Investigation of involved tissue in axial spondyloarthritis – what have we learnt from immunohistochemical studies?

The principal clinical and radiological feature of all axial spondyloarthritis is an involvement of the axial skeleton (sacroiliac joints and spine) and, to a lesser extent, the hip joints. Immunohistochemical studies provide worthwhile information regarding disease mechanisms in axial spondyloarthritis. Immunohistochemical investigation of sacroiliac joints, spine and hip joints suggested an important role of T-cells in the development of acute inflammatory lesions. Furthermore, activated angiogenesis and macrophage/osteoclast activation also play a relevant role in the development of early active inflammatory lesions. Nonetheless, the mechanisms leading to activation of osteoproliferation with subsequent syndesmophyte and ankylosis formation in patients with spondyloarthritis remains unclear. It is suggested that the Wnt pathway is actively involved in this process due to decreased expression of new bone formation inhibitors such as sclerostin. However, to understand the full picture of the interrelationship between inflammation and new bone formation, and to explore new treatment targets for suppression of the excessive bone formation as well, further investigations are needed.