Abstract: Objective:To investigate the effect of costimulatory signal 4-1BBL/4-1 BB in the activation of the T lymphocytes and the expression of costimulatory molecule of patients with (SLE). Methods:The peripheral blood mononuclear cells (PBMCs) were stimulated with anti-CD3 and the costimulatory signal of 4-1BBL/4-1BB was blocked by anti-4-1BBL. The expressions of 4-1BB, CD40L,4-1BBL,CD40 and CD69 of T cell and B cell before blocked and after blocked by anti-4-1BBL were detected by flow cytometry. Results:The expression of CD69 on T cells of SLE patients showed significant difference(P<0.01) with healthy controls. The expressions of 4-1BB,CD40L,4-1BBL and CD40 on the lymphocytes of SLE patients were significantly higher than that of normal control(P<0.01). There were obviously increased expression of 4-1BB and CD40L on the T lymphocytes stimulated by anti-CD3 antibody in SLE patients than healthy controls (P<0.01). With the anti-4-1BBL monoantibody blocked, there were decreased expression of 4-1BB CD40L (P<0.01). The expression of CD40 on B cell was significantly decreased, which is less than SLE unstimulted group and SLE stimulated group(P<0.05). Conclusions:T cell and B cell activation level of SLE patients were increased and 4-1BBL/4-1BB signals play a role in keeping the activation.