The paradoxic effect of proteasome inhibition with bortezomib on the alteration of GVHD development and mortality as determined by the timing of bortezomib administration. (A-B) B6 (H2^b) recipients of BALB/c (H2^d) 15 million bone marrow and 20 million spleen cells were treated with or without 15 μg bortezomib per dose daily. Bortezomib protected mice from GVHD mortality when administration was daily from day 0 through +2 after BMT. Significant increases in survival were observed in early bortezomib-treated mice (•) compared with GVHD control (vehicle control-treated) mice (▪; panels A and B, P < .001). Conversely, bortezomib accelerated GVHD mortality when administration was from day +5 through +7 (A) or bortezomib treatment from day 0 through +2 and day +5 through +7 (B) after BMT. Significant decreases in survival were observed in late bortezomib-treated mice (•) compared with GVHD control (vehicle control-treated) mice (▪; panel A, P < .001) and in continuous bortezomib-treated mice (▴) compared with GVHD control (vehicle control-treated) mice (▪; panel B, P < .001). Results from 1 of 3 independent experiments are presented for panels A and B. Each experiment consists of 5 to 10 mice per treatment group. (C) C.B-17 SCID (H2^d) recipients of B6 (H2^b) 40 million spleen cells were treated with or without 15 μg bortezomib per dose for 1 day. Significant decreases in survival were observed in day +5 bortezomib-treated mice (▾) compared with GVHD control (vehicle control-treated) mice (▪; panel A, P < .001) and in day +12 bortezomib-treated mice (▴) compared with GVHD control (vehicle control-treated) mice (▪; panel B, P < .001). Results from 1 of 3 independent experiments are presented. Each experiment consists of 4 to 10 mice per treatment group.