Objective. To investigate mechanisms involved in inflammation and new bone formation in the sacroiliac (SI) joints of patients with ankylosing spondylitis (AS).

Patients and methods. Computed tomography–assisted biopsy of the SI joint was performed in 5 patients with AS with a mean disease duration of 4.5 years and radiographic stage 2–3 disease. Immuno‐histologic studies were performed with the alkaline phosphatase–anti–alkaline phosphatase technique, and cytokine messenger RNA (mRNA) was detected by in situ hybridization.

Results. Dense cellular infiltrates with varying amounts of CD3+ cells (mean ± SD 53.3 ± 24.1%), CD4+ cells (29.7 ± 17.6%), CD8+ cells (15.8 ± 11.4%), CD14+ cells (23.6 ± 16.9%), CD45RO+ cells (48.4 ± 23.6%), and CD45RA+ cells (4.5 ± 2.9%) were found in the synovial portion of the SI joints of all 5 patients. In these infiltrates a high amount of tumor necrosis factor α (TNFα) mRNA and, near the site of new bone formation, a lower amount of transforming growth factor β (TGF β) mRNA, were detected, while no message for interleukin‐1 was found in the 3 patients examined by this technique.

Conclusion. The presence of T cells and macrophages was demonstrated in cellular infiltrates in the SI joints of 5 patients with active AS. The finding of abundant TNFα message in these joints could have implications regarding potential immunotherapeutic approaches to this disease. TGFβ might be involved in new bone formation in AS.