Insights into the pathophysiology of ankylosing spondylitis: contributions from animal models

The introduction of anti-tumor necrosis factor strategies has significantly changed the perspective and outcome of patients with ankylosing spondylitis and related spondyloarthritides. This breakthrough has also boosted further research efforts into the mechanisms of disease. As human tissue specimens of the spine and sacroiliac joints are very difficult to obtain and rarely allow mechanistic studies, most of the new concepts have emerged from different animal models of disease. In this review, we summarize insights into the role of HLA-B27 based on transgenic rat and mouse models, efforts into the identification of cell populations stimulating inflammation and molecular studies of pathological bone formation leading to ankylosis. Important progress has been made and novel hypotheses were put forward. These include the impact of HLA-B27 on endoplasmic reticulum stress and the unfolded protein response, the role of stromal cells in inflammation, the entheseal stress hypothesis and the identification of the bone morphogenetic protein and WNT signaling pathways as therapeutic targets for ankylosis.