Production and Function of Immunoglobulin A.

Among antibodies, IgA is unique because it has evolved to be secreted onto mucosal surfaces. The structure of IgA and the associated secretory component allow IgA to survive the highly proteolytic environment of mucosal surfaces but also substantially limit IgA's ability to activate effector functions on immune cells. Despite these characteristics, IgA is critical for both preventing enteric infections and shaping the local microbiome. IgA's function is determined by a distinct antigen-binding repertoire, composed of antibodies with a variety of specificities, from permissive polyspecificity to cross-reactivity to exquisite specificity to a single epitope, which act together to regulate intestinal bacteria. Development of the unique function and specificities of IgA is shaped by local cues provided by the gut-associated lymphoid tissue, driven by the constantly changing environment of the intestine and microbiota.

Gut-Innervating Nociceptor Neurons Regulate Peyer's Patch Microfold Cells and SFB Levels to Mediate Salmonella Host Defense.

Gut-innervating nociceptor sensory neurons respond to noxious stimuli by initiating protective responses including pain and inflammation; however, their role in enteric infections is unclear. Here, we find that nociceptor neurons critically mediate host defense against the bacterial pathogen Salmonella enterica serovar Typhimurium (STm). Dorsal root ganglia nociceptors protect against STm colonization, invasion, and dissemination from the gut. Nociceptors regulate the density of microfold (M) cells in ileum Peyer's patch (PP) follicle-associated epithelia (FAE) to limit entry points for STm invasion. Downstream of M cells, nociceptors maintain levels of segmentous filamentous bacteria (SFB), a gut microbe residing on ileum villi and PP FAE that mediates resistance to STm infection. TRPV1+ nociceptors directly respond to STm by releasing calcitonin gene-related peptide (CGRP), a neuropeptide that modulates M cells and SFB levels to protect against Salmonella infection. These findings reveal a major role for nociceptor neurons in sensing and defending against enteric pathogens.

Fasting-Refeeding Impacts Immune Cell Dynamics and Mucosal Immune Responses.

Nutritional status potentially influences immune responses; however, how nutritional signals regulate cellular dynamics and functionality remains obscure. Herein, we report that temporary fasting drastically reduces the number of lymphocytes by ∼50% in Peyer's patches (PPs), the inductive site of the gut immune response. Subsequent refeeding seemingly restored the number of lymphocytes, but whose cellular composition was conspicuously altered. A large portion of germinal center and IgA+ B cells were lost via apoptosis during fasting. Meanwhile, naive B cells migrated from PPs to the bone marrow during fasting and then back to PPs during refeeding when stromal cells sensed nutritional signals and upregulated CXCL13 expression to recruit naive B cells. Furthermore, temporal fasting before oral immunization with ovalbumin abolished the induction of antigen-specific IgA, failed to induce oral tolerance, and eventually exacerbated food antigen-induced diarrhea. Thus, nutritional signals are critical in maintaining gut immune homeostasis.

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.

M cell maturation and cDC activation determine the onset of adaptive immune priming in the neonatal Peyer's patch.

Early-life immune development is critical to long-term host health. However, the mechanisms that determine the pace of postnatal immune maturation are not fully resolved. Here, we analyzed mononuclear phagocytes (MNPs) in small intestinal Peyer's patches (PPs), the primary inductive site of intestinal immunity. Conventional type 1 and 2 dendritic cells (cDC1 and cDC2) and RORgt+ antigen-presenting cells (RORgt+ APC) exhibited significant age-dependent changes in subset composition, tissue distribution, and reduced cell maturation, subsequently resulting in a lack in CD4+ T cell priming during the postnatal period. Microbial cues contributed but could not fully explain the discrepancies in MNP maturation. Type I interferon (IFN) accelerated MNP maturation but IFN signaling did not represent the physiological stimulus. Instead, follicle-associated epithelium (FAE) M cell differentiation was required and sufficient to drive postweaning PP MNP maturation. Together, our results highlight the role of FAE M cell differentiation and MNP maturation in postnatal immune development.

The cholesterol metabolite 25-hydroxycholesterol restrains the transcriptional regulator SREBP2 and limits intestinal IgA plasma cell differentiation.

