Background: The synovial membrane is the primary target tissue in rheumatoid arthritis (RA). Oxylipins, metabolites derived from polyunsaturated fatty acids, play critical roles in both inflammation and resolution. While synovial lipidomic profiles have been linked to synovitis severity in active RA, the expression and functional role of omega-6- and omega-3-derived oxylipins in RA remission remain unclear.

Objectives: Our study aimed to comprehensively quantify oxylipins and their precursors in synovial tissue specimens from RA patients across the disease spectrum. Additionally, we evaluated their potential as biomarkers for predicting disease flare in patients achieving sustained remission.

Methods: Sixty-two patients fulfilling the ACR/EULAR 2010 classification criteria for RA underwent ultrasound-guided minimally invasive synovial tissue biopsy [(n=30 patients with DAS28≥4.2 and n=32 patients in sustained (≥12 months) clinical (DAS28≤2.6) and ultrasound remission (Power Doppler negative)]. Each tissue was oxylipins detection using liquid chromatography with tandem mass spectrometry (LC-MS-MS). Detected oxylipins were classified into two groups as pro-inflammatory (Infl/OL) or anti-inflammatory and pro-resolving (Res/OL), according to their biological properties. Each sample was processed for histology and the synovitis degree was assessed using H&E-based scoring by pathologist blinded on clinical characteristics. Finally, a scRNAseq dataset of 228961 synovial tissue cells from RA patients in different disease phases including sustained clinical and imaging remission was analyzed to identify the expression of enzymes involved in oxylipins metabolism within synovial cellular components.

Results: In the entire cohort, 84 distinct oxylipins were detected by LC-MS/MS in at least one sample, with 55 oxylipins remaining after excluding those present in less than 50% of the samples. When stratified by disease activity, the synovial tissue of RA patients in sustained clinical and imaging remission exhibited lower enrichment of both pro-resolving oxylipins (Res/OL) and inflammatory oxylipins (Infl/OL) compared to patients with active RA (p< 0.0001 for both comparisons). Particularly, Infl/OL concentrations correlated with ESR (r= 0.406, p= 0.001), CRP (r= 0.484, p< 0.0001) and synovitis score (r= 0.563, p< 0.0001) and Res/OL with ESR (r= 0.408, p= 0.001) and synovitis score (r= 0.416, p= 0.001), regardless to age and BMI. In RA synovium, synovial tissue macrophages (STMs) show the highest expression of ALOX5, 15-PGDH and CYP but with different distribution between STM clusters. ALOX5 is expressed in TREM2 ^pos , FOLR2 ^high LYVE1 ^pos , SPP1 ^pos and SPP1 ^pos TREM2 ^low STMs. 15-PGDH, an enzyme involved in prostaglandins catabolism is highly expressed only in TREM2 ^pos and FOLR2 ^high LYVE1 ^pos STMs. Among RA patients in remission, 11 (34.4%) experienced a disease flare following treatment withdrawal, while 21 maintained sustained clinical and ultrasound remission over 12 months of follow-up. Synovial tissue from RA patients in remission at the time of treatment withdrawal who subsequently experienced a flare showed a significant enrichment of inflammatory oxylipins (Infl/OL), particularly arachidonic acid (AA) (p=0.0003) and docosahexaenoic acid (DHA) (p=0.0039), compared to those who maintained remission. In addition, at scRNAseq resolution, STMs of patients maintaining sustained remission showed a significant enrichment of 15-HPGD (a repressor of prostaglandins function) compared to patients experiencing disease flare. Using ROC curve analysis, cutoff values were established for AA (≥33.35 pmol/mg, AUC: 0.874 [95% CI: 0.756–0.992]) and DHA (≥15.28 pmol/mg, AUC: 0.732 [95% CI: 0.522–0.941]), which significantly distinguish RA patients who experience disease flare from those who maintain remission prior to treatment withdrawal. Notably, RA patients in remission with synovial AA ≥33.35 pmol/mg and DHA ≥15.28 pmol/mg exhibited a 100% flare rate, compared to 0% (p=0.002) in patients without such synovial enrichment.

Conclusion: Distinct STMs contribute to oxylipin metabolism in RA synovium, demonstrating differential enrichment of oxylipins and their precursors based on disease status and the degree of synovitis. Furthermore, synovial concentrations of AA and DHA, along with the expression levels of 15-HPGD, effectively distinguish RA patients in sustained clinical and ultrasound remission who experience disease flare. These findings highlight potential novel biomarkers to guide personalized RA disease management.

REFERENCES: NIL.

Acknowledgements: NIL.

Disclosure of Interests: None declared.

© The Authors 2025. This abstract is an open access article published in Annals of Rheumatic Diseases under the CC BY-NC-ND license ( http://creativecommons.org/licenses/by-nc-nd/4.0/ ). Neither EULAR nor the publisher make any representation as to the accuracy of the content. The authors are solely responsible for the content in their abstract including accuracy of the facts, statements, results, conclusion, citing resources etc.