Staphylococcus aureus is an opportunistic pathogen that can cause life-threatening infections, particularly in immunocompromised individuals. The high-level virulence of S. aureus largely relies on its diverse and variable collection of virulence factors and immune evasion proteins, including the six serine protease-like proteins SplA to SplF. Spl proteins are expressed by most clinical isolates of S. aureus, but little is known about the molecular mechanisms by which these proteins modify the host's immune response for the benefit of the bacteria. Here, we identify SplB as a protease that inactivates central human complement proteins, i.e., C3, C4, and the activation fragments C3b and C4b, by preferentially cleaving their α-chains. SplB maintained its proteolytic activity in human serum, degrading C3 and C4. SplB further cleaved the components of the terminal complement pathway, C5, C6, C7, C8, and C9. In contrast, the important soluble human complement regulators factor H and C4b-binding protein (C4BP), as well as C1q, were left intact. Thereby, SplB reduced C3b-mediated opsonophagocytosis by human neutrophils as well as C5b-9 deposition on the bacterial surface. In conclusion, we identified the first physiological substrates of the S. aureus extracellular protease SplB. This enzyme inhibits all three complement pathways and blocks opsonophagocytosis. Thus, SplB can be considered a novel staphylococcal complement evasion protein. IMPORTANCE The success of bacterial pathogens in immunocompetent humans depends on the control and inactivation of host immunity. S. aureus, like many other pathogens, efficiently blocks host complement attack early in infection. Aiming to understand the role of the S. aureus-encoded orphan proteases of the Spl operon, we asked whether these proteins play a role in immune escape. We found that SplB inhibits all three complement activation pathways as well as the lytic terminal complement pathway. This blocks the opsonophagocytosis of the bacteria by neutrophils. We also clarified the molecular mechanisms: SplB cleaves the human complement proteins C3, C4, C5, C6, C7, C8, and C9 as well as factor B but not the complement inhibitors factor H and C4BP. Thus, we identify the first physiological substrates of the extracellular protease SplB of S. aureus and characterize SplB as a novel staphylococcal complement evasion protein.
Keywords: complement; immune evasion; innate immunity.