Oncolytic Virus M1 Research Articles

Enhancing antitumor efficacy of oncolytic virus M1 via albendazole-sustained CD8+ T cell activation

The immune response plays a crucial role in the functionality of oncolytic viruses. In this study, Albendazole, an antihelminthic drug known to modulate the immune checkpoint PD-L1, was combined with the oncolytic virus M1 (OVM1) to treat mice with either prostate cancer (RM-1) or glioma (GL261) tumors. This combination therapy enhanced anti-tumor effects in immunocompetent mice, but not in immunodeficient ones, without increasing OVM1 replication. Instead, it led to an increase in the number of CD8+ T cells within the tumor, downregulated the expression of PD1 on CD8+ T cells, and upregulated activation markers such as Ki67, CD44, and CD69 and the secretion of cytotoxic factors including interferon (IFN)-γ, granzyme B, and tumor necrosis factor (TNF)-α. Consistently, it enhanced the invitro tumor-killing activity of lymphocytes from tumor-draining lymph nodes or spleens. The synergistic effect of Albendazole on OVM1 was abolished by depleting CD8+ Tcells, suggesting a CD8+ Tcell-dependent mechanism. In addition, Albendazole and OVM1 therapy increased CTLA4 expression in the spleen, and the addition of CTLA4 antibodies further enhanced the anti-tumor efficacy invivo. In summary, Albendazole can act synergistically with oncolytic viruses via CD8+ T cell activation, and the Albendazole/OVM1 combination can overcome resistance to CTLA4-based immune checkpoint blockade therapy.

Pharmacokinetic enhancement of oncolytic virus M1 by inhibiting JAK‒STAT pathway

Oncolytic viruses (OVs), a group of replication-competent viruses that can selectively infect and kill cancer cells while leaving healthy cells intact, are emerging as promising living anticancer agents. Unlike traditional drugs composed of non-replicating compounds or biomolecules, the replicative nature of viruses confer unique pharmacokinetic properties that require further studies. Despite some pharmacokinetics studies of OVs, mechanistic insights into the connection between OV pharmacokinetics and antitumor efficacy remain vague. Here, we characterized the pharmacokinetic profile of oncolytic virus M1 (OVM) in immunocompetent mouse tumor models and identified the JAK‒STAT pathway as a key modulator of OVM pharmacokinetics. By suppressing the JAK‒STAT pathway, early OVM pharmacokinetics are ameliorated, leading to enhanced tumor-specific viral accumulation, increased AUC and Cmax, and improved antitumor efficacy. Rather than compromising antitumor immunity after JAK‒STAT inhibition, the improved pharmacokinetics of OVM promotes T cell recruitment and activation in the tumor microenvironment, providing an optimal opportunity for the therapeutic outcome of immune checkpoint blockade, such as anti-PD-L1. Taken together, this study advances our understanding of the pharmacokinetic-pharmacodynamic relationship in OV therapy.

STT3A-mediated viral N-glycosylation underlies the tumor selectivity of oncolytic virus M1.

Oncolytic viruses are emerging as promising anticancer agents. Although the essential biological function of N-glycosylation on viruses are widely accepted, roles of N-glycan and glycan-processing enzyme in oncolytic viral therapy are remain elusive. Here, via cryo-EM analysis, we identified three distinct N-glycans on the envelope of oncolytic virus M1 (OVM) as being necessary for efficient receptor binding. E1-N141-glycan has immediate impact on the binding of MXRA8 receptor, E2-N200-glycan mediates the maturation of E2 from its precursor PE2 which is unable to bind with MXRA8, and E2-N262-glycan slightly promotes receptor binding. The necessity of OVM N-glycans in receptor binding make them indispensable for oncolysis in vitro and in vivo. Further investigations identified STT3A, a key catalytic subunit of oligosaccharyltransferase (OST), as the determinant of OVM N-glycosylation, and STT3A expression in tumor cells is positively correlated with OVM-induced oncolysis. Increased STT3A expression was observed in various solid tumors, pointing to a broad-spectrum anticancer potential of OVM. Collectively, our research supports the importance of STT3A-mediated N-glycosylation in receptor binding and oncolysis of OVM, thus providing a novel predictive biomarker for OVM.

