In situ immunofluorescence analysis of IL-17+ cells. In situ analysis of IL-17+ cells in facet joints of ankylosing spondylitis (AS) patients and patients with osteoarthritis (OA) by using immunofluorescence microscopy. Double-staining reveals that myeloperoxidase-positive (MPO+) and CD15+ cells are the major source of IL-17 expression. The frequency of these cells was significantly higher in AS than in OA (P < 0.05 in both cases). The population of MPO+ cells included mononuclear cells and cells with polysegmental nuclei. Th17 cells and mast cells are also a source for IL-17+ expression, both of which were significantly higher in AS patients than in OA patients (P < 0.05 in both cases).