The main approach to reduce graft rejection has been focused on the development of immunosuppressive agents at present. Although these strategies have reportedly reduced graft rejection, there has been a reciprocal increase in more severe immunosuppression and lethal infections, as well as severe side effects. Blockade of costimulatory T cell response has been proved as one of useful strategies to reduce graft rejection. Furthermore,it has been shown that infusion of dendritic cells (DCs) with a potent negative regulatory ability for T cells could prolong allograft survival. In this study mouse DCs (mDCs) were transfected with the recombinant plasmid pcDNA3.0 containing mouse inducible costimulator-Ig (mICOS-Ig) cDNA by electroporation. The transient expression of mICOS-Ig in mDC could be detected by ELISA and SDS-PAGE. Mouse ICOS-Ig fusion protein expressed in mDC and mICOS-Ig gene-modified mDC could inhibit lymphocyte proliferation in mixed lymphocyte culture (MLC) in vitro. Furthermore, mICOS-Ig gene-modified mDC could inhibit lymphocyte proliferation in recipient mice. These results suggested that mICOS-Ig gene-modified mDC exerted inhibitory effects on T cells, and might be suitable for treatment or prevention of graft rejection and immunopathologicdiseases.