THU0035 A Key Role for Platelet-Derived Clec-2 in the Regulation of Synovial Inflammation
Authors: G.E. Desanti, A.J. Naylor, L. Navarro-Nunez, J.D. Turner, A.N. Saghir, K.L. Lowe, A. Filer, S.P. Watson, C.D. Buckley
Journal: Annals of the Rheumatic Diseases (2015)

<h3>Background</h3> It is becoming clear that platelets play a more significant role in the immune system than previously recognized<sup>1-3</sup>. Furthermore they are known to play a role in the induction of inflammation but the mechanisms by which they do so remain unclear. Both CLEC-2, which is highly expressed by platelets, and its ligand PDPN (gp38) are found in the joints of patients with rheumatoid arthritis (RA)<sup>4</sup>. However little is known about the role of CLEC-2-PDPN axis in synovial inflammation. <h3>Objectives</h3> To determine whether platelet-derived CLEC-2 induces synovial inflammation by triggering PDPN-expressing fibroblasts to adopt a pro-inflammatory phenotype. <h3>Methods</h3> Using flow cytometry we analyzed the PDPN expression in biopsies from RA patients as well as in mouse joints using the TNFΔare/+ and the anti-collagen antibodies induced arthritis (CAIA) models. We also induced arthritis in tamoxifen-induced CLEC-2 deficient mice to monitor the disease onset and resolution. After resolution, bones erosion and remodelling was assessed by micro-CT. We generated mouse PDPN<sup>Pos.</sup> and PDPN<sup>Neg.</sup> synovial fibroblasts (SF) that we co-cultured with wild-type or CLEC-2 deficient platelets. After incubation pro- and anti-inflammatory makers were measured at the mRNA (qPCR) and the protein (Luminex) levels. <h3>Results</h3> In human and mouse synovium, high levels of PDPN expression are found on SF as well as on a subset of haematopoietic cells. PDPN expression on mouse SF and myeloid cells is dynamic and depends on the level of inflammation. <i>In vivo</i> studies in which CLEC-2 is selectively deleted in platelets show that arthritis is more severe in absence of platelet-derived CLEC-2 suggesting that platelet-derived CLEC-2 plays a protective role in physiological conditions. However, <i>in vitro</i> co-culture data indicate that platelet-derived CLEC-2 induce SF to adopt a pro-inflammatory phenotype. This discrepancy between our <i>in vitro</i> and <i>in vivo</i> data might be explained by the fact that platelet-derived CLEC-2 induces myeloid cells to adopt an anti-inflammatory phenotype. <h3>Conclusions</h3> Our observations suggest that CLEC-2-PDPN interactions play a dual role in arthritis depending on whether they occur between platelets and fibroblasts or platelets and macrophages. <h3>References</h3> Benezech C, et al. CLEC-2 is required for development and maintenance of lymph nodes. Blood. 2014;123:3200-3207. Acton SE, et al. Dendritic cells control fibroblastic reticular network tension and lymph node. Nature. 2014;514:498-502 LID - 410.1038/nature13814 [doi]. Astarita JL, et al. The CLEC-2-podoplanin axis controls the contractility of fibroblastic reticular cells and lymph node microarchitecture. Nat Immunol. 2014:doi: 10.1038/ni.3035. Del Rey MJ, et al. Clinicopathological correlations of podoplanin (gp38) expression in rheumatoid synovium and its potential contribution to fibroblast platelet crosstalk. PLoS One. 2014;9:e99607. <h3>Disclosure of Interest</h3> None declared