din WS, Davis-Smith T, Anderson DM, Gimpel SD, Sato TA, Maliszewski CR, Brannan CI, Copeland NG, Jenkins NA, Farrah T, Armitage RJ, Fanslow WC, Smith CA: Molecular cloning of a ligand for the inducible T cell gene 4–1BB: a member of an emerging family of cytokines with homology to tumor necrosis factor. Eur J Immunol 23: 2631–2641, 1993
                            110. Melero I, Johonston JV, Shufford WW, Mittler RS, Chen L: NK1.1 cells express 4–1BB (CDw137) costimulatory molecule and are required for tumor immunity elicited by anti-4–1BB monoclonal antibodies. Cell Immunol 190: 167–172, 1998
                            111. Hurtado JC, Kim YJ, Kwon BS: Signals through 4–1BB are costimulatory to previously activated T cells and inhibit activation-induced cell death. J Immunol 158: 2600–2609, 1996
                            112. DeBenedette MA, Shahinian A, Mak TW, Watts TH: Costimulation of CD28- Tlymphocytes by 4–1BB ligand. J Immunol 158: 551–559, 1997
                            113. Pollok KE, Kim YJ, Hurtado J, Zhou Z, Kim KK, Kwon BS: 4–1BB T-cell antigen binds to mature B cells and macrophages, and costimulates anti-mu-primed splenic B cells. Eur J Immunol 24: 367–374, 1994
                            114. Michel J, Pauly S, Langstein J, Krammer PH, Schwarz H: CD137-induced apoptosis is independent of CD95. Immunology 98: 42–46, 1999
                            115. Shuford WW, Klussman K, Tritchler DD, Loo DT, Chalupny J, Siadak AW, Brown TJ, Emswiler J, Raecho H, Larsen CP, Pearson TC, Ledbetter JA, Aruffo A, Mittler RS: 4–1BB costimulatory signals preferentially induce CD8+ T cell proliferation and lead to the amplification in vivo of cytotoxic T cell response. J Exp Med 186: 47–55, 1997
                            116. Takahashi T, Mittler RS, Vella AT: Cutting Edge: 4–1BB is a bona fide CD8 T cell survival signal. J Immunol 162: 5037–5040, 1999
                            117. DeBenedette MA, Wen T, Bachmann MF, Ohashi PS, Barber BH, Stocking KL, Peschon JJ, Watts TH: Analysis of 4–1BB ligand (4–1BBL)-deficient mice and of mice lacking both 4–1BBL and CD28 reveals a role for 4–1BBL in skin allograft rejection and in the cytotoxic T cell response to influenza virus. J Immunol 163: 4833–4841, 1999
                            118. Gramaglia I, Cooper D, Miner KT, Kwon BS, Croft M: Co-stimulation of antigen-specific CD4 T cells by 4–1BB ligand. Eur J Immunol 30: 392–402, 2000
                            119. Mittler RS, Bailey TS, Klussman K, Trailsmith MD, Hoffmann MK: Anti-4–1BB monoclonal antibodies abrogate T cell-dependent humoral immune responses in vivo through the induction of helper T cell anergy. J Exp Med 190: 1535–1540, 1999
                            120. Kim YJ, Kim SH, Mantel P, S. KB: Human 4–1BB regulates CD28 co-stimulation to promote Th1 cell responses. Eur J Immunol 28: 881–890, 1998
                            121. Abe R, Vandenberghe P, Craighead N, Smoot DS, Lee KP, June CH: Distinct signal transduction in mouse CD4+ and CD8+ splenic T cells after CD28 receptor ligation. J Immunol 154: 985–997, 1995
                            122. Cannons JL, Lau P, Ghumman B, DeBenedette MA, Yagita H, Okumura K, Watts TH: 4–1Bb ligand induces cell division, sustains survival, and enhances effector function of cd4 and cd8 t cells with similar efficacy. J Immunol 167: 1313–1324, 2001
                            123. Chu NR, DeBenedette MA, Stiernholm BJN, Barber BH, Watts TH: Role of IL-12 and 4–1BB ligand in cytokine production by CD28+ and CD28+ T cells. J Immunol 158: 3081–3089, 1997
