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 autoimmune myocarditis. A T cell hybridoma specific for mouse cardiac myosin was generated from A/J mice and used to demonstr ate that endogenous myosin/I-A(k) complexes are constitutively express ed on antigen-presenting cells in the heart. This T cell hybridoma, Se u.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 alpha (myhc alpha) sequences were synthesized and t ested for their ability to stimulate Seu.5 T cells. One peptide, myhc alpha(325-357) strongly stimulated the Seu.5 T cells, localizing the e pitope to this region of the myhc alpha molecule. Using truncated pept ides, the epitope was further localized to residues 331-352. The myhc alpha(334-352) peptide strongly induced myocarditis when administered to A/J mice, which was histologically indistinguishable from that indu ced by myosin. The myhc alpha(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 m yocarditis by this epitope was restricted to the myhc alpha isoform an d not the myhc beta isoform, suggesting there may be a difference in t he efficiency of generating tolerance to these isoforms of cardiac myo sin, which are differentially developmentally regulated. The myhc alph a(334-352) epitope bound to purified I-A(k) molecules in a similar man ner to other I-A(k)-restricted immunogenic epitopes, HEL(48-61) and RN ase(43-56). Importantly, the myhc alpha(334-352) epitope was able to b ind to I-A(k) molecules on the surface of antigen-presenting cells in a stable manner. These findings demonstrate that autoantigenic epitope s can behave in a dominant manner and constitutively bind to class II molecules in the target organ in a similar manner to foreign immunogen ic epitopes.