Interleukin (IL)-7 is a potent survival factor for T cells acting through the maintenance of basic cellular homeostasis (i.e. transport mechanisms and metabolic activity) and the regulation of anti-apoptotic and proapoptotic Bcl-2 family member proteins . IL-7 acts as a costimulatory molecule for T cell activation induced by cognate antigens, stimulates homeostatic peripheral expansion of T cells in response to low-affinity antigens in lymphopenic hosts [2,3] and promotes memory formation [4,5].
Increased IL-7 levels have been detected in conditions characterized by abnormally low T cell numbers, including HIV-1 infection [6–9], idiopathic CD4+ T cell lymphocytopenia  or cytoreductive therapies for cancer, autoimmune diseases or bone marrow transplantation [6,11,12]. Associated with these lymphopenic conditions, a negative correlation has been repeatedly observed between serum IL-7 levels and peripheral blood CD4+ T cell counts. These findings contributed to the formulation of a homeostatic model of peripheral T cell regulation predicting that T cell depletion leads to high IL-7 levels, which, in turn, would accelerate T cell regeneration through increased survival and proliferation.
In chronic HIV-1 infection, high IL-7 levels can be observed in the blood predominantly when CD4+ T cell count falls below 200 cells/μl, a late stage of HIV-1 infection when T cells seem to be incapable of spontaneous regeneration and high IL-7 levels may not be sufficient to counteract T cell depletion (Fig. 1) [6,7,9]. Therefore, the regenerative effects of lymphopenia-induced IL-7 in HIV-1-infected individuals remain mostly speculative, based on animal models and in-vitro data. By contrast to endogenously elevated IL-7 levels, using high doses of exogenous IL-7 as a short-term therapy may have beneficial effects on peripheral T cells as suggested by the first human trials. In the present review, we discuss the potential effects of elevated IL-7 levels in T cell restoration and immune activation during HIV-1 infection, with particular emphasis on the mechanisms that may restrain T cell recovery despite the high levels of this cytokine.
The cellular source for interleukin-7 production during HIV-1 infection
Multiple sources of IL-7 have been described, including bone marrow stromal cells, thymic epithelial cells, dendritic cells, keratinocytes and the intestinal epithelium . Recently, specialized stromal cells – the T zone fibroblastic reticular cells (FRCs) – were identified as the main source of IL-7 in lymph node, and these cells effectively supported T cell survival in vitro, partly by IL-7 production . FRCs might play an important role in peripheral T cell maintenance because of their close contact with circulating lymphocytes. HIV-1 infection leads to an altered structure of T cell niches in lymphoid tissues, disrupted by fibrosis related to chronic immune activation and inflammation [30,31], which may interfere with T cell trafficking within the IL-7-producing reticular cell network and, consequently, the access to IL-7.
To date, the mechanisms that modulate circulating IL-7 levels remain to be clarified. IL-7 levels could increase because of an enhanced IL-7 production in response to lymphopenia, implying a feedback loop in which T cell loss would induce the production of factors that could stimulate lymphocyte repopulation. Antigen-presenting cells (APCs) , as well as bone marrow-derived stromal cells , have been implicated in inducible IL-7 production. Whether FRCs modulate IL-7 production during HIV-1 infection is yet to be further analyzed, although it has already been indicated that the level of IL-7 production in the lymph nodes might be similar in HIV-1-infected and non-infected individuals .
As an alternative scenario, IL-7 may accumulate due to reduced consumption caused by the declining number of IL-7R-expressing cells [2,33]. According to the latter, the production of IL-7 by stromal cells, occurring at a fixed constitutive rate, provides an amount of IL-7 that would be enough to support the survival of a determined number of T cells, limiting, in this way, the size of the lymphocyte pool.
Regulation of interleukin-7 sensitivity during HIV-1 infection through downregulation of interleukin-7Rα
For a better understanding of the possible effects of high IL-7 doses, reached via the homeostatic response to T cell depletion or administered as part of a T cell regenerative therapy, it is important to analyze which cells are targeted by high IL-7 levels. IL-7Rα, a molecule that comprises the receptor for IL-7 together with the common γ chain, is lost on up to 60–70% of peripheral T cells in HIV-1-infected individuals, and these cells are most probably unable to benefit from high IL-7 doses (Fig. 1) [14–16]. In addition to chronic HIV-1 infection, IL-7Rα downregulation has been detected in chronic hepatitis C virus (HCV) infection  and in aged individuals .
The IL-7Rα-low T cells mostly include CD8+ T cells that express activation or memory markers, and these cells often lack the CD28 coreceptor molecule, a phenotype previously associated with defective proliferative abilities [14,15]. CD4+ T cells of HIV-1-infected individuals are also characterized by IL-7Rα downregulation [15,17], although to a lower extent than CD8+ T cells. Part of the CD4+IL-7Rα-low T cells probably comprises forkhead box P3+ (FoxP3+) regulatory T cells , a population that does not exist in the CD8+ T cell subset .
The IL-7Rα-low T cells express lower levels of the anti-apoptotic Bcl-2 molecule compared with the IL-7Rα-high T cell counterpart in the same donors, possibly reflecting altered maintenance strategies for these cells not involving IL-7-mediated Bcl-2 induction . Although T cells of HIV-1-infected individuals, analyzed as a whole without distinguishing IL-7Rα-high and IL-7Rα-low populations, are able to benefit from IL-7-induced survival signals in vitro [15,38], such effects of IL-7 on T cell maintenance are clearly decreased in HIV-1-infected individuals compared with controls .
IL-7Rα downregulation accelerated in line with disease progression measured by CD4+ T cell depletion and HIV-1 viremia in several cohorts of HIV-1-infected patients [14–16]. As indicated previously [39,40], antiretroviral therapy (ART) induced only partial restoration of IL-7 sensitivity of T cells. Preserved IL-7Rα levels, however, may be indicative of better CD4+ T cell maintenance following treatment interruption .
The mechanism leading to IL-7Rα downregulation during HIV-1 infection is yet unknown. Attempts to explain low IL-7Rα levels as the consequence of a high IL-7 concentration or alternatively, through the specific effect of the Tat viral polypeptide on IL-7Rα expression, have been made [42,43]. These studies, however, ignore the fact that both IL-7 or Tat reduces IL-7Rα level transiently, whereas the low IL-7Rα level is stable when T cells from HIV-1-infected patients are cultured in vitro . Moreover, IL-7 or Tat do not affect IL-7Rα expression on specific subsets of T cells as found when T cells of HIV-1-infected individuals were analyzed [14,15]. In addition to potential regulatory mechanisms specifically targeting IL-7Rα expression in HIV-1-infected individuals, alternative models suggest either the preferential infection of CD4+IL-7Rα-high T cells versus the IL-7Rα-low counterparts  or survival advantages for the IL-7Rα-low T cells. It has been previously shown that IL-7 facilitates HIV-1 infection of T cells in vitro [44–48] and, in this way. T cells not sensitive for IL-7 may be better preserved. This scenario was, however, not confirmed by another study showing that CD4+IL-7Rα-low T cells were infected by HIV-1 as efficiently as IL-7Rα-high memory T cells . In any case, a preferential infection model could not explain the robust increase in CD8+IL-7Rα-low T cells during HIV-1 infection [14–16].
