Protective Immunogenicity of Chemically Synthesized Peptide Fragments of Group A Streptococcal M Proteins

Summary

The M protein on the surface of group A streptococci contains epitopes that evoke type-specific protective immunity against subsequent encounters with the related serotypes of streptococci. Because some of the M proteins are closely associated with potentially harmful epitopes that give rise to antibodies cross-reactive with host tissues, especially the heart, we have undertaken studies of the covalent structures of M protein polypeptide fragments extracted with pepsin at pH 5.8 from rheumatogenic strains of group A streptococci. We have identified 35-residue peptide fragments of type 24 M protein that elicit protective, but not heart-cross-reactive antibodies. Chemically synthesized copies of a 35-residue peptide and subpeptides containing as few as 13 M protein amino acid residues were shown to be immunogenic when covalently conjugated to poly-L-lysine or tetanus toxoid. A 20-residue NH₂ terminal peptide of type 5 M protein was shown to be similarly immunogenic. The immune responses in each case were protective, but not tissue cross-reactive. These results show that the whole M protein molecule is not necessary for the production of protective immunity. Moreover, selected synthetic peptide copies of the natural M protein molecule provide effective immunogens without the potential fear of tissue damage.