Exposure to host-type apoptotic cells 48 hours prior, but not after BMT improves survival in GVHD models. (A) ECP treatment 48 hours prior to BMT in C57BL/6→BALB/c mice improves survival. BALB/c mice injected with C57BL/6 TCD-BM plus Tcon only (○, n = 29) or with prior injection of ECP-treated BALB/c cells (△, n = 32) or with prior injection of 8MOP but no UV light-treated BALB/c cells (▪, n = 9). ○ vs △, P < .0001 (median survival, 26 vs 59 days). (B) ECP treatment 2 or 4 days after BMT does not improve survival. BALB/c with TCD-BM plus Tcon only (●, n = 10) or followed by ECP treatment with C57BL/6 cells at day +2 (▿, n = 10) or day +4 (□, n = 10). (C) ECP treatment 48 hours prior to BMT across minor histocompatibility barriers in AKR/J→BALB.K improves outcome. BALB.K injected with purified AKR/J HSC and Tcon alone (○, n = 10) or with prior injection of ECP-treated BALB.K cells (△, n = 10). ○ vs △, P = .0002 (median survival, 8 vs 37 days). Significance was assessed using the log-rank test.