Effects of strain variation, serotype, and structural modification on kinetics for activation and binding of C3 to Cryptococcus neoformans

Abstract

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 determined whether the variation in capsular structure that occurs among the four major cryptococcal serotypes was reflected in the kinetics for activation and binding of C3. We also examined the effects on activation kinetics of de-O-acetylation or periodate oxidation of the capsule. Binding kinetics were characterized in terms of the time required to deposit 5% of the maximal amount of C3 on the yeast (t5%), the first-order rate constant for amplification of C3 deposition (k'), and the maximum amount of C3 that could be deposited in the capsule (C3max). Our results showed that variations in the capsular structure that characterized each serotype had no significant influence on C3max but that the rate of C3 deposition depended significantly on the serotype. C3 accumulated 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 capsular volume and C3max, although the relationship was not linear. Periodate treatment of encapsulated cryptococci of all four serotypes led to decapsulation. Periodate-oxidized encapsulated cells displayed kinetics for activation and binding of C3 that were identical to kinetics observed with nonencapsulated cryptococci. Finally, de-O-acetylation led to a significant but relatively minor increase in C3max.

Anti‐Chemotactic Activity of Capsular Polysaccharide of Cryptococcus neoformans In Vitro

“…Since the active C3 thioester group at the initiation of ACP has been shown to bind efficiently to mannose (19), it is reasonable that de-O-acetylation of CPS caused an increase in the capacity of CPS to activate ACP. In contrast with our observations, it has been reported that removal of O-acetyl substitution of the mannose backbone has a significant but relatively minor effect on the activation and binding of C3 to whole cells of C. neoformans (27,32). The discrepancy may be due to the form of activator used, soluble CPS or whole cells.…”

“…In recent comprehensive studies, Kozel and coworkers showed that encapsulated strains of C. neoformans are powerful activators of the complement system (18,32), and the activation and binding of C3 fragments to encapsulated cryptococci are due solely to the action of the ACP (17,31). On the other hand, Laxalt and Kozel (20) and Diamond and Erickson (3) previously reported that purified capsular polysaccharide was unable to activate ACP.…”

Complement and fungal pathogens: an update

“…107 Fortunately, the capsule represents an extremely powerful activator of the complement cascade, achieving a deposition of up to 10 7 -10 8 C3 molecules per yeast cell while acapsular cryptococcal cells bind at least ten times less C3. [108][109][110] The predominant pathway as well as the precise localisation of opsonising C3 is species-specific; human complement induces a deposition of C3 at the outermost capsular surface, mainly via activation of the alternative pathway. 111 The localisation of C3 is of major importance for the efficacy of phagocytosis: whereas deposition inside the capsule is characteristic for poor phagocytosis, an opsonisation at the outer capsule edge allows efficient ingestion.…”

Cas1p is a membrane protein necessary for the O‐acetylation of the Cryptococcus neoformans capsular polysaccharide

“…In vitro, various GXM produce various effects in terms of antiphagocytic activity (Small and Mitchell, 1989) or induction of cytokine synthesis by different host cells (Chaka et al, 1997;Lipovsky et al, 1998). Finally, capsule structure seems to affect the in vitro binding of C3 to C. neoformans cells (Washburn et al, 1991;Young and Kozel, 1993). However, how the capsule influences yeast virulence is unknown.…”