Active immunization of combined β1-adrenoceptor and M 2-muscarinic receptor peptides induces cardiac hypertrophy in rabbits

Abstract

Background: The high prevalence of patients with dilated cardiomyopathy (DCM) with anti- β 1-adrenoceptor and/or anti-M 2-Muscarinic receptor autoantibodies in their sera has been observed. However, the pathophysiological role of these autoantibodies in the development of cardiomyopathy is unknown. We previously reported an experimental model of early-stage DCM-like cardiomyopathy induced by immunizing rabbits for 1 year with synthetic peptides corresponding to the sequence of the second extracellular loop of either β 1-adrenoceptor or M 2-Muscarinic receptor. Because approximately half the sera of patients with DCM that recognize one of the two receptor sequences also recognize the second sequence, a model was created in rabbits simultaneously immunized with the synthetic peptides corresponding to the second extracellular loop of the β 1-adrenoceptor and M 2-Muscarinic receptor. Methods and Results: All rabbits (n = 8) immunized with both peptides had a high titer of both anti- β 1-adrenoceptor and anti-M 2-Muscarinic receptor autoantibodies in their sera, whereas none of the sera from control rabbits injected with saline (n = 9) was positive. No significant cross-reaction with peptides other than those used for immunization was found. The weight of the hearts of immunized rabbits increased significantly. The hearts of immunized rabbits showed marked concentric left ventricular hypertrophy with mild inflammatory cell infiltration. In these rabbits, mild or moderate interstitial fibrosis was also observed. In electron micrographs, immunized rabbits showed focal myofibrillar lysis, loss of myofilament, and a marked increase in the number of mitochondria and deposition of dense granules in both sarcoplasm and myofibrils. Conversely, one of the control rabbits showed scant mononuclear cell infiltration. However, in this control rabbit, no significant alteration was found by electron microscopy. Conclusion: Our results showed the coexistence of both anti- β 1-adrenoceptor and anti-M 2-muscarinic receptor autoantibodies in the sera has pathophysiological importance, shown by their ability to induce cardiac hypertrophy in rabbits.