We often think of viruses as the enemy to mankind, the invisible agent that can infect all kinds of life forms and spread many infectious diseases. The public is even growing more fearful of viruses since the COVID-19 pandemic has emerged. However, have you ever thought that we could turn this enemy into our ally, using it as a therapy for cancer treatments?
Oncolytic viruses are natural or genetically engineered viruses that selectively kill cancer cells without affecting healthy cells in the patients. The earliest reports of the oncolytic virus appeared at the beginning of the last century, but these viruses were not explored or developed further due to the lack of advanced technology and understanding of virology. However, 21st-century developments in genome editing tools and the accumulation of clinical experiences of using viruses as a vector brought the topic of oncolytic viruses back on stage, and scientists began to re-explore oncolytic viruses.
As immunotherapies are becoming increasingly popular in cancer treatments, oncolytic viruses undoubtedly have an irreplaceable role in the next generation of cancer therapy. This article will introduce the general mechanism of action of oncolytic viruses, how they are used to make drugs, and the current challenges in this field.
Why should you care about oncolytic viruses?
Cancer is one of the leading causes of death globally and it has always been a major area of research focus. Conventional cancer therapies such as chemotherapy and radiotherapy have lots of limitations, and the treatment process is often unpleasant with many side effects. There is also a chance of developing resistance for the treatment. Oncolytic virus, as a type of immunotherapy, has an advantage of selective targeting, which conventional therapy lacks, as well as the ability to create a long-term immunity in patients by reducing the chances of relapse. Although there are several challenges to overcome, an increasing number of researchers are interested in oncolytic viruses and their development as this is a promising field that can help change the lives of cancer patients for the better.
Mechanism of action of oncolytic viruses
In order to dive into the mechanism of the oncolytic virus, let’s first review some basic information about viruses. Viruses are classified by their genetic material (i.e. DNA or RNA). Although different types of viruses infect their host cell using diverse strategies, a typical virus replication and infection life cycle has several stages including attachment, entry, uncoating, genome replication, assembly, maturation, and release of the matured virus particles (Figure 1). Viruses use receptors on cell surfaces to attach to the host cell and enter the cytoplasm. The virus envelope will then be removed to expose the viral genome, which is replicated and transcribed by the host machinery to make new viral proteins. All the necessary components including the replicated genome and new viral proteins will assemble into new virus particles. After these progenies are assembled in the cell cytoplasm, they bud off or ‘shed’ from the cell (figure 1). Sometimes this release of the virus particles lyses the cell, while other times the virus can leave without directly killing the host cell.
Figure 1: Virus replication cycle. The stages in the virus life cycle shown using HIV as an example. "HIV Virus Replication Cycle" by NIAID is licensed with CC BY 2.0. To view a copy of this license, visit https://creativecommons.org/licenses/by/2.0/
Upon virus invasion, the innate defence of your immune system comes into play by first recognizing viral PAMPs (pathogen-associated molecular patterns) with PRRs (pathogen recognition receptors) on first-line cells such as macrophages and neutrophils. This recognition activates several intracellular signalling pathways and promotes the production of a wide range of molecules such as cytokines (groups of proteins that regulate cell signalling in immune responses) to trigger a pro-inflammatory response to clear the virus. Type I interferons are the classic cytokines produced to stimulate several antiviral machineries in infected cells, such as the JAK-STAT pathway which plays an important role in cell proliferation, survival, and differentiation. The adaptive immune response will also be activated to kill infected cells, prevent the virus from spreading and establish long-term immunity using several strategies, including cytotoxic T cell killing (cytotoxic T cells induce apoptosis of the targeted cell) and antibody production.
So what’s special about oncolytic viruses?
