New assay for measuring binding of platelet glycoprotein IIb/IIIa to unpurified von Willebrand factor

Among the numerous variants of vWD, no patient with an abnormal vWF binding to GPIIb/IIIa has been described to date. To search for such potential var iants, we developed a two-site assay for measuring the binding of purified GPIIb/IIIa to vWF in biological fluids and we used it to study a large seri es of plasmas from various types of von Willebrand disease (vWD) and recomb inant vWF (rvWF). vWF in plasma or rvWF in culture medium was immobilized onto anti-vWF monoc lonal antibodies (MoAb)-coated wells of microtiter plates. After incubation with either unlabeled GPIIb/IIIa and a I-125-anti-GPIIb/IIIa MoAb or I-125 -GPIIb/IIIa, binding curves and binding isotherms were respectively establi shed. Normal pool plasma and wild-type rvWF were used as reference samples. We tested plasmas from 85 normal subjects, 115 patients with different typ es of vWD (64 type 1, 2 type 3, 9 type 2A, 4 type 2M, 16 type 2B, 15 type 2 N, 3 type IID and 2 acquired forms) and 50 patients with various bleeding d isorders. Four mutated rvWF with 2A (Glu875Lys and Pro885Ser) or 2B (Dupl.M et540 and Val551Phe) substitutions and one rvWF mutated in the RGD domain o f the C-terminal part of vWF-subunit (Asp1746Gly) were also studied. Among the various samples tested, only rvWF Asp1746Gly had no affinity for GPIIb/IIIa. In contrast, GPIlb/IIIa similarly bound to the other vWF, indep endently of the proteic environment, the factor VIII level. the degree of m ultimerization or the mutation of vWF. Our results indicate that subjects w ith an abnormal vWF binding to GPIIb/IIIa are probably rare and difficult t o target for a specific screening.