Protective and nonprotective epitopes of chemically synthesized peptides of the NH2-terminal region of type 6 streptococcal M protein

Abstract

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.