Graft rejection requires host T-cell expression of STAT1 but not STAT4 or STAT6. (A-C) B6-into-BALB/c transplantation consisted of lethal host irradiation (1050 cGy; day −2), and some combination, as indicated, of wild-type (WT; “HT”), STAT4-deficient (^−/−), or STAT6^−/− host T cells (T-cell dose, 10⁶; day −1); and B6 donor BM cells (day 0). Results shown are mean plus or minus SEM of n = 5 to 10 per cohort. On day 8 after BMT, spleen cells were isolated and stimulated with syngeneic (BALB/c) or allogeneic (B6) DCs for 24 hours. The total number of host CD4⁺ and CD8⁺ cells producing allospecific IFN-γ was determined by flow cytometry (panel A). Resultant supernatants were tested for cytokine content (panel B; pg/mL). *P < .05; **P < .01; ***P < .001. Overall survival of recipient mice is shown (panel C). (D-F) BALB/c-into-B6 transplantation consisted of lethal irradiation (1100 cGy; day −2), and some combination, as indicated, of WT (“HT”) or STAT1^−/− host T cells (T-cell dose, 10⁶; day −1) and BALB/c donor BM cells (day 0). Results shown are mean plus or minus SEM of n = 5 to 10 per cohort. On day 8 after BMT, spleen cells were isolated and stimulated with syngeneic or allogeneic DCs; total number of host CD4⁺ and CD8⁺ cells producing allospecific IFN-γ was determined by flow cytometry (panel D) and resultant supernatants were tested for cytokine content (panel E; pg/mL). *P < .05; **P < .01; ***P < .001. (F) Overall survival is shown (left panel; n = 10 per cohort); day 90 after BMT donor chimerism in spleen, BM, and blood is shown (right panel).