Critical Role of MyD88 in Phagocytosis and Cell Signaling During E. faecalis Infection of Macrophages

Macrophages are the most abundant leukocytes in the lamina propria of the intestine and are involved in both induction of protective immunity as well as immune tolerance to endogenous bacterial flora. Enterococcus spp. are commensal bacteria residing in the gastrointestinal tract of mammals but can also cause serious antibiotic-resistant opportunistic infections. One of the mechanisms by which these commensals cause life-threatening infections is through bacterial translocation from the intestinal lumen to extraintestinal sites. Under such circumstances E. faecalis must interact with the host immune system including macrophages. Understanding the macrophage activation program during enterococcal infection, and the bacterial components that elicit it, will be useful to explore non-conventional therapies to combat enterococcal infections. 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. 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 appeared to involve the ERK and JNK signaling pathways regulated by adaptor protein MyD88. These findings have important implications for understanding the interactions of E. faecalis at the gut mucosal interface.