                            124. Tan JT, Ha J, Cho HR, Tucker-Burden C, Hendrix RC, Mittler RS, Pearson TC, Larsen CP: Analysis of expression and function of the costimulatory molecule 4–1BB in alloimmune responses. Transplantation 70: 175–183, 2000
                            125. Guinn BA, DeBenedette MA, Watts TH, Berinstein NL: 4–1BBL cooperates with B7–1 and B7–2 in converting a B cell lymphoma cell line into a long-lasting antitumor vaccine. J Immunol 162: 5003–5010, 1999
                            126. Guinn BA, Bertram EM, DeBenedette MA, Berinstein NL, Watts TH: 4–1BBL enhances anti-tumor responses in the presence or absence of CD28 but CD28 is required for protective immunity against parental tumors. Cell Immunol 210: 56–65, 2001
                            127. Hurtado JC, Kim SH, Pollok KE, Lee ZH, Kwon BS: Potential role of 4–1BB in T cell activation. J Immunol 155: 3360–3367, 1995
                            128. DeBenedette MA, Chu NR, Pollok KE, Hurtado J, Wade WF, Kwon BS, Watts TH: Role of 4–1BB ligand in costimulation of T lymphocyte growth and its upregulation on M12 B lymphomas by cAMP. J Exp Med 181: 985–992, 1995
                            129. Schwab U, Stein H, Gerdes J, Lemke H, Kirchner H, Schaadt M, Diehl V: Production of a monoclonal antibody specific for Hodgkin and Sternberg-Reed cells of Hodgkin’s disease and a subset of normal lymphoid cells. Nature 299: 65–67, 1982
                            130. Bowen MA, Lee RK, Miragliotta G, Nam SY, Podack ER: Structure and expression of murine CD30 and its role in cytokine production. J Immunol 156: 442–449, 1996
                            131. Gilfillan MC, Noel PJ, Podack ER, Reiner SL, Thompson CB: Expression of the costimulatory receptor CD30 is regulated by both CD28 and cytokines. J Immunol 160: 2180–2187, 1998
                            132. Schwarting R, Gerdes J, Durkop H, Falini B, Pileri S, Stein H: BER-H2: A new anti-Ki-1 (CD30) monoclonal antibody directed at a formol-resistant epitope. Blood 74: 1678–1689, 1989
                              133. Telford WG, Nam SY, Podack ER, Miller RA: CD30-regulated apoptosis in murine CD8 T cells after cessation of TCR signals. Cell Immunol 182: 125–136, 1997
                              134. Hamann D, Hilkens CM, Grogan JL, Lens SM, Kapsenberg ML, Yazdanbakhsh M, van Lier RA: CD30 expression does not discriminate between human Th1- and Th2-type T cells. J Immunol 156: 1387–1391, 1996
                              135. Gruss HJ, Boiani N, Williams DE, Armitage RJ, Smith CA, Goodwin RG: Pleiotropic effects of the CD30 ligand on CD30-expressing cells and lymphoma cell lines. Blood 83: 2045–2056, 1994
                              136. Smith CA, Gruss HJ, Davis T, Anderson D, Farrah T, Baker E, Sutherland GR, Brannan CI, Copeland NG, Jenkins NA, Grabstein KH, Gliniak B, McAlister IB, Fanslow W, Alderson M, Falk B, Gimpel S, Gillis S, Din WS, Goodwin RG, Armitage RJ: CD30 antigen, a marker for Hodgkin’s lymphoma, is a receptor whose ligand defines an emerging family of cytokines with homology to TNF. Cell 73: 1349–1360, 1993
                              137. Amakawa R, Hakem A, Kundig TM, Matsuyama T, Simard JJ, Timms E, Wakeham A, Mittruecker HW, Griesser H, Takimoto H, Schmits R, Shahinian A, Ohashi P, Penninger JM, Mak TW: Impaired negative selection of T cells in Hodgkin’s disease antigen CD30-deficient mice. Cell 84: 551–562, 1996
                              138. Lee SY, Kandala G, Liou ML, Liou HC, Choi Y: CD30/TNF receptor-associated factor interaction: NF-kappa B activation and binding specificity. Proc Natl Acad Sci USA 93: 9699–9703, 1996
                              139. Lee SY, Park CG, Choi Y: T cell receptor-dependent cell death of T cell hybridomas mediated by the CD30 cytoplasmic domain in association with tumor necrosis factor receptor-associated factors. J Exp Med 183: 669–674, 1996
