Messages from the Inside. The Dynamic Environment that Favors Intestinal Homeostasis

Abstract

An organism is defined as “an individual living thing capable of responding to stimuli, growing, reproducing, and maintaining homeostasis.” Early during evolution multicellular organisms explored the advantages of a symbiotic life. Mammals harbor a complex aggregate of microorganisms (called microbiota) that includes bacteria, fungi, and archaea. Some of these bacteria have already defined beneficial roles for the human host that include the ability to break down nutrients that could not otherwise be digested, preventing the growth of harmful species, as well as the ability to produce vitamins or hormones. It is intuitive that along the evolutionary path several mechanisms favored bacteria that provided advantages to the host which, in return, avoided launching an aggressive immunological response against them. The intestinal immunological response does not ignore the lumenal content, on the contrary, immune surveillance is favored by continuous antigen sampling. Some intestinal epithelial cells (ECs) are crucial during the sampling process, others actively participate in the defense mechanism. In essence the epithelium acts as a traffic light, communicating to the inside world whether conditions are safe or dangerous, and thus influencing immunological response. In this review we will discuss the dynamic factors that act on the intestinal ECs and how they directly or indirectly influence immune cells during states of health and disease.

Claudin expression in follicle‐associated epithelium of rat Peyer's patches defines a major restriction of the paracellular pathway

“…Primary functions of the intestinal mucosal villous epithelium are absorption, secretion and barrier function (Turner ). The main function of the FAE of PP is considered to capture and present antigens to the underlying immunocompetent cells (Eri & Chieppa ). Therefore, it seems probable that the epithelial barrier is a prerequisite for the specific transcellular transport.…”

Immune-directed support of rich microbial communities in the gut has ancient roots

“…The association of stable microbial communities is governed by epithelial homeostasis and is influenced by the physical process of cellular turnover (Boehm et al, 2012; Bosch, 2013; Fraune et al, 2009). In mammals, for comparison, stem-cell guided gut epithelial turnover helps to promote not only healing of injured surfaces but to re-establish the complex layering of mucin (Johansson, 2012; Johansson et al, 2011a; Johansson et al, 2011b), re-concentrate host-derived factors such as AMPs and prompt re-colonization of some adherent bacterial communities (Eri and Chieppa, 2013; Marchiando et al, 2010; Mukherjee et al, 2008; Sommer and Backhed, 2013). Thus, even in the most basic epithelial surface of the Hydra system, a vastly complex interplay between an array of factors, including local physiological properties, host-derived immune mediators, and microbiome-derived factors, governs the establishment, maintenance and displacement of adherent microbial communities.…”

Microbial sensing by goblet cells controls immune surveillance of luminal antigens in the colon

“…The non-follicle bearing epithelium lining the gastrointestinal (GI) tract is the largest environmentally exposed surface in the body. This epithelium functions as a site for nutrient absorption, as a barrier separating the underlying immune system from potential pathogens, and as a site for immune surveillance of substances derived from the lumen 1 , 2 . How immune surveillance at the non-follicle bearing epithelium is regulated to allow education of the immune system to the luminal contents while preventing unwanted or unnecessary exposures is poorly understood.…”