Functional resetting occurs even in NK cells that do not divide after transfer. (a) CFSE-labeled WT splenocytes were transferred to irradiated WT or β2m-deficient hosts. (b) CFSE-labeled β2m^−/− splenocytes were transferred to irradiated β2m^−/− or MHC I–expressing NK cell–deficient (NKD) hosts. 10 d after transfer, NK cells were stimulated and assayed as in Fig. 1, except that the donor cells were gated based on CFSE level, distinguishing cells that had divided 0, 1, 2, 3, or 4 times after initial labeling. Experiments were repeated two to four times with n = 3–6 mice for each. (c) WT splenocytes were transferred to unirradiated, CD8a- and CD4-depleted WT, or β2m-deficient hosts. 9 d after transfer, NK cells were stimulated and assayed as in Fig. 1. The experiment was performed twice with n = 3–5 mice. For some comparisons, statistical significance was determined with a two-tailed unpaired Student’s t test (*, P < 0.05; **, P < 0.005; ***, P < 0.0005). Data represent means ± SEM.