Receptor-mediated recognition of Cryptococcus neoformans.

Cryptococcus neoformans, a facultative intracellular pathogen of macrophages, is unique among medically important fungi in its possession of a polysaccharide capsule. Capsule represents the organism's major virulence factor. In the absence of opsonins, binding of encapsulated C. neoformans to macrophages is minimal. Following incubation in serum, C. neoformans potently activates complement, resulting in surface deposition of the third component of complement. Macrophages bind and phagocytose opsonized C. neoformans via three major complement receptors (CR) for C3 fragments, designated CD35 (CR1), CD11b/CD18 (CR3), and CD11c/CD18 (CR4). Antibody in normal human serum generally lacks opsonic activity, although vaccination can elicit anticapsular antibodies that are opsonic. The major component of cryptococcal capsule, glucuronoxylomannan (GXM), is shed from the fungus and circulates in the blood and cerebrospinal fluid of patients with cryptococcosis. Cellular receptors defined for GXM include CD14, toll-like receptor-2, toll-like receptor-4, and CD18. GXM binding to macrophage receptors triggers activation of nuclear factor-kB, but not mitogen-activated protein kinases. This results in no proinflammatory gene expression or release. C. neoformans also secretes mannoproteins, which are recognized by mannose receptors as well as by mannose-binding lectin, perhaps in conjunction with CD14. Strategies directed at modulating how intact C. neoformans and its released components are recognized by phagocytes could lead to novel approaches to treating cryptococcosis