Oncolytic virus M1 functions as a bifunctional checkpoint inhibitor to enhance the antitumor activity of DC vaccine

Although promising, dendritic cell (DC) vaccines still provide limited clinical benefits, mainly due to the immunosuppressive tumor microenvironment (TME) and the lack of tumor-associated antigens (TAAs). Oncolytic virus therapy is an ideal strategy to overcome immunosuppression and expose TAAs; therefore, they may work synergistically with DC vaccines. In this study, we demonstrate that oncolytic virus M1 (OVM) can enhance the antitumor effects of DC vaccines across diverse syngeneic mouse tumor models by increasing the infiltration of CD8+ effector Tcells in the TME. Mechanically, we show that tumor cells counteract DC vaccines through the SIRPα-CD47 immune checkpoint, while OVM can downregulate SIRPα in DCs and CD47 in tumor cells. Since OVM upregulates PD-L1 in DCs, combining PD-L1 blockade with DC vaccines and OVM further enhances antitumor activity. Overall, OVM strengthens the antitumor efficacy of DC vaccines by targeting the SIRPα-CD47 axis, which exerts dominant immunosuppressive effects on DC vaccines.

Directed natural evolution generates a next-generation oncolytic virus with a high potency and safety profile

Oncolytic viruses (OVs) represent a type of encouraging multi-mechanistic drug for the treatment of cancer. However, attenuation of virulence, which is generally required for the development of OVs based on pathogenic viral backbones, is frequently accompanied by a compromised killing effect on tumor cells. By exploiting the property of viruses to evolve and adapt in cancer cells, we perform directed natural evolution on refractory colorectal cancer cell HCT-116 and generate a next-generation oncolytic virus M1 (NGOVM) with an increase in the oncolytic effect of up to 9690-fold. The NGOVM has a broader antitumor spectrum and a more robust oncolytic effect in a range of solid tumors. Mechanistically, two critical mutations are identified in the E2 and nsP3 genes, which accelerate the entry of M1 virus by increasing its binding to the Mxra8 receptor and antagonize antiviral responses by inhibiting the activation of PKR and STAT1 in tumor cells, respectively. Importantly, the NGOVM is well tolerated in both rodents and nonhuman primates. This study implies that directed natural evolution is a generalizable approach for developing next-generation OVs with an expanded scope of application and high safety.

Overcoming resistance to oncolytic virus M1 by targeting PI3K-γ in tumor-associated myeloid cells

Oncolytic viruses (OVs) have become a category of promising anticancer immunotherapeutic agents over the last decade. However, the fact that many individuals fail to respond to OVs highlights the importance of defining the barely known immunosuppressive mechanisms that lead to treatment resistance. Here we found that the immunosuppression mediated by tumor-associated myeloid cells (TAMCs) directly quenches the antitumor effect of oncolytic virus M1 (OVM). OVM induces myeloid cells to migrate into tumors and strengthens their immunosuppressive phenotypes. Mechanically, tumor cells treated with OVM secrete interleukin-6 (IL-6) to activate the phosphatidylinositol 3-kinase (PI3K)-γ/Akt axis in TAMCs, promoting infiltration of TAMCs and aggravating their inhibition on cytotoxic CD8+ T lymphocytes. Pharmacologically targeting PI3K-γ relieves TAMC-mediated immunosuppression and enhances the efficacy of OVM. Additional treatment with immune checkpoint antibodies eradicates multiple refractory solid tumors and induces potent long-term antitumor immune memory. Our findings indicate that OVM functions as a double-edged sword in antitumor immunity and provide insights into the rationale for liberating Tcell-mediated antitumor activity by abolishing TAMC-mediated immunosuppression.

