Synovial tissue from sites of joint pain in knee osteoarthritis patients exhibits a differential phenotype with distinct fibroblast subsets

Abstract

Background: Synovial inflammation is associated with pain severity in patients with knee osteoarthritis (OA). The aim here was to determine in a population with knee OA, whether synovial tissue from areas associated with pain exhibited different synovial fibroblast subsets, compared to synovial tissue from sites not associated with pain. A further aim was to compare differences between early and end-stage disease synovial fibroblast subsets. Methods: Patients with early knee OA (n = 29) and end-stage knee OA (n = 22) were recruited. Patient reported pain was recorded by questionnaire and using an anatomical knee pain map. Proton density fat suppressed MRI axial and sagittal sequences were analysed and scored for synovitis. Synovial tissue was obtained from the medial and lateral parapatellar and suprapatellar sites. Fibroblast single cell RNA sequencing was performed using Chromium 10X and analysed using Seurat. Transcriptomes were functionally characterised using Ingenuity Pathway Analysis and the effect of fibroblast secretome on neuronal growth assessed using rat DRGN. Findings: Parapatellar synovitis was significantly associated with the pattern of patient-reported pain in knee OA patients. Synovial tissue from sites of patient-reported pain exhibited a differential transcriptomic phenotype, with distinct synovial fibroblast subsets in early OA and end-stage OA. Functional pathway analysis revealed that synovial tissue and fibroblast subsets from painful sites promoted fibrosis, inflammation and the growth and activity of neurons. The secretome of fibroblasts from early OA painful sites induced greater survival and neurite outgrowth in dissociated adult rodent dorsal root ganglion neurons. Interpretation: Sites of patient-reported pain in knee OA exhibit a different synovial tissue phenotype and distinct synovial fibroblast subsets. Further interrogation of these fibroblast pathotypes will increase our understanding of the role of synovitis in OA joint pain and provide a rationale for the therapeutic targeting of fibroblast subsets to alleviate pain in patients.

Synovial inflammation in osteoarthritis progression

“…The transcriptome of synovium from pain zones in patients with early OA was characterized by upregulated expression of pro-fibrotic and pro-inflammatory genes, whereas the transcriptome of both early and end-stage OA showed upregulation of several nociceptive signalling pathways and neuronal growth genes 52 . Interestingly, scRNA-seq analysis of synovial explant FLS revealed that the gene expression profile of an FLS cluster representing the end-stage OA pain zone was associated with eicosanoid signalling, and the most active functions in these cells were "migration of cells" and "cell viability" 52 . Eicosanoid signalling was also associated with a FLS cluster related to early OA pain zone.…”

“…Nature reviews | Rheumatology R e v i e w s 0123456789();: different functional associated phenotypes in FLS isolated from fresh synovial tissue from patients with OA compared with patients with RA who underwent joint replacement 90 (Table 1). In a 2021 study, the transcriptomic profiles of synovia and FLS isolated from patients with OA were distinct between patients with early or end-stage OA as well as between patient-reported pain zones and pain-free zones 52 . The transcriptome of synovium from pain zones in patients with early OA was characterized by upregulated expression of pro-fibrotic and pro-inflammatory genes, whereas the transcriptome of both early and end-stage OA showed upregulation of several nociceptive signalling pathways and neuronal growth genes 52 .…”

“…In a 2021 study, the transcriptomic profiles of synovia and FLS isolated from patients with OA were distinct between patients with early or end-stage OA as well as between patient-reported pain zones and pain-free zones 52 . The transcriptome of synovium from pain zones in patients with early OA was characterized by upregulated expression of pro-fibrotic and pro-inflammatory genes, whereas the transcriptome of both early and end-stage OA showed upregulation of several nociceptive signalling pathways and neuronal growth genes 52 . Interestingly, scRNA-seq analysis of synovial explant FLS revealed that the gene expression profile of an FLS cluster representing the end-stage OA pain zone was associated with eicosanoid signalling, and the most active functions in these cells were "migration of cells" and "cell viability" 52 .…”

“…This distribution may be clinically relevant, as different locations and scores of synovial inflammation determined by CE-MRI correlate differently with pain and radiographic OA severity 51 . Synovitis can be present at any disease stage 49 , and a study reports a correlation between the patterns of patient-reported knee pain and the location of synovitis; specifically, suprapatellar pain was highly associated with suprapatellar synovitis on MRI 52 . Joint effusions and synovitis may be detected by MRI in subjects with OA joint pain and normal or very minimal damage by joint radiography, indicating that synovitis is not restricted to late stages of disease 11 .…”

Differential Metabotypes in Synovial Fibroblasts and Synovial Fluid in Hip Osteoarthritis Patients Support Inflammatory Responses

“…Osteoarthritis (OA) is usually seen in older individuals and thus has long been regarded as a “wear and tear” disease in contrast to the immune driven, systemic inflammatory joint disease rheumatoid arthritis (RA). However, increasingly, OA is now being regarded as a localised inflammatory joint disease [ 1 , 2 , 3 ], in which the inflammation contributes significantly to disease progression by driving degradation of articular cartilage [ 4 , 5 ] and also promotes symptomatic pain [ 6 , 7 , 8 ]. Synovitis, typified by synovial lining hyperplasia, inflammatory cell infiltration [ 9 ] and stromal vascularisation [ 10 , 11 ], is present in OA patients early in the disease course [ 9 ] prior to radiographic signs of cartilage damage, suggesting synovitis is amongst the early pathophysiological changes in OA.…”