Since the oncolytic virus kills cancer cells without affecting healthy cells in our body, the two basic properties it has are cell lysis activity and selectivity towards cancer cells. Cell lytic activity is generally achieved by choosing a lytic virus species as oncolytic virus candidates. Oncolytic viruses utilise the intrinsic properties of tumour cells. Some antiviral machinery and immune defence pathways are often abnormal or impaired in tumour cells, thus giving the virus a selective advantage for replication. For example, the JAK-STAT pathway mentioned above is often mutated and constantly active in cancer cells. This can lead to uncontrolled cell division thus cancer. Therefore, the uncontrolled cell proliferation and impaired immune defence against viruses in cancer cells provide a perfect environment for an oncolytic virus to survive and replicate (Figure 2). In addition, selectivity towards cancer cells is also achieved by recognising overexpressed or uniquely expressed receptors on cancer cells.
Figure 2: Oncolytic virus mechanism of action. After administration of oncolytic viruses, they replicate specifically in tumour cells, lyse them to release more virus particles and tumour associated antigens which expose tumour cells to the immune system and activate a systemic anti-tumour response. Immune cells such as T lymphocytes, macrophages, DC (dendritic cells) and NK (natural killer) cells come to the infection site, produce cytokines and recruit other immune cells to target tumour cells and the virus. This action leads to the elimination of tumour cells at distant sites not injected with the oncolytic virus and immunological memory to avoid relapse of cancer. Adapted from Marelli et al. (2018).
After killing the tumour cells, the oncolytic virus therapy enters the second and more important action phase: inducing a systemic antitumour immune response in patients. Instead of exposing viral antigens (viral proteins that cause an immune response in the host) to your immune system, which is what vaccines do, lysed tumour cells tend to release tumour-associated antigens and more viral PAMPs. These signals activate immune cells including APC (antigen-presenting cells), T cells, and NK (natural killer) cells and formulate an immune response fighting against tumour cells. Since tumour cells are very good at hiding themselves from the immune system by utilising immune-inhibitory receptors and recruiting immune-suppressing cells, oncolytic viruses are able to modify tumour microenvironment through its mechanism of action and cleverly reveal tumour cells to the immune system. In ideal cases, the activated immune response can also kill tumour cells at distant sites that are not exposed to an oncolytic virus injection (Figure 2).
Although all these described functions of oncolytic virus make it seem like a very versatile and promising therapy, there are many variables and uncertainties in one’s body that could hinder the virus’s therapeutic effects. As you might have noticed, oncolytic viruses are essentially live viruses, a replicating foreign entity that will be detected by one’s immune system. The immune system, in this case, is essentially a double-edged sword; it is helping the therapy to remove tumour cells, but it is also acting as an opposing force and neutralises the virus (Figure 2). If the neutralisation of virus prevails, oncolytic viruses will be eliminated prematurely, and the therapeutic effect is largely reduced. However, we also don’t want the immune system to be completely unresponsive to the oncolytic virus, which means the virus may stay in the patient’s body for too long and cause other problems. Therefore, there needs to be a delicate balance of the action of the immune system on these antitumour and antiviral responses. This balance is hard to control as it is not fully understood. Additionally, controlling this balance varies for every case as it depends on the type of tumour being targeted, the tumour microenvironment, the type of oncolytic virus used and various other factors.
Apart from the immune response to the oncolytic virus, physical barriers such as the blood-brain barrier also have an impact on the distribution of the virus in the body. This border of cells that protects the brain from toxins and pathogenic invasions makes it difficult for the oncolytic virus to reach the brain, thus limiting its ability to kill brain tumours. Studies are being conducted to overcome this challenge through ways such as direct injection of oncolytic virus into the central nervous system, and promising results are expected in the future to tackle this problem.
Developing oncolytic viruses as a drug
The FDA (United States Food and Drug Administration) regulations state that quality, safety, and efficacy are the general principles of a drug. Among these three, safety would be the first consideration in drug preclinical and clinical trials. As oncolytic viruses are still a virus that is toxic and might cause infectious diseases in patients, it is even more critical in this case to ensure its safety as a therapy.
