Toll-Like Receptor 2- and MyD88-Dependent Phosphatidylinositol 3-Kinase and Rac1 Activation Facilitates the Phagocytosis ofListeria monocytogenesby Murine Macrophages

Toll-like receptors (TLRs) play a key role in the innate immune response by sensing bacterial ligands. The mechanisms involved in the TLR-mediated cytokine response are well established; however, the possible contribution of TLR-dependent recognition of bacteria to macrophage phagocytosis remains unclear.Listeria monocytogenesis an intracellular, parasitic, Gram-positive bacterium recognized mainly by TLR2. In this study, we investigated whether TLR2-dependent signaling is involved in the phagocytosis ofL. monocytogenesby macrophages. We found no difference in the number ofL. monocytogenescells associating with wild-type (WT) and TLR2−/−macrophages 1 h after infection. However, the number ofL. monocytogenescells phagocytosed in TLR2−/−and MyD88−/−macrophages was significantly lower than that of WT macrophages. In addition, lipopolysaccharide (LPS) treatment restored impaired phagocytic activity of TLR2−/−macrophages but did not enhance the activity of MyD88−/−macrophages. The efficiency of phagocytosis was suppressed by inhibitors of phosphatidylinositol 3-kinase (PI3K) and the small Rho GTPases but not by cycloheximide. Moreover, functional activation of PI3K and Rac1 was impaired in TLR2−/−and MyD88−/−macrophages. In anin vivoinfection model, we found significantly lower numbers ofL. monocytogenescells phagocytosed in peritoneal macrophages of TLR2−/−and MyD88−/−mice after intraperitoneal infection. Moreover, a lower number of bacteria were detected in the spleens of TLR2−/−mice 1 day after intravenous infection than in WT mice. These results clearly indicated that TLR2-MyD88-dependent signaling enhances the basal level of phagocytosis ofL. monocytogenesby macrophages through activation of PI3K and Rac1, not by synthesis of proinflammatory cytokines or expression of phagocytic receptors.