Secondary Logo

Transplant Tolerance: Converging on a Moving Target

Newell, Kenneth A.1,3; Larsen, Christian P.1; Kirk, Allan D.2

doi: 10.1097/01.tp.0000179149.12979.13
Editorials and Perspectives: Overview
Free

Enthusiasm for tolerance induction has been tempered by the realization that it is more difficult to achieve clinically than was predicted by experimental models. Unlike the view that the immune response to an allograft is ordered and thus predictable, we view alloimmunity as highly plastic and molded by previous and ongoing experiences with allogeneic and environmental antigens. This implies that an individual’s response to an allograft changes over time and that responses of seemingly similar individuals may vary greatly. This variability highlights the need to develop assays for monitoring the recipient immune response as well as individualized methods for therapeutic immune modulation.

1 The Emory Transplant Center, Emory University, Atlanta, GA.

2 Transplantation Branch, National Institute of Diabetes, Digestive and Kidney Diseases, National Institutes of Health, Department of Health and Human Services, Bethesda, MD.

3 Address correspondence to: Kenneth A. Newell, M.D., Ph.D., The Emory Transplant Center, Emory University, Atlanta, GA 30345.

E-mail: kenneth_newell@emoryhealthcare.org.

Submitted 3 April 2004. Revision requested 7 June 2004.

Accepted 4 July 2005.

The publisher apologizes for the delay of the publication of this article.

Back to Top | Article Outline

The Target: Transplantation Tolerance

The ability to consistently induce robust, sustained, donor-specific tolerance would offer many benefits to transplant recipients. However, although the theoretical basis for acquired tolerance was established in the mid-20th century, clinical success has been rare and unpredictable. Advances in our understanding of the mechanisms that mediate rejection and tolerance increasingly allow the induction of tolerance in many experimental models. This, together with a rapidly expanding armamentarium of more selective biologic immunosuppressants, has rekindled interest in transitioning tolerance to the clinic.

Balancing this enthusiasm is the realization that achieving tolerance has proven to be significantly more difficult in humans than in animals. Together with the improving outcome of clinical transplantation, this has caused some to question the need for transplantation tolerance. Nevertheless, chronic immunosuppression remains associated with expense, risks of infections and malignancies, and drug-specific toxicities including hypertension, glucose intolerance, and hypercholesterolemia. These unwanted consequences of chronic immunosuppression result in a 5–10 fold increase in the all-cause mortality of transplant recipients relative to the general population independent of the effects of rejection (1). The nephrotoxicity of calcineurin inhibitors also affects recipients of renal and nonrenal allografts and contributes to the increasingly common development of chronic allograft nephropathy and end-stage renal disease in recipients of extrarenal organs (2). Lastly, recent data suggest that, unlike immediate outcomes, long-term outcomes of transplantation may not be improving (3). Viewed as a whole, these problems continue to fuel enthusiasm for tolerance.

Back to Top | Article Outline

How is the Target Moving?

Barriers to Attaining Transplantation Tolerance

Several features of the immune system conspire against the development of transplantation tolerance. First, the innate immune system is designed to detect threats to homeostasis. The act of transplantation itself causes injury which in turn triggers responses by multiple components of the innate immune system. Two models have been proposed to describe how innate immune responses are initiated. The danger model hypothesizes that cellular components of the innate immune system respond to tissue injury by producing soluble mediators that perpetuate the inflammatory state and promote the maturation of adaptive immune responses (4). The pattern recognition receptor model postulates that highly conserved molecules expressed on the surface of injured cells engage receptors expressed by cells comprising the innate immune system (e.g., toll-like receptors) thereby triggering innate immunity (5).

A second factor that likely poses a barrier to tolerance is the unusually large proportion of the T-cell repertoire capable of recognizing alloantigens. Although it has been estimated that fewer than one in 100,000 T cells recognize a given nominal antigen, it has been reported that 7% of the T cell repertoire undergoes proliferation in response to alloantigens (6). This large clone size may mediate an early, aggressive immune response that irreversibly damages a transplanted organ before potentially protective responses can develop. Recognition of alloantigens can occur via two distinct pathways. The first and dominant process begins upon organ reperfusion when passenger leukocytes or APCs migrate to recipient secondary lymphoid organs where direct donor antigen recognition occurs (7, 8). It is now clear that naïve responses are critically dependent on secondary lymphoid organs and rejection is greatly attenuated in their absence (9, 10). As donor APCs are unlikely to be replenished, the direct response would be predicted to dissipate over time. At later time points, the recognition of alloantigens is believed to be primarily via the indirect pathway of antigen presentation (11). Indirect presentation refers to the recognition of donor peptides presented by recipient MHC molecules and APCs. The magnitude of the indirect response is significantly less than that of the direct response, possibly reflecting the lower clone size of T cells capable of recognizing indirectly presented antigens. It has been suggested that the indirect pathway plays a particularly important role in chronic rejection (12). Distinguishing between these two methods of alloantigen recognition may have therapeutic implications. For example, costimulation blockade has been reported to inhibit T cell priming via the indirect, but not direct, pathway (13).

