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 therap y trial by repeated intratumoral injections with different doses of xenogen ic fibroblasts secreting high amounts of human interleukin-2 (Vero-IL2). Tr eatments in a total of 14 courses were well tolerated and resulted in clini cal responses and measurable biological effects. Together with increases in serum interleukin-2 (IL-2), modifications of the V-beta T cell receptor re pertoire and induction of intratumoral T-cell infiltration were observed. W hen the intratumoral expression of endogenous cytokine genes and the persis tence 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 10(5) nontransfected cells w ere not able to demonstrate significant expression of exogenous IL-2 (i.e. the transgene or transgene-carrying cells) in tumor biopsies on: blood at d ifferent times. Therefore, further studies were performed to evaluate the m echanism(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 t he 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 C D3(-)/CD16(+)/CD56(+) peripheral blood NK cell clones kill both K562 and Ve ro-IL2 target cells. The failure of other mechanisms (complement, antibody- dependent cell cytotoxicity or cytotoxic lr lymphocytes) to destroy xenogen ic, 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 o f importance for some aspects of the current discussion of xenotransplantat ion.