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

Multipathogen Analysis of IgA and IgG Antigen Specificity for Selected Pathogens in Milk Produced by Women From Diverse Geographical Regions: The INSPIRE Study

Breastfeeding provides defense against infectious disease during early life. The mechanisms underlying this protection are complex but likely include the vast array of immune cells and components, such as immunoglobulins, in milk. Simply characterizing the concentrations of these bioactives, however, provides only limited information regarding their potential relationships with disease risk in the recipient infant. Rather, understanding pathogen and antigen specificity profiles of milk-borne immunoglobulins might lead to a more complete understanding of how maternal immunity impacts infant health and wellbeing. Milk produced by women living in 11 geographically dispersed populations was applied to a protein microarray containing antigens from 16 pathogens, including diarrheagenic E. coli, Shigella spp., Salmonella enterica serovar Typhi, Staphylococcus aureus, Streptococcus pneumoniae, Mycobacterium tuberculosis and other pathogens of global health concern, and specific IgA and IgG binding was measured. Our analysis identified novel disease-specific antigen responses and suggests that some IgA and IgG responses vary substantially within and among populations. Patterns of antibody reactivity analyzed by principal component analysis and differential reactivity analysis were associated with either lower-to-middle-income countries (LMICs) or high-income countries (HICs). Antibody levels were generally higher in LMICs than HICs, particularly for Shigella and diarrheagenic E. coli antigens, although sets of S. aureus, S. pneumoniae, and some M. tuberculosis antigens were more reactive in HICs. Differential responses were typically specific to canonical immunodominant antigens, but a set of nondifferential but highly reactive antibodies were specific to antigens possibly universally recognized by antibodies in human milk. This approach provides a promising means to understand how breastfeeding and human milk protect (or do not protect) infants from environmentally relevant pathogens. Furthermore, this approach might lead to interventions to boost population-specific immunity in at-risk breastfeeding mothers and their infants.

Production of germ-free mosquitoes via transient colonisation allows stage-specific investigation of host-microbiota interactions

The mosquito microbiota impacts the physiology of its host and is essential for normal larval development, thereby influencing transmission of vector-borne pathogens. Germ-free mosquitoes generated with current methods show larval stunting and developmental deficits. Therefore, functional studies of the mosquito microbiota have so far mostly been limited to antibiotic treatments of emerging adults. In this study, we introduce a method to produce germ-free Aedes aegypti mosquitoes. It is based on reversible colonisation with bacteria genetically modified to allow complete decolonisation at any developmental stage. We show that, unlike germ-free mosquitoes previously produced using sterile diets, reversibly colonised mosquitoes show no developmental retardation and reach the same size as control adults. This allows us to uncouple the study of the microbiota in larvae and adults. In adults, we detect no impact of bacterial colonisation on mosquito fecundity or longevity. In larvae, data from our transcriptome analysis and diet supplementation experiments following decolonisation suggest that bacteria support larval development by contributing to folate biosynthesis and by enhancing energy storage. Our study establishes a tool to study the microbiota in insects and deepens our knowledge on the metabolic contribution of bacteria to mosquito development.

Tissues: the unexplored frontier of antibody mediated immunity

Pathogen-specific immunity evolves in the context of the infected tissue. However, current immune correlates analyses and vaccine efficacy metrics are based on immune functions from peripheral cells. Less is known about tissue-resident mechanisms of immunity. While antibodies represent the primary correlate of immunity following most clinically approved vaccines, how antibodies interact with localized, compartment-specific immune functions to fight infections, remains unclear. Emerging data demonstrate a unique community of immune cells that reside within different tissues. These tissue-specific immunological communities enable antibodies to direct both expected and unexpected local attack strategies to control, disrupt, and eliminate infection in a tissue-specific manner. Defining the full breadth of antibody effector functions, how they selectively contribute to control at the site of infection may provide clues for the design of next-generation vaccines able to direct the control, elimination, and prevention of compartment specific diseases of both infectious and non-infectious etiologies.

Associations between the Gut Microbiota, Immune Reconstitution, and Outcomes of Allogeneic Hematopoietic Stem Cell Transplantation

Immune reconstitution following allogeneic hematopoietic stem cell transplantation (allo-HSCT) sets the stage for the goal of a successful transplant—the prevention of disease relapse without graft versus host disease (GVHD) and opportunistic infection. In both epidemiologic studies and in controlled animal studies, it is known that the gut microbiome (GM) can profoundly influence normal innate and adaptive immune development and can be altered by microbial transfer and antibiotics. Following allo-HSCT the GM has been shown to influence clinical outcomes but published associations between the GM and immune reconstitution post-allo-HSCT are lacking. In this viewpoint we propose that the extensive knowledge garnered from studying normal immune development can serve as a framework for studying immune development post-allo-HSCT. We summarize existing studies addressing the effect of the GM on immune ontogeny and draw associations with immune reconstitution and the GM post-allo-HSCT.

Exploring the Effect of Probiotics, Prebiotics, and Postbiotics in Strengthening Immune Activity in the Elderly

Vaccination is the easiest way to stimulate the immune system to confer protection from disease. However, the inefficacy of vaccination in the elderly, especially those under nutritional control such as individuals receiving artificial nutrition after cerebral infarction or during dementia, has led to the search for an adjuvant to augment the acquired immune response in this population. The cross-talk between the gut microbiota and the host immune system is gaining attention as a potential adjuvant for vaccines. Probiotics, prebiotics, and postbiotics, which are commonly used to modulate gut health, may enhance the immune response and the effectiveness of vaccination in the elderly. This review summarizes the use of these gut modulators as adjuvants to boost both the innate and acquired immune responses in the elderly under nutritional control. Although the clinical evidence on this topic is limited and the initial findings await clarification through future studies with large sample sizes and proper study designs, they highlight the necessity for additional research in this field, especially in light of the ongoing COVID-19 pandemic, which is disproportionately affecting the elderly.

Transcriptional and Metabolic Control of Memory B Cells and Plasma Cells

For many infections and almost all vaccines, neutralizing-antibody-mediated immunity is the primary basis and best functional correlate of immunological protection. Durable long-term humoral immunity is mediated by antibodies secreted by plasma cells that preexist subsequent exposures and by memory B cells that rapidly respond to infections once they have occurred. In the midst of the current pandemic of coronavirus disease 2019, it is important to define our current understanding of the unique roles of memory B cells and plasma cells in immunity and the factors that control the formation and persistence of these cell types. This fundamental knowledge is the basis to interpret findings from natural infections and vaccines. Here, we review transcriptional and metabolic programs that promote and support B cell fates and functions, suggesting points at which these pathways do and do not intersect.

miR-148a controls metabolic programming and survival of mature CD19-negative plasma cells in mice

Long-lived antibody-secreting plasma cells are essential to establish humoral memory against pathogens. While a regulatory transcription factor network has been established in plasma cell differentiation, the regulatory role of miRNAs remains enigmatic. We have recently identified miR-148a as the most abundant miRNA in primary mouse and human plasma cells. To determine whether this plasma cell signature miRNA controls the in vivo development of B cells into long-lived plasma cells, we established mice with genomic, conditional, and inducible deletions of miR-148a. The analysis of miR-148a-deficient mice revealed reduced serum Ig, decreased numbers of newly formed plasmablasts and reduced CD19-negative, CD93-positive long-lived plasma cells. Transcriptome and metabolic analysis revealed an impaired glucose uptake, a reduced oxidative phosphorylation-based energy metabolism, and an altered abundance of homing receptors CXCR3 (increase) and CXCR4 (reduction) in miR-148a-deficient plasma cells. These findings support the role of miR-148a as a positive regulator of the maintenance of long-lived plasma cells.

