Anti-mitochondrial antibodies in patients with dilated cardiomyopathy (anti-M7) are directed against flavoenzymes with covalently bound FAD

Pathological findings with vacuoles in anti-mitochondrial antibody-positive inflammatory myopathy

BACKGROUND

A few patients with inflammatory myopathy showed anti-mitochondrial antibody (AMA) positivity. This study aimed to report the clinical and pathological findings with vacuoles in 3 cases of such patients.

METHODS

Three cases with myositis from the Myositis Clinical Database of Peking University First Hospital were identified with AMA positivity. Their clinical records were retrospectively reviewed and the data was extracted. All the 3 cases underwent muscle biopsy.

RESULTS

Three middle-aged patients presented with chronic-onset weakness of proximal limbs, marked elevation of creatine kinase, and AMA-positivity. Two of the 3 cases meet the criteria of primary biliary cholangitis. All the 3 cases presented with cardiac involvement and proteinuria. Two cases developed type 2 respiratory failure. MRI of the thigh muscle showed multiple patches of edema bilaterally in both cases, mostly in the adductor magnus. Pathological findings include degeneration of muscle fibers, diffused MHC-I positivity, and complement deposits on cell membranes. Vacuoles without rims of different sizes were discovered under the membrane of the muscle fibers. A few RBFs were discovered in case 1, while a diffused proliferation of endomysium and perimysium was shown in case 2.

CONCLUSIONS

AMA-positive inflammatory myopathy is a disease that could affect multiple systems. Apart from inflammatory changes, the pathological findings of muscle can also present vacuoles.

The role of mitochondria in rheumatic diseases

The mitochondrion is an intracellular organelle thought to originate from endosymbiosis between an ancestral eukaryotic cell and an α-proteobacterium. Mitochondria are the powerhouses of the cell, and can control several important processes within the cell, such as cell death. Conversely, dysregulation of mitochondria possibly contributes to the pathophysiology of several autoimmune diseases. Defects in mitochondria can be caused by mutations in the mitochondrial genome or by chronic exposure to pro-inflammatory cytokines, including type I interferons. Following the release of intact mitochondria or mitochondrial components into the cytosol or the extracellular space, the bacteria-like molecular motifs of mitochondria can elicit pro-inflammatory responses by the innate immune system. Moreover, antibodies can target mitochondria in autoimmune diseases, suggesting an interplay between the adaptive immune system and mitochondria. In this Review, we discuss the roles of mitochondria in rheumatic diseases such as systemic lupus erythematosus, antiphospholipid syndrome and rheumatoid arthritis. An understanding of the different contributions of mitochondria to distinct rheumatic diseases or manifestations could permit the development of novel therapeutic strategies and the use of mitochondria-derived biomarkers to inform pathogenesis.

PhIP-Seq Reveals Autoantibodies for Ubiquitously Expressed Antigens in Viral Myocarditis

Simple Summary

Myocarditis is the inflammation of the heart muscle, and viral infections are a common cause of this disease. Myocarditis in some patients can progress to dilated cardiomyopathy (DCM). The mouse model of coxsackievirus B3 (CVB3) is commonly used to understand this disease progression in DCM patients. In this paper, we have attempted to analyze antibodies for heart antigens that could be produced as a result of heart damage in animals infected with CVB3 using a technique called Phage ImmunoPrecipitation Sequencing (PhIP-Seq). The analyses led us to identify antibodies for several proteins that were not previously reported that may have relevance to human disease.

