The fate and function of NK cells are Differentially regulated by T-bet, eomes, and IL-15

Problem: We have previously shown that IL-15 activated NK cells can mediate acute allograft rejection in the absence of adaptive immunity. As NK cells are functionally diverse, we hypothesized that different NK subsets may respond differentially to IL-15, based on their regulation by distinct transcription factors. Methods: In B6 mice, NK cells (NK1.1+CD3-) can be divided into two major subsets based on differential expression of CD27: CD27+ and CD27-NK cells. We used a polychromatic approach to study the biology of these subsets in vivo. To stimulate NK cells with IL-15 in vivo, we treated host mice with IL-15 pre-complexed with IL-15Receptor-α (IL-15:Rα), which has been shown to increase half-life and effectiveness. Results: We observed that treatment of B6 mice with IL-15:Rα induced vigorous proliferation of NK cells as determined by BrdU-uptake-assay in vivo. Interestingly, we found that CD27+ NK cells preferentially responded to IL-15:Rα, suggesting that CD27+ NK cells contributed significantly to the expanded pool of NK cells after activation. To prove this, we used an adoptive transfer model and showed that CD27+ but not CD27- NK cells expanded extensively upon IL-15:Rα treatment. Importantly, the CD27+ subset expressed high levels of IFN-γ, Granzyme B, and Perforin, and was highly cytotoxic against allogeneic targets in vivo. Next, we studied the mechanisms that account for this differential responsiveness to IL-15. We found that expression of IL-15Receptor-β is strikingly different among the NK subsets, with CD27+ NK cells expressing the highest levels. This difference is due to reciprocal expression of T-Bet and Eomes among the subsets, two transcription factors that have been shown to regulate the expression of IL-15Receptor-β in CD8+ memory T cells. Conclusion: We show that NK cells consist of functionally diverse subsets with distinct responsiveness to IL-15, due to differential expression of IL-15Receptor-β, T-Bet, and Eomes. Our studies may have important clinical implications in targeting innate immunity in transplantation.