Plasticity of Th17 cells in Peyer's patches is responsible for the induction of T cell-dependent IgA responses. Nat Immunol. 2013;14:372-379. [PMID: 23475182 DOI: 10.1038/ni.2552]

Regulation of the Germinal Center Response

The germinal center (GC) is a specialized microstructure that forms in secondary lymphoid tissues, producing long-lived antibody secreting plasma cells and memory B cells, which can provide protection against reinfection. Within the GC, B cells undergo somatic mutation of the genes encoding their B cell receptors which, following successful selection, can lead to the emergence of B cell clones that bind antigen with high affinity. However, this mutation process can also be dangerous, as it can create autoreactive clones that can cause autoimmunity. Because of this, regulation of GC reactions is critical to ensure high affinity antibody production and to enforce self-tolerance by avoiding emergence of autoreactive B cell clones. A productive GC response requires the collaboration of multiple cell types. The stromal cell network orchestrates GC cell dynamics by controlling antigen delivery and cell trafficking. T follicular helper (Tfh) cells provide specialized help to GC B cells through cognate T-B cell interactions while Foxp3⁺ T follicular regulatory (Tfr) cells are key mediators of GC regulation. However, regulation of GC responses is not a simple outcome of Tfh/Tfr balance, but also involves the contribution of other cell types to modulate the GC microenvironment and to avoid autoimmunity. Thus, the regulation of the GC is complex, and occurs at multiple levels. In this review we outline recent developments in the biology of cell subsets involved in the regulation of GC reactions, in both secondary lymphoid tissues, and Peyer's patches (PPs). We discuss the mechanisms which enable the generation of potent protective humoral immunity whilst GC-derived autoimmunity is avoided.

In Vitro Th17-Polarized Human CD4+ T Cells Exacerbate Xenogeneic Graft-versus-Host Disease

Acute graft-versus-host disease (aGVHD) is a severe complication of allogeneic hematopoietic stem cell transplantation. The role of Th17 cells in its pathophysiology remains a matter of debate. In this study, we assessed whether enrichment of human peripheral blood mononuclear cells (PBMCs) with in vitro Th17-polarized CD4⁺ T cells would exacerbate xenogeneic GVHD (xGVHD) into NOD-scid IL-2Rγ null (NSG) mice. Naive human CD4⁺ T cells were stimulated under Th17-skewing conditions for 8 to 10 days and then coinjected in NSG mice with fresh PBMCs from the same donor. We observed that Th17-polarized cells engrafted and migrated toward xGVHD target organs. They also acquired a double-expressing IL-17A⁺IFNγ⁺ profile in vivo. Importantly, cotransfer of Th17-polarized cells (1 × 10⁶) with PBMCs (1 × 10⁶) exacerbated xGVHD compared with transplantation of PBMCs alone (2 × 10⁶). Furthermore, PBMC cotransfer with Th17-polarized cells was more potent for xGVHD induction than cotransfer with naive CD4⁺ T cells stimulated in nonpolarizing conditions (Th0 cells, 1 × 10⁶ + 1 × 10⁶ PBMCs) or with Th1-polarized cells (1 × 10⁶ + 1 × 10⁶ PBMCs). In summary, our results suggest that human Th17-polarized cells can cooperate with PBMCs and be pathogenic in the NSG xGVHD model.

Gut microbiota: Role in pathogen colonization, immune responses, and inflammatory disease

The intestinal tract of mammals is colonized by a large number of microorganisms including trillions of bacteria that are referred to collectively as the gut microbiota. These indigenous microorganisms have co-evolved with the host in a symbiotic relationship. In addition to metabolic benefits, symbiotic bacteria provide the host with several functions that promote immune homeostasis, immune responses, and protection against pathogen colonization. The ability of symbiotic bacteria to inhibit pathogen colonization is mediated via several mechanisms including direct killing, competition for limited nutrients, and enhancement of immune responses. Pathogens have evolved strategies to promote their replication in the presence of the gut microbiota. Perturbation of the gut microbiota structure by environmental and genetic factors increases the risk of pathogen infection, promotes the overgrowth of harmful pathobionts, and the development of inflammatory disease. Understanding the interaction of the microbiota with pathogens and the immune system will provide critical insight into the pathogenesis of disease and the development of strategies to prevent and treat inflammatory disease.

Advances in T follicular helper and T follicular regulatory cells in transplantation immunity

B cells play a crucial role in alloreactivity of organ transplant rejection and graft versus host diseases (GVHD). Over the past decade, it has been well recognized that B-cell infiltration in allografts and de novo donor-specific antibodies (DSA) were strongly associated with severe graft rejection and loss, as well as glucocorticoid resistance. Emerging evidence has demonstrated that Follicular T helper (Tfh) cells are key effectors to promote the proliferation and differentiation of B cells into antibody-producing plasmablasts and memory B cells. T-follicular regulatory (Tfr) cells are a recently recognized cell population that has a negative regulatory role on Tfh cells in the follicle, preventing incessant antibody production. It is still less clear how those humoral immunoreactive cells affect transplant rejection and allograft loss. This review focuses on the production and function of Tfr/Tfh cells in the transplant environment. Better understanding of the functions and mechanisms of Tfr/Tfh cells will help to design new strategies to prevent allograft rejection and prolong graft survival.

Transient T-bet expression functionally specifies a distinct T follicular helper subset

The mechanisms underlying the differentiation of T follicular helper (Tfh) cell subsets are poorly understood. Here, Fang et al. show that the NKG2D^high Tfh cells in germinal centers with a history of T-bet expression represent the IFN-γ–producing Tfh subset.

T follicular helper (Tfh) cells express transcription factor BCL-6 and cytokine IL-21. Mature Tfh cells are also capable of producing IFN-γ without expressing the Th1 transcription factor T-bet. Whether this IFN-γ–producing Tfh population represents a unique Tfh subset with a distinct differentiation pathway is poorly understood. By using T-bet fate–mapping mouse strains, we discovered that almost all the IFN-γ–producing Tfh cells have previously expressed T-bet and express high levels of NKG2D. DNase I hypersensitivity analysis indicated that the Ifng gene locus is partially accessible in this “ex–T-bet” population with a history of T-bet expression. Furthermore, multicolor tissue imaging revealed that the ex–T-bet Tfh cells found in germinal centers express IFN-γ in situ. Finally, we found that IFN-γ–expressing Tfh cells are absent in T-bet–deficient mice, but fully present in mice with T-bet deletion at late stages of T cell differentiation. Together, our findings demonstrate that transient expression of T-bet epigenetically imprints the Ifng locus for cytokine production in this Th1-like Tfh cell subset.

