Background: Polymyalgia rheumatica (PMR) is characterized by elevated serum IL-6 levels and an increased proportion of Th17 cells in peripheral blood. During PMR disease activity, IL-6 may originate from circulating monocytes or from synovial tissue itself. To clarify the role of synovial tissue, we performed experiments using synovial fibroblast (SFs) cultures derived from synovial bursae, which are the main structures affected in PMR. As secukinumab (SEC) is currently being evaluated in glucocorticoid-dependent PMR, we investigated the impact of IL-17A and SEC on SFs activation.

Methods: SFs were obtained from ex vivo cultures of synovial tissue collected from subacromial bursae during arthroscopy in patients with rotator cuff injury. SFs were cultured in vitro with IL-17A, IFN-γ, and/or SEC for 24 hours. mRNA expression was analyzed using bulk RNA sequencing and RT-PCR. The impact of IL-17A on T-cell polarization was assessed using co-cultures of SFs and allogeneic peripheral blood mononuclear cells (PBMCs) activated with anti-CD3/CD28 beads. SFs were stimulated with IL-17A, IL-17A + SEC, or an IgG isotype control for 72 hours then co-cultures with PBMCs were performed over 7 days. T-cell polarization was analyzed by flow cytometry.

Results: Cells obtained from synovial tissue cultures exhibited a fibroblast phenotype (CD90 ⁺ CD105 ⁺ CD73 ^int CD34 ⁻ CD45 ⁻ ). Bulk RNA sequencing revealed increased expression of genes involved in the IL-17, PI3K-Akt, and NF-κB signaling pathways. IL-17A stimulation significantly upregulated 28 genes, notably IL6 , CSF2 (encoding GM-CSF), and CCL2 . Addition of SEC significantly reduced the expression of these mRNAs, restoring a transcriptomic profile similar to that of unstimulated SFs. RT-PCR confirmed increased expression of IL6 , CSF2 , and CCL2 mRNA in response to IL-17A, whereas no significant changes were observed for IL6ST , IFNGR1 / 2 , IL12A , IL23A , FAP , IL17RA , or IL17RC mRNA expression. Co-stimulation with IL-17A and IFN-γ induced a strong increase of IL6 , IL6ST , and CCL2 mRNA expression (Figure 1A), suggesting a synergistic effect potentially mediated by IFN-γ-induced upregulation of IL17RA gene (Figure 1B). In co-culture experiments, SFs pretreated with IL-17A promoted Th17 polarization compared with IgG or IL-17A + SEC conditions, without significant increases in the proportions of Th1, Tc1 or Tc17 cells while contact with T cells and SFs were able to promote Th1 polarization (Figure 2).

Conclusions: These findings highlight a potential role of SFs in PMR through their ability to strongly express IL6 . SFs are responsive to IL-17A and IFN-γ, two key cytokines in PMR. IL-17A stimulation, particularly in synergy with IFN-γ, induces robust IL6 expression and promotes Th17 polarization. SFs also express chemokines such as CCL2 , suggesting a role in CCR2 ⁺ monocyte recruitment. Finally, SEC restores a non-inflammatory phenotype in activated SFs, supporting IL-17A as a potential therapeutic target in PMR.

Figure 1.

Level of mRNA expression of IL6, IL6ST, CCL2 and IL17RA by synovial fibroblasts after stimulation with IL-17A and/or IFN-γ (n = 9).

A: Level of mRNA expression of IL6 , IL6ST and CCL2 in synovial fibroblasts after stimulation with IL-17A and/or IFN-γ. B: Effect of IFN-γ on mRNA expression of IL17RA by synovial fibroblasts. Columns represent the median, the “whiskers” represent quartiles Q1 and Q3. *: p < 0.05, **: p < 0.01, ***: p < 0.001, ****: p < 0.0001.

Figure 2.

Study of the effect of synovial fibroblasts on T-cell polarization.

Percentage of Th1 (CD4 ⁺ CD8 ^- IFN-γ ⁺ ), Tc1 (CD4 ^- CD8 ⁺ IFN-γ ⁺ ), Th17 (CD4 ⁺ CD8 ^- IL-17 ⁺ ), and Tc17 (CD4 ^- CD8 ⁺ IL-17 ⁺ ) cells. Columns represent the median, the “whiskers” represent quartiles Q1 and Q3. *: p < 0.05, ***: p < 0.001. NS = not significant . SEC: secukinumab. Cocultures were performed using 14 samples of synovial tissue.

REFERENCES: NIL.

Acknowledgments: NIL.

Disclosure of Interests: André Ramon Novartis, Chugai, Lilly, Fresnius Kabbi, UCB, Johnson&Johnson, Novartis (This study received support from Novartis (grant and secukinumab supply). Novartis was not involved in the conduct or interpretation of results)., Helene GREIGERT: None declared, Baptiste Lamarthée: None declared, Corentin Richard: None declared, Alexis Varin: None declared, Claudie Cladière: None declared, Noémie Kloppfenstein: None declared, Ludovic Labattut: None declared, Alice Bordet: None declared, Sylvain Audia: None declared, Romain Boidot: None declared, jean Francis Maillefert: None declared, Paul Ornetti: None declared, Bernard Bonnotte: None declared, Maxime Samson: None declared.