Innate immune defence defines susceptibility of gastrointestinal and non-gastrointestinal sarcomas to measles vaccine virus based virotherapy

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 suicide gene (SCD) was inserted encoding a fusion protein of the enzymes yeast cytosine deaminase and yeast uracil phosphoribosyltransferase (MeV ld-SCD). 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 suitable 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. Six cell lines were susceptible to MeV ld-SCD mediated oncolysis whereas three showed a primary resistance. In the resistant cell lines, we observed an inhibition of viral replication going along with a strong upregulation of the intracellular sensing molecule RIG-I and of the interferon-stimulated gene (ISG) IFIT1. Expression of IFIT1 and phosphorylation of STAT1 took place rapidly and were found to be persistent over time. In contrast, susceptible cell lines showed a weaker, delayed or missing expression of IFIT1 as well as a delayed or only transient phosphorylation of STAT1. Stimulation with exogenous interferon beta (IFN-ß) resulted in activation of STAT1 and expression of IFIT1 in all cell lines. Pretreatment with IFN-ß rendered four of the susceptible cell lines more resistant to MeV mediated oncolysis. In a therapeutic perspective, we were able to overcome resistances to MeV by increasing the multiplicity of infection (MOI) and by addition of 5-FC thereby exploiting the suicide gene function of MeV ld-SCD.

Conclusion: These data suggest that differences in innate immune defence account for differenzial susceptibility of gastrointestinal and non-gastrointestinal sarcomas to MeV mediated oncolysis.