Monocytes mediate inhibition of T-cell proliferative responses after HSCT. (A) Total PBMCs or T cells highly enriched from total PBMCs obtained from healthy controls (□) or HSCT recipients (▪) were stimulated by immobilized anti-CD3 (250 ng/mL) for 72 hours and the proliferative response was quantified by net [³H]thymidine incorporation. Each bar depicts the median proliferative response of 15 experiments. Proliferation of post-HSCT PBMCs was significantly lower than that of control PBMCs (P < .001). The asterisk indicates that in HSCT recipients proliferation of enriched T cells was significantly increased over that of total PBMCs (P < .001). (B) T cells were cocultured with graded proportions of autologous monocytes as indicated and stimulated as in panel A. The bars depict net [³H]thymidine incorporation of control cultures (□) and cultures obtained from HSCT recipients (▪) as percent of the response of the respective purified T cells alone (horizontal line = 100%). ^*Significant (P < .05) differences of post-HSCT and control cultures. Error bars indicate standard error of the mean (SEM).