Myocarditis-inducing epitope of myosin binds constitutively and stably to I-A^k on antigen-presenting cells in the heart

Immune interactions in the heart were studied using a murine model of myosin-induced autoimnume myocarditis. A T cell hybridoma specific for mouse cardiac myosin was generated from A/J mice and used to demonstrate that endogenous myosin/I-A^k complexes are constitutively expressed on antigen-presenting cells in the heart. This T cell hybridoma, Seu.5, was used as a functional probe to identify a myocarditis-inducing epitope of cardiac myosin. Overlapping peptides based on the cardiac myosin heavy chain α (myhcα) sequences were synthesized and tested for their ability to stimulate Seu.5 T cells. One peptide, myhcα(325-357) strongly stimulated the Seu.5 T cells, localizing the epitope to this region of the myhcα molecule. Using truncated peptides, the epitope was further localized to residues 334-352. The myhcα(334-352) peptide strongly induced myocarditis when administered to A/J mice, which was histologically indistinguishable from that induced by myosin. The myhcα(334-352) epitope was present in cardiac myosin and not skeletal muscle myosins, providing a biochemical basis for the cardiac specificity of this autoimmune disease. Induction of myocarditis by this epitope was restricted to the myhcα isoform and not the myhcβ isoform, suggesting there may be a difference in the efficiency of generating tolerance to these isoforms of cardiac myosin, which are differentially developmentally regulated. The myhcα(334-352) epitope bound to purified I-A^k molecules in a similar manner to other I-A^k-restricted immunogenic epitopes, HEL(48-61) and RNase(43-56). Importantly, the myhcα(334-352) epitope was able to bind to I-A^k molecules on the surface of antigen-presenting cells in a stable manner. These findings demonstrate that autoantigenic epitopes can behave in a dominant manner and constitutively bind to class II molecules in the target organ in a similar manner to foreign immunogenic epitopes.