Inflammasomes and the microbiota--partners in the preservation of mucosal homeostasis

Role of gut microbiota in tumorigenesis and antitumoral therapies: an updated review

Gut microbiota plays a prominent role in regulation of host nutrientmetabolism, drug and xenobiotics metabolism, immunomodulation and defense against pathogens. It synthesizes numerous metabolites thatmaintain the homeostasis of host. Any disbalance in the normalmicrobiota of gut can lead to pathological conditions includinginflammation and tumorigenesis. In the past few decades, theimportance of gut microbiota and its implication in various diseases, including cancer has been a prime focus in the field of research. Itplays a dual role in tumorigenesis, where it can accelerate as wellas inhibit the process. Various evidences validate the effects of gutmicrobiota in development and progression of malignancies, wheremanipulation of gut microbiota by probiotics, prebiotics, dietarymodifications and faecal microbiota transfer play a significant role.In this review, we focus on the current understanding of theinterrelationship between gut microbiota, immune system and cancer,the mechanisms by which they play dual role in promotion andinhibition of tumorigenesis. We have also discussed the role ofcertain bacteria with probiotic characteristics which can be used tomodulate the outcome of the various anti-cancer therapies under theinfluence of the alteration in the composition of gut microbiota.Future research primarily focusing on the microbiota as a communitywhich affect and modulate the treatment for cancer would benoteworthy in the field of oncology. This necessitates acomprehensive knowledge of the roles of individual as well asconsortium of microbiota in relation to physiology and response ofthe host.

Potential effects of gut microbiota on host cancers: focus on immunity, DNA damage, cellular pathways, and anticancer therapy

The symbiotic bacteria that live in the human gut and the metabolites they produce have long influenced local and systemic physiological and pathological processes of the host. The gut microbiota are increasingly being recognized for its impact on a range of human diseases, including cancer, it may play a key role in the occurrence, progression, treatment, and prognosis of many types of cancer. Understanding the functional role of the gut microbiota in cancer is crucial for the development of the era of personalized medicine. Here, we review recent advances in research and summarize the important associations and clear experimental evidence for the role of the gut microbiota in a variety of human cancers, focus on the application and possible challenges associated with the gut microbiota in antitumor therapy. In conclusion, our research demonstrated the multifaceted mechanisms of gut microbiota affecting human cancer and provides directions and ideas for future clinical research.

The Development of Gut Microbiota and Its Changes Following C. jejuni Infection in Broilers

The gut is home to more than millions of bacterial species. The gut bacteria coexist with the host in a symbiotic relationship that can influence the host's metabolism, nutrition, and physiology and even module various immune functions. The commensal gut microbiota plays a crucial role in shaping the immune response and provides a continuous stimulus to maintain an activated immune system. The recent advancements in high throughput omics technologies have improved our understanding of the role of commensal bacteria in developing the immune system in chickens. Chicken meat continues to be one of the most consumed sources of protein worldwide, with the demand expected to increase significantly by the year 2050. Yet, chickens are a significant reservoir for human foodborne pathogens such as Campylobacter jejuni. Understanding the interaction between the commensal bacteria and C. jejuni is essential in developing novel technologies to decrease C. jejuni load in broilers. This review aims to provide current knowledge of gut microbiota development and its interaction with the immune system in broilers. Additionally, the effect of C. jejuni infection on the gut microbiota is addressed.

