Modulating T cell Homeostasis with IL-7: Preclinical and Clinical Studies

Abstract

Interleukin-7 (IL-7) is required for the development and survival of T cells and plays a critical role in modulating T cell homeostasis. This review will address current understanding of IL-7 biology, review recent clinical experiences and discuss potential future clinical applications of IL-7, or IL-7 blockade, in the setting of disease.

IL-7 production and IL-7 receptor signaling

The human IL-7 gene was identified in 1988[1] and is located on chromosome 8q12-13[2]. Homology between human and murine IL-7 coding sequences is 81% [3]. Human IL-7 can bind and signal via the murine IL-7 receptor, and visa versa[4, 5]. IL-7 is produced primarily by non-hematopoietic stromal and epithelial cells, including fibroblastic reticular cells in the T cell zone of lymphoid organs, bone marrow stromal cells, major histocompatibility complex (MHC) class II⁺ thymic epithelial cells, liver and intestinal epithelial cells, endothelial cells, fibroblasts, keratinocytes, follicular dendritic cells and smooth muscle cells[6–8]. Dendritic cells and macrophages also produce IL-7 in lesser amounts[3] and IL-7 is expressed in most organs including the brain[9].

IL-7 binds to the IL-7 receptor (IL-7R), a heterodimer consisting of the IL-7 receptor alpha chain (IL-7Rα or CD127) and the common gamma (γc or CD132) chain. Both CD127 and CD132 are also components of other cytokine receptors. CD127 is shared by thymic stromal lymphopoietin (TSLP), and IL-2, IL-4, IL-9, IL-15 and IL-21 share CD132. The IL-7R heterodimer is expressed at high levels on most mature T cells excluding effector T cells and senescent memory populations (Figure 1). IL-7R is also expressed on dendritic cells and monocytes[10], on subsets of developing B cells and T cells, but not on mature B cells[3]. Stimulation of IL-7R activates the Janus kinase/signal transducers and activators of transcription (JAK/STAT - mainly JAK1, JAK3 and STAT5), src family kinases and phosphatidylinositol 3-kinase (PI3K-AKT) signaling pathways[11, 12].

Figure 1. Expression levels of IL-7 receptor subunits across T cell development.

CD132 (γc-receptor) is expressed throughout T cell differentiation, whereas CD127 (IL-7Rα) expression is seen early in thymopoiesis, then on recent thymic emigrants and naïve T cells. Effector T cells have low levels of CD127 expression, but increase levels once they acquire a memory phenotype during early antigen exposure. Because CD132 expression is relatively constant, regulation of CD127 expression largely dictates availability of the IL-7 receptor. Subsets with high levels of CD127 expression are potential targets for expansion with IL-7 therapy.

Because IL-7 is present in most tissues and nearly all peripheral T cells express IL-7R, the mechanisms by which IL-7 signaling are regulated are of great interest. IL-7 production was previously described as “constitutive” but recent data has demonstrated direct control of IL-7 production via a simple IL-7R mediated feedback loop[13]. Under normal circumstances, physiologic utilization of IL-7 by the T cell pool leads to stable levels despite continuous production, and IL-7 is measurable in the serum of normal humans[14]. However, when T cell numbers are reduced, circulating and tissue IL-7 levels rise due to diminished utilization, and this is paradoxically associated with diminished production of IL-7. Other axes through which IL-7 production is regulated may also exist. Murine studies have reported that transforming growth factor-beta (TGF-β) levels relate inversely to IL-7 production[15, 16], and nocturnal sleep has been shown to be associated with increased serum concentrations of IL-7, particularly during the late phase of sleep[17].

