Natural α-galactoside- and β-glucoside-specific antibody immune complexes bound to surface O-glycoproteins of circulating platelets prevent aggregation, but dislodge at diabetic glucose levels

Abstract

Human anti-α-galactoside (anti-Gal) and anti-β-glucoside (ABG) antibodies were reported to recognize as surrogate antigens the serine- and threonine-rich peptide sequences (STPS) in newly detected albumin-associated O-glycoproteins AOP1 and AOP2 to form circulating antibody-AOP1/AOP2-albumin triplets. Since triplet antibodies still possessed unoccupied binding sites, their binding to the O-glycoprotein-rich platelet surface was examined. Upon treatment with sugars specific to the above antibodies, freshly harvested normal human platelets released triplets identical with plasma triplets. The resulting denuded platelets, unless pre-treated with fibrinogen or the O-glycan-binding lectin jacalin, recaptured the sugar-extracted triplets, plasma triplets and isolated triplet antibodies. Antibody-specific sugars inhibited recapture, confirming that recognition of STPS on O-glycosylated fibrinogen receptors by antibodies facilitated triplet attachment to platelets. More triplets were platelet-bound than in plasma. The dominant jacalin-binding subunit in triplet-free platelet membrane had size (116 kDa), close to around 120 kDa reported for the O-glycosylated GPIIb subunit of GPIIb/IIIa, the most expressed and fibrinogen-binding platelet protein. Denuded but not native platelets were prone to slow spontaneous aggregation and to quick ADP-mediated GPIIb/IIIa-dependent aggregation unless pre-treated with jacalin. Amyloid β (Aβ-42 monomer), reported to recognize STPS in plasma AOP1 and AOP2, but not albumin, bound to triplets in normal platelets and to membrane O-glycoproteins in denuded ones. ABG-specific sugar glucose at diabetic concentrations denuded the platelets since sugar specific to either antibody released triplets of both. Data suggest a molecular mechanism for diabetes-driven platelet aggregation and how platelet-leukocyte adhesion and cerebral amyloid angiopathy, both requiring exposed GPIIb/IIIa on platelets, may accompany diabetes.