Canine inflammatory myopathy: analysis of cellular infiltrates.

Inflammatory myopathies (IMs) occur relatively frequently in dogs, and, with the exception of masticatory muscle myositis (MMM), have not been characterized. This study analyzed the distribution and types of cellular infiltrates in 21 cases of generalized IM, 3 cases of focal IM (MMM), and 1 case with features of both generalized and focal IM, using a panel of monoclonal antibodies to cell surface markers. In generalized IM, mononuclear cells showed an endomysial and perimysial distribution with invasion of non-necrotic fibers similar to human IM. T lymphocytes with T-cell receptor (TCR)alphabeta predominated. Distinct differences were seen in MMM including prominent B-cell infiltration, dendritic cells and macrophages in greater numbers than T cells, and numerous T cells with TCRgammadelta. Thus, generalized IM and MMM appear to be distinct diseases with different mechanisms. Canine generalized IM may be an important animal model for human IM.