Immunobiochemical mechanisms and pathophysiological significance of β₁-adrenergic receptor autoantibodies in dilated cardiomyopathy

Abstract

Idiopathic dilated cardiomyopathy (IDC) is a heart muscle disease of unknown origin and the second most common reason for heart transplantation. Autoimmune processes are involved in the progression of the disease. Circulating autoantibodies against several cardiac proteins have been identified in these patients, e.g. autoantibodies against the b1-adrenergic receptor (b1-AR). The second extracellular loop of the human b1-AR (LII) has been identified as one of the immunogenic targets. The disulfide bridge between the first and the second extracellular loops plays an important role in the receptor activation and signal transduction mechanisms.The focus of this study was to investigate the immunological properties of the extracellular domains of the human b1-AR by raising antipeptide antibodies against the N-terminal, first and second extracellular loops of the receptor. By immunological, pharmacological and cellular physiological studies of these antipeptide antibodies, the second extracellular loop of the receptor was determined to be an important epitope for the receptor recognition and activation. A monoclonal antibody (mAbM16) against this domain was produced. MAbM16 showed a partial agonistic activity by its ability to induce positive chronotropic and inotropic effects on rat cardiomyocytes, effects that were similar to those shown by autoantibodies found in IDC patients. A 10-amino-acid sequence on the Loop II was identified as the specific binding site for the mAbM16. This sequence has been the predominant binding site recognized by affinity-purified autoantibodies from IDC patients.The involvement of anti-b1-AR autoantibodies in IDC as a pathogenic factor has evoked the hypothesis that the removal of these autoantibodies may have beneficial effect in the treatment of IDC. Immunoadsorption (IA) therapy has shown to result in remarkable cardiac improvement in patients with dilated cardiomyopathy (DCM). However, the therapy is nonspecific and the underlying mechanism is unknown. The present study provides evidence for the existence of physiologically active anti-b1-AR autoantibodies in IA-treated DCM patients and shown that the removal of these autoantibodies may have an acute therapeutic effect towards cardiac improvement in DCM patients. The prevalence of anti-b1-AR autoantibodies could act as an immunological marker of the DCM and, in turn, as a mean of evaluating the efficacy of IA therapy. This will unambiguously open new therapeutic approach in IDC.