Interaction of human chagasic IgG with the second extracellular loop of the human heart muscarinic acetylcholine receptor: Functional and pathological implications

Circulating antibodies from human and murine chagasic sera are able to interact with myocardium-activating neurotransmitter receptors. Here we reported the presence of autoantibodies against the second extracellular loop of the human heart muscarinic acetylcholine receptors (mAChR) in patients with Chagas' disease by using a synthetic 24-mer peptide in immunoblotting and enzyme immunoassay. Affinity-purified antipeptide IgG from chagasic patients, similar to monoclonal antihuman M-2 mAChR, recognized bands with a molecular weight corresponding to the cardiac mAChR. The binding was inhibited by the peptide, assessing the specificity of the interaction. The antipeptide autoantibody also displayed an ''agonist-like'' activity modifying the intracellular events associated with mAChR activation, i.e., decreased contractility, increased cGMP, and decreased cAMP production. All of these effects on rat atria by chagasic antipeptide autoantibodies resemble the effects of the authentic agonist and those of the total polyclonal chagasic IgG, being selectively blunted by atropine and neutralized by the synthetic peptide corresponding in amino acid sequence to the second extracellular loop of the human M-2 mAChR. A clinical relevance of these findings is demonstrated by a strong association between the existence of circulating antipeptide antoantibodies in chagasic patients and the presence of dysautonomic symptoms, making these antoantibodies a proper early marker of heart autonomic dysfunction.