Responsive NK cells reset their functional potential downward in the absence of MHC I contact. Responses of WT NK cells after transfer to WT or β2m-deficient hosts. (a and b) 10 d after transfer, splenic NK cells were stimulated for 5 h with 5 µg/ml of plate-bound NKG2D or 50 µg/ml NKR-P1C antibody as indicated. The percentage of IFN-γ–producing NK cells among gated donor NK cells (a) or gated donor NK cells expressing one or more of Ly49C, Ly49I, and/or CD94/NKG2A (CIN⁺) or lacking those three receptors (CIN⁻; b) was determined by flow cytometry. Responses of NK cells from unmanipulated WT and β2m^−/− mice are shown for comparison. Experiments were repeated five to six times with n = 3–5 each for transfer groups. For relevant comparisons, statistical significance was determined with a two-tailed unpaired (a) or paired (b) Student’s t test (*, P < 0.05; ***, P < 0.0005). ctrls, controls. (c) At day 12 after transfer, groups of adoptive transfer recipients (n = 3–4) received grafts of CFSE^high-labeled β2m^−/− spleen cells mixed with an equal number of CFSE^low-labeled WT spleen cells. Rejection of β2m^−/− target cells was determined by flow cytometry of spleen cells 18 h later. Some groups of transfer recipients were pretreated i.p. twice (on days −2 and −1 relative to the time of engraftment) with 200 µg PK136 (NK1.1) antibody to deplete NK cells, as indicated. The experiment was repeated three times with n = 3–6 mice for each (***, P = 0.0002). Data represent means ± SEM.