Retinoic acid, acting as a highly specific IgA isotype switch factor, cooperates with TGF-β1 to enhance the overall IgA response

Association of Vitamin A and D Deficiencies with Infectious Outcomes in Children Undergoing Intensive Induction Therapy for Acute Lymphoblastic Leukemia

OBJECTIVE

To evaluate the association between deficiency of vitamin A or D at diagnosis of pediatric acute lymphoblastic leukemia (ALL) and subsequent infectious complications during induction therapy.

STUDY DESIGN

An IRB-approved, retrospective cohort study of children diagnosed with newly-diagnosed ALL from 2007 to 2017 at St. Jude Children's Research Hospital. We measured vitamin D, vitamin D binding protein, retinol binding protein as a surrogate for vitamin A, and immunoglobulin isotypes in serum obtained at ALL diagnosis, and we assessed the association between vitamin deficiencies or levels and infection-related complications during the 6-week induction phase using Cox regression models.

RESULTS

Among 378 evaluable participants, vitamin A and D deficiencies were common (43% and 17% respectively). Vitamin D deficiency was associated with higher risks of febrile neutropenia (adjusted hazard ratio [aHR] 1.7; p=0.0072), clinically-documented infection (aHR 1.73; p=0.025), and likely bacterial infection (aHR 1.86; p=0.008). Conversely, vitamin A deficiency was associated solely with a reduced risk of sepsis (aHR 0.19; p=0.027).

CONCLUSIONS

In this retrospective study, vitamin D deficiency was associated with an increased risk of common infection-related complications during induction therapy for ALL. Additional studies are warranted to evaluate whether vitamin D supplementation could mitigate this effect.

Cellular and micro-environmental responses influencing the antitumor activity of all-trans retinoic acid in breast cancer

All-trans retinoic acid (ATRA) is the most relevant and functionally active metabolite of Vitamin-A. From a therapeutic standpoint, ATRA is the first example of pharmacological agent exerting its anti-tumor activity via a cell differentiating action. In the clinics, ATRA is used in the treatment of Acute Promyelocytic Leukemia, a rare form of myeloid leukemia with unprecedented therapeutic results. The extraordinary effectiveness of ATRA in the treatment of Acute Promyelocytic Leukemia patients has raised interest in evaluating the potential of this natural retinoid in the treatment of other types of neoplasias, with particular reference to solid tumors.The present article provides an overview of the available pre-clinical and clinical studies focussing on ATRA as a therapeutic agent in the context of breast cancer from a holistic point of view. In detail, we focus on the direct effects of ATRA in breast cancer cells as well as the underlying molecular mechanisms of action. In addition, we summarize the available information on the action exerted by ATRA on the breast cancer micro-environment, an emerging determinant of the progression and invasive behaviour of solid tumors. In particular we discuss the recent evidences of ATRA activity on the immune system. Finally, we analyse and discuss the results obtained with the few ATRA-based clinical trials conducted in the context of breast cancer.

Therapeutic targeting of gut-originating regulatory B cells in neuroinflammatory diseases

Regulatory B cells (Bregs) are immunosuppressive cells that support immunological tolerance by the production of IL-10, IL-35, and TGF-β. Bregs arise from different developmental stages in response to inflammatory stimuli. In that regard, mounting evidence points towards a direct influence of gut microbiota on mucosal B cell development, activation, and regulation in health and disease. While an increasing number of diseases are associated with alterations in gut microbiome (dysbiosis), little is known about the role of microbiota on Breg development and induction in neuroinflammatory disorders. Notably, gut-originating, IL-10- and IgA-producing regulatory plasma cells have recently been demonstrated to egress from the gut to suppress inflammation in the CNS raising fundamental questions about the triggers and functions of mucosal-originating Bregs in systemic inflammation. Advancing our understanding of Bregs in neuroinflammatory diseases could lead to novel therapeutic approaches. Here, we summarize the main aspects of Breg differentiation and functions and evidence about their involvement in neuroinflammatory diseases. Further, we highlight current data of gut-originating Bregs and their microbial interactions and discuss future microbiota-regulatory B cell-targeted therapies in immune-mediated diseases.

Retinoic Acid Signaling Is Required for Dendritic Cell Maturation and the Induction of T Cell Immunity

Vitamin A and its biologically active metabolites, all-trans and 9-cis retinoic acid (RA), are thought to be important in generating and modulating immune function. However, RA modulates the function of many types of immune cells, and its specific role in dendritic cell (DC) activation, Ag presentation, and T cell effector function has not been fully characterized. Because RA works primarily through RA receptor (RAR)α, we examined mice with a myeloid cell-specific defect in RA signaling. These transgenic mice have a CD11c-cre-driven expression of a truncated form of RARα that specifically blocks the signaling of all forms of RARs in myeloid cells. This defect results in abnormal DC function, with impaired DC maturation and activation, and reduced Ag uptake and processing. These DC abnormalities were associated with a reduced ability to mount Ag-specific T cell responses to immunization despite having normally functioning T cells. In contrast, the loss of DC-specific RA signaling did not significantly alter levels of Ag-specific Abs postimmunization and resulted in an increase in bronchial IgA. Our findings indicate that RA signaling in DCs is crucial for immune activation, and its absence impairs the development of Ag-specific effector functions of T cell immunity.

Immunomodulatory Effects of Microbiota-Derived Short-Chain Fatty Acids in Autoimmune Liver Diseases

Nonpathogenic commensal microbiota and their metabolites and components are essential to maintain a tolerogenic environment and promote beneficial health effects. The metabolic environment critically impacts the outcome of immune responses and likely impacts autoimmune and allergic responses. Short-chain fatty acids (SCFAs) are the main metabolites produced by microbial fermentation in the gut. Given the high concentration of SCFAs in the gut and portal vein and their broad immune regulatory functions, SCFAs significantly influence immune tolerance and gut-liver immunity. Alterations of SCFA-producing bacteria and SCFAs have been identified in a multitude of inflammatory diseases. These data have particular significance in primary biliary cholangitis, primary sclerosing cholangitis, and autoimmune hepatitis because of the close proximity of the liver to the gut. In this focused review, we provide an update on the immunologic consequences of SCFA-producing microbiota and in particular on three dominant SCFAs in autoimmune liver diseases.

The intestine: A highly dynamic microenvironment for IgA plasma cells

To achieve longevity, IgA plasma cells require a sophisticated anatomical microenvironment that provides cytokines, cell-cell contacts, and nutrients as well as metabolites. The intestinal epithelium harbors cells with distinct functions and represents an important defense line. Anti-microbial peptide-producing paneth cells, mucus-secreting goblet cells and antigen-transporting microfold (M) cells cooperate to build a protective barrier against pathogens. In addition, intestinal epithelial cells are instrumental in the transcytosis of IgA to the gut lumen, and support plasma cell survival by producing the cytokines APRIL and BAFF. Moreover, nutrients are sensed through specialized receptors such as the aryl hydrocarbon receptor (AhR) by both, intestinal epithelial cells and immune cells. However, the intestinal epithelium is highly dynamic with a high cellular turn-over rate and exposure to changing microbiota and nutritional factors. In this review, we discuss the spatial interplay of the intestinal epithelium with plasma cells and its potential contribution to IgA plasma cell generation, homing, and longevity. Moreover, we describe the impact of nutritional AhR ligands on intestinal epithelial cell-IgA plasma cell interaction. Finally, we introduce spatial transcriptomics as a new technology to address open questions in intestinal IgA plasma cell biology.

