THU0061 Restricted axl synovial expression and increased cleavage of axl ectodomain correlate with highly inflamed synovitis and more severe rheumatoid arthritis
Authors: A. Nerviani, D. Mauro, S. Pagani, G. Lliso-Ribera, F. Rivellese, M. Lewis, M. Bombardieri, F. Humby, C. Pitzalis
Journal: Annals of the Rheumatic Diseases (2018)

<h3>Background</h3> Tyrosine kinase Axl, member of the TAM family, is expressed by antigen presenting cells and behaves as a negative regulator of the inflammatory cascade. Inflammatory stimuli up-regulate Axl expression in bone-marrow-derived macrophages <sup>1</sup>. Soluble (s) Axl, generated by ADAM10, is a potent decoy for the TAM-ligand Gas6 and can impair TAM axis activation in lupus <sup>2</sup>. In rheumatoid arthritis (RA) dendritic cells, Axl is epigenetically down-regulated <sup>3</sup>. Emerging evidence has emphasised the significant role of Axl/sAxl in the pathogenesis and progression of autoimmune diseases, but little is known about TAM expression and regulation within the rheumatoid synovium. <h3>Objectives</h3> We aimed to quantify Axl/ADAM10 in synovial tissue (ST) and sAxl/Gas6 in synovial fluid (SF) and to correlate Axl/sAxl expression with synovial inflammation and disease severity in RA patients. <h3>Methods</h3> ST/SF were sampled from early treatment-naïve RA patients undergoing ultrasound (US)-guided synovial biopsy of the most inflamed accessible joint. RA was diagnosed according to ACR/EULAR2010 criteria. The Krenn's synovitis score of inflammation was determined by H and E. Immunohistochemistry (IHC) staining of CD3/CD20/CD138/CD68 allowed to define the synovial immune infiltrate. Axl/ADAM10/CD68 were assessed in ST by IHC/immunofluorescence (30 patients). sAxl/Gas6 were quantified in 17 SF by ELISA. Synovial gene expression data were obtained by next-generation sequencing (80 patients). <h3>Results</h3> Axl was predominantly expressed only by CD68-positive macrophages of the lining and co-localised with ADAM10. AXL synovial mRNA significantly negatively correlated with the Krenn's score and with the degree of infiltration by B/T lymphocytes, plasma cells, and sub-lining macrophages, but not lining macrophages. In SF, sAxl positively correlated with Gas6 and was significantly more abundant in patients with highly inflamed synovitis and more active disease. AXL synovial gene expression showed strong negative correlation with indexes of disease severity, including ESR, CRP, DAS28, and US synovial thickening/power-doppler, and with the mRNA of pro-inflammatory cytokines, e.g. TNF and IL6. <h3>Conclusions</h3> Our data demonstrate that, despite the highly-inflamed environment characterising RA synovia, Axl expression is restricted to the synovial lining, where it can be cleaved and released into the synovial fluid by ADAM10. There, sAXL can bind Gas6 with high affinity, preventing its interaction with transmembrane functional TAM receptors. Raised levels of sAxl and down-regulated AXL mRNA expression correlate with highly inflamed synovitis and more severe disease. Defects in the TAM system could provide an original mechanistic explanation of the persistent inflammation in the RA joints, representing a novel therapeutic target exploitable to regain tissue homeostasis. <h3>References</h3> [1] Zagórska A, et al. Diversification of TAM receptor tyrosine kinase function. Nat Immunol2014. [2] Orme JJ, et al. Heightened Cleavage of Axl Receptor Tyrosine Kinase by ADAM metalloproteases may contribute to disease pathogenesis in SLE. Clinical Immunology2016. [3] Kurowska-Stolarska M, et al. MicroRNA-34a dependent regulation of AXL controls the activation of dendritic cells in inflammatory arthritis. Nat Commun. Nature Publishing Group 2017. <h3>Disclosure of Interest</h3> None declared