In situ analysis and quantification of IL-17+ cells in patients with ankylosing spondylitis or osteoarthritis. (a) In situ analysis of IL-17+ cells in facet joints of ankylosing spondylitis (AS) (top) and osteoarthritis (OA) (bottom) patients. The specificity of IL-17 staining (top) is shown by blocking the anti-IL-17 antibody with recombinant IL-17 (rIL-17) (middle). (b) and (c) The frequency of IL-17-secreting mononuclear cells and cells with polysegmental nuclei in the bone marrow of AS facet joints was significantly higher in AS than in OA facet joints. *P < 0.001.