Gut Commensal Microbes and the Gut Immune System

Abstract

An enormous number of bacteria, the gut commensal microbiota, inhabit in the intestinal tract of animals. The microbiota forms a unique and complex gut ecosystem by interacting with the host, and this affects the health and diseases of the host in various ways, including the modification of the immune system. To protect from the gut commensal microbiota, animals have evolved a unique gut immune system, which is by far the largest component of the peripheral immune system, harboring ~70 % of peripheral immune cells. The gut immune system has evolved to sense the quality and quantity of the gut microbiota to contain as well as maintain them and has several distinguishing features such as unique antigen-sampling epithelial M cells and a preponderance of secretory immunoglobulin A in the gut. On the other hand, certain gut commensal microbes promote the development of particular subsets of CD4+ T cells, such as Th17 and regulatory T cells. As such, the host gut immune system and gut commensal microbiota have coevolved and influence each other to maintain gut ecosystem homeostasis.