THU0054 THE SYNOVIAL FLUID FROM RHEUMATOID ARTHRITIS PATIENTSCONTAINS SOLUBLE TAM RECEPTOR TYROSINE KINASES OF WHICH SOLUBLE TYRO3 AND ITS LIGAND GAS6 CORRELATES WITH THE PROINFLAMMATORY CYTOKINE LEVELS

Background We recently showed that the tyrosine-protein kinase receptors MER and AXL, both members of the TAM (including TYRO3) receptor family, play a protective role in mouse models of rheumatoid arthritis (RA) [1, 2]. In both humans and mice, TAM receptors can be proteolytically cleaved from the cell membrane and shed as a soluble (s) receptors. These sTAM receptors may act as decoy receptors, bind their ligands Growth Arrest-Specific 6 (GAS6) and Protein S (PROS1), and thereby inhibit the immunoregulatory and anti-inflammatory effect of TAM receptor activation, and prevent TAM receptor-mediated phagocytic clearance of dying cells. Objectives To determine the soluble TAM receptor levels in synovial fluids from RA and osteoarthritis (OA) patients and correlate this to the inflammatory process. Methods The level of sMER, sAXL, sTYRO3, GAS6, IL-8, TNFα and IL-1β was measured in synovial fluids of RA (n = 28) and OA patients (n = 12) by ELISA or multiplex array. Synovial explants were cultured for 24 hrs and the release of sMER and GAS6 was measured by ELISA. Human synovial explants of RA (n = 15) and osteoarthritis (OA) (n = 17) patients were immunostained for MER. Results Soluble MER and sTYRO3 were significantly enhanced in synovial fluids (P<0.0001 and P=0.0042) of RA patients as compared to OA patients. The sAXL and GAS6 levels in synovial fluid of RA and OA patients was equally high (P = 0.267 and P = 0.074). In RA patients, but not in OA patients, the synovial fluid levels of sTYRO3 and GAS6 (a ligand for all three TAM receptors) positively correlated with TNFα (r=0.555, r=0.477), IL-1β (r=0.597, r=0.481) and IL-8 (r=0.567, r=0.529). Soluble MER levels, however, did not correlated with local inflammatory markers. Although synovial explants from RA released more sMER into the culture media than explants from OA (P = 0.0045), the explants of both RA and OA patients contained comparable amounts of immunopositive MER cells. Conclusion We showed that synovial fluid sTYRO3 and GAS6 levels correlates with higher proinflammatory cytokine levels in RA but not in OA patients. This is in line with the recent observation done by Xu et al. [3], that circulating levels of sTYRO3, but not sMER or sAXL, correlates with disease activity and bone destruction in RA patients. In accordance to serum, the synovial sMER levels were significantly higher in RA - than in OA patients with sAXL levels comparable between both patient groups. Interestingly, only for sMER the synovial fluid levels were much higher than in serum suggesting local production as we confirmed in the synovial explants cultures. This illustrates that release of soluble TAM receptors is either the results of different processes (enzymatic shedding or alternative splicing isoforms) or reflects the different cell populations in the arthritic joint. We found that naïve macrophages express high MER and inducible AXL, while synovial fibroblast express high AXL and TYRO3. Further studies are warranted to determine if sMER levels impair the clearance of apoptotic cells in the arthritic joint as we have found in our mice studies. Our study shows that synovial sTYRO3 is a marker for joint inflammation and possibly a novel therapeutic target for RA. References [1] Waterborg, C.E., et al., Protective role of the Mer Tyrosine Kinase via efferocytosis in rheumatoid arthritis Models. Frontiers in immunology, 2018. 9. [2] Waterborg, C.E., et al., 03.05 The tam receptors axl and mer play a protective role in a temporal and spatial manner in inflammatory arthritis. 2017, BMJ Publishing Group Ltd. [3] Xu, L., et al., Soluble TAM receptor tyrosine kinases in rheumatoid arthritis: correlation with disease activity and bone destruction. Clinical & Experimental Immunology, 2018. 192(1): p. 95-103. Disclosure of Interests None declared