Selective removal of alpha heavy-chain glycosylation sites causes immunoglobulin A degradation and reduced secretion

TNFRSF13B Diversification Fueled by B Cell Responses to Environmental Challenges-A Hypothesis

B cell differentiation and memory are controlled by the transmembrane activator and CAML interactor (TACI), a receptor encoded by TNFRSF13B. TNFRSF13B mutations are frequently found in common variable immunodeficiency (CVID) and in IgA -deficiency; yet, ~98% of those with mutant TNFRSF13B are healthy. Indeed, TNFRSF13B is among the 5% most polymorphic genes in man. Other mammals evidence polymorphism at comparable loci. We hypothesize that TNFRSF13B diversity might promote rather than detract from well-being by controlling key elements of innate immunity. We shall discuss how extraordinary diversity of TNFRSF13B could have evolved and persisted across diverse species of mammals by controlling innate and adaptive B cell responses in apparently paradoxical ways.

Immunoglobulin A N-glycosylation Presents Important Body Fluid-specific Variations in Lactating Mothers

Secretory Immunoglobulin A (SIgA) is central to mucosal immunity: represents one of the main immunological mechanisms of defense against the potential attack of pathogens. During lactation SIgA is produced by plasmablasts in the mammary gland and is present in breast milk, playing a vital role in the passive immunity of the newborn. Interestingly, the different components of SIgA are highly N-glycosylated, and these N-Glycans have an essential role in health maintenance. In this work, we performed a glycomic study to compare N-glycosylation of SIgA purified from mature breast milk and saliva, and plasma IgA from the same lactating participants. Our results revealed a greater diversity than previously reported, with 89 glycan compositions that may correspond to over 250 structures. Among these glycans, 54 glycan compositions were characterized as body-fluid specific. Most of these unique N-Glycan compositions identified in SIgA from mature milk and IgA from plasma were fucosylated and both fucosylated and sialylated species, whereas in salivary SIgA the unique structures were mainly undecorated complex N-Glycans. In addition, we evaluated the effect of delivery mode on (S)IgA glycosylation. Lactating participants who had given birth by vaginal delivery presented an increased proportion of high mannose and fucosylated glycans in salivary SIgA, and selected high mannose, fucosylated, sialylated, and both fucosylated and sialylated glycans in plasma IgA, indicating that the hormonal changes during vaginal delivery could affect plasma and saliva IgA. These results reveal the structural details that provide a new dimension to the roles of (S)IgA N-Glycans in different tissues, and especially in maternal and new-born protection and infant development. The design of optimal recombinant IgA molecules specifically targeted to protect mucosal surfaces will need to include this dimension of structural detail.

Magnesium transporter 1 (MAGT1) deficiency causes selective defects in N-linked glycosylation and expression of immune-response genes

Magnesium transporter 1 (MAGT1) critically mediates magnesium homeostasis in eukaryotes and is highly-conserved across different evolutionary branches. In humans, loss-of-function mutations in the MAGT1 gene cause X-linked magnesium deficiency with Epstein-Barr virus (EBV) infection and neoplasia (XMEN), a disease that has a broad range of clinical and immunological consequences. We have previously shown that EBV susceptibility in XMEN is associated with defective expression of the antiviral natural-killer group 2 member D (NKG2D) protein and abnormal Mg²⁺ transport. New evidence suggests that MAGT1 is the human homolog of the yeast OST3/OST6 proteins that form an integral part of the N-linked glycosylation complex, although the exact contributions of these perturbations in the glycosylation pathway to disease pathogenesis are still unknown. Using MS-based glycoproteomics, along with CRISPR/Cas9-KO cell lines, natural killer cell-killing assays, and RNA-Seq experiments, we now demonstrate that humans lacking functional MAGT1 have a selective deficiency in both immune and nonimmune glycoproteins, and we identified several critical glycosylation defects in important immune-response proteins and in the expression of genes involved in immunity, particularly CD28. We show that MAGT1 function is partly interchangeable with that of the paralog protein tumor-suppressor candidate 3 (TUSC3) but that each protein has a different tissue distribution in humans. We observed that MAGT1-dependent glycosylation is sensitive to Mg²⁺ levels and that reduced Mg²⁺ impairs immune-cell function via the loss of specific glycoproteins. Our findings reveal that defects in protein glycosylation and gene expression underlie immune defects in an inherited disease due to MAGT1 deficiency.

