Treatment with IFN-α before transplantation inhibits Th1 differentiation and protects from GVHD. Recipient B6.WT or B6.IFNAR1^−/− mice received either IFN-α or saline immediately after total-body irradiation and were transplanted the following day with BM + T cells from BALB/c.CD45.1⁺ donors. (A) Serum cytokine levels of IFN-γ were assessed (pooled data from 2 experiments; ^#P < .005, saline vs IFN-α, n = 11, 11, 7, 7) and donor CD4⁺ T cells were enumerated in the spleen 4 days later (pooled data from 2 experiments; **P < .01, saline vs IFN-α; n = 6 per group). (B) IFN-γ production from CD4⁺ T cells stimulated ex vivo for 5 hours was determined by intracellular cytokine staining. Plots shown are representative of 3 experiments and displayed as mean ± SEM, n = 9 per group. (C) CD4⁺IFN-γ–producing cells were enumerated in the spleen and MFI determined. Plots shown are representative of 3 experiments and displayed as mean ± SEM, n = 9 per group. Lethally irradiated B6D2F1 recipients received B6.WT BM (10⁷) + B6^luc+ T cells (2 × 10⁶). (D) Luminescence was quantified at day 7 after transplant (*P < .05, saline vs IFN-α treatment in mLN and iLN; **P < .01, saline vs IFN-α treatment in spleen, gastrointestinal tract, liver and lung, n = 5 per group). (E) Representative biophotonic images are shown as labeled. (F) Donor CD4⁺ T-cell expansion was quantified on day 4 (**P < .01, saline vs IFN-α treatment, n = 10 per group) and (G) serum IFN-γ analyzed at day 3 after transplantation (^#P < .005, saline vs IFN-α, n = 10 per group).