The majority of intestinal IgA+ and IgG+ plasmablasts in the human gut are antigen-specific

Expression Cloning of Antibodies from Single Human B Cells

The majority of lymphomas originate from B cells at the germinal center stage. Preferential selection of B-cell clones by a limited set of antigens has been suggested to drive lymphoma development. While recent studies in B-cell chronic lymphocytic leukemia (CLL) have shown that self-reactive B-cell receptors (BCR) can generate cell-autonomous signaling and proliferation, our knowledge about the role of BCRs for the development or survival of other lymphomas remains limited. Here, we describe a strategy to characterize the antibody reactivity of human B cells. The approach allows for unbiased characterization of the human antibody repertoire on single-cell level through the generation of recombinant monoclonal antibodies from primary human B cells of defined origin. This protocol offers a detailed description of the method starting from the flow cytometric isolation of single human B cells to the RT-PCR-based amplification of the expressed immunoglobulin (Ig) transcripts (IGH, IGK, and IGL) and their subsequent cloning into expression vectors for the in vitro production of recombinant monoclonal antibodies. The strategy may be used to obtain information about the clonal evolution of B-cell lymphomas by single-cell sequencing of Ig transcripts and on the antibody reactivity of human lymphoma B cells.

Increased abundance of Firmicutes and depletion of Bacteroidota predicts poor outcome in chronic lymphocytic leukemia

Evidence indicates that there are significant alterations in gut microbiota diversity and composition in patients with hematological malignancies. The present study investigated the oral and intestinal microbiome in patients with chronic lymphocytic leukemia (CLL) (n=81) and age-matched healthy volunteers (HVs; n=21) using 16S ribosomal RNA next-generation sequencing. Changes in both oral and gut microbiome structures were identified, with a high abundance of Proteobacteria and depletion of Bacteroidetes in CLL as compared to HVs. Oral and stool samples of patients with CLL revealed a significant change in the abundance of short-chain fatty acid-producing genera in comparison with HVs. Furthermore, the relative abundance of oral and intestine Bacteroidetes was significantly decreased in patients with CLL with negative prognostic features, including unmutated immunoglobulin heavy chain gene (IGHV). Notably, an increased abundance of gut Firmicutes was found to be associated with high expression of CD38. Finally, the present study suggested the log Firmicutes/Bacteroidota ratio as a novel intestinal microbiome signature associated with a shorter time to first treatment in individuals with CLL. The findings indicate that oral and gut microbial diversity in CLL might point to the inflammatory-related modulation of the clinical course of the disease.

All roads lead to IgA: Mapping the many pathways of IgA induction in the gut

The increasing prevalence of food allergy and related pathologies in recent years has underscored the need to understand the factors affecting adverse reactions to food. Food allergy is caused when food-specific IgE triggers the release of histamine from mast cells. However, other food-specific antibody isotypes exist as well, including IgG and IgA. IgA is the main antibody isotype in the gut and mediates noninflammatory reactions to toxins, commensal bacteria, and food antigens. It has also been thought to induce tolerance to food, thus antagonizing the role of food-specific IgE. However, this has remained unclear as food-specific IgA generation is poorly understood. Particularly, the location of IgA induction, the role of T cell help, and the fates of food-specific B cells remain elusive. In this review, we outline what is known about food-specific IgA induction and highlight areas requiring further study. We also explore how knowledge of food-specific IgA induction can be informed by and subsequently contribute to our overall knowledge of gut immunity.

Cracking the type 1 diabetes code: Genes, microbes, immunity, and the early life environment

Type 1 diabetes (T1D) results from a complex interplay of genetic predisposition, immunological dysregulation, and environmental triggers, that culminate in the destruction of insulin-secreting pancreatic β cells. This review provides a comprehensive examination of the multiple factors underpinning T1D pathogenesis, to elucidate key mechanisms and potential therapeutic targets. Beginning with an exploration of genetic risk factors, we dissect the roles of human leukocyte antigen (HLA) haplotypes and non-HLA gene variants associated with T1D susceptibility. Mechanistic insights gleaned from the NOD mouse model provide valuable parallels to the human disease, particularly immunological intricacies underlying β cell-directed autoimmunity. Immunological drivers of T1D pathogenesis are examined, highlighting the pivotal contributions of both effector and regulatory T cells and the multiple functions of B cells and autoantibodies in β-cell destruction. Furthermore, the impact of environmental risk factors, notably modulation of host immune development by the intestinal microbiome, is examined. Lastly, the review probes human longitudinal studies, unveiling the dynamic interplay between mucosal immunity, systemic antimicrobial antibody responses, and the trajectories of T1D development. Insights garnered from these interconnected factors pave the way for targeted interventions and the identification of biomarkers to enhance T1D management and prevention strategies.

Idiotopes of antibodies against HIV-1 CD4-induced epitope shared with those against microorganisms

Induction of antibodies (Abs) against the conformational CD4-induced (CD4i) epitope is frequent in HIV-1 infection. However, the mechanism of development of anti-CD4i Abs is unclear. We used anti-idiotypic (aID) monoclonal Abs (mAbs) of anti-CD4i mAbs to isolate anti-CD4i mAbs from infected subjects and track the causative antigens. One anti-aID mAb sorted from infected subjects by aID mAbs had the characteristics of anti-CD4i Abs, including IGHV1-69 usage and ability to bind to HIV-1 Env enhanced by sCD4. Critical amino acid sequences for the binding of six anti-aID mAbs, with shared idiotope to anti-CD4i mAbs, were analysed by phage display. The identified amino acid sequences showed similarity to proteins from human microbiota and infectious agents. Peptides synthesized from Caudoviricetes sp and Vibrio vulnificus based on the identified sequences were reactive to most anti-aID and some anti-CD4i mAbs. These results suggest that anti-CD4i Abs may evolve from B cells primed by microorganisms.

Intergenerational protective anti-gut commensal immunoglobulin G originates in early life

Maternal immunoglobulins of the class G (IgGs) protect offspring from enteric infection, but when, where, and how these antibodies are physiologically generated and confer protection remains enigmatic. We found that circulating IgGs in adult mice preferentially bind early-life gut commensal bacteria over their own adult gut commensal bacteria. IgG-secreting plasma cells specific for early-life gut bacteria appear in the intestine soon after weaning, where they remain into adulthood. Manipulating exposure to gut bacteria or plasma cell development before, but not after, weaning reduced IgG-secreting plasma cells targeting early-life gut bacteria throughout life. Further, the development of this anti-gut commensal IgG response coincides with the early-life interval in which goblet cell-associated antigen passages (GAPs) are present in the colon. Offspring of dams "perturbed" by B cell ablation or reduced bacterial exposure in early life were more susceptible to enteric pathogen challenge. In contrast to current concepts, protective maternal IgGs targeted translocating gut commensals in the offspring, not the enteric pathogen. These early-life events affecting anti-commensal IgG production have intergenerational effects for protection of the offspring.

