Reversible microbial colonization of germ-free mice reveals the dynamics of IgA immune responses

Diet-microbial cross-talk underlying increased visceral perception

Visceral hypersensitivity, a fundamental mechanism of chronic visceral pain disorders, can result from both central or peripheral factors, or their combination. As an important regulator of normal gut function, the gut microbiota has been implicated as a key peripheral factor in the pathophysiology of visceral hypersensitivity. Patients with chronic gastrointestinal disorders, such as irritable bowel syndrome, often present with abdominal pain secondary to adverse reactions to dietary components. As both long- and short-term diets are major determinants of gut microbiota configuration that can result in changes in microbial metabolic output, it is becoming increasingly recognized that diet-microbiota interactions play an important role in the genesis of visceral sensitivity. Changes in pain signaling may occur via diet-induced changes in secretion of mediators by both the microbiota and/or host cells. This review will examine the peripheral influence of diet-microbiota interactions underlying increased visceral sensitivity.

Immunomodulatory role of oral microbiota in inflammatory diseases and allergic conditions

In recent years, the interplay between oral microbiota and systemic disease has gained attention as poor oral health is associated with several pathologies. The oral microbiota plays a role in the maintenance of overall health, and its dysbiosis influences chronic inflammation and the pathogenesis of gum diseases. Periodontitis has also been associated with other diseases and health complications such as cancer, neurogenerative and autoimmune disorders, chronic kidney disease, cardiovascular diseases, rheumatic arthritis, respiratory health, and adverse pregnancy outcomes. The host microbiota can influence immune cell development and immune responses, and recent evidence suggests that changes in oral microbiota composition may also contribute to sensitization and the development of allergic reactions, including asthma and peanut allergies. Conversely, there is also evidence that allergic reactions within the gut may contribute to alterations in oral microbiota composition. Here we review the current evidence of the role of the oral microbiota in inflammatory diseases and health complications, as well as its future relevance in improving health and ameliorating allergic disease.

Maternal microbe-specific modulation of the offspring microbiome and development during pregnancy and lactation

The maternal microbiome is essential for the healthy growth and development of offspring and has long-term effects later in life. Recent advances indicate that the maternal microbiome begins to regulate fetal health and development during pregnancy. Furthermore, the maternal microbiome continues to affect early microbial colonization via birth and breastfeeding. Compelling evidence indicates that the maternal microbiome is involved in the regulation of immune and brain development and affects the risk of related diseases. Modulating offspring development by maternal diet and probiotic intervention during pregnancy and breastfeeding could be a promising therapy in the future. In this review, we summarize and discuss the current understanding of maternal microbiota development, perinatal microbial metabolite transfer, mother-to-infant microbial transmission during/after birth and its association with immune and brain development as well as corresponding diseases.

Intestinal mucin-type O-glycans: the major players in the host-bacteria-rotavirus interactions

Rotavirus (RV) causes severe diarrhea in young children and animals worldwide. Several glycans terminating in sialic acids (SAs) and histo-blood group antigens (HBGAs) on intestinal epithelial cell (IEC) surface have been recognized to act as attachment sites for RV. IECs are protected by the double layer of mucus of which O-glycans (including HBGAs and SAs) are a major organic component. Luminal mucins, as well as bacterial glycans, can act as decoy molecules removing RV particles from the gut. The composition of the intestinal mucus is regulated by complex O-glycan-specific interactions among the gut microbiota, RV and the host. In this review, we highlight O-glycan-mediated interactions within the intestinal lumen prior to RV attachment to IECs. A better understanding of the role of mucus is essential for the development of alternative therapeutic tools including the use of pre- and probiotics to control RV infection.

Gut microbiome in type 1 diabetes: the immunological perspective

INTRODUCTION

Type 1 diabetes (T1D) is a prevalent, and yet uncurable, autoimmune disease targeting insulin-producing pancreatic β-cells. Despite a known genetic component in T1D onset, genetics alone cannot explain the alarming worldwide rise in T1D incidence, which is attributed to a growing impact of environmental factors, including perturbations of the gut microbiome.

AREAS COVERED

Intestinal commensal bacteria plays a crucial role in host physiology in health and disease by regulating endocrine and immune functions. An aberrant gut microbiome structure and metabolic function have been documented prior and during T1D onset. In this review, we summarize and discuss the current studies depicting the taxonomic profile and role of the gut microbial communities in murine models of T1D, diabetic patients and human interventional trials.

EXPERT OPINION

Compelling evidence have shown that the intestinal microbiota is instrumental in driving differentiation and functions of immune cells. Therefore, any alterations in the intestinal microbiome composition or microbial metabolite production, particularly early in life, may impact disease susceptibility and amplify inflammatory responses and hence accelerate the course of T1D pathogenesis.

Antibiotic-resistant bacteria originating from the gut may modulate the mucosal immune response during sepsis and septic shock

The enrichment and diversity of gut microbiota play an important role in sepsis, but the role of gut microbiota composition and early-life colonization in sepsis and septic shock has not yet been characterized. The impact of gut microbiota diversity on host immunological disorders and future treatments of inflammatory diseases are not yet fully elucidated. Further, the association between the microbiota and immune development in sepsis remains unknown, and the underlying mechanisms are not well understood. The altered composition of gut microbiota during sepsis is profoundly associated with a loss of commensal bacteria and an overgrowth of potentially pathogenic bacteria, especially AMR bacteria. Disruptions of gut microbiota diversity are directly associated with susceptibility to sepsis and a higher risk of adverse outcomes. Several studies have confirmed that a mutual association between gut microbiota and the host is important for the metabolism of essential nutrients for the organism, for gut development, and for the maturation and development of a fully functional immune system. Therefore, understanding the gut microbiota diversity, composition, and function during various inflammatory conditions and sepsis may provide a comprehensive knowledge of the mechanisms behind the pathogenesis of gut-derived infection in diseases and the design of new treatment options (e.g., probiotics or fecal microbiota transplantation).

Emerging evidence displays an important role of gut microbiota and their derived metabolites in modulating the host mucosal immune response and determining the susceptibility to, as well as outcomes of sepsis.

Effects of rearing system and antibiotic treatment on immune function, gut microbiota and metabolites of broiler chickens

BACKGROUND

In China, cage systems with a high space utilization have gradually replaced ground litter systems, but the disease incidence of chickens in cages is higher. Broilers in the ground litter pens may be stimulated by more environmental microbes during the growth process and show strong immune function and status, but knowledge of which microbes and their metabolites play an immunomodulatory role is still limited. This study aimed to explore the differences and correlations in the immune function, gut microbiota and metabolites and the importance of gut microbiota of broilers raised in cages and ground litter pens.

METHODS

The experiment involved a 2 × 2 factorial arrangement, with rearing systems (cages or ground litter pens) and antibiotic treatment (with or without broad-spectrum antibiotics in drinking water) as factors.

