Overexpression of thrombin receptor on SFs from patients with RA. (a) Histograms representing thrombin receptor expression on synovial fibroblasts (SFs) from rheumatoid arthritis (RA) and osteoarthritis (OA) patients. Cells were stained with the antithrombin receptor mAb ATAP2. Flow-cytometric analyses were performed using FACScan. Open histograms represent the number of cells stained with ATAP2 in each logarithmic scale on a fluorescence amplifier. Shaded histograms represent profiles of anti-Thy1.2 mAb as a negative control. (b) Comparison of thrombin receptor expression in RA and OA SFs. The expression of thrombin receptor was analyzed by FACScan. Each value represents the number of molecules expressed per cell, calculated using standard QIFKIT beads from five similar experiments, as described in Materials and methods. Data are expressed as mean ± standard deviation for five independent donors. FACS, fluorescence-activated cell sorter. **P < 0.01.