We have previously shown that regulatory CD25+CD4+ T cells are resistant to clonal deletion induced by viral superantigen in vivo. In this work we report that isolated CD25+CD4+ T cells activated in vitro by anti-CD3 Ab are resistant to Fas-induced apoptosis, in contrast to their CD25−CD4+ counterparts. Resistance of CD25+CD4+ T cells to Fas-dependent activation-induced cell death is not linked to their inability to produce IL-2 or to their ability to produce IL-10. The sensitivity of both populations to Fas-induced apoptosis can be modulated in vitro by changing the CD25+CD4+:CD25−CD4+ T cell ratio. The sensitivity of CD25−CD4+ T cells to apoptosis can be reduced, while the sensitivity of CD25+CD4+ T cells can be enhanced. Modulation of Fas-dependent apoptosis is associated with changes in cytokine production. However, while CD25−CD4+ T cell apoptosis is highly dependent on IL-2 (production of which is inhibited by CD25+CD4+ T cells in coculture), modulation of CD25+CD4+ T cell apoptosis is IL-2 independent. Taken together, these results suggest that CD25+CD4+ and CD25−CD4+ T cell sensitivity to Fas-dependent apoptosis is dynamically modulated during immune responses; this modulation appears to help maintain a permanent population of regulatory T cells required to control effector T cells.

+ regulatory T cells have major roles in controlling immune responses, and use heterogeneous regulatory mechanisms. It is possible that these different activities are mediated by different subsets. Here we show that CD103 + CD25 + CD4 + T cells (that control inflammatory bowel disease) are highly enriched in gut-associated lymphoid tissue and have unique functional properties. In vivo, only this subpopulation is able to control wasting disease and peripheral T cell homeostasis. In vitro, only this subpopulation is able to regulate IL-10 secretion, and it might also mediate infectious suppression. These results demonstrate that regulatory T cells can be divided into discrete subpopulations with defined functional properties and regulatory mechanisms.

Immune regulation plays an important role in the establishment and maintenance of self-tolerance. Nevertheless, it has been difficult to conclude whether regulation is Ag specific because studies have focused on polyclonal populations of regulatory T cells. We have used in this study a murine transgenic model that generates self-reactive, regulatory T cells of known Ag specificity to determine their capacity to suppress naive T cells specific for other Ags. We show that these regulatory cells can regulate the responses of naive T cells with the same TCR specificity, but do not inhibit T cell proliferation or differentiation of naive T cells specific for other Ags. These results demonstrate that immune regulation may be more Ag specific than previously proposed.