Ectopic expression of CD9 suppressed pulmonary metastasis and lung retention of HT1080 cells. (A) Stably transfected polyclonal HT1080/mock (left panel) and HT1080/CD9 (right panel) cells were treated with a control mouse IgG (red line) or an anti-CD9 antibody (blue line). After incubation with Oregon Green 488–conjugated second antibody, CD9 expression was estimated by flow cytometer. (B) HT1080/mock, HT1080/CD9, and HT1080/CD44 cells were intravenously injected into C.B-17/Icr-scid mice (n = 10). At 28 days after tumor inoculation, lung surface nodules were counted. Representative pictures of the lungs (left panels) and detailed numbers of the lung metastasis nodules in each mouse (right panel) are shown. Data are means plus or minus SD. **P < .005 by Student t test. (C) Stably transfected polyclonal HT1080/AcGFP+mock, HT1080/AcGFP+CD9, and HT1080/AcGFP+CD44 cells were intravenously injected into BALB/c-nu/nu mice (n = 6). After 5 minutes or 4 hours, the lungs were extirpated. The fluorescence was visualized using an IV100 imaging system, and the fluorescence area was quantitated. Representative pictures of the lungs (left panels) and detailed fluorescence area (right panels) are shown. Data are means plus or minus SD. **P < .005; ***P < .001 using the Student t test.