When T cell numbers are normal, IL-7 signaling may also be regulated via competition for limiting amounts of IL-7, a process that may be impacted by relative levels of IL7R expression. Indeed, IL-7R modulation is likely to be a primary factor regulating T cell responses to this ubiquitous cytokine. Because CD132 expression is relatively constant, regulation of CD127 expression largely dictates availability of the IL-7R (Figure 2). IL-7 signaling downregulates CD127 expression in mature T cells, and such downregulation may serve to maximize the number of T cells that can respond to the limiting amounts of IL-7 available when normal T cell numbers are present[18]. IL-7 mediated downregulation of CD127 expression may also serve to limit uncontrolled T cell proliferation when IL-7 levels are available in excess. This is illustrated by findings from recent IL-7 clinical trials wherein supraphysiologic levels of IL-7 led to downmodulation of CD127 expression, which limited IL-7 mediated proliferation despite continued high levels of the cytokine[19].

Figure 2. Naïve T cell production is hampered through a normal aging-associated involution of the thymus.

While children have the capacity to generate large numbers of naïve T cells by thymopoiesis, thymic-dependent T cell production in adults is limited, rendering lymphopenic adults dependent upon homeostatic peripheral expansion for T cell regeneration. A similar dependence of peripheral expansion to maintain T cell numbers occurs with advancing age, and leads to limited T cell receptor repertoire diversity. IL-7 therapy may selectively expand small numbers of recent thymic emigrants and naïve T cells, which would lead to increased diversity of the peripheral T cell repertoire. IL-7 therapy may also enhance thymic throughput in some settings, also potentially increasing TCR repertoire diversity. Figure adapted from the 2007 NIH Annual Report.

Stimuli that augment T cell activation, including IL-2, IL-4 and IL-15, as well as IL-6 also generally diminish CD127 transcription[18, 20], and memory and chronically activated T cells express lower levels of CD127 than naïve cells. One important exception to this rule occurs during the natural evolution of an immune response, where upregulation of CD127 occurs on a small number of effector cells during the contraction phase, and these effectors are selectively recruited to the central memory T cell pool[21]. Thus, dynamic regulation of CD127 expression plays a critical role in the effector-memory T cell transition. CD127 expression is also inversely correlated with FoxP3 expression on regulatory T cells, and as a result, low CD127 expression is now an important marker for the regulatory CD4⁺CD25⁺ subset[22, 23]. Several stimuli can increase expression of CD127, including type 1 interferons[24], FLT3 ligand[25], tumor necrosis factor[26, 27], and glucocorticoids [28–30]. Indeed, a glucocorticoid response element in the IL-7Rα gene is conserved in humans, mice and rats[10]. Finally, recent genetic studies have suggested that polymorphisms in CD127 are important susceptibility factors for autoimmune disease, specifically multiple sclerosis (discussed below). Together, it is clear that CD127 signaling plays central role in IL-7 biology, with regulation of CD127 expression serving as a dynamic and sensitive modulator of IL-7 bioactivity in vivo.

IL-7 in Lymphocyte Development

T cell progenitors emigrate from the bone marrow to the thymus, where their developmental stages can be identified by changes in expression of surface markers. Within the thymus, there is a tightly controlled spectrum of CD127 expression on thymocytes at various stages of T cell differentiation[31]. Thymocytes begin as a CD4⁻CD8⁻ “double negative” (DN) population which gradually show diminished expression of CD44, and acquisition then loss of CD25, as they progress through well defined DN1, DN2, DN3 and DN4 subsets. The DN1 subset that has T lineage potential does not express CD127[32], however upregulation of CD127 is observed in the DN2 to DN3 compartments, followed by low to absent expression in the DN4 subset[33, 34]. With the loss of CD44, which occurs at the DN2-3 transition, cells undergo rearrangement of the T cell receptor (TCR)β, γ, and δ genes. During this phase, IL-7R-mediated survival signals and recombinase activity appear to be critical[35]. Subsequently, DN cells acquire expression of CD3 and CD4/CD8, to become so-called double positive thymocytes. Double positive thymocytes comprise the majority of the thymocyte population, but only a minority of these cells is finally selected to become CD4⁺ or CD8⁺ single positive thymocytes and ultimately emigrate from the thymus. CD127 is not expressed by CD4⁺CD8⁺ double positive thymocytes, but is expressed again by mature single positive cells[3] and is highly expressed on recent thymic emigrants[36, 37].

