Pathogenic anti-β2-glycoprotein I antibodies recognize domain I of β2-glycoprotein I only after a conformational change

Recently, we published the existence of 2 populations of anti-beta(2)-glycoprotein I (beta(2)-GPI) IgG antibodies. Type A antibodies recognize epitope G40-R43 in domain I of beta(2)-GPI and are strongly associated with thrombosis. Type B antibodies recognize other parts of beta(2)-GPI and are not associated with thrombosis. In this study we demonstrate that type A antibodies only recognize plasma-purified beta(2)-GPI when coated onto a negatively charged surface and not when coated onto a neutrally charged surface. The affinity of type B antibodies toward plasma-purified beta(2)-GPI was independent of the charge of the surface to which beta 2-GPI was coated. Type A antibodies did not recognize plasma-purified beta 2-GPI in solution, whereas they did recognize recombinant beta 2-GPI both in solution and coated onto a neutrally charged plate. When the carbohydrate chains were removed from plasma-purified beta 2-GPI, we found that type A antibodies did recognize the protein in solution. This supports the hypothesis that the difference in recognition of plasma-purified and recombinant beta 2-GPI is caused by the difference in glycosylation and that epitope G40-R43 of plasma-purified beta(2)-GPI is covered by a carbohydrate chain. Type A anti-beta(2)-GPI antibodies can only recognize this epitope when this carbohydrate chain is displaced as a result of a conformational change. This finding has major implications both for the detection of pathogenic anti-beta(2)-GPI antibodies and the comprehension of the pathophysiology of the antiphospholipid syndrome.