The sensing of environmental stimuli by follicular dendritic cells promotes immunoglobulin A generation in the gut

Microbiome's Universe: Impact on health, disease and cancer treatment

The human microbiome is a diverse ecosystem of microorganisms that reside in the body and influence various aspects of health and well-being. Recent advances in sequencing technology have brought to light microbial communities in organs and tissues that were previously considered sterile. The gut microbiota plays an important role in host physiology, including metabolic functions and immune modulation. Disruptions in the balance of the microbiome, known as dysbiosis, have been linked to diseases such as cancer, inflammatory bowel disease and metabolic disorders. In addition, the administration of antibiotics can lead to dysbiosis by disrupting the structure and function of the gut microbial community. Targeting strategies are the key to rebalancing the microbiome and fighting disease, including cancer, through interventions such as probiotics, fecal microbiota transplantation (FMT), and bacteria-based therapies. Future research must focus on understanding the complex interactions between diet, the microbiome and cancer in order to optimize personalized interventions. Multidisciplinary collaborations are essential if we are going to translate microbiome research into clinical practice. This will revolutionize approaches to cancer prevention and treatment.

The role of the gut microbiota in regulating responses to vaccination: current knowledge and future directions

Antigen-specific B and T cell responses play a critical role in vaccine-mediated protection against infectious diseases, but these responses are highly variable between individuals and vaccine immunogenicity is frequently sub-optimal in infants, the elderly and in people living in low- and middle-income countries. Although many factors such as nutrition, age, sex, genetics, environmental exposures, and infections may all contribute to variable vaccine immunogenicity, mounting evidence indicates that the gut microbiota is an important and targetable factor shaping optimal immune responses to vaccination. In this review, we discuss evidence from human, preclinical and experimental studies supporting a role for a healthy gut microbiota in mediating optimal vaccine immunogenicity, including the immunogenicity of COVID-19 vaccines. Furthermore, we provide an overview of the potential mechanisms through which this could occur and discuss strategies that could be used to target the microbiota to boost vaccine immunogenicity where it is currently sub-optimal.

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.

Critical role of the gut microbiota in immune responses and cancer immunotherapy

The gut microbiota plays a critical role in the progression of human diseases, especially cancer. In recent decades, there has been accumulating evidence of the connections between the gut microbiota and cancer immunotherapy. Therefore, understanding the functional role of the gut microbiota in regulating immune responses to cancer immunotherapy is crucial for developing precision medicine. In this review, we extract insights from state-of-the-art research to decipher the complicated crosstalk among the gut microbiota, the systemic immune system, and immunotherapy in the context of cancer. Additionally, as the gut microbiota can account for immune-related adverse events, we discuss potential interventions to minimize these adverse effects and discuss the clinical application of five microbiota-targeted strategies that precisely increase the efficacy of cancer immunotherapy. Finally, as the gut microbiota holds promising potential as a target for precision cancer immunotherapeutics, we summarize current challenges and provide a general outlook on future directions in this field.

Oral Administration of Cancer Vaccines: Challenges and Future Perspectives

Cancer vaccines, a burgeoning strategy in cancer treatment, are exploring innovative administration routes to enhance patient and medical staff experiences, as well as immunological outcomes. Among these, oral administration has surfaced as a particularly noteworthy approach, which is attributed to its capacity to ignite both humoral and cellular immune responses at systemic and mucosal tiers, thereby potentially bolstering vaccine efficacy comprehensively and durably. Notwithstanding this, the deployment of vaccines through the oral route in a clinical context is impeded by multifaceted challenges, predominantly stemming from the intricacy of orchestrating effective oral immunogenicity and necessitating strategic navigation through gastrointestinal barriers. Based on the immunogenicity of the gastrointestinal tract, this review critically analyses the challenges and recent advances and provides insights into the future development of oral cancer vaccines.

Intranasal Liposomal Formulation of Spike Protein Adjuvanted with CpG Protects and Boosts Heterologous Immunity of hACE2 Transgenic Mice to SARS-CoV-2 Infection

Mucosal vaccination appears to be suitable to protect against SARS-CoV-2 infection. In this study, we tested an intranasal mucosal vaccine candidate for COVID-19 that consisted of a cationic liposome containing a trimeric SARS-CoV-2 spike protein and CpG-ODNs, a Toll-like receptor 9 agonist, as an adjuvant. In vitro and in vivo experiments indicated the absence of toxicity following the intranasal administration of this vaccine formulation. First, we found that subcutaneous or intranasal vaccination protected hACE-2 transgenic mice from infection with the wild-type (Wuhan) SARS-CoV-2 strain, as shown by weight loss and mortality indicators. However, when compared with subcutaneous administration, the intranasal route was more effective in the pulmonary clearance of the virus and induced higher neutralizing antibodies and anti-S IgA titers. In addition, the intranasal vaccination afforded protection against gamma, delta, and omicron virus variants of concern. Furthermore, the intranasal vaccine formulation was superior to intramuscular vaccination with a recombinant, replication-deficient chimpanzee adenovirus vector encoding the SARS-CoV-2 spike glycoprotein (Oxford/AstraZeneca) in terms of virus lung clearance and production of neutralizing antibodies in serum and bronchial alveolar lavage (BAL). Finally, the intranasal liposomal formulation boosted heterologous immunity induced by previous intramuscular vaccination with the Oxford/AstraZeneca vaccine, which was more robust than homologous immunity.

Vitamin A and vitamin D induced nuclear hormone receptor activation and its impact on B cell differentiation and immunoglobulin production

Vitamin A and vitamin D metabolites are ligands to nuclear receptors - namely RAR, RXR and VDR. The activation of these receptors in human B cells impacts B cell maturation and function. In this review, we discuss how 9-cis retinoic acid (9cRA) and 1,25-dihydroxyvitamin D3 (calcitriol) individually or in conjunction, signal through their nuclear receptors and thereby impact B cell differentiation, immunoglobulin class switching to IgA at the expense of IgE, and also B cell migration and homing. Impact of the vitamin metabolites individually on B cell survival factors are well elucidated, be it the regulation of BAFF and APRIL, the induction of TGF-β or suppression of NF-κB. Very little is known about the impact of 9cRA and calcitriol together on B cells. Recently our group revealed that 9cRA and calcitriol together in the context of the B cell differentiation, induces naïve B cell differentiation into IgA+ plasmablasts, the functional and underlying molecular regulations however require further investigation. In conclusion, the conjunctional impact of these nuclear receptor ligands on B cell functionality is important to better understand B cell dependent clinical outcomes in allergy and autoimmunity. Within this review, we hypothesize that a balance between both vitamins is of utmost importance to provide a robust humoral immune response and a better treatment of disorders characterised by dysregulated immune responses such as IgE-dependent allergy or autoimmunity such as lupus erythematosus.

