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 ß2-adrenergic receptor (ß2AR)-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 ß2AR. Surface plasmon resonance analysis revealed high binding affinity for the ß2AR ECL2 peptide. Immunoblotting and immunofluorescence demonstrated specific binding to ß2AR in H9c2 cardiomyocytes, CHO cells expressing human ß2AR, and rat aorta. C5F2 stimulated cyclic AMP production in ß2AR-transfected CHO cells and induced potent dilation of isolated rat cremaster arterioles, both of which were specifically blocked by the ß2AR-selective antagonist ICI-118551 and by the ß2AR 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.

Agonistic autoantibodies as vasodilators in orthostatic hypotension: a new mechanism.

Agonistic autoantibodies to the ß-adrenergic and muscarinic receptors are a novel investigative and therapeutic target for certain orthostatic disorders. We have identified the presence of autoantibodies to ß2-adrenergic and/or M3 muscarinic receptors by ELISA in 75% (15 of 20) of patients with significant orthostatic hypotension. Purified serum IgG from all 20 of the patients and 10 healthy control subjects were examined in a receptor-transfected cell-based cAMP assay for ß2 receptor activation and ß-arrestin assay for M3 receptor activation. There was a significant increase in IgG-induced activation of ß2 and M3 receptors in the patient group compared with controls. A dose response was observed for both IgG activation of ß2 and M3 receptors and inhibition of their activation with the nonselective ß blocker propranolol and muscarinic blocker atropine. The antibody effects on ß2 and/or M3 (via production of NO) receptor-mediated vasodilation were studied in a rat cremaster resistance arteriole assay. Infusion of IgG from patients with documented ß2 and/or M3 receptor agonistic activity produced a dose-dependent vasodilation. Sequential addition of the ß-blocker propranolol and the NO synthase inhibitor N(G)-nitro-L-arginine methyl ester partially inhibited IgG-induced vasodilation (percentage of maximal dilatory response: from 57.7±10.4 to 35.3±4.6 and 24.3±5.8, respectively; P<0.01; n=3), indicating that antibody activation of vascular ß2 and/or M3 receptors may contribute to systemic vasodilation. These data support the concept that circulating agonistic autoantibodies serve as vasodilators and may cause or exacerbate orthostatic hypotension.

Autoantibody activation of beta-adrenergic and muscarinic receptors contributes to an "autoimmune" orthostatic hypotension.

BACKGROUND: Orthostatic hypotension (OH) is characterized by an abnormal autonomic response to upright posture. Activating autoantibodies to ß1/2-adrenergic (AAß1/2AR) and M2/3 muscarinic receptors (AAM2/3R) produce vasodilative changes in the vasculature that may contribute to OH. METHODS: Immunoglobulin (Ig)G from 6 patients with idiopathic OH harboring autoantibodies and from 10 healthy control subjects were examined for: 1) ß1AR and M2R activity with a perfused Purkinje fiber assay and PKA assay in H9c2 cells and 2) vasodilator ß2AR and M3R activity using a pressurized cremaster resistance arteriole assay. Changes in IgG activity with and without propranolol, atropine, and L-NAME were used to estimate AAßAR, AAM2R, and AAM3R activation of their respective functions. RESULTS: All six patients had elevated enzyme-linked immunosorbent assay titers to at least one of the receptors compared with controls. ßAR-mediated contractility activity and M2R activity were increased in five of the six patients. IgG from all six patients produced a direct vasodilator effect on cremaster arterioles. ßAR and nitric oxide synthase blockade led to near normalization of IgG-induced vasodilation. CONCLUSION: AAß1/2AR and AAM2/3R are present in some patients with idiopathic OH compatible with an in vivo effect. These autoantibodies and their cardiovascular effects provide new mechanistic insights into the pathophysiology of OH.

Postural orthostatic tachycardia syndrome: the Mayo clinic experience.

OBJECTIVE: To evaluate the prevalence and pathogenetic mechanisms of postural orthostatic tachycardia syndrome (POTS). PATIENTS AND METHODS: We reviewed the medical records of patients with POTS seen at the Mayo Clinic in Rochester, Minn, from January 1, 1993, through December 31, 2003. All patients were required to have had a full autonomic reflex screen. The results of the following additional tests were evaluated: thermoregulatory sweat test, plasma catecholamine measurement, serum ganglionic (a3) acetylcholine receptor antibody detection, and 24-hour urinary sodium measurement. RESULTS: We identified 152 patients (86.8% female; mean +/- SD age, 30.2+/-10.3 years) with a mean duration of symptoms of 4.1 years. The mean orthostatic heart rate increment was 44 beats/min. Half the patients had sudomotor abnormalities (apparent on both the quantitative sudomotor axon reflex test and thermoregulatory sweat test), and 34.9% had significant adrenergic impairment, indicating that at least half of the patients had a neuropathic pattern of POTS. In 13.8% of patients, onset was subacute, and ganglionic acetylcholine receptor antibody was detected in 14.6%, suggesting an autoimmune origin in at least 1 in 7 patients. Hyperadrenergic status was documented in 29.0% of patients (standing plasma norepinephrine level 2600 pg/mL), and at least 28.9% were presumably hypovolemic (24-hour urinary sodium level <100 mEq/24h). The lack of correlation between urinary sodium and standing norepinephrine levels suggests that mechanisms other than hypovolemia accounted for the hyperadrenergic state. CONCLUSION: Our findings suggest a neuropathic basis for at least half the cases of POTS and that a substantial percentage of cases may be autoimmune. Hyperadrenergic and hypovolemic correlates are likely compensatory or exacerbating.

L-DOPS therapy for refractory orthostatic hypotension in autoimmune autonomic neuropathy.

The authors report a 46-year-old woman with antibodies to the nicotinic acetylcholine receptor (NiAchR) of the autonomic ganglia. She presented with severe orthostatic intolerance refractory to treatment with midodrine, fludrocortisone, erythropoietin, vasopressin, salt, and fluid loading. Addition of L-threo-3,4-dihidroxyphenylserine (L-DOPS) substantially improved blood pressure and orthostatic tolerance. L-DOPS may benefit patients with severe orthostatic intolerance and be particularly effective in patients with ganglionic NiAchR antibodies.

Genetic control of progressive autonomic failure: evidence for an association with an HLA antigen.

In 16 patients with progressive autonomic failure, a rare disease of unknown cause, the frequency of the HLA antigen Aw32 was 13 times more common than in healthy controls, giving a relative risk of progressive autonomic failure with Aw32 of 28.7. Such genetic predisposition to the disease might cause a defect of catecholamine metabolism In the brain or possibly affect the immune response to a virus.