Structural features of the human bradykinin B2 receptor probed by agonists, antagonists, and anti-idiotypic antibodies.

The human bradykinin B2 receptor belongs to the family of G-protein-coupled receptors. To characterize the receptor protein, we have solubilized the membranes of cultured human foreskin fibroblasts bearing the B2 receptor. Affinity cross-linking of the solubilized receptor with the labeled agonist, 125I-Tyr0-bradykinin, or the labeled antagonist, 125I-(4-hydroxy-phenyl-propionyl)-HOE140, revealed major bands of apparent molecular mass of 69 kDa in SDS-polyacrylamide gel electrophoresis under reducing conditions, and of 59 kDa under non-reducing conditions. A 1000-fold molar excess of each of the unlabeled ligands quenched the specific labeling suggesting that the agonist and the antagonist compete for overlapping binding site(s). Covalent coupling of the receptor to bradykinin or HOE140, followed by Western blotting and immunoprinting with specific anti-ligand antibodies confirmed that the major ligand-binding form of the receptor is of 69 kDa. Anti-idiotypic antibodies which bear the internal image of bradykinin (Haasemann, M., Buschko, J., Faussner, A., Roscher, A.A., Hoebeke, J., Burch, R.M., and Müller-Esterl, W. (1991) J. Immunol. 147, 3882-3892) immunoprecipitated the 125I-labeled receptor as a major band of 68 kDa and a minor band of 47 kDa indicative of partial proteolysis. Chemical deglycosylation of the 125I-labeled receptor shifted the apparent molecular mass from 69 to 44 kDa demonstrating that the receptor is heavily glycosylated. Two-dimensional electrophoresis of the affinity-purified receptor revealed overlapping spots of 69 kDa and of pI 6.8-7.1 pointing to a microheterogeneity of the carbohydrate moiety. Elucidation of the key structural features of the B2 receptor protein will aid in understanding the structure-function relationships governing this prototypic peptide receptor.

Anti-idiotypic antibodies against the kinin receptor.

Three sets of monoclonal antibodies against bradykinin (MBK1, MBK2, MBK3) were generated by somatic cell fusion, characterized by their peptide specificity and compared to the known ligand specificity of the kinin receptor subtypes. By these criteria the paratope of MBK3 resembled the B2 receptor binding site whereas MBK1 shared principal binding characteristics with the B1 recrptor. Anti-idiotypic antibodies against MBK1, MBK2 and MBK3 were raised in rabbit and sheep. Specificity of the network components was verified by inhibition experiments on the level of peptide, idiotype and anti-idiotype. Anti-idiotypic antibodies against MBK3 recognized a conformation-dependent epitope which was binding site-related. Binding studies on human foreskin fibroblasts and guinea pig ileum showed mutual displacement of the anti-idiotypic antibody and bradykinin at the binding site pointing to a specific interaction of the antibody with the receptor from various species. An agonist activity of the antibodies, demonstrated in human (inositolphosphate pathway) and mouse (prostaglandin pathway) fibroblasts indicated that the anti-idiotypes bear an internal image of the ligand epitope. This molecular mimicry which was further substantiated by the detection of bradykinin specific anti-idiotypic antibodies, provides the structural basis for the observed cross-reactivity over species borders.

Anti-idiotypic antibodies against the kinin receptor.

Three sets of monoclonal antibodies against bradykinin (MBK1, MBK2, and MBK3) were generated by somatic cell fusion, characterized by their peptide specificity, and compared with the known ligand specificity of the kinin receptor subtypes. By these criteria, the paratope of MBK3 resembled the B2 receptor binding site, whereas MBK1 shared principal binding characteristics with the B1 receptor. Anti-idiotypic antibodies against MBK1, MBK2, and MBK3 were raised in rabbit and sheep. Specificity of the network components was verified by inhibition experiments on the level of peptide, idiotype, and anti-idiotype. Anti-idiotypic antibodies against MBK3 recognized a conformation-dependent epitope which was binding site-related. Binding studies on human foreskin fibroblasts and guinea pig ileum showed mutual displacement of the anti-idiotypic antibody and bradykinin at the binding site pointing to a specific interaction of the antibody with the receptor from various species. An agonist activity of the antibodies, demonstrated in human (inositolphosphate pathway) and mouse (prostaglandin pathway) fibroblasts indicated that the anti-idiotypes bear an internal image of the ligand epitope. This molecular mimicry, which was further substantiated by the detection of bradykinin-specific anti-anti-idiotypic antibodies, provides the structural basis for the observed cross-reactivity over species borders.

Anti-idiotypic antibodies bearing the internal image of a bradykinin epitope. Production, characterization, and interaction with the kinin receptor.

mAb against bradykinin, the prototypic member of the kinin family of vasodilator peptides, were generated by somatic cell fusion. The antibodies were isotyped as IgG1, kappa-type, and their target epitopes mapped with bradykinin, lysyl-bradykinin (kallidin), kinin receptor antagonists, and fragments thereof, revealing three distinct sets of mAb, i.e., mAb against bradykinin (MBK)1, MBK2, and MBK3. Comparison of the immunologic binding affinities and the known pharmacologic binding specificities of bradykinin derivatives disclosed a striking similarity in the binding profiles of mAb MBK3 and the B2 type of the kinin receptor. Anti-idiotypic antibodies against MBK1, MBK2, and MBK3 were raised in rabbit and sheep. Inhibition and competition experiments on the level of the Ag (ligand), the idiotype, and the anti-idiotype demonstrated the mutual specificity of the network system components. Anti-idiotypic antibodies against MBK3 recognized a particular idiotope that was conformation-dependent and associated with the Ag binding site of the antibody. Binding of anti-idiotypic antibodies to the B2 receptor expressed by human foreskin fibroblasts and guinea pig ileum demonstrated that the anti-idiotypes cross-react with the corresponding receptor across species. Specific stimulation of the inositol phosphate pathway in human fibroblasts and of the PG pathway in mouse fibroblasts, respectively, and inhibition of the latter effect by the B2 kinin receptor antagonist NPC 567 indicated that the anti-idiotypes bear the internal image of a bradykinin epitope. Furthermore, antibodies of the third generation (anti-anti-idiotypic antibodies) recognized the authentic Ag, i.e., bradykinin. Hence, the anti-idiotypic approach provides powerful tools to probe for the hitherto poorly characterized B2 kinin receptor.