RESULTS

The results showed that, compared with the cage group, the ground litter broilers had stronger nonspecific immune function (Macrophages% and NO in blood), humoral immune function (IgG in blood, LPS stimulation index in ileum) and cellular immune function (T%, Tc%, ConA stimulation index and cytokines in blood). Antibiotic (ABX) treatment significantly reduced nonspecific immune function (Macrophages% and NO in blood, iNOS and Mucin2 mRNA expression in ileum), humoral immune function (IgG in blood and sIgA in ileum) and cellular immune function (T% and cytokines in blood, Th and Tc ratio, TLRs and cytokines mRNA expression in ileum). Furthermore, the ground litter broilers had higher α diversity of microbiota in ileum. The relative abundance of Staphylococcus, Jeotgalicoccus, Jeotgalibaca and Pediococcus in the ileum of ground litter broilers were higher. ABX treatment significantly reduced the α diversity of ileal microbiota, with less Chloroplast and Mitochondria. In addition, the levels of acetic acid, isobutyric acid, kynurenic acid and allolithocholic acid in the ileum of ground litter broilers were higher. Spearman correlation analysis showed that Jeotgalibaca, Pediococcus, acetic acid, kynurenic acid and allolithocholic acid were related to the immune function.

CONCLUSIONS

There were more potential pathogens, litter breeding bacteria, short-chain fatty acids, kynurenine, allolithocholic acid and tryptophan metabolites in the ileum of broilers in ground litter pens, which may be the reason for its stronger immune function and status.

GLP-1 and GLP-2 Orchestrate Intestine Integrity, Gut Microbiota, and Immune System Crosstalk

The intestine represents the body's largest interface between internal organs and external environments except for its nutrient and fluid absorption functions. It has the ability to sense numerous endogenous and exogenous signals from both apical and basolateral surfaces and respond through endocrine and neuronal signaling to maintain metabolic homeostasis and energy expenditure. The intestine also harbours the largest population of microbes that interact with the host to maintain human health and diseases. Furthermore, the gut is known as the largest endocrine gland, secreting over 100 peptides and other molecules that act as signaling molecules to regulate human nutrition and physiology. Among these gut-derived hormones, glucagon-like peptide 1 (GLP-1) and -2 have received the most attention due to their critical role in intestinal function and food absorption as well as their application as key drug targets. In this review, we highlight the current state of the literature that has brought into light the importance of GLP-1 and GLP-2 in orchestrating intestine-microbiota-immune system crosstalk to maintain intestinal barrier integrity, inflammation, and metabolic homeostasis.

B cell responses to the gut microbiota

Most antibody produced by humans originates from mucosal B cell responses. The rules, mechanisms, and outcomes of this process are distinct from B cell responses to infection. Within the context of the intestine, we discuss the induction of follicular B cell responses by microbiota, the development and maintenance of mucosal antibody-secreting cells, and the unusual impacts of mucosal antibody on commensal bacteria. Much remains to be learned about the interplay between B cells and the microbiota, but past and present work hints at a complex, nuanced relationship that may be critical to the way the mammalian gut fosters a beneficial microbial ecosystem.

Sequestration of gut pathobionts in intraluminal casts, a mechanism to avoid dysregulated T cell activation by pathobionts

T cells that express the transcription factor RORγ, regulatory (Treg), or conventional (Th17) are strongly influenced by intestinal symbionts. In a genetic approach to identify mechanisms underlying this influence, we performed a screen for microbial genes implicated, in germfree mice monocolonized with Escherichia coli Nissle. The loss of capsule-synthesis genes impaired clonal expansion and differentiation of intestinal RORγ⁺ T cells. Mechanistic exploration revealed that the capsule-less mutants remained able to induce species-specific immunoglobulin A (IgA) and were highly IgA-coated. They could still trigger myeloid cells, and more effectively damaged epithelial cells in vitro. Unlike wild-type microbes, capsule-less mutants were mostly engulfed in intraluminal casts, large agglomerates composed of myeloid cells extravasated into the gut lumen. We speculate that sequestration in luminal casts of potentially harmful microbes, favored by IgA binding, reduces the immune system's actual exposure, preserving host-microbe equilibrium. The variable immunostimulation by microbes that has been charted in recent years may not solely be conditioned by triggering molecules or metabolites but also by physical limits to immune system exposure.

Selective IgA Deficiency in Spontaneously Hypertensive Rats With Gut Dysbiosis

BACKGROUND

The spontaneously hypertensive rat (SHR) is extensively used to study hypertension. Gut microbiota dysbiosis is a notable feature in SHR for reasons unknown. Immunoglobulin A (IgA) is a major host factor required for gut microbiota homeostasis. We hypothesized that inadequate IgA contributes to gut microbiota dysbiosis in SHR.

METHODS

IgA was measured in feces, cecum, serum, liver, gut-associated lymphoid tissue, and milk from SHR and Wistar Kyoto rats. IgA regulatory factors like IgM, IgG, and pIgR (polymeric immunoglobulin receptor) were analyzed. IgA and IgG antibodies and blood pressure (BP) were measured before and after administrating a bacterial antigen (ie, flagellin).

RESULTS

Compared with Wistar Kyoto rats, SHR displayed remarkably near-deficient IgA levels accompanied by compensatory increases in serum IgM and IgG and gut-liver pIgR expression. Inadequate milk IgA in SHR emphasized this immune defect stemmed from the neonatal stage. Reduced IgA+ B cells in circulation and Peyer patches indicated a possible reason for the lower IgA in SHR. Noteworthy, a genetic insufficiency was unlikely because administering flagellin to SHR induced anti-flagellin IgA antibodies. This immune response surprisingly accelerated hypertension development in SHR, suggesting IgA quiescence may help maintain lower BP.

CONCLUSIONS

This study is the first to reveal IgA deficiency in SHR as one host factor associated with gut microbiota dysbiosis and invigorates future research to determine the pathophysiological role of IgA in hypertension.

Altered Immunoglobulin Repertoire and Decreased IgA Somatic Hypermutation in the Gut during Chronic HIV-1 Infection

Humoral immune perturbations contribute to pathogenic outcomes in persons with HIV-1 infection (PWH). Gut barrier dysfunction in PWH is associated with microbial translocation and alterations in microbial communities (dysbiosis), and IgA, the most abundant immunoglobulin (Ig) isotype in the gut, is involved in gut homeostasis by interacting with the microbiome. We determined the impact of HIV-1 infection on the antibody repertoire in the gastrointestinal tract by comparing Ig gene utilization and somatic hypermutation (SHM) in colon biopsies from PWH (n = 19) versus age and sex-matched controls (n = 13). We correlated these Ig parameters with clinical, immunological, microbiome and virological data. Gene signatures of enhanced B cell activation were accompanied by skewed frequencies of multiple Ig Variable genes in PWH. PWH showed decreased frequencies of SHM in IgA and possibly IgG, with a substantial loss of highly mutated IgA sequences. The decline in IgA SHM in PWH correlated with gut CD4+ T cell loss and inversely correlated with mucosal inflammation and microbial translocation. Diminished gut IgA SHM in PWH was driven by transversion mutations at A or T deoxynucleotides, suggesting a defect not at the AID/APOBEC3 deamination step but at later stages of IgA SHM. These results expand our understanding of humoral immune perturbations in PWH that could have important implications in understanding mucosal immune defects in individuals with chronic HIV-1 infection. IMPORTANCE The gut is a major site of early HIV-1 replication and pathogenesis. Extensive CD4+ T cell depletion in this compartment results in a compromised epithelial barrier that facilitates the translocation of microbes into the underlying lamina propria and systemic circulation, resulting in chronic immune activation. To date, the consequences of microbial translocation on the mucosal humoral immune response (or vice versa) remains poorly integrated into the panoply of mucosal immune defects in PWH. We utilized next-generation sequencing approaches to profile the Ab repertoire and ascertain frequencies of somatic hypermutation in colon biopsies from antiretroviral therapy-naive PWH versus controls. Our findings identify perturbations in the Ab repertoire of PWH that could contribute to development or maintenance of dysbiosis. Moreover, IgA mutations significantly decreased in PWH and this was associated with adverse clinical outcomes. These data may provide insight into the mechanisms underlying impaired Ab-dependent gut homeostasis during chronic HIV-1 infection.

