Dimerize and die

Adoptive immunotherapy with genetically modified T cells has many potential applications in treating viral infections and malignancy. However, this modality of therapy carries the risk of alloreactivity when donor cells are used after hematopoietic stem cell transplantation. In addition, there is a risk of autoimmune reactions if the target antigen is expressed on normal tissue and a risk of abnormal cell function induced by the transgene. An inducible suicide gene that can be activated in vivo in case of toxicity would increase the safety and broaden the application of this approach.

Initial studies focused on the herpes simplex virus 1–thymidine kinase (HSK-TK) gene, which renders transduced cells sensitive to ganciclovir. This strategy has been used in several clinical trials and has not been associated with any acute toxicity. However, this approach has been limited by immunogenicity of the TK gene product, which leads to the inadvertent destruction of TK-expressing lymphocytes.¹  More recently, an alternate strategy relying on novel artificial death switches based on chemical inducers of dimerization (CIDs) and endogenous proapoptotic molecules has been described.^2,3  In this approach, human apoptosis molecules can be linked to FK506 binding proteins that contain a binding site for a CID. Administration of this drug then results in the formation of a complex of 2 apoptosis fusion molecules, which leads to their activation and thus, apoptosis.³ 

In this issue, Berger and colleagues (page 1261) provide the first in vivo demonstration of elimination of T cells expressing an inducible Fas molecule. Their construct consists of a chimeric human protein expressing the Fas intracellular domain with 2 copies of an FK506-binding protein, which should not be immunogenic as it contains only human components.⁴  Transduced cells rapidly undergo apoptosis in vitro with the addition of subnanomolar concentrations of AP1903, a bivalent “dimerizer” drug that binds FK506 binding protein and induces Fas cross-linking and that has proved safe in testing in healthy volunteers. In the current study, Berger et al evaluate this approach in a primate model and show administration of AP1903 results in elimination of autologous T cells transduced with this construct.

One potential drawback is that a small number of transduced cells were unresponsive to CIDs and survived. While it is unclear what the clinical significance of this will be, the authors make a compelling argument that the surviving cells are resting T cells with a low level of transgene expression and are as such less likely to be alloreactive. Alternatively, it may reflect an effect of cellular inhibitors of apoptosis and could potentially be overcome by using a downstream-acting molecule in the caspase family.^3,5  A second issue is that a transgene-specific immune response was generated, but epitope mapping showed that it was directed at sequences in the Fas suicide construct that differed between human and macaque, so it is unlikely to be a problem in human clinical studies. The results described in Berger et al are therefore encouraging that CIDs can be used to induce death if necessary in adoptively transferred T cells transduced with human apoptosis molecules.