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BACKGROUND: Pigmented villonodular synovitis (PVNS) is characterized by hypervascular proliferative synovium containing multinucleated giant cells, macrophages, and hemosiderin. The destruction of articular cartilage and erosion of periarticular bone is thought to be mediated by matrix metalloproteinases (MMPs). Expression of MMPs in some tumors appears to be stimulated through local production of cytokines, and several specific cytokines (TNF alpha, IL-1, and IL-6) play an important role in the stimulation of osteoclastic bone resorption. The role of cytokine secretion and regulation of MMP production in PVNS has not been investigated. DESIGN: In the present study, ten specimens from eight patients (ages 19 to 80) were evaluated histologically according to a modified Mirra classification and immunohistochemically (IHC) for the expression of MMP-9 (92 kDa gelatinase B), tumor necrosis factor alpha (TNF alpha), interleukin 1-beta (IL-1 beta), and interleukin 6 (IL-6). Localization of IL-6 and TNF alpha production was confirmed by in situ hybridization (ISH) for mRNA. RESULTS: All specimens, regardless of location (six knees, one ankle, one subtalar joint), showed diffuse and intense immunoreactivity for cytokines in the giant cells and synovial cells, and less intense and diffuse staining in the activated macrophages. Staining in the fibroblastic elements was minimal. In situ hybridization for TNF alpha and IL-6 mRNA mirrored the immunohistochemistry results, although the IL-6 staining was weaker than that for TNF alpha. Immunoreactivity for MMP-9 was diffuse and strong in giant cells, diffuse and moderate in synovial cells, and focal and moderate to strong in macrophages. In contrast, normal synovium demonstrated focal, moderate immunoreactivity for IL-1 beta, IL-6, TNF alpha and MMP-9 localized in the synovial lining cells. CONCLUSION: These findings suggest that periarticular bone resorption and cartilage destruction which characterize PVNS may be related to the expression of inflammatory cytokines, which in turn stimulate MMP production.