Strain variation in antiphagocytic activity of capsular polysaccharides from Cryptococcus neoformans serotype A

Strains of Cryptococcus neoformans vary in resistance to phagocytosis in vitro. The binding of isolated capsular polysaccharide (CPS) to a capsule-free mutant of C. neoformans confers resistance to phagocytosis. The importance of capsule composition to differences among strains in susceptibility to phagocytosis was evaluated. CPSs from five strains of C. neoformans serotype A, designated 6, 15, 98, 110, and 145, which had previously been isolated and characterized as to molecular size, composition, and binding properties, were evaluated for relative antiphagocytic potencies. In the presence of 5% normal isologous serum, murine thioglycolate-elicited peritoneal macrophages phagocytized (i.e., attached to or engulfed) 80% of 51Cr-labeled cells of C. neoformans 602, a capsule-free mutant. Added CPS inhibited the uptake of these yeast cells. CPS from strain 110 was most potent, followed in decreasing order of inhibitory activity by CPSs from strains 6, 145, 98, and 15. The presence of 100 micrograms of strain 110 CPS per ml reduced uptake of cells of strain 602 from 80 to 50%. CPS had no effect on the uptake of 51Cr-labeled Saccharomyces cerevisiae. Cells of strain 602 that were preincubated with CPS and then washed were more resistant to phagocytosis than nonpretreated control cells, indicating the importance of bound, not free, CPS. Added CPS did not affect the uptake of wild-type, encapsulated cells of C. neoformans. Addition of endotoxin had no effect on phagocytosis. CPSs from strains of C. neoformans serotype A varied widely in their abilities to inhibit the uptake of capsule-free cells. The antiphagocytic activity of CPS did not correlate with the ability to bind to capsule-free mutant but was somewhat related to the capsule size of the strain from which the CPS was isolated.