The fact that the majority of IL-7Rα-low T cells represent previously activated antigen-specific T cell clones in late differentiation stages in both HIV-infected and elderly individuals suggests that chronic antigenic stimulation may provide a driving force for IL-7Rα downregulation and, at the same time, survival and proliferative signals that are independent of IL-7 [14,15,35]. Expansion of IL-7Rα-low T cells has been suggested to take place in vivo in HIV-1-infected and aged individuals [14,35], suggesting that the increasing number of IL-7Rα-low T cells may not only occur because of the loss of IL-7Rα+ T cell subsets.
Interestingly, studies on mouse with lymphocytic choriomeningitis virus (LCMV) infection supported a scenario when long-term antigenic signals would lead to cytokine-independent but activation-dependent maintenance of antigen-specific T cells. Acute LCMV infection resulted in long-lasting memory T cells maintained by IL-7 and IL-15, whereas, in chronic LCMV infection, antigen-specific T cells were not sensitive for these homeostatic cytokines but were maintained through repeated antigen-specific stimulation [49,50]. Acute LCMV infection in mice lacking CD4+ T cells resulted in a higher ratio of IL-7Rα-low CD8+ memory T cells compared with wild type mice, suggesting that CD4+ T cell depletion in HIV-1-infected individuals may further facilitate IL-7R downmodulation .
The association of persistent antigenic signals and IL-7Rα downregulation indicates that T cells, specific for pathogens that the host is unable to clear, will loose the ability to receive survival signals from IL-7. Indeed, a high proportion of T cells specific for HIV-1, HCV, cytomegalovirus and Epstein–Barr virus antigens are IL-7Rα low, whereas IL-7Rα expression is better preserved on T cells specific for vaccinia virus or flu [14,34,52,53]. In aged individuals, T cells with low IL-7Rα expression are typically CD8+CD28− effector memory populations with limited T cell receptor (TCR) diversity, implicating long-term T cell activation as the driving force of IL-7R downmodulation in human aging as well . An increased IL-7 concentration may efficiently improve peripheral T cell maintenance by supporting the IL-7Rα-high naive or transiently activated T cells, whereas T cells already committed to HIV-1 or to other chronic pathogens enter an IL-7-independent differentiation pathway with decreased sensitivity to IL-7.
It is noteworthy that a CD8+ T cell subset was also identified with an apparently naive phenotype that expressed low levels of IL-7Rα . When these cells were cultured in vitro, the IL-7Rα expression readily increased and reached levels comparable to T cells that were originally IL-7Rα high. Such transient downregulation of IL-7Rα on naive T cells might reflect the effect of IL-7 or other common gamma chain cytokines on IL-7Rα gene expression [14,15,56].
Can the high interleukin-7 levels present during HIV-1 infection support T cell maintenance and proliferation?
A major question regarding the potential actions of IL-7 elevated in response to lymphopenia is whether the IL-7 levels present in T cell-depleted individuals are able to confer more efficient maintenance of T cell counts than baseline IL-7 levels. The serum concentration of IL-7 is in the pg/ml range, and even the highest IL-7 levels observed in T cell-depleted individuals are far beyond the concentrations (usually in the ng/ml range) used to identify IL-7 action on T cells in vitro. Although such a discrepancy is yet to be solved, some assumptions and findings may favor the scenario that IL-7 increase triggered by lymphopenia could indeed influence T cell homeostasis. First, blood is probably not a major source of IL-7, and serum IL-7 likely represents a leakage from solid tissues where IL-7 is produced by stromal cells and APCs. Accordingly, IL-7 measurement in serum may not reflect the absolute cytokine levels available for T cells in vivo but could rather serve as an indicatory tool to detect alterations in IL-7 availability. In addition, IL-7 can be deposited on extracellular matrix proteoglycans, thereby increasing local availability and effectiveness in action [57,58].
The finding that IL-7Rα is transiently downregulated by its own ligand has been interpreted as an altruistic regulatory process, in that T cells receiving signals through IL-7Rα downmodulate the receptor to let other cells benefit from the survival effects of IL-7 [33,56]. Indeed, several studies have demonstrated that IL-7 is available at limited concentrations for T cells, and increased IL-7 levels resulted in elevated T cell numbers and increased T cell reactivity in several animal models [59,60], as well as in humans [61,62]. By contrast increased IL-7Rα expression resulted in decreased T cell numbers in thymus as well as in the periphery, possibly through increased IL-7 consumption [56,63]. These studies indicate that an increase in IL-7 concentration by up to ten-fold, predicted by blood IL-7 measurements in HIV-1-infected individuals, could greatly influence IL-7 actions on T cells.
Interleukin-7 and T cell regeneration during HIV-1 infection
Although IL-7 is often considered to be a cytokine that might contribute to a better T cell maintenance in HIV-1-infected individuals, such a role of IL-7 has been questioned by several studies. First, longitudinal studies on simian immunodeficiency virus (SIV)-infected macaques  and HIV-1-infected individuals  showed that the increase in IL-7 levels in response to T cell depletion might be inefficient to counteract T cell depletion, and a potentially beneficial effect of IL-7 on T cell preservation has been observed only in sporadic cases [64,65]. High IL-7 levels in chronically HIV-1-infected individuals are associated with late disease stages and profound CD4+ T cell depletion [6,7,9], a finding that may argue against IL-7-induced T cell recovery. In addition, IL-7 levels were significantly lower in long-term nonprogressors (LTNPs) with stable immune status compared with patients who lost the LTNP status during follow-up . Thus, in LTNPs, a high IL-7 concentration appears to predict an accelerated disease progression, rather than increased T cell maintenance. Therefore, whether indications are available to postulate a role of endogenous IL-7 in T cell regeneration in HIV-1-infected individuals is open to question.
During the natural course of HIV-1 infection, a limited degree of T cell regeneration can be observed in the early stage of infection following the initial control of virus replication; IL-7, elevated during primary infection, may promote this transient increase in T cell numbers. A correlation between baseline IL-7 levels and the efficiency of CD4+ T cell regeneration has been observed when ART was initiated during primary infection . However, because limited information is available about IL-7 regulation during primary infection, different IL-7 levels may not only reflect donor variability, but also a different timing of ART initiation that may obviously influence treatment success.
