Toll-Like Receptor Stimulation Enhances Phagocytosis and Intracellular Killing of Nonencapsulated and EncapsulatedStreptococcus pneumoniaeby Murine Microglia

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 (Pam 3 CSK 4 ), 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.

In VitroCharacterization of the Microglial Inflammatory Response toStreptococcus suis, an Important Emerging Zoonotic Agent of Meningitis

“…It was hypothesized that these cells would be critically implicated in the CNS inflammatory response induced by S. suis (18). Microglial cells have recently been shown to be able to phagocytose and kill Gram-positive bacteria, including wellencapsulated pathogenic Streptococcus pneumoniae (52). In the case of S. suis, previous studies carried out with murine, porcine, and human phagocytes indicated that the S. suis capsule is critical for bacterial resistance to phagocytosis (12,56,61).…”

Bright Fluorescent Streptococcus pneumoniae for Live-Cell Imaging of Host-Pathogen Interactions

“…To our knowledge, the only known published examples of GFP-labeled S. pneumoniae used for imaging host interactions are strains constitutively expressing GFP from a multicopy plasmid (plasmids named pGFP1 [21] and pMV158GFP [22,42]). The pGFP1-carrying strain was used to image S. pneumoniae in mouse bronco-epithelial cells (21).…”

“…The method of choice would therefore be to have strains expressing fluorescent proteins, yet there are only very few examples of such imaging of S. pneumoniae published. These examples include the work of Kadioglu et al (21), who studied pneumococcal invasion of bronco-epithelial cells in mice, and Ribes et al (22), who imaged pneumococcal interactions with murine microglial cells. In both of these cases, the S. pneumoniae strains contained a green fluorescent protein (GFP) expressed from a multicopy plasmid.…”

Vitamin D Deficiency Reduces the Immune Response, Phagocytosis Rate, and Intracellular Killing Rate of Microglial Cells

“…Stimulation of microglial cells with TLR agonists can increase the phagocytic and intracellular killing properties of microglial cells (26). Recent studies described the key mechanisms associated with vitamin D metabolism and signaling for both innate and adaptive immune responses (14,27).…”