The development of immunologic memory poses a third potential impediment to the development of transplantation tolerance. Memory cells differ from naïve cells in that they have higher functional avidity, lower activation thresholds, the ability to rapidly engage effector functions, and the potential to circulate widely through peripheral tissues. By virtue of these properties, allospecific memory cells represent a clear threat to transplanted organs. Until recently, many have viewed one’s initial encounter with an alloantigen as a naïve response. However, the growing recognition that alloimmunity is strongly influenced by heterologous responses to previously encountered antigens suggests that even first exposures to alloantigen provoke responses by crossreactive memory cells (14, 15). Thus, memory cells may be important mediators of allograft damage even in “naïve” recipients. Heterologous immunity is likely an important factor contributing to the different success rates of tolerance regimens in clinical transplantation versus experimental models that typically use young, specific-pathogen-free animals.

Homeostatic proliferation following therapies that cause massive T-cell depletion may represent a fourth barrier to tolerance (16). T cells from lymphopenic hosts undergo extensive proliferation. The phenotype and function of the re-emerging T cells is similar to that of memory T cells. Furthermore, memory T cells have been reported to be relatively resistant to depletion (17). Thus, following massive T-cell depletion, recipients may be selectively repopulated by homeostatically proliferating memory or memory-like T cells that are resistant to maneuvers that typically inhibit naïve responses. Taken together, these four factors produce barriers that vary not only between similar individuals, but also vary over time in the same individual. Thus, the route to tolerance is highly variable.

Back to Top | Article Outline

Mechanisms of Tolerance

Based upon our current approach to organ transplantation, tolerance appears an infrequent occurrence that requires the appropriately timed disruption of numerous immune mechanisms. However, tolerance is the default response to a multitude of self and environmental antigens, and many of the mechanisms that maintain self-tolerance may also be capable of promoting allograft tolerance. Tolerance mechanisms can be broadly classified as either central or peripheral. Central tolerance refers to the deletion within the thymus of T cells whose affinity for self antigens is inappropriately high and thus likely to result in autoimmunity. The tolerance displayed by neonatal mice to a transplanted organ demonstrates the robustness of these central mechanisms. Similarly, central deletion is an important mechanism promoting tolerance to organ allografts in mice displaying mixed hematopoietic chimerism following bone marrow transplantation (18).

In addition to central deletion, a number of mechanisms operating in the periphery have been reported to contribute to the tolerant state. Peripheral mechanisms for maintaining tolerance include ignorance (9), anergy (19), regulation or suppression (20–22), and apoptosis or peripheral deletion (23, 24).

Ignorance as a mechanism mediating tolerance was demonstrated by Lakkis et al. who showed that naïve mice lacking secondary lymphoid organs accepted skin or heart allografts indefinitely (9). Impaired rejection in this model resulted from the failure of T cells to be primed when they encountered donor antigens outside of lymphoid organs. The findings that in some settings recipient T cells can be primed by alloantigens encountered within the allograft (25) and that memory T cells can be reactivated after encountering antigen outside of secondary lymphoid organs (26) suggest that the role of this mechanism may be limited.

Active, antigen-specific suppression of immune responses was first reported in the 1970s (27,28). Though interest in this phenomenon waned, it was rekindled by the demonstration that CD4+CD25+ T cells from rats bearing long-term surviving cardiac allografts could transfer tolerance to untreated recipients (29). A growing body of experimental and clinical evidence suggests a role for regulatory T cells in the induction and maintenance of tolerance (reviewed in 21). While CD4+CD25+ cells constitute the most widely recognized phenotype of regulatory cells, cells of other lineages including NK1.1, C8+CD28-, and CD3+CD4-CD8- cells may display regulatory properties (30–32). Other markers of regulatory T cells may include CD45RB, GITR, CTLA4, CD103, and FOXP3 (33–41).

At least three mechanisms appear to be important for tolerance mediated by regulatory T cells. Preclinical and clinical evidence suggests that TR1, TH3, and NKT cells mediate regulatory effects at least in part via production of the cytokines IL-10 and TGFβ (33, 42–45). The function of CD4+ CD25+ T reg also appears to be affected by the expression of the cell surface molecules GITR (36, 37) and CTLA4 (42) which may contribute to the contact dependent effects of regulatory cells (reviewed in 21,22). Finally, anergic CD4+ T cells may mediate their regulatory effects by inhibiting the maturation and function of dendritic cells (46).