Gut-associated lymphoid tissue: a microbiota-driven hub of B cell immunity

The diverse gut microbiota, which is associated with mucosal health and general wellbeing, maintains gut-associated lymphoid tissues (GALT) in a chronically activated state, including sustainment of germinal centers in a context of high antigenic load. This influences the rules for B cell engagement with antigen and the potential consequences. Recent data have highlighted differences between GALT and other lymphoid tissues. For example, GALT propagates IgA responses against glycans that show signs of having been generated in germinal centers. Other findings suggest that humans are among those species where GALT supports the diversification, propagation, and possibly selection of systemic B cells. Here, we review novel findings that identify GALT as distinctive, and able to support these processes.

Specific gut pathobionts escape antibody coating and are enriched during flares in patients with severe Crohn's disease

OBJECTIVE

Patients with Crohn's disease (CD) exhibit great heterogeneity in disease presentation and treatment responses, where distinct gut bacteria and immune interactions may play part in the yet unresolved disease aetiology. Given the role of antibodies in the barrier defence against microbes, we hypothesised that gut bacterial antibody-coating patterns may influence underlying disease-mediated processes.

DESIGN

Absolute and relative single and multicoating of gut bacteria with IgA, IgG1, IgG2, IgG3 and IgG4 in patients with CD and healthy controls were characterised and compared with disease activity. IgG2-coated and non-coated taxa from patients with severe CD were identified, profiled for pathogenic characteristics and monitored for enrichment during active disease across cohorts.

RESULTS

Patients with severe CD exhibited higher gut bacterial IgG2-coating. Supervised clustering identified 25 bacteria to be enriched in CD patients with high IgG2-coating. Sorting, sequencing and in silico-based assessments of the virulent potential of IgG2-coated and bulk stool bacteria were performed to evaluate the nature and pathogenicity of IgG2-coated and non-coated bacteria. The analyses demonstrated IgG2-coating of both known pathogenic and non-pathogenic bacteria that co-occurred with two non-coated pathobionts, Campylobacter and Mannheimia. The two non-coated pathobionts exhibited low prevalence, rarely coincided and were strongly enriched during disease flares in patients with CD across independent and geographically distant cohorts.

CONCLUSION

Distinct gut bacterial IgG2-coating was demonstrated in patients with severe CD and during disease flares. Co-occurrence of non-coated pathobionts with IgG2-coated bacteria points to an uncontrolled inflammatory condition in severe CD mediated via escape from antibody coating by two gut pathobionts.

Who goes there?

How B cells assess risk in the intestine.

Transferrin Is Up-Regulated by Microbes and Acts as a Negative Regulator of Immunity to Induce Intestinal Immunotolerance

Cross-talks (e.g., host-driven iron withdrawal and microbial iron uptake between host gastrointestinal tract and commensal microbes) regulate immunotolerance and intestinal homeostasis. However, underlying mechanisms that regulate the cross-talks remain poorly understood. Here, we show that bacterial products up-regulate iron-transporter transferrin and transferrin acts as an immunosuppressor by interacting with cluster of differentiation 14 (CD14) to inhibit pattern recognition receptor (PRR) signaling and induce host immunotolerance. Decreased intestinal transferrin is found in germ-free mice and human patients with ulcerative colitis, which are characterized by impaired intestinal immunotolerance. Intestinal transferrin and host immunotolerance are returned to normal when germ-free mice get normal microbial commensalism, suggesting an association between microbial commensalism, transferrin, and host immunotolerance. Mouse colitis models show that transferrin shortage impairs host's tolerogenic responses, while its supplementation promotes immunotolerance. Designed peptide blocking transferrin-CD14 interaction inhibits immunosuppressive effects of transferrin. In monkeys with idiopathic chronic diarrhea, transferrin shows comparable or even better therapeutic effects than hydrocortisone. Our findings reveal that by up-regulating host transferrin to silence PRR signaling, commensal bacteria counteract immune activation induced by themselves to shape host immunity and contribute for intestinal tolerance.

IgG and IgM cooperate in coating of intestinal bacteria in IgA deficiency

Immunoglobulin A (IgA) is acknowledged to play a role in the defence of the mucosal barrier by coating microorganisms. Surprisingly, IgA-deficient humans exhibit few infection-related complications, raising the question if the more specific IgG may help IgM in compensating for the lack of IgA. Here we employ a cohort of IgA-deficient humans, each paired with IgA-sufficient household members, to investigate multi-Ig bacterial coating. In IgA-deficient humans, IgM alone, and together with IgG, recapitulate coating of most bacterial families, despite an overall 3.6-fold lower Ig-coating. Bacterial IgG coating is dominated by IgG1 and IgG4. Single-IgG2 bacterial coating is sparse and linked to enhanced Escherichia coli load and TNF-α. Although single-IgG2 coating is 1.6-fold more prevalent in IgA deficiency than in healthy controls, it is 2-fold less prevalent than in inflammatory bowel disease. Altogether we demonstrate that IgG assists IgM in coating of most bacterial families in the absence of IgA and identify single-IgG2 bacterial coating as an inflammatory marker.

Gut immune responses and evolution of the gut microbiome-a hypothesis

The gut microbiome is an assemblage of microbes that have profound effects on their hosts. The composition of the microbiome is affected by bottom-up, among-taxa interactions and by top-down, host effects, which includes the host immune response. While the high-level composition of the microbiome is generally stable over time, component strains and genotypes will constantly be evolving, with both bottom-up and top-down effects acting as selection pressures, driving microbial evolution. Secretory IgA is a major feature of the gut's adaptive immune response, and a substantial proportion of gut bacteria are coated with IgA, though the effect of this on bacteria is unclear. Here we hypothesize that IgA binding to gut bacteria is a selection pressure that will drive the evolution of IgA-bound bacteria, so that they will have a different evolutionary trajectory than those bacteria not bound by IgA. We know very little about the microbiome of wild animals and even less about their gut immune responses, but it must be a priority to investigate this hypothesis to understand if and how host immune responses contribute to microbiome evolution.

Secretory-IgA binding to intestinal microbiota attenuates inflammatory reactions as the intestinal barrier of preterm infants matures

Previous work has shown that Secretory-IgA (SIgA) binding to the intestinal microbiota is variable and may regulate host inflammatory bowel responses. Nevertheless, the impact of the SIgA functional binding to the microbiota remains largely unknown in preterm infants whose immature epithelial barriers make them particularly susceptible to inflammation. Here, we investigated SIgA binding to intestinal microbiota isolated from stools of preterm infants <33 weeks gestation with various levels of intestinal permeability. We found that SIgA binding to intestinal microbiota attenuates inflammatory reactions in preterm infants. We also observed a significant correlation between SIgA affinity to the microbiota and the infant's intestinal barrier maturation. Still, SIgA affinity was not associated with developing host defenses, such as the production of mucus and inflammatory calprotectin protein, but it depended on the microbiota shifts as the intestinal barrier matures. In conclusion, we reported an association between the SIgA functional binding to the microbiota and the maturity of the preterm infant's intestinal barrier, indicating that the pattern of SIgA coating is altered as the intestinal barrier matures.

