Roles of TLR/MyD88/MAPK/NF-κB Signaling Pathways in the Regulation of Phagocytosis and Proinflammatory Cytokine Expression in Response to E. faecalis Infection

Enterococcus faecalis is a commensal bacterium residing in the gastrointestinal tract of mammals, but in certain situations it is also an opportunistic pathogen which can cause serious disease. Macrophages have been shown to play a critical role in controlling infections by commensal enterococci and also have an important role in mediating chromosomal instability and promoting colon cancer during high-level enterococcal colonization in genetically susceptible mice. However, the molecular mechanisms involved in the interaction of macrophages with enterococci during infection are not fully understood. In this study, using BMDM and RAW264.7 macrophages we show that enterococcal infection activates ERK, JNK and p38 MAPK as well as NF-κB, and drives polarization of macrophages towards the M1 phenotype. Inhibition of NF-κB activation significantly reduced the expression of TNF-α and IL-1β, as did the inhibition of ERK, JNK and p38 MAPK, although to differing extent. Enterococci-induced activation of these pathways and subsequent cytokine expression was contact dependent, modest compared to activation by E. coli and, required the adaptor protein MyD88. Phagocytosis of enterococci by macrophages was enhanced by preopsonization with E. faecalis antiserum and involved the ERK and JNK signaling pathways, with the adaptor protein MyD88 as an important mediator. This study of the interaction of macrophages with enterococci could provide a foundation for studying the pathogenesis of infection by this opportunistic pathogen and to developing new therapeutic approaches to combat enterococcal infection.