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Frozen shoulder is a painful inflammatory fibrotic disease of the glenohumeral joint capsule. While it’s frequently self-limiting, patients can be symptomatic for years. The underlying pathophysiology of frozen shoulder is not fully understood, hampering the development of an effective therapy. Increased expression of inflammatory mediators has been identified in frozen shoulder, suggesting a potential role for these mediators in disease. However, the precise cellular interactions and the cell types expressing inflammatory mediators have not been identified. A single cell atlas revealed that the rotator interval of the glenohumeral joint capsule is comprised of fibroblasts, myeloid cells, lymphocytes, endothelial cells, and mural cells. Clustering of myeloid cells revealed a CD48+IL1A+ macrophage subset which showed increased expression of several pro-inflammatory genes, including PTGS2, S100A8, EREG, IL1A, IL1B, IL1R1, IL1RN. In frozen shoulder, CD48+IL1A+ macrophages show increased expression of several pro-inflammatory genes, including IL-1 genes (IL1A, IL1B, IL1R1). CD48+IL1A+ macrophages were localised to both the lining and sub-lining regions of the rotator interval of the glenohumeral joint capsule. The expression (immunohistochemistry) of CD68, CD48, IL-1α, IL-1β, and IL-1R was increased in frozen shoulder relative to comparator patient tissues. As the IL1R1 gene was mainly expressed by fibroblasts, ligand-receptor interaction analysis predicted several important IL-1 ligand-receptor pairs between CD48+IL1A+ macrophages and fibroblast populations, including IL1A-IL1R1 and IL1BIL1R1. In turn, fibroblast populations were predicted to communicate with myeloid cell populations via the CSF1-CSF1R pathway. Fibroblasts derived from frozen shoulder capsular tissues acquire an inflammatory fibrotic phenotype upon IL-1α and/or IL-1β stimulation, which was characterised by an increase in inflammation (CCL2, CCL5, CCL8, CCL20, CXCL3, CXCL5, NFKBIA, NFKBIZ, IGF1, FGF7, IL6, CSF2, CSF3) and fibrosis related genes (TGFB1, TGFBR1, WNT5A, COL3A1, MMP2, TIMP1), as well as an increased secretion of inflammatory mediators (CXCL-12, GMCSF, IL-1RA, IL-33, IL-6). This thesis provides novel insight into the cellular basis of frozen shoulder, revealing a distinct CD48+IL1A+ macrophage subset enriched for inflammation in advanced-stage frozen shoulder. CD48+IL1A+ macrophages release IL-1α and IL-1β, which activate surrounding macrophages and fibroblasts via IL-1R to release chemokines and other inflammatory mediators, further promoting inflammation and fibrosis. This data supports the hypothesis that macrophage-fibroblast crosstalk contributes to the underlying pathophysiology of frozen shoulder. Importantly, this thesis provides sufficient data to identify the IL-1R pathway as a potentially important therapeutic target. |
