Bidirectional regulatory potentials of short-chain fatty acids and their G-protein-coupled receptors in autoimmune neuroinflammation

Microbial metabolites, produced in the intestine, have significant effects on inflammatory diseases throughout the body. Short-chain fatty acids (SCFAs) have protective effects on experimental autoimmune encephalitis (EAE) responses but the detailed roles of SCFAs and their receptors in regulating autoimmune CNS inflammation have been unclear. SCFAs metabolically regulate T cells and change the phenotype of antigen presenting cells to efficiently induce IL-10(+) regulatory T cells. In line with the overall protective effect, blood levels of major SCFAs, such as acetate, propionate and butyrate, are significantly decreased in long-term active progressive multiple sclerosis (MS) patients. Importantly, SCFAs can induce CD4(+) effector T cells, which are highly inflammatory when transferred into mice, suggesting that the direct effect of SCFAs on T cells can even be pro-inflammatory in the CNS. In contrast to the moderate protective effect of SCFAs, mice deficient in GPR41 or GPR43 are more resistant to EAE pathogenesis. Thus, despite the overall protective function of SCFAs, SCFAs and their receptors have the potential to regulate autoimmune CNS inflammation both positively and negatively.