The Bystander Effect Mediated by the New Murine Gammaherpesvirus 72 — Thymidine Kinase/5′-Fluoro-2′-Deoxyuridine (MHV72-TK/5-FUdR) System in Vitro

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Abstract
To investigate the potential of murine gammaherpesvirus 72 thymidine kinase (MHV-72-TK) to act as a suicide gene, we used a mammalian expression vector on rat fibroblastoid cells deficient in the cellular TK gene. Substrate specificity was assessed in vitro in cells with stable expression of MHV-72-TK. The Herpes simplex virus 1-TK (HSV-1-TK) was used as a reference suicide gene. Unlike HSV-1-TK modified cells, which were sensitive to ganciclovir (GCV) (IC50=9.7 μM), cells modified by MHV-72-TK did not show sensitivity to this drug. The use of 3′-azido-3′-deoxythymidine (AZT) and (E)-5-(2-bromovinyl)-2′-deoxyuridine (BVDU) did not affect the growth of cells expressing either MHV-72-TK or HSV-1-TK in the range of concentration used for AZT (0–375 μM) and for BVDU (0–50 μM). In contrast, 5′-fluoro-2′-deoxyuridine (5-FUdR) was extremely cytotoxic and effectively killed MHV-72-TK expressing cells (IC50 value 2.1 μM). This value was 16 times lower than that required to kill cells expressing HSV1-TK. To test whether the bystander effect between two heterologous cell types could be mediated by the MHV-72-TK/5-FUdR system in vitro, cells expressing MHV-72-TK were co-cultured with the tumour fibroblastoid cell line NAD for 48 hours before the drug (10.8 μM) was added. The cell mixtures contained various ratios of cells expressing MHV-72-TK (0 to 50% of total cells). Only 1% of MHV-72-TK-expressing cells were needed to enhance mouse tumour cell killing and to decrease the survival rate to 25.6%. The bystander effect was more pronounced when 10% of cells expressing MHV-72-TK were used, decreasing survival to 17.4%. In parallel, the same concentration of 5-FUdR dose only marginally inhibited tumour cell growth in the absence of exogenous TK activity (84% survival). These results demonstrate the efficiency of MHV-72-TK as a suicide gene when 5-FUdR is used as a pro-drug. When sequenced, MHV-72-TK proved to be identical to MHV-68 strain TK.

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