Immunolocalization of bradykinin receptors on human synovial tissue

Kinins have been implicated in the pathogenesis of experimental and clinical inflammatory arthritis. Previous studies have reported increased amounts of plasma and tissue kallikreins in synovial fluid, raised kinin levels and an upregulation of kinin B2 receptors on synovial fluid neutrophils in rheumatoid arthritis. Bradykinin binding sites have been identified on human synovial cells by autoradiographic localization and Scatchard analysis. This study was undertaken to localize immunohistochemically kinin B1 and B2 receptors on human synovial tissue. Synovial tissue was obtained at the time of joint replacement surgery or arthroscopic synovectomy in six patients (two RA, two OA and two with avascular necrosis). Tissue sections were immunolabelled for kinin B1 and B2 receptors and viewed by light and confocal microscopy. No immunolabelling of the kinin receptors was observed in the method controls. In all patients labelling for kinin B2 receptors was observed in the synovial lining cells, fibroblasts and endothelial lining cells of blood vessels. There was no immunolabelling for kinin B1 receptors in all samples. These findings further support a role for the B2 receptors in joint diseases. There did not appear to be an induction of the kinin B1 receptor in human synovial tissue obtained from patients with chronic arthritis. However, further studies are required to assess the role of B1 receptors in active joint inflammation.