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In polymyositis and sporadic inclusion body myositis, clonal expansion of CD8+ cells which are primed to recognize previously unknown muscle antigens occurs. Compared with sporadic inclusion body myositis, however, in which the T-cell response may not be antigen driven, there is in polymyositis an overexpression of certain T-cell receptor gene families among the autoinvasive T-cells. Although studies on the endomysial expression of cytokines and cell adhesion molecules have provided additional support for the concept of an ongoing immune process, the site of sensitization and the mechanism by which the autoimmune process is triggered remains to be established. In dermatomyositis, a multiorgan disease, evidence exists that the complement-mediated microvascular injury by the putative antibody may not be limited to the endomysial vessels but may also involve the blood vessels in the dermis. The antigenic target on the endothelial cell in dermatomyositis patients and the pathogenic role of the recently studied anti-Mi-2 antibody directed against a helicase are still to be determined.