Atrial tachycardia provoked in the presence of activating autoantibodies to β2-adrenergic receptor in the rabbit

Association among β2-adrenergic receptor autoantibodies and proximal left anterior descending artery lesions in patients with initial ST-segment elevation myocardial infarction

BACKGROUND

β2 -adrenergic receptor autoantibody (β2 -AA) are widely present in patients with many different types of cardiovascular diseases. Proximal left anterior descending (LAD) artery lesions are associated with adverse prognostic events in patients with ST-segment elevation myocardial infarction (STEMI).

HYPOTHESIS

β2 -AA is associated with the presence of proximal LAD lesions in patients with STEMI.

METHODS

A cohort of 153 patients with STEMI who underwent primary percutaneous coronary intervention (PPCI) was enrolled in the study. Baseline characteristics were compared between the proximal LAD group (n = 62) and the nonproximal LAD group (n = 91). Admission serum of patients was collected to detect the level of β2 -AA. Data for echocardiogram within 24 hours after PPCI and at the 6-month follow-up were recorded.

RESULTS

The optical density values and positive rates of β2 -AA in the proximal LAD group were higher than those in the nonproximal LAD group (p < 0.05). β2 -AA positively correlated with high sensitivity C-reactive protein and peak N-terminal pro-B type natriuretic peptide levels in the proximal LAD group, but those were not relevant in the nonproximal LAD group. Multivariate logistic regression analysis revealed that high β2 -AA levels was independently associated with the presence of proximal LAD lesions in patients with STEMI. Furthermore, a receiver operating characteristic curve was used to show the efficiency of β2 -AA levels to detect proximal LAD lesions, and the AUC of the β2-AA OD value was 0.658 (95% confidence interval 0.568-0.749; p = 0.001).

CONCLUSIONS

The STEMI patients with high β2 -AA levels had a greater possibility having proximal LAD lesions.

Autoantibodies in Atrial Fibrillation-State of the Art

Atrial fibrillation (AF) is the most common type of cardiac arrhythmia. To date, a lot of research has been conducted to investigate the underlying mechanisms of this disease at both molecular and cellular levels. There is increasing evidence suggesting that autoimmunity is an important factor in the initiation and perpetuation of AF. Autoantibodies are thought to play a pivotal role in the regulation of heart rhythm and the conduction system and, therefore, are associated with AF development. In this review, we have summarized current knowledge concerning the role of autoantibodies in AF development as well as their prognostic and predictive value in this disease. The establishment of the autoantibody profile of separate AF patient groups may appear to be crucial in terms of developing novel treatment approaches for those patients; however, the exact role of various autoantibodies in AF is still a matter of ongoing debate.

PAGln, an Atrial Fibrillation-Linked Gut Microbial Metabolite, Acts as a Promoter of Atrial Myocyte Injury

Phenylacetylglutamine (PAGln), a gut microbiota (GM)-derived metabolite, is associated with cardiovascular disease. Studies have shown that disordered GM participated in the progression of atrial fibrillation (AF), but the relationship between PAGln and AF is unclear. This study investigated the characteristics of PAGln in AF patients and its impact on atrial myocytes. Based on our previous metagenomic data, the relative abundance of porA, a critical bacterial enzyme for PAGln synthesis, exhibited an increased tendency in AF. In an independent cohort consisting of 42 controls without AF and 92 AF patients, plasma PAGln levels were higher in AF patients than in controls (p < 0.001) by immunoassay. Notably, PAGln exerted a predictive potential of AF with an AUC of 0.774 (p < 0.001), and a predictive model constructed based on the PAGln and Taiwan AF score further improved the predictive potential. Furthermore, a positive correlation was determined between PAGln and LA diameter. Subsequently, the effect of PAGln intervention was examined on HL-1 cells in vitro, revealing that PAGln increased apoptosis, reactive oxygen species (ROS) production, CaMKII and RyR2 activation and decreased cell viability. In conclusion, increased PAGln was associated with AF, and PAGln might contribute to the AF pathogenesis by promoting oxidative stress and apoptosis in atrial myocytes.

M2 muscarinic autoantibodies and thyroid hormone promote susceptibility to atrial fibrillation and sinus tachycardia in an autoimmune rabbit model

NEW FINDINGS

What is the central question of this study? Do autoantibodies to the M2 muscarinic receptor (M2R-AAbs) have the potential to facilitate specific sustained tachyarrhythmias in the presence of thyroxine (T4 ) in rabbits? What is the main finding and its importance? The M2R-AAb and T4 jointly destabilized the electrophysiological properties, thus promoting the occurrence of atrial and sinus tachyarrhythmias in rabbits. These findings provide a practical basis for understanding the pathophysiological role of M2R-AAb alone and with T4 in arrhythmia induction and might provide an innovative option for treatment of Graves' disease with rhythm disturbance.