Effects of Early Transplantation of the Faecal Microbiota from Tibetan Pigs on the Gut Development of DSS-Challenged Piglets

The present study was conducted to investigate the effects of early transplantation of the faecal microbiota from Tibetan pigs on the gut development of dextran sulphate sodium- (DSS-) challenged piglets. In total, 24 3-day-old DLY piglets were divided into four groups (n = 6 per group); a 2 × 2 factorial arrangement was used, which included faecal microbiota transplantation (FMT) (from Tibetan pigs) and DSS challenge. The whole trial lasted for 55 days. DSS infusion increased the intestinal density, serum diamine oxidase (DAO) activity, and colonic Escherichia coli count (P < 0.05), and decreased the Lactobacillus spp. count and mRNA abundances of epidermal growth factor (EGF), glucagon-like peptide-2 (GLP-2), insulin-like growth factor 1 (IGF-1), occludin, mucin 2 (MUC2), regeneration protein IIIγ (RegIIIγ), and interleukin-10 (IL-10) in the colon (P < 0.05). FMT increased the Lactobacillus spp. count and mRNA abundances of GLP-2, RegIIIγ, and IL-10 in the colon (P < 0.05), and decreased the intestinal density, serum DAO activity, and colonic E. coli number (P < 0.05). In addition, in DSS-challenged piglets, FMT decreased the disease activity index (P < 0.05) and attenuated the effect of DSS challenge on the intestinal density, serum DAO activity, and colonic E. coli number (P < 0.05). These data indicated that the faecal microbiota from Tibetan pigs could attenuate the negative effect of DSS challenge on the gut development of piglets.

Effects of vasoactive drugs on hepatic and intestinal circulation and intestinal barrier in patients with septic shock

In this study, 60 patients with septic shock were selected over the course of 1 year, and the effects of dopamine and norepinephrine combined with dobutamine on hepatic and intestinal circulation and intestinal barrier in patients with septic shock were studied by comparison between the control group and the experimental group. All patients received mechanical ventilation to maintain breathing at 14 to 20 times/min. The experimental group was treated with vascular active drugs after adequate rehydration, and the control group only received adequate rehydration. There were extremely significant differences (p<0.01) in the total effective rate of each group. There were significant differences in the hemodynamic indexes in each group (p<0.05). There was a significant difference in total 24-hour bile output (p<0.01). There were significant differences in liver function and blood lipid values in patients (p<0.01). There were significant differences in the repair of epithelial injury at 0 hour, 48 hours and 96 hours (p<0.01). There were significant differences in the transmembrane resistance of monolayer cells (p<0.01). The expression differences of three proteins ZO-1, occludin and β-actin were also significant, among which the three proteins in the control group were weak, while those in groups A and B were strong. The expression of tight junction protein in monolayer cells was weakly positive in expression and strong in other proteins. In conclusion, vasoactive drugs had significant effects on hepatic and intestinal circulation and intestinal barrier in patients with septic shock.

Role of IgA in the early-life establishment of the gut microbiota and immunity: Implications for constructing a healthy start

Colonization and maturation of the gut microbiota (GM) during early life is a landmark event that fundamentally influences the (early) immunity and later-life health of various mammals. This is a delicate, systematic process that is biologically actively regulated by infants and their mothers, where (secretory) IgA, an important regulator of microbes found in breast milk and generated actively by infants, may play a key role. By binding to microbes, IgA can inhibit or enhance their colonization, influence their gene expression, and regulate immune responses. IgA dysfunction during early life is associated with disrupted GM maturation and various microbe-related diseases, such as necrotizing enterocolitis and diarrhea, which can also have a lasting effect on GM and host health. This review discusses the process of early GM maturation and its interaction with immunity and the role of IgA (focusing on milk secretory IgA) in regulating this process. The possible application of this knowledge in promoting normal GM maturation processes and immune education has also been highlighted.

Robust microbe immune recognition in the intestinal mucosa

The mammalian mucosal immune system acts as a multitasking mediator between bodily function and a vast diversity of microbial colonists. Depending on host-microbial interaction type, mucosal immune responses have distinct functions. Immunity to pathogen infection functions to limit tissue damage, clear or contain primary infection, and prevent or lower the severity of a secondary infection by conferring specific long-term adaptive immunity. Responses to nonpathogenic commensal or mutualistic microbes instead function to tolerate continuous colonization. Mucosal innate immune and epithelial cells employ a limited repertoire of innate receptors to program the adaptive immune response accordingly. Pathogen versus nonpathogen immune discrimination appears to be very robust, as most individuals successfully maintain life-long mutualism with their nonpathogenic microbiota, while mounting immune defense to pathogenic microbe infection specifically. However, the process is imperfect, which can have immunopathological consequences, but may also be exploited medically. Normally innocuous intestinal commensals in some individuals may drive serious inflammatory autoimmunity, whereas harmless vaccines can be used to fool the immune system into mounting a protective anti-pathogen immune response. In this article, we review the current knowledge on mucosal intestinal bacterial immune recognition focusing on T_H17 responses and identify commonalities between intestinal pathobiont and vaccine-induced T_H17 responses.

The Interface of Vibrio cholerae and the Gut Microbiome

The bacterium Vibrio cholerae is the etiologic agent of the severe human diarrheal disease cholera. The gut microbiome, or the native community of microorganisms found in the human gastrointestinal tract, is increasingly being recognized as a factor in driving susceptibility to infection, in vivo fitness, and host interactions of this pathogen. Here, we review a subset of the emerging studies in how gut microbiome structure and microbial function are able to drive V. cholerae virulence gene regulation, metabolism, and modulate host immune responses to cholera infection and vaccination. Improved mechanistic understanding of commensal-pathogen interactions offers new perspectives in the design of prophylactic and therapeutic approaches for cholera control.

