Obesity is accompanied by chronic, low-grade inflammation of adipose tissue, which promotes insulin resistance and type-2 diabetes. How does fat inflammation escape the powerful armamentarium of cells and molecules normally responsible for guarding against a run-away immune response? Regulatory CD4 + T cells expressing the transcription factor Foxp3 (termed T reg cells) are a lymphocyte lineage specialized in controlling immunologic reactivity. T reg cells with a unique phenotype were highly enriched in the abdominal fat of normal mice, but were strikingly and specifically reduced at this site in insulin-resistant models of obesity. In loss-offunction and gain-of-function experiments, T reg cells regulated the inflammatory state of adipose tissue and insulin resistance. Cytokines differentially synthesized by fat-resident regulatory and conventional T cells directly impacted on the synthesis of inflammatory mediators and glucose uptake by cultured adipocytes. These findings open the door to harnessing the anti-inflammatory properties of T reg cells to inhibit elements of the metabolic syndrome.

Humans expressing a defective form of the transcription factor AIRE (autoimmune regulator) develop multiorgan autoimmune disease. We used aire- deficient mice to test the hypothesis that this transcription factor regulates autoimmunity by promoting the ectopic expression of peripheral tissue- restricted antigens in medullary epithelial cells of the thymus. This hypothesis proved correct. The mutant animals exhibited a defined profile of autoimmune diseases that depended on the absence of aire in stromal cells of the thymus. Aire-deficient thymic medullary epithelial cells showed a specific reduction in ectopic transcription of genes encoding peripheral antigens. These findings highlight the importance of thymically imposed "central" tolerance in controlling autoimmunity.