Sialylation regulates peripheral tolerance in CD4+ T cells

Genetic Variants in the ST6GAL1 Gene Are Associated with Thyroglobulin Plasma Level in Healthy Individuals

Background: Thyroglobulin (Tg) is a 660 kDa iodoglycoprotein that serves as a scaffold for thyroid hormone synthesis. Although a twin study showed that variability of serum Tg levels has a substantial genetic basis, no genome-wide association study (GWAS) of serum/plasma Tg levels has been performed to date. The aim of this study was to identify genetic variants associated with plasma Tg levels among healthy individuals. Methods: A GWAS was conducted on two Croatian cohorts, and a combined analysis was performed. The analyses included 1094 individuals. A total of 7,597,379 variants, imputed using the 1000 Genomes reference panel, were analyzed for association. GWAS was performed under an additive model, controlling for age, sex, and relatedness within each data set. Combined analysis was conducted using the inverse-variance fixed-effects method. Results: Sixteen variants located on chromosome 3, within the ST6GAL1 gene, reached genome-wide significance. The lead SNP was rs4012172 ( \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\usepackage{upgreek}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} $$p = 1.29 \times {10^{ - 10}}$$ \end{document} ), which explained 3.19% of the variance in Tg levels. ST6GAL1 belongs to the sialyltransferase protein family, which has a fundamental role in the synthesis of specific sialylated structures on various glycoproteins, including Tg. It is known that only immature Tg (poorly sialylated or desialylated) can be transferred to the bloodstream. Conclusions: A highly biologically plausible locus was identified that could have a role in the regulation of plasma Tg levels in healthy individuals.

Increased Polysialylation of the Neural Cell Adhesion Molecule in a Transgenic Mouse Model of Sialuria

Sialuria is a rare autosomal dominant disorder of mammalian metabolism, caused by defective feedback inhibition of the UDP-N-acetylglucosamine-2-epimerase N-acetylmannosamine kinase (GNE), the key enzyme of sialic acid biosynthesis. Sialuria is characterized by overproduction of free sialic acid in the cell cytoplasm. Patients exhibit vastly increased urinary excretion of sialic acid and show differently pronounced developmental delays. The physiopathology of sialuria is not well understood. Here we established a transgenic mouse line that expresses GNE containing the sialuria mutation R263L, in order to investigate the influence of an altered sialic acid concentration on the organism. The transgenic mice that expressed the mutated RNA excreted up to 400 times more N-acetylneuraminic acid than wild-type mice. Additionally, we found higher sialic acid concentration in the brain cytoplasm. Analyzing the (poly)sialylation of neural cell adhesion molecule (NCAM) revealed increased polysialylation in brains of transgenic mice compared to wild-type. However, we found only minor changes in membrane-bound sialylation in various organs but, surprisingly, a significant increase in surface sialylation on leukocytes. Our results suggest that the intracellular sialic acid concentration regulates polysialylation on NCAM in vivo; this could play a role in the manifestation of the developmental delays in sialuria patients.

Siglec-E Negatively Regulates the Activation of TLR4 by Controlling Its Endocytosis

TLR4 signaling is critical for providing effective immune protection, but it must be tightly controlled to avoid inflammation-induced pathology. Previously, we reported extensive and direct interactions between TLR and Siglec families of pattern recognition receptors. In this study, we examined the biological significance of this interaction during infection. We show that Siglec-E is required for Escherichia coli-induced endocytosis of TLR4. Siglec-E-deficient dendritic cells infected with E. coli fail to internalize TLR4. This leads to sustained TLR4 on the cell surface and activation of NF-κB and MAPK p38, resulting in high levels of TNF-α and IL-6 compared with wild-type dendritic cells. In contrast to the signaling events occurring at the plasma membrane, as a result of the inability to internalize TLR4, Siglec-E-deficient dendritic cells were also defective for TRIF-mediated IFN-β production in response to E. coli infection. Furthermore, we found that accumulation of ubiquitinated TLR4 and binding of E3 ubiquitin ligase Triad3A to TLR4 was increased significantly in bone marrow-derived dendritic cells from wild-type mice, but not from Siglec-E-deficient mice, after E. coli infection. This represents a newly discovered mechanism that regulates the signaling of TLR4 during E. coli infection.

