Search

The antiphospholipid syndrome (APS) is associated with production of autoantibodies with lupus anticoagulant (LA) activity. These antibodies cause prolongation of in vitro clotting tests by inhibition of the conversion of prothrombin to thrombin in the presence of anionic phospholipid (PL). The extent to which this inhibition reflects antibody binding to, or functional inhibition of, phospholipids alone, prothrombin alone, or a prothrombin-phospholipid complex is pertinent to our understanding of the pathophysiology of this syndrome. Immunoglobulin fractions (IgG) from 18 patients with LA activity were tested for inhibitory activity against prothrombin activation by either factor Xa, in a purified prothrombinase system, or by purified fractions of snake venoms (E. carinatus, E. multisquamatus) which cleave prothrombin at the same initial site as the prothrombinase complex but do not require anionic phospholipid as a cofactor. Parallel testing of the same IgG samples for prothrombin binding by immunoassay was performed. Although all IgG samples inhibited the prothrombinase reaction, only three exhibited any inhibition of venom protease prothrombin activation in either the presence or absence of PL. Only one sample exhibited prothrombin binding by Western blot. These results suggest that lupus anticoagulant antibodies are heterogenous and that many, if not most, of the autoantibody populations responsible for LA activity impair prothrombin activation by interaction either with phospholipid alone or with a restricted range of prothrombin-phospholipid epitopes expressed by prothrombin only as part of the intact prothrombin-prothrombinase complex.

Lupus anticoagulants, commonly found in the immunoglobulin fraction of patients with the antiphospholipid syndrome (APS), and the normal plasma protein beta 2-glycoprotein 1 (beta 2GP1) may both contribute to the in vitro impairment of prothrombin activation associated with the APS. We examined the effects upon prothrombin activation supported by phospholipid vesicles of plasma IgG preparations from APS patients in the presence and absence of beta 2GP1. Using a purified system for measurement of prothrombin activation to thrombin, we demonstrated significant phospholipid concentration-dependent inhibition of prothrombin activation in the absence of beta 2GP1 by 11 consecutive patient IgG preparations. The degree of inhibition of prothrombin activation by equivalent concentrations of patient IgG correlated well with the extent of prolongation of the plasma clotting time in lupus anticoagulant assays of whole patient plasma. Additional studies with eight patient IgG preparations indicated that the addition of beta 2GP1 to patient IgG-phospholipid vesicle mixtures resulted in either independently additive inhibition by the two protein species (six cases) or potential inhibition of beta 2GP1 of the IgG inhibitory activity demonstrable in the absence of beta 2 GP1 (two cases). In addition, beta 2GP1-independent inhibition of prothrombin activation also occurred with three patient IgG preparations obtained by affinity binding to cardiolipin.

In contrast to previously reported studies, no evidence could be adduced for the activation of Hageman factor (factor XII) by platelets, whether or not these cells had been incubated with adenosine diphosphate (ADP).

An inhibitor of plasma thromboplastin antecedent (PTA, factor XI), measured in coagulant and radioimmunoassays, was detected in a 60-year-old man with carcinoma of the prostate who had no evidence of a bleeding tendency. Family studies indicated that the patient was either a homozygote or a heterozygote for hereditary factor XI deficiency. In contrast to earlier described patients with factor XI deficiency in whom inhibitors were detected, the patient was unaware of having been transfused with blood or blood products at any time before the discovery of the inhibitor. The inhibitor of factor XI in the patient's plasma appeared to be predominantly in the IgG4 fraction and to be directed at a locus on the factor XI molecule other than the active site; it did not block the amidolytic properties of activated factor XI (XIa). Rather, it appeared to block adsorption of factor XI to negatively charged surfaces. The inhibitor interfered with measurement of other components of the intrinsic pathway of thrombin formation, perhaps explaining the low titres of other coagulation factors of the intrinsic system reported in patients with strong inhibitors directed against factor XI.