Although chronic HIV-1 infection leads to progressive T cell death, it is evident that T cell depletion occurs in the presence of a T cell stimulatory mechanism reflected by the increased expression of activation markers and an increased ratio of circulating effector and memory T cells. A potential role for T cell regenerative factors during HIV-1 infection may be indicated by CD8+ T cell expansion that compensates for CD4+ T cell depletion and leads to relatively stable T cell numbers for up to several years . In this scenario, competition might take place between peripheral T cells for shared survival and proliferative factors, and IL-7 may serve as a potential candidate for these functions. Depletion of CD4+ T cells may decrease competition for IL-7, and the increased availability of IL-7 may allow an increase of the CD8+ T cell pool, whereas HIV-1-induced killing mechanisms restrain CD4+ T cell regeneration. The exhaustion of CD8+ T cell regenerative potential due to the acquisition of inhibitory receptors [67,68], replicative senescence [69,70] and the loss of IL-7Rα [14,15,71] may explain why IL-7 levels increase only at late stages of HIV-1 infection – as IL-7 consumer T cells are not regenerated any more – and CD8+ T cell exhaustion may also explain the low efficiency of IL-7 to rescue T cells in patients who progress to AIDS. Initiation of ART in chronically infected individuals may open new possibilities for IL-7-mediated T cell stimulation. In patients responding well to therapy, ART often leads to exacerbated T cell responses against previously asymptomatic infections, a phenomenon termed as immune reconstitution inflammatory syndrome (IRIS) . The hyper-responsiveness of T cells in HIV-1-infected individuals suggests the presence of lymphopenia-associated T cell costimulatory factors, and IL-7 is obviously a potential candidate for such an effect .
In addition, it has been reported that baseline IL-7 levels may be indicatory for ART efficiency in improving CD4+ T cell numbers [18–21] or viral control [22,23] in chronically infected patients. Contradictory results, however, indicate a less efficient control on virus replication in donors with higher baseline levels of IL-7 . From these findings, it can be concluded that T cell regenerative processes are detectable during HIV-1 infection, independently or as a result of ART, and IL-7 may act as a T cell stimulatory cytokine in these processes.
As we have already discussed, high IL-7 doses may not contribute to a better control of HIV-1 replication and to stable CD4+ T cell numbers in LTNPs . In other contexts, however, IL-7 might play a role in a more efficient T cell preservation. In rhesus macaques infected with SIV, IL-7 concentrations increased in parallel with progression to AIDS, similarly to human HIV-1 infection. By contrast, SIV infection of sooty mangabeys, a species that tolerates SIV with minimal clinical symptoms, resulted in an early increase in serum IL-7 levels right after the transient CD4+ T cell decline upon primary infection. High IL-7 levels were followed by increased T cell proliferation and CD4+ T cell count stabilization, indicating a possible role for early IL-7 increase in rescuing T cell homeostasis from an irreversible damage .
In humans, HIV-2 infection is associated with a better disease prognosis due to the slower rate of CD4+ T cell decline compared with HIV-1 infection. In HIV-2-infected individuals, IL-7 levels are inversely correlated with peripheral CD4+ T cell decline, similar to that observed in HIV-1 infection . IL-7Rα expression in HIV-2 infection was better preserved on several T cell subsets, especially on CD8+ cells , and higher IL-7Rα expression of T cells may indicate that more T cells could benefit from increasing IL-7 concentrations. Altogether, SIV infection of sooty mangabeys and HIV-2 infection of humans may represent conditions in which the increase in IL-7 concentrations might contribute to a better stability of peripheral CD4+ T cells.
High interleukin-7 levels may influence B cell differentiation
In HIV-1-infected individuals, several B cell dysfunctions have been identified, including hyperactivation, priming for apoptosis and increased ratio of immature/transitional B cells in circulation. The direct action of IL-7 on peripheral B cells can be debated because of the undetectable IL-7Rα expression on mature B cells. In addition, IL-7 therapy in primate models or humans did not lead to increased peripheral B cell numbers [61,62,75]. Although there are no indications available for a modulatory effect of IL-7 on B cell numbers during HIV infection, IL-7 has been implicated in the perturbation of B cell differentiation that leads to an increased ratio of circulating transitional B lymphocytes, characterized as CD27–CD10+ B cells with low receptor diversity and weak proliferative abilities upon B cell receptor cross-linking. The ratio of immature/transitional B cells correlated with CD4+ T cell depletion as well as with serum IL-7 levels, indicating that IL-7 may influence the peripheral expansion of CD10+ transitional B cells . It is noteworthy that, in idiopathic CD4+ T lymphopenia, unrelated to HIV-1 infection, the prevalence of immature/transitional B cells in blood correlated positively with serum IL-7 concentrations and negatively with CD4+ T cell counts . It is tempting to speculate that factors produced in lymphopenia, including IL-7, may provide modulatory feedback signals for B cell differentiation, although such mechanisms are yet to be better defined.
The effect of interleukin-7 on virus replication
IL-7 has been shown to facilitate HIV-1 replication, a finding not confirmed in SIV-infected macaques in vivo but repeatedly observed in vitro in thymocytes, resting T cells and peripheral blood mononuclear cells [45–48]. In addition, IL-7 increased the transfection efficiency of naive CD4+ T lymphocytes using HIV-1-derived vectors . In accordance with the potential effects of IL-7 on HIV-1 replication, high IL-7 levels were associated with rapidly replicating syncytium-inducing HIV-1 strains, characterized by the use of chemokine (C-X-C motif) receptor 4 (CXCR4) as a coreceptor [7,25]. These studies suggested that higher IL-7 doses may somehow facilitate the appearance of syncytium-inducing viral strains and not just passively follow the accelerated T cell depletion associated with the emergence of such variants. Although it remains to be further defined whether higher IL-7 levels predispose to a viral coreceptor switch, patients characterized by syncytium-inducing viral phenotype expressed more IL-7 in the serum than non-syncytium-inducing virus-bearing individuals, even when patients with similar levels of CD4+ T cell depletion were compared [7,25]. IL-7 induced CXCR4 upregulation in T cell cultures [7,78,79], and this property of IL-7 may increase the emergence of CXCR4 using viral strains.
Interestingly, when proviral reactivation was compared with resting CD4+ T cells of the same donors in the presence of IL-7 or the combination of phytohemagglutinin and IL-2, it was found that these signals activated different strains of the same proviral repertoire, providing further indication for the modulatory effects of cytokines on virus evolution in parallel with disease progression . Such strain-specific sensitivity of HIV-1 for IL-7 may contribute to the controversial experimental results demonstrating a positive effect of IL-7 on viral infection and replication in vitro [45–48] and the lack of such an effect in vivo (discussed further below). As the latent HIV-1 reservoir represents a major problem in HIV-1 eradication, the property of IL-7 to induce proviral reactivation in resting T cells has suggested the possibility of using IL-7 as a combinatory tool to increase ART efficiency [48,80]. IL-7 in this scenario would possibly extend ART effects towards the latent pool not sensitive to therapy.
Fas-mediated T cell apoptosis: a potential feedback mechanism that counteracts T cell stimulation by interleukin-7
Apoptotic signals transmitted by Fas molecules contribute to immunological tolerance by depleting repeatedly activated antigen-specific T cells as well as dendritic cells and activated B lymphocytes . Increased Fas triggering may contribute to T cell depletion in HIV-1-infected patients because the expression of Fas on T cells, the levels of membrane bound and soluble FasL molecules and sensitivity to apoptosis are all increased in parallel with disease progression [82–89]. Fas expression and sensitivity of T cells to activation-induced apoptosis is also increased in non-HIV-1-related lymphopenic conditions induced by cytoreductive therapies [90,91].
