Objective：To construct a recombinant retroviral vector carrying human inducible costimulator (ICOS) gene,screen for CHO cell line stably expressing ICOS protein and to study its biological activity．Methods: ICOS cDNA was obtained from human peripheral blood mononuclear cells (PBMC) through RT-PCR and was cloned into retroviral vector to construct retroviral recombinant pMSCV-ICOS; the latter was then packed and the high-titer virus producing cells were screened．Then CHO cell was infected by this high-titer virus and the stable cell line was screened．CHO-ICOS cells were co-cultured with PBMC (the ratio of CHO-ICOS to PBMC being 11,12,15, and 110) in presence of substimulating dose of anti-human CD3 antibody.The proliferation of PBMC and the CD25 expression on T cells were examined by 3H-TdR incorporation method and flow cytometry，respectively.CHO-pMSCV cells co-cultured with PBMC (11) served as the negative control and PBMC served as blank control.Results: We successfully constructed the retroviral recombinant pMSCV-ICOS and obtained CHO cell line stably expressing ICOS protein．3H-TdR incorporation method and flow cytometry showed that,compared with the negative control group and the blank control group,co-culture with CHO-ICOS cells significantly inhibited the anti-CD3 antibody-induced activation and proliferation of PBMC(P<0.05)；the maximal inhibitory rates of activation and proliferation were obtained when the ratio of CHO-ICOS to PBMC was 11,being (68±5.9)% and (44.08±3.26)%，respectively．Conclusion: CHO cell line stably expressing ICOS protein has been successfully established,which lays a foundation for future study.