A self-sustaining layer of early-life-origin B cells drives steady-state IgA responses in the adult gut

The adult immune system consists of cells that emerged at various times during ontogeny. We aimed to define the relationship between developmental origin and composition of the adult B cell pool during unperturbed hematopoiesis. Lineage tracing stratified murine adult B cells based on the timing of output, revealing that a substantial portion originated within a restricted neonatal window. In addition to B-1a cells, early-life time-stamped B cells included clonally interrelated IgA plasma cells in the gut and bone marrow. These were actively maintained by B cell memory within gut chronic germinal centers and contained commensal microbiota reactivity. Neonatal rotavirus infection recruited recurrent IgA clones that were distinct from those arising by infection with the same antigen in adults. Finally, gut IgA plasma cells arose from the same hematopoietic progenitors as B-1a cells during ontogeny. Thus, a complex layer of neonatally imprinted B cells confer unique antibody responses later in life.

Comparison and Correlation Analysis of Immune Function and Gut Microbiota of Broiler Chickens Raised in Double-Layer Cages and Litter Floor Pens

This study aimed to compare the immune function and gut microbiota between double-layer caged and litter floor pen-raised broiler chickens. Eighty meaty male chicks were selected and divided into cage group and litter floor group, with 20 replicates in each group. The broilers were raised in the same chicken house. The rearing density of the two rearing systems was same. The broilers were sampled on days 13 and 34. The results showed that compared with the cage group, the litter floor broilers had worse growth performance (23.24% increase in feed conversion ratio) in the early stage; better slaughter performance at day 42; stronger peripheral immune function (including higher lysozyme activity, T-cell ratio, Th-cell ratio, Tc-cell ratio, CD4/CD8, IL-10, B-cell ratio, IgG and IgA levels; and spleen immune-related gene expression); and stronger intestinal immune function (including higher ileum CD80, AvBD2, Mucin2, NF-κB, IL-8, IFN-γ/IL-4, and IgA mRNA expression levels and ileal mucosa sIgA levels). Compared with the cage group, the alpha diversity of ileum microbiota of the litter floor broilers was higher, and the relative abundance levels of litter breeding bacteria (Facklamia, Globicatella, and Jeotgalicoccus) and potential pathogenic bacteria (Streptococcus and Staphylococcus) were higher (P < 0.05). Through Spearman correlation analysis, it was found that enriched microbes in the ileum of litter floor broilers were positively correlated with immune function. In summary, compared with cage broilers, litter floor broilers had more potential pathogenic bacteria and litter breeding bacteria in the ileum, which may be one of the important reasons for the stronger immune function status. IMPORTANCE In China, the three-dimensional rearing system (cage) for broilers has gradually become a trend. In production, it was found that the incidence of disease in broiler chickens raised in cage systems was significantly higher than that of ground litter. Given that broilers raised on ground litter systems may be exposed to more environmental microbes, it is important to understand whether the rearing environment affects the function and status of the host immune system by altering the gut microbiota. In this study, rearing environment-derived gut microbes associated with stronger immune function in ground litter broilers were provided, which will provide new insights into strategies to target gut microbes to promote immune function and status in broilers raised in cages.

Influence of Gut–Liver Axis on Portal Hypertension in Advanced Chronic Liver Disease: The Gut Microbiome as a New Protagonist in Therapeutic Management

Clinically significant portal hypertension is associated with most complications of advanced chronic liver disease (ACLD), including variceal bleeding, ascites, spontaneous bacterial peritonitis, hepatorenal syndrome, and hepatic encephalopathy. Gut dysbiosis is a hallmark of ACLD with portal hypertension and consists of the overgrowth of potentially pathogenic bacteria and a decrease in autochthonous bacteria; additionally, congestion makes the intestinal barrier more permeable to bacteria and their products, which contributes to the development of complications through inflammatory mechanisms. This review summarizes current knowledge on the role of the gut–liver axis in the pathogenesis of portal hypertension, with a focus on therapies targeting portal hypertension and the gut microbiota. The modulation of the gut microbiota on several levels represents a major challenge in the upcoming years; in-depth characterization of the molecular and microbiological mechanisms linking the gut–liver axis to portal hypertension in a bidirectional relationship could pave the way to the identification of new therapeutic targets for innovative therapies in the management of ACLD.

Approaches to discern if microbiome associations reflect causation in metabolic and immune disorders

Our understanding of microorganisms residing within our gut and their roles in the host metabolism and immunity advanced greatly over the past 20 years. Currently, microbiome studies are shifting from association and correlation studies to studies demonstrating causality of identified microbiome signatures and identification of molecular mechanisms underlying these interactions. This transformation is crucial for the efficient translation into clinical application and development of targeted strategies to beneficially modulate the intestinal microbiota. As mechanistic studies are still quite challenging to perform in humans, the causal role of microbiota is frequently evaluated in animal models that need to be appropriately selected. Here, we provide a comprehensive overview on approaches that can be applied in addressing causality of host-microbe interactions in five major animal model organisms (Caenorhabditis elegans, Drosophila melanogaster, zebrafish, rodents, and pigs). We particularly focused on discussing methods available for studying the causality ranging from the usage of gut microbiota transfer, diverse models of metabolic and immune perturbations involving nutritional and chemical factors, gene modifications and surgically induced models, metabolite profiling up to culture-based approached. Furthermore, we addressed the impact of the gut morphology, physiology as well as diet on the microbiota composition in various models and resulting species specificities. Finally, we conclude this review with the discussion on models that can be applied to study the causal role of the gut microbiota in the context of metabolic syndrome and host immunity. We hope this review will facilitate important considerations for appropriate animal model selection.

Dietary protein increases T-cell-independent sIgA production through changes in gut microbiota-derived extracellular vesicles

Secretory IgA is a key mucosal component ensuring host-microbiota mutualism. Here we use nutritional geometry modelling in mice fed 10 different macronutrient-defined, isocaloric diets, and identify dietary protein as the major driver of secretory IgA production. Protein-driven secretory IgA induction is not mediated by T-cell-dependent pathways or changes in gut microbiota composition. Instead, the microbiota of high protein fed mice produces significantly higher quantities of extracellular vesicles, compared to those of mice fed high-carbohydrate or high-fat diets. These extracellular vesicles activate Toll-like receptor 4 to increase the epithelial expression of IgA-inducing cytokine, APRIL, B cell chemokine, CCL28, and the IgA transporter, PIGR. We show that succinate, produced in high concentrations by microbiota of high protein fed animals, increases generation of reactive oxygen species by bacteria, which in turn promotes extracellular vesicles production. Here we establish a link between dietary macronutrient composition, gut microbial extracellular vesicles release and host secretory IgA response.

