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ackground/Aims: The pro-angiogenic factors vascular endothelial growth factor (VEGF) and angiopoietins (Angs) play a prominent role in synovial angiogenesis, an early and critical event in the pathogenesis of rheumatoid arthritis (RA). Interleukin (IL)-35 is an anti-inflammatory cytokine that attenuates collagen-induced arthritis, however, the mechanisms involved are not fully understood. Methods: The effects of IL-35 on endothelial cell migration, adhesion, and tube formation were examined using human umbilical vein endothelial cells (HUVEC) in vitro. The effects of IL-35 on vessel formation in vivo were examined using a murine Matrigel plugs model. MMP2/MMP9 and IL-6/IL-8 secretion were assessed by zymography and ELISA, respectively. The crosstalk between IL-35, VEGF, and Ang2 in HUVECs and RA synovial tissue explants was investigated. Results: IL-35 inhibited basal and VEGF-induced HUVEC migration and adhesion in vitro as well as tube formation in vitro and in vivo. VEGF increased Ang2 secretion by HUVECs and RA synovial tissue explants, and exogenous Ang2 promoted HUVEC migration, adhesion, and tube formation with similar potency to VEGF. Blocking the Ang/Tie2 pathway with a Tie2 kinase antibody inhibited the proangiogenic effects of exogenous Ang2 and VEGF in HUVECs. IL-35 inhibited basal and VEGF-induced Ang2 secretion by HUVECs and RA synovial tissue explants; it also antagonized the proangiogenic effects of exogenous Ang2 in HUVECs. Moreover, IL-35 reduced basal and VEGF/Ang2-induced MMP2/MMP9 and IL-6/IL-8 secretion. Conclusion: These results suggested that IL-35 restrains RA angiogenesis and inflammation by downregulating basal and VEGF-induced Ang2 secretion as well as disrupting Ang2/Tie2 signal transduction. Our findings extend current understanding of mechanisms regulating RA angiogenesis and may support development of novel angiogenesis-targeting therapeutics for RA treatment. [ABSTRACT FROM AUTHOR]