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Signs of inflammation in both symptomatic and asymptomatic muscles from patients with polymyositis and dermatomyositis

C Dorph¹, P Englund¹, I Nennesmo², I E Lundberg¹

¹ Rheumatology Unit, Department of Medicine, Karolinska University Hospital, Karolinska Institutet, Solna, Sweden
² Department of Laboratory Medicine, Division of Pathology, Karolinska University Hospital, Huddinge, Sweden

Correspondence to:
C Dorph
Rheumatology Clinic, D2:01, Karolinska University Hospital, Solna, 171 76 Stockholm, Sweden; Christina.Dorph{at}karolinska.se

Objectives: To determine whether muscle weakness is correlated with inflammation, expression of interleukin 1 (IL1) and major histocompatibility complex (MHC) class I and II antigens on muscle fibres.

Methods: Biopsy specimens from clinically symptomatic (proximal muscles) and asymptomatic (all distal but two proximal) muscles in eight patients with polymyositis, three patients with dermatomyositis and six healthy controls were analysed by immunohistochemistry for the presence of T cells and macrophages, and expression of IL1 and of MHC class I and II antigens. Results were evaluated by conventional light microscopy and by computerised image analysis.

Results: Inflammatory infiltrates with T cells and macrophages were observed to an equal degree in both symptomatic and asymptomatic muscle. The numbers of capillaries with IL1 expression were significantly higher (p<0.05) in the symptomatic and asymptomatic muscles of patients than in controls. The total IL1 expression per tissue section assessed by computerised image analysis was significantly higher in symptomatic muscles but not in asymptomatic muscles compared with that in controls. Neither the number of IL1-positive capillaries nor the total IL1 expression differed significantly between symptomatic and asymptomatic muscles. Expression of MHC class I and II antigens on muscle fibres was detected in both symptomatic and asymptomatic muscles but rarely in healthy controls.

Conclusions: Presence of inflammatory infiltrates, T cells and macrophages, and expression of MHC class I and II antigens and of IL1 on muscle fibres were independent of clinical symptoms, and were present to an equal degree in both proximal and distal muscles. Thus, other factors seem to determine the development of clinical symptoms. One such factor could be variations in physical demands.

Abbreviations: EMG, electromyography; HLA, human leucocyte antigen; MHC, major histocompatibility complex; MRI, magnetic resonance imaging

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