Engineered bacteria launch and control an oncolytic virus

The ability of bacteria and viruses to selectively replicate in tumors has led to synthetic engineering of new microbial therapies. Here we design a cooperative strategy whereby S. typhimurium bacteria transcribe and deliver the Senecavirus A RNA genome inside host cells, launching a potent oncolytic viral infection. "Encapsidated" by bacteria, the viral genome can further bypass circulating antiviral antibodies to reach the tumor and initiate replication and spread within immune mice. Finally, we engineer the virus to require a bacterially delivered protease to achieve virion maturation, demonstrating bacterial control over the virus. This work extends bacterially delivered therapeutics to viral genomes, and shows how a consortium of microbes can achieve a cooperative aim.

Development and characterization of a recombinant Senecavirus A expressing enhanced green fluorescent protein

Introduction

Senecavirus A (SVA), belonging to the genus Senecavirus in the family Picornaviridae, is an emerging pathogen causing vesicular disease in pigs. The main clinical manifestations of SVA infection include high mortality in neonatal piglets, skin ulceration, and vesicular lesions. So far, there is no commercially available vaccines or drugs against SVA. Construction of SVA infectious clones carrying reporter genes will help understand the characteristics of SVA and promote vaccine development.

Methods

In this study, we established a reverse genetics system for a local SVA isolate and used it to rescue a recombinant SVA, rSVA-eGFP, expressing the enhanced green fluorescent protein (eGFP) by inserting eGFP, GSG linker and the P2A sequence between 2A and 2B genes.

Results

We found that rSVA-eGFP exhibited a high replication efficiency comparable to the parental virus, was able to express the eGFP reporter efficiently and stable in maintaining the reporter gene up to six rounds of serial passages in BHK-21 cells. In mice, rSVA-eGFP also showed similar replication kinetics and pathogenicity to the parental virus, both causing mild lung lesions. In addition, a high-throughput viral neutralization assay was developed using eGFP as a surrogate readout in a fluorescence-based direct titration (FBT) assay based on rSVA-eGFP, facilitating rapid and accurate determination of the neutralizing antibody (nAb) titers.

Discussion

The successful establishment of an SVA reverse genetics system and the rescue of rSVA-eGFP would create a powerful tool for future studies of SVA replication mechanisms and pathogenicity as well as for antiviral development.

Engineering Non-Human RNA Viruses for Cancer Therapy

Alongside the development and progress in cancer immunotherapy, research in oncolytic viruses (OVs) continues advancing novel treatment strategies to the clinic. With almost 50 clinical trials carried out over the last decade, the opportunities for intervention using OVs are expanding beyond the old-fashioned concept of "lytic killers", with promising breakthrough therapeutic strategies focused on leveraging the immunostimulatory potential of different viral platforms. This review presents an overview of non-human-adapted RNA viruses engineered for cancer therapy. Moreover, we describe the diverse strategies employed to manipulate the genomes of these viruses to optimize their therapeutic capabilities. By focusing on different aspects of this particular group of viruses, we describe the insights into the promising advancements in the field of virotherapy and its potential to revolutionize cancer treatment.

NOG1 downregulates type I interferon production by targeting phosphorylated interferon regulatory factor 3

The innate immune system is the first line of the host's defense, and studying the mechanisms of the negative regulation of interferon (IFN) signaling is important for maintaining the balance of innate immune responses. Here, we found that the host GTP-binding protein 4 (NOG1) is a negative regulator of innate immune responses. Overexpression of NOG1 inhibited viral RNA- and DNA-mediated signaling pathways, and NOG1 deficiency promoted the antiviral innate immune response, resulting in the ability of NOG1 to promote viral replication. Vesicular stomatitis virus (VSV) and herpes simplex virus type 1 (HSV-1) infection induced a higher level of IFN-β protein in NOG1 deficient mice. Meanwhile, NOG1-deficient mice were more resistant to VSV and HSV-1 infection. NOG1 inhibited type I IFN production by targeting IRF3. NOG1 was also found to interact with phosphorylated IFN regulatory factor 3 (IRF3) to impair its DNA binding activity, thereby downregulating the transcription of IFN-β and downstream IFN-stimulated genes (ISGs). The GTP binding domain of NOG1 is responsible for this process. In conclusion, our study reveals an underlying mechanism of how NOG1 negatively regulates IFN-β by targeting IRF3, which uncovers a novel role of NOG1 in host innate immunity.