IgA-producing B cells in lung homeostasis and disease

Immunoglobulin A (IgA) is the most abundant Ig in mucosae where it plays key roles in host defense against pathogens and in mucosal immunoregulation. Whereas intense research has established the different roles of secretory IgA in the gut, its function has been much less studied in the lung. This review will first summarize the state-of-the-art knowledge on the distribution and phenotype of IgA⁺ B cells in the human lung in both homeostasis and disease. Second, it will analyze the studies looking at cellular and molecular mechanisms of homing and priming of IgA⁺ B cells in the lung, notably following immunization. Lastly, published data on observations related to IgA and IgA⁺ B cells in lung and airway disease such as asthma, cystic fibrosis, idiopathic pulmonary fibrosis, or chronic rhinosinusitis, will be discussed. Collectively it provides the state-of-the-art of our current understanding of the biology of IgA-producing cells in the airways and identifies gaps that future research should address in order to improve mucosal protection against lung infections and chronic inflammatory diseases.

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.

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.

Vitamin A in resistance to and recovery from infection: relevance to SARS-CoV2

SARS-CoV2 infects respiratory epithelial cells via its cellular receptor angiotensin-converting enzyme 2, causing a viral pneumonia with pronounced inflammation resulting in significant damage to the lungs and other organ systems, including the kidneys, though symptoms and disease severity are quite variable depending on the intensity of exposure and presence of underlying conditions that may affect the immune response. The resulting disease, coronavirus disease 2019 (COVID-19), can cause multi-organ system dysfunction in patients requiring hospitalisation and intensive care treatment. Serious infections like COVID-19 often negatively affect nutritional status, and the resulting nutritional deficiencies may increase disease severity and impair recovery. One example is the viral infection measles, where associated vitamin A (VA) deficiency increases disease severity and appropriately timed supplementation during recovery reduces mortality and hastens recovery. VA may play a similar role in COVID-19. First, VA is important in maintaining innate and adaptive immunity to promote clearance of a primary infection as well as minimise risks from secondary infections. Second, VA plays a unique role in the respiratory tract, minimising damaging inflammation, supporting repair of respiratory epithelium and preventing fibrosis. Third, VA deficiency may develop during COVID-19 due to specific effects on lung and liver stores caused by inflammation and impaired kidney function, suggesting that supplements may be needed to restore adequate status. Fourth, VA supplementation may counteract adverse effects of SARS-CoV2 on the angiotensin system as well as minimises adverse effects of some COVID-19 therapies. Evaluating interactions of SARS-CoV2 infection with VA metabolism may thus provide improved COVID-19 therapy.

Lactoferrin Potentiates Inducible Regulatory T Cell Differentiation through TGF-β Receptor III Binding and Activation of Membrane-Bound TGF-β

Lactoferrin (LF) is known to possess anti-inflammatory activity, although its mechanisms of action are not well-understood. The present study asked whether LF affects the commitment of inducible regulatory T cells (Tregs). LF substantially promoted Foxp3 expression by mouse activated CD4+T cells, and this activity was further enhanced by TGF-β1. Interestingly, blocking TGF-β with anti-TGF-β Ab completely abolished LF-induced Foxp3 expression. However, no significant amount of soluble TGF-β was released by LF-stimulated T cells, suggesting that membrane TGF-β (mTGF-β) is associated. Subsequently, it was found that LF binds to TGF-β receptor III, which induces reactive oxygen species production and diminishes the expression of mTGF-β-bound latency-associated peptide, leading to the activation of mTGF-β. It was followed by phosphorylation of Smad3 and enhanced Foxp3 expression. These results suggest that LF induces Foxp3+ Tregs through TGF-β receptor III/reactive oxygen species-mediated mTGF-β activation, triggering canonical Smad3-dependent signaling. Finally, we found that the suppressive activity of LF-induced Tregs is facilitated mainly by CD39/CD73-induced adenosine generation and that this suppressor activity alleviates inflammatory bowel disease.

MicroRNA-directed pathway discovery elucidates an miR-221/222-mediated regulatory circuit in class switch recombination

MicroRNAs (miRNAs, miRs) regulate cell fate decisions by post-transcriptionally tuning networks of mRNA targets. We used miRNA-directed pathway discovery to reveal a regulatory circuit that influences Ig class switch recombination (CSR). We developed a system to deplete mature, activated B cells of miRNAs, and performed a rescue screen that identified the miR-221/222 family as a positive regulator of CSR. Endogenous miR-221/222 regulated B cell CSR to IgE and IgG1 in vitro, and miR-221/222-deficient mice exhibited defective IgE production in allergic airway challenge and polyclonal B cell activation models in vivo. We combined comparative Ago2-HITS-CLIP and gene expression analyses to identify mRNAs bound and regulated by miR-221/222 in primary B cells. Interrogation of these putative direct targets uncovered functionally relevant downstream genes. Genetic depletion or pharmacological inhibition of Foxp1 and Arid1a confirmed their roles as key modulators of CSR to IgE and IgG1.

Pharmacological Modulation of Immune Responses by Nutritional Components

The incidence of noncommunicable diseases (NCDs) has increased over the last few decades, and one of the major contributors to this is lifestyle, especially diet. High intake of saturated fatty acids and low intake of dietary fiber is linked to an increase in NCDs. Conversely, a low intake of saturated fatty acids and a high intake of dietary fiber seem to have a protective effect on general health. Several mechanisms have been identified that underlie this phenomenon. In this review, we focus on pharmacological receptors, including the aryl hydrocarbon receptor, binding partners of the retinoid X receptor, G-coupled protein receptors, and toll-like receptors, which can be activated by nutritional components and their metabolites. Depending on the nutritional component and the receptors involved, both proinflammatory and anti-inflammatory effects occur, leading to an altered immune response. These insights may provide opportunities for the prevention and treatment of NCDs and their inherent (sub)chronic inflammation. SIGNIFICANCE STATEMENT: This review summarizes the reported effects of nutritional components and their metabolites on the immune system through manipulation of specific (pharmacological) receptors, including the aryl hydrocarbon receptor, binding partners of the retinoid X receptor, G-coupled protein receptors, and toll-like receptors. Nutritional components, such as vitamins, fibers, and unsaturated fatty acids are able to resolve inflammation, whereas saturated fatty acids tend to exhibit proinflammatory effects. This may aid decision makers and scientists in developing strategies to decrease the incidence of noncommunicable diseases.