Induction of Siglec-1 by Endotoxin Tolerance Suppresses the Innate Immune Response by Promoting TGF-β1 Production

Sepsis is one of the leading causes of death worldwide. Although the prevailing theory for the sepsis syndrome is a condition of uncontrolled inflammation in response to infection, sepsis is increasingly being recognized as an immunosuppressive state known as endotoxin tolerance. We found sialylation of cell surface was significantly increased on LPS-induced tolerant cells; knockdown of Neu1 in macrophage cell line RAW 264.7 cells resulted in enhanced LPS-induced tolerance, whereas overexpression of Neu1 or treatment with sialidase abrogated LPS-induced tolerance, as defined by measuring TNF-α levels in the culture supernatants. We also found that the expression of Siglec-1 (a member of sialic acid-binding Ig (I)-like lectin family members, the predominant sialic acid-binding proteins on cell surface) was specifically up-regulated in endotoxin tolerant cells and the induction of Siglec-1 suppresses the innate immune response by promoting TGF-β1 production. The enhanced TGF-β1 production by Siglec-1 was significantly attenuated by spleen tyrosine kinase (Syk) inhibitor. Knockdown of siglec-1 in RAW 264.7 cells resulted in inhibiting the production of TGF-β1 by ubiquitin-dependent degradation of Syk. Mechanistically, Siglec-1 associates with adaptor protein DNAX-activation protein of 12 kDa (DAP12) and transduces a signal to Syk to control the production of TGF-β1 in endotoxin tolerance. Thus, Siglec-1 plays an important role in the development of endotoxin tolerance and targeted manipulation of this process could lead to a new therapeutic opportunity for patients with sepsis.

Intrinsically de-sialylated CD103(+) CD8 T cells mediate beneficial anti-glioma immune responses

BACKGROUND

Cancer vaccines reproducibly cure laboratory animals and reveal encouraging trends in brain tumor (glioma) patients. Identifying parameters governing beneficial vaccine-induced responses may lead to the improvement of glioma immunotherapies. CD103(+) CD8 T cells dominate post-vaccine responses in human glioma patients for unknown reasons, but may be related to recent thymic emigrant (RTE) status. Importantly, CD8 RTE metrics correlated with beneficial immune responses in vaccinated glioma patients.

METHODS

We show by flow cytometry that murine and human CD103(+) CD8 T cells respond better than their CD103(-) counterparts to tumor peptide-MHC I (pMHC I) stimulation in vitro and to tumor antigens on gliomas in vivo.

RESULTS

Glioma responsive T cells from mice and humans both exhibited intrinsic de-sialylation-affecting CD8 beta. Modulation of CD8 T cell sialic acid with neuraminidase and ST3Gal-II revealed de-sialylation was necessary and sufficient for promiscuous binding to and stimulation by tumor pMHC I. Moreover, de-sialylated status was required for adoptive CD8 T cells and lymphocytes to decrease GL26 glioma invasiveness and increase host survival in vivo. Finally, increased tumor ST3Gal-II expression correlated with clinical vaccine failure in a meta-analysis of high-grade glioma patients.

CONCLUSIONS

Taken together, these findings suggest that de-sialylation of CD8 is required for hyper-responsiveness and beneficial anti-glioma activity by CD8 T cells. Because CD8 de-sialylation can be induced with exogenous enzymes (and appears particularly scarce on human T cells), it represents a promising target for clinical glioma vaccine improvement.