Thymic cellularity is reduced 20-fold in IL-7−/− mice, and to 0.01–10% of normal in CD127−/− mice[3]. Despite diminished thymocyte numbers in these mice, all of the major thymic subsets are present in a similar proportion to wild type controls, and in IL-7−/− mice, thymopoiesis can be augmented by IL-7 therapy. These results are due to a requirement for IL-7 in thymocyte expansion during the early phases of thymic development as well as IL-7’s contribution to expression of the Rag-1 gene, which is required for TCR recombination. IL-7’s effects on recombinase expression are non-redundant for development of γδ T cells[8, 38] as this subset does not develop in IL-7−/− mice[14]. IL-7 signaling also plays an essential role in preventing apoptosis in developing thymocytes. In fact, thymocytes of CD127−/− mice show low bcl-2 levels, and forced expression of a Bcl-2 transgene in these mice largely rescues T cell development and function[39].

In humans, severe combined immunodeficiency characterized by profound T cell deficiency is observed in patients with genetic errors of IL-7 signaling. For example, one of the major causes of severe combined immunodeficiency syndrome (SCID) in humans is JAK3 deficiency [40] and mutations in CD132 result in the X-linked form of SCID, characterized by profound deficits in T cell and natural killer (NK) cell development[41] due to absence of IL-7 and IL-15 signaling respectively. Mutations in CD127 have also been reported and lead to T cell deficiency but relatively normal NK and B cell numbers[42]. Thus, IL-7 is an essential, nonredundant cytokine for human T cell development but is not required for B cell and NK cell development in humans.

Peripheral T Cell Homeostasis

IL-7 is a key survival factor for naïve T cells and well as a modulator of homeostatic peripheral expansion (HPE). HPE is a phenomenon whereby adoptively transferred or residual T cells expand in T cell-depleted hosts as a means for restoring T cell number and function. While high-affinity antigens drive some of this process, low affinity antigens play an important role in HPE, since expansion of T cells to a wide array of antigens is important for maintaining adequate T cell receptor diversity during conditions of lymphopenia[43–45]. As discussed above, IL-7 levels are increased in clinical conditions associated with lymphopenia, and strong inverse relationships have been observed between serum IL-7 levels and CD4⁺ counts in chemotherapy-induced lymphopenia, HIV-associated lymphopenia, idiopathic lymphopenia, following bone marrow transplantation, and following monoclonal antibody-mediated T cell-depleting therapy for autoimmune disease[46–48]. The elevated IL-7 levels present in these settings induce T cell expansion to self-antigens by providing costimulatory signals that allow low affinity antigen to trigger T cell receptor signaling. In addition, IL-7 provides anti-apoptotic signals by upregulating Bcl-2 and Mcl-1[49], inhibiting pro-apoptotic signals like BAX or BIM[40, 50], and maintaining longer telomeres in T cells[51]. Indeed, anti-apoptotic effects are important contributors to IL-7 biology as exemplified by data showing that IL-7 can partially protect T cells from apoptosis induced by glucocorticoids[52], cytokine withdrawal[53] and radiation[54].

Preclinical Studies and Early Clinical Studies of rhIL-7

When IL-7 is injected into normal mice, it leads to increased numbers of T cells in the peripheral lymphoid organs, and when injected into mice rendered lymphopenic by cyclophosphamide or total body irradiation in the context of BMT, it accelerates lymphoid regeneration leading to increased T cell numbers in the spleen and lymph nodes [55–57]. Studies such as these provided evidence that IL-7 enhances both thymic-dependent and thymic-independent immune reconstitution[58]. In some cases, enhanced thymic recovery was observed in recipients of IL-7, however thymectomized mice also show enhanced immune reconstitution with IL-7[59].