                              140. Del Prete G, De Carli M, Almerigogna F, Daniel CK, D’Elios MM, Zancuoghi G, Vinante F, Pizzolo G, Romagnani S: Preferential expression of CD30 by human CD4+ T cells producing Th2-type cytokines. Faseb J 9: 81–86, 1995
                              141. Akiba H, Miyahira Y, Atsuta M, Takeda K, Nohara C, Futagawa T, Matsuda H, Aoki T, Yagita H, Okumura K: Critical contribution of OX40 ligand to T helper cell type 2 differentiation in experimental leishmaniasis. J Exp Med 191: 375–380, 2000
                              142. Kurts C, Carbone FR, Krummel MF, Koch KM, Miller JF, Heath WR: Signalling through CD30 protects against autoimmune diabetes mediated by CD8 T cells. Nature 398: 341–344, 1999
                              143. Beckmann J, Kurts C, Klebba I, Bayer B, Klempnauer J, Hoffmann MW: The role of CD30 in skin and heart allograft rejection in the mouse. Transplant Proc 33: 140–141, 2001
                              144. Mallett S, Fossum S, Barclay AN: Characterization of the MRC OX40 antigen of activated CD4 positive T lymphocytes–a molecule related to nerve growth factor receptor. Embo J 9: 1063–1068, 1990
                              145. Calderhead DM, Buhlmann JE, van den Eertwegh AJ, Claassen E, Noelle RJ, Fell HP: Cloning of mouse Ox40: A T cell activation marker that may mediate T-B cell interactions. J Immunol 151: 5261–5271, 1993
                              146. Latza U, Durkop H, Schnittger S, Ringeling J, Eitelbach F, Hummel M, Fonatsch C, Stein H: The human OX40 homolog: cDNA structure, expression and chromosomal assignment of the ACT35 antigen. Eur J Immunol 24: 677–683, 1994
                              147. Baum PR, Gayle RB,3rd, Ramsdell F, Srinivasan S, Sorensen RA, Watson ML, Seldin MF, Clifford KN, Grabstein K, Alderson MR, Goodwin RG, Fanslow WC: Identification of OX40 ligand and preliminary characterization of its activities on OX40 receptor. Circ Shock 44: 30–34, 1994
                              148. Godfrey WR, Fagnoni FF, Harara MA, Buck D, Engleman EG: Identification of a human OX-40 ligand, a costimulator of CD4+ T cells with homology to tumor necrosis factor. J Exp Med 180: 757–762, 1994
                              149. Weinberg AD: Antibodies to OX-40 (CD134) can identify and eliminate autoreactive T cells: implications for human autoimmune disease. Mol Med Today 4: 76–83, 1998
                              150. Durkop H, Latza U, Himmelreich P, Stein H: Expression of the human OX40 (hOX40) antigen in normal and neoplastic tissues. Br J Haematol 91: 927–931, 1995
                              151. Stuber E, Neurath M, Calderhead D, Fell HP, Strober W: Cross-linking of OX40 ligand, a member of the TNF/NGF cytokine family, induces proliferation and differentiation in murine splenic B cells. Immunity 2: 507–521, 1995
                              152. Ohshima Y, Tanaka Y, Tozawa H, Takahashi Y, Maliszewski C, Delespesse G: Expression and function of OX40 ligand on human dendritic cells. J Immunol 159: 3838–3848, 1997
                              153. Imura A, Hori T, Imada K, Ishikawa T, Tanaka Y, Maeda M, Imamura S, Uchiyama T: The human OX40/gp34 system directly mediates adhesion of activated T cells to vascular endothelial cells. J Exp Med 183: 2185–2195, 1996
                              154. Akiba H, Atsuta M, Yagita H, Okumura K: Identification of rat OX40 ligand by molecular cloning. Biochem Biophys Res Commun 251: 131–136, 1998
                              155. Gramaglia I, Weinberg AD, Lemon M, Croft M: Ox-40 ligand: A potent costimulatory molecule for sustaining primary CD4 T cell responses. J Immunol 161: 6510–6517, 1998
