Gas6 negatively regulates the Staphylococcus aureus‐induced inflammatory response via TLR signaling in the mouse mammary gland

Abstract Staphylococcus aureus ( S. aureus )‐induced mastitis is the most frequent, pathogenic, and prevalent infection of the mammary gland. The ligand growth arrest‐specific 6 (Gas6) is a secretory protein that binds to and activates Tyro3, Axl, and MerTK receptors. This study explored the role of Gas6 in S. aureus ‐induced mastitis. Our results revealed that TLR receptors initiate the innate immune response in mammary gland tissues and epithelial cells and that introducing S. aureus activates TLR2 and TLR6 to drive multiple intracellular mitogen‐activated protein kinase (MAPK) and nuclear factor kappa‐B (NF‐κB) pathways. Moreover, S. aureus also induces Gas6, which then activates the TAM receptor kinase pathway, which is related to the inhibition of TLR2‐ and TLR6‐mediated inflammatory pathways through SOCS1 and SOCS3 induction. Gas6 absence alone was found to be involved in the downregulation of TAM receptor‐mediated anti‐inflammatory effects by inducing significantly prominent expression of TRAF6 and low protein and messenger RNA expression of SOCS1 and SOCS3. S. aureus ‐induced MAPK and NF‐ĸB p65 phosphorylation were also dependent on Gas6, which negatively regulated the production of Pro‐inflammatory cytokines (IL‐1β, IL‐6, and TNF‐α) in S. aureus ‐treated mammary tissues and mammary epithelial cells. Our in vivo and in vitro study uncovered the Gas6‐mediated negative feedback mechanism, which inhibits TLR2‐ and TLR6‐mediated MAPK and NF‐ĸB signaling by activating TAM receptor kinase (MerTK, Axl, and Tyro3) through the induction of SOCS1/SOCS3 proteins.