Some antiphospholipid antibodies inhibit phospholipase A₂ activity

Certain antiphospholipid antibodies, particularly those associated with arterial thrombosis, reduce vascular prostacyclin production. Studies were conducted to determine whether antibody-mediated inhibition of phospholipase A₂ accounts for this effect. In this report we present evidence that purified antiphospholipid antibodies reduce phospholipase A₂ activity toward phospholipid substrates, both in vitro and in a defined system. Purified immunoglobulin, obtained from patients at risk for thrombosis who had plasma antiphospholipid antibodies, Impaired prostacyclin generation after endothelial stimulation with thrombin or the calcium ionophore A23187. The release of arachldonate in response to A23187 was reduced in endothelial cells pretreated with antibody; the metabolism of exogenous arachidonate to prostacyclin was normal. Thrombin-induced synthesis of platelet-activating factor, which follows phospholipase A₂-mediated generation of lysophosphatidylcholine, was also inhibited in parallel with the inhibition of prostacyclin generation. Phospholipase A₂ activity was determined in a defined test system with two phosphollpases A₂. The hydrolysis of fatty acid was less in the presence of patient immunoglobulin than in buffer alone or with normal immunoglobulin. Inhibition by antibody was present at a range of phospholipase concentrations. Antiphospholipid antibodies, purified from patient serum by adsorption to and subsequent elution from immobilized cardiolipin or phosphatidylserine, also inhibited phospholipase A₂ activity. The data support our conclusions that purified antiphospholipid antibodies inhibit endothelial phospholfpase A₂ activity in response to thrombin or ionophore and that phosphatidylcholine is a common metabolic precursor of both prostacyclin and platelet-activating factor. In a defined enzyme assay, inhibition by antiphospholipid antibody of phospholipase A₂ activity does not require additional cofactors.