Diets high in cholesterol alter intestinal immunity. Here, we examined how the cholesterol metabolite 25-hydroxycholesterol (25-HC) impacts the intestinal B cell response. Mice lacking cholesterol 25-hydroxylase (CH25H), the enzyme generating 25-HC, had higher frequencies of immunoglobulin A (IgA)-secreting antigen-specific B cells upon immunization or infection. 25-HC did not affect class-switch recombination but rather restrained plasma cell (PC) differentiation. 25-HC was produced by follicular dendritic cells and increased in response to dietary cholesterol. Mechanistically, 25-HC restricted activation of the sterol-sensing transcription factor SREBP2, thereby regulating B cell cholesterol biosynthesis. Ectopic expression of SREBP2 in germinal center B cells induced rapid PC differentiation, whereas SREBP2 deficiency reduced PC output in vitro and in vivo. High-cholesterol diet impaired, whereas Ch25h deficiency enhanced, the IgA response against Salmonella and the resulting protection from systemic bacterial dissemination. Thus, a 25-HC-SREBP2 axis shapes the humoral response at the intestinal barrier, providing insight into the effect of high dietary cholesterol in intestinal immunity.

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.

TGF-ß1 impairs IgA class switch recombination and production in porcine Peyer's patches B cells.

Secretory IgA is crucial for preventing the invasion of entero-pathogens via intestinal mucosa. While it is well-established that Transforming growth factor ß1 (TGF-ß1) regulates IgA production in human and mouse B cells, our previous investigation revealed different functions of TGF-ß1 in IgA generation in pigs compared with humans and mice, with the underlying mechanism remaining elusive. In this study, IgM+ B cells from porcine Peyer's patches (PPs) were isolated and stimulated with recombinant porcine TGF-ß1 to evaluate the effect of TGF-ß1 on pigs. The results showed that antibody production from B cells of PPs was impaired by TGF-ß1 ex vivo. Furthermore, TGF-ß1 treatment led to a decrease in the expression of germ-line transcript αand postswitch transcript α. Moreover, we observed that TGF-ß1 predominantly inhibited the phosphorylation of p38-mitogen-activated protein kinases (MAPK), confirming the involvement of the p38-MAPK pathway in porcine IgA generation and IgA class switch recombination. The application of p38-MAPK inhibitor resulted in decreased B-cell differentiation levels. Collectively, this study demonstrates that exogenous TGF-ß1 restrains the production and class switch recombination of IgA antibodies by inhibiting p38-MAPK signaling in porcine PPs B cells, which may constitute a component of TGF-ß1-mediated inhibition of B-cell activation.

Lessons of Vascular Specialization From Secondary Lymphoid Organ Lymphatic Endothelial Cells.

Secondary lymphoid organs, such as lymph nodes, harbor highly specialized and compartmentalized niches. These niches are optimized to facilitate the encounter of naive lymphocytes with antigens and antigen-presenting cells, enabling optimal generation of adaptive immune responses. Lymphatic vessels of lymphoid organs are uniquely specialized to perform a staggering variety of tasks. These include antigen presentation, directing the trafficking of immune cells but also modulating immune cell activation and providing factors for their survival. Recent studies have provided insights into the molecular basis of such specialization, opening avenues for better understanding the mechanisms of immune-vascular interactions and their applications. Such knowledge is essential for designing better treatments for human diseases given the central role of the immune system in infection, aging, tissue regeneration and repair. In addition, principles established in studies of lymphoid organ lymphatic vessel functions and organization may be applied to guide our understanding of specialization of vascular beds in other organs.

CD200R activation on naïve T cells by B cells induces suppressive activity of T cells via IL-24.

