Autoantibodies and their clinical significance in idiopathic inflammatory myopathies; polymyositis/dermatomyositis and related conditions.

The inflammatory muscle diseases, polymyositis (PM) and dermatomyositis (DM) are systemic connective tissue disorders characterized by chronic inflammation in skeletal muscle and involvement of various systemic organs. The pathogenesis of these heterogeneous diseases is unknown, but appear to mediate an autoimmune disorder that culminates in the tissue damage. Autoantibodies directed against various cellular constituents have been detected in patients with PM/DM, and 40-50% of patients have autoantibodies (myositis-specific antibodies : MSAs) that are found specifically in myositis patients. These autoantibodies are closely associated with characteristic clinical features and therefore provide us useful information for diagnosis, patient classification as well as predict of signs, symptoms of myositis, response to treatment, and prognosis. Autoantibodies to the cytoplasmic antigens, that are involved in protein synthesis or translation related proteins, are seen in patients with PM. Autoantibodies to eight of the aminoacyl tRNA synthetases are each associated with a similar syndrome marked by myositis, interstitial lung disease, arthritis, and other features constituting an "anti-synthetase syndrome." However, certain differences of the clinical features associated with each anti-synthetase have been noted, although their similarity is impressive. Anti-signal recognition particle antibodies are associated with severe, refractory myositis that differs significantly from anti-synthetase syndrome. Autoantibodies to the nuclear antigen, Mi-2 that is a transcription-regulating protein, are specifically seen in patients with DM responsive to corticosteroid therapy. In recent years, novel MSAs have been identified in clinically amyopathic dermatomyositis (anti-CADM-140 antibodies) and malignancy-associated myositis (anti-p155 and p155/p140 antibodies), in which autoantibodies have been thought to be negative. For understanding the pathogenic mechanisms of PM/DM, it is important to elucidate the relationship between these novel MSAs and their related clinical entities. Recently the nature of the target MSA autoantigens has been characterized using molecular biology and proteomic techniques. However, the mechanism of development of MSAs remains unknown. Further analysis of the molecular structure and biological function of target autoantigens recognized by these MSAs might provide the clues to the understanding of the etiology and pathogenesis of these disorders.