Administration of NK-cell targets induces strong CD4⁺ T- and B-cell responses. (A) OVA_323-339-specific IFN-γ production as measured by ELISPOT by isolated splenocytes from mice immunized with either 10⁶ live K^b-sufficient or -deficient act-mOVA cells or with 10⁶ irradiated act-mOVA.K^b+/+ cells. Data are expressed as the average number of IFN-γ-producing cells per spleen (n = 4 per group). (B) Absence of CD4 help either through CD4 depletion via Ab treatment or using class II MHC-deficient recipient mice leads to complete abrogation of CD8⁺ T-cell responses as measured by antigen-specific intracellular IFN-γ production. (C) Anti-asialo GM1 treatment abrogates CD4⁺ T-cell response in mice immunized with 10⁷ act-mOVA.K^b−/−-deficient target cells. (D,E) OVA-specific total IgG titers after immunization of mice with various doses of live K^b-sufficient or -deficient act-mOVA cells or irradiated act-mOVA.K^b+/+ cells. (F-H) Polarization of antibody responses as measured by OVA-specific IgG1 and IgG2c levels induced by 10⁶ act-mOVA.K^b−/− in the presence or absence of NK cells. Sera are collected at day 14 after immunization. Data represent mean values ± SEM (n = 8). * P < .05; ** P < .01.