Potential Replacements for Antibiotic Growth Promoters in Poultry: Interactions at the Gut Level and Their Impact on Host Immunity

The chicken gastrointestinal tract (GIT) has a complex, biodiverse microbial community of ~ 9 million bacterial genes plus archaea and fungi that links the host diet to its health. This microbial population contributes to host physiology through metabolite signaling while also providing local and systemic nutrients to multiple organ systems. In a homeostatic state, the host-microbial interaction is symbiotic; however, physiological issues are associated with dysregulated microbiota. Manipulating the microbiota is a therapeutic option, and the concept of adding beneficial bacteria to the intestine has led to probiotic and prebiotic development. The gut microbiome is readily changeable by diet, antibiotics, pathogenic infections, and host- and environmental-dependent events. The intestine performs key roles of nutrient absorption, tolerance of beneficial microbiota, yet responding to undesirable microbes or microbial products and preventing translocation to sterile body compartments. During homeostasis, the immune system is actively preventing or modulating the response to known or innocuous antigens. Manipulating the microbiota through nutrition, modulating host immunity, preventing pathogen colonization, or improving intestinal barrier function has led to novel methods to prevent disease, but also resulted in improved body weight, feed conversion, and carcass yield in poultry. This review highlights the importance of adding different feed additives to the diets of poultry in order to manipulate and enhance health and productivity of flocks.

IL-18 maintains the homeostasis of mucosal immune system via inflammasome-independent but microbiota-dependent manner

Inflammasomes and their product interleukin 18 (IL-18) play important roles in gut microbiota monitoring and homeostasis, and their loss of function could lead to microbiota dysbiosis and accelerate disease progression. However, the impacts of the resulting microbiota dysbiosis on the mucosal immune system are largely unknown. Here, we show that dysbiotic microbiota from Il18^-/- mice induced immune cell loss in the small intestine (SI) in an inflammasome-independent manner. Cohousing experiments revealed that the immunotoxic phenotype of these microbiota was transferable to wild type (WT) mice and induced immune cell death through the receptor-interacting protein kinase 3 (RIP3)-mixed lineage kinase domain like pseudokinase (MLKL) pathway. Analysis of microbiota composition identified two types of bacteria at the genus level, Ureaplasma and Parasutterella, that accumulated in Il18^-/- mice and negatively mediated changes in immune cells in the SI. Furthermore, dysbiosis in Il18^-/- mice also contributed to increased susceptibility to Listeria infection. Collectively, our results demonstrate that IL-18 is essential to microbiota homeostasis and that dysbiotic microbiota could significantly shape the landscape of the immune system.

The Differential Expression of the Inflammasomes in Adipose Tissue and Colon Influences the Development of Colon Cancer in a Context of Obesity by Regulating Intestinal Inflammation

BACKGROUND

Inflammasomes maintain tissue homeostasis and their altered regulation in the colon, and the adipose tissue (AT) leads to chronic activation of inflammatory pathways promoting colon cancer (CC) development. We aimed to analyze the potential involvement of inflammasomes in obesity-associated CC.

METHODS

Ninety-nine volunteers [61 with obesity (OB) and 38 normoponderal (NP)] further subclassified according to the approved protocol for the diagnosis of CC (58 without CC and 41 with CC) were included in the case-control study.

RESULTS

CC (P<0.01) and obesity (P<0.01) were accompanied by increased mRNA levels of NLRP3, NLRP6, ASC, IL1B and NOD2 in VAT. Contrarily, patients with CC exhibited a downregulation of NLRP6 and IL18 in their colon. Additionally, we revealed that the decreased Nlrp1 (P<0.05), Nlrp3 (P<0.01) and Nlrp6 (P<0.01) mRNA levels in the colon from obese rats significantly increase (P<0.05) after caloric restriction. Adipocyte-conditioned media obtained from subjects with obesity reduced (P<0.01) the mRNA of NLRP3 as well as molecules involved in maintaining the intestinal integrity (MUC2, CLDN1 and TJP1) and the anti-inflammatory factors FGF21, KLF4, and IL33 and in HT-29 cells. We also found that the knockdown of NLRP6 in HT-29 cells significantly upregulated (P<0.05) the mRNA of NLRP1 and NLRP3 and inhibited (P<0.05) the expression levels of MUC2. Finally, we showed that the incubation of HT-29 with Akkermansia muciniphila influence (P<0.05) the inflammasome expression profile as well as intestinal integrity-related genes and aberrant inflammation.

