THE ANTI-BETA(2)-GLYCOPROTEIN-I ACTIVITY IN HUMAN ANTIPHOSPHOLIPID SYNDROME SERA IS DUE TO MONOREACTIVE LOW-AFFINITY AUTOANTIBODIES DIRECTED TO EPITOPES LOCATED ON NATIVE BETA(2)-CLYCOPROTEIN I AND PRESERVED DURING SPECIES EVOLUTION

To characterize the reactivity pattern of Abs directed to beta(2)-glyc oprotein I (anti-beta(2)GPI) in patients with anti-phospholipid syndro me, we have purified anti-beta(2)GPI Abs by affinity chromatography us ing the IgG fractions from sera of five different anti-phospholipid sy ndrome patients, Affinity-purified anti-beta(2)GPI were shown to be re presentative of Abs found in human sera because their activity could b e virtually abolished from the IgG preparations after repeated absorpt ions on immobilized human beta(2)GPI column, Our results show that aff inity-purified anti-beta(2)GPI: 1) do react with beta(2)GPI in the abs ence of any phospholipid, as demonstrated by the lack of phosphorus co ntaminant in the employed reagents, as well as by their comparable bin ding activity before and after extensive delipidation procedure; 2) ca n recognize beta(2)GPI regardless of its origin from different animal species; 3) are able to bind soluble beta(2)GPI with a mean K-d value of 4.65 x 10(-6) M (range 3, 4-7, 2 x 10(-6) M); 4) significantly enha nce their binding avidity when beta(2)GPI is linked to a solid support ; and 5) appear to be mainly monoreactive autoantibodies, In conclusio n, we have shown that human polyclonal anti-beta(2)GPI are low affinit y, mainly monoreactive autoantibodies directed to an epitope located o n native beta(2)GPI, preserved along the species evolution.