Naturally occurring CD4(+)CD25(+) regulatory T cells (Treg) suppress in vitro the proliferation of other T cells in a cell-contact-dependent manner. Dendritic cells (DCs) appear to be a target of Treg-mediated immune suppression. We show here that, in coculture of dye-labeled Treg cells and CD4(+)CD25(-) naïve T cells in the presence of T cell receptor stimulation, Treg cells, which are more mobile than naïve T cells in vitro, out-compete the latter in aggregating around DCs. Deficiency or blockade of leukocyte function-associated antigen-1 (LFA-1) (CD11a/CD18) abrogates Treg aggregation, whereas that of cytotoxic T lymphocyte-associated antigen 4 (CTLA-4) (CD152) does not. After forming aggregates, Treg cells specifically down-regulate the expression of CD80/86, but not CD40 or class II MHC, on DCs in both a CTLA-4- and LFA-1-dependent manner. Notably, Treg exerts this CD80/86-down-modulating effect even in the presence of strong DC-maturating stimuli, such as GM-CSF, TNF-alpha, IFN-gamma, type I IFN, and lipopolysaccharide. Taken together, as a possible mechanism of in vitro Treg-mediated cell contact-dependent suppression, we propose that antigen-activated Treg cells exert suppression by two distinct steps: initial LFA-1-dependent formation of Treg aggregates on immature DCs and subsequent LFA-1- and CTLA-4-dependent active down-modulation of CD80/86 expression on DCs. Both steps prevent antigen-reactive naïve T cells from being activated by antigen-presenting DCs, resulting in specific immune suppression and tolerance.