CONCLUSIONS

These findings provide evidence that the downregulated levels of NLRP6 and IL18 in the colon from patients with CC may be responsible for a reduced intestinal-barrier integrity, triggering local inflammation, which in turn acts on the dysfunctional AT in obesity, increasing the expression of different inflammasome components and flaring up a vicious cycle of uncontrollable inflammatory cascades that favours a pro-tumorigenic microenvironment.

Exposure to environmental black carbon exacerbates nasal epithelial inflammation via the reactive oxygen species (ROS)-nucleotide-binding, oligomerization domain-like receptor family, pyrin domain containing 3 (NLRP3)-caspase-1-interleukin 1β (IL-1β) pathway

BACKGROUND

Allergic rhinitis(AR) is an increasing challenge to public health worldwide. Exposure to environmental black carbon (BC) is associated with increased risk of allergic rhinitis, but the molecular mechanisms underlying its toxicity have not been fully elucidated. The aims of the present study were therefore to determine the effect of BC on the expression of interleukin 1β (IL-1β) and to investigate the mechanism underlying BC-induced IL-1β production in pollen-sensitized human nasal epithelial cells (hNECs).

METHODS

Nasal mucosal samples collected from 10 patients undergoing nasal surgery were used to isolate and culture epithelial cells as air-liquid interface (ALI) cultures. Cultures exposed to BC ± pollen allergen for 24 hours were assessed for the presence of IL-1β, the production of reactive oxygen species (ROS), and activation of the nucleotide-binding, oligomerization domain-like receptor family, pyrin domain containing 3 (NLRP3) inflammasome. Furthermore, the mechanisms underlying BC ± pollen allergen-induced IL-1β in hNECs were evaluated.

RESULTS

Exposure to BC significantly increased the production of IL-1β and ROS and the expression of NLRP3 in hNECs, compared with control, all of which were significantly increased further by exposure to a combination of BC and pollen. Incubation of hNECs with N-acetyl-L-cysteine (NAC) significantly attenuated BC ± pollen-induced expression of ROS, NLRP3, and IL-1β. NLRP3 and Caspase-1 inhibitors (MCC950 and YVAD) significantly inhibited IL-1β expression and NLRP3 activation, but not NLRP3 expression following exposure to BC ± pollen.

CONCLUSION

These findings suggest that exposure to BC and pollen can exaggerate oxidative stress and significantly increase the expression of IL-1β in hNECs, and that this may involve a pathway integrating ROS-NLRP3-Caspase-1-IL-1β signaling.

ASIC1a inhibits cell pyroptosis induced by acid-induced activation of rat hepatic stellate cells

The activation of hepatic stellate cells (HSCs) is associated with liver fibrosis, the pathological feature of most forms of chronic hepatic damage, and is accompanied by abnormal deposition of the extracellular matrix (ECM). During the pathological process, acid-sensing ion channel 1a (ASIC1a), which is responsible for Ca2+ transportation, is involved in the activation of HSCs. It has previously been identified that ASIC1a is related to pyroptosis in articular chondrocytes. However, it remains unclear whether ASIC1a restrains pyroptosis during liver fibrosis. Here, we determined that the levels of pyroptosis-associated speck-like protein, gasdermin D, caspase-1, nucleotide-binding oligomerization domain (NOD)-like receptor 3, and apoptosis-associated speck-like protein (ASC) decreased, while the level of α-smooth muscle actin and collagen-I increased upon introduction of ASIC1a into an acid-induced model. Inhibition or silencing of ASIC1a and the use of Ca2+ -free medium were able to promote the pyroptosis of activated HSCs, which reduced their deposition. In summary, our study indicates that ASIC1a inhibits pyroptosis of HSCs and that inhibition of ASIC1a may be able to promote pyroptosis to relieve liver fibrosis.

