Autoantibodies against cardiac G-protein-coupled receptors define different populations with cardiomyopathies but not with hypertension

Frequency of autoantibodies and their associated clinical characteristics and outcomes in patients with dilated cardiomyopathy: A systematic review and meta-analysis

BACKGROUND

Dilated cardiomyopathy (DCM) is a prevalent myocardial disorder characterized by impaired cardiac function affecting either the left ventricle or both ventricles. Accumulating evidence suggests that autoimmunity represents a key mechanism implicated in its pathogenesis, as several abundant autoantibodies have been identified in patients with the condition. However, the prevalence of these antibodies (Abs) in patients with DCM compared to that in both healthy controls (HCs) and those with ischemic cardiomyopathy (ICM), as well as their potential association with DCM, remains unclear. This study aimed to elucidate the prevalence of certain autoantibodies in patients with DCM compared to that in HCs and patients with ICM, as well as to evaluate their correlation with clinical characteristics and outcomes.

METHODS

A comprehensive literature search of the PubMed, Web of Science, EMBASE, the Cochrane Library, and Scopus was conducted up to March 26, 2024, and any article that fulfilled our inclusion criteria was reviewed. A meta-analysis was then conducted, using both random- and fixed-effects models.

RESULTS

A total of 38 studies met the inclusion criteria and were pulled for this analysis. Significantly higher prevalence rates of autoantibodies targeting the anti-β1 adrenergic receptor (β1-AR; odds ratio [OR] = 4.96, p = 0.000), M2 muscarinic receptor (M2-R; OR = 4.07, p = 0.000), adenine nucleotide translocator (ANT; OR = 21.18, p = 0.001) and myosin (OR = 12.26, p = 0.000) were observed in patients with DCM compared to HCs. Moreover, patients with DCM exhibited a significantly higher frequency of positive ANT Abs (OR = 34.52, p = 0.005) compared to those with ICM. Regarding clinical characteristics and outcomes, seropositivity for β1-AR Abs was found to be significantly correlated with New York Heart Association (NYHA) classification (standardized mean difference [SMD] = 0.78, p = 0.006), left ventricular ejection fraction (LVEF) (SMD = -1.38, p = 0.001), and heart rate (HR) (SMD = 1.505, p = 0.022). Seropositivity for anti‑calcium channel Abs was significantly associated with sudden cardiac death (SCD; OR = 3.17, p = 0.000) and all-cause mortality (OR = 2.06, p = 0.008), while anti-troponin I (TnI) Abs were associated with atrial fibrillation (OR = 0.21, p = 0.042). In terms of Ab prevalence rates, significant heterogeneity in the frequency of anti-β1-AR Abs between studies investigating DCM and ICM may be partially explained by the detection methods used and the mean ages of the patients. Meta-regression analysis suggested that the patients' ages may partially explain the observed heterogeneity between studies regarding β1-AR Ab seropositivity and HR. However, the heterogeneity observed in the studies comparing the prevalences of Abs in patients with DCM vs HCs and ICM, as well as their associated clinical characteristics, could not be explained by subgroup analyses or demographic factors such as age and sex-nor by cardiac function.

CONCLUSIONS

Patients with DCM are more likely to have elevated levels of anti- β1-AR, M2-R, ANT and myosin autoantibodies compared to HCs, as well as higher levels of ANT Abs compared to patients with ICM. Anti-β1-AR, calcium channel, and TnI Abs may play an essential role in DCM severity and poor prognosis. This study represents the first comprehensive meta-analysis regarding autoantibody prevalence and DCM and may thus potentially guide the clinical management of such patients. However, further research is warranted to evaluate the accuracy of this presumed role of autoantibodies in DCM, owing to the small number of the studies included and their high degree of heterogeneity.

The knowns and unknowns of cardiac autoimmunity in viral myocarditis

Myocarditis can result from various infectious and non-infectious causes that can lead to dilated cardiomyopathy (DCM) and heart failure. Among the infectious causes, viruses are commonly suspected. But the challenge is our inability to demonstrate infectious viral particles during clinical presentations, partly because by that point, the viruses would have damaged the tissues and be cleared by the immune system. Therefore, viral signatures such as viral nucleic acids and virus-reactive antibodies may be the only readouts pointing to viruses as potential primary triggers of DCM. Thus, it becomes hard to explain persistent inflammatory infiltrates that might occur in individuals affected with chronic myocarditis/DCM manifesting myocardial dysfunctions. In these circumstances, autoimmunity is suspected, and antibodies to various autoantigens have been demonstrated, suggesting that immune therapies to suppress the autoimmune responses may be necessary. From this perspective, we endeavoured to determine whether or not the known viral causes are associated with development of autoimmune responses to cardiac antigens that include both cardiotropic and non-cardiotropic viruses. If so, what their nature and significance are in developing chronic myocarditis resulting from viruses as primary triggers.

The Time Has Come to Explore Plasma Biomarkers in Genetic Cardiomyopathies

For patients with hypertrophic cardiomyopathy (HCM), dilated cardiomyopathy (DCM) or arrhythmogenic cardiomyopathy (ACM), screening for pathogenic variants has become standard clinical practice. Genetic cascade screening also allows the identification of relatives that carry the same mutation as the proband, but disease onset and severity in mutation carriers often remains uncertain. Early detection of disease onset may allow timely treatment before irreversible changes are present. Although plasma biomarkers may aid in the prediction of disease onset, monitoring relies predominantly on identifying early clinical symptoms, on imaging techniques like echocardiography (Echo) and cardiac magnetic resonance imaging (CMR), and on (ambulatory) electrocardiography (electrocardiograms (ECGs)). In contrast to most other cardiac diseases, which are explained by a combination of risk factors and comorbidities, genetic cardiomyopathies have a clear primary genetically defined cardiac background. Cardiomyopathy cohorts could therefore have excellent value in biomarker studies and in distinguishing biomarkers related to the primary cardiac disease from those related to extracardiac, secondary organ dysfunction. Despite this advantage, biomarker investigations in cardiomyopathies are still limited, most likely due to the limited number of carriers in the past. Here, we discuss not only the potential use of established plasma biomarkers, including natriuretic peptides and troponins, but also the use of novel biomarkers, such as cardiac autoantibodies in genetic cardiomyopathy, and discuss how we can gauge biomarker studies in cardiomyopathy cohorts for heart failure at large.

β1-adrenergic and Muscarinic Acetylcholine Type 2 Receptor Antibodies are Increased in Graves' Hyperthyroidism and Decrease During Antithyroid Therapy

OBJECTIVE

To determine the association between autoantibodies to G-protein-coupled receptors with effect on the cardiovascular system and the cardiac biomarker N-terminal pro-brain natriuretic peptide reflecting heart function in Graves' disease.

DESIGN AND METHODS

Sixty premenopausal women with Graves' disease were analyzed for IgG autoantibodies against β₁-adrenergic, muscarinic acetylcholine type 2 and angiotensin II type 1 receptors using enzyme-linked immunosorbent assays based on cell membranes overexpressing receptors in their native conformations. N-terminal pro-brain natriuretic peptide and heart symptoms were analyzed in hyperthyroidism and after 7.5 months of antithyroid treatment. Matched thyroid healthy controls were also assessed.

RESULTS

Serum levels of antibodies against the β₁-adrenergic and the muscarinic acetylcholine type 2 receptors were higher in hyperthyroid patients than in controls (median β₁-adrenergic receptor antibodies 1.9 [IQR 1.3-2.7] vs. 1.1 [0.8-1.7] μg/mL, P<0.0001; muscarinic acetylcholine type 2 receptor 20.5 [14.0-38.3] vs. 6.0 [3.2-9.9] U/mL, P<0.0001). These antibodies decreased in euthyroidism (P<0.01), but were still higher than in controls (P<0.01). Angiotensin II type 1 receptor levels did not differ. N-terminal pro-brain natriuretic peptide was higher in hyperthyroidism (240 [134-372] vs. <35 [<35-67] ng/L, P<0.0001), normalized after treatment and did not correlate with autoantibodies.

CONCLUSION

Autoantibodies against the β₁-adrenergic and the muscarinic acetylcholine type 2 receptors were increased in Graves' patients, decreased with treatment, but did not correlate with cardiac function. However, an autoimmune effect on the heart cannot be excluded in subpopulations, as the functional properties of the analyzed antibodies remain to be determined.

β1-Adrenoceptor antibodies induce PPCM via inhibition of PGC-1α related pathway

OBJECTIVES

Peripartum cardiomyopathy (PPCM) is a pregnancy-associated and life-threatening cardiac disease. However, the causes and pathogenesis are not fully understood. Accumulating studies show that cardiomyopathy often appears to be associated with elevated levels of β1-adrenoceptor (β1AR) antibodies, indicating a possible involvement of β1AR antibodies in the development of PPCM.