Although regulation has been extensively studied in experimental systems, far less is known about its role in clinical tolerance. Salama et al. reported that CD25+ regulatory cells developed as early as three months after renal transplantation and persisted for years (47). Furthermore, evidence of regulation in vitro as detected by ELISPOT analysis was more commonly observed in rejection-free recipients than those who had experienced rejection. Analysis of a limited number of operationally tolerant transplant recipients using the trans-vivo DTH assay has also suggested a role for regulation that was dependent upon the production of TGFβ and/or IL-10 and the expression of CTLA4 (48, 49).

Although transplant tolerance may develop as a result of spontaneous regulatory mechanisms, clinical application of regulation is likely to require interventions that purposely produce regulatory cells. Many of these approaches (i.e., DST and in vitro generation of T reg) depend upon exposure to donor antigen, and are facilitated by the demonstration that recipient T cells need not be tolerized to each foreign antigen expressed by the transplanted organ. Rather, linked, dominant suppression has been described (50–52). In rodent and large animal models, the expression of one MHC molecule recognized by regulatory T cells effectively suppresses the response to other mismatched MHC molecules. A final consideration pertinent to the clinical application of regulatory mechanisms is the possible effect of currently used immunosuppressive agents on T regs. Although still controversial, some agents such as calcineurin inhibitors may inhibit the development of tolerance by preventing the activation or function of regulatory T cells (53).

In addition to ignorance and regulation, peripheral deletion of alloreactive T cells may contribute to tolerance. This can occur in the setting of chronic alloantigen stimulation or when alloantigen is encountered under suboptimal conditions for T-cell activation. Clonal exhaustion has been reported after liver transplantation as an example of how chronic stimulation may induce peripheral T cell deletion (54). Alternatively, the blockade of costimulatory signals such as CD28 or CD154 at the time T cells encounter alloantigens has been reported to result in incomplete T-cell activation followed by anergy and apoptosis (55). Several groups have now demonstrated that peripheral deletion mediated by apoptosis is an important event contributing to long-term allograft acceptance (23, 24). Apoptosis of T cells can be mediated either by cell surface death receptors or cytokine withdrawal. The death receptors mediating apoptosis are comprised of TNF receptor superfamily members that have a cytoplasmic death domain that binds cell-signaling proteins such as TRADD, FADD, FLICE, and caspase-3 leading to apoptosis. Cytokines promoting T-cell survival include the common γ chain cytokines IL-2, IL-4, IL-7, and IL-15. Their absence predisposes cells to active, apoptotic death.

Back to Top | Article Outline

Moving Targets

From this review, it should be obvious that there are multiple barriers to tolerance and multiple mechanisms that, in the correct setting, may promote tolerance. For example, both genetic factors such as CCR5 gene polymorphisms (56) and acquired factors such as the development of allo-reactive memory populations will differ between recipients and fundamentally affect the nature of the immune response to transplanted organs. It is also likely that the organ transplanted affects the nature of the immune response and the probability of developing tolerance (47, 57). How these multiple variables interact will determine the outcome of transplantation. The multiplicity of possible interactions explains our current inability to predict the outcome of transplantation in seemingly similar recipients.

In addition to variations between recipients in anti- and pro-tolerogenic factors, it is increasingly accepted that tolerance may be dependent upon the sequential development of more than one mechanism (58,59). For example, profound T-cell depletion eliminates the large population of cells with direct alloantigen specificity. However, other pro-tolerant mechanisms are required to maintain tolerance as recipient T cells re-emerge. This is exemplified by the finding that tolerance in NOD and IL-2−/− recipients treated with sirolimus, an agonist IL-2 and antagonist IL-15-related fusion protein was associated with the early deletion of alloreactive T cells followed by the development of CD4+CD25+ regulatory T cells (60). Deletion of donor-reactive T cells has also been shown to occur in recipients rendered tolerant to organ allografts following bone marrow transplantation (61). However, even in this model it may not be accurate to ascribe the development of donor-specific tolerance entirely to deletion. Tolerance developing after the infusion of donor bone marrow in mice treated with busulfan and short-term blockade of CD28 and CD154 is initially dependent upon CD4+ T cells (62). Although tolerance is eventually associated with deletion of donor-reactive T cells, this process requires months for completion, implying that mechanisms other than depletion are responsible for early graft acceptance. Using the trans-vivo DTH assay, we have recently demonstrated the existence of regulatory cells that produce IL-10 and/or TGFβ at early time points following transplantation (63). Interestingly, these regulatory cells were not detected at later time points, suggesting that they may be deleted together with alloaggressive recipient T cells. Thus, the mechanisms which induce tolerance may be distinct from those that maintain tolerance. Finally, mechanisms that protect organ allografts at one point in time may be harmful at later times. For instance, treatment of mice with an anti-CD4 antibody or gallium nitrate allows the long-term acceptance of heart allografts. This effect is mediated by regulatory cells that produce TGFβ (64). However, with time, heart grafts from anti-CD4 and gallium nitrate treated recipients develop chronic rejection, which has been postulated to be the result of the pro-fibrotic effects of TGFβ (65).

