Inhibition of C1-mediated immune hemolysis by monomeric and dimeric peptides from the second constant domain of human immunoglobulin G

Activation of the classical complement cascade by immunglobulin G involves binding of the first complement component (C1) to a multivalent antigen-IgG complex. Binding occurs by interaction of a site on the surface of the C(γ)2 domain IgG with a globular head of the C1q subcomponent of C1. Previously we found that the synthetic decapeptide consisting of residues 281-290 from the second constant domain of the γ-chain of the human IgG1 protein Eu inhibited the binding of human C1 to sensitized erythrocytes. The present study describes inhibition by monomeric and dimeric peptides containing residues 289-292 or 282-292: 282 285 288 290 292 (Val-Gln-Val-His-Asn-Ala-Lys-Thr-Lys-Pro-Arg). On a molar basis, monomeric peptide 282-292 is just as active an inhibitor of C1 binding as peptide 281-290, whereas monomeric peptide 289-292 (tuftsin) is 4 times less active. Peptides 282-292 and 289-292 were each cross-linked at the amino terminus through terephthaloyl-bis(iminodiacetic acid) (Tid). Each dimeric peptide is twice as active on a molar basis as the corresponding monomeric peptide. Dimeric peptide Tid(282-292)₂ is just as active on a molar basis as the monomeric 7S form of human IgG1 and 60% as active as the Fc fragment of IgG in inhibiting the binding of human C1 to sensitized erythrocytes. These results suggest that the positively charged residues His-285, Lys-288, Lys-290, and Arg-292, which are located on the outer surface of the C(γ)2 domain, may be involved in the C1q-binding site of human IgG.