Pneumococcal surface protein A (PspA) and
Pneumolysin derivatives (Pds) are important
vaccine candidates, which can confer protection in different models of
pneumococcal infection. Furthermore, the combination of these two
proteins was able to increase protection against pneumococcal
sepsis in mice. The present study investigated the potential of hybrid
proteins generated by genetic fusion of PspA fragments to Pds to increase cross-protection against fatal
pneumococcal infection. Pneumolisoids were fused to the N-terminus of clade 1 or clade 2 pspA gene fragments. Mouse immunization with the fusion
proteins induced high levels of
antibodies against PspA and Pds, able to bind to intact pneumococci expressing a homologous PspA with the same intensity as
antibodies to rPspA alone or the co-administered
proteins. However, when antibody binding to pneumococci with heterologous PspAs was examined,
antisera to the PspA-Pds fusion molecules showed stronger antibody binding and C3 deposition than
antisera to co-administered
proteins. In agreement with these results,
antisera against the hybrid
proteins were more effective in promoting the phagocytosis of bacteria bearing heterologous PspAs in vitro, leading to a significant reduction in the number of bacteria when compared to co-administered
proteins. The respective
antisera were also capable of neutralizing the lytic activity of
Pneumolysin on sheep red blood cells. Finally, mice immunized with fusion
proteins were protected against fatal challenge with pneumococcal strains expressing heterologous PspAs. Taken together, the results suggest that PspA-Pd fusion
proteins comprise a promising
vaccine strategy, able to increase the immune response mediated by cross-reactive
antibodies and
complement deposition to heterologous strains, and to confer protection against fatal challenge.