New insights into IL-7 signaling pathways during early and late T cell development

IL-7, a cytokine secreted mainly by stromal cells in the bone marrow and thymus, has been recognized to be critical for the development of all lymphocytes.1 However, the underlying mechanisms through which IL-7-mediated signaling contributes to early T-cell development and to mature T-cell maintenance are not yet completely understood. Two recently published studies in Nature Immunology reported that a newly identified IL-7 signaling pathway acts during early T-cell development and documented how IL-7 signaling interacts with the T-cell antigen receptor (TCR) to influence mature T-cell survival.2,3 IL-7 signaling through the IL-7 receptor (IL-7R) plays a critical role in the survival of CD4 and CD8 double-negative (DN) thymocytes during early T-cell development.1,4 DN thymocytes can be divided into four populations known as DN1 (CD44+CD25−), DN2 (CD44+CD25+), DN3 (CD44−CD25+) and DN4 (CD44−CD25−).1,5 The development of T lymphocytes can also be divided into pre-TCR and TCR-dependent stages based on their TCR differentiation status.6 During the pre-TCR stage, T lymphocytes are CD4− and CD8− DN, which consists of the four previously described populations. After the TCR subunits are rearranged and constructed in DN3 and DN4, T lymphocytes progress to the TCR-dependent stage.5,7 T cells with the αβ TCR are generally single positive CD4+ or CD8+ cells, which later become members of the helper or cytotoxic/effector subsets.5,7 Those cells expressing the alternate γδ TCR are negative for T lineage markers and recognize non-peptidic antigens.5,7,8 It has been widely recognized that interaction between IL-7 and the IL-7R activates the Jak1 and JAK3/STAT5 signaling pathway, leading to the production of effector molecules, such as Bcl-2, which plays key roles in T-cell progression and proliferation.9 In the thymus, only the IL-7-STAT5 pathway is recognized to be critical for the pre-TCR development of T cells.1 The existence of a STAT5-independent IL-7 signaling pathway during this stage of T cell development has been suspected, but has not been experimentally documented. Patra et al.2 recently identified the IL-7/NFATc1 pathway as a novel alternative signaling pathway that cooperates with the IL-7/STAT5 pathway in guiding thymocyte development. NFATc1 is a lymphoid-specific member of the NFAT family of transcription factors that is involved in the differentiation and function of T cells.6 It has been widely accepted that in T cells, signaling through the TCR induces the dephosphorylation and activation of NFAT, resulting in the translocation of NFAT to the nucleus where it binds to the promoter region of lymphokine genes, such as IL-2.7 However, the role of NFAT factors during the development of pre-TCR-negative T lymphocytes (thymocytes) has not been previously explored. Patra et al.2 found that in DN thymocytes from both wild-type and Rag−/− (deficient in pre-TCR) mice, there was a gradual increase in nuclear NFATc1 from DN1 to DN3 cells. Specifically, pre-TCR-negative DN2 and DN3 cells had more nuclear NFATc1 than pre-TCR-positive DN4 cells.2 This expression pattern was similar to the pattern of IL-7R expression observed in these cells in both wild-type mice and transgenic mice with calcineurin overexpression (ΔCam mice), which led to the activation of NFAT. Furthermore, deficiency in IL-2, a predominant cytokine regulated by NFAT activation, did not change the expression pattern of NFATc.2 In contrast, the absence of IL-7 signaling in IL-7R deficient mice resulted in decreased nuclear NFATc1 in DN thymocytes. IL-7 signaling was found to activate NFATc1 and facilitate its translocation to the nucleus, which was independent of STAT5 but dependent on Tyr371 phosphorylation of Jak3.2 Previously, IL-7 had been demonstrated to induce Bcl-2 expression in pre-T cells, thereby prolonging their survival.8 The effect of NFATc1 on Bcl-2 expression was further investigated in wild-type DN1–DN4 cells, and silencing NFATc1 with shRNA resulted in decreased Bcl-2 production and induced massive cell death (90% dead cells). Higher Bcl-2 expression in IL-7−/− DN thymocytes was induced with recombinant IL-7 through the simultaneous activation of both STAT3 and NFATc1 pathways.2 These results indicate that NFATc1 increases Bcl-2 expression by cooperating with STAT5, which subsequently affects the survival of pre-TCR-negative thymocytes. Furthermore, activation of NFATc1 by chemical and genetic approaches enabled to rescue the defective phenotype observed in IL-7R−/− mice in early T-cell development.2 Defective thymocyte development was also consistently found in mice deficient in the NFATc1 gene, specifically during the early stage of T cell development.2 Taken together, these results indicate that IL-7/Jak3/NFATc1 signaling, which represents a new alternative and independent pathway, acts cooperatively with IL-7/Jak3/STAT5 signaling in regulating the survival of pre-TCR cells (Figure 1a). Figure 1 Roles of IL-7 signals in pre-TCR development and the homeostasis of T cells. (a) The IL-7/Jak3/NFATc1 pathway functions as an alternative independent mechanism behind early T-cell development. Binding between IL-7 and the IL-7R induces the activation ... Another study published in the same issue of Nature Immunology reported that intermittent but not continuous IL-7 signaling promoted the quiescence and survival of naive CD8+ T cells.3 Under physiological conditions, intermittent TCR signaling blocks IL-7 signal transduction, and both intermittent homeostatic engagement of TCRs and IL-7 signaling are essential for the long-term survival of mature T cells.10,11 However, the underlying mechanism by which the IL-7 and TCR pathways participate in the survival CD8+ T cells remains unclear. In this study, Kimura et al.3 documented that continuous IL-7 signaling did not contribute to the maintenance of naive CD8+ T cells, but instead, induced them to proliferate, produce IFN-γ and die. This phenomenon was described as cytokine-induced cell death (CICD) and is discordant with the traditional view that IL-7 maintains the homeostasis of naive CD8+ T cells by triggering their proliferation.3 This CICD was found to be triggered by IFN-γ released from CD8+ T cells with continuous IL-7 signaling activation.3 Blocking IFN-γ production using gene-deficient cells or a transgenic model promoted cell survival in the absence of TCRs. Further, they documented that the homeostatic engagement of TCRs prevented CD8+ T cells from expressing IFN-γ in vivo, while, the weaker homeostatic engagement of the TCR led to uninterrupted IL-7 signaling and increased expression of IFN-γ and FasL.3 In contrast, intermittent IL-7 signaling maintained naive CD8+ T cells as viable and quiescent cells even in the absence of TCRs. Thus, IL-7 signaling functions to maintain naive CD8+ T cell homeostasis in a duration-dependent manner. These findings help us better understand why intermittent IL-7 is crucial for the homeostasis of T cells, how apoptosis of post-TCR T cells can be induced and how IL-7 and the TCR interact with each other to maintain the homeostasis of mature T cells (Figure 1b). In summary, these two studies described novel mechanisms outlining the involvement of IL-7 signaling in the development and homeostasis of immature pre-TCR and TCR-dependent T cells. Although the IL-7/Jak3/NFATc1 pathway and the induction of CICD by IL-7 signaling were both well documented in these two studies, the following questions warrant further investigation: (i) when and how does this newly identified IL-7/NFATc1 pathway interact with the IL-7/STAT5 pathway to induce Bcl-2 production?; (ii) how does IL-7 signaling interact with TCRs to activate and influence the fate of T cell subsets other than CD8+ T cells?; and (iii) because IgH rearrangement in B lymphocytes is also dependent on IL-7/STAT5 pathway activation12, does the IL-7/NFATc1 pathway also exert any influence on B-cell development?