Long-term retention of antigens in germinal centers is controlled by the spatial organization of the follicular dendritic cell network

Germinal centers (GCs) require sustained availability of antigens to promote antibody affinity maturation against pathogens and vaccines. A key source of antigens for GC B cells are immune complexes (ICs) displayed on follicular dendritic cells (FDCs). Here we show that FDC spatial organization regulates antigen dynamics in the GC. We identify heterogeneity within the FDC network. While the entire light zone (LZ) FDC network captures ICs initially, only the central cells of the network function as the antigen reservoir, where different antigens arriving from subsequent immunizations colocalize. Mechanistically, central LZ FDCs constitutively express subtly higher CR2 membrane densities than peripheral LZ FDCs, which strongly increases the IC retention half-life. Even though repeated immunizations gradually saturate central FDCs, B cell responses remain efficient because new antigens partially displace old ones. These results reveal the principles shaping antigen display on FDCs during the GC reaction.

Elevated levels of enteric IgA in an unimmunised mouse model of Hyper IgM syndrome derived from gut-associated secondary lymph organs even in the absence of germinal centres

Introduction

Patients with Human Hyper IgM syndromes (HIGM) developed pulmonary and gastrointestinal infections since infancy and most patients have mutations in the CD40 ligand (CD40L) gene. Most HIGM patients compared to healthy subjects have higher/similar IgM and lower IgG, and IgA serum concentrations but gut antibody concentrations are unknown. CD40L on activated T-cells interacts with CD40 on B-cells, essential for the formation of germinal centres (GCs) inside secondary lymphoid organs (SLOs), where high-affinity antibodies, long-lived antibody-secreting plasma cells, and memory B-cells, are produced. C57BL6-CD40 ligand deficient mice (C57BL6-cd40l ^-/-), are a model of HIGM, because serum immunoglobulin concentrations parallel levels observed in HIGM patients and have higher faecal IgA concentrations. In mice, TGFβ and other cytokines induce IgA production.

Aims

To compare and evaluate B-cell populations and IgA-producing plasma cells in peritoneal lavage, non-gut-associated SLOs, spleen/inguinal lymph nodes (ILN), and gut-associated SLOs, mesenteric lymph nodes (MLN)/Peyer´s patches (PP) of unimmunised C57BL6-cd40l ^-/- and C57BL6-wild-type (WT) mice.

Material and methods

Peritoneal lavages, spleens, ILN, MLN, and PP from 8-10 weeks old C57BL6-cd40l ^-/- and WT mice, were obtained. Organ cryosections were analysed by immunofluorescence and B-cell populations and IgA-positive plasma cell suspensions by flow cytometry.

Results

In unimmunised WT mice, GCs were only observed in the gut-associated SLOs, but GCs were absent in all C57BL6-cd40l ^-/- SLOs. PP and MLN of C57BL6-cd40l ^-/- mice exhibited a significantly higher number of IgA-producing cells than WT mice. In the spleen and ILN of C57BL6-cd40l^{- /-} mice IgA-producing cells significantly decreased, while IgM-positive plasma cells increased. C57BL6-cd40l ^-/- B-1 cells were more abundant in all analysed SLOs, whereas in WT mice most B-1 cells were contained within the peritoneal cavity. C57BL6-cd40l ^-/- B-cells in MLN expressed a higher TGFβ receptor-1 than WT mice. Mouse strains small intestine microvilli (MV), have a similar frequency of IgA-positive cells.

Discussion

Together our results confirm the role of PP and MLN as gut inductive sites, whose characteristic features are to initiate an IgA preferential immune response production in these anatomical sites even in the absence of GCs. IgA antibodies play a pivotal role in neutralising, eliminating, and regulating potential pathogens and microorganisms in the gut.

IgA Nephropathy: Pleiotropic impact of Epstein-Barr virus infection on immunopathogenesis and racial incidence of the disease

IgA nephropathy (IgAN) is an autoimmune disease in which poorly galactosylated IgA1 is the antigen recognized by naturally occurring anti-glycan antibodies, leading to formation of nephritogenic circulating immune complexes. Incidence of IgAN displays geographical and racial disparity: common in Europe, North America, Australia, and east Asia, uncommon in African Americans, many Asian and South American countries, Australian Aborigines, and rare in central Africa. In analyses of sera and cells from White IgAN patients, healthy controls, and African Americans, IgAN patients exhibited substantial enrichment for IgA-expressing B cells infected with Epstein-Barr virus (EBV), leading to enhanced production of poorly galactosylated IgA1. Disparities in incidence of IgAN may reflect a previously disregarded difference in the maturation of the IgA system as related to the timing of EBV infection. Compared with populations with higher incidences of IgAN, African Americans, African Blacks, and Australian Aborigines are more frequently infected with EBV during the first 1-2 years of life at the time of naturally occurring IgA deficiency when IgA cells are less numerous than in late childhood or adolescence. Therefore, in very young children EBV enters "non-IgA" cells. Ensuing immune responses prevent infection of IgA B cells during later exposure to EBV at older ages. Our data implicate EBV-infected cells as the source of poorly galactosylated IgA1 in circulating immune complexes and glomerular deposits in patients with IgAN. Thus, temporal differences in EBV primo-infection as related to naturally delayed maturation of the IgA system may contribute to geographic and racial variations in incidence of IgAN.

GATA4 as a maestro of gut regionalization

How gut regionalization impacts microbiota and immunity is unclear. In this issue of Immunity, Earley et al. show that jejumal GATA4 expression controls bacteria colonization through retinol metabolism and IgA production. This metabolic-immune axis limits intestinal Th17 responses and immunopathology.

Diet-induced gut dysbiosis and inflammation: Key drivers of obesity-driven NASH

Sucrose, the primary circulating sugar in plants, contains equal amounts of fructose and glucose. The latter is the predominant circulating sugar in animals and thus the primary fuel source for various tissue and cell types in the body. Chronic excessive energy intake has, however, emerged as a major driver of obesity and associated pathologies including nonalcoholic fatty liver diseases (NAFLD) and the more severe nonalcoholic steatohepatitis (NASH). Consumption of a high-caloric, western-style diet induces gut dysbiosis and inflammation resulting in leaky gut. Translocation of gut-derived bacterial content promotes hepatic inflammation and ER stress, and when either or both of these are combined with steatosis, it can cause NASH. Here, we review the metabolic links between diet-induced changes in the gut and NASH. Furthermore, therapeutic interventions for the treatment of obesity and liver metabolic diseases are also discussed with a focus on restoring the gut-liver axis.

