Synovial expression of osteopontin correlates with angiogenesis in juvenile idiopathic arthritis

Objective. To evaluate the synovial expression of osteopontin (OPN) and its possible correlation with the degree of synovial angiogenesis in human chronic idiopathic arthritis. Methods. Forty-five patients with active juvenile idiopathic arthritis (JIA) were studied. All patients underwent SF aspiration before steroid injection. A paired plasma sample was collected from 22 JIA patients. Plasma from 15 age-matched healthy subjects was used as control. Plasma and SF were tested by ELISA for OPN and vascular endothelial growth factor (VEGF). Synovial tissue was obtained at synovectomy from 10 JIA patients. Immunohistochemistry was performed according to a standard technique with anti-OPN, anti-CD68, anti-CD31 anti-VEGF and anti-alpha(v)beta(3) antibodies. Results. OPN levels were significantly higher in SF than in paired plasma samples (P<0.001). The same pattern was observed for VEGF (P<0.001). A positive correlation between OPN and VEGF concentrations was found in SF (r = 0.6, P = 0.001). In synovial tissue, OPN was expressed at the level of the lining and sublining layers with a distribution similar to that observed for VEGF. OPN expression in the lining layer correlated with the number of vessels present in the areas underlying the sublining layer. Conclusions. Synovial expression of OPN correlates with parameters of angiogenesis in JIA. These data support, in human disease, the possible role of OPN in the vascularization of inflamed synovial tissue, as previously shown in OPN-deficient animal models of arthritis.