Nanoscopic characterization of immunogenic b-glucan exposure on Candida species pathogens

Abstract

Abstract The cell walls of Candida spp. are composed largely of polysaccharides. Here we focus on β-glucan, an immunogenic cell wall polysaccharide whose surface exposure is masked from the innate immunoreceptor Dectin-1 to evade immunity. Previous research suggested that the physical presentation geometry of β-glucan may determine whether or not it can be recognized by Dectin-1. We used super resolution imaging to measure the fine structure of glucan exposure on C. albicans cell walls before and after glucan unmasking with the antimycotic drug caspofungin. Most exposed glucan on C. albicans yeast and hyphae exists as single Dectin-1 binding sites. Caspofungin-mediated glucan unmasking results in nanoscale remodeling of glucan exposures, resulting in increased density and size of glucan exposure sites. The less studied pathogen C. glabrata exhibits significantly greater glucan exposure than C. albicans in type strains, but recent clinical isolates of C. glabrata have lower exposure, suggesting that C. glabrata also masks its glucan. We used C. glabrata cell wall mutants to define the role of N-mannan structure in maintaining glucan masking. The less complex N-mannan structures of mutant strains provided poorer glucan masking, increasing the density of nanoscale glucan exposure sites. a-(1,6)-mannan backbone length was a major structural determinant of glucan masking, while a-(1,2;1,3)-mannan side chain density was less critical to masking function. This work provides the first characterization of changing glucan exposure nanostructure at the pathogenic cell wall surface that participates in host-Candida interaction. Further studies will examine the functional impact of glucan nanostructure on Dectin-1 activation.