Back to Top | Article Outline

How Can We Converge on the Target?

The Impact of Early Tolerance Studies

Our current concept of transplantation tolerance remains heavily influenced by the original studies of acquired tolerance (66). These studies predicted that tolerance should be achievable following a brief intervention that fundamentally changes the immune system. Ideally, this intervention would be initiated at the time of transplantation and result in permanent, antigen-specific organ acceptance. However, it is important to recall that even in the landmark study of Billingham et al., only two of the five recipients were tolerant (two developed acute rejection and one developed chronic rejection). Nevertheless, the view that tolerance could be acutely induced has been reinforced by experiments demonstrating that prolonged, if not indefinite, allograft survival could be achieved following brief treatments with agents such as CTLA4-Ig, anti-CD154, or anti-CD4 (65, 67).

Back to Top | Article Outline

Moving Transplantation Tolerance to the Clinic

For some time, nonhuman primate (NHP) models have been used to evaluate the safety and efficacy of the most promising tolerogenic regimens developed in rodent models prior to their use in humans. However, when compared to the results obtained using rodents, virtually all tolerogenic strategies have proven far less effective in NHPs (68). This may be the result of the homogeneity of rodent models (i.e., uniform age, environmental exposure, and genetic background) and the reductionist design of studies aimed at determining the role of a specific pathway/molecule. Attempts to closely replicate these approaches in large animal preclinical or clinical transplantation have been largely unsuccessful, likely due to the more diverse environmental exposure and genetic background of primate transplant recipients.

One of the more promising approaches tested in NHPs is brief therapy with an antibody against the CD3 molecule coupled to a diphtheria-derived immunotoxin (69) that mediates profound but reversible T cell depletion. Approximately 40% of the treated animals develop operational tolerance following pretransplant T-cell depletion, as indicated by the absence of rejection and the presence of donor-specific hyporesponsiveness following T cell repopulation. This approach was made more reproducible by the addition of deoxyspergualin, a polyamine antibiotic that inhibits APC function (70). Based on this approach, a number of groups have undertaken clinical trials utilizing early recipient T-cell depletion. Several investigators have used alemtuzumab (an anti-CD52 monoclonal antibody) to induce profound T-cell depletion. Despite achieving depletion that is equivalent to that obtained using anti-CD3-immunotoxin with respect to kinetics, magnitude, and effectiveness within the secondary lymphoid tissues, treatment with alemtuzumab alone or in combination with deoxyspergualin is not sufficient to induce tolerance in adult humans (71, 72). This experience has led other groups to combine the T-cell depletion with other agents such as sirolimus (73). Although this approach remains promising, initial studies have shown a significant incidence of acute rejection (73) and an unusually high incidence of antibody-mediated rejection. The failure of these T-cell-centric approaches suggests that other components of the immune system, such as B cells, NK cell, or monocytes, may need to be specifically targeted to achieve tolerance.

Back to Top | Article Outline

Chimerism and Tolerance

Combined nonmyeloablative bone marrow and solid organ transplantation has been shown to induce robust tolerance in rodents (18, 62). In humans, successful bone marrow transplantation allows the acceptance of subsequent organ allografts from the same donor in the absence of immunosuppression (74). These observations form the basis for clinical trials, sponsored by the Immune Tolerance Network, of combined bone marrow and kidney transplantation. Using a nonmyeloablative conditioning regimen (cyclophosphamide, thymic irradiation, and antithymocyte globulin) and a short course of cyclosporine, two patients have been reported to display functional tolerance and sustained antitumor responses at 2 and 4 years following combined bone marrow and kidney transplantation from the same donor (75). To date, this approach has proven less effective for non-HLA identical donor and recipient pairs. The effectiveness of this approach may be in large part related to its ability to target multiple mechanisms of rejection and tolerance. There is experimental evidence to suggest that these types of regimens control alloreactive recipient T cells by central deletion and peripheral mechanisms including anergy, deletion, and regulation (18, 76, 77). Despite targeting multiple mechanisms, combined bone marrow/solid organ transplantation does not invariably prevent the development of chronic allograft vasculopathy (78, 79). This suggests that in addition to inhibiting multiple mechanisms that promote acute rejection, tolerogenic strategies will also need to target a discreet group of alloimmune responses that cause chronic allograft injury, or alternatively promote protective responses that suppress injurious responses.

