Hematopoietic stem and progenitor cell (HSPC) maintenance and the differentiation of various lineages is a highly complex but precisely regulated process. Multiple signaling pathways and an array of transcription factors influence HSPC maintenance and the differentiation of individual lineages to constitute a functional hematopoietic system. Nuclear factor of activated T cell (NFAT) family transcription factors have been studied in the context of development and function of multiple mature hematopoietic lineage cells. However, until now their contribution in HSPC physiology and HSPC differentiation to multiple hematopoietic lineages has remained poorly understood. Here, we show that NFAT proteins, specifically NFATc1, play an indispensable role in the maintenance of HSPCs. In the absence of NFATc1, very few HSPCs develop in the bone marrow, which are functionally defective. In addition to HSPC maintenance, NFATc1 also critically regulates differentiation of lymphoid, myeloid, and erythroid lineage cells from HSPCs. Deficiency of NFATc1 strongly impaired, while enhanced NFATc1 activity augmented, the differentiation of these lineages, which further attested to the vital involvement of NFATc1 in regulating hematopoiesis. Hematopoietic defects due to lack of NFATc1 activity can lead to severe pathologies such as lymphopenia, myelopenia, and a drastically reduced lifespan underlining the critical role NFATc1 plays in HSPC maintenance and in the differentaion of various lineages. Our findings suggest that NFATc1 is a critical component of the myriad signaling and transcriptional regulators that are essential to maintain normal hematopoiesis.

The transcription factors of the nuclear factor of activated T cell (NFAT) family play a crucial role in multiple aspects of T cell function. It has recently been reported that NFATs play an important role in the suppressive function of CD4+CD25+Foxp3+ regulatory T (Treg) cells. In this study, we have investigated the role of NFATs in the thymic development of Treg cells in mice. We show that NFAT factors are dispensable for the development of Foxp3+ Treg cells in the thymus but are critical for the maintenance of both the phenotype and survival of Treg cells in the thymus as well as in peripheral lymphoid organs. Specifically, the homeostasis of CD4+CD25+Foxp3+ but not the CD4+CD25−Foxp3+ fraction is severely perturbed when NFAT signaling is blocked, leading to a strongly reduced Treg population. We underscored this intriguing effect of NFAT on CD4+CD25+Foxp3+ Treg cells to the disruption of survival signals provided by interleukin 2 (IL-2). Accordingly, blocking Treg cell death by abolishing the activity of pro-apoptotic Bcl-2 family member Bim, compensated for the survival defects induced due to a lack of NFAT-IL-2-IL-2R signaling. Inhibition of NFAT activity led to a strong reduction in the number of Foxp3+ Treg cells; however, it did not influence the level of Foxp3 expression on an individual cell basis. In addition, we show a differential effect of IL-2 and IL-7 signaling on Foxp3+ Treg versus CD4+CD25− T cell development, again underlining the dispensability of NFAT signaling in the development, but not in the maintenance of Foxp3+ Treg cells.