Oncolytic Rodent Protoparvoviruses Evade a TLR- and RLR-Independent Antiviral Response in Transformed Cells

Area of Science:

• Immunology
• Virology
• Oncology

Background:

• Oncolytic rodent protoparvoviruses (PVs), such as minute virus of mice (MVMp) and H-1 parvovirus (H-1PV), are investigated for cancer viro-immunotherapy due to their oncolytic activity and ability to induce anticancer immune responses (AIRs).
• Type-I interferon (IFN) production is critical for activating effective AIRs against cancer.
• Understanding how PVs modulate IFN induction in host cells is key to enhancing their therapeutic potential.

Purpose of the Study:

• To investigate the molecular mechanisms by which MVMp and H-1PV modulate type-I IFN induction in host cells.
• To characterize the differences in IFN production between normal and transformed/tumor cells upon PV infection.
• To identify viral factors or mechanisms responsible for the evasion of IFN induction in neoplastic cells.

Main Methods:

• Infection of semi-permissive normal mouse embryonic fibroblasts (MEFs) and human peripheral blood mononuclear cells (PBMCs) with MVMp and H-1PV.
• Assessment of IFN production and its dependence on viral replication and pattern recognition receptors (PRRs) like Toll-like (TLR) and RIG-like (RLR) receptors.
• Analysis of transcription factor nuclear translocation (NFκB, IRF3) and dsRNA accumulation.
• Transfection of dsRNAs into naïve cells to assess MAVS-dependent RLR signaling.
• Comparison of IFN production in normal versus immortalized/transformed MEFs and tumor cells.
• Pre-infection experiments to evaluate the impact of PV on RLR ligand-induced IFN production.

Main Results:

• MVMp and H-1PV triggered IFN production in semi-permissive normal cells (MEFs, PBMCs) but not in permissive transformed/tumor cells.
• IFN production in normal cells required PV replication and was independent of TLRs and RLRs, though PV infection led to NFκB and IRF3 nuclear translocation.
• PV replication generated dsRNAs that could activate MAVS-dependent RLR signaling in naïve cells, but this signaling was aborted in PV-infected neoplastic cells, preventing IFN production.
• MEF immortalization significantly reduced PV-induced IFN production.
• Pre-infection of tumor cells with PVs inhibited IFN production induced by classical RLR ligands, suggesting a viral evasion mechanism.

Conclusions:

• Rodent PVs employ a complex mechanism to regulate the innate antiviral immune response in host cells.
• PV replication in normal cells engages a PRR pathway that is distinct from TLR/RLR signaling, leading to IFN production.
• Transformed/tumor cells exhibit an arrested PRR signaling pathway, preventing IFN production due to a viral factor(s) that inhibits IFN induction.
• These findings suggest the development of second-generation PVs engineered to overcome this viral evasion mechanism, thereby enhancing their immunostimulatory potential and therapeutic efficacy in cancer treatment.