DESIGN

We injected the antigen peptide segment of the β1AR into the postpartum Wistar rats to make the immune models and their cardiac function was detected by echocardiography. Also, the concentration of β1AR antibodies and apoptosis rate of left ventricular myocytes was tested by SA-ELISA, TUNEL, HE staining, qRT-PCR and western blot methods. Finally, the expression of peroxisome proliferator-activated receptor γ coactivator-1α (PGC-1α) and its related proteins were examined by qRT-PCR and western blot methods.

RESULTS

We found that the level of β1AR antibodies in the serum was significantly increased and the postpartum rats exhibited symptoms of PPCM after autoimmunity. Moreover, the expression of peroxisome PGC-1α, which was a master regulator of mitochondrial metabolism, and its downstream transcript vascular endothelial growth factor (VEGF), was decreased in autoimmune perinatal rats. In addition, the expression of the apoptosis factor caspase 3 as well as the apoptosis rate of left ventricular myocytes was significantly increased.

CONCLUSIONS

The results suggested that the symptoms of PPCM that appeared in autoimmune perinatal rats may be due to the increase of β1AR antibodies, which inhibited the pathway associated with peroxisome PGC-1α.

Cardiomyopathy - An approach to the autoimmune background

Autoimmunity is increasingly accepted as the origin or amplifier of various diseases. In contrast to classic autoantibodies (AABs), which induce immune responses resulting in the destruction of the affected tissue, an additional class of AABs is directed against G-protein-coupled receptors (GPCRs; GPCR-AABs). GPCR-AABs functionally affect their related GPCRs for activation of receptor mediated signal cascades. Diseases which are characterized by the presence of GPCR-AABs with evidence for disease-specific pathogenic activity could be named "functional autoantibody disease". We briefly summarize here the historical view on autoimmunity in cardiomyopathy, followed by an approach to the mechanistic autoimmunity background. Furthermore, autoantibodies with outstanding importance for cardiomyopathies as a functional autoantibody disease, such as GPCR-AABs, and mainly those directed against the beta1-adrenergic and muscarinic 2 receptor autoantibodies, are introduced. Anti-cardiac myosin and anti-cardiac troponin autoantibodies, as further potential players in autoimmune cardiomyopathy, are additionally taken into account. The basic view on the autoantibodies, their related receptor interactions and pathogenic consequences are presented. Focused specifically on GPCR-AABs, "pros and cons" of assays such as indirect assays (functional changes of cell preparations are monitored after GPCR-AAB receptor binding) and direct assays based on the ELISA technologies (GPCR epitope mimics for GPCR-AAB binding) are critically discussed. Last but not least, treatment strategies for "functional autoantibody disease", such as for GPCR-AAB removal (therapeutic plasma exchange, immunoadsorption) and in vivo GPCR-AAB attack such as intravenous IgG treatment (IVIG), B-cell depletion and GPCR-AAB binding and neutralization, are critically reflected with respect to their patient benefits.

Identification of novel antigens contributing to autoimmunity in cardiovascular diseases

In myocarditis and dilated cardiomyopathy (DCM) patients the immune system may play an important role in disease progression. In this study, we aimed to identify new antigens as a target for autoimmune response that might play a crucial role in these diseases. Therefore, a peptide-array was used to investigate antibody binding profiles in patients with autoimmune myocarditis or DCM compared to healthy controls and thus to identify disease relevant antigens. To analyze the pathogenicity of the identified antigens, an experimental autoimmune myocarditis (EAM) model was used. Hereby, 3 peptide sequences, derived from myosin-binding-protein-C (MYBPC) fast-type, RNA-binding-protein 20 (RBM20), and dystrophin, showed pathogenic effects on the myocardium of mice. In summary, 3 potentially cardiopathogenic peptides (MYBPC fast-type, RBM20, dystrophin) were identified. Thus, this study could serve as a basis for future investigations aimed at determining further antigens leading to pathogenic effects on the myocardium of DCM as well as myocarditis patients.

[Cardiovascular effects of beta1 and beta3-adrenergic receptor autoantibodies in Lewis rat]

PURPOSE

To analyze vascular reactivity changes in response to immunization protocols with antigens corresponding to the second extracellular loop of -β3 and -β1 and 3 adrenergic receptors (AR).

METHODS

Lewis rats were immunized for 3months with peptidic sequences corresponding to the second extracellular loop of β3-AR or β1 and 3-AR. Specific β3-AR antibodies were characterized by Elisa and purified using "Proteus Protein G" kit. Their functionality were tested in rabbit isolated ventricular cardiomyocytes. Aortic and mesenteric artery rings isolated from control or immunized rats were mounted in organ baths and precontracted with phenylephrine. Then, relaxant curves were established.

RESULTS

SR58611A (10nM), a preferential β3-AR agonist and purified β3-AR antibodies (25μg/mL) induced a decrease of cell shortening (-39.56±4.4% [n=11] and -18.45±3.9% [n=10] respectively) in isolated cardiomyocytes. This decrease was significantly inhibited when the cardiomyocytes were pre-incubated with the L-748337 (1μM), a selective β3-AR antagonist (P<0.05). In contrast with what was observed in rats immunized against the β1-AR, vasorelaxations induced by acetylcholine and SR58611A in both aorta and mesenteric arteries were unaltered in rats immunized against the β3-AR and β1 and 3-AR.

CONCLUSION

These results show, for the first time, that β3-AR antibodies induced a β3-AR agonist-like activity. They would not have a vascular pathogenic action but would offset the endothelial dysfunction caused by β1-AR antibodies.

Relationship between the autoantibody and expression of β3-adrenoceptor in lung and heart

BACKGROUND

Evidences suggest that β3 -adrenoceptor (β3-AR) plays an important role in heart failure (HF), although no data is reported indicating how these effects may change with the increasing age. Pulmonary congestion and edema are the major life-threatening complications associated with HF. The purpose of this study is to explore the relationship between the anti-β3-AR autoantibody and the expression of β3-AR in the lungs and heart for both aged patients and rats with HF.

METHODS

Synthetic β3-AR peptides served as the target antigens in ELISA were used to screen the anti-β3-AR autoantibody in aged patients and rats. Two aged rat models were constructed based on aortic banding and sham-operation. The expression of β3-AR mRNA and protein in the lung and heart was measured in intervention and non-intervention groups by Western blot analysis at the baseline, 5(th), 7(th), 9(th) and 11(th) week, respectively.

RESULTS

The frequency and titer of anti-β3-AR autoantibody in aged patients and rats with HF were higher than those in the control group (p<0.05). The expression of β3-AR mRNA and protein in pulmonary tissues decreased continually from the 7(th) week (p<0.05), followed by HF observed during the 9(th) week. The expression of β3-AR in myocardial tissues continued to increase after the 9(th) week (p<0.05), and the expression of both β3-AR mRNA and protein in the BRL group [HF group with BRL37344 (4-[-[2-hydroxy-(3-chlorophenyl)ethyl-amino] phenoxyacetic acid) (a β3-AR agonist) injection] was positively correlated with BRL37344 when compared with non-BRL group (HF group without BRL37344 injection) (p<0.05).

CONCLUSION

Anti-β3-AR autoantibody was detected in aged patients and rats with HF. The expression of β3-AR mRNA and protein in pulmonary tissues decreased continually, and began earlier than in the heart, but its expression in myocardial tissues increased continually and could be further promoted by β3-AR agonist.

Association between the presence of autoantibodies against adrenoreceptors and severe pre-eclampsia: a pilot study

Background

Pre-eclampsia is the leading cause of maternal and neonatal morbidity and mortality with incompletely understood etiopathogenesis. The purpose of the current study is to determine whether there is a relationship between the presence of autoantibodies against β₁, β₂ and α₁ adrenoreceptors and severe pre-eclampsia.