The role of transforming growth factor β in T helper 17 differentiation

T helper 17 (Th17) cells play critical roles in inflammatory and autoimmune diseases. The lineage-specific transcription factor RORγt is the key regulator for Th17 cell fate commitment. A substantial number of studies have established the importance of transforming growth factor β (TGF-β) -dependent pathways in inducing RORγt expression and Th17 differentiation. TGF-β superfamily members TGF-β1 , TGF-β3 or activin A, in concert with interleukin-6 or interleukin-21, differentiate naive T cells into Th17 cells. Alternatively, Th17 differentiation can occur through TGF-β-independent pathways. However, the mechanism of how TGF-β-dependent and TGF-β-independent pathways control Th17 differentiation remains controversial. This review focuses on the perplexing role of TGF-β in Th17 differentiation, depicts the requirement of TGF-β for Th17 development, and underscores the multiple mechanisms underlying TGF-β-promoted Th17 generation, pathogenicity and plasticity. With new insights and comprehension from recent findings, this review specifically tackles the involvement of the canonical TGF-β signalling components, SMAD2, SMAD3 and SMAD4, summarizes diverse SMAD-independent mechanisms, and highlights the importance of TGF-β signalling in balancing the reciprocal conversion of Th17 and regulatory T cells. Finally, this review includes discussions and perspectives and raises important mechanistic questions about the role of TGF-β in Th17 generation and function.

Transcriptional regulation of CD4+ TH cells that mediate tissue inflammation

Acquired and genetic immunodeficiencies have revealed an indispensable role for CD4⁺ T cells in the induction of protective host immune responses against a myriad of microbial pathogens. Influenced by the cytokines present in the microenvironment, activated CD4⁺ T cells may differentiate into several highly-specialized helper subsets defined by the production of distinct signature cytokines tailored to combat diverse classes of pathogens. The process of specification and differentiation is controlled by networks of core, master, and accessory transcription factors, which ensure that CD4⁺ T helper (T_H ) cell responses mounted against an invading microbe are of the correct specificity and type. However, aberrant activation or inactivation of transcription factors can result in sustained and elevated expression of immune-related genes, leading to chronic activation of CD4⁺ T_H  cells and organ-specific autoimmunity. In this review, we provide an overview of the molecular basis of CD4⁺ T_H  cell differentiation and examine how combinatorial expression of transcription factors, which promotes genetic plasticity of CD4⁺ T_H  cells, can contribute to immunological dysfunction of CD4⁺ T_H responses. We also discuss recent studies which highlight the potential of exploiting the genetic plasticity of CD4⁺ T_H  cells in the treatment of autoimmune and other immune-mediated disorders.

Several Follicular Regulatory T Cell Subsets With Distinct Phenotype and Function Emerge During Germinal Center Reactions

An efficient B cell immunity requires a dynamic equilibrium between positive and negative signals. In germinal centers (GCs), T follicular helper cells are supposed to be the positive regulator while T follicular regulatory (Tfr) cells were assigned to be the negative regulators. Indeed, Tfr cells are considered as a homogenous cell population dedicated to dampen the GC extent. Moreover, Tfr cells prevent autoimmunity since their dysregulation leads to production of self-reactive antibodies (Ab). However, a growing corpus of evidence has revealed additional and unexpected functions for Tfr cells in the regulation of B cell responses. This review provides an overview of the Tfr cell contribution and presents Tfr cell proprieties in the context of vaccination.

T cell-intrinsic IL-1R signaling licenses effector cytokine production by memory CD4 T cells

Innate cytokines are critical drivers of priming and differentiation of naive CD4 T cells, but their functions in memory T cell response are largely undefined. Here we show that IL-1 acts as a licensing signal to permit effector cytokine production by pre-committed Th1 (IFN-γ), Th2 (IL-13, IL-4, and IL-5) and Th17 (IL-17A, IL-17F, and IL-22) lineage cells. This licensing function of IL-1 is conserved across effector CD4 T cells generated by diverse immunological insults. IL-1R signaling stabilizes cytokine transcripts to enable productive and rapid effector functions. We also demonstrate that successful lineage commitment does not translate into productive effector functions in the absence of IL-1R signaling. Acute abrogation of IL-1R signaling in vivo results in reduced IL-17A production by intestinal Th17 cells. These results extend the role of innate cytokines beyond CD4 T cell priming and establish IL-1 as a licensing signal for memory CD4 T cell function.

CD4 T cell polarizations and functions are regulated by cytokines from innate cells. Here the authors show that IL-1 deficiency does not impair the differentiation of Th1, Th2 and Th17, but IL-1 signaling is required for maintaining the expressions of their respective key cytokines to ‘license’ the functions of these T cell subsets.

A model of TH17-associated ileal hyperplasia that requires both IL-17A and IFNγ to generate self-tolerance and prevent colitis

Homeostasis in the ileum, which is commonly disrupted in patients with Crohn's disease, involves ongoing immune responses. To study how homeostatic processes of the ileum impact CD4⁺T cell responses, we used TCR transgenic tools to breed mice that spontaneously produced CD4⁺T cells reactive to an antigen expressed in the ileum. At an early age, the ilea of these mice exhibit crypt hyperplasia and accumulate increased numbers of T_H17 cells bearing non-transgenic clonotypes. Half of these mice subsequently developed colitis linked to broad mucosal infiltration by T_H17 and T_H1 cells expressing non-transgenic clonotypes, chronic wasting disease and loss of ileal crypt hyperplasia. By contrast, adult mice with normal growth continued to exhibit T_H17-associated ileal crypt hyperplasia and additionally accumulated ileal-reactive Treg cells. Both IL-17A and IFNγ were protective, as their deficiency precluded ileal-reactive Treg accumulation and exacerbated colitic disease. IL-23R blockade prevented progression to colitis, whereas nTreg cell transfers prevented colitic disease, ileal crypt hyperplasia and ileal-reactive Treg accumulation. Thus, our studies identify an IL-17A and IFNγ-dependent homeostatic process that mobilizes ileal-reactive Treg cells and is disrupted by IL-23.

Suppression of IL-17F, but not of IL-17A, provides protection against colitis by inducing Treg cells through modification of the intestinal microbiota

The cytokines IL-17A and IL-17F have 50% amino-acid identity and bind the same receptor; however, their functional differences have remained obscure. Here we found that Il17f^-/- mice resisted chemically induced colitis, but Il17a^-/- mice did not, and that Il17f^-/- CD45RB^hiCD4⁺ T cells induced milder colitis in lymphocyte-deficient Rag2^-/- mice, accompanied by an increase in intestinal regulatory T cells (T_reg cells). Clostridium cluster XIVa in colonic microbiota capable of inducing T_reg cells was increased in both Il17f^-/- mice and mice given transfer Il17f^-/- T cells, due to decreased expression of a group of antimicrobial proteins. There was substantial production of IL-17F, but not of IL-17A, not only by naive T cells but also by various colon-resident cells under physiological conditions. Furthermore, antibody to IL-17F suppressed the development of colitis, but antibody to IL-17A did not. These observations suggest that IL-17F is an effective target for the treatment of colitis.

