Objective: To study the synergistic effect of ICOS-Ig combined with cyclosporine (CsA) on mouse heart transplantation and explore its therapeutic potential. Methods: ICOS-Ig fusion protein was generated by fusing the extracellular portion of human ICOS and Fc portion of human IgG. To investigate the effect of ICOS-Ig on T-cell proliferation in vitro, ICOS-Ig or IgG was added to the primary MLR cultures (BALB/c spleen T cells as responder cells and irradiated C57BL/6 spleen cells as stimulator cells). The cells responsiveness rates were detected by 3H-TdR methods. Then the T cells of each group in primary MLR were cultured as responder cells for secondary MLR, and irradiated C57BL/6 (donor) or C3H (third party) spleen cells as stimulator cells. To study the effect of ICOS-Ig on T-cell proliferation in vivo, CFSE-labeled C57BL/6 spleen cells were transferred to irradiated BALB/c mice. Mice were then treated with IgG, ICOS-Ig or CsA. Seventy two hours after transfer, the spleen cells of the mice were harvested for the detection of CD4+CFSE+ and CD8+CFSE+ by FACS. C57BL/6 mouse underwent transplantation of the hearts of BALB/c mouse and were then randomly divided into five equal groups: no treatment group, control IgG treated group (250 μg i.p. d2, 4, 6), ICOS-Ig treated group (250 μg i.p. d2, 4, 6), CsA treated group (10 mg/kg i.p. d0-6), ICOS-Ig combined with CsA group. The cardiac allograft survival was monitored by daily palpation. Results: In primary MLR, ICOS-Ig inhibited T-cell proliferation, (inhibition ratio 58±8.2% in 50 μg/ml). In secondary MLR, ICOS-Ig specifically inhibited donor spleen cells, which suggested ICOS-Ig could induce donor-specific hyporesponsiveness. In the CFSE dye assay, CD4+CFSE+ and CD8+CFSE+ in ICOS-Ig and CsA group was stronger than those in control group, which showed ICOS-Ig and CsA could inhibit the proliferation of allo-reactive T cells in vivo. In mouse heart transplantation model, survival was significantly prolonged in animals treated with ICOS-Ig or CsA as compared with controls. Moreover, ICOS-Ig combined with CsA group had even longer engraftment (>100 d) than ICOS-Ig or CsA used alone. In histological examination, it was found that there were congestions and edemas in no treatment and IgG treated recipients, together with a lot of inflammatory cells infiltrated. Allogeneic hearts from ICOS-Ig and/or CsA immunized recipients revealed milder histological changes. It was revealed in mechanical analysis that splenic T cells from recipients also exhibited depressed mixed leukocyte reactions (MLR) and cytotoxic lymphocyte reactions (CTL). Conclusion: These data suggest that ICOS-Ig combined with CsA induces a long-term survival of mouse cardiac allografts, whereas monotherapy is less effective in this regard. Thus, ICOS-Ig combined with CsA treatment may be a novel regimen to combat allograft rejection.