Background/Purpose: Background: A substantial proportion of Rheumatoid Arthritis patients do not respond to treatment and only a small proportion achieve sustained disease remission. We showed previously that human synovial tissue macrophages (STMs) are heterogeneous; and a sub-population of CD206⁺MertK⁺ positive STMs predominates in RA patients in sustained remission as compared to patients with active RA. Their surface receptors, e.g. MerTK, and distinct transcriptome suggest that they may play a role in re-enforcing joint homeostasis (1). Thus, we hypothesise that activation of CD206⁺MerTK⁺ human synovial tissue macrophages contributes to the resolution of inflammation. 

Objectives: We aimed to investigate whether activation of MerTK in STM sub-population promotes an anti-inflammatory environment in the synovium.

Methods: Methods: Using flow cytometric techniques and with a specific antibody panel design (1), STMs from digested RA synovial biopsies were phenotyped and/or harvested for functional studies. The study included patients with active disease and patients in remission (DAS28 ESR< 2.6 and power doppler negative). After excluding all other cell types, macrophages were gated based on the expression of CD64 and CD11b (CD45posCD64posCD11bposHLA-DRpos) (1). Two distinct subpopulations: CD206+MerTK+ and CD206-MerTK- were sorted and cultured on a collagen coated 96-well plate at 1000 cells per well in the presence of with LPS (10ng/ml) ± Gas 6, a MerTK ligand (100ng/ml), for 24h. Cytokine production was measured using a high sensitivity UPLEX assay. To test the role of MerTK expressing macrophages on the activation of synovial fibroblasts, a trans-well membrane coculture system was used. Macrophages were pre-treated with LPS (1ng/ml)  ± MertK inhibitor (UNC1062, 100um-250um) for 4 and 2 hours respectively, and then co-cultured with fibroblasts for 48hours. MMPs production were measured by multiplex.

Results: Results: As previously shown (1), RA patients in sustained remission have a majority CD206⁺MerTK⁺ STMs, whilst patients with active RA show an increased number of CD206^–MerTK^– macrophages.  The percentage of CD206⁺MerTK⁺ macrophages negatively correlated with the disease activity score in RA.  Stimulation of FACS-Aria sorted CD206⁺MerTK⁺ and CD206^–MerTK^– macrophages with TLR4 ligand induced a different cytokine pattern:  the CD206^–MerTK^– macrophages mostly produced TNF, IL-6 and IL-1b while CD206⁺MerTK⁺ produced IL-10.  Inhibition of MerTK in macrophages increased the production of MMP1 and MMP3 by synovial fibroblasts in the co-culture system. 

Conclusion: Conclusion: CD206⁺MERTK⁺ macrophages, which predominate in RA patients in remission, have Gas6-mediated negative feedback mechanism limiting inflammatory cytokine production and activation of synovial fibroblasts. Thus, Gas6/MerTK pathway in synovial tissue macrophages could drive the resolution of inflammation and synovial tissue homeostasis.

References:

1. Elmesmari A et al., Synovial tissue of RA patients in remission contains a unique population of regulatory macrophages. Annals of the Rheumatic Diseases, 2017;76 (Suppl2) 138-139.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/presence-of-the-mertk-receptor-on-human-synovial-tissue-macrophages-lowers-inflammatory-cytokine-production-and-activation-of-the-mertkcd206-subpopulation-limits-the-inflammatory-response-of-synovia/