Protective activity of a protein fragment identified by screening of a phage display genomic library of Streptococcus pneumoniae

Objective: Streptococcus pneumoniae is a major cause of morbidity and mortality worldwide. The pathogenicity of pneumococci has been attributed to various virulence factors, most of which are located on its surface. The present study was undertaken to identify novel virulence factors and antigens with immunoprotective activity by using a phage display genomic library of Streptococcus pneumoniae. Methods. A lambda phage display library from the D39 S. pneumoniae strain was selected using sera from patients convalescing from invasive pneumococcal disease. For virulence studies, adult BALB/c mice (Charles River) were inoculated i.v. with the wild type D39 strain or with an equal number of CFUs from a mutant lacking spr1875 (Δspr1875). Results. Using one a convalescent serum, we selected from the phage display library an insert encoding for a fragment designated as R4, 161 amino acid in length, whose sequence matched ORF spr1875 D39 genome. This ORF encoded for a putative surface protein with a signal peptide and a LysM domain, which is found in a number of important virulence factors. To evaluate whether the spr1875 protein is involved in pneumococcal pathogenicity we constructed the Δspr1875. Using 7 x 106 CFUs for challenge, all mice inoculated with D39 died in six days, while 80% of the mice challenged with Δspr1875 survived. The R4 fragment was capable of inducing significant, albeit partial, protection in mice inoculated with a wide variety of virulent streptococcal strains. Conclusion. We have identified a novel virulence factor of S. pneumoniae that showed protective activity in a mouse model of systemic pneumococcal disease. Further studies are needed to characterize the biological activities of this antigen.