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Objective Fibroblast‐like synoviocytes (FLS) play a pivotal role in rheumatoid arthritis (RA) by contributing to synovial inflammation and progressive joint damage. An imprinted epigenetic state is associated with the FLS aggressive phenotype. We identified CASP8 (encoding for caspase‐8) as a differentially marked gene and evaluated its pathogenic role in RA FLSs. Methods RA FLS lines were obtained from synovial tissues at arthroplasty and used at passage 5‐8. Caspase‐8 was silenced using small interfering RNA, and its effect was determined in cell adhesion, migration and invasion assays. Quantitative reverse transcription PCR and western blot were used to assess gene and protein expression, respectively. A caspase‐8 selective inhibitor was used determine the role of enzymatic activity on FLS migration and invasion. Caspase‐8 isoform transcripts and epigenetic marks in FLSs were analyzed in FLS public databases. Crystal structures of caspase‐8B and G were determined. Results Caspase‐8 deficiency in RA FLSs reduced cell adhesion, migration, and invasion independent of its catalytic activity. Epigenetic and transcriptomic analyses of RA FLSs revealed that a specific caspase‐8 isoform, variant G, is the dominant isoform expressed (~80% of total caspase‐8) and induced by PDGF. The crystal structures of caspase‐8 variant G and B were identical except for a unique unstructured 59 amino acid N‐terminal domain in variant G. Selective knockdown of caspase‐8G was solely responsible for the effects of caspase‐8 on calpain activity and cell invasion in FLS. Conclusion Blocking caspase‐8 variant G could decrease cell invasion in diseases like RA without the potential deleterious effects of nonspecific caspase‐8 inhibition.

Objective To study epigenetic patterns in T lymphocytes that accumulate in the rheumatoid arthritis (RA) synovium, we characterized DNA methylation of CD3+ T cells in peripheral blood and synovial tissue in patients with RA and osteoarthritis (OA). Methods Genomic DNA of CD3+ T cells was isolated from patients with RA (n = 8) and OA (n = 5) from blood or the synovium at the time of an arthroplasty using antibodies and magnetic beads. Methylation was measured by using the Illumina Infinium MethylationEPIC Kit. Differentially methylated loci (DML) and differentially methylated genes (DMGs) were identified by using Welch’s t‐test. Principal component analysis, hierarchical clustering, and pathway analysis were used to determine relationships among groups. Results When we compared DNA methylation of CD3+ T cells between peripheral blood and synovial tissue within each disease, 4615 and 164 DML were identified in RA and OA samples, respectively, resulting in 832 and 36 DMGs. A principal component analysis showed that methylation differences in T cells were greater on the basis of on location (blood vs synovium) rather than disease (RA vs OA). Differentially modified pathways were significantly enriched between RA blood and synovial T cells, especially in genes related to complement, integrin cell surface interactions, and the P53 pathway. The limited number of DMGs identified between OA blood and synovial T cells did not conform to biologic pathways. Conclusion The patterns of DNA methylation in RA show location‐specific differences related to immune pathways, whereas methylation differences in OA are limited. The RA joint‐specific signatures could be due to selective accumulation of T‐cell populations or expansion of differentially marked adaptive immune cells. Understanding epigenetic patterns could provide clues to the types of T cells that accumulate in the RA joint and identify potential therapeutic targets.

Objective Rheumatoid arthritis (RA) fibroblast‐like synoviocytes (FLS) derived from hip and knee have distinctive DNA methylation and transcriptome patterns in interleukin (IL)‐6 signaling and Janus kinase (JAK)–signal transducers and activators of transcription (STAT) pathways. To determine the functional effects of these joint‐specific signatures, we evaluated how RA hip and knee FLS differ in their response to IL‐6. Methods Hip or knee RA FLS were obtained after arthroplasty. Previously published datasets on epigenetic landscape of FLS were mined to identify joint‐specific IL‐6–related epigenomic differences. RNA sequencing was performed on five RA hip and five knee FLS treated with or without IL‐6. Differential gene expression was determined using edgeR software. STAT3 phosphorylation was measured using bead assays. Sensitivity to tofacitinib was evaluated by measuring CCL2 inhibition using quantitative polymerase chain reaction. Results Assay for Transposase‐Accessible Chromatin sequencing and histone chromatin immunoprecipitation sequencing datasets from RA FLS were analyzed to identify epigenomic differences between hip and knee. Differential chromatin accessibility was associated with IL‐6, IL‐6R, and JAK1 genes. H3K27ac was also differentially marked at other JAK‐STAT–related genes, including STAT3‐STAT5A region. Principal component analysis of RNA sequencing data confirmed segregation between RA hip and knee FLS under basal conditions, that persisted following IL‐6 treatment. STAT3 phosphorylation after IL‐6 was significantly higher in knee than hip FLS and was highly correlated with JAK1 protein levels. Knee FLS were less sensitive to the JAK inhibitor tofacitinib than hip FLS. Conclusion RA hip and knee FLS have distinct transcriptomes, epigenetic marks, and STAT3 activation patterns in the IL‐6 pathway. These joint‐specific differences might contribute to a differential clinical response in individual joints to targeted therapies such as JAK inhibitors.