Some ‘antiphospholipid antibodies’ bind to β2‐glycoprotein I in the absence of phospholipid

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Abstract
Summary. Some antiphospholipid antibodies (aPL) only bind to anionic phospholipids in the presence of a serum cofactor, β2‐glycoprotein I (β2GPI). Whether these aPL can bind to β2GPI in the absence of phospholipid is controversial. We have purified anticardiolipin antibodies (aCL) from the plasma of four patients and β2GPI from normal plasma by solid phase affinity methods. All four aCL bound to cardiolipin and phosphatidylserine in the presence of β2GPI but not in its absence. The binding of two of the antibodies to cardiolipin and phosphatidylserine at various concentrations of human β2GPI was compared with that obtained using 10% bovine serum. The two antibodies responded differently to increasing β2GPI concentrations, and binding to phosphatidylserine was relatively greater than to cardiolipin using human β2GPI. All four aCL bound to plastic plates coated with β2GPI in the absence of phospholipid, and β2GPI in the fluid phase had no effect on binding. Binding to β2GPI coated plates was increased equally when bovine serum or bovine albumin were used as the sample diluent in place of gelatine. These findings and those of others have important implications for the design of assays for antiphospholipid antibodies.