Microbiota and host immune responses: a love–hate relationship

Abstract

SummaryA complex relationship between the microbiota and the host emerges early at birth and continues throughout life. The microbiota includes the prokaryotes, viruses and eukaryotes living among us, all of which interact to different extents with various organs and tissues in the body, including the immune system. Although the microbiota is most dense in the lower intestine, its influence on host immunity extends beyond the gastrointestinal tract. These interactions with the immune system operate through the actions of various microbial structures and metabolites, with outcomes ranging from beneficial to deleterious for the host. These differential outcomes are dictated by host factors, environment, and the type of microbes or products present in a specific ecosystem. It is also becoming clear that the microbes are in turn affected and respond to the host immune system. Disruption of this complex dialogue between host and microbiota can lead to immune pathologies such as inflammatory bowel diseases, diabetes and obesity. This review will discuss recent advances regarding the ways in which the host immune system and microbiota interact and communicate with one another.

<b>Possible involvement of mucosal-associated invariant T cells in the progression of inflammatory bowel </b><b>diseases </b>

“…petuation of IBD (2,16). Both the host innate and adaptive immune systems work together to determine the class of a microbial threat and direct the type and degree of immune response to the exposure (25). Therefore, it is likely that both the innate and adaptive intestinal immune systems contribute to the pathology of IBD.…”

The Surface-Associated Exopolysaccharide of Bifidobacterium longum 35624 Plays an Essential Role in Dampening Host Proinflammatory Responses and Repressing Local T H 17 Responses

“…Furthermore, accumulating evidence suggests that the composition and metabolic activity of the gut microbiota has profound effects on proinflammatory activity and the induction of immune tolerance within mucosal tissue (3)(4)(5). Certain microbes induce regulatory responses, while others induce effector responses, resulting in the case of healthy individuals in a balanced homeostatic immunological state that protects against infection and controls aberrant, tissue-damaging inflammatory responses (6).…”

The interaction between invariant Natural Killer T cells and the mucosal microbiota

“…For example, the polysaccharide A from Bacteroides fragilis and the exopolysaccharide from Bifidobacterium longum both can expand regulatory T (Treg) cells in vivo by inducing interleukin-10 (IL-10) production in APCs. 123,124 In some cases, receptors for IgA can facilitate the uptake of bacteria. 125 Cytokines produced by PAMP-activated APCs can also stimulate cytokine production by iNKT cells (see text and Figure 1).…”

“…Short-chain fatty acids (SCFAs), like acetate, propionate, butyrate; biogenic amines, like taurine and histamine; tryptophan catabolites, are all known to modulate metabolic processes in APCs and thereby the host immune response. 123,124 However, we are not aware of data that link these metabolites to iNKT cells.…”