Methodology/Principal Findings

Synthetic peptides corresponding to amino acid sequences of the second extracellular loops of β₁, β₂ and α₁ adrenoreceptors were synthesized as antigens to test 34 patients with severe pre-eclampsia, 36 normal pregnancy women and 40 non-pregnant controls for the presence of autoantibodies using enzyme-linked immunosorbent assay. The respective frequencies of autoantibodies against β₁, β₂ and α₁ adrenoreceptors were 50.0% (17/34), 52.9% (18/34) and 55.9% (19/34) in patients with severe pre-eclampsia, 19.4% (7/36) (p = 0.011), 19.4% (7/36) (p = 0.006) and 17.6% (6/36) (p = 0.001) in normal pregnancy women and 10% (4/40), 7.5% (3/40) and 10% (4/40) (p<0.001) in non-pregnant controls. Titers of these autoantibodies were also significantly increased in patients with severe pre-eclampsia. By logistic regression analysis, the presence of these three autoantibodies significantly increased the risk of neonatal death (odds ratio, 13.5; 95% confidence interval, 1.3–141.3; p = 0.030) and long-term neonatal hospitalization (odds ratio, 5.0; 95% confidence interval, 1.3–19.1; p = 0.018). The risk of hypertension and fetal distress were also associated with the presence of these three autoantibodies.

Conclusions/Significance

This novel pilot study demonstrated for the first time that the presence of autoantibodies against β₁, β₂ and α₁ adrenoreceptors are increased in patients with severe pre-eclampsia. Pregnant women who are positive for the three autoantibodies are at increased risks of neonatal mortality and morbidity. We posit that these autoantibodies may be involved in the pathogenesis of severe pre-eclampsia.

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.

Autoantibodies in Heart Failure and Cardiac Dysfunction

Human heart failure is a disease with multifactorial causes, considerable morbidity, and high mortality. Several circulating autoantibodies, some of them being heart-specific, play a crucial role in the progression and induction of heart failure. However the precise mechanisms on how these autoantibodies perpetuate or even induce an organ specific autoimmune response are not yet fully understood. Also it is being a matter of current research to elucidate a potential pathophysiological role of the innate immune system in generating auto-reactive antibodies. In this review we will summarize the current available literature on circulating autoantibodies which are related to human heart failure. We will present clinical and animal studies that demonstrate the occurrence and pathophysiological relevance of several autoantibodies in heart failure, as well as point out biological mechanisms on molecular and cellular level. Finally the beneficial therapeutic effects of numerous clinical studies that target the humoral arm of the immune system by using either intravenous immunoglobulins and/or immunoadsorption will be critically discussed.

Transfer of immune components from rabbit autoimmune cardiomyopathy into severe combined immunodeficiency (SCID) mice induces cardiomyopathic changes

BACKGROUND

Growing evidence suggests that autoimmune mechanism plays an important role in the pathogenesis of cardiomyopathy. The purpose of this study was to investigate whether passive transfer of IgG and/or lymphocytes from rabbits with autoimmune cardiomyopathy is able to reproduce cardiomyopathic changes in severe combined immunodeficiency (SCID) mice.

METHODS AND RESULTS

SCID mice were injected intraperitoneally with IgG and/or peripheral blood lymphocytes (PBL) from either rabbits immunized with both beta1-adrenoceptor peptide and M2-muscarinic receptor peptide (beta1+M2 group) or rabbits with adjuvant (N group). Thirty five SCID mice were divided into seven groups; N-IgG, N-PBL, N-IgG & PBL, (beta1+M2)-IgG, (beta1+M2)-PBL, (beta1+M2)-IgG & PBL and control groups. Heart weight in three (beta1+M2) groups were significantly increased. All mice in three (beta1+M2) groups showed high titer of rabbit anti-beta1 adrenoceptor autoantibodies, and 4 mice in the (beta1+M2)-PBL group and 3 mice in the (beta1+M2)-IgG & PBL group showed a significant increase in titer of rabbit anti-M2-muscarinic receptor autoantibodies. Focal infiltration of inflammatory cells in the myocardium was observed in the (beta1+M2)-IgG & PBL group. In the (beta1+M2)-PBL group and (beta1+M2)-IgG & PBL group, cardiomyocyte diameters were significantly increased. Some myocytes of the (beta1+M2)-IgG & PBL group exhibited intracellular edema, clumps of Z-band and increased numbers of mitochondria by using electron microscopy.

CONCLUSION

Transfer of IgG and PBL from rabbits immunized with combined beta1 and M2 peptides was able to reproduce the early stage of cardiomyopathic changes in SCID mice.

Throat infection, neck and chest pain and cardiac response: a persistent infection-related clinical syndrome

Dizziness, chest discomfort, chest depression and dyspnea are a group of symptoms that are common complaints in clinical practice. Patients with these symptoms are usually informed that while neurosis consequent to coronary heart disease is excluded nonetheless they remain unhealthy with no rational explanation or treatment. 165 cases of these symptoms and 85 control subjects were reviewed and underwent further medical history inquiry, routine EKG test and cardiac ultrasound examination. Thirty-five patients received coronary artery angiography to exclude coronary heart disease. Serum myocardial autoantibodies against beta(1)-adrenoceptor, alpha-myosin heavy chain, M(2)-muscarinic receptor and adenine-nucleotide translocator were tested, and inflammatory cytokines and high sensitivity C-reaction protein were measured and lymphocyte subclass was assayed by flow cytometry. All patients had a complex of four symptoms or tetralogy: (1) persistent throat or upper respiratory tract infection, (2) neck pain, (3) chest pain and (4) chest depression or dyspnea, some of them with anxiety. Anti-myocardial autoantibodies (AMCAs) were present in all patients vs. 8% in controls. TNF-alpha, IL-1 and IL-6 were significantly higher in patients than in controls (P<0.01). CD3(+) and CD4-CD8(+) lymphocytes were significantly higher and CD56(+) lymphocytes lower in patients than those in controls (P<0.01). The ratio of serum pathogen antibodies positive against Coxsackie virus-B, cytomegalovirus, Mycoplasma pneumoniae and Chlamydia pneumoniae were all markedly higher in patients. These data led to identification of a persistent respiratory infection-related clinical syndrome, including persistent throat infection, neck spinal lesion, rib cartilage inflammation, symptoms of cardiac depression and dyspnea with or without anxiety.

Autoimmunological features in inflammatory cardiomyopathy

During recent years, increasing evidence has been obtained that cellular as well as humoral autoimmunity is involved in the pathogenesis of dilated cardiomyopathy (DCM). The immune system is generally activated by viral infections with the objective of virus elimination from the myocardium. However, a relevant number of patients demonstrate viral persistence and/or chronic inflammation in the myocardium. This chronic myocardial inflammation, defined by chronic inflammation, is termed "inflammatory cardiomyopathy" according to the WHO classification of cardiomyopathies. Chronic inflammation is frequently followed by the development of autoimmunity. A breakdown in the control mechanisms protecting against autoimmune reactions by both presentation of normally not accessible self-antigens and bystander- activation, induced by the pathogen, leads to the formation of autoreactive antibodies and T cells. The auto-reactive antibodies interact directly with heart tissue resulting in altered signal transduction or complement activation, whereas the T cell-mediated mechanisms include direct attack by cytotoxic T cells or indirect effects of cytotoxic cytokines released by stimulated T cells or macrophages.

Autoantibody against beta1-adrenergic receptor and left ventricular remodeling changes in response to metoprolol treatment

Autoantibodies specific for the beta(1)-adrenoceptor (beta(1)-AR) have been implicated in the pathology of congestive heart failure (CHF). We hypothesized that the presence of autoantibodies against beta(1)-AR (anti-beta(1)-AR) is associated with left ventricular (LV) remodelling in response to metoprolol. Synthetic beta(1)-AR peptides served as the target antigen in an ELISA (enzyme-linked immunosorbent assay) were used to screen the sera of 106 CHF patients. Patients were separated into positive (+) anti-beta(1)-AR or negative (-) anti-beta(1)-AR groups according to their anti-beta(1)-AR reactivity. Echocardiography (ECG) was performed at baseline and after one year of metoprolol therapy in combination with standard treatment regime for CHF, that is, digoxin, diuretics and an ACEI (angiotensin-converting enzyme inhibitor). The dose of metoprolol was doubled on a biweekly basis up to 50 mg x 2 daily (b.i.d./day) or attainment of maximum tolerated dose. Ninety-six patients completed final data analysis. Fifty-four patients with (+) anti-beta(1)-AR had greater improvements than 42 patients with (-) anti-beta(1)-AR in LVEDD (left ventricular end-diastolic dimension) (P < 0.01, from 69 +/- 0.8 to 58.0 +/- 0.5 mm vs. 69.0 +/- 0.8-63.6 +/- 0.9 mm) and LVESD (left ventricular end-systolic dimension) (P < 0.01, from 57.1 +/- 1.4 to 43.9 +/- 0.8 mm vs. 56.2 +/- 0.9-48.6 +/- 1.0 mm), and LVEF (left ventricular ejection fraction) (P < 0.01, from 35.4 +/- 1.3 to 49.8 +/- 0.6% vs. 34.4 +/- 1.0-44.3 +/- 1.1%) by metoprolol therapy in combination with standard treatment regime for one year. Of the CHF patients with (+) anti-beta(1)-AR, 65.4% responded to target metoprolol dose as compared to 21.4% of CHF patients without anti-beta(1)-AR (P < 0.01). Response to target metoprolol dose occurred more rapidly in (+) anti-beta(1)-AR than (-) anti-beta(1)-AR of CHF patients (67.5 +/- 2.4 vs. 100.8 +/- 3.0 days, P < 0.01). These results demonstrated that CHF patients with (+) anti-beta(1)-AR had greater improvements in LV remodelling and heart function by metoprolol as compared to (-) anti-beta(1)-AR patients. Moreover, patients with (+) anti-beta(1)-AR have better tolerance to metoprolol therapy than patients without anti-beta(1)-AR.