Identification of the receptor of oncolytic virus M1 as a therapeutic predictor for multiple solid tumors

Over the last decade, oncolytic virus (OV) therapy has shown its promising potential in tumor treatment. The fact that not every patient can benefit from it highlights the importance for defining biomarkers that help predict patients’ responses. As particular self-amplifying biotherapeutics, the anti-tumor effects of OVs are highly dependent on the host factors for viral infection and replication. By using weighted gene co-expression network analysis (WGCNA), we found matrix remodeling associated 8 (MXRA8) is positively correlated with the oncolysis induced by oncolytic virus M1 (OVM). Consistently, MXRA8 promotes the oncolytic efficacy of OVM in vitro and in vivo. Moreover, the interaction of MXRA8 and OVM studied by single-particle cryo-electron microscopy (cryo-EM) showed that MXRA8 directly binds to this virus. Therefore, MXRA8 acts as the entry receptor of OVM. Pan-cancer analysis showed that MXRA8 is abundant in most solid tumors and is highly expressed in tumor tissues compared with adjacent normal ones. Further study in cancer cell lines and patient-derived tumor tissues revealed that the tumor selectivity of OVM is predominantly determined by a combinational effect of the cell membrane receptor MXRA8 and the intracellular factor, zinc-finger antiviral protein (ZAP). Taken together, our study may provide a novel dual-biomarker for precision medicine in OVM therapy.

Cytotoxic T lymphocyte-associated protein 4 antibody aggrandizes antitumor immune response of oncolytic virus M1 via targeting regulatory T cells.

Oncolytic virotherapies are perceived as remarkable immunotherapies coming into view and represent highly promising cancer treatments, yet to figure out its specific immune responses and underlying barriers remains critical. Albeit recent studies have demonstrated that oncolytic viruses (OVs) could fine tune tumor microenvironment (TME) to elicit tumor suppression mainly due to effective T-cell responses, the interaction between suppressive T cells and OVs is barely undetermined. Herein, we found that regulatory T cells (Treg cells) were increased in the TME following systemic administration of oncolytic virus M1 along with the higher expression of relative cytokines and chemokines in both mouse RM-1 prostatic carcinoma model and mouse B16F10 melanoma model. Besides, Treg cells expressed high levels of CD25 post-M1 treatment, and its suppressive effect on CD8+ T cells was also elevated. Depletion of Treg cells in M1-treated groups significantly reinforced antitumor effect of M1. Specific targeting of Treg cells using cytotoxic T lymphocyte-associated protein 4 (CTLA-4) antibody (Ab) in combination with M1 treatment elicited a more profound tumor suppression and longer overall survival time than M1 alone in both tumor models. Moreover, CTLA-4 Ab further aggrandized antitumor immune response elicited by M1, including increased infiltration of CD45+ immune cells and CD8+ or CD4+ T lymphocytes, decreased ratio of Treg cells to CD4+ T lymphocytes, the intensified lymphocytotoxicity and elevated secretion of cytotoxic cytokines like interferon-γ, granzyme B and perforin. Therefore, our findings constituted a suggestive evidence that targeting Treg cells in M1-based oncolytic virotherapy may achieve a highly response in clinical cancer research.

Real-Time Visualization and Quantification of Oncolytic M1 Virus In Vitro and In Vivo

Alphavirus M1 is a promising oncolytic virus for cancer therapy. Here, we constructed a fluorescent reporter virus for real-time visualization and quantification of M1 virus both in vitro and in vivo. The reporter-encoding M1 virus maintained the characteristics of parental virus in the aspects of structure, replication capacity, the feature to induce cytopathic cell death, and the property of tumor targeting. The fluorescence is positively correlated with virus replication both in vitro and in vivo. More importantly, the reporter can be stably expressed for at least 10 generations in a serial passage assay. In summary, we successfully constructed stable and authentic reporter viruses for studying M1 virus and provided a feasible technical route for gene modification of oncolytic virus M1.

