Fc-binding components: a virulence factor in Actinobacillus actinomycetemcomitans?

Actinobacillus actinomycetemcomitans (ATCC 33384) can produce and release components that bind to the Fc part of IgG. Fc-binding components were observed in whole bacteria, capsular material and medium from broth cultures. The components were separated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and immunoblotted with biotinylated Fc-fragments and myeloma proteins. In a phagocytosis assay with human granulocytes and sheep erythrocytes, preincubation of opsonized erythrocytes with protein A reduced phagocytosis by 90%. In contrast, preincubation of the opsonizing antibody with medium components from a culture of A. actinomycetemcomitans enhanced the opsonizing effect of the antibody. The enhanced binding of erythrocytes may be caused by formation of aggregates between opsonizing antibody and bacterial Fc-binding components. Aggregated IgG can bind to low-affinity Fc gamma II and gamma III receptors that cannot bind monomeric IgG. Release of Fc-binding components from bacteria may contribute to the periodontal lesion through interference with the phagocytic activity of granulocytes and with the complement system. Fc-binding components may also interfere with downregulation of the B-cell response.