Background: Rheumatoid arthritis (RA) is characterized by impaired regulatory T cell (Treg) function. Treg identity is influenced by their microenvironment, and Treg stability reflects their ability to maintain function and characteristics in pro-inflammatory conditions. Investigating Treg within the joint—the primary site of inflammation in RA—is essential, as these cells display distinct features and functions compared to peripheral blood (PB) Treg.

Objectives: This study aims to compare the stability and suppressive phenotype of peripheral versus local Treg in RA patients.

Methods: Treg from the PB and synovial fluid (SF) of RA patients were analyzed by flow cytometry. The frequency of Treg expressing stability markers, such as the methyltransferase EZH2, and suppressive markers, including CD39 and TNFR2, was assessed. CD4 ⁺ CD25 ⁺ CD127 ^- Treg were isolated by biomagnetic separation. In vitro experiments were conducted by culturing Treg isolated from healthy individuals with acellular SF from RA patients to evaluate the impact on EZH2 expression. Additionally, a mouse model of autoimmune inflammation, experimental autoimmune encephalomyelitis (EAE), was utilized to compare Treg from the inflammatory tissue (central nervous system, CNS) with those from systemic compartments (lymph nodes, and spleen).

Results: The frequency of Treg was significantly higher in the SF compared to PB Treg expressing the stability marker EZH2 were overrepresented in the SF. Additionally, the frequencies of Treg expressing suppressive markers, such as CD39 and TNFR2, were also higher in the SF. Preliminary in vitro results showed that SF from RA patients enhanced the frequency of Treg expressing EZH2. We used the EAE model to compare Treg from the local site of inflammation to those from the periphery in an autoimmune inflammatory context. Similar results were observed in this model, as we demonstrated increased expression of stability markers, such as EZH2, and suppressive markers (CD39 and TNFR2, among others) in Treg from the inflammatory site (CNS) compared to those from the lymph nodes.

Conclusion: The study suggests that Treg at the inflammatory site in RA are more stable and suppressive compared to peripheral Treg. The increase in EZH2 expression in Treg from the synovial fluid may be due to the inflammatory environment. Understanding why more stable and suppressive Treg are unable to resolve inflammation locally requires further investigation.

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.