(A–F) Lethally irradiated B6D2F1 mice were transplanted with 5 × 10⁶ TCD-BM cells from CD45.1 or CD45.2 B6 donors supplemented or not with 3 × 10⁶ purified total T cells from CD45.2 B6 donors. Three of the 4 groups of mice were also supplemented with 5,000 P815 at the time of BMT and received vehicle, 0.25 mg/kg CPT only, mature T cells plus vehicle (T+Veh), or mature T cells plus 0.25 mg/kg CPT (T+CPT) on day 0 and then every other day until day 28 after BMT. Recipient survival (A) and aGVHD clinical scores (B) were monitored following BMT. At the experimental endpoint (~day 80) donor splenic H2K^d–B220⁺ (C) and thymic CD4⁺CD8⁺ double-positive (DP) (D) cell populations were analyzed by flow cytometry. An IVIS 200 imager was used to periodically monitor firefly-luciferase expression in transplanted P815 cells in recipient mice, which were injected with d-luciferin substrate at each imaging time point (E). Western blot analysis of the indicated proteins and tumor cell lines after 24 hours in culture (F). α, anti-. Data in A–E represent 3 independent experiments (BM only, n = 6; BM + P815 + vehicle, n = 9; BM + P815 + CPT, n = 7; BM + P815 + T cells + vehicle, n = 14; BM + P815 + T cells + CPT, n = 11). Data in F are from an individual Western blot. (G–I) Purified T cells isolated from Fli1^WT, Fli1^fl/WT, or Fli1^fl/fl donor mice plus WT TCD-BM were transferred into lethally irradiated B6D2F1 mice. On the day of BMT, 5,000 luciferase-transduced P815 cells were i.v. injected into these recipients. Recipient survival rates (G), clinical scores (H), and P815 tumor growth (I) were monitored following BMT (BM only, n = 3; BM + P815, n = 3; BM + P815 + Fli1^WT, n = 5; BM + P815 + Fli1^fl/WT, n = 5; BM + P815 + Fli1^fl/fl, n = 5). Significance was determined using mixed-model tests for clinical scores, a log-rank test for survival data, and a 1-way ANOVA for all other data. *P < 0.05, **P < 0.01, and ***P < 0.001.