PspK of Nonencapsulated Streptococcus pneumoniae: Mechanisms of Virulence and Immunization Potential.

摘要

Streptococcus pneumoniae (the pneumococcus) is a significant human pathogen causing over one million deaths a year. The diverse polysaccharide capsule types have long been thought to be necessary for colonization and disease. Since 2010 there have been increased reports of pneumococcal disease caused by nonencapsulated S. pneumoniae (NESp). Understanding how NESp are able to cause infections is important because current pneumococcal vaccines are only effective against encapsulated strains. A gene encoding the novel protein, pneumococcal surface protein K (PspK), was found to replace the capsule locus of some NESp carriage isolates. It was found that PspK increases nasopharyngeal colonization through unknown mechanisms. Various methods were used to determine the functional role of PspK during colonization, virulence, and its potential as a protection eliciting immunogen. In vitro methods included; protein purification and isolation, flow cytometry, Western blot analysis, opsonphagocytosis, epithelial cell adhesion and invasion assays, genetic complementation, cytokine expression analysis, and ELISA. In vivo methods included; murine colonization, immunization, and a chinchilla model of otitis media. PspK was found to increase epithelial cell adhesion, but not invasion and bind human sIgA independent of genetic background. Immunization with PspK produced antibodies protective against colonization and epithelial cell adhesion. Virulence in a chinchilla model of otitis media was enhanced by the expression of PspK and epithelial cell adhesion was associated with increased virulence in PspKNESp. NESp are a significant proportion of the pneumococcal population, and the expression of PspK aids in colonization and virulence. Understanding mechanisms of NESp disease and the ability to protect against NESp infections is vital to preventing future episodes of disease. The current work explains how a subset of NESp cause disease and provides a potential immunogen for next generation pneumococcal vaccines.