Role of Optical Configuration in the Immunogenicity and Specificity of Synthetic Antigens Derived from Multichain Polyproline

Multichain polyprolines of either optical configuration, prepared in dimethyl sulfoxide from polylysine and N-carboxyproline anhydride, served as intermediates in the synthesis of a series of polymers of the general formulas, poly(Tyr, Glu)-polyPro-polyLys and poly(Phe, Glu)-poly-Pro-polyLys, in which the amino acids appear in different optical configurations. The various multichain copolymers obtained were used to elucidate whether, and to what extent, the optical configuration of the component amino acids in a macromolecule contributes to its immunogenicity and affects the relative immunopotency of different regions within the molecule. Multichain polyprolines of either optical configuration did not provoke antibody formation in rabbits, even though poly L Pro-poly L Lys was immunogenic in guinea pigs. Attachment of short peptides composed of ( L Tyr, L Glu) or ( L Phe, L Glu) to each of them led to materials provoking a strong immune response in rabbits. Thus, the attachment of as little as 8.5% of L -amino acid residues on the outside of a macromolecule composed exclusively of D -amino acids converted it into a good immunogen. The specificity of the antibodies formed was directed mainly to the peptides attached. Materials obtained upon attachment of short peptides of ( D Tyr, D Glu) or ( D Phe, D Glu) to either multichain poly- D -proline or poly- L -proline did not lead to a detectable immune response in rabbits when the usual high dose was used for immunization, but were weakly immunogenic at low dosage. Thus, a macromolecule containing as much as 95.5% of L -amino acids, but having all its polymeric side chains terminating in D -amino acids, is as poor an immunogen as a similar macromolecule composed exclusively of D -amino acids. It is suggested that this is due to the lack of endopeptidases capable of splitting peptide bonds between two L -proline residues, and that proteolysis may play a role in antibody biosynthesis, even though not at the stage of antigenic “recognition” but rather as a step in the control of the rate at which antibodies are formed. Whereas antigens containing L -peptides on the outside led mainly to antibodies specific toward these peptides, antigens containing D -peptides on the outside caused mainly the formation of stereospecific antibodies against the polyproline moiety.