Immunoglobulin A antibody composition is sculpted to bind the self gut microbiome

Despite being the most abundantly secreted immunoglobulin isotype, the pattern of reactivity of immunoglobulin A (IgA) antibodies toward each individual's own gut commensal bacteria still remains elusive. By colonizing germ-free mice with defined commensal bacteria, we found that the binding specificity of bulk fecal and serum IgA toward resident gut bacteria resolves well at the species level and has modest strain-level specificity. IgA hybridomas generated from lamina propria B cells of gnotobiotic mice showed that most IgA clones recognized a single bacterial species, whereas a small portion displayed cross-reactivity. Orally administered hybridoma-produced IgAs still retained bacterial antigen binding capability, implying the potential for a new class of therapeutic antibodies. Species-specific IgAs had a range of strain specificities. Given the distinctive bacterial species and strain composition found in each individual's gut, our findings suggest the IgA antibody repertoire is shaped uniquely to bind "self" gut bacteria.

Association between intestinal microbiota and inflammatory bowel disease

Inflammatory bowel disease (IBD), which includes Crohn's disease (CD) and ulcerative colitis (UC), has emerged as a global disease with high incidence, long duration, devastating clinical symptoms, and low curability (relapsing immune response and barrier function defects). Mounting studies have been performed to investigate its pathogenesis to provide an ever‐expanding arsenal of therapeutic options, while the precise etiology of IBD is not completely understood yet. Recent advances in high‐throughput sequencing methods and animal models have provided new insights into the association between intestinal microbiota and IBD. In general, dysbiosis characterized by an imbalanced microbiota has been widely recognized as a pathology of IBD. However, intestinal microbiota alterations represent the cause or result of IBD process remains unclear. Therefore, more evidences are needed to identify the precise role of intestinal microbiota in the pathogenesis of IBD. Herein, this review aims to outline the current knowledge of commonly used, chemically induced, and infectious mouse models, gut microbiota alteration and how it contributes to IBD, and dysregulated metabolite production links to IBD pathogenesis.

Structure induced laminar vortices control anomalous dispersion in porous media

Natural porous systems, such as soil, membranes, and biological tissues comprise disordered structures characterized by dead-end pores connected to a network of percolating channels. The release and dispersion of particles, solutes, and microorganisms from such features is key for a broad range of environmental and medical applications including soil remediation, filtration and drug delivery. Yet, owing to the stagnant and opaque nature of these disordered systems, the role of microscopic structure and flow on the dispersion of particles and solutes remains poorly understood. Here, we use a microfluidic model system that features a pore structure characterized by distributed dead-ends to determine how particles are transported, retained and dispersed. We observe strong tailing of arrival time distributions at the outlet of the medium characterized by power-law decay with an exponent of 2/3. Using numerical simulations and an analytical model, we link this behavior to particles initially located within dead-end pores, and explain the tailing exponent with a hopping across and rolling along the streamlines of vortices within dead-end pores. We quantify such anomalous dispersal by a stochastic model that predicts the full evolution of arrival times. Our results demonstrate how microscopic flow structures can impact macroscopic particle transport.

Osteoarthritis Can Also Start in the Gut: The Gut-Joint Axis

Background

Osteoarthritis is a common cause of pain and disability with an increasing prevalence among the global population (Hunter and Bierma-Zeinstra in Lancet 393(10182):1745-1759, 2019; Zhang and Jordan in Clinics in Geriatric Medicine 26(3):355-369, 2010). Altered immune responses and low-grade systemic inflammation driven by gut dysbiosis are being increasingly recognized as contributing factors to the pathophysiology of OA (Tan et al. in International Journal of Rheumatic Diseases. https://doi.org/10.1111/1756-185X.14123, 2021; Binvignat et al. in Joint, Bone, Spine 88(5):105203, 2021; Ramasamy et al. in Nutrients 13(4):1272, 2021), which increased the interest in the so-called "gut-joint axis". The various microbiota in the gastrointestinal tract is commonly referred to as the gut microbiome. The gut microbiome is affected by age, sex, and immune system activity as well as medications, environment, and diet (Arumugam in Nature. https://doi.org/10.1038/nature09944, 2011). The microbiome is pivotal to maintain host health and contributes to nutrition, host defense, and immune development (Nishida et al. in Clinical Journal of Gastroenterology 11:1-10, 2018). Alterations in this microbiome can induce dysbiosis, which is associated with many human disease states including allergies, autoimmune disease, diabetes, and cancer (Lin and Zhang in BMC Immunology 18(1):2, 2017). A gut-joint axis is proposed as a link involving the gastrointestinal microbiome, the immune response that it induces, and joint health.

Results

Emerging evidence has shown that there are specific changes in the microbiome that are associated with osteoarthritis, including increased Firmicutes/Bacteroides ratio, Streptococcus spp. prevalence, and local inflammation (Collins in Osteoarthritis and Cartilage. https://doi.org/10.1016/j.joca.2015.03.014, 2015; Rios in Science and Reports. https://doi.org/10.1038/s41598-019-40601-x, 2019; Schott in JCI insight. https://doi.org/10.1172/jci.insight.95997, 2018; Boer et al. in Nature Communications 10:4881, 2019). Both the innate and adaptive immune systems are affected by the gut microbiome and can become dysregulated in dysbiosis which ultimately triggers events associated with joint OA.

Conclusions

The gut is an intriguing and novel target for OA therapy. Dietary modification or supplementation with fiber, probiotics, or prebiotics could provide a positive impact on the gut joint axis.

Aeromonas hydrophila Induces Skin Disturbance through Mucosal Microbiota Dysbiosis in Striped Catfish (Pangasianodon hypophthalmus)

Bacterial pathogens are well equipped to adhere to and initiate infection in teleost fish. Fish skin mucus serves as the first barrier against environmental pathogens. The mucus harbors commensal microbes that impact host physiological and immunological responses. However, how the skin mucosal microbiota responds to the presence of pathogens remains largely unexplored. Thus, little is known about the status of skin mucus prior to infection with noticeable symptoms. In this study, we investigated the interactions between pathogens and the skin mucosal microbiota as well as the fish skin immune responses in the presence of pathogens. Striped catfish (Pangasianodon hypophthalmus) were challenged with different concentrations of the bacterial pathogen Aeromonas hydrophila (AH), and the skin immune response and the mucosal microbiota were examined by quantitative PCR (qPCR) and 16S rRNA gene sequence analysis. We determined that the pathogen concentration needed to stimulate the skin immune response was associated with significant mucosal microbiota changes, and we reconfirmed these observations using an ex vivo fish skin model. Further analysis indicated that changes in the microbiota were attributed to a significant increase in opportunistic pathogens over AH. We concluded that the presence and increase of AH result in dysbiosis of the mucosal microbiota that can stimulate skin immune responses. We believe that our work sheds light on host-pathogen-commensal microbiota interactions and therefore contributes to aquaculture fish health.

IMPORTANCE The fish skin mucosal microbiota is essential in modulating the host response to the presence of pathogens. Our study provides a platform to study both the correlation and causation of the interactions among the pathogen, fish skin, and the skin mucosal microbiota. Based on these findings, we provide the first mechanistic information on how mucosal microbiota changes induced by the pathogen AH result in skin disturbance with immune stimulation in striped catfish in the natural state and a potential direction for early-infection screening. Thus, this study is highly significant in the prevention of fish disease.

