Rationale: We have argued already 10 years ago that autoimmunity plays an important role in the pathogenesis of ankylosing spondylitis (AS). Although there is no direct evidence, as in nearly all ‘suspected’ autoimmune diseases, of an autoimmune response in AS it has been proposed repeatedly over the last years that the cartilage is the most likely target of an autoimmune response in AS. Histological studies 4,5 and magnet resonance imaging investigations suggest that the primary site of inflammation is the cartilage/bone interphase. Mononuclear cell infiltrates are mainly found in cartilage and the subchondral bone. In early and active sacroiliitis, T cells and macrophages are dominant in these infiltrates underlining the relevance of a specific cellular immune response 5.Furthermore, T cell responses have been demonstrated against proteoglycan (an important cartilage protein) in human arthritides including ankylosing spondylitis. We could also recently demonstrate both a CD4+ and a CD8+ T cell response to proteogkycan (aggrecan) derived peptides in the peripheral blood and a CD8+ T cell response against a collagen VI derived peptide in the synovial fluid from AS patients. Thus, all these findings suggest that a chronic, probably T cell mediated, immune response against cartilage is relevant in the pathogenesis of AS.This was further backed by recent studies from our group demonstrating mononuclear infiltrates of cartilage by investigating femoral heads and facette joints (small joints of the spine) obtained by surgery from a number of AS patients). The presence of mononuclear cell infiltrates was strongly dependent on the presence of cartilage on the surface of the femoral heads, suggesting that cartilage could be indeed the stimulus and target of a cellular immune response. However, rather surprisingly there were also dense infiltrations of B cells in the subchondral bone marrow in these patients. In comparison to immunohistological stainings from controls without spinal disease, the number of B cells in AS was even higher than the T cells. At the moment it is not clear whether this indicates that autoantibodies do play a role in the pathogenesis or whether these B cells might rather act as important local antigen presenting cells. In any case, given the assumed autoimmune pathogenesis in AS and the presence of B cells aggregates in inflammatory cellular infiltrates the study of potential effects of an immunotherapy which targets B cells in AS is justified and needed.