Anti-β2-glycoprotein I (GPI) autoantibodies, annexin V binding and the anti-phospholipid syndrome

We examined the role of autoantibodies to beta(2)-GPI and prothrombin (PT) in the inhibition of annexin V binding to cardiolipin (CL) and the association with clinical manifestations of the anti-phospholipid syndrome (APS). Plasma samples from 59 patients with anti-phospholipid (aPL) antibodies were studied. Affinity purification of total IgG and IgG anti-beta(2)-GPI antibodies was performed using staphylococcal protein A and phospholipid liposomes. Annexin V binding to CL was significantly inhibited by 31/59 (53%) aPL(+) plasma samples. There was a significant association between annexin V inhibition and elevated levels of IgG anti-cardiolipin (aCL) (r = -0.62; P < 0.001), IgG anti-beta(2)-GPI (r = -0.67; P < 0.001) and a weaker association with lupus anti-coagulant (r = -0.27; P = 0.05). There was no association with other isotypes of aCL and anti-beta(2)-GPI or with anti-PT of any isotype. In patients with clinical manifestations of the APS there were higher levels of Ige aCL (median (range) Z score): 10.0 (0-17.6) versus 5.0 (0-16.1); P = 0.03), IgG anti-beta(2)-GPI (4.5 (0-11.3) versus 0.9 (0-9.7); P = 0.02) and greater inhibition of annexin V binding to CL (-3.4 (-11.4-0.6) versus -1.1 (-10.8-1.2); P = 0.22). Odds ratios for the laboratory assays and the presence of clinical manifestations of the APS varied between 0.38 and 4.16, with the highest values for IgG aCL (4.16), IgG anti-beta(2)-GPI (3.28) and annexin V inhibition (2.85). Additional experiments with affinity-purified IgG antibodies indicated that inhibition of annexin V binding was dependent upon the concentration of beta(2)-GPI and anti-beta(2)-GPI antibodies. These results indicate that inhibition of annexin V binding to procoagulant phospholipid surfaces is dependent upon anti-beta(2)-GPI antibodies and suggest a role for annexin V in the pathogenesis of the APS.