Immune regulation of autoimmune disease can function at two sites: at the secondary lymphoid organs or in the target organ itself. In this study, we investigated the natural resolution of autoimmune pathology within the CNS using murine experimental autoimmune encephalomyelitis (EAE). Recovery correlates with the accumulation of IL-10-producing CD4+CD25+ T cells within the CNS. These CD4+CD25+ cells represent as many as one in three of CD4+ cells in the CNS during recovery, they are FoxP3+ and express other markers associated with regulatory cells (CTLA-4, GITR, and αEβ7), and they have regulatory function ex vivo. Depletion of CD25+ cells inhibits the natural recovery from EAE. Also, depletion of CD25+ cells after recovery removes the resistance to reinduction of EAE observed in this model. Furthermore, passive transfer of CNS-derived CD4+CD25+ cells in low numbers provides protection from EAE in recipient mice. These are the first data demonstrating the direct involvement of CD4+CD25+ regulatory T cells in the natural resolution of autoimmune disease within the target organ.

Transfer of CD4+ T cells to immune-deficient mice in the absence of the CD25+ subset leads to the development of colitis, indicating that regulatory cells capable of controlling a bacteria-driven inflammatory response are present in normal mice. Cells with this function are present in the thymus as well as in the periphery of germ-free mice, suggesting they may be reactive with self-antigen. These cells resemble CD4+CD25+ cells that inhibit organ-specific autoimmunity, suggesting that a similar subset of regulatory T cells may control responses to self and foreign antigens. Development of colitis is dependent on accumulation of activated CD134L+ dendritic cells (DC) in the mesenteric lymph nodes, which is inhibited by CD4+CD25+ cells, indicating that regulatory T cells may control DC activation in vivo. Whilst inhibition of T-cell activation in vitro by CD4+CD25+ cells does not involve interleukin-10 and transforming growth factor-beta, these cytokines are required for the suppression of colitis. It may be that control of responses that activate the innate immune system requires multiple mechanisms of immune suppression. Recently, we identified CD4+CD25+ cells with immune suppressive activity in the thymus and peripheral blood of humans, raising the possibility that dysfunction in this mechanism of immune regulation may be involved in the development of autoimmune and inflammatory diseases.

CD4+CD25+ T cells in mice and rats are capable of transferring protection against organ‐specific autoimmune disease and colitis and suppressing the proliferation of other T cells after polyclonal stimulation in vitro. Here we describe the existence in humans of CD4+CD25+ T cells with the same in vitro characteristics. CD4+CD8–CD25+ T cells are present in both the thymus and peripheral blood of humans (∼ 10 % of CD4+CD8– T cells), proliferate poorly in response to mitogenic stimulation and suppress the proliferation of CD4+CD25– cells in co‐culture. This suppression requires cell contact and can be overcome by the addition of exogenous IL‐2. CD4+CD25+ cells from thymus and blood were poor producers of IL‐2 and IFN‐γ, and suppressed the levels of these cytokines produced by CD4+CD25– cells. However, CD4+CD25+ PBL produced higher levels of IL‐4 and similar amounts of IL‐10 as CD4+CD25– cells. Regulatory CD4+CD25+ T cells have an activated phenotype in the thymus with expression of CTLA‐4 and CD122 (IL‐2Rβ). The fact that CD4+CD25+ regulatory T cells are present with a similar frequency in the thymus of humans, ratsand mice, suggests that the role of these cells in the maintenance of immunological tolerance is an evolutionarily conserved mechanism.

Previously we have shown that autoimmune diabetes, induced in rats by a protocol of adult thymectomy and split-dose gamma irradiation, can be prevented by the transfer of a subset of CD4+ T cells with a memory phenotype (CD45RC−), as well as by CD4+CD8− thymocytes, from syngeneic donors. Further studies now reveal that in the thymus the regulatory cells are observed in the CD25+ subset of CD4+CD8− cells, whereas transfer of the corresponding CD25− thymocyte subset leads to acceleration of disease onset in prediabetic recipients. However, in the periphery, not all regulatory T cells were found to be CD25+. In thoracic duct lymph, cells that could prevent diabetes were found in both CD25− and CD25+ subsets of CD4+CD45RC− cells. Further, CD25− regulatory T cells were also present within the CD4+CD45RC− cell subset from spleen and lymph nodes, but were effective in preventing diabetes only after the removal of CD25− recent thymic emigrants. Phenotypic analysis of human thymocytes showed the presence of CD25+ cells in the same proportions as in rat thymus. The possible developmental relationship between CD25+ and CD25− regulatory T cells is discussed.