THU0344 CLINICAL AND HISTOLOGICAL SPECTRUM OF ANTI-MI2 DERMATOMYOSITIS: A MULTICENTRE RETROSPECTIVE COHORT
Authors: M. Fornaro, F. Girolamo, L. Cavagna, F. Franceschini, M. Giannini, G. Zanframundo, M. Fredi, M. Tampoia, A. Amati, L. Serlenga, A. Lia, L. Moschetti, D. Dabbicco, L. Coladonato, F. Iannone
Journal: Annals of the Rheumatic Diseases (2020)

Background: Dermatomyositis (DM) shows a wide clinical spectrum that seems to be different based on the type of autoantibody status. Furthermore, less is known regarding to the histopathology of different serological subsets of DM Objectives: The aim of our study was to investigate clinical and histopathological hallmarks in adult DM patients positive for anti-Mi2 (Mi2+) antibody compared to DM patients negative for anti-Mi2 (Mi2-) Methods: All clinical data of adult DM patients admitted in three tertiary Rheumatology Units, who fulfilled EULAR/ACR 2017 classification criteria 1 , were included in this study. Overlap syndrome and antisynthetase syndrome were exclusion criterion. Serum samples were tested in the local reference laboratories using line immunoassays methods for Myositis specific and associated antibodies. Histopathological study was carried out from muscle biopsies performed for diagnostic purpose in outpatient clinic of Bari (Italy) University. Quantitative analysis was performed for myofiber and capillary features, whereas semi-quantitative analysis (score from 0 to 3) was performed for inflammatory cells infiltrate, both at endomysial and perimysial sites Results: A total of 95 DM patients, followed for a median (IQR) follow-up of 28 (9-85) months, were analyzed. Of these, 23 (24.2%) patients (87% female, mean age at onset 55.4±16.2 years) were anti-Mi2+, while 72 (75.8%) patients were Mi2- (72.2% female, mean age at onset 55.2±17 years). All Mi2+ patients showed muscle involvement. Moreover, Mi2+ DM showed higher levels of serum creatine kinase (CK) at onset compared to Mi2- (CK (IQR): 2649 UI/l (1130-6000) vs 575 UI/l (164-1617), p&lt;0.001). Prevalence of interstitial lung disease (ILD) was lower in Mi2+ patients (8.7% vs 30.6%, p=0.05), and no case of rapidly progressive ILD (RP-ILD) was found. Survival analysis at 5-years follow-up highlighted good survival for Mi2+ patients, but not different from Mi2- (95.7% vs 83.1%, p=0.151). Multivariate analysis showed that age at onset (HR:1.07), RP-ILD (HR:36.2) and cancer associated myositis (HR:6.1) correlated with a poor prognosis. Finally, a total of 26 biopsies (12 Mi2+ and 14 Mi2-) were included into the histological analysis showing higher prevalence of necrotic/degenerating myofibers (median (IQR) 2.6 (0.7-11)% vs 0.6 (0.4-1.1)%, p=0.009) and sarcoplasmic deposit of membrane attack complex (MAC) (median (IQR) 0.2 (0-1.2)% vs (0(0-0)%,p=0.009) in Mi2+ patients. In addition, the endomysial macrophage score was higher in Mi-2+ patients (median 1.5 (0.25-2) vs (0.5 (0-1), p=0.031) Conclusion: Mi2+ patients represent a specific DM subset with higher muscle damage, sarcoplasmic MAC deposits and endomysial macrophages infiltration as histological hallmarks References: [1]Ann Rheum Dis. 2017 Dec;76(12):1955-1964 Disclosure of Interests: Marco Fornaro: None declared, Francesco Girolamo: None declared, Lorenzo Cavagna: None declared, Franco Franceschini: None declared, margherita giannini: None declared, Giovanni Zanframundo: None declared, Micaela Fredi: None declared, Marilin Tampoia: None declared, Angela Amati: None declared, Luigi Serlenga: None declared, Anna Lia: None declared, Liala Moschetti: None declared, Dario Dabbicco: None declared, Laura Coladonato: None declared, Florenzo Iannone Consultant of: Speaker and consulting fees from AbbVie, Eli Lilly, Novartis, Pfizer, Roche, Sanofi, UCB, MSD, Speakers bureau: Speaker and consulting fees from AbbVie, Eli Lilly, Novartis, Pfizer, Roche, Sanofi, UCB, MSD