SUMMARY Development of T helper (Th) 17 cells requires transforming growth factor (TGF)-β and interleukin (IL)-6 and is independent of the Th1 pathway. Although T cells that produce interferon (IFN)-γ are a recognized feature of Th17 cell responses, mice deficient for STAT4 and T-bet—two prototypical Th1 transcription factors—are protected from autoimmunity associated with Th17 pathogenesis. To examine the fate and pathogenic potential of Th17 cells and origin of IFN-γ-producing T cells that emerge during Th17 immunity, we developed IL-17F reporter mice that identify cells committed to expression of IL-17F and IL-17A. Th17 cells required TGF-β for sustained expression of IL-17F and IL-17A. In the absence of TGF-β, both IL-23 and IL-12 acted to suppress IL-17 and enhance IFN-γ production in a STAT4- and T-bet-dependent manner, albeit with distinct efficiencies. These results support a model of late Th17 developmental plasticity with implications for auto-immunity and host defense.

CD4(+) regulatory T cells (T(reg) cells) that produce interleukin 10 (IL-10) are important contributors to immune homeostasis. We generated mice with a 'dual-reporter' system of the genes encoding IL-10 and the transcription factor Foxp3 to track T(reg) subsets based on coordinate or differential expression of these genes. Secondary lymphoid tissues, lung and liver had enrichment of Foxp3(+)IL-10(-) T(reg) cells, whereas the large and small intestine had enrichment of Foxp3(+)IL-10(+) and Foxp3(-)IL-10(+) T(reg) cells, respectively. Although negative for Il10 expression, both Foxp3(+) and Foxp3(-) CD4(+) thymic precursor cells gave rise to peripheral IL-10(+) T(reg) cells, with only Foxp3(-) precursor cells giving rise to all T(reg) subsets. Each T(reg) subset developed in IL-10-deficient mice, but this was blocked by treatment with antibody to transforming growth factor-beta. Thus, Foxp3(+) and Foxp3(-) precursor cells give rise to peripheral IL-10-expressing T(reg) cells by a mechanism dependent on transforming growth factor-beta and independent of IL-10.

A hallmark of adaptive immunity is the generation of memory T cells that confer long-lived, antigen-specific protection against repeat challenges by pathogens. Understanding the mechanisms by which memory T cells arise is important for rational vaccination strategies and improved therapeutic interventions for chronic infections and autoimmune disorders. The large clonal expansion of CD8 T cells in response to some infections has made the development of CD8 T-cell memory more amenable to study, giving rise to a model of memory cell differentiation in which a fraction of fully competent effector T cells transition into long-lived memory T cells. Delineation of CD4 T-cell memory development has proved more difficult as a result of limitations on tracking the smaller populations of CD4 effector T cells generated during a pathogenic challenge, complicating efforts to determine whether CD4 memory T cells are direct descendants of effector T cells or whether they develop by alternative pathways. Here, using two complementary cytokine reporter mouse models to identify interferon (IFN)-gamma-positive effector T cells and track their fate, we show that the lineage relationship between effector and memory CD4 T cells resembles that for CD8 T cells responding to the same pathogen. We find that, in parallel with effector CD8 T cells, IFN-gamma-positive effector CD4 T cells give rise to long-lived memory T cells capable of anamnestic responses to antigenic rechallenge.

Chromatin dynamics that regulate Ifng gene expression are incompletely understood. By using cross-species comparative sequence analyses, we have identified conserved noncoding sequences (CNSs) upstream of the Ifng gene, one of which, located -22 kb from the transcriptional start site, contains clustered consensus binding sequences of transcription factors that function in T cell differentiation. CNS-22 was uniquely associated with histone modifications typical of accessible chromatin in both T helper 1 (Th1) and Th2 cells and demonstrated significant and selective T-bet (T-box transcription factor expressed in T cells, Tbx21)-dependent binding and enhancer activity in Th1 cells. Deletion of CNS-22 in the context of an Ifng reporter transgene ablated T cell receptor-dependent and -independent Ifng expression in Th1 effectors and similarly blocked expression by cytotoxic T lymphocytes and natural killer cells. Thus, a single distal element may be essential for Ifng gene expression by both innate and adaptive immune effector cell lineages.

Accumulating evidence indicates that clinically relevant vitamin D deficiency may be widespread throughout the human population (1). Insofar as exposure to sunshine provides much more vitamin D than can be obtained from dietary sources (1), this deficiency follows from the relatively recent transition in employment practices from hunting, gathering, and farming to the indoor activities characteristic of industrialized economies. The racial disparity in vitamin D status that is seen in the United States (2) along with the seasonal fluctuation of vitamin D levels measured in the United Kingdom (3) and elsewhere (4) highlights the contribution made by sunlight to vitamin D status.A large body of epidemiological and animal data suggests that vitamin D deficiency promotes autoinflammatory disease. The incidence of several human autoimmune diseases has been reported to correlate with increased geographical latitude, low vitamin D intake, and low vitamin D status (1). Systemically administered 1␣,25-dihydroxyvitamin D 3 (1,25-D 3 ), 2 the active form of vitamin D, protects mice against many experimental forms of autoimmune disease (5-9), and deficiency in the vitamin D receptor (VDR) aggravates inflammatory bowel disease in the CD45RB transfer and IL-10-null models (10). Topically applied vitamin D analogs are effective against human psoriasis (1), and a recent report suggests that VDR is a "master regulator" of mouse skin inflammation (11).The contributions made by CD4 T cells to the pathogenesis of many autoimmune diseases were historically ascribed to the activity of the Th1 subset. These cells express IFN-␥ and develop from naïve precursors by activation in the presence of IL-12 and by induction or activation of the transcription factors T-bet, STAT1 and STAT4 (12). More recently, a third effector T cell subset, Th17, has been linked to immunopathology in some forms of autoimmunity previously thought to be Th1-mediated (12). Th17 cells express IL-17A, IL-17F and IL-22 and develop in response to TGF-␤1 and IL-6 and by the induction or activation of the transcription factors STAT3, ROR␥t and ROR␣, with IL-21 and IL-23 providing subsequent reinforcement of this lineage (12). IL-1␤ enhances the development of Th17 cells (13), whereas IL-23 enhances the development of cells that express both IL-17A and IL-22 when TGF-␤1 is limiting (14 -16). Still more recently, a subset of IL-9-expressing T cells (i.e. "Th9" cells) has been implicated in autoimmunity (17)(18)(19). The development of these cells is directed by and is increased in the presence of IL-17E (known also as IL-25) (22). In light of recent insights into the pathogenicity of Th17 and Th9 cells and the longstanding appreciation of the protective effects of vitamin D, we reevaluated the impact of vitamin D signaling on the development of CD4 effector T cells. * This work was supported, in whole or in part, by National Institutes of Health Grants AI035783, AI057956, and DK064400 (to C. T. W.) and Postdoctoral Training Grant AR053458. This work was also supported b...