Gene therapy with cytokine-transfected xenogenic cells (Vero-IL-2) in patients with metastatic solid tumors: mechanism(s) of elimination of the transgene-carrying cells

Eleven patients with advanced cancer were treated in a clinical gene therapy trial by repeated intra- tumoral injections with different doses of xenogenic fibroblasts secreting high amounts of human interleukin-2 (Vero-IL2). Treatments in a total of 14 courses were well tolerated and resulted in clinical responses and measurable biological effects. Together with increases in serum interleukin-2 (IL-2), modifications of the V-β T cell receptor repertoire and induction of intratumoral T-cell infiltration were observed. When the intratumoral expression of endogenous cytokine genes and the persistence of the IL-2 transgene at the application site and in peripheral blood were investigated, rapid disappearance of the transgene at the application site appeared to be the most prominent biological effect. Tests detecting a single Vero-IL2 cell against a background of 10⁵ non-transfected cells were not able to demonstrate significant expression of exogenous IL-2 (i.e. the transgene or transgene-carrying cells) in tumor biopsies or blood at different times. Therefore, further studies were performed to evaluate the mechanism(s) involved in the rapid disappearance of xenogenic carrier cells in more detail. We show here that significant in vitro cytotoxicity against transgene-carrying Vero cells can be observed in peripheral blood of all the patients before treatment as well as in healthy controls. “Cold” target inhibition shows that significant killing of Vero-IL2 cells is mediated by natural killer (NK) cells. This was confirmed by showing that established CD3⁻/CD16 ⁺ /CD56 ⁺ peripheral blood NK cell clones kill both K562 and Vero-IL2 target cells. The failure of other mechanisms (complement, antibody-dependent cell cytotoxicity or cytotoxic T lymphocytes) to destroy xenogenic, histoincompatible Vero cells in vitro suggests that NK cells also might be responsible for the killing of Vero-IL2 in vivo and for the failure to detect the transgene at the application site. These results might also be of importance for some aspects of the current discussion of xenotransplantation. Received: 9 April 1999 / Accepted: 14 June 1999