Development of 5-valent conjugate pneumococcal protein A – Capsular polysaccharide pneumococcal vaccine against invasive pneumococcal disease

In this study, we synthesized a 5-valent pneumococcal conjugate vaccine, which was prepared with the pneumococcal capsular polysaccharides (PCPs) (from Streptococcus pneumoniae 1, 5, 6B, 19F, 23F) and pneumococcal surface protein A (PspA) mediated by 1,4-butanediol diglycidyl ether. The PspA cloned from serotype 19 strain showed good cross-immune response to 1, 5, 6B, and 23F serotypes of Streptococcus pneumonia (S. pneumoniae). Analysis of the maturation process of conjugate polyclonal antibody showed that conjugation with the protein carrier converted the polysaccharide from a weak T cell-independent (TI) antigen to a T cell-dependent (TD) antigen, although antibodies affinity to polysaccharide was not as strong as it to PspA in conjugate. We used an invasive disease mouse model to evaluate the protective efficacy of this conjugate vaccine. Active and passive protection against intraperitoneal challenge with virulent type 6B strain showed that the median survival times for mice immunized with conjugate were significantly longer than that of mice treated with capsular polysaccharides or PspA alone. Our study's results showed that immunization of the 5-valent PspA-capsular polysaccharides conjugate vaccine could afford strong protection to mice against the invasion of 1, 5, 6B, 19F, 23F serotypes S. pneumoniae.