Retrovirus-mediated transfer of the herpes simplex virus thymidine kinase and enhanced green fluorescence protein genes in primary T lymphocytes

Functional expression of thymidine kinase in human leukaemic and colorectal cells, delivered as EGFP fusion protein by herpesvirus saimiri-based vector

Herpesvirus saimiri (HVS) has the capacity to incorporate large amounts of heterologous DNA and can infect many different human cell types. To develop its potential as a gene therapy vector, we cloned herpes simplex virus thymidine kinase (TK) gene into the HVS genome in the form of an enhanced green fluorescent protein (EGFP) fusion protein, using a cosmid-based approach. At multiplicity of infection = 100 over 90% of human leukemic K562 and Jurkat cells were transduced with HVS/EGFP-TK. Conditions of no selective pressure expression were maintained at > 92% per cell division. Expression of the EGFP-TK fusion protein rendered transfected leukaemic cells sensitive to cytotoxic treatment with the prodrugs ganciclovir (GCV) and (E)-5-(2-bromovinyl)-2'deoxyuridine (BVDU) at concentrations as low as 10 ng/ml. The viral vector was also screened against a panel of colorectal and pancreatic carcinoma cell lines. All cell lines were transduced but showed a range of sensitivity to infection. Three of the most easily transduced cell lines: Mia PaCa, HCT116 and SW948 transduced with HVS/EGFP-TK were effectively ablated by subsequent treatment with GCV or BVDU. Our results show that in its current form HVS/EGFP-TK could be utilized as an antitumour agent, or it could be developed further by inclusion of a therapeutic gene, with TK presence ensuring a mechanism of controlled removal of modified cells when no longer necessary. These results suggest that HVS/EGFP-TK has a great potential for a number of gene therapy applications.

In Vitro and In Vivo Characterization of a Dual-Function Green Fluorescent Protein–HSV1-Thymidine Kinase Reporter Gene Driven by the Human Elongation Factor 1α Promoter

Toward the goal of monitoring activity of native mammalian promoters with molecular imaging techniques, we stably transfected DU145 prostate carcinoma cells with a fusion construct of enhanced green fluorescent protein (EGFP) and wild-type herpes simplex virus-1 thymidine kinase (HSV1-TK) as a reporter gene driven by the promoter for human elongation factor 1α (EF-1α-EGFP-TK). Using this model system, expression of EGFP was quantified by flow cytometry and fluorescence microscopy, while the HSV1-TK component of the reporter was quantified with 8-[3H]ganciclovir (8-[3H]GCV). As analyzed by flow cytometry, passage of EGFP-TK-DU145 transfected cells (ETK) in vitro resulted in populations of cells with high and low expression of EGFP over time. High and low ETK cells retained 23-fold and 5-fold more GCV, respectively, than control. While differences in uptake and retention of GCV corresponded to relative expression of the reporter gene in each subpopulation of cells as determined by both flow cytometry (EGFP) and quantitative RT-PCR, the correlation was not linear. Furthermore, in high ETK cells, net retention of various radiolabeled nucleoside analogues varied; the rank order was 8-[3H]GCV < 9-(4-fluoro-3-hydroxymethylbutyl)guanine ([18F]FHBG) ≈ 8-[3H]penciclovir (8-[3H]PCV) < 2′-fluoro-2′-deoxy-5-iodouracil-beta-d-arabinofuranoside (2-[14C]FIAU). Xenograft tumors of ETK cells in vivo accumulated 2.5-fold more 8-[3H]GCV per gram of tissue and showed greater fluorescence from EGFP than control DU145 cells, demonstrating that the reporter gene functioned in vivo. These data extend previous reports by showing that a human promoter can be detected in vitro and in vivo with a dual-function reporter exploiting optical and radiotracer techniques.