IL-7 therapy also augments responses to immunization, with a preferential enhancement of the immune response to weak or low affinity antigens. IL-7 given with a vaccine against the minor antigen H-Y expanded T cells that react to both CD4⁺ and CD8⁺ dominant antigens, increased expansion of T cells against subdominant CD8⁺ antigens, and enhanced the generation of CD8⁺ memory T cells, which persisted for several months after cessation of IL-7[43]. Given that most tumor antigens are self-antigens that are weak and poorly immunogenic, this feature made IL-7 an attractive candidate for adjuvant therapy for potentiating cancer immunotherapies.

Two phase I clinical trials in cancer patients have now been completed at the National Cancer Institute with recombinant human (rh) IL-7 (Table 1) provided by Cytheris, Inc (Rockville, MD). The first trial enrolled 16 adults, (12 men, 4 women) ranging from 20 – 71 years with refractory malignancy. In addition to standard phase I inclusion criteria, subjects were required to have a peripheral CD3⁺ count above 300/mm³ and biologic activity of rhIL7 was defined a priori as a 50% increase in peripheral blood CD3⁺ T cells over baseline[42]. The 4 tested doses of rhIL-7 were 3, 10, 30, and 60μg/kg per dose given subcutaneously every other day for 14 days. Doses of 10, 30 and 60μg/kg showed biologic activity. An MTD was not reached and the therapy was well tolerated, without evidence for capillary leak syndrome or significant inflammatory signs or symptoms, which have been seen with several other cytokines. Five out of 15 subjects developed non-neutralizing antibodies, but none developed neutralizing antibodies. No anti-tumor effects were observed.

Table 1.

Comparison of the first two Phase I trials with rhIL-7 at the National Cancer Institute

	Sportes et al. [19]	Rosenberg et al. [60]
Number of patients	16	12
Age range	20–71	20–67
rhIL-7 doses tested (mcg/kg/dose)	3, 10, 30, 601	3, 10, 30, 602
Total CD4 and CD8 expansion	↑	↑
Regulatory T cells	↓	↓
Naïve T cells	↑ CD4 and CD8	↑ trend only
Memory T cells	↑ Central > effector	↓ trend only
Peripheral B cells	No change	No change
Non-neutralizing antibodies	31%	100%
Neutralizing antibodies	0%	0%

¹every other day × 14 days

²every 3 days × 8 doses

In a dose dependent fashion, patients receiving IL-7 showed increases in both CD4⁺ and CD8⁺ T cells, coincident with increased rates of T cell cycling. The increases were observed both in peripheral blood and in secondary lymphoid tissues monitored by radiographic imaging. Interestingly however, thymic size did not increase with rhIL-7 therapy. In general, both CD4⁺ and CD8⁺ cells were increased to similar extents, and the increases persisted for several weeks after discontinuation of the drug. Despite CD4⁺ expansion, there was no significant increase in regulatory T cell numbers as monitored by FoxP3 expression. Lymphocyte expansion showed no significant relationship to age, yet naïve T cells, and in particular naïve CD8+ T cells, were preferentially induced to cycle, and as a result, their numbers increased to a greater extent than memory populations. Notably, TCR spectratyping revealed that there was increased T cell receptor diversity of the peripheral T cell pool following IL-7 therapy compared to that observed prior to IL-7 therapy. Significant changes in T cell receptor excision circle (TREC) numbers were not observed, suggesting that the primary mechanism of IL-7-mediated increase in T cell repertoire diversity was peripheral cycling. Central memory cells were also increased, but IL-7 therapy induced minimal expansion of effector memory cells. Therefore, IL-7 therapy selectively expands populations with the highest levels of CD127 expression, resulting in measurable increases in T cell receptor repertoire diversity. No significant expansion of mature B cell populations was observed[19].

