Armed oncolytic measles vaccine virus – a novel therapeutic agent for virotherapy of solid tumors

Introduction: The oncolytic potential of measles vaccine virus (MeV) has been demonstrated in several tumor models but it has also been shown that primary and secondary resistances exist or may arise. To improve the oncolytic potential of MeV and to overcome resistances, a transgene armed measles vaccine virus was generated (MeV ld-SCD). A suicide gene (SCD) was inserted encoding a fusion protein consisting of the enzymes yeast cytosine deaminase and yeast uracil phosphoribosyltransferase. This allows the systemic application of the prodrug 5-fluorocytosine (5-FC) which is locally converted into the chemotherapeutic 5-fluorouracil (5-FU). 5-FU is diffusible and can kill primarily noninfected neighbouring cells.

Aims: It should be investigated if MeV ld-SCD in combination with the prodrug 5-FC is a suitable tool to overcome primary resistances of solid tumors towards MeV mediated oncolysis. In addition, the molecular mechanisms of resistance should be elucidated.

Results: MeV ld-SCD was used to infect nine sarcoma cell lines of gastrointestinal and non-gastrointestinal origin in the presence or absence of the prodrug 5-FC. Five cell lines were found to be susceptible to MeV ld-SCD mediated oncolysis whereas four showed a primary resistance which however could be overcome by employment of an increased input of viral particles and application of the prodrug 5-FC. To elucidate the molecular mechanism of tumor cell resistance versus susceptibility we had a closer look at one MeV resistant (SRH) and one MeV susceptible (BRZ) cell line. Viral growth curves revealed an inhibition of viral replication in the resistant cell line. Real time PCR analyses showed a strong induction of TLR3 and MDA5 as well as interferon beta mRNA in the resistant cell line whereas there was no induction of these transcripts in the susceptible cell line. These data were corroborated by Western Blot analyses showing constitutive phosphorylation of Stat1 in the resistant cell line which was increased after infection with MeV ld-SCD. No Stat1 phosphorylation/Stat1 activation could be detected in the susceptible cell line.

Conclusion: In conclusion, differences in interferon signaling were found to account for resistance or susceptibility of tumor cells towards MeV mediated oncolysis.