Sublingual Adjuvant Delivery by a Live Attenuated Vibrio cholerae-Based Antigen Presentation Platform

Diarrheal disease is the most common infectious disease of children in the developing world. Our goal is to develop a diarrheal antigen presentation platform based on whole Vibrio cholerae cells that does not depend on protein purification. We have previously shown the feasibility of genetically fusing antigens to the V. cholerae biofilm matrix protein RbmA for presentation on the cell surface. A mucosal adjuvant could improve immunogenicity of such a vaccine at the mucosal surface. Here we engineer a live attenuated V. cholerae vaccine to constitutively synthesize mmCT, a nontoxic form of cholera toxin. When this vaccine is delivered sublingually, in vivo-synthesized mmCT acts as both an adjuvant and antigen. This could greatly increase the magnitude and duration of the immune response elicited by codelivered heterologous antigens.

A sublingually delivered heterologous antigen presentation platform that does not depend on antigen or adjuvant purification would be of great benefit in protection against diarrheal disease. In proof-of-concept studies, we previously showed that when a fusion protein comprised of the Vibrio cholerae biofilm matrix protein RbmA and the B subunit of cholera toxin (R-CTB) is expressed from a plasmid within V. cholerae, R-CTB is sequestered in the biofilm matrix, leading to decoration of the cell surface. Sublingual delivery of live attenuated R-CTB-decorated cells results in a mucosal immune response to CTB. To improve the immune response to diarrheal antigens presented by this platform, we have engineered our live attenuated vaccine to express the mucosal adjuvant mmCT (i.e., multiply mutated CT). Here we report that delivery of this adjuvant via sublingual administration of our vaccine enhances the mucosal immune response to V. cholerae LPS and elicits a systemic and mucosal immune response to CTB. However, provision of R-CTB with mmCT selectively blunts the mucosal immune response to CTB. We propose that mmCT delivered by this live attenuated Vibrio cholerae vaccine platform may serve as a mucosal adjuvant for heterologous antigens, provided they are not too similar to mmCT.

IMPORTANCE Diarrheal disease is the most common infectious disease of children in the developing world. Our goal is to develop a diarrheal antigen presentation platform based on whole Vibrio cholerae cells that does not depend on protein purification. We have previously shown the feasibility of genetically fusing antigens to the V. cholerae biofilm matrix protein RbmA for presentation on the cell surface. A mucosal adjuvant could improve immunogenicity of such a vaccine at the mucosal surface. Here we engineer a live attenuated V. cholerae vaccine to constitutively synthesize mmCT, a nontoxic form of cholera toxin. When this vaccine is delivered sublingually, in vivo-synthesized mmCT acts as both an adjuvant and antigen. This could greatly increase the magnitude and duration of the immune response elicited by codelivered heterologous antigens.

Insight Into Non-Pathogenic Th17 Cells in Autoimmune Diseases

Th17 cells are generally considered to be positive regulators of immune responses because they produce pro-inflammatory cytokines, including IL-17A, IL-17F, and IL-22. Cytokine production not only promotes accumulation of immune cells, such as macrophages, neutrophils and lymphocytes, at inflammatory sites but can also cause tissue pathologies. Conversely, certain Th17 cells can also negatively regulate immune responses by secreting immunosuppressive factors, such as IL-10; these cells are termed non-pathogenic Th17 cells. In this review, we summarize recent advances in the development and regulatory functions of non-pathogenic Th17 cells in autoimmune diseases.

ADP-ribosylating enterotoxins as vaccine adjuvants

Most infections are caused by pathogens that access the body at mucosal sites. Hence, development of mucosal vaccines to prevent local infection or invasion of pathogens appears highly warranted, especially since only mucosal immunization will stimulate strong local IgA responses and tissue resident memory CD4 and CD8 T cells. The most significant obstacle to developing such vaccines is the lack of approved adjuvants that can effectively and safely enhance relevant mucosal and systemic immune responses. The most potent mucosal adjuvants known today are the adenosine diphosphate (ADP)-ribosylating bacterial enterotoxins cholera toxin (CT) and Escherichia coli heat-labile toxins (LTs). Unfortunately, these molecules are also very toxic, which precludes their clinical use. However, much effort has been devoted to developing derivatives of these enterotoxins with low or no toxicity and retained adjuvant activity. Although it is fair to say that we know more about how these toxins affect the immune system than ever before, we still lack a detailed understanding of how and why these toxins are effective adjuvants. In the present review, we provide a state-of-the-art overview of the mechanism of action of the holotoxins and the strategies used for improving the toxin-based adjuvants.

Shared and Unique Features Distinguishing Follicular T Helper and Regulatory Cells of Peripheral Lymph Node and Peyer's Patches

Follicular helper (TFH) and regulatory (TFR) cells are critical players in managing germinal center (GC) reactions that accomplish effective humoral immune responses. Transcriptome analyses were done comparing gene regulation of TFH and TFR cells isolated from Peyer’s Patches (PP) and immunized peripheral lymph nodes (pLNs) revealing many regulatory patterns common to all follicular cells. However, in contrast to TFH cells, the upregulation or downregulation of many genes was attenuated substantially in pLN TFR cells when compared to those of PP. Additionally, PP but not pLN TFR cells were largely unresponsive to IL2 and expressed Il4 as well as Il21. Together with fundamental differences in gene expression that were found between cells of both compartments this emphasizes specific adaptations of follicular T cell functions to their micro-milieu. Moreover, although GL7 expression distinguishes matured follicular T cells, GL7⁺ as well as GL7⁻ cells are present in the GC.

Gut microbiome modulates efficacy of immune checkpoint inhibitors

Immune checkpoint inhibitors (ICIs) therapy is a novel strategy for cancer treatments in recent years. However, it was observed that most patients treated with ICIs could not get benefit from the therapy, which led to the limitation of clinical application. Motivated by potent and durable efficacy of ICIs, oncologists endeavor to explore the mechanisms of resistance to ICIs and increase the drug sensitivity. It is known that heterogeneity of gut microbiome in populations may result in different outcomes of therapy. In xenograft model, bacteria in gut have been proved as a crucial factor regulating immunotherapy efficacy. And the similar phenomenon was obtained in patients. In this review, we summarized relevant advancements about gut microbiome and ICIs. Furthermore, we focused on modulatory function of gut microbiome in ICIs therapy and possible antitumor mechanism of specific commensals in ICIs treatment. We propose that gut microbiome is an important predictive factor, and manipulation of gut microbiome is feasible to elevate response rate in ICIs therapy.