Differences in N-glycan structures found on recombinant IgA1 and IgA2 produced in murine myeloma and CHO cell lines

The development and production of recombinant monoclonal antibodies is well established. Although most of these are IgGs, there is also great interest in producing recombinant IgAs since this isotype plays a critical role in providing immunologic protection at mucosal surfaces. The choice of expression system for production of recombinant antibodies is crucial because they are glycoproteins containing at least one N-linked carbohydrate. These glycans have been shown to contribute to the stability, pharmacokinetics and biologic function of antibodies. We have produced recombinant human IgA1 and all three allotypes of IgA2 in murine myeloma and CHO cell lines to systematically characterize and compare the N-linked glycans. Recombinant IgAs produced in murine myelomas differ significantly from IgA found in humans in that they contain the highly immunogenic Galalpha(1,3)Gal epitope and N-glycolylneuraminic acid residues, indicating that murine myeloma is not the optimal expression system for the production of human IgA. In contrast, IgAs produced in CHO cells contained glycans that were more similar to those found on human IgA. Expression of IgA1 and IgA2 in Lec2 and Lec8 cell lines that are defective in glycan processing resulted in a less complex pool of N-glycans. In addition, the level of sialylation of rIgAs produced in murine and CHO cells was significantly lower than that previously reported for serum IgA1. These data underscore the importance of choosing the appropriate cell line for the production of glycoproteins with therapeutic potential.

In vitro comparison of the antigen-binding and stability properties of the various molecular forms of IgA antibodies assembled and produced in CHO cells

The hallmark of a mucosal immune response is the production of antigen-specific secretory IgA (S-IgA) antibodies in external secretions. S-IgA consists of ten polypeptides produced in two different cell lineages. The heavy and light chains in plasma cells assemble into IgA, which on association with J chain become polymerized, whereas secretory component (SC) is added during transport across the epithelium. Recombinant chimeric mouse-human monomeric, dimeric, and S-IgA antibodies have been produced in a single CHO cell sequentially transfected with expression vectors carrying three independent selective markers for chimeric heavy and light chains, human J chain, and human SC, respectively. Biochemical characterization of the various molecular forms indicates that the assembly of the various polypeptides resulted in species of the expected size and covalence. All chimeric IgA antibodies retained the antigen-binding capacity of the parent mouse IgA antibody. The resistance of S-IgA to protease-rich intestinal washes was enhanced when compared with dimeric IgA lacking associated SC. Up to 20 micrograms of recombinant S-IgA per 1 x 10(6) cells were recovered in 24 h with the best producing clones. We conclude that CHO cells programmed de novo with four different genetic elements can assemble functional chimeric S-IgA.

Role of glycosylation in transport and enzymic activity of neutral endopeptidase-24.11

Neutral endopeptidase (NEP, EC 3.4.24.11) is a major ectoenzyme of the brush-border membrane. The ectodomain of NEP contains five putative N-glycosylation sites. In order to determine the role of the addition of sugar moieties on the activity and intracellular transport of NEP, we have used site-directed mutagenesis to remove all or some of the five potential sites of sugar addition in membrane-bound and secreted forms of the enzyme. Expression of NEP glycosylation mutants in COS-1 cells showed that all five sites are used for sugar addition. Immunoblotting of NEP in COS-1 cell extracts or culture media indicated that total expression of normal membrane-bound NEP was not affected by mutations at glycosylation sites, whereas this expression level appeared to be strictly dependent on the number of glycosylation sites retained on the soluble form. The transport to the cell surface was also reduced by decreased glycosylation, but again the phenomenon appeared more drastic in the case of the soluble form than for the membrane-bound enzyme. Enzyme activity was decreased by deglycosylation. However, the presence of either of two crucial sites (sites 1 and 5; numbered from the N-terminus of the protein) was sufficient to recover close-to-normal enzymic activities. Transport to the cell surface and enzyme activity of NEP are thus both dependent on sugar residues, probably through different conformational constraints. These constraints seem to be local for enzyme activity but more global for transport to the cell surface.

How N-linked oligosaccharides affect glycoprotein folding in the endoplasmic reticulum

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Glycosylation requirements for intracellular transport and function of the hemagglutinin of influenza virus

The contribution of each of the seven asparagine-linked oligosaccharide side chains on the hemagglutinin of the A/Aichi/68 (X31) strain of influenza virus was assessed with respect to its effect on the folding, intracellular transport, and biological activities of the molecule. Twenty mutant influenza virus hemagglutinins were constructed and expressed, each of which had one or more of the seven glycosylation sites removed. Investigations of these mutant hemagglutinins indicated that (i) no individual oligosaccharide side chain is necessary or sufficient for the folding, intracellular transport, or function of the molecule, (ii) at least five oligosaccharide side chains are required for the X31 hemagglutinin molecule to move along the exocytic pathway to the plasma membrane, and (iii) mutant hemagglutinins having less than five oligosaccharide side chains form intracellular aggregates and are retained in the endoplasmic reticulum.

