Aims: To evaluate tissue eosinophil density, localization of eosinophil cytotoxic products, histopathological muscle changes and inflammatory cell types in different eosinophilia-associated myopathies, which are clinicopathologically heterogeneous; -to assess an immunohistological range of tissue eosinophil density in non-eosinophilic inflammatory myopathies.
Methods: Muscle biopsies from 7 patients with blood and/or tissue eosinophilia and clinico-laboratory myopathic signs (5 chronic course myopathies, 1 subacute onset fasciitis/myositis, 1 acute myositis), and from 18 non-eosinophilic inflammatory myopathies, underwent routine stains, inflammatory infiltrate immunophenotyping, immunostain for eosinophil major basic protein (MBP) and transmission electron microscopy examination. Eosinophils and total inflammatory cells counts were statistically analyzed.
Results: Histology evidentiated in all cases occasional or no infiltrating eosinophils. By MBP stain, tissue eosinophils density and percentages in eosinophilia-associated myopathies were significantly higher than in idiopathic myositides. Extracellular MBP diffusion, the hallmark of eosinophil cytotoxicity, was recurrent on sarcolemma and endothelium. Electron microscopy showed eosinophils in close adjacency to sarcolemma, abundant mastcells and capillary endothelial swellings. Immunostaining detected higher mean eosinophil density in idiopathic myositides than previously histologically assessed.
Conclusions: MBP immunohistology on skeletal muscle, previously performed only in acute eosinophilic polymyositis, suggests that eosinophil-mediated injury of muscle cells may occur in a wider spectrum of less aggressive eosinophilia-associated myopathies. As conventional histology is likely to underestimate this leukocyte subset, MBP stain might therefore be a tool in analysis of tissue infiltration of eosinophils, as a possible treatment target.