Transplantation with IL-23^−/− marrow grafts is associated with reduced GVHD severity. (A) Lethally irradiated Balb/c mice (900 cGy) were transplanted with either BM (10 × 10⁶) alone from B6 (■, n = 6) or IL-23^−/− (□, n = 6) donors or BM and spleen cells (1.0-1.6 × 10⁶) from B6 (●, n = 15) or IL-23^−/− animals (○, n = 15). Because the percentage of αβ⁺ T cells in the spleens of donor IL-23^−/− mice was observed to be higher in some instances (by approximately 5%-10%), the spleen cell dose was adjusted in these experiments so that the administered number of T cells was equivalent. Overall survival is depicted. Data are cumulative results from 3 experiments. (B) Lethally irradiated Balb/c mice (900 cGy) were transplanted with either BM (10 × 10⁶) alone from B6 (■, n = 8) or IL-23^−/− (□, n = 11) donors or BM and a reduced dose of spleen cells (0.4-0.6 × 10⁶) from B6 (●, n = 20) or IL-23^−/− animals (○, n = 20). The percentage of weight change over time of mice from all 4 groups is depicted. Data are cumulative results from 5 experiments. (C-G) Percentage of donor cells in the spleen (actual values ± standard error of the mean [SEM] are 99 ± 1, 99 ± 0, 96 ± 1, and 99 ± 0 for all 4 groups, respectively) (C), spleen cellularity (D), total number of splenic B cells (E), thymus size (F), and absolute number of double positive (CD4⁺ CD8⁺) thymocytes (G) in mice 3 to 4 weeks posttransplantation with B6 BM alone (n = 4, ▫), IL-23^−/− BM alone (n = 3, ), B6 BM plus spleen cells (n = 14-15, ■), or IL-23^−/− BM and spleen cells (n = 12-14, □). Data are presented as the mean plus or minus SEM. *P ≤ .05; **P < .01.