Fluctuating selection on bacterial iron regulation in the mammalian gut

Iron is critical in host-microbe interactions, and its availability is tightly regulated in the mammalian gut. Antibiotics and inflammation can perturb iron availability in the gut, which could alter host-microbe interactions. Here, we show that an adaptive allele of iscR, a major regulator of iron homeostasis of Escherichia coli, is under fluctuating selection in the mouse gut. In vivo competitions in immune-competent, immune-compromised, and germ-free mice reveal that the selective pressure on an iscR mutant E. coli is modulated by the presence of antibiotics, the microbiota, and the immune system. In vitro assays show that iron availability is an important mediator of the iscR allele fitness benefits or costs. We identify Lipocalin-2, a host's immune protein that prevents bacterial iron acquisition, as a major host mechanism underlying fluctuating selection of iscR. Our results provide a remarkable example of strong fluctuating selection acting on bacterial iron regulation in the mammalian gut.

Deciphering the Interdependent Labyrinth between Gut Microbiota and the Immune System

The human gut microbiome interacts with each other and the host, which has significant effects on health and disease development. Intestinal homeostasis and inflammation are maintained by the dynamic interactions between gut microbiota and the innate and adaptive immune systems. Numerous metabolic products produced by the gut microbiota play a role in mediating cross-talk between gut epithelial and immune cells. In the event of an imbalance between the immune system and microbiota, the body becomes susceptible to infections, and homeostasis is compromised. This review mainly focuses on the interplay between microbes and the immune system, such as, T-cell and B-cell mediated adaptive responses to microbiota and signaling pathways for effective communication between the two. We have also highlighted the role of microbes in the activation of the immune response, the development of memory cells, and how the immune system determines the diversity of human gut microbiota. The review also explains the relationship of commensal microbiota and their relation in the production of immunoglobulins.

Altered Pattern of Immunoglobulin A-Targeted Microbiota in Inflammatory Bowel Disease After Fecal Transplantation

Adaptive immune response to the gut microbiota is one of the main drivers of inflammatory bowel disease (IBD). Under inflammatory conditions, immunoglobulin (Ig)-targeted bacteria are altered. However, changes in Ig-targeted bacteria in Asian patients with IBD with ulcerative colitis (UC) remain unclear. Furthermore, changes in IgA-targeted bacteria in patients with UC treated with fecal microbiota transplantation (FMT) are unclear. Here, we analyzed fecal samples of patients with IBD and patients with UC before and after FMT by flow cytometry. We found that the percentage of IgA/G-coated bacteria can be used to assess the severity of IBD. Besides oral pharyngeal bacteria such as Streptococcus, we hypothesized that Megamonas, Acinetobacter, and, especially, Staphylococcus might play an important role in IBD pathogenesis. Moreover, we evaluated the influence of FMT on IgA-coated bacteria in patients with UC. We found that IgA-bacterial interactions were re-established in human FMT recipients and resembled those in the healthy fecal donors. Additionally, the IgA targeting was not influenced by delivery methods: gastroscopy spraying and colonic transendoscopic enteral tubing (TET). Then, we established an acute dextran sulfate sodium (DSS)-induced mouse model to explore whether FMT intervention would impact IgA/G memory B cell in the intestine. We found that after FMT, both IgA/G memory B cell and the percentage of IgA/G-targeted bacteria were restored to normal levels in DSS mice.

Roles of Secretory Immunoglobulin A in Host-Microbiota Interactions in the Gut Ecosystem

In the gastrointestinal tract (GIT), the immune system interacts with a variety of microorganisms, including pathogens as well as beneficial symbionts that perform important physiological functions for the host and are crucial to sustain intestinal homeostasis. In normal conditions, secretory immunoglobulin A (SIgA) is the principal antibody produced by B cells in the GIT mucosa. Polyreactivity provides certain SIgA molecules with the ability of binding different antigens in the bacterial surface, such as O-antigens and teichoic acids, while cross-species reactivity allows them to recognize and interact with different types of bacteria. These functions may be crucial in allowing SIgA to modulate the complex gut microbiota in an efficient manner. Several studies suggest that SIgA can help with the retention and proliferation of helpful members of the gut microbiota. Gut microbiota alterations in people with IgA deficiency include the lack of some species that are known to be normally coated by SIgA. Here, we discuss the different ways in which SIgA behaves in relation to pathogens and beneficial bacteria of the gut microbiota and how the immune system might protect and facilitate the establishment and maintenance of certain gut symbionts.

Where Do We Stand in the Behavioral Pathogenesis of Inflammatory Bowel Disease? The Western Dietary Pattern and Microbiota-A Narrative Review

Despite the increasing knowledge with regard to IBD (inflammatory bowel disease), including ulcerative colitis (UC) and Crohn’s disease (CD), the etiology of these conditions is still not fully understood. Apart from immunological, environmental and nutritional factors, which have already been well documented, it is worthwhile to look at the possible impact of genetic factors, as well as the composition of the microbiota in patients suffering from IBD. New technologies in biochemistry allow to obtain information that can add to the current state of knowledge in IBD etiology.

Instructing durable humoral immunity for COVID-19 and other vaccinable diseases

Many aspects of SARS-CoV-2 have fully conformed with the principles established by decades of viral immunology research, ultimately leading to the crowning achievement of highly effective COVID-19 vaccines. Nonetheless, the pandemic has also exposed areas where our fundamental knowledge is thinner. Some key unknowns are the duration of humoral immunity post-primary infection or vaccination and how long booster shots confer protection. As a corollary, if protection does not last as long as desired, what are some ways it can be improved? Here, I discuss lessons from other infections and vaccines that point to several key features that influence durable antibody production and the perseverance of immunity. These include (1) the specific innate sensors that are initially triggered, (2) the kinetics of antigen delivery and persistence, (3) the starting B cell receptor (BCR) avidity and antigen valency, and (4) the memory B cell subsets that are recalled by boosters. I further highlight the fundamental B cell-intrinsic and B cell-extrinsic pathways that, if understood better, would provide a rational framework for vaccines to reliably provide durable immunity.

More Active Intestinal Immunity Developed by Obese Mice Than Non-Obese Mice After Challenged by Escherichia coli

Obese mice presented lower mortality to non-fatal pneumonia induced by Escherichia coli (E. coli) than the non-obese mice. However, it remained obscure whether the intestine contributed to the protective effect of obese mice with infection. The 64 non-obese (NOB) mice were divided into NOB-uninfected and NOB-E. coli groups, while 64 high-fat diet-induced obesity (DIO) mice were divided into DIO-uninfected and DIO-E. coli groups. Mice in E. coli groups were intranasally instilled with 40 μl E. coli (4.0 ×109 colony-forming units [CFUs]), while uninfected groups with the same volume of phosphate buffer saline (PBS). The T subsets of Intraepithelial lymphocytes (IELs) and lamina propria lymphocytes (LPLs) in the intestine were collected for flow cytometry analysis at 0, 12, 24, and 72 h post-infection, also the duodenum and colon were harvested to survey histopathological change. The results showed that the percentage of CD3+T cells in LPLs in DIO-E. coli group was significantly lower than that in the DIO-uninfected group after infection (p &lt; 0.05). The percentage of CD4+T cells in IELs in NOB-E. coli was significantly lower than that in DIO-E. coli after infection (p &lt; 0.05). The percentage of CD8+T cells in LPLs in NOB-E. coli was significantly lower than that in DIO-E. coli at 12 and 24 h (p &lt; 0.05). The immunoglobulin A (IgA)+ cells in DIO-uninfected were higher than that in NOB-uninfected at all time points (p &lt; 0.05). The IgA+ cells in DIO-E. coli were higher than that in DIO-uninfected at 12, 24, and 72 h (p &lt; 0.05). The results revealed that the level of intestinal mucosal immunity in obese mice was more active than that in non-obese mice.