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

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

Clickable Vitamins as a New Tool to Track Vitamin A and Retinoic Acid in Immune Cells

The vitamin A derivative, retinoid acid (RA) is key player in guiding adaptive mucosal immune responses. However, data on the uptake and metabolism of vitamin A within human immune cells has remained largely elusive because retinoids are small, lipophilic molecules which are difficult to detect. To overcome this problem and to be able to study the effect of vitamin A metabolism in human immune cell subsets, we have synthesized novel bio-orthogonal retinoid-based probes (clickable probes), which are structurally and functionally indistinguishable from vitamin A. The probes contain a functional group (an alkyne) to conjugate to a fluorogenic dye to monitor retinoid molecules in real-time in immune cells. We demonstrate, by using flow cytometry and microscopy, that multiple immune cells have the capacity to internalize retinoids to varying degrees, including human monocyte-derived dendritic cells (DCs) and naïve B lymphocytes. We observed that naïve B cells lack the enzymatic machinery to produce RA, but use exogenous retinoic acid to enhance CD38 expression. Furthermore, we showed that human DCs metabolize retinal into retinoic acid, which in co-culture with naïve B cells led to of the induction of CD38 expression. These data demonstrate that in humans, DCs can serve as an exogenous source of RA for naïve B cells. Taken together, through the use of clickable vitamins our data provide valuable insight in the mechanism of vitamin A metabolism and its importance for human adaptive immunity.

Circulating Adaptive Immune Cells Expressing the Gut Homing Marker α4β7 Integrin Are Decreased in COVID-19

Background

Infection with the novel severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) causes a wide range of symptoms including gastrointestinal manifestations, and intestinal epithelial cells are a target of the virus. However, it is unknown how the intestinal immune system contributes to systemic immune responses in coronavirus disease 2019 (COVID-19).

Methods

We characterized peripheral blood lymphocytes from patients with active COVID-19 and convalescent patients as well as healthy controls by flow cytometry.

Results

The frequency and absolute number of circulating memory T and B cells expressing the gut homing integrin α4β7 integrin was reduced during COVID-19, whether gastrointestinal symptoms were present or not. While total IgA-expressing B cells were increased, gut-imprinted B cells with IgA expression were stable.

Conclusion

COVID-19 is associated with a decrease in circulating adaptive immune cells expressing the key gut homing marker α4β7 suggesting that these cells are preferentially recruited to extra-intestinal tissues independently of α4β7 or that the systemic immune response against SARS-CoV-2 is at least numerically dominated by extraintestinal, particularly pulmonary, immune cell priming.

The Alteration of Salivary Immunoglobulin A in Autism Spectrum Disorders

Objectives: Autism spectrum disorders (ASD) are neurodevelopmental disorders with changes in the gut and oral microbiota. Based on the intimate relationship between the oral microbiota and oral mucosal immunity, this study aimed to investigate changes in salivary immunoglobulin A (IgA) level in ASD and the underlying mechanism for any such changes. Methods: We recruited 36 children diagnosed with ASD and 35 normally developing children and measured their salivary IgA content using enzyme-linked immunosorbent assay (ELISA). The valproate (VPA) -treated ASD mouse model was established by prenatal exposure to valproate and mouse salivary IgA content was also quantified by ELISA. The submandibular glands of VPA and control mice were isolated and analyzed using qRT-PCR, immunofluorescence staining, and flow cytometry. ASD-related Streptococci were co-incubated with the human salivary gland (HSG) cell line, and western blotting was used to detect the levels of relevant proteins. Results: We found that salivary IgA content was significantly decreased in patients with ASD and had a significant ASD diagnostic value. The salivary IgA content also decreased in VPA mice and was significantly correlated with autistic-like behaviors among them. The mRNA and protein levels of the polymeric immunoglobulin receptor (Pigr) were downregulated in the submandibular glands of VPA mice and the Pigr mRNA level was positively correlated with mouse salivary IgA content. HSG cells treated with ASD-related Streptococci had reduced PIGR protein level. Conclusion: Therefore, protective IgA levels were reduced in the saliva of individuals with ASD, which correlated with the bacteria-induced downregulation of Pigr in salivary glands. This study suggests a new direction for ASD diagnosis and prevention of oral diseases in ASD cohorts and provides evidence for the ASD mucosal immunophenotype in the oral cavity.

αvβ8 integrin-expression by BATF3-dependent dendritic cells facilitates early IgA responses to Rotavirus

Secretory intestinal IgA can protect from re-infection with rotavirus (RV), but very little is known about the mechanisms that induce IgA production during intestinal virus infections. Classical dendritic cells (cDCs) in the intestine can facilitate both T cell-dependent and -independent secretory IgA. Here, we show that BATF3-dependent cDC1, but not cDC2, are critical for the optimal induction of RV-specific IgA responses in the mesenteric lymph nodes. This depends on the selective expression of the TGFβ-activating integrin αvβ8 by cDC1. In contrast, αvβ8 on cDC1 is dispensible for steady state immune homeostasis. Given that cDC2 are crucial in driving IgA during steady state but are dispensable for RV-specific IgA responses, we propose that the capacity of DC subsets to induce intestinal IgA responses reflects the context, as opposed to an intrinsic property of individual DC subsets.

Effects of supplementation with β-carotene on the growth performance and intestinal mucosal barriers in layer-type cockerels

β-carotene is a robust modulator of mucosal barriers, and it can amplify the immunoglobulin A (IgA) response via the retinoic acid (RA)-mediated pathway. We investigated the influence of β-carotene on intestinal barriers in layer-type cockerels. In this study, β-carotene has a positive influence on growth performance and intestinal morphology. β-carotene remarkably enhanced serum secretory immunoglobulin A (sIgA) levels, jejunal mucosal sIgA, and IgA concentrations. β-Carotene significantly enhanced mRNA expression levels of IgA, CC chemokine receptor-9 (CCR9), polymeric immunoglobulin receptor (pIgR), and retinoic acid receptor α (RARα) in the ileal tissues and pIgR in the jejunal tissues. β-Carotene improves mRNA expression of intestinal barrier-related proteins including: mucin-2 (MUC-2), zonula occludens-2 (ZO-2), occludins (OCLN), and zonula occludens-1 (ZO-1) in the ileal tissues. Moreover, β-carotene decreased the levels of Escherichia coli and elevates the levels of Lactobacillus. The results indicate that β-carotene can promote growth performance and contribute to the gradual development of intestinal barriers in Hyline Brown chicks. This study enriches our knowledge about the effects of β-carotene on intestinal barrier and highlights a theoretical basis of β-carotene application in the poultry industry.