Microbiome and pathogen interaction with the immune system

The intestinal tract harbors a diverse community of microbes that have co-evolved with the host immune system. Although many of these microbes execute functions that are critical for host physiology, the host immune system must control the microbial community so that the dynamics of this interdependent relationship is maintained. To facilitate host homeostasis, the immune system ensures that the microbial load is tolerated, but anatomically contained, while remaining reactive to microbial invasion. Although the microbiota is required for intestinal immune development, immune responses regulate the structure and composition of the intestinal microbiota by evolving unique immune adaptations that manage this high-bacterial load. The immune mechanisms work together to ensure that commensal bacteria rarely breach the intestinal barrier and that any that do invade should be killed rapidly to prevent penetration to systemic sites. The communication between microbiota and the immune system is mediated by the interaction of bacterial components with pattern recognition receptors expressed by intestinal epithelium and various antigen-presenting cells resulting in activation of both innate and adaptive immune responses. Interaction between the microbial community and host plays a crucial role in the mucosal homeostasis and health status of the host. In addition to providing a home to numerous microbial inhabitants, the intestinal tract is an active immunological organ, with more resident immune cells than anywhere else in the body, organized in lymphoid structures called Peyer's patches and isolated lymphoid follicles such as the cecal tonsils. Macrophages, dendritic cells, various subsets of T cells, B cells and the secretory immunoglobulin A (IgA) they produce, all contribute to the generation of a proper immune response to invading pathogens while keeping the resident microbial community in check without generating an overt inflammatory response to it. IgA-producing plasma cells, intraepithelial lymphocytes, and γδT cell receptor-expressing T cells are lymphocytes that are uniquely present in the mucosa. In addition, of the γδT cells in the intestinal lamina propria, there are significant numbers of IL-17-producing T cells and regulatory T cells. The accumulation and function of these mucosal leukocytes are regulated by the presence of intestinal microbiota, which regulate these immune cells and enhance the mucosal barrier function allowing the host to mount robust immune responses against invading pathogens, and simultaneously maintains immune homeostasis.

Infectious Threats, the Intestinal Barrier, and Its Trojan Horse: Dysbiosis

The ecosystem of the gut microbiota consists of diverse intestinal species with multiple metabolic and immunologic activities and it is closely connected with the intestinal epithelia and mucosal immune response, with which it builds a complex barrier against intestinal pathogenic bacteria. The microbiota ensures the integrity of the gut barrier through multiple mechanisms, either by releasing antibacterial molecules (bacteriocins) and anti-inflammatory short-chain fatty acids or by activating essential cell receptors for the immune response. Experimental studies have confirmed the role of the intestinal microbiota in the epigenetic modulation of the gut barrier through posttranslational histone modifications and regulatory mechanisms induced by epithelial miRNA in the epithelial lumen. Any quantitative or functional changes of the intestinal microbiota, referred to as dysbiosis, alter the immune response, decrease epithelial permeability and destabilize intestinal homeostasis. Consequently, the overgrowth of pathobionts (Staphylococcus, Pseudomonas, and Escherichia coli) favors intestinal translocations with Gram negative bacteria or their endotoxins and could trigger sepsis, septic shock, secondary peritonitis, or various intestinal infections. Intestinal infections also induce epithelial lesions and perpetuate the risk of bacterial translocation and dysbiosis through epithelial ischemia and pro-inflammatory cytokines. Furthermore, the decline of protective anaerobic bacteria (Bifidobacterium and Lactobacillus) and inadequate release of immune modulators (such as butyrate) affects the release of antimicrobial peptides, de-represses microbial virulence factors and alters the innate immune response. As a result, intestinal germs modulate liver pathology and represent a common etiology of infections in HIV immunosuppressed patients. Antibiotic and antiretroviral treatments also promote intestinal dysbiosis, followed by the selection of resistant germs which could later become a source of infections. The current article addresses the strong correlations between the intestinal barrier and the microbiota and discusses the role of dysbiosis in destabilizing the intestinal barrier and promoting infectious diseases.

