IL-10-competent regulatory T cells: Development, phenotype and function

In order to avoid chronic cell activation and inflammation the immune system has developed numerous mechanisms that cooperate to induce and/or maintain what is known as peripheral tolerance. One such mechanism involves active suppression of immune cells by regulatory CD4 T cells (Treg). Treg can generally be categorized as either natural Treg that develop intrathymically during positive selection and suppress the function of autoreactive T cells, or adaptive Treg that develop in the periphery following antigenic stimulation. One widely studied subset of adaptive Treg is the T regulatory 1 (Tr1) cell that is able to suppress immune responses in vivo and in vitro in an IL-10-dependent manner. Although it now appears that Tr1 develop in vitro in the absence of natural CD25+ Treg, it is still unknown whether or not there exists any in vivo lineage relationship between the two subsets. Despite certain overlapping phenotypic and functional features between natural Treg and Tr1, the specific role of Tr1 in immunoregulation has not been defined.; Natural Treg were originally identified by expression of the IL-2 receptor-alpha chain (CD25), a transiently expressed marker of activation. This means that a CD4+CD25+ T cell population is a heterogeneous mix of natural Treg and one or more effector subsets. Consequently, it is still unclear which specific cells within this population are responsible for the regulatory phenomena observed. The transcription factor Foxp3 has been identified as the natural Treg lineage specification factor, and despite being a nuclear protein, is currently the most effective marker for this subset. Isolation of Tr1 has been even more difficult primarily because no unique surface marker has been identified. Instead, Tr1 are often studied following ex vivo isolation of CD4 T cells induced in an antigen-specific manner to secrete large quantities of IL-10.; We have developed IL-10 and Foxp3 dual-reporter mice that allow us to simultaneously identify cells that express these two genes. Using this model we found that in vivo in the steady state, based solely on co-ordinate or differential expression of these genes, there exists three bona fide subsets of regulatory T cells---Foxp3+IL-10-, Foxp3-IL-10+, and Foxp3+IL-10+---all of which can develop extrathymically from naive precursors. Despite very striking polarized distribution of the three subsets, all subsets were found in every lymphoid and non-lymphoid compartment examined. In contrast to what has been suggested for Tr1 cells in particular, we found no role for IL-10 or ICOS in the development of either any of the two IL-10+ subsets. Instead, TGF-beta appears to be a critical player in the developmental cascade of all three subsets.