Opsonization of Encapsulated Cryptococcus Neoformans by Specific Anticapsular Antibody

Antisera prepared in rabbits against either whole encapsulated cells of Cryptococcus neoformans or purified cryptococcal polysaccharide were opsonic for the encapsulated yeast. The opsonic activity was removed by absorption with whole cryptococci and was inhibited by free polysaccharide. As little as 0.13 microgram of cryptococcal polysaccharide produced a 50% inhibition of opsonization. Various degrees of neutralization by polysaccharides from the four cryptococcal serotypes suggested that the opsonins were type specific. Fractionation of antiserum on Bio-Gel A-5m (Bio-Rad Laboratories) and diethylaminoethyl cellulose showed that the opsonins were antibodies of the immunoglobulin G class. These opsonizing antibodies did not require heat-labile serum components for optimal phagocytosis of the yeast. Inhibition studies using 2-deoxy-D-glucose demonstrated that ingestion of encapsulated cryptococci opsonized with anticapsular antibody was a 2-deoxy-D-glucose-inhibitable process. This result differed from similar studies with non-encapsulated cryptococci which showed that ingestion of non-encapsulated cryptococci opsonized with normal serum was not inhibited by 2-deoxy-D-glucose.