SPP1 and arthritic joint disease: It was found to be expressed in higher levels in arthritic joints compared to nonarthritic joints in mice, SPP1/osteopontin-deficiency protected against joint destruction in collagen-antibody-induced arthritis, and increased expression was also observed in the synovial tissue of RA patients [31–34], but other studies found no influence of SPP1/osteopontin on murine arthritis models [35, 36].