IFN-γ by NKs Enhances Bacterial Pathogenesis in a Murine Model of Meningitis by Type 4 Streptococcus

INTRODUCTION Streptococcus pneumoniae is the leading cause of bacterial meningitis. Pneumococcal meningitis (PM) in a life-threatening disease with high rates of mortality and neurological sequelae. Immune targeting of S. pneumoniae is essential for clearance of infection; however, within the brain, the attendant inflammatory response contributes to pathogenesis. In this study we investigate the local and systemic inflammatory response and the role of IFN-γ in a murine model of PM induced by type 4 S. pneumoniae. MATERIALS AND METHODS PM was induced in C57BL/6 mice by intracranial injection (subarachnoidal) of bacteria. Wild type strain TIGR4 (type 4) and its derivatives lacking two major virulence factors (capsule and surface protein PspC) were used for the experiments. Lymphoid and myeloid cell populations involved in meningitis were studied by flow cytometric analysis and cytokines/chemokines mRNA levels were detected by RT-qPCR in brains 48 hours after infection. The presence of cytokines/chemokines in brains was assessed by intracellular cytokine staining whereas in blood analysis was by Luminex technology. Animals were treated with a monoclonal antibody specific for murine IFN-γ to evaluate its role in animal survival and clearance of bacteria in the brain. RESULTS Intracranial inoculation of 3x104 colony-forming units of wild type TIGR4 caused 70% of mice to develop meningitis within 48 hours. The amount of neutrophils and NK cells in the brain increased following infection. NK cells were found to be the dominant source of IFN-γ, with mRNA levels 200-fold higher in brains of infected mice compared to uninfected controls. Pro-inflammatory cytokines such as IL-1β and TNF-α were also upregulated. In vivo treatment with anti-IFN-γ antibody increased survival of infected mice and enhanced local bacterial clearance. Mutants lacking the two major virulence factors of pneumococcus were tested. The unencapsulated mutant did not cause meningitis and did not produce IFN-γ, whereas the PspC- mutant behaved as the wild type. DISCUSSION AND CONCLUSIONS The murine model is an essential tool to study local and systemic inflammatory responses and cytokines involvement during PM pathogenesis. IFN-γ was found to enhance bacterial pathogenesis in a murine model of meningitis induced by type 4 S. pneumoniae. This study shows that IFN-γ produced by NK cells during meningitis by S. pneumoniae exerts a negative effect on the disease outcome.