Monitoring of developing graft-versus-host disease mediated by herpes simplex virus thymidine kinase gene-transduced T cells.

Introduction of the HSV-Tk suicide gene into allogeneic T cells offers the possibility to control developing host-reactive cells within the context of allogeneic bone marrow transplantation (BMT). Sensitive quantitative detection methods are a prerequisite to monitor genetically modified T cells in peripheral blood and tissues to study their involvement in graft-versus-host disease (GVHD)-induced lesions as well as their disappearance or persistence after ganciclovir (GCV)-induced suicide. We monitored the alloreactivity of HSV-Tk-transduced T cells after BMT by studying their in vivo distribution and quantity in peripheral blood and in tissues in a WAG/Rij into Brown Norway fully mismatched rat allogeneic BMT model. Genetically modified T cells were quantified in blood and tissues by fluorescence-activated cell sorting, immunohistochemical analysis, and real-time quantitative polymerase chain reaction (PCR) analysis. A significant increase in the number of allogeneic HSV-Tk(+) T cells was found in particular in spleen and lymph nodes and large numbers were found in tongue, skin, and intestines. In blood, an increase in HSV-Tk(+) T cells closely preceded clinical symptoms of GVHD. Real-time quantitative PCR proved to be a fast and accurate tool by which to quantify transduced T cells both in blood and tissues. This enables the study of the in vivo alloreactivity of retrovirus-transduced cells and the response of HSV-Tk-expressing T cells to GCV-induced suicide therapy. Furthermore, we showed the potential use to study specific cause-effect relationships in a broad range of animal and clinical studies involving genetically engineered cells.