Although the etiologic stimulus has not been identified, there is considerable evidence that cell-mediated immune mechanisms play an important role in the pathogenesis of polymyositis-dermatomyositis: 1) a discrete chronic mononuclear cell infiltrate is almost always present in the affected muscles of patients with polymyositis-dermatomyositis; 2) the predominant cell of this chronic mononuclear cell infiltrate is the T lymphocyte; 3) the infiltrating T cells appear to be activated because they express activation antigens, such as major histocompatibility complex class II molecules; 4) peripheral blood lymphocytes express activation markers, they are sensitized to muscle, and they seem to be cytotoxic to musclein vitro; and 5) pathologic similarities reminiscent of polymyositis are found in animal models of experimental myositis. Recent observations on cell-mediated immunity in polymyositis-dermatomyositis, discussed in this review, provide new insights into the pathogenesis of the inflammatory myopathies.
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