ABSTRACT

Activating autoantibodies toward the β1/2 -adrenergic receptors (β1/2AR-AAbs) and M2 muscarinic receptor (M2R-AAbs) are present in a high proportion of patients with Graves' disease. We previously demonstrated that β1/2AR-AAbs with or without the presence of M2R-AAbs in combination with excessive thyroxine (T4 ) increased the induction of sustained tachyarrhythmias in an autoimmune rabbit model. However, the separate role of M2R-AAbs and their interaction with T4 are not clear. The aim of this study was to investigate the impact of M2R-AAbs and T4 on the induction of cardiac arrhythmias in a similar rabbit model. Ten New Zealand White rabbits were randomly divided into two groups. In group A (n = 6), the rabbits were immunized with the second extracellular loop peptide of M2R and subjected to 2 weeks of T4 treatment. In group B (n = 4), the rabbits were treated only with T4 for 2 weeks. After induction of general anaesthesia, rabbits were subjected to an electrophysiological study at 0 (pre-immune), 6 (post-immune) and 8 weeks (post-immune+T4 treatment) in group A and at 0 (baseline) and 8 weeks (T4 treatment) in group B. Each rabbit served as its own control. In group A, high levels and activity of M2R-AAbs were detected in all immunized animals. Thyroxine in combination with immunization significantly increased induction of sustained sinus tachycardia and atrial fibrillation in comparison to the pre-immune state. In group B, T4 predominantly induced sustained sinus tachycardia. This study demonstrated that M2R-AAbs and T4 jointly increased the susceptibility to both sinus and atrial tachyarrhythmias. The data supported the pathophysiological role of M2R-AAbs in hyperthyroidism-associated supraventricular tachyarrhythmias.

The Role of Autoantibodies in Arrhythmogenesis

PURPOSE OF REVIEW

The role of autoantibodies in arrhythmogenesis has been the subject of research in recent times. This review focuses on the rapidly expanding field of autoantibody-mediated cardiac arrhythmias.

RECENT FINDINGS

Since the discovery of cardiac autoantibodies more than three decades ago, a great deal of effort has been devoted to understanding their contribution to arrhythmias. Different cardiac receptors and ion channels were identified as targets for autoantibodies, the binding of which either initiates a signaling cascade or serves as a biomarker of underlying remodeling process. Consequently, the wide spectrum of heart rhythm disturbances may emerge, ranging from atrial to ventricular arrhythmias as well as conduction diseases, irrespective of concomitant structural heart disease or manifest autoimmune disorder. The time has come to acknowledge autoimmune cardiac arrhythmias as a distinct disease entity. Establishing the autoantibody profile of patients will help to develop novel treatment approaches for patients.

Adrenergic Autoantibody-Induced Postural Tachycardia Syndrome in Rabbits

Background Previous studies have demonstrated that functional autoantibodies to adrenergic receptors may be involved in the pathogenesis of postural tachycardia syndrome. The objective of this study was to examine the impact of these autoantibodies on cardiovascular responses to postural changes and adrenergic orthosteric ligand infusions in immunized rabbits. Methods and Results Eight New Zealand white rabbits were coimmunized with peptides from the α1-adrenergic receptor and β1-adrenergic receptor (β1AR). Tilt test and separate adrenergic agonist infusion studies were performed on conscious animals before and after immunization and subsequent treatment with epitope-mimetic peptide inhibitors. At 6 weeks after immunization, there was a greater percent increase in heart rate upon tilting compared with preimmune baseline. No significant difference in blood pressure response to tilting was observed. The heart rate response to infusion of the β-adrenoceptor agonist isoproterenol was significantly enhanced in immunized animals, suggesting a positive allosteric effect of β1AR antibodies. In contrast, the blood pressure response to infusion of the α1-adrenergic receptor agonist phenylephrine was attenuated in immunized animals, indicating a negative allosteric effect of α1-adrenergic receptor antibodies. Injections of antibody-neutralizing peptides suppressed the postural tachycardia and reversed the altered heart rate and blood pressure responses to orthosteric ligand infusions in immunized animals at 6 and 30 weeks. Antibody production and suppression were confirmed with in vitro bioassays. Conclusions The differential allosteric effect of α1-adrenergic receptor and β1AR autoantibodies would lead to a hyperadrenergic state and overstimulation of cardiac β1AR. These data support evidence for an autoimmune basis for postural tachycardia syndrome.

