Streptococcus mutans is associated with the initiation and progression of human dental caries and is occasionally isolated from the blood of patients with bacteremia and infective endocarditis. For the pathogen to survive in the infected host, surface lipoproteins of S. mutans are likely to play important roles in interactions with the innate immune system. To clarify the role that a putative lipoprotein, peptidyl-prolyl cis/transisomerase (PpiA), of S. mutans plays in the macrophage response, we investigated the response of THP-1-derived macrophages to S. mutans challenge. The deletion of the gene encoding Lgt eliminated PpiA on the cell surface of S. mutans, which implies that PpiA is a lipoprotein that is lipid anchored in the cell membrane by Lgt. Human and murine peritoneal macrophages both showed higher phagocytic activities for the ppiA and lgt mutants than the wild type, which indicates that the presence of PpiA reduces S. mutans phagocytosis. In addition, infection with S. mutans markedly induced mRNAs of macrophage receptor with collagenous structure (MARCO) and scavenger receptor A (SR-A) in human macrophages. In particular, transcriptional and translational levels of MARCO in human macrophages infected with the ppiA mutant were higher than those in macrophages infected with the wild type. Phagocytosis of S. mutans by human macrophages markedly decreased after treatment with anti-MARCO IgG. These results demonstrate that the S. mutans lipoprotein PpiA contributes to suppression of MARCO-mediated phagocytosis of this bacterium by macrophages.

Streptococcus gordonii is a commensal gram-positive bacterium that resides in the human oral cavity, and is one of the most common causes of infective endocarditis (IE). Bacterial surface molecules play an important role in establishing IE, and several S. gordonii proteins have been implicated in binding to host cells during the establishment of IE. In this study, we identified a putative lipoprotein, peptidyl-prolyl cis/trans isomerase (PpiA), and clarified its role in evasion of phagocytosis by macrophages. Attenuation of the gene encoding prolipoprotein diacylglyceryl transferase (Lgt) altered the localization of PpiA from the cell surface to the culture supernatant, indicating that PpiA is lipid-anchored in the cell membrane by Lgt. Both human and murine macrophages showed higher phagocytic activity towards ppiA and lgt mutants than the wild-type, indicating that the presence of PpiA suppresses phagocytosis of S. gordonii. Human macrophages treated with dextran sulfate had significantly impaired phagocytosis of S. gordonii, suggesting that class A scavenger receptors in human macrophages are involved in the phagocytosis of S. gordonii. These results provide evidence that S. gordonii lipoprotein PpiA plays an important role in inhibiting phagocytic engulfment and in evasion of the host immune response.

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