NOD-like receptor family, pyrin domain containing 3 (NLRP3) contributes to inflammation, pyroptosis, and mucin production in human airway epithelium on rhinovirus infection

BACKGROUND

The airway epithelium maintains mucosal homeostasis and effectively responds to pathogens. The roles of the epithelial NOD-like receptor family, pyrin domain containing 3 (NLRP3) inflammasome in human rhinovirus (hRV) infection and its effects mediating epithelial functional changes remain poorly understood.

OBJECTIVE

We investigated the mechanisms and cellular functions mediated by the epithelial NLRP3 inflammasome on hRV infection.

METHODS

Using models of primary human nasal epithelial progenitor cells and differentiated human nasal epithelial cells (hNECs) infected by hRV, we functionally examined key factors for NLRP3 inflammasome activation, cell death, and mucus production. Furthermore, NLRP3 and IL-1β in human epithelium from nasal mucosal inflammation induced by hRV were evaluated.

RESULTS

The inflammasome-mediated IL-1β secretion and pyroptosis in human nasal epithelial progenitor cells and hNECs on hRV infection were dependent on the DDX33/DDX58-NLRP3-caspase-1-GSDMD axis. In differentiated hNECs hRV could also promote major airway epithelial mucin (MUC5AC) production through this axis. Our results further confirmed that the NLRP3 inflammasome signaling pathway was responsible for suppressing hRV replication in airway epithelium. Finally, hRV infection in chronically inflamed nasal mucosa was associated with epithelial mucus hyperproduction, whereas NLRP3 and IL-1β expression levels were significantly increased in hRV-infected epithelium with goblet cell hyperplasia compared with normal epithelium without viral infection.

CONCLUSION

The current study showed that the NLRP3 inflammasome signaling axis could functionally mediate hRV-induced inflammation, pyroptosis, and mucus production in airway epithelium, which might be an essential mechanism associated with hRV-induced airway remodeling.

The role of Ca2+ in acid-sensing ion channel 1a-mediated chondrocyte pyroptosis in rat adjuvant arthritis

Rheumatoid arthritis is an autoimmune disease with a poor prognosis. Pyroptosis is a type of proinflammatory programmed cell death that is characterised by the activation of caspase-1 and secretion of the proinflammatory cytokines interleukin (IL)-1β/18. Previous reports have shown that pyroptosis is closely related to the development of some autoimmune diseases, such as rheumatoid arthritis. The decrease in the pH of joint fluid is a main pathogenic feature of RA and leads to excessive apoptosis in chondrocytes. Acid-sensitive ion channels (ASICs) are extracellular H⁺-activated cation channels that mainly influence Na⁺ and Ca²⁺ permeability. In this study, we investigated the role of Ca²⁺ in acid-sensing ion channel 1a-mediated chondrocyte pyroptosis in an adjuvant arthritis rat model. The expression of apoptosis-associated speck-like protein, NLRP3, caspase-1, ASIC 1a, IL-1β and IL-18 was upregulated in the joints of rats compared with that in normal rats, but the expression of Col2a in cartilage was decreased. However, these changes were reversed by amiloride, which is an inhibitor of ASIC1a. Extracellular acidosis significantly increased the expression of ASIC1a, IL-1β, IL-18, ASC, NLRP3 and caspase-1 and promoted the release of lactate dehydrogenase. Interestingly, Psalmotoxin-1 (Pctx-1) and BAPTA-AM inhibited these effects. These results indicate that ASIC1a mediates pyroptosis in chondrocytes from AA rats. The underlying mechanism may be associated with the ability of ASIC1a to promote [Ca²⁺]_i and upregulate the expression of the NLRP3 inflammasome.