Susceptibility for Fas-mediated apoptosis is a hallmark of activated T cells, whereas these cells downregulate the IL-7Rα , suggesting that IL-7 and Fas may act at different stages of T cell differentiation. However, in lymphopenic individuals, the increase in IL-7 concentrations and the sensitivity of T cells to Fas-mediated apoptosis coexist, indicating a possible interaction of IL-7 and Fas signals. Indeed, as we and others have shown, IL-7 increases Fas expression on naive and memory T cells and induces a cytoskeleton-dependent Fas polarization on the cell surface [26,27]. T cells, when treated with IL-7, undergo apoptosis upon experimental Fas cross-linking [26,27], and IL-7 increased Fas-mediated apoptosis of CD4+ T cells in HIV-1-infected cell cultures as well . High IL-7 levels in the circulation of HIV-1-infected patients were associated with increased Fas expression on T cells and enhanced sensitivity to Fas-mediated apoptosis .
Fas, on the other hand, is a dual-function molecule that stimulates T cell proliferation when triggered on the surface of suboptimally activated T cells. Also, administration of high doses of IL-7 to lymphopenic hosts, as well as to T cell cultures, induces T cell maintenance or restoration rather than apoptosis [59–62], suggesting that IL-7-induced sensitivity to Fas-mediated apoptosis may not ultimately lead to T cell depletion. Indeed, as we showed in a recent study, T cells of HIV-1-infected individuals responded with an enhanced proliferation when Fas triggering accompanied suboptimal TCR signals . IL-7 treatment increased the costimulatory activity of Fas molecules on T cells, suggesting a model in which IL-7 and Fas may promote both apoptosis and proliferation depending on the activation status of the cells (Fig. 2). In this scenario, high IL-7 doses increase the sensitivity to Fas signals in T cells that may be followed by apoptosis when Fas is triggered on nonactivated T cells. By contrast, Fas signals, when induced in T cells weakly activated by low-affinity antigens, may rather contribute to an enhanced proliferation.
Interleukin-7 administration stimulates T cell expansions in simian immunodeficiency virus-infected macaques
The positive effects of IL-7 on T cell survival and proliferation, as well as the natural increase in serum IL-7 concentrations in response to T cell depletion, suggested that this cytokine could be useful to stimulate T cell regeneration in clinical conditions associated with lymphopenia. When recombinant human or simian IL-7 was administered to SIV-infected macaques, a consistent, but transient, increase in peripheral blood T cell count was detected [59,60,75]. Notably, the doses of IL-7 administered in these studies may significantly exceed in-vivo concentrations, even in lymphopenic individuals, as suggested by the almost complete downregulation of IL-7Rα on peripheral T cells in response to therapy . The numbers of CD4+ and CD8+ naive and memory T cells were evenly increased in the circulation of IL-7-treated animals. The expression of T cell activation markers, human leukocyte antigen-DR, CD25 and Fas, was found to be elevated in some of the studies [59,60,26], reflecting the activated/memory phenotype observed upon homeostatic T cells expansion in lympopenic humans or mice [91,93]. Peripheral expansion of all T cell subsets was evident [59,60,75] and, by using the sj:βTREC ratio, the stimulatory effect of IL-7 on intrathymic proliferation of T cell progenitors was also detected . By contrast to studies conducted with T cell cultures, IL-7 therapy did not induce viral replication in the infected animals [59,60]. The increase in T cell numbers by exogenous IL-7 was transient [59,60,75] and, in line with T cell decline, a neutralizing anti-IL-7 antibody response appeared, indicating potential limitations for long-term IL-7 therapies . The transient nature of T cell stimulation suggests that exogenous IL-7 may serve as a potential supplement for other approaches aiming at long-term T cell regeneration, such as ART for HIV-1-infected individuals, or as an adjuvant improving vaccine efficiency.
Interleukin-7 therapy during lymphopenic conditions in humans
In many different contexts, the lymphopenic condition induced upon chemotherapy of cancer patients has been correlated with HIV-1-induced immunopathology. Thus, when trying to unravel the potential use of IL-7 therapy to ameliorate T cell regeneration during HIV-1 infection, it is interesting to learn how IL-7 therapy improves the T cell pool in chemotherapy-treated cancer patients. A trial was conducted by administering recombinant IL-7 every third day, eight times in total, to 11 patients with metastatic melanoma and one with metastatic sarcoma . This study indicated that high IL-7 doses induced a consistent but transient increase in both CD4+ and CD8+ T cell counts and a decreased ratio of FoxP3+ regulatory T cells, possibly reflecting the inefficiency of IL-7 to stimulate T regulatory cells characterized by low IL-7Rα expression. Lack of regulatory T cell stimulation by IL-7, in contrast to the effect of IL-2 therapy [94,95], may be an important factor for therapy design when an improved antiviral or antitumor immune response is desired in addition to increasing overall T cell numbers.
Another more detailed study that enrolled 16 individuals with nonhematologic, nonlymphoid cancer confirmed the increase in both peripheral CD4+ and CD8+ T cell numbers in response to IL-7 therapy . Increased peripheral expansion, as measured by Ki67 expression and TREC dilution, upregulated expression of the antiapoptotic protein Bcl-2 and, possibly, an increased egress of newly developed lymphocytes from the thymus may all contribute to the beneficiary effects of IL-7 on peripheral T cell numbers. Importantly, apart from an effect on cell numbers, IL-7 therapy also induced qualitative changes in peripheral T cell repertoire by increasing the ratio of naive cells and by promoting TCR repertoire diversity in four out of six individuals tested and decreasing the ratio of terminally differentiated effector and memory populations as well as regulatory T cells. Two additional ongoing clinical trials, the Adult Clinical Trials Group (ACTG) 5214 and at the Agence Nationale de Recherche sur le Sida et les Hepatitis Virales, on IL-7 administration to HIV-1-infected patients have reported favorable results regarding safety, tolerability and T cell expansion.
Although the initial data are encouraging, questions about long-term benefits of exogenously administered IL-7 remain to be addressed. First, an immediate decrease in T cell numbers has been repeatedly observed (in ACTG 5214 as well as in another study ) together with the increase in Fas expression on the surface of peripheral T cells, indicating the activation of T cell-depleting mechanisms by high IL-7 doses. As IL-7 primed T cells to Fas-mediated apoptosis in vitro [26,27], it will be important to analyze whether T cells are lost in IL-7-treated patients through Fas activation and whether therapy can be optimized in order to avoid unnecessary T cell loss. It also remains to be analyzed whether patients benefit from the IL-7-induced T cell boost longer than a few weeks following termination of the therapy or whether, as observed in primates, the IL-7-induced increase in peripheral T cell numbers is only transient. Administration of high IL-7 doses may also increase the risk for autoimmunity as T cells that recognize low-affinity antigens (potentially self-antigens) have the ability to undergo activation and homeostatic expansions. In this respect, a link between lymphopenia and increased risk of autoimmunity has already been recognized . IL-7 has been implicated in joint destruction in rheumatoid arthritis patients [96,97] and in the activation of auto-reactive T cells in a mouse diabetes model . Furthermore, HIV-1 infection is associated with a wide range of autoimmune disorders , possibly indicating a further risk of autoimmune reactions boosted by IL-7. An additional complication that might be associated with IL-7 therapy is the induction of an IL-7-specific antibody response, as observed in several of the studies on primates and cancer patients [59,61,62]. However, IL-7-neutralizing antibody responses have not been detected in the first human studies.
