The mechanisms of genetically modified vaccinia viruses for the treatment of cancer

https://doi.org/10.1016/j.critrevonc.2015.04.001Get rights and content

Highlights

  • We provide an overview of the use of genetically modified VACVs for the treatment of cancer.

  • VACVs have an inherent affinity toward malignant cells.

  • Genetic modifications to VACVs have improved tumor cell selectivity and death.

  • Some modified VACVs are in clinical trials with preliminary results being favorable.

  • Continued work will be needed to explore the utility of additional VACVs as chemotherapeutics.

Abstract

The use of oncolytic viruses for the treatment of cancer is an emerging field of cancer research and therapy. Oncolytic viruses are designed to induce tumor specific immunity while replicating selectively within cancer cells to cause lysis of the tumor cells.

While there are several forms of oncolytic viruses, the use of vaccinia viruses for oncolysis may be more beneficial than other forms of oncolytic viruses. For example, vaccinia viruses have been shown to exert their anti-tumor effects through genetic engineering strategies which enhance their therapeutic efficacy. This paper will address some of the most common forms of genetically modified vaccinia viruses and will explore the mechanisms whereby they selectively target, enter and destroy cancer cells. Furthermore, this review will highlight how vaccinia viruses activate host immune responses against cancer cells and will address clinical trials evaluating the tumor-directed and killing efficacy of these viruses against solid tumors.

Introduction

In the early twentieth century, clinicians introduced the radical concept that viruses may be used to treat cancer [1], [2]. Although revolutionary, the notion died down due to concerns from side effects and the lack of substantial findings [1]. Termed virotherapy, it wasn’t until the late 1990s that clinicians became re-interested in the use of viruses to target and treat cancer [1]. Modern technology as well as the introduction of gene therapy provided new enlightenment. As a result, the use of oncolytic viruses for the treatment of cancer has now become an emerging field of cancer research and treatment. Of the viruses currently under investigation, the Oncolytic Vaccinia Virus (VACV) has been one of the most extensively studied. The vaccinia virus belongs to the poxviridae family, which are large, double stranded DNA viruses closely related to cowpox and monkeypox. Infection with VACV is characterized by the formation of pock lesions on skin [3]. With a genome the size of ∼190 kbp, VACV is considered to be a large and complex animal virus and consists of many strains of which the most prominent include: Lister, Wyeth, and Western Reserve. With regard to the treatment of cancer, oncolytic VACV has been shown to replicate and lyse tumor cells within 72 h post-infection [4]. It has also been shown to exhibit broad tumor tropism and can move through the bloodstream to target distant tumors [4]. Importantly, recombinant oncolytic VACV, which has been genetically modified, selectively targets tumor cells while sparing non-malignant cells, making it an ideal agent as it minimizes damage to healthy tissues [5]. Non-genetically modified VACV indiscriminately targets both tumor cells and healthy cells. Furthermore, VACV has been reported to replicate in the cytoplasm of cells, preventing integration of viral DNA into host chromosomes and thus passage of viral progeny to daughter cells [6]. These features of VACV have made it an ideal agent for the treatment of cancer.

There are four infectious forms of VACV produced during the virus's life cycle [7], [8], [9]. These forms include: Intracellular Mature Virion (MV), Intracellular Enveloped Virion (IEV), Cell-associated Enveloped Virion (CEV) and Extracellular Enveloped Virion (EEV). Out of the four infectious forms, MV and EEV are the most common produced during assembly. Assembly takes place in cytoplasmic factories and involves the usage of non-infectious precursors called “crescents” [8].

The MV has been suggested to be the most abundant form of VACV, a feature which may be related to its early assembly during maturation [6]. Consisting of a single lipid bilayer, the MV is the simplest form of VACV and as such, is principally used in research [6]. Sometimes MV will morph into an EEV, a process that is accomplished as a result of the MV exiting cytoplasmic crescents via microtubules to undergo additional modifications [8]. These new modifications include the assembly of an additional membrane, which is formed by viral transport through endosomal or trans-golgi cisternae [6]. While EEV's outer envelope is unstable ex-vivo, it is able to spread more rapidly than MV due to its early release from cells following viral replication [10]. Furthermore, EEV is unique from MV in that it has fewer viral antigens exposed on its outer surface and additionally incorporates host cell proteins, enabling it to go undetected by the host's immune system [11]. This feature thus limits its destruction prior to its arrival at the tumor [11]. Importantly, VACVs, including MV and EEV, are able to accommodate multiple large transgenes [4] improving selective tumor targeting and killing. In this manner, the mechanisms surrounding the endogenous features of VACV in addition to genetic modifications enabling VACV tumor entry and tumor cell death will be discussed in this paper. It is anticipated that this information will give light to new mechanisms whereby VACVs may be used as a frontline form of chemotherapy in the future.

