Toll-Like Receptor Stimulation Enhances Phagocytosis and Intracellular Killing of Nonencapsulated and Encapsulated Streptococcus pneumoniae by Murine Microglia

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.