The Need to Consider Context in the Evaluation of Anti-infectious and Immunomodulatory Effects of Vitamin A and its Derivatives

Vitamin A and its derivatives (retinoids) act as potent regulators in many aspects of mammalian reproduction, development, repair, and maintenance of differentiated tissue functioning. Unlike other vitamins, Vitamin A and retinoids, which have hormonal actions, present significant toxicity, which plays roles in clinically relevant situations, such as hypervitaminosis A and retinoic acid ("differentiation") syndrome. Although clinical presentation is conspicuous in states of insufficient or excessive Vitamin A and retinoid concentration, equally relevant effects on host resistance to specific infectious agents, and in the general maintenance of immune homeostasis, may go unnoticed, because their expression requires either pathogen exposure or the presence of inflammatory co-morbidities. There is a vast literature on the roles played by retinoids in the maintenance of a tolerogenic, noninflammatory environment in the gut mucosa, which is considered by many investigators representative of a general role played by retinoids as anti-inflammatory hormones elsewhere. However, in the gut mucosa itself, as well as in the bone marrow and inflammatory sites, context determines whether one observes an anti-inflammatory or proinflammatory action of retinoids. Both interactions between specialized cell populations, and interactions between retinoids and other classes of mediators/regulators, such as cytokines and glucocorticoid hormones, must be considered as important factors contributing to this overall context. We review evidence from recent studies on mucosal immunity, granulocyte biology and respiratory allergy models, highlighting the relevance of these variables as well as their possible contributions to the observed outcomes.

All-trans retinoic acid increases NF-κB activity in PMA-stimulated THP-1 cells upon unmethylated CpG challenge by enhancing cell surface TLR9 expression

An active metabolite of vitamin A, all-trans retinoic acid (ATRA), is known to exert immunomodulatory functions. This study investigates the possible immune potentiating effect of ATRA on NF-κB activity in human monocytic THP-1 cells after exposure to unmethylated CpG DNA ODN2006. We observed that challenge with ODN2006 significantly enhanced the NF-κB activity of PMA-differentiated THP-1 cells. ATRA synergistically enhanced NF-κB activity of cells, in a concentration- and time-dependent manner. The enhanced NF-κB activity of PMA-differentiated THP-1 cells after ODN2006 challenge was dependent on the RAR/RXR pathway. To determine the mechanism involved in increasing in the NF-κB activity of stimulated THP-1 cells, we examined the effects of PMA and ATRA on the expression of TLR9 (a receptor of ODN2006) in THP-1 cells. PMA treatment significantly enhanced both the intracellular and cell surface expression of TLR9, while ATRA alone showed no effect. However, ATRA synergistically enhanced the cell surface TLR9 expression of PMA-differentiated cells. To determine whether the ATRA-enhanced NF-κB activity is due to the enhanced cell surface TLR9 expression, we examined NF-κB activity after treatment with anti-TLR9 blocking antibody. Results revealed that the anti-TLR9 antibody treatment almost completely reverses the ATRA-enhanced NF-κB activity, suggesting that ATRA enhances NF-κB activity through upregulation of the cell surface TLR9 expression in PMA-differentiated and unmethylated CpG challenged THP-1 cells.

T lymphocytes in IgA nephropathy

Immunoglobulin A nephropathy (IgAN), the most common primary glomerulonephritis worldwide, is the main cause of end-stage renal disease. IgAN is characterized by the accumulation of immune complexes in the circulation, which contain abnormal levels of IgA. IgAN primarily results from galactose-deficient IgA1 (Gd-IgA1) and Gd-IgA1 deposition in the renal mesangium, causing local proliferation and matrix expansion. Gd-IgA1 has been confirmed as one of the key effectors in the pathogenesis of IgAN, but the origin of Gd-IgA1 is not clear. Recent studies have shown that Gd-IgA1 deposition could be the result of mucosally primed plasma cells and is associated with T cell dysregulation. T cells contribute to the IgA response and play an important role in the development of IgAN. In the present review, the latest discoveries regarding the role of T lymphocytes in the pathogenesis of IgAN have been summarized. Understanding these advances will allow novel therapeutic strategies for the treatment of IgAN.

IL-21 and IL-5 coordinately induce surface IgA+ cells

Intestinal IgA is induced by microbes and food antigens. Peyer's patches (PPs) are known as one of the inductive sites for intestinal IgA production. However, the precise mechanism of IgA induction is as yet unknown. IgA secretion was induced from IgD⁺ B cells in vitro by stimulus with lipopolysaccharide in the presence of only retinoic acid (RA) and low doses of TGF-β1. Surface IgA⁺ cells were effectively induced from IgD⁺ B cells in vitro by the mixture of RA and the cytokines TGF-β1, APRIL, IL-5 and IL-21. rIL-21 upregulated surface IgA⁺ but impaired the proliferation of stimulated B cells in the presence of rTGF-β1, RA and rAPRIL, in vitro. The addition of rIL-5 restored the impaired proliferation by rIL-21, resulting in the expansion of IgA⁺ cells. rIL-21 induced the expression of Aicda and Prdm1, and impaired Rel in IgD⁺ B cells. Blockade of IL-21R signaling by a neutralizing mAb in vivo led to lower frequencies of IgA⁺ and IgG2b⁺ cells and lower germinal center B cells in PPs in a homeostatic condition. Although amounts of small intestinal IgA and titers of anti-dsDNA, the major target of intestinal IgA, in these mice were not altered, anti-OVA IgA titers induced by OVA drinking in OVA-specific T-cell receptor (TCR) transgenic mice were decreased. PP-deficient TCR transgenic mice showed diminished anti-OVA IgA induction. Blockade of IL-5R signaling in vivo led to similar results with relatively weaker effects than that of IL-21R mAb administration. These results suggest that IL-21 and IL-5 play cooperative roles in surface expression of IgA in PPs.

The role of innate lymphoid cells in response to microbes at mucosal surfaces

Innate lymphoid cells (ILCs) are a lymphocyte population that is mostly resident at mucosal surfaces. They help to induce an appropriate immune response to the microbiome at homeostasis. In healthy people, the mucosal immune system works symbiotically with organisms that make up the microbiota. ILCs play a critical role in orchestrating this balance, as they can both influence and in turn be influenced by the microbiome. ILCs also are important regulators of the early response to infections by diverse types of pathogenic microbes at mucosal barriers. Their rapid responses initiate inflammatory programs, production of antimicrobial products and repair processes. This review will focus on the role of ILCs in response to the microbiota and to microbial infections of the lung and intestine.

