Mice made unresponsive by repeated injection of staphylococcal enterotoxin B (SEB) contained SEB-specific CD25(+)CD4(+)TCRBV8(+) T cells that were able to transfer their state of unresponsiveness to primary-stimulated T cells. About one-half of these cells stably up-regulated the expression of CD152. We undertook the present study to determine whether CD152(high) cells seen in this system were T regulatory cells responsible for suppression or whether they represented SEB-activated CD4(+) T effector cells. Our results show that, among SEB-specific TCRBV8(+) T cells isolated from unresponsive mice, all CD152(high)CD25(+)CD4(+) T cells expressed Foxp3, the NF required for differentiation and function of natural T regulatory cells. Moreover, suppression by CD25(+)CD4(+)TCRBV8(+) T cells was fully inhibited by anti-CD152 Abs. Following stimulation by soluble CD152-Ig, dendritic cells (DC) isolated from unresponsive mice strongly increased the expression and the function of indoleamine-2,3-dioxygenase (IDO), the enzyme responsible for the catabolism of tryptophan. This capacity to activate IDO was independent of IFN-gamma production by DC because CD152-Ig stimulation of DC isolated from SEB-treated IFN-gamma-deficient animals activated IDO expression and function. Finally, adding 1-methyl-tryptophan, an inhibitor of tryptophan catabolism, increased substantially the capacity of DC from unresponsive animals to stimulate primary T cell response toward SEB. Thus, we conclude that IFN-gamma-independent CD152-mediated activation of tryptophan catabolism by Foxp3(+)CD25(+) T regulatory cells provides DC with immune regulatory activity in mice unresponsive to SEB.