Autoantibodies to Vascular Heparan Sulfate Proteoglycan in Systemic Lupus Erythematosus React with Endothelial Cells and Inhibit the Formation of Thrombin-Antithrombin III Complexes

Vascular heparan sulfate proteoglycan (vHSPG) is an important functional component of the microvasculature. Previous studies have demonstrated autoimmunity to vHSPG in systemic lupus erythematosus (SLE). In the current studies, we further investigated the immunospecificity of anti-vHSPG antibodies in SLE sera by enzyme-linked immunoassay (ELISA). In direct binding assays, SLE sera contained IgG antibodies reactive with native vHSPG and with heparan sulfate (HS) glycosaminoglycan in significantly higher titers than controls. Employing purified SLE IgG in liquid-phase competitive immunoinhibition ELISAs, SLE IgG anti-HS antibodies cross-reacted with heparin and DNA, but not with other glycosaminoglycans or anionic phospholipid antigens. Immunochemical studies demonstrated that the immunodominant site on HS recognized by SLE IgG contained 2-O-sulfated uronic acid. Removal of N-sulfated and 6-O-sulfated residues primarily on N-acetylglucosamine had no effect on antigenicity, further demonstrating that nonspecific charge interactions which are the result of sulfation do not solely account for the antigenicity of HS. SLE IgG from patients with active SLE was further affinity purified on DNA-cellulose and HS-Sepharose columns for immunospecificity studies. After affinity purification of both anti-DNA and anti-HS antibodies, significant enhancement of direct binding reactivity with HS was noted. In addition, anti-DNA and anti-HS IgG antibody reacted with the cell surface of endothelial cells by a cellular ELISA (CELISA). Immunoinhibition studies of CELISA reactivity confirmed that affinity-purified SLE IgG anti-DNA anti-HS antibody were reactive with endothelial cell surface HS antigens. Furthermore, SLE IgG anti-DNA antibody reactivity with endothelial cells was not reduced by DNase treatment of the cells, but was significantly reduced by heparitinase digestion. Since HS plays an important role in the maintenance of normal anticoagulation on the endothelial cell surface by binding antithrombin III, we investigated the inhibition of heparin-accelerated thrombin-antithrombin III complex formation by SLE IgG. Purified IgG from patients with active SLE, but not from normal controls, inhibited heparin-accelerated formation of TAT complexes. These studies demonstrate the presence of IgG autoantibodies to HS in patients with SLE. Anti-HS antibodies recognize an antigenic site also present in heparin, but not other glycosaminoglycans, bind to the endothelial cell surface, and inhibit the formation of TAT complexes. SLE IgG anti-HS antibodies recognize a sulfated uronic acid epitope containing 2-O-sulfate which is important in certain functions of HS, including antithrombin III binding. Thus, anti-HS antibodies may promote a procoagulant state at the endothelial cell surface. Anti-DNA antibodies which cross-react with high avidity with anticoagulant sites on cell surface HS may represent a subpopulation of anti-DNA antibody with the capacity to cause thrombosis.