Abstract

Enteroviruses such as group B coxsackieviruses (CVB) are commonly suspected as causes of myocarditis that can lead to dilated cardiomyopathy (DCM), and the mouse model of CVB3 myocarditis is routinely used to understand DCM pathogenesis. Mechanistically, autoimmunity is suspected due to the presence of autoantibodies for select antigens. However, their role continues to be enigmatic, which also raises the question of whether the breadth of autoantibodies is sufficiently characterized. Here, we attempted to comprehensively analyze the autoantibody repertoire using Phage ImmunoPrecipitation Sequencing (PhIP-Seq), a versatile and high-throughput platform, in the mouse model of CVB3 myocarditis. First, PhIP-Seq analysis using the VirScan library revealed antibody reactivity only to CVB3 in the infected group but not in controls, thus validating the technique in this model. Second, using the mouse peptide library, we detected autoantibodies to 32 peptides from 25 proteins in infected animals that are ubiquitously expressed and have not been previously reported. Third, by using ELISA as a secondary assay, we confirmed antibody reactivity in sera from CVB3-infected animals to cytochrome c oxidase assembly factor 4 homolog (COA4) and phosphoinositide-3-kinase adaptor protein 1 (PIK3AP1), indicating the specificity of antibody detection by PhIP-Seq technology. Fourth, we noted similar antibody reactivity patterns in CVB3 and CVB4 infections, suggesting that the COA4- and PIK3AP1-reactive antibodies could be common to multiple CVB infections. The specificity of the autoantibodies was affirmed with influenza-infected animals that showed no reactivity to any of the antigens tested. Taken together, our data suggest that the autoantibodies identified by PhIP-Seq may have relevance to CVB pathogenesis, with a possibility that similar reactivity could be expected in human DCM patients.

Role of apoptosis repressor with caspase recruitment domain (ARC) in cell death and cardiovascular disease

Apoptosis repressor with caspase recruitment domain (ARC) is a highly effective and multifunctional inhibitor of apoptosis that is mainly expressed in postmitotic cells such as cardiomyocytes and skeletal muscle cells. ARC contains a C-terminal region rich in proline and glutamic acid residues and an N-terminal caspase recruitment domain (CARD). The CARD is originally described as a protein-binding motif that interacts with caspase through a CARD-CARD interaction. Initially, the inhibitory effect of ARC was only found in apoptosis, however, it was later found that ARC also played a regulatory role in other types of cell death. As a powerful cardioprotective factor, ARC can protect the heart by inhibiting the death of cardiomyocytes in various ways. ARC can reduce the cardiomyocyte apoptotic response to various stresses and injuries, including extrinsic apoptosis induced by death receptor ligands, cellular Ca²⁺ homeostasis and the dysregulation of endoplasmic reticulum (ER) stress, oxidative stress and hypoxia. In addition, changes in ARC transcription and translation levels in the heart can cause a series of physiological and pathological changes, and ARC can also perform corresponding functions through interactions with other molecules. Although there has been much research on ARC, the functional redundancy among proteins shows that ARC still has much research value. This review summarizes the molecular characteristics of ARC, its roles in the various death modes in cardiomyocytes and the roles of ARC in cardiac pathophysiology. This article also describes the potential therapeutic effect and research prospects of ARC.

Anti-mitochondrial Antibodies in Patients with Dilated Cardiomyopathy

Objective It has been reported that anti-mitochondrial antibodies (AMAs) recognize mitochondrial antigens and are associated with some diseases involving multiple organs, such as primary biliary cholangitis, Sjögren syndrome, Hashimoto's thyroiditis, systemic sclerosis, interstitial pneumoniae, dilated cardiomyopathy, and tubulointerstitial nephritis. In the current study, we examined the prevalence of AMAs in patients with dilated cardiomyopathy (DCM) and their clinical characteristics. Methods We enrolled 270 patients with DCM. We measured serum AMAs and analyzed the associated factors. Out of the 270 patients, positive AMAs were detected in 3 patients (1.1%; mean age, 68 years old; 2 men). These three patients had a significantly higher prevalence of primary biliary cholangitis and myopathy and levels of alanine alkaline phosphatase than those who were negative for said antibodies. There were no significant differences in the levels of B-type natriuretic peptide, aspartate transaminase, and left ventricular ejection fraction between these groups of patients. During the follow-up period, two of the three patients died due to respiratory failure. The other patient survived but experienced type II respiratory failure. Conclusion The prevalence of AMAs in 270 DCM patients was only 1.1%, and these patients suffered from respiratory failure.