Role of the microbiome and its metabolites in ankylosing spondylitis

Ankylosing spondylitis (AS), a chronic condition that commonly influences the spine and sacroiliac joints, usually progresses to stiffness and progressive functional limitation. Its fundamental etiology and pathogenesis are likely multifactorial and remain elusive. As environmental factors, gut microbiota performs critical functions in the pathogenesis of AS through various mechanisms, including interacting with genes, enhancing intestinal permeability, activating the gut mucosa immune system, and affecting the intestinal microbiota metabolites. This review provides an overview of recent advances in investigating gut microbiota in AS pathogenesis and discusses potential methods for future therapeutic intervention.

Multiple rather than specific autoantibodies were identified in irritable bowel syndrome with HuProt™ proteome microarray

Immune activation and several autoantibodies might be involved in the pathophysiology of irritable bowel syndrome (IBS). We aimed to identify serum biomarkers for IBS by HuProt™ microarray. IBS patients met Rome III criteria were enrolled. Control groups included healthy controls (HCs) and disease controls (DCs). In stage I, we profiled sera from IBS and control groups with HuProt™ microarrays. Based on significant different proteins in stage I, IBS focused microarrays were constructed and validated in a larger cohort in stage II, then decision tree models were generated to establish a combination of biomarkers. In stage III, 4 purified proteins were verified by ELISA. Finally, we analyzed the correlation of autoantibodies with symptoms. In stage I, we identified 47 significant different proteins including 8 autoantibodies of IgG, 2 of IgA between IBS and HCs; 13 autoantibodies of IgG, 13 of IgA between IBS and DCs. In stage II, we found the positive rates of 14 IgG and IgA autoantibodies in IBS were significantly higher than HCs. Five autoantibodies of IgG and 7 IgA were comprehensively involved in differentiating IBS and HCs with the sensitivity and specificity to diagnose IBS as 40%–46.7% and 79.4%–86.3%. The median optical density value of ELAVL4 (IgG) and PIGP (IgA) were significantly higher in IBS than HCs. Parts of autoantibodies above were related to IBS symptoms. We found a combination of autoantibodies to differentiate IBS with HCs, but no specific autoantibodies could serve as serum biomarkers for IBS.

New perspectives on the regulation of germinal center reaction via αvβ8- mediated activation of TGFβ

Transforming growth factor-β (TGFβ) is a long-known modulator of immune responses but has seemingly contradictory effects on B cells. Among cytokines, TGFβ has the particularity of being produced and secreted in a latent form and must be activated before it can bind to its receptor and induce signaling. While the concept of controlled delivery of TGFβ signaling via αvβ8 integrin-mediated activation has gained some interest in the field of mucosal immunity, the role of this molecular mechanism in regulating T-dependent B cell responses is just emerging. We review here the role of TGFβ and its activation, in particular by αvβ8 integrin, in the regulation of mucosal IgA responses and its demonstrated and putative involvement in regulating germinal center (GC) B cell responses. We examine both the direct effect of TGFβ on GC B cells and its ability to modulate the functions of helper cells, namely follicular T cells (Tfh and Tfr) and follicular dendritic cells. Synthetizing recently published works, we reconcile apparently conflicting data and propose an innovative and unified view on the regulation of the GC reaction by TGFβ, highlighting the role of its activation by αvβ8 integrin.

Investigating the Mechanism of Germinal Center Shutdown

Germinal centers (GCs) are transient structures where affinity maturation of B cells gives rise to high affinity plasma and memory cells. The mechanism of GC shutdown is unclear, despite being an important phenomenon maintaining immune homeostasis. In this study, we used a mathematical model to identify mechanisms that can independently promote contraction of GCs leading to shutdown. We show that GC shutdown can be promoted by antigen consumption by B cells, antigen masking by soluble antibodies, alterations in follicular dendritic cell (FDC) network area, modulation of immune complex cycling rate constants, alterations in T follicular helper signaling, increased terminal differentiation and reduced B cell division capacity. Proposed mechanisms promoted GC contraction by ultimately decreasing the number of B cell divisions and recycling cells. Based on the in-silico predictions, we suggest a combination of experiments that can be potentially employed by future studies to unravel the mechanistic basis of GC shutdown such as measurements of the density of pMHC presentation of B cells, FDC network size per B cell, fraction of cells expressing differentiation markers. We also show that the identified mechanisms differentially affect the efficiency of GC reaction estimated based on the quantity and quality of resulting antibodies.

Cecal Patches Generate Abundant IgG2b-Bearing B Cells That Are Reactive to Commensal Microbiota

Gut-associated lymphoid tissue (GALT), such as Peyer's patches (PPs), are key inductive sites that generate IgA⁺ B cells, mainly through germinal center (GC) responses. The generation of IgA⁺ B cells is promoted by the presence of gut microbiota and dietary antigens. However, the function of GALT in the large intestine, such as cecal patches (CePs) and colonic patches (CoPs), and their regulatory mechanisms remain largely unknown. In this study, we demonstrate that the CePs possess more IgG2b⁺ B cells and have fewer IgA⁺ B cells than those in PPs from BALB/c mice with normal gut microbiota. Gene expression analysis of postswitched transcripts supported the differential expression of dominant antibody isotypes in B cells in GALT. Germ-free (GF) mice showed diminished GC B cells and had few IgA⁺ or IgG2b⁺ switched B cells in both the small and large intestinal GALT. In contrast, myeloid differentiation factor 88- (MyD88-) deficient mice exhibited decreased GC B cells and presented with reduced numbers of IgG2b⁺ B cells in CePs but not in PPs. Using ex vivo cell culture, we showed that CePs have a greater capacity to produce total and microbiota-reactive IgG2b, in addition to microbiota-reactive IgA, than the PPs. In line with the frequency of GC B cells and IgG2b⁺ B cells in CePs, there was a decrease in the levels of microbiota-reactive IgG2b and IgA in the serum of GF and MyD88-deficient mice. These data suggest that CePs have a different antibody production profile compared to PPs. Furthermore, the innate immune signals derived from gut microbiota are crucial for generating the IgG2b antibodies in CePs.

TCDD exposure alters fecal IgA concentrations in male and female mice

Background

Activation of the aryl hydrocarbon receptor (AhR) can alter diurnal rhythms including those for innate lymphoid cell numbers, cytokine and hormone levels, and feeding behaviors. Because immune responses and antibody levels are modulated by exposure to AhR agonists, we hypothesized that some of the variation previously reported for the effects of AhR activation on fecal secretory immunoglobulin A (sIgA) levels could be explained by dysregulation of the diurnal sIgA rhythm.

