Antibody-mediated BM rejection in the primed recipient is dependent on host FcR⁺ cells. B6 and B6.Fcϵr1γ^−/− mice were primed against BALB/c on day −28, lethally irradiated (8.0 Gy) on day −1, and infused with 20 × 10⁶ BALB/c T-cell–depleted BM cells on day 0. A cohort received anti-CD4 and anti-CD8 mAbs around the time of transplantation to ensure depletion of host T cells. (A) Overlay histograms illustrate equivalent serum alloantibody levels at time of BMT in primed Fcϵr1γ^−/− as wild-type mice in a thymocyte-binding assay. Serum from nonprimed mice (histograms on the left) illustrates low-level binding. Histograms on the right illustrate high degree of thymocyte binding by serum from primed mice. n = 5/group. Negative control (no serum) is indicated by the thin dotted line. (B) Survival is shown. n = 10/group; P < .001 for primed B6 versus primed B6.Fcϵr1γ^−/−. With the exception of the group of primed B6.FcϵrIγ^−/− mice that received anti-CD4 and anti-CD8 mAbs, data were reproduced in a second experiment. Each bold line represents a different mouse.