Impaired salivary SIgA antibodies elicit oral dysbiosis and subsequent induction of alveolar bone loss

OBJECTIVE

Secreted IgA (SIgA) plays a central role in preventing bacterial and viral infections on mucosal surfaces by neutralizing toxins and viruses and inhibiting bacterial attachment to epithelial cells. However, the role of salivary SIgA antibodies (Abs) in regulating oral flora is still unknown. This study aimed to evaluate the association among oral bacteria, their metabolites and periodontitis in IgA-deficient (IgA KO) and wild-type (WT) control mice.

METHODS

Microcomputed tomography (micro-CT) analysis was used to assess alveolar bone resorption as a development of periodontitis. The bacterial profiles of saliva were determined using the next-generation sequencing assays. Furthermore, the metabolites in saliva were measured and compared using CE-TOFMS.

RESULTS

Salivary microbiota of IgA KO mice revealed a remarkably decreased frequency of Streptococcus, and increased percentages of Aggregatibacer, Actinobacillus, and Prevotella at the genus level when compared with those of WT. Compared to WT control mice of the same age, the level of alveolar bone loss was significantly increased in IgA KO mice, and infiltration of osteoclasts was found on the surface of the alveolar bone. The metabolome profile indicated that the metabolites of IgA KO mice had greater variability in carbon metabolic, urea cycle, and lipid pathways than WT mice.

CONCLUSION

These results suggest that salivary SIgA plays an important role in regulating and maintaining normal oral microflora to prevent the development of periodontal disease.

Divergent T follicular helper cell requirement for IgA and IgE production to peanut during allergic sensitization

Immunoglobulin A (IgA) is the dominant antibody isotype in the gut and has been shown to regulate microbiota. Mucosal IgA is also widely believed to prevent food allergens from penetrating the gut lining. Even though recent work has elucidated how bacteria-reactive IgA is induced, little is known about how IgA to food antigens is regulated. Although IgA is presumed to be induced in a healthy gut at steady state via dietary exposure, our data do not support this premise. We found that daily food exposure only induced low-level, cross-reactive IgA in a minority of mice. In contrast, induction of significant levels of peanut-specific IgA strictly required a mucosal adjuvant. Although induction of peanut-specific IgA required T cells and CD40L, it was T follicular helper (T_FH) cell, germinal center, and T follicular regulatory (T_FR) cell-independent. In contrast, IgG1 and IgE production to peanut required T_FH cells. These data suggest an alternative paradigm in which the cellular mechanism of IgA production to food antigens is distinct from IgE and IgG1. We developed an equivalent assay to study this process in stool samples from healthy, nonallergic humans, which revealed substantial levels of peanut-specific IgA that were stable over time. Similar to mice, patients with loss of CD40L function had impaired titers of gut peanut-specific IgA. This work challenges two widely believed but untested paradigms about antibody production to dietary antigens: (i) the steady state/tolerogenic response to food antigens includes IgA production and (ii) T_FH cells drive food-specific gut IgA.

Early life antibiotic exposure and host health: Role of the microbiota-immune interaction

The neonatal population is at high risk for infections secondary to a unique, developing immune system. While a multitude of factors direct the development of the immune system, the role of environmental exposures on the microbiota may play a critical and potentially modifiable role. Recent evidence suggests that the disruption of the microbiota through the use of antibiotics not only leads to an immediately increased risk for neonatal complications but also long-term health issues related to autoimmune and inflammatory diseases. The exact cellular and molecular mechanisms behind these associations between the microbiota and neonatal outcomes are still under investigation. This review will examine the mechanistic interactions between the microbiota and the immune system, particularly in early life, along with how antibiotic mediated aberrations of the microbiome potentially lead to disease.

Intestinal microbial metabolites in human metabolism and type 2 diabetes

Humans with the metabolic syndrome and type 2 diabetes have an altered gut microbiome. Emerging evidence indicates that it is not only the microorganisms and their structural components, but also their metabolites that influences the host and contributes to the development of the metabolic syndrome and type 2 diabetes. Here, we discuss some of the mechanisms underlying how microbial metabolites are recognised by the host or are further processed endogenously in the context of type 2 diabetes. We discuss the possibility that gut-derived microbial metabolites fuel the development of the metabolic syndrome and type 2 diabetes. Graphical abstract.

Adhesive Bacteria in the Terminal Ileum of Children Correlates With Increasing Th17 Cell Activation

Humans and symbiotic bacteria are interdependent and co-evolved for millions of years. These bacteria communicate with human hosts in the gut in a contact-independent metabolite. Because most intestinal bacteria are non-adhesive, they do not penetrate the mucus layer and are not directly in contact with epithelial cells (ECs). Here, we found that there are adhesive bacteria attached to the Children's terminal ileum. And we compared the immune factors of non-adhesive bacteria in the children ileum with adhesive bacteria as well. Stimulated Th17 cell associated with adherent bacteria in the ileum ECs. SIgA responses are similar to those roles in mouse experiments. Immunohistochemical analysis confirmed that the expression of SAA1, IL-2, IL-17A, foxp3, RORγt, TGFβ, and protein increased in Th17 cells. Finally, we used 16S rRNA genes 454 pyrosequencing to analyze the differences in bacterial communities between adhesive and non-adhesive bacteria in the ileum. Ileum with adherent bacteria demonstrated increased mucosa-related bacteria, such as Clostridium, Ruminococcus, Veillonella, Butyricimonas, and Prevotella. We believe that adhesive bacteria in children’s terminal ileum associated with an increased Th17 cell activation and luminal secretory IgA. Adhesive bacteria very closely adhere to terminal ileum of children. They may play important role in human gut immunity and Crohn’s disease.

Zebrafish Models for Pathogenic Vibrios

Vibrio is a large and diverse genus of bacteria, of which most are nonpathogenic species found in the aquatic environment. However, a subset of the Vibrio genus includes several species that are highly pathogenic, either to humans or to aquatic animals. In recent years, Danio rerio, commonly known as the zebrafish, has emerged as a major animal model used for studying nearly every aspect of biology, including infectious diseases. Zebrafish are especially useful because the embryos are transparent, larvae are small and facilitate imaging studies, and numerous transgenic fish strains have been constructed. Zebrafish models for several pathogenic Vibrio species have been described, and indeed a fish model is highly relevant for the study of aquatic bacterial pathogens. Here, we summarize the zebrafish models that have been used to study pathogenic Vibrio species to date.