Elevated concentrations of endogenous IL-7 seem to be unable to rescue peripheral T cells when HIV-1-infected patients progress to AIDS. Several factors might contribute to the inefficiency of high IL-7 doses in T cell restoration, including T cell-depleting signals and T cell exhaustion induced by HIV-1, directly or through chronic T cell activation (Table 1). Whether IL-7, when applied at very high doses for a short period as a T cell regenerative therapy, may revert T cell depletion and functional impairments in HIV-1-infected individuals is a question that should be answered by currently ongoing trials.
Accelerated aging of T cells reflected by a narrowed TCR diversity, shift from naive cells towards a limited set of effector clones, replicative senescence and the acquisition of inhibitory receptors are all implicated in the immunopathology of HIV-1 infection as well as in other chronic diseases and aging [54,67,69,70,100,101]. Therefore, IL-7 therapy, by increasing peripheral T cell numbers and reversing T cell aging and exhaustion, may have the potential to significantly improve T cell functionality in these conditions.
Bence Rethi, Nancy Vivar, Stefano Sammicheli and Francesca Chiodi contributed to the writing of the text. The work of the authors is supported by grants received from the Swedish MRC, the Swedish International Development Agency (SIDA-SAREC), and the regional agreement on medical training and clinical research (ALF) between Stockholm County Council and the Karolinska Institutet. Francesca Chiodi is a member of the EU Fp6 Network of Excellence Europrise, and Stefano Sammicheli is a fellow of the Fp6 EU Marie-Curie training program on HIV and tuberculosis coinfections. Bence Rethi is supported by the Magyary Zoltan Postdoctoral Fellowship and by the Hungarian Scientific Research Fund (K72532).
1. Jiang Q, Li WQ, Aiello FB, Mazzucchelli R, Asefa B, Khaled AR, Durum SK. Cell biology of IL-7, a key lymphotrophin. Cytokine Growth Factor Rev 2005; 16:513–533.
2. Fry TJ, Mackall CL. The many faces of IL-7: from lymphopoiesis to peripheral T cell maintenance. J Immunol 2005; 174:6571–6576.
3. Fry TJ, Mackall CL. Interleukin-7: master regulator of peripheral T-cell homeostasis? Trends Immunol 2001; 22:564–571.
4. Kaech SM, Tan JT, Wherry EJ, Konieczny BT, Surh CD, Ahmed R. Selective expression of the interleukin 7 receptor identifies effector CD8 T cells that give rise to long-lived memory cells. Nat Immunol 2003; 4:1191–1198.
5. Li J, Huston G, Swain SL. IL-7 promotes the transition of CD4 effectors to persistent memory cells. J Exp Med 2003; 198:1807–1815.
6. Fry TJ, Connick E, Falloon J, Lederman MM, Liewehr DJ, Spritzler J, et al. A potential role for interleukin-7 in T-cell homeostasis. Blood 2001; 97:2983–2990.
7. Llano A, Barretina J, Gutierrez A, Blanco J, Cabrera C, Clotet B, Este JA. Interleukin-7 in plasma correlates with CD4 T-cell depletion and may be associated with emergence of syncytium-inducing variants in human immunodeficiency virus type 1-positive individuals. J Virol 2001; 75:10319–10325.
8. Mastroianni CM, Forcina G, d'Ettorre G, Lichtner M, Mengoni F, D'Agostino C, Vullo V. Circulating levels of interleukin-7 in antiretroviral-naive and highly active antiretroviral therapy-treated HIV-infected patients. HIV Clin Trials 2001; 2:108–112.
9. Napolitano LA, Grant RM, Deeks SG, Schmidt D, De Rosa SC, Herzenberg LA, et al. Increased production of IL-7 accompanies HIV-1-mediated T-cell depletion: implications for T-cell homeostasis. Nat Med 2001; 7:73–79.
10. Malaspina A, Moir S, Chaitt DG, Rehm CA, Kottilil S, Falloon J, Fauci AS. Idiopathic CD4+ T lymphocytopenia is associated with increases in immature/transitional B cells and serum levels of IL-7. Blood 2007; 109:2086–2088.
11. Cox AL, Thompson SA, Jones JL, Robertson VH, Hale G, Waldmann H, et al. Lymphocyte homeostasis following therapeutic lymphocyte depletion in multiple sclerosis. Eur J Immunol 2005; 35:3332–3342.
12. Bolotin E, Annett G, Parkman R, Weinberg K. Serum levels of IL-7 in bone marrow transplant recipients: relationship to clinical characteristics and lymphocyte count. Bone Marrow Transplant 1999; 23:783–788.
13. Isgro A, Aiuti F, Mezzaroma I, Franchi F, Mazzone AM, Lebba F, Aiuti A. Interleukin 7 production by bone marrow-derived stromal cells in HIV-1-infected patients during highly active antiretroviral therapy. AIDS 2002; 16:2231–2232.
14. Paiardini M, Cervasi B, Albrecht H, Muthukumar A, Dunham R, Gordon S, et al. Loss of CD127 expression defines an expansion of effector CD8+ T cells in HIV-infected individuals. J Immunol 2005; 174:2900–2909.
15. Rethi B, Fluur C, Atlas A, Krzyzowska M, Mowafi F, Grutzmeier S, et al. Loss of IL-7Ralpha is associated with CD4 T-cell depletion, high interleukin-7 levels and CD28 down-regulation in HIV infected patients. AIDS 2005; 19:2077–2086.
16. Sasson SC, Zaunders JJ, Zanetti G, King EM, Merlin KM, Smith DE, et al. Increased plasma interleukin-7 level correlates with decreased CD127 and Increased CD132 extracellular expression on T cell subsets in patients with HIV-1 infection. J Infect Dis 2006; 193:505–514.
17. Koesters SA, Alimonti JB, Wachihi C, Matu L, Anzala O, Kimani J, et al. IL-7Ralpha expression on CD4+ T lymphocytes decreases with HIV disease progression and inversely correlates with immune activation. Eur J Immunol 2006; 36:336–344.
18. Resino S, Perez A, Leon JA, Gurbindo MD, Munoz-Fernandez MA. Interleukin-7 levels before highly active antiretroviral therapy may predict CD4+ T-cell recovery and virological failure in HIV-infected children. J Antimicrob Chemother 2006; 57:798–800.
