196: Interleukin-7-mediated NFAT activation sets the threshold for T cell development

Abstract

Interleukin-7 (IL-7), a member of the common cytokine gamma-chain family, plays critical role in T cell biology. Studies using gene knock out mice for IL-7 or the components of IL-7 signaling have unequivocally established that IL-7-Jak3-STAT5 signaling is an absolute necessity for development of a normal T cell repertoire. However, subtle differences in the phenotype of STAT5 deficient mice to that of IL-7 or IL-7R deficient mice implicate that transcription factors other than STAT5 might be activated upon IL-7 signaling. Transcription factors of NFAT family play prominant role in T cell development and function. However, the role of NFAT family proteins in the early cytokine-dependent stages of thymocyte development is unclear. Additionally, whether the activity of individual NFAT family members during thymocyte development are complementary, antagonistic or redundant is unknown. Differentiation of DN thymocytes from the early thymic progenitor (ETP) cells is critically dependent on IL-7 signaling. We have elucidated a novel aspect of IL-7 signaling, whereby IL-7 signals activate NFATc1 in the DN1-DN3 thymocytes by Jak3-mediated tyrosine phosphorylation, which occurs parallel to that of STAT5 activation. Conventionally, in T cells NFAT proteins are activated via calcineurin-mediated multiple serine/threonine dephosphorylation in its regulatory domain upon TCR signaling. However the alternative NFAT activation pathway we have elucidated is distinct from that of the calcineurin-mediated NFAT activation pathway. First, instead of ser/thr dephosphorylation, IL-7 activates NFAT via tyrosine (tyr) phosphorylation. Second, JAK3 activated downstream to IL-7 signals is the kinase responsible for NFAT activation. Third, IL-7 induced NFAT activation is insensitive to cyclosporin A treatment and finally, this alternative NFAT activation pathway is vital for the survival of the early DN thymocytes which have not yet acquired the cell surface pre-TCR. We show that NFATc1 is the pre-dominant NFAT protein over NFATc2 or NFATc3 in regulating early thymocyte development and synergistically with STAT5, it controls the gene expression programs associated with early T cell survival. Accordingly, ablation of NFATc1, and not NFATc2 or NFATc3, severely impaired the T cell development resulting in T-cell lymphopenia similar to that observed in mice deficient for STAT5 or IL-7 signaling. Thus by elucidating a previously unknown aspect of IL-7 signaling, we demonstrate that in addition to STAT5, NFATc1 activation by IL-7/JAK3 sets the threshold between development of lymphopenia or that of a normal functional T cell repertoire during the cytokine-dependent phases of thymocyte development.