Sortase A Confers Protection against Streptococcus pneumoniae in Mice

Streptococcus pneumoniae sortase A (SrtA) is a transpeptidase that is highly conserved among pneumococcal strains, whose involvement in adhesion/colonization has been reported. We found that intraperitoneal immunization with recombinant SrtA conferred to mice protection against S. pneumoniae intraperitoneal challenge and that the passive transfer of immune serum before intraperitoneal challenge was also protective. Moreover, by using the intranasal challenge model, we observed a significant reduction of bacteremia when mice were intraperitoneally immunized with SrtA, while a moderate decrease of lung infection was achieved by intranasal immunization, even though no influence on nasopharynx colonization was seen. Taken together, our results suggest that SrtA is a good candidate for inclusion in a multicomponent, protein-based, pneumococcal vaccine.Streptococcus pneumoniae colonizes the nasopharynx of humans and represents a leading cause of severe diseases, such as otitis media, pneumonia, and meningitis. S. pneumoniae is one of the major causes of bacterial pneumonia in developing countries (19). It is estimated that each year, nearly 1 million children worldwide die because of pneumococcal diseases (10). Besides children, groups at high risk of pneumococcal infection are immunocompromised subjects and the elderly, for whom a high case fatality rate is also observed. The last decades have seen an increase in investigations of protein antigens, and several protein candidates have been proposed for a vaccine for S. pneumoniae (2) to overcome the problems inherent to the currently available polysaccharide-based vaccines. In fact, the 23-valent polysaccharide pneumococcal vaccine is not effective in children under 2 years of age, whose immune systems are unable to mount a T-independent response to polysaccharides. On the other hand, the 7-valent polysaccharide conjugate vaccine, although efficacious, induces serotype replacement (5, 20). Moreover, while more than 90 S. pneumoniae serotypes are presently known, both polysaccharide pneumococcal vaccines and polysaccharide conjugate vaccines are effective only against the serotypes included in the vaccine. Efforts to identify new S. pneumoniae factors that play a role in colonization and pathogenesis may contribute to the indication of possible targets of either new therapeutic agents or vaccines.Sortase A (SrtA) is a membrane-anchored transpeptidase expressed by gram-positive bacteria (12). The role of SrtA in the processing of sorting signals at the LPXTG motif to anchor surface proteins to the cell wall envelope was first described for Staphylococcus aureus (21), in which an isogenic SrtA mutation resulted in a strongly reduced ability to infect animals (13, 23). SrtA has been shown to participate in the colonization and/or pathogenesis of several Streptococcus species (1, 6, 8, 22, 24).S. pneumoniae SrtA has been described as playing a role in adhesion to human pharyngeal cells in vitro (7), in nasopharyngeal colonization in chinchilla (3), and in pneumonia, bacteremia, and nasopharyngeal colonization in murine models (15). Although SrtA seems to be dispensable in pilus biogenesis, its possible role in repressing pilus islet expression has been very recently proposed (9). SrtA has been found to be widely expressed among S. pneumoniae isolates and highly conserved, with a DNA identity of 99 to 100% (15). Although all of these findings suggest that pneumococcal SrtA might be useful as a protein vaccine, to the best of our knowledge no data have been provided so far on the protective efficacy afforded by SrtA immunization in animal models. Thus, we investigated the protective role of SrtA in murine models of S. pneumoniae infection.