Perforin-dependent fratricide causes defective NK function in the absence of 2B4-CD48 interactions. (A) WT and perforin KO LAK cells were stimulated with anti-NK1.1 mAb–coated plates in the presence of anti-2B4 or anti-CD48 blocking antibodies. After 6 hours of stimulation, culture supernatants were measured for granzyme secretion via the BLT esterase assay. *P < .05. Results are representative of 3 independent experiments. (B) WT and perforin KO NK cells were cultured in complete media supplemented with IL-2 in the presence of anti-2B4 or anti-CD48 blocking antibodies. Thymidine incorporation was measured at different times during culture. Results are representative of 3 independent experiments. (C) Five days after injection with CpG, NK cells from WT, 2B4 KO, perf KO, and perf/2B4 DKO mice were counted. Fold expansions were calculated by dividing NK numbers of mice injected with CpG by NK numbers in noninjected control mice; n=3 mice per group, and data are representative of 2 independent experiments. Error bars are SD.