Commensal-bacteria-derived butyrate promotes the T-cell-independent IgA response in the colon

Secretory immunoglobulin A (SIgA), the most abundant antibody isotype in the body, maintains a mutual relationship with commensal bacteria and acts as a primary barrier at the mucosal surface. Colonization by commensal bacteria induces an IgA response, at least partly through a T-cell-independent process. However, the mechanism underlying the commensal-bacteria-induced T-cell-independent IgA response has yet to be fully clarified. Here, we show that commensal-bacteria-derived butyrate promotes T-cell-independent IgA class switching recombination (CSR) in the mouse colon. Notably, the butyrate concentration in human stools correlated positively with the amount of IgA. Butyrate up-regulated the production of transforming growth factor β1 and all-trans retinoic acid by CD103+CD11b+ dendritic cells, both of which are critical for T-cell-independent IgA CSR. This effect was mediated by G-protein-coupled receptor 41 (GPR41/FFA3) and GPR109a/HCA2, and the inhibition of histone deacetylase. The butyrate-induced IgA response reinforced the colonic barrier function, preventing systemic bacterial dissemination under inflammatory conditions. These observations demonstrate that commensal-bacteria-derived butyrate contributes to the maintenance of the gut immune homeostasis by facilitating the T-cell-independent IgA response in the colon.

Antigen-Specific Mucosal Immunity Regulates Development of Intestinal Bacteria-Mediated Diseases

BACKGROUND & AIMS

Dysregulation of the microbiome has been associated with development of complex diseases, such as obesity and diabetes. However, no method has been developed to control disease-associated commensal microbes. We investigated whether immunization with microbial antigens, using CpG oligodeoxynucleotides and/or curdlan as adjuvants, induces systemic antigen-specific IgA and IgG production and affects development of diseases in mice.

METHODS

C57BL/6 mice were given intramuscular injections of antigens (ovalbumin, cholera toxin B-subunit, or pneumococcal surface protein A) combined with CpG oligodeoxynucleotides and/or curdlan. Blood and fecal samples were collected weekly and antigen-specific IgG and IgA titers were measured. Lymph nodes and spleens were collected and analyzed by enzyme-linked immunosorbent assay for antigen-specific splenic T-helper 1 cells, T-helper 17 cells, and memory B cells. Six weeks after primary immunization, mice were given a oral, nasal, or vaginal boost of ovalbumin; intestinal lamina propria, bronchial lavage, and vaginal swab samples were collected and antibodies and cytokines were measured. Some mice were also given oral cholera toxin or intranasal Streptococcus pneumoniae and the severity of diarrhea or pneumonia was analyzed. Gnotobiotic mice were gavaged with fecal material from obese individuals, which had a high abundance of Clostridium ramosum (a commensal microbe associated with obesity and diabetes), and were placed on a high-fat diet 2 weeks after immunization with C ramosum. Intestinal tissues were collected and analyzed by quantitative real-time polymerase chain reaction.

RESULTS

Serum and fecal samples from mice given injections of antigens in combination with CpG oligodeoxynucleotides and curdlan for 3 weeks contained antigen-specific IgA and IgG, and splenocytes produced interferon-gamma and interleukin 17A. Lamina propria, bronchial, and vaginal samples contained antigen-specific IgA after the ovalbumin boost. This immunization regimen prevented development of diarrhea after injection of cholera toxin, and inhibited lung colonization by S pneumoniae. In gnotobiotic mice colonized with C ramosum and placed on a high-fat diet, the mice that had been immunized with C ramosum became less obese than the nonimmunized mice.

CONCLUSIONS

Injection of mice with microbial antigens and adjuvant induces antigen-specific mucosal and systemic immune responses. Immunization with S pneumoniae antigen prevented lung infection by this bacteria, and immunization with C ramosum reduced obesity in mice colonized with this microbe and placed on a high-fat diet. This immunization approach might be used to protect against microbe-associated disorders of intestine.

BLT1 mediates commensal bacteria-dependent innate immune signals to enhance antigen-specific intestinal IgA responses

Leukotriene B₄ receptor 1 (BLT1) triggers the migration of granulocytes and activated T cells; however, its role in B-cell function remains unclear. Here we report that BLT1 is required to induce the production of antigen-specific IgA against oral vaccine through mediating innate immune signals from commensal bacteria. B cells acquire BLT1 expression during their differentiation to IgA⁺ B cells and plasma cells in Peyer's patches and the small intestinal lamina propria, respectively. BLT1 KO mice exhibited impaired production of antigen-specific fecal IgA to oral vaccine despite normal IgG responses to systemically immunized antigen. Expression of MyD88 was decreased in BLT1 KO gut B cells and consequently led to diminished proliferation of commensal bacteria-dependent plasma cells. These results indicate that BLT1 enhances the proliferation of commensal bacteria-dependent IgA⁺ plasma cells through the induction of MyD88 and thereby plays a key role in the production of antigen-specific intestinal IgA.

Gut-associated IgA+ immune cells regulate obesity-related insulin resistance

The intestinal immune system is emerging as an important contributor to obesity-related insulin resistance, but the role of intestinal B cells in this context is unclear. Here, we show that high fat diet (HFD) feeding alters intestinal IgA⁺ immune cells and that IgA is a critical immune regulator of glucose homeostasis. Obese mice have fewer IgA⁺ immune cells and less secretory IgA and IgA-promoting immune mediators. HFD-fed IgA-deficient mice have dysfunctional glucose metabolism, a phenotype that can be recapitulated by adoptive transfer of intestinal-associated pan-B cells. Mechanistically, IgA is a crucial link that controls intestinal and adipose tissue inflammation, intestinal permeability, microbial encroachment and the composition of the intestinal microbiome during HFD. Current glucose-lowering therapies, including metformin, affect intestinal-related IgA⁺ B cell populations in mice, while bariatric surgery regimen alters the level of fecal secretory IgA in humans. These findings identify intestinal IgA⁺ immune cells as mucosal mediators of whole-body glucose regulation in diet-induced metabolic disease.

The effect of diet-induced obesity on intestinal B cell populations is not well understood despite emerging evidence of a critical role for the intestinal immune system in contributing to insulin resistance. Here, the authors show important functions of IgA in regulating metabolic disease and for intestinal immunity in modulating systemic glucose metabolism.

Influences of Vitamin A on Vaccine Immunogenicity and Efficacy

Vitamin A deficiencies and insufficiencies are widespread in developing countries, and may be gaining prevalence in industrialized nations. To combat vitamin A deficiency (VAD), the World Health Organization (WHO) recommends high-dose vitamin A supplementation (VAS) in children 6–59 months of age in locations where VAD is endemic. This practice has significantly reduced all-cause death and diarrhea-related mortalities in children, and may have in some cases improved immune responses toward pediatric vaccines. However, VAS studies have yielded conflicting results, perhaps due to influences of baseline vitamin A levels on VAS efficacy, and due to cross-regulation between vitamin A and related nuclear hormones. Here we provide a brief review of previous pre-clinical and clinical data, showing how VAD and VAS affect immune responses, vaccines, and infectious diseases. We additionally present new results from a VAD mouse model. We found that when VAS was administered to VAD mice at the time of vaccination with a pneumococcal vaccine (Prevnar-13), pneumococcus (T4)-specific antibodies were significantly improved. Preliminary data further showed that after challenge with Streptococcus pneumoniae, all mice that had received VAS at the time of vaccination survived. This was a significant improvement compared to vaccination without VAS. Data encourage renewed attention to vitamin A levels, both in developed and developing countries, to assist interpretation of data from vaccine research and to improve the success of vaccine programs.

