Plasma glycomics predict cardiovascular disease in patients with ART-controlled HIV infections

Despite effective control of HIV infection with antiretroviral drugs, individuals with HIV have high incidences of secondary diseases. These sequelae, such as cardiovascular disease (CVD), are poorly understood and represent a major health burden. To date, predictive biomarkers of HIV-associated secondary disease have been elusive, making preventative clinical management essentially impossible. Here, we applied a newly developed and easy to deploy, multitarget, and high-throughput glycomic analysis to banked HIV+ human plasma samples to determine whether the glycome may include biomarkers that predict future HIV-associated cardiovascular events or CVD diagnoses. Using 324 patient samples, we identified a glycomic fingerprint that was predictive of future CVD events but independent of CD4 counts, diabetes, age, and birth sex, suggesting that the plasma glycome may serve as a biomarker for specific HIV-associated sequelae. Our findings constitute the discovery of novel glycan biomarkers that could classify patients with HIV with elevated risk for CVD and reveal the untapped prognostic potential of the plasma glycome in human disease.-Oswald, D. M., Sim, E. S., Baker, C., Farhan, O., Debanne, S. M., Morris, N. J., Rodriguez, B. G., Jones, M. B., Cobb, B. A. Plasma glycomics predict cardiovascular disease in patients with ART-controlled HIV infections.

Antibody receptors steal the sweet spotlight

Immunoglobulin G (IgG) antibodies function, in part, through ligation of cell-surface Fc receptors such as FcγRIIIA (also known as CD16A). IgG glycosylation is known to impact antibody function, but the role of FcγRIIIA glycans, if any, is unclear. Patel et al. now reveal that these glycans do impact protein conformation and IgG affinity and display cell-specific glycosylation patterns, leading to a potential model in which the affinity and possibly function of Fc receptors is dictated by the cell type and its surface glycome.

Polysaccharide-experienced effector T cells induce IL-10 in FoxP3+ regulatory T cells to prevent pulmonary inflammation

Inhibition of peripheral inflammatory disease by carbohydrate antigens derived from normal gut microbiota has been demonstrated for the GI tract, brain, peritoneum, and most recently the airway. We have demonstrated that polysaccharide A (PSA) from the commensal organism Bacteroides fragilis activates CD4+ T cells upon presentation by the class II major histocompatibility complex, and that these PSA-experienced T cells prevent the development of lung inflammation in murine models. While the PSA-responding T cells themselves are not canonical FoxP3+ regulatory T cells (Tregs), their ability to prevent inflammation is dependent upon the suppressive cytokine IL-10. Using an adoptive T cell transfer approach, we have discovered that PSA-experienced T cells require IL-10 expression by PSA-naïve recipient animals in order to prevent inflammation. A cooperative relationship was found between PSA-activated effector/memory T cells and tissue-resident FoxP3+ Tregs both in vivo and in vitro, and it is this cooperation that enables the suppressive activity of PSA outside of the gut environment where exposure takes place. These findings suggest that carbohydrate antigens from the normal microbiota communicate with peripheral tissues to maintain homeostasis through T cell-to-T cell cooperation.

The direct and indirect effects of glycans on immune function

The biological impact of glycans is as diverse and complex as the impact of proteins on biology. Familiar roles include those as a protein folding checkpoint in the endoplasmic reticulum and as a modulator of the serum half-life of secreted glycoproteins, but it has become clear over the last several decades that glycans are key signaling moieties, participate in cell-cell interactions and modulate the function of individual proteins, to name but a few examples. In the immune system, the majority of microbial "patterns" are glycans or glycoconjugates, while virtually all cell surface receptors are glycoproteins, and antibody glycosylation critically influences antibody function. In order to provide a simple contextual framework to understand the myriad roles, glycans play in immunity, we propose that glycan effects are considered direct or indirect, depending on their direct participation or their indirect effects on other components in a given biological process or pathway. Here, we present the published evidence that supports this framework, which ultimately leads to the conclusion that we should learn to embrace the complexity inherent to the glycome and its potential as a largely uncharted but target rich area of new therapeutic investigation.