Role of TFH Cells in Promoting T Helper 17-Induced Neuroinflammation

Both T cells and B cells are implicated in the pathology of multiple sclerosis (MS), but how these cells cooperate to drive disease remains unclear. Recent studies using experimental autoimmune encephalomyelitis (EAE) demonstrated that the TH17 pathway is correlated with increased numbers of ectopic B-cell follicles in the central nervous system (CNS). As follicular T helper (TFH) cells are regulators of B cell responses, we sought to examine the role of TFH cells in EAE induced by the transfer of myelin-specific TH17 cells (TH17-EAE). In this study, we first confirmed previous reports that B-cells are a major cell type infiltrating the CNS during TH17-EAE. In addition, we found that B cells contribute to the severity of TH17-EAE. Class-switched B-cells in the CNS were positively correlated with disease and, strikingly, the severity TH17-EAE was diminished in B cell deficient mice. We next focused on the role TFH cells play in TH17-EAE. We found substantial numbers of CXCR5⁺PD1⁺CD4⁺ TFH cells in the CNS tissue of TH17-EAE mice and that at the peak of disease, the number of infiltrating TFHs was correlated with the number of infiltrating B-cells. Using congenic CD45.1⁺ donor mice and CD45.2⁺ recipient mice, we determined that the TFH cells were recipient-derived, whereas IL-17⁺ cells were donor-derived. We assessed whether myelin-specific TFH cells are capable of inducing EAE in recipient mice and found that transferring TFH cells failed to induce EAE. Finally, we tested the effects of blocking TFH trafficking in TH17-EAE using an antagonistic antibody against CXCL13, the chemokine ligand for CXCR5 on TFH cells. We found anti-CXCL13 treatment significantly reduced TH17-EAE disease. This treatment blocked CD4⁺ T cells from entering the CNS, but had no effect on infiltration of B cells. Strikingly, this antibody treatment had no measurable effect on TH17 disease in B cell-deficient mice. These data demonstrate that infiltrating TFH cells are a key cell type that contributes to an inflammatory B cell response in TH17-EAE and provide evidence for targeting TFH cells as a treatment for neuro-autoimmune diseases like MS.

Commensal Microbes Induce Serum IgA Responses that Protect against Polymicrobial Sepsis

Serum immunoglobulin A (IgA) antibodies are readily detected in mice and people, but the mechanisms underlying the induction of serum IgA and its role in host protection remain uncertain. We report that select commensal bacteria induce several facets of systemic IgA-mediated immunity. Exposing conventional mice to a unique but natural microflora that included several members of the Proteobacteria phylum led to T cell-dependent increases in serum IgA levels and the induction of large numbers of IgA-secreting plasma cells in the bone marrow. The resulting serum IgA bound to a restricted collection of bacterial taxa, and antigen-specific serum IgA antibodies were readily induced after intestinal colonization with the commensal bacterium Helicobacter muridarum. Finally, movement to a Proteobacteria-rich microbiota led to serum IgA-mediated resistance to polymicrobial sepsis. We conclude that commensal microbes overtly influence the serum IgA repertoire, resulting in constitutive protection against bacterial sepsis.

Understanding Subset Diversity in T Cell Memory

Considerable advances have been made in recent years in understanding the generation and function of memory T cells. Memory T cells are typically parsed into discreet subsets based on phenotypic definitions that connote distinct roles in immunity. Here we consider new developments in the field and focus on how emerging differences between memory cells with respect to their trafficking, metabolism, epigenetic regulation, and longevity may fail to fit into small groups of "memory subsets." Rather, the properties of individual memory T cells fall on a continuum within each of these and other parameters. We discuss how this continuum influences the way that the efficacy of vaccination is assessed, as well as the suitability of a memory population for protective immunity.

Galectin-9 Is Critical for Mucosal Adaptive Immunity through the T Helper 17-IgA Axis

Impairment of the intestinal mucosal immunity significantly increases the risk of acute and chronic diseases. IgA plays a major role in humoral mucosal immunity to provide protection against pathogens and toxins in the gut. Here, we investigated the role of endogenous galectin-9, a tandem repeat-type β-galactoside-binding protein, in intestinal mucosal immunity. By mucosal immunization of Lgals9^-/- and littermate control mice, it was found that lack of galectin-9 impaired mucosal antigen-specific IgA response in the gut. Moreover, Lgals9^-/- mice were more susceptible to developing watery diarrhea and more prone to death in response to high-dose cholera toxin. The results indicate the importance of galectin-9 in modulating intestinal adaptive immunity. Furthermore, bone marrow chimera mice were established, and galectin-9 in hematopoietic cells was found to be critical for adaptive IgA response. In addition, immunized Lgals9^-/- mice exhibited lower expression of Il17 and fewer T helper 17 (Th17) cells in the lamina propria, implying that the Th17-IgA axis is involved in this mechanism. Taken together, these findings suggest that galectin-9 plays a role in mucosal adaptive immunity through the Th17-IgA axis. By manipulating the expression or activity of galectin-9, intestinal mucosal immune response can be altered and may benefit the development of mucosal vaccination.

An immune-related gene expression atlas of the shrimp digestive system in response to two major pathogens brings insights into the involvement of hemocytes in gut immunity

Much of our current knowledge on shrimp immune system is restricted to the defense reactions mediated by the hemocytes and little is known about gut immunity. Here, we have investigated the transcriptional profile of immune-related genes in different organs of the digestive system of the shrimp Litopenaeus vannamei. First, the tissue distribution of 52 well-known immune-related genes has been assessed by semiquantitative analysis in the gastrointestinal tract (foregut, midgut and hindgut) and in the hepatopancreas and circulating hemocytes of shrimp stimulated or not with heat-killed bacteria. Then, the expression levels of 18 genes from key immune functional categories were quantified by fluorescence-based quantitative PCR in the midgut of animals experimentally infected with the Gram-negative Vibrio harveyi or the White spot syndrome virus (WSSV). Whereas the expression of some genes was induced at 48 h after the bacterial infection, any of the analyzed genes showed to be modulated in response to the virus. Whole-mount immunofluorescence assays confirmed the presence of infiltrating hemocytes in the intestines, indicating that the expression of some immune-related genes in gut is probably due to the migratory behavior of these circulating cells. This evidence suggests the participation of hemocytes in the delivery of antimicrobial molecules into different portions of the digestive system. Taken all together, our results revealed that gut is an important immune organ in L. vannamei with intimate association with hemocytes.

IgA Function in Relation to the Intestinal Microbiota

IgA is the dominant immunoglobulin isotype produced in mammals, largely secreted across the intestinal mucosal surface. Although induction of IgA has been a hallmark feature of microbiota colonization following colonization in germ-free animals, until recently appreciation of the function of IgA in host-microbial mutualism has depended mainly on indirect evidence of alterations in microbiota composition or penetration of microbes in the absence of somatic mutations in IgA (or compensatory IgM). Highly parallel sequencing techniques that enable high-resolution analysis of either microbial consortia or IgA sequence diversity are now giving us new perspectives on selective targeting of microbial taxa and the trajectory of IgA diversification according to induction mechanisms, between different individuals and over time. The prospects are to link the range of diversified IgA clonotypes to specific antigenic functions in modulating the microbiota composition, position and metabolism to ensure host mutualism.

