Surface expression of a glycolytic enzyme, alpha-enolase, recognized by autoantibodies in connective tissue disorders

In systemic autoimmune diseases, autoantibodies specific for alpha-enolase are detected more frequently in patients with active renal involvement. To analyze the properties of anti-alpha-enolase antibodies and the distribution of the enzyme in the cell, mouse monoclonal and polyclonal antibodies were obtained from mice immunized with a glutathione-S-transferase-alpha-enolase fusion protein. Anti-alpha-enolase antibodies were purified from patient sera on enolase from human kidney. Using these antibodies, the distribution of alpha-enolase in the cell was analyzed in subcellular fractions and in the cell membrane by flow cytometry and immunoprecipitation. Plasminogen binding was studied by an immunoenzymatic assay. We observed that alpha-enolase was present in the cytosol and membrane fractions obtained from kidney and U937 cells. By flow cytometry, mouse polyclonal anti-enolase antibodies, one monoclonal and 7/9 human anti-enolase antibodies bound the membrane of U937 cells. One monoclonal antibody and mouse polyclonal anti-enolase antibodies immunoprecipitated a 48-kDa molecule from surface-labeled U937 cells and this molecule was recognized by rabbit anti-enolase antibodies. Both immunization-induced antibodies and 7/9 autoantibodies from patient sera inhibited the binding of plasminogen to alpha-enolase. The results show that alpha-enolase, an autoantigen in connective tissue diseases, is a cytoplasmic enzyme which is also expressed on the cell membrane, with which it is strongly associated. Anti-alpha-enolase autoantibodies isolated from patient sera recognize the membrane-associated form of the enzyme and/or interfere with its receptor function, thus inhibiting the binding of plasminogen. Autoantibodies specific for alpha-enolase could play a pathogenic role, either by a cytopathic effect or by interfering with membrane fibrinolytic activity.