Seneca Valley virus replicons are packaged in trans and have the capacity to overcome the limitations of viral transgene expression

Oncolytic viruses (OVs) promote the anti-tumor immune response as their replication, and the subsequent lysis of tumor cells, triggers the activation of immune-sensing pathways. Arming OVs by expressing transgenes with the potential to promote immune cell recruitment and activation is an attractive strategy to enhance OVs' therapeutic benefit. For picornaviruses, a family of OVs with clinical experience, the expression of a transgene is limited by multiple factors: genome physical packaging limits, high rates of recombination, and viral-mediated inhibition of transgene secretion. Here, we evaluated strategies for arming Seneca Valley virus (SVV) with relevant immunomodulatory transgenes. Specificially in the contex of arming SVV, we evaluated transgene maximum size and stabiltity, transgene secretion, and the impact of transgene inclusion on viral fitness. We find that SVV is not capable of expressing secreted payloads and has a transgene packaging capacity of ∼10% of viral genome size. To enable transgene expression, we developed SVV replicons with greater transgene size capacity and secretion capabilities. SVV replicons can be packaged in trans by virus in co-infected cells to express immunomodulatory transgenes in surrounding cells, thus providing a means to enhance the potential of this therapeutic to augment the anti-tumor immune response.

Procedures and potential pitfalls for constructing a bee-infecting RNA virus clone

Viruses are factors that can fluctuate insect populations, including honey bees. Most honey bee infecting viruses are single positive-stranded RNA viruses that may not specifically infect honey bees and can be hazardous to other pollinator insects. In addition, these viruses could synergize with other stressors to worsen the honey bee population decline. To identify the underlying detailed mechanisms, reversed genetic studies with infectious cDNA clones of the viruses are necessary. Moreover, an infectious cDNA clone can be applied to studies as an ideal virus isolate that consists of a single virus species with a uniform genotype. However, only a few infectious cDNA clones have been reported in honey bee studies since the first infectious cDNA clone was published four decades ago. This article discusses steps, rationales, and potential issues in bee-infecting RNA virus cloning. In addition, failed experiences of cloning a Deformed wing virus isolate that was phylogenetically identical to Kakugo virus were addressed. We hope the information provided in this article can facilitate further developments of reverse-genetic studies of bee-infecting viruses to clarify the roles of virus diseases in the current pollinator declines.

Evolving role of seneca valley virus and its biomarker TEM8/ANTXR1 in cancer therapeutics

Oncolytic viruses have made a significant inroad in cancer drug development. Numerous clinical trials are currently investigating oncolytic viruses both as single agents or in combination with various immunomodulators. Oncolytic viruses (OV) are an integral pillar of immuno-oncology and hold potential for not only delivering durable anti-tumor responses but also converting “cold” tumors to “hot” tumors. In this review we will discuss one such promising oncolytic virus called Seneca Valley Virus (SVV-001) and its therapeutic implications. SVV development has seen seismic evolution over the past decade and now boasts of being the only OV with a practically applicable biomarker for viral tropism. We discuss relevant preclinical and clinical data involving SVV and how bio-selecting for TEM8/ANTXR1, a negative tumor prognosticator can lead to first of its kind biomarker driven oncolytic viral cancer therapy.

N-Linked Glycosylation on Anthrax Toxin Receptor 1 Is Essential for Seneca Valley Virus Infection

Seneca Valley virus (SVV) is a picornavirus with potency in selectively infecting and lysing cancerous cells. The cellular receptor for SVV mediating the selective tropism for tumors is anthrax toxin receptor 1 (ANTXR1), a type I transmembrane protein expressed in tumors. Similar to other mammalian receptors, ANTXR1 has been shown to harbor N-linked glycosylation sites in its extracellular vWA domain. However, the exact role of ANTXR1 glycosylation on SVV attachment and cellular entry was unknown. Here we show that N-linked glycosylation in the ANTXR1 vWA domain is necessary for SVV attachment and entry. In our study, tandem mass spectrometry analysis of recombinant ANTXR1-Fc revealed the presence of complex glycans at N166, N184 in the vWA domain, and N81 in the Fc domain. Symmetry-expanded cryo-EM reconstruction of SVV-ANTXR1-Fc further validated the presence of N166 and N184 in the vWA domain. Cell blocking, co-immunoprecipitation, and plaque formation assays confirmed that deglycosylation of ANTXR1 prevents SVV attachment and subsequent entry. Overall, our results identified N-glycosylation in ANTXR1 as a necessary post-translational modification for establishing stable interactions with SVV. We anticipate our findings will aid in selecting patients for future cancer therapeutics, where screening for both ANTXR1 and its glycosylation could lead to an improved outcome from SVV therapy.

