MeV-RII_HepG2, a persistently measles virus (MeV) infected human hepatoma cell line, displays resistance towards secondary infections with MeV virotherapeutics
Introduction: The oncolytic potential of measles vaccine virus (MeV) has been shown both in vitro and in vivo. On the contrary, resistances to MeV-mediated oncolysis were demonstrated to either pre-exist or arise in the course of MeV virotherapy. To investigate mechanisms of secondary resistance we have generated a hepatoma cell line being persistently infected with MeV (MeV-RII_HepG2) which was generated by two rounds of infection with the GFP reporter gene encoding vector MeV-GFP followed by expansion of surviving (resistant) cell clones.
Aims: To characterize phenomena of secondary resistance to MeV-based virotherapeutics in persistently MeV infected MeV-RII_HepG2 human hepatoma cells. Further, susceptibility towards secondary infections with MeV was tested in these cells.
Results: MeV-RII_HepG2 cells were found to constitutively express both the GFP transgene as well as the viral N protein. Furthermore, a constant release of MeV was demonstrated by infection of parental (infection naïve) HepG2 cells resulting in syncytia formation being typical for the cytopathic effect of MeV. When using a dsRed encoding reporter vector (MeV-dsRed) only a very low number of MeV-RII_HepG2 cells was found to be susceptible indicating a high grade resistance towards such secondary MeV infections. Also, secondary infections with suicide gene armed vector MeV-SCD (Berchtold et al., 2013) did not result in any significant oncolysis even when employed at an MOI of as high as 10. However, addition of the prodrug 5-FC was able to break this acquired resistance resulting in a reduction of the targeted tumor cell mass to numbers below 20%. Characterization of MeV-RII_HepG2 cells on a molecular level revealed a downregulation of the MeV receptor CD46. In addition we observed a strong upregulation of the intracellular sensing molecules RIG-I and MDA5, of interferon beta (IFN-b) and of the interferon-stimulated gene (ISG) IFIT1.
Conclusion: These data reveal that persistently infected MeV-RII_HepG2 cells display a resistance towards secondary infections with MeV; this phenomenon is mediated at least in part by an upregulation of innate immune defence pathways. We are currently investigating whether this resistance to MeV does extend also to other nonrelated oncolytic viruses.