Inflammatory myopathy with abundant macrophages (IMAM): the immunology revisited

We describe a patient with a clinically atypical presentation of inflammatory myopathy with abundant macrophages (IMAM) but with convincing muscle biopsy features of this subform of inflammatory myopathy. IMAM is characterized mainly by a conspicuous infiltration of muscle and connective tissue by numerous macrophages remote from necrotic and basophilic regenerating muscle fibers. Typically few, mostly CD4(+) T helper (Th) cells are also present. Here, we report a patient with IMAM and demonstrate, that most macrophages express the macrophage mannose receptor 1 (CD206) corresponding to alternatively activated (M2) polarization. Accordingly, signal transducer and activator of transcription 6 (STAT6), involved in Th2-M2 immunity, was expressed at high levels in skeletal muscle. However, TNFα, IFNγ and STAT1, mediators of the T helper 1-classically activated (M1) response were elevated in skeletal muscle and in blood, while expression of CD206 was elevated in skeletal muscle only. Our results argue that IMAM could be a distinct entity between the inflammatory myopathies rather than a subform of dermatomyositis.