EFFECTS OF STRAIN VARIATION, SEROTYPE, AND STRUCTURAL MODIFICATION ONKINETICS FOR ACTIVATION AND BINDING OF C3 TO CRYTOCOCCUS-NEOFORMANS

Incubation of encapsulated cells of Cryptococcus neoformans in normal human serum leads to activation Of the alternative complement pathway and deposition of opsonic fragments of C3 into the capsule. We determi ned whether the variation in capsular structure that occurs among the four major cryptococcal serotypes was reflected in the kinetics for ac tivation and binding of C3. We also examined the effects on activation kinetics of de-0-acetylation or periodate oxidation of the capsule. B inding kinetics were characterized in terms of the time required to de posit 5% of the maximal amount of C3 on the yeast (t5%), the first-ord er rate constant for amplification of C3 deposition (k'), and the maxi mum amount of C3 that could be deposited in the capsule (C3max). Our r esults showed that variations in the capsular structure that character ized each serotype had no significant influence on C3max but that the rate of C3 deposition depended significantly on the serotype. C3 accum ulated at a higher rate on cells of serotypes A and D than on cells of serotypes B and C. There was a significant correlation between capsul ar volume and C3max, although the relationship was not linear. Perioda te treatment of encapsulated cryptococci of all four serotypes led to decapsulation. Periodate-oxidized encapsulated cells displayed kinetic s for activation and binding of C3 that were identical to kinetics obs erved with nonencapsulated cryptococci. Finally, de-O-acetylation led to a significant but relatively minor increase in C3max.