A second rhIL-7 phase I trial enrolled 12 patients, 11 with metastatic melanoma and 1 with metastatic sarcoma, aged 20–67. The 4 tested doses were 3, 10, 30, and 60μg/kg per dose given subcutaneously every 3 days for a total of 8 doses. Patients also received 2 melanoma antigen peptides, gp100 and MART-1, in incomplete Freund’s adjuvant subcutaneously. Similar to the trial discussed above, no anti-tumor effects were observed, and all patients developed non-neutralizing antibodies to rhIL-7, but none developed neutralizing antibodies[60]. CD4⁺ and CD8⁺ T cell subsets increased in a dose-dependent manner, with a relative decrease in regulatory T cells and there was a trend toward an increase of naïve relative to memory T cells. B cell precursors in the bone marrow increased in some patients, but they were not reflected in the peripheral circulation. There was no evidence for increased immune reactivity against the melanoma peptides compared to historical controls, although two doses of such peptides were well below the doses required to induce measurable responses. Therefore the limited duration of IL-7 therapy rendered and the limited number of vaccines administered in this trial did not allow an adequate assessment of the activity of rhIL7 as a vaccine adjuvant.

Potential Future Applications of IL-7 Therapy

IL-7 Therapy to “Rejuvenate” the Peripheral T Cell Repertoire

As we age, the thymus involutes and the contribution of recent thymic emigrants to the peripheral naïve T cell pool decreases substantially. Further, as naïve T cells encounter foreign antigens, they undergo an irreversible differentiation to memory cells. While memory cells enhance immune competence to recall antigens, the diminished repertoire diversity of the memory pool compared to the naïve T cell pool can limit immune competence to new antigenic challenges[61, 62]. Indeed, the near absence of a naïve cell pool in the elderly results in a highly restricted repertoire of T cells available to respond to new antigens, and this likely contributes to the limited immune competence of this population[63]. Similar changes occur during lymphopenia, when HPE renders lymphopenic hosts devoid of naive cells, with a predominance of activated memory cells and highly restricted TCR repertoire diversity. IL-7 maintains the naive T cell population by inducing telomerase, allowing the T cell to bypass TCR stimulation[51, 63]. Thus, IL-7’s biologic effects on peripheral T cells pools appear to counteract the normal aging process (Figure 2), raising the prospect that IL-7 therapy could enhance overall immune competence in elderly populations or individuals who experience lymphopenia as a result of bone marrow transplantation, cytotoxic chemotherapy, HIV infection, monoclonal antibody therapy for autoimmune disease, etc. [31, 59]. Notably, among cytokines currently under study, IL-7 is unique in its capacity to selectively expand naïve T cell pools.

IL-7 therapy in HIV Infection

The hallmark of HIV infection is CD4⁺ T cell depletion, but the mechanisms responsible for this remain incompletely understood. Cytopathogenic effects of the virus accounts for some of the CD4⁺ T cell loss, but mathematical models demonstrate that the absolute number of infected CD4⁺ T cells is too small to account for the degree of cell death. A combination of indirect viral effects, such as chronic activation of the immune system leading to exhaustion of the T cell pool[64], and inadequate regeneration of lost T cells[65] are therefore also implicated. Furthermore, the capacity to respond specifically to the virus is lost and the absence of HIV-specific CD4⁺ and CD8⁺ responses is only partially restored by highly active anti-retroviral therapy (HAART).

Since IL-7 therapy potently enhances CD4⁺ T cell numbers, IL-7 is currently being explored as a candidate for therapeutic and vaccine adjuvant applications in HIV disease. The capacity of IL-7 to enhance naïve T cell numbers raises the prospect that IL-7 therapy could potentially serve as a first step in restoring HIV-specific CD4⁺ T cells, known to be lost in the early stages of viral infection[39]. Indeed, in SIV-infected monkeys, IL-7 therapy induced a remarkable CD4⁺ increase, particularly in naïve subsets[46, 66]. IL-7 has also been shown to enhance HIV-1-specific CD8⁺ proliferation and cytolytic activity in vitro [39], and CD4⁺ dependent humoral responses and cytotoxic CD8⁺ activity in mice immunized with HIV-1 envelope protein[66]. IL-7 can also protect CD4⁺ and CD8⁺ T cells against apoptosis induced by HIV-1[67], enhance the effects of IFN-α on HIV replication, and protect against CD4 depletion when given alone or with IFN-α by upregulating bcl-2[68].

