Myxoma Virus M083 Is a Virulence Factor Which Mediates Systemic Dissemination

Oncolytic myxoma virus is effective in murine models of triple negative breast cancer despite poor rates of infection

Oncolytic viruses are being heavily investigated as novel methods to treat cancers; however, predicting their therapeutic efficacy remains challenging. The most commonly used predictive tests involve determining the in vitro susceptibility of a tumor's malignant cells to infection with an oncolytic agent. Whether these tests are truly predictive of in vivo efficacy, however, remains unclear. Here we demonstrate that a recombinant, oncolytic myxoma virus shows efficacy in two murine models of triple negative breast cancer despite extremely low permissivity of these models to viral infection. These data demonstrate that in vitro infectivity studies are not an accurate surrogate for therapeutic efficacy and suggest that other tests need to be developed.

High Levels of Extracellular Potassium Can Delay Myxoma Virus Replication by Preventing Release of Virions from the Endosomes

Potassium (K+) is one of the most abundant cations in the human body. Under normal conditions, the vast majority of K+ is found within cells, and the extracellular [K+] is tightly regulated to within 3.0 to 5.0 mM. However, it has recently been shown that high levels of localized necrosis can increase the extracellular concentration of K+ to above 50 mM. This raises the possibility that elevated extracellular K+ might influence a variety of biological processes that occur within regions of necrotic tissue. For example, K+ has been shown to play a central role in the replication cycles of numerous viral families, and in cases of lytic infection, localized regions containing large numbers of necrotic cells can be formed. Here, we show that the replication of the model poxvirus myxoma virus (MYXV) is delayed by elevated levels of extracellular K+. These increased K+ concentrations alter the cellular endocytic pathway, leading to increased phagocytosis but a loss of endosomal/lysosomal segregation. This slows the release of myxoma virus particles from the endosomes, resulting in delays in genome synthesis and infectious particle formation as well as reduced viral spread. Additionally, mathematical modeling predicts that the extracellular K+ concentrations required to impact myxoma virus replication can be reached in viral lesions under a variety of conditions. Taken together, these data suggest that the extracellular [K+] plays a role in determining the outcomes of myxoma infection and that this effect could be physiologically relevant during pathogenic infection. IMPORTANCE Intracellular K+ homeostasis has been shown to play a major role in the replication of numerous viral families. However, the potential impact of altered extracellular K+ concentrations is less well understood. Our work demonstrates that increased concentrations of extracellular K+ can delay the replication cycle of the model poxvirus MYXV by inhibiting virion release from the endosomes. Additionally, mathematical modeling predicts that the levels of extracellular K+ required to impact MYXV replication can likely be reached during pathogenic infection. These results suggest that localized viral infection can alter K+ homeostasis and that these alterations might directly affect viral pathogenesis.

Reduced cellular binding affinity has profoundly different impacts on the spread of distinct poxviruses

Poxviruses are large enveloped viruses that replicate exclusively in the cytoplasm. Like all viruses, their replication cycle begins with virion adsorption to the cell surface. Unlike most other viral families, however, no unique poxviral receptor has ever been identified. In the absence of a unique receptor, poxviruses are instead thought to adhere to the cell surface primarily through electrostatic interactions between the positively charged viral envelope proteins and the negatively charged sulfate groups on cellular glycosaminoglycans (GAGs). While these negatively charged GAGs are an integral part of all eukaryotic membranes, their specific expression and sulfation patterns differ between cell types. Critically, while poxviral binding has been extensively studied using virally centered genetic strategies, the impact of cell-intrinsic changes to GAG charge has never been examined. Here we show that loss of heparin sulfation, accomplished by deleting the enzyme N-Deacetylase and N-Sulfotransferase-1 (NDST1) which is essential for GAG sulfation, significantly reduces the binding affinity of both vaccinia and myxoma viruses to the cell surface. Strikingly, however, while this lowered binding affinity inhibits the subsequent spread of myxoma virus, it actually enhances the overall spread of vaccinia by generating more diffuse regions of infection. These data indicate that cell-intrinsic GAG sulfation plays a major role in poxviral infection, however, this role varies significantly between different members of the poxviridae.

