Abstract: Objective: To observe the function of the 4-1BB/4-1BBL signal in production of TGF-β in vitro,as well as the influence of HL-60 cells' culture supernatant on T lymphocyte activation,proliferation and inducing apoptosis after blocking 4-1BB/4-1BBL signal.To explore the mechanism by which tumors escape from immunosurveillance.Methods: The expression of 4-1BB ligand and the production of TGF-β in HL-60 tumor cells were analyzed by flow cytometry (FCM);using different concentrations of HL-60 tumor cells culture supernatant which been blocked 4-1BB/4-1BBL signal on lymphocyte,the cell proliferation rate was determined by methyl thiazoly tetrazolium (MTT),the cell apoptosis and CD69 were analyzed by FCM.Results: The level of 4-1BBL expressed on HL-60 was 92.6%.Compared with non-blocking 4-1BB/4-1BBL signal,the production of TGF-β in tumor cells was decreased (P<0.05),the effect of culture supernatant of HL-60 cells inhibited lymphocyte proliferation and induced it apoptosis was decreased (P<0.05 and P<0.01),but there was no significant influence on activation of lymphocytes (P>0.05).Conclusions: 4-1BBL has a reversely regulatory function to induce TGF-β production of HL-60 cells and enhance HL-60 tumor cells to inhibit lymphocyte activity,by which tumor cells can escape from the immune surveillance of tumor-bearing host.