A Liposome-Based Adjuvant Containing Two Delivery Systems with the Ability to Induce Mucosal Immunoglobulin A Following a Parenteral Immunization

Worldwide, enteric infections rank third among all causes of disease burdens, and vaccines able to induce a strong and long-lasting intestinal immune responses are needed. Parenteral immunization generally do not generate intestinal IgA. Recently, however, injections of retinoic acid (RA) dissolved in oil, administered multiple times before vaccination to precondition the vaccine-draining lymph nodes, enabled a parenteral vaccine strategy to induce intestinal IgA. As multiple injections of RA before vaccination is not an attractive strategy for clinical practice, we aimed to develop a "one injection" vaccine formulation that upon parenteral administration induced intestinal IgA. Our vaccine formulation contained two liposomal delivery systems. One delivery system, based on 1,2-distearoyl- sn-glycero-3-phosphocholine stabilized with PEG, was designed to exhibit fast drainage of RA to local lymph nodes to precondition these for a mucosal immune response before being subjected to the vaccine antigen. The other delivery system, based on the cationic liposomal adjuvant CAF01 stabilized with cholesterol, was optimized for prolonged delivery of the antigen by migratory antigen-presenting cells to the preconditioned lymph node. Combined we call the adjuvant CAF23. We show that CAF23, administered by the subcutaneous route induces an antigen specific intestinal IgA response, making it a promising candidate adjuvant for vaccines against enteric diseases.

TGFβ Superfamily Members as Regulators of B Cell Development and Function-Implications for Autoimmunity

The TGFβ superfamily is composed of more than 33 growth and differentiation factors, including TGFβ1, β2, β3, BMPs, GDFs, nodal-related proteins, and activins. These members usually exert pleiotropic actions on several tissues and control multiple cellular processes, such as cell growth, cell survival, cell migration, cell fate specification, and differentiation, both during embryonic development and postnatal life. Although the effects of these factors on immune responses were elucidated long ago, most studies have been focused on the actions of TGFβs on T cells, as major regulators of adaptive immunity. In this review, we discuss new findings about the involvement of TGFβ superfamily members in the control of B cell development and function. Moreover, the potential contribution of TGFβ signaling to control B cell-mediated autoimmune diseases and its utility in the design of new therapies are also discussed.

Dietary antioxidant micronutrients alter mucosal inflammatory risk in a murine model of genetic and microbial susceptibility

Inflammatory bowel diseases (IBD) are caused by the convergence of microbial, environmental, and genetic factors. Diet significantly alters these interactions by affecting both the host and microbiome. Using a mucosal inflammatory model that resembles the human condition of ileal pouchitis, we investigated the effects of Control (CONT) or Antioxidant (AOX) diet, containing pharmacologically relevant levels of 4 micronutrients, on disease risk in wild-type and IL-10-/- animals following surgical self-filling (SF) ileal blind loop placement. Although no differences were found in body weight change or survival, IL-10-/- CONT animals had significantly larger lymphoid organs compared with IL-10-/- AOX or with WT. SF loops from IL-10-/- CONT loop mucosa demonstrated histological inflammation, characterized by goblet cell depletion, increased mucosal myeloperoxidase (MPO), and elevated IFNγ, TNFα, and IL-17α gene expression, which AOX attenuated. AOX elevated luminal IgA in IL-10-/- animals, but not significantly in WT. In IL-10-/- animals, AOX significantly decreased the percentage of CD4 + T-bet and CD4 + RORγ T-cells compared with CONT, with no changes in CD4 + Foxp3+ Treg cells. 16S rRNA gene sequencing demonstrated AOX increased microbial alpha diversity compared with CONT in both genotypes. Notably, colonizing germ-free IL-10-/- hosts with CONT bacterial communities, but not AOX, recapitulated the inflammatory phenotype. Collectively, these findings highlight that common dietary antioxidant micronutrients reshape the gut microbial community to mitigate intestinal inflammatory profiles in genetically susceptible hosts. Insights into the dietary-immune-microbial nexus may improve understanding for recurrent inflammatory episodes in susceptible patient populations and opportunities for practical therapeutics to restore immune and microbial homeostasis.

Impact of Childhood Malnutrition on Host Defense and Infection

The global impact of childhood malnutrition is staggering. The synergism between malnutrition and infection contributes substantially to childhood morbidity and mortality. Anthropometric indicators of malnutrition are associated with the increased risk and severity of infections caused by many pathogens, including viruses, bacteria, protozoa, and helminths. Since childhood malnutrition commonly involves the inadequate intake of protein and calories, with superimposed micronutrient deficiencies, the causal factors involved in impaired host defense are usually not defined. This review focuses on literature related to impaired host defense and the risk of infection in primary childhood malnutrition. Particular attention is given to longitudinal and prospective cohort human studies and studies of experimental animal models that address causal, mechanistic relationships between malnutrition and host defense. Protein and micronutrient deficiencies impact the hematopoietic and lymphoid organs and compromise both innate and adaptive immune functions. Malnutrition-related changes in intestinal microbiota contribute to growth faltering and dysregulated inflammation and immune function. Although substantial progress has been made in understanding the malnutrition-infection synergism, critical gaps in our understanding remain. We highlight the need for mechanistic studies that can lead to targeted interventions to improve host defense and reduce the morbidity and mortality of infectious diseases in this vulnerable population.

Mechanism underlying the suppressor activity of retinoic acid on IL4-induced IgE synthesis and its physiological implication

The present study extends an earlier report that retinoic acid (RA) down-regulates IgE Ab synthesis in vitro. Here, we show the suppressive activity of RA on IgE production in vivo and its underlying mechanisms. We found that RA down-regulated IgE class switching recombination (CSR) mainly through RA receptor α (RARα). Additionally, RA inhibited histone acetylation of germ-line ε (GL ε) promoter, leading to suppression of IgE CSR. Consistently, serum IgE levels were substantially elevated in vitamin A-deficient (VAD) mice and this was more dramatic in VAD-lecithin:retinol acyltransferase deficient (LRAT^-/-) mice. Further, serum mouse mast cell protease-1 (mMCP-1) level was elevated while frequency of intestinal regulatory T cells (Tregs) were diminished in VAD LRAT^-/- mice, reflecting that deprivation of RA leads to allergic immune response. Taken together, our results reveal that RA has an IgE-repressive activity in vivo, which may ameliorate IgE-mediated allergic disease.

