Possible molecular basis for macromolecular antigen attachment to host cells: their immune complex with plasma antibodies have unoccupied binding sites enabling binding to smaller ligands

Abstract

Macromolecules such as lipoprotein(a) and antigens of Streptococcus mutans that cause dental infections accumulate in perivascular cells in inflammatory vascular disorders though they do not bind host cells directly. Suspecting a role for cognate circulating antibodies in the molecular mechanisms of macromolecular antigen deposition in tissues we investigated the ligand binding properties of de novo immune complexes (IC) formed between these two entities. Negatively charged Streptococcus mutans antigens (NSMAg) from cultured bacteria and apo(a) subunit separated from human plasma lipoprotein (a) were used as antigens to interact with anti β-glucan (ABG) and anti-α-galactoside (anti-Gal) antibodies respectively of normal plasma. Binding of antigen to antibody was measured in terms of the enhancement of fluorescence of FITC-labeled antibody that accompanies antigen binding. Unoccupied binding sites on macromolecular antigen-antibody ICs were demonstrated by capturing them on smaller ligands immobilized on microplates. Biotin-labeled macromolecular antigens were detected in ICs using HRP-labeled avidin. While total Streptococcus mutans antigens and a synthetic glycoprotein in which an ABG-specific disaccharide (cellobiose) had been covalently attached were equal in avidity towards ABG, NSMAg was superior. When binding of antibody to plate-coated ligands was monitored using HRP-labeled anti-immunoglobulin preincubation of ABG or anti-Gal with respective macromolecular antigens significantly enhanced binding response whereas antibodies preincubated with small sugars were fully inhibited from binding, indicating that macromolecular antigen binding had activated the Fc part of antibodies and that the resulting ICs in turn bound to other ligands using binding sites in the antibody spared by the large antigens. Presence of biotinylated macromolecular antigen in IC bound to immobilized ligands was confirmed using avidin-HRP probe. Unoccupied binding sites in the antibodies involved may enable homing of macromolecule ICs on vascular cells possessing smaller ligands. This along with an activated Fc could enhance their inflammatory potential.