Lactobacillus plantarum BSGP201683 Improves the Intestinal Barrier of Giant Panda Microbiota-Associated Mouse Infected by Enterotoxigenic Escherichia coli K88

Giant pandas often suffered from gastrointestinal disease, especially the captive sub-adult one. Our study aims to investigate whether L. plantarum G83, a good panda-derived probiotic, can improve the intestinal barrier against the enterotoxigenic Escherichia coli K88 (E. coli K88) infection in giant panda microbiota-associated mice (GPAM). We treated SPF mice with antibiotics cocktail and transplanted the giant panda intestinal microbiota to set up a GPAM. Our results demonstrated that the microbiota of GPAM changed over time and was relatively stable in the short-term experiment (2-4 weeks). Whereafter, the GPAM pretreated with L. plantarum G83 for 15 days and infected with enterotoxigenic E. coli K88. The result indicated that the number of Bifidobacteria spp. increased in GPAM-G and GPAM-GE groups; the Lactobacillus spp. only increased in the GPAM-G group. Although the abundance of Enterobacteriaceae spp. only decreased in the GPAM-G group, the copy number of Escherichia coli in the GPAM-E group was significantly lower than that in the other groups. Meanwhile, the L. plantarum G83-induced alteration of microbiota could increase the mRNA expression of Claudin-1, Zo-1, and Occludin-1 in the GPAM-G group in the ileum; only Occludin-1 was increased in the GPAM-GE group. The sIgA in the ileum showed a positive response, also the result of body weight and histology in both the GPAM-G and GPAM-GE group. These results indicated that the L. plantarum G83 could improve the intestinal barrier to defense the enterotoxigenic E. coli K88 invasion.

Gut-educated IgA plasma cells defend the meningeal venous sinuses

The central nervous system has historically been viewed as an immune-privileged site, but recent data have shown that the meninges-the membranes that surround the brain and spinal cord-contain a diverse population of immune cells1. So far, studies have focused on macrophages and T cells, but have not included a detailed analysis of meningeal humoral immunity. Here we show that, during homeostasis, the mouse and human meninges contain IgA-secreting plasma cells. These cells are positioned adjacent to dural venous sinuses: regions of slow blood flow with fenestrations that can potentially permit blood-borne pathogens to access the brain2. Peri-sinus IgA plasma cells increased with age and following a breach of the intestinal barrier. Conversely, they were scarce in germ-free mice, but their presence was restored by gut re-colonization. B cell receptor sequencing confirmed that meningeal IgA+ cells originated in the intestine. Specific depletion of meningeal plasma cells or IgA deficiency resulted in reduced fungal entrapment in the peri-sinus region and increased spread into the brain following intravenous challenge, showing that meningeal IgA is essential for defending the central nervous system at this vulnerable venous barrier surface.

Pediatrics Consequences of Caesarean Section-A Systematic Review and Meta-Analysis

BACKGROUND

Cesarean section is a surgical procedure, which is the most frequently performed in gynecology and obstetrics. It is commonly believed that an operative delivery is a less painful and safer mode of delivery, which translates into an increasing number of the procedures performed without medical indications. The maternal sequelae of cesarean sections are well elucidated and widely discussed in the literature, while long-term neonatal consequences still remain the issue of research and scientific dispute. The aim of the present paper was to perform a systematic review of current literature regarding pediatrics consequences of cesarean section.

METHODS

We reviewed available data from PubMed, Science Direct as well as Google Scholar bases concerning early and long-term neonatal sequelae of operative deliveries. The following key words were used: "cesarean section", "caesarean section", "neonatal outcomes", "respiratory disorders", "asthma", "obesity", "overweight", and "neurological disorders". A total of 1636 papers were retrieved out of which 27 were selected for the final systematic review whereas 16 articles provided data for meta-analysis. Statistical analyses were performed using RevMan 5.4. To determine the strength of association between the caesarean section and respiratory tract infections, asthma, diabetes type 1 as well as obesity the pooled odds ratios (OR) with the 95% confidence intervals (CI) were calculated.

RESULTS

Conducted meta-analyses revealed that caesarean section is a risk factor for respiratory tract infections (pooled OR = 1.30 95%CI 1.06-1.60, p = 0.001), asthma (pooled OR = 1.23 95%CI 1.14-1.33, p < 0.00001) as well as obesity (pooled OR = 1.35 95%CI 1.29-1.41, p < 0.00001) in offspring.

CONCLUSIONS

The results of the studies included indicated that children delivered by cesarean section more commonly developed respiratory tract infections, obesity and the manifestations of asthma than children delivered vaginally. The risk of developing diabetes mellitus type 1 or neurological disorders in offspring after caesarean section is still under discussion.

Regulation of Gut Microbiota on Immune Reconstitution in Patients With Acquired Immunodeficiency Syndrome

Human immunodeficiency virus type 1 (HIV-1) infection of CD4⁺ T cells in the gut plays an insidious role in acquired immunodeficiency syndrome (AIDS) pathogenesis. Host immune function is closely related to gut microbiota. Changes in the gut microbiota cause a different immune response. Previous studies revealed that HIV-1 infection caused changes in gut microbiota, which induced immune deficiency. HIV-1 infection results in an abnormal composition and function of the gut microbiota, which may disrupt the intestinal epithelial barrier and microbial translocation, leading to long-term immune activation, including inflammation and metabolic disorders. At the same time, an abnormal gut microbiota also hinders the effect of antiviral therapy and affects the immune reconstruction of patients. However, studies on the impact of the gut microbiota on immune reconstitution in patients with HIV/AIDS are still limited. In this review, we focus on changes in the gut microbiota caused by HIV infection, as well as the impact and regulation of the gut microbiota on immune function and immune reconstitution, while we also discuss the potential impact of probiotics/prebiotics and fecal microbiota transplantation (FMT) on immune reconstitution.

Gut Microbiota as a Trigger for Metabolic Inflammation in Obesity and Type 2 Diabetes

The gut microbiota has been linked to the development of obesity and type 2 diabetes (T2D). The underlying mechanisms as to how intestinal microbiota may contribute to T2D are only partly understood. It becomes progressively clear that T2D is characterized by a chronic state of low-grade inflammation, which has been linked to the development of insulin resistance. Here, we review the current evidence that intestinal microbiota, and the metabolites they produce, could drive the development of insulin resistance in obesity and T2D, possibly by initiating an inflammatory response. First, we will summarize major findings about immunological and gut microbial changes in these metabolic diseases. Next, we will give a detailed view on how gut microbial changes have been implicated in low-grade inflammation. Lastly, we will critically discuss clinical studies that focus on the interaction between gut microbiota and the immune system in metabolic disease. Overall, there is strong evidence that the tripartite interaction between gut microbiota, host immune system and metabolism is a critical partaker in the pathophysiology of obesity and T2D.