Pathological importance of anti-G-protein coupled receptor autoantibodies

Anti-cardiac receptor autoantibodies have been studied intensively last 10 years. A growing evidence, both in vitro and in vivo and both clinical and experimental studies, have supported that these anti-cardiac receptor autoantibodies, mainly anti-beta1 adrenoceptor autoantibodies, play a very important pathophysiological role, at least partly, in the development of idiopathic dilated cardiomyopathy. This concise review article will only summarize studies done in rabbits with consideration of other articles in this focused issue.

Autoantibody against cardiac beta1-adrenoceptor induces apoptosis in cultured neonatal rat cardiomyocytes

To clarify whether apoptosis is involved in the injury processes induced by autoantibody against cardiac beta1-adrenoceptor, we investigated the biological and apoptotic effects of antibodies on cultured neonatal rat cardiomyocytes. Wistar rats were immunized with peptides corresponding to the second extracellular loop of the beta1-adrenoceptor to induce the production of anti-beta1-adrenoceptor antibodies in the sera. Immunoglobulin (Ig) G in the sera was detected using synthetic antigen enzyme-linked immunosorbent assay and purified using the diethylaminoethyl cellulose ion exchange technique. Apoptosis of cardiomyocytes was evaluated using agarose gel electrophoresis and flow cytometry. Our results showed that the positive serum IgG greatly increased the beating rates of cardiomyocytes and showed an agonist-like activity. Furthermore, positive serum IgG induced cardiomyocyte apoptosis after treatment with beta1-adrenoceptor overstimulation for 48 h. The effects of monoclonal antibody against beta1-adrenoceptor were also found to be similar to those of positive serum IgG. It was suggested that the autoantibody could induce cardiomyocyte apoptosis by excessive stimulation of beta1-adrenoceptor.

Profiling humoral autoimmune repertoire of dilated cardiomyopathy (DCM) patients and development of a disease-associated protein chip

Dilated cardiomyopathy (DCM) is a myocardial disease characterized by progressive depression of myocardial contractile function and ventricular dilatation. Thirty percent of DCM patients belong to the inherited genetic form; the rest may be idiopathic, viral, autoimmune, or immune-mediated associated with a viral infection. Disturbances in humoral and cellular immunity have been described in cases of myocarditis and DCM. A number of autoantibodies against cardiac cell proteins have been identified in DCM. In this study, we have profiled the autoantibody repertoire of plasma from DCM patients against a human protein array consisting of 37,200 redundant, recombinant human proteins and performed qualitative and quantitative validation of these putative autoantigens on protein microarrays to identify novel putative DCM specific autoantigens. In addition to analyzing the whole IgG autoantibody repertoire, we have also analyzed the IgG3 antibody repertoire in the plasma samples to study the characteristics of IgG3 subclass antibodies. By combining screening of a protein expression library with protein microarray technology, we have detected 26 proteins identified by the IgG antibody repertoire and 6 proteins bound by the IgG3 subclass. Several of these autoantibodies found in plasma of DCM patients, such as the autoantibody against the Kv channel-interacting protein, are associated with heart failure.

A high-affinity monoclonal antibody with functional activity against the 5-hydroxytryptaminergic (5-HT4) receptor

Splenocytes from a BALB/c mouse immunised with a synthetic peptide corresponding to the second extracellular loop of the 5-HT4 receptor were fused with SP2/O myeloma cells to produce a monoclonal antibody. The monoclonal antibody was of the IgG2b isotype. The antibody recognised the human 5-HT4(g) (h5-HT4(g)) receptor by immunoblots and by immunofluorescence on chinese hamster ovary (CHO) cells expressing this 5-HT4 receptor isoform. Epitope mapping of the antibody suggested the recognition of a conformational epitope, encompassing the N- and C-terminal fragments of the second extracellular loop. Kinetic experiments using surface plasmon resonance showed that the antibody had a picomolar affinity for its cognate peptide. Inhibition experiments using the same methodology confirmed the specificity of the interaction. The antibody at a concentration of 500 pM competitively inhibited inverse agonist GR113808 binding and showed an inverse agonist effect on the basal activity of CHO cells expressing the 5-HT4(g) receptor. The antibody decreased the effect of 5-HT at 500 and 50 pM concentrations but it increased 5-HT-induced cAMP levels at 5 pM. The dual effect of the monoclonal antibody could be ascribed to mono- or bivalent recognition of the receptor. The antibody described here is the first example of a high-affinity modulator of the 5-HT4 receptor.

Pathogenic roles of cardiac autoantibodies in dilated cardiomyopathy

Whether autoimmunity could cause dilated cardiomyopathy (DCM) was disputed for more than half a century. Autoantibodies against various cardiac antigens have been found in the sera of patients with DCM but none of these autoantibodies has been shown to have a substantial role in the development of DCM. It was recently reported that the injection of autoantibodies against cardiac troponin I (cTnI) can induce DCM in normal mice. This observation showed that autoantibodies can cause DCM and put an end to the controversy. Clinical trials of immunoglobulin-adsorption therapy for DCM have already started in Germany and the results seem promising. Here, we discuss the recent findings and possibilities of immunoglobulin-adsorption therapy for this deadly disease.

Transfer of Rabbit Autoimmune Cardiomyopathy into Severe Combined Immunodeficiency Mice

Growing evidence suggests that the autoimmune mechanism plays an important role in the pathogenesis of dilated cardiomyopathy. The purpose of this study was to evaluate the effect on the cardiac structure and function by the transfer of immunoglobulin G (IgG) and/or lymphocytes from rabbits immunized with a synthetic peptide corresponding to the sequence of the second extracellular loop of beta1-adrenoceptor (beta peptide) into severe combined immunodeficiency (SCID) mice. CB-17 SCID mice were injected intraperitoneally with 2 mg of IgG and/or 1 x 10(7) peripheral blood lymphocytes (PBL) from either rabbits immunized with both beta1 peptide and adjuvant (beta group), and adjuvant or rabbits with adjuvant only (N group). Thirty-five SCID mice were divided into seven groups: (1) N-IgG group; (2) N-PBL group; (3) N-IgG+PBL group; (4) beta-IgG group; (5) beta-PBL group; (6) beta-IgG+PBL group; and (7) control group. Morphological, serological and endocrinological studies were performed 70 days after the transfer. Results showed that heart weight and heart weight/body weight ratio in the beta-IgG+PBL group tended to be increased as compared with those in other groups. All mice in the beta-IgG group, two in the beta-PBL group and four in the beta-IgG+PBL group showed high titer of rabbit anti-beta1-adrenoceptor antibodies. Brain natriuretic peptide in the beta-IgG+PBL group showed a significant increase as compared with those in the control group and N-IgG+PBL. Pathohistologically, focal infiltration of inflammatory cells in the myocardium was observed in one mouse of the beta-IgG+PBL group. Rabbit CD3-positive T-lymphocytes in the myocardium were observed in two mice of the beta group. In conclusion, transfer of IgG and PBL from rabbits immunized with beta1 peptide was able to induce the early stages of myocardial damage in SCID mice. These data provide direct evidence that the autoimmune mechanism is important in the pathogenesis of dilated cardiomyopathy.