Anti-mitochondrial M2 Antibodies Enhance the Risk of Supraventricular Arrhythmias in Patients with Elevated Hepatobiliary Enzyme Levels

Objective Supraventricular arrhythmias are commonly detected in patients with anti-mitochondrial antibody M2 (AMA-M2)-associated myopathy. However, the prevalence of supraventricular arrhythmias in unselected AMA-M2-positive patients and the impact of AMA-M2 on supraventricular arrhythmias have yet to be fully investigated. Methods We analyzed 384 patients (116 men; age, 60 [48-69] years), who underwent AMA-M2 testing following the detection of elevated hepatobiliary enzymes. Supraventricular arrhythmias involving atrial fibrillation, atrial flutter, atrial tachycardia, sick sinus syndrome, and atrial standstill were confirmed by a 12-lead electrocardiogram, 24-hour ambulatory monitoring, and physician-assigned diagnoses within the three years before and two years after the AMA-M2 test. Results Seventy-seven (20%) patients were positive for AMA-M2. The prevalence of supraventricular arrhythmias among AMA-M2-positive patients was higher than that among AMA-M2-negative patients (14% vs. 6%, p=0.008). A univariate analysis showed that supraventricular arrhythmias were associated with AMA-M2 positivity, aging, congestive heart failure, and the CHADS₂ score. The multivariate analysis determined that AMA-M2 positivity was an independent risk factor for supraventricular arrhythmias (odds ratio 3.52, p=0.011). Among the AMA-M2-positive patients, the AMA-M2 titer did not differ to a statistically significant extent, regardless of the presence or absence of supraventricular arrhythmias. Multiple supraventricular arrhythmias with extremely low atrial deflections was a characteristic finding in AMA-M2-positive patients with supraventricular arrhythmias. Conclusion AMA-M2 enhances the risk of supraventricular arrhythmias, indicating the possible involvement of the atrial myocardium and the formation of an arrhythmogenic substrate. The results highlight the need for clinical attention to supraventricular arrhythmias in AMA-M2-positive patients.

What's in a covalent bond? On the role and formation of covalently bound flavin cofactors

Many enzymes use one or more cofactors, such as biotin, heme, or flavin. These cofactors may be bound to the enzyme in a noncovalent or covalent manner. Although most flavoproteins contain a noncovalently bound flavin cofactor (FMN or FAD), a large number have these cofactors covalently linked to the polypeptide chain. Most covalent flavin-protein linkages involve a single cofactor attachment via a histidyl, tyrosyl, cysteinyl or threonyl linkage. However, some flavoproteins contain a flavin that is tethered to two amino acids. In the last decade, many studies have focused on elucidating the mechanism(s) of covalent flavin incorporation (flavinylation) and the possible role(s) of covalent protein-flavin bonds. These endeavors have revealed that covalent flavinylation is a post-translational and self-catalytic process. This review presents an overview of the known types of covalent flavin bonds and the proposed mechanisms and roles of covalent flavinylation.

Anti-heart autoantibodies in familial dilated cardiomyopathy

Familial aggregation is a feature of myocarditis and dilated cardiomyopathy (DCM). Myocarditis, a clinically polymorphic inflammatory disease of the myocardium, is diagnosed by endomyocardial biopsy (EMB) and may lead to DCM. Mutations in several genes encoding myocyte structural proteins are known monogenic DCM causes, but because of high etiologic and genetic heterogeneity, the gene defects identified so far account for a minority of cases. In the last decade, it has been discovered that autoimmunity plays a pivotal role in myocarditis and DCM that are thought to represent different stages of an organ-specific autoimmune disease in genetically predisposed individuals. None of the available genetic studies in familial DCM has taken into account the autoimmune phenotype markers in the characterization of index patients and relatives, thus it is not known whether or not the described gene defects are involved in the autoimmune form of the disease. In animal models autoimmune myocarditis/DCM can be induced by viral infection, immunization with heart-specific autoantigens, or develop spontaneously in genetically predisposed strains. It may be cell or antibody-mediated; susceptibility is based upon multiple MHC and non-MHC genes. In patients, the diagnosis of autoimmune myocarditis/DCM requires exclusion of viral genome on EMB and detection of serum heart-reactive autoantibodies. They are found in index patients and relatives from about 60% of both familial and non-familial pedigrees and predict DCM development among healthy relatives. Some antibodies have functional effects on cardiac myocytes in vitro, in animal models and possibly in a DCM subset without inflammation, responsive to extracorporeal immunoadsorption. Cardiac-specific autoantibodies, which are shown to be disease-specific for myocarditis/DCM, can be used as biomarkers for identifying patients in whom, in the absence of active infection of the myocardium, immunosuppression and/or immunomodulation may be beneficial and their relatives at risk. Future studies should clarify genetic basis of human autoimmune myocarditis/DCM as well as genotype/immune phenotype correlations.