The Impact of Western Diet and Nutrients on the Microbiota and Immune Response at Mucosal Interfaces

Recent findings point toward diet having a major impact on human health. Diets can either affect the gut microbiota resulting in alterations in the host’s physiological responses or by directly targeting the host response. The microbial community in the mammalian gut is a complex and dynamic system crucial for the development and maturation of both systemic and mucosal immune responses. Therefore, the complex interaction between available nutrients, the microbiota, and the immune system are central regulators in maintaining homeostasis and fighting against invading pathogens at mucosal sites. Westernized diet, defined as high dietary intake of saturated fats and sucrose and low intake of fiber, represent a growing health risk contributing to the increased occurrence of metabolic diseases, e.g., diabetes and obesity in countries adapting a westernized lifestyle. Inflammatory bowel diseases (IBD) and asthma are chronic mucosal inflammatory conditions of unknown etiology with increasing prevalence worldwide. These conditions have a multifactorial etiology including genetic factors, environmental factors, and dysregulated immune responses. Their increased prevalence cannot solely be attributed to genetic considerations implying that other factors such as diet can be a major contributor. Recent reports indicate that the gut microbiota and modifications thereof, due to a consumption of a diet high in saturated fats and low in fibers, can trigger factors regulating the development and/or progression of both conditions. While asthma is a disease of the airways, increasing evidence indicates a link between the gut and airways in disease development. Herein, we provide a comprehensive review on the impact of westernized diet and associated nutrients on immune cell responses and the microbiota and how these can influence the pathology of IBD and asthma.

Tissue adaptation: Implications for gut immunity and tolerance

Faria et al. discuss the concept that immune cells undergo specialized adaptation to tissue-specific conditions and its potential implications for tolerance and immunity.

Tissue adaptation is an intrinsic component of immune cell development, influencing both resistance to pathogens and tolerance. Chronically stimulated surfaces of the body, in particular the gut mucosa, are the major sites where immune cells traffic and reside. Their adaptation to these environments requires constant discrimination between natural stimulation coming from harmless microbiota and food, and pathogens that need to be cleared. This review will focus on the adaptation of lymphocytes to the gut mucosa, a highly specialized environment that can help us understand the plasticity of leukocytes arriving at various tissue sites and how tissue-related factors operate to shape immune cell fate and function.

An expanding stage for commensal microbes in host immune regulation

Gastrointestinal commensal microbiota is a concentrated mix of microbial life forms, including bacteria, fungi, archaea and viruses. These life forms are targets of host antimicrobial defense in order to establish a homeostatic symbiosis inside the host. However, they are also instrumental in shaping the functions of our immune system via a diverse set of communication mechanisms. In the gut, T helper 17, regulatory T and B cells are continuously tuned by specific microbial strains and metabolic processes. These cells in return help to establish a mutually beneficial exchange with the gut microbial contents. Imbalances in this symbiosis lead to dysregulations in the host's ability to control infections and the development of autoimmune diseases. In addition, the commensal microbiota has a significant and obligatory role in shaping both gut intrinsic and distal lymphoid organs, casting a large impact on the overall immune landscape in the host. This review discusses the major components of the microbial community in the gut and how its members collectively and individually exert regulatory roles in the host immune system and lymphoid structure development, as well as the functions of several major immune cell types.

Peyer's patches: organizing B-cell responses at the intestinal frontier

Secondary lymphoid tissues share the important function of bringing together antigens and rare antigen-specific lymphocytes to foster induction of adaptive immune responses. Peyer's patches (PPs) are unique compared to other secondary lymphoid tissues in their continual exposure to an enormous diversity of microbiome- and food-derived antigens and in the types of pathogens they encounter. Antigens are delivered to PPs by specialized microfold (M) epithelial cells and they may be captured and presented by resident dendritic cells (DCs). In accord with their state of chronic microbial antigen exposure, PPs exhibit continual germinal center (GC) activity. These GCs not only contribute to the generation of B cells and plasma cells producing somatically mutated gut antigen-specific IgA antibodies but have also been suggested to support non-specific antigen diversification of the B-cell repertoire. Here, we review current understanding of how PPs foster B-cell encounters with antigen, how they favor isotype switching to the secretory IgA isotype, and how their GC responses may uniquely contribute to mucosal immunity.

The dual nature of retinoic acid in pemphigus and its therapeutic potential: Special focus on all-trans Retinoic Acid

The efficient treatment of pemphigus with no certain side effect remained a controversial issue. Although there are various options for controlling disease severity, the majority of them may cause serious side effects. Retinoic acid (RA), an active metabolite converted from vitamin A, plays an active role in immune functions. Effects of RA, especially all-trans-Retinoic Acid (ATRA) on different types of cells involved in immune responses were analyzed in vitro and in vivo. RAs could affect the differentiation of T helper (Th) cells, B cells responses, stabilization of both natural regulatory T cells (nTregs) and regulatory B cells (Bregs) populations, and regulating the expression of critical genes in immune responses. The role of RA, based on major immune cells involved in pemphigus has not been addressed so far. In this study, we sought to determine the possible effects of RA, with a special focus on ATRA in pemphigus. All the evidences of ATRA effects on the immune system were collected and their association with the pemphigus was analyzed. According to the previous results, ATRA causes a decline in Th17 populations; increase in CD4+ induced regulatory T cells (iTregs), stabilization of nTregs, and promotion of suppressive B cells, which are critical in the improvement of pemphigus. Nevertheless, it also causes shifting of the Th1:Th2 balance toward Th2 cells, which is not favorable for pemphigus patients. In conclusion, ATRA acts via different ways in pemphigus. Due to increase in the suppressive function via iTregs, nTregs, and Bregs, it is suggested that patients with pemphigus may benefit from systemic ATRA therapy. To clarify this issue, further studies, such as clinical trials are needed.

Aberrantly methylated DNA regions lead to low activation of CD4+ T-cells in IgA nephropathy

IgAN (IgA nephropathy) is the most common form of primary glomerulonephritis worldwide and has a strong genetic component. In this setting, DNA methylation could also be an important factor influencing this disease. We performed a genome-wide screening for DNA methylation in CD4(+) T-cells from IgAN patients and found three regions aberrantly methylated influencing genes involved in the response and proliferation of CD4(+) T-cells. Two hypomethylated regions codified genes involved in TCR (T-cell receptor) signalling, TRIM27 (tripartite motif-containing 27) and DUSP3 (dual-specificity phosphatase 3), and an hypermethylated region included the VTRNA2-1 (vault RNA 2-1) non-coding RNA, also known as miR-886 precursor. We showed that the aberrant methylation influences the expression of these genes in IgAN patients. Moreover, we demonstrated that the hypermethylation of the miR-886 precursor led to a decreased CD4(+) T-cell proliferation following TCR stimulation and to the overexpression of TGFβ (transforming growth factor β). Finally, we found a Th1/Th2 imbalance in IgAN patients. The IL (interleukin)-2/IL-5 ratio was notably higher in IgAN patients and clearly indicated a Th1 shift. In conclusion, we identified for the first time some specific DNA regions abnormally methylated in IgAN patients that led to the reduced TCR signal strength of the CD4(+) T-cells and to their anomalous response and activation that could explain the T-helper cell imbalance. The present study reveals new molecular mechanisms underlying the abnormal CD4(+) T-cell response in IgAN patients.