Aldosteronism and peripheral blood mononuclear cell activation: a neuroendocrine-immune interface

Aldosteronism eventuates in a proinflammatory/fibrogenic vascular phenotype of the heart and systemic organs. It remains uncertain whether peripheral blood mononuclear cells (PBMCs) are activated before tissue invasion by monocytes/macrophages and lymphocytes, as is the case for responsible pathogenic mechanisms. Uninephrectomized rats treated for 4 weeks with dietary 1% NaCl and aldosterone (ALDOST, 0.75 microg/h) with or without spironolactone (Spi, 100 mg/kg per daily gavage) were compared with unoperated/untreated and uninephrectomized/salt-treated controls. Before intramural coronary vascular lesions appeared at week 4 of ALDOST, we found (1) a reduction of PBMC cytosolic free [Mg2+]i, together with intracellular Mg2+ and Ca2+ loading, whereas plasma and cardiac tissue Mg2+ were no different from controls; (2) increased H2O2 production by monocytes and lymphocytes together with upregulated PBMC gene expression of oxidative stress-inducible tyrosine phosphatase and Mn2+-superoxide dismutase and the presence of 3-nitrotyrosine in CD4+ and ED-1-positive inflammatory cells that had invaded intramural coronary arteries; (3) B-cell activation, including transcription of immunoglobulins, intracellular adhesion molecule-1, and CC and CXC chemokines and their receptors; (4) expansion of B lymphocyte subset and myosin heavy chain class II-expressing lymphocytes; and (5) autoreactivity with gene expression for antibodies to acetylcholine receptors and a downregulation of RT-6.2, which is in keeping with cell activation and associated with autoimmunity. Spi cotreatment attenuated the rise in intracellular Ca2+, the appearance of oxidative/nitrosative stress in PBMCs and invading inflammatory cells, and alterations in PBMC transcriptome. Thus, aldosteronism is associated with an activation of circulating immune cells induced by iterations in PBMC divalent cations and transduced by oxidative/nitrosative stress. ALDO receptor antagonism modulates this neuroendocrine-immune interface. The full text of this article is available online at http://www.circresaha.org.

Beta1-adrenoceptor antibodies induce apoptosis in adult isolated cardiomyocytes

beta(1)-Adrenoceptor autoantibodies are present in about 30% of patients suffering from dilated cardiomyopathy. The apoptotic effects mediated by beta(1)-adrenoceptor antibodies remain to be studied. Monoclonal antibodies were raised against a synthetic peptide corresponding to the second extracellular loop of the human beta(1)-adrenoceptor in balb/C mouse, and were characterized by enzyme immunoassay. Purified immunoglobulin G from nonimmunized animals (controls) did not influence the rate of apoptosis. beta(1)-Adrenoceptor antibodies caused a dose-related increase in apoptotic cells: annexin test (dilution 1:2: 21+/-1.1% apoptotic cells vs. 4+/-0.4% apoptotic cells in controls; p<0.01); TdT-mediated dUTP nick end labeling (TUNEL) test (dilution 1:2: 26+/-2% apoptotic cells vs. 10+/-2% apoptotic cells in controls; p<0.01). The effect of the beta(1)-adrenoceptor antibodies was blocked by the antigenic peptide and by the antagonist metoprolol (10 micromol/l). The apoptotic effect induced by isoproterenol was attenuated by the beta(1)-adrenoceptor antibody. After pre-incubation of cardiomyocytes with the protein kinase A inhibitor Rp-Adenosine-3',5'-cyclic monophosphothioate triethylamine (RpcAMPS), beta(1)-adrenoceptor antibody was not capable of inducing an increase of the rate of apoptosis. beta(1)-Adrenoceptor antibodies induced apoptosis in adult rat cardiomyocytes via the protein kinase A cascade.

Potential role of autoantibodies belonging to the immunoglobulin G-3 subclass in cardiac dysfunction among patients with dilated cardiomyopathy

BACKGROUND

Immunoadsorption capable of removing circulating autoantibodies represents an additional therapeutic approach in dilated cardiomyopathy (DCM). The role played by autoantibodies belonging to the immunoglobulin (Ig) subclass G-3 in cardiac dysfunction remains to be elucidated.

METHODS AND RESULTS

Patients with DCM (left ventricular ejection fraction <30%) participated in this case-control study. Nine patients underwent immunoadsorption with protein A (low affinity to IgG-3), and 9 patients were treated with anti-IgG, which removes all IgG subclasses. Immunoadsorption was performed in 4 courses at 1-month intervals until month 3. In the 2 groups, immunoadsorption induced comparable reduction of total IgG (>80%). IgG-3 was effectively eliminated only by anti-IgG adsorption (eg, during the first immunoadsorption course; protein A, -37+/-4%; anti-IgG, -89+/-3%; P<0.001 versus protein A). The beta1-receptor autoantibody was effectively reduced only by anti-IgG (P<0.01 versus protein A). Hemodynamics did not change in the protein A group. In the anti-IgG group during the first immunoadsorption course, cardiac index increased from 2.3+/-0.1 to 3.0+/-0.1 L x min(-1) x m(-2) (P<0.01 versus protein A). After 3 months, before the last immunoadsorption course, cardiac index was 2.2+/-0.1 L x min(-1) x m(-2) in the protein A group and 3.0+/-0.2 L x min(-1) x m(-2) in the anti-IgG group (P<0.01 versus protein A). Left ventricular ejection fraction increased only in the anti-IgG group (P<0.05 versus protein A).

CONCLUSIONS

Autoantibodies belonging to IgG-3 may play an important role in cardiac dysfunction of DCM. The removal of antibodies of the IgG-3 subclass may represent an essential mechanism of immunoadsorption in DCM.

Relationship of myocardial remodeling to the genesis of serum autoantibodies to cardiac beta(1)-adrenoceptors and muscarinic type 2 acetylcholine receptors in rats

OBJECTIVES

We sought to investigate the mechanism responsible for the occurrence of anticardiac receptor autoantibodies.

BACKGROUND

Increasing evidence suggests the involvement of autoimmune mechanisms in the pathogenesis of a number of cardiovascular diseases. Among them, the biologic, functional and pathogenic properties of anticardiac receptor antibodies have been extensively investigated. However, the mechanism responsible for the occurrence of anticardiac receptor autoantibodies remains poorly understood.

METHODS

Two rat models (aortic banding [AB] and adriamycin [ADR] groups) were constructed. Determination of cardiac function and morphology and T-lymphocyte subtypes, enzyme-linked immunosorbent assay and cardiomyocyte cultures were performed.

RESULTS

It was shown, in the AB and ADR groups, that the frequency and titer of autoantibodies to beta(1) and muscarinic type 2 receptors were increased when myocardial remodeling occurred, as evidenced by significant cardiac morphologic changes, deposition of collagen and obvious functional impairment. This suggests that cardiac remodeling itself, in two disparate models of heart failure and cardiomyopathy, was able to trigger the genesis of anticardiac receptor autoantibodies. These autoantibodies have biologic effects similar to those seen in human autoantibodies. They have also shown a characteristic self-growth, as well as a time-course decline, suggesting that a negative finding of anticardiac receptor autoantibodies in sera of patients with heart disease does not necessarily imply that there is no autoimmune reaction involved in the pathogenesis.

CONCLUSIONS

Our results demonstrated that myocardial damage was able to trigger the occurrence of an autoimmune reaction, resulting in the genesis of anticardiac receptor autoantibodies with properties similar to those seen in patients with idiopathic dilated cardiomyopathy.

[Dilated cardiomyopathy and autoimmunity: an overview of current knowledge and perspectives]

The diagnosis of idiopathic dilated cardiomyopathy is assigned to patients with left ventricular systolic dysfunction and dilatation in the absence of any other documented cause. Idiopathic dilated cardiomyopathy is presumed to have a multifactorial origin, possibly including autoimmune mechanisms. We reviewed the current state of knowledge of this topic, including a pathophysiological hypothesis postulating a relation between an autoimmune process and sympathetic over-stimulation and systolic dysfunction. The implications for therapy are considered in the light of experience with other autoimmune diseases. The results of immunosuppressant treatment and preliminary experiences with immunoadsorption are reviewed and their future perspectives are discussed.

cGMP-mediated inhibition of cardiac L-type Ca(2+) current by a monoclonal antibody against the M(2) ACh receptor

The effects of a monoclonal antibody (B8E5) directed against the second extracellular loop of the muscarinic M(2) receptor were studied on the L-type Ca(2+) currents (I(Ca,L)) of guinea pig ventricular myocytes using the whole cell patch-clamp technique. Similar to carbachol, B8E5 reduced the isoproterenol (ISO)-stimulated I(Ca,L) but did not significantly affect basal I(Ca,L). Atropine blocked the inhibitory effect of B8E5. The electrophysiological parameters of ISO-stimulated I(Ca,L) were not modified in presence of B8E5. Inhibition of I(Ca,L) by B8E5 was still observed when intracellular cAMP was either enhanced by forskolin or maintained constant by using a hydrolysis-resistant cAMP analog (8-bromoadenosine 3',5'-cyclic monophosphate) or by applying the phosphodiesterase inhibitor IBMX. The effect of B8E5 was mimicked by 8-bromoguanosine 3',5'-cyclic monophosphate, a potent stimulator of cGMP-dependent protein kinase, and prevented by a selective inhibitor of nitric oxide-sensitive guanylyl cyclase [1H-(1,2,4)oxadiazolo[4,3-a]quinoxaline-1-one]. These results indicate that the antibody B8E5 inhibits the beta-adrenergic-stimulated I(Ca,L) through activation of the M(2) muscarinic receptor and further suggest that the antibody acts not via the classical pathway of decreasing intracellular cAMP, but rather by increasing cGMP.