Littermate-Controlled Experiments Reveal Eosinophils Are Not Essential for Maintaining Steady-State IgA and Demonstrate the Influence of Rearing Conditions on Antibody Phenotypes in Eosinophil-Deficient Mice

Conflicting data has emerged regarding a role for eosinophils in IgA production, with some reports that eosinophils support both secretory and circulating IgA levels during homeostasis. Previous studies have compared antibody levels between wildtype and eosinophil-deficient mice, but these mice were obtained from different commercial vendors and/or were not littermates. Thus, the possibility remains that extrinsic environmental factors, rather than an intrinsic lack of eosinophils, are responsible for the reports of reduced IgA in eosinophil-deficient mice. Here we used wild-type and eosinophil-deficient (ΔdblGATA) mice that were purchased from a single vendor, subsequently bred in-house and either co-housed as adults, co-reared from birth or raised as littermates. We found no differences in the levels of secretory IgA or in the numbers of small intestinal IgA-producing plasma cells between wild-type and ΔdblGATA mice, demonstrating that under controlled steady-state conditions eosinophils are not essential for the maintenance of secretory IgA in the intestinal tract. While we found that levels of IgM and IgE were significantly elevated in the serum of ΔdblGATA mice compared to co-reared or co-housed wild-type mice, no significant differences in these or other circulating antibody isotypes were identified between genotypes in littermate-controlled experiments. Our results demonstrate that eosinophils are not required to maintain secretory or circulating IgA production and the absence of eosinophils does not impact circulating IgG1, IgG2b, IgM, or IgE levels during homeostasis. These findings emphasize the importance of optimally controlling rearing and housing conditions throughout life between mice of different genotypes.

Perspectives on how mucosal immune responses, infections and gut microbiome shape IgA nephropathy and future therapies

Infections have been considered to play a critical role in the pathogenesis of IgA nephropathy (IgAN) because synpharyngitic hematuria is a common feature in IgAN. However, how infections participate in this process is still debated. More recent studies have also revealed that the alteration of the gut microbiome exerts a profound effect on host immune responses, contributing to the etiology or progression of autoimmunity. Considering IgA as the first line of defense against bacterial and viral antigens, this review evaluates the relationships among intestinal infections, gut microbiome, and IgA for a better understanding of the pathogenesis of IgAN. Moreover, as a prototype of IgA immunity, we provide detailed clarification of IgAN pathogenesis to shed light on other diseases in which IgA plays a role. Finally, we discuss potential therapies focusing on microbes and mucosal immune responses in IgAN.

Curcumin improves necrotising microscopic colitis and cell pyroptosis by activating SIRT1/NRF2 and inhibiting the TLR4 signalling pathway in newborn rats

This study aimed to explore comprehensively the biological function of curcumin, and its underlying mechanism, in protecting from necrotising microscopic colitis in newborn rats. A total of 20 normal healthy rats were selected, and a necrotising enterocolitis (NEC) model was established. After hypoxia and hypothermia stimulation, these rats were treated with different doses of curcumin (control group, NEC model group, NEC+20 mg/kg curcumin and NEC+50 mg/kg curcumin). Inflammation was identified using hematoxylin and eosin staining, and inflammatory factors were detected via ELISA. The mRNA and protein levels of SIRT1, NRF2, TLR4, NLRP3 and caspase-1 were determined by quantitative RT-PCR and Western blotting, respectively. Curcumin improved the inflammatory condition of NEC and inhibited the expression of inflammatory factors in NEC newborn rat intestinal tissue. Furthermore, the SIRT1/NRF2 pathway was inhibited in the intestinal tissue of NEC newborn rats, whereas curcumin treatment induced the activation of the SIRT1/NRF2 pathway and inhibited TLR4 expression in these animals. In addition, curcumin could also inhibit the expression of inflammatory factors and alleviate the LPS/ATP-induced focal death pathway in intestinal epithelial cells through the SIRT1 pathway. Curcumin can improve necrotising microscopic colitis and cell pyroptosis by attenuating NEC-induced inhibition of SIRT1/NRF2 and inhibiting the TLR4 signalling pathway in newborn rats.

Human Respiratory and Gut Microbiomes-Do They Really Contribute to Respiratory Health?

Human gastrointestinal and respiratory tracts are colonized by diverse polymicrobial communities shortly after birth, which are continuously molded by environmental exposure. The development of the resident microbiota in early life is a critical factor in the maturation of a healthy immune system. Disturbances to the intricate relationship between environmental exposure and maturation of the infant microbiome have been increasingly identified as a potential contributor to a range of childhood diseases. This review details recent evidence that implicates the contribution of gut and airway microbiome to pediatric respiratory health.

Gut Microbiota between Environment and Genetic Background in Familial Mediterranean Fever (FMF)

The gastrointestinal tract hosts the natural reservoir of microbiota since birth. The microbiota includes various bacteria that establish a progressively mutual relationship with the host. Of note, the composition of gut microbiota is rather individual-specific and, normally, depends on both the host genotype and environmental factors. The study of the bacterial profile in the gut demonstrates that dominant and minor phyla are present in the gastrointestinal tract with bacterial density gradually increasing in oro-aboral direction. The cross-talk between bacteria and host within the gut strongly contributes to the host metabolism, to structural and protective functions. Dysbiosis can develop following aging, diseases, inflammatory status, and antibiotic therapy. Growing evidences show a possible link between the microbiota and Familial Mediterranean Fever (FMF), through a shift of the relative abundance in microbial species. To which extent such perturbations of the microbiota are relevant in driving the phenotypic manifestations of FMF with respect to genetic background, remains to be further investigated.

Abundance and nuclear antigen reactivity of intestinal and fecal Immunoglobulin A in lupus-prone mice at younger ages correlate with the onset of eventual systemic autoimmunity

Our recent studies, using (SWRxNZB)F1 (SNF1) mice, showed a potential contribution of the gut microbiota and pro-inflammatory immune responses of the gut mucosa to systemic autoimmunity in lupus. Here, using this mouse model, we determined the abundance and the nAg reactivity of IgA antibody produced in the intestine under lupus susceptibility. Intestinal lymphoid tissues from SNF1 mice, females particularly, showed significantly higher frequencies of nAg (dsDNA and nucleohistone) reactive IgA producing B cells compared to B6 females. Most importantly, younger age fecal IgA -abundance and -nAg reactivity of lupus-prone mice showed a positive correlation with eventual systemic autoimmunity and proteinuria onset. Depletion of gut microbiota in SNF1 mice resulted in the diminished production of IgA in the intestine and the nAg reactivity of these antibodies. Overall, these observations show that fecal IgA features, nuclear antigen reactivity particularly, at preclinical stages/in at-risk subjects could be predictive of autoimmune progression.

IL-21 Promotes Intestinal Memory IgA Responses

The role of IL-21, produced mainly by Th17 cells and T follicular helper cells, has been intensively investigated in B cell differentiation and Ab class switch. However, how IL-21 regulates memory IgA⁺ B cell development and memory IgA responses in the intestines is still not completely understood. In this study, we found the total IgA⁺ B cells as well as CD38⁺CD138^-IgA⁺ memory B cells were significantly increased in intestinal lamina propria (LP) of TCRβxδ^-/- mice after transfer of microbiota Ag-specific Th17 cells but not Th1 cells. Although IL-21R^-/- mice or IL-17R^-/- mice showed decreased Ag-specific memory IgA production in the intestines upon infection with Citrobacter rodentium, the percentage of IgA⁺CD38⁺CD138^- memory B cells in Peyer's patches and LP was decreased only in IL-21R^-/- mice, but not in IL-17R^-/- mice, after reinfection with C. rodentium compared with wild-type mice. Blockade IL-21 in vivo suppressed intestinal C. rodentium-specific IgA production as well as IgA⁺CD38⁺CD138^- memory B cells in Peyer's patches and LP. Furthermore, IL-21 significantly induced B cell IgA production in vitro, with the increased expression of genes related with class-switching and memory B cell development, including Aicda, Ski, Bmi1, and Klf2. Consistently, Aicda and Ski expression was decreased in B cells of IL-21R^-/- mice after C. rodentium reinfection. In conclusion, our study demonstrated that IL-21 promotes intestinal memory IgA B cell development, possibly through upregulating differentiation-related and class switching-related genes, indicating a potential role of IL-21 in memory IgA⁺ B cell responses in the intestines.