19. Ruiz-Mateos E, de la Rosa R, Franco JM, Martinez-Moya M, Rubio A, Soriano N, et al. Endogenous IL-7 is associated with increased thymic volume in adult HIV-infected patients under highly active antiretroviral therapy. AIDS 2003; 17:947–954.
20. Beq S, Rannou MT, Fontanet A, Delfraissy JF, Theze J, Colle JH. HIV infection: prehighly active antiretroviral therapy IL-7 plasma levels correlate with long-term CD4 cell count increase after treatment. AIDS 2004; 18:563–565.
21. Mussini C, Pinti M, Borghi V, Nasi M, Amorico G, Monterastelli E, et al. Features of ‘CD4-exploders’, HIV-positive patients with an optimal immune reconstitution after potent antiretroviral therapy. AIDS 2002; 16:1609–1616.
22. Boulassel MR, Smith GH, Gilmore N, Klein M, Murphy T, MacLeod J, et al. Interleukin-7 levels may predict virological response in advanced HIV-1-infected patients receiving lopinavir/ritonavir-based therapy. HIV Med 2003; 4:315–320.
23. Boulassel MR, Samson J, Khammy A, Lapointe N, Soudeyns H, Routy JP. Predictive value of interleukin-7 levels for virological response to treatment in HIV-1-infected children. Viral Immunol 2007; 20:649–656.
24. Muthukumar A, Zhou D, Paiardini M, Barry AP, Cole KS, McClure HM, et al. Timely triggering of homeostatic mechanisms involved in the regulation of T-cell levels in SIVsm-infected sooty mangabeys. Blood 2005; 106:3839–3845.
25. Kopka J, Mecikovsky D, Aulicino PC, Mangano AM, Rocco CA, Bologna R, Sen L. High IL-7 plasma levels may induce and predict the emergence of HIV-1 virulent strains in pediatric infection. J Clin Virol 2005; 33:237–242.
26. Fluur C, De Milito A, Fry TJ, Vivar N, Eidsmo L, Atlas A, et al. Potential role for IL-7 in Fas-mediated T cell apoptosis during HIV infection. J Immunol 2007; 178:5340–5350.
27. Jaleco S, Swainson L, Dardalhon V, Burjanadze M, Kinet S, Taylor N. Homeostasis of naive and memory CD4+ T cells: IL-2 and IL-7 differentially regulate the balance between proliferation and Fas-mediated apoptosis. J Immunol 2003; 171:61–68.
28. Lelievre JD, Petit F, Arnoult D, Ameisen JC, Estaquier J. Interleukin 7 increases human immunodeficiency virus type 1 LAI-mediated Fas-induced T-cell death. J Virol 2005; 79:3195–3199.
29. Link A, Vogt TK, Favre S, Britschgi MR, Acha-Orbea H, Hinz B, et al. Fibroblastic reticular cells in lymph nodes regulate the homeostasis of naive T cells. Nat Immunol 2007; 8:1255–1265.
30. Schacker TW, Nguyen PL, Beilman GJ, Wolinsky S, Larson M, Reilly C, Haase AT. Collagen deposition in HIV-1 infected lymphatic tissues and T cell homeostasis. J Clin Invest 2002; 110:1133–1139.
31. Schacker TW, Reilly C, Beilman GJ, Taylor J, Skarda D, Krason D, et al. Amount of lymphatic tissue fibrosis in HIV infection predicts magnitude of HAART-associated change in peripheral CD4 cell count. AIDS 2005; 19:2169–2171.
32. Biancotto A, Grivel JC, Iglehart SJ, Vanpouille C, Lisco A, Sieg SF, et al. Abnormal activation and cytokine spectra in lymph nodes of people chronically infected with HIV-1. Blood 2007; 109:4272–4279.
33. Mazzucchelli R, Durum SK. Interleukin-7 receptor expression: intelligent design. Nat Rev Immunol 2007; 7:144–154.
34. Golden-Mason L, Burton JR Jr, Castelblanco N, Klarquist J, Benlloch S, Wang C, Rosen HR. Loss of IL-7 receptor alpha-chain (CD127) expression in acute HCV infection associated with viral persistence. Hepatology 2006; 44:1098–1109.
35. Kim HR, Hong MS, Dan JM, Kang I. Altered IL-7Ralpha expression with aging and the potential implications of IL-7 therapy on CD8+ T-cell immune responses. Blood 2006; 107:2855–2862.
36. Dunham RM, Cervasi B, Brenchley JM, Albrecht H, Weintrob A, Sumpter B, et al. CD127 and CD25 expression defines CD4+ T cell subsets that are differentially depleted during HIV infection. J Immunol 2008; 180:5582–5592.
37. Vivar N, Thang PH, Atlas A, Chiodi F, Rethi B. Potential role of CD8+CD28− T lymphocytes in immune activation during HIV-1 infection. AIDS 2008; 22:1083–1086.
38. Vassena L, Proschan M, Fauci AS, Lusso P. Interleukin 7 reduces the levels of spontaneous apoptosis in CD4+ and CD8+ T cells from HIV-1-infected individuals. Proc Natl Acad Sci USA 2007; 104:2355–2360.
39. Colle JH, Moreau JL, Fontanet A, Lambotte O, Delfraissy JF, Theze J. The correlation between levels of IL-7Ralpha expression and responsiveness to IL-7 is lost in CD4 lymphocytes from HIV-infected patients. AIDS 2007; 21:101–103.
40. Colle JH, Moreau JL, Fontanet A, Lambotte O, Joussemet M, Jacod S, et al. Regulatory dysfunction of the interleukin-7 receptor in CD4 and CD8 lymphocytes from HIV-infected patients: effects of antiretroviral therapy. J Acquir Immune Defic Syndr 2006; 42:277–285.
41. Nemes E, Lugli E, Nasi M, Ferraresi R, Pinti M, Bugarini R, et al. Immunophenotype of HIV+ patients during CD4 cell-monitored treatment interruption: role of the IL-7/IL-7 receptor system. AIDS 2006; 20:2021–2032.
42. Faller EM, McVey MJ, Kakal JA, MacPherson PA. Interleukin-7 receptor expression on CD8 T-cells is downregulated by the HIV Tat protein. J Acquir Immune Defic Syndr 2006; 43:257–269.
43. Vranjkovic A, Crawley AM, Gee K, Kumar A, Angel JB. IL-7 decreases IL-7 receptor alpha (CD127) expression and induces the shedding of CD127 by human CD8+ T cells. Int Immunol 2007; 19:1329–1339.
44. Zaunders JJ, Ip S, Munier ML, Kaufmann DE, Suzuki K, Brereton C, et al. Infection of CD127+ (interleukin-7 receptor+) CD4+ cells and overexpression of CTLA-4 are linked to loss of antigen-specific CD4 T cells during primary human immunodeficiency virus type 1 infection. J Virol 2006; 80:10162–10172.
45. Chene L, Nugeyre MT, Guillemard E, Moulian N, Barre-Sinoussi F, Israel N. Thymocyte-thymic epithelial cell interaction leads to high-level replication of human immunodeficiency virus exclusively in mature CD4(+) CD8(−) CD3(+) thymocytes: a critical role for tumor necrosis factor and interleukin-7. J Virol 1999; 73:7533–7542.