Methods

C57Bl/6 J mice were exposed to peanut oil or 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD, 10 or 40 μg/Kg) and fecal sIgA levels were determined in samples collected every 4 h over 4 days.

Results

Fecal sIgA concentrations were not significantly different between light and dark phases of the photoperiod in either male or female mice, and there were no significant circadian rhythms observed, but TCDD exposure significantly altered both fecal mesor sIgA and serum IgA concentrations, in parallel, in male (increased) and female (biphasic) mice.

Conclusions

AhR activation can contribute to the regulation of steady state IgA/sIgA concentrations.

Supplementary Information

The online version contains supplementary material available at 10.1186/s40360-022-00563-9.

The role of retinoic acid in the production of immunoglobulin A

Vitamin A and its derivative retinoic acid (RA) play important roles in the regulation of mucosal immunity. The effect of vitamin A metabolism on T lymphocyte immunity has been well documented, but its role in mucosal B lymphocyte regulation is less well described. Intestinal immunoglobulin A (IgA) is key in orchestrating a balanced gut microbiota composition. Here, we describe the contribution of RA to IgA class switching in tissues including the lamina propria, mesenteric lymph nodes, Peyer's patches and isolated lymphoid follicles. RA can either indirectly skew T cells or directly affect B cell differentiation. IgA levels in healthy individuals are under the control of the metabolism of vitamin A, providing a steady supply of RA. However, IgA levels are altered in inflammatory bowel disease patients, making control of the metabolism of vitamin A a potential therapeutic target. Thus, dietary vitamin A is a key player in regulating IgA production within the intestine, acting via multiple immunological pathways.

Gut microbiota and immunity relevance in eubiosis and dysbiosis

Human gut is colonized by numerous microorganisms, in which bacteria present the highest proportion of this colonization that live in a symbiotic relationship with the host. This microbial collection is commonly known as the microbiota. The gut microbiota can mediate gut epithelial and immune cells interaction through vitamins synthesis or metabolic products. The microbiota plays a vital role in growth and development of the main components of human’s adaptive and innate immune system, while the immune system regulates host-microbe symbiosis. On the other hand, negative alteration in gut microbiota composition or gut dysbiosis, can disturb immune responses. This review highlights the gut microbiota-immune system cross-talk in both eubiosis and dysbiosis.

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.

Impact of gut microbiome on skin health: gut-skin axis observed through the lenses of therapeutics and skin diseases

The human intestine hosts diverse microbial communities that play a significant role in maintaining gut-skin homeostasis. When the relationship between gut microbiome and the immune system is impaired, subsequent effects can be triggered on the skin, potentially promoting the development of skin diseases. The mechanisms through which the gut microbiome affects skin health are still unclear. Enhancing our understanding on the connection between skin and gut microbiome is needed to find novel ways to treat human skin disorders. In this review, we systematically evaluate current data regarding microbial ecology of healthy skin and gut, diet, pre- and probiotics, and antibiotics, on gut microbiome and their effects on skin health. We discuss potential mechanisms of the gut-skin axis and the link between the gut and skin-associated diseases, such as psoriasis, atopic dermatitis, acne vulgaris, rosacea, alopecia areata, and hidradenitis suppurativa. This review will increase our understanding of the impacts of gut microbiome on skin conditions to aid in finding new medications for skin-associated diseases.

Mechanism of the Gut Microbiota Colonization Resistance and Enteric Pathogen Infection

The mammalian gut microbial community, known as the gut microbiota, comprises trillions of bacteria, which co-evolved with the host and has an important role in a variety of host functions that include nutrient acquisition, metabolism, and immunity development, and more importantly, it plays a critical role in the protection of the host from enteric infections associated with exogenous pathogens or indigenous pathobiont outgrowth that may result from healthy gut microbial community disruption. Microbiota evolves complex mechanisms to restrain pathogen growth, which included nutrient competition, competitive metabolic interactions, niche exclusion, and induction of host immune response, which are collectively termed colonization resistance. On the other hand, pathogens have also developed counterstrategies to expand their population and enhance their virulence to cope with the gut microbiota colonization resistance and cause infection. This review summarizes the available literature on the complex relationship occurring between the intestinal microbiota and enteric pathogens, describing how the gut microbiota can mediate colonization resistance against bacterial enteric infections and how bacterial enteropathogens can overcome this resistance as well as how the understanding of this complex interaction can inform future therapies against infectious diseases.

Visualization and functional characterization of lymphoid organ fibroblasts

Fibroblastic reticular cells (FRCs) are specialized stromal cells of lymphoid organs that generate the structural foundation of the tissue and actively interact with immune cells. Distinct FRC subsets position lymphocytes and myeloid cells in specialized niches where they present processed or native antigen and provide essential growth factors and cytokines for immune cell activation and differentiation. Niche‐specific functions of FRC subpopulations have been defined using genetic targeting, high‐dimensional transcriptomic analyses, and advanced imaging methods. Here, we review recent findings on FRC‐immune cell interaction and the elaboration of FRC development and differentiation. We discuss how imaging approaches have not only shaped our understanding of FRC biology, but have critically advanced the niche concept of immune cell maintenance and control of immune reactivity.

The Microbial Composition of Penaeid Shrimps' Hepatopancreas Is Modulated by Hemocyanin

Aquatic organisms have to produce proteins or factors that help maintain a stable relationship with microbiota and prevent colonization by pathogenic microorganisms. In crustaceans and other aquatic invertebrates, relatively few of these host factors have been characterized. In this study, we show that the respiratory glycoprotein hemocyanin is a crucial host factor that modulates microbial composition and diversity in the hepatopancreas of penaeid shrimp. Diseased penaeid shrimp (Penaeus vannamei), had an empty gastrointestinal tract with atrophied hepatopancreas, expressed low hemocyanin, and high total bacterial abundance, with Vibrio as the dominant bacteria. Similarly, shrimp depleted of hemocyanin had mitochondrial depolarization, increased reactive oxygen species (ROS) levels, and dysregulation of several energy metabolism-related genes. Hemocyanin silencing together with ROS scavenger (N-acetylcysteine) treatment improved microbial diversity and decreased Vibrio dominance in the hepatopancreas. However, fecal microbiota transplantation after hemocyanin knockdown could not restore the microbial composition in the hepatopancreas. Collectively, our data provide, to our knowledge, new insight into the pivotal role of hemocyanin in modulating microbial composition in penaeid shrimp hepatopancreas via its effect on mitochondrial integrity, energy metabolism, and ROS production.