TH17 cell plasticity: The role of dendritic cells and molecular mechanisms

Upon interaction with dendritic cells (DCs), naïve CD4 T cells differentiate into distinct subsets and orchestrate the development of a physiological immune response. When uncontrolled by cellular and molecular mechanisms, CD4 T cells can also lead to immune mediated inflammatory diseases (IMIDs). Initially, these distinct CD4 T-cell subsets were defined according to the expression of a limited number of cytokines. Later it was revealed that CD4 T cells can acquire much more complex functional phenotypes than previously thought. Experimental data showed that the CD4 T-cell subset T_H17 can secrete IFN-γ and IL-4, which are signature molecules of other T-cell subsets. Furthermore, some T_H17 cells can also explore an anti-inflammatory fate and participate in the resolution of the immune response. A more flexible theory has therefore evolved with the scope to better represent the plastic biology of CD4 T cells. In this context, several aspects still remain unclear. The goal of this review is to discuss the role of extrinsic and intrinsic cellular and molecular mechanisms, which can drive the plasticity of T_H17 cells. In particular, we will outline the role of DCs and the function of transcriptional factors in shaping the fate of T_H17 cells towards either a pathogenic or a regulatory phenotype. Finally, we will discuss whether T_H17 cell plasticity could be a target for new therapies for IMIDs. We indeed envision that when the cellular and molecular mechanisms controlling T_H17 plasticity are known, new therapies, which aim to reset the immune system, will be developed. This will be achieved by either selectively depleting only the pathogenic T_H17 cells or, if possible, re converting these cells from pathogenic to regulatory. This will overcome the challenge posed by the immune suppressive side effects caused by the current therapies, which impair the function of CD4 cells or delete all of them, to the detriment of the patient.

Basic Aspects of T Helper Cell Differentiation

CD4+ T helper cells orchestrate the immune response and play a pivotal role during infection, chronic inflammatory, autoimmune diseases, and carcinogenesis. CD4+ T helper cells can be subdivided into different subsets, which are characterized by a specific network of transcriptional regulators and unique cytokine profiles: Th17 cells express RORγt that in turn promotes the transcription of Il17a, Il17f; Th1 cells, expresses T-bet and produces IFN-γ, IL-2, and TNF-α; Th2 cells express GATA-3 and secrete IL-4, IL-5, and IL-13. The two most studied regulatory T cell subtypes are Foxp3+ regulatory T cells, which can be generated either in the thymus (tTreg) or induced in peripheral lymphoid organs (pTregs) and type 1 regulatory T cells (Tr1), which are induced in the periphery. These T helper cell subsets can be differentiated from naïve T cells. In addition, recent findings indicate that some T helper cell subsets can emerge from other T helper cells, suggesting a certain degree of plastiticy. Here we report basic aspects of T helper cell differentiation and function while underlining some still open questions.

A cellular and molecular view of T helper 17 cell plasticity in autoimmunity

Since the original identification of the T helper 17 (Th17) subset in 2005, it has become evident that these cells do not only contribute to host defence against pathogens, such as bacteria and fungi, but that they are also critically involved in the pathogenesis of many autoimmune diseases. In contrast to the classic Th1 and Th2 cells, which represent rather stably polarized subsets, Th17 cells display remarkable heterogeneity and plasticity. This has been attributed to the characteristics of the key transcription factor that guides Th17 differentiation, retinoic acid receptor-related orphan nuclear receptor gamma (RORγ). Unlike the 'master regulators' T-bet and GATA3 that orchestrate Th1 and Th2 differentiation, respectively, RORγ controls transcription at relatively few loci in Th17 cells. Moreover, its expression is not stabilized by positive feedback loops but rather influenced by environmental cues, allowing for substantial functional plasticity. Importantly, a subset of IL-17/IFNγ double-producing Th17 cells was identified in both human and mouse models. Evidence is accumulating that these IL-17/IFNγ double-producing cells are pathogenic drivers in autoimmune diseases, including rheumatoid arthritis, multiple sclerosis and inflammatory bowel disease. In addition, IL-17/IFNγ double-producing cells have been identified in disorders in which the role of autoimmunity remains unclear, such as sarcoidosis. The observed plasticity of Th17 cells towards the Th1 phenotype can be explained by extensive epigenetic priming of the IFNG locus in Th17 cells. In fact, Th17 cells display an IFNG chromatin landscape that is remarkably similar to that of Th1 cells. On the other hand, pathogenic capabilities of Th17 cells can be restrained by stimulating IL-10 production and transdifferentiation into IL-10 producing T regulatory type 1 (Tr1) cells. In this review, we discuss recent advances in our knowledge on the cellular and molecular mechanisms involved in Th17 differentiation, heterogeneity and plasticity. We focus on transcriptional regulation of the Th17 expression program, the epigenetic dynamics involved, and how genetic variants associated with autoimmunity may affect immune responses through distal gene regulatory elements. Finally, the implications of Th17 cell plasticity for the pathogenesis and treatment of human autoimmune diseases will be discussed.

Plasticity and heterogeneity of Th17 in immune-mediated kidney diseases

Anti-neutrophil cytoplasmatic antibody (ANCA)-associated glomerulonephritis, anti-glomerular basement membrane (GBM) glomerulonephritis and lupus nephritis are the most common causes of rapid progressive glomerulonephritis (RPGN) in the Western world. These aggressive forms of autoimmune kidney diseases significantly contribute to end-stage renal disease and are associated with high morbidity and mortality. Moreover, patients show significant heterogeneity with respect to clinical outcome and response to therapy. T cell infiltration is a morphological hallmark of RPGN and it is a critical driver of kidney injury. Different CD4⁺ T cell subsets that are endowed with distinct regulatory and effector functions are involved in this detrimental inflammatory process. In particular, the identification and functional characterization of IL-17-expressing CD4⁺ Th17 cells have substantially advanced our understanding of organ-specific autoimmunity. In experimental models of crescentic and proliferative GN, including ANCA-associated GN, anti-GBM-GN and lupus nephritis, the Th17/IL-17 axis significantly contributes to renal tissue damage. In patients with ANCA-associated GN or lupus nephritis, IL-17 serum levels correlated with disease activity. Moreover, Th17 cells are present in the kidneys of these patients and represents a topic of intense ongoing clinical and basic research. Importantly, recent studies have challenged the view of CD4⁺ T cells subsets as terminally differentiated homogenous cells, showing that T cells, in particular Th17 cells, are much more flexible and heterogeneous than previously thought. However, analysis of Th17 cell fate in mouse models of autoimmune kidney disease revealed a high degree of stability within these cells, an observation that is in contrast to Th17 cells in other models of autoimmune diseases including experimental autoimmune encephalomyelitis. Interestingly, anti-CD3 treatment interferes with stable Th17 cells and induces a potential regulatory phenotype in Th17 cells, highlighting the therapeutic potential of targeting pathogenic Th17 cells in autoimmunity. In this review, we discuss the current knowledge of Th17 cell plasticity and heterogeneity in autoimmune kidney diseases with a special focus on the underlying mechanisms of this process and debate if Th17 cell plasticity is beneficial or harmful to renal inflammation.