Back to Top | Article Outline

CONCLUSION

A New Mind-Set Towards Tolerance Induction

Fifty years after the initial description of acquired transplant tolerance, the occurrence of tolerance in clinical transplantation remains largely accidental and unpredictable. However, the argument in favor of continued efforts to design, test, and implement tolerogenic regimens in transplantation remains as compelling today as at any time in the past. The heterogeneous nature of clinical transplantation together with the redundancy and plasticity of the immune system conspire against the acquisition of tolerance, and suggest that when tolerance does develop, it may be impermanent. In this review, we propose that successful tolerogenic strategies will need to inhibit multiple destructive immune responses while promoting immune responses that protect the transplanted organ. Furthermore, a single such regimen might not prove to be uniformly effective as the targeted mechanisms may vary significantly between recipients and within a given recipient over time. The clinical observation that transplantation tolerance may be lost, often after several years or even decades of drug-free allograft acceptance, supports the notion that the immune response of tolerant patients mutates over time. Thus, the timing, as well as the nature, of tolerance-promoting interventions may need to be varied. Consequently, as we converge on tolerance, we are not focusing on one static target, but many moving targets. If long-term allograft acceptance requires a series of discreet immunologic responses or nonresponses, it seems unlikely that a single, brief intervention will result in long-term allograft acceptance. Rather, a longer period of treatment sequentially targeting developing immune responses as they occur may afford the greatest likelihood of achieving drug-free allograft acceptance. If we are to apply such an approach to transplantation, we will need to develop a battery of monitoring techniques that are capable of quantifying the balance of destructive and protective mechanisms following transplantation.

Back to Top | Article Outline

ACKNOWLEDGMENTS

The authors thank Lisa Carlson for critical reading of this manuscript.

