Constitutive nuclear localization of NFAT in FoxP3+ regulatory T cells independent of calcineurin activity (179.4)

Abstract

Abstract FoxP3 plays an essential role in conferring suppressive functionality to CD4+/FoxP3+ regulatory T (Treg) cells. While studies have shown that FoxP3 has to form cooperative complexes with NFAT to bind to target genes, it remains unclear whether NFAT is available in the nucleus of primary Treg cells for FoxP3 access. It is generally believed that NFAT in resting cells resides in the cytoplasm and its nuclear translocation depends on calcineurin activation. We report that a fraction NFAT protein constitutively localizes in the nucleus of primary Treg cells, where it selectively binds to FoxP3 target genes. Treating Treg cells with calcineurin (CN) inhibitor does not induce export of NFAT from the nucleus, indicating that its nuclear translocation is independent of CN activity. Consistently, Treg cells are resistant to CN-inhibitors in the presence of IL-2 and continue to proliferate in response to anti-CD3 stimulation, while proliferation of non-Treg cells is abrogated by CN-inhibitors. In addition, PMA, which activates other transcription factors required for T cell activation but not NFAT, selectively induces Treg cell proliferation in the absence of ionomycin. TCR-interaction with self-MHC class II is not required for PMA-induced Treg cell proliferation. Treg cells expanded by PMA or in the presence of CN-inhibitors maintain Treg cell phenotype and functionality. These findings shed light on Treg cell biology, paving the way for strategies to selectively activate Treg cells.