                              156. Walker LS, Gulbranson-Judge A, Flynn S, Brocker T, Raykundalia C, Goodall M, Forster R, Lipp M, Lane P: Compromised OX40 function in CD28-deficient mice is linked with failure to develop CXC chemokine receptor 5-positive CD4 cells and germinal centers. J Exp Med 190: 1115–1122, 1999
                              157. Gramaglia I, Jember A, Pippig SD, Weinberg AD, Killeen N, Croft M: The OX40 costimulatory receptor determines the development of CD4 memory by regulating primary clonal expansion. J Immunol 165: 3043–3050, 2000
                              158. Chen AI, McAdam AJ, Buhlmann JE, Scott S, Lupher ML, Jr, Greenfield EA, Baum PR, Fanslow WC, Calderhead DM, Freeman GJ, Sharpe AH: Ox40-ligand has a critical costimulatory role in dendritic cell: T cell interactions. Immunity 11: 689–698, 1999
                              159. Kopf M, Ruedl C, Schmitz N, Gallimore A, Lefrang K, Ecabert B, Odermatt B, Bachmann MF: OX40-deficient mice are defective in Th cell proliferation but are competent in generating B cell and CTL Responses after virus infection. Immunity 11: 699–708, 1999
                                160. Murata K, Ishii N, Takano H, Miura S, Ndhlovu LC, Nose M, Noda T, Sugamura K: Impairment of antigen-presenting cell function in mice lacking expression of OX40 ligand. J Exp Med 191: 365–374, 2000
                                  161. Pippig SD, Pena-Rossi C, Long J, Godfrey WR, Fowell DJ, Reiner SL, Birkeland ML, Locksley RM, Barclay AN, Killeen N: Robust B cell immunity but impaired T cell proliferation in the absence of CD134 (OX40). J Immunol 163: 6520–6529, 1999
                                  162. Stuber E, Von Freier A, Marinescu D, Folsch UR: Involvement of OX40-OX40L interactions in the intestinal manifestations of the murine acute graft-versus-host disease. Gastroenterology 115: 1205–1215, 1998
                                  163. Brocker T, Gulbranson-Judge A, Flynn S, Riedinger M, Raykundalia C, Lane P: CD4 T cell traffic control: in vivo evidence that ligation of OX40 on CD4 T cells by OX40-ligand expressed on dendritic cells leads to the accumulation of CD4 T cells in B follicles. Eur J Immunol 29: 1610–1616, 1999
                                  164. Jember AG, Zuberi R, Liu FT, Croft M: Development of allergic inflammation in a murine model of asthma is dependent on the costimulatory receptor OX40. J Exp Med 193: 387–392, 2001
                                  165. Flynn S, Toellner KM, Raykundalia C, Goodall M, Lane P: CD4 T cell cytokine differentiation: The B cell activation molecule, OX40 ligand, instructs CD4 T cells to express interleukin 4 and upregulates expression of the chemokine receptor, Blr-1. J Exp Med 188: 297–304, 1998
                                  166. Bansal-Pakala P, Jember AG, Croft M: Signaling through OX40 (CD134) breaks peripheral T-cell tolerance. Nat Med 7: 907–912, 2001
                                  167. Weinberg AD, Lemon M, Jones AJ, Vainiene M, Celnik B, Buenafe AC, Culbertson N, Bakke A, Vandenbark AA, Offner H: OX-40 antibody enhances for autoantigen specific V beta 8.2+ T cells within the spinal cord of Lewis rats with autoimmune encephalomyelitis. J Neurosci Res 43: 42–49, 1996
                                  168. Weinberg AD, Wegmann KW, Funatake C, Whitham RH: Blocking OX-40/OX-40 ligand interaction in vitro and in vivo leads to decreased T cell function and amelioration of experimental allergic encephalomyelitis. J Immunol 162: 1818–1826, 1999