β1-Adrenergic and M2 Muscarinic Autoantibodies and Thyroid Hormone Facilitate Induction of Atrial Fibrillation in Male Rabbits

Activating autoantibodies to the β1-adrenergic and M2 muscarinic receptors are present in a very high percentage of patients with Graves' disease and atrial fibrillation (AF). The objective of this study was to develop a reproducible animal model and thereby to examine the impact of these endocrine-like autoantibodies alone and with thyroid hormone on induction of thyroid-associated atrial tachyarrhythmias. Five New Zealand white rabbits were coimmunized with peptides from the second extracellular loops of the β1-adrenergic and M2 muscarinic receptors to produce both sympathomimetic and parasympathomimetic antibodies. A catheter-based electrophysiological study was performed on anesthetized rabbits before and after immunization and subsequent treatment with thyroid hormone. Antibody expression facilitated the induction of sustained sinus, junctional and atrial tachycardias, but not AF. Addition of excessive thyroid hormone resulted in induced sustained AF in all animals. AF induction was blocked acutely by the neutralization of these antibodies with immunogenic peptides despite continued hyperthyroidism. The measured atrial effective refractory period as one parameter of AF propensity shortened significantly after immunization and was acutely reversed by peptide neutralization. No further decrease in the effective refractory period was observed after the addition of thyroid hormone, suggesting other cardiac effects of thyroid hormone may contribute to its role in AF induction. This study demonstrates autonomic autoantibodies and thyroid hormone potentiate the vulnerability of the heart to AF, which can be reversed by decoy peptide therapy. These data help fulfill Witebsky's postulates for an increased autoimmune/endocrine basis for Graves' hyperthyroidism and AF.

A peptidomimetic inhibitor suppresses the inducibility of β1-adrenergic autoantibody-mediated cardiac arrhythmias in the rabbit

PURPOSE

Previous studies demonstrated that burst pacing and subthreshold infusion of acetylcholine in β1-adrenergic receptor (β1AR)-immunized rabbits induced sustained sinus tachycardia. The aim of this study was to examine the anti-arrhythmogenic effect of a newly designed retro-inverso (RI) peptidomimetic inhibitor that specifically targets the β1AR antibodies in the rabbit.

METHODS

Six New Zealand white rabbits were immunized with a β1AR second extracellular loop peptide to produce sympathomimetic β1AR antibodies. A catheter-based electrophysiological study was performed on anesthetized rabbits before and after immunization and subsequent treatment with the RI peptide inhibitor. Each rabbit served as its own control.

RESULTS

No sustained arrhythmias were induced at preimmune baseline. At 6 weeks after immunization, there was a marked increase in induced sustained tachyarrhythmias, predominantly sinus tachycardia, which was largely suppressed by the RI peptide. The atrial effective refractory period was shortened significantly in immunized rabbits compared to their preimmune state. The RI peptide reversed and prolonged this shortening. β1AR antibody levels were negatively correlated with the atrial effective refractory period. Postimmune sera-induced β1AR activation in transfected cells in vitro was also blocked by the RI peptide.

CONCLUSIONS

β1AR-activating autoantibodies are associated with reduction of the atrial effective refractory period and facilitate arrhythmia induction in this model. The RI peptide reversal may have important therapeutic implications in subjects who harbor these autoantibodies.

Activating autoantibodies to the β1/2-adrenergic and M2 muscarinic receptors associate with atrial tachyarrhythmias in patients with hyperthyroidism

We have previously demonstrated that activating autoantibodies to β1-adrenergic receptor (β1AR) and M2 muscarinic receptor (M2R) facilitate atrial fibrillation (AF) in patients with Graves' disease (GD). The objectives of this expanded study were to examine the prevalence of β1AR, β2AR, and M2R autoantibodies in hyperthyroidism subjects. Sera from 81 patients including 31 with GD and AF, 36 with GD and sinus rhythm, 9 with toxic multinodular goiter, 5 with subacute thyroiditis, and 10 control subjects were examined for these autoantibodies by ELISA. Sera from 20 ELISA-positive GD subjects, 10 with AF and 10 with sinus rhythm, were assayed for autoantibody bioactivity using cell-based bioassays. In patients with GD and AF, 45, 65, and 77 % were ELISA positive for β1AR, M2R, and β2AR autoantibodies, respectively. In patients with GD and sinus rhythm, 17, 39, and 75 % were ELISA positive for β1AR, M2R, and β2AR autoantibodies, respectively. β1AR and M2R autoantibodies were co-present in 39 % of patients with GD and AF compared to 14 % in GD with sinus rhythm (p = 0.026). Patients with toxic multinodular goiter or subacute thyroiditis had a low prevalence of autoantibodies. The mean β1AR and M2R autoantibody activity was elevated in both GD groups but higher in those with AF than those with sinus rhythm. β2AR autoantibody activity was also increased in both groups. In conclusion, β1AR, β2AR, and M2R autoantibodies were elevated in GD. β1AR and M2R autoantibodies appear to be related to concurrent AF, while β2AR autoantibodies were equally prevalent in those with a sinus tachycardia and those with AF.

