PP2A negatively regulates NK cell T-bet expression and anti-tumor effector function.

The transcription factor T-bet is essential for the anti-tumor effector function of natural killer (NK) cells, but the mechanism regulating its expression in NK cells remains unclear. In this study, we aimed to identify an NK cell-intrinsic regulator that controls T-bet expression. Using T-bet-luciferase reporter assay screening, we identified a protein phosphatase inhibitor as a potential activator of T-bet expression. A series of protein phosphatase 2A (PP2A)-specific inhibitors (PP2Ai) or PP2A siRNA induced the expression of T-bet. In PP2Ai-treated mice, the expression of T-bet and its downstream effector molecules, granzyme B and IFN-γ, was also upregulated in NK cells. Mechanistically, PP2Ai increased the phosphorylation of mTOR and ribosomal protein S6 in NK cells, and mTOR inhibitor canceled the effects of PP2Ai in NK cells. Importantly, NK cells isolated from PP2Ai-treated mice showed higher cytotoxicity and IFN-γ production; therefore, they increased the anti-tumor effector function of NK cells. Accordingly, PP2Ai treatment inhibited lung metastasis of B16 melanoma by NK cell- and mTOR-dependent mechanisms. These results suggest that PP2A negatively regulates NK cell T-bet expression and effector function by an mTOR-dependent mechanism.