Protection elicited by nasal immunization with pneumococcal surface protein A (PspA) adjuvanted with bacterium-like particles against Streptococcus pneumoniae infection in mice

Streptococcus pneumoniae is a major respiratory tract pathogen causing high levels of mortality and morbidity in infants and the elderly. In spite of the multitude of capsular polysaccharide vaccines used to guard against pneumococcal disease, fatal pneumococcal disease remains epidemic. Immunization with pneumococcal surface protein A (PspA), a highly immunogenic surface protein present in all strains of S. pneumoniae, can elicit protection against deadly pneumococcal infection. We have previously evaluated PspA in systemic vaccination. However, the mucosal immune system, as a first line of defense against respiratory infection, plays the most important role against the invasion of S. pneumoniae. In this study, we employed bacterium-like particles (BLPs) as an adjuvant for a PspA mucosal vaccine. The BLPs served as a carrier for PspA proteins bound to their surface. Mice were immunized intranasally with the PspA-BLP pneumococcal vaccine consisting of PspA3 from pneumococcal family 2. Not only did the immunized mice show a high level of serum IgG antibodies but also a high level of SIgA antibodies in the respiratory tract. After immunization with the PspA3-BLP vaccine, the mice were broadly protected against fatal intranasal challenge with homologous and heterogenous pneumococcal strains of different PspA families regardless of serotype, and the colony count was notably decreased in the lungs. Therefore, the PspA3-BLP pneumococcal vaccine has the potential to serve as a novel mucosal vaccine to enhance both systemic and mucosal immune responses to this disease.