Host-microbe interactions and outcomes in multiple myeloma and hematopoietic stem cell transplantation

Microbiota are essential to normal immune development and there is growing recognition of its importance to human health and disease and deepening understanding of the complexity of host-microbe interactions in the human gut and other tissues. Commensal microbes not only can influence host immunity locally through impacts of bioactive microbial metabolites and direct interactions with epithelial cells and innate immune receptors but also can exert systemic immunomodulatory effects via impacts on host immune cells capable of trafficking beyond the gut. Emerging data suggest microbiota influence the development of multiple myeloma (MM), a malignancy of the immune system derived from immunoglobulin-producing bone marrow plasma cells, through the promotion of inflammation. Superior treatment outcomes for MM correlate with a higher abundance of commensal microbiota capable of influencing inflammatory responses through the production of butyrate. In patients with hematologic malignancies, higher levels of diversity of the gut microbiota correlate with superior outcomes after hematopoietic stem cell transplantation. Correlative data support the impact of commensal microbiota on survival, risk of infection, disease relapse, and graft-versus-host disease (GVHD) after transplant. In this review, we will discuss the current understanding of the role of host-microbe interactions and the inflammatory tumor microenvironment of multiple myeloma, discuss data describing the key role of microbiota in hematopoietic stem cell transplantation for treatment of hematologic malignancies, and highlight several possible concepts for interventions directed at the gut microbiota to influence treatment outcomes.

From germ-free to wild: modulating microbiome complexity to understand mucosal immunology

The gut microbiota influences host responses at practically every level, and as research into host-microbe interactions expands, it is not surprising that we are uncovering similar roles for the microbiota at other barrier sites, such as the lung and skin. Using standard laboratory mice to assess host-microbe interactions, or even host intrinsic responses, can be challenging, as slight variations in the microbiota can affect experimental outcomes. When it comes to designing and selecting an appropriate level of microbial diversity and community structure for colonization of our laboratory rodents, we have more choices available to us than ever before. Here we will discuss the different approaches used to modulate microbial complexity that are available to study host-microbe interactions. We will describe how different models have been used to answer distinct biological questions, covering the entire microbial spectrum, from germ-free to wild.

Making Sense of Quorum Sensing at the Intestinal Mucosal Interface

The gut microbiome can produce metabolic products that exert diverse activities, including effects on the host. Short chain fatty acids and amino acid derivatives have been the focus of many studies, but given the high microbial density in the gastrointestinal tract, other bacterial products such as those released as part of quorum sensing are likely to play an important role for health and disease. In this review, we provide of an overview on quorum sensing (QS) in the gastrointestinal tract and summarise what is known regarding the role of QS molecules such as auto-inducing peptides (AIP) and acyl-homoserine lactones (AHL) from commensal, probiotic, and pathogenic bacteria in intestinal health and disease. QS regulates the expression of numerous genes including biofilm formation, bacteriocin and toxin secretion, and metabolism. QS has also been shown to play an important role in the bacteria–host interaction. We conclude that the mechanisms of action of QS at the intestinal neuro–immune interface need to be further investigated.

Gut Microbiome as a Mediator of Stress Resilience: A Reactive Scope Model Framework

Stress resilience is defined as the ability to rebound to a homeostatic state after exposure to a perturbation. Organisms modulate various physiological mediators to respond to unpredictable changes in their environment. The gut microbiome is a key example of a physiological mediator that coordinates a myriad of host functions including counteracting stressors. Here, we highlight the gut microbiome as a mediator of host stress resilience in the framework of the reactive scope model. The reactive scope model integrates physiological mediators with unpredictable environmental changes to predict how animals respond to stressors. We provide examples of how the gut microbiome responds to stressors within the four ranges of the reactive scope model (i.e., predictive homeostasis, reactive homeostasis, homeostatic overload, and homeostatic failure). We identify measurable metrics of the gut microbiome that could be used to infer the degree to which the host is experiencing chronic stress, including microbial diversity, flexibility, and gene richness. The goal of this perspective piece is to highlight the underutilized potential of measuring the gut microbiome as a mediator of stress resilience in wild animal hosts.

Gastrointestinal Microbiota of Spiny Lobster: A Review

The gastrointestinal (GI) microbiota is a group of complex and dynamic microorganisms present in the GI tract of an organism that live in symbiosis with the host and benefit the host with various biological functions. The communities of GI microbiota are formed by various aerobic, anaerobic, and facultatively anaerobic bacteria in aquatic species. In spiny lobsters, common GI microorganisms found in the GI tract are Vibrio, Pseudomonas, Bacillus, Micrococcus, and Flavobacterium, where the structure and abundance of these microbes are varied depending on the environment. GI microbiotas hold an important role and significantly affect the overall condition of spiny lobsters, such as secreting digestive enzymes (lipase, protease, and cellulase), helping in digesting food intake, providing nutrition and synthesising vitamins needed by the host system, and protecting the host against infection from pathogens and diseases by activating an immune mechanism in the GI tract. The microorganisms in the water column, sediment, and diet are primarily responsible for altering, manipulating, and shaping GI microbial structures and communities. This review also highlights the possibilities of isolating the indigenous GI microbiota as a potential probiotic strain and introducing it to spiny lobster juveniles and larvae for better health management.

Immunity to enteric viruses

Pathogenic enteric viruses are a major cause of morbidity and mortality, particularly among children in developing countries. The host response to enteric viruses occurs primarily within the mucosa, where the intestinal immune system must balance protection against pathogens with tissue protection and tolerance to harmless commensal bacteria and food. Here, we summarize current knowledge in natural immunity to enteric viruses, highlighting specialized features of the intestinal immune system. We further discuss how knowledge of intestinal anti-viral mechanisms can be translated into vaccine development with particular focus on immunization in the oral route. Research reveals that the intestine is a complex interface between enteric viruses and the host where environmental factors influence susceptibility and immunity to infection, while viral infections can have lasting implications for host health. A deeper mechanistic understanding of enteric anti-viral immunity with this broader context can ultimately lead to better vaccines for existing and emerging viruses.

Gut Microbiota Disruption in COVID-19 or Post-COVID Illness Association with severity biomarkers: A Possible Role of Pre / Pro-biotics in manipulating microflora

Coronavirus disease (COVID-19), a coronavirus-induced illness attributed to severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) transmission, is thought to have first emerged on November 17, 2019. According to World Health Organization (WHO). COVID-19 has been linked to 379,223,560 documented occurrences and 5,693,245 fatalities globally as of 1st Feb 2022. Influenza A virus that has also been discovered diarrhea and gastrointestinal discomfort was found in the infected person, highlighting the need of monitoring them for gastro intestinal tract (GIT) symptoms regardless of whether the sickness is respiration related. The majority of the microbiome in the intestines is Firmicutes and Bacteroidetes, while Bacteroidetes, Proteobacteria, and Firmicutes are found in the lungs. Although most people overcome SARS-CoV-2 infections, many people continue to have symptoms months after the original sickness, called Long-COVID or Post COVID. The term "post-COVID-19 symptoms" refers to those that occur with or after COVID-19 and last for more than 12 weeks (long-COVID-19). The possible understanding of biological components such as inflammatory, immunological, metabolic activity biomarkers in peripheral blood is needed to evaluate the study. Therefore, this article aims to review the informative data that supports the idea underlying the disruption mechanisms of the microbiome of the gastrointestinal tract in the acute COVID-19 or post-COVID-mediated elevation of severity biomarkers.

