[Anti-beta(2) glycoprotein I beta(2) glycoprotein I] immune complexes in patients with antiphospholipid syndrome and other autoimmune diseases

Antiphospholipid syndrome (APS) is defined by the presence of aPL antibodie s in patients with thromboembolic phenomena. Some antiphospholipid (aPL) an tibodies, such as those directed against beta(2)-glycoprotein I (beta(2)GPI ), are associated with thromboembolism, possess Lupus Anticoagulant (LA) ac tivity and recognize their target antigen only when bound to specific surfa ces or to phospholipids (PL). To ascertain whether both free and antibody-b ound beta(2)GPI circulate in APS, we set up an ELISA to detect [IgG anti-be ta(2)GPI-beta(2)GPI] immune complexes. In this system, rabbit anti-human be ta(2)GPI antibodies were adsorbed onto plastic plates, incubated with patie nt plasma, and bound complexes were detected by means of alkaline phosphata se-labeled goat anti-human IgG; each assay was stopped when positive contro ls consisting of in vitro generated immune complexes reached an Optical Den sity (OD) of 0.5 at 405 nm. Plasma from 16 patients with APS showed a mean OD405 of 0.291 (range 0.115-0.558), not statistically different from the me an obtained for 15 age- and sex-matched healthy volunteers (mean OD405=0.16 9, range 0.066-0.264). Surprisingly, levels of immune complexes in 14 patie nts with other autoimmune diseases and no circulating anti-beta(2)GPI antib odies were statistically higher (mean OD405 = 0.552, range 0.204-0.991) tha n those of healthy subjects and patients with APS. These data indicate that while autoantibodies to beta(2)GPI are mainly unbound in plasma of patient s with APS, they are complexed with their antigen in patients with other au toimmune diseases, possibly reflecting a higher binding affinity.