Streptococcus pneumoniae, a leading cause of community-acquired pneumonia, often evades host immunity and causes systemic diseases, such as sepsis and meningitis. Pneumococcal pneumonia is accompanied by an excessive inflammatory response at infection sites. In the present study, we investigated the role of PfbA, a pneumococcal parallel β-helix protein, in infection.
A homology search indicated that the pfbA gene is highly conserved among pneumococcal strains, while other streptococci do not contain such an orthologous gene. In human neutrophil bactericidal assay findings, a pfbA mutant strain showed significantly reduced survival as compared to the wild-type strain, while exogenous addition of recombinant PfbA recovered survival of the mutant strain incubated with neutrophils. Next, we performed time-lapse microscopic analysis of the interaction between S. pneumoniae and human neutrophils. Within 1 minute, neutrophils phagocytosed the pfbA mutant strain, whereas after 5 minutes they were unable to capture the wild-type strain. In addition, flow cytometric analysis revealed that recombinant PfbA-coated beads were captured less often by neutrophils as compared to non-coated beads. Furthermore, a TLR2/4 inhibitor peptide significantly enhanced survival of the pfbA mutant strain incubated with neutrophils, whereas it had no effect on survival of the wild type. Finally, 24 hours after CD-1 mice underwent intratracheal infection with S. pneumoniae, the wild-type strain showed a significantly greater number of colony forming units in bronchial lavage fluid as compared to the pfbA mutant strain.
Our results indicate that PfbA is specific to S. pneumoniae and functions as an anti-phagocytic factor.