Characteristics of TCR/CD3 complex CD3{varepsilon} chains of regulatory CD4+ T (Treg) lymphocytes: role in Treg differentiation in vitro and impact on Treg in vivo

Tregs are anergic CD4(+)CD25(+)Foxp3(+) T lymphocytes exerting active suppression to control immune and autoimmune responses. However, the factors in TCR recognition underlying Treg differentiation are unclear. Based on our previous data, we hypothesized that Treg TCR/CD3 antigen receptor complexes might differ from those of CD4(+)CD25(-) Tconv. Expression levels of TCR/CD3, CD3ε,ζ chains, or other molecules involved in antigen signaling and the characteristics of CD3ε chains were analyzed in thymus or spleen Treg cells from normal mice. Tregs had quantitative and qualitatively distinct TCR/CD3 complexes and CD3ε chains. They expressed significantly lower levels of the TCR/CD3 antigen receptor, CD3ε chains, TCR-ζ chain, or the CD4 coreceptor than Tconv. Levels of kinases, adaptor molecules involved in TCR signaling, and early downstream activation pathways were also lower in Tregs than in Tconv. Furthermore, TCR/CD3 complexes in Tregs were enriched in CD3ε chains conserving their N-terminal, negatively charged amino acid residues; this trait is linked to a higher activation threshold. Transfection of mutant CD3ε chains lacking these residues inhibited the differentiation of mature CD4(+)Foxp3(-) T lymphocytes into CD4(+)Foxp3(+) Tregs, and differences in CD3ε chain recognition by antibodies could be used to enrich for Tregs in vivo. Our results show quantitative and qualitative differences in the TCR/CD3 complex, supporting the hyporesponsive phenotype of Tregs concerning TCR/CD3 signals. These differences might reconcile avidity and flexible threshold models of Treg differentiation and be used to implement therapeutic approaches involving Treg manipulation.

Quorum sensing regulatory cascades control Vibrio fluvialis pathogenesis

Quorum sensing (QS) is a process by which individual bacteria are able to communicate with one another, thereby enabling the population as a whole to coordinate gene regulation and subsequent phenotypic outcomes. Communication is accomplished through production and detection of small molecules in the extracellular milieu. In many bacteria, particularly Vibrio species, multiple QS systems result in multiple signals, as well as cross talk between systems. In this study, we identify two QS systems in the halophilic enteric pathogen Vibrio fluvialis: one acyl-homoserine lactone (AHL) based and one CAI-1/AI-2 based. We show that a LuxI homolog, VfqI, primarily produces 3-oxo-C10-HSL, which is sensed by a LuxR homolog, VfqR. VfqR-AHL is required to activate vfqI expression and autorepress vfqR expression. In addition, we have shown that similar to that in V. cholerae and V. harveyi, V. fluvialis produces CAI-1 and AI-2 signal molecules to activate the expression of a V. cholerae HapR homolog through LuxO. Although VfqR-AHL does not regulate hapR expression, HapR can repress vfqR transcription. Furthermore, we found that QS in V. fluvialis positively regulates production of two potential virulence factors, an extracellular protease and hemolysin. QS also affects cytotoxic activity against epithelial tissue cultures. These data suggest that V. fluvialis integrates QS regulatory pathways to play important physiological roles in pathogenesis.

Increasing functional avidity of TCR-redirected T cells by removing defined N-glycosylation sites in the TCR constant domain

Adoptive transfer of T lymphocytes transduced with a T cell receptor (TCR) to impart tumor reactivity has been reported as a potential strategy to redirect immune responses to target cancer cells (Schumacher, T.N. 2002. Nat. Rev. Immunol. 2:512–519). However, the affinity of most TCRs specific for shared tumor antigens that can be isolated is usually low. Thus, strategies to increase the affinity of TCRs or the functional avidity of TCR-transduced T cells might be therapeutically beneficial. Because glycosylation affects the flexibility, movement, and interactions of surface molecules, we tested if selectively removing conserved N-glycoslyation sites in the constant regions of TCR α or β chains could increase the functional avidity of T cells transduced with such modified TCRs. We observed enhanced functional avidity and improved recognition of tumor cells by T cells harboring TCR chains with reduced N-glycosylation (ΔTCR) as compared with T cells with wild-type (WT) TCR chains. T cells transduced with WT or ΔTCR chains bound tetramer equivalently at 4°C, but tetramer binding was enhanced at 37°C, predominantly as a result of reduced tetramer dissociation. This suggested a temperature-dependent mechanism such as TCR movement in the cell surface or structural changes of the TCR allowing improved multimerization. This strategy was effective with mouse and human TCRs specific for different antigens and, thus, should be readily translated to TCRs with any specificity.