T cell-intrinsic TLR2 stimulation promotes IL-10 expression and suppressive activity by CD45RbHi T cells

While Toll-like receptors (TLRs) represent one of the best characterized innate immune pathways, evidence suggests that TLRs are not restricted to innate leukocytes and some epithelial cells, but are also expressed in T cells. Specifically, published evidence focusing on FoxP3⁺ regulatory T cells demonstrate that they express functional TLR2, which is already known among the TLR family for its association with immune suppression; however, little is known about the relationship between T cell-intrinsic TLR2 binding and cytokine production, T cell differentiation, or T cell receptor (TCR) stimulation. Here, we demonstrate that TCR and TLR2 co-stimulation provides a T cell-intrinsic signal which generates a dramatic, synergistic cytokine response dominated by IL-10. Importantly, the response was not seen in either CD4⁺CD25⁺ or CD4⁺FoxP3⁺ Tregs, yet resulted in the expansion of a suppressive CD4⁺CD25⁺CD62L^-CD44⁺CD45Rb^hi effector/memory T cell subset not typically associated with immune inhibition. This study reveals the striking ability of a prototypical innate immune receptor to trigger a potent and suppressive IL-10 response in effector/memory T cells, supporting the notion that TLR2 is a co-regulatory receptor on T cells.

Amelioration of pulmonary inflammation through T cell activation in murine asthma model

Asthma is an increasingly common chronic inflammatory airway disorder with phenotypic heterogeneity driven by age, gender, atopy, lung function, lifestyle, body mass indexes, and the underlying cellular and molecular pathways. In asthma, T helper (Th) cells drive inflammatory cascades via cytokine/inflammatory mediators and recruit leukocytes into the airway. While treatments are available, such as inhaled corticosteroids, individuals with severe asthma are often steroid resistant and have a poor quality of life. It is therefore important to develop our understanding of the underlying T cell driven mechanisms leading to pulmonary inflammation such that novel therapies may be developed. Previous work from our lab has demonstrated the importance of commensal microbiota in the development and maintenance of peripheral homeostasis. Exposure to bacterial polysaccharides, such as PSA from Bacteroides fragilis, inhibits Th1 and Th2 activity in murine models by activation of CD4+ FoxP3− CD45Rblo CD62Llo CD44hi T effector memory (Rblo Tem) cells in an IL-10 dependent fashion. Here, we show that activated Rblo Tem cells can reverse established pulmonary inflammatory disease in a murine house dust mite (HDM) model. Adoptive transfer of these cells nearly eliminates lung pathology and severely limits tissue infiltration by leukocytes. These results provide a rationale for the use of autologous cell transfer as a means to prevent and reverse ongoing inflammatory lung disease.

Dendritic cell-specific Mgat2 knockout mice show antigen presentation defects but reveal an unexpected CD11c expression pattern

Zwitterionic polysaccharide antigens such as polysaccharide A (PSA) from Bacteroides fragilis have been shown to activate CD4⁺ T cells upon presentation by class II major histocompatibility complex (MHCII) on professional antigen presenting cells. For T cell recognition and activation, high affinity binding between MHCII and PSA is required, and complex N-glycans on conserved MHCII asparagine residues play a central role in controlling this interaction. By truncating these glycans in a myeloid-specific knockout of Mgat2, created using the LyzM-CRE mouse (M-cKO), we previously reported defects in PSA responses in vivo. Unfortunately, the M-cKO also showed a propensity to develop common variable immunodeficiency with autoimmune hemolytic anemia features. Here, we describe a novel murine model in which Mgat2 was targeted for ablation using the dendritic cell (DC)-specific CD11c-CRE-GFP strain in order to develop a more specific and robust in vivo model of PSA presentation defects (DC-cKO). This study shows that Mgat2 deficient DCs from DC-cKO mice show ablation of PSA presentation and downstream T cell activation in vitro. However, the CD11c promoter was unexpectedly active and triggered Mgat2 deletion within multiple hematopoietic lineages, showed remarkably poor penetrance within native DC populations, and produced almost undetectable levels of green fluorescent protein signal. These findings show that the CD11c promoter is not DC-specific, and extreme care should be taken in the interpretation of data using any mouse created using the CD11c-CRE model.