Protection of cardiomyocytes from ischemic/hypoxic cell death via Drbp1 and pMe2GlyDH in cardio-specific ARC transgenic mice

The ischemic death of cardiomyocytes is associated in heart disease and heart failure. However, the molecular mechanism underlying ischemic cell death is not well defined. To examine the function of apoptosis repressor with a caspase recruitment domain (ARC) in the ischemic/hypoxic damage of cardiomyocytes, we generated cardio-specific ARC transgenic mice using a mouse alpha-myosin heavy chain promoter. Compared with the control, the hearts of ARC transgenic mice showed a 3-fold overexpression of ARC. Langendoff preparation showed that the hearts isolated from ARC transgenic mice exhibited improved recovery of contractile performance during reperfusion. The cardiomyocytes cultured from neonatal ARC transgenic mice were significantly resistant to hypoxic cell death. Furthermore, the ARC C-terminal calcium-binding domain was as potent to protect cardiomyocytes from hypoxic cell death as ARC. Genome-wide RNA expression profiling uncovered a list of genes whose expression was changed (>2-fold) in ARC transgenic mice. Among them, expressional regulation of developmentally regulated RNA-binding protein 1 (Drbp1) or the dimethylglycine dehydrogenase precursor (pMe(2)GlyDH) affected hypoxic death of cardiomyocytes. These results suggest that ARC may protect cardiomyocytes from hypoxic cell death by regulating its downstream, Drbp1 and pMe(2)GlyDH, shedding new insights into the protection of heart from hypoxic damages.

Genetically determined myocarditis: clinical presentation and immunological characteristics

PURPOSE OF REVIEW

Myocarditis is a clinically heterogeneous myocardial inflammatory disease, diagnosed by endomyocardial biopsy; it may be idiopathic, infectious, or autoimmune and may lead to dilated cardiomyopathy. Myocarditis and dilated cardiomyopathy represent different stages of an organ-specific autoimmune disease in genetically predisposed individuals.

RECENT FINDINGS

In animal models, cell-mediated or antibody-mediated autoimmune myocarditis/dilated cardiomyopathy can be induced by viral infection or immunization with heart-specific autoantigens, or can develop spontaneously in genetically predisposed strains. Susceptibility is based on multiple major histocompatibility complex and nonmajor histocompatibility complex genes. In patients the diagnosis of autoimmune myocarditis/dilated cardiomyopathy requires exclusion of viral genome on endomyocardial biopsy and detection of serum heart-reactive autoantibodies. They are directed against multiple antigens that are found in patients and relatives from about 60% of familial and nonfamilial pedigrees. They predict dilated cardiomyopathy development among relatives, years before disease. Some antibodies have functional effects on cardiac myocytes in vitro, in animals and possibly in a dilated cardiomyopathy subset, responsive to extracorporeal immunoadsorption.

SUMMARY

In myocarditis/dilated cardiomyopathy, cardiac-specific and disease-specific antibodies of IgG class are potential biomarkers for identifying 'at risk' relatives as well as those patients in whom, in the absence of active infection of the myocardium, immunosuppression, and/or immunomodulation may be beneficial. Future studies should better define the genetic basis of human autoimmune myocarditis/dilated cardiomyopathy.

The purified recombinant precursor of rat mitochondrial dimethylglycine dehydrogenase binds FAD via an autocatalytic reaction

The precursor of the rat mitochondrial flavoenzyme dimethylglycine dehydrogenase (Me(2)GlyDH) has been produced in Escherichia coli as a C-terminally 6-His-tagged fusion protein, purified by one-step affinity chromatography and identified by ESI-MS/MS. It was correctly processed into its mature form upon incubation with solubilized rat liver mitoplasts. The purified precursor was mainly in its apo-form as demonstrated by immunological and fluorimetric detection of covalently bound flavin adenine dinucleotide (FAD). Results described here definitively demonstrate that: (i) covalent attachment of FAD to Me(2)GlyDH apoenzyme can proceed in vitro autocatalytically, without third reactants; (ii) the removal of mitochondrial presequence by mitochondrial processing peptidase is not required for covalent autoflavinylation.

