Molecular Aspects of Myocarditis

c-FLIP-Short reduces type I interferon production and increases viremia with coxsackievirus B3

Cellular FLIP (c-FLIP) is an enzymatically inactive paralogue of caspase-8 and as such can block death receptor-induced apoptosis. However, independent of death receptors, c-FLIP-Long (c-FLIP_L) can heterodimerize with and activate caspase-8. This is critical for promoting the growth and survival of T lymphocytes as well as the regulation of the RIG-I helicase pathway for type I interferon production in response to viral infections. Truncated forms of FLIP also exist in mammalian cells (c-FLIP_S) and certain viruses (v-FLIP), which lack the C-terminal domain that activates caspase-8. Thus, the ratio of c-FLIP_L to these short forms of FLIP may greatly influence the outcome of an immune response. We examined this model in mice transgenically expressing c-FLIP_S in T cells during infection with Coxsackievirus B3 (CVB3). In contrast to our earlier findings of reduced myocarditis and mortality with CVB3 infection of c-FLIP_L-transgenic mice, c-FLIP_S-transgenic mice were highly sensitive to CVB3 infection as manifested by increased cardiac virus titers, myocarditis score, and mortality compared to wild-type C57BL/6 mice. This observation was paralleled by a reduction in serum levels of IL-10 and IFN-α in CVB3-infected c-FLIP_S mice. In vitro infection of c-FLIP_S T cells with CVB3 confirmed these results. Furthermore, molecular studies revealed that following infection of cells with CVB3, c-FLIP_L associates with mitochondrial antiviral signaling protein (MAVS), increases caspase-8 activity and type I IFN production, and reduces viral replication, whereas c-FLIP_S promotes the opposite phenotype.

Coxsackievirus B3-induced myocarditis: infection of females during the estrus phase of the ovarian cycle leads to activation of T regulatory cells

Transgenic female mice expressing the TNFalpha gene under the cardiac myosin promoter (TNF1.6) develop substantially increased myocarditis and increased numbers of CD4+Th1 (interferon gamma+) cells when infected with coxsackievirus B3 (CVB3) during the diestrus and proestrus phases of the estrus cycle compared to females infected during the estrus and metestrus phases. Cardiac virus titers were increased in females infected in estrus compared to females infected during the other phases. T regulatory cells (CD4+CD25+FoxP3+) were increased in both peripheral blood and inflammatory cells in the heart in females infected during estrus. Exogenous administration of 200 ng/mouse 17-beta-estradiol to females protected against CVB3 induced myocarditis and increased CD4+CD25+FoxP3+ cells. These results demonstrate that hormonal fluctuations occurring in normally cycling females can determine T regulatory cell response and control virus-induced pathogenesis.

Factors associated with establishing a causal diagnosis for children with cardiomyopathy

OBJECTIVE

The goal was to identify the clinical variables associated with establishing a cause of cardiomyopathy in children.

METHODS

The Pediatric Cardiomyopathy Registry contains clinical and causal testing information for 916 children who were diagnosed as having cardiomyopathy in North America between 1990 and 1995. Children with a causal diagnosis were compared with those without with respect to several demographic, clinical, and causal testing variables.

RESULTS

Cardiomyopathy was 1 of 4 types, hypertrophic (34.2%), dilated (53.8%), restrictive (3.2%), or other or mixed (8.9%). Only one third of cases had a known cause. Children with a known cause for hypertrophic cardiomyopathy were more likely to be female, to be relatively smaller, to present with congestive heart failure, and to have increased left ventricular posterior wall thickness without outflow tract obstruction. For dilated cardiomyopathy, a known cause was associated with older age, lower heart rate, smaller left ventricular dimensions, and greater shortening fraction. Family history of cardiomyopathy predicted a significantly higher rate of causal diagnoses for all cardiomyopathy types, whereas family histories of genetic syndromes and sudden death were also predictive of a cause for hypertrophic and dilated cardiomyopathies. For hypertrophic cardiomyopathy, only blood and urine testing was associated with a causal diagnosis, whereas both viral serologic testing or culture and endomyocardial biopsy were independent predictors of a causal diagnosis in dilated cardiomyopathy.

CONCLUSIONS

Certain patient characteristics, family history, echocardiographic findings, laboratory testing, and biopsy were associated significantly with establishing a cause of pediatric cardiomyopathy. Early endomyocardial biopsy should be considered strongly for children with dilated cardiomyopathy, for definitive diagnosis of viral myocarditis. Although not widely used, skeletal muscle biopsy may yield a cause for some patients with hypertrophic cardiomyopathy and for patients suspected of having a mitochondrial disorder.

