Highly Alkaline-Resistant Enterococcus faecalis Induces Compromised M1 Polarization and Phagocytosis of Macrophage via Z-DNA Binding Protein 1.

Persistent apical periodontitis (PAP) of human teeth is related to Enterococcus faecalis (E. faecalis) with higher alkaline resistance. This study aimed to investigate how highly alkaline-resistant (HAR) E. faecalis modulates macrophage M1 polarization and phagocytosis via Z-DNA-binding protein 1 (ZBP1). HAR E. faecalis was generated through serial alkaline passaging. RAW264.7 macrophages were infected with standard or HAR E. faecalis. M1 polarization markers and Cd274 (Programmed death-ligand 1 (PD-L1)) were profiled by RT-qPCR. ZBP1 was detected by RT-qPCR and immunofluorescence staining, and was silenced using small interfering RNA (siRNA). iNOS, ZBP1 and PD-L1 proteins were analyzed by Western blotting. The phagocytosis of CFDA-SE-labeled bacteria was then quantified by confocal microscopy and flow cytometry. The results showed that HAR E. faecalis induced significantly lower expression of ZBP1, M1 polarization markers and Cd274 in macrophages than the standard strain. After ZBP1 knock-down, expression of these markers decreased. Macrophages phagocytosed much fewer HAR E. faecalis than the standard strain. After ZBP1 knock-down, the differences between the two strains disappeared. In conclusion, HAR E. faecalis induced compromised M1 polarization and phagocytosis of macrophage via ZBP1. These findings may provide new insights into the pathogenesis and treatment of PAP.