Virus attenuation to remove its pathogenicity could be carried out through several ways using recombinant engineering. Genes that encode toxic viral products such as those blocking type I interferon pathways for antiviral immune response could be deleted from the viral genome. Cancer-cell or tissue-specific promoter could also be incorporated upstream of the pathogenic viral genes so that the virus only becomes active and pathogenic in targeted sites.
To ensure the quality and minimize the side effects of the treatment, targeting specificity is a critical factor to consider. As we have seen before, oncolytic viruses have a selective replication advantage in cancer cells; some of them also have a natural tropism for the receptors on cancer cell surfaces. These receptors are usually overexpressed on cancer cells. Depending on the type of cancer, it could be possible to engineer oncolytic viruses with genes that target some uniquely expressed receptors on cancer cells. The oncolytic virus could also be engineered to target specific oncogenic signalling pathways.
A successful switch from antiviral response to anti-tumour immune response is regarded as the gold standard for oncolytic virus efficacy. This is the most ingenious part of the oncolytic virus therapy, but also one of the hardest challenges to undertake. Therefore, limiting antiviral response and promoting a long-lasting antitumour response is a research priority. One way to reduce viral neutralisation is introducing polymer coating on the virus to prevent antibody binding. The induction of antitumour responses could be enhanced through expressing proinflammatory cytokines in the virus, or including ‘suicide genes’ in viruses to boost direct killing of tumour cells.
Many types of viruses have already been selected as oncolytic virus candidates and modified to treat cancer in clinical trials. T-VEC (Talimogene Laherparepvec) is the most famously known example and a revolutionary step of the oncolytic virus. T-VEC is a modified HSV-1 (herpes simplex viruses) virus with the neurovirulence gene ICP34.5 deleted to remove HSV-1 virus infectivity in neurons and a human GM-CSF gene inserted to promote a tumour-specific immune response. GM-CSF (granulocyte-macrophage colony-stimulating factor) is a cytokine that helps recruit immune cells such as APC (antigen-presenting cells) to the tumour sites and increases cytotoxic T cell killings. The successful results in its phase III trials make it the first oncolytic virus therapy approved by the FDA in 2015. Other types of virus including adenoviruses, vaccinia virus, measles virus and poliovirus have also been manipulated in research and studied in clinical trials.
Current challenges and future directions
As a new class of cancer treatment, oncolytic viruses are still in the early development stage. There are indeed some challenges regarding the primary mechanism of action, clinical applications, and also regulatory issues in this field.
Apart from the difficulties in finding the balance between antiviral and anti-tumour responses, another major problem is the determination of an effective dose. The unique nature of oncolytic viruses make it different from a regular drug in terms of dosage; the dosage is not proportional to the therapeutic effect. A high dosage might kill the tumour cells too fast and leave insufficient host cells for virus replication. The antiviral and antitumour responses generated in patients also differ on an individual basis due to the difference in the amount of pre-existing neutralising antibody in the body, T cell responses, tumour microenvironment, and other variables. This also leads to another challenge, which is the difficulties for preclinical animal models to accurately reflect conditions in the human body. Therefore, animal trials and clinical experiments should be designed carefully to validate the effectiveness of oncolytic viruses in patients.
Biosafety is another issue in this field. Since oncolytic viruses are a replicative entity in the body, concerns have been raised about how to precisely control their actions in patients. Clear instructions on the preparation, handling, and administration of the virus are also required to avoid exposure of the virus to clinicians. In addition, as this is a relatively new area of research, more regulatory guidelines on oncolytic virus trial evaluations are required for further application and commercialization of the therapy.
Conclusion
Although there are several challenges to overcome, oncolytic viruses still represent a promising direction for cancer therapy research. Theoretically, it can be applied to treat all kinds of cancer and be personalised based on patients’ needs. It is now believed that combination therapy is the trend of cancer treatment, and oncolytic viruses obviously have a role to play in this to help change patients’ lives.
Author: Esme Fan, BSc Biochemistry