Development of follicular dendritic cells in lymph nodes depends on retinoic acid-mediated signaling

Specialized stromal cells occupy and help define B- and T-cell domains, which are crucial for proper functioning of our immune system. Signaling through lymphotoxin and TNF receptors is crucial for the development of different stromal subsets, which are thought to arise from a common precursor. However, mechanisms that control the selective generation of the different stromal phenotypes are not known. Using in vitro cultures of embryonic mouse stromal cells, we show that retinoic acid-mediated signaling is important for the differentiation of precursors towards the Cxcl13pos follicular dendritic cell (FDC) lineage, and also blocks lymphotoxin-mediated Ccl19pos fibroblastic reticular cell lineage differentiation. Accordingly, at the day of birth we observe the presence of Cxcl13posCcl19neg/low and Cxcl13neg/lowCcl19pos cells within neonatal lymph nodes. Furthermore, ablation of retinoic acid receptor signaling in stromal precursors early after birth reduces Cxcl13 expression, and complete blockade of retinoic acid signaling prevents the formation of FDC networks in lymph nodes.

Characterization of human FDCs reveals regulation of T cells and antigen presentation to B cells

Stromal-derived follicular dendritic cells (FDCs) are essential for germinal centers (GCs), the site where B cells maturate their antibodies. FDCs present native antigen to B cells and maintain a CXCL13 gradient to form the B cell follicle. Yet despite their essential role, the transcriptome of human FDCs remains undefined. Using single-cell RNA sequencing and microarray, we provided the transcriptome of these enigmatic cells as a comprehensive resource. Key genes were validated by flow cytometry and microscopy. Surprisingly, marginal reticular cells (MRCs) rather than FDCs expressed B cell activating factor (BAFF). Furthermore, we found that human FDCs expressed TLR4 and can alter antigen availability in response to pathogen-associated molecular patterns (PAMPs). High expression of PD-L1 and PD-L2 on FDCs activated PD1 on T cells. In addition, we found expression of genes related to T cell regulation, such as HLA-DRA, CD40, and others. These data suggest intimate contact between human FDCs and T cells.

Mega doses of retinol: A possible immunomodulation in Covid-19 illness in resource-limited settings

Of all the nutrients, vitamin A has been the most extensively evaluated for its impact on immunity. There are three main forms of vitamin A, retinol, retinal and retinoic acid (RA) with the latter being most biologically active and all-trans-RA (ATRA) its main derivative. Vitamin A is a key regulator of the functions of various innate and adaptive immune cells and promotes immune-homeostasis. Importantly, it augments the interferon-based innate immune response to RNA viruses decreasing RNA virus replication. Several clinical trials report decreased mortality in measles and Ebola with vitamin A supplementation.During the Covid-19 pandemic interventions such as convalescent plasma, antivirals, monoclonal antibodies and immunomodulator drugs have been tried but most of them are difficult to implement in resource-limited settings. The current review explores the possibility of mega dose vitamin A as an affordable adjunct therapy for Covid-19 illness with minimal reversible side effects. Insight is provided into the effect of vitamin A on ACE-2 expression in the respiratory tract and its association with the prognosis of Covid-19 patients. Vitamin A supplementation may aid the generation of protective immune response to Covid-19 vaccines. An overview of the dosage and safety profile of vitamin A is presented along with recommended doses for prophylactic/therapeutic use in randomised controlled trials in Covid-19 patients.

The Gut Microbiome, Metformin, and Aging

Metformin has been extensively used for the treatment of type 2 diabetes, and it may also promote healthy aging. Despite its widespread use and versatility, metformin's mechanisms of action remain elusive. The gut typically harbors thousands of bacterial species, and as the concentration of metformin is much higher in the gut as compared to plasma, it is plausible that microbiome-drug-host interactions may influence the functions of metformin. Detrimental perturbations in the aging gut microbiome lead to the activation of the innate immune response concomitant with chronic low-grade inflammation. With the effectiveness of metformin in diabetes and antiaging varying among individuals, there is reason to believe that the gut microbiome plays a role in the efficacy of metformin. Metformin has been implicated in the promotion and maintenance of a healthy gut microbiome and reduces many age-related degenerative pathologies. Mechanistic understanding of metformin in the promotion of a healthy gut microbiome and aging will require a systems-level approach. Expected final online publication date for the Annual Review of Pharmacology and Toxicology, Volume 62 is January 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Redefining the Role of Lymphotoxin Beta Receptor in the Maintenance of Lymphoid Organs and Immune Cell Homeostasis in Adulthood

Lymphotoxin beta receptor (LTβR) is a promising therapeutic target in autoimmune and infectious diseases as well as cancer. Mice with genetic inactivation of LTβR display multiple defects in development and organization of lymphoid organs, mucosal immune responses, IgA production and an autoimmune phenotype. As these defects are imprinted in embryogenesis and neonate stages, the impact of LTβR signaling in adulthood remains unclear. Here, to overcome developmental defects, we generated mice with inducible ubiquitous genetic inactivation of LTβR in adult mice (iLTβR^Δ/Δ mice) and redefined the role of LTβR signaling in organization of lymphoid organs, immune response to mucosal bacterial pathogen, IgA production and autoimmunity. In spleen, postnatal LTβR signaling is required for development of B cell follicles, follicular dendritic cells (FDCs), recruitment of neutrophils and maintenance of the marginal zone. Lymph nodes of iLTβR^Δ/Δ mice were reduced in size, lacked FDCs, and had disorganized subcapsular sinus macrophages. Peyer`s patches were smaller in size and numbers, and displayed reduced FDCs. The number of isolated lymphoid follicles in small intestine and colon were also reduced. In contrast to LTβR^-/- mice, iLTβR^Δ/Δ mice displayed normal thymus structure and did not develop signs of systemic inflammation and autoimmunity. Further, our results suggest that LTβR signaling in adulthood is required for homeostasis of neutrophils, NK, and iNKT cells, but is dispensable for the maintenance of polyclonal IgA production. However, iLTβR^Δ/Δ mice exhibited an increased sensitivity to C. rodentium infection and failed to develop pathogen-specific IgA responses. Collectively, our study uncovers new insights of LTβR signaling in adulthood for the maintenance of lymphoid organs, neutrophils, NK and iNKT cells, and IgA production in response to mucosal bacterial pathogen.

