Cryptococcus neoformans serotype A virulence and pathogenicity are capsular glucuronoxylomannan (GXM) motif composition dependent

Abstract

The Cryptococcus neoformans capsular extracellular polysaccharide (EPS) plays an essential role in virulence and host immunomodulation. In this study, we analyzed the virulence of four serotype A C. neoformans strains with different polysaccharide structures: two multi-motif (H99 and KN99α) and two single-motif (Mu-1 and ATCC-24064 (24064)). Mice were infected by the intranasal (IN) or intravenous (IV) route. Survival, fungal load (colony-forming unit), cell lung population, cytokines, and histology were analyzed. No differences in cell size or capsule size were found between the four strains. However, polysaccharide sizes were highly variable, and no significant differences were observed for all other virulence factors. Nitrite production in cell culture dendritic cells was reduced for the H99 or KN99α strains of EPS. The reactive oxygen species in cell culture M1 macrophages and dendritic cells were increased for the H99 or KN99α strain's EPS. Cytokines differed from the IN and IV infections. Lung histology showed increased inflammatory cell presence after infection with Mu-1 or 24064 strains, with no cryptococcal cells observed in the lung parenchyma or alveoli, whereas the opposite pattern was seen after challenges with H99 or KN99α. There were differences in lung neutrophil infiltration between the analyzed strains, and a higher presence of activated macrophages and CD4+ lymphocytes for the Mu-1 or 24064 strains. Polysaccharides from single and multi-motif strains elicited different effects on macrophage-like cells in vitro. These results suggest that variation in polysaccharide composition and structure can translate into differences in virulence and pathogenicity.

Importance: Cryptococcosis is a systemic fungal infection that causes approximately 1 million cases globally, leading to approximately 625,000 annual deaths. Two species are responsible for the majority of cases, Cryptococcus neoformans and Cryptococcus gattii. C. neoformans usually causes disease in immunosuppressed hosts, whereas C. gattii can infect and cause disease in immunocompetent hosts. The capsule of Cryptococcus spp. is one of its major virulence factors, due to its immunomodulation abilities. The capsule is composed of three major different molecules: glucuronoxylomannan (GXM), galactoxylomannan (GalXM), and mannoproteins. GXM is composed of six different structures called motifs. The exact mechanism and structures associated with its immunomodulation are still not well elucidated. Here, we looked at different immune responses based on the capsule composition. Our results strongly suggest that the capsule motif composition can affect the virulence, pathogenicity, and immunomodulatory capabilities of the Cryptococcus capsule.

Keywords: Cryptococcus neoformans; GXM; capsular polysaccharide; capsule; exopolysaccharide; extracellular polysaccharide; glucuronoxylomannan; motifs; pathogenesis; virulence.