Mechanistic insights into the oncolytic activity of vesicular stomatitis virus in cancer immunotherapy

Abstract

Immunotherapy and oncolytic virotherapy have both shown anticancer efficacy in the clinic as monotherapies but the greatest promise lies in therapies that combine these approaches. Vesicular stomatitis virus is a prominent oncolytic virus with several features that promise synergy between oncolytic virotherapy and immunotherapy. This review will address the cytotoxicity of vesicular stomatitis virus in transformed cells and what this means for antitumor immunity and the virus' immunogenicity, as well as how it facilitates the breaking of tolerance within the tumor, and finally, we will outline how these features can be incorporated into the rational design of new treatment strategies in combination with immunotherapy.

Keywords: anti-tumor immunity; natural killer cell; rhabdovirus; t cell; therapeutic vaccine; virotherapy.

Figure 1

A schematic representation of the vesicular stomatitis virus virion and genome showing subgenomic mRNA and relative transcript abundance. Notes: The negative-sense RNA genome is completely coated in the viral nucleoprotein (N protein) and is transcribed by the viral RNA-dependent RNA polymerase consisting of L and P subunits. Attachment, entry into the host cell, and membrane fusion are mediated by the glycoprotein (G protein). The matrix protein (M protein) plays a number of vital roles in the infection cycle. It is responsible for the budding of progeny virions, regulation of viral genome replication, and genome packaging, in addition to allowing VSV to shut down host cells’ innate antiviral responses through a variety of mechanisms. Dissociation of the viral polymerase in the intergenic regions of the genome controls the transcriptional abundance of five subgenomic monocistronic mRNAs. The full arrow indicates the direction of transcription of the viral genome, and the dashed arrows indicate the points at which the viral polymerase can dissociate from the viral genome. Abbreviations: mRNA, messenger RNA; VSV, vesicular stomatitis virus.

Figure 2

VSV M protein mediates the shutdown of host cell gene expression through three mechanisms. Notes: Association of M protein with nuclear pore complexes inside the nucleus via binding to Nup98/Rae1 heterodimers, thereby physically blocking the export of host cell mRNA. M protein can induce the indirect inactivation of host cell DNA-dependent RNA polymerase II via inactivation of the TATA-binding subunit of TFIID, which results in the shutdown of host gene transcription. By altering the phosphorylation states of components of the eIF4F cap-binding complex, M protein acts to prevent the translation of host mRNAs. Abbreviations: RNAP, RNA polymerase; TBP, TATA-binding protein; TFIID, transcription factor IID; VSV, vesicular stomatitis virus; mRNA, messenger RNA.

Figure 3

Wild-type and mutant VSV induce apoptosis via distinct pathways, but cross-talk does occur. Notes: Wild-type VSV M protein-mediated shutdown of host gene expression induces the intrinsic apoptotic pathway involving the release of cytochrome C from mitochondria into the cytosol. Formation of the apoptosome allows for the activation of caspase-9 and downstream caspases, which culminates in the death of the infected cell. VSV∆M51 induces apoptosis via the extrinsic pathway as a consequence of the detection of viral replication intermediates by double-stranded RNA-regulated protein kinase (PKR) and/or via type I interferon signaling. Downstream effects of PKR signaling induce the dimerization of FasR and Daxx to facilitate the phosphorylation of ASK-1. Phosphorylated ASK-1 activates the JNK signaling cascade, leading to the upregulation of proapoptotic factors and increased surface expression of death receptors (eg, the death-inducing signaling complex, DISC, which can activate caspase-3). Additionally, the production of viral proteins induces the endoplasmic reticulum stress response and leads to the activation of caspase-12. Blue arrows indicate points of cross–talk between the intrinsic and extrinsic pathways. The dashed arrows in the figure indicate multi-step processes. Abbreviations: ASK, apoptosis signal-regulating kinase 1; Daxx, Fas death domain-associated-xx; dsRNA, double-stranded RNA; IFN, interferon; MAC, mitochondrial apoptosis-induced channel; MKK4, mitogen-activated protein kinase kinase 4; pASK, phosphorylated ASK; pJNK, phosphorylated JNK; PKR, RNA-regulated protein kinase; pMKK4, phosphorylated mitogen-activated protein kinase kinase 4; VSV, vesicular stomatitis virus.

Figure 4

VSV oncolytic virotherapy can engage antitumor immunity as a consequence of viral infection of transformed cells. Notes: Tumor debulking is achieved via direct and indirect means. Viral replication in tumor cells will result in cytopathic effects and culminate in the death of the infected cell. Similarly, the innate antiviral response of infected cells will result in the death of tumor cells through the downstream effects of type I interferon signaling. This, coupled with the release of tumor antigens and danger-associated molecular patterns (DAMPs), stimulates the maturation of local dendritic cells. Mature dendritic cells secrete proinflammatory cytokines to recruit innate effector cells to mediate tumor killing, and they migrate to tumor-draining and distal lymph nodes to present tumor antigens to tumor-reactive CD8+ T-cells. Activated T-cells migrate to the tumor and mediate antigen-specific attack on tumor cells. Abbreviations: DAMPS, danger-associated molecular patterns; DC, dendritic cell; IFN, interferon; IL, interleukin; MHC, major histocompatibility complex; NK cell, natural killer cell; TNF, tumor necrosis factor; VSV, vesicular stomatitis virus.

Figure 5

The engagement of innate and adaptive immunity against tumors. Notes: In an effort to enhance the immunogenicity of VSV, a number of groups have explored the incorporation of a transgene into the VSV genome, most commonly between the G and L genes. The expression of cytokines to enhance the antigen-presenting and co-stimulatory abilities of dendritic cells, and the expression of cytokines to enhance effector lymphocyte survival and function have shown great promise in preclinical models. Some transgenes may also improve the safety profile of the engineered virus while simultaneously enhancing the function of antigen-presenting and effector cells (eg, VSV-driven expression of type I interferons). Abbreviations: Ag, antigen; APC, antigen-presenting cell; DC, dendritic cell; FLT3L, FMS-related tyrosine kinase-3 ligand; GM-CSF, granulocyte-monocyte colony stimulating factor; IFN, interferon; IL, interleukin; NK, natural killer cells; VSV, vesicular stomatitis virus.

Figure 6

A representation of the rationale behind a heterologous prime-boost therapeutic vaccination strategy using two oncolytic viruses expressing a tumor antigen. Notes: (A) An oncolytic virus (eg, adenovirus) expressing the target tumor antigen (Ag) is administered intravenously to the patient to engage a primary adaptive immune response and generate memory lymphocytes specific to the tumor. (B) Following the contraction of the primary immune response and the generation of memory, a booster vaccine consisting of VSV expressing the same tumor antigen is administered intravenously to engage the memory T-cells generated by the priming vaccine. This leads to a secondary response against the tumor that is much greater in magnitude than the primary response. Abbreviations: Ag, antigen; VSV, vesicular stomatitis virus.