Inflammatory myopathy with abundant macrophages and dermatomyositis: two stages of one disorder or two distinct entities?

Inflammatory myopathy with abundant macrophages (IMAM) and dermatomyositis (DM) are considered to represent related disorders, since they share inflammatory infiltrates and skin alterations. In order to get more insight into these disorders, we addressed the cellular composition of the inflammatory infiltrates in muscle biopsies of 11 patients with IMAM and DM. In IMAM, inflammatory infiltrates predominantly consisted of CD68+ MRP14+ macrophages which weakly expressed TNF-α, a few CD3+ T cells with a prominent IL-10 expression, and single CD123+ plasmacytoid dendritic cells. In DM, infiltrates were mainly composed of CD3+ CD4+ T cells which expressed IL-10, numerous CD123+ plasmacytoid dendritic cells, and CD20+ B cells. The low number of CD68+ macrophages was of 25F9+ late inflammatory phenotype. Membrane attack complex was expressed in necrotic muscle fibers in IMAM and on endomysial capillaries in DM, respectively. Thus, in IMAM, the inflammatory reaction markedly differed from DM. These observations may lend support to the hypothesis that IMAM and DM are distinct with respect to their pathogenesis. Whether, alternatively, these differences in the cellular composition of the infiltrates and the cytokine profile rather reflect different stages of disease, will require the analysis of a larger series.