Agonistic antibodies directed at cell surface receptors and cardiovascular disease

Development of cardiomyopathy and atrial tachyarrhythmias associated with activating autoantibodies to beta-adrenergic and muscarinic receptors

A 71-year-old male with well-controlled hypertension developed atrial tachyarrhythmias in 2002 and a restrictive cardiomyopathy in 2006 to 2007. Sera from 1992, 2001, and 2006 to 2008 demonstrated activating autoantibodies against beta-adrenergic (AAbetaAR) and M2 muscarinic receptors (AAM2R). These sera have been characterized for bioactivity using in vitro assays of cardiac contractility and automaticity using a canine cardiac Purkinje fiber assay as well as protein kinase assay activation in H9c2 cells. These assays demonstrated concurrent positive betaAR and inhibitory M2R effects that were blocked by nadolol and atropine, respectively. In a canine pulmonary vein atrial sleeve preparation, sera diluted 1:100 produced atrial hyperpolarization that was blocked by atropine. Atrial tachyarrhythmias developed in 2002 in the presence of a persistent bradycardia. Serial echocardiograms demonstrated progressive diastolic dysfunction in the absence of cardiac hypertrophy between 2006 and 2007. A dual-chamber pacemaker was installed with combined betaAR (nadolol) and M2<3R (oxybutynin) blockade, resulting in marked suppression of atrial ectopy and improved diastolic function. The estimated pulmonary artery pressure decreased and exercise tolerance returned. Blood pressure has remained normal with beta-blockade. AAbetaAR and AAM2R prospectively influenced atrial and ventricular function in this patient, and specific receptor blockade was associated with improved cardiac function.

Potential relevance of alpha(1)-adrenergic receptor autoantibodies in refractory hypertension

Background

Agonistic autoantibodies directed at the α₁-adrenergic receptor (α₁-AAB) have been described in patients with hypertension. We implied earlier that α₁-AAB might have a mechanistic role and could represent a therapeutic target.

Methodology/Principal Findings

To pursue the issue, we performed clinical and basic studies. We observed that 41 of 81 patients with refractory hypertension had α₁-AAB; after immunoadsorption blood pressure was significantly reduced in these patients. Rabbits were immunized to generate α₁-adrenergic receptor antibodies (α₁-AB). Patient α₁-AAB and rabbit α₁-AB were purified using affinity chromatography and characterized both by epitope mapping and surface plasmon resonance measurements. Neonatal rat cardiomyocytes, rat vascular smooth muscle cells (VSMC), and Chinese hamster ovary cells transfected with the human α_1A-adrenergic receptor were incubated with patient α₁-AAB and rabbit α₁-AB and the activation of signal transduction pathways was investigated by Western blot, confocal laser scanning microscopy, and gene expression. We found that phospholipase A2 group IIA (PLA2-IIA) and L-type calcium channel (Cacna1c) genes were upregulated in cardiomyocytes and VSMC after stimulation with both purified antibodies. We showed that patient α₁-AAB and rabbit α₁-AB result in protein kinase C alpha activation and transient extracellular-related kinase (EKR1/2) phosphorylation. Finally, we showed that the antibodies exert acute effects on intracellular Ca²⁺ in cardiomyocytes and induce mesentery artery segment contraction.

Conclusions/Significance

Patient α₁-AAB and rabbit α₁-AB can induce signaling pathways important for hypertension and cardiac remodeling. Our data provide evidence for a potential clinical relevance for α₁-AAB in hypertensive patients, and the notion of immunity as a possible cause of hypertension.