Normal Human Plasma Anti-β-Glucoside Antibody Has Markedly Elevated IgA Content and Binds Fungal and Yeast Polysaccharides

High glucose removes natural anti-α-galactoside and anti-β-glucoside antibody immune complexes adhering to surface O-glycoproteins of normal platelets and enhances platelet aggregation

Human natural anti-α-galactoside (anti-Gal) and anti-β-glucoside (ABG) antibodies were previously reported to recognize the serine- and threonine-rich peptide sequences (STPS) of albumin-associated O-glycoproteins (AOP1 and AOP2) as surrogate antigens, forming anti-Gal/ABG-AOP1/AOP2-albumin triplet immune complexes in plasma. Since antibodies in these triplets still possessed unoccupied binding sites, the presence of triplets on human platelets that abound in surface O-glycoproteins was examined. Upon treatment with α-galactosides and β-glucosides, normal platelets freshly isolated from young healthy individuals released triplets identical with plasma triplets according to ELISA results. The resulting denuded platelets, unless pre-treated with fibrinogen or the O-glycan-binding lectin jacalin, recaptured these sugar-extracted triplets in the absence of antibody-specific sugars, suggesting that the triplet antibodies recognized the STPS of O-glycosylated receptors on platelets. Molecular weight of the dominant jacalin-binding subunit on triplet-free platelet membrane was 116 kDa, close to the ~120 kDa reported for the IIb subunit of the most abundant fibrinogen-binding platelet O-glycoprotein, GPIIb/IIIa. Denuded, but not native, platelets underwent slow spontaneous aggregation and rapid ADP-mediated GPIIb/IIIa-dependent aggregation according to spectrophotometric assay. Pre-treatment of denuded platelets with jacalin significantly reduced their ADP-mediated aggregation. Amyloid β (Aβ-42 monomer) was reported to bind triplet O-glycoproteins through their STPS but not to albumin or the antibodies. This peptide bound to the triplets on normal platelets and to surface membrane O-glycoproteins on denuded platelets, suggesting that the surface O-glycoproteins on the normal platelets were engaged and masked by the triplets. The ABG-specific sugar glucose denuded the platelets at concentrations typically reached in diabetic sera, since anti-Gal specific or ABG-specific sugar released the triplets of both the antibodies from the platelets. In conclusion, the present study offered rationale for the presence of anti-Gal/ABG-O-glycoprotein-albumin triplets on normal platelets, for the role of triplets in platelet physiology amidst circulating platelet-activating factors such as ADP, and for platelet vulnerability during diabetes.

Human IgM Inhibits the Formation of Titan-Like Cells in Cryptococcus neoformans

Human studies have shown associations between cryptococcal meningitis and reduced IgM memory B cell levels, and studies in IgM- and/or B cell-deficient mice have demonstrated increased Cryptococcus neoformans dissemination from lungs to brain. Since immunoglobulins are part of the immune milieu that C. neoformans confronts in a human host, and its ability to form titan cells is an important virulence mechanism, we determined the effect of human immunoglobulins on C. neoformans titan cell formation in vitro (i) Fluorescence microscopy showed normal human IgG and IgM bind C. neoformans (ii) C. neoformans grown in titan cell-inducing medium with IgM, not IgG, inhibited titan-like cell formation. (iii) Absorption of IgM with laminarin or curdlan (branched and linear 1-3-beta-d-glucans, respectively) decreased this effect. (iv) Transmission electron microscopy revealed that cells grown with IgM had small capsules and unique features not seen with cells grown with IgG. (v) Comparative transcriptional analysis of cell wall, capsule, and stress response genes showed that C. neoformans grown with IgM, not IgG or phosphate-buffered saline (PBS), had decreased expression of chitin synthetase, CHS1, CHS2, and CHS8, and genes encoding cell wall carbohydrate synthetases α-1-3-glucan (AGS1) and β-1,3-glucan (FKS1). IgM also decreased expression of RIM101 and HOG1, genes encoding central regulators of C. neoformans stress response pathways and cell morphogenesis. Our data show human IgM affects C. neoformans morphology in vitro and suggest that the hypothesis that human immunoglobulins may affect C. neoformans virulence in vivo warrants further investigation.

Activity of MUC1 cancer antigen-binding plasma anti-α-galactoside antibody correlates inversely with size of autologous lipoprotein(a)

Variation in ligand-binding affinity of natural plasma anti-α-galactoside antibody (anti-Gal) is a plausible reason for differing anti-cancer defense among individuals since serine- and threonine-rich peptide sequences (STPS) in the cancer-specific MUC-1 antigen are surrogate ligands for this antibody. As affinity of a natural antibody could be modulated by systemic antigens by processes including affinity maturation, we examined the contribution of the size of lipoprotein(a) [Lp(a)], an efficient autologous anti-Gal-binding macromolecule that possesses variable numbers of STPS due to genetically determined size polymorphism, towards the specific activity (activity per unit mass) of anti-Gal. Binding of purified Lp(a) to FITC-labeled anti-Gal, measured in terms of increase in fluorescence of the latter, was inhibited by LDL in proportion to Lp(a) size presumably because LDL molecules also bind noncovalently and in proportion to Lp(a) size at the O-glycosylated and STPS-rich region of Lp(a). For the same reason, circulating forms of smaller Lp(a) which carried fewer or no noncovalently attached LDL molecules were more efficient ligands for the antibody than the same number of larger ones ( P < 0.0001). Result suggested that smaller Lp(a), with their STPS ligands less obstructed by adhering LDL, would be more effective systemic antigens for anti-Gal. In confirmation of this, the specific activity of anti-Gal decreased with Lp(a) size (r − 0.5443; P < 0.0001) but increased with Lp(a) concentration (r 0.6202; P < 0.0001) among 73 normal plasma samples. IgG to IgM ratio, an index of immunoglobulin class switching characteristic of affinity maturation, was decidedly higher for anti-Gal in small Lp(a) individuals than in their large Lp(a) counterparts ( P = 0.0014). Results indicated that modulation of activity of anti-Gal by Lp(a) size may account for the lower incidence of cancer reported in people carrying more plasma Lp(a) which are generally smaller as well.

