PD-L1 expression on parenchymal cells is critical for suppression of acute GVHD. Lethally irradiated wt B6 or PD-L1^−/− recipients were given BM cells from PD-L1^−/− or wt B6 mice, respectively, to create chimeras. We also created control chimeras (wt→wt). After 3 months, these chimeras were reirradiated and infused with allogeneic BALB/c BM cells with 10 × 10⁶ BALB/c splenocytes. Kaplan–Meier survival curve of transplanted mice (n = 8–11/group). wt→ wt vs PD-L1^−/−→wt chimeras [P = .06]; wt→wt vs wt→PD-L1^−/− chimeras [P < .0001]; PD-L1^−/−→wt vs wt→ PD-L1^−/− chimeras [P = .0009]). Data are representative of 2 independent experiments.