Inflammation and the pathogenesis of atrial fibrillation

Atrial fibrillation (AF) is the most common cardiac arrhythmia. However, the development of preventative therapies for AF has been disappointing. The infiltration of immune cells and proteins that mediate the inflammatory response in cardiac tissue and circulatory processes is associated with AF. Furthermore, the presence of inflammation in the heart or systemic circulation can predict the onset of AF and recurrence in the general population, as well as in patients after cardiac surgery, cardioversion, and catheter ablation. Mediators of the inflammatory response can alter atrial electrophysiology and structural substrates, thereby leading to increased vulnerability to AF. Inflammation also modulates calcium homeostasis and connexins, which are associated with triggers of AF and heterogeneous atrial conduction. Myolysis, cardiomyocyte apoptosis, and the activation of fibrotic pathways via fibroblasts, transforming growth factor-β and matrix metalloproteases are also mediated by inflammatory pathways, which can all contribute to structural remodelling of the atria. The development of thromboembolism, a detrimental complication of AF, is also associated with inflammatory activity. Understanding the complex pathophysiological processes and dynamic changes of AF-associated inflammation might help to identify specific anti-inflammatory strategies for the prevention of AF.

Induced KCNQ1 autoimmunity accelerates cardiac repolarization in rabbits: potential significance in arrhythmogenesis and antiarrhythmic therapy

BACKGROUND

Autoantibodies directed against various cardiac receptors have been implicated in cardiomyopathy and heart rhythm disturbances. In a previous study among patients with dilated cardiomyopathy, autoantibodies targeting the cardiac voltage-gated KCNQ1 K(+) channel were associated with shortened corrected QT intervals (QTc). However, the electrophysiologic actions of KCNQ1 autoimmunity have not been assessed experimentally in a direct fashion.

OBJECTIVE

The purpose of this study was to investigate the cardiac electrophysiologic effects of KCNQ1 autoantibody production induced by vaccination in a rabbit model.

METHODS

Rabbits were immunized with KCNQ1 channel peptide. ECG recordings were obtained during a 1-month follow-up period. Rabbits then underwent in vivo electrophysiologic study, after which cardiomyocytes were isolated for analysis of slow delayed rectifier current (IKs) and action potential properties via patch-clamp.

RESULTS

KCNQ1-immunized rabbits exhibited shortening of QTc compared to sham-immunized controls. Reduced ventricular effective refractory periods and increased susceptibility to ventricular tachyarrhythmia induction were noted in KCNQ1-immunized rabbits upon programmed ventricular stimulation. Action potential durations were shortened in cardiomyocytes isolated from KCNQ1-immunized rabbits compared to the sham group. IKs step and tail current densities were enhanced after KCNQ1 immunization. Functional and structural changes of the heart were not observed. The potential therapeutic significance of KCNQ1 immunization was then explored in a dofetilide-induced long QT rabbit model. KCNQ1 immunization prevented dofetilide-induced QTc prolongation and attenuated long QT-related arrhythmias.

CONCLUSION

Induction of KCNQ1 autoimmunity accelerates cardiac repolarization and increases susceptibility to ventricular tachyarrhythmia induction through IKs enhancement. On the other hand, vaccination against KCNQ1 ameliorates drug-induced QTc prolongation and might be useful therapeutically to enhance repolarization reserve in long QT syndrome.

Atrial tachyarrhythmias induced by the combined effects of β1/2-adrenergic autoantibodies and thyroid hormone in the rabbit

Activating autoantibodies (AAb) to β-adrenergic receptors (βAR) are associated with atrial fibrillation in patients with Graves' disease. In the present study, we examined the interaction of thyroid hormone with β1/2AR-AAb in inducing atrial tachyarrhythmias in the rabbit. Immunization of rabbits with a β1AR or β2AR second extracellular loop peptide produced high titers of β1AR-AAb or β2AR-AAb. Thyroid hormone in combination with β1AR-AAb or β2AR-AAb induced a significant number of sustained sinus tachycardia and atrial tachycardia, respectively. Both combinations resulted in significantly increased inductions of sustained arrhythmias compared to AAb alone. Thyroid hormone alone induced sustained sinus and junctional tachycardia. Sera from immunized rabbits specifically bound to and activated β1AR or β2AR in transfected cells in vitro. This study demonstrates thyroid hormone qualitatively accentuates the specific arrhythmogenic action of these AAb and quantitatively enhances their rate. Our data support a dual role of AAb and thyroid hormone in Graves'-associated tachyarrhythmias.