Antigen receptor signaling and cell death resistance controls intestinal humoral response zonation

Immunoglobulin A (IgA) maintains commensal communities in the intestine while preventing dysbiosis. IgA generated against intestinal microbes assures the simultaneous binding to multiple, diverse commensal-derived antigens. However, the exact mechanisms by which B cells mount broadly reactive IgA to the gut microbiome remains elusive. Here, we have shown that IgA B cell receptor (BCR) is required for B cell fitness during the germinal center (GC) reaction in Peyer's patches (PPs) and for generation of gut-homing plasma cells (PCs). We demonstrate that IgA BCR drove heightened intracellular signaling in mouse and human B cells, and as a consequence, IgA+ B cells received stronger positive selection cues. Mechanistically, IgA BCR signaling offset Fas-mediated death, possibly rescuing low-affinity B cells to promote a broad humoral response to commensals. Our findings reveal an additional mechanism linking BCR signaling, B cell fate, and antibody production location, which have implications for how intestinal antigen recognition shapes humoral immunity.

HIV-1 treatment timing shapes the human intestinal memory B-cell repertoire to commensal bacteria

HIV-1 infection causes severe alterations of gut mucosa, microbiota and immune system, which can be curbed by early antiretroviral therapy. Here, we investigate how treatment timing affects intestinal memory B-cell and plasmablast repertoires of HIV-1-infected humans. We show that only class-switched memory B cells markedly differ between subjects treated during the acute and chronic phases of infection. Intestinal memory B-cell monoclonal antibodies show more prevalent polyreactive and commensal bacteria-reactive clones in late- compared to early-treated individuals. Mirroring this, serum IgA polyreactivity and commensal-reactivity are strongly increased in late-treated individuals and correlate with intestinal permeability and systemic inflammatory markers. Polyreactive blood IgA memory B cells, many of which egressed from the gut, are also substantially enriched in late-treated individuals. Our data establish gut and systemic B-cell polyreactivity to commensal bacteria as hallmarks of chronic HIV-1 infection and suggest that initiating treatment early may limit intestinal B-cell abnormalities compromising HIV-1 humoral response.

B cells and the intestinal microbiome in time, space and place

The gut immune system is shaped by the continuous interaction with the microbiota. Here we dissect temporal, spatial and contextual layers of gut B cell responses. The microbiota impacts on the selection of the developing pool of pre-immune B cells that serves as substrate for B cell activation, expansion and differentiation. However, various aspects of the gut B cell response display unique features. In particular, occurrence of somatically mutated B cells, chronic gut germinal centers in T cell-deficient settings and polyreactive binding of gut IgA to the microbiota questioned the nature and microbiota-specificity of gut germinal centers. We propose a model to reconcile these observations incorporating recent work demonstrating microbiota-specificity of gut germinal centers. We speculate that adjuvant effects of the microbiota might modify permissiveness for B cell to enter and exit gut germinal centers. We propose that separating aspects of time, space and place facilitate the occasionally puzzling discussion of gut B cell responses to the microbiota.

The Underappreciated Role of Secretory IgA in IBD

Eighty percent of antibody secreting cells (ASCs) are found in the intestine, where they produce grams of immunoglobulin (Ig) A daily. immunoglobulin A is actively transcytosed into the lumen, where it plays a critical role in modulating the gut microbiota. Although loss of immune tolerance to bacterial antigens is the likely trigger of the dysregulated immune response that characterizes inflammatory bowel disease (IBD), little effort has been placed on understanding the interface between B cells, IgA, and the microbiota during initiation or progression of disease. This may be in part due to the misleading fact that IgA-deficient humans are mostly asymptomatic, likely due to redundant role of secretory (S) IgM. Intestinal B cell recruitment is critically dependent on integrin α4β7-MAdCAM-1 interactions, yet antibodies that target α4β7 (ie, vedolizumab), MAdCAM-1 (ie, ontamalimab), or both β7 integrins (α4β7 and αE [CD103] β7; etrolizumab) are in clinical use or development as IBD therapeutics. The effect of such interventions on the biology of IgA is largely unknown, yet a single dose of vedolizumab lowers SIgA levels in stool and weakens the oral immunization response to cholera vaccine in healthy volunteers. Thus, it is critical to further understand the role of these integrins for the migration of ASC and other cellular subsets during homeostasis and IBD-associated inflammation and the mode of action of drugs that interfere with this traffic. We have recently identified a subset of mature ASC that employs integrin αEβ7 to dock with intestinal epithelial cells, predominantly in the pericryptal region of the terminal ileum. This role for the integrin had not been appreciated previously, nor the αEβ7-dependent mechanism of IgA transcytosis that it supports. Furthermore, we find that B cells more than T cells are critically dependent on α4β7-MAdCAM-1 interactions; thus MAdCAM-1 blockade and integrin-β7 deficiency counterintuitively hasten colitis in interleukin-10-deficient mice. In both cases, de novo recruitment of IgA ASC to the intestinal lamina propria is compromised, leading to bacterial overgrowth, dysbiosis, and lethal colitis. Thus, despite the safe and effective use of anti-integrin antibodies in patients with IBD, much remains to be learned about their various cell targets.

Remodeling of colon plasma cell repertoire within ulcerative colitis patients

Plasma cells (PCs) constitute a significant fraction of colonic mucosal cells and contribute to inflammatory infiltrates in ulcerative colitis (UC). While gut PCs secrete bacteria-targeting IgA antibodies, their role in UC pathogenesis is unknown. We performed single-cell V(D)J- and RNA-seq on sorted B cells from the colon of healthy individuals and patients with UC. A large fraction of B cell clones is shared between different colon regions, but inflammation in UC broadly disrupts this landscape, causing transcriptomic changes characterized by an increase in the unfolded protein response (UPR) and antigen presentation genes, clonal expansion, and isotype skewing from IgA1 and IgA2 to IgG1. We also directly expressed and assessed the specificity of 152 mAbs from expanded PC clones. These mAbs show low polyreactivity and autoreactivity and instead target both shared bacterial antigens and specific bacterial strains. Altogether, our results characterize the microbiome-specific colon PC response and how its disruption might contribute to inflammation in UC.

Altered B cell immunoglobulin signature exhibits potential diagnostic values in human colorectal cancer

Antibody-secreting B cells have long been considered the central element of gut homeostasis; however, tumor-associated B cells in human colorectal cancer (CRC) have not been well characterized. Here, we show that the clonotype, phenotype, and immunoglobulin subclasses of tumor-infiltrating B cells have changed compared to adjacent normal tissue B cells. Remarkably, the tumor-associated B cell immunoglobulin signature alteration can also be detected in the plasma of patients with CRC, suggesting that a distinct B cell response was also evoked in CRC. We compared the altered plasma immunoglobulin signature with the existing method of CRC diagnosis. Our diagnostic model exhibits improved sensitivity compared to the traditional biomarkers, CEA and CA19-9. These findings disclose the altered B cell immunoglobulin signature in human CRC and highlight the potential of using the plasma immunoglobulin signature as a non-invasive method for the assessment of CRC.

