A suicide gene encoding fas-estrogen receptor chimeric molecule for inducible apoptosis of cytotoxic t lymphocytes

One of the therapeutic strategies for relapsed leukemia after allo-BMT is donor lymphocyte infusion (DLI), in which graft-versus-leukemia (GVL) reaction via allo-reactive cytotoxic T lymphocytes (CTL) is a major factor contributing to the eradication of malignant cells. However, the most serious obstacle is life-threatening GVHD. One strategy to regulate harmful CTL activity is to incorporate a suicide gene into donor lymphocytes prior to DLI, allowing on-demand removal of these cells when severe GVHD occurs. For this purpose, HSV-TK gene has been used in combination with ganciclovir, but there are several problems in this system. We have developed a distinct apoptosis induction system by using a gene encoding Fas-estrogen receptor chimeric molecule (MfasER gene from Dr. A. Kakizuka) for eliminating transduced cells. We investigated the feasibility of using MfasER gene to control CTL activity. A murine CTL line, CTLL-2, was used as an effector in model experiments. MfasER gene transfer itself did not affect the viability and cytotoxic activity of CTLL-2. The addition of estrogen into the culture media eradicated more than 90% of the transduced CTLL-2 with significant decrease in the cytotoxicity. Considering in vivo applications, the MfasER/estrogen system has several desirable characteristics such as least immunogenicity of the fusion protein, low toxicity of the reagent, and cell cycle-independent apoptosis induction. Furthermore, Fas-mediated apoptosis pathway would work more effectively in activated T-lymphocytes than in resting cells, which provides a rationale for this approach.