McNally, Tracy Jane; (1995) An investigation of the role of antiphospholipid antibodies and [beta]2 glycoprotein-I in the modulation of haemostatic reactions. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The purpose of this thesis was to study the interactions of antiphospholipid antibodies (aPAs) and [beta]2 glycoprotein-I ([beta]2GPI) and examine their effects on haemostatic reactions occurring on plasma lipids. Antiphospholipid antibodies are now well recognised to be associated with thrombosis and recurrent fetal loss and some aPAs have recently been shown to require a cofactor, [beta]2GPI, for binding to phospholipids. [beta]2GPI has demonstrated in vitro anticoagulant properties, so modulation of [beta]2GPI function may therefore result in altered haemostatic regulation. Methods for study of [beta]2GPI were first established. An enzyme linked immunosorbent assay (ELISA) was developed and compared to immunoelectrophoretic (IEP) assay, an assay for estimation of [beta]2GPI aPA cofactor activity was also developed. [beta]2GPI was purified and used for measurement of antibodies directed against [beta]2GPI ([alpha][beta]2GPI). [beta]2GPI has been shown to bind to plasma lipids and work presented in this thesis has demonstrated increased levels of [beta]2GPI in patients with hyperlipidaemia. These patients had reduced [beta]2GPI aPA cofactor activity, suggesting that[beta]2GPI is compromised. Measurement of [beta]2GPI in lipoprotein fractions, separated by density ultracentrifugation, of normal subjects and hyperlipidaemia patients demonstrated [beta]2GPI in association with HDL but none was detected in LDL or VLDL fractions. [beta]2GPI antigen, aPA cofactor activity and [beta]2GPI were measured in the following groups: primary anti phospholipid syndrome patients (PaPS); SLE patients with and without aPAs and normal healthy subjects. Elevated [beta]2GPI antigen and [alpha][beta]2GPI levels and modified [beta]2GPI function were associated with a history of thrombosis in these patients. Whole IgG and affinity purified anticardiolipin antibodies (APaCL) were isolated from the plasmas of patients with aPAs in order to examine the specific effects of these antibodies on haemostatic reactions. Factor XIIa (FXIIa) generation on VLDL was measured using a test system employing purified FXII, VLDL and an amidolytic substrate assay for FXIIa. FXIIa generation was dependent on lipoprotein lipase treatment of VLDL. [beta]2GPI caused a dose dependent inhibition of FXIIa generation but no consistent effects were demonstrated with the IgG fractions from aPA patients. Inhibition of the [beta]2GPI activity was demonstrated by some antibodies, indicating that this may contribute to the pathogenic mechanism for thrombosis in some patients with aPAs. However, the results suggest that a spectra of antibody reactivities exists in PaPS and SLE patients and as such are consistent with the results of previous studies examining their effect on haemostatic reactions.

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