CD200 is an anti-inflammatory protein that facilitates signal transduction through its receptor, CD200R, in cells, resulting in immune response suppression. This includes reducing M1-like macrophages, enhancing M2-like macrophages, inhibiting NK cell cytotoxicity, and downregulating CTL responses. Activation of CD200R has been found to modulate dendritic cells, leading to the induction or enhancement of Treg cells expressing Foxp3. However, the precise mechanisms behind this process are still unclear. Our previous study demonstrated that B cells in Peyer's patches can induce Treg cells, so-called Treg-of-B (P) cells, through STAT6 phosphorylation. This study aimed to investigate the role of CD200 in Treg-of-B (P) cell generation. To clarify the mechanisms, we used wild-type, STAT6 deficient, and IL-24 deficient T cells to generate Treg-of-B (P) cells, and antagonist antibodies (anti-CD200 and anti-IL-20RB), an agonist anti-CD200R antibody, CD39 inhibitors (ARL67156 and POM-1), a STAT6 inhibitor (AS1517499), and soluble IL-20RB were also applied. Our findings revealed that Peyer's patch B cells expressed CD200 to activate the CD200R on T cells and initiate the process of Treg-of-B (P) cells generation. CD200 and CD200R interaction triggers the phosphorylation of STAT6, which regulated the expression of CD200R, CD39, and IL-24 in T cells. CD39 regulated the expression of IL-24, which sustained the expression of CD223 and IL-10 and maintained the cell viability. In summary, the generation of Treg-of-B (P) cells by Peyer's patch B cells was through the CD200R-STAT6-CD39-IL-24 axis pathway.

Fantastic IgA plasma cells and where to find them.

IgA is produced in large quantities at mucosal surfaces by IgA+ plasma cells (PC), protecting the host from pathogens, and restricting commensal access to the subepithelium. It is becoming increasingly appreciated that IgA+ PC are not constrained to mucosal barrier sites. Rather, IgA+ PC may leave these sites where they provide both host defense and immunoregulatory function. In this review, we will outline how IgA+ PC are generated within the mucosae and how they subsequently migrate to their "classical" effector site, the gut lamina propria. From there we provide examples of IgA+ PC displacement from the gut to other parts of the body, referencing examples during homeostasis and inflammation. Lastly, we will speculate on mechanisms of IgA+ PC displacement to other tissues. Our aim is to provide a new perspective on how IgA+ PC are truly fantastic beasts of the immune system and identify new places to find them.

Peyer's Patch: Possible target for modulating the Gut-Brain-Axis through microbiota.

The gastrointestinal (GI) tract and the brain form bidirectional nervous, immune, and endocrine communications known as the gut-brain axis. Several factors can affect this axis; among them, various studies have focused on the microbiota and imply that alterations in microbiota combinations can influence both the brain and GI. Also, many studies have shown that the immune system has a vital role in varying gut microbiota combinations. In the current paper, we will review the multidirectional effects of gut microbiota, immune system, and nervous system on each other. Specifically, this review mainly focuses on the impact of Peyer's patches as a critical component of the gut immune system on the gut-brain axis through affecting the gut's microbial composition. In this way, some factors were discussed as proposed elements of missing gaps in this field.

Enteric-Coated Capsules Providing Reliable Site-Specific Drug Delivery to the Distal Ileum.

Nefecon, a targeted-release capsule formulation of budesonide approved for the reduction of proteinuria in adults with primary immunoglobulin A nephropathy, targets overproduction of galactose-deficient immunoglobulin A type 1 in the Peyer's patches at the gut mucosal level. To investigate whether the commercial formulation of Nefecon capsules reliably releases budesonide to the distal ileum, a human study was conducted with test capsules reproducing the delayed-release function of Nefecon capsules. Caffeine was included in the test capsules as a marker for capsule opening in the gut since it appears rapidly in saliva after release from orally administered dosage forms. Magnetic resonance imaging with black iron oxide was used to determine the capsule's position in the gut at the time caffeine was first measured in saliva and additionally to directly visualize dispersion of the capsule contents in the gut. In vitro dissolution results confirmed that the test capsules had the same delayed-release characteristics as Nefecon capsules. In 10 of 12 human volunteers, the capsule was demonstrated to open in the distal ileum; in the other two subjects, it opened just past the ileocecal junction. These results compared favorably with the high degree of variability seen in other published imaging studies of delayed-release formulations targeting the gut. The test capsules were shown to reliably deliver their contents to the distal ileum, the region with the highest concentration of Peyer's patches.

Two cases of small bowel necrosis due to intussusception secondary to abnormal proliferation of intestinal Peyer's patches in infants after MMR vaccination.

