Autoantibodies directed against labile epitopes on cell surface proteins in autoimmune disease patients: Proposal of a novel ELISA for the detection of anti-endothelial cell antibodies

This article describes a novel method for detecting anti-endothelial cell antibodies (AECAs). Sera from patients with systemic vasculitis or inflammatory conditions have been reported to contain antibodies (Abs) that bind to endothelial cells (EC), i.e., AECAs. AECAs are known to play immunogenic effects by triggering EC activation and vascular damage, but the immunopathological role of AECAs is not clear. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and Western blotting have previously been used for detecting target antigens of AECAs. However, we assumed that these methods are not appropriate for searching genuine target antigens (Ags) on cell surface, and developed a novel solubilized cell surface protein-capture ELISA (CSP-ELISA). Ags were obtained as cell surface proteins from the plasma membrane of human umbilical vein endothelial cells (HUVECs); these cell surface proteins were biotinylated, solubilized with detergent, and captured on ELISA wells coated with NeutrAvidin™ biotin binding protein (NeuAvi). AECA titers in serum from 126 autoimmune disease patients and 122 healthy donors were tested. AECAs were detected in 28 of 36 (78%) of systemic lupus erythematosus (SLE) patients; in 13 of 16 (81%) of mixed connective tissue disease (MCTD) patients; and in 5 of 9 (56%) of systemic sclerosis (SSc) patients. Relatively weak denaturation of antigens on ELISA wells caused loss of binding of these autoantibodies (autoAbs). Thus, this newly developed CSP-ELISA method enables the detection of Abs to the labile epitopes of autoantigens (autoAgs) such as membrane proteins, and this method is generally applicable to various kinds of membrane proteins and the Abs against them. We propose CSP-ELISA for measuring AECAs in serum samples for routine laboratory testing.