The impact of metabolites derived from the gut microbiota on immune regulation and diseases.

The gut microbiota strongly impacts the physiology and pathology in the host. To understand the complex interactions between host and gut microbiota, an 'integrated omics' approach has been employed, where exhaustive analyses for the different layers of cellular functions, such as epigenomics, transcriptomics and metabolomics, in addition to metagenomics, are combined. With this approach, the mechanisms whereby short-chain fatty acids (SCFAs) regulate host defense and the immune system have been elucidated. In a gnotobiotic mouse model of enterohemorrhagic Escherichia coli infection, Bifidobacterium-derived acetate can protect from infection-mediated death by changing the gene expression profile of colonic epithelial cells. It has also been shown that gut microbiota-derived butyrate enhances colonic regulatory T-cell differentiation through its epigenetic modulatory ability via histone deacetylase inhibition. SCFAs are involved in many other immunomodulatory effects as well as host pathophysiological conditions. Dysbiosis in the gut has been implicated in the pathogenesis of many diseases. Although the causal relationship of gut microbial dysbiosis and/or metabolites with pathogenesis is mostly unknown, mechanistic insights have been elucidated in some cases. Metabolism in the gut microbiota and host liver produces trimethylamine N-oxide, which is known to aggravate atherosclerosis, and a secondary bile acid deoxycholate, which reportedly induces non-alcoholic steatohepatitis-related hepatocellular carcinoma. It has been reported that secondary bile acids could also induce the differentiation of peripherally derived regulatory T cells in the gut. Further studies on the interactions between the host and gut microbiota could lead to the development of new therapeutic strategies as well as in preventive medicine.