Back to Top | Article Outline

REFERENCES

1.Annual Report of the U.S. Organ Procurement and Transplantation Network and the Scientific Registry for Transplant Recipients: Transplant Data 1992-2001. Rockville, MD: Department of Health and Human Services, 2002.
2.Ojo AO, Held PJ, Port FK, et al. Chronic renal failure after transplantation of a nonrenal organ. N Engl J Med 2003; 349(10): 931.
3.Meier-Kriesche HU, Schold JD, Srinivas TR, Kaplan B. Lack of improvement in renal allograft survival despite a marked decrease in acute rejection rates over the most recent era. Am J Transplant 2004; 4(3): 378.
4.Matzinger P. The danger model: a renewed sense of self. Science 2002; 296(5566): 301.
5.Medzhitov R, Janeway Jr. CA. Decoding the patterns of self and nonself by the innate immune system. Science 2002; 296(5566): 298.
6.Suchin EJ, Langmuir PB, Palmer E, et al. Quantifying the frequency of alloreactive T cells in vivo: new answers to an old question. J Immunol 2001; 166(2): 973.
7.Larsen CP, Morris PJ, Austyn JM. Migration of dendritic leukocytes from cardiac allografts into host spleens. A novel pathway for initiation of rejection. J Exp Med 1990; 171(1): 307.
8.Benichou G, Valujskikh A, Heeger PS. Contributions of direct and indirect T cell alloreactivity during allograft rejection in mice. J Immunol 1999; 162(1): 352.
9.Lakkis FG, Arakelov A, Konieczny BT, Inoue Y. Immunologic ’ignorance’ of vascularized organ transplants in the absence of secondary lymphoid tissue. Nat Med 2000; 6(6): 686.
10.Zhou P, Hwang KW, Palucki D, et al. Secondary lymphoid organs are important but not absolutely required for allograft responses. Am J Transplant 2003; 3(3): 259.
11.Lechler RI, Batchelor JR. Immunogenicity of retransplanted rat kidney allografts. Effect of inducing chimerism in the first recipient and quantitative studies on immunosuppression of the second recipient. J Exp Med 1982; 156(6): 1835.
12.Baker RJ, Hernandez-Fuentes MP, Brookes PA, et al. Loss of direct and maintenance of indirect alloresponses in renal allograft recipients: implications for the pathogenesis of chronic allograft nephropathy. J Immunol 2001; 167(12): 7199.
13.Yamada A, Chandraker A, Laufer TM, et al. Recipient MHC class II expression is required to achieve long-term survival of murine cardiac allografts after costimulatory blockade. J Immunol 2001; 167(10): 5522.
14.Welsh RM, Selin LK. No one is naive: the significance of heterologous T-cell immunity. Nat Rev Immunol 2002; 2(6): 417.
15.Adams AB, Pearson TC, Larsen CP. Heterologous immunity: an overlooked barrier to tolerance. Immunol Rev 2003; 196: 147.
16.Wu Z, Bensinger SJ, Zhang J, et al. Homeostatic proliferation is a barrier to transplantation tolerance. Nat Med 2004; 10(1): 87.
17.Pearl JP, Parris J, Hale DA, et al. Immunocompetent T-cells with a memory-like phenotype are the dominant cell type following antibody-mediated T-cell depletion. Am J Transplant 2005; 5(3): 465.
18.Wekerle T, Kurtz J, Ito H, et al. Allogeneic bone marrow transplantation with co-stimulatory blockade induces macrochimerism and tolerance without cytoreductive host treatment. Nat Med 2000; 6(4): 464.
19.Jones LA, Chin LT, Merriam GR, et al. Failure of clonal deletion in neonatally thymectomized mice: tolerance is preserved through clonal anergy. J Exp Med 1990; 172(5): 1277.
20.Walsh PT, Strom TB, Turka LA. Routes to transplant tolerance versus rejection; the role of cytokines. Immunity 2004; 20(2): 121.
21.Wood KJ, Sakaguchi S. Regulatory T cells in transplantation tolerance. Nat Rev Immunol 2003; 3(3): 199.
22.Jiang S, Lechler RI. Regulatory T cells in the control of transplantation tolerance and autoimmunity. Am J Transplant 2003; 3(5): 516.
23.Li Y, Li XC, Zheng XX, et al. 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 1999; 5(11): 1298.
24.Wells AD, Li XC, Li Y, et al. Requirement for T-cell apoptosis in the induction of peripheral transplantation tolerance. Nat Med 1999; 5(11): 1303.
25.Kreisel D, Krupnick AS, Gelman AE, et al. Non-hematopoietic allograft cells directly activate CD8+ T cells and trigger acute rejection: an alternative mechanism of allorecognition. Nat Med 2002; 8(3): 233.
26.Chalasani G, Dai Z, Konieczny BT, et al. Recall and propagation of allospecific memory T cells independent of secondary lymphoid organs. Proc Natl Acad Sci U S A 2002; 99(9): 6175.
27.Gershon RK, Kondo K. Infectious immunological tolerance. Immunology 1971; 21(6): 903.
28.Kilshaw PJ, Brent L, Pinto M. Suppressor T cells in mice made unresponsive to skin allografts. Nature 1975; 255(5508): 489.
