Functional long-term thymidine kinase suicide gene expression in human T cells using a herpesvirus saimiri vector

Herpesvirus saimiri transforms human T lymphocytes to stable growth and per sists episomally without genomic integration and without virus production. The transformed T cells retain essential features of their parental cells i ncluding the MHC-restricted antigen specificity which may be useful for app lications in adoptive immunotherapy. In order to improve the biological saf ety of such vectors, the prodrug activating gene thymidine kinase of herpes simplex virus was inserted into the genome of herpesvirus saimiri by homol ogous recombination. After infection with wild-type or cloned recombinant v iruses, T cells from tamarin monkeys and from humans were transformed to st able growth. Thymidine kinase-expressing transformed T cells were efficient ly eliminated in the presence of low concentrations of ganciclovir. This el imination mechanism remained fully functional over an observation period of 12 months. The potentially immunogenic neomycin resistance gene expression cassette was deleted from the genome of established mutant viruses by usin g the prokaryotic Cre/LoxP recombination system. At any time during the cou rse of a therapeutic application, thymidine kinase-expressing transformed h uman T cells might be eliminated after administration of ganciclovir. In pr inciple, this function could be useful for the T cell-dependent immunothera py of resistant blood cancer while avoiding the risk of uncontrolled graft- versus-host disease.