A Synthetic Peptide Corresponding to the Second Extracellular Loop of the Human M2 Acetylcholine Receptor Induces Pharmacological and Morphological Changes in Cardiomyocytes by Active Immunization after 6 Months in Rabbits

We have previously shown the presence of functional autoantibodies against the second extracellular loop of the human M2 acetylcholine receptor as an autoimmune epitope in 39% of patients with idiopathic dilated cardiomyopathy (DCM). To see if this autoimmune DCM can be experimentally reproduced, a synthetic peptide corresponding to the above autoimmune epitope was used as an autoantigen to monthly immunize rabbits for 6 months. Affinity-purified antibodies were able to display the negative chronotropic effects on neonatal rat cardiomyocytes in culture. The density of the M2 acetylcholine receptors was increased in the immunized rabbits compared to controls but the receptors showed only low-affinity binding sites for the agonist carbachol. Ultrastructural changes such as moderate sarcolemma alterations, mitochondrial swelling, disruption of cristae, and formation of myelin-like structures in the myocardial cells of rabbits immunized for 6 months suggest an immune-induced cardiotoxicity. With only a limited infiltration of inflammatory cells, these results point toward a pathophysiological role of anti M2 acetylcholine receptor autoantibodies.