A) T cell mediated suppression of secondary allogeneic MLR. Autologous T eels primed in the presence of the IDO inhibitor. 1-metnyl-D-tryptophan (1-MT), do not develop as much suppressor activity as T cells primed in the absence of 1-MT, resulting in loss inhibition of the secondary MLR Data presented are representative of 6 experiments. B) CD4⁻CD25***** T cells art potent inhibitors of T cell proliferation. FACS-sorted CD4⁻CD25******, CD4⁻CD25*****, and CD4⁻CD25***** T cells (‘suppressor T cells’) were added to allogeneic MLRs composed of autologous DCs + allogeneic T cells. DC to T cell ratio was 1:30. Suppressor T cell to responder T cell ratios were 1:25, 1:5, and 1:1. After 4–5 days in culture, responder T cell proliferation was assessed by measuring ³HTdR incorporation and comparing with controls containing no suppressor cells (black bar). CD4⁻CD25***** cells inhibited T cell proliferation in a dose-dependent manner. CD4⁻CD25***** cells inhibited proliferation to a lesser degree, and CD4-CD25***** cells showed no inhibition. While the CD4⁻CD25***** cells and to a lesser extent the CD4⁻CD25***** cells suppressed the proliferative response of alloreactive T cells in the MLRs, they were themselves anergic to the allogeneic DCs in proportion to CD25 and FOXP3 expression (data not shown). Values are pooled from 3 independent experiments (N=4 for each condition), and error bars indicate standard deviation of the mean.