Stenzel, Werner

ABSTRACT Aims Sarcoid myopathy (SaM) is characterised by granulomatous myositis (GM) and can overlap with inclusion body myositis (IBM), a late‐onset chronic idiopathic inflammatory myopathy with a still enigmatic pathogenesis. As GM can occur in different clinical contexts, we aimed to examine the histomorphologic features and gene expression profiles in cases of definite SaM that may inform diagnostic and therapeutic considerations. Methods We performed a multidimensional characterisation of muscle biopsy specimens from patients with ‘pure SaM’ (n=17), SaM with concomitant IBM (SaM‐IBM) (n=2), including histopathologic and ultrastructural analysis in addition to quantitative real‐time polymerase chain reaction. Additionally, bulk RNA sequencing was performed on 38 muscle biopsy specimens from patients with SaM (including SaM‐IBM) (n=30) and NGI (n=8). Results Histopathological analysis revealed a pattern of endomysial and perimysial granulomatous inflammation frequently extending to the fascia, endomysial fibrosis, muscle fibre atrophy, variation in muscle fibre size and capillary thickening. Findings from immunohistochemical studies established Chitinase 1 as a pure giant cell marker in SaM. On a subcellular level, ‘pure SaM’ was characterised by focal accumulation of large swollen mitochondria with rare cristae but an absence of irregular cristae. Transcriptomic analysis of patients with SaM confirmed markedly elevated expression of both type 1 and type 2 human leukocyte antigen molecules. Macrophage activity markers were highly elevated. Consistent with histologic findings, CHIT1 was specifically overexpressed in SaM samples but not in ‘pure IBM’ muscle biopsy specimens. Conclusions SaM is characterised by a stereotypical appearance at the histopathologic level and disease‐specific immune dysregulation that involves macrophage function and maturation. SaM‐IBM represents a noteworthy overlap syndrome that shares multiple dysregulated immune pathways with ‘pure SaM’.

Abstract Objective Myositis is a heterogeneous family of inflammatory myopathies. We sought to define the differential expression of cytokines, cytokine receptors and immune checkpoint genes in muscle biopsies from patients with different forms of myositis in order to characterize patterns of inflammation in each. Methods Bulk RNA-sequencing was performed on muscle biopsy samples from 669 patients, including 105 with DM, 80 with immune-mediated necrotizing myopathy (IMNM), 65 with anti-synthetase syndrome, 53 with IBM, 19 with anti-PM/Scl myositis, 310 with other inflammatory or genetic myopathies and 37 controls with normal tissue (NT). Myositis clinical groups and autoantibody subgroups were analysed separately. Expression data were analysed for 338 genes encoding cytokines, cytokine receptors and immune checkpoints. Myositis group-specific genes were identified from this list by finding genes that were specifically differentially expressed in one group compared with all samples and compared with NT (α < 0.001). Results IBM patients had the most differentially overexpressed genes (71) among all clinical groups, including 37 that were IBM-specific. Among the top genes were several involved in type 1 inflammation, including CCL5, CXCR3, CCR5, CXCL9 and IFNG. Anti-Jo1 and anti-PM/Scl patients exhibited differential overexpression of a similar set of genes, while DM patients exhibited differential overexpression of a different set of genes involved in type 1 inflammation. IMNM patients had the least number of differentially overexpressed genes with no predominant inflammatory pattern. Conclusion Each myositis clinical group and autoantibody subgroup had differentially overexpressed inflammatory mediators, including a strong type 1 inflammatory gene signature in IBM.