46. Steffens CM, Managlia EZ, Landay A, Al-Harthi L. Interleukin-7-treated naive T cells can be productively infected by T-cell-adapted and primary isolates of human immunodeficiency virus 1. Blood 2002; 99:3310–3318.
47. Uittenbogaart CH, Boscardin WJ, Anisman-Posner DJ, Koka PS, Bristol G, Zack JA. Effect of cytokines on HIV-induced depletion of thymocytes in vivo. AIDS 2000; 14:1317–1325.
48. Wang FX, Xu Y, Sullivan J, Souder E, Argyris EG, Acheampong EA, et al. IL-7 is a potent and proviral strain-specific inducer of latent HIV-1 cellular reservoirs of infected individuals on virally suppressive HAART. J Clin Invest 2005; 115:128–137.
49. Lang KS, Recher M, Navarini AA, Harris NL, Lohning M, Junt T, et al. Inverse correlation between IL-7 receptor expression and CD8 T cell exhaustion during persistent antigen stimulation. Eur J Immunol 2005; 35:738–745.
50. Wherry EJ, Barber DL, Kaech SM, Blattman JN, Ahmed R. Antigen-independent memory CD8 T cells do not develop during chronic viral infection. Proc Natl Acad Sci USA 2004; 101:16004–16009.
51. Sun JC, Williams MA, Bevan MJ. CD4+ T cells are required for the maintenance, not programming, of memory CD8+ T cells after acute infection. Nat Immunol 2004; 5:927–933.
52. Boutboul F, Puthier D, Appay V, Pelle O, Ait-Mohand H, Combadiere B, et al. Modulation of interleukin-7 receptor expression characterizes differentiation of CD8 T cells specific for HIV, EBV and CMV. AIDS 2005; 19:1981–1986.
53. Wherry EJ, Day CL, Draenert R, Miller JD, Kiepiela P, Woodberry T, et al. HIV-specific CD8 T cells express low levels of IL-7Ralpha: implications for HIV-specific T cell memory. Virology 2006; 353:366–373.
54. Pawelec G, Akbar A, Caruso C, Effros R, Grubeck-Loebenstein B, Wikby A. Is immunosenescence infectious? Trends Immunol 2004; 25:406–410.
55. Alves NL, van Leeuwen EM, Remmerswaal EB, Vrisekoop N, Tesselaar K, Roosnek E, et al. A new subset of human naive CD8+ T cells defined by low expression of IL-7R alpha. J Immunol 2007; 179:221–228.
56. Park JH, Yu Q, Erman B, Appelbaum JS, Montoya-Durango D, Grimes HL, Singer A. Suppression of IL7Ralpha transcription by IL-7 and other prosurvival cytokines: a novel mechanism for maximizing IL-7-dependent T cell survival. Immunity 2004; 21:289–302.
57. Borghesi LA, Yamashita Y, Kincade PW. Heparan sulfate proteoglycans mediate interleukin-7-dependent B lymphopoiesis. Blood 1999; 93:140–148.
58. Kimura K, Matsubara H, Sogoh S, Kita Y, Sakata T, Nishitani Y, et al. Role of glycosaminoglycans in the regulation of T cell proliferation induced by thymic stroma-derived T cell growth factor. J Immunol 1991; 146:2618–2624.
59. Beq S, Nugeyre MT, Ho Tsong Fang R, Gautier D, Legrand R, Schmitt N, et al. IL-7 induces immunological improvement in SIV-infected rhesus macaques under antiviral therapy. J Immunol 2006; 176:914–922.
60. Nugeyre MT, Monceaux V, Beq S, Cumont MC, Ho Tsong Fang R, Chene L, et al. IL-7 stimulates T cell renewal without increasing viral replication in simian immunodeficiency virus-infected macaques. J Immunol 2003; 171:4447–4453.
61. Rosenberg SA, Sportes C, Ahmadzadeh M, Fry TJ, Ngo LT, Schwarz SL, et al. IL-7 administration to humans leads to expansion of CD8+ and CD4+ cells but a relative decrease of CD4+ T-regulatory cells. J Immunother 2006; 29:313–319.
62. Sportes C, Hakim FT, Memon SA, Zhang H, Chua KS, Brown MR, et al. Administration of rhIL-7 in humans increases in vivo TCR repertoire diversity by preferential expansion of naive T cell subsets. J Exp Med 2008; 205:1701–1714.
63. Munitic I, Williams JA, Yang Y, Dong B, Lucas PJ, El Kassar N, et al. Dynamic regulation of IL-7 receptor expression is required for normal thymopoiesis. Blood 2004; 104:4165–4172.
64. Muthukumar A, Wozniakowski A, Gauduin MC, Paiardini M, McClure HM, Johnson RP, et al. Elevated interleukin-7 levels not sufficient to maintain T-cell homeostasis during simian immunodeficiency virus-induced disease progression. Blood 2004; 103:973–979.
65. Fluur C, Rethi B, Thang PH, Vivar N, Mowafi F, Lopalco L, et al. Relationship between serum IL-7 concentrations and lymphopenia upon different levels of HIV immune control. AIDS 2007; 21:1048–1050.
66. Margolick JB, Munoz A, Donnenberg AD, Park LP, Galai N, Giorgi JV, et al. Failure of T-cell homeostasis preceding AIDS in HIV-1 infection. The Multicenter AIDS Cohort Study. Nat Med 1995; 1:674–680.
67. Trautmann L, Janbazian L, Chomont N, Said EA, Gimmig S, Bessette B, et al. Upregulation of PD-1 expression on HIV-specific CD8+ T cells leads to reversible immune dysfunction. Nat Med 2006; 12:1198–1202.
68. Kaufmann DE, Kavanagh DG, Pereyra F, Zaunders JJ, Mackey EW, Miura T, et al. Upregulation of CTLA-4 by HIV-specific CD4(+) T cells correlates with disease progression and defines a reversible immune dysfunction. Nat Immunol 2007; 8:1246–1254.
69. Effros RB. Impact of the Hayflick Limit on T cell responses to infection: lessons from aging and HIV disease. Mech Ageing Dev 2004; 125:103–106.
70. Papagno L, Spina CA, Marchant A, Salio M, Rufer N, Little S, et al. Immune activation and CD8+ T-cell differentiation towards senescence in HIV-1 infection. PLoS Biol 2004; 2:E20.
71. Carini C, McLane MF, Mayer KH, Essex M. Dysregulation of interleukin-7 receptor may generate loss of cytotoxic T cell response in human immunodeficiency virus type 1 infection. Eur J Immunol 1994; 24:2927–2934.
72. Lipman M, Breen R. Immune reconstitution inflammatory syndrome in HIV. Curr Opin Infect Dis 2006; 19:20–25.
73. Krupica T Jr, Fry TJ, Mackall CL. Autoimmunity during lymphopenia: a two-hit model. Clin Immunol 2006; 120:121–128.
74. Albuquerque AS, Cortesao CS, Foxall RB, Soares RS, Victorino RM, Sousa AE. Rate of increase in circulating IL-7 and loss of IL-7Ralpha expression differ in HIV-1 and HIV-2 infections: two lymphopenic diseases with similar hyperimmune activation but distinct outcomes. J Immunol 2007; 178:3252–3259.