Polysaccharide A from the capsule of Bacteroides fragilis induces clonal CD4+ T cell expansion

For 3 decades, the view of MHCII-dependent antigen presentation has been completely dominated by peptide antigens despite our 2004 discovery in which MHCII was shown to present processed fragments of zwitterionic capsular polysaccharides to T cells. Published findings further demonstrate that polysaccharide A (PSA) from the capsule of Bacteroides fragilis is a potent activator of CD4(+) T cells and that these T cells have important biological functions, especially in the maintenance of immunological homeostasis. However, little is known about the nature of T cell recognition of the polysaccharide-MHCII complex or the phenotype of the resulting activated cells. Here, we use next-generation sequencing of the αβT cell receptor of CD4(+) T cells from mice stimulated with PSA in comparison with protein antigen simulation and non-immunized controls and found that PSA immunization induced clonal expansion of a small subset of suppressive CD4(+)CD45RB(low) effector/memory T cells. Moreover, the sequences of the complementarity-determining region 3 (CDR3) loop from top clones indicate a lack of specific variable β and joining region use and average CDR3 loop length. There was also a preference for a zwitterionic motif within the CDR3 loop sequences, aligning well with the known requirement for a similar motif within PSA to enable T cell activation. These data support a model in which PSA, and possibly other T cell-dependent polysaccharide antigens, elicits a clonal and therefore specific CD4(+) T cell response often characterized by pairing dual-charged CDR3 loop sequences with dual-charged PSA.

Bacterial capsular polysaccharide prevents the onset of asthma through T-cell activation

Over the last four decades, increases in the incidence of immune-mediated diseases in the Western world have been linked to changes in microbial exposure. It is becoming increasingly clear that the normal microbiota in the gut can profoundly alter susceptibility to a wide range of diseases, such as asthma, in which immune homeostasis is disrupted, yet the mechanisms governing this microbial influence remains poorly defined. In this study, we show that gastrointestinal exposure to PSA, a capsular polysaccharide derived from the commensal bacterium Bacteroides fragilis, significantly limits susceptibility to the induction of experimental asthma. We report that direct treatment of mice with PSA generates protection from asthma, and this effect can be given to a naïve recipient by adoptive transfer of CD4(+) T cells from PSA-exposed mice. Remarkably, we found that these PSA-induced T cells are not canonical FoxP3(+) regulatory T cells, but that they potently inhibit both Th1 and Th2 models of asthma in an IL-10-dependent fashion. These findings reveal that bacterial polysaccharides link the microbiota with the peripheral immune system by activating CD4(+)Foxp3(-) T cells upon exposure in the gut, and they facilitate resistance to unnecessary inflammatory responses via the production of IL-10.

Myeloid glycosylation defects lead to a spontaneous common variable immunodeficiency-like condition with associated hemolytic anemia and antilymphocyte autoimmunity

Common variable immunodeficiency (CVID), the most frequent symptomatic primary immune deficiency in humans, is a heterogeneous group of immunologic disorders estimated to affect 1:10,000-1:50,000. Although a clear disease etiology remains elusive, a common characteristic of CVID is deficient IgG Ab production in response to infection or vaccination. Patients often also exhibit autoimmune cytopenias with symptoms of abnormal T cell function, including reductions in naive T cells, which correlate with clinical severity. In this study, we discovered that targeted alterations in the glycome of the myeloid lineage lead to spontaneous immunodeficiency characteristic of both humoral and T cell dysfunction regularly found in human CVID. Mice carrying a myeloid-specific knockout of the Mgat2 gene encoding UDP-GlcNAc:α-6-d-mannoside β-1,2-N-acetylglucosaminyltransferase II enzyme exhibit deficiencies in IgG responses to both protein and polysaccharide conjugate vaccines. Interestingly, the immunodeficiency is associated with decreased T cell activity because of a persistent autoimmune-mediated depletion of naive T cells, which is induced by changes in erythrocyte surface glycosylation. The N-glycosylation dependent autoepitopes that emerge on erythrocytes lead to autoimmune hemolytic anemia, and the causative auto-IgM cross-reacts with naive T cells despite the lack of glycan change on T cells. These findings demonstrate that alterations in erythrocyte glycosylation trigger the development of autoantibodies directed at both erythrocytes and naive T cells, revealing a possible mechanistic link between the induction of autoimmune hemolytic anemia, the reduction in naive T cells, and poor Ab responses to vaccine in severe CVID patients.