Clinical implications of anti-heart autoantibodies in myocarditis and dilated cardiomyopathy

Dilated cardiomyopathy (DCM), a leading cause of heart failure and heart transplantation in younger adults, is characterized by dilatation and impaired contraction of the left or both ventricles; it may be idiopathic, familial/genetic (20-30%), viral, and/or immune. On endomyocardial biopsy there is chronic inflammation in 30-40% of cases. Mutations in genes encoding myocyte structural proteins, cardiotoxic noxae and infectious agents are known causes; due to high aetiologic and genetic heterogeneity, the gene defects identified so far account for a tiny proportion of the familial cases. In at least two thirds of patients, DCM remains idiopathic. Myocarditis may be idiopathic, infectious or autoimmune and may heal or lead to DCM. Circulating heart-reactive autoantibodies are found in myocarditis/DCM patients and symptom-free relatives at higher frequency than in normal or noninflammatory heart disease control groups. These autoantibodies are directed against multiple antigens, some of which are expressed only in the heart (organ-specific); some autoantibodies have functional effects on cardiac myocytes in vitro as well as in animal models. Depletion of nonantigen-specific antibodies by extracorporeal immunoadsorption is associated with improved ventricular function and reduced cardiac symptoms in some DCM patients, suggesting that autoantibodies may also have a functional role in humans. Immunosuppression seems beneficial in patients who are virus-negative and cardiac autoantibody positive. Prospective family studies have shown that cardiac-specific autoantibodies are present in at least 60% of both familial and non familial pedigrees and predict DCM development among asymptomatic relatives, years before clinical and echocardiographic evidence of disease. Animal models have shown that autoimmune myocarditis/DCM can be induced by virus as well as reproduced by immunization with a well-characterized autoantigen, cardiac myosin. Thus, in a substantial proportion of patients, myocarditis and DCM represent different stages of an organ-specific autoimmune disease, that represents the final common pathogenetic pathway of infectious and noninfectious myocardial injuries in genetically predisposed individuals.

Prospective familial assessment in dilated cardiomyopathy: cardiac autoantibodies predict disease development in asymptomatic relatives

BACKGROUND

In autoimmune disorders, circulating autoantibodies identify healthy relatives at risk years before clinical presentation. Healthy relatives of patients with dilated cardiomyopathy (DCM) who have echocardiographic changes, including left ventricular enlargement or depressed fractional shortening at baseline, have increased medium-term risk for DCM development. Approximately one third of relatives have serum anti-heart autoantibodies (AHAs) at baseline; we intended to assess their potential role in predicting DCM development.

METHODS AND RESULTS

Baseline evaluation, including electrocardiography, echocardiography, and AHA, was performed in 592 asymptomatic relatives of 169 consecutive DCM patients (291 males and 301 females; mean age 36+/-16 years). Relatives were classified in accordance with published echocardiographic criteria; those who did not have DCM were followed up (median of 58 months). DCM among relatives was diagnosed by echocardiography at follow-up. Of the 592 individuals evaluated, 77% were assessed as normal, 4.4% as having DCM, and 19% as possibly affected on the basis of depressed fractional shortening without ventricular dilatation in 17 and left ventricular enlargement without systolic dysfunction in 94. Five-year follow-up of 311 relatives revealed that 26 had progressed (13 to DCM, 11 to left ventricular enlargement, and 2 to depressed fractional shortening). Relatives who developed DCM were more frequently AHA-positive than those who did not (69% versus 37%, P=0.02). Five-year probability of progression to DCM, among normal or possibly affected relatives, was higher in AHA-positive cases (P=0.03). By Cox regression, positive AHAs at baseline were independent predictors of progression (RR 2.26, CI 1 to 5.1, P=0.03).

CONCLUSIONS

Among healthy relatives of DCM patients, AHAs are independent predictors of disease development within 5 years.

Clinical implications of anti-cardiac immunity in dilated cardiomyopathy

Criteria of organ-specific autoimmunity are fulfilled in a subset of patients with myocarditis/dilated cardiomyopathy (DCM). In particular, circulating heart-reactive autoantibodies are found in such patients and symptom-free relatives. These autoantibodies are directed against multiple antigens, some of which are expressed in the heart (organ-specific), others in heart and some skeletal muscle fibres (partially heart-specific) or in heart and skeletal muscle (muscle-specific). Distinct autoantibodies have different frequency in disease and normal controls. Different techniques detect one or more antibodies, thus they cannot be used interchangeably for screening. It is unknown whether the same patients produce more antibodies or different patient groups develop autoimmunity to distinct antigens. IgG antibodies, shown to be cardiac- and disease-specific for myocarditis/DCM, can be used as autoimmune markers for relatives at risk as well as for identifying patients in whom immunosuppression may be beneficial. Some autoantibodies may also have a functional role, but further work is needed.

