Schneider, Udo

Abstract Objectives Granulomatous myositis (GM) is defined by focal collections of activated macrophages that fuse to form multinucleated cells that aggregate into granulomas within skeletal muscle. This study aimed to elucidate the pathophysiology of GM by defining its specific transcriptomic profile. Methods Bulk RNA sequencing was performed on 722 muscle biopsies, including 38 from patients with GM, other myopathies and healthy comparators. Spatial transcriptomics and immunohistochemistry assays were used to identify the location of specific transcripts and proteins within the tissue. Results We conducted a systematic analysis of the inflammatory pathways implicated in GM. This analysis revealed that GM muscle biopsies are characterized by elevated levels of IFNγ, IFNγ-inducible genes and proinflammatory cytokine genes including IL1B, TNF and TGFB1. Additionally, we identified a specific gene signature in GM, comprising 1293 genes that were significantly upregulated and 256 significantly downregulated. These genes were strongly correlated with transcriptomic markers of disease activity. Using this gene signature, support vector machine models accurately identified GM, achieving an area under the curve of 99.6% (99.0–99.9%) and an accuracy of 98.6% (98.2–98.9%). Among the identified genes, CHIT1 emerged as the most significantly upregulated gene, with its expression strongly correlating with disease severity. Furthermore, CHIT1 was detected at both the RNA and protein levels in granulomas and giant cells. Conclusion GM is transcriptomically characterized by a strong IFNγ signature and overexpression of proinflammatory cytokines, including IL1B, TNF and TGFB1. Additionally, it exhibits a unique transcriptomic profile, including CHIT1, which correlates with disease activity.

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’.