Objective:To observe the expression of BTLA on T cells during activation and further analyze its inhibitory effects on T cell activation in different phases.Methods: T cells from PBMC were enriched by negative selection using magnetic beads.Expression of BTLA,CTLA-4 and PD-1 on freshly isolated human T cells and kinetics expression of BTLA,CILA-4 and PD-1 on CD3 mAb stimulated T cells were examined by flow cytometry.T cells were stimulated by anti-CD3 mAb combined with anti-CD28 mAb in the presence of anti-BTLA mAb 8H9,then T cell proliferation was tested by MTT assay in the different culture time.Immature DCs were generated from monocytes cultured in the medium containing GM-CSF and IL-4, and further driven to maturation by anti-CD40 mAb.Expression of HVEM on DCs was measured by flow cytometry.T cells were co-cultured with DCs in the presence of soluble 8H9 or anti-HVEM antibody to block HVEM-BTLA interaction,T cell proliferation was measured by MTT assay.Results:Freshly isolated T cells exhibited high levels of BTLA expression, but not CTLA-4 and PD-1.After T cell activation, BTLA expression decreased on first 2 days, with rapidly increasing to high levels.Unlike BTLA, expression of CTLA-4 and PD-1 was gradually increased during T cell activation.8H9 significantly inhibited the proliferation of T cell stimulated by CD3 mAb and CD28 mAb.8H9 could still exhibit inhibitory effect on T cell proliferation after 24 h or 48 h of preactivation by CD3 mAb plus CD28 mAb stimulation.HVEM was highly expressed on immature DCs, and down-regulated on mature DCs.Blockade of BTLA by soluble 8H9 or anti-HVEM antibody enhanced DC-mediated T cell proliferation within 48 h.Conclusion: BTLA signal enhances the threshold of T cell activation and plays importantly negative regulatory role in the initiation and early phase of T cell activation.