Background: Tumor necrosis factor (TNF) and interleukin (IL)-17 are central drivers of axial spondyloarthritis (AxSpA) pathology. We recently identified activated CD4 ⁺ tissue-resident memory Th17 (TRM17) cells as the primary source of IL17 A/F within AxSpA synovium [1] .

Objectives: This study aimed to identify the cellular sources of TNF and the factors driving the CCL20 ⁺ myeloid signature in the joint and blood.

Methods: Single-cell RNA sequencing (scRNA-seq) was performed on synovial tissue from eight AxSpA patients. To model synovial interactions, healthy control monocyte-derived macrophages (MDMs) were stimulated with activated autologous CD4 ⁺ T cells. Blocking antibodies and cytokine screens identified drivers of the CCL20 ⁺ macrophage phenotype. ScRNA-seq and functional assays characterized circulating monocyte subsets in AxSpA blood.

Results: scRNA-seq identified a macrophage subset as the predominant sources of TNF in AxSpA synovium, expressing high levels of CCL20 known to recruit Th17 via CCR6. Activated CD4 ⁺ T cells induced a CCL20 ⁺ MDM phenotype mirroring synovial macrophages; this was driven by CD40L, IL-1 and TNF-α. Furthermore, CCL20 ⁺ CD14 ⁺ monocytes - the peripheral equivalent of synovial CCL20 ⁺ macrophages - were expanded in AxSpA blood, hyper-responsive to stimulation and could be induced in vitro from healthy monocytes by IL-1β and TNF-α.

Conclusions: We identify CCL20 ⁺ macrophages as the primary source of TNF in AxSpA synovium. Their induction by activated T cells suggests an inflammatory circuit where CCL20 production likely recruits circulating Th17 cells to replenish the synovial TRM17 pool. These findings identify synovial CCL20 ⁺ macrophages as a potential therapeutic target and circulating CCL20 ⁺ monocytes as a candidate biomarker for AxSpA.

REFERENCES: [1] Liu F, Shi H, Turner J.D, Anscomble R, …, Croft A, Filer A, Bowness P, Chen L *. CD4+ tissue-resident memory Th17 cells are a major source of IL-17A in Spondyloarthritis synovial tissue. Ann Rheum Dis. 2025 . doi: 10.1016/j.ard.2025.04.018.

Acknowledgments: NIL.

Disclosure of Interests: Jinyi Zhao: None declared, Feng Liu: None declared, Jiaqi Li: None declared, Hui Shi: None declared, Claudia Worth: None declared, Lili Jiang: None declared, Davide Simone UCB, Frank Penkava: None declared, Antoni Chan: None declared, LIHUA DUAN: None declared, Paul Bowness: None declared, Liye Chen: None declared.