Complement C3 deposition restricts the proliferation of internalized Staphylococcus aureus by promoting autophagy

Complement C3 (C3) is usually deposited spontaneously on the surfaces of invading bacteria prior to internalization, but the impact of C3 coating on cellular responses is largely unknown. Staphylococcus aureus (S. aureus) is a facultative intracellular pathogen that subverts autophagy and replicates in both phagocytic and nonphagocytic cells. In the present study, we deposited C3 components on the surface of S. aureus by complement opsonization before cell infection and confirmed that C3-coatings remained on the surface of the bacteria after they have invaded the cells, suggesting S. aureus cannot escape or degrade C3 labeling. We found that the C3 deposition on S. aureus notably enhanced cellular autophagic responses, and distinguished these responses as xenophagy, in contrast to LC3-associated phagocytosis (LAP). Furthermore, this upregulation was due to the recruitment of and direct interaction with autophagy-related 16-like 1 (ATG16L1), thereby resulting in autophagy-dependent resistance to bacterial growth within cells. Interestingly, this autophagic effect occurred only after C3 activation by enzymatic cleavage because full-length C3 without cleavage of the complement cascade reaction, although capable of binding to ATG16L1, failed to promote autophagy. These findings demonstrate the biological function of intracellular C3 upon bacterial infection in enhancing autophagy against internalized S. aureus.