Staphylococcus aureus SaeRS impairs macrophage immune functions through bacterial clumps formation in the early stage of infection.

The Staphylococcus aureus (S. aureus) SaeRS two-component system (TCS) regulates over 20 virulence factors. While its impact on chronic infection has been thoroughly discussed, its role in the early stage of infection remains elusive. Since macrophages serve as the primary immune defenders at the onset of infection, this study investigates the influence of SaeRS on macrophage functions and elucidates the underlying mechanisms. Macrophage expression of inflammatory and chemotactic factors, phagocytosis, and bactericidal activity against S. aureus were assessed, along with the evaluation of cellular oxidative stress. SaeRS was found to impair macrophage function. Mechanistically, SaeRS inhibited NF-κB pathway activation via toll-like receptor 2 (TLR2). Its immune-modulating effect could partially be explained by the strengthened biofilm formation. More importantly, we found SaeRS compromised macrophage immune functions at early infection stages even prior to biofilm formation. These early immune evasion effects were dependent on bacterial clumping as cytokine secretion, phagocytosis, and bactericidal activity were repaired when clumping was inhibited. We speculate that the bacterial clumping-mediated antigen mask is responsible for SaeRS-mediated immune evasion at the early infection stage. In vivo, ΔsaeRS infection was cleared earlier, accompanied by early pro-inflammatory cytokines production, and increased tissue oxidative stress. Subsequently, macrophages transitioned to an anti-inflammatory state, thereby promoting tissue repair. In summary, our findings underscore the critical role of the SaeRS TCS in S. aureus pathogenicity, particularly during early infection, which is likely initiated by SaeRS-mediated bacterial clumping.