Cellular mechanisms in inflammatory myopathies

Cell-mediated immune mechanisms play a prominent role in inclusion body myositis (IBM) and polymyositis (PM). In both IBM and PM, CD8+ cytotoxic T cells expressing the alpha/beta receptor surround and focally invade non-necrotic muscle fibres. This lesion can be considered the hallmark of cell-mediated myocytotoxicity. Essentially the same type of lesion is observed in a variant form of PM, in which CD4-CD8- T cells bearing the gamma/delta receptor surround and invade non-necrotic muscle fibres. In both IBM and PM, all of the invaded and some of the non-invaded muscle fibres strongly express HLA class I molecules. This is consistent with the hypothesis that the CD8+ autoinvasive cytotoxic T cells recognize antigenic peptide(s) bound to HLA class I molecules on the muscle fibre surface. According to the rules of antigen processing, these peptides derive from proteins synthesized in the muscle fibre. Theoretically, the proteins could be viral components or self proteins that resemble viral components. Most attempts to demonstrate viral antigens or genome in muscle fibres have failed. On the other hand, the majority of HLA class I molecules expressed on the surface of any cell are loaded with endogenous self peptides. It seems plausible that muscle-specific autoantigen(s) could be recognized by autoaggressive T cells in the inflammatory myopathies, but the precise reasons for the recognition event remains elusive. Recently, it has become possible to study the interactions of muscle cells and cytotoxic effector cells in vitro. Myoblasts and myotubes can be induced to express a variety of immunologically relevant histocompatibility and cell adhesion molecules. Myotubes are highly susceptible to lysis by allogeneic CD8+ cytotoxic T cells sensitized against HLA class I alloantigens. Interestingly, cultured myotubes are also susceptible to lysis by antigen-nonspecific natural killer cells. Further, myoblasts stimulated by IFN gamma express HLA class II and acquire the full potential to process and present complex protein antigens to CD4+ T cells. This may indicate that myoblasts can actively participate in local immune reactions by presenting (auto) antigens to helper/inducer T cells. In different inflammatory myopathies, CD8+ T cells have been expanded directly from muscle and their interactions with autologous myotubes have been investigated in vitro. In several cases, a low but significant autoreactive cytotoxic effect was observed. This is consistent with the hypothesis that some cytotoxic effector T cells recognize an autoantigen on myotubes. One of the major goals for future studies is to define the autoantigens that are relevant in the pathogenesis of the inflammatory myopathies.