Abstract: The invention provides cell-mediated gene therapy for cancer using mesenchymal stem cells (MSC) expressing a suicide gene. The MSC are administered to a subject having a tumor, the MSC are allowed to migrate to the tumor site, and then the subject is administered with a prodrug. Expression of the suicide gene by the MSC at the tumor site converts the prodrug to a drug that is lethal to cells of the tumor. The impact of the tumor on the subject's health can thereby be reduced or eliminated. The MSC of this invention may be optimized for use as an off-the-shelf product by using an immortalized MSC line, and selecting MSC with immunological characteristics to forestall a graft-versus-host response.