Objective. To purify a protein inhibitor from rheumatoid arthritis (RA) synovial fluids which suppresses the apparent incorporation of 3H‐thymidine into fibroblasts and synovial cells, and to define its biochemical features that have clinical relevance to the pathogenesis of RA. Methods. Several standard chromatographic techniques were employed for the purification of the protein. Immunochemical methods with monoclonal antibody were used to quantify and visualize the protein in sera, synovial fluids, and tissues from RA patients. Results. The chemical properties of purified inhibitor from RA synovial fluids confirmed its identity as gliostatin/platelet‐derived endothelial cell growth factor (PD‐ECGF), a potent angiogenic factor. The gliostatin/ PD‐ECGF level in synovial fluid and serum was higher in RA patients than in osteoarthritis controls. Conclusion. These findings strongly suggest that gliostatin/PD‐ECGF might play an important role in the aberrant neovascularization of rheumatoid synovium.
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