NLRP6 inflammasome regulates colonic microbial ecology and risk for colitis

Caspase deficiency alters the murine gut microbiome

Caspases are aspartate-specific cysteine proteases that have an essential role in apoptosis and inflammation, and contribute to the maintenance of homeostasis in the intestine. These facts, together with the knowledge that caspases are implicated in host-microbe crosstalk, prompted us to investigate the effect of caspase (Casp)1, -3 and -7 deficiency on the composition of the murine gut microbiota. We observed significant changes in the abundance of the Firmicutes and Bacteroidetes phyla, in particular the Lachnospiraceae, Porphyromonodaceae and Prevotellacea families, when comparing Casp-1, -7 and -3 knockout mice with wild-type mice. Our data point toward an intricate relationship between these caspases and the composition of the murine gut microflora.

Emerging immunological targets in inflammatory bowel disease

Crohn's disease (CD) and ulcerative colitis (UC) are the major forms of inflammatory bowel diseases (IBD) in man. They are caused by damage to the lining of the intestine and deeper layers, due to an excessive immune response directed against components of the gut microflora and poorly controlled by counter-regulatory mechanisms. CD and UC are however immunologically distinct. CD-related inflammation is characterized by a marked mucosal infiltration of T lymphocytes secreting T helper type (Th) 1 and Th17 cytokines. In UC, the local immune response is less polarized but may show enhanced production of IL-5, IL-13 and Th17 cytokines. Downstream however CD and UC share important end-stage effector pathways of intestinal injury, mediated by an active cross-talk between immune and non-immune cells. The clarification of the complex networks of immune-inflammatory mediators operating in the gut of IBD patients has led to the identification of new targets that should facilitate the development of novel biological therapies.

Role of microbiota in postnatal maturation of intestinal T-cell responses

PURPOSE OF REVIEW

Taking advantage of their rapid growth and capacity for continuous genetic adaptation, prokaryotes have colonized all possible ecological environments on earth, including the body surfaces of eukaryotes and their gastrointestinal tract. The mammalian gut contains a complex community of 10 bacteria with a meta-genome containing 1500-fold more genes than the human genome. The forces that control the relationships between eukaryotic hosts and their intestinal bacterial symbionts have, thus, become a major focus of interest.

RECENT FINDINGS

Recent data have highlighted how the dialogue between mammalian hosts and their microbiota stimulates the postnatal maturation of an efficient intestinal barrier that promotes niche colonization by symbiotic bacteria and opposes colonization by pathogens. Herein, we review microbiota-induced T-cell responses and discuss how individual bacteria may shape the balance between regulatory and inflammatory responses. We will also show how host factors might influence the outcome of gut immune responses and affect the structure of the microbiota.

SUMMARY

Deciphering host-microbiota reciprocal influence may not only help in understanding the recent outburst of intestinal inflammatory diseases but also point to strategies able to maintain or restore intestinal homeostasis.

Immunoglobulin A: a bridge between innate and adaptive immunity

PURPOSE OF REVIEW

The review summarizes the recent progress that has been made in understanding the function of immunoglobulin A (IgA) in promoting a healthy mutualism with the commensal microbiota and protecting against pathogens. Although IgA is by far the most abundant antibody produced by mammals, direct experimental evidence for its function is still lacking.

RECENT FINDINGS

IgA is the predominant antibody induced in response to intestinal colonization with commensal bacteria: even fish have been shown to have a mucosal immunoglobulin (IgT), which is produced in the mucosa and coats commensals in the intestinal lumen. Recent studies indicate that intestinal IgA can be highly specific to the inducing commensals. Priming of IgA also appears to be a long-lasting response dependent on the overall dose (integral) of the bacteria sampled rather than exhibiting prime-boost effects normally observed with systemic immunoglobulin responses. Not only is human IgA highly mutated, but a mouse model with deficient hypermutation but intact class-switch recombination also shows that this mutation process (presumably leading to better anticommensal affinities) is important for IgA protection at the mucosal surface. It has been shown that some IgA can be induced independently of T cells through stimulation by epithelial cell and plasmacytoid dendritic cell cytokines including BAFF and APRIL, although the relative roles of the T-dependent and T-independent IgA pathways in generating mucosal protection are still unclear.

SUMMARY

Protection at mucosal surfaces through the secretion of antibodies is a phylogenetically ancient function. Mammals can produce high and low-affinity IgA against their commensal microbes via T-cell-dependent and T-cell-independent pathways to contribute to host microbial mutualism. The process of improving IgA affinity to intestinal luminal contents through somatic hypermutation of immunoglobulin genes improves the level of protection at the mucosal surface and such mutations are abundant in human IgA sequences.

Metagenomics and personalized medicine

The microbiome is a complex community of Bacteria, Archaea, Eukarya, and viruses that infect humans and live in our tissues. It contributes the majority of genetic information to our metagenome and, consequently, influences our resistance and susceptibility to diseases, especially common inflammatory diseases, such as type 1 diabetes, ulcerative colitis, and Crohn's disease. Here we discuss how host-gene-microbial interactions are major determinants for the development of these multifactorial chronic disorders and, thus, for the relationship between genotype and phenotype. We also explore how genome-wide association studies (GWAS) on autoimmune and inflammatory diseases are uncovering mechanism-based subtypes for these disorders. Applying these emerging concepts will permit a more complete understanding of the etiologies of complex diseases and underpin the development of both next-generation animal models and new therapeutic strategies for targeting personalized disease phenotypes.

