FRI0397 Peculiar expression of autophagy biomarkers in necrotizing autoimmune myopathy muscle
Authors: M. Giannini, F. Girolamo, A. Amati, A. Lia, L. Serlenga, D. D’Abbicco, M. Tampoia, F. Iannone
Journal: Annals of the Rheumatic Diseases (2018)

<h3>Background</h3> Immune mediated necrotizing myopathy (IMNM) is a recently recognised pathology within the spectrum of idiopathic inflammatory myopathies (IIMs). Specific autoantibodies and the response to immunosuppressants aid to make the diagnosis and suggest immune-mediated pathogenesis, although histopathological features are not specific for IMNM. Autophagy and ubiquitin-proteasome system are two interacting systems by which dysfunctional cellular components are degraded in the cell. Their dysregulation, in sporadic Inclusion Body Myositis (sIBM), seems to be responsible for the protein aggregates. The autophagy dysfunction in IMNM was not widely investigated. <h3>Objectives</h3> To investigate autophagy marker expression, macrophages localization and accumulation of misfolded proteins in non-necrotic fibres of IMNM muscle in comparison with Dermatomyositis (DM), Polymyositis (PM) and sIBM. <h3>Methods</h3> Among 52 IIMs diagnosed from January 2015 to June 2017, we reviewed muscle biopsies and stored sera. Six subjects were included in the IMNM group, characterised by many necrotic muscle fibres, regenerating muscle fibres and no significant inflammation despite of numerous but scattered macrophages removing necrotic muscle fibres. Two patients had anti-signal recognition particle (SRP) autoantibodies, two patients anti-3-hydroxy-3-methylglutaryl-coenzyme A reductase (HMGCR), the others tested negative for specific autoantibodies. All IMNM patients had a positive response to immunosuppressants. Muscle sections were immunolabelled with the following antigens: ubiquitin, autophagy markers LC3b, p62 (a receptor of autophagy), TDP-43 (a marker of ubiquitinated proteic inclusions), SMI31 and SMI310 (Phosphorylated Neurofilaments), CD31 (endothelial cell marker), C5b-9 (membrane attack complex), CD4 (T-helper lymphocytes), CD8 (T-suppressor lymphocytes), CD68 (macrophages), CD20 (B-lymphocytes), CD56 (NK lymphocytes and regenerating muscle fibres), MHC I, MHC II. Quantitative results were compared among IMNM (n=6), DM (n=4), sIBM (n=4), PM (n=5) and healthy controls (n=4). <h3>Results</h3> In IMNM, inflammation was mild compared with DM, PM, sIBM, and consisted in sporadic endomysial and/or perivascular cells CD68+. Skeletal muscle fibres (SMFs) containing LC3b+puncta were significantly higher in IMNM and IBMs than in DM or PM. In all IMNM, the greater proportion of LC3b+puncta was localised in CD56 +fibres (figure 1), instead, sIBM showed a high number of LC3b+puncta in vacuolated SMFs with low expression of CD56 +SMFs. As expected, P62 and SMI31 aggregates were significantly higher in sIBM than in the other IIMs, even if, also in IMNM, there were moderate p62 accumulations and a little proportion of SMFs stained by SMI31. Finally, in IMNM there was the highest number of ubiquitin +SMFs. <h3>Conclusions</h3> These findings suggest an involvement of cellular clearance systems in the pathophysiology of IMNM similarly to sIBM. Nevertheless, LC3b+puncta in regenerating fibres can be considered a peculiar biomarker in IMNM. Further studies of larger cohort of patients are needed to better define IMNM. <h3>Disclosure of Interest</h3> None declared