Opsonization of Cryptococcus neoformans by human immunoglobulin G: masking of immunoglobulin G by cryptococcal polysaccharide.

Previous studies have shown that attachment of non-encapsulated cryptococci to macrophages is highly dependent on opsonizing immunoglobulin G (IgG) and that cryptococcal polysaccharide inhibits the attachment phase of phagocytosis. We investigated various mechanisms by which cryptococcal polysaccharide might interfere with the opsonizing action of IgG. Cryptococcal polysaccharide did not appreciably prevent binding of opsonizing IgG to the yeast. Furthermore, cryptococcal polysaccharide acted as a noncompetitive inhibitor with respect to the opsonizing action of IgG. These experiments suggested that cell wall-bound IgG is masked in some manner such that it is unable to participate in Fc-mediated phagocytosis. This appeared to be the case, since cryptococcal polysaccharode inhibited agglutination of IgG-opsonized yeast cells by antiserum to IgG. There was good dose-response correlation between the amount of polysaccharide needed to inhibit phagocytosis of non-encapsulated Cryptococcus neoformans and the amount of polysaccharide needed to prevent agglutination of IgG-opsonized cryptococci by antiserum to IgG. The ability of cryptococcal polysaccharide to prevent agglutination of IgG-opsonized cryptococci by antiserum to IgG was lost if dextran, a substance known to enhance agglutination of several particles, was incorporated into the medium.