Differences in the immunoglobulin gene repertoires of IgG versus IgA multiple myeloma allude to distinct immunopathogenetic trajectories

The analysis of the immunogenetic background of multiple myeloma (MM) has proven key to understanding disease ontogeny. However, limited information is available regarding the immunoglobulin (IG) gene repertoire in MM cases carrying different heavy chain isotypes. Here, we studied the IG gene repertoire in a series of 523 MM patients, of whom 165 and 358 belonged to the IgA and IgG MM groups, respectively. IGHV3 subgroup genes predominated in both groups. However, at the individual gene level, significant (p<0.05) differences were identified regarding IGHV3-21 (frequent in IgG MM) and IGHV5-51 (frequent in IgA MM). Moreover, biased pairings were identified between certain IGHV genes and IGHD genes in IgA versus IgG MM. Turning to the imprints of somatic hypermutation (SHM), the bulk of rearrangements (IgA: 90.9%, IgG: 87.4%) were heavily mutated [exhibiting an IGHV germline identity (GI) <95%]. SHM topology analysis disclosed distinct patterns in IgA MM versus IgG MM cases expressing B cell receptor IG encoded by the same IGHV gene: the most pronounced examples concerned the IGHV3-23, IGHV3-30 and IGHV3-9 genes. Furthermore, differential SHM targeting was also identified between IgA MM versus IgG MM, particularly in cases utilizing certain IGHV genes, alluding to functional selection. Altogether, our detailed immunogenetic evaluation in the largest to-date series of IgA and IgG MM patients reveals certain distinct features in the IGH gene repertoires and SHM. These findings suggest distinct immune trajectories for IgA versus IgG MM, further underlining the role of external drive in the natural history of MM.

Tango of B cells with T cells in the making of secretory antibodies to gut bacteria

Polymeric IgA and IgM are transported across the epithelial barrier from plasma cells in the lamina propria to exert a function in the gut lumen as secretory antibodies. Many secretory antibodies are reactive with the gut bacteria, and mounting evidence suggests that these antibodies are important for the host to control gut bacterial communities. However, we have incomplete knowledge of how bacteria-reactive secretory antibodies are formed. Antibodies from gut plasma cells often show bacterial cross-species reactivity, putting the degree of specificity behind anti-bacterial antibody responses into question. Such cross-species reactive antibodies frequently recognize non-genome-encoded membrane glycan structures. On the other hand, the T cell epitopes are peptides encoded in the bacterial genomes, thereby allowing a higher degree of predictable specificity on the T cell side of anti-bacterial immune responses. In this Perspective, we argue that the production of bacteria-reactive secretory antibodies is mainly controlled by the antigen specificity of T cells, which provide help to B cells. To be able to harness this system (for instance, for manipulation with vaccines), we need to obtain insight into the bacterial epitopes recognized by T cells in addition to characterizing the reactivity of the antibodies.

Intestinal IgA-Coated Bacteria in Healthy- and Altered-Microbiomes (Dysbiosis) and Predictive Value in Successful Fecal Microbiota Transplantation

IgA-coated bacteria in the gut (IgA-biome) provide a homeostatic function in healthy people through inhibition of microbial invaders and by protecting the epithelial monolayer of the gut. The laboratory methods used to detect this group of bacteria require flow cytometry and DNA sequencing (IgA-Seq). With dysbiosis (reduced diversity of the microbiome), the IgA-biome also is impaired. In the presence of enteric infection, oral vaccines, or an intestinal inflammatory disorder, the IgA-biome focuses on the pathogenic bacteria or foreign antigens, while in other chronic diseases associated with dysbiosis, the IgA-biome is reduced in capacity. Fecal microbiota transplantation (FMT), the use of fecal product from well-screened, healthy donors administered to patients with dysbiosis, has been successful in engrafting the intestine with healthy microbiota and metabolites leading to improve health. Through FMT, IgA-coated bacteria have been transferred to recipients retaining their immune coating. The IgA-biome should be evaluated in FMT studies as these mucosal-associated bacteria are more likely to be associated with successful transplantation than free luminal organisms. Studies of the microbiome pre- and post-FMT should employ metagenomic methods that identify bacteria at least at the species level to better identify organisms of interest while allowing comparisons of microbiota data between studies.

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.

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.

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.

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.

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.

Luminal bacteria coated with IgA and IgG during intestinal inflammation as a new and abundant stimulus for colonic macrophages

In the gut, secretory immunoglobulin A is the predominant humoral response against commensals, although healthy hosts also produce microbiota-specific IgG antibodies. During intestinal inflammation, the content of IgG in the lumen increases along with the proportion of commensal bacteria coated with this antibody, suggesting signalling through the IgG-CD64 axis in the pathogenesis of inflammatory bowel diseases. In this work, we evaluated day by day the frequency of faecal bacteria coated with IgA and IgG during the development of DSS colitis. We studied the phenotype and phagocytic activity of F4/80⁺ CD64⁺ colonic macrophages, as well as the production of cytokines and nitric oxide by lamina propria or bone marrow-derived macrophages after stimulation with IgA⁺ , IgG⁺ and IgA⁺ IgG⁺ bacteria. We found that the percentage of faecal IgA⁺ IgG⁺ double-coated bacteria increased rapidly during DSS colitis. Also, analysis of the luminal content of mice with colitis showed a markedly superior ability to coat fresh bacteria. IgA⁺ IgG⁺ bacteria were the most potent stimulus for phagocytic activity involving CD64 and Dectin-1 receptors. IgA⁺ IgG⁺ bacteria observed during the development of DSS colitis could represent a new marker to monitor permeability and inflammatory progression. The interaction of IgA⁺ IgG⁺ bacteria with CD64⁺ F4/80⁺ macrophages could be part of the complex cascade of events in colitis. Interestingly, after stimulation, CD64⁺ colonic macrophages showed features similar to those of restorative macrophages that are relevant for tissue repair and healing.

Effects of Gut Microbiota on Host Adaptive Immunity Under Immune Homeostasis and Tumor Pathology State

Gut microbiota stimulate and shape the body’s adaptive immune response through bacterial components and its active metabolites, which orchestrates the formation and maintenance of the body’s immune homeostasis. In addition, the imbalances in microbiota-adaptive immunity contribute to the development of tumor and the antitumor efficiency of a series of antitumor therapies at the preclinical and clinical levels. Regardless of significant results, the regulation of gut microbiota on adaptive immunity in immune homeostasis and tumors needs a more thorough understanding. Herein, we highlighted the comprehensive knowledge, status, and limitations in the mechanism of microbiome interaction with adaptive immunity and put forward the prospect of how to translate these insights in inhibiting tumor progression and enhancing the efficacy of antitumor interventions.

