Apoptosis-mediated regulation of recombinant human granulocyte colony-stimulating factor production by genetically engineered fibroblasts

Reduction of CTLL-2 cytotoxicity by induction of apoptosis with a Fas-estrogen receptor chimera

Allogeneic bone marrow transplantation and donor lymphocyte infusion are powerful treatments for chemotherapy-resistant leukemia. Tumor eradication is attributed to a graft-versus-leukemia reaction by the donor-derived cytotoxic T lymphocytes (CTLs), but the same cell population may cause severe graft-versus-host disease. One strategy to suppress harmful CTL activity is to incorporate a suicide gene into the donor lymphocytes prior to infusion, and to destroy these cells when they aggressively attack nonmalignant host tissues. In this study, we investigated the feasibility of using a Fas-estrogen receptor fusion protein (MfasER) to control T cell-mediated cytotoxicity, based on our previous finding that the chimera transmits a Fas-mediated death signal through activation by estrogen binding. A murine CTL line CTLL-2 was transfected with a vector encoding MfasER, and the growth, viability and cytotoxic activity of the transfected cells (CTLL/MfasER) were analyzed. The expression of apoptosis-related proteins such as Fas ligand and perforin was also investigated. In the absence of estrogen, CTLL/MfasER showed similar growth to parental CTLL-2, and the killing activity was preserved. Addition of 10 (-7) M estrogen induced a rapid apoptosis of CTLL/MfasER, and the cytotoxicity was severely impaired. A decrease of Fas ligand and perforin in the estrogen-treated CTLL/MfasER was seen in an immunoblot analysis. These functional and biochemical analyses showed that the estrogen-inducible apoptosis in MfasER-expressing CTLs rapidly terminated their target cell killing. The feasibility of using the MfasER-estrogen system to control graft-versus-host disease was demonstrated.

Fas and mutant estrogen receptor chimeric gene: a novel suicide vector for tamoxifen-inducible apoptosis

Several cancer gene therapy strategies involve suicide genes to kill the neoplasm, or to regulate effector cells such as lymphocytes. We have developed an inducible apoptosis system with a Fas-estrogen receptor fusion protein (MfasER) for rapid elimination of transduced cells. In the present study, we further improved this molecular switch for estrogen-inducible apoptosis to overcome concerns with the wild-type estrogen receptor and its natural ligand, 17beta-estradiol (E2). The ligand-binding domain of MfasER was replaced with that of a mutant estrogen receptor which is unable to bind estrogen yet retains affinity for a synthetic ligand, 4-hydroxytamoxifen (Tm). The resultant fusion protein (MfasTmR) and MfasER were expressed in L929 cells for examination of their ligand specificities. Tm induced apoptosis in MfasTmR-expressing cells (L929MfasTmR) at 10(-8) M or higher concentrations, but induced no apoptosis in MfasER-expressing cells (L929MfasER) at up to 10(-6) M. On the other hand, E2 induced apoptosis in L929MfasER at concentrations as low as 10(-10)-10(-9) M, while it did so partially in L929MfasTmR at concentrations greater than 10(-7) M. Thus, L929MfasTmR cells were highly susceptible to Tm, but refractory to E2, with 100-1,000 times more tolerance than L929MfasER. These results suggest that the MfasTmR/Tm system would induce apoptosis in the target cells more safely in vivo, working independently of endogenous estrogen.