A major involvement of the immune system and of microbial flora in the HLA-B27 transgenic rat model of spondylarthropathy was demonstrated. The role of inflammatory pathways, such as cytokines and inducible nitric oxide synthase (iNOS) was investigated. Treatment with IL-10 failed to improve established disease, whereas such improvement was achieved with IL-11. In contrast, aggravation was observed after treatment with selective inhibitor of iNOS.