Role of glucan and surface protein BAD1 in complement activation by Blastomyces dermatitidis yeast

Our previous studies showed that Blastomyces dermatitidis yeast activates t he human complement system, leading to deposition of opsonic complement fra gments onto the yeast surface. This report examines the influence of altere d surface expression of glucan or BAD1 protein (formerly WI-1) on the yeast 's ability to activate and bind C3. Compared to the wild type, a glucan-def icient mutant yeast delayed initiation of C3 deposition and reduced C3-bind ing capacity by 50%. Linkage of baker's-yeast beta -glucan to the glucan-de ficient yeast restored initial C3 deposition kinetics to the wild-type leve l and partially restored C3-binding capacity, suggesting that P-glucan is a n initiator of complement activation and a C3 acceptor. The role of BAD1 in B. dermatitidis yeast-complement interaction was also assessed. BAD1 knock out yeast initiated faster C3 deposition and increased C3-binding capacity compared to the wild-type yeast or a BAD1-reconstituted yeast, suggesting e ither a lack of an intrinsic ability in BAD1 or an inhibitory role of BAD1 in complement activation and binding. However, both complement activation a nd the capacity for C3 binding by the wild-type yeast were enhanced in norm al human serum supplemented with an anti-BAD1 monoclonal antibody (MAb) or in immune sera from blastomycosis patients. Microscopic analysis revealed t hat more initial C3-binding sites were formed on yeast in the presence of b oth naturally occurring complement initiators and exogenous anti-BAD1 MAb, suggesting that anti-BAD1 antibody enhanced the ability of B. dermatitidis yeast to interact with the host complement system. Thus, glucan and BAD1 ha ve distinctly different regulatory effects on complement activation by B. d ermatitidis.