Abstract
BACKGROUND: Platelets play a crucial role not only in thrombosis and hemostasis but also in inflammation, including rheumatoid arthritis. Although synovial cells in the joints express the membrane protein podoplanin, and platelets express the podoplanin receptor C-type lectin-like receptor 2 (CLEC-2), the role of CLEC-2 in arthritis has not been elucidated. OBJECTIVES: This study investigated the role of CLEC-2 in arthritis. METHODS: Fibroblast-like synoviocytes (FLSs) from patients with rheumatoid arthritis were cocultured with washed human platelets, activated platelet supernatants, or recombinant CLEC-2 (rCLEC-2) to assess FLS proliferation and inflammatory cytokine mRNA levels. A KRN/B6xNOD (K/BxN) mice serum-transfer arthritis (STA) mouse model was used to study the in vivo roles of CLEC-2/podoplanin. Radiation bone marrow chimeric mice were generated by transplanting fetal liver cells from platelet/megakaryocyte-specific CLEC-2 conditional knockout (cKO) or wild-type (WT) embryos, to which serum from K/BxN was transferred. Arthritis was assessed by measuring the thicknesses of all 4 limbs. RESULTS: Coculture with platelets or rCLEC-2 significantly enhanced FLS proliferation and increased the mRNA levels of inflammatory cytokines including IL-6, IL-8, CXCL-2, CXCL-3, IL-1β, and tumour necrosis factor-α. Furthermore, platelets from wild-type mice showed increased mRNA levels of these cytokines in mouse FLSs. CLEC-2-deficient platelets partially, but significantly, inhibited this effect. In the STA model, CLEC-2 cKO mice showed reduced arthritis compared to WT mice. Immunohistological analysis confirmed the presence of platelets in the proliferative synovium. CONCLUSION: CLEC-2 amplified arthritic platelets by stimulating the production of inflammatory cytokines and cell proliferation in FLSs.