CD4+CD25+ T cells have been shown to inhibit experimentally induced organ‐specific autoimmune disease and depletion of these regulatory T cells from normal mice results in development of such conditions. Furthermore, CD4+CD25+ T cells suppress the IL‐2 production and thereby the proliferation of polyclonally activated CD4+CD25– T cells in vitro. The suppression in vitro is independent of secreted factors but requires interactions between CD4+CD25– and CD4+CD25+ T cells and antigen‐presenting cells (APC). We have now further investigated the function of CD4+CD25+ T cells in vitro and have focused on their interactions with APC. We found that CD4+CD25+ T cells down‐regulated the expression of the co‐stimulatory molecules CD80 and CD86 on dendritic cells. The steady‐state level of CD80 mRNA was also decreased, while the steady‐state level of CD86 mRNA was not, suggesting that distinct mechanisms regulate the expression of these molecules. The down‐regulation occurred even in the presence of stimuli that would normally increase the expression of CD80 and CD86 molecules. Thus, down‐regulation of co‐stimulatory molecules may be an additional effector function of these regulatory T cells.

We have previously demonstrated that CD4+ CD25+ natural regulatory T cells (Treg cells) induce down-modulation of CD80 and CD86 (B7) molecules on dendritic cells (DCs) in vitro. In this report we show that the extent of down-modulation is functionally significant because Treg-cell conditioned DCs induced poor T-cell proliferation responses. Further, we report that down-modulation was induced rapidly and was inhibited by blocking cytotoxic T lymphocyte antigen-4 (CTLA-4), which is constitutively expressed by the Treg cells. Even though Treg cells have previously been reported to kill antigen-presenting cells, the down-modulation was not due to selective killing of DCs expressing high level of the costimulatory molecules. We propose that Treg cells down-modulate B7-molecules on DCs in a CTLA-4-dependent way, thereby enhancing suppression of T-cell activity.

Repeated exposures to both microbial and innocuous Ags in vivo have been reported to both eliminate and tolerize T cells after their initial activation and expansion. The remaining tolerant T cells have been shown to suppress the response of naive T cells in vitro. This feature is reminiscent of natural CD4+CD25+ regulatory T cells. However, it is not known whether the regulatory function of in vivo-tolerized T cells is similar to the function of natural CD4+CD25+ regulatory T cells. In this study, we demonstrate that CD4+CD25+ as well as CD4+CD25− T cells isolated from mice treated with superantigen three consecutive times to induce tolerance were functionally comparable to natural CD4+CD25+ regulatory T cells, albeit more potent. The different subpopulations of in vivo-tolerized CD4+ T cells efficiently down-modulated costimulatory molecules on dendritic cells, and their suppressive functions were strictly cell contact dependent. Importantly, we demonstrate that conventional CD4+CD25− T cells could also be induced to acquire regulatory functions by the same regimen in the absence of natural regulatory T cells in vivo, but that such regulatory cells were functionally different.

Most T cells in a normal non-immunized individual are in a resting state. However, a small proportion of splenic T cells are large activated cells both in specific pathogen-free and antigen-free mice. To further elucidate the effector functions associated with these "naturally" activated CD4+ T cells, we have characterized the expression of various membrane markers, cytokine production and T helper activity by these cells. We show that naturally activated CD4+ T cells express activation markers and contain tenfold higher proportions of cells producing IL-4, IL-10 and IFN-gamma as compared to small CD4+ T cells. Despite the high proportion of IFN-gamma producers, naturally activated CD4+ T cells still induce B cell proliferation and differentiation. These results are discussed in the context of normal physiological autoreactivity.