| Abstract: | Toll-like receptors (TLRs) are crucial pattern recognition receptors in innate immunity that are expressed in microglia, the resident macrophages of the brain. TLR2, -4, and -9 are important in the responses against Streptococcus pneumoniae, the most common agent causing bacterial meningitis beyond the neonatal period. Murine microglial cultures were stimulated with agonists for TLR1/2 (Pam3CSK4), TLR4 (lipopolysaccharide), and TLR9 (CpG oligodeoxynucleotide) for 24 h and then exposed to either the encapsulated D39 (serotype 2) or the nonencapsulated R6 strain of S. pneumoniae. After stimulation, the levels of interleukin-6 and CCL5 (RANTES [regulated upon activation normal T-cell expressed and secreted]) were increased, confirming microglial activation. The TLR1/2, -4, and -9 agonist-stimulated microglia ingested significantly more bacteria than unstimulated cells (P < 0.05). The presence of cytochalasin D, an inhibitor of actin polymerizaton, blocked >90% of phagocytosis. Along with an increased phagocytic activity, the intracellular bacterial killing was also increased in TLR-stimulated cells compared to unstimulated cells. Together, our data suggest that microglial stimulation by these TLRs may increase the resistance of the brain against pneumococcal infections.Immunocompromised patients have a higher risk of developing bacterial infections in the central nervous system (CNS) (34, 37, 42). The list of the pathogens includes many organisms with low pathogenicity in the immunocompetent host (34, 37). Moreover, the distribution of the pathogens also differs from the immunocompetent host and depends on the nature of the immune defect. Patients with a decrease in B-lymphocyte function or with a loss of splenic function have an increased risk of meningitis caused by encapsulated bacteria, while patients with an impaired T-lymphocyte-macrophage system are more susceptible to CNS infections caused by intracellular pathogens (7, 42). One additional cause of this increased susceptibility to CNS infections probably is a decreased local immune defense (33).CNS infections not only are more frequent but also are associated with higher mortality rates and more severe long-term sequelae in immunocompromised than in immunocompetent individuals (9, 17, 34, 44). Polymicrobial infections, multiple organ system presentation, and the absence of typical clinical manifestations subsequent to the host''s diminished inflammatory response are challenging aspects in the management of these infections (34, 37, 42).The brain tissue shows a well-organized innate immune reaction in response to bacteria in the cerebrospinal fluid (CSF) (3, 21). Microglial cells, the resident phagocytes of the CNS, express Toll-like receptors (TLRs) that identify pathogen-associated molecular patterns (PAMPs) (41). The receptor-ligand interactions activate microglia to undergo morphological transformation as well as functional changes, such as the production of proinflammatory cytokines, chemokines, and reactive oxygen species, enhanced phagocytic activity, and antigen presentation (15, 39). This immune reaction cannot eliminate high amounts of pneumococci from the CSF but does prevent or minimize the invasion of these pathogens into the brain tissue, thereby limiting tissue destruction and neuronal injury.TLR2, -4, and -9 contribute to the recognition and response to Streptococcus pneumoniae in the CNS (31). A deficiency of TLR2, -4, or -9 or of the coreceptor CD14, which is necessary for TLR4 signaling increases the susceptibility of mice to S. pneumoniae (1, 11, 12, 40).Here, we hypothesized that activation of the innate immune response in microglia could increase the resistance of the brain tissue against CNS pneumococcal infections (14). This may be of particular interest in immunocompromised patients, whose outcome after S. pneumoniae meningitis is worse than that of immunocompetent individuals (9, 44). The aim of the present study was to investigate whether the stimulation of microglia by respective PAMPs can increase their ability to phagocytose and kill intracellular nonencapsulated and encapsulated S. pneumoniae strains, thereby protecting the brain during meningitis. Moreover, by using an encapsulated and a nonencapsulated pneumococcal strain, we assessed the protective effect of the capsule against phagocytosis by microglial cells. |
