The effect of phospholipids on the formation of immune complexes between autoantibodies and β2-glycoprotein I or prothrombin

In the last decennium, it became clear that antiphospholipid antibodies found in patients with antiphospholipid syndrome (APS) are in fact antibodies against lipid-bound plasma proteins. The most frequently occurring antigens are β2-glycoprotein I and prothrombin, although several other lipid-bound plasma proteins have been reported as antigen for antiphospholipid antibodies. Both proteins bind to anionic phospholipids, mainly phosphatidylserine, which becomes exposed at the surface of activated platelets, apoptotic cells, or cell-derived microparticles. The binding of β2-glycoprotein I and prothrombin to these cell surfaces or to artificial lipid vesicles with comparable amounts of anionic phospholipids is rather weak. Antiphospholipid antibodies from patients are predominantly of low affinity regarding their interaction with β2-glycoprotein I or prothrombin in solution. In the presence of a suitable phospholipid surface, however, this interaction is strongly enhanced. There is now strong evidence that formation of bivalent, trimolecular immune complexes at the lipid membrane essentially contributes to the binding of these intrinsically low affinity patient antibodies. Depending on the affinity, the epitope specificity, and the polyclonality of a particular IgG preparation, multimeric structures of lipid-bound immune complexes may form a lattice with multiple interactions on the lipid (cell) surface. It is hypothesized that the functional activity, that is, the ability of antibodies to interfere with lipid-dependent reactions, not only depends on their affinity for the antigen, but also on their ability to form multiple interconnected bivalent trimolecular complexes at the lipid (or cell) surface. It is further proposed that the rate of desorption of immune complexes may present a better indicator for the functional properties of the antibodies than the amount of adsorbed immune complexes.