Ulcerative colitis is characterized by a plasmablast-skewed humoral response associated with disease activity

B cells, which are critical for intestinal homeostasis, remain understudied in ulcerative colitis (UC). In this study, we recruited three cohorts of patients with UC (primary cohort, n = 145; validation cohort 1, n = 664; and validation cohort 2, n = 143) to comprehensively define the landscape of B cells during UC-associated intestinal inflammation. Using single-cell RNA sequencing, single-cell IgH gene sequencing and protein-level validation, we mapped the compositional, transcriptional and clonotypic landscape of mucosal and circulating B cells. We found major perturbations within the mucosal B cell compartment, including an expansion of naive B cells and IgG⁺ plasma cells with curtailed diversity and maturation. Furthermore, we isolated an auto-reactive plasma cell clone targeting integrin αvβ6 from inflamed UC intestines. We also identified a subset of intestinal CXCL13-expressing TFH-like T peripheral helper cells that were associated with the pathogenic B cell response. Finally, across all three cohorts, we confirmed that changes in intestinal humoral immunity are reflected in circulation by the expansion of gut-homing plasmablasts that correlates with disease activity and predicts disease complications. Our data demonstrate a highly dysregulated B cell response in UC and highlight a potential role of B cells in disease pathogenesis.

B cell class switching in intestinal immunity in health and disease

The gastrointestinal tract is colonized by trillions of commensal microorganisms that collectively form the microbiome and make essential contributions to organism homeostasis. The intestinal immune system must tolerate these beneficial commensals, whilst preventing pathogenic organisms from systemic spread. Humoral immunity plays a key role in this process, with large quantities of immunoglobulin (Ig)A secreted into the lumen on a daily basis, regulating the microbiome and preventing bacteria from encroaching on the epithelium. However, there is an increasing appreciation of the role of IgG antibodies in intestinal immunity, including beneficial effects in neonatal immune development, pathogen and tumour resistance, but also of pathological effects in driving chronic inflammation in inflammatory bowel disease (IBD). These antibody isotypes differ in effector function, with IgG exhibiting more proinflammatory capabilities compared with IgA. Therefore, the process that leads to the generation of different antibody isotypes, class-switch recombination (CSR), requires careful regulation and is orchestrated by the immunological cues generated by the prevalent local challenge. In general, an initiating signal such as CD40 ligation on B cells leads to the induction of activation-induced cytidine deaminase (AID), but a second cytokine-mediated signal determines which Ig heavy chain is expressed. Whilst the cytokines driving intestinal IgA responses are well-studied, there is less clarity on how IgG responses are generated in the intestine, and how these cues might become dysfunctional in IBD. Here, we review the key mechanisms regulating class switching to IgA vs IgG in the intestine, processes that could be therapeutically manipulated in infection and IBD.

A Direct MS-Based Approach to Profile Human Milk Secretory Immunoglobulin A (IgA1) Reveals Donor-Specific Clonal Repertoires With High Longitudinal Stability

Recently, a mass spectrometry-based approach was introduced to directly assess the IgG1 immunoglobulin clonal repertoires in plasma. Here we expanded upon this approach by describing a mass spectrometry-based technique to assess specifically the clonal repertoire of another important class of immunoglobulin molecules, IgA1, and show it is efficiently and robustly applicable to either milk or plasma samples. Focusing on two individual healthy donors, whose milk was sampled longitudinally during the first 16 weeks of lactation, we demonstrate that the total repertoire of milk sIgA1 is dominated by only 50-500 clones, even though the human body theoretically can generate several orders of magnitude more clones. We show that in each donor the sIgA1 repertoire only changes marginally and quite gradually over the monitored 16-week period of lactation. Furthermore, the observed overlap in clonal repertoires between the two individual donors is close to non-existent. Mothers provide protection to their newborn infants directly by the transfer of antibodies via breastfeeding. The approach introduced here, can be used to visualize the clonal repertoire transferred from mother to infant and to detect changes in-time in that repertoire adapting to changes in maternal physiology.

Potential role of tumor-infiltrating T-, B-lymphocytes, tumor-associated macrophages and IgA-secreting plasma cells in long-term survival in the rectal adenocarcinoma patients

AIMS

Many studies investigated the associations between the role of immune cells of rectal cancer microenvironment and survival during the first 5 years post-surgery. This is problematic as this disease has the potential to progress even after 5 years after relapse and infiltrating immune cells could play key roles. Therefore, this retrospective study investigates expression and roles of tumor-infiltrating T-lymphocytes (TIL-T), tumor-infiltrating B-lymphocytes (TILB), IgA⁺ plasma cells (IgA⁺ PC) and tumor-associated macrophages (TAM) in patients with or without progression over 5 years survival with rectal adenocarcinoma.

MAIN METHODS

Here we used immunohistochemical staining of CD3, CD20, IgA, CD68 positive cells and its detection in rectal cancer stroma. Data was analyzed using Mann Whitney U test, ROC, survival and Cox's regression analysis.

KEY FINDINGS

The number of TIL-T (p = 0.0276), TIL-B (p < 0.0001) and IgA⁺ PC (p = 0.015) immune cells was significantly higher in rectal cancer stroma of patients with favorable outcome. Univariate Cox's regression analysis revealed a predictive role of TIL-T (HR = 0.482; 95% CI, 0.303 to 0.704; p < 0.0001), TIL-B (HR = 0.301; 95% CI, 0.198 to 0.481; p < 0.0001) and IgA⁺-PC (HR = 0.488; 95% CI, 0.322 to 0.741; p < 0.0001). Multivariate Cox's regression analysis showed prognostic role of TIL-B (HR = 0.940; 95% CI, 0.914 to 0.968; p < 0.0001) and IgA⁺-PC (HR = 0.985; 95% CI, 0.975 to 0.996; p = 0.006) play role in long time survival.

SIGNIFICANCE

CD20⁺ TIL-B and IgA⁺ cells have significant associations with long -term survival of patients with rectal cancer, with potential therapeutic intervention in cancer immunotherapy.

Affinity-based clonal selection in Peyer's patches

Effective long-lasting immunity depends on the generation of protective antibodies that restrict the invasion of harmful pathogens. The germinal center (GC) is a microanatomical site at which B cells acquire random somatic mutations in their immunoglobulin genes followed by affinity-based selection. Whereas this process was extensively studied in lymph nodes and spleen, less is known about GCs located in mucosal tissues lymphoid organs, such as the Peyer's patches (PPs). These lymphoid organs have a special structure and host a unique niche known as the subepithelial dome (SED), where B cell activation and class switch recombination to IgA take place before GC seeding. As opposed to typical lymph-nodes, the PPs host chronic GC reactions that are driven by gut-bacteria. Direct evidence for antibody affinity maturation in PPs, and competition between B cells for T cell help was recently obtained. Here, we discuss these findings and how they complement each other.