Hotspots for Vitamin-Steroid-Thyroid Hormone Response Elements Within Switch Regions of Immunoglobulin Heavy Chain Loci Predict a Direct Influence of Vitamins and Hormones on B Cell Class Switch Recombination

Vitamin A deficiencies are common throughout the world and have a significant negative influence on immune protection against viral infections. Mouse models demonstrate that the production of IgA, a first line of defense against viruses at mucosal sites, is inhibited in the context of vitamin A deficiency. In vitro, the addition of vitamin A to activated B cells can enhance IgA expression, but downregulate IgE. Previous reports have demonstrated that vitamin A modifies cytokine patterns, and in so doing may influence antibody isotype expression by an indirect mechanism. However, we have now discovered hundreds of potential response elements among Sμ, Sɛ, and Sα switch sites within immunoglobulin heavy chain loci. These hotspots appear in both mouse and human loci and include targets for vitamin receptors and related proteins (e.g., estrogen receptors) in the nuclear receptor superfamily. Full response elements with direct repeats are relatively infrequent or absent in Sγ regions although half-sites are present. Based on these results, we pose a hypothesis that nuclear receptors have a direct effect on the immunoglobulin heavy chain class switch recombination event. We propose that vitamin A may alter S site accessibility to activation-induced deaminase and nonhomologous end-joining machinery, thereby influencing the isotype switch, antibody production, and protection against viral infections at mucosal sites.

Microbiota regulate the ability of lung dendritic cells to induce IgA class-switch recombination and generate protective gastrointestinal immune responses

Ruane et al. demonstrate a role for the microbiota in modulating protective immunity to intranasal vaccination via the ability of lung dendritic cells to induce B cell IgA class switching.

Protective immunoglobulin A (IgA) responses to oral antigens are usually orchestrated by gut dendritic cells (DCs). Here, we show that lung CD103⁺ and CD24⁺CD11b⁺ DCs induced IgA class-switch recombination (CSR) by activating B cells through T cell–dependent or –independent pathways. Compared with lung DCs (LDC), lung CD64⁺ macrophages had decreased expression of B cell activation genes and induced significantly less IgA production. Microbial stimuli, acting through Toll-like receptors, induced transforming growth factor-β (TGF-β) production by LDCs and exerted a profound influence on LDC-mediated IgA CSR. After intranasal immunization with inactive cholera toxin (CT), LDCs stimulated retinoic acid–dependent up-regulation of α4β7 and CCR9 gut-homing receptors on local IgA-expressing B cells. Migration of these B cells to the gut resulted in IgA-mediated protection against an oral challenge with active CT. However, in germ-free mice, the levels of LDC-induced, CT–specific IgA in the gut are significantly reduced. Herein, we demonstrate an unexpected role of the microbiota in modulating the protective efficacy of intranasal vaccination through their effect on the IgA class-switching function of LDCs.

Retinoic acid enhances lactoferrin-induced IgA responses by increasing betaglycan expression

Lactoferrin (LF) and retinoic acid (RA) are enriched in colostrum, milk, and mucosal tissues. We recently showed that LF-induced IgA class switching through binding to betaglycan (transforming growth factor-beta receptor III, TβRIII) and activation of canonical TGF-β signaling. We investigated the combined effect of LF and RA on the overall IgA response. An increase in IgA production by LF was further augmented by RA. This combination effect was also evident in Ig germ-line α (GLα) transcription and GLα promoter activity, indicating that LF in cooperation with RA increased IgA isotype switching. We subsequently found that RA enhanced TβRIII expression and that this increase contributed to LF-stimulated IgA production. In addition to the IgA response, LF and RA in combination also enhanced the expression of the gut-homing molecules C-C chemokine receptor 9 (CCR9) and α4β7 on B cells. Finally, peroral administration of LF and RA enhanced the frequency of CCR9⁺IgA⁺ plasma cells in the lamina propria. Taken together, these results suggest that LF in cooperation with RA can contribute to the establishment of gut IgA responses.

Lactoferrin causes IgA and IgG2b isotype switching through betaglycan binding and activation of canonical TGF-β signaling

Lactoferrin (LF), a pleiotropic iron-binding glycoprotein, is known to modulate the humoral immune response. However, its exact role in Ig synthesis has yet to be elucidated. In this study, we investigated the effect of LF on Ig production by mouse B cells and its underlying mechanisms. LF, like transforming growth factor (TGF)-β1, stimulated B cells to produce IgA and IgG2b, while downregulating other isotypes. Using limiting dilution analysis, LF was shown to increase the frequency of IgA-secreting B-cell clones. This was paralleled by an increase in Ig germ-line α (GLα) transcripts, indicating that LF plays a role as an IgA switch factor. Interestingly, LF directly interacted with betaglycan (TGF-β receptor III, TβRIII) and in turn induced phosphorylation of TβRI and Smad3 through formation of the TβRIII/TβRII/TβRI complex, leading to IgA isotype switching. Peroral administration of LF increased intestinal/serum IgA production as well as number of IgA plasma cells in lamina propria. Finally, we found that LF has an adjuvant activity when nontoxigenic Salmonella typhimurium was inoculated perorally, conferring protection against intragastrical infection of toxigenic S. typhimurium. These results suggest that LF has an important effect on the mucosal/systemic IgA response and can contribute to protection against intestinal pathogens.

Lactoferrin Combined with Retinoic Acid Stimulates B1 Cells to Express IgA Isotype and Gut-homing Molecules

It is well established that TGF-β1 and retinoic acid (RA) cause IgA isotype switching in mice. We recently found that lactoferrin (LF) also has an activity of IgA isotype switching in spleen B cells. The present study explored the effect of LF on the Ig production by mouse peritoneal B cells. LF, like TGF-β1, substantially increased IgA production in peritoneal B1 cells but little in peritoneal B2 cells. In contrast, LF increased IgG2b production in peritoneal B2 cells much more strongly than in peritoneal B1 cells. LF in combination with RA further enhanced the IgA production and, interestingly, this enhancement was restricted to IgA isotype and B1 cells. Similarly, the combination of the two molecules also led to expression of gut homing molecules α4β7 and CCR9 on peritoneal B1 cells, but not on peritoneal B2 cells. Thus, these results indicate that LF and RA can contribute to gut IgA response through stimulating IgA isotype switching and expression of gut-homing molecules in peritoneal B1 cells.

Intestinal IgA production and its role in host-microbe interaction

Complex and diverse communities of bacteria establish mutualistic and symbiotic relationships with the gut after birth. The intestinal immune system responds to bacterial colonization by acquiring a state of hypo-responsiveness against commensals and active readiness against pathogens. The resulting homeostatic balance involves a continuous dialog between the microbiota and lymphocytes with the intermediation of epithelial and dendritic cells. This dialog causes massive production of immunoglobulin A (IgA), a non-inflammatory antibody specialized in mucosal protection. Here, we discuss recent advances on the regulation of intestinal IgA responses and their role in host-microbe interaction.