CD4+ T cells naturally expressing CD25 molecules (natural T regulatory cells 
(Tregs)) have a role in maintaining self tolerance and in regulating responses 
to infectious agents, transplantation Ags, and tumor Ags. CD4+ Tregs induced 
from CD4+CD25- precursors (induced Tregs) also regulate immune responses in the 
periphery. However, which of these Tregs is a major impediment in generating 
antitumor CTL responses is not clear. We show that although the CD4+CD25+ 
subsets isolated from peripheral blood-derived lymphocytes do suppress the 
proliferation of CD4+CD25- effector T cells, they do not suppress the activation 
and expansion of the self but melanoma-associated, melanoma Ag-reactive T cell 1 
(MART-1)27-35-specific CD8+ T cells stimulated by the respective peptide-loaded 
matured dendritic cells in vitro. The CD4+CD25- counterparts, in contrast, lead 
to the generation of CD25+ glucocorticoid-inducible TNFR+-Forkhead/winged helix 
transcription factor+ populations and efficiently suppress the activation and 
expansion of the MART-127-35 epitope-specific CTLs. Our data suggest that when 
CTL precursors are optimally stimulated, natural Tregs are not a formidable 
constraint toward generating a robust antitumor CTL response, but induced Tregs 
could be.