Effects of molecular weight of chitosan on anti-inflammatory activity and modulation of intestinal microflora in an ulcerative colitis model

This study investigated the therapeutic effects and mechanisms of chitosans (CSs) with different molecular weights on ulcerative colitis (UC). Three size classes of CSs (Mw ≤ 3, 50, and 200 kDa) were used in this study. The effect of large CSs (Mw ≤ 200 kDa) on UC was the best, followed by that of medium CSs (Mw ≤ 50 kDa), and that of small CSs (Mw ≤ 3 kDa) was the least in the LPS-induced Raw 264.7 cell model and DSS-induced UC mice model. The therapeutic mechanisms of three CSs are related to anti-oxidation, anti-inflammation, and regulation of immunoglobulin and intestinal flora by attenuating body weight loss, decreasing the disease activity index (DAI) and MPO activity, suppressing proinflammatory cytokines and IgG levels, down-regulating the level of oxidative stress, increasing anti-inflammatory cytokines, SOD activity and Prevotellaceae_UCG-001 levels, and reducing the abundance of Proteobacteria, Actinobacteria, and Escherichia-Shigella. In general, the molecular weight of CSs influences their efficacy against UC. CSs with an optimal molecular weight demonstrate good development prospects for ameliorating UC.

Traditional Farming Lifestyle in Old Older Mennonites Modulates Human Milk Composition

Background

In addition to farming exposures in childhood, maternal farming exposures provide strong protection against allergic disease in their children; however, the effect of farming lifestyle on human milk (HM) composition is unknown.

Objective

This study aims to characterize the maternal immune effects of Old Order Mennonite (OOM) traditional farming lifestyle when compared with Rochester (ROC) families at higher risk for asthma and allergic diseases using HM as a proxy.

Methods

HM samples collected at median 2 months of lactation from 52 OOM and 29 ROC mothers were assayed for IgA₁ and IgA₂ antibodies, cytokines, endotoxin, HM oligosaccharides (HMOs), and targeted fatty acid (FA) metabolites. Development of early childhood atopic diseases in children by 3 years of age was assessed. In addition to group comparisons, systems level network analysis was performed to identify communities of multiple HM factors in ROC and OOM lifestyle.

Results

HM contains IgA₁ and IgA₂ antibodies broadly recognizing food, inhalant, and bacterial antigens. OOM HM has significantly higher levels of IgA to peanut, ovalbumin, dust mites, and Streptococcus equii as well TGF-β2, and IFN-λ3. A strong correlation occurred between maternal antibiotic use and levels of several HMOs. Path-based analysis of HMOs shows lower activity in the path involving lactoneohexaose (LNH) in the OOM as well as higher levels of lacto-N-neotetraose (LNnT) and two long-chain FAs C-18OH (stearic acid) and C-23OH (tricosanoic acid) compared with Rochester HM. OOM and Rochester milk formed five different clusters, e.g., butyrate production was associated with Prevotellaceae, Veillonellaceae, and Micrococcaceae cluster. Development of atopic disease in early childhood was more common in Rochester and associated with lower levels of total IgA, IgA₂ to dust mite, as well as of TSLP.

Conclusion

Traditional, agrarian lifestyle, and antibiotic use are strong regulators of maternally derived immune and metabolic factors, which may have downstream implications for postnatal developmental programming of infant's gut microbiome and immune system.

Immunoglobulin A, an Active Liaison for Host-Microbiota Homeostasis

Mucosal surfaces in the gastrointestinal tract are continually exposed to native, commensal antigens and susceptible to foreign, infectious antigens. Immunoglobulin A (IgA) provides dual humoral responses that create a symbiotic environment for the resident gut microbiota and prevent the invasion of enteric pathogens. This review features recent immunological and microbial studies that elucidate the underlying IgA and microbiota-dependent mechanisms for mutualism at physiological conditions. IgA derailment and concurrent microbiota instability in pathological diseases are also discussed in detail. Highlights of this review underscore that the source of IgA and its structural form can dictate microbiota reactivity to sustain a diverse niche where both host and bacteria benefit. Other important studies emphasize IgA insufficiency can result in the bloom of opportunistic pathogens that encroach the intestinal epithelia and disseminate into circulation. The continual growth of knowledge in these subjects can lead to the development of therapeutics targeting IgA and/or the microbiota to treat life threatening diseases.

Parallelism of intestinal secretory IgA shapes functional microbial fitness

Dimeric IgA secreted across mucous membranes in response to nonpathogenic taxa of the microbiota accounts for most antibody production in mammals. Diverse binding specificities can be detected within the polyclonal mucosal IgA antibody response^1-10, but limited monoclonal hybridomas have been studied to relate antigen specificity or polyreactive binding to functional effects on microbial physiology in vivo^11-17. Here we use recombinant dimeric monoclonal IgAs (mIgAs) to finely map the intestinal plasma cell response to microbial colonization with a single microorganism in mice. We identify a range of antigen-specific mIgA molecules targeting defined surface and nonsurface membrane antigens. Secretion of individual dimeric mIgAs targeting different antigens in vivo showed distinct alterations in the function and metabolism of intestinal bacteria, largely through specific binding. Even in cases in which the same microbial antigen is targeted, microbial metabolic alterations differed depending on IgA epitope specificity. By contrast, bacterial surface coating generally reduced motility and limited bile acid toxicity. The overall intestinal IgA response to a single microbe therefore contains parallel components with distinct effects on microbial carbon-source uptake, bacteriophage susceptibility, motility and membrane integrity.

Immunoglobulin subtype-coated bacteria are correlated with the disease activity of inflammatory bowel disease

Immune response involving various immunoglobulin (Ig) isotypes and subtypes to microbiome is involved in the pathogenesis and disease activity of inflammatory bowel diseases (IBDs). To clarify the presence of Ig-coated bacteria in the intestine and its association with disease activity in ulcerative colitis (UC) and Crohn’s disease (CD), we extracted and classified Ig-coated bacteria from fecal samples of 42 patients with IBD and 12 healthy controls (HCs) using flow cytometry and 16S ribosomal RNA sequence analysis. The percentage of bacteria coated with IgA and IgM was higher in patients with IBD than in HCs, and IgG-coated bacteria were found only in patients with IBD. Moreover, the percentages of bacteria coated with IgG1, IgG2, IgG3, and IgM in UC samples and IgG3, IgG4, and IgM in CD samples were correlated with disease activities. The proportions of Bacteroides ovatus and Streptococcus increased during the active phase of CD. Hence, the detailed analysis of Ig-coated bacteria and Ig subtypes using flow cytometry could aid in developing useful indicators of disease activity and identifying more disease-related bacteria, which could become novel treatment targets for IBDs.

