Type I-IFN signaling in the hematopoietic compartment of recipients of BM transplantation protects from GVHD. B6 chimeras were generated as described in “BM transplantation” and underwent transplantation 4 months later with 10⁷ BM and 5 × 10⁶ T cells from BALB/c.WT donors. (A) Serum cytokine levels were assessed at day 4 (IFNγ): *P < .05, B6 (WT→WT) vs B6 (IFNAR1^−/−→IFNAR1^−/−), n = 6 per group, combined from 2 experiments) or day 7 (TNF: *P < .05, B6 (WT→WT) vs B6 (IFNAR1^−/−→WT) and B6 (IFNAR1^−/−→IFNAR1^−/−), n = 4-7 per group) after transplantation. (B) Semiquantitative colon histopathology at day 7 after BM transplantation (*P < .05, B6 (WT→WT) vs B6 (IFNAR1^−/−→WT) and **P < .01, B6 (WT→WT) vs B6 (IFNAR1^−/−→IFNAR1^−/−), n = 4-7 per T cell–replete group). (C) Representative images are shown (× 250). (D) B6.WT or B6.IFNAR1^−/− mice were lethally irradiated and 24 hours later, cDC enumerated, sorted, and cultured with BALB/c CD4⁺ T cells at 1:10 ratio for 5 days. Tissue culture supernatant was harvested (for IFNγ) and proliferation quantified by tritiated thymidine uptake during the final 18 hours. Representative plots from 2 duplicate experiments and displayed as mean ± SEM of triplicate wells.