Cryptococcus neoformans produces a life-threatening meningitis in patients who are immunocompromised by AIDS. A striking feature of cryptococcosis in AIDS is high serum levels of the major capsular polysaccharide, glucuronoxylomannan (GXM). Soluble GXM has numerous biologic activities that may contribute to the pathogenesis of infection. The objective of the study was to further understand in vivo processing of GXM. Mice were injected intravenously with GXM, and the tissue distribution was determined. A macrophage suicide technique that used liposome-encapsulated dichloromethylene diphosphonate determined the role of macrophages. GXM was cleared from serum with a half-life of 24-48 h but was retained for an indefinite period in tissues rich in cells of the mononuclear phagocyte system. Ablation of macrophages decreased GXM in the liver and spleen and increased serum GXM. The results identify a key role for macrophages in the clearance of GXM from serum and identify macrophages as a long-term reservoir for storage.

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Candida albicans is a potent activator of the complement system. The objective of this study was to characterize factors that influence the kinetics for activation of C3 and binding of C3 fragments to C. albicans. Factors that were examined included the surface properties of the yeast and contributions of the classical and alternative complement pathways. The results showed that incubation of hydrophobic, hydrophilic, or germinating yeast cells in normal human serum (NHS) containing radiolabeled C3 led to immediate accumulation of C3 on all three cell types, although the rate of accumulation of C3 on germinating cells was lower. An examination of the sites for early C3 binding showed that classical pathway initiation led to immediate, synchronous binding over the entire cell surface. A blockade of the classical pathway by absorption of putative classical pathway initiators or by chelation of calcium limited activation to the alternative pathway. Binding of C3 solely via the alternative pathway was characterized by a significant lag in the initial binding kinetics. In the absence of classical pathway initiation, the early cellular sites for C3 binding appeared as random, asynchronous foci of C3 that appeared to expand with time. The factor(s) mediating rapid deposition of C3 that was characteristic of the classical pathway initiation was reciprocally cross-absorbed by hydrophilic and hydrophobic C. albicans but was not removed by absorption of NHS with Saccharomyces cerevisiae, encapsulated Cryptococcus neoformans, or nonencapsulated C. neoformans. Delayed binding of C3 produced by absorption of serum was largely reversed by addition to the absorbed serum of immunoglobulin G isolated from NHS, indicating a significant role for a naturally occurring anti-C. albicans immunoglobulin G in classical pathway initiation.

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The extracellular polysaccharide capsule of Cryptococcus neoformans is a well-recognized virulence factor. Strain 602 is an acapsular clinical isolate of unknown serotype which has been widely used in studies of virulence and host-parasite interactions. In previous studies, strain 602 was compared with genetically unrelated strains of various serotypes because the wild-type equivalent of strain 602 was not available. We created an encapsulated strain, TYCC38-602, by transforming strain 602 with the CAP64 gene which was isolated from a serotype D strain. Serological tests and chemical analysis of the major polysaccharide capsule of TYCC38-602 indicated that strain 602 was originally derived from a serotype A strain. Restoration of the ability to produce a capsule enabled strain 602 to cause fatal infection in mice, whereas the acapsular strain 602 remained avirulent. Capsule-restored yeast cells of strain 602 activated the human complement system and bound C3 fragments in a manner that is characteristic of encapsulated cryptococci. In addition, the capsule in TYCC38-602 masked the ability of the organism to induce tumor necrosis factor alpha and subsequent nitric oxide synthase production in primed macrophage-like cells. These results indicate that the lack of capsule in strain 602 is the reason for its inability to cause fatal infection. Moreover, the acapsular phenotype accounts for differences in various biological activities of strain 602 compared to encapsulated strains. The results also indicate that the gene product of CAP64 does not contribute to serotype specificity of capsules in C. neoformans.