Homeostasis and inflammation in the intestine

The immune system and the gut microbiota: friends or foes?

The mammalian intestine is home to a complex community of trillions of bacteria that are engaged in a dynamic interaction with the host immune system. Determining the principles that govern host-microbiota relationships is the focus of intense research. Here, we describe how the intestinal microbiota is able to influence the balance between pro-inflammatory and regulatory responses and shape the host's immune system. We suggest that improving our understanding of the intestinal microbiota has therapeutic implications, not only for intestinal immunopathologies but also for systemic immune diseases.

Obesity, diabetes, and gut microbiota: the hygiene hypothesis expanded?

The connection between gut microbiota and energy homeostasis and inflammation and its role in the pathogenesis of obesity-related disorders are increasingly recognized. Animals models of obesity connect an altered microbiota composition to the development of obesity, insulin resistance, and diabetes in the host through several mechanisms: increased energy harvest from the diet, altered fatty acid metabolism and composition in adipose tissue and liver, modulation of gut peptide YY and glucagon-like peptide (GLP)-1 secretion, activation of the lipopolysaccharide toll-like receptor-4 axis, and modulation of intestinal barrier integrity by GLP-2. Instrumental for gut microbiota manipulation is the understanding of mechanisms regulating gut microbiota composition. Several factors shape the gut microflora during infancy: mode of delivery, type of infant feeding, hospitalization, and prematurity. Furthermore, the key importance of antibiotic use and dietary nutrient composition are increasingly recognized. The role of the Western diet in promoting an obesogenic gut microbiota is being confirmation in subjects. Following encouraging results in animals, several short-term randomized controlled trials showed the benefit of prebiotics and probiotics on insulin sensitivity, inflammatory markers, postprandial incretins, and glucose tolerance. Future research is needed to unravel the hormonal, immunomodulatory, and metabolic mechanisms underlying microbe-microbe and microbiota-host interactions and the specific genes that determine the health benefit derived from probiotics. While awaiting further randomized trials assessing long-term safety and benefits on clinical end points, a healthy lifestyle--including breast lactation, appropriate antibiotic use, and the avoidance of excessive dietary fat intake--may ensure a friendly gut microbiota and positively affect prevention and treatment of metabolic disorders.

Genes and environment: how will our concepts on the pathophysiology of IBD develop in the future?

Inflammatory bowel disease (IBD) has long been known to arise from the interplay between host and environmental factors. From this, a picture is currently emerging in which IBD is likely the result of a continuum of diseases that range from mono- and oligogenically inherited familial forms at one extreme to sporadic forms at the other extreme, which are polygenic in origin and strongly influenced by environmental factors and especially those of infectious origin. The recent expansion of knowledge on the genetic underpinning of IBD has revealed several converging and inter-related functional host pathways that are central to the pathogenesis of these disorders. These include pathways such as autophagy, intracellular bacterial sensing and the unfolded protein response, which play specific roles at the interface between the host and the highly complex microbial communities within the intestines. As such they focus on the functional relationship between the intestinal epithelium and the unique microbial and immune environments along its luminal and abluminal surfaces. Thus, the genetic and environmental factors which are relevant to IBD seem to have the common property of influencing disease by virtue of their specific impact upon the functional relationship between these microbial communities and the intestinal immune system.

Human intestinal microbiota: cross-talk with the host and its potential role in colorectal cancer

In this review, we discuss the multifactorial role of intestinal microbiota in colorectal cancer. The peculiar metabolism of dietary compounds of the individual microbiota complement, its overall immunostimulation and immunomodulatory activity, and eventually the production of toxins that perturb the regulation of cell growth, define the balance of positive and negative risk factors for colorectal cancer development. Moreover, shaping the composition of the human intestinal microbiota, diet has an indirect impact in determining the balance between health and disease. The integration of diet, microbial, and host factors in a system approach is mandatory to determine the overall balance of risk and protective factors for colorectal cancer onset.

IgT, a primitive immunoglobulin class specialized in mucosal immunity

Teleost fish are the most primitive bony vertebrates that contain immunoglobulins. In contrast to mammals and birds, these species are devoid of immunoglobulin A (IgA) or a functional equivalent. This observation suggests that specialization of immunoglobulin isotypes into mucosal and systemic responses took place during tetrapod evolution. Challenging that paradigm, here we show that IgT, an immunoglobulin isotype of unknown function, acts like a mucosal antibody. We detected responses of rainbow trout IgT to an intestinal parasite only in the gut, whereas IgM responses were confined to the serum. IgT coated most intestinal bacteria. As IgT and IgA are phylogenetically distant immunoglobulins, their specialization into mucosal responses probably occurred independently by a process of convergent evolution.

Pharmabiotic manipulation of the microbiota in gastrointestinal disorders, from rationale to reality

The viewpoints of enthusiasts and skeptics in relation to the role of probiotics should not be allowed to distract clinicians from the bigger issue, which is the pivotal role of the microbiota in the protection against many disorders and in the pathogenesis of others. However, all probiotics, like all bacteria, are not created equal, and therapeutic deployment in a generic sense is as absurd as the administration of pills or tablets without regard for the nature of the active ingredient and the intended effect. The rationale for therapeutic manipulation or supplementation of the microbiota is sound in conditions where the intestinal ecosystem is poorly developed, such as in low birth weight neonates, or where it is profoundly disturbed, such as after broad-spectrum antibiotics. In other conditions, such as irritable bowel disorder (IBD), the efficacy of some, but not all, probiotics has been a welcome surprise. However, the impact of probiotics is likely to be modest and is probably more complicated in IBD. In choosing a probiotic strategy, clinicians should adhere to the principles of evidence-based therapeutics. These include: selection from a reputable supplier, with appropriate documentation of contents and shelf life; anticipation of strain-specific effects; avoidance of cocktails without documentation of the activities of each ingredient with absence of interstrain antagonism; and published evidence of efficacy from clinical trials.