Mgat2 ablation in the myeloid lineage leads to defective glycoantigen T cell responses

N-linked glycosylation is a central regulatory factor that influences the immune system in varied and profound ways, including leukocyte homing, T cell receptor signaling and others. Moreover, N-glycan branching has been demonstrated to change as a function of infection and inflammation. Our previous findings suggest that complex-type N-glycans on the class II major histocompatibility complex play an important role in antigen selection within antigen presenting cells (APCs) such that highly branched N-glycans promote polysaccharide (glycoantigen, GlyAg) presentation following Toll-like receptor 2 (TLR2)-dependent antigen processing. In order to explore the impact of N-glycan branching on the myeloid-derived APC population without the confounding problems of altering the branching of lymphocytes and non-hematopoietic cells, we created a novel myeloid-specific knockout of the β-1,2-N-acetylglucosaminyltransferase II (Mgat2) enzyme. Using this novel mouse, we found that the reduction in multi-antennary N-glycans characteristic of Mgat2 ablation had no impact on GlyAg-mediated TLR2 signaling. Likewise, no deficits in antigen uptake or cellular homing to lymph nodes were found. However, we discovered that Mgat2 ablation prevented GlyAg presentation and T cell activation in vitro and in vivo without apparent alterations in protein antigen response or myeloid-mediated protection from infection. These findings demonstrate that GlyAg presentation can be regulated by the N-glycan branching pattern of APCs, thereby establishing an in vivo model where the T cell-dependent activity of GlyAgs can be experimentally distinguished from GlyAg-mediated stimulation of the innate response through TLR2.

Interaction of the Capsular Polysaccharide A from Bacteroides fragilis with DC-SIGN on Human Dendritic Cells is Necessary for Its Processing and Presentation to T Cells

The zwitterionic capsular polysaccharide A (PSA) of Bacteroides fragilis is the first carbohydrate antigen described to be presented in major histocompatibility complex (MHC) class II for the induction of CD4⁺ T cell responses. However, the identity of the receptor mediating binding and internalization of PSA in antigen presenting cells remains elusive. C-type lectins are glycan-binding receptors known for their capacity to target ligands for antigen presentation to T cells. Here, we investigated whether C-type lectins were involved in the internalization of PSA and identified dendritic cell-specific intercellular adhesion molecule-3-grabbing non-integrin (DC-SIGN) as the main receptor for PSA on human dendritic cells (DC). The induction of PSA-specific T cell proliferation appeared to be completely dependent on DC-SIGN. These data reveal a crucial role for DC-SIGN in the endocytosis and routing of PSA in human DC for the efficient stimulation of PSA-specific CD4⁺ T cells.

Neutrophils confer T cell resistance to myeloid-derived suppressor cell-mediated suppression to promote chronic inflammation

Low-grade chronic inflammation can persist in aging humans unnoticed for years or even decades, inflicting continuous damage that can culminate later in life as organ dysfunction, physical frailty, and some of the most prominent debilitating and deadly age-associated diseases, including rheumatoid arthritis, diabetes, heart disease, and cancer. Despite the near universal acceptance of these associations, the mechanisms underlying unresolved inflammation remain poorly understood. In this study, we describe a novel inducible method to examine systemic chronic inflammation using susceptible animal models. Induced inflammation results in unresolved innate cellular responses and persistence of the same serum proinflammatory molecules used as diagnostic biomarkers and therapeutic targets for chronic inflammation in humans. Surprisingly, we found long-term persistence of an inflammation-associated neutrophil cell population constitutively producing the proinflammatory IFN-γ cytokine, which until now has only been detected transiently in acute inflammatory responses. Interestingly, these cells appear to confer T cell resistance to the otherwise potent anti-inflammatory function of myeloid-derived suppressor cells, revealing a novel mechanism for the maintenance of chronic inflammatory responses over time. This discovery represents an attractive target to resolve inflammation and prevent the inflammation-induced pathologies that are of critical concern for the well-being of the aging population.

