CTLA4 and cancer: Expression of PD-1 on tumor-associated murine and human NK cells is well established,14, 17 and there is also some evidence for CTLA-4 expression on NK cells.18, 20 Both PD-1 and CTLA-4 on NK cells have been shown to inhibit their cytotoxic activity towards cancer cells and their cytokine production.14, 20 Thus, anti-PD-1 and anti-CTLA-4 in our model are likely acting on NK cells directly.