Objective:To express human ICOS extracellular region and IgG Fc fusion protein using euckaryotic vector and to analyze its biological activity.Methods: Human ICOS extracellular region and IgG Fc fragment were cloned by RT-PCR and inserted into eukaryotic expression vector pSecTag2.The constructed plasmid pSecTag2-ICOS-Ig was stably transfected into CHO cells.Soluble ICOS-Ig fusion protein was collected from serum-free medium and purified with protein A affinity chromatography.The purified product was analyzed by SDS-PAGE,Western blotting assay, and ELISA.Fluorescence-activated cell sorting (FACS) and mixed lymphocyte reaction (MLR) were used to study the activity of the fusion protein.Results: ICOS extracellular region and IgG Fc fragment were successfully cloned into expression vector; ICOS-Ig fusion protein was expressed and purified in mammal cells.The purified fusion protein specifically bound to ICOSL and inhibited mixed lymphocyte reaction.Conclusion: A ICOS-Ig fusion protein expression system has been successfully constructed,and bioactive ICOS-Ig fusion protein has been obtained.