The Propensity for Inducing Atrial Fibrillation: A Comparative Study on Old versus Young Rabbits

It is well established that atrial fibrillation (AF) is far more common in elderly humans. Autonomic activation is thought to be an operative mechanism for AF propensity. The aim of the study was to investigate the impact of age on atrial tachyarrhythmia induction in a rabbit model. Six old (aged 4-6 years) and 9 young (aged 3-4 months) New Zealand white rabbits were subjected to a catheter-based electrophysiological study. Atrial tachyarrhythmia susceptibility was tested by burst pacing before and after infusion of increasing concentrations of acetylcholine. Both young and old rabbits were in normal sinus rhythm at the beginning of the infusion/burst pacing protocol. The old rabbits had faster heart rates and a marked increase in atrial tachyarrhythmias compared to the young rabbits. Nonsustained and sustained AF events were more frequent in the old rabbits. No significant fibrosis was observed in the atria of either young or old rabbits. In conclusion, the old rabbits have a greater propensity for induction of AF. The significantly faster heart rates in the old rabbits suggest that dominant sympathetic activity may play an important role in the propensity for AF in this group.

Inducible cardiac arrhythmias caused by enhanced β1-adrenergic autoantibody expression in the rabbit

Previous studies demonstrated burst pacing and intravenous infusion of ACh induced sustained atrial tachycardia when rabbits were immunized to produce β2-adrenergic receptor (β2AR)-activating autoantibodies. The objective of this study was to examine the arrhythmogenic effect of β1-adrenergic receptor (β1AR)-activating autoantibodies in the rabbit. Eight New Zealand white rabbits were immunized with a β1AR second extracellular loop peptide to raise β1AR antibody titers. A catheter-based electrophysiological study was performed on anesthetized rabbits before and after immunization. Arrhythmia occurrence was determined in response to burst pacing before and after ACh infusion in incremental concentrations of 10 μM, 100 μM, and 1 mM. The baseline sinus heart rate before and after immunization averaged 149 ± 17 per min and 169 ± 16 per min, respectively (P < 0.05). In the preimmune studies, there were five sustained (≥10 s) arrhythmias in 32 induction attempts, which occurred in only four of eight rabbits. In the postimmune studies, there were 22 sustained arrhythmias in 32 induction attempts, which occurred in all eight rabbits (P < 0.0001 for the independent effect of immunization). Of the 22 sustained arrhythmias postimmunization, 15 were sinus tachycardia compared with only two before immunization (P < 0.01 for the independent effect of immunization). Postimmune (but not preimmune) rabbit sera demonstrated specific binding to β1AR and induced significant β1AR activation in transfected cells in vitro. No cross-reactivity with β2AR was observed. In conclusion, in contrast with rabbits with β2AR-activating autoantibodies that demonstrate predominantly atrial tachycardias, enhanced autoantibody activation of β1AR in the rabbit leads to tachyarrhythmias mainly in the form of sustained sinus tachycardia.

Implications of a vasodilatory human monoclonal autoantibody in postural hypotension

Functional autoantibodies to the autonomic receptors are increasingly recognized in the pathophysiology of cardiovascular diseases. To date, no human activating monoclonal autoantibodies to these receptors have been available. In this study, we describe for the first time a β2-adrenergic receptor (β2AR)-activating monoclonal autoantibody (C5F2) produced from the lymphocytes of a patient with idiopathic postural hypotension. C5F2, an IgG3 isotype, recognizes an epitope in the N terminus of the second extracellular loop (ECL2) of β2AR. Surface plasmon resonance analysis revealed high binding affinity for the β2AR ECL2 peptide. Immunoblotting and immunofluorescence demonstrated specific binding to β2AR in H9c2 cardiomyocytes, CHO cells expressing human β2AR, and rat aorta. C5F2 stimulated cyclic AMP production in β2AR-transfected CHO cells and induced potent dilation of isolated rat cremaster arterioles, both of which were specifically blocked by the β2AR-selective antagonist ICI-118551 and by the β2AR ECL2 peptide. This monoclonal antibody demonstrated sufficient activity to produce postural hypotension in its host. Its availability provides a unique opportunity to identify previously unrecognized causes and new pharmacological management of postural hypotension and other cardiovascular diseases.