BACKGROUND: Intussusception is one of the most common acute abdominal conditions in pediatric patients, and if left untreated, it may result in intestinal necrosis and even death. The etiology of the disease is unknown and may be related to a variety of factors, and there are only limited reports of small bowel necrosis secondary to abnormal Peyer's node hyperplasia after MMR vaccination. CASE PRESENTATION: In this report, we present two infants who had an abnormal proliferation of Peyer's nodes secondary to intussusception eventually leading to small bowel necrosis after MMR vaccination. CONCLUSIONS: Intestinal necrosis and infectious shock are the most common causes of infant mortality, and early detection and management are critical.

Quercetin Improves Barrier Properties in Porcine Small Intestine but Not in Peyer's Patches.

Peyer's patches (PPs) are part of the gut-associated lymphatic tissue (GALT) and represent the first line of the intestinal immunological defense. They consist of follicles with lymphocytes and an overlying subepithelial dome with dendritic cells and macrophages, and they are covered by the follicle-associated epithelium (FAE). A sealed paracellular pathway in the FAE is crucial for the controlled uptake of luminal antigens. Quercetin is the most abundant plant flavonoid and has a barrier-strengthening effect on tight junctions (TJs), a protein complex that regulates the paracellular pathway. In this study, we aimed to analyze the effect of quercetin on porcine PPs and the surrounding villus epithelium (VE). We incubated both tissue types for 4 h in Ussing chambers, recorded the transepithelial electrical resistance (TEER), and measured the unidirectional tracer flux of [3H]-mannitol. Subsequently, we analyzed the expression, protein amount, and localization of three TJ proteins, claudin 1, claudin 2, and claudin 4. In the PPs, we could not detect an effect of quercetin after 4 h, neither on TEER nor on the [3H]-mannitol flux. In the VE, quercetin led to a higher TEER value, while the [3H]-mannitol flux was unchanged. The pore-forming claudin 2 was decreased while the barrier-forming claudin 4 was increased and the expression was upregulated. Claudin 1 was unchanged and all claudins could be located in the paracellular membrane by immunofluorescence microscopy. Our study shows the barrier-strengthening effect of quercetin in porcine VE by claudin 4 upregulation and a claudin 2 decrease. Moreover, it underlines the different barrier properties of PPs compared to the VE.

T cell help to B cells: Cognate and atypical interactions in peripheral and intestinal lymphoid tissues.

Enduring immunity against harmful pathogens depends on the generation of immunological memory. Serum immunoglobulins are constantly secreted by long-lived antibody-producing cells, which provide extended protection from recurrent exposures. These cells originate mainly from germinal center structures, wherein B cells introduce mutations to their immunoglobulin genes followed by affinity-based selection. Generation of high-affinity antibodies relies on physical contacts between T and B cells, a process that facilitates the delivery of fate decision signals. T-B cellular engagements are mediated through interactions between the T cell receptor and its cognate peptide presented on B cell major histocompatibility class II molecules. Here, we describe the cellular and molecular aspects of these cognate T-B interactions, and highlight exceptional cases, especially those arising at intestinal lymphoid organs, at which T cells provide help to B cells in an atypical manner, independent of T cell specificity.

Transnuclear mice reveal Peyer's patch iNKT cells that regulate B-cell class switching to IgG1.

Tissue-resident iNKT cells maintain tissue homeostasis and peripheral surveillance against pathogens; however, studying these cells is challenging due to their low abundance and poor recovery from tissues. We here show that iNKT transnuclear mice, generated by somatic cell nuclear transfer, have increased tissue resident iNKT cells. We examined expression of PLZF, T-bet, and RORγt, as well as cytokine/chemokine profiles, and found that both monoclonal and polyclonal iNKT cells differentiated into functional subsets that faithfully replicated those seen in wild-type mice. We detected iNKT cells from tissues in which they are rare, including adipose, lung, skin-draining lymph nodes, and a previously undescribed population in Peyer's patches (PP). PP-NKT cells produce the majority of the IL-4 in Peyer's patches and provide indirect help for B-cell class switching to IgG1 in both transnuclear and wild-type mice. Oral vaccination with α-galactosylceramide shows enhanced fecal IgG1 titers in iNKT cell-sufficient mice. Transcriptional profiling reveals a unique signature of PP-NKT cells, characterized by tissue residency. We thus define PP-NKT as potentially important for surveillance for mucosal pathogens.

