Although the development of regulatory T cells (T(reg) cells) in the thymus is defined by expression of the lineage marker Foxp3, the precise function of Foxp3 in T(reg) cell lineage commitment is unknown. Here we examined T(reg) cell development and function in mice with a Foxp3 allele that directs expression of a nonfunctional fusion protein of Foxp3 and enhanced green fluorescent protein (Foxp3DeltaEGFP). Thymocyte development in Foxp3DeltaEGFP male mice and Foxp3DeltaEGFP/+ female mice recapitulated that of wild-type mice. Although mature EGFP(+) CD4(+) T cells from Foxp3DeltaEGFP mice lacked suppressor function, they maintained the characteristic T(reg) cell 'genetic signature' and failed to develop from EGFP(-) CD4(+) T cells when transferred into lymphopenic hosts, indicative of their common ontogeny with T(reg) cells. Our results indicate that T(reg) cell effector function but not lineage commitment requires the expression of functional Foxp3 protein.

SUMMARY Although both natural and induced regulatory T (nTreg and iTreg) cells can enforce tolerance, the mechanisms underlying their synergistic actions have not been established. We examined the functions of nTreg and iTreg cells by adoptive transfer immunotherapy of newborn Foxp3-deficient mice. As monotherapy, only nTreg cells prevented disease lethality, but did not suppress chronic inflammation and autoimmunity. Provision of Foxp3-sufficient conventional T cells with nTreg cells reconstituted the iTreg pool and established tolerance. In turn, acute depletion of iTreg cells in rescued mice resulted in weight loss and inflammation. Whereas the transcriptional signatures of nTreg and in vivo derived iTreg cells were closely matched, there was minimal overlap in their T cell receptor (TCR) repertoires. Thus, iTreg cells are an essential non-redundant regulatory subset that supplements nTreg cells, in part by expanding TCR diversity within regulatory responses.

The population dynamics that enable a small number of regulatory T (T(R)) cells to control the immune responses to foreign Ags by the much larger conventional T cell subset were investigated. During the primary immune response, the expansion and contraction of conventional and T(R) cells occurred in synchrony. Importantly, the relative accumulation of T(R) cells at peak response significantly exceeded that of conventional T cells, reflecting extensive cell division within the T(R) cell pool. Transfer of a polyclonal T(R) cell population before immunization antagonized both polyclonal and TCR transgenic responses, whereas blocking T(R) cell function enhanced those responses. These results define an inverse quantitative relationship between T(R) and conventional T cells that controls the magnitude of the primary immune response. The high frequency of dividing T(R) cells suggests degenerate TCR specificity enabling activation by a broad spectrum of Ags.

Multiple sclerosis (MS) is an incurable autoimmune neurodegenerative disease. Environmental factors may be key to MS prevention and treatment. MS prevalence and severity decrease with increasing sunlight exposure and vitamin D 3 supplies, supporting our hypothesis that the sunlight-dependent hormone, 1,25-dihydroxyvitamin D 3 (1,25-(OH) 2 D 3 ), inhibits autoimmune T-cell responses in MS. Moreover, 1,25-(OH) 2 D 3 inhibits and reverses experimental autoimmune encephalomyelitis (EAE), an MS model. Here, we investigated whether 1,25-(OH) 2 D 3 inhibits EAE via the vitamin D receptor (VDR) in T lymphocytes. Using bone marrow chimeric mice with a disrupted VDR only in radiosensitive hematopoietic cells or radio-resistant non-hematopoietic cells, we found that hematopoietic cell VDR function was necessary for 1,25-(OH) 2 D 3 to inhibit EAE. Furthermore, conditional targeting experiments showed that VDR function in T cells was necessary. Neither 1,25-(OH) 2 D 3 nor T-cell-specific VDR targeting influenced CD4 1 Foxp3 1 T-cell proportions in the periphery or the CNS in these studies. These data support a model wherein 1,25-(OH) 2 D 3 acts directly on pathogenic CD41 T cells to inhibit EAE.