Proteolytic shedding of ST6Gal-I by BACE1 regulates the glycosylation and function of alpha4beta1 integrins

Differentiation of monocytes into macrophages is accompanied by increased cell adhesiveness, due in part to the activation of alpha4beta1 integrins. Here we report that the sustained alpha4beta1 activation associated with macrophage differentiation results from expression of beta1 integrin subunits that lack alpha2-6-linked sialic acids, a carbohydrate modification added by the ST6Gal-I sialyltransferase. During differentiation of U937 monocytic cells and primary human CD14(+) monocytes, ST6Gal-I is down-regulated, leading to beta1 hyposialylation and enhanced alpha4beta1-dependent VCAM-1 binding. Importantly, ST6Gal-I down-regulation results from cleavage by the BACE1 secretase, which we show is dramatically up-regulated during macrophage differentiation. BACE1 up-regulation, ST6Gal-I shedding, beta1 hyposialylation, and alpha4beta1-dependent VCAM-1 binding are all temporally correlated and share the same signaling mechanism (protein kinase C/Ras/ERK). Preventing ST6Gal-I down-regulation (and therefore integrin hyposialylation), through BACE1 inhibition or ST6Gal-I constitutive overexpression, eliminates VCAM-1 binding. Similarly, preventing integrin hyposialylation inhibits a differentiation-induced increase in the expression of an activation-dependent conformational epitope on the beta1 subunit. Collectively, these results describe a novel mechanism for alpha4beta1 regulation and further suggest an unanticipated role for BACE1 in macrophage function.

Superantigen-induced CD4+ T cell tolerance is associated with DNA methylation and histone hypo-acetylation at cytokine gene loci

Anergy is an important mechanism of peripheral tolerance in which T cells lose the capacity to produce proinflammatory cytokines such as interleukin-2 (IL-2) and interferon-gamma (IFNgamma). To determine whether the induction of T-cell anergy in vivo is associated with epigenetic changes that oppose cytokine gene expression, we measured DNA methylation and histone acetylation at the IL2 and IFNgamma loci in CD4+ T cells from mice tolerant to a viral superantigen. Tolerant T cells exhibited more DNA methylation and less histone acetylation at the regulatory regions of the IL2 and IFNgamma genes than effector T cells, which are able to produce IL-2 and IFNgamma. These data show that T-cell anergy in this model is associated with epigenetic modifications that oppose gene expression, and suggest that these mechanisms may be important in the maintenance of tolerance.

Differential glycosylation of TH1, TH2 and TH-17 effector cells selectively regulates susceptibility to cell death

Regulated glycosylation controls T cell processes, including activation, differentiation and homing by creating or masking ligands for endogenous lectins. Here we show that stimuli promoting T helper type 1 (TH1), TH2 or interleukin 17-producing T helper (TH-17) differentiation can differentially regulate the glycosylation pattern of T helper cells and modulate their susceptibility to galectin-1, a glycan-binding protein with anti-inflammatory activity. Although TH1- and TH-17-differentiated cells expressed the repertoire of cell surface glycans critical for galectin-1-induced cell death, TH2 cells were protected from galectin-1 through differential sialylation of cell surface glycoproteins. Consistent with those findings, galectin-1-deficient mice developed greater TH1 and TH-17 responses and enhanced susceptibility to autoimmune neuroinflammation. Our findings identify a molecular link among differential glycosylation of T helper cells, susceptibility to cell death and termination of the inflammatory response.

FOXP3 ensembles in T-cell regulation

Our recent studies have identified dynamic protein ensembles containing forkhead box protein 3 (FOXP3) that provide insight into the molecular complexity of suppressor T-cell activities, and it is our goal to determine how these ensembles regulate FOXP3's transcriptional activity in vivo. In this review, we summarize our current understanding of how FOXP3 expression is induced and how FOXP3 functions in vivo as a transcriptional regulator by assembling a multisubunit complex involved in histone modification as well as chromatin remodeling.