Interactions Between the Gut Microbiota and the Host Innate Immune Response Against Pathogens

The mammalian intestine is colonized by over a trillion microbes that comprise the "gut microbiota," a microbial community which has co-evolved with the host to form a mutually beneficial relationship. Accumulating evidence indicates that the gut microbiota participates in immune system maturation and also plays a central role in host defense against pathogens. Here we review some of the mechanisms employed by the gut microbiota to boost the innate immune response against pathogens present on epithelial mucosal surfaces. Antimicrobial peptide secretion, inflammasome activation and induction of host IL-22, IL-17, and IL-10 production are the most commonly observed strategies employed by the gut microbiota for host anti-pathogen defense. Taken together, the body of evidence suggests that the host gut microbiota can elicit innate immunity against pathogens.

Gut Microbiota and Cancer: From Pathogenesis to Therapy

Cancer is a multifactorial pathology and it represents the second leading cause of death worldwide. In the recent years, numerous studies highlighted the dual role of the gut microbiota in preserving host’s health. Gut resident bacteria are able to produce a number of metabolites and bioproducts necessary to protect host’s and gut’s homeostasis. Conversely, several microbiota subpopulations may expand during pathological dysbiosis and therefore produce high levels of toxins capable, in turn, to trigger both inflammation and tumorigenesis. Importantly, gut microbiota can interact with the host either modulating directly the gut epithelium or the immune system. Numerous gut populating bacteria, called probiotics, have been identified as protective against the genesis of tumors. Given their capability of preserving gut homeostasis, probiotics are currently tested to help to fight dysbiosis in cancer patients subjected to chemotherapy and radiotherapy. Most recently, three independent studies show that specific gut resident species may potentiate the positive outcome of anti-cancer immunotherapy. The highly significant studies, uncovering the tight association between gut microbiota and tumorigenesis, as well as gut microbiota and anti-cancer therapy, are here described. The role of the Lactobacillus rhamnosus GG (LGG), as the most studied probiotic model in cancer, is also reported. Overall, according to the findings here summarized, novel strategies integrating probiotics, such as LGG, with conventional anti-cancer therapies are strongly encouraged.

Microbiome, pattern recognition receptor function in health and inflammation

The innate immune system plays an important role in shaping the microbiota into configurations that are tolerated and beneficial to the host, thereby playing a crucial role in human health. Innate immunity is based on the fundamental principle that Pattern Recognition Receptors (PRRs) recognise pathogen associated molecular patterns as non-self-entities and trigger intracellular signalling pathways that lead to the induction of numerous cytokines and chemokines that help maintain host resistance to infections. Dysregulation of this interaction has been identified as the core defect that leads to chronic intestinal inflammation allowing certain microbiota to be harmful to host health. This dysbiosis of the microbiome is found associated with numerous chronic diseases. A logical explanation would be that genetic defects in the recognition and response pathways that the host uses to identify these microbial pathogens could lead to altered microbial colonisation or mis-recognition of normal bacteria leading to diseases. The interaction between pattern recognition receptors, microbial traits and human health with respect to the gut are now rapidly resolved and will be the subject of this review.

Host Genotype and Microbiota Contribute Asymmetrically to Transcriptional Variation in the Threespine Stickleback Gut

Recent studies of interactions between hosts and their resident microbes have revealed important ecological and evolutionary consequences that emerge from these complex interspecies relationships, including diseases that occur when the interactions go awry. Given the preponderance of these interactions, we hypothesized that effects of the microbiota on gene expression in the developing gut—an important aspect of host biology—would be pervasive, and that these effects would be both comparable in magnitude to and contingent on effects of the host genetic background. To evaluate the effects of the microbiota, host genotype, and their interaction on gene expression in the gut of a genetically diverse, gnotobiotic host model, the threespine stickleback (Gasterosteus aculeatus), we compared RNA-seq data among 84 larval fish. Surprisingly, we found that stickleback population and family differences explained substantially more gene expression variation than the presence of microbes. Expression levels of 72 genes, however, were affected by our microbiota treatment. These genes, including many associated with innate immunity, comprise a tractable subset of host genetic factors for precise, systems-level study of host–microbe interactions in the future. Importantly, our data also suggest subtle signatures of a statistical interaction between host genotype and the microbiota on expression patterns of genetic pathways associated with innate immunity, coagulation and complement cascades, focal adhesion, cancer, and peroxisomes. These genotype-by-environment interactions may prove to be important leads to the understanding of host genetic mechanisms commonly at the root of sometimes complex molecular relationships between hosts and their resident microbes.