Glycoform-selective prion formation in sporadic and familial forms of prion disease

The four glycoforms of the cellular prion protein (PrP(C)) variably glycosylated at the two N-linked glycosylation sites are converted into their pathological forms (PrP(Sc)) in most cases of sporadic prion diseases. However, a prominent molecular characteristic of PrP(Sc) in the recently identified variably protease-sensitive prionopathy (VPSPr) is the absence of a diglycosylated form, also notable in familial Creutzfeldt-Jakob disease (fCJD), which is linked to mutations in PrP either from Val to Ile at residue 180 (fCJD(V180I)) or from Thr to Ala at residue 183 (fCJD(T183A)). Here we report that fCJD(V180I), but not fCJD(T183A), exhibits a proteinase K (PK)-resistant PrP (PrP(res)) that is markedly similar to that observed in VPSPr, which exhibits a five-step ladder-like electrophoretic profile, a molecular hallmark of VPSPr. Remarkably, the absence of the diglycosylated PrP(res) species in both fCJD(V180I) and VPSPr is likewise attributable to the absence of PrP(res) glycosylated at the first N-linked glycosylation site at residue 181, as in fCJD(T183A). In contrast to fCJD(T183A), both VPSPr and fCJD(V180I) exhibit glycosylation at residue 181 on di- and monoglycosylated (mono181) PrP prior to PK-treatment. Furthermore, PrP(V180I) with a typical glycoform profile from cultured cells generates detectable PrP(res) that also contains the diglycosylated PrP in addition to mono- and unglycosylated forms upon PK-treatment. Taken together, our current in vivo and in vitro studies indicate that sporadic VPSPr and familial CJD(V180I) share a unique glycoform-selective prion formation pathway in which the conversion of diglycosylated and mono181 PrP(C) to PrP(Sc) is inhibited, probably by a dominant-negative effect, or by other co-factors.

Fungal antioxidant pathways promote survival against neutrophils during infection

Filamentous fungi are a common cause of blindness and visual impairment worldwide. Using both murine model systems and in vitro human neutrophils, we found that NADPH oxidase produced by neutrophils was essential to control the growth of Aspergillus and Fusarium fungi in the cornea. We demonstrated that neutrophil oxidant production and antifungal activity are dependent on CD18, but not on the β-glucan receptor dectin-1. We used mutant A. fumigatus strains to show that the reactive oxygen species-sensing transcription factor Yap1, superoxide dismutases, and the Yap1-regulated thioredoxin antioxidant pathway are each required for protection against neutrophil-mediated oxidation of hyphae as well as optimal survival of fungal hyphae in vivo. We also demonstrated that thioredoxin inhibition using the anticancer drug PX-12 increased the sensitivity of fungal hyphae to both H2O2- and neutrophil-mediated killing in vitro. Additionally, topical application of PX-12 significantly enhanced neutrophil-mediated fungal killing in infected mouse corneas. Cumulatively, our data reveal critical host oxidative and fungal anti-oxidative mediators that regulate hyphal survival during infection. Further, these findings also indicate that targeting fungal anti-oxidative defenses via PX-12 may represent an efficacious strategy for treating fungal infections.

Host glycans and antigen presentation

The cell-mediated adaptive immune response depends upon the activation of T cells via recognition of antigen in the context of a major histocompatibility complex (MHC) molecule. Although studies have shown that alterations in T cell receptor glycosylation reduces the activation threshold, the data on MHC is far less definitive. Here, we discuss the data on MHC glycosylation and the role the glycans might play during the adaptive host response.

