The protective immunogenicity of chemically synthesized copies of the NH2-terminal region of type 6
streptococcal M protein was investigated. Four overlapping
peptides were synthesized by copying residues 1-20, 10-20, 12-31, and 22-31. Rabbit
antisera raised against whole cells of type 6 streptococci reacted at high dilutions (1/12,800 to 1/51,200) with S-M6(1-20) and S-M6(10-20), and at low dilutions (1/100-1/800) with S-M6(12-31) and S-M6(22-31), indicating that the NH2-terminal region of type 6 M
protein bears
immunodominant epitopes. When covalently linked to
tetanus toxoid and emulsified in complete
Freund's adjuvant, the synthetic
peptides S-M6(1-20), S-M6(10-20), and S-M6(12-31), but not S-M6(22-31), evoked type-specific opsonic
antibodies against type 6 streptococci. Although the
immune sera reacted in low dilutions by
enzyme linked immunoabsorbent assay (ELISA) with the heterologous M
protein polypeptides pep M5, pep M19, and pep M24, they failed to opsonize the streptococci from which these M
protein polypeptides were derived. Each of the
immune sera reacted in high dilution by ELISA with the respective immunizing
peptides. All except those against S-M6(22-31) also reacted with pep M6. None of the
immune sera reacted with human cardiac tissue by immunofluorescence or with muscle
myosin by ELISA. The pattern of the inhibition of opsonization by each of the synthetic
peptides of each of the
immune sera indicates the presence of at least three protective
epitopes in the NH2-terminal region of type 6 M
protein. Our results indicate that the NH2-terminal region of type 6 M
protein contains both protective and nonprotective
epitopes, and chemically synthesized copies of this region lack cardiac tissue cross-reactive
epitopes. These studies hold promise for the development of safe and effective
vaccines against group A streptococci, especially against the strains giving rise to
rheumatic fever and
rheumatic heart disease.