Functional studies of Willebrand factor using monoclonal antibodies

Previously described monoclonal antibodies to porcine Willebrand factor were used to study various functional assays of Willebrand factor. These antibodies comprise at least five separate specificities as determined by differential reactivity in three distinct binding assays. The antibodies were tested for their effect on the in vivo bleeding time, in vitro bleeding time, and ristocetin cofactor activity. The titers of the antibodies in a binding assay did not correlate with any inhibitory activities, and the ability to inhibit a given functional test was independent of inhibitory activity in the other assays. These data suggest that the antibodies are able to detect specific structures on Willebrand factor involved in specific functional assays and that the structure(s) involved in ristocetin cofactor activity is not identical to those mediating the in vivo or in vitro bleeding time.