Streptococcus pyogenes-induced acute rheumatic fever (ARF) is one of the best examples of postinfectious autoimmunity due to molecular mimicry between host and pathogen. Sydenham chorea is the major neurological manifestation of ARF but its pathogenesis has remained elusive, with no candidate autoantigen or mechanism of pathogenesis described. Chorea monoclonal antibodies showed specificity for mammalian lysoganglioside and N-acetyl-beta-D-glucosamine (GlcNAc), the dominant epitope of the group A streptococcal (GAS) carbohydrate. Chorea antibodies targeted the surface of human neuronal cells, with specific induction of calcium/calmodulin-dependent protein (CaM) kinase II activity by monoclonal antibody 24.3.1 and sera from active chorea. Convalescent sera and sera from other streptococcal diseases in the absence of chorea did not activate the kinase. The new evidence implicates antibody-mediated neuronal cell signaling in the immunopathogenesis of Sydenham chorea and will lead to a better understanding of other antibody-mediated neurological disorders.

The mechanisms by which autoantibodies against cardiac myosin (CM) may lead to heart dysfunction is unknown. We show that autoantibodies to CM in anti-CM sera and mAbs derived from experimental autoimmune myocarditis targeted the heart cell surface and induced Ab-mediated cAMP-dependent protein kinase A activity. Ab-mediated cell signaling of protein kinase A was blocked by CM, anti-IgG, or by specific inhibitors of the β-adrenergic receptor (β-AR) pathway. mAbs confirmed mimicry between CM and the β-AR. Passive transfer of purified Ab (IgG) from CM-immunized rats resulted in IgG deposition and apoptosis in the heart, leading to a cardiomyopathic heart disease phenotype in recipients. Our novel findings link anti-CM Ab with the β-AR and subsequent Ab-mediated cell signaling in the heart.

Myocarditis, often initiated by viral infection, may progress to autoimmune inflammatory heart disease, dilated cardiomyopathy and heart failure. Although cardiac myosin is a dominant autoantigen in animal models of myocarditis and is released from the heart during viral myocarditis, the characterization, role and significance of anti-cardiac myosin autoantibodies is poorly defined. In our study, we define the human cardiac myosin epitopes in human myocarditis and cardiomyopathies and establish a mechanism to explain how anti-cardiac myosin autoantibodies may contribute to heart disease. We show that autoantibodies to cardiac myosin in sera from myocarditis and dilated cardiomyopathies in humans targeted primarily epitopes in the S2 hinge region of cardiac myosin. In addition, anti-cardiac myosin antibodies in sera or purified IgG from myocarditis and cardiomyopathy targeted the beta-adrenergic receptor and induced antibody-mediated cAMP-dependent protein kinase A (PKA) cell signaling activity in heart cells. Antibody-mediated PKA activity in sera was abrogated by absorption with anti-human IgG. Antibody-mediated cell signaling of PKA was blocked by antigen-specific inhibition by human cardiac myosin or the beta-adrenergic receptor but not the alpha adrenergic receptor or bovine serum albumin. Propranolol, a beta blocker and inhibitor of the beta-adrenergic receptor pathway also blocked the antibody-mediated signaling of the beta-adrenergic receptor and PKA. The data suggest that IgG antibody against human cardiac myosin reacts with the beta-adrenergic receptor and triggers PKA signaling in heart cells. In-summary, we have identified a new class of crossreactive autoantibodies against human cardiac myosin and the beta-adrenergic receptor in the heart. In addition, we have defined disease specific peptide epitopes in the human cardiac myosin rod S2 region in human myocarditis and cardiomyopathy as well as a mechanistic role of autoantibody in the pathogenesis of disease.

Myocarditis is a common cause of dilated cardiomyopathy leading to heart failure. Chronic stages of myocarditis may be initiated by autoimmune responses to exposed cardiac Ags after myocyte damage. Cardiac myosin, a heart autoantigen, induced experimental autoimmune myocarditis (EAM) in susceptible animals. Although cardiac myosin-induced myocarditis has been reported in Lewis rats, the main pathogenic epitope has not been identified. Using overlapping synthetic peptides of the S2 region of human cardiac myosin, we identified an amino acid sequence, S2–16 (residues 1052–1076), that induced severe myocarditis in Lewis rats. The myocarditic epitope was localized to a truncated S2–16 peptide (residues 1052–1073), which contained a sequence identical in human and rat cardiac myosin. The S2–16 peptide was not myocarditic for three other strains of rats, in which the lack of myocarditis was accompanied by the absence of strong S2–16-specific lymphocyte responses in vitro. For Lewis rats, S2–16 was characterized as a cryptic epitope of cardiac myosin because it did not recall lymphocyte and Ab responses after immunization with cardiac myosin. Lymphocytes from S2–16 immunized rats recognized not only S2–16, but also peptides in the S2–28 region. Furthermore, peptide S2–28 was the dominant epitope recognized by T cells from cardiac myosin immunized rats. S2–16 was presented by Lewis rat MHC class II molecules, and myocarditis induction was associated with an up-regulation of inflammatory cytokine production. S2–16-induced EAM provides a defined animal model to investigate mechanisms of EAM and modulation of immune responses to prevent autoimmune myocarditis.

Experimental autoimmune myocarditis (EAM) can be induced in the LewisMyocarditis is an inflammatory heart disease that can be initiated by infectious pathogens. 1-3 Dilated cardiomyopathy, which may follow myocarditis and represent the chronic stage of disease, is a major cause of heart failure and heart transplantation. 4 -6 Evidence suggests that autoimmune responses to cardiac antigens exposed after heart damage may play an important role in prolonged damage of myocardium. 3,7-9 Nevertheless, little progress has been made in treating myocarditis by immunosuppression, because a complete understanding of key factors that regulate the pathogenic immune responses in autoimmune myocarditis are not well established.Experimental autoimmune myocarditis (EAM) generated in susceptible mouse and rat strains by immunization with purified cardiac myosin or a specific pathogenic cardiac myosin peptide in adjuvant has been used to investigate the pathogenesis of myocarditis induced by autoimmune mechanisms. 10 -20 Many studies have shown that cardiac antigen-induced myocarditis is a Tcell-mediated disease. 18,[21][22][23][24] However, the active induction of EAM relies on the use of bacterial adjuvants [complete Freund's adjuvant (CFA)] during immunization, suggesting that activation of the innate immune system is important in disease induction. [25][26][27] Inflammatory cytokines such as interleukin (IL)-1, tumor necrosis factor (TNF)-␣, and IL-12 promote myocarditis development in animals, 28 -31 whereas mice that lack TNF-Rp55 or are deficient in IL-12 signaling were protected from EAM. 32,33 In vivo inhibition of co-stimulatory molecule B7-1 and CD40 also markedly decreased myocardial inflammation. 34,35 A recent study directly demonstrated that cardiac antigen-loaded dendritic cells (DCs) induced autoimmune myocarditis when they were activated and transferred. 36 Taken together, these studies suggest that EAM induction is closely associated with not only the myocarditic epitopes of cardiac myosin and their reactive T cells, but also with the activation of antigen-presenting cells (APCs) such as DCs by inflammatory cytokines.