The Hyphal and Yeast Forms ofCandida albicansBind the Complement Regulator C4b-Binding Protein

Abstract

Candida albicans, an important pathogenic yeast, activates all three pathways of the complement system. To understand how this yeast evades the effects of the activated system, we have analyzed the binding of the classical pathway inhibitor C4b-binding protein (C4BP) by C. albicans. Purified native as well as recombinant C4BP bound dose dependently to the yeast and hyphal forms, as shown by multiple methods, such as confocal microscopy, flow cytometry, a novel enzyme-linked immunosorbent assay, absorption from human serum, and direct binding assays with purified proteins. A prominent binding site was identified at the tip of the germ tube, a structure that is considered important for tissue penetration and pathogenesis. The binding site in C4BP was localized to the two N-terminal complement control protein domains by using recombinant deletion constructs and site-specific monoclonal antibodies. As the alternative pathway inhibitors factor H and FHL-1 also bind to C. albicans, the binding of all three plasma proteins was compared. Simultaneous binding of the classical regulator C4BP and the alternative pathway regulator factor H was demonstrated by confocal microscopy. In addition, FHL-1 competed for binding with C4BP, suggesting that these two related complement regulators bind to the same structures on the yeast surface. The surface-attached C4BP maintains its complement regulatory activities and inactivates C4b. The surface-attached human C4BP serves multiple functions relevant for immune evasion and likely pathogenicity. It inhibits complement activation at the yeast surface and, in addition, mediates adhesion of C. albicans to host endothelial cells.

Haemophilus influenzae Interacts with the Human Complement Inhibitor Factor H

“…In addition to inhibition of the alternative pathway, several bacteria (e.g., Moraxella catarrhalis, S. pyogenes, E. coli K1, and H. influenzae) bind C4BP and are shielded against the classical and lectin pathways (49 -51). Interestingly, many species, C. albicans, Borrelia recurrentis, and S. pyogenes, among others, have been shown to bind both FH and C4BP (26,(52)(53)(54)(55). We have recently demonstrated that non-typeable H. influenzae binds C4BP (30).…”

“…Different regulators of the complement system have been shown to mediate adherence and ingestion to epithelial and endothelial cells. The binding of C4BP by C. albicans has been shown to mediate adherence to endothelial cells, and bound FHL-1 to S. pyogenes mediates and enhances the ingestion of the pathogen into epithelial cells (54,57). In fact, several studies have indicated that complement proteins and regulators are present in the respiratory tract (50,60,61).…”

Complement evasion by human pathogens

“…In fact, of all the mechanisms of complement evasion that are used by various pathogens, trapping RCA is by far the most widely disseminated strategy for avoiding the complement response. RCA recruitment is common in bacteria 16 (for example, Escherichia coli, Borrelia burgdorferi and streptococci), but has also been described for viruses 17 (for example, HIV-1), fungi 18,19 (for example, Candida albicans) and parasites 20 (for example, Echinococcus spp.) (Supplementary information S1 (table)).…”

Human C4b-binding Protein, Structural Basis for Interaction with Streptococcal M Protein, a Major Bacterial Virulence Factor

“…Expression of M protein is important for the ability of S. pyogenes to cause the diseases associated with this pathogen: acute pharyngitis and impetigo (6). Many other pathogens share the ability to sequester C4BP (7)(8)(9)(10)(11)(12). Thus escape from complement attack by hijacking of C4BP is emerging as a widespread contributor to immune evasion strategies and is a therapeutic target.…”