Germinal Centre Shutdown

Germinal Centres (GCs) are transient structures in secondary lymphoid organs, where affinity maturation of B cells takes place following an infection. While GCs are responsible for protective antibody responses, dysregulated GC reactions are associated with autoimmune disease and B cell lymphoma. Typically, ‘normal’ GCs persist for a limited period of time and eventually undergo shutdown. In this review, we focus on an important but unanswered question – what causes the natural termination of the GC reaction? In murine experiments, lack of antigen, absence or constitutive T cell help leads to premature termination of the GC reaction. Consequently, our present understanding is limited to the idea that GCs are terminated due to a decrease in antigen access or changes in the nature of T cell help. However, there is no direct evidence on which biological signals are primarily responsible for natural termination of GCs and a mechanistic understanding is clearly lacking. We discuss the present understanding of the GC shutdown, from factors impacting GC dynamics to changes in cellular interactions/dynamics during the GC lifetime. We also address potential missing links and remaining questions in GC biology, to facilitate further studies to promote a better understanding of GC shutdown in infection and immune dysregulation.

Nutritional Modulation of the Microbiome and Immune Response

The evolution of the immune system, diet, and the microbiome are interconnected. Dietary metabolites modulate the cells of the immune system both directly and indirectly via shifts in the composition of the intestinal microbiota and its products. As a result, overconsumption and malnutrition can have substantial effects on immune responses and inflammation. In resource-rich nations, diets high in processed foods, fat, and sugar can contribute to chronic inflammatory conditions, which are on the rise worldwide. Conversely, in resource-poor countries, malnutrition associated with food insecurity can lead to immunodeficiencies and shifts in the microbiome that drive intestinal inflammation. Developing a deeper understanding of the relationship between diet, microbiota, and the immune system is of huge importance, given its impact on inflammatory diseases and its potential as an easily modifiable mediator of immunomodulation.

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 Microbiota-Host Interactions in Inborn Errors of Immunity

Inborn errors of immunity (IEI) are a group of disorders that are mostly caused by genetic mutations affecting immune host defense and immune regulation. Although IEI present with a wide spectrum of clinical features, in about one third of them various degrees of gastrointestinal (GI) involvement have been described and for some IEI the GI manifestations represent the main and peculiar clinical feature. The microbiome plays critical roles in the education and function of the host's innate and adaptive immune system, and imbalances in microbiota-immunity interactions can contribute to intestinal pathogenesis. Microbial dysbiosis combined to the impairment of immunosurveillance and immune dysfunction in IEI, may favor mucosal permeability and lead to inflammation. Here we review how immune homeostasis between commensals and the host is established in the gut, and how these mechanisms can be disrupted in the context of primary immunodeficiencies. Additionally, we highlight key aspects of the first studies on gut microbiome in patients affected by IEI and discuss how gut microbiome could be harnessed as a therapeutic approach in these diseases.

Associations of Genetic Variants Contributing to Gut Microbiota Composition in Immunoglobin A Nephropathy

The gut microbiota and host genetics are implicated in the pathogenesis of IgAN. Recent studies have confirmed that microbial compositions are heritable (microbiome quantitative trait loci [QTL]).

The gut microbiota has been implicated in immunoglobin A nephropathy (IgAN) because the intestinal immune response is assumed to be one of the disease triggers. Since the microbial composition is heritable, we hypothesize that genetic variants controlling gut microbiota composition may be associated with susceptibility to IgAN or clinical phenotypes. A total of 175 gut-microbiome-associated genetic variants were retrieved from the Genome-Wide Association Study (GWAS) Catalog. Genetic associations were examined in 1,511 patients with IgAN and 4,469 controls. Subphenotype associations and microbiome annotations were undertaken for a better understanding of how genes shaped phenotypes. Likely candidate microbes suggested in genetic associations were validated using 16S rRNA gene sequencing in two independent data sets with 119 patients with IgAN and 45 controls in total. Nine genetic variants were associated with susceptibility to IgAN. Risk genotypes of LYZL1 were associated with higher serum levels of galactose-deficient IgA1 (Gd-IgA1). Other significant findings included the associations between the risk genotype of SIPA1L3 and early age at onset, PLTP and worse kidney function, and AL365503.1 and more severe hematuria. Besides, risk genotypes of LYZL1 and SIPA1L3 were associated with decreased abundances of Dialister and Bacilli, respectively. Risk genotypes of PLTP and AL365503.1 were associated with increased abundances of Erysipelotrichaceae and Lachnobacterium, respectively. 16S data validated a decreased tendency for Dialister and an increased tendency for Erysipelotrichaceae in IgAN. In this pilot study, our results provided preliminary evidence that the gut microbiota in IgAN was affected by host genetics and shed new light on candidate bacteria for future pathogenesis studies.

IMPORTANCE The gut microbiota and host genetics are implicated in the pathogenesis of IgAN. Recent studies have confirmed that microbial compositions are heritable (microbiome quantitative trait loci [QTL]). The relationship between host genetics and the microbiota and the role of the microbiota in IgAN are unclear. We retrieved the GWAS Catalog and associated microbiome QTL in IgAN, observing that nine genetic variants were associated with IgAN susceptibility and some clinical phenotypes. In a consistent way, the decreased abundance of Dialister was associated with higher serum levels of Gd-IgA1, and 16S rRNA gene analysis confirmed the decreased abundance of Dialister in IgAN. These data provided preliminary evidence that the gut microbiota in IgAN was affected by host genetics, which is a new strategy for future pathogenesis and intervention studies.

Recent Insights into Cellular Crosstalk in Respiratory and Gastrointestinal Mucosal Immune Systems

The human body is continuously threatened by pathogens, and the immune system must maintain a balance between fighting infection and becoming over-activated. Mucosal surfaces cover several anatomically diverse organs throughout the body, such as the respiratory and gastrointestinal tracts, and are directly exposed to the external environment. Various pathogens invade the body through mucosal surfaces, making the mucosa the frontline of immune defense. The immune systems of various mucosal tissues display distinctive features that reflect the tissues' anatomical and functional characteristics. This review discusses the cellular components that constitute the respiratory and gastrointestinal tracts; in particular, it highlights the complex interactions between epithelial and immune cells to induce Ag-specific immune responses in the lung and gut. This information on mucosal immunity may facilitate understanding of the defense mechanisms against infectious agents that invade mucosal surfaces, such as severe acute respiratory syndrome coronavirus 2, and provide insight into effective vaccine development.

