Streptococcus gordonii induces nitric oxide production through its lipoproteins stimulating Toll-like receptor 2 in murine macrophages

Streptococcus gordonii, a Gram-positive commensal in the oral cavity, is an opportunistic pathogen that can cause endodontic and systemic infections resulting in infective endocarditis. Lipoteichoic acid (LTA) and lipoprotein are major virulence factors of Gram-positive bacteria that are preferentially recognized by Toll-like receptor 2 (TLR2) on immune cells. In the present study, we investigated the effect of S. gordonii LTA and lipoprotein on the production of the representative inflammatory mediator nitric oxide (NO) by the mouse macrophages. Heat-killed S. gordonii wild-type and an LTA-deficient mutant (Delta ltaS) but not a lipoprotein-deficient mutant (Delta lgt) induced NO production in mouse primary macrophages and the cell line, RAW 264.7.S. gordonii wild-type and Delta ItaS also induced the expression of inducible NO synthase (iNOS) at the mRNA and protein levels. In contrast, the Delta lgt mutant showed little effect under the same condition. Furthermore, S. gordonii wild-type and Delta ItaS induced NF-kappa B activation, STAT1 phosphorylation, and IFN-beta expression, which are important for the induction of iNOS gene expression, with little activation by Delta lgt. S. gordonii wild-type and Delta ltaS showed an increased adherence and internalization to RAW 264.7 cells compared to Delta lgt. In addition, S. gordonii wild-type and AIMS, but not Delta lgt, substantially increased TLR2 activation while none of these induced NO production in TLR2-deficient macrophages. Triton X-114-extracted lipoproteins from S. gordonii were sufficient to induce NO production. Collectively, we suggest that lipoprotein is an essential cell wall component of S. gordonii to induce NO production in macrophages through TLR2 triggering NF-kappa B and STAT1 activation. (C) 2016 Elsevier Ltd. All rights reserved.