Mutational analysis of N-linked glycosylation sites of Friend murine leukemia virus envelope protein

The roles played by the N-linked glycans of the Friend murine leukemia virus envelope proteins were investigated by site-specific mutagenesis. The surface protein gp70 has eight potential attachment sites for N-linked glycan; each signal asparagine was converted to aspartate, and mutant viruses were tested for the ability to grow in NIH 3T3 fibroblasts. Seven of the mutations did not affect virus infectivity, whereas mutation of the fourth glycosylation signal from the amino terminus (gs4) resulted in a noninfectious phenotype. Characterization of mutant gene products by radioimmunoprecipitation confirmed that glycosylation occurs at all eight consensus signals in gp70 and that gs2 carries an endoglycosidase H-sensitive glycan. Elimination of gs2 did not cause retention of an endoglycosidase H-sensitive glycan at a different site, demonstrating that this structure does not play an essential role in envelope protein function. The gs3- mutation affected a second posttranslational modification of unknown type, which was manifested as production of gp70 that remained smaller than wild-type gp70 after removal of all N-linked glycans by peptide N-glycosidase F. The gs4- mutation decreased processing of gPr80 to gPr90, completely inhibited proteolytic processing of gPr90 to gp70 and Pr15(E), and prevented incorporation of envelope products into virus particles. Brefeldin A-induced mixing of the endoplasmic reticulum and parts of the Golgi apparatus allowed proteolytic processing of wild-type gPr90 to occur in the absence of protein transport, but it did not overcome the cleavage defect of the gs4- precursor, indicating that gs4- gPr90 is resistant to the processing protease. The work reported here demonstrates that the gs4 region is important for env precursor processing and suggests that gs4 may be a critical target in the disruption of murine leukemia virus env product processing by inhibitors of N-linked glycosylation.

Characterization of a recombinant herpes simplex virus which expresses a glycoprotein D lacking asparagine-linked oligosaccharides

Glycoprotein D (gD) is an envelope component of herpes simplex virus essential for virus penetration. gD contains three sites for addition of asparagine-linked carbohydrates (N-CHO), all of which are utilized. Previously, we characterized mutant forms of herpes simplex virus type 1 gD (gD-1) lacking one or all three N-CHO addition sites. All of the mutants complemented the infectivity of a gD-minus virus, F-gD beta, to the same extent as wild-type gD. Here, we show that recombinant viruses containing mutations in the gD-1 gene which eliminate the three N-CHO signals are viable. Two such viruses, called F-gD(QAA)-1 and F-gD(QAA)-2, were independently isolated, and the three mutations in the gD gene in one of these viruses were verified by DNA sequencing. We also verified that the gD produced in cells infected by these viruses is devoid of N-CHO. Plaques formed by both mutants developed more slowly than those of the wild-type control virus, F-gD(WT), and were approximately one-half the size of the wild-type. One mutant, F-gD(QAA)-2, was selected for further study. The QAA mutant and wild-type gD proteins extracted from infected cells differed in structure, as determined by the binding of monoclonal antibodies to discontinuous epitopes. However, flow cytometry analysis showed that the amount and structure of gD found on infected cell surfaces was unaffected by the presence or absence of N-CHO. Other properties of F-gD(QAA)-2 were quite similar to those of F-gD(WT). These included (i) the kinetics of virus production as well as the intracellular and extracellular virus titers; (ii) the rate of virus entry into uninfected cells; (iii) the levels of gB, gC, gE, gH, and gI expressed by infected cells; and (iv) the turnover time of gD. Thus, the absence of N-CHO from gD-1 has some effect on its structure but very little effect on its function in virus infection in cell culture.

