TOPOLOGY OF PLATELET GPIb INVESTIGATED BY LOCATION OF MONOCLONAL ANTIBODY EPITOPES

Abstract

A large number of monoclonal antibodies to platelet membrane glycoprotein lb (GPIb) have been described but for most of these the position of the epitope is not known. Since many of these influence platelet function, a better understanding of struc-ture-function relationships requires this knowledge. The position of the epitopes for the monoclonal antibodies API (Dr. T.J. Kunicki), AN51 and SZ-2 (Dr. C-G. Ruan), WM23 (Dr. M.C. Berndt) and PI were determined by analysis of proteolytic cleavage fragments of glycocalicin via affinity chromatography on the monoclonal antibodies coupled to Sepharose, elution with diethyl ami ne solution, separation on SDS-gel electrophoresis and detection by silver-staining. First, intact glycocalicin was examined and was found to bind to all monoclonals with the exception of PI. All monoclonals bound intact GPIb. WM23 bound a 70 kDa glycopeptide from the highly-glycosylated 90 kDa tryptic fragment of glycocalicin. API, AN51 and SZ-2 all bound to 45 kDa and 40 kDa, poorly glycosylated tryptic fragments. The 40 kDa fragment is derived from the 45 kDa fragment and has been shown to be the N-terminal region of GPIb. All these monoclonals have been shown to inhibit von Willebrand factor induced platelet agglutination. Platelets were treated with either elastase or calcium activated protease and monoclonal binding checked by immunofluorescence. The immunofluorescence with API, AN51 and SZ-2 was minimal compared to control platelets whereas that of PI remained as strong as the controls. This indicates that the epitope for PI lies on GPIb in a region other than glycocalicin and its absence from glycocalicin is not simply due to conformational changes in that fragment. Since PI inhibits platelet activation by thrombin and ADP it must act via conformational effects and not by blocking the thrombin receptor which lies on the 45 kDa region of glycocalicin. These results support a more complex role for GPIb in platelet activation.