Host Th1/Tc1 cells mediate increased rejection relative to host Th2/Tc2 cells. (A) BALB/c host T cells were costimulated and expanded in IL-4 or IL-12 to generate Th2/Tc2 or Th1/Tc1 cells, respectively. On culture day 6, cells were restimulated; supernatants were tested for cytokine content (ng/mL). (B-D) B6-into-BALB/c transplantation consisted of lethal host irradiation (1050 cGy; day −2), and some combination, as indicated, of host T cells that were nonpolarized [“HT”], polarized to type I cytokines [“HTh1/Tc1”], or polarized to type II cytokines [“HTh2/Tc2”] (T-cell dose of 0.1 × 10⁶; day −1); and donor BM cells (day 0). 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 using anti–H-2^d, anti–H-2^b, -CD4, or -CD8, and antibody capture of IFN-γ (panel B). Resultant supernatants were tested for cytokine content (panel C; pg/mL). Results are mean plus or minus SEM of n = 10 per cohort. *P < .05; **P < .01; ***P < .001. (D) Recipient mice were followed for 90 days after BMT: data shown are overall survival (left panel) and peripheral blood donor chimerism (right panel). Survival data were pooled from 2 independent experiments (total of n = 10 recipients per cohort).