Mechanisms of Innate Immune Activation by a Hybrid Alphavirus-Rhabdovirus Vaccine Platform.

Virus-like vesicles (VLV) are infectious, self-propagating alphavirus-vesiculovirus hybrid vaccine vectors that can be engineered to express foreign antigens to elicit a protective immune response. VLV are highly immunogenic and nonpathogenic in vivo, and we hypothesize that the unique replication and structural characteristics of VLV efficiently induce an innate antiviral response that enhances immunogenicity and limits replication and spread of the vector. We found that VLV replication is inhibited by interferon (IFN)-α, IFN-γ, and IFN-λ, but not by tumor necrosis factor-α. In cell culture, VLV infection activated IFN production and expression of IFN-stimulated genes (ISGs), such as MXA, ISG15, and IFI27, which were dependent on replication of the evolved VLV-encoded Semliki Forest virus replicon. Knockdown of the pattern recognition receptors, retinoic acid-inducible gene I and melanoma differentiation-associated protein 5 or their intermediary signaling protein mitochondrial antiviral-signaling protein (MAVS) blocked IFN production. Furthermore, ISG expression in VLV-infected cells was dependent on IFN receptor signaling through the Janus kinase (JAK) tyrosine kinases and phosphorylation of the STAT1 protein, and JAK inhibition restored VLV replication in otherwise uninfectable cell lines. This work provides new insight into the mechanism of innate antiviral responses to a hybrid virus-based vector and provides the basis for future characterization of the platform's safety and adjuvant-like effects in vivo. [Figure: see text].