The plethora of
virulence factors associated with Staphylococcus aureus make this bacterium an attractive candidate for a molecularly-designed
epitope-focused
vaccine. This approach, which necessitates the identification of neutralizing
epitopes for incorporation into a
vaccine construct, is being evaluated for pathogens where conventional approaches have failed to elicit protective humoral responses, like HIV-1 and
malaria, but may also hold promise for pathogens like S. aureus, where the elicitation of humoral immunity against multiple
virulence factors may be required for development of an effective
vaccine. Among the
virulence factors employed by S. aureus, animal model and epidemiological data suggest that alpha toxin, a multimeric β-pore forming toxin like protective
antigen from Bacillus anthracis, is particularly critical, yet no candidate neutralizing
epitopes have been delineated in alpha toxin to date. We have previously shown that a linear determinant in the 2β2-2β3 loop of the pore forming domain of B. anthracis protective
antigen is a linear neutralizing
epitope. Antibody against this site is highly potent for neutralizing
anthrax lethal toxin in vitro and for protection of rabbits in vivo from virulent B. anthracis. We hypothesized that sequences in the β-pore of S. aureus alpha toxin that share structural and functional homology to β-pore sequences in protective
antigen would contain a similarly critical neutralizing
epitope. Using an in vivo mapping strategy employing
peptide immunogens, an optimized in vitro toxin neutralization assay, and an in vivo dermonecrosis model, we have now confirmed the presence of this
epitope in alpha toxin, termed the pore neutralizing determinant. Antibody specific for this determinant neutralizes alpha toxin in vitro, and is highly effective for mitigating dermonecrosis and bacterial growth in a mouse model of S. aureus USA300 skin
infection. The delineation of this linear neutralizing determinant in alpha toxin could facilitate the development of an
epitope-focused
vaccine against S. aureus.