Interleukin‐1 expression in inflammatory myopathies: evidence of marked immunoreactivity in sarcoid granulomas and muscle fibres showing ischaemic and regenerative changes

The most frequent autoimmune adult inflammatory myopathies are dermatomyositis, polymyositis, inclusion body myositis, and sarcoid myopathy. Interleukin‐1 (IL‐1) is a pleiotropic molecule, implicated in the inflammatory process, but also in tissue protection and remodelling. We evaluated the immunocytochemical expression of IL‐1α and β in frozen muscle biopsy specimens from patients with dermatomyositis (15 cases), polymyositis (five cases), inclusion body myositis (five cases) and sarcoid myopathy (five cases). Positive immunoreactivities, were observed in both inflammatory cells and muscle fibres. Specificity of the immunostaining was assessed by Western blot experiments. IL‐1 positive inflammatory cells were rare in polymyositis and inclusion body myositis, moderately abundant in dermatomyositis, and prominent in sarcoid myopathy granulomas. In sarcoid myopathy, 24.6±4.1% inflammatory cells were IL‐1α‐positive and 45.2±2.6% were IL‐1β‐positive. IL‐1 positive muscle fibres were mainly observed in dermatomyositis, usually remote from inflammatory infiltrates. Positive immunostaining for IL‐1 was observed in fibres showing ischaemic punched‐out vacuoles, that correspond to areas of myosinolysis, in atrophic perifascicular fibres, and in fibres located within healing microinfarcts. All NCAM‐positive regenerating fibres were IL‐1 positive. We conclude that: (i) IL‐1 is expressed in granulomas of sarcoid myopathy, which is in keeping with the role ascribed to IL‐1 in the formation of granulomas; (ii) IL‐1 is expressed by muscle fibres undergoing ischaemic damage; and (iii) IL‐1 expression by muscle fibres is associated with myofibrillar protein breakdown and regeneration.