Beneficial effect of muscarinic-2 antagonist on dilated cardiomyopathy induced by autoimmune mechanism against muscarinic-2 receptor

We have previously shown that a peptide corresponding to the sequence of the second extracellular loop of the human muscarinic-2 (M2) receptor (M2-peptide) was able to induce an autoimmune cardiomyopathy in rabbits. In this study, we investigated the effect of M2-antagonist (otenzepad) on M2-peptide-induced cardiomyopathy in rabbits. New Zealand White rabbits were divided into four groups: 1) control group, saline injection; 2) M2-peptide group, M2-peptide injection; 3) M2-antagonist group, otenzepad (30 mg/day) orally and saline injection; and (4) M2-antagonist + M2-peptide group, otenzepad (30 mg/day) orally and M2-peptide injection. The study duration was 1 year. Saline or peptide was injected once a month. All rabbits in both the M2-peptide group and the M2-antagonist + M2-peptide group had high titers of anti-M2-autoantibodies in their sera. Rabbits in the M2-peptide group showed an increase in heart weight, wall thinning and dilatation of the right ventricle. On the contrary, rabbits in the M2-antagonist + M2-peptide group had normal heart weight and shape. All rabbits in the M2-peptide group showed multifocal degeneration and necrosis of myocardial cells with moderate infiltration of inflammatory cells, while four rabbits in the M2-antagonist + M2-peptide group showed slight infiltration of inflammatory cells with normal myocardial cells and interstitium, and another three showed no histological changes in the hearts. In conclusion, M2-antagonist protects the myocardium from injury induced by autoimmune mechanism against M2-muscarinic receptor.

beta(1)-Adrenoceptor antibodies induce positive inotropic response in isolated cardiomyocytes

beta(1)-Adrenoceptor autoantibodies are present in approximately 30% of patients suffering from dilated cardiomyopathy. The inotropic effects mediated by these antibodies remain to be studied. Monoclonal antibodies were raised against a peptide corresponding to the second extracellular loop of the human beta(1)-adrenoceptor in balb/C mouse (n=6), and were characterized by enzyme immunoassay after purification by protein A. Purified immunoglobulin G from non-immunized animals (controls) did not influence Ca(2+) transient and cell shortening of rat cardiomyocytes measured by confocal-laser-scanning-microscopy. beta(1)-adrenoceptor antibodies caused a dose-related increase in Ca(2+) transient (dilution 1:2: +35.3+/-5.1%), and in cell shortening (dilution 1:2: +40.5+/-6.3%) (P<0.01 vs. controls). The effect of the beta(1)-adrenoceptor antibodies was blocked by the antigenic peptide and by the antagonist metoprolol. In addition, beta(1)-adrenoceptor antibodies induced a dose-dependent increase of the cyclic adenosine monophosphate. The inotropic response induced by isoproterenol was attenuated by the beta(1)-adrenoceptor antibody. beta(1)-adrenoceptor antibodies as partial agonists induce a specific positive inotropic effect via the protein-kinase-A-cascade.

Beneficial effect of angiotensin-converting enzyme inhibitor on dilated cardiomyopathy induced by autoimmune mechanism against beta1-adrenoceptor

We have shown that a peptide corresponding to the sequence of the second extracellular loop of the human beta1-adrenoceptor (beta1-peptide) was able to induce an autoimmune cardiomyopathy in rabbits. In this study, we examined the effect of angiotensin-converting enzyme inhibitor (ACEI) on beta1-peptide-induced cardiomyopathy. Rabbits were divided into four groups: (1) control group (n= 6) receiving saline injection; (2) beta1-peptide group (n = 8) immunized with beta1-peptide; (3) ACEI group (n = 6), lisinopril (3 mg/day) given orally and receiving saline injection; and (4) ACEI + beta1-peptide group (n = 7), lisinopril (3 mg/day) given orally and immunized with beta1-peptide. Our results showed that, after 1 year, all rabbits in the beta1-peptide group had an increase in heart weight, wall thinning and dilatations of both ventricles as compared with rabbits in the ACEI + beta1-peptide group that had normal heart weight and shape. All rabbits in the beta1-peptide group exhibited multifocal degeneration and necrosis of myocardial cells with moderate infiltration of inflammatory cells. In the ACEI + beta1-peptide group, three rabbits showed focal degeneration and necrosis of myocardial cells accompanied by mononuclear cells. The lesions in this group were apparently less marked than those in the beta1-peptide group. In conclusion, ACEI protects the myocardium from injury induced by an autoimmune mechanism against beta1-adrenoceptor.

Induction of cardiomyopathy in severe combined immunodeficiency mice by transfer of lymphocytes from patients with idiopathic dilated cardiomyopathy

Growing evidence suggests that autoimmune mechanisms play an important role in the pathogenesis of idiopathic dilated cardiomyopathy (DCM). The aim of the study was to evaluate the effects of transfer of lymphocytes from patients with DCM into severe combined immunodeficiency (SCID) mice on the heart structure and function. Thirty CB-17 SCID (6-8 weeks old) mice were used and divided into 3 groups (n = 10). Mice were injected intraperitoneally with up to 25 x 10(6) peripheral blood lymphocytes (PBL) from either patients with DCM which contain human autoantibodies against cardiac beta1-adrenergic receptors and M2-muscarinic receptors (DCM group) or PBL from healthy controls (control-H group). Ten mice did not receive any injections and were used as baseline controls (control-N group). Echocardiography and morphological studies were performed seventy five days after the transfer. Results showed that in DCM group, left ventricle dimensions (LVD) in diastole were increased (4.2 +/- 0.1mm) as compared to both control-H group (3.8 +/- 0.1mm) and control-N group (3.6 +/- 0.1 mm) (p < 0.01). Further, there was a trend for increased LVD in systole. Fractional shortening was not different between groups. Histological evaluation revealed accumulation of human lymphocytes in the capillaries and scarce infiltration of the lymphocytes in the hearts from DCM group. Diffuse fibrosis was significant increased in DCM mice as compared to mice receiving PBL from normal subjects (2.2 +/- 0.3% vs. 0.8 +/- 0.1%, p < 0.01). In conclusion, transfer of the PBL from the patients with DCM was able to induce early stage of heart dilatation in SCID mice. These data provide for the first time the direct evidence supporting that the autoimmune mechanism is important in the pathogenesis of human DCM.

Autoantibodies against the angiotensin receptor (AT1) in patients with hypertension

Sera from patients with malignant essential hypertension (n = 14), malignant secondary hypertension mainly attributable to renovascular diseases (n = 12) and renovascular diseases without malignant hypertension (n = 11) and from normotensive healthy blood donors (n = 35) were studied for the presence of autoantibodies against G-protein-coupled cardiovascular receptors. Autoantibodies against the angiotensin II receptor (AT1) were detected in 14, 33, 18 and 14% of patients with malignant essential hypertension, malignant secondary hypertension, renovascular diseases and control patients, respectively. Sensitivity of the enzyme immunoassay was assessed as 5 microg/ml IgG. Patients did not show antibodies against bradykinin (B2) or angiotensin II subtype 2 (AT2) receptors. Autoantibodies affinity-purified from positive patients localized AT receptors in Chinese hamster ovary transfected cells, and displayed a positive chronotropic effect on cultured neonatal rat cardiomyocytes. These results demonstrate the existence of autoantibodies against a functional extracellular domain of human AT1 receptors in patients with malignant hypertension, and suggest that these autoantibodies might be involved in the pathogenesis of malignant hypertension.