Over-expression in Escherichia coli, functional characterization and refolding of rat dimethylglycine dehydrogenase

Dimethylglycine dehydrogenase (Me(2)GlyDH) is a mitochondrial enzyme that catalyzes the oxidative demethylation of dimethylglycine to sarcosine. The enzyme requires flavin adenine dinucleotide (FAD), which is covalently bound to the apoprotein via a histidyl(N3)-(8alpha)FAD linkage. In the present study, the mature form of rat Me(2)GlyDH has been over-expressed in Escherichia coli as an N-terminally 6-His-tagged fusion protein. The over-expressed protein distributed almost equally between the soluble and insoluble (inclusion bodies) cell fraction. By applying the soluble cell lysate to a nickel-chelating column, two fractions were eluted, both containing a nearly homogeneous protein with a molecular mass of 93 kDa, on SDS-PAGE. The first protein fraction was identified by Western blotting analysis as the covalently flavinylated Me(2)GlyDH. It showed optical properties and specific activity (240 nmol/min/mg protein) similar to those of the native holoenzyme. The second fraction was identified as an underflavinylated (apo-) form of Me(2)GlyDH, with a 70% lower specific activity. The recombinant holoenzyme exhibited optimal activity at pH 8.5, an activation energy of about 80 kJ/mol, and two KM values for N,N-dimethylglycine (KM1 = 0.05 mM and KM2 = 9.4 mM), as described for the native holoenzyme. Starting from the inclusion bodies, the unfolded flavinylated enzyme was solubilized by SDS treatment and refolded by an 80-fold dilution step, with a reactivation yield of 50-60%.

Specific stimulation of peripheral blood mononuclear cells from patients with acute myocarditis by peptide-bound flavin adenine dinucleotide (FAD), a naturally occurring autologous hapten

The tryptic FAD-peptide carrying the flavin in 8alpha-(N3)histidyl linkage as natural hapten was isolated by HPLC from the bacterial enzyme 6-hydroxy-d-nicotine oxidase. The same flavin protein linkage is found in the mitochondrial succinate dehydrogenase flavoprotein subunit, the predominant flavoprotein with covalently bound FAD in mitochondria of cardiomyocytes. Peripheral blood mononuclear cells (PBMC) were isolated from four patients with acute myocarditis, seven patients with dilated cardiomyopathy (DCM) and from four healthy control individuals. The response of PBMC to the FAD-peptide was evaluated by measuring proliferation ([3H]-dThd incorporation) and cytokine secretion [interferon (IFN)-gamma]. PBMC from all patients with acute myocarditis showed positive responses to the FAD-peptide, in contrast to PBMC from patients with DCM or control individuals. Following the recovery of the patients from the acute inflammation of the heart, PBMC no longer exhibited a proliferation response to the FAD-peptide. A chemically synthesized FAD-free peptide with identical amino acid sequence induced no response of PBMC. The results are consistent with a recall response by activated T cells, specific for the normally cryptic mitochondrial flavin-hapten, which may be liberated following cardiomyocyte destruction during the inflammation of the heart.

Circulating cardiac autoantibodies in dilated cardiomyopathy and myocarditis: pathogenetic and clinical significance

