Monocyte/macrophage differentiation in dermatomyositis and polymyositis

Recent advances have revealed significant differences in the pathogenesis of inflammatory myopathies. To determine whether different patterns of macrophage differentiation are a useful tool to delineate the major groups of inflammatory myopathies, the muscle biopsies of 11 patients with dermatomyositis and 12 patients with polymyositis were studied using different macrophage markers. In polymyositis, the early-activation markers MRP14 and 27E10 stained the majority of macrophages, which were recognized by the pan-macrophage marker Ki-M1P and which were located primarily in the endomysium. In dermatomyositis, macrophages predominantly expressed the late-activation marker 25F9 and were found mainly in the perimysium. Thus, the location and presence of different subsets of macrophages distinguish dermatomyositis and polymyositis. The predominance of early-activated macrophages in polymyositis indicates a more acute disease process. The findings in dermatomyositis, by contrast, suggest a role of persistent monocytes/macrophages in the disease process.