Contribution of gut bacteria to liver pathobiology

Emerging evidence suggests a strong interaction between the gut microbiota and health and disease. The interactions of the gut microbiota and the liver have only recently been investigated in detail. Receiving approximately 70% of its blood supply from the intestinal venous outflow, the liver represents the first line of defense against gut-derived antigens and is equipped with a broad array of immune cells (i.e., macrophages, lymphocytes, natural killer cells, and dendritic cells) to accomplish this function. In the setting of tissue injury, whereby the liver is otherwise damaged (e.g., viral infection, toxin exposure, ischemic tissue damage, etc.), these same immune cell populations and their interactions with the infiltrating gut bacteria likely contribute to and promote these pathologies. The following paper will highlight recent studies investigating the relationship between the gut microbiota, liver biology, and pathobiology. Defining these connections will likely provide new targets for therapy or prevention of a wide variety of acute and chronic liver pathologies.

Programming dendritic cells to induce T(H)2 and tolerogenic responses

A fundamental puzzle in immunology is how the immune system decides what types of immune responses to launch against different stimuli. Although much is known about control of T helper type 1 (T(H)1) and T(H)17 responses, the mechanisms that initiate T(H)2 and T regulatory (T(reg)) responses remain obscure. Emerging studies suggest a fundamental role for the innate immune system, particularly dendritic cells (DCs), in this process. We review these studies, and suggest that the innate control of T(H)2 and T(reg) responses can be viewed as different hierarchies of organization, in which DCs, their innate receptors and signaling networks, and their interactions with other cells and local microenvironments represent different levels of the hierarchy.

The two mucus layers of colon are organized by the MUC2 mucin, whereas the outer layer is a legislator of host-microbial interactions

The normal intestinal microbiota inhabits the colon mucus without triggering an inflammatory response. The reason for this and how the intestinal mucus of the colon is organized have begun to be unraveled. The mucus is organized in two layers: an inner, stratified mucus layer that is firmly adherent to the epithelial cells and approximately 50 μm thick; and an outer, nonattached layer that is usually approximately 100 μm thick as measured in mouse. These mucus layers are organized around the highly glycosylated MUC2 mucin, forming a large, net-like polymer that is secreted by the goblet cells. The inner mucus layer is dense and does not allow bacteria to penetrate, thus keeping the epithelial cell surface free from bacteria. The inner mucus layer is converted into the outer layer, which is the habitat of the commensal flora. The outer mucus layer has an expanded volume due to proteolytic activities provided by the host but probably also caused by commensal bacterial proteases and glycosidases. The numerous O-glycans on the MUC2 mucin not only serve as nutrients for the bacteria but also as attachment sites and, as such, probably contribute to the selection of the species-specific colon flora. This observation that normal human individuals carry a uniform MUC2 mucin glycan array in colon may indicate such a specific selection.

Functional intestinal microbiome, new frontiers in prebiotic design

In this review we focus on the revision of the prebiotic concept in the context of the new metagenomic era. Functional metagenomic data provided by the Human Microbiome Project are revolutionizing the view of the symbiotic relationship between the intestinal microbiota and the human host. A deeper knowledge of the mechanisms that govern the dynamic interplay between diet, intestinal microbiota and host nutrition opens the way to better information on the prebiotic structure-function relationships, tailoring prebiotic formula into specific health attributes. On the other hand, functional genomic studies of the sourdough microbial communities allow to scan the environmental variability to identify novel metabolic traits for the biosynthesis of new potential prebiotic molecules. The integration of the functional analyses provided by the massive sequencing of bacterial genomes and metagenomes will allow the rational production of a desired prebiotic molecule with specific functional properties.

Characterization of mice with targeted deletion of the gene encoding core 2 beta1,6-N-acetylglucosaminyltransferase-2

The three glycosyltransferases of the Core 2 beta1,6-N-acetylglucosaminyltransferase (C2GnT) family, C2GnT1, C2GnT2, and C2GnT3, are able to initiate the Core 2 branch of O-glycans. However, C2GnT2, which is highly expressed in the digestive tract, has a broader acceptor substrate specificity that allows it to also generate Core 4 O-glycans and I branches. We discovered that C2GnT2 KO mice have decreased mucosal barrier function in the digestive tract, reduced levels of circulating IgGs and fecal IgA, and increased susceptibility to experimental colitis. Mass spectrometric analyses also revealed that C2GnT2 KO mice had a reduction in Core 2 O-glycans in the digestive tract with a corresponding increase in elongated Core 1 O-glycans. Unexpectedly, we saw that the loss of C2GnT2 and especially the loss of all three C2GnTs resulted in the expression of elongated O-mannose structures in the stomach, suggesting that the elongation of these structures is controlled by competition for UDP-GlcNAc [Stone, E. L., Ismail, M. N., Lee, S. H., Luu, Y., Ramirez, K., Haslam, S. M., Ho, S. B., Dell, A., Fukuda, M. and Marth, J. D. (2009). Glycosyltransferase function in Core 2-type protein O-glycosylation. Mol. Cell. Biol. 29, 3370-3782].

Immune responses to gut microbiota-commensals and pathogens

The mammalian alimentary tract harbors hundreds of species of commensal microorganisms that intimately interact with the host immune system. Within the gut, the immune system actively reacts with potentially pathogenic microbes, while simultaneously remaining ignorant towards the vast majority of non-pathogenic microbiota. The disruption of this delicate balance results in inflammatory bowel diseases. In this review, we describe the recent advances in our understanding of how host-microbiota interactions shape the immune system and how they affect the responses against pathogenic bacteria.