Dilated cardiomyopathy (DCM) is a relevant cause of heart failure and a common indication for heart transplantation. It may be idiopathic, familial/genetic, viral, autoimmune or immune-mediated associated with a viral infection. Myocarditis is an inflammatory disease of the myocardium; it may be idiopathic, infectious or autoimmune and may heal or lead to DCM. Thus, in a patient subset, myocarditis and DCM are thought to represent the acute and chronic stages of an organ-specific autoimmune disease of the myocardium. In keeping with this hypothesis, autoimmune features in patients with myocarditis/DCM include: familial aggregation; a weak association with HLA-DR4; abnormal expression of HLA class II on cardiac endothelium on endomyocardial biopsy; and detection of organ- and disease-specific cardiac autoantibodies of the IgG class in the sera of affected patients and symptom-free relatives. The cardiac autoantibodies detected by immunofluorescence are directed against multiple antigens. Two of these, first identified using immunoblotting and confirmed by ELISA, are the atrial-specific alpha- and the ventricular and skeletal muscle beta-heavy chain isoform. The alpha-myosin isoform fulfils the expected criteria for organ-specific autoimmunity, in that immunization with cardiac, but not skeletal myosin reproduces, in susceptible mouse strains, the human disease phenotype of myocarditis/DCM; in addition, alpha-myosin is entirely cardiac-specific. Additional antigenic targets of heart-reactive autoantibodies include unknown sarcolemmal proteins, mitochondrial enzymes, beta-adrenergic and muscarinic receptors. For some of these antibodies, there is in vitro evidence for a functional role. The organ-specific cardiac autoantibodies detected by immunofluorescence in symptom-free relatives were associated with echocardiographic features suggestive of early disease. Mid-term follow-up suggests that these antibodies are predictive markers of progression to DCM among symptom-free relatives with or without abnormal echocardiographic findings.

[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.

Analysis of mitochondrial antigens reveals inner membrane succinate dehydrogenase flavoprotein subunit as autoantigen to antibodies in anti-M7 sera

The role of mitochondrial proteins as antigens to antibodies of anti-M7 sera was analysed by flavin fluorescence, one- and two-dimensional Western blots and blue native gel electrophoresis. Flavin fluorescence of succinate dehydrogenase (SucDH, complex II of the respiratory chain) of rat liver inner mitochondrial membranes correlated with the immunoreactivity of a representative anti-M7 myocarditis serum. Antigens of isolated bovine heart mitochondria reacting with antibodies of myocarditis serum on two-dimensional Western blots were identified by MALDI-TOF and NanoESI mass spectrometry as myosin heavy chain beta and as dihydrolipoamide dehydrogenase of the mitochondrial 2-oxoacid dehydrogenase complexes. The SucDH-flavoprotein was not resolved as a discrete protein spot on two-dimensional polyacrylamide gels. However, separation of the rat liver inner mitochondrial membrane complexes by blue native gel electrophoresis followed by Western blotting, and Western blots of purified Escherichia coli SucDH complex revealed that anti-M7 sera contained antibodies directed against the SucDH-flavoprotein subunit.

Eye muscle antibodies and subtype of thyroid-associated ophthalmopathy

The eye changes associated with Graves' hyperthyroidism can be classified into two subtypes, congestive ophthalmopathy (CO), in which inflammatory changes in the periorbital tissues predominate, and ocular myopathy (OM), in which eye muscle damage is the main feature. Antibodies against the flavoprotein (Fp) subunit of succinate dehydrogenase (SDH), the 64-kd protein, and G2s, a thyroid and eye muscle shared protein of unknown function, are good markers of eye muscle cell damage in patients with OM. Another antigen associated with ophthalmopathy is the flavine adenine nucleotide (FAD) cofactor of several mitochondrial enzymes, including SDH. We tested for serum antibodies against purified human recombinant Fp, FAD, and a G2s fusion protein, in patients with thyroid-associated ophthalmopathy (TAO) and control patients and subjects, in enzyme-linked immunosorbent assay. Antibodies against Fp were detected in 32% of patients with TAO, 30% with Graves' hyperthyroidism (GH), 16% with Hashimoto's thyroiditis (HT), in 14% of patients with multi-nodular goiter (MNG), and in 6% of normal subjects. Antibodies against FAD were found in 24%, 30%, 24%, and 14%, respectively, of these patients and in 12% of the normals, while antibodies against G2s were detected in 50% of patients with TAO, 40% with GH, 40% with HT, in 29% of patients with MNG, and in 7% of normals. We also tested for antibodies against SDH, FAD, and G2s in 12 patients with GH who developed CO (6 patients) or OM (6 patients) after treatment with antithyroid drugs. Of the 6 patients who developed OM, antibodies against SDH preceded the onset of eye disease in 4 and coincided with it in 2, antibodies against G2s preceded eye muscle disease in 5 and coincided with it in 1 patient while antibodies against FAD preceded the development of OM in 5 patients. Of the 6 patients who developed CO, antibodies against SDH were detected in only one patient and borderline levels were demonstrated in 1, while anti-FAD and anti-G2s each preceded the onset of eye signs in 6 patients. Positive sera from another group of patients with TAO, and a second group of normal subjects, were tested at increasing serum dilutions. Sera from the two groups showed similar dilution patterns, except for a few patients with TAO in whom increasing dilutions was associated with increased, then decreased, antibody levels. In this experiment the prevalences of the two antibodies were much greater in patients with TAO namely, 67% for anti-Fp and 89% for anti-G2s, while the prevalences in the normals were 11% and 22%, respectively. The reason for this apparent discrepancy is not clear but may reflect subject and assay differences. Because Fp is found within the mitochondrial membrane it is likely that the corresponding antibodies are produced after eye muscle necrosis, and do not play a role in its pathogenesis. The primary reaction in the eye muscle may be T-cell autoimmunity against G2s, although this has not been proven. The mechanism for the production of antibodies against G2s, FAD, and Fp in subjects who do not have ophthalmopathy is unclear. The significance of such antibodies in control subjects is presently being addressed in our laboratory.

