Synovial fibroblasts from obese hip osteoarthritis patients exhibit distinct single-cell subsets characterised by pathological inflammatory functions

Inflammation of synovial tissue (synovitis) is a hallmark of osteoarthritis (OA) pathogenesis and is pronounced in obese individuals. The aim of this study was therefore to perform scRNA-seq analysis of synovial fibroblast from hip OA patients who were either obese or normal-weight to identify specific fibroblast subsets that exhibit disease-associated inflammatory functions. : scRNA-seq identified eight OA synovial fibroblast clusters, with distinct differences between obese and normal-weight patients. Fibroblast clusters in obese OA patients highly expressed gene signatures related to immune cell regulation, fibrosis and inflammatory signalling, including Chitinase3-like 1 (CHI3L1), CXCL12, osteonectin (SPARC), SMOC2 and Galectin-1 (LGALS1). Pseudotemporal expression dynamics demonstrated a transition in the expression of the transcriptional regulator MYC and Inhibin A in normal-weight clusters, and the expression of FOS and CHI3L1 in obese fibroblast clusters. Analysis of fibroblast conditioned media showed that obese OA fibroblasts secreted significantly greater amounts of CHI3L1, whilst normal-weight fibroblasts secreted greater amounts of Inhibin. Synovial joint tissue was obtained from n=4 obese hip OA patients (BMI >30) and n=4 normal-weight hip OA patients (BMI 18.5-24.9) who were undergoing elective joint replacement surgery. Synovial fibroblasts were isolated, and subjected to scRNA-seq, using Chromium 10X and analysed using Seurat and Monocle software.