Reactions of group 1 patient sera with CHO cells stably transfected with cDNAs from various parts of the GPIb-IX complex and inhibition of quinine-dependent antibody binding by anti-GPIX MoAbs. MAIPA studies were performed in the presence and absence of quinine with patient serum and a MoAb as indicated in the figure. (A) The results of one representative patient's serum binding to mock-transfected CHO, CHO βIX, and CHO α cells in the presence of quinine. Binding was not observed in the absence of quinine (not shown). In the presence of quinine, binding did not occur with mock-transfected CHO cells but was observed with CHO βIX and CHO α cells. (B) The results of one representative patient's serum binding to CHO βIX cells expressing GPIbβ and GPIX in the presence (▩) or absence (▧) of quinine and a competitive MoAb (FMC25 or SZ1). No binding was observed in the absence of quinine. Competition with FMC25 did not inhibit the quinine-dependent antibody binding. Conversely, competition with SZ1 (specific for GPIX) inhibited binding of the quinine-dependent antibody.