A monoclonal antibody against the platelet fibrinogen receptor contains a sequence that mimics a receptor recognition domain in fibrinogen

The binding of fibrinogen to its platelet receptor, the glycoprotein IIb-IIIa complex, is mediated, in part, by an Arg-Gly-Asp (RGD) sequence within the fibrinogen A alpha chain. PAC1 is an IgM-kappa murine monoclonal antibody that binds to the platelet fibrinogen receptor, and its binding is inhibited by both fibrinogen and RGD-containing peptides. To identify the regions of PAC1 that interact with the fibrinogen receptor, we determined the mRNA sequences of PAC1 immunoglobulin heavy and light chain variable regions. Five out of the six complementarity-determining regions (CDRs) of PAC1 had entirely germline sequences with no regions of similarity to fibrinogen. However, CDR3 of the PAC1 heavy chain (H-CDR3) was very large and unique due to the insertion of a novel D region segment. H-CDR3 contained a sequence, Arg-Tyr-Asp (RYD), that, if present in the proper conformation, might behave like the RGD sequence in fibrinogen. A 21-residue synthetic peptide encompassing the H-CDR3 region inhibited fibrinogen-dependent platelet aggregation as well as the binding of PAC1 (Ki = 10 microM) and fibrinogen (Ki = 5 microM) to activated platelets. The RYD region of H-CDR3 appeared to be central to its function, because substitution of the tyrosine with glycine increased the inhibitory potency of the peptide by 10-fold, while replacing the tyrosine with D-alanine or inverting the RYD sequence sharply reduced the inhibitory potency. Thus, the linear sequence, RYD, within H-CDR3 of PAC1 appears to mimic the RGD receptor recognition sequence in fibrinogen. This type of immunologic approach could be useful in studying the structural basis of other receptor-ligand interactions.