Early-Life Human Microbiota Associated With Childhood Allergy Promotes the T Helper 17 Axis in Mice

The intestinal microbiota influences immune maturation during childhood, and is implicated in early-life allergy development. However, to directly study intestinal microbes and gut immune responses in infants is difficult. To investigate how different types of early-life gut microbiota affect immune development, we collected fecal samples from children with different allergic heredity (AH) and inoculated germ-free mice. Immune responses and microbiota composition were evaluated in the offspring of these mice. Microbial composition in the small intestine, the cecum and the colon were determined by 16S rRNA sequencing. The intestinal microbiota differed markedly between the groups of mice, but only exposure to microbiota associated with AH and known future allergy in children resulted in a T helper 17 (Th17)-signature, both systemically and in the gut mucosa in the mouse offspring. These Th17 responses could be signs of a particular microbiota and a shift in immune development, ultimately resulting in an increased risk of allergy.

T Helper 17 Promotes Induction of Antigen-Specific Gut-Mucosal Cytotoxic T Lymphocytes following Adenovirus Vector Vaccination

Few current vaccines can establish antigen (Ag)-specific immune responses in both mucosal and systemic compartments. Therefore, development of vaccines providing defense against diverse infectious agents in both compartments is of high priority in global health. Intramuscular vaccination of an adenovirus vector (Adv) has been shown to induce Ag-specific cytotoxic T lymphocytes (CTLs) in both systemic and gut-mucosal compartments. We previously found that type I interferon (IFN) signaling is required for induction of gut-mucosal, but not systemic, CTLs following vaccination; however, the molecular mechanism involving type I IFN signaling remains unknown. Here, we found that T helper 17 (Th17)-polarizing cytokine expression was down-regulated in the inguinal lymph nodes (iLNs) of Ifnar2^−/− mice, resulting in the reduction of Ag-specific Th17 cells in the iLNs and gut mucosa of the mice. We also found that prior transfer of Th17 cells reversed the decrease in the number of Ag-specific gut-mucosal CTLs in Ifnar2^−/− mice following Adv vaccination. Additionally, prior transfer of Th17 cells into wild-type mice enhanced the induction of Ag-specific CTLs in the gut mucosa, but not in systemic compartments, suggesting a gut mucosa-specific mechanism where Th17 cells regulate the magnitude of vaccine-elicited Ag-specific CTL responses. These data suggest that Th17 cells translate systemic type I IFN signaling into a gut-mucosal CTL response following vaccination, which could promote the development of promising Adv vaccines capable of establishing both systemic and gut-mucosal protective immunity.

IL-23 signaling in Th17 cells is inhibited by HIV infection and is not restored by HAART: Implications for persistent immune activation

OBJECTIVES

HIV infection causes a profound depletion of gut derived Th17 cells, contributing to loss of mucosal barrier function and an increase in microbial translocation, thus driving systemic immune activation. Despite normalization of circulating CD4+ T cell counts with highly active antiretroviral therapy (HAART), Th17 frequency and function often remain impaired. Given the importance of interleukin (IL)-23 in the generation and stabilization of Th17 cells we hypothesized that impaired IL-23 signaling causes persistent Th17 dysfunction in HIV infection.

METHODS

The effects of in vitro HIV infection on responses to IL-23 in Th17 cells were examined. These included the production of IL-17, phosphorylated STAT3 (pSTAT3) and the transcription of retinoic acid orphan receptor C (RORC) gene. Blood derived Th17 cells from untreated and HAART-treated HIV-infected individuals were also examined for the IL-23 induced production of phosphorylated STAT3 (pSTAT3) and the expression of the IL-23 receptors.

RESULTS

In vitro HIV infection significantly inhibited IL-17 production and IL-23 induced pSTAT3 while expression of RORC RNA was unaffected. Th17 cells isolated from untreated and HAART-treated HIV-infected individuals showed complete loss of IL-23 induced pSTAT3 without a decrease in the expression of the IL-23 receptors.

CONCLUSIONS

This study is the first to demonstrate an effect of HIV on the IL-23 signaling pathway in Th17 cells. We show that in vitro and in vivo HIV infection results in impaired IL-23 signaling which is not reversed by HAART nor is it a result of reduced receptor expression, suggesting that HIV interferes with IL-23-activated signaling pathways. These findings may explain the inability of HAART to restore Th17 frequency and function and the resulting persistent chronic immune activation observed in HIV infected individuals.

Antigen-specific regulatory T-cell responses to intestinal microbiota

The mammalian gastrointestinal tract can harbor both beneficial commensal bacteria important for host health, but also pathogenic bacteria capable of intestinal damage. It is therefore important that the host immune system mount the appropriate immune response to these divergent groups of bacteria-promoting tolerance in response to commensal bacteria and sterilizing immunity in response to pathogenic bacteria. Failure to induce tolerance to commensal bacteria may underlie immune-mediated diseases such as human inflammatory bowel disease. At homeostasis, regulatory T (Treg) cells are a key component of the tolerogenic response by adaptive immunity. This review examines the mechanisms by which intestinal bacteria influence colonic T-cells and B-cell immunoglobulin A (IgA) induction, with an emphasis on Treg cells and the role of antigen-specificity in these processes. In addition to discussing key primary literature, this review highlights current controversies and important future directions.

The regulation of gut mucosal IgA B-cell responses: recent developments

The majority of activated B cells differentiate into IgA plasma cells, with the gut being the largest producer of immunoglobulin in the body. Secretory IgA antibodies have numerous critical functions of which protection against infections and the role for establishing a healthy microbiota appear most important. Expanding our knowledge of the regulation of IgA B-cell responses and how effective mucosal vaccines can be designed are of critical importance. Here we discuss recent developments in the field that shed light on the uniqueness and complexity of mucosal IgA responses and the control of protective IgA responses in the gut, specifically.

The paradox of Th17 cell functions in tumor immunity

Immune system acts as a host defensive mechanism protecting against attacking pathogens and transformed cells, including cancer cells. Th17 cells are a specific subset of T helper lymphocytes determined by high secretion of IL-17 and other inflammatory cytokines. Th17 cells increase tumor progression by activating angiogenesis and immunosuppressive activities. They can also mediate antitumor immune responses through recruiting immune cells into tumors, stimulating effector CD8+ T cells, or surprisingly by altering toward Th1 phenotype and producing IFN-γ, so Th17 cells are supposed as a double-edged sword in cancer. A comprehensive approach to indicating the activity of Th17 cells in tumor progression could help in the planning of new therapeutic approaches specially targeting Th17 cells in cancer.

The Th17 Lineage: From Barrier Surfaces Homeostasis to Autoimmunity, Cancer, and HIV-1 Pathogenesis

The T helper 17 (Th17) cells represent a subset of CD4+ T-cells with unique effector functions, developmental plasticity, and stem-cell features. Th17 cells bridge innate and adaptive immunity against fungal and bacterial infections at skin and mucosal barrier surfaces. Although Th17 cells have been extensively studied in the context of autoimmunity, their role in various other pathologies is underexplored and remains an area of open investigation. This review summarizes the history of Th17 cell discovery and the current knowledge relative to the beneficial role of Th17 cells in maintaining mucosal immunity homeostasis. We further discuss the concept of Th17 pathogenicity in the context of autoimmunity, cancer, and HIV infection, and we review the most recent discoveries on molecular mechanisms regulating HIV replication/persistence in pathogenic Th17 cells. Finally, we stress the need for novel fundamental research discovery-based Th17-specific therapeutic interventions to treat pathogenic conditions associated with Th17 abnormalities, including HIV infection.

