This study is a prospective, open-label, proof-of-concept clinical trial that will be conducted in a referral center for patients with AS in Berlin. Eligible patients will be treated with ustekinumab 90 mg given subcutaneously at weeks 0, 4, and 16. The entire study period accounts 28 weeks. Assessment of the primary outcome parameter will be performed at week 24. The patients will be closely monitored throughout the study on a total of 9 visits.