Role of Glucan and Surface Protein BAD1 in Complement Activation byBlastomyces dermatitidisYeast

Abstract

Our previous studies showed that Blastomyces dermatitidis yeast activates the human complement system, leading to deposition of opsonic complement fragments onto the yeast surface. This report examines the influence of altered 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-deficient mutant yeast delayed initiation of C3 deposition and reduced C3-binding capacity by 50%. Linkage of baker's-yeast ␤-glucan to the glucan-deficient yeast restored initial C3 deposition kinetics to the wild-type level and partially restored C3-binding capacity, suggesting that ␤-glucan is an initiator of complement activation and a C3 acceptor. The role of BAD1 in B. dermatitidis yeast-complement interaction was also assessed. BAD1 knockout yeast initiated faster C3 deposition and increased C3-binding capacity compared to the wild-type yeast or a BAD1-reconstituted yeast, suggesting either a lack of an intrinsic ability in BAD1 or an inhibitory role of BAD1 in complement activation and binding. However, both complement activation and the capacity for C3 binding by the wild-type yeast were enhanced in normal human serum supplemented with an anti-BAD1 monoclonal antibody (MAb) or in immune sera from blastomycosis patients. Microscopic analysis revealed that more initial C3-binding sites were formed on yeast in the presence of both 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 have distinctly different regulatory effects on complement activation by B. dermatitidis.Blastomyces dermatitidis is the etiological agent of blastomycosis, which is one of the principal endemic systemic mycoses of humans and other mammals (22). Host defense mechanisms against B. dermatitidis infection include the complement system. Complement promotes attachment of human phagocytes to B. dermatitidis and is required for neutrophil-mediated killing of B. dermatitidis yeast (5). Our previous studies showed that B. dermatitidis yeast cells activate the human complement system via both the classical and alternative pathways, leading to deposition of C3b and iC3b complement fragments on the yeast surface (26). Furthermore, we found that naturally occurring anti-␤-glucan antibodies promote activation of the classical complement pathway by the yeast (26). How yeast cell surface structures modulate the yeast's ability to interact with the human complement system remains unknown.The cell surface of B. dermatitidis yeast is associated with a dynamic array of proteins and carbohydrates. Prominent among them are glucan and surface protein BAD1, formerly termed WI-1 (21), both of which have been implicated as virulence factors (2, 10). About 95% of the glucan components are ␣-glucan, and the remainder are ␤-glucan (8). BAD1 is a 120-kDa immunodominant antigen; it is also an adhesin that binds the yeast to CD14 a...

Interplay between Protective and Inhibitory Antibodies Dictates the Outcome of Experimentally Disseminated Candidiasis in Recipients of aCandida albicansVaccine

“…Our data, however, invite us to reconsider this aspect, as anti-␤-glucan antibodies might well play a role, mostly when other blocking antibodies are absent, as discussed above. Anti-␤-glucan antibodies have been involved in rapid opsonizing complement activation by Blastomyces dermatitidis yeast cells (61), which expose ␤-glucan on their surfaces (35). However, no evidence has been provided that these anti-␤-glucan antibodies are indeed protective.…”

Influence of Mannan and Glucan on Complement Activation and C3 Binding byCandida albicans

“…Purified glucans from different sources have been demonstrated to activate the alternative pathway (10,15,21). The role of ␤-glucan in complement activation has also been studied with zymosan (11), B. dermatitidis (48,49), and nonencapsulated C. neoformans (46). Common to these systems are the display of ␤-glucan on the surface and activation of the alternative pathway in the absence of anti-glucan antibody.…”

“…This masking is analogous to the interplay between BAD1 and ␤-glucan in B. dermatitidis. BAD1 is a cell surface-displayed protein antigen and adhesin (4,24) and is inert to complement activation (48). Genetic disruption of BAD1 expression or chemical addition of exogenous ␤-glucan to the cell surface enhanced C3 deposition onto B. dermatitidis yeast cells (48).…”

“…However, C. albicans glucan may become exposed during infection (45) or by treatment with caspofungin (44,45). Glucan of nonencapsulated C. neoformans (46) or B. dermatitidis (48,49) is cell surface displayed and contributes to complement activation. The activity of C. albicans glucan and the interplay of mannan and glucan in modulation of complement activation have not been studied.…”

“…Glucan of nonencapsulated Cryptococcus neoformans (46) or Blastomyces dermatitidis (48,49) is cell surface displayed and contributes to initiation of complement activation. Therefore, C. albicans glucan that is exposed during infection may influence the outcome of the interaction of Candida with the human complement system.…”