Mucosal or systemic microbiota exposures shape the B cell repertoire

Colonization by the microbiota causes a marked stimulation of B cells and induction of immunoglobulin, but mammals colonized with many taxa have highly complex and individualized immunoglobulin repertoires^1,2. Here we use a simplified model of defined transient exposures to different microbial taxa in germ-free mice³ to deconstruct how the microbiota shapes the B cell pool and its functional responsiveness. We followed the development of the immunoglobulin repertoire in B cell populations, as well as single cells by deep sequencing. Microbial exposures at the intestinal mucosa generated oligoclonal responses that differed from those of germ-free mice, and from the diverse repertoire that was generated after intravenous systemic exposure to microbiota. The IgA repertoire-predominantly to cell-surface antigens-did not expand after dose escalation, whereas increased systemic exposure broadened the IgG repertoire to both microbial cytoplasmic and cell-surface antigens. These microbial exposures induced characteristic immunoglobulin heavy-chain repertoires in B cells, mainly at memory and plasma cell stages. Whereas sequential systemic exposure to different microbial taxa diversified the IgG repertoire and facilitated alternative specific responses, sequential mucosal exposure produced limited overlapping repertoires and the attrition of initial IgA binding specificities. This shows a contrast between a flexible response to systemic exposure with the need to avoid fatal sepsis, and a restricted response to mucosal exposure that reflects the generic nature of host-microbial mutualism in the mucosa.

Gut T cell-independent IgA responses to commensal bacteria require engagement of the TACI receptor on B cells

The gut mounts secretory immunoglobulin A (SIgA) responses to commensal bacteria through nonredundant T cell-dependent (TD) and T cell-independent (TI) pathways that promote the establishment of mutualistic host-microbiota interactions. SIgAs from the TD pathway target penetrant bacteria, and their induction requires engagement of CD40 on B cells by CD40 ligand on T follicular helper cells. In contrast, SIgAs from the TI pathway bind a larger spectrum of bacteria, but the mechanism underpinning their production remains elusive. Here, we show that the intestinal TI pathway required CD40-independent B cell-activating signals from TACI, a receptor for the innate CD40 ligand-like factors BAFF and APRIL. TACI-induced SIgA responses targeted a fraction of the gut microbiota without shaping its overall composition. Of note, TACI was dispensable for TD induction of IgA in gut-associated lymphoid organs. Thus, BAFF/APRIL signals acting on TACI orchestrate commensal bacteria-specific SIgA responses through an intestinal TI program.

Gut Microbiota Regulation of T Cells During Inflammation and Autoimmunity

The intestinal microbiota plays a crucial role in influencing the development of host immunity, and in turn the immune system also acts to regulate the microbiota through intestinal barrier maintenance and immune exclusion. Normally, these interactions are homeostatic, tightly controlled, and organized by both innate and adaptive immune responses. However, a combination of environmental exposures and genetic defects can result in a break in tolerance and intestinal homeostasis. The outcomes of these interactions at the mucosal interface have broad, systemic effects on host immunity and the development of chronic inflammatory or autoimmune disease. The underlying mechanisms and pathways the microbiota can utilize to regulate these diseases are just starting to emerge. Here, we discuss the recent evidence in this area describing the impact of microbiota-immune interactions during inflammation and autoimmunity, with a focus on barrier function and CD4⁺ T cell regulation.

Rethinking mucosal antibody responses: IgM, IgG and IgD join IgA

Humoral immune responses at mucosal surfaces have historically focused on IgA. Growing evidence highlights the complexity of IgA-inducing pathways and the functional impact of IgA on mucosal commensal bacteria. In the gut, IgA contributes to the establishment of a mutualistic host-microbiota relationship that is required to maintain homeostasis and prevent disease. This Review discusses how mucosal IgA responses occur in an increasingly complex humoral defence network that also encompasses IgM, IgG and IgD. Aside from integrating the protective functions of IgA, these hitherto neglected mucosal antibodies may strengthen the communication between mucosal and systemic immune compartments.

Interaction between microbiota and immunity in health and disease

The interplay between the commensal microbiota and the mammalian immune system development and function includes multifold interactions in homeostasis and disease. The microbiome plays critical roles in the training and development of major components of the host's innate and adaptive immune system, while the immune system orchestrates the maintenance of key features of host-microbe symbiosis. In a genetically susceptible host, imbalances in microbiota-immunity interactions under defined environmental contexts are believed to contribute to the pathogenesis of a multitude of immune-mediated disorders. Here, we review features of microbiome-immunity crosstalk and their roles in health and disease, while providing examples of molecular mechanisms orchestrating these interactions in the intestine and extra-intestinal organs. We highlight aspects of the current knowledge, challenges and limitations in achieving causal understanding of host immune-microbiome interactions, as well as their impact on immune-mediated diseases, and discuss how these insights may translate towards future development of microbiome-targeted therapeutic interventions.

Interaction between microbiota and immunity in health and disease

The interplay between the commensal microbiota and the mammalian immune system development and function includes multifold interactions in homeostasis and disease. The microbiome plays critical roles in the training and development of major components of the host's innate and adaptive immune system, while the immune system orchestrates the maintenance of key features of host-microbe symbiosis. In a genetically susceptible host, imbalances in microbiota-immunity interactions under defined environmental contexts are believed to contribute to the pathogenesis of a multitude of immune-mediated disorders. Here, we review features of microbiome-immunity crosstalk and their roles in health and disease, while providing examples of molecular mechanisms orchestrating these interactions in the intestine and extra-intestinal organs. We highlight aspects of the current knowledge, challenges and limitations in achieving causal understanding of host immune-microbiome interactions, as well as their impact on immune-mediated diseases, and discuss how these insights may translate towards future development of microbiome-targeted therapeutic interventions.

