Redistribution of a glucuronoxylomannan epitope towards the capsule surface coincides with Titanisation in the human fungal pathogen Cryptococcus neoformans

Abstract

Disseminated infections with the fungal species Cryptococcus neoformans or, less frequently, C. gattii, are a leading cause of mortality in immunocompromised individuals. Central to the virulence of both species is an elaborate polysaccharide capsule that consists predominantly of glucuronoxylomannan (GXM). Due to its abundance, GXM is an ideal target for host antibodies, and several monoclonal antibodies (mAbs) have previously been derived using purified GXM or whole capsular preparations as antigen. In addition to their application in the diagnosis of cryptococcosis, anti-GXM mAbs are invaluable tools for studying capsule structure. In this study, we report the production and characterisation of a novel anti-GXM mAb, Crp127, that unexpectedly reveals a role for GXM remodelling during the process of fungal Titanisation. We show that Crp127 recognises a GXM epitope in an O-acetylation dependent, but xylosylation-independent, manner. The epitope is differentially expressed by the four main serotypes of Cryptococcus neoformans and gattii, is heterogeneously expressed within clonal populations of C. gattii serotype B strains and is typically confined to the central region of the enlarged capsule. Uniquely, however, this epitope redistributes to the capsular surface in Titan cells, a recently recognised subset of giant fungal cells that are produced in the host lung and are critical for successful infection. Crp127 therefore highlights hitherto unexpected features of cryptococcal morphological change and may hold significant therapeutic potential in differentially identifying cryptococcal strains and subtypes.\n\nImportanceCryptococcus neoformans and Cryptococcus gattii are the etiological agents of cryptococcosis, an invasive fungal infection responsible for approximately 200,000 deaths each year and 15% of AIDS-related deaths annually. Whilst the main virulence factor for both species is a highly variable polysaccharide capsule, formation of Titan cells also underlies the pathogenesis of C. neoformans. Previous studies have shown that capsule composition differs between yeast and Titan cells, however no clear distinctions in the expression or localisation of specific capsular epitopes have been made. In this study, we characterise a novel monoclonal antibody (mAb) specific to a capsular epitope that is differentially distributed throughout the capsules produced by yeast and Titan cells. Whilst this epitope is found within the midzone of yeast capsules, the presentation of this epitope on the surface of Titan cell capsules may represent a way in which these cell types are perceived differently by the immune system.