Male accessory gland infection frequency in infertile patients with chronic microbial prostatitis and irritable bowel syndrome

Recently, we reported an increased prevalence of chronic bacterial prostatitis (CBP) in patients with prostatitis syndromes (PS) and irritable bowel syndrome (IBS) compared with patients with PS alone. The aim of this study was to evaluate the frequency of male accessory gland infections (MAGI) in patients with CBP plus IBS and to compare the sperm parameters of patients with or without MAGI. Fifty consecutive patients with the following criteria were enrolled: (i) infertility; (ii) diagnosis of CBP; and (iii) diagnosis of IBS according to the Rome III criteria. The following two aged-matched control groups were also studied: infertile patients with CBP alone (n = 56) and fertile men (n = 30) who fathered a child within the previous 3 months. Patients and controls underwent to an accurate anamnesis, administration of the NIH-Chronic Prostatitis Symptom Index (NIH-CPSI) and the Rome III questionnaires for prostatitis and IBS, respectively, physical examination, and semen analysis. A significantly higher frequency of MAGI was found in patients with CBP plus IBS (82.0%) compared with the patients with CBP alone (53.6%) or the fertile men (0%). The presence of MAGI in the patients with CBP plus IBS was associated with a significantly lower sperm concentration, total number, and forward motility, and with a higher seminal leucocyte concentration compared with the patients with CBP alone and MAGI. Sperm normal morphology was similar in the groups of patients. All sperm parameters did not differ significantly in both the groups of patients without MAGI. The patients with CBP plus IBS had a significantly higher frequency of MAGI compared with the patients with CBP alone. This was associated with worse sperm parameters and, hence, poorer reproductive prognosis. We suggest to search for the presence of IBS in the patients with PS and in particular when CBP and/or worse sperm parameters are present.

The gut microbiota: challenging immunology

For several decades the intestinal microbiota was mainly studied by those investigating infections and diseases associated with gut health, usually from a microbiology point of view. In the past few years, however, it has become apparent that the intestinal microbiota has widespread implications in the field of immunology, and researchers are being compelled to explain how the microbiota contributes to and/or affects their studies.

The microbiome and rheumatoid arthritis

Humans are not (and have never been) alone. From the moment we are born, millions of micro-organisms populate our bodies and coexist with us rather peacefully for the rest of our lives. This microbiome represents the totality of micro-organisms (and their genomes) that we necessarily acquire from the environment. Micro-organisms living in or on us have evolved to extract the energy they require to survive, and in exchange they support the physiological, metabolic and immune capacities that have contributed to our evolutionary success. Although currently categorized as an autoimmune disorder and regarded as a complex genetic disease, the ultimate cause of rheumatoid arthritis (RA) remains elusive. It seems that interplay between predisposing genetic factors and environmental triggers is required for disease manifestation. New insights from DNA sequence-based analyses of gut microbial communities and a renewed interest in mucosal immunology suggest that the microbiome represents an important environmental factor that can influence autoimmune disease manifestation. This Review summarizes the historical clues that suggest a possible role for the microbiota in the pathogenesis of RA, and will focus on new technologies that might provide scientific evidence to support this hypothesis.

Pathobionts of the gastrointestinal microbiota and inflammatory disease

Our immune system is charged with the vital mission of identifying invading pathogens and mounting proper inflammatory responses. During the process of clearing infections, the immune system often causes considerable tissue damage. Conversely, if the target of immunity is a member of the resident microbiota, uncontrolled inflammation may lead to host pathology in the absence of infectious agents. Recent evidence suggests that several inflammatory disorders may be caused by specific bacterial species found in most healthy hosts. Although the mechanisms that mediate pathology remain largely unclear, it appears that genetic defects and/or environmental factors may predispose mammals to immune-mediated diseases triggered by potentially pathogenic symbionts of the microbiota. We have termed this class of microbes 'pathobionts', to distinguish them from acquired infectious agents. Herein, we explore burgeoning hypotheses that the combination of an immunocompromised state with colonization by pathobionts together comprise a risk factor for certain inflammatory disorders and gastrointestinal (GI) cancer.

The innate immune system in transplantation

The vertebrate innate immune system consists of inflammatory cells and soluble mediators that comprise the first line of defense against microbial infection and, importantly, trigger antigen-specific T and B cell responses that lead to lasting immunity. The molecular mechanisms responsible for microbial non-self recognition by the innate immune system have been elucidated for a large number of pathogens. How the innate immune system recognizes non-microbial non-self, such as organ transplants, is less clear. In this review, we approach this question by describing the principal mechanisms of non-self, or 'damaged' self, recognition by the innate immune system (pattern recognition receptors, the missing self theory, and the danger hypothesis) and discussing whether and how these mechanisms apply to allograft rejection.