Nonencapsulated Streptococcus pneumoniaeavoid innate immunity clearance through reactive oxygen species resistance

Abstract

Neutrophils provide an important mechanism of clearance for Streptococcus pneumoniae. Therefore, evasion of the innate immune response is essential for pneumococci to cause disease. The pneumococcal capsule is protective against phagocytosis but nonencapsulated Streptococcus pneumoniae (NESp) lack this capsule. Despite the lack of capsule NESp effectively colonize and cause invasive and noninvasive disease. All invasive isolates of NESp express the oligopeptide transporters AliC and AliD, which are required for virulence in multiple animal models. AliC and AliD alter downstream gene expression, and it is hypothesized that one of these regulated genes is responsible for increased immune evasion of NESp. A mutant library of AliC/AliD regulated genes was created and tested for virulence in a Galleria mellonella model of infection. Genes of interest from the in vivo screen were tested for the ability to resist phagocytic killing by neutrophil-like cells. Resistance to killing through reactive oxygen species (ROS) was examined. Of the seven AliC/AliD regulated genes tested, LytFN1 and MgtC mutants were found to have significantly reduced virulence compared to wildtype. A 40% decrease in resistance to phagocytic killing was observed in both mutants, as well as significantly decreased survival during exposure to ROS from growth in 2.5 mM hydrogen peroxide. Expressing AliD in a pneumococcal background that does not natively have the oligopeptide transporter significantly increased virulence and ROS resistance. Regulation of gene expression by AliC and AliD promote pneumococcal survival and virulence by increasing resistance to ROS mediated clearance and partially complements the lack of the opsonophagocytosis resistant capsule. Institutional funds