The Spr1875 protein of Streptococcus pneumoniae protects the bacterium from microglia-mediated killing

Objectives: S. pneumoniae is a major cause of morbidity and mortality worldwide. For the development of vaccines effective in preventing pneumococcal infection identification/characterization of bacterial antigens involved in host immune response is crucial. In this study, we employed the unencapsulated DP1004 strain and its isogenic spr1875-ko mutant to investigate the role of the Spr1875 protein in the interaction of S. pneumoniae with microglia, the resident brain macrophages. Methods: by screening a whole genome phage display library with sera from convalescent patients we identified clones carrying pneumococcal B-cell epitopes. Among these, we isolated an antigenic fragment, designated R4, encoded by the ORF spr1875 in the R6 strain genomic sequence. To evaluate whether Spr1875 is involved in pneumococcal pathogenicity a mutant strain lacking this protein was constructed. Here, by using an in vitro infection model and an antibiotic protection assay, we investigated the ability of microglial BV2 cells to phagocytose and kill the spr1875-ko mutant, as well as the survival of these bacteria inside BV2 cells. Results: both strains were internalized by microglia efficiently and to a similar extent. However, the survival of the spr1875-ko strain inside the cells was significantly lower than that observed with the parental DP1004. Accordingly, the percentage of acidic phagosomes in BV2 cells bearing spr1875-ko pneumococci was markedly higher than that containing DP1004 bacteria. A different susceptibility between the two strains was also observed when S. pneumoniae-microglia interaction was assessed by a bactericidal assay. Conclusion: these findings indicate that the Spr1875 antigen confers resistance to microglia-mediated killing of S. pneumoniae, suggesting that this protein may play an important role in the pathogenesis of pneumococcal meningitis.