Fatty Acids and NLRP3 Inflammasome-Mediated Inflammation in Metabolic Tissues

Worldwide obesity rates have reached epidemic proportions and significantly contribute to the growing prevalence of metabolic diseases. Chronic low-grade inflammation, a hallmark of obesity, involves immune cell infiltration into expanding adipose tissue. In turn, obesity-associated inflammation can lead to complications in other metabolic tissues (e.g., liver, skeletal muscle, pancreas) through lipotoxicity and inflammatory signaling networks. Importantly, although numerous signaling pathways are known to integrate metabolic and inflammatory processes, the nucleotide-binding and oligomerization domain-like receptor, leucine-rich repeat and pyrin domain-containing 3 (NLRP3) inflammasome is now noted to be a key regulator of metabolic inflammation. The NLRP3 inflammasome can be influenced by various metabolites, including fatty acids. Specifically, although saturated fatty acids may promote NLRP3 inflammasome activation, monounsaturated fatty acids and polyunsaturated fatty acids have recently been shown to impede NLRP3 activity. Therefore, the NLRP3 inflammasome and associated metabolic inflammation have key roles in the relationships among fatty acids, metabolites, and metabolic disease. This review focuses on the ability of fatty acids to influence inflammation and the NLRP3 inflammasome across numerous metabolic tissues in the body. In addition, we explore some perspectives for the future, wherein recent work in the immunology field clearly demonstrates that metabolic reprogramming defines immune cell functionality. Although there is a paucity of information about how diet and fatty acids modulate this process, it is possible that this will open up a new avenue of research relating to nutrient-sensitive metabolic inflammation.

Metabolites: messengers between the microbiota and the immune system

In this review, Levy et al. highlight examples of how microbiota-modulated metabolites control the development, differentiation, and activity of the immune system and classify them into functional categories that illustrate the spectrum of ways by which microbial metabolites influence host physiology.

The mammalian intestine harbors one of the largest microbial densities on Earth, necessitating the implementation of control mechanisms by which the host evaluates the state of microbial colonization and reacts to deviations from homeostasis. While microbial recognition by the innate immune system has been firmly established as an efficient means by which the host evaluates microbial presence, recent work has uncovered a central role for bacterial metabolites in the orchestration of the host immune response. In this review, we highlight examples of how microbiota-modulated metabolites control the development, differentiation, and activity of the immune system and classify them into functional categories that illustrate the spectrum of ways by which microbial metabolites influence host physiology. A comprehensive understanding of how microbiota-derived metabolites shape the human immune system is critical for the rational design of therapies for microbiota-driven diseases.

NLRP6: A Multifaceted Innate Immune Sensor

NLRP6, a member of the nucleotide-binding domain, leucine-rich repeat-containing (NLR) innate immune receptor family, regulates inflammation and host defense against microorganisms. Similar to other NLRs, NLRP6 not only participates in inflammasome formation, but is also involved in nuclear factor-κB (NF-κB) and mitogen-activated protein kinase (MAPK) signaling regulation and facilitation of gastrointestinal antiviral effector functions. Additionally, NLRP6 contributes to the regulation of mucus secretion and antimicrobial peptide production, thereby impacting intestinal microbial colonization and associated microbiome-related infectious, autoinflammatory, metabolic, and neoplastic diseases. However, several of the mechanisms attributed to the functions of NLRP6 remain debatable, leaving open questions as to the relevant molecular mechanisms and interacting partners, and putative human relevance. We herein discuss recent findings related to NLRP6 activity, while highlighting outstanding questions and future perspectives in elucidating its roles in health and disease.

