Intestinal bacteria and development of the antibody repertoire

Extract: The function of the immune system is to protect the host from invasion by pathogenic microorganisms. Accordingly, most studies of host-microbial interactions have focused on pathogenic agents. The most common and intricate host-microbial interactions are, however, those between the host and non-pathogenic microorganisms that have taken up residence within the host, especially within the gastrointestinal tract. The lower gastrointestinal tract of mammals provides one of the best examples in nature of mutually beneficial association between host and colonizing foreign microorganisms. Between 500 and 1,000 microbial species colonize the mammalian colon to a density of approximately 1,000,000,000,000 bacteria per gram of content, comprising in total 100 times more cells than those that make up the host itself. Over the period of evolution, mammals have not only developed immunological tolerance mechanisms to accommodate this resident community, but they also have come to rely on these intestinal commensals for their own normal development. Studies of germfree and gnotobiotic animals revealed that the intestinal microbiota influences many aspects of immunity including development and function of mucosal immunity, development of oral tolerance and generation of the antibody repertoire. A recent study demonstrated that capsular polysaccharide produced by the commensal bacteria, Bacteroides fragilis, stimulates maturation of the developing immune system by contributing to the generation of CD4+ T cells and the proper T helper 1/T helper 2 cytokine balance.