Gut microbiota and metabolites in the pathogenesis of endocrine disease

Type 1 diabetes (T1D) and Hashimoto's thyroiditis (HT) are the two most common autoimmune endocrine diseases that have rising global incidence. These diseases are caused by the immune-mediated destruction of hormone-producing endocrine cells, pancreatic beta cells and thyroid follicular cells, respectively. Both genetic predisposition and environmental factors govern the onset of T1D and HT. Recent evidence strongly suggests that the intestinal microbiota plays a role in accelerating or preventing disease progression depending on the compositional and functional profile of the gut bacterial communities. Accumulating evidence points towards the interplay between the disruption of gut microbial homeostasis (dysbiosis) and the breakdown of host immune tolerance at the onset of both diseases. In this review, we will summarize the major recent findings about the microbiome alterations associated with T1D and HT, and the connection of these changes to disease states. Furthermore, we will discuss the potential mechanisms by which gut microbial dysbiosis modulates the course of the disease, including disruption of intestinal barrier integrity and microbial production of immunomodulatory metabolites. The aim of this review is to provide broad insight into the role of gut microbiome in the pathophysiology of these diseases.

Microbiota-Induced Type I Interferons Instruct a Poised Basal State of Dendritic Cells

Environmental signals shape host physiology and fitness. Microbiota-derived cues are required to program conventional dendritic cells (cDCs) during the steady state so that they can promptly respond and initiate adaptive immune responses when encountering pathogens. However, the molecular underpinnings of microbiota-guided instructive programs are not well understood. Here, we report that the indigenous microbiota controls constitutive production of type I interferons (IFN-I) by plasmacytoid DCs. Using genome-wide analysis of transcriptional and epigenetic regulomes of cDCs from germ-free and IFN-I receptor (IFNAR)-deficient mice, we found that tonic IFNAR signaling instructs a specific epigenomic and metabolic basal state that poises cDCs for future pathogen combat. However, such beneficial biological function comes with a trade-off. Instructed cDCs can prime T cell responses against harmless peripheral antigens when removing roadblocks of peripheral tolerance. Our data provide fresh insights into the evolutionary trade-offs that come with successful adaptation of vertebrates to their microbial environment.

Resident Memory B Cells

In mammals, adaptive immunity is mediated by a broadly diverse repertoire of naive B and T lymphocytes that recirculate between secondary lymphoid organs. Initial antigen exposure promotes lymphocyte clonal expansion and differentiation, including the formation of memory cells. Antigen-specific memory cells are maintained at higher frequencies than their naive counterparts and have different functional and homing abilities. Importantly, a subset of memory cells, known as tissue-resident memory cells, is maintained without recirculating in nonlymphoid tissues, often at barrier surfaces, where they can be reactivated by antigen and rapidly perform effector functions that help protect the tissue in which they reside. Although antigen-experienced B cells are abundant at many barrier surfaces, their characterization as tissue-resident memory B (BRM) cells is not well developed. In this study, we describe the characteristics of memory B cells in various locations and discuss their possible contributions to immunity and homeostasis as bona fide BRM cells.

Maternal H. pylori is associated with differential fecal microbiota in infants born by vaginal delivery

Helicobacter pylori colonization may affect the mucosal immune system through modification of microbiota composition and their interactions with the host. We hypothesized that maternal H. pylori status affects the maternal intestinal microbiota of both mother and newborn. In this study, we determine the structure of the fecal microbiota in mothers and neonates according to maternal H. pylori status and delivery mode. We included 22 mothers and H. pylori infection was determined by fecal antigen test. Eleven mothers (50%) were H. pylori-positive (7 delivering vaginally and 4 by C-section), and 11 were negative (6 delivering vaginally and 5 by C-section). Stool samples were obtained from mothers and infants and the fecal DNA was sequenced. The fecal microbiota from mothers and their babies differed by the maternal H. pylori status, only in vaginal birth, not in C-section delivery. All 22 infants tested negative for fecal H. pylori at 15 days of age, but those born vaginally -and not those by C-section- showed differences in the infant microbiota by maternal H. pylori status (PERMANOVA, p = 0.01), with higher abundance of Enterobacteriaceae and Veillonella, in those born to H. pylori-positive mothers. In conclusion, the structure of the infant fecal microbiota is affected by the maternal H. pylori status only in infants born vaginally, suggesting that the effect could be mediated by labor and birth exposures.

Uncoupling of invasive bacterial mucosal immunogenicity from pathogenicity

There is the notion that infection with a virulent intestinal pathogen induces generally stronger mucosal adaptive immunity than the exposure to an avirulent strain. Whether the associated mucosal inflammation is important or redundant for effective induction of immunity is, however, still unclear. Here we use a model of auxotrophic Salmonella infection in germ-free mice to show that live bacterial virulence factor-driven immunogenicity can be uncoupled from inflammatory pathogenicity. Although live auxotrophic Salmonella no longer causes inflammation, its mucosal virulence factors remain the main drivers of protective mucosal immunity; virulence factor-deficient, like killed, bacteria show reduced efficacy. Assessing the involvement of innate pathogen sensing mechanisms, we show MYD88/TRIF, Caspase-1/Caspase-11 inflammasome, and NOD1/NOD2 nodosome signaling to be individually redundant. In colonized animals we show that microbiota metabolite cross-feeding may recover intestinal luminal colonization but not pathogenicity. Consequent immunoglobulin A immunity and microbial niche competition synergistically protect against Salmonella wild-type infection.

The Current and Future State of Vaccines, Antivirals and Gene Therapies Against Emerging Coronaviruses

Emerging coronaviruses (CoV) are constant global public health threats to society. Multiple ongoing clinical trials for vaccines and antivirals against CoVs showcase the availability of medical interventions to both prevent and treat the future emergence of highly pathogenic CoVs in human. However, given the diverse nature of CoVs and our close interactions with wild, domestic and companion animals, the next epidemic zoonotic CoV could resist the existing vaccines and antivirals developed, which are primarily focused on Severe Acute Respiratory Syndrome Coronavirus (SARS-CoV) and Middle East Respiratory Syndrome Coronavirus (MERS CoV). In late 2019, the novel CoV (SARS-CoV-2) emerged in Wuhan, China, causing global public health concern. In this review, we will summarize the key advancements of current vaccines and antivirals against SARS-CoV and MERS-CoV as well as discuss the challenge and opportunity in the current SARS-CoV-2 crisis. At the end, we advocate the development of a "plug-and-play" platform technologies that could allow quick manufacturing and administration of broad-spectrum countermeasures in an outbreak setting. We will discuss the potential of AAV-based gene therapy technology for in vivo therapeutic antibody delivery to combat SARS-CoV-2 outbreak and the future emergence of severe CoVs.

