One concern about using CTLA-4/Ig in gene therapy is that discontinuation of treatment exposes patients to immunization against the transgene product. The present results illustrate this point since initial CTLA-4/Ig efficiently blocks the anti-Ova cellular and humoral responses while sustained sOva production finally elicits this undesired response when CTLA-4/Ig is probably cleared from the circulation. Our experimental conditions have been purposely designed to model this very situation where the immune response escapes the initial immunoregulatory regimen in order to evaluate if a second strategy could provide additional protection of transduced cells, i.e., PD-L1/2 gene therapy.