Recruitment of C4b-binding protein is not a complement evasion strategy employed by Staphylococcus aureus

Complement offers a first line of defence against infection through the opsonization of microbial pathogens, recruitment of professional phagocytes to the infection site and the coordination of inflammatory responses required for the resolution of infection. Staphylococcus aureus is a successful pathogen that has developed multiple mechanisms to thwart host immune responses. Understanding the precise strategies employed by S. aureus to bypass host immunity will be paramount for the development of vaccines and or immunotherapies designed to prevent or limit infection. To gain a better insight into the specific immune evasion mechanisms used by S. aureus we examined the pathogen's interaction with the soluble complement inhibitor, C4b-binding protein (C4BP). Previous studies indicated that S. aureus recruits C4BP using a specific cell-wall-anchored surface protein and that bound C4BP limits complement deposition on the staphylococcal surface. Using flow-cytometric-based bacterial-protein binding assays we observed no interaction between S. aureus and C4BP. Moreover, we offer a precautionary warning that C4BP isolated from plasma can be co-purified with minute quantities of human IgG, which can distort binding analysis between S. aureus and human-derived proteins. Combined our data indicates that recruitment of C4BP is not a complement evasion strategy employed by S. aureus.