29.Hall BM, Pearce NW, Gurley KE, Dorsch SE. Specific unresponsiveness in rats with prolonged cardiac allograft survival after treatment with cyclosporine. III. Further characterization of the CD4+ suppressor cell and its mechanisms of action. J Exp Med 1990; 171(1): 141.
30.Bendelac A, Rivera MN, Park SH, Roark JH. Mouse CD1-specific NK1 T cells: development, specificity, and function. Annu Rev Immunol 1997; 15: 535.
31.Chang CC, Ciubotariu R, Manavalan JS, et al. Tolerization of dendritic cells by T(S) cells: the crucial role of inhibitory receptors ILT3 and ILT4. Nat Immunol 2002; 3(3): 237.
32.Zhang ZX, Yang L, Young KJ, et al. Identification of a previously unknown antigen-specific regulatory T cell and its mechanism of suppression. Nat Med 2000; 6(7): 782.
33.Hara M, Kingsley CI, Niimi M, et al. IL-10 is required for regulatory T cells to mediate tolerance to alloantigens in vivo. J Immunol 2001; 166(6): 3789.
34.Powrie F, Leach MW, Mauze S, et al. Phenotypically distinct subsets of CD4+ T cells induce or protect from chronic intestinal inflammation in C. B-17 scid mice. Int Immunol 1993; 5(11): 1461.
35.Davies JD, O’Connor E, Hall D, et al. CD4+ CD45RB low-density cells from untreated mice prevent acute allograft rejection. J Immunol 1999; 163(10): 5353.
36.Shimizu J, Yamazaki S, Takahashi T, et al. Stimulation of CD25(+)CD4(+) regulatory T cells through GITR breaks immunological self-tolerance. Nat Immunol 2002; 3(2): 135.
37.McHugh RS, Whitters MJ, Piccirillo CA, et al. CD4(+)CD25(+) immunoregulatory T cells: gene expression analysis reveals a functional role for the glucocorticoid-induced TNF receptor. Immunity 2002; 16(2): 311.
38.Takahashi T, Tagami T, Yamazaki S, et al. Immunologic self-tolerance maintained by CD25(+)CD4(+) regulatory T cells constitutively expressing cytotoxic T lymphocyte-associated antigen 4. J Exp Med 2000; 192(2): 303.
39.Read S, Malmstrom V, Powrie F. Cytotoxic T lymphocyte-associated antigen 4 plays an essential role in the function of CD25(+)CD4(+) regulatory cells that control intestinal inflammation. J Exp Med 2000; 192(2): 295.
40.Lehmann J, Huehn J, de la Rosa M, et al. Expression of the integrin alpha Ebeta 7 identifies unique subsets of CD25+ as well as CD25- regulatory T cells. Proc Natl Acad Sci U S A 2002; 99(20): 13031.
41.Brunkow ME, Jeffery EW, Hjerrild KA, et al. Disruption of a new forkhead/winged-helix protein, scurfin, results in the fatal lymphoproliferative disorder of the scurfy mouse. Nat Genet 2001; 27(1): 68.
42.Kingsley CI, Karim M, Bushell AR, Wood KJ. CD25+CD4+ regulatory T cells prevent graft rejection: CTLA-4- and IL-10-dependent immunoregulation of alloresponses. J Immunol 2002; 168(3): 1080.
43.Josien R, Douillard P, Guillot C, et al. A critical role for transforming growth factor-beta in donor transfusion-induced allograft tolerance. J Clin Invest 1998; 102(11): 1920.
44.Torrealba J, Katayama M, Fechner JH, et al. Metastable tolerance to rhesus monkey renal transplants is correlated with allograft TGF-beta+ CD4+ T regulatory cell infiltrates. In press.
45.Cai J, Lee J, Jankowska-Gan E, et al. Minor H antigen HA-1-specific regulator and effector CD8+ T cells, and HA-1 microchimerism, in allograft tolerance. J Exp Med, in press.
46.Vendetti S, Chai JG, Dyson J, et al. Anergic T cells inhibit the antigen-presenting function of dendritic cells. J Immunol 2000; 165(3): 1175.
47.Salama AD, Remuzzi G, Harmon WE, Sayegh MH. Challenges to achieving clinical transplantation tolerance. J Clin Invest 2001; 108(7): 943.
48.van Essen D, Kikutani H, Gray D. CD40 ligand-transduced co-stimulation of T cells in the development of helper function. Nature 1995; 378(6557): 620.
49.VanBuskirk AM, Burlingham WJ, Jankowska-Gan E, et al. Human allograft acceptance is associated with immune regulation. J Clin Invest 2000; 106(1): 145.
50.Madsen JC, Superina RA, Wood KJ, Morris PJ. Immunological unresponsiveness induced by recipient cells transfected with donor MHC genes. Nature 1988; 332(6160): 161.
51.Honey K, Cobbold SP, Waldmann H. CD40 ligand blockade induces CD4+ T cell tolerance and linked suppression. J Immunol 1999; 163(9): 4805.
52.Sonntag KC, Emery DW, Yasumoto A, et al. Tolerance to solid organ transplants through transfer of MHC class II genes. J Clin Invest 2001; 107(1): 65.
53.Furtado GC, Curotto de Lafaille MA, Kutchukhidze N, Lafaille JJ. Interleukin 2 signaling is required for CD4(+) regulatory T cell function. J Exp Med 2002; 196(6): 851.
54.Bishop GA, Sun J, Sheil AG, McCaughan GW. High-dose/activation-associated tolerance: a mechanism for allograft tolerance. Transplantation 1997; 64(10): 1377.