The Identification and Development of a Novel Oncolytic Virus: Alphavirus M1.

Oncolytic virotherapy represents an ideal therapeutic platform for cancer in which natural or engineered viruses selectively replicate in and destroy tumor cells, whereas sparing normal cells. Oncolytic virotherapy is considered as a key contributor in modern immunotherapy. However, several challenges remain with regard to exploiting the potential of oncolytic viruses, such as the lack of biomarkers for precise treatment, the difficulty of systemic administration because of pre-existing neutralizing antibodies to popular oncolytic viral vectors in human serum and the lack of mature lyophilization technology for convenient transport and preservation of viral preparations. The M1 strain, which was isolated on Hainan Island of China in the 1960s, is a member of the alphavirus genus Togaviridae family. It was identified as a novel oncolytic virus in 2014. During the development of M1 virus, many challenges have been overcome: several biomarkers have been identified for precise treatment; systematic administration of M1 is suitable and feasible because of the extremely low percentage of pre-existing neutralizing antibodies in the general population, and a lyophilized powder that maintains high biological stability has been developed. This review provides an encyclopedia of studies supporting M1 as an oncolytic virus, including the biological characteristics, tumor selectivity and its mechanism, tumor killing mechanism, combination therapy, and nonclinical pharmacokinetics of M1 virus. The future development direction of oncolytic virus M1 is also discussed at the end of the review.

Suppression of CCDC6 sensitizes tumor to oncolytic virus M1

Oncolytic virus is an effective therapeutic strategy for cancer treatment, which exploits natural or manipulated viruses to selectively target and kill cancer cells. However, the innate antiviral system of cancer cells may resistant to the treatment of oncolytic virus. M1 virus is a newly identified oncolytic virus belonging to alphavirus species, but the molecular mechanisms underlying its anticancer activity are largely unknown. Cell viability was measured by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT) assays. RNA seq analysis was used to analyze the gene alternation after M1 virus infection. Small interfering RNAs transfection for gene knockdown was used for gene functional tests. Caspase-3/7 activity was detected by Caspase-Glo Assay Systems. A mice model of orthotopic bladder tumor was established to determine the oncolytic effectiveness of the M1 virus. The expression of cleaved-Caspase 3 as well as Ki-67 in tumor cells were detected by immunohistochemical analysis. To further define the molecular factors involved in M1 virus-mediated biological function, we knocked down genes related to alphavirus’ activity and found that CCDC6 plays an important role in the oncolytic activity of M1 virus. Moreover, knocked down of CCDC6 augments the reproduction of M1 virus and resulted in endoplasmic reticulum (ER) stress-induced cell apoptosis in vitro as well as in vivo orthotopic bladder cancer model. Our research provides a rational new target for developing new compounds to promote the efficacy of oncolytic virus therapy.

Intravenous injection of the oncolytic virus M1 awakens antitumor T cells and overcomes resistance to checkpoint blockade

Reversing the highly immunosuppressive tumor microenvironment (TME) is essential to achieve long-term efficacy with cancer immunotherapy. Despite the impressive clinical response to checkpoint blockade in multiple types of cancer, only a minority of patients benefit from this approach. Here, we report that the oncolytic virus M1 induces immunogenic tumor cell death and subsequently restores the ability of dendritic cells to prime antitumor T cells. Intravenous injection of M1 disrupts immune tolerance in the privileged TME, reprogramming immune-silent (cold) tumors into immune-inflamed (hot) tumors. M1 elicits potent CD8+ T cell-dependent therapeutic effects and establishes long-term antitumor immune memory in poorly immunogenic tumor models. Pretreatment with M1 sensitizes refractory tumors to subsequent checkpoint blockade by boosting T-cell recruitment and upregulating the expression of PD-L1. These findings reveal the antitumor immunological mechanism of the M1 virus and indicated that oncolytic viruses are ideal cotreatments for checkpoint blockade immunotherapy.