Roles of gastrointestinal polypeptides in intestinal barrier regulation

The intestinal barrier is a dynamic entity that is organized as a multilayer system and includes various intracellular and extracellular elements. The gut barrier functions in a coordinated manner to impede the passage of antigens, toxins, and microbiome components and simultaneously preserves the balanced development of the epithelial barrier and the immune system and the acquisition of tolerance to dietary antigens and intestinal pathogens.Numerous scientific studies have shown a significant association between gut barrier damage and gastrointestinal and extraintestinal diseases such as inflammatory bowel disease, celiac disease and hepatic fibrosis. Various internal and external factors regulate the intestinal barrier. Gastrointestinal peptides originate from enteroendocrine cells in the luminal digestive tract and are critical gut barrier regulators. Recent studies have demonstrated that gastrointestinal peptides have a therapeutic effect on digestive tract diseases, enhancing epithelial barrier activity and restoring the gut barrier. This review demonstrates the roles and mechanisms of gastrointestinal polypeptides, especially somatostatin (SST) and vasoactive intestinal peptide (VIP), in intestinal barrier regulation.

Microbiota in health and diseases

The role of microbiota in health and diseases is being highlighted by numerous studies since its discovery. Depending on the localized regions, microbiota can be classified into gut, oral, respiratory, and skin microbiota. The microbial communities are in symbiosis with the host, contributing to homeostasis and regulating immune function. However, microbiota dysbiosis can lead to dysregulation of bodily functions and diseases including cardiovascular diseases (CVDs), cancers, respiratory diseases, etc. In this review, we discuss the current knowledge of how microbiota links to host health or pathogenesis. We first summarize the research of microbiota in healthy conditions, including the gut-brain axis, colonization resistance and immune modulation. Then, we highlight the pathogenesis of microbiota dysbiosis in disease development and progression, primarily associated with dysregulation of community composition, modulation of host immune response, and induction of chronic inflammation. Finally, we introduce the clinical approaches that utilize microbiota for disease treatment, such as microbiota modulation and fecal microbial transplantation.

Oral S2-Ag85 DNA Vaccine Activated Intestinal Cell dsDNA and RNA Sensors to Promote the Presentation of Intestinal Antigen

Objective

To explore the molecular mechanism by which oral S2-Ag85DNA vaccines present intestinal antigens. The oral S2-Ag85 vaccine has been shown to protect the human body and effectively improve the titration of the vaccine by acting on intestinal mucosa cells and enhancing their immunogenicity.

Method

Mice were immunized with the recombinant S2-Ag85 vaccine, and antibody secretion was then detected in the intestinal tissue. The molecular mechanisms of in vitro detection sensor molecules RIG-1, Pol III, and related conductor transductor molecules DAI, STING, AIM2, IRF3, and IRF7 were determined by separating intestinal IEC, DC, and IELC cells.

Results

The S2-Ag85A vaccine was effective in activating dsDNA and RNA transduction pathways in intestinal cells and improving intestinal antigen presentation in mice.

Roles of Microbiota in Cancer: From Tumor Development to Treatment

Cancer as a second leading cause of death arises from multifactorial pathology. The association of microbiota and their products with various pathologic conditions including cancer is receiving significant attention over the past few years. Mounting evidence showed that human microbiota is an emerging target in tumor onset, progression, prevention, and even diagnosis. Accordingly, modulating this composition might influence the response to tumor therapy and therapeutic resistance as well. Through this review, one could conceive of complex interaction between the microbiome and cancer in either positive or negative manner by which may hold potential for finding novel preventive and therapeutic strategies against cancer.

Understanding the Complexities and Changes of the Astronaut Microbiome for Successful Long-Duration Space Missions

During space missions, astronauts are faced with a variety of challenges that are unique to spaceflight and that have been known to cause physiological changes in humans over a period of time. Several of these changes occur at the microbiome level, a complex ensemble of microbial communities residing in various anatomic sites of the human body, with a pivotal role in regulating the health and behavior of the host. The microbiome is essential for day-to-day physiological activities, and alterations in microbiome composition and function have been linked to various human diseases. For these reasons, understanding the impact of spaceflight and space conditions on the microbiome of astronauts is important to assess significant health risks that can emerge during long-term missions and to develop countermeasures. Here, we review various conditions that are caused by long-term space exploration and discuss the role of the microbiome in promoting or ameliorating these conditions, as well as space-related factors that impact microbiome composition. The topics explored pertain to microgravity, radiation, immunity, bone health, cognitive function, gender differences and pharmacomicrobiomics. Connections are made between the trifecta of spaceflight, the host and the microbiome, and the significance of these interactions for successful long-term space missions.

Gut microbiota and allergic diseases in children

The gut microbiota resides in the human gastrointestinal tract, where it plays an important role in maintaining host health. The human gut microbiota is established by the age of 3 years. Studies have revealed that an imbalance in the gut microbiota, termed dysbiosis, occurs due to factors such as cesarean delivery and antibiotic use before the age of 3 years and that dysbiosis is associated with a higher risk of future onset of allergic diseases. Recent advancements in next-generation sequencing methods have revealed the presence of dysbiosis in patients with allergic diseases, which increases attention on the relationship between dysbiosis and the development of allergic diseases. However, there is no unified perspective on the characteristics on dysbiosis or the mechanistic link between dysbiosis and the onset of allergic diseases. Here, we introduce the latest studies on the gut microbiota in children with allergic diseases and present the hypothesis that dysbiosis characterized by fewer butyric acid-producing bacteria leads to fewer regulatory T cells, resulting in allergic disease. Further studies on correcting dysbiosis for the prevention and treatment of allergic diseases are warranted.

The Maternal-Fetal Gut Microbiota Axis: Physiological Changes, Dietary Influence, and Modulation Possibilities

The prenatal period and the first years of life have a significant impact on the health issues and life quality of an individual. The appropriate development of the immune system and the central nervous system are thought to be major critical determining events. In parallel to these, establishing an early intestinal microbiota community is another important factor for future well-being interfering with prenatal and postnatal developmental processes. This review aims at summarizing the main characteristics of maternal gut microbiota and its possible transmission to the offspring, thereby affecting fetal and/or neonatal development and health. Since maternal dietary factors are potential modulators of the maternal-fetal microbiota axis, we will outline current knowledge on the impact of certain diets, nutritional factors, and nutritional modulators during pregnancy on offspring's microbiota and health.

Gut Microbiota: A Promising Milestone in Enhancing the Efficacy of PD1/PD-L1 Blockade Therapy

In the past few decades, immunotherapy has emerged as one of the most promising strategies among current treatments of cancer. In particular, the field of PD1/PD-L1 inhibitors has been boosted, widely applied into clinical practice with potent therapeutic efficacy and remarkable survival benefits on various cancers such as melanoma, non-small cell lung cancer (NSCLC), and urothelial carcinoma (UC). However, the application of PD1/PD-L1 blockade therapy is still quite restricted because of unexpected toxicities, limited response rate, as well as associated resistance. In consequence, searching for potential strategies that possibly resolve the existing limitations and enhance the therapeutic responsiveness of PD1/PD-L1 blockade is of great significance. Fortunately, the gut microbiome has been demonstrated to serve as a pivotal regulator in anti-PD1/PD-L1 therapy, providing an applicable tool to improve anti-PD1/PD-L1 clinical efficacy. In this review, we summarized published advancements about how microbiota modulated in anti-PD1/PD-L1 therapy and illustrated its underlying mechanisms, giving insights into putative manipulation of gut microbiota to facilitate PD1/PD-L1 blockade.