Inflammatory cardiomyopathy: there is a specific matrix destruction in the course of the disease

Cardiomyopathies are responsible for a high proportion of cases of congestive heart failure and sudden death, as well as for the need for transplantation. Understanding of the causes of these disorders has been sought in earnest over the past decade. We hypothesized that DCM is a disease of the cytoskeleton/sarcolemma, which affects the sarcomere. Evaluation of the sarcolemma in DCM and other forms of systolic heart failure demonstrates membrane disruption; and, secondarily, the extracellular matrix architecture is also affected. Disruption of the links from the sarcolemma to ECM at the dystrophin C-terminus and those to the sarcomere and nucleus via N-terminal dystrophin interactions could lead to a "domino effect" disruption of systolic function and development of arrhythmias. We also have suggested that dystrophin mutations play a role in idiopathic DCM in males. The T-cap/MLP/alpha-actinin/titin complex appears to stabilize Z-disc function via mechanical stretch sensing. Loss of elasticity results in the primary defect in the endogenous cardiac muscle stretch sensor machinery. The over-stretching of individual myocytes leads to activation of cell death pathways, at a time when stretch-regulated survival cues are diminished due to defective stretch sensing, leading to progression of heart failure. Genetic DCM and the acquired disorder viral myocarditis have the same clinical features including heart failure, arrhythmias, and conduction block, and also similar mechanisms of disease based on the proteins targeted. In dilated cardiomyopathy, the process of progressive ventricular dilation and changes of the shape of the ventricle to a more spherical shape, associated with changes in ventricular function and/or hypertrophy, occurs without known initiating disturbance. In those cases in which resolution of cardiac dysfunction does not occur, chronic DCM results. It has been unclear what the underlying etiology of this long-term sequela could be, but viral persistence and autoimmunity have been widely speculated.

Genome-environment interactions in the molecular pathogenesis of dilated cardiomyopathy

Dilated cardiomyopathy (DCM) is a heart muscle disease characterized by impaired contractility and dilation of the ventricles. In a subset of DCM patients, classical inheritance patterns occur (familial DCM), which have led to the identification of specific genomic loci and gene defects causing monogenic DCM subtypes. In the majority of DCM patients, however, there is no evidence for a monogenic etiology of the disorder (sporadic DCM), and in the absence of other recognizable etiological factors, these cases were classified as "idiopathic". Recent research suggests that cardiotropic viruses are important environmental factors in the pathogenesis of "idiopathic" cases and that DCM commonly results from interactions between genetic and environmental factors, whereas "pure" genetic forms are rather rare. Regarding genetics, the clinical cardiomyopathic phenotype associated with single gene defects may be highly variable for unknown reasons. Furthermore, a novel class of genetic defects was identified recently which provide a molecular basis for abnormal reactions of cardiomyocytes to environmental stress. These defects are paradigms of specific molecular links between genome and environment during the pathogenesis of DCM. Regarding environmental factors, a recent molecular virological study based on myocardial biopsies in a large series of sporadic DCM patients has detected cardiac viral infections in the majority of patients, with a broad spectrum of virus species being involved. Apparently, DCM does not only occur as a late sequela of acute viral myocarditis, but also in patients without clinical history of cardiac viral disease. Cardiotropic viruses thus emerge as prevalent environmental factors which may cause or influence the course of DCM in a large fraction of cases. Synopsis of current data suggests that a comprehensive picture of DCM pathogenesis can only be drawn if both genetic and environmental pathogenetic factors are considered. The course of cardiac viral infections depends strongly on genetic host factors and may range from rapid and complete virus elimination or silencing without clinical symptoms, to rapidly progressive or fatal disease. Viruses interact not only with genetically heterogenous host systems of virus uptake, migration, and antiviral immunity, but, due to their prevalence in DCM hearts, are also likely to encounter multiple structural proteins of cardiac cells known to be defective in familial DCM. The combined knowledge on DCM-associated gene defects and viruses therefore suggests in-depth studies on genome-environment interactions in DCM pathogenesis which may underlie the high clinical variability observed both in monogenic and virus-associated DCM and have implications for the clinical management of DCM patients.

High prevalence of viral genomes and multiple viral infections in the myocardium of adults with "idiopathic" left ventricular dysfunction

BACKGROUND

For a long time, enteroviruses have been considered to be the most common cause of acute viral myocarditis (MC), with possible transition from MC to dilated cardiomyopathy (DCM). Recent investigations have shown, however, that other viruses are also frequently encountered in MC patients, suggesting that persistence of various virus species may play a pathogenic role in the transition from MC to DCM. The purpose of this study was to screen endomyocardial biopsies (EMBs) from patients with "idiopathic" DCM for the presence of viral genomes by using polymerase chain reaction (PCR) to assess the frequency of cardiac viral infections that may be involved in the pathogenesis of the disease.

