Objective: To address the hypothesis that endothelial cells and/or muscle fibers are primary targets in the disease process by analysis of muscle tissue from patients with polymyositis (PM) and dermatomyositis (DM).
Methods: We included patients with laboratory signs and clinical symptoms typical of myositis, but without detectable infiltration of clusters of inflammatory cells in their muscle biopsy samples. An immunohistochemical technique was applied to identify CD3, CD68, lymphocyte function-associated antigen 1alpha, CD11b, very late activation antigen 4, endothelium 4, interleukin-1alpha (IL-1alpha), intercellular adhesion molecule 1, vascular cell adhesion molecule 1, IgG, IgM, IgA, and HLA-A/B/C in muscle tissue. Fiber type was defined by ATPase staining.
Results: IL-1alpha expression was detected in endothelial cells of capillaries to a greater extent in patients than in controls, and class I major histocompatibility complex (MHC) expression was significantly increased in muscle fibers. We also observed that class I MHC expression was mainly confined to type II muscle fibers.
Conclusion: Our findings imply that defined molecular changes of blood vessels and muscle fibers are both independent of adjacent inflammatory infiltrates and could thus be primary events in the development of myositis. Moreover, both IL-1alpha and class I MHC molecules might be important for the development of clinical symptoms in PM and DM patients.