An Overview of the Innate and Adaptive Immune System in Inflammatory Bowel Disease

Inflammatory bowel diseases (IBDs) are thought to develop as a result of complex interactions between host genetics, the immune system and the environment including the gut microbiome. Although an improved knowledge of the immunopathogenesis of IBDs has led to great advances in therapy such as the highly effective anti-tumor necrosis factor class of medications, a significant proportion of patients with Crohn's disease and ulcerative colitis do not respond to anti-tumor necrosis factor antibodies. Further understanding of the different immune pathways involved in the genesis of chronic intestinal inflammation is required to help find effective treatments for IBDs. In this review, the role of the mucosal innate and adaptive immune system in IBD is summarized, highlighting new areas of discovery which may hold the key to identifying novel predictive or prognostic biomarkers and new avenues of therapeutic discovery.

Microbiome, metabolites and host immunity

In the intestine, the microbial genomes and repertoire of biochemical reactions outnumber those of the host and significantly contribute to many aspects of the host's health, including metabolism, immunity, development and behavior, while microbial community imbalance is associated with disease. The crosstalk between the host and its microbiome occurs in part through the secretion of metabolites, which have a profound effect on host physiology. The immune system constantly scans the intestinal microenvironment for information regarding the metabolic state of the microbiota as well as the colonization status. Recent studies have uncovered a major role for microbial metabolites in the regulation of the immune system. In this review, we summarize the central findings of how microbiota-modulated metabolites control immune development and activity.

Modulation of inflammasome activity by Porphyromonas gingivalis in periodontitis and associated systemic diseases

Inflammasomes are large multiprotein complexes localized in the cytoplasm of the cell. They are responsible for the maturation of pro-inflammatory cytokines such as interleukin-1β (IL-1β) and IL-18 as well as for the activation of inflammatory cell death, the so-called pyroptosis. Inflammasomes assemble in response to cellular infection, cellular stress, or tissue damage; promote inflammatory responses and are of great importance in regulating the innate immune system in chronic inflammatory diseases such as periodontitis and several chronic systemic diseases. In addition to sensing cellular integrity, inflammasomes are involved in the homeostatic mutualism between the indigenous microbiota and the host. There are several types of inflammasomes of which NLRP3 is best characterized in microbial pathogenesis. Many opportunistic bacteria try to evade the innate immune system in order to survive in the host cells. One of these is the periodontopathogen Porphyromonas gingivalis which has been shown to have several mechanisms of modulating innate immunity by limiting the activation of the NLRP3 inflammasome. Among them, ATP-/P2X₇- signaling is recently associated not only with periodontitis but also with development of several systemic diseases. The present paper reviews multiple mechanisms through which P. gingivalis can modify innate immunity by affecting inflammasome activity.

Metagenomic cross-talk: the regulatory interplay between immunogenomics and the microbiome

The human microbiome, often referred to as the 'second genome', encompasses up to 100-fold more genes than the host genome. In contrast to the human genome, the microbial genome is flexible and amenable to change during the host's lifetime. As the composition of the microbial metagenome has been associated with the development of human disease, the mechanisms controlling the composition and function of the metagenome are of considerable interest and therapeutic potential. In the past few years, studies have revealed how the host immune system is involved in determining the microbial metagenome, and, in turn, how the microbiota regulates gene expression in the immune system. This species-specific bidirectional interaction is required for homeostatic health, whereas aberrations in the tightly controlled regulatory circuits that link the host immunogenome and the microbial metagenome drive susceptibility to common human diseases. Here, we summarize some of the major principles orchestrating this cross-talk between microbial and host genomes, with a special focus on the interaction between the intestinal immune system and the gut microbiome. Understanding the reciprocal genetic and epigenetic control between host and microbiota will be an important step towards the development of novel therapies against microbiome-driven diseases.