Irradiation upregulates the expression of antigen-presenting and costimulatory molecules on DCs. Splenic DCs were isolated from (a) irradiated mice at 6 hours and 24 hours after irradiation or (b) irradiated mice infused with donor CD8⁺ T cells or donor CD8⁺ T cells + T^–BM cells. These purified DCs were double stained with anti-CD11c Ab coupled with anti-Ia Ab or anti-CD86 Ab as described in Methods. The expression of Ia and CD86 antigens on the CD11c⁺ cell population was analyzed by flow cytometry. Isotype-matched IgG was used as control. (c) IL-12 in serum from nonirradiated and irradiated B6 mice at 6 hours, 24 hours, and 5 days following TBI or in the supernatants of cultured host DCs was examined by ELISA. Host splenic DCs were separately purified from the spleen of B6 mice at 6 and 24 hours following TBI or from nonirradiated mice as described in Methods. The supernatants were collected from the cultures of DCs at 48 hours. *P < 0.05 compared with samples from nonirradiated mice or irradiated mice at 24 hours and 5 days after TBI. (d) Host DCs were isolated from nonirradiated B6 mice (diamonds) and from B6 mice at 6 hours (triangles) and 24 hours (circles) after irradiation. These cells were cocultured with BALB/c mouse–derived CD4⁺ T cells to examine their capacity to stimulate an allogeneic mixed lymphocyte reaction. Activated DCs that were induced by culturing naive B6 DCs in the presence of GM-CSF + TNF-α for 48 hours were used as positive control (squares). One representative experiment of three is shown.