Localization on encapsulated Cryptococcus neoformans of serum components opsonic for phagocytosis by macrophages and neutrophils.

Previous studies have shown that the cryptococcal capsule inhibits phagocytosis of Cryptococcus neoformans by macrophages and neutrophils. In this study, the binding sites of potential serum opsonins in immune and nonimmune sera were determined by immunoelectron microscopy, and the results were compared with the results of phagocytosis of the yeasts by mouse peritoneal macrophages and human neutrophils. Immunoglobulin G (IgG) from normal human serum showed low-density binding at the capsular surface and at sites throughout the capsule. Complement component C3 from normal serum bound heavily at the capsular surface. IgG from rabbit capsular antiserum showed relatively dense deposition at the capsular surface and at sites throughout the capsule. Cells opsonized with heat-inactivated human serum were engulfed poorly by both macrophages and neutrophils, indicating that the low-density deposition of IgG produced by normal serum was not adequate for opsonization. Yeasts opsonized with normal human serum were engulfed in large numbers by neutrophils and to a lesser extent by macrophages, indicating that neutrophils in particular were able to effectively utilize the opsonically active C3 which normal human serum deposited at the capsular surface. Yeasts opsonized with rabbit anticapsular serum were engulfed by both macrophages and neutrophils, indicating that the high density of surface IgG produced by capsular antiserum is an effective opsonin for both cells. These results suggest that the complement-neutrophil system is a possible defense mechanism in the nonimmune host.