Genetically modified CD34(+) cells exert a cytotoxic bystander effect on human endothelial and cancer cells

We and others have proposed mammalian cells as gene delivery vehicles with the potential for overcoming physiological barriers to viral vectors. To th at end, we previously have shown the potential of CD34(+) endothelial proge nitors for systemic gene delivery in a primate angiogenesis model. Here we seek to explore the utility of CD34+ cells of human origin as vehicles for toxin genes and, in particular, to measure their capacity to effect a cytot oxic bystander effect in human endothelium and tumor cells. To this end, CD 34+ cells were transduced with TOZ.1, a nonreplicative herpes simplex vecto r encoding thymidine kinase, To test the capacity of CD34+ cells to induce a cytotoxic bystander effect in target cells, we performed mixing experimen ts, whereby TOZ.1-transduced CD34(+) cells were mixed with either human vas cular endothelial cells or human ovarian tumor cells (SKOV3.ip1). Cell viab ility was measured by the MTS assay. Lastly, mixtures of TOZ.1-transduced C D34+ cells and SKOV3.ip1 tumor cells were injected s.c. to evaluate the bys tander effect in vivo. After transduction of CD34(+) cells with TOZ.1, trea tment with ganciclovir induced the killing of 99% of cells. In cell-mixing experiments, a linear correlation was observed between the percentages of T OZ.1-transduced CD34+ cells and total cell killing. For example, when 50% o f CD34(+) transduced cells were mixed with nontransduced SKOV3.ip1, >70% of all cells died. Similarly, when the same percentage was mixed with human v ascular endothelial cells, >80% of the total number of cells died. In vivo studies showed an abrogation of tumor formation when TOZ.1-transduced CD34( +) cells and ganciclovir were administered. Our observations establish the feasibility of a method for cell-based toxin gene delivery into disseminate d areas of tumor angiogenesis.