Mixed populations of IFN-c + and IFN-c -Tregs are able to suppress proliferation of both anti-CD3 mAb and MHV peptidestimulated T cells [8] . However, it was not possible from these experiments to determine whether IFN-c expression attenuated suppressive function. To address this question, we treated spleen and lymph node-derived lymphocytes isolated from Foxp3 gfp mice with anti-CD3 mAb in the presence or absence of IL-12. GFP + Tregs were then sorted from the mixed cultures. In the presence of IL-12, .90% of all GFP + cells expressed IFN-c while 90-98% IFN-c -Tregs were detected in cultures treated with anti-CD3 mAb alone. Both sets of Tregs (IFN-c + or IFN-c -) were then mixed with CFSE-labeled Tconvs. Cells were then stimulated with anti-CD3 antibody in the absence of IL-12. As shown in Fig. 4 , IFN-c + and IFN-c -Tregs inhibited the proliferation of CFSE-labeled Tconv to the same extent.