Microbiota-Driven Activation of Intrahepatic B Cells Aggravates NASH Through Innate and Adaptive Signaling

BACKGROUND AND AIMS

Nonalcoholic steatohepatitis is rapidly becoming the leading cause of liver failure and indication for liver transplantation. Hepatic inflammation is a key feature of NASH but the immune pathways involved in this process are poorly understood. B lymphocytes are cells of the adaptive immune system that are critical regulators of immune responses. However, the role of B cells in the pathogenesis of NASH and the potential mechanisms leading to their activation in the liver are unclear.

APPROACH AND RESULTS

In this study, we report that NASH livers accumulate B cells with elevated pro-inflammatory cytokine secretion and antigen-presentation ability. Single-cell and bulk RNA sequencing of intrahepatic B cells from mice with NASH unveiled a transcriptional landscape that reflects their pro-inflammatory function. Accordingly, B-cell deficiency ameliorated NASH progression, and adoptively transferring B cells from NASH livers recapitulates the disease. Mechanistically, B-cell activation during NASH involves signaling through the innate adaptor myeloid differentiation primary response protein 88 (MyD88) as B cell-specific deletion of MyD88 reduced hepatic T cell-mediated inflammation and fibrosis, but not steatosis. In addition, activation of intrahepatic B cells implicates B cell-receptor signaling, delineating a synergy between innate and adaptive mechanisms of antigen recognition. Furthermore, fecal microbiota transplantation of human NAFLD gut microbiotas into recipient mice promoted the progression of NASH by increasing the accumulation and activation of intrahepatic B cells, suggesting that gut microbial factors drive the pathogenic function of B cells during NASH.

CONCLUSION

Our findings reveal that a gut microbiota-driven activation of intrahepatic B cells leads to hepatic inflammation and fibrosis during the progression of NASH through innate and adaptive immune mechanisms.

Diversity and dynamism of IgA-microbiota interactions

IgA mediates microbial homeostasis at the intestinal mucosa. Within the gut, IgA acts in a context-dependent manner to both prevent and promote bacterial colonization and to influence bacterial gene expression, thus providing exquisite control of the microbiota. IgA-microbiota interactions are highly diverse across individuals and populations, yet the factors driving this variation remain poorly understood. In this Review, we summarize evidence for the host, bacterial and environmental factors that influence IgA-microbiota interactions. Recent advances have helped to clarify the antigenic specificity and immune selection of intestinal IgA and have highlighted the importance of microbial glycan recognition. Furthermore, emerging evidence suggests that diet and nutrition play an important role in shaping IgA recognition of the microbiota. IgA-microbiota interactions are disrupted during both overnutrition and undernutrition and may be altered dynamically in response to diet, with potential implications for host health. We situate this research in the context of outstanding questions and future directions in order to better understand the fascinating paradigm of IgA-microbiota homeostasis.

Know your enemy or find your friend?-Induction of IgA at mucosal surfaces

Most antibodies produced in the body are of the IgA class. The dominant cell population producing them are plasma cells within the lamina propria of the gastrointestinal tract, but many IgA-producing cells are also found in the airways, within mammary tissues, the urogenital tract and inside the bone marrow. Most IgA antibodies are transported into the lumen by epithelial cells as part of the mucosal secretions, but they are also present in serum and other body fluids. A large part of the commensal microbiota in the gut is covered with IgA antibodies, and it has been demonstrated that this plays a role in maintaining a healthy balance between the host and the bacteria. However, IgA antibodies also play important roles in neutralizing pathogens in the gastrointestinal tract and the upper airways. The distinction between the two roles of IgA - protective and balance-maintaining - not only has implications on function but also on how the production is regulated. Here, we discuss these issues with a special focus on gut and airways.

Influenza A Virus Hemagglutinin Trimer, Head and Stem Proteins Identify and Quantify Different Hemagglutinin-Specific B Cell Subsets in Humans

Antibody responses against the influenza A virus hemagglutinin (HA)-protein are studied intensively because they can protect against (re)infection. Previous studies have focused on antibodies targeting the head or stem domains, while other possible specificities are often not taken into account. To study such specificities, we developed a diverse set of HA-domain proteins based on an H1N1pdm2009-like influenza virus strain, including monomeric head and trimeric stem domain, as well as the full HA-trimer. These proteins were used to study the B cell and antibody responses in six healthy human donors. A large proportion of HA-trimer B cells bound exclusively to HA-trimer probe (54-77%), while only 8-18% and 9-23% were able to recognize the stem or head probe, respectively. Monoclonal antibodies (mAbs) were isolated and three of these mAbs, targeting the different domains, were characterized in-depth to confirm the binding profile observed in flow cytometry. The head-directed mAb, targeting an epitope distinct from known head-specific mAbs, showed relatively broad H1N1 neutralization and the stem-directed mAb was able to broadly neutralize diverse H1N1 viruses. Moreover, we identified a trimer-directed mAb that did not compete with known head or stem domain specific mAbs, suggesting that it targets an unknown epitope or conformation of influenza virus' HA. These observations indicate that the described method can characterize the diverse antibody response to HA and might be able to identify HA-specific B cells and antibodies with previously unknown specificities that could be relevant for vaccine design.

Intestinal Microbiota in Common Chronic Inflammatory Disorders Affecting Children

The incidence and prevalence rate of chronic inflammatory disorders is on the rise in the pediatric population. Recent research indicates the crucial role of interactions between the altered intestinal microbiome and the immune system in the pathogenesis of several chronic inflammatory disorders in children, such as inflammatory bowel disease (IBD) and autoimmune diseases, such as type 1 diabetes mellitus (T1DM) and celiac disease (CeD). Here, we review recent knowledge concerning the pathogenic mechanisms underlying these disorders, and summarize the facts suggesting that the initiation and progression of IBD, T1DM, and CeD can be partially attributed to disturbances in the patterns of composition and abundance of the gut microbiota. The standard available therapies for chronic inflammatory disorders in children largely aim to treat symptoms. Although constant efforts are being made to maximize the quality of life for children in the long-term, sustained improvements are still difficult to achieve. Additional challenges are the changing physiology associated with growth and development of children, a population that is particularly susceptible to medication-related adverse effects. In this review, we explore new promising therapeutic approaches aimed at modulation of either gut microbiota or the activity of the immune system to induce a long-lasting remission of chronic inflammatory disorders. Recent preclinical studies and clinical trials have evaluated new approaches, for instance the adoptive transfer of immune cells, with genetically engineered regulatory T cells expressing antigen-specific chimeric antigen receptors. These approaches have revolutionized cancer treatments and have the potential for the protection of high-risk children from developing autoimmune diseases and effective management of inflammatory disorders. The review also focuses on the findings of studies that indicate that the responses to a variety of immunotherapies can be enhanced by strategic manipulation of gut microbiota, thus emphasizing on the importance of proper interaction between the gut microbiota and immune system for sustained health benefits and improvement of the quality of life of pediatric patients.

