Epitope-specific protective immunogenicity of chemically synthesized 13-, 18-, and 23-residue peptide fragments of streptococcal M protein.

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Abstract
The ability of chemically synthesized subpeptides of type 24 streptococcal M protein to evoke protective antibodies in rabbits was investigated. Copies of the COOH-terminal 13, 18 and 23 amino acid residues of CNBr fragment 7 (CB7) of pepsin-extracted type 24 M protein were synthesized; methionine was substituted for homoserine as the COOH-terminal residue. An additional residue of cysteine was added at the COOH terminus of the 13-residue peptide. Each of the peptides, designated S-CB7-(23-35)-Cys, S-CB7-(18-35) and S-CB7-(13-35), when conjugated to lysylated tetanus toxoid with glutaraldehyde, stimulated formation of protective anti-type 24 M protein antibodies in rabbits. The smallest peptide, S-CB7-(23-35)-Cys, elicited immune responses equally as strong, if not stronger, than those to the longer peptides. A single Lys/Gly substitution in this 13 residue peptide resulted in its failure to stimulate protective antibodies. None of the antisera reacted with heterologous serotypes of M protein; none reacted with frozen sections of human heart tissue. Thus, a chemically synthesized peptide fragment corresponding to as few as 13 amino acid residues of streptococcal M protein is capable of evoking protective anti-streptococcal antibodies without evoking antibodies crossreactive with cardiac tissue.

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