Coxsackievirus B3 infection induces anti-flavoprotein antibodies in mice

Enteroviruses, the most common cause of acute myocarditis, are also supposed aetiological agents of dilated cardiomyopathy. Autoantibodies (anti-M7; Klein & Berg, Clin Exp Immunol 1990; 58:283-92) directed against flavoproteins with covalently bound flavin (alphaFp-Ab; Otto et al., Clin Exp Immunol 1998; 111:541-2) are detected in up to 30% of sera of patients with myocarditis and idiopathic dilated cardiomyopathy (IDCM). Mice inoculated with a myocarditic variant of coxsackievirus B3 (CVB3) were employed to study the occurrence of serum alphaFp-Ab following viral infection. The presence of alphaFp-Ab was analysed by Western blotting with the flavoprotein antigens 6-hydroxy-D-nicotine oxidase (6HDNO) and sarcosine oxidase (SaO). Of 10 sera from CVB3-infected mice, five showed a strong reaction with both antigens. The sera were reactive also to the mitochondrial covalently flavinylated proteins dimethylglycine dehydrogenase and sarcosine dehydrogenase. Sera of non-infected mice did not react with these antigens. A 6HDNO mutant protein with non-covalently bound FAD no longer reacted on Western blots with sera of CVB3-infected mice. Preincubation with FAD abolished or reduced the reaction of the sera with the 6HDNO antigen. At 2 weeks p.i. the alphaFp-Ab were of the IgM and IgG isotypes, at 7 and 9 weeks p.i. of the IgG isotype. The sera of CVB3-infected mice reproduced closely the antigenic specificity of the anti-M7 sera of patients, lending further support to the role of coxsackieviruses in the pathogenesis of IDCM.

Anti-mitochondrial flavoprotein autoantibodies of patients with myocarditis and dilated cardiomyopathy (anti-M7): interaction with flavin-carrying proteins, effect of vitamin B2 and epitope mapping

Vitamin B2 and flavin cofactors are transported tightly bound to immunoglobulin in human serum. We reasoned that anti-mitochondrial flavoprotein autoantibodies (alpha Fp-AB) present in the serum of patients with myocarditis and cardiomyopathy of unknown aetiology may form immunoglobulin aggregates with these serum proteins. However, immunodiffusion and Western blot assays demonstrated that the flavin-carrying proteins were not recognized by alpha Fp-AB. Apparently the flavin moiety in the native protein conformation was inaccessible to alpha Fp-AB. This conclusion was supported by the absence of an immunoreaction between the riboflavin-binding protein from egg white and alpha FP-AB. Intravenous application of vitamin B2 to rabbits immunized with 6-hydroxy-D-nicotine oxidase, a bacterial protein carrying covalently attached FAD, did not neutralize alpha Fp-AB which had been raised in the serum of the animals. FAD-carrying peptides generated from 6-hydroxy-D-nicotine oxidase by trypsin and chymotrypsin treatment were not recognized by the alpha Fp-AB, but those generated by endopeptidase Lys were. This demonstrates that the epitope recognized by alpha Fp-AB comprises, besides the flavin moiety, protein secondary structure elements.