Characterization of the human immunoglobulin G Fc-binding activity in Prevotella intermedia.

Many pathogenic bacteria possess cell surface receptors which can bind immunoglobulins via the Fc portion. The aim of this study was to characterize the human immunoglobulin G (IgG) Fc-binding activity of Prevotella intermedia, a suspected etiologic agent of adult chronic periodontitis. The Fc-binding activity of P. intermedia on whole cells and on extracellular vesicles was demonstrated. Incubation of P. intermedia cells in the presence of Zwittergent 3-14 allowed complete solubilization of the Fc receptor from the cell surface. This cell envelope extract was thus used to characterize the Fc-binding activity. A microtiter plate assay using alkaline phosphatase-labeled Fc fragments showed that preincubation of the cell envelope extract with human IgG, human IgG Fc fragments, or human serum completely inhibited the Fc-binding activity. Partial inhibition was obtained with human IgG F(ab')2 fragments, whereas no inhibition occurred following preincubation with human IgA, carbohydrates, and selected proteins. Preincubation of the cell envelope extract with IgG from a variety of animals demonstrated that rabbit, mouse, rat, goat, and sheep IgG did not inhibit Fc-binding activity, whereas cow, pig, and dog IgG partially inhibited Fc-binding activity. A strong inhibition comparable to that obtained with human IgG was noted with monkey IgG. The Fc receptor of P. intermedia is thus different from the six types previously reported in other nonoral bacteria. Polyacrylamide gel electrophoresis and Western blotting (immunoblotting) analysis of the cell envelope extract revealed a major band with a molecular mass of approximately 65 kDa which reacted with peroxidase-labeled human IgG Fe fragments. Transmission electron microscopy showed a uniform distribution of the Fc receptor on the bacterial surface, as revealed by gold labeling. The Fc-binding activity demonstrated in this study may act as an additional virulence factor for P. intermedia by reducing IgG reactions with the bacterial cell.