Infectious recombinant Senecavirus A expressing novel reporter proteins

Senecavirus A (SVA) is an emerging picornavirus that has been associated with vesicular disease and neonatal mortality in swine. The construction of SVA virus carrying foreign reporter gene provides a powerful tool in virus research. However, it is often fraught with rescuing a recombinant picornavirus harboring a foreign gene or maintaining the stability of foreign gene in the virus genome. Here, we successfully generated recombinant SVA GD05/2017 viruses (V-GD05-clone) expressing the green fluorescent protein (iLOV), red fluorescent protein (RFP), or NanoLuc luciferase (Nluc). These recombinant viruses have comparable growth kinetics to the parental virus. Genetic stability analysis indicated that V-GD05-iLOV was highly stable in retaining iLOV gene for more than 10 passages, while V-GD05-RFP and V-GD05-Nluc lost the foreign genes in five passages. In addition, high-intensity fluorescent signals were found in the V-GD05-RFP- and V-GD05-iLOV-infected cells by fluorescence observation and flow cytometry analysis, and the luciferase activity assay could quantitatively monitor the replication of V-GD05-Nluc. In order to identify the porcine cell receptor for SVA, anthrax toxin receptor 1 (ANTXR1) was knocked out or overexpressed in the ST-R cells. The ANTXR1 knock-out cells lost the ability for SVA infection, while overexpression of ANTXR1 significantly increased the cell permissivity. These results confirmed that ANTXR1 was the receptor for SVA to invade porcine cells as reported in the human cells. Overall, this study suggests that these SVA reporter viruses will be useful tools in elucidating virus pathogenesis and developing control measures. KEY POINTS: • We successfully generated SVA viruses expressing the iLOV, RFP, or Nluc. • The iLOV was genetically stable in the V-GD05-iLOV genome over ten passages. • ANTXR1 was the receptor for SVA to invade porcine cells.

Development of an improved dual-promoter-based reverse genetics system for emerging Senecavirus A

Senecavirus A (SVA), a recently emerging picornavirus, poses a great threat to the swine industry because it causes swine idiopathic vesicular disease and epidemic transient neonatal losses. Thus far, the progress in SVA viral pathogenesis studies and vaccine development remains sluggish, and an available and convenient reverse genetics system would undoubtedly promote relevant research. Herein, we established an improved universal dual-promoter reverse genetics system with an SVA-specific hammerhead ribozyme and hepatitis delta virus ribozyme at both terminals of the viral genome; this system could be applied to rescue all SVA strains by both eukaryotic and prokaryotic RNA polymerase systems. The genome of the clone-derived Chinese field strain CH/HeN-2018 was assembled into the universal vector pcDNA-rSVAuni through the Gibson assembly technique. Moreover, two silent mutations, G6848C and C7163 G, were separately engineered into the full-length cDNA clone with one step site-directed mutagenesis to create a KpnI restriction enzyme site, which served as a unique genetic marker. The viruses, designated rCH/HeN-2018-T7, rCH/HeN-2018-CMV, rCH/HeN-2018-6484 m and rCH/HeN-2018-7163 m, were successfully rescued through both CMV- and T7-dependent pathways, and their biological properties were further evaluated. The results showed that all four viruses grew rapidly in PK-15 cells and exhibited viral titers and growth kinetics similar to those of parental wtCH/HeN-2018. The established reverse genetics system is easily operated and can be applied to rescue all SVA strains in a short time, which will be helpful for studying SVA biology, including viral pathogenesis, antiviral therapies and vaccine development.

Comprehensive review on immunopathogenesis, diagnostic and epidemiology of Senecavirus A

Senecavirus A (SVA), formerly known as Seneca Valley virus, is a single-strand, positive-sense RNA virus in the family Picornaviridae. This virus has been associated with recent outbreaks of vesicular disease (SVA-VD) and epidemic transient neonatal losses (ETNL) in several swine-producing countries. The clinical manifestation of and lesion caused by SVA are indistinguishable from other vesicular diseases. Pathogenicity studies indicate that SVA could regulate the host innate immune response to facilitate virus replication and the spread of the virus to bystander cells. SVA infection can induce specific humoral and cellular responses that can be detected within the first week of infection. However, SVA seems to produce persistent infection, and the virus can be shed in oral fluids for a month and detected in tissues for approximately two months after experimental infection. SVA transmission could be horizontal or vertical in infected herds of swine, while positive animals can also remain subclinical. In addition, mice seem to act as reservoirs, and the virus can persist in feed and feed ingredients, increasing the risk of introduction into naïve farms. Besides the pathological effects in swine, SVA possesses cytolytic activity, especially in neoplastic cells. Thus, SVA has been evaluated in phase II clinical trials as a virotherapy for neuroendocrine tumors. The goal of this review is summarize the current SVA-related research in pathogenesis, immunity, epidemiology and advances in diagnosis as well as discuses current challenges with subclinical/persistent presentation.

