IDO-expressing, mature moDCs prime autologous T cells to suppress allogeneic T-cell responses. (A) T cells were cultured twice for 6 days, each with fresh autologous, mature moDCs, with or without the IDO inhibitor, dl-1MT. These primed T cells (Auto) from either the untreated or dl-1MT–treated group were harvested, washed, and added to separate allogeneic MLRs composed of fresh autologous moDCs with new allogeneic responder T cells (Allo), but no dl-1MT. MoDC to new allogeneic responder T-cell ratio was 1:30; and candidate suppressor T-cell (Auto) to responder allogeneic T-cell (Allo) ratios were 1:10 (□), 1:2 (▩), and 1:1 (▤). Controls were MLRs without suppressor cells (■). After 4 to 5 days in culture, responder T-cell proliferation was measured by [³H]TdR incorporation (average of triplicate means ± SEM, n = 6 independent experiments; P = NS). (B) T-cell viability was determined by trypan blue exclusion and was compared in autologous priming cultures with or without dl-1MT or l-1MT. T-cell counts were determined at day 0 and after 6 days of culture (average of triplicate means ± SEM, n = 6 independent experiments; P = NS).