METHODS AND RESULTS

EMBs were obtained for PCR analysis from 245 consecutive patients (median left ventricular ejection fraction, 35.0%; range, 9% to 59%). PCR and reverse transcription-PCR were performed to detect the genomic sequences of enterovirus (EV), adenovirus (ADV), human cytomegalovirus (HCMV), herpes simplex virus, Epstein-Barr virus (EBV), human herpesvirus 6 (HHV-6), parvovirus B19 (PVB19), and influenza A and B viruses. Myocardial inflammation was assessed by histological and immunohistological analyses. Viral genomes could be amplified from EMBs of 165 (67.4%) of the 245 DCM patients: EV=23 (9.4%), ADV=4 (1.6%), PVB19=126 (51.4%), HHV-6=53 (21.6%), EBV=5 (2.0%), HCMV=2 (0.8%), including n=45 cases (27.3%) with multiple infections. Active or borderline myocarditis according to the Dallas classification did not exist in any case. Lymphocyte and macrophage infiltrates were not significantly different in virus-positive versus virus-negative patients.

CONCLUSIONS

Viral genomes were frequently detected in EMBs of patients with systolic left ventricular dysfunction. Our data suggest that myocardial persistence of various viruses, often presenting as multiple infections, may play a role in the pathogenesis of DCM far more frequently than suspected so far.

Cardiotropic viruses in the myocardium of children with end-stage heart disease

BACKGROUND

Transplantation has become a lifesaving procedure for children with end-stage heart failure. The long-term outcome for children who undergo transplantation has been of considerable interest, but the causes of graft failure and death are largely unknown, and the role of pre-transplant viral infection is unclear.

METHODS

Myocardial samples from 80 explanted hearts from children with end-stage heart disease caused by congenital heart disease (CHD), cardiomyopathy, or chronic rejection were analyzed using polymerase chain reaction and reverse-transcriptase polymerase chain reaction for cardiotropic viruses using virus-specific primers. We used immunohistochemical analysis of cytoskeletal proteins to evaluate myocyte architecture.

RESULTS

We identified parvoviral genomes in 6 patients (3 with CHD and 3 with cardiomyopathy). We detected no other viruses. Immunohistochemistry showed normal staining for key components of the cytoskeleton/sarcolemma, sarcomere, and nuclear membrane in the 6 virus-positive samples. The clinical outcome of these children was worse (4 long-term survivors, but 2 deaths) than for individuals without the genome.

CONCLUSIONS

Detecting viruses within the myocardium at the point of end-stage heart failure is not common, regardless of the primary pathology. However, the presence of viruses may result in poor outcome for the patient.

Increased susceptibility of male BALB/c mice to coxsackievirus B3-induced myocarditis: role for CD1d

BALB/c male mice infected with the H3 variant of coxsackievirus B3 (CVB3) develop fulminant myocarditis. Age-matched female mice show little myocarditis due to decreased virus receptor expression on cardiac cells. TNFalpha and IL-1beta levels were increased in males by 3 days after infection. IFNgamma levels increased more slowly throughout the 7-day observation period. CD4+, CD8+, macrophage (Mac3+) and gammadelta+ cells all accumulated in male hearts, with gammadelta+ cells showing early (day 3) infiltration. Females also accumulated CD4+ cells, but few of the other cell types. CD4+ cells in male hearts predominately produced IFNgamma, indicating a Th1 cell phenotype, whereas CD4+ cells in females produced IL-4, but little IFNgamma, indicating a Th2 phenotype. CD1d, a major histocompatibility complex I-like molecule often implicated in innate immunity, was increased in CVB3-infected male but not female cardiocytes both in vivo and in vitro. These results demonstrate that CVB3 infections produce gender-specific differences in both innate and adaptive immunity, which may explain the difference in disease susceptibility.

Acute myocarditis

Myocarditis is defined as inflammation of the myocardium accompanied by myocellular necrosis. Acute myocarditis must be considered in patients who present with recent onset of cardiac failure or arrhythmia. Often there is a history of an antecedent flu-like illness. Fulminant myocarditis is a distinct entity characterized by sudden onset of severe congestive heart failure or cardiogenic shock, usually following a flu-like illness. Giant cell myocarditis is a rare, frequently fatal disorder of unknown origin characterized by presence of giant cell inflammatory infiltrate in the myocardium. In recent years we have made good progress in understanding the causes, pathogenesis, natural history, diagnosis, and treatment of myocarditis. However, our knowledge is still far from complete. New information that extends our understanding of myocarditis is being reported constantly. This review summarizes recent advances in myocarditis, with an emphasis on the literature during the last year.