Immunopathogenesis of myositis

These inflammatory muscular disorders include dermatomyositis (DM), polymyositis (PM) and inclusion body myositis (IBM). One of the characteristic histological features of DM is ''perifascicular atrophy''. Immunohistochemical analyses in DM show deposition of complement proteins in capillaries. The patients probably produce pathological antibodies that react with components of the muscle endothelial cells, thus causing secondary deposition of complement. In contrast, PM is marked by a cellular injury mechanism. The characteristic histological feature is endomysial T cell infiltration, consisting predominantly of CD8-positive T lymphocytes. These cytotoxic T cells surround and invade individual muscle fibres. This culminates in destruction of the muscle fibre. The attacked muscle fibres express class I (HLA-A, B, C) histocompatibility antigens. It is likely that the CD-8 positive T lymphocytes recognise muscular autoantigens as HLA-bound peptides. The active autoantigens are still unknown. In IBM - though not in PM - electron-microscopy reveals intranuclear and intracytoplasmic filaments that resemble myxoviruses. Hence, although it is possible that IBM is based on a virus infection, this has not been proved. Other investigations have shown interesting immunohistochemical similarities between IBM and Alzheimer's disease.