Immunization with components of two iron uptake ABC transporters protects mice against systemic Streptococcus pneumoniae infection

Abstract

There has been considerable recent research into protein based Streptococcus pneumoniae vaccines as alternatives to the existing capsular antigen vaccines. PiuA and PiaA (formerly Pit1A and Pit2A) are recently identified lipoprotein components of S. pneumoniae iron uptake ABC transporters which are required for full virulence and are likely to be expressed on the surface of the bacterial cell membrane. We investigated the efficacy of recombinant PiuA and PiaA proteins at eliciting protective immunity in mice against systemic infection with S. pneumoniae. Both recombinant PiuA and PiaA generated antibody responses that cross-reacted with each other but not with pneumolysin and reacted with identical proteins from nine different S. pneumoniae serotypes. Mice immunized with recombinant PiuA and PiaA were protected against systemic challenge to a degree similar to those immunized with an existing protein vaccine candidate, PdB (a genetically modified pneumolysin toxoid). Immunization with a combination of both PiuA and PiaA resulted in additive protection and was highly protective against systemic infection with S. pneumoniae. PiuA and PiaA are therefore promising additional candidates for a novel S. pneumoniae vaccine using protein antigens.

FIG. 1

(A) Coomassie blue-stained SDS-PAGE gel of purified His₆-PiuA and His₆-PiaA proteins showing their relative masses and purity. (B) Western blot of purified His₆-PiuA and His₆-PiaA proteins probed with monoclonal anti-His₆ tag antibody (1 in 3,000 dilution).

FIG. 2

Western blots of purified His₆-PiuA, His₆-PiaA, and PdB proteins (A) and S. pneumoniae whole-cell lysates (B) probed with anti-alum, anti-PiuA, anti-PiaA, antiPiuA-PiaA, or anti-PdB sera (1 in 3,000 dilution). The relative molecular mass is indicated on the left.

FIG. 3

Survival curves for groups of 12 to 15 mice vaccinated with His₆-PiuA (triangles), His₆-PiaA (circles), PdB (diamonds), His₆-PiuA combined with His₆-PiaA (open squares) or alum-PBS alone (filled squares) after IP challenge with 10⁶ CFU of S. pneumoniae. Symbols: ▪, alum; ▵, PiuA; ⋄, PdB; ○, PiaA; □, PiuA + PiaA.