In spite of a very robust body of literature and definitive data demonstrating the importance of the programmed cell death receptor-1 (PD-1) pathway in T cells and their function, the data on NK cell PD-1 expression have been highly variable and, particularly in the case of mouse NK cells, scarce. In this issue of the JCI, Hsu et al. present data demonstrating PD-1 expression on mouse NK cells only within tumors and show that PD-1 blockade elicits an antitumor NK cell-mediated response. This study indicates that, given the complexity of both the biology and study of NK cells, further work is needed to more clearly determine the role of the PD-1/PD-1 ligand (PD-L1) on NK cells.
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