Anti-α-galactoside and Anti-β-glucoside Antibodies are Partially Occupied by Either of Two Albumin-bound O-glycoproteins and Circulate as Ligand-binding Triplets

Two heavily O-glycosylated proteins and albumin co-purified with anti-α-galactoside (anti-Gal), the chief xenograft-rejecting antibody and anti-β-glucan (ABG) antibody isolated from human plasma by affinity chromatography on respective ligand-bearing matrices. Both antibodies and O-glycoproteins co-purified with plasma albumin eluted from albumin-specific matrix. Using components of affinity-purified antibody samples separated by electrophoresis binding of either albumin or antibody to the affinity matrix of the other or binding of O-glycoprotein to either matrix was ruled out. Enzyme-linked immunoassay and ligand-induced fluorescence enhancement of fluorolabeled antibody showed that O-glycoproteins occupied sugar-binding sites of anti-Gal and ABG. Neither antibody recognized albumin. O-Glycoprotein-albumin complexes free in plasma, or released from antibodies by specific sugars, were captured on microwell-coated O-glycan-specific lectin jacalin and detected using labeled anti-albumin. We conclude that circulating anti-Gal and ABG form protein triplets in which either O-glycoprotein bridges between antibody and albumin by binding simultaneously to both. Bound albumin restricted O-glycoprotein occupation on antibodies enabling triplets to bind other ligands using spared binding sites. Free anti-Gal and ABG were undetectable in plasma. Jacalin treatment, but not de-O-glycosylation of O-glycoproteins abolished their recognition by anti-Gal or ABG indicating that antibodies recognized serine- and threonine-rich peptide sequences that underlie the O-glycans and are reported surrogate ligands for anti-Gal. The albumin- and antibody-binding O-glycoproteins AOP1 and AOP2 were single polypeptide proteins of size 107 kDa and 98 kDa, containing 54% and 51% carbohydrate respectively and conformed to no known plasma protein in properties. Prospects of triplet-mediated modulations in autologous tissues expressing antibody ligands are discussed.

High polymeric IgA content facilitates recognition of microbial polysaccharide-natural serum antibody immune complexes by immobilized human galectin-1

Dextran-binding immunoglobulin (DIg) and anti-β-glucan antibody (ABG) are naturally occurring human serum antibodies specific to α- and β-glucoside epitopes respectively of polysaccharide antigens and heavily enriched in IgA. ABG and DIg are shown here to have much more of their IgA in polymeric form than does serum IgA in general. Cell wall β-glucans and glycoproteins of the widely consumed yeast (Saccharomyces cerevisiae) offered several hundred fold better ligands for ABG than did small β-glucosides. Candida albicans cell wall antigen (CCA), a commonly encountered polysaccharide-rich fungal antigen was recognized by normal human serum anti-carbohydrate antibodies to precipitate maximally at a definite stoichiometry typical of immune complexes (IC). IC formed in serum in vitro on addition of CCA contained a significantly higher percentage of IgA than did either naturally occurring IC or serum. Polymeric IgA was far better ligand than monomeric IgA for both anti-IgA antibody and the most widely expressed human tissue lectin galectin-1 which recognizes O-linked oligosaccharides characteristic of IgA, in contrast to N-linked oligosaccharides present in all immunoglobulins. Moreover, desialylation by neuraminidase, an enzyme released into circulation during many microbial infections and diabetes, increased lectin-binding activity of polymeric IgA much more than that of monomeric IgA. Human galectin-1 immobilized in active form in vitro sugar-specifically captured IgA and IgA-containing IC formed by CCA in serum but not IgG. Results suggest that while high IgA content especially in polymeric form may render polysaccharide IC more susceptible to tissue uptake, desialylation of IgA in IC could enhance the process.

IgA1 desialylated by microbial neuraminidase forms immune complex with naturally occurring anti-T antibody in human serum

IgA1 was identified as the most prominent O-glycosylated protein of human serum. Desialylation by bacterial (Clostridium perfringens) neuraminidase rendered dot-blotted IgA1 recognizable by the naturally occurring serum antibody (anti-T) directed against Thomsen-Friedenreich antigen, Galbeta1-->3GalNAc-alpha-. On Western blot of serum O-glycosylated proteins anti-T recognized nearly all the bands including IgA1 as did the T antigen-specific animal lectin galectin-1 but only after their desialylation. Agglutination of desialylated human erythrocytes by anti-T was effectively inhibited by desialylated IgA1, but not by native IgA1 or other immunoglobulins. Desialylation of serum by neuraminidase led to significantly increased formation of immune complexes containing IgM, the major immunoglobulin type in anti-T on one hand and O-glycosylated proteins/IgA1 on the other. In further evidence for anti-T-desialylated IgA1 immune complex formation, purified anti-T added to desialylated, but not native serum led to formation of additional IgA-IgM immune complexes. Also neuraminidase treatment significantly reduced the titre of free (non-immune complexed) anti-T in serum, while selective removal of anti-T by affinity absorption resulted in considerable decrease in the amount of IgA1 that got converted to immune complexes following enzymatic desialylation of serum. Formation of immune complex between anti-T and neuraminidase-treated IgA1 in serum may be significant since many disease pathogens release neuraminidase and since IgA1 is a powerful ligand for tissue galectin-1 more so after desialylation. Diabetes also raises serum IgA and neuraminidase levels.