Influenza A virus infection during pregnancy causes immunological changes in gut-associated lymphoid tissues of offspring mice.

Maternal influenza A virus (IAV) infection during pregnancy can affect offspring immune programming and development. Offspring born from influenza-infected mothers are at increased risk of neurodevelopmental disorders and have impaired respiratory mucosal immunity against pathogens. The gut-associated lymphoid tissue (GALT) represents a large proportion of the immune system in the body and plays an important role in gastrointestinal (GI) homeostasis. This includes immune modulation to antigens derived from food or microbes, gut microbiota composition, and gut-brain axis signaling. Therefore, in this study, we investigated the effect of maternal IAV infection on mucosal immunity of the GI tract in the offspring. There were no major anatomical changes to the gastrointestinal tract of offspring born to influenza-infected dams. In contrast, maternal IAV did affect the mucosal immunity of offspring, showing regional differences in immune cell profiles within distinct GALT. Neutrophils, monocytes/macrophages, CD4+ and CD8+ T cells infiltration was increased in the cecal patch offspring from IAV-infected dams. In the Peyer's patches, only activated CD4+ T cells were increased in IAV offspring. IL-6 gene expression was also elevated in the cecal patch but not in the Peyer's patches of IAV offspring. These findings suggest that maternal IAV infection perturbs homeostatic mucosal immunity in the offspring gastrointestinal tract. This could have profound ramifications on the gut-brain axis and mucosal immunity in the lungs leading to increased susceptibility to respiratory infections and neurological disorders in the offspring later in life.NEW & NOTEWORTHY Influenza A virus (IAV) infection during pregnancy is associated with changes in gut-associated lymphoid tissue (GALT) in the offspring in a region-dependent manner. Neutrophils and monocytes/macrophages were elevated in the cecal patch of offspring from infected dams. This increase in innate immune cell infiltration was not observed in the Peyer's patches. T cells were also elevated in the cecal patch but not in the Peyer's patches.

Diurnal changes in bacterial settlement on the Peyer's patch and surrounding mucosa in the rat ileum and its effect against the intestinal immune system.

Our previous study revealed the diurnal change in the indigenous bacteria settling on the terminal region of the rat ileum. In the present study, we investigated the diurnal change in indigenous bacteria on the most distal ileal Peyer's patch (PP) and surrounding ileal mucosa and explored how stimulation from indigenous bacteria for a day affects the intestinal immune system at the beginning of the light phase. Histological measurement revealed that bacteria adjacent to the follicle-associated epithelium of PP and to the villous epithelium of the surrounding ileal mucosa are more abundant at zeitgeber time (ZT)0 and ZT18 than at ZT12. On the other hand, tissue-section 16S rRNA amplicon sequencing revealed no significant difference between ZT0 and ZT12 in the bacterial composition on the ileal tissue including the PP. One-day treatment with an antibiotic (Abx) successfully impaired the settlement of bacteria around the ileal PP. In transcriptome analysis, 1-day Abx treatment led to the downregulation of several chemokines in both PP and ordinary ileal mucosa at ZT0. Histological analysis of the 1-day Abx group revealed decreases in both CD68+ macrophages in PP and naphthol AS-D chloroacetate esterase stain-positive mast cells in the ileal villi. Together, these findings suggest that the colonies of indigenous bacteria on the distal ileal PP and surrounding mucosa expand during the dark phase, which might lead to the expression of genes to regulate the intestinal immune system and contribute to the homeostasis of at least macrophages in PP and mast cells in the ileal mucosa.

Maternal Leukocytes and Infant Immune Programming during Breastfeeding.

The fetal immune system develops in a rather sterile environment relative to the outside world and, therefore, lacks antigenic education. Soon after birth, the newborn is exposed to the hostile environment of pathogens. Recently, animal- and limited human-based studies have indicated that help from the mother, upon transfer of leukocytes and their products via breast milk feeding, greatly assists the newborn's immune system. Here, I discuss the newest advances on how milk leukocytes impact early life immunity, with an emphasis on the development of the infant T cell repertoire and early immune responses in the periphery and gut-associated lymphoid tissue. A deeper understanding of these novel mechanistic insights may inform potential translational approaches to improving immunity in infants.