Construction of eGFP-Tagged Senecavirus A for Facilitating Virus Neutralization Test and Antiviral Assay

Senecavirus A (SVA), also known as Seneca Valley virus, is an emerging virus that causes vesicular disease in pigs. This virus belongs to the genus Senecavirus in the family Picornaviridae. The SVA CH-LX-01-2016 was isolated from Guangdong Province of China in 2016. In this study, a recombinant SVA CH-LX-01-2016 was constructed using reverse genetics, and proven to be able to express efficiently an enhanced green fluorescent protein (eGFP) in vitro. This eGFP-tagged recombinant SVA (rSVA-eGFP) exhibited a high capacity for viral replication. Its fluorescence-tracked characteristics greatly facilitated both virus neutralization test (VNT) and antiviral assay. The rSVA-eGFP-based VNT was used to detect eight porcine serum samples, out of which four were determined to be neutralization titer-positive. Subsequently, two antiviral drugs, ribavirin and apigenin, were assayed for evaluating both effects against the rSVA-eGFP in vitro. The result showed that only the ribavirin exhibited an anti-SVA activity.

Visualizing Sacbrood Virus of Honey Bees via Transformation and Coupling with Enhanced Green Fluorescent Protein

Sacbrood virus (SBV) of honey bees is a picornavirus in the genus Iflavirus. Given its relatively small and simple genome structure, single positive-strand RNA with only one ORF, cloning the full genomic sequence is not difficult. However, adding nonsynonymous mutations to the bee iflavirus clone is difficult because of the lack of information about the viral protein processes. Furthermore, the addition of a reporter gene to the clones has never been accomplished. In preliminary trials, we found that the site between 3′ untranslated region (UTR) and poly(A) can retain added sequences. We added enhanced green fluorescent protein (EGFP) expression at this site, creating a SBV clone with an expression tag that does not affect virus genes. An intergenic region internal ribosome entry site (IRES) from Black queen cell virus (BQCV) was inserted to initiate EGFP expression. The SBV-IRES-EGFP clone successfully infected Apis cerana and Apis mellifera, and in A. cerana larvae, it was isolated and passaged using oral inoculation. The inoculated larvae had higher mortality and the dead larvae showed sacbrood symptoms. The added IRES-EGFP remained in the clone through multiple passages and expressed the expected EGFP in all infected bees. We demonstrated the ability to add gene sequences in the site between 3′-UTR and poly(A) in SBV and the potential to do so in other bee iflaviruses; however, further investigations of the mechanisms are needed. A clone with a desired protein expression reporter will be a valuable tool in bee virus studies.

Developing Picornaviruses for Cancer Therapy

Oncolytic viruses (OVs) form a group of novel anticancer therapeutic agents which selectively infect and lyse cancer cells. Members of several viral families, including Picornaviridae, have been shown to have anticancer activity. Picornaviruses are small icosahedral non-enveloped, positive-sense, single-stranded RNA viruses infecting a wide range of hosts. They possess several advantages for development for cancer therapy: Their genomes do not integrate into host chromosomes, do not encode oncogenes, and are easily manipulated as cDNA. This review focuses on the picornaviruses investigated for anticancer potential and the mechanisms that underpin this specificity.