A monoclonal antibody directed against an autoimmune epitope on the human beta1-adrenergic receptor recognized in idiopathic dilated cardiomyopathy

A monoclonal antibody (MAb M16) was obtained by immunizing Balb/C mice with free peptide H26R, corresponding to the second extracellular loop of the human beta1-adrenergic receptor (beta1AR), against which functional autoantibodies have been detected in patients with idiopathic dilated cardiomyopathy. The MAb was found to be of IgG2b type and directed against a conformational epitope, encompassing the sequence recognized by the human autoantibodies. BIAcore measurements yielded an equilibrium constant of 6.5 X 10(7) M1 with an association rate constant (kon) of 6.5 X 10(4) M(-1) sec(-1) and a dissociation rate constant (koff) of 1.0 X 10(-3) sec(-1). It immunoprecipitated only poorly the solubilized beta1AR of Sf9 cell membranes. Functionally, the MAb was capable of not only reducing the number of the maximal binding sites to the beta1-adrenergic receptor of transfected Sf9 cell membranes, but also of displaying a positive chronotropic effect on cultured neonatal rat cardiomyocytes. These properties, which the MAb shares with the human autoantibodies, makes it an interesting tool for passive transfer studies in mice.

Presence of antibodies against the third intracellular loop of the m2 muscarinic receptor in the sera of chronic chagasic patients

Patients in the chronic phase of Chagas' disease suffer from a slowly evolving inflammatory cardiomyopathy that can lead to severe cardiac dilatation, congestive heart failure, and death. This process appears to be caused by autoimmune recognition of heart tissue by a mononuclear cell infiltrate decades after infection with the parasite Trypanosoma cruzi. Recent evidence suggests that there are circulating antibodies in chronic chagasic patients that alter the physiological behavior of the heart on binding to G-protein-coupled cardiovascular receptors, including beta1-adrenergic and m2 muscarinic receptors. A 42 kDa fusion protein was constructed that contains the central part of the third intracellular loop (i3; Arg(267)-Arg(381)) of the human m2 muscarinic receptor, linked to glutathione S-transferase. This fusion protein was overexpressed in Escherichia coli and subsequently purified by affinity chromatography. Based on Western blots, the i3 loop is specifically recognized by the sera of chronic chagasic patients who have reached advanced stages of cardiac failure (according to the Los Andes classification). Analysis of the prevalence and distribution of these antibodies shows a strong association between seropositive patients and moderate (group II) to severe (group III) heart dysfunction.

Activating beta-1-adrenoceptor antibodies are not associated with cardiomyopathies secondary to valvular or hypertensive heart disease

OBJECTIVES

We investigated whether autoantibodies against the human beta-adrenergic receptor (beta-AR) might be involved in cardiomyopathies secondary to valvular heart disease (VHD) or hypertensive heart disease (HHD).

BACKGROUND

Autoimmunity to beta-AR has been proposed as a pathogenic principle in human cardiomyopathy. Recently, by the use of intact recombinant human beta-AR, we were able to confirm the existence of functionally active anti-beta-1-AR autoantibodies in patients with dilated cardiomyopathy (26% prevalence) or ischemic cardiomyopathy (10% prevalence); however, their prevalence in other (secondary) cardiomyopathies remained to be determined.

METHODS

Immunoglobulin G (IgG) was prepared from the sera of 28 VHD and 19 HHD patients and first screened by a peptide-based enzyme-linked immunosorbent assay (antigens: aminoterminus, second extracellular loop [ECII] and carboxyterminus of human beta-1- and beta-2-AR). IgG from 108 gender- and age-matched healthy subjects served to define the threshold for positive immunoreactions. Positive sera were further screened for their ability to recognize and activate native human beta-AR situated in a cell membrane.

RESULTS

Twenty-five percent (VHD) or 11% (HHD) of the patients and 4% of the healthy controls had IgG antibodies randomly directed against all the three domains tested and both beta-AR subtypes. Only one patient with aortic valve and concomitant coronary heart disease and one healthy subject had functionally active anti-b1-AR (targeting beta-1-ECII). Moreover, one HHD patient with concomitant collagenosis had IgG that was cross-reacting with recombinant beta-AR in immunological assays but was unable to affect receptor function.

CONCLUSIONS

Autoimmune reactions against the human beta-AR do not appear to be associated with cardiomyopathies secondary to VHD or HHD.

Active immunization of combined beta1-adrenoceptor and M2-muscarinic receptor peptides induces cardiac hypertrophy in rabbits

BACKGROUND

The high prevalence of patients with dilated cardiomyopathy (DCM) with anti-beta1-adrenoceptor and/or anti-M2-muscarinic receptor autoantibodies in their sera has been observed. However, the pathophysiological role of these autoantibodies in the development of cardiomyopathy is unknown. We previously reported an experimental model of early-stage DCM-like cardiomyopathy induced by immunizing rabbits for 1 year with synthetic peptides corresponding to the sequence of the second extracellular loop of either beta1-adrenoceptor or M2-muscarinic receptor. Because approximately half the sera of patients with DCM that recognize one of the two receptor sequences also recognize the second sequence, a model was created in rabbits simultaneously immunized with the synthetic peptides corresponding to the second extracellular loop of the beta1-adrenoceptor and M2-muscarinic receptor.

METHODS AND RESULTS

All rabbits (n = 8) immunized with both peptides had a high titer of both anti-beta1-adrenoceptor and anti-M2-muscarinic receptor autoantibodies in their sera, whereas none of the sera from control rabbits injected with saline (n = 9) was positive. No significant cross-reaction with peptides other than those used for immunization was found. The weight of the hearts of immunized rabbits increased significantly. The hearts of immunized rabbits showed marked concentric left ventricular hypertrophy with mild inflammatory cell infiltration. In these rabbits, mild or moderate interstitial fibrosis was also observed. In electron micrographs, immunized rabbits showed focal myofibrillar lysis, loss of myofilament, and a marked increase in the number of mitochondria and deposition of dense granules in both sarcoplasm and myofibrils. Conversely, one of the control rabbits showed scant mononuclear cell infiltration. However, in this control rabbit, no significant alteration was found by electron microscopy.

CONCLUSION

Our results showed the coexistence of both anti-beta1-adrenoceptor and anti-M2-muscarinic receptor autoantibodies in the sera has pathophysiological importance, shown by their ability to induce cardiac hypertrophy in rabbits.

The frequency of occurrence of anti-cardiac receptor autoantibodies and their correlation with clinical manifestation in patients with hypertrophic cardiomyopathy

The purpose of this study was to investigate the frequency of occurrence of autoantibodies against G-protein coupled cardiovascular receptors and their relation to the clinical manifestation of hypertrophic cardiomyopathy (HCM). Autoantibodies against beta1-receptors, Muscarin-2-receptors, Angiotensin-II-receptor subtype 1 and alpha1-receptors were determined with ELISA in 52 patients with HCM (37 male, 15 female, mean age 55 +/- 15 years) and 40 healthy, age and sex matched controls. The clinical characterization of the HCM-patients included ECG, 24-h Holter, and echocardiography. The results showed that there is no significant difference in the frequency of a single autoantibody between HCM-patients and controls. However, if the number of patients who have autoantibodies against beta1-receptors and/or Muscarin-2-receptors were counted together, there are significantly more autoantibodies in HCM compared to controls (11 vs. 2, p = 0.035). Analysis of clinical data from this pooled group of patients showed that in patients with autoantibodies, heart rate variability (HRV), ultra low frequency (ULF) and very low frequency (VLF) were decreased (HRV by 20%, ULF by 50%, and VLF by 46%, p < 0.008) whereas the QTc-interval was increased by 8% (p < 0.02 each). The ratio of septal to posterior wall thickness was increased by 23% (p = 0.05), and the preejection period was prolonged by 46% in patients with autoantibodies (p < 0.001). These results suggest that the existence of these autoantibodies could be associated with an advanced stage or a severe manifestation of HCM.

Autoantibodies against M2 muscarinic receptors in patients with cardiomyopathy display non-desensitized agonist-like effects

Circulating autoantibodies against the human M2 muscarinic receptors have been previously shown in 38% of patients with idiopathic dilated cardiomyopathy. The functional properties of these autoantibodies are reported herein. They were able to decrease the cell beating frequency of myocytes in cultured neonatal rat heart cells in a dose-dependent manner without desensitization over a period of more than 5 hours whereas the non-specific muscarinic receptor agonist carbachol also inhibited the heart cell beating frequency but was desensitized within 1 hour. In the same cell culture, anti-M2 muscarinic receptor autoantibodies were not able to induce internalization of muscarinic receptor whereas carbachol did. These results demonstrate for the first time that anti-M2 muscarinic receptor autoantibodies from patients with idiopathic dilated cardiomyopathy have stimulatory muscarinic activity in vitro, which differ from normal muscarinic agonists by non-desensitization.

