CD4⁺ T cells from [H2-Ab1^−/− → C3H] mice were predominantly B6 reactive. (A) C3H mice were irradiated and underwent transplantation with TCD-BM from either control WT or H2-AB1^−/− B6 donors. Six weeks after BMT, splenic CD4⁺ T cells isolated from [WT → C3H] (□) or [H2-Ab1^−/− → C3H] (▪) mice were stimulated with irradiated DCs from B6, C3H, and BALB/c mice. Seventy-two hours later, proliferation was determined by a thymidine uptake assay. Data are shown as the mean ± SD. (B) Similarly, BALB/c mice underwent transplantation, and [WT → BALB/c] (□) or [H2-Ab1^−/− → BALB/c] (▪) CD4⁺ T cells were stimulated with irradiated DCs. Proliferation was determined by a thymidine uptake assay after 72 hours. Data are shown as the mean ± SD. (C) CFSE-labeled [WT → C3H] and [H2-Ab1^−/− → C3H] CD4⁺ T cells were stimulated with irradiated splenocytes in culture. Cell division determined by dilution of CFSE is shown for CD4⁺ cells. (D-F) A total of 1 × 10⁷ [WT → C3H] (open symbols) or [H2-Ab1^−/− → C3H] (closed symbols) CD4⁺ T cells were transferred to lethally irradiated C3H (squares) or B6 (circles) mice together with host-type TCD-BM. Clinically acute GVHD scores as the mean ± SE (D) and survivals (E) of mice after transfer are shown; n = 3 to 6 per group. The histologic findings of the small intestine (F, left panel) and liver (F, right panel) from B6 recipients of [H2-Ab1^−/− → C3H] CD4⁺ T cells are shown (original magnification, × 200). *P < .05, **P < .001.