Despite the promising features of IL-7 for potential utility in HIV infection, some investigators have also wondered if the elevated IL-7 contribute to HIV-1-associated immune pathophysiology, and several issues need to be considered in the context of considering IL-7-induced immune restoration in HIV infection. As discussed above, HIV infection-associated lymphopenia results in elevations in serum and tissue IL-7 levels[69] in both HIV-infected children and adults[39, 47]. As CD4 counts increase with HAART, the elevated IL-7 levels in the serum decrease. IL-7 stimulates HIV replication in vitro, as has been shown in thymic organ cultures[70–72] and IL-7 also enhances HIV entry into cells through CXCR4[73]. Finally, chronically elevated levels of IL-7 upregulate Fas (or CD95) expression, a death receptor, on naïve T cells, and stimulate Fas-induced apoptosis in T cell cultures infected with HIV-1. In HIV-infected patients, only the CD127⁺ naïve and memory subsets show elevated Fas expression. Thus, the Fas pathway has been implicated in the process of T cell depletion following HIV infection[74] and it remains possible that chronically elevated IL-7 can actually diminish CD4 numbers by predisposing to Fas-mediated cell death.

While IL-7-induced increases in viral load can largely be prevented by coadministration of effective antiviral therapy[75], IL-7 therapy has also been proposed as a means of activating latently infected CD4⁺ T cells by increasing the turnover rate of the latent viral reservoir to promote viral clearance. IL-7 has also been combined with an immunotoxin to activate and deplete viral residual disease[66].

IL-7 in Cancer Therapy

There are several potential features of IL-7 that could be exploited in the context of cancer therapy, and IL-7 can prolong survival of tumor-bearing hosts in several preclinical tumor models[76, 77]. First is a potential role for IL-7 in enhancing immune reconstitution in cancer patients following cytotoxic chemotherapy. Lymphopenia occurs commonly following dose-intensive cytotoxic regimens, and T cell counts, most notably CD4⁺ T cells, typically remain well below the normal range for months to years following chemotherapy[78–80]. Age plays a critical role in determining the rate and degree of recovery since children with a functioning thymus can regenerate a naïve T cell repertoire more quickly and more completely than adults[81]. Given that IL-7 therapy enhances immune reconstitution and can augment even limited thymic function by facilitating peripheral expansion of even small numbers of recent thymic emigrants, IL-7 therapy could potentially repair the immune system of patients who have been depleted by cytotoxic chemotherapy[69].

Another strategy for the use of IL-7 in the context of anti-tumor therapy is to utilize IL-7 in the context of adoptive immunotherapy, wherein cells are activated and expanded ex vivo, then infused back into the patient. Current technologies can readily expand T cells ex vivo 4–5 logs, and IL-7 is among the cytokines that can be used to sustain or optimize T cell expansion ex vivo. In one example, addition of IL-7 and IL-15 to agonist antibodies against CD3 and CD28 enhanced expansion of CD4⁺ T cells specific for hepatitis B surface antigen to more than 4000-fold in 14 days, and resulted in up-regulation of NKG2D, a co-stimulatory molecule whose ligand has been expressed on tumors[82]. IL-7 was more effective in this regard than IL-2, which induced less expansion and increased sensitivity to apoptosis.

Systemic therapy with IL-7 may also be able to enhance survival of adoptively transferred T cells in vivo or augment other immune-based therapies. When IL-7 was combined with local hyperthermia, enhanced anti-tumour activity was attained in mice with melanoma[83] and in a colon cancer xenograft model, administration of human IL-7 and lymphocytes resulted in prolonged survival[84]. Several investigators have used IL-7 to improve the cytotoxicity of cytokine-induced killer cells[12]. In a preclinical xenograft model using human neuroblastoma cells, IL-7 added to a regimen of gamma-delta T cells and an anti-GD2 antibody (hu14.18) significantly improved survival[85]. In humans, melanoma cells engineered to express IL-7 were reinfused into patients in a phase I trial, leading to increased melanoma reactive T cells in 3 out of 6 patients, and minor anti-tumor responses in 2 patients[86]. Finally, in a recent clinical study of adoptive immunotherapy in lymphopenic hosts, CD127 expression was one of only a few features which correlated with T cell persistence following adoptive T cell therapy[87].

