Brief priming by host DCs triggers the committed differentiation of donor naive CD8⁺ T cells into effectors. (a) Naive C3H.SW CFSE-CD8⁺ T cells were intravenously injected into lethally irradiated B6/SJL (CD45.1⁺) mice. After 24 to 36 hours, these primed C3H.SW CFSE-CD8⁺ T cells were then purified and adoptively transferred into secondary irradiated β₂m^–/– recipient mice. Six days later, cells from the spleens and lymph nodes of secondary recipients were cultured in the presence or absence of CD3 Ab plus MC57SV cells irradiated with 30 Gy for measuring IFN-γ production as described in Methods. Dot plots shown represent IFN-γ and CFSE intensity measured in gated CD8⁺ T cells. Naive unprimed C3H.SW CFSE-CD8⁺ T cells were injected into lethally irradiated B6/SJL mice (CD45.1⁺) and β₂m^–/– mice as control. (b) C3H.SW CFSE-CD8⁺ T cells were stimulated ex vivo by host CD11c⁺ DCs for 24 hours, recovered, sorted, and adoptively transferred into irradiated β₂m^–/– mice. Six days later, cells from spleens and lymph nodes were separated from the secondary recipients and stained with anti-CD8 Ab for measuring donor CD8⁺ T cell divisions. These cells were also cultured for 16 hours as described in a, to measure IFN-γ secretion. (c) Survival rate of β₂m^–/– B6 → B6 BM chimeric mice that received either C3H.SW T^–BM alone, C3H.SW T^–BM + C3H.SW CD8⁺ T cells, or C3H.SW T^–BM + C3H.SW CD8⁺ T cells that were primed ex vivo by B6 CD11c⁺ DCs. Representative results from two independent experiments are shown.