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 demonstrate that endogenous
myosin/I-Ak 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 alpha (myhc alpha) sequences were synthesized and tested for their ability to stimulate Seu.5 T cells. One
peptide, myhc alpha (325-357) strongly stimulated the Seu.5 T cells, localizing the
epitope to this region of the myhc alpha molecule. Using truncated
peptides, the
epitope was further localized to residues 334-352. The myhc alpha (334-352)
peptide strongly induced
myocarditis when administered to A/J mice, which was histologically indistinguishable from that induced 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
myocarditis by this
epitope was restricted to the myhc alpha
isoform and not the myhc beta
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 myhc alpha (334-352)
epitope bound to purified I-Ak molecules in a similar manner to other I-Ak-restricted immunogenic
epitopes, HEL(48-61) and RNase(43-56). Importantly, the myhc alpha (334-352)
epitope was able to bind to I-Ak 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.