Unique Tailoring of Th17 at the Gingival Oral Mucosal Barrier

Our recent work highlights unique requirements for the induction of Th17 cells at the oral/gingival mucosal barrier. Unlike other barrier sites, such as the skin and gastrointestinal tract, we found that Th17 cells can develop at the gingiva independently of commensal microbiota colonization. Instead, we identified that damage, which occurs physiologically due to mastication, promotes induction of Th17 cells and tones homeostatic immunity at the gingiva.

Dietary Fiber and Bacterial SCFA Enhance Oral Tolerance and Protect against Food Allergy through Diverse Cellular Pathways

The incidence of food allergies in western countries has increased dramatically in recent decades. Tolerance to food antigens relies on mucosal CD103(+) dendritic cells (DCs), which promote differentiation of regulatory T (Treg) cells. We show that high-fiber feeding in mice improved oral tolerance and protected from food allergy. High-fiber feeding reshaped gut microbial ecology and increased the release of short-chain fatty acids (SCFAs), particularly acetate and butyrate. High-fiber feeding enhanced oral tolerance and protected against food allergy by enhancing retinal dehydrogenase activity in CD103(+) DC. This protection depended on vitamin A in the diet. This feeding regimen also boosted IgA production and enhanced T follicular helper and mucosal germinal center responses. Mice lacking GPR43 or GPR109A, receptors for SCFAs, showed exacerbated food allergy and fewer CD103(+) DCs. Dietary elements, including fiber and vitamin A, therefore regulate numerous protective pathways in the gastrointestinal tract, necessary for immune non-responsiveness to food antigens.

Inducing Mucosal IgA: A Challenge for Vaccine Adjuvants and Delivery Systems

Mucosal IgA or secretory IgA (SIgA) are structurally equipped to resist chemical degradation in the harsh environment of mucosal surfaces and enzymes of host or microbial origin. Production of SIgA is finely regulated, and distinct T-independent and T-dependent mechanisms orchestrate Ig α class switching and SIgA responses against commensal and pathogenic microbes. Most infectious pathogens enter the host via mucosal surfaces. To provide a first line of protection at these entry ports, vaccines are being developed to induce pathogen-specific SIgA in addition to systemic immunity achieved by injected vaccines. Mucosal or epicutaneous delivery of vaccines helps target the inductive sites for SIgA responses. The efficacy of such vaccines relies on the identification and/or engineering of vaccine adjuvants capable of supporting the development of SIgA alongside systemic immunity and delivery systems that improve vaccine delivery to the targeted anatomic sites and immune cells.

Autoimmune Renal Disease Is Exacerbated by S1P-Receptor-1-Dependent Intestinal Th17 Cell Migration to the Kidney

Th17 cells are most abundant in the gut, where their presence depends on the intestinal microbiota. Here, we examined whether intestinal Th17 cells contribute to extra-intestinal Th17 responses in autoimmune kidney disease. We found high frequencies of Th17 cells in the kidneys of patients with antineutrophil cytoplasmatic antibody (ANCA)-associated glomerulonephritis. We utilized photoconversion of intestinal cells in Kaede mice to track intestinal T cell mobilization upon glomerulonephritis induction, and we found that Th17 cells egress from the gut in a S1P-receptor-1-dependent fashion and subsequently migrate to the kidney via the CCL20/CCR6 axis. Depletion of intestinal Th17 cells in germ-free and antibiotic-treated mice ameliorated renal disease, whereas expansion of these cells upon Citrobacter rodentium infection exacerbated pathology. Thus, in some autoimmune settings, intestinal Th17 cells migrate into target organs, where they contribute to pathology. Targeting the intestinal Th17 cell "reservoir" may present a therapeutic strategy for these autoimmune disorders.

Phenotype and Antimicrobial Activity of Th17 Cells Induced by Propionibacterium acnes Strains Associated with Healthy and Acne Skin

Studies of the human skin microbiome suggest that Propionibacterium acnes strains may contribute differently to skin health and disease. However, the immune phenotype and functions of T helper type 17 (Th17) cells induced by healthy (P_H) versus acne (P_A) skin-associated P. acnes strains are currently unknown. We stimulated peripheral blood mononuclear cells from healthy donors and observed that P_A strains induce higher IL-17 levels than P_H strains. We next generated P_H and P_A strain-specific Th17 clones and show that P. acnes strains induce Th17 cells of varied phenotype and function that are stable in the presence of IL-2 and IL-23. Although P_H- and P_A-specific clones expressed similar levels of LL-37 and DEFB4, only P_H-specific clones secreted molecules sufficient to kill P. acnes. Furthermore, electron microscopic studies showed that supernatants derived from activated P_H and not P_A-specific clones exhibited robust bactericidal activity against P. acnes, and complete breaches in the bacterial cell envelope were observed. This antimicrobial activity was independent of IL-26, because both natural IL-26 released by Th17 clones and rhIL-26 lacked antimicrobial potency against P. acnes. Overall, our data suggest that P. acnes strains may differentially modulate the CD4⁺ T-cell responses, leading to the generation of Th17 cells that may contribute to either homeostasis or acne pathogenesis.

JunB promotes Th17 cell identity and restrains alternative CD4+ T-cell programs during inflammation

T helper 17 (Th17) cell plasticity contributes to both immunity and autoimmunity; however, the factors that control lineage flexibility are mostly unknown. Here we show the activator protein-1 (AP-1) factor JunB is an essential regulator of Th17 cell identity. JunB activates expression of Th17 lineage-specifying genes and coordinately represses genes controlling Th1 and regulatory T-cell fate. JunB supports Th17 cell identity by regulating key AP-1 complex constituents. In particular, JunB limits the expression of the subset repressor IRF8, and impedes access of JunD to regulatory regions of alternative effector loci. Although dispensable for homeostatic Th17 cell development, JunB is required for induction and maintenance of Th17 effector responses in the inflammatory contexts of both acute infection and chronic autoimmunity in mice. Through regulatory network analysis, we show that JunB is a core regulator of global transcriptional programs that promote Th17 cell identity and restrict alternative CD4⁺ T-cell potential.

AP-1 family transcription factors regulate CD4⁺ T helper cell differentiation. Here the authors show that the AP-1 member JunB is a nonredundant regulator of transcriptional programs that support Th17 cell identity and restrain alternative Th1 and Treg cell fates in inflammatory contexts of acute fungal infection and chronic autoimmunity.

