It adds to the prior reports identifying bleeding that is out of proportion to inhibition of fVIII in standard assays. Paullinic acid below a poorly defined epitope that was previously identified (yellow). It is at a distance from membrane-interactive residues of the C2 domain name (also blue). The second C1 epitope, group AB, is usually colored green and overlaps with previously recognized membrane-interactive residues. Professional illustration by Somersault18:24. In contrast to most inhibitory antibodies, some anti-C1 domain name antibodies increase clearance of fVIII while only modestly affecting fVIII activity. Inhibitory Lyl-1 antibody anti-fVIII antibodies are the most severe complication of hemophilia A therapy. Because inhibition of fVIII occurs through a variety of mechanisms, detailed studies of these antibodies has also provided amazing insights into fVIII biology. Inhibitory antibodies against major epitopes around the A2 and C2 domains have confirmed the clinical importance of fVIII binding to fIXa2 and to a phospholipid surface through the respective domains (observe figure panel A).3 Some anti-C1 domain name antibodies interfere with uptake of fVIII by scavenger receptors and processing by dendritic cells, identifying a surface involved in clearance.4 One anti-C1 antibody that blocks von Willebrand factor (VWF) binding prolongs plasma blood circulation to the same extent as VWF, confirming the relationship of the C1 domain name to the clearance pathway and indicating the potential of antibodies to prolong blood circulation of fVIII.5 Most inhibitory antibodies only partially block fVIII activity. Paullinic acid A paradox of hemophilia A patient care is usually that the degree Paullinic acid to which inhibitory antibodies inhibit fVIII activity in standard assays has poor predictive value for the risk of bleeding.6,7 Thus, the assays are used to measure the titer of antibodies but not to assess bleeding risk. This lack of correlation indicates that our current fVIII assays do not measure crucial components of fVIII function. Recent studies show that most clinically important inhibitory antibodies mature slowly into high-affinity immunoglobulin G4 inhibitors, and lower-affinity forms of these antibodies may be detectable many months prior to clinical bleeding. 8 This raises the intriguing possibility that dangerous antibodies might be detected before bleeding occurs. Detecting the risk of bleeding before it occurs will require assay(s) that better measure the risk of bleeding. One approach to improving fVIII activity assays may be to measure platelet-dependent activity rather than, or in addition to, activity on phospholipid vesicles. Our laboratory recently found that fVIII binds to a complex of soluble fibrin and the IIb3 integrin on activated platelets rather than to phosphatidylserine. This enabled testing that showed that the degree of inhibition by 2 prototype antibodies varies 10- to 100-fold compared with phospholipid vesicle-based activity.9 Batsuli and coworkers have analyzed a panel of monoclonal antibodies against the fVIII C1 domain.1 Many of these antibodies recognize epitopes that are at least partially unique from those that were previously characterized (observe figure panel B). Paullinic acid These are adjacent to, but unique from, regions that participate phospholipid membranes and VWF. They found that >60% of plasmas from a group of hemophilia patients with inhibitors contained antibodies that compete with anti-C1 antibodies. Thus, antibodies against this domain name are likely to be much more frequent than previously anticipated. Several of the antibodies caused bleeding, from snipped mouse tails, that was nearly as severe as total deficiency of fVIII, even though the inhibition of Paullinic acid fVIII activity was modest. Most prevented binding to VWF. These antibodies accelerated fVIII clearance (observe figure panel A) presumably by separating fVIII from VWF and enabling clearance by scavenger receptors in the established clearance pathway. The accelerated clearance contributes to, and appears to be the major cause of, bleeding risk. This work makes it obvious that.