Distinct roles for the IIId2 sub-domain in pestivirus and picornavirus internal ribosome entry sites

Viral internal ribosomes entry site (IRES) elements coordinate the recruitment of the host translation machinery to direct the initiation of viral protein synthesis. Within hepatitis C virus (HCV)-like IRES elements, the sub-domain IIId(1) is crucial for recruiting the 40S ribosomal subunit. However, some HCV-like IRES elements possess an additional sub-domain, termed IIId2, whose function remains unclear. Herein, we show that IIId2 sub-domains from divergent viruses have different functions. The IIId2 sub-domain present in Seneca valley virus (SVV), a picornavirus, is dispensable for IRES activity, while the IIId2 sub-domains of two pestiviruses, classical swine fever virus (CSFV) and border disease virus (BDV), are required for 80S ribosomes assembly and IRES activity. Unlike in SVV, the deletion of IIId2 from the CSFV and BDV IRES elements impairs initiation of translation by inhibiting the assembly of 80S ribosomes. Consequently, this negatively affects the replication of CSFV and BDV. Finally, we show that the SVV IIId2 sub-domain is required for efficient viral RNA synthesis and growth of SVV, but not for IRES function. This study sheds light on the molecular evolution of viruses by clearly demonstrating that conserved RNA structures, within distantly related RNA viruses, have acquired different roles in the virus life cycles.

Transgene expression in the genome of Middle East respiratory syndrome coronavirus based on a novel reverse genetics system utilizing Red-mediated recombination cloning

Middle East respiratory syndrome coronavirus (MERS-CoV) is a high-priority pathogen in pandemic preparedness research. Reverse genetics systems are a valuable tool to study viral replication and pathogenesis, design attenuated vaccines and create defined viral assay systems for applications such as antiviral screening. Here we present a novel reverse genetics system for MERS-CoV that involves maintenance of the full-length viral genome as a cDNA copy inserted in a bacterial artificial chromosome amenable to manipulation by homologue recombination, based on the bacteriophage λ Red recombination system. Based on a full-length infectious MERS-CoV cDNA clone, optimal genomic insertion sites and expression strategies for GFP were identified and used to generate a reporter MERS-CoV expressing GFP in addition to the complete set of viral proteins. GFP was genetically fused to the N-terminal part of protein 4a, from which it is released during translation via porcine teschovirus 2A peptide activity. The resulting reporter virus achieved titres nearly identical to the wild-type virus 48 h after infection of Vero cells at m.o.i. 0.001 (1×10⁵ p.f.u. ml^-1 and 3×10⁵ p.f.u. ml^-1, respectively), and allowed determination of the 50 % inhibitory concentration for the known MERS-CoV inhibitor cyclosporine A based on fluorescence readout. The resulting value was 2.41 µM, which corresponds to values based on wild-type virus. The reverse genetics system described herein can be efficiently mutated by Red-mediated recombination. The GFP-expressing reporter virus contains the full set of MERS-CoV proteins and achieves wild-type titres in cell culture.

Anthrax toxin receptor 1 is the cellular receptor for Seneca Valley virus

Seneca Valley virus (SVV) is an oncolytic picornavirus with selective tropism for neuroendocrine cancers. It has shown promise as a cancer therapeutic in preclinical studies and early-phase clinical trials. Here, we have identified anthrax toxin receptor 1 (ANTXR1) as the receptor for SVV using genome-wide loss-of-function screens. ANTXR1 is necessary for permissivity in vitro and in vivo. However, robust SVV replication requires an additional innate immune defect. We found that SVV interacts directly and specifically with ANTXR1, that this interaction is required for SVV binding to permissive cells, and that ANTXR1 expression is necessary and sufficient for infection in cell lines with decreased expression of antiviral IFN genes at baseline. Finally, we identified the region of the SVV capsid that is responsible for receptor recognition using cryoelectron microscopy of the SVV-ANTXR1-Fc complex. These studies identify ANTXR1, a class of receptor that is shared by a mammalian virus and a bacterial toxin, as the cellular receptor for SVV.

Oncolytic Seneca Valley Virus: past perspectives and future directions

Seneca Valley Virus isolate 001 (SVV-001) is an oncolytic RNA virus of the Picornaviridae family. It is also the first picornavirus discovered of the novel genus Senecavirus. SVV-001 replicates through an RNA intermediate, bypassing a DNA phase, and is unable to integrate into the host genome. SVV-001 was originally discovered as a contaminant in the cell culture of fetal retinoblasts and has since been identified as a potent oncolytic virus against tumors of neuroendocrine origin. SVV-001 has a number of features that make it an attractive oncolytic virus, namely, its ability to target and penetrate solid tumors via intravenous administration, inability for insertional mutagenesis, and being a self-replicating RNA virus with selective tropism for cancer cells. SVV-001 has been studied in both pediatric and adult early phase studies reporting safety and some clinical efficacy, albeit primarily in adult tumors. This review summarizes the current knowledge of SVV-001 and what its future as an oncolytic virus may hold.