One potential caveat of the use of IL-7 in cancer therapy is the possibility that IL-7 may signal tumors themselves. While CD127 expression has been reported on some adult solid tumors[88], we have not seen CD127 receptor or mRNA expression on most pediatric tumor cell lines (unpublished observations). There also has not been good data to demonstrate that a functional IL-7 receptor complex is expressed on solid tumors or that IL-7 contributes to tumor growth. Of two Ewing sarcoma lines that were positive for CD127 expression, despite also expressing CD132, none showed phosphorylation of STAT5 after stimulation with IL-7 in vitro (data not shown), implying the IL-7 receptor was nonfunctional. In contrast, it is likely that IL-7 plays a role in either initiation or maintenance of some lymphocyte-derived tumors. This has been studied at length in pediatric ALL, where interruption of PI3K signaling via rapamycin can transiently diminish leukemic growth, but escape from this therapy occurs via an IL-7-dependent pathway[89]. Thus while many promising and beneficial effects of IL-7 therapy exist in the literature, these studies highlight the potential for IL-7 blockade as a therapeutic modality as well.

IL-7 and Hematopoietic Stem Cell Transplant

Regeneration of T cells after hematopoietic stem cell transplant (HSCT) can occur de novo through the thymus, or through thymic-independent peripheral expansion of mature T cells given with the donor stem cell graft[90]. De novo generation of T cells offers an added advantage of negative selection in the thymus, thus decreasing the possibility for graft-versus-host-disease (GVHD)[91]. However, the preparative regimen, as well as GVHD, can severely damage the thymic epithelium and impair thymopoiesis post-transplant[92, 93]. As a result, immune deficiency after HSCT is typically severe and prolonged [94]. IL-7 offers a number of exciting prospects after bone marrow transplantation, including improving immunocompetence through thymic-dependent and thymic-independent restoration of the peripheral T cell pool[58, 95]. In a number of preclinical studies, injection of IL-7 after HSCT results in normalization of thymocyte and peripheral T cell numbers, subsets and T cell proliferative responses to antigens[90, 96]. In irradiated non-human primate recipients of HSCT, IL-7 increased CD4⁺ T cell numbers[12], a population that demonstrates delayed recovery after infusion of hematopoietic progenitor cells[97].

Importantly however, there is ample evidence that the IL-7 signaling pathway contributes to GVHD. Mice receiving T cell-replete HSCT show increased incidence of GVHD when limiting numbers of alloreactive T cells are provided[98]. Furthermore, IL-7−/− mice do not develop GVHD[56], and monoclonal antibodies that neutralize IL-7 signaling improve mortality from GVHD[99]. In humans, the CD127 polymorphism +1237 on donor cells leads to higher transplant-related mortality after matched unrelated HSCT[100], and Day +14 IL-7 levels predict GVHD in humans receiving matched-sibling donor HSCT[101]. Thus, the capacity for IL-7 to costimulate T cells and augment immune reactivity can potentially lead to adverse effects in the context of T cell-replete BMT. On the other hand, the capacity of IL-7 to augment naïve T cell expansion and potentially increase thymic output could greatly reduce immune incompetence in the setting of T cell-depleted HSCT. Current clinical studies are underway to address clinical outcomes in patients treated in this manner. Furthermore, creative approaches could be developed to simultaneously prevent GVHD and augment immune reconstitution. For instance, donor mesenchymal stem cells (MSCs) transduced with the IL-7 gene following T cell-depleted HSCT enhanced thymopoiesis and expanded peripheral T cells while GVHD was prevented[102].