LT-K63 Enhances B Cell Activation and Survival Factors in Neonatal Mice That Translates Into Long-Lived Humoral Immunity

Adjuvants enhance magnitude and duration of immune responses induced by vaccines. In this study we assessed in neonatal mice if and how the adjuvant LT-K63 given with a pneumococcal conjugate vaccine, Pnc1-TT, could affect the expression of tumor necrosis factor receptor (TNF-R) superfamily members, known to be involved in the initiation and maintenance of antibody responses; B cell activating factor receptor (BAFF-R) and B cell maturation antigen (BCMA) and their ligands, BAFF, and a proliferation inducing ligand (APRIL). Initially we assessed the maturation status of different B cell populations and their expression of BAFF-R and BCMA. Neonatal mice had dramatically fewer B cells than adult mice and the composition of different subsets within the B cell pool differed greatly. Proportionally newly formed B cells were most abundant, but they had diminished BAFF-R expression which could explain low proportions of marginal zone and follicular B cells observed. Limited BCMA expression was also detected in neonatal pre-plasmablasts/plasmablasts. LT-K63 enhanced vaccine-induced BAFF-R expression in splenic marginal zone, follicular and newly formed B cells, leading to increased plasmablast/plasma cells, and their enhanced expression of BCMA in spleen and bone marrow. Additionally, the induction of BAFF and APRIL expression occurred early in neonatal mice immunized with Pnc1-TT either with or without LT-K63. However, BAFF⁺ and APRIL⁺ cells in spleens were maintained at a higher level in mice that received the adjuvant. Furthermore, the early increase of APRIL⁺ cells in bone marrow was more profound in mice immunized with vaccine and adjuvant. Finally, we assessed, for the first time in neonatal mice, accessory cells of the plasma cell niche in bone marrow and their secretion of APRIL. We found that LT-K63 enhanced the frequency and APRIL expression of eosinophils, macrophages, and megakaryocytes, which likely contributed to plasma cell survival, even though APRIL⁺ cells showed a fast decline. All this was associated with enhanced, sustained vaccine-specific antibody-secreting cells in bone marrow and persisting vaccine-specific serum antibodies. Our study sheds light on the mechanisms behind the adjuvanticity of LT-K63 and identifies molecular pathways that should be triggered by vaccine adjuvants to induce sustained humoral immunity in early life.

Dendritic Cell Subsets in Intestinal Immunity and Inflammation

The mammalian intestine is a complex environment that is constantly exposed to Ags derived from food, microbiota, and metabolites. Intestinal dendritic cells (DC) have the responsibility of establishing oral tolerance against these Ags while initiating immune responses against mucosal pathogens. We now know that DC are a heterogeneous population of innate immune cells composed of classical and monocyte-derived DC, Langerhans cells, and plasmacytoid DC. In the intestine, DC are found in organized lymphoid tissues, such as the mesenteric lymph nodes and Peyer's patches, as well as in the lamina propria. In this Brief Review, we review recent work that describes a division of labor between and collaboration among gut DC subsets in the context of intestinal homeostasis and inflammation. Understanding relationships between DC subtypes and their biological functions will rationalize oral vaccine design and will provide insights into treatments that quiet pathological intestinal inflammation.

Past, Present, and Future of Gastrointestinal Microbiota Research in Cats

The relationship between microbial community and host has profound effects on the health of animals. A balanced gastrointestinal (GI) microbial population provides nutritional and metabolic benefits to its host, regulates the immune system and various signaling molecules, protects the intestine from pathogen invasion, and promotes a healthy intestinal structure and an optimal intestinal function. With the fast development of next-generation sequencing, molecular techniques have become standard tools for microbiota research, having been used to demonstrate the complex intestinal ecosystem. Similarly to other mammals, the vast majority of GI microbiota in cats (over 99%) is composed of the predominant bacterial phyla Firmicutes, Bacteroidetes, Actinobacteria, and Proteobacteria. Many nutritional and clinical studies have shown that cats' microbiota can be affected by several different factors including body condition, age, diet, and inflammatory diseases. All these factors have different size effects, and some of these may be very minor, and it is currently unknown how important these are. Further research is needed to determine the functional variations in the microbiome in disease states and in response to environmental and/or dietary modulations. Additionally, further studies are also needed to explain the intricate relationship between GI microbiota and the genetics and immunity of its host. This review summarizes past and present knowledge of the feline GI microbiota and looks into the future possibilities and challenges of the field.

Early-life programming of mesenteric lymph node stromal cell identity by the lymphotoxin pathway regulates adult mucosal immunity

Redundant mechanisms support immunoglobulin A (IgA) responses to intestinal antigens. These include multiple priming sites [mesenteric lymph nodes (MLNs), Peyer's patches, and isolated lymphoid follicles] and various cytokines that promote class switch to IgA, even in the absence of T cells. Despite these backup mechanisms, vaccination against enteric pathogens such as rotavirus has limited success in some populations. Genetic and environmental signals experienced during early life are known to influence mucosal immunity, yet the mechanisms for how these exposures operate remain unclear. Here, we used rotavirus infection to follow antigen-specific IgA responses through time and in different gut compartments. Using genetic and pharmacological approaches, we tested the role of the lymphotoxin (LT) pathway-known to support IgA responses-at different developmental stages. We found that LT-β receptor (LTβR) signaling in early life programs intestinal IgA responses in adulthood by affecting antibody class switch recombination to IgA and subsequent generation of IgA antibody-secreting cells within an intact MLN. In addition, early-life LTβR signaling dictates the phenotype and function of MLN stromal cells to support IgA responses in the adult. Collectively, our studies uncover new mechanistic insights into how early-life LTβR signaling affects mucosal immune responses during adulthood.

The Gut Microbiome: Human Health and Inflammatory Skin Diseases

The human microbiome is a rich environment consisting of bacteria, fungi and other commensal microorganisms of the gut, mucosa and skin. The functional role of the gut microbiome includes facilitation in metabolism of macronutrients, maturation of the immune system, and production of pro- or anti-inflammatory signaling molecules and peptides. The identification of these resident organisms has brought about a new understanding of disease processes. Nevertheless, more questions remain regarding the interactions within the microbiome, its interactions with the host, and its contributions to the pathophysiology of disease. The purpose of this review is to examine the existing medical literature to highlight the role of the gut microbiome in human health, also paying attention to its role in several inflammatory skin diseases, namely atopic dermatitis, psoriasis, and rosacea.