Different roles of individual N-linked oligosaccharide chains in folding, assembly, and transport of the simian virus 5 hemagglutinin-neuraminidase

The role of N-linked glycosylation in protein maturation and transport has been studied by using the simian virus 5 hemagglutinin-neuraminidase (HN) protein, a model class II integral membrane glycoprotein. The sites of N-linked glycosylation on HN were identified by eliminating each of the potential sites for N-linked glycosylation by oligonucleotide-directed mutagenesis on a cDNA clone. Expression of the mutant HN proteins in eucaryotic cells indicated that four sites are used in the HN glycoprotein for the addition of N-linked oligosaccharide chains. These functional glycosylation sites were systematically eliminated in various combinations from HN to form a panel of mutants in which the roles of individual carbohydrate chains and groups of carbohydrate chains could be analyzed. Alterations in the normal glycosylation pattern resulted in the impairment of HN protein folding and assembly which, in turn, affected the intracellular transport of HN. The severity of the consequences on HN maturation depended on both the number of deleted carbohydrate sites and their position in the HN molecule. Analysis of the reactivity pattern of HN conformation-specific monoclonal antibodies with the mutant HN proteins indicated that one specific carbohydrate chain plays a major role in promoting the correct folding of HN. Another carbohydrate chain, which is not essential for the initial folding of HN was found to play a role in preventing the aggregation of HN oligomers. The HN molecules which were misfolded, owing to their altered glycosylation pattern, were retained in the endoplasmic reticulum. Double-label immunofluorescence experiments indicate that misfolded HN and folded HN are segregated in the same cell. Misfolded HN forms disulfide-linked aggregates and is stably associated with the resident endoplasmic reticulum protein, GRP78-BiP, whereas wild-type HN forms a specific and transient complex with GRP78-BiP during its folding process.

Different roles of individual N-linked oligosaccharide chains in folding, assembly, and transport of the simian virus 5 hemagglutinin-neuraminidase

The role of N-linked glycosylation in protein maturation and transport has been studied by using the simian virus 5 hemagglutinin-neuraminidase (HN) protein, a model class II integral membrane glycoprotein. The sites of N-linked glycosylation on HN were identified by eliminating each of the potential sites for N-linked glycosylation by oligonucleotide-directed mutagenesis on a cDNA clone. Expression of the mutant HN proteins in eucaryotic cells indicated that four sites are used in the HN glycoprotein for the addition of N-linked oligosaccharide chains. These functional glycosylation sites were systematically eliminated in various combinations from HN to form a panel of mutants in which the roles of individual carbohydrate chains and groups of carbohydrate chains could be analyzed. Alterations in the normal glycosylation pattern resulted in the impairment of HN protein folding and assembly which, in turn, affected the intracellular transport of HN. The severity of the consequences on HN maturation depended on both the number of deleted carbohydrate sites and their position in the HN molecule. Analysis of the reactivity pattern of HN conformation-specific monoclonal antibodies with the mutant HN proteins indicated that one specific carbohydrate chain plays a major role in promoting the correct folding of HN. Another carbohydrate chain, which is not essential for the initial folding of HN was found to play a role in preventing the aggregation of HN oligomers. The HN molecules which were misfolded, owing to their altered glycosylation pattern, were retained in the endoplasmic reticulum. Double-label immunofluorescence experiments indicate that misfolded HN and folded HN are segregated in the same cell. Misfolded HN forms disulfide-linked aggregates and is stably associated with the resident endoplasmic reticulum protein, GRP78-BiP, whereas wild-type HN forms a specific and transient complex with GRP78-BiP during its folding process.

Influence of asparagine-linked oligosaccharides on antigenicity, processing, and cell surface expression of herpes simplex virus type 1 glycoprotein D

Glycoprotein D (gD) is an envelope component of herpes simplex virus types 1 and 2. gD-1 contains three sites for the addition of N-linked carbohydrate (N-CHO), all of which are used. Three mutants were constructed by site-directed mutagenesis, each of which altered one N-CHO addition site from Asn-X-Thr/Ser to Asn-X-Ala. A fourth mutant was altered at all three sites. The mutant genes were inserted into an expression vector, and the expressed protein was analyzed in transiently transfected COS-1 cells. The mutant protein lacking N-CHO at site 1 (Asn-94) had a reduced affinity for monoclonal antibodies (MAbs) to discontinuous epitopes, suggesting that the conformation of the protein had been altered. However, the protein was processed and transported to the cell surface. The absence of N-CHO at site 2 (Asn-121) had no apparent effect on processing or transport of gD-1 but resulted in reduced binding of two MAbs previously shown to be in group VI. Binding of other MAbs to discontinuous epitopes (including other group VI MAbs) was not affected. The absence of N-CHO at site 3 (Asn-262) had no effect on processing, transport, or conformation of the gD-1 protein. The absence of N-CHO from site 1 or from all three sites resulted in the formation of high-molecular-weight aggregates or complexes and a reduction in MAb binding. However, these proteins were modified by the addition of O-glycans and transported to the cell surface. We conclude that the absence of the first or all N-linked carbohydrates alters the native conformation of gD-1 but does not prevent its transport to the cell surface.