75. Fry TJ, Moniuszko M, Creekmore S, Donohue SJ, Douek DC, Giardina S, et al. IL-7 therapy dramatically alters peripheral T-cell homeostasis in normal and SIV-infected nonhuman primates. Blood 2003; 101:2294–2299.
76. Malaspina A, Moir S, Ho J, Wang W, Howell ML, O'Shea MA, et al. Appearance of immature/transitional B cells in HIV-infected individuals with advanced disease: correlation with increased IL-7. Proc Natl Acad Sci USA 2006; 103:2262–2267.
77. Unutmaz D, KewalRamani VN, Marmon S, Littman DR. Cytokine signals are sufficient for HIV-1 infection of resting human T lymphocytes. J Exp Med 1999; 189:1735–1746.
78. Jourdan P, Vendrell JP, Huguet MF, Segondy M, Bousquet J, Pene J, Yssel H. Cytokines and cell surface molecules independently induce CXCR4 expression on CD4+ CCR7+ human memory T cells. J Immunol 2000; 165:716–724.
79. Schmitt N, Chene L, Boutolleau D, Nugeyre MT, Guillemard E, Versmisse P, et al. Positive regulation of CXCR4 expression and signaling by interleukin-7 in CD4+ mature thymocytes correlates with their capacity to favor human immunodeficiency X4 virus replication. J Virol 2003; 77:5784–5793.
80. Scripture-Adams DD, Brooks DG, Korin YD, Zack JA. Interleukin-7 induces expression of latent human immunodeficiency virus type 1 with minimal effects on T-cell phenotype. J Virol 2002; 76:13077–13082.
81. Stranges PB, Watson J, Cooper CJ, Choisy-Rossi CM, Stonebraker AC, Beighton RA, et al. Elimination of antigen-presenting cells and autoreactive T cells by Fas contributes to prevention of autoimmunity. Immunity 2007; 26:629–641.
82. Baumler CB, Bohler T, Herr I, Benner A, Krammer PH, Debatin KM. Activation of the CD95 (APO-1/Fas) system in T cells from human immunodeficiency virus type-1-infected children. Blood 1996; 88:1741–1746.
83. Boudet F, Lecoeur H, Gougeon ML. Apoptosis associated with ex vivo down-regulation of Bcl-2 and up-regulation of Fas in potential cytotoxic CD8+ T lymphocytes during HIV infection. J Immunol 1996; 156:2282–2293.
84. Grelli S, Campagna S, Lichtner M, Ricci G, Vella S, Vullo V, et al. Spontaneous and anti-Fas-induced apoptosis in lymphocytes from HIV-infected patients undergoing highly active antiretroviral therapy. AIDS 2000; 14:939–949.
85. Hosaka N, Oyaizu N, Than S, Pahwa S. Correlation of loss of CD4 T cells with plasma levels of both soluble form Fas (CD95) Fas ligand (FasL) in HIV-infected infants. Clin Immunol 2000; 95:20–25.
86. Katsikis PD, Wunderlich ES, Smith CA, Herzenberg LA, Herzenberg LA. Fas antigen stimulation induces marked apoptosis of T lymphocytes in human immunodeficiency virus-infected individuals. J Exp Med 1995; 181:2029–2036.
87. Mitra D, Steiner M, Lynch DH, Staiano-Coico L, Laurence J. HIV-1 upregulates Fas ligand expression in CD4+ T cells in vitro and in vivo: association with Fas-mediated apoptosis and modulation by aurintricarboxylic acid. Immunology 1996; 87:581–585.
88. Silvestris F, Cafforio P, Frassanito MA, Tucci M, Romito A, Nagata S, Dammacco F. Overexpression of Fas antigen on T cells in advanced HIV-1 infection: differential ligation constantly induces apoptosis. AIDS 1996; 10:131–141.
89. Li Q, Duan L, Estes JD, Ma ZM, Rourke T, Wang Y, et al. Peak SIV replication in resting memory CD4+ T cells depletes gut lamina propria CD4+ T cells. Nature 2005; 434:1148–1152.
90. Brugnoni D, Airo P, Pennacchio M, Carella G, Malagoli A, Ugazio AG, et al. Immune reconstitution after bone marrow transplantation for combined immunodeficiencies: down-modulation of Bcl-2 and high expression of CD95/Fas account for increased susceptibility to spontaneous and activation-induced lymphocyte cell death. Bone Marrow Transplant 1999; 23:451–457.
91. Hakim FT, Cepeda R, Kaimei S, Mackall CL, McAtee N, Zujewski J, et al. Constraints on CD4 recovery postchemotherapy in adults: thymic insufficiency and apoptotic decline of expanded peripheral CD4 cells. Blood 1997; 90:3789–3798.
92. Rethi B, Vivar N, Sammicheli S, Fluur C, Ruffin N, Atlas A, et al. Priming of T cells to Fas-mediated proliferative signals by interleukin-7. Blood 2008; 112:1195–1204.
93. Murali-Krishna K, Ahmed R. Cutting edge: naive T cells masquerading as memory cells. J Immunol 2000; 165:1733–1737.
94. Ahmadzadeh M, Rosenberg SA. IL-2 administration increases CD4+ CD25(hi) Foxp3+ regulatory T cells in cancer patients. Blood 2006; 107:2409–2414.
95. Sereti I, Imamichi H, Natarajan V, Imamichi T, Ramchandani MS, Badralmaa Y, et al. In vivo expansion of CD4CD45RO-CD25 T cells expressing foxP3 in IL-2-treated HIV-infected patients. J Clin Invest 2005; 115:1839–1847.
96. Churchman SM, Ponchel F. Interleukin-7 in rheumatoid arthritis. Rheumatology (Oxford) 2008; 47:753–759.
97. van Roon JA, Lafeber FP. Role of interleukin-7 in degenerative and inflammatory joint diseases. Arthritis Res Ther 2008; 10:107.
98. Calzascia T, Pellegrini M, Lin A, Garza KM, Elford AR, Shahinian A, et al. CD4 T cells, lymphopenia, and IL-7 in a multistep pathway to autoimmunity. Proc Natl Acad Sci USA 2008; 105:2999–3004.
99. Zandman-Goddard G, Shoenfeld Y. HIV and autoimmunity. Autoimmun Rev 2002; 1:329–337.
100. Appay V, Rowland-Jones SL. Premature ageing of the immune system: the cause of AIDS? Trends Immunol 2002; 23:580–585.
101. Ouyang Q, Wagner WM, Wikby A, Walter S, Aubert G, Dodi AI, et al. Large numbers of dysfunctional CD8+ T lymphocytes bearing receptors for a single dominant CMV epitope in the very old. J Clin Immunol 2003; 23:247–257.
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