Regulation of tissue-resident memory T cells by the Microbiota

Resident memory T cells (Trms) predominantly reside within tissue and are critical for providing rapid protection against invasive viruses, fungi and bacteria. Given that tissues are heavily impacted and shaped by the microbiota, it stands to reason that Trms are also influenced by the microbiota that inhabits barrier sites. The influence of the microbiota is largely mediated by microbial production of metabolites which are crucial to the immune response to both viral infection and cancerous tumors. In addition to the effects of metabolites, antigens derived from the microbiota can activate T cell responses. While microbiota-specific T cells may assist in tissue repair, control of infection and anti-tumor immunity, the actual 'memory' potential of these cells remains unclear. Here, we hypothesize that memory responses to antigens from the microbiota must be 'licensed' by inflammatory signals activated by invasion of the host by microorganisms.

Ifnar gene variants influence gut microbial production of palmitoleic acid and host immune responses to tuberculosis

Both host genetics and the gut microbiome have important effects on human health, yet how host genetics regulates gut bacteria and further determines disease susceptibility remains unclear. Here, we find that the gut microbiome pattern of participants with active tuberculosis is characterized by a reduction of core species found across healthy individuals, particularly Akkermansia muciniphila. Oral treatment of A. muciniphila or A. muciniphila-mediated palmitoleic acid strongly inhibits tuberculosis infection through epigenetic inhibition of tumour necrosis factor in mice infected with Mycobacterium tuberculosis. We use three independent cohorts comprising 6,512 individuals and identify that the single-nucleotide polymorphism rs2257167 'G' allele of type I interferon receptor 1 (encoded by IFNAR1 in humans) contributes to stronger type I interferon signalling, impaired colonization and abundance of A. muciniphila, reduced palmitoleic acid production, higher levels of tumour necrosis factor, and more severe tuberculosis disease in humans and transgenic mice. Thus, host genetics are critical in modulating the structure and functions of gut microbiome and gut microbial metabolites, which further determine disease susceptibility.

Preclinical stage abundance and nuclear antigen reactivity of fecal Immunoglobulin A (IgA) varies among males and females of lupus-prone mouse models

Systemic lupus erythematosus (SLE) is characterized by the production of pathogenic autoantibodies with nuclear antigen (nAg) specificity. Using (SWRxNZB)F1 (SNF1) mice, we showed higher levels of IgA production in the intestine and the nAg reactivity of fecal IgA under lupus susceptibility. Here, we determined if the fecal IgA abundance and nAg reactivity are higher in, different among, various lupus-prone preclinical models (MRL/lpr, NZBxNZW-F1, SNF1, NZM2410 and NZM2328). We also determined if the fecal IgA nAg reactivity at pre-seropositive ages correlates with the eventual serum autoantibody levels in males and females of these mouse models. We show that age dependent increase in the abundance and nAg reactivity of fecal IgA can vary among different lupus-prone mouse models. Importantly, fecal IgA in these mice show significant levels of nAg reactivity, starting as early as at juvenile age. Furthermore, the pre-seropositive stage nAg reactivity of fecal IgA in most lupus-prone strains correlates well with that of eventual, seropositive stage systemic autoantibody levels. Gender differences in serum autoantibody levels were preceded by similar differences in the fecal IgA abundance and nAg reactivity. These observations suggest that fecal IgA features, nAg reactivity particularly, could serve as a biomarker for early prediction of the eventual systemic autoimmunity in lupus-prone subjects.

The Gut Microbiome as a Regulator of the Neuroimmune Landscape

The gut microbiome influences many host physiologies, spanning gastrointestinal function, metabolism, immune homeostasis, neuroactivity, and behavior. Many microbial effects on the host are orchestrated by bidirectional interactions between the microbiome and immune system. Imbalances in this dialogue can lead to immune dysfunction and immune-mediated conditions in distal organs including the brain. Dysbiosis of the gut microbiome and dysregulated neuroimmune responses are common comorbidities of neurodevelopmental, neuropsychiatric, and neurological disorders, highlighting the importance of the gut microbiome–neuroimmune axis as a regulator of central nervous system homeostasis. In this review, we discuss recent evidence supporting a role for the gut microbiome in regulating the neuroimmune landscape in health and disease.

Expected final online publication date for the Annual Review of Immunology, Volume 40 is April 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Microbial regulation of intestinal motility provides resistance against helminth infection

Soil-transmitted helminths cause widespread disease, infecting ~1.5 billion people living within poverty-stricken regions of tropical and subtropical countries. As adult worms inhabit the intestine alongside bacterial communities, we determined whether the bacterial microbiota impacted on host resistance against intestinal helminth infection. We infected germ-free, antibiotic-treated and specific pathogen-free mice, with the intestinal helminth Heligmosomoides polygyrus bakeri. Mice harboured increased parasite numbers in the absence of a bacterial microbiota, despite mounting a robust helminth-induced type 2 immune response. Alterations to parasite behaviour could already be observed at early time points following infection, including more proximal distribution of infective larvae along the intestinal tract and increased migration in a Baermann assay. Mice lacking a complex bacterial microbiota exhibited reduced levels of intestinal acetylcholine, a major excitatory intestinal neurotransmitter that promotes intestinal transit by activating muscarinic receptors. Both intestinal motility and host resistance against larval infection were restored by treatment with the muscarinic agonist bethanechol. These data provide evidence that a complex bacterial microbiota provides the host with resistance against intestinal helminths via its ability to regulate intestinal motility.

The hidden universe of human milk microbiome: origin, composition, determinants, role, and future perspectives

Although traditionally considered sterile, human milk is currently recognized as an alive ecosystem that harbors not only bacteria, but also viruses, fungi and yeasts, and minor genera, collectively known as the human milk microbiome (HMM). The seeding of HMM is a complex phenomenon whose dynamics are still a matter of research. Many factors contribute to its determination, both maternal, neonatal, environmental, and related to human milk itself. The transmission of microorganisms to the infant through breastfeeding may impact its present and future health, mainly shaping the GI tract microbiome and immune system. The existence and persistence of HMM as a conserved feature among different species may also have an evolutionary meaning, which will become apparent only in evolutionary times.

CONCLUSION

The complexities of HMM warrant further research in order to deepen our knowledge on its origin, determinants, and impact on infants' health. The practical and translational implications of research on HMM (e.g., reconstitution of donor human milk through inoculation of infant's own mother milk, modulation of HMM through maternal dietary supplementation) should not be overlooked.

WHAT IS KNOWN

• Human milk harbors a wide variety of microorganisms, ranging from bacteria to viruses, fungi and yeasts, and minor genera. • Human milk microbiome is shaped over time by many factors: maternal, neonatal, environmental, and related to human milk itself. • The transmission of microorganisms through breastfeeding may impact the infant's present and future health.

WHAT IS NEW

• We provide an overview on human milk microbiome, hopefully encouraging physicians to consider it among the other better-known breastfeeding benefits. • Further studies, with standardized and rigorous study designs to enhance accuracy and reproducibility of the results, are needed to deepen our knowledge of the human milk microbiota and its role in newborn and infant's health.