Group A streptococcus (GAS) is an important human pathogen that produces several extracellular
exotoxins to facilitate invasion and
infection.
Streptococcal pyrogenic exotoxin B (SPE B) has been demonstrated to be an important
virulence factor of GAS. Our previous studies indicate that SPE B cleaves
complement 3 (C3) and inhibits the activation of
complement pathways. In this study, we constructed and expressed recombinant fragments of SPE B to examine the C3-binding site of SPE B. Using
enzyme-linked
immunosorbent assays and pull-down assays, we found that the C-terminal domain, containing
amino-acid residues 345-398, of SPE B was the major binding site of human serum C3. We further identified a major, Ala376-Pro398, and a minor C3-binding motif, Gly346-Gly360, that both mediated the binding of
C3 complement. Immunization with the C3-binding motifs protected mice against challenge with a lethal dose of non-invasive M49 strain GAS but not invasive M1 strains. To achieve higher efficiency against invasive M1 GAS
infection, a combination of synthetic
peptides derived from C-terminal
epitope of
streptolysin S (SLSpp) and from the major C3-binding motif of SPE B (PP6, Ala376-Pro398) was used to elicit specific immune response to those two important streptococcal
exotoxins. Death rates and the severity of skin lesions decreased significantly in PP6/SLSpp-immunized mice that were infected with invasive M1 strains of GAS. These results indicate a combination of the C3-binding motif of SPE B and the protective
epitope of SLS could be used as a
subunit vaccine against invasive M1 strains
group A streptococcal infection.