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In mice, provision of butyrate--a short-chain fatty acid produced by commensal microorganisms during starch fermentation--facilitates extrathymic generation and differentiation of Foxp3.sup.+ regulatory T cells, demonstrating that metabolic by-products are sensed by cells of the immune system and affect the balance between pro- and anti-inflammatory cells. Bacterial butyrate influences immune balance Several lines of evidence indicate that subsets of commensal microbes shape the gut immune system. For instance, colonization with Clostridia promotes extrathymic generation of regulatory T (T.sub.reg) cells that have a central role in the suppression of inflammatory and allergic responses. However, the molecular basis of such microbe-mediated T.sub.reg induction remains unknown. Two papers in this issue of Nature show that the colonic microbial fermentation product butyrate significantly accelerates the differentiation of colonic T.sub.reg cells and ameliorates colitis in conjunction with an increase in histone H3 acetylation at the Foxp3 promoter. This finding links butyrate to the induction of functional T.sub.reg cells in the colonic mucosa, and also provides molecular insight into the therapeutic application of butyrate. Intestinal microbes provide multicellular hosts with nutrients and confer resistance to infection. The delicate balance between pro- and anti-inflammatory mechanisms, essential for gut immune homeostasis, is affected by the composition of the commensal microbial community. Regulatory T cells (T.sub.reg cells) expressing transcription factor Foxp3 have a key role in limiting inflammatory responses in the intestine.sup.1. Although specific members of the commensal microbial community have been found to potentiate the generation of anti-inflammatory T.sub.reg or pro-inflammatory T helper 17 (T.sub.H17) cells.sup.2,3,4,5,6, the molecular cues driving this process remain elusive. Considering the vital metabolic function afforded by commensal microorganisms, we reasoned that their metabolic by-products are sensed by cells of the immune system and affect the balance between pro- and anti-inflammatory cells. We tested this hypothesis by exploring the effect of microbial metabolites on the generation of anti-inflammatory T.sub.reg cells. We found that in mice a short-chain fatty acid (SCFA), butyrate, produced by commensal microorganisms during starch fermentation, facilitated extrathymic generation of T.sub.reg cells. A boost in T.sub.reg-cell numbers after provision of butyrate was due to potentiation of extrathymic differentiation of T.sub.reg cells, as the observed phenomenon was dependent on intronic enhancer CNS1 (conserved non-coding sequence 1), essential for extrathymic but dispensable for thymic T.sub.reg-cell differentiation.sup.1,7. In addition to butyrate, de novo T.sub.reg-cell generation in the periphery was potentiated by propionate, another SCFA of microbial origin capable of histone deacetylase (HDAC) inhibition, but not acetate, which lacks this HDAC-inhibitory activity. Our results suggest that bacterial metabolites mediate communication between the commensal microbiota and the immune system, affecting the balance between pro- and anti-inflammatory mechanisms., Author(s): Nicholas Arpaia [sup.1] [sup.2] , Clarissa Campbell [sup.1] [sup.2] , Xiying Fan [sup.1] [sup.2] , Stanislav Dikiy [sup.1] [sup.2] , Joris van der Veeken [sup.1] [sup.2] , Paul deRoos [...]