Staphylococcus aureus Metalloprotease Aureolysin Cleaves Complement C3 To Mediate Immune Evasion

Abstract

Complement is one of the first host defense barriers against bacteria. Activated complement attracts neutrophils to the site of infection and opsonizes bacteria to facilitate phagocytosis. The human pathogen Staphylococcus aureus has successfully developed ways to evade the complement system, for example by secretion of specific complement inhibitors. However, the influence of S. aureus proteases on the host complement system is still poorly understood. In this study, we identify the metalloprotease aureolysin as a potent complement inhibitor. Aureolysin effectively inhibits phagocytosis and killing of bacteria by neutrophils. Furthermore, we show that aureolysin inhibits the deposition of C3b on bacterial surfaces and the release of the chemoattractant C5a. Cleavage analyses show that aureolysin cleaves the central complement protein C3. Strikingly, there was a clear difference between the cleavages of C3 in serum versus purified conditions. Aureolysin cleaves purified C3 specifically in the α-chain, close to the C3 convertase cleavage site, yielding active C3a and C3b. However, in serum we observe that the aureolysin-generated C3b is further degraded by host factors. We pinpointed these factors to be factor H and factor I. Using an aureolysin mutant in S. aureus USA300, we show that aureolysin is essential and sufficient for C3 cleavage by bacterial supernatant. In short, aureolysin acts in synergy with host regulators to inactivate C3 thereby effectively dampening the host immune response.

Pseudomonas aeruginosa Alkaline Protease Blocks Complement Activation via the Classical and Lectin Pathways

“…Interestingly, we found that AprA cleaves C2 very close to the cleavage site of its natural proteases C1s and MASP. Strikingly, this is similar to the Staphylococcus aureus metalloprotease aureolysin, which cleaves the C3 molecule 2 aa apart from the natural protease-cleavage site (34). This suggests that the natural cleavage site in these proteins is an accessible part of the protein that makes it vulnerable for cleavage by bacterial proteases.…”

Neisseria meningitidis NalP cleaves human complement C3, facilitating degradation of C3b and survival in human serum

“…Similar to NalP, S. aureus aureolysin also cleaves C3 at a single site. Unlike NalP, aureolysin cleaves C3 2 aa C-terminal to the C3 convertase cleavage site to generate functionally active C3b and C3a (28). In contrast to NalP, the streptococcal cysteine protease SpeB fully degrades C3 as a result of cleavage at multiple sites (25).…”

Galectin-3 Is a Target for Proteases Involved in the Virulence of Staphylococcus aureus

“…These activities promote detachment from colonized tissues, dissipation of biofilms, and hiding from detection by immune cells (5,37,38). Many immunomodulating mechanisms of the proteases have been identified (3), including effects on phagocyte chemotaxis and phagocytosis of S. aureus (37,(39)(40)(41)(42). Furthermore, the proteases have been shown to inactivate human protease inhibitors (e.g., ␣ 1 -antitrypsin), to cleave phagocyte "donot-eat-me" surface receptors (CD31), to inhibit complement activation (9), and to induce phagocyte cell death (42,43).…”