Impact of Induced Syncytia Formation on the Oncolytic Potential of Myxoma Virus

Introduction

Cancer has become one of the most critical health issues of modern times. To overcome the ineffectiveness of current treatment options, research is being done to explore new therapeutic modalities. One such novel treatment is oncolytic virotherapy (OV) which uses tumor tropic viruses to specifically target and kill malignant cells. While OV has shown significant promise in recent clinical trials, the therapeutic use of viruses poses a number of unique challenges. In particular, obtaining effective viral spread throughout the tumor microenvironment remains problematic. Previous work has suggested this can be overcome by forcing oncolytic viruses to induce syncytia formation.

Methods

In the current work, we generated a series of recombinant myxoma viruses expressing exogenous fusion proteins from other viral genomes and examined their therapeutic potential in vitro and in vivo.

Results

Similar to previous studies, we observed that the expression of these fusion proteins during myxoma infection induced the formation of multinucleated syncytia which increased viral spread and lytic potential compared to non-fusogenic controls. Contrary to expectations, however, the treatment of established tumors with these viruses resulted in decreased therapeutic efficacy which corresponded with reduced viral persistence.

Discussion

These findings indicate that enhanced viral spread caused by syncytia formation can actually reduce the efficacy of OV and supports a number of previous works suggesting that the in vitro properties of viruses frequently fail to predict their in vivo efficacy.

Oncolytic myxoma virus synergizes with standard of care for treatment of glioblastoma multiforme

Background

Glioblastoma multiforme (GBM) is an aggressive form of brain cancer which is associated with poor prognosis. A variety of oncolytic viruses have previously shown positive efficacy against GBM, potentially offering new treatment options for patients. One such oncolytic virus is Myxoma virus (MYXV), a rabbit-specific poxvirus that has been shown to be efficacious against a variety of tumor models including GBM.

Purpose

The purpose of this study was to test the efficacy of MYXV combined with current treatment regimens for GBM in both established cell lines as well as patient biopsy samples.

Materials and methods

U118 gliobastoma cell lines were treated under various standard of care combinations (untreated, radiation and chemotherapeutic) prior to infection with MYXV. Infection was then monitored for differences in rate of infection, titer and rate of spread. Cellular death was measured by MTT assay and Caspase-3 colorimetric assay. Patient biopsies were harvested and treated under similar treatment conditions.

Results

The addition of GBM standard of care to MYXV infection resulted in an increased rate of spread compared to single treatment with either radiation or chemotherapeutic alone. SOC did not alter viral replication or infection rates. Similar effects were seen in ex vivo patient biopsies. Cellular viability was significantly decreased with the combination therapy of SOC and MYXV infection compared to any other treatment outcome. Caspase-3 activity was also significantly increased in samples treated with combination therapy when compared to any other treatment combination.

Conclusion

Our results show that the combination of MYXV with current SOC results in both increased killing of GBM cells compared to either treatment regime alone as well as increased spread of MYXV infection. These findings lay the foundation for future in vivo studies on combining MYXV with GBM SOC.

Myxoma Virus Induces Ligand Independent Extrinsic Apoptosis in Human Myeloma Cells

INTRODUCTION

Multiple myeloma is a clonal malignancy of plasma B cells. Although recent advances have improved overall prognosis, virtually all myeloma patients still succumb to relapsing disease. Therefore, novel therapies to treat this disease remain urgently needed. We have recently shown that treatment of human multiple myeloma cells with an oncolytic virus known as myxoma results in rapid cell death even in the absence of viral replication; however, the specific mechanisms and pathways involved remain unknown.

MATERIALS AND METHODS

To determine how myxoma virus eliminates human multiple myeloma cells, we queried the apoptotic pathways that were activated after viral infection using immunoblot analysis and other cell biology approaches.

RESULTS

Our results indicate that myxoma virus infection initiates apoptosis in multiple myeloma cells through activation of the extrinsic initiator caspase-8. Caspase-8 activation subsequently results in cleavage of BH3 interacting-domain death agonist and loss of mitochondrial membrane potential causing secondary activation of caspase-9. Activation of caspase-8 appears to be independent of extrinsic death ligands and instead correlates with depletion of cellular inhibitors of apoptosis. We hypothesize that this depletion results from virally mediated host-protein shutoff because a myxoma construct that overexpresses the viral decapping enzymes displays improved oncolytic potential.

CONCLUSION

Taken together, these results suggest that myxoma virus eliminates human multiple myeloma cells through a pathway unique to oncolytic poxviruses, making it an excellent therapeutic option for the treatment of relapsed or refractory patients.