Objectives: Several studies reported an association between Sjögren's disease (SjD) and inclusion body myositis (IBM). However, the potential specificities of IBM when associated with SjD have been poorly investigated. Here, we compared the muscular inflammatory infiltrates between IBM patients with or without associated SjD.

Materials and methods: Formalin-fixed and paraffin-embedded muscle biopsies of patients with IBM, associated with SjD (IBM-SjD) and sporadic (sIBM) forms, from 6 French expert centers, were collected. Imaging mass cytometry (IMC) multiplex immunostaining (34 markers) was used to quantify and analyze inflammatory infiltrate composition. Supervised and unsupervised descriptive and comparative analyses were performed.

Results: Fourteen IBM-SjD and 7 sIBM muscle samples were analyzed. No statistically significant difference was encountered but some trends were pointed. IBM-SjD samples had a broader inflammatory infiltrate surface (median 4.8%, IQR: 1.4-8.6) than sIBM samples (median 1.6% IQR: 1.2-2.4). In both groups, the main inflammatory cells in muscle infiltrate were primarily macrophages and T cells. However, the proportion of plasma cells (14.7% IQR: 5.4-24.6 vs 8.5% IQR: 4.6-9.8) and B cells (3.1% IQR: 0.4-5.6 vs 0.5% IQR: 0.0-3.2) were higher in IBM-SjD patients.

Conclusion: Using IMC on muscle biopsies, IBM-SjD and sIBM patients share common histological features, but there are notable distinctions (more extensive infiltrate, high numbers of B cells and plasma cells in IBM-SjD). These observations were exploratory and based on a small number of patients. but may suggest IBM-SjD has distinct SjD-related pathophysiology compared with sIBM, and open to further research with potential diagnostic and therapeutic implications.