Anticardiolipin antibodies and lupus anticoagulants comprise separate antibody subgroups with different phospholipid binding characteristics

Summary Autoantibodies to phospholipid antigens can be characterized using solid phase immunoassays to detect anticardiolipin antibodies (ACA) or in phospholipid‐dependent clotting tests where lupus anticoagulant (LA) activity can be demonstrated. It has not been established whether each activity is due to the same or separate antibody subgroups. Plasma from two patients with high levels of both activities were used for purification of ACA and LA using sequential ion‐exchange, gel‐filtration, and anti‐Ig affinity chromatography. Plasma could be separated into fractions containing each activity in the absence of the other. In these fractions, antibodies responsible for LA activity do not bind to isolated phospholipids in solid phase immunoassays, and conversely antibodies binding in these assays (ACA) do not possess LA activity, suggesting LA are directed against a more complex lipid epitope. In addition, in one patient ACA was of IgG isotype only, whilst LA was due to IgG and IgM isotypes. In this patient, the IgG‐ACA was heterogenous with three peaks clearly separated on ion‐exchange chromatography. Affinity purified antiphospholipid antibodies have been previously prepared from a number of patients using a phospha‐tidyl‐serine column and antibodies purified in this manner possess ACA but not LA activity. Taken together, these data indicate that tests for ACA and LA define separable subgroups of phospholipid binding antibodies, thus explaining the discordance often seen between the two activities.