Construction and characterization of a full-length cDNA infectious clone of emerging porcine Senecavirus A

A full-length cDNA infectious clone, pKS15-01-Clone, was constructed from an emerging Senecavirus A (SVA; strain KS15-01). To explore the potential use as a viral backbone for expressing marker genes, the enhanced green fluorescent protein (EGFP)-tagged reporter virus (vKS15-01-EGFP) was generated using reverse genetics. Compared to the parental virus, the pKS15-01-Clone derived virus (vKS15-01-Clone) replicated efficiently in vitro and in vivo, and induced similar levels of neutralizing antibody and cytokine responses in infected animals. In contrast, the vKS15-01-EGFP virus showed impaired growth ability and induced lower level of immune response in infected animals. Lesions on the dorsal snout and coronary bands were observed in all pigs infected by parental virus KS15-01, but not in pigs infected with vKS15-01-Clone or vKS15-01-EGFP viruses. These results demonstrated that the infectious clone and EGFP reporter virus could be used as important tools in further elucidating the SVA pathogenesis and development of control measures.

Therapeutic Use of Native and Recombinant Enteroviruses

Research on human enteroviruses has resulted in the identification of more than 100 enterovirus types, which use more than 10 protein receptors and/or attachment factors required in cell binding and initiation of the replication cycle. Many of these “viral” receptors are overexpressed in cancer cells. Receptor binding and the ability to replicate in specific target cells define the tropism and pathogenesis of enterovirus types, because cellular infection often results in cytolytic response, i.e., disruption of the cells. Viral tropism and cytolytic properties thus make native enteroviruses prime candidates for oncolytic virotherapy. Copy DNA cloning and modification of enterovirus genomes have resulted in the generation of enterovirus vectors with properties that are useful in therapy or in vaccine trials where foreign antigenic epitopes are expressed from or on the surface of the vector virus. The small genome size and compact particle structure, however, set limits to enterovirus genome modifications. This review focuses on the therapeutic use of native and recombinant enteroviruses and the methods that have been applied to modify enterovirus genomes for therapy.

Seneca Valley Virus 3Cpro Substrate Optimization Yields Efficient Substrates for Use in Peptide-Prodrug Therapy

The oncolytic picornavirus Seneca Valley Virus (SVV-001) demonstrates anti-tumor activity in models of small cell lung cancer (SCLC), but may ultimately need to be combined with cytotoxic therapies to improve responses observed in patients. Combining SVV-001 virotherapy with a peptide prodrug activated by the viral protease 3Cpro is a novel strategy that may increase the therapeutic potential of SVV-001. Using recombinant SVV-001 3Cpro, we measured cleavage kinetics of predicted SVV-001 3Cpro substrates. An efficient substrate, L/VP4 (kcat/KM = 1932 ± 183 M(-1)s(-1)), was further optimized by a P2' N→P substitution yielding L/VP4.1 (kcat/KM = 17446 ± 2203 M(-1)s(-1)). We also determined essential substrate amino acids by sequential N-terminal deletion and substitution of amino acids found in other picornavirus genera. A peptide corresponding to the L/VP4.1 substrate was selectively cleaved by SVV-001 3Cpro in vitro and was stable in human plasma. These data define an optimized peptide substrate for SVV-001 3Cpro, with direct implications for anti-cancer therapeutic development.

Selective tropism of Seneca Valley virus for variant subtype small cell lung cancer

We assessed the efficacy of Seneca Valley virus (SVV-001), a neuroendocrine cancer-selective oncolytic picornavirus, in primary heterotransplant mouse models of small cell lung cancer (SCLC), including three lines each of classic and variant SCLC. Half-maximal effective concentrations for cell lines derived from three variant heterotransplants ranged from 1.6×10(-3) (95% confidence interval [CI] = 1×10(-3) to 2.5×10(-3)) to 3.9×10(-3) (95% CI = 2.8×10(-3) to 5.5×10(-3)). Sustained tumor growth inhibition in vivo was only observed in variant lines (two-sided Student t test, P < .005 for each). Doses of 10(14) vp/kg were able to completely and durably eradicate tumors in a variant SCLC heterotransplant model in two of six mice. Gene expression profiling revealed that permissive lines are typified by lower expression of the early neurogenic transcription factor ASCL1 and, conversely, by higher expression of the late neurogenic transcription factor NEUROD1. This classifier demonstrates a sensitivity of .89, specificity of .92, and accuracy of .91. The NEUROD1 to ASCL1 ratio may serve as a predictive biomarker of SVV-001 efficacy.