T helper type 17 cells in immune-mediated glomerular disease

CD4⁺ T cells are important drivers of tissue damage in immune-mediated renal diseases, such as anti-glomerular basement membrane glomerulonephritis, anti-neutrophil cytoplasmic antibody-associated glomerulonephritis, and lupus nephritis. The discovery of a distinct, IL-17-producing CD4⁺ T-cell lineage termed T helper type 17 (T_H17) cells has markedly advanced current understanding of the pathogenic mechanisms of organ-specific immunity and the pathways that lead to target organ damage. T_H17 cells are characterized by the expression of the transcription factor RORγt, the production of the pro-inflammatory cytokines IL-17A, IL-17F, IL-22, and high expression of the chemokine receptor C-C-motif chemokine receptor 6 (CCR6). An emerging body of evidence from experimental models and human studies supports a key role for these cells in the development of renal damage, and has led to the identification of targets to inhibit the production of T_H17 cells in the intestine, their migration, or their actions within the kidney. Here, we describe the identification, regulation, and function of T_H17 cells and their associated pathways in immune-mediated kidney diseases, with a particular focus on the mechanisms underlying renal tissue injury. We also discuss the rationale for the translation of these findings into new therapeutic approaches in patients with autoimmune kidney disease.

A rapid IL-17 response to Cryptosporidium parvum in the bovine intestine

Cryptosporidium parvum causes diarrhoea, due to villi damage, in livestock and humans globally. Immunity develops after repeated infections but initial infections can be severe, highlighting the importance of early infection dynamics. We have modelled early C. parvum infection in bovine jejunum biopsies. IL-17A accumulated over time peaking at 9 h post-infection, with no effect of infection on IL-1β; antibiotics positively influenced IL-17A as higher levels were found in cultures with antibiotics. Infection of primary fibroblasts resulted in lower plaque formation when fibroblasts were primed with IL-17A. Our results indicate a role for IL-17A in reducing C. parvum-dependent host cell damage.

CCR6 Deficiency Impairs IgA Production and Dysregulates Antimicrobial Peptide Production, Altering the Intestinal Flora

Intestinal immunity exists as a complex relationship among immune cells, epithelial cells, and microbiota. CCR6 and its ligand-CCL20 are highly expressed in intestinal mucosal tissues, such as Peyer's patches (PPs) and isolated lymphoid follicles (ILFs). In this study, we investigated the role of the CCR6-CCL20 axis in intestinal immunity under homeostatic conditions. CCR6 deficiency intrinsically affects germinal center reactions in PPs, leading to impairments in IgA class switching, IgA affinity, and IgA memory B cell production and positioning in PPs, suggesting an important role for CCR6 in T-cell-dependent IgA generation. CCR6 deficiency impairs the maturation of ILFs. In these follicles, group 3 innate lymphoid cells are important components and a major source of IL-22, which stimulates intestinal epithelial cells (IECs) to produce antimicrobial peptides (AMPs). We found that CCR6 deficiency reduces IL-22 production, likely due to diminished numbers of group 3 innate lymphoid cells within small-sized ILFs. The reduced IL-22 levels subsequently decrease the production of AMPs, suggesting a critical role for CCR6 in innate intestinal immunity. Finally, we found that CCR6 deficiency impairs the production of IgA and AMPs, leading to increased levels of Alcaligenes in PPs, and segmented filamentous bacteria in IECs. Thus, the CCR6-CCL20 axis plays a crucial role in maintaining intestinal symbiosis by limiting the overgrowth of mucosa-associated commensal bacteria.

Differences in serum IgA responses to HIV-1 gp41 in elite controllers compared to viral suppressors on highly active antiretroviral therapy

Mechanisms responsible for natural control of human immunodeficiency type 1 (HIV) replication in elite controllers (EC) remain incompletely defined. To determine if EC generate high quality HIV-specific IgA responses, we used Western blotting to compare the specificities and frequencies of IgA to HIV antigens in serum of gender-, age- and race-matched EC and aviremic controllers (HC) and viremic noncontrollers (HN) on highly active antiretroviral therapy (HAART). Concentrations and avidity of IgA to HIV antigens were measured using ELISA or multiplex assays. Measurements for IgG were performed in parallel. EC were found to have stronger p24- and V1V2-specific IgG responses than HN, but there were no IgG differences for EC and HC. In contrast, IgA in EC serum bound more frequently to gp160 and gag proteins than IgA in HC or HN. The avidity of anti-gp41 IgA was also greater in EC, and these subjects had stronger IgA responses to the gp41 heptad repeat region 1 (HR1), a reported target of anti-bacterial RNA polymerase antibodies that cross react with gp41. However, EC did not demonstrate greater IgA responses to E. coli RNA polymerase or to peptides containing the shared LRAI sequence, suggesting that most of their HR1-specific IgA antibodies were not induced by intestinal microbiota. In both EC and HAART recipients, the concentrations of HIV-specific IgG were greater than HIV-specific IgA, but their avidities were comparable, implying that they could compete for antigen. Exceptions were C1 peptides and V1V2 loops. IgG and IgA responses to these antigens were discordant, with IgG reacting to V1V2, and IgA reacting to C1, especially in EC. Interestingly, EC with IgG hypergammaglobulinemia had greater HIV-specific IgA and IgG responses than EC with normal total IgG levels. Heterogeneity in EC antibody responses may therefore be due to a more focused HIV-specific B cell response in some of these individuals. Overall, these data suggest that development of HIV-specific IgA responses and affinity maturation of anti-gp41 IgA antibodies occurs to a greater extent in EC than in subjects on HAART. Future studies will be required to determine if IgA antibodies in EC may contribute in control of viral replication.

Intestinal alkaline phosphatase at the crossroad of intestinal health and disease - a putative role in type 1 diabetes

BACKGROUND

Patients with type 1 diabetes have shown an increase in circulating cytokines, altered lipoprotein metabolism and signs of vascular dysfunction in response to high-fat meals. Intestinal alkaline phosphatase (IAP) regulates lipid transport and inflammatory responses in the gastrointestinal tract. We therefore hypothesized that changes in IAP activity could have profound effects on gut metabolic homeostasis in patients with type 1 diabetes.

METHODS

Faecal samples of 41 nondiabetic controls and 46 patients with type 1 diabetes were analysed for IAP activity, calprotectin, immunoglobulins and short-chain fatty acids (SCFAs). The impact of oral IAP supplementation on intestinal immunoglobulin levels was evaluated in C57BL/6 mice exposed to high-fat diet for 11 weeks.

RESULTS

Patients with type 1 diabetes exhibited signs of intestinal inflammation. Compared to controls, patients with diabetes had higher faecal calprotectin levels, lower faecal IAP activities accompanied by lower propionate and butyrate concentrations. Moreover, the amount of faecal IgA and the level of antibodies binding to oxidized LDL were decreased in patients with type 1 diabetes. In mice, oral IAP supplementation increased intestinal IgA levels markedly.

CONCLUSION

Deprivation of protective intestinal factors may increase the risk of inflammation in the gut - a phenomenon that seems to be present already in patients with uncomplicated type 1 diabetes. Low levels of intestinal IgA and antibodies to oxidized lipid epitopes may predispose such patients to inflammation-driven complications such as cardiovascular disease and diabetic nephropathy. Importantly, oral IAP supplementation could have beneficial therapeutic effects on gut metabolic homeostasis, possibly through stimulation of intestinal IgA secretion.