β₁-adrenergic receptor contains multiple IA^k and IE^k binding epitopes that induce T cell responses with varying degrees of autoimmune myocarditis in A/J mice

Myocarditis/dilated cardiomyopathy (DCM) patients can develop autoantibodies to various cardiac antigens and one major antigen is β₁-adrenergic receptor (β₁AR). Previous reports indicate that animals immunized with a β₁AR fragment encompassing, 197-222 amino acids for a prolonged period can develop DCM by producing autoantibodies, but existence of T cell epitopes, if any,were unknown. Using A/J mice that are highly susceptible to lymphocytic myocarditis, we have identified β₁AR 171-190, β₁AR 181-200, and β₁AR 211-230 as the major T cell epitopes that bind major histocompatibility complex class II/IA^k or IE^k alleles, and by creating IA^k and IE^k dextramers, we demonstrate that the CD4 T cell responses to be antigen-specific. Of note, all the three epitopes were found also to stimulate CD8 T cells suggesting that they can act as common epitopes for both CD4 and CD8 T cells. While, all epitopes induced only mild myocarditis, the disease-incidence was enhanced in animals immunized with all the three peptides together as a cocktail. Although, antigen-sensitized T cells produced mainly interleukin-17A,their transfer into naive animals yielded no disease. But, steering for T helper 1 response led the T cells reacting to one epitope, β₁AR 181-200 to induce severe myocarditis in naive mice. Finally, we demonstrate that all three β₁AR epitopes to be unique for T cells as none of them induced antibody responses. Conversely, animals immunized with a non-T cell activator, β₁AR 201-220, an equivalent of β₁AR 197-222, had antibodies comprising of all IgG isotypes and IgM except, IgA and IgE. Thus, identification of T cell and B cell epitopes of β₁AR may be helpful to determine β₁AR-reactive autoimmune responses in various experimental settings in A/J mice.