The effect of antibiotic cocktails on host immune status is dynamic and does not always correspond to changes in gut microbiota

The disruption of the gut microbiota by treatment with an antibiotic cocktail (ABx) can trigger an imbalance in immune homeostasis. However, whether the changes in the intestinal microbiota always correspond to the changes in the physiology and immune homeostasis of the host remains unclear. Here, we analyzed the effects of ABx on immune homeostasis by analyzing the colonic transcriptome with 16S rRNA analysis of the gut microbiota on the 7th and 21st days of continuous treatment with ABx. Our results showed that the composition profile of the gut microbiota was similar after 7 and 21 days of ABx treatment. However, after 21 days of ABx treatment, the intestinal inflammation did not deteriorate further. Instead, the inflammation of the host was relieved, and half of the differentially expressed genes in the colon were restored compared with the 7 days of ABx treatment. Furthermore, the enrichment and network analysis of these restored genes indicated that expression of regenerating islet-derived protein 3β (Reg3b) and expression of regenerating islet-derived protein 3γ (Reg3g), especially Reg3b, may participate in the regulation of the inflammatory response and affect the changes in host immune homeostasis during continuous ABx treatment. Finally, Spearman's correlation analysis showed that the expression of Reg3b is correlated with the growth of Escherichia-Shigella. Our data demonstrated that even though the disruption of the gut microbiota profile induced by ABx treatment is similar, the host response and immune status will be different at different times.Key Points• Host immune status can change in different ABx treatment times.• Gut microbiota showed same exhaustion state in different ABx treatment times.• Host tried to revert to a certain extent after long-term ABx treatment.• Reg3b may affect the changes in host immune homeostasis during continuous ABx treatment.• The expression of Reg3b correlated with the growth of Escherichia-Shigella.

Gut Microbiome, Intestinal Permeability, and Tissue Bacteria in Metabolic Disease: Perpetrators or Bystanders?

The emerging evidence on the interconnectedness between the gut microbiome and host metabolism has led to a paradigm shift in the study of metabolic diseases such as obesity and type 2 diabetes with implications on both underlying pathophysiology and potential treatment. Mounting preclinical and clinical evidence of gut microbiota shifts, increased intestinal permeability in metabolic disease, and the critical positioning of the intestinal barrier at the interface between environment and internal milieu have led to the rekindling of the "leaky gut" concept. Although increased circulation of surrogate markers and directly measurable intestinal permeability have been linked to increased systemic inflammation in metabolic disease, mechanistic models behind this phenomenon are underdeveloped. Given repeated observations of microorganisms in several tissues with congruent phylogenetic findings, we review current evidence on these unanticipated niches, focusing specifically on the interaction between gut permeability and intestinal as well as extra-intestinal bacteria and their joint contributions to systemic inflammation and metabolism. We further address limitations of current studies and suggest strategies drawing on standard techniques for permeability measurement, recent advancements in microbial culture independent techniques and computational methodologies to robustly develop these concepts, which may be of considerable value for the development of prevention and treatment strategies.

Microbiome in Multiple Sclerosis; Where Are We, What We Know and Do Not Know

An increase of multiple sclerosis (MS) incidence has been reported during the last decade, and this may be connected to environmental factors. This review article aims to encapsulate the current advances targeting the study of the gut-brain axis, which mediates the communication between the central nervous system and the gut microbiome. Clinical data arising from many research studies, which have assessed the effects of administered disease-modifying treatments in MS patients to the gut microbiome, are also recapitulated.

Microbiota-Dependent Regulation of Antimicrobial Immunity in the Lung

Several body sites, including the intestinal and respiratory tracts, are colonized with a myriad of bacteria, archaea, fungi, and viruses, which are collectively referred to as the "microbiota." The bacterial component of the microbiota in particular has been recognized to influence a multitude of physiological functions, including innate and adaptive immune responses. Germ-free and microbiota-depleted animals display an impaired antimicrobial defense and are therefore highly susceptible to various infections, including those affecting the lung. In this review, we summarize current understanding of how the microbiota affects antimicrobial immunity and disease tolerance during viral and bacterial pulmonary infections. A better understanding of these mechanisms could help to refine clinical approaches to preserve or rescue the microbiota-immune system interplay and protect patients against lung infections.

Fecal IgA Levels Are Determined by Strain-Level Differences in Bacteroides ovatus and Are Modifiable by Gut Microbiota Manipulation

Fecal IgA production depends on colonization by a gut microbiota. However, the bacterial strains that drive gut IgA production remain largely unknown. Here, we assessed the IgA-inducing capacity of a diverse set of human gut microbial strains by monocolonizing mice with each strain. We identified Bacteroides ovatus as the species that best induced gut IgA production. However, this induction varied bimodally across different B. ovatus strains. The high IgA-inducing B. ovatus strains preferentially elicited more IgA production in the large intestine through the T cell-dependent B cell-activation pathway. Remarkably, a low-IgA phenotype in mice could be robustly and consistently converted into a high-IgA phenotype by transplanting a multiplex cocktail of high IgA-inducing B. ovatus strains but not individual ones. Our results highlight the critical importance of microbial strains in driving phenotype variation in the mucosal immune system and provide a strategy to robustly modify a gut immune phenotype, including IgA production.

Unraveling the mysteries of plasma cells

Antibody-secreting plasma cells are the central pillars of humoral immunity. They are generated in a fundamental cellular restructuring process from naive B cells upon contact with antigen. This outstanding process is guided and controlled by a complex transcriptional network accompanied by a fascinating morphological metamorphosis, governed by the combined action of Blimp-1, Xbp-1 and IRF-4. The survival of plasma cells requires the intimate interaction with a specific microenvironment, consisting of stromal cells and cells of hematopoietic origin. Cell-cell contacts, cytokines and availability of metabolites such as glucose and amino acids modulate the survival abilities of plasma cells in their niches. Moreover, plasma cells have been shown to regulate immune responses by releasing cytokines. Furthermore, plasma cells are central players in autoimmune diseases and malignant transformation of plasma cells can result in the generation of multiple myeloma. Hence, the development of sophisticated strategies to deplete autoreactive plasma cells and myeloma cells represents a challenge for current and future research.

Specific microbiota enhances intestinal IgA levels by inducing TGF-β in T follicular helper cells of Peyer's patches in mice

In humans and mice, mucosal immune responses are dominated by IgA antibodies and the cytokine TGF-β, suppressing unwanted immune reactions but also targeting Ig class switching to IgA. It had been suggested that eosinophils promote the generation and maintenance of mucosal IgA-expressing plasma cells. Here, we demonstrate that not eosinophils, but specific bacteria determine mucosal IgA production. Co-housing of eosinophil-deficient mice with mice having high intestinal IgA levels, as well as the intentional microbiota transfer induces TGF-β expression in intestinal T follicular helper cells, thereby promoting IgA class switching in Peyer's patches, enhancing IgA⁺ plasma cell numbers in the small intestinal lamina propria and levels of mucosal IgA. We show that bacteria highly enriched for the genus Anaeroplasma are sufficient to induce these changes and enhance IgA levels when adoptively transferred. Thus, specific members of the intestinal microbiota and not the microbiota as such regulate gut homeostasis, by promoting the expression of immune-regulatory TGF-β and of mucosal IgA.