Activation of the complement system by Cryptococcus neoformans leads to binding of iC3b to the yeast

The complement system plays a key role in resistance to cryptococcosis. In the present study, we examined several factors that influence the binding of C3 cleavage fragments to Cryptococcus neoformans. Binding of C3 was determined by using normal human serum supplemented with 125I-labeled C3. Incubation of encapsulated cryptococci in 20% serum led to the binding of approximately 3.2 X 10(6) molecules of C3 to each cell. The binding of C3 was markedly inhibited by heating the serum at 56 degrees C for 30 min or by chelation of the serum with EDTA. Chelation of the serum with EGTA [ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid] reduced binding of C3 by 37%. These results indicated that activation of C3 cleavage fragments and their binding to C. neoformans was primarily dependent upon the alternative pathway. Bound C3 could be removed by incubation with 1.0 M hydroxylamine (pH 10) but not by incubation with 3.5 M NaSCN or with phosphate-buffered saline containing 0.1% sodium dodecyl sulfate. These results suggested that C3 fragments were bound to C. neoformans by ester bonds. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of C3 fragments eluted from the yeast showed the presence of protein bands consistent with the presence of iC3b. C3b was not detected on the yeast after incubation with serum for time intervals as short as 2.5 min, indicating a rapid conversion of cell-bound C3b to iC3b. These results indicate that iC3b is the ligand which most likely interacts with the phagocyte C3 receptors involved in the phagocytosis of C. neoformans.