Screening of serum autoantibodies to cardiac beta1-adrenoceptors and M2-muscarinic acetylcholine receptors in 408 healthy subjects of varying ages

Autoantibodies to cardiac beta1-adrenoceptors and M2-muscarinic receptors have mainly been found in the sera of patients with idiopathic dilated cardiomyopathy (DCM). In order to elucidate the pathological significance of these autoantibodies in DCM, it is necessary to understand their characteristic distribution in a healthy population of different genders and ages. The peptides corresponding to the sequences of the second extracellular loops of the human beta1-adrenoceptor and M2-muscarinic receptors were therefore used as antigens to screen the sera of 408 healthy subjects of different ages (ranging from 0.5 to 85 years). Of 408 sera, 41 (10.0%) and 46 (11.3%) recognized the beta1-adrenoceptor and M2-muscarinic receptor peptides respectively. Of the positive sera for beta1-adrenoceptors and M2-muscarinic receptors, up to 63.4% and 56.5% had both anti-beta1-adrenoceptor and anti-M2-muscarinic receptor autoantibodies respectively. The antibody titres of the positive sera of healthy subjects were all of a low level, with a geometric mean titre of 1:42+/-1.9 for anti-beta1-adrenoceptor antibodies and 1:51+/-1.7 for anti-M2-muscarinic receptor antibodies. The frequency of occurrence of autoantibodies to both receptors in the sera of healthy subjects increased significantly with age. In conclusion, the autoantibodies to beta1-adrenoceptors and M2-muscarinic receptors in the sera of healthy subjects are characterized by a low frequency of occurrence and low titre, with the frequency of occurrence increasing with age.

Transfer of human serum IgG to nonobese diabetic Igmu null mice reveals a role for autoantibodies in the loss of secretory function of exocrine tissues in Sjögren's syndrome

The NOD (nonobese diabetic) mouse has been studied as an animal model for autoimmune insulin-dependent diabetes and Sjögren's syndrome. NOD.Igmu null mice, which lack functional B lymphocytes, develop progressive histopathologic lesions of the submandibular and lachrymal glands similar to NOD mice, but in the absence of autoimmune insulitis and diabetes. Despite the focal appearance of T cells in salivary and lachrymal tissues, NOD.Igmu null mice fail to lose secretory function as determined by stimulation of the muscarinic/cholinergic receptor by the agonist pilocarpine, suggesting a role for B cell autoantibodies in mediating exocrine dryness. Infusion of purified serum IgG or F(ab')2 fragments from parental NOD mice or human primary Sjögren's syndrome patients, but not serum IgG from healthy controls, alters stimulated saliva production, an observation consistent with antibody binding to neural receptors. Furthermore, human patient IgG fractions competitively inhibited the binding of the muscarinic receptor agonist, [3H]quinuclidinyl benzilate, to salivary gland membranes. This autoantibody activity is lost after preadsorption with intact salivary cells. These findings indicate that autoantibodies play an important part in the functional impairment of secretory processes seen in connection with the autoimmune exocrinopathy of Sjögren's syndrome.

Anti-M2 muscarinic receptor antibodies inhibit beta-adrenoceptor-mediated inotropic response in rat myocardium

The modulation of the inotropic effect by affinity-purified antibodies against a synthetic peptide corresponding to the second extracellular loop of the human muscarinic M2 receptors was studied in adult rat ventricular myocardium. These anti-muscarinic M2 receptor antibodies shifted the dose-response relationship of the beta-adrenoceptor agonist isoproterenol to higher concentrations whereas preimmune rabbit immunoglobulin G (IgG) or antibodies against the N-terminus of the beta 1-adrenoceptor had no effect. This effect of anti-muscarinic M2 receptor antibodies was fully blocked after preincubation with the antigenic peptide. No significant change of maximal inotropic response to isoproterenol was observed in the presence of anti-muscarinic M2 receptor antibodies. The anti-muscarinic M2 receptor antibodies did apparently not hamper the access of the muscarinic receptor agonist carbachol. The muscarinic receptor antagonist atropine attenuated the effect of the anti-muscarinic M2 receptor antibodies. The present study demonstrates for the first time in intact adult ventricular myocardium a specific stimulatory muscarinic activity of antibodies raised against a part of the muscarinic M2 receptor protein.

Alpha 1-adrenergic receptor antibodies in patients with primary hypertension

Autoimmune mechanisms have been proposed to play a role in the pathogenesis of primary (essential) hypertension. Autoantibodies against the alpha 1-adrenergic receptor have been described in patients with malignant and secondary hypertension. To investigate the incidence of autoantibodies against the alpha 1-adrenoceptor in patients with primary hypertension, we examined the immunoglobulin fractions of sera from 54 patients with primary hypertension and 26 normotensive control subjects for the presence of autoantibodies against the alpha 1-adrenoceptor. Sera from 24 patients (44%) and 3 subjects (12%) were positive. An epitope analysis of 16 autoantibody-positive immunoglobulin fractions revealed that in two thirds of the cases, the antibodies were directed against the first extracellular loop of the alpha 1-adrenoceptor and in one third, against the second. The autoantibodies had a positive chronotropic effect on isolated neonatal rat cardiomyocytes, an effect that was blocked by alpha 1-adrenergic antagonists. Since the functional characteristics of the autoantibodies showed no desensitization phenomena, they may play a role in elevating peripheral vascular resistance and promoting cardiac hypertrophy in patients with primary hypertension.

Effects of anti-peptide antibodies against the second extracellular loop of human M2 muscarinic acetylcholine receptors on transmembrane potentials and currents in guinea pig ventricular myocytes

The effects of anti-peptide antibodies against the second extracellular loop of human M2 muscarinic receptor on transmembrane potentials and currents in guinea pig single ventricular cells were analyzed using whole-cell patch clamp technique. These effects were compared with those of the muscarinic receptor agonists carbachol and acetylcholine. The antibodies shortened the action potential duration in a dose-dependent manner. By using a ramp or step rectangular pulse protocol, it was found that the antibodies increased the outward K+ current and decreased the inward basal ICa significantly. The reversal potential of both carbachol- and antibody-induced extra currents were close to -80 mV, being in proximity to the calculated Ek of -90 mV. A beta-adrenergic receptor agonist, isoprenaline, prolonged the action potential and increased the overshoot which could be inhibited by both antibody and carbachol. Isoprenaline increased inward ICa and outward Ik simultaneously. Both antibody and carbachol could significantly reduce the isoprenaline-stimulated ICa but not the isoprenaline-stimulated Ik. The antibody- or carbachol-induced outward K+ current and the depressant effects of antibody and carbachol on isoprenaline-stimulated ICa were partially antagonized by atropine. These results suggest that the anti-M2 muscarinic receptor antibodies display a stimulatory activity similar to muscarinic receptor agonist on the receptor-mediated electrophysiological events.

Antibodies from Trypanosoma cruzi infected mice recognize the second extracellular loop of the beta 1-adrenergic and M2-muscarinic receptors and regulate calcium channels in isolated cardiomyocytes

Sera from T. cruzi infected mice were tested in an enzyme immunoassay on peptides corresponding to the second extracellular loops of the beta 1-, the beta 2-adrenergic receptor and the M2 muscarinic receptor. All sera of mice (4/4) in the acute phase recognized the beta 1-adrenergic receptor and the M2 muscarinic receptor peptides but not the beta 2-adrenergic receptor peptide. The same peptides were recognized during the chronic phase in half of the mice (6/12). The immunoglobulin fractions of the mice were tested for their activity on L-type Ca++ channels of isolated guinea-pig cardiomyocytes using the whole-cell patch clamp technique. The immunoglobulin fractions of acute phase mice were able to activate the Ca++ channels by stimulation of the beta-adrenergic receptors, as assessed by inhibition with propranolol. Those of the chronic phase mice reduced the Ca++ current by stimulation of the muscarinic receptors, as assessed by inhibition with atropine. These results confirm the existence of functional epitopes on the second extracellular loops of both receptors. They suggest that, as in humans, the parasite is able to elicit functional autoantibodies against these epitopes. They give evidence that these autoantibodies mediate their physiological effects by modulating the cAMP activated Ca++ channels.

Characterization of anti-heart M2 muscarinic receptor antibodies--a combined clinical and experimental study

To elucidate the relationship between autoimmunity and idiopathic dilated cardiomyopathy has been one of today's heated topics in the field of heart research. So far it has been identified that there are a variety of autoantibodies including antireceptor autoantibodies. However, the role of these autoantibodies in the development of dilated cardiomyopathy has not been defined. An increasing number of in vitro studies showed that these autoantibodies had different functions, suggesting that they may play different roles in the pathogenesis of cardiomyopathy. The main purpose of this article is to briefly go through the results obtained from both clinical and experimental in vitro studies on anti-M2 muscarinic receptor antibodies to see where we stand in the understanding of the role of these autoantibodies in the pathogenesis of idiopathic dilated cardiomyopathy.