Tumors driven by RAS signaling harbor a natural vulnerability to oncolytic virus M1.

Oncolytic viruses are potent anticancer agents that replicate within and kill cancer cells rather than normal cells, and their selectivity is largely determined by oncogenic mutations. M1, a novel oncolytic virus strain, has been shown to target cancer cells, but the relationship between its cancer selectivity and oncogenic signaling pathways is poorly understood. Here, we report that RAS mutation promotes the replication and oncolytic effect of M1 in cancer, and we further provide evidence that the inhibition of the RAS/RAF/MEK signaling axis suppresses M1 infection and the subsequent cytopathic effects. Transcriptome analysis revealed that the inhibition of RAS signaling upregulates the type I interferon antiviral response, and further RNA interference screen identified CDKN1A as a key downstream factor that inhibits viral infection. Gain- and loss-of-function experiments confirmed that CDKN1A inhibited the replication and oncolytic effect of M1 virus. Subsequent TCGA data mining and tissue microarray (TMA) analysis revealed that CDKN1A is commonly deficient in human cancers, suggesting extensive clinical application prospects for M1. Our report indicates that virotherapy is feasible for treating undruggable RAS-driven cancers and provides reliable biomarkers for personalized cancer therapy.

Liposome Encapsulation of Oncolytic Virus M1 To Reduce Immunogenicity and Immune Clearance in Vivo.

Oncolytic viral therapy is an attractive novel strategy for cancer therapy. As a natural alphavirus, oncolytic virus M1 is able to infect and kill various zinc finger antiviral protein (ZAP)-deficient tumor cells selectively, while leaving normal cells undamaged. However, M1 can trigger the production of neutralizing antibodies that dramatically weaken its antitumor effect. In order to attenuate immunogenicity of the therapeutic M1 virus, we encapsulated it into liposomes (referred to as M-LPO) using the thin-film hydration method. The effect of anti-M1 neutralizing antibody on M-LPO was examined in LoVo and Hep 3B cell lines. In the absence of neutralizing antibodies, treating cells with naked M1, blank liposomes (LPO), M-LPO, or a simple mixture of M1 and liposomes (LPO+M1) inhibited cell growth. In the presence of neutralizing antibodies, only M-LPO inhibited cell growth. After intravenous administration, M-LPO reduced the production of the M1-neutralizing antibody and the corresponding immune response. Analysis of the M-LPO uptake by cells was examined by confocal microscopy using M1 labeled with FITC and liposomal shells labeled with RhB. The results suggest that M1 may be released from liposomes before or after M-LPO internalization. Taken together, our results suggest that encapsulating oncolytic virus M1 in liposomes may reduce intrinsic viral immunogenicity for improved anticancer therapy.

DNA-PK inhibition synergizes with oncolytic virus M1 by inhibiting antiviral response and potentiating DNA damage

Oncolytic virotherapy is a promising therapeutic strategy that uses replication-competent viruses to selectively destroy malignancies. However, the therapeutic effect of certain oncolytic viruses (OVs) varies among cancer patients. Thus, it is necessary to overcome resistance to OVs through rationally designed combination strategies. Here, through an anticancer drug screening, we show that DNA-dependent protein kinase (DNA-PK) inhibition sensitizes cancer cells to OV M1 and improves therapeutic effects in refractory cancer models in vivo and in patient tumour samples. Infection of M1 virus triggers the transcription of interferons (IFNs) and the activation of the antiviral response, which can be abolished by pretreatment of DNA-PK inhibitor (DNA-PKI), resulting in selectively enhanced replication of OV M1 within malignancies. Furthermore, DNA-PK inhibition promotes the DNA damage response induced by M1 virus, leading to increased tumour cell apoptosis. Together, our study identifies the combination of DNA-PKI and OV M1 as a potential treatment for cancers.