                                  169. Higgins LM, McDonald SA, Whittle N, Crockett N, Shields JG, MacDonald TT: Regulation of T cell activation in vitro and in vivo by targeting the OX40-OX40 ligand interaction: amelioration of ongoing inflammatory bowel disease with an OX40-IgG fusion protein, but not with an OX40 ligand-IgG fusion protein. J Immunol 162: 486–493, 1999
                                  170. Nohara C, Akiba H, Nakajima A, Inoue A, Koh CS, Ohshima H, Yagita H, Mizuno Y, Okumura K: Amelioration of experimental autoimmune encephalomyelitis with anti- OX40 ligand monoclonal antibody: A critical role for OX40 ligand in migration, but not development, of pathogenic T cells. J Immunol 166: 2108–2115, 2001
                                  171. Ndhlovu LC, Ishii N, Murata K, Sato T, Sugamura K: Critical involvement of ox40 ligand signals in the t cell priming events during experimental autoimmune encephalomyelitis. J Immunol 167: 2991–2999, 2001
                                  172. Akiba H, Oshima H, Takeda K, Atsuta M, Nakano H, Nakajima A, Nohara C, Yagita H, Okumura K: CD28-independent costimulation of T cells by OX40 ligand and CD70 on activated B cells. J Immunol 162: 7058–7066, 1999
                                  173. Gravestein LA, Amsen D, Boes M, Calvo CR, Kruisbeek AM, Borst J: The TNF receptor family member CD27 signals to Jun N-terminal kinase via Traf-2. Eur J Immunol 28: 2208–2216, 1998
                                  174. Takeda K, Oshima H, Hayakawa Y, Akiba H, Atsuta M, Kobata T, Kobayashi K, Ito M, Yagita H, Okumura K: CD27-mediated activation of murine NK cells. J Immunol 164: 1741–1745, 2000
                                  175. Tesselaar K, Gravestein LA, van Schijndel GM, Borst J, van Lier RA: Characterization of murine CD70, the ligand of the TNF receptor family member CD27. J Immunol 159: 4959–4965, 1997
                                  176. Oshima H, Nakano H, Nohara C, Kobata T, Nakajima A, Jenkins NA, Gilbert DJ, Copeland NG, Muto T, Yagita H, Okumura K: Characterization of murine CD70 by molecular cloning and mAb. Int Immunol 10: 517–526, 1998
                                  177. Hendriks J, Gravestein LA, Tesselaar K, van Lier RA, Schumacher TN, Borst J: CD27 is required for generation and long-term maintenance of T cell immunity. Nat Immunol 1: 433–440, 2000
                                  178. Schmitter D, Bolliger U, Hallek M, Pichert G: Involvement of the CD27-CD70 co-stimulatory pathway in allogeneic T- cell response to follicular lymphoma cells. Br J Haematol 106: 64–70, 1999
                                  179. Jacquot S: CD27/CD70 interactions regulate T dependent B cell differentiation. Immunol Res 21: 23–30, 2000
                                  180. Nakajima A, Oshima H, Nohara C, Morimoto S, Yoshino S, Kobata T, Yagita H, Okumura K: Involvement of CD70-CD27 interactions in the induction of experimental autoimmune encephalomyelitis. J Neuroimmunol 109: 188, 196., 2000
                                  181. Braun-Falco M, Hallek M: Recombinant adeno-associated virus (rAAV) vector-mediated cotransduction of CD70 and CD80 into human malignant melanoma cells results in an additive T-cell response. Arch Dermatol Res 293: 12–17, 2001
                                  182. Douin-Echinard V, Bornes S, Rochaix P, Tilkin AF, Peron JM, Bonnet J, Favre G, Couderc B: The expression of CD70 and CD80 by gene-modified tumor cells induces an antitumor response depending on the MHC status. Cancer Gene Ther 7: 1543–1556, 2000
                                  183. Salama AD, Remuzzi G, Harmon WE, Sayegh MH: Challenges to achieving clinical transplantation tolerance. J Clin Invest 108: 943–948, 2001
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