Negligible role for pneumococcal surface protein A (PspA) and pneumococcal surface protein C (PspC) in the nasopharyngeal colonization of mice with a serotype 6B pneumococcal strain

Abstract

Streptococcus pneumoniae colonizes the human nasopharynx asymptomatically, but it can also cause several diseases, including otitis media, pneumonia, bacteremia, and meningitis. The colonization of the nasopharynx by the bacteria is an essential step for the pneumococcus to invade other sites and cause diseases. Pneumococcal surface protein A (PspA) and Pneumococcal surface Protein C (PspC) are important virulence factors and have been described to play roles in adhesion and immune evasion. In this study, we immunized mice subcutaneously with the recombinant α-helical region of PspA and/or PspC combined with different adjuvants to assess protection against colonization with the serotype 6B strain BHN418. Though high serum levels of specific IgG were detected, none of the formulations led to reduction in the colonization of the nasopharynx. The negative result may be due to the poor induction of IgG2c, which has been previously correlated with protection against pneumococcal colonization in mice. Furthermore, BHN418 pspA and pspC single and double knockouts were evaluated in colonization experiments and no differences in bacterial load were observed. In competition assays with the wild-type strain, borderline to no reduction was observed in the loads of the knockouts. Our results contrast with data from the literature using other pneumococcal strains, showing that the role of PspA and PspC in colonization can vary depending on the background of the knockout strain studied. BHN418 has been selected for its capacity to colonize humans in experimental challenge studies and may have redundant factors that compensate for the lack of PspA and PspC during nasopharyngeal colonization of mice.