MerTK Does Not Mediate Phagocytosis of Staphylococcus aureus but Attenuates Inflammation Induced by Staphylococcal Lipoteichoic Acid Through Blocking NF-κB Activation

Mer receptor tyrosine kinase (MerTK) expressed in macrophages is essential for phagocytosis of apoptotic cells. Here, we investigate whether MerTK is involved in the phagocytosis of Staphylococcus aureus (S. aureus) and regulation of staphylococcal lipoteichoic acid (LTA)-induced inflammatory response in macrophages. We found that stimulating RAW264.7 macrophages with S. aureus activated multiple signaling pathways including toll-like receptor 2 (TLR2), scavenger receptor A (SR-A), and MerTK. Meanwhile, S. aureus stimulation also induced activation of proteins focal adhesion kinase (FAK) and Rac1, which are related to phagocytosis. Pretreatment with a specific Mer-blocking antibody significantly inhibited S. aureus-induced phosphorylation of MerTK, while it had no effect on S. aureus-induced activation of FAK and Rac1. Moreover, by confocal laser microscope, we observed that the antibody blockade of MerTK had little impact on the phagocytosis of S. aureus by RAW264.7 macrophages. Additionally, pretreatment with this antibody further promoted LTA-induced phosphorylation of nuclear factor κB (NF-κB) p65 subunit and production of pro-inflammatory cytokines, such as TNF-α, IL-6, IL-1β, and macrophage inflammatory protein-2 (MIP-2). Collectively, these results suggest that MerTK does not play an essential role in the phagocytosis of S. aureus but attenuates inflammation induced by staphylococcal LTA through blocking NF-κB activation.