Abstract: OBJECTIVE: To investigate the effect of indoleamine 2，3-dioxygenase(IDO) on the expression of human leucocyte antigen-G(HLA-G) in hepatocellular carcinoma cells. METHODS： Hepatocellular carcinoma cells SMMC-7721 transfected with pcDNA3.1- IDO recombinant plasmid by lipofectamineTM 2000 reagent as experimental group (pcDNA3.1 -IDO group)， SMMC-7721 transfected with blank plasmid( pcDNA3.1 group) and blank SMMC-7721 (non-transfected group) were used as control groups. The expression of IDO in expreimental group and control groups were tested by RT-PCR and Western blot， the expression of HLA-G mRNA and protein in cells were tested by real time fluorescence quantitative PCR and Western blot. Then pcDNA 3.1-IDO plasmid group cells were treated with IDO competitive inhibitior( 1-methyl-D-trypophan) and its substrate (L-tryptophan) intervention to investigate the effect of drugs on the expression of HLA-G. RESULTS：IDO was highly expressed in pcDNA3.1-IDO plasmid group cells shown by RT-PCR and Western blot. Expression of HLA-G mRNA and protein in pcDNA3.1-IDO plasmid group were higher than the other control groups, the intervention of L-tryptophan and 1-methyl-D-trypophan could reverse this effect. CONCLUSION：IDO could up regulate the expression of HLA-G in SMMC- 7721 cells with tryptophan degradation pathway involved in this regulation.

Key words: indoleamine 2, 3-dioxygenase, human leucocyte antigen-G, SMMC-7721cells