Transfusion of mHA-mismatched leukoreduced blood induces rejection of subsequent BMT. Transfusion with mHA-mismatched leukoreduced blood induces rejection of BMT from BALB.B donors. (A) The number of mice with engraftment was significantly reduced after 4 transfusions with mHA-mismatched blood (group BALB.B LR) compared with mice transfused with syngeneic blood (group B6 LR; P < .001, 2-tailed Fisher exact test). (B,D,F) Chimerism in engrafted animals was somewhat variable from experiment to experiment but resulted in a clear determination of engraftment versus rejection. (C) Use of BALB/c RBC donors induces rejection of BALB.B bone marrow, suggesting that the indirect pathway is active for donor RBCs (P < .001, 2-tailed Fisher exact test). (E) Transfusion-induced mHA-mismatched BMT rejection is not the result of alloimmunization against H-2^d MHC molecules, as no rejection is seen in mice transfused with whole blood from B6.H2d mice (group B6.H2d WB). (G) Specific lysis against target cells from B6.H2d donor mice measured in an in vivo killing assay, indicating that anti-H2d alloimmunization occurred. Data shown are the combined results of at least 3 separate experiments in each figure. *The data corresponding to the no transfusion and syngeneic transfusion groups were combined for all the experiments and are presented in multiple panels.