B7-H3^−/− recipients have accelerated GVHD lethality. (A) A mixed leukocyte reaction (MLR) was performed by coculturing BALB/c-purified T cells that were carboxyfluorescein diacetate succinimidyl ester (CFSE) –labeled with irradiated B6 or B7-H3^−/− DC stimulators (10:1). Cells were analyzed by flow cytometry on day 9. Cells were gated on H2K^b-positive, viability dye–negative CD4⁺ or CD8⁺ events and were analyzed for dilution of CFSE (n = 5). One of 2 representative experiments is shown. (B) B6 or B7-H3^−/− mice were lethally irradiated and infused with 10⁷ BALB/c NTCD BM and 2 × 10⁶ BALB/c-purified T cells. Survival plot of B6-WT (green solid circle) vs B7-H3^−/− (red open circle) is shown (n = 16 per group; P < .0001). B6 or B7-H3^−/− mice were lethally irradiated and infused with 10⁷ BALB/c NTCD BM and 1 × 10⁶ CD25-depleted BALB/c-purified T cells. Survival plot of WT (solid circle) vs B7-H3^−/− (open circle) is shown (n = 16 per group pooled from 2 experiments with comparable results; P < .0001). (C) B6 or B7-H3^−/− mice were lethally irradiated and infused with 10⁷ BALB/c NTCD BM and 2 × 10⁶ BALB/c-purified T cells. Mice were analyzed for clinical (left panel) and weight (right panel) scores (n = 8). One experiment was performed. (D) Mice were transplanted as in (C). Twenty-one days after transplant, colons were harvested, sectioned, and stained by hematoxylin and eosin (H&E). Sections were scored for pathology (n = 4). One of 2 representative experiments is shown. (E) B6 or B7-H3^−/− mice were lethally irradiated and infused with 10⁷ BALB/c NTCD BM and 1 × 10⁶ BALB/c-purified T cells. Mice were sacrificed on day 7, and splenocytes were analyzed for percentage of α₄β₇, IL-2, and Ki-67 expression (n = 5). One experiment was performed. *P < .05; **P < .01; ***P < .001.