Enhanced parasympathetic activity in Chagas disease still stands in need of proof

The diagnostic and clinical significance of anti-muscarinic receptor autoantibodies

The role of autoimmunity in cardiovascular diseases has become one of the focal points of research studies. Autoimmune response and autoreactive autoantibodies have been found in dilated cardiomyopathy, heart failure, rheumatic fever, myocarditis, atherosclerosis, and other diseases. Autoantibodies may appear due to tissue injury and exposure of autoantigens, in addition to molecular mimicry and cross-reactivity with antigens found in infectious agents in predisposed individuals. In the early 1990s, autoantibodies reacting with the M2 muscarinic receptor were found in patients with dilated cardiomyopathy and subsequently, in patients with Chagas heart disease and arrhythmic disorders. Immunization of animals with the corresponding antigen triggered cardiac abnormalities also appearing in dilated cardiomyopathy of humans. It has been suggested that antibodies against M2 muscarinic receptors play a role in the pathogenesis of cardiac diseases and may also alter the electrophysiological properties of cardiac tissue. Herein, we review the current knowledge of antibodies against M2 muscarinic receptors and the possible use of a targeted therapy against these autoantibodies.

Comments on the article by Ribeiro et al. 'Enhanced parasympathetic activity in Chagas' disease still stands in need of proof': definitions and biases

It has been hypothesized that impaired cardiac autonomic function in chagasic subject carriers of anti-muscarinic type 2 antibodies (Anti-M2) is a consequence of tonic stimulation (enhancement) of M2 receptors, thus damping high frequency spontaneous heart rate fluctuations. However, expected reduction in baseline average heart rate response as a consequence of M2 receptors stimulation has not been confirmed, imposing a query on current hypothesis for explaining autonomic impairment in Chagas' disease. In this context, possible explanations for the lack of heart rate reduction in the setting of chronic muscarinic stimulation in Chagas' disease have been discussed, based on current knowledge. Among possible factors implicated, stands out: i) spontaneous heart rate recovery after chronic vagal stimulation, ii) simultaneous anti-M2 and anti-beta 1 adrenergic receptors antibodies environments, iii) underlying cardiac function as assessed by both mechanical and humoral indexes, iv) age, v) immune status. Rigorous experimental settings may be necessary to mechanistically demonstrate eventually the association between agonistic anti-M2 environments and reduced autonomic modulation of the heart, in both humans and animal models, associated with short term and long term singular average resting heart rate balance secondary to parasympathetic stimulation.