Indirect alloAg presentation by CD34⁺ cell-derived APCs induced upon contact with allogeneic T cells. CD34-APCs obtained after culture with irradiated allogeneic T cells were incubated for 48 hours with autologous (Auto MLC + AutoAg) or allogeneic apoptotic cells (Auto MLC + AlloAg) or without apoptotic cells (Auto MLC; see “Indirect antigen presentation”). CB Mo-DCs were incubated with allogeneic apoptotic cells (Mo-DC Auto MLC + AlloAg). CD34-APCs or Mo-DCs were then irradiated and cultured with autologous CB responders in primary MLC at a 1:2 ratio. Autologous (AutoAg) or allogeneic (AlloAg) apoptotic cells in the absence of irradiated APCs were also tested as stimulators. T-cell proliferation was measured by 3H-Thymidine incorporation assay. The CB T-cell response to alloantigen indirectly presented by autologous CD34-APCs was significantly greater (P = .02) than the response to autoantigen or to either apoptotic cell population alone. Results are the mean SI ± SD of 4 separate experiments. ^*P < .05.