, 1993; Dyson et al., 1999). In this regard, epidemiological studies have shown the presence of oral streptococcal species including S. sanguinis in clinical specimens of heart valve and atheromatous plaque (Chiu, 1999; Nakano et al., 2006; Koren et al., 2011). One of the earliest events in atherogenesis is foam cell formation of blood macrophages induced by the uptake of low-density lipoprotein (LDL) (Erridge, 2008). In addition, cell death of macrophages

is also considered learn more to be associated with atherosclerosis, because dead macrophages are found in atheromatous plaque (Tabas, 2010). Macrophages and monocytes present in the bloodstream are major contributors to host immune responses against bacterial infections. It is known that periodontal disease-related oral pathogens such as Porphyromonas gingivalis are involved in atherosclerosis (Hajishengallis et al., 2002; Gibson et al., 2005). In vitro studies have also shown that P. gingivalis elicits foam cell formation of macrophages (Qi et al., 2003; Giacona et al., 2004). Although S. sanguinis is known to induce infectious endocarditis, its possible contribution

to atherosclerosis has not been Alectinib studied. In the present study, we investigated whether S. sanguinis infection induces foam cell formation and cell death of human macrophages. Streptococcus sanguinis strain SK36 (Kilian et al., 1989) was provided by Dr M. Kilian (Aarhus University, Denmark), and cultured in brain heart infusion (BHI) broth (Becton Dickinson, Sparks, MD) supplemented with 0.2% yeast extract (Becton Dickinson). Heat-inactivated S. sanguinis SK36 was prepared by heating the bacterial suspension in phosphate-buffered saline (PBS; pH 7.4) at 60 °C for 30 min (Okahashi et al., 2003). In some experiments, a cariogenic bacterial strain, Org 27569 Streptococcus mutans UA159, was used as a negative control. Human monocyte cell

line THP-1 cells were purchased from RIKEN Bioresorce Center (Tsukuba, Japan) and cultured in RPMI1640 medium (Invitrogen, Carlsbad, CA) supplemented with 5% fetal bovine serum (FBS) (Invitrogen) (5% FBS RPMI1640), penicillin (100 U mL−1), and streptomycin (100 μg mL−1). Differentiated THP-1 macrophages were prepared by treating THP-1 cells with 100 nM phorbol myristate acetate (Sigma Aldrich, St. Louis, MO) for 2 days. For infection, differentiated THP-1 cells (5 × 104 cells in 100 μL of 5% FBS RPMI1640 without antibiotics) in 96-well culture plates (Asahi Glass, Tokyo, Japan) were infected with viable S. sanguinis SK36 at a multiplicity of infection (MOI) of 10, 20, or 50 for 2 h. The cells were washed with PBS to remove extracellular nonadherent bacteria, and cultured for 2 days in the presence of human LDL (100 μg mL−1; Sigma Aldrich) and antibiotics. The cells were also stimulated with lipopolysaccharide (LPS) of Escherichia coli O127 (Sigma Aldrich) or heat-inactivated S. sanguinis SK36 whole cells for 2 days.