Perturbed Microbiota/Immune Homeostasis in Multiple Sclerosis

Objective

Based on animal models and human studies, there is now strong suspicion that host/microbiota mutualism in the context of gut microbial dysbiosis could influence immunity and multiple sclerosis (MS) evolution. Our goal was to seek evidence of deregulated microbiota-induced systemic immune responses in patients with MS.

Methods

We investigated gut and systemic commensal-specific antibody responses in healthy controls (n = 32), patients with relapsing-remitting MS (n = 30), and individuals with clinically isolated syndromes (CISs) (n = 15). Gut microbiota composition and diversity were compared between controls and patients by analysis of 16S ribosomal ribonucleic acid (rRNA) sequencing. Autologous microbiota and cultivable bacterial strains were used in bacterial flow cytometry assays to quantify autologous serum IgG and secretory IgA responses to microbiota. IgG-bound bacteria were sorted by flow cytometry and identified using 16S rRNA sequencing.

Results

We show that commensal-specific gut IgA responses are drastically reduced in patients with severe MS, disease severity being correlated with the IgA-coated fecal microbiota fraction (r = −0.647, p < 0.0001). At the same time, IgA-unbound bacteria elicit qualitatively broad and quantitatively increased serum IgG responses in patients with MS and CIS compared with controls (4.1% and 2.5% vs 1.9%, respectively, p < 0.001).

Conclusions

Gut and systemic microbiota/immune homeostasis are perturbed in MS. Our results argue that defective IgA responses in MS are linked to a breakdown of systemic tolerance to gut microbiota leading to an enhanced triggering of systemic IgG immunity against gut commensals occurring early in MS.

Ecological and network analyses identify four microbial species with potential significance for the diagnosis/treatment of ulcerative colitis (UC)

BACKGROUND

Ulcerative colitis (UC) is one of the primary types of inflammatory bowel disease (IBD), the occurrence of which has been increasing worldwide. Although IBD is an intensively studied human microbiome-associated disease, research on Chinese populations remains relatively limited, particularly on the mucosal microbiome. The present study aimed to analyze the changes in the mucosal microbiome associated with UC from the perspectives of medical ecology and complex network analysis.

RESULTS

In total, 56 mucosal microbiome samples were collected from 28 Chinese UC patients and their healthy family partners, followed by amplicon sequencing. Based on sequencing data, we analyzed species diversity, shared species, and inter-species interactions at the whole community, main phyla, and core/periphery species levels. We identified four opportunistic "pathogens" (i.e., Clostridium tertium, Odoribacter splanchnicus, Ruminococcus gnavus, and Flavonifractor plautii) with potential significance for the diagnosis and treatment of UC, which were inhibited in healthy individuals, but unrestricted in the UC patients. In addition, we also discovered in this study: (i) The positive-to-negative links (P/N) ratio, which measures the balance of species interactions or inhibition effects in microbiome networks, was significantly higher in UC patients, indicating loss of inhibition against potentially opportunistic "pathogens" associated with dysbiosis. (ii) Previous studies have reported conflicting evidence regarding species diversity and composition between UC patients and healthy controls. Here, significant differences were found at the major phylum and core/periphery scales, but not at the whole community level. Thus, we argue that the paradoxical results found in existing studies are due to the scale effect.

CONCLUSIONS

Our results reveal changes in the ecology and network structure of the gut mucosal microbiome that might be associated with UC, and these changes might provide potential therapeutic mechanisms of UC. The four opportunistic pathogens that were identified in the present study deserve further investigation in future studies.

CD27hiCD38hi plasmablasts are activated B cells of mixed origin with distinct function

Clinically important broadly reactive B cells evolve during multiple infections, with B cells re-activated after secondary infection differing from B cells activated after a primary infection. Here we studied CD27^highCD38^high plasmablasts from patients with a primary or secondary dengue virus infection. Three transcriptionally and functionally distinct clusters were identified. The largest cluster 0/1 was plasma cell-related, with cells coding for serotype cross-reactive antibodies of the IgG1 isotype, consistent with memory B cell activation during an extrafollicular response. Cells in clusters 2 and 3 expressed low levels of antibody genes and high levels of genes associated with oxidative phosphorylation, EIF2 pathway, and mitochondrial dysfunction. Clusters 2 and 3 showed a transcriptional footprint of T cell help, in line with activation from naive B cells or memory B cells. Our results contribute to the understanding of the parallel B cell activation events that occur in humans after natural primary and secondary infection.

Control of Immunity by the Microbiota

The immune system has coevolved with extensive microbial communities living on barrier sites that are collectively known as the microbiota. It is increasingly clear that microbial antigens and metabolites engage in a constant dialogue with the immune system, leading to microbiota-specific immune responses that occur in the absence of inflammation. This form of homeostatic immunity encompasses many arms of immunity, including B cell responses, innate-like T cells, and conventional T helper and T regulatory responses. In this review we summarize known examples of innate-like T cell and adaptive immunity to the microbiota, focusing on fundamental aspects of commensal immune recognition across different barrier sites. Furthermore, we explore how this cross talk is established during development, emphasizing critical temporal windows that establish long-term immune function. Finally, we highlight how dysregulation of immunity to the microbiota can lead to inflammation and disease, and we pinpoint outstanding questions and controversies regarding immune system-microbiota interactions.

ICOS ligand and IL-10 synergize to promote host-microbiota mutualism

Genome-wide association studies have identified ICOSLG, which encodes the inducible costimulator ligand (ICOSLG or ICOSL) as a susceptibility locus for inflammatory bowel disease. ICOSL has been implicated in the enhancement of pattern recognition receptor signaling in dendritic cells, induction of IL-10 production by CD4 T cells, and the generation of high-affinity antibodies to specific antigens-all of which can potentially explain its involvement in gastrointestinal inflammation. Here, we show that murine ICOSL deficiency results in significant enrichment of IL-10-producing CD4 T cells particularly in the proximal large intestine. Transient depletion of IL-10-producing cells from adult ICOSL-deficient mice induced severe colonic inflammation that was prevented when mice were first treated with metronidazole. ICOSL-deficient mice displayed reduced IgA and IgG antibodies in the colon mucus and impaired serum antibody recognition of microbial antigens, including flagellins derived from mucus-associated bacteria of the Lachnospiraceae family. Confirming the synergy between ICOSL and IL-10, ICOSL deficiency coupled with CD4-specific deletion of the Il10 gene resulted in juvenile onset colitis that was impeded when pups were fostered by ICOSL-sufficient dams. In this setting, we found that both maternally acquired and host-derived antibodies contribute to the life anti-commensal antibody repertoire that mediates this protection in early life. Collectively, our findings reveal a partnership between ICOSL-dependent anti-commensal antibodies and IL-10 in adaptive immune regulation of the microbiota in the large intestine. Furthermore, we identify ICOSL deficiency as an effective platform for exploring the functions of anti-commensal antibodies in host-microbiota mutualism.