Infection, inflammation and host carbohydrates: a Glyco-Evasion Hypothesis

Microbial immune evasion can be achieved through the expression, or mimicry, of host-like carbohydrates on the microbial cell surface to hide from detection. However, disparate reports collectively suggest that evasion could also be accomplished through the modulation of the host glycosylation pathways, a mechanism that we call the "Glyco-Evasion Hypothesis". Here, we will summarize the evidence in support of this paradigm by reviewing three separate bodies of work present in the literature. We review how infection and inflammation can lead to host glycosylation changes, how host glycosylation changes can increase susceptibility to infection and inflammation and how glycosylation impacts molecular and cellular function. Then, using these data as a foundation, we propose a unifying hypothesis in which microbial products can hijack host glycosylation to manipulate the immune response to the advantage of the pathogen. This model reveals areas of research that we believe could significantly improve our fight against infectious disease.

Glycobiology of immune responses

Unlike their protein "roommates" and their nucleic acid "cousins," carbohydrates remain an enigmatic arm of biology. The central reason for the difficulty in fully understanding how carbohydrate structure and biological function are tied is the nontemplate nature of their synthesis and the resulting heterogeneity. The goal of this collection of expert reviews is to highlight what is known about how carbohydrates and their binding partners-the microbial (non-self), tumor (altered-self), and host (self)-cooperate within the immune system, while also identifying areas of opportunity to those willing to take up the challenge of understanding more about how carbohydrates influence immune responses. In the end, these reviews will serve as specific examples of how carbohydrates are as integral to biology as are proteins, nucleic acids, and lipids. Here, we attempt to summarize general concepts on glycans and glycan-binding proteins (mainly C-type lectins, siglecs, and galectins) and their contributions to the biology of immune responses in physiologic and pathologic settings.

Roles for major histocompatibility complex glycosylation in immune function

The major histocompatibility complex (MHC) glycoprotein family, also referred to as human leukocyte antigens, present endogenous and exogenous antigens to T lymphocytes for recognition and response. These molecules play a central role in enabling the immune system to distinguish self from non-self, which is the basis for protective immunity against pathogenic infections and disease while at the same time representing a serious obstacle for tissue transplantation. All known MHC family members, like the majority of secreted, cell surface, and other immune-related molecules, carry asparagine (N)-linked glycans. The immune system has evolved increasing complexity in higher-order organisms along with a more complex pattern of protein glycosylation, a relationship that may contribute to immune function beyond the early protein quality control events in the endoplasmic reticulum that are commonly known. The broad MHC family maintains peptide sequence motifs for glycosylation at sites that are highly conserved across evolution, suggesting importance, yet functional roles for these glycans remain largely elusive. In this review, we will summarize what is known about MHC glycosylation and provide new insight for additional functional roles for this glycoprotein modification in mediating immune responses.

Adaptive immune defects against glycoantigens in chronic granulomatous disease via dysregulated nitric oxide production

Chronic granulomatous disease (CGD) is a primary immunodeficiency defined by mutations in the NADPH oxidase complex leading to reduced superoxide production, increased susceptibility to infection, chronic inflammation, and recurring abscess and granuloma formation. Here, we found that CGD mice were hyperresponsive to abscess-inducing T-cell-dependent carbohydrate antigens (glycoantigens) due to a ten-fold increase in NO production within APCs, which is known to be necessary for glycoantigen presentation on MHC class II. CGD mice exhibited increased Th1 pro-inflammatory T-cell responses in vitro and in vivo, characterized by more severe abscess pathology. This phenotype was also seen in WT animals following adoptive transfer of neutrophil-depleted APCs from CGD animals, demonstrating that this phenotype was independent of neutrophil and T-cell defects. Finally, pharmacological attenuation of NO production to WT levels in vivo reduced abscess incidence and severity in CGD without overt increases in inflammation or the ability to clear infection, suggesting a potential new treatment option for early stage CGD-associated infections.

MHCII glycosylation modulates Bacteroides fragilis carbohydrate antigen presentation

N-linked glycans on class II MHC molecules are required for the presentation of glycoantigens, but not peptide antigens.

