Abstract-Experimental autoimmune myocarditis (EAM) resembles the giant cell myocarditis seen in humans, and recurrent forms lead to dilated cardiomyopathy. EAM has been shown to be a T cell-mediated autoimmune myocarditis. We have previously shown that cDNA encoding V complementarity-determining region (CDR) 3 from heart-and pericardial space-infiltrating T cells in EAM induced by rod cardiac myosin contains more restricted sequences than that from normal spleen T cells. Recently, it has become apparent that several epitopes of EAM exist in rod cardiac myosin; therefore, T cells infiltrating into lesions may recognize certain epitopes in EAM induced by rod cardiac myosin. In this study, we examined heart-and pericardial space-infiltrating T-cell clonotypes in EAM induced by synthetic peptides of cardiac myosin. EAM was produced by immunization with synthetic peptides corresponding to N-terminally acetylated amino acids 1539 to 1555 of rat cardiac myosin ␣ heavy chain. Five of 12 rats receiving synthetic peptides developed macroscopic signs of myocarditis. To examine T-cell receptor (TCR) V expression and CDR3 of the TCR  chain of lesion-infiltrating T cells in EAM, total RNA was isolated from heart, pericardial effusion, spleen, lymph node, and peripheral blood. TCR V expression of the T cells infiltrating the lesions revealed a predominance of V4. On the basis of single-strand conformation polymorphism analysis for CDR3 of the TCR V4 chain, heart-and pericardial space-infiltrating T cells were considered to be oligoclonal, whereas spleen, lymph node, and peripheral blood in a rat with EAM and spleen in a native rat were considered to be polyclonal. In the same rat, clonotypes of heart-infiltrating T cells were almost the same as those of pericardial space-infiltrating T cells. Furthermore, on sequence analysis for CDR3 of the TCR V4 chain, the amino acid motifs were similar among T cells infiltrating into lesions of different EAM rats. In the present study, TCR  chains of heart-and pericardial space-infiltrating T cells in EAM induced by synthesized peptide consisting of 17 amino acids were examined. V4ϩ T cells with similar V CDR3 motifs that infiltrate the heart and pericardial space may recognize the same epitope. (Circ Res. 1998;83:133-140.)Key Words: myocarditis Ⅲ T-cell receptor Ⅲ epitope Ⅲ cardiomyopathy Ⅲ immune system E AM induced by injecting cardiac myosin into susceptible strains of rats and mice resembles the giant cell myocarditis seen in humans, and the recurrent forms lead to dilated cardiomyopathy.1-3 EAM in rats is characterized by cardiomegaly, pericardial effusion, and extensive myocardial necrosis. Mononuclear cells, which are almost exclusively macrophages and CD4ϩ T cells, massively infiltrate the heart and pericardial space.4,5 EAM has been shown to be a T cell-mediated autoimmune disease by adoptive transfer, and the prevention of EAM by anti-␣TCR antibody indicates the important role of ␣T cells. [6][7][8] ␣T cells recognize antigenic peptides in the context of major histo...