Caveats to IL-7 Therapy

Beyond the general prediction that IL-7 therapy may rejuvenate immunity in the aged population and may hasten recovery of immune competence in lymphopenic hosts, the question of how much IL-7 is needed is less clear. Transgenic mice that stably overexpress IL-7 show atrophy of the thymic cortex and a 3.9-fold reduction in thymocytes, with a dramatic decline in double positive cells[103]. Furthermore, when sustained dosing of IL-7 was administered, downmodulation of CD127 abrogated the proliferative effects of the cytokine, suggesting that dynamic regulatory effects may dictate the most effective dosing regimen. While it appears likely that such regulation will prevent unwanted or uncontrolled lymphoproliferation, mice with sustained IL-7 expression via transgene develop autoimmune-mediated toxicity and lymphomas[104–106]. Thus, caution should be exercised in developing IL-7 based regimens to augment peripheral T cell homeostasis, and while short term IL-7 administration appears safe in small studies, careful future studies are needed before chronic dosing regimens can be advocated.

IL-7 Blockade in Autoimmunity and Lymphoproliferative Disease

While IL-7 has the capacity to expand T cell clones that react with foreign antigens, the possibility of expanding self-reactive T cells remains[107]. Several studies on multiple sclerosis (MS), Type I diabetes, rheumatoid arthritis and ulcerative colitis have linked lymphodepletion with the development of autoimmunity[107, 108]. Furthermore, polymorphisms in CD127 predisopose to MS among individuals of European ancestry, implicating dysfunctional IL-7 signaling in autoimmunity[109] and T cell reactivity against myelin basic protein is augmented in those MS patients with elevated IL-7 levels[110]. In a murine model of Type I diabetes, lymphopenia induced by cyclophosphamide led to 100% of the mice developing diabetes, but blocking IL-7 receptor with a monoclonal antibody returned the incidence to background levels[107]. Also, administration of IL-7 with CD4⁺ T cells transgenic for a viral epitope expressed in pancreatic beta-islet cells caused diabetes at the same rate as mice that received cyclophosphamide alone, implicating IL-7 as the mediator of cyclophosphamide induced diabetes in this model. In rheumatoid arthritis, elevated levels of IL-7 are present in the serum and synovial fluid of these patients[8] and chronic elevations in IL-7 levels that occur in IL-7 transgenic mice lead to autoimmune inflammatory manifestations, including dermatitis and colitis[111, 112]. In the colitis model, selective elimination of T cells expressing high levels of IL-7R with an immunotoxin completely reversed established, ongoing colitis[113]. Thus, preclinical models suggest that IL-7 signaling may predispose to initiation of autoimmune disease and in some cases, may be required for ongoing autoimmune inflammation, thus raising the prospect of using IL-7 blockade therapeutically for autoimmune disease.

As discussed above, the potent effects of IL-7 signaling on developing lymphocytes of both the B cell and T cell lineage also raise the prospect that the IL-7 axis could potentially serve as a growth pathway for lymphoid malignancies. This is most well studied in childhood acute lymphoblastic leukemia where IL-7 signaling clearly induces lymphoblast proliferation and interruption of IL-7 signaling can lead to enhanced leukemic control[114]. Thus, future work aimed at developing agents to block IL-7 signaling in lymphoid malignancies could have important anti-neoplastic effects. As a corollary, therapeutic regimens wherein IL-7 is administered chronically will need to carefully monitor for predisposition to lymphoproliferative syndrome or lymphoid malignancies.

Conclusion

Modulating the IL-7 axis holds promise for a variety of clinical scenarios spanning aging-related declines in T cell immunity, to repairing damaged immune systems from HIV disease or cancer chemotherapy. As this field moves forward, combinations of multiple therapies may be needed to optimize immune reconstitution or anti-cancer effects in the context of specific immunotherapy, but IL-7 holds promise as a central component of such regimens. In addition, autoimmune disease and lymphoid malignancies that depend upon IL-7 signaling present an opportunity to explore blockade of IL-7 and/or its receptor in modulating these diseases.