Autoimmune basis for postural tachycardia syndrome

Abstract

Background: Patients with postural tachycardia syndrome (POTS) have exaggerated orthostatic tachycardia often following a viral illness, suggesting autoimmunity may play a pathophysiological role in POTS. We tested the hypothesis that they harbor functional autoantibodies to adrenergic receptors (AR).

Methods and results: Fourteen POTS patients (7 each from 2 institutions) and 10 healthy subjects were examined for α1AR autoantibody-mediated contractility using a perfused rat cremaster arteriole assay. A receptor-transfected cell-based assay was used to detect the presence of β1AR and β2AR autoantibodies. Data were normalized and expressed as a percentage of baseline. The sera of all 14 POTS patients demonstrated significant arteriolar contractile activity (69±3% compared to 91±1% of baseline for healthy controls, P<0.001) when coexisting β2AR dilative activity was blocked; and this was suppressed by α1AR blockade with prazosin. POTS sera acted as a partial α1AR antagonist significantly shifting phenylephrine contractility curves to the right. All POTS sera increased β1AR activation (130±3% of baseline, P<0.01) and a subset had increased β2AR activity versus healthy subjects. POTS sera shifted isoproterenol cAMP response curves to the left, consistent with enhanced β1AR and β2AR agonist activity. Autoantibody-positive POTS sera demonstrated specific binding to β1AR, β2AR, and α1AR in transfected cells.

Conclusions: POTS patients have elevated α1AR autoantibodies exerting a partial peripheral antagonist effect resulting in a compensatory sympathoneural activation of α1AR for vasoconstriction and concurrent βAR-mediated tachycardia. Coexisting β1AR and β2AR agonistic autoantibodies facilitate this tachycardia. These findings may explain the increased standing plasma norepinephrine and excessive tachycardia observed in many POTS patients.

Keywords: adrenergic receptor; autoantibody; autonomic function; postural tachycardia syndrome.

Figure 1.

Effects of serum/IgG from POTS patients and healthy control subjects on activation of β2AR, β1AR, and M3R in receptor‐transfected CHO cells. Serum IgG (100 μg/mL) from 7 OU POTS (POU), serum (1:50) from 7 VU POTS (PVU), and serum/IgG from 10 control (CTRL) subjects were measured for activation of cAMP in cultured CHO cells expressing β2AR (A) or β1AR (C). The cAMP values are expressed as a percentage of buffer baseline. Samples from each respective institution were run simultaneously to eliminate inter‐assay variation. There was a significant increase in β2AR (B) and β1AR (D) activity in both POTS groups compared to control values. The addition of the specific β2 blocker ICI‐118551 (ICI) or β‐blocker propranolol (PROP) during the incubation period significantly suppressed the values to control levels. IgG from the 7 OU POTS and 10 control subjects were also measured for M3R activation in cultured CHO cells expressing M3R (E). The M3R‐mediated β‐arrestin recruitment levels are expressed as a percentage of buffer baseline. These samples failed to demonstrate significant M3R activation (F). AR indicates adrenergic receptors; ATR, atropine; CHO, Chinese hamster ovary; OU, Oklahoma University; POTS, postural tachycardia syndrome; VU, Vanderbilt University.

Figure 2.

Dose effects of serum and IgG from 3 POTS patients on β2AR and β1AR activation of cAMP production in receptor‐transfected CHO cells. Values are expressed as a percentage of buffer baseline. The serum (A and C) and IgG (B and D) demonstrated similar dosage effects. *P<0.05, **P<0.01 vs buffer baseline. AR indicates adrenergic receptors; CHO, Chinese hamster ovary; POTS, postural tachycardia syndrome.

Figure 3.

Effects of serum/IgG from POTS patients and healthy control subjects on basal spontaneous myogenic tone of rat cremaster arterioles. Values are expressed as % decrease in arteriolar diameter from baseline. IgG (100 μg/mL) from the 7 OU POTS patients (A) and serum (1:50) from the 7 VU POTS patients (B) caused significant vasoconstriction compared to serum/IgG from the 10 control subjects. This assay was performed after blocking β2AR activity by addition of ICI‐118551 (5 μmol/L). The addition of the α1AR blocker prazosin (PRAZ, 10 μmol/L) markedly suppressed the contractility to control levels. There was a significant dosage‐dependent vasoconstrictive effect for both IgG (C) and serum (D) from the POTS patients. *P<0.05, **P<0.01 vs baseline, n=3. AR indicates adrenergic receptors; OU, Oklahoma University; POTS, postural tachycardia syndrome; VU, Vanderbilt University.

Figure 4.

Vasoconstrictive effect for the combined POTS groups. The vasoconstriction for the combined POTS groups was significantly greater than in control subjects. This constriction was enhanced with withdrawal of β2AR‐mediated vasodilatation with ICI‐118551. Prazosin reversed this to baseline. No significant vasoconstriction was observed for the control subjects. AR indicates adrenergic receptors; POTS, postural tachycardia syndrome; PRAZ, prazosin.

Figure 5.

Allosteric effects of POTS serum/IgG on phenylephrine and isoproterenol dose response. A, Serum (1:200) from 3 POTS subjects with α1AR activating activity shifted the phenylephrine (PE) dosage response curve to the right in a cremaster vasoconstriction assay. Control serum failed to alter the curve. B, Serum (1:200) from 3 POTS subjects was incubated with prazosin (10 μmol/L, to block coexisting α1AR pressor activity) and used to construct an isoproterenol (ISO) vasodilator response curve in the cremaster arteriole assay. In contrast to the α1AR autoantibody dosage response, the β2AR activation curve was shifted significantly to the left compatible with a positive allosteric effect. C, Likewise, IgG (50 μg/mL) from 3 β2AR antibody‐positive POTS subjects significantly shifted to the left an isoproterenol activation curve of cAMP production in β2AR‐transfected CHO cells. D, IgG (50 μg/mL) from 3 POTS subjects containing β1AR autoantibodies produced a similar leftward shift of an isoproterenol activation curve in β1AR‐transfected CHO cells. *P<0.05, **P<0.01. AR indicates adrenergic receptors; CHO, Chinese hamster ovary; PRAZ, prazosin; POTS, postural tachycardia syndrome.

Figure 6.

Immunofluorescence staining of β2AR, β1AR and α1AR in transfected cells with POTS serum. CHO cells expressing human β2AR/β1AR and Chem‐1 cells expressing human α1AR were stained with POTS serum, rabbit polyclonal antibodies to β2AR, β1AR, or α1AR (positive control), or control serum and FITC (green)‐labeled secondary antibodies. Nuclei were counterstained with DAPI (blue). Both POTS serum and the positive control antibodies demonstrated strong reactivity, while the control serum did not show any significant binding. POTS serum binding was abolished by preabsorption with the second extracellular loop (ECL2) peptide of the respective receptors. All images were obtained at ×40 magnification. AR indicates adrenergic receptors; DAPI, 4′,6‐diamidino‐2‐phenylindole; FITC, fluorescein isothiocyanate; POTS, postural tachycardia syndrome.

Figure 7.

Diagram depicting the proposed effects of activating autoantibodies (AAb) on the pathophysiology of POTS. These conclusions are supported by data demonstrating the consistent presence of autoantibodies to the α1AR, the β1AR, and variably the β2AR. The differing allosteric impact on their respective receptors will lead to an exaggerated tachycardia characteristic of this common syndrome. AR indicates adrenergic receptors; BP, blood pressure; NE, norepinephrine; POTS, postural tachycardia syndrome.