Impact of birth weight and neonatal nutritional interventions with micronutrients and bovine colostrum on the development of piglet immune response during the peri-weaning period

Immune system development of piglets is influenced by birth weight and colostrum and milk intake. Moreover, the dam transfer to piglets of vitamins A and D and copper, which play important role in immunity, is limited during lactation. In this study, we evaluated the potential of maternal and neonatal supplementations with vitamins A and D and copper, with or without neonatal supplementation of bovine colostrum (BC), to modulate the immune system development of low birth weight (LBW) and high birth weight (HBW) piglets during the peri-weaning period. Litters from 23 control sows (CONT) were assigned to one of the following treatments: 1) control (C); 2) oral administration at 2 and 8 days (d) of age of retinol-acetate, 25-hydroxyvitamin D and CuSO₄ and exposure to UVB light for 15 min every second day from d 5 to d 21 (ADCu); 3) oral administration of dehydrated BC (4 g/d) from d 5 to d 10 (BC); 4) ADCu + BC. This experimental design was repeated with 24 sows fed extra daily supplements of 25-hydroxyvitamin D (4,000 IU), β-carotene (30,000 IU) and Cu-yeast (equivalent 45 mg of Cu) from 90 d of gestation until weaning at d 21 (SUPPL). Within each litter, 2 LBW and 2 HBW piglets were euthanized at d 16 and d 23 in order to characterize leukocyte subsets in mesenteric lymph nodes (MLN) and blood by flow cytometry, and to measure gene expression in the MLN and jejunal mucosa by qPCR. At d 16, results revealed that the percentages of γδ and cytotoxic T lymphocytes were significantly reduced in LBW compared to HBW piglets. The jejunal expression of interleukin (IL) 22 was also up-regulated, along with MLN expression of C-C Motif Chemokine Ligand 23, bone morphogenetic protein 2 and secreted phosphoprotein 1 (SPP1), whereas jejunal expression of tumor necrosis factor α was decreased in LBW piglets. At d 23, LBW piglets showed lower amounts of γδ T lymphocytes, higher percentages of CD3^- and CD3^-CD8α⁺CD16⁺ leukocytes (which include Natural killer cells) and lower jejunal expression of IL18. Furthermore, supplementation with BC increased the blood percentage of CD3^-CD16⁺ leukocytes and reduced jejunal IL5 and MLN IL15 expression whereas supplementation with ADCu + BC increased jejunal TNF superfamily 13B and MLN SPP1 expression. Our results suggest that immune system development after birth differed between LBW and HBW piglets and that early dietary supplementation with BC and ADCu has the potential to modulate development of immune functions.

The CTA1-DD adjuvant strongly potentiates follicular dendritic cell function and germinal center formation, which results in improved neonatal immunization

Vaccination of neonates and young infants is hampered by the relative immaturity of their immune systems and the lack of safe and efficacious vaccine adjuvants. Immaturity of the follicular dendritic cells (FDCs), in particular, appears to play a critical role for the inability to stimulate immune responses. Using the CD21mT/mG mouse model we found that at 7 days of life, FDCs exhibited a mature phenotype only in the Peyer´s patches (PP), but our unique adjuvant, CTA1-DD, effectively matured FDCs also in peripheral lymph nodes following systemic, as well as mucosal immunizations. This was a direct effect of complement receptor 2-binding to the FDC and a CTA1-enzyme-dependent enhancing effect on gene transcription, among which CR2, IL-6, ICAM-1, IL-1β, and CXCL13 encoding genes were upregulated. This way we achieved FDC maturation, increased germinal center B-cell- and Tfh responses, and enhanced specific antibody levels close to adult magnitudes. Oral priming immunization of neonates against influenza infection with CTA1-3M2e-DD effectively promoted anti-M2e-immunity and significantly reduced morbidity against a live virus challenge infection. To the best of our knowledge, this is the first study to demonstrate direct effects of an adjuvant on FDC gene transcriptional functions and the subsequent enhancement of neonatal immune responses.

Stromal cells and immune cells involved in formation of lymph nodes and their niches

Secondary lymphoid organs are critical for efficient interaction between innate antigen presenting cells and adaptive lymphocytes in order to start adaptive immune responses. The efficiency by which these cellular subsets meet is highly increased by the orchestrating role of stromal cells within the secondary lymphoid organs. These cells provide cytokines, chemokines and cell surface receptors necessary for survival and guided migration. This increases the likelihood that antigen specific adaptive immune responses occur. Already from initial formation of secondary lymphoid organs, the interaction of immune cells with stromal cells is crucial and this interaction continues during immune activation. With the recent discovery of many stromal cell subsets new immune micro-niches with specific functions that are orchestrated by stromal cells will be discovered. Here, we will discuss how the development of lymph nodes as well as their specific niches is guided by the interaction of immune cells and stromal cells.

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

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

Remodeling of reactive lymph nodes: Dynamics of stromal cells and underlying chemokine signaling

Lymph nodes (LNs) are secondary immune organs dispersed throughout the body. They are primarily composed of lymphocytes, "transient passengers" that are only present for a few hours. During this time, they extensively interact with a meshwork of stromal cells. Although these cells constitute less than 5% of all LN cells, they are integral to LN function: Stromal cells create a three-dimensional network that provides a rigid backbone for the transport of lymph and generates "roads" for lymphocyte migration. Beyond structural support, the LN stroma also produces survival signals for lymphocytes and provides nutrients, soluble factors, antigens, and immune cells collectively required for immune surveillance and the generation of adaptive immune responses. A unique feature of LNs is their ability to considerably and rapidly change size: the volume and cellularity of inflamed LNs can increase up to 20-fold before returning to homeostatic levels. This cycle will be repeated many times during life and is accommodated by stromal cells. The dynamics underlying this dramatic remodeling are subject of this review. We will first introduce the main types of LN stromal cells and explain their known functions. We will then discuss how these cells enable LN growth during immune responses, with a particular focus on underlying cellular mechanisms and molecular cues. Similarly, we will elaborate on stromal dynamics mediating the return to LN homeostasis, a process that is mechanistically much less understood than LN expansion.

Diversified IgA-Bacteria Interaction in Gut Homeostasis

Immunoglobulin A (IgA) is the major immunoglobulin isotype produced by the gut immune system, and many studies revealed key roles of IgA in establishing host-bacteria mutualism. This chapter will review current understandings for the function of gut IgA in regulating commensal microbiota. IgA specifically recognizes bacterial species that strongly stimulate host's immune responses, and suppresses their overgrowth or reduces the expressions of bacterial pro-inflammatory genes. On the other hand, IgA coatings on symbiotic bacteria enhance bacteria-mucus and bacteria-bacteria interactions, which induce production of metabolites enforcing mucosal barrier functions. Such diversified effects suggest that multiple factors may be involved in the mechanisms of IgA-bacteria interactions, including IgA specificity to microbial epitopes, mode of cellular responses of IgA synthesis (T-dependent and T-independent) and post-translational modifications of IgA proteins, such as glycosylation.