Deficiency of the IRE1α-Autophagy Axis Enhances the Antitumor Effects of the Oncolytic Virus M1.

Oncolytic virotherapy is an emerging treatment modality that uses replication-competent viruses to destroy cancer cells. M1 is a naturally occurring alphavirus (Togaviridae) which shows potent oncolytic activities against many cancers. Accumulation of unfolded proteins during virus replication leads to a transcriptional/translational response known as the unfolded protein response (UPR), which might counteract the antitumor effect of the oncolytic virus. In this report, we show that either pharmacological or biological inhibition of IRE1α or PERK, but not ATF6, substantially increases the oncolytic effects of the M1 virus. Moreover, inhibition of IRE1α blocks M1 virus-induced autophagy, which restricts the antitumor effects of the M1 virus through degradation of viral protein, in glioma cells. In addition, IRE1α suppression significantly increases the oncolytic effect of M1 virus in an orthotopic glioma model. From a molecular pathology study, we found that IRE1α is expressed at lower levels in higher-grade gliomas, suggesting greater antitumor efficacy of the oncolytic virus M1. Taken together, these findings illustrate a defensive mechanism of glioma cells against the oncolytic virus M1 and identify possible approaches to enhance the oncolytic viral protein accumulation and the subsequent lysis of tumor cells.IMPORTANCE Although oncolytic virotherapy is showing great promise in clinical applications, not all patients are benefiting. Identifying inhibitory signals in refractory cancer cells for each oncolytic virus would provide a good chance to increase the therapeutic effect. Here we describe that infection with the oncolytic virus M1 triggers the unfolded protein response (UPR) and subsequent autophagy, while blocking the UPR-autophagy axis significantly potentiates the antitumor efficacy of M1 in vitro and in vivo A survey of cancer tissue banks revealed that IRE1α, a key element in the UPR pathway, is commonly downregulated in higher-grade human gliomas, suggesting favorable prospects for the application of M1. Our work provides a potential predictor and target for enhancement of the therapeutic effectiveness of the M1 virus. We predict that the mechanism-based combination therapy will promote cancer virotherapy in the future.

Intravenous injections of the oncolytic virus M1 as a novel therapy for muscle-invasive bladder cancer

Muscle-invasive bladder cancer (MIBC) is associated with low survival and high recurrence rates even in cases in which patients receive systemic treatments, such as surgery and chemotherapy. Here, we found that a naturally existing alphavirus, namely, M1, selectively kills bladder cancer cells but not normal cells, findings supported by our observations of changes in viral replication and MIBC and patient-derived MIBC cell apoptosis. Transcriptome analysis revealed that interferon-stimulated genes (ISGs) are expressed at low levels in sensitive bladder cancer cells and high levels in resistant cells. Knocking down ZC3HAV1 (ZAP), an antiviral factor in ISGs, restores M1 virus reactivity in resistant cells, and overexpressing ZAP partially reverses M1 virus-induced decreases in cell viability in sensitive cells. In orthotopic MIBC mice, tail vein injections of M1 significant inhibit tumor growth and prolong survival period, antitumor effects of M1 are stronger than those of the first-line chemotherapy agent cisplatin (CDDP). Treated tumors display enhanced cleaved-caspase-3 signals, which are representative of cell apoptosis, and decreased Ki-67 signals, which are representative of cell proliferation. Moreover, tissue microarray (TMA) analyses of clinical tumor specimens revealed that up to 45.6% of cases of MIBC presented with low ZAP expression, a finding that is prevalent in advanced MIBC. Our results indicate that the oncolytic virus M1 is a novel agent capable of functioning as a precise and effective therapy for MIBC.

Targeting VCP enhances anticancer activity of oncolytic virus M1 in hepatocellular carcinoma.