55.Appleman LJ, Boussiotis VA. T cell anergy and costimulation. Immunol Rev 2003; 192: 161.
56.Fischereder M, Luckow B, Hocher B, et al. CC chemokine receptor 5 and renal-transplant survival. Lancet 2001; 357(9270): 1758.
57.Zhang Z, Zhu L, Quan D, et al. Pattern of liver, kidney, heart, and intestine allograft rejection in different mouse strain combinations. Transplantation 1996; 62: 1267.
58.Lechler RI, Garden OA, Turka LA. The complementary roles of deletion and regulation in transplantation tolerance. Nat Rev Immunol 2003; 3(2): 147.
59.Zheng XX, Sanchez-Fueyo A, Domenig C, Strom TB. The balance of deletion and regulation in allograft tolerance. Immunol Rev 2003; 196: 75.
60.Zheng XX, Sanchez-Fueyo A, Sho M, et al. Favorably tipping the balance between cytopathic and regulatory T cells to create transplantation tolerance. Immunity 2003; 19(4): 503.
61.Wekerle T, Sayegh MH, Hill J, et al. Extrathymic T cell deletion and allogeneic stem cell engraftment induced with costimulatory blockade is followed by central T cell tolerance. J Exp Med 1998; 187(12): 2037.
62.Adams AB, Durham MM, Kean L, et al. Costimulation blockade, busulfan, and bone marrow promote titratable macrochimerism, induce transplantation tolerance, and correct genetic hemoglobinopathies with minimal myelosuppression. J Immunol 2001; 167(2): 1103.
63.Orosz CG, Bickerstaff AA, Wang J, et al. Allograft tolerance, immune regulation, and chronic rejection. Am J Transplant 2003; 3(Suppl 5): S319.
64.Orosz CG, Wakely E, Bergese SD, et al. Prevention of murine cardiac allograft rejection with gallium nitrate. Comparison with anti-CD4 monoclonal antibody. Transplantation 1996; 61(5): 783.
65.Orosz CG, Wakely E, Sedmak DD, et al. Prolonged murine cardiac allograft acceptance: characteristics of persistent active alloimmunity after treatment with gallium nitrate versus anti-CD4 monoclonal antibody. Transplantation 1997; 63(8): 1109.
66.Billingham RE, Brent L, Medawar PB. "Actively acquired tolerance" of foreign cells. Nature 1953; 172: 603.
67.Larsen CP, Elwood ET, Alexander DZ, et al. Long-term acceptance of skin and cardiac allografts after blocking CD40 and CD28 pathways. Nature 1996; 381(6581): 434.
68.Elster EA, Hale DA, Mannon RB, et al. The road to tolerance: renal transplant tolerance induction in nonhuman primate studies and clinical trials. Transpl Immunol 2004; 13(2): 87.
69.Contreras JL, Wang PX, Eckhoff DE, et al. Peritransplant tolerance induction with anti-CD3-immunotoxin: a matter of proinflammatory cytokine control. Transplantation 1998; 65(9): 1159.
70.Thomas JM, Contreras JL, Jiang XL, et al. Peritransplant tolerance induction in macaques: early events reflecting the unique synergy between immunotoxin and deoxyspergualin. Transplantation 1999; 68(11): 1660.
71.Kirk AD, Hale DA, Mannon RB, et al. Results from a human renal allograft tolerance trial evaluating the humanized CD52-specific monoclonal antibody alemtuzumab (CAMPATH-1H). Transplantation 2003; 76(1): 120.
72.Kirk AD, Mannon RB, Kleiner DE. Results from a human renal allograft tolerance trial evaluating T-cell depletion with alemtuzumab combined with deoxyspergualin. Transplantation, in press.
73.Knechtle SJ, Pirsch JD, Fechner JJ, et al. Campath-1H induction plus rapamycin monotherapy for renal transplantation: results of a pilot study. Am J Transplant 2003; 3(6): 722.
74.Sellers MT, Deierhoi MH, Curtis JJ, et al. Tolerance in renal transplantation after allogeneic bone marrow transplantation-6-year follow-up. Transplantation 2001; 71(11): 1681.
75.Buhler LH, Spitzer TR, Sykes M, et al. Induction of kidney allograft tolerance after transient lymphohematopoietic chimerism in patients with multiple myeloma and end-stage renal disease. Transplantation 2002; 74(10): 1405.
76.Kurtz J, Wekerle T, Sykes M. Tolerance in mixed chimerism - a role for regulatory cells? Trends Immunol 2004; 25(10): 518.
77.Kurtz J, Shaffer J, Lie A, et al. Mechanisms of early peripheral CD4 T-cell tolerance induction by anti-CD154 monoclonal antibody and allogeneic bone marrow transplantation: evidence for anergy and deletion but not regulatory cells. Blood 2004; 103(11): 4336.
78.Russell PS, Chase CM, Sykes M, et al. Tolerance, mixed chimerism, and chronic transplant arteriopathy. J Immunol 2001; 167(10): 5731.
79.Kawai T, Cosimi AB, Wee SL, et al. Effect of mixed hematopoietic chimerism on cardiac allograft survival in cynomolgus monkeys. Transplantation 2002; 73(11): 1757.
Keywords:

Tolerance; Transplantation; Immunosuppression; Regulation; Deletion

© 2006 Lippincott Williams & Wilkins, Inc.