Characterization and immunologic studies of the Cryptococcus neoformans capsular components

Abstract

Cryptococcus neoformans is an encapsulated fungal pathogen that causes life-threatening infections in humans that usually manifests clinically as meningoencephalitis. As a result of the AIDS pandemic, the medical importance of this microbe has increased dramatically. C. neoformans is unusual among eukaryotic microbes because it contains a polysaccharide capsule with functional similarities to those of encapsulated bacteria such as Streptococcus pneumoniae and Haemophilus influenzae. The capsule is composed of at least three components: glucuronoxylomannan (GXM), galactoxylomannan (GalXM), and mannoprotein (MP). Although extensive studies have focused on the structural, chemical and immunological analysis for GXM, little is known about the effects of GalXM in vivo and its role and location in the capsule. This thesis describes the immunological and biological effects of GalXM in mice and elucidates its capsular location. By utilizing a hyperimmune serum to a GalXM-PA conjugate allowed us to identify GalXM's location in the capsule and determined that GalXM is an exopolysaccharide that may play a role in budding. Additionally, we explored the role of mannoproteins in the capsule, specifically that of mannoprotein 98 (MP98). By generating mAb's we able to infer that MP98 is located near the capsular edge and is also a secreted capsular component. Because microbial polysaccharides such as GXM remain in host tissues and are often immune modulators, we also investigated the role of the splenic marginal zone macrophage receptor SIGN-related 1 (SIGN-R1) receptor in the detection and removal of GXM in host tissue. The results indicated that there is a selective localization of these polysaccharides to different receptors such as SIGN-R1 for FITC-dextran in marginal zone macrophages and an unidentified receptor selectively expressed by red pulp macrophages for GXM.