Streptococcus pneumoniae is an important pathogen accounting for a large number of deaths worldwide. Due to drawbacks of the current polysaccharide-based vaccine, the most promising way to generate an improved vaccine may be to utilize protection-eliciting pneumococcal proteins. Pneumococcal surface adhesin A (PsaA) and pneumococcal surface protein A (PspA) are two vaccine candidates which have been evaluated against S. pneumoniae infection in animal models or human clinical trials with encouraging results. In this study, the efficacy of the fusion protein PsaA-PspA, which includes PsaA part and PspA part, in inducing immunoprotective effects against fatal pneumococcal challenge was evaluated in an animal model. PspA part of PsaA-PspA fusion protein contains both family1 N-terminal region and family 2 N-terminal clade-defining region of PspA. Immunization with the PsaA-PspA fusion protein induced high levels of antibodies against both PsaA and PspA, which could bind to intact S. pneumoniae strains bearing different PspAs. Ex vivo stimulation of splenocytes from mice immunized with PsaA-PspA induced IL-17A secretion. Mice immunized with PsaA-PspA showed reduced S. pneumoniae levels in the blood and lungs compared with the PBS group after intranasal infection. Finally, mice immunized with PsaA-PspA fusion proteins were protected against fatal challenge with pneumococcal strains expressing different PspAs regardless of the challenge route. These results support the PsaA-PspA fusion protein as a promising vaccine strategy, as demonstrated by its ability to enhance the immune response and stimulate production of high titer antibodies against S. pneumoniae strains bearing heterologous PspAs, as well as confer protection against fatal challenge with PspA family 1 and family 2 strains.

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Streptococcus pneumoniae is a major cause of infectious disease and complications worldwide, such as pneumonia, otitis media, bacteremia and meningitis. New generation protein-based pneumococcal vaccines are recognized as alternative vaccine candidates. Pneumolysin (Ply) is a cholesterol-dependent cytolysin produced by all clinical isolates of S. pneumoniae. Our research group previously developed a highly detoxified Ply mutant designated Plym2 by replacement of two animo acids (C428G and W433F). Exhibiting undetectable levels of cytotoxicity, Plym2 could still elicit high titer neutralizing antibodies against the native toxin. However, evaluation of the active immunoprotective effects of Plym2 by subcutaneous immunization and lethal challenge with S. pneumoniae in mice did not yield favorable results. In the present work, we confirmed the previous observations by using passive immunization and systemic challenge. Results of the passive immunization were consistent with those of active immunization. Further experiments were conducted to explain the inability of high titer neutralizing antibodies against Ply to protect mice from S. pneumoniae challenge. Pneumococcal Ply is known to be the major factor responsible for the induction of inflammation that benefits the host. Proinflammatory cytokines facilitate the clearance of invaders by the recruitment and activation of leukocytes at the early infection stage. We demonstrated that Plym2 could induce proinflammatory cytokines similarly to wild-type Ply. A systemic infection model was used to clarify that Plym2 lacking cytolytic activity could protect mice from intraperitoneal challenge directly, while antibodies to the mutant had no effect. Therefore, the protective function of Plym2 may be due to its induction of proinflammatory cytokines. When used in the systemic infection model, Plym2 antibodies may block the induction of proinflammatory cytokines by Ply. These findings demonstrate that a Ply-based vaccine would not be an effective primary vaccine component, but it may be beneficial as an adjuvant to stimulate cytokine production.

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