Oncolytic virotherapy is rapidly progressing through clinical evaluation. However, the therapeutic efficacy of oncolytic viruses in humans has been less than expected from preclinical studies. We describe an anticancer drug screen for compounds that enhance M1 oncolytic virus activity in hepatocellular carcinoma (HCC). An inhibitor of the valosin-containing protein (VCP) was identified as the top sensitizer, selectively increasing potency of the oncolytic virus up to 3600-fold. Further investigation revealed that VCP inhibitors cooperated with M1 virus-suppressed inositol-requiring enzyme 1α (IRE1α)-X-box binding protein 1 (XBP1) pathway and triggered irresolvable endoplasmic reticulum (ER) stress, subsequently promoting robust apoptosis in HCC. We show that VCP inhibitor improved the oncolytic efficacy of M1 virus in several mouse models of HCC and primary HCC tissues. Finally, this combinatorial therapeutic strategy was well tolerated in nonhuman primates. Our study identifies combined VCP inhibition and oncolytic virus as a potential treatment for HCC and demonstrates promising therapeutic potential.

Selective Antagonism of Bcl-xL Potentiates M1 Oncolysis by Enhancing Mitochondrial Apoptosis.

Oncolytic virotherapy is a novel and intriguing treatment strategy for cancer therapy. However, the clinical potential of oncolytic virus as single agent is limited. M1 virus is a promising oncolytic virus that has been tested in preclinical studies. In this study, we investigated the effect of the combination use of M1 virus and Bcl-2 family inhibitors. A chemical compounds screening including ten Bcl-2 family inhibitors demonstrated that pan-Bcl-2 inhibitors selectively augmented M1 virus oncolysis in cancer cells at very low doses. The mechanism of the enhanced antitumor effect of pan-Bcl-2 inhibitors with M1 virus is mainly due to the inhibition of Bcl-xL, which synergizes with M1-induced upregulation of Bak to trigger apoptosis. In xenograft mouse models and patient-derived tumor tissues, the combination of M1 and pan-Bcl-2 inhibitors significantly inhibited tumor growth and prolonged survival, suggesting the potential therapeutic value of this strategy. These findings offer insights into the synergy between Bcl-xL inhibition and oncolytic virus M1 as a combination anticancer treatment modality.

Selective replication of oncolytic virus M1 results in a bystander killing effect that is potentiated by Smac mimetics

Oncolytic virotherapy is a treatment modality that uses native or genetically modified viruses that selectively replicate in and kill tumor cells. Viruses represent a type of pathogen-associated molecular pattern and thereby induce the up-regulation of dozens of cytokines via activating the host innate immune system. Second mitochondria-derived activator of caspases (Smac) mimetic compounds (SMCs), which antagonize the function of inhibitor of apoptosis proteins (IAPs) and induce apoptosis, sensitize tumor cells to multiple cytokines. Therefore, we sought to determine whether SMCs sensitize tumor cells to cytokines induced by the oncolytic M1 virus, thus enhancing a bystander killing effect. Here, we report that SMCs potentiate the oncolytic effect of M1 in vitro, in vivo, and ex vivo. This strengthened oncolytic efficacy resulted from the enhanced bystander killing effect caused by the M1 virus via cytokine induction. Through a microarray analysis and subsequent validation using recombinant cytokines, we identified IL-8, IL-1A, and TRAIL as the key cytokines in the bystander killing effect. Furthermore, SMCs increased the replication of M1, and the accumulation of virus protein induced irreversible endoplasmic reticulum stress- and c-Jun N-terminal kinase-mediated apoptosis. Nevertheless, the combined treatment with M1 and SMCs had little effect on normal and human primary cells. Because SMCs selectively and significantly enhance the bystander killing effect and the replication of oncolytic virus M1 specifically in cancer cells, this combined treatment may represent a promising therapeutic strategy.