Everyone knows that time is of the essence when you have a train to catch. Recent work by Cohen and colleagues in the journal Blood shows that timing is also essential when it comes to the application of suicide gene therapy of graft-versus-host disease (GVHD), a life-threatening complication of allogeneic haematopoietic stem-cell transplantation (HSCT).

Mature donor T cells present in an allograft after HSCT improve engraftment and T-cell reconstitution, and provide a graft-versus-leukaemia (GVL) effect. In addition to these beneficial effects, activation of donor T cells that are specific for recipient alloantigens (termed alloreactive) result in GVHD.

Is it possible to selectively eliminate the alloreactive donor T cells, but spare the T cells which mediate immune reconstitution and the GVL effect? A strategy has been developed to eliminate donor T cells. Before transplantation, donor T cells are transduced with the herpes simplex type I thymidine kinase (TK) suicide gene. Treatment with gancyclovir (GCV; a thymidine analogue that is toxic to dividing cells expressing TK) allows these donor T cells to be eliminated. However, as the TK–GCV system is based on the cell-cycle status of donor TK-expressing T cells, and not on their alloreactivity, its therapeutic usefulness was thought to be limited.

Cohen et al. assessed the kinetics of T-cell expansion after semiallogeneic bone marrow transplantation (BMT) (when both alloreactive and homeostatic T-cell expansion occur) and syngeneic BMT (when only homeostatic expansion occurs). T cells from double-transgenic mice expressing TK in CD4+ and CD8+ T cells and a marker (human CD4) were stained with carboxyfluorescein diacetate succinimide ester (CFSE) and infused with wild-type BM into lethally irradiated recipients. Spleen cells were then collected from the recipient mice at different time points after BMT, and donor T-cell division was assessed on the basis of CFSE fluorescence. In semiallogenic hosts, the donor T cells proliferated rapidly, and after 88 hours most donor T cells had divided at least once. In syngeneic hosts, T-cell divisions were significantly delayed. These results indicate that alloreactive T cells divide earlier than non-alloreactive T cells.

The authors then investigated the persistence and expansion of donor T cells in the semiallogeneic BMT setting after GCV treatment. This treatment resulted in the death of most donor T cells, with surviving T cells being those that had not divided. After a 7-day GCV course, a pool of donor T cells persist, which significantly expands after GCV treatment is stopped. Finally, the authors show that the surviving T cells contribute to the replenishment of the T-cell compartment and provide a diversified T-cell receptor repertoire.

This study has identified a therapeutic window when GCV treatment can be administered to specifically kill alloreactive donor T cells (and so control GVHD), but spare non-dividing, non-alloreative T cells, which enables T-cell reconstitution to be maintained.