Section snippets

VACV targeting of tumor cells

The targeting of tumor cells while sparing normal, healthy cells is crucial when it comes to finding a treatment for cancer. Since several wild-type VACVs have been reported to possess inherent affinity to tumor cells [12], genetic modifications have been introduced into VACVs to further improve selective tumor cell infection and death and/or viral replication. The most common form of genetically modified oncolytic VACV is VVdd, a double-deletion mutant [13].

VVdd, originating from the Western

Carrier cells for viral delivery

Carrier cells are immune cells or tumor cells, which are utilized to both protect the virus from inactivation by neutralizing antibodies and deliver the virus to tumors [25]. In one study, researchers investigated the use of carrier cells as a means of viral delivery to tumor-bearing mice [26]. In this study, a triad of immunosuppressive drugs was combined with murine colon adenocarcinoma (MC 38) cancer carrier cells to deliver a recombinant VACV to tumor-bearing mice previously infected with

VACV tumor cell entry

While the mechanisms of oncolytic VACV cell entry are not completely understood, a number of cell signaling factors and pathways have been shown to contribute to its entry into cancer cells. Depending on the form or strain of VACV, the mechanism of entry can be different and encompasses endocytic routes or direct fusion to the plasma membrane (Fig. 1).

Regarding direct fusion, it has been reported that 4 proteins are responsible for the attachment of the MV form of VACV to the plasma membrane

Mechanisms of enhanced VACV therapeutic efficacy

Certain modifications to VACV lead to increased viral replication and enhanced therapeutic efficacy of VACVs. GLV-1h68, for example, is a recombinant Lister strain VACV which possesses enhanced tumor targeting as a result of 3 deleted genes [44]. These genes include: F14.5L, which encodes a secretory signal peptide; J2R, which encodes TK, and A56R, which encodes hemagglutinin [44]. Inactivation of F14.5L and A56R leads to an attenuated virus while inactivation of J2R leads to selective

VACV-mediated anti-tumor immune responses

The use of chemokines in conjunction with VACV has been shown to facilitate both viral-mediated targeting of the tumor and host-directed immune responses against the tumor [11], [51]. As such, several groups have engineered the expression of chemokines in VACVs to enhance therapeutic effects. Since the chemokine CCL5 (RANTES) is responsible for attracting granulocytes and agranulocytes to sites of inflammation or infection [51], [52], researchers created a recombinant VACV of Western Reserve

Mechanisms of viral-mediated cell death

It has been determined that the type of cell death that occurs in tumors may be dependent on the virus. Most cancer cells have the ability to evade apoptosis [60]. Researchers reported that although cancer cells have apoptosis evasion mechanisms, they can be induced to die by mechanisms that are not dependent on apoptosis. Some of these examples include but are not limited to: necrosis, senescence, mitotic catastrophe, a special case of apoptosis in which cytogenic aberrations are suppressed

VACV in the clinic

Since VACVs have successfully targeted and disrupted tumor growth in-vitro and in-vivo, clinical trials are underway to address the therapeutic potential of VACVs in humans. One phase I/phase II clinical trial used JX-594 from the Wyeth strain to determine tumor selective killing in a wide range of solid tumors [65]. In this study, 23 patients with advanced stage refractory solid tumors were given intravenous infusions of 1 of 6 different dosages of JX-594 ranging from 1 × 105 plaque forming

Conclusions

In conclusion, the use of oncolytic vaccinia virus appears to be an effective method for treating a variety of cancers. The ability to insert or delete genes into VACV allows for recombinant VACVs to selectively target, enter and destroy cancer cells. Selective targeting and killing of tumor cells has been shown to be accomplished as a result of genetic engineering strategies to improve VACV tumor infectivity. This research has culminated in current phase I and phase II clinical trials and

Financial support

We gratefully acknowledge the PCOM Division of Research for funding of this project.

Conflict of interest

The authors declare no conflicts of interest.

Reviewers

Robert Jeffrey Hogan, Riverbend Research Laboratories South, 220 Riverbend Road, Athens, GA 30602, United States.

Artrish Jefferson (M.Sc.) recently graduated with her master's degree in Biomedical Sciences from the Georgia-Philadelphia College of Osteopathic Medicine. This paper was fulfilled as part of her requirements for her degree.

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    Artrish Jefferson (M.Sc.) recently graduated with her master's degree in Biomedical Sciences from the Georgia-Philadelphia College of Osteopathic Medicine. This paper was fulfilled as part of her requirements for her degree.

    Valerie E. Cadet (Ph.D.) is an assistant professor of microbiology and immunology in the Biomedical Sciences Department at the Georgia-Philadelphia College of Osteopathic Medicine. Her research has focused on small molecule screening to examine the modulation of poxvirus replication and viral gene expression with the goal of therapeutic target identification and validation.

    Abigail Hielscher (Ph.D.) is an assistant professor of anatomy in the Biomedical Sciences Department at the Georgia-Philadelphia College of Osteopathic Medicine. Her research interests are primarily centered on the role of the microenvironment, specifically the extracellular matrix, in breast cancer initiation, growth and metastasis.

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