Objective: Transfer of the CTLA4IgG gene induces long-term and high levels of CTLA4IgG expression, which can result in generalized immunosuppression. In this study, we utilized Cre/loxP-mediated on–off switch recombination to eliminate transgene expression of CTLA4IgG following acceptance of murine cardiac allografts. Methods: Fully MHC-mismatched hearts from BALB/c donor mice were transplanted into C3H/He recipient mice. Adenovirus-containing CTLA4IgG flanked between two loxP sites was administered via a recipient tail vein immediately after transplantation. Cre-recombinase gene was subsequently transferred at day 30 posttransplantation. Results: Long-term allograft survival was observed in recipients that received the CTLA4IgG gene. Cre-mediated recombination reduced CTLA4IgG gene expression without any adverse effect on the graft survival. Secondary skin grafts of donor type and of third party were promptly rejected in the recipients that accepted cardiac allografts. In addition, the B cell response against ovalbumin was suppressed during high levels of serum CTLA4IgG, but recovered after Cre-mediated inactivation of CTLA4IgG gene. Conclusion: CTLA4IgG gene transfer promoted long-term survival of murine cardiac allografts; however, this was not sufficient to induce tolerance. Cre/loxP-mediated on–off switch recombination was useful to inactivate the CTLA4IgG gene so that recipients’ immune responses against neoantigens were restored without an influence on the allograft survival. This system may open novel strategies to orchestrate clinically relevant immunosuppression.