Involvement of NFAT Transcription Factors in NK Cell Reactivity

NK cells are lymphoid components of innate immunity and play an important role in tumor immunosurveillance. One of the major transcriptional regulators in lymphoid cells is NFAT (Nuclear Factor of Activated T Cells), as highlighted by its important role in T and B cell development and function. With regard to NK cells, available data indicate that NFAT is dispensable for development. However, several lines of evidence including the observation that the immunosuppressive drugs cyclosporin A and tacrolimus, which mediate their effects through inhibition of calcineurin and consecutively NFAT, influence NK reactivity implicate a role of this family of transcription factors in NK cell reactivity and function. Here we employed different genetic mouse models on the C57BL/6 background to directly study the functional role of NFAT in NK cells.

We found that except for NFAT3 mRNA and protein of all family members (NFAT 1, 2, 4 and 5) was expressed in resting NK cells of wild type (WT) mice with NFAT1, 2 and 4 being most abundantly detectable. When we employed NK cells with knockout (KO) of NFAT 1, 2, and 4 in comparative in vitro analyses, we surprisingly found that lack of NFAT resulted in enhanced NK cell activation, degranulation and release of immunomodulatory cytokines like IFN-γ after co-culture with YAC-1 target cells as well as increased production of granzyme B and perforin after cytokine activation. The inhibitory effect of NFAT on NK cell effector function was further confirmed in vivo by employing WT and germ line NFAT KO animals in the syngeneic B16 melanoma model, which revealed a significantly reduced metastatic burden in NFAT KO mice. Depletion of NK cells in this model system in turn resulted in increased metastasis, however, with WT animals displaying significantly higher metastatic burden compared to NFAT KO mice. As this pointed to the fact that NFAT influences metastasis via both NK-dependent and independent mechanisms, we further generated mice with a NK cell-specific (conditional Ncr1-Cre dependent) NFAT2 KO. When these animals were employed again in analyses of B16 lung metastasis, comparative analyses with WT animals confirmed the inhibitory effect of NFAT on NK tumor immunosurveillance.

Taken together, these results provide the first direct evidence for the functional involvement of NFAT in NK cell antitumor reactivity and, in contrast to T and B cells, identify NFAT as a negative regulator of NK cell function.