N-linked glycans are thought to protect class II major histocompatibility complex (MHC) molecules (MHCII) from proteolytic cleavage and assist in arranging proteins within the immune synapse, but were not thought to directly participate in antigen presentation. Here, we report that antigen-presenting cells (APCs) lacking native complex N-glycans showed reduced MHCII binding and presentation of the T cell activating glycoantigen (GlyAg) polysaccharide A from Bacteroides fragilis but not conventional peptides. APCs lacking native N-glycans also failed to mediate GlyAg-driven T cell activation but activated T cells normally with protein antigen. Mice treated with the mannosidase inhibitor kifunensine to prevent the formation of complex N-glycans were unable to expand GlyAg-specific T cells in vivo upon immunization, yet adoptive transfer of normally glycosylated APCs into these animals overcame this defect. Our findings reveal that MHCII N-glycosylation directly impacts binding and presentation of at least one class of T cell–dependent antigen.

Glycoantigens induce human peripheral Tr1 cell differentiation with gut-homing specialization

The carbohydrate antigen (glycoantigen) PSA from an intestinal commensal bacteria is able to down-regulate inflammatory bowel disease in model mice, suggesting that stimulation with PSA results in regulatory T cell (Treg) generation. However, mechanisms of how peripheral human T cells respond and home in response to commensal antigens are still not understood. Here, we demonstrate that a single exposure to PSA induces differentiation of human peripheral CD4(+) T cells into type-Tr1 Tregs. This is in contrast to mouse models where PSA induced the production of Foxp3(+) iTregs. The human PSA-induced Tr1 cells are profoundly anergic and exhibit nonspecific bystander suppression mediated by IL-10 secretion. Most surprisingly, glycoantigen exposure provoked expression of gut homing receptors on their surface. These findings reveal a mechanism for immune homeostasis in the gut whereby exposure to commensal glycoantigens provides the requisite information to responding T cells for proper tissue localization (gut) and function (anti-inflammatory/regulatory).

Structural characterization and MHCII-dependent immunological properties of the zwitterionic O-chain antigen of Morganella morganii

Morganella morganii is a commensal Gram-negative bacterium that has long been known to produce an antigen bearing phosphocholine groups. We determined the structure of this O-chain antigen and found that its repeating unit also contains a free amino group and a second phosphate: This alternating charge character places the M. morganii O-chain polysaccharide into a small family of zwitterionic polysaccharides (ZPSs) known to induce T-cell-dependent immune responses via presentation by class II major histocompatibility complex (MHCII) molecules. In vitro binding assays demonstrate that this O-chain interacts with MHCII in a manner that competes with binding of the prototypical ZPS antigen PSA from Bacteroides fragilis, despite its lack of a helical structure. Cellular studies also showed that the M. morganii polysaccharide induces activation of CD4(+) T-cells. Antibody binding experiments using acid hydrolyzed fragments representing the monomer and higher oligomers of the repeating unit showed that the phosphocholine group was the dominant element of the epitope with an overall affinity (K(D)) of about 5 × 10(-5) M, a typical value for an IgM anti-carbohydrate antibody but much lower than the affinity for phosphocholine itself. These data show that the structure of the M. morganii polysaccharide contains a unique zwitterionic repeating unit which allows for immune recognition by T-cells, making it the first identified T-cell-dependent O-chain antigen.

Carbohydrate oxidation acidifies endosomes, regulating antigen processing and TLR9 signaling

Phagocytes kill encapsulated microbes through oxidative cleavage of surface carbohydrates, releasing glycan fragments and microbial contents that serve as ligands for immune receptors, which tailor the immune response against the offending pathogen. The glycan fragments serve as MHC class II (MHC II) ligands and innate receptor agonists, whereas microbial proteins serve as substrates for proteolytic cleavage and MHC II presentation, and released nucleic acids activate innate pattern-recognition receptors (e.g., TLR9). In the current study, confocal microscopy of live macrophages and dendritic cells revealed that endocytosis of carbohydrates lead to vesicular acidification independent of proton pump activity. Acidification was dependent on NO-mediated oxidation in the presence of the ingested carbohydrate and was sufficient to negatively regulate T cell-dependent polysaccharide Ag cleavage, promote acid-dependent protein Ag processing, and facilitate CpG-mediated TLR9 signaling. Our findings lead to a model in which oxidation of carbohydrates from encapsulated microbes facilitates adaptive immune responses against microbial protein and carbohydrate Ags through promoting Ag processing for MHC II-mediated presentation as well as innate responses against released microbial DNA via TLR9 signaling.