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].

Virus-Like Vesicles of Kaposi's Sarcoma-Associated Herpesvirus Activate Lytic Replication by Triggering Differentiation Signaling

Virus-like vesicles (VLVs) are membrane-enclosed vesicles that resemble native enveloped viruses in organization but lack the viral capsid and genome. During the productive infection of tumor-associated gammaherpesviruses, both virions and VLVs are produced and are released into the extracellular space. However, studies of gammaherpesvirus-associated VLVs have been largely restricted by the technical difficulty of separating VLVs from mature virions. Here we report a strategy of selectively isolating VLVs by using a Kaposi's sarcoma-associated herpesvirus (KSHV) mutant that is defective in small capsid protein and is unable to produce mature virions. Using mass spectrometry analysis, we found that VLVs contained viral glycoproteins required for cellular entry, as well as tegument proteins involved in regulating lytic replication, but lacked capsid proteins. Functional analysis showed that VLVs induced the expression of the viral lytic activator RTA, initiating KSHV lytic gene expression. Furthermore, employing RNA sequencing, we performed a genomewide analysis of cellular responses triggered by VLVs and found that PRDM1, a master regulator in cell differentiation, was significantly upregulated. In the context of KSHV replication, we demonstrated that VLV-induced upregulation of PRDM1 was necessary and sufficient to reactivate KSHV by activating its RTA promoter. In sum, our study systematically examined the composition of VLVs and demonstrated their biological roles in manipulating host cell responses and facilitating KSHV lytic replication.IMPORTANCE Cells lytically infected with tumor-associated herpesviruses produce a high proportion of virus-like vesicles (VLVs). The composition and function of VLVs have not been well defined, largely due to the inability to efficiently isolate VLVs that are free of virions. Using a cell system capable of establishing latent KSHV infection and robust reactivation, we successfully isolated VLVs from a KSHV mutant defective in the small capsid protein. We quantitatively analyzed proteins and microRNAs in VLVs and characterized the roles of VLVs in manipulating host cells and facilitating viral infection. More importantly, we demonstrated that by upregulating PRDM1 expression, VLVs triggered differentiation signaling in targeted cells and facilitated viral lytic infection via activation of the RTA promoter. Our study not only demonstrates a new strategy for isolating VLVs but also shows the important roles of KSHV-associated VLVs in intercellular communication and the viral life cycle.