At the turn of the 20th century, physicians observed that some cancer patients who contracted an infectious viral disease saw their tumors spontaneously regress. A particularly compelling case report, published in 1904 by the American pathologist and clinician George Dock, told the story of a female leukemia patient he had treated some years earlier. Though she had failed to respond to conventional therapies, her white blood cell counts dropped sharply, and her leukemia went into remission after she contracted influenza in February 1897. Sadly, within a year her counts climbed back to preinfection levels, and she ultimately succumbed to her disease. 1 But her case revealed a link between cancer regression, infection, and the body’s antiviral response.

Virus versus tumor

Cancer is a disease in which genetic mutations lead to dysregulated cell behavior and biology. Messages become garbled, cellular pathways go into overdrive, and control mechanisms are taken offline. Yet the very pathways that a tumor cell corrupts to ensure its own growth and survival also render it susceptible to viral infection.

In 1923, Romanian physicians Constantin Levaditi and Stefan Nicolau were among the first to report on a “tumor-parasitizing” virus. They postulated that, “tumors are more susceptible to [certain] viruses than normal cells, and tumors act as a sponge attracting viral replication.”2 Researchers now know that viruses can and do target cancer, and their ability to distinguish between normal and cancerous cells depends on the collective effects of the mutations within a tumor.

Tumors have alluring features that offer viruses a much greater chance of replication success. (See infographic.) The surfaces of tumor cells express a more diverse repertoire of receptors, providing viruses with extra grappling hooks that facilitate binding. Tumor cells can also quiet innate immune responses, such as interferon signaling, to avoid immune detection, affording viruses the same escape. Moreover, to support their prolific growth, tumors boost levels of nucleotides, which can be siphoned off by viruses to generate their own infectious progeny. Finally, tumors can deactivate cell-death pathways to become immortal, providing a long-term residence for viral invaders.

But the relationship between viruses and cancer is not mutually beneficial. Once viruses have reached tumors, they lyse and destroy cancerous cells over the course of the viral life cycle. They also shut down tumor blood supply, activate tumor-specific immune cells, and reverse immunosuppressive tumor microenvironments. As was first suspected more than a century ago, these viruses, termed oncolytic viruses, are efficient and potent killers of cancer.

Oncolytic viruses not only kill cancer cells directly, they also help train the immune system to view cancer as a threat.

In the mid-1950s, researchers at the National Institutes of Health (NIH) were among the first to develop oncolytic virotherapy as a controlled and clinical treatment for cancer, after observing that adenovirus could kill HeLa cells in culture. They administered oncolytic adenovirus to cervical cancer patients, and observed that many tumors began to slough away and regress. But ultimately none of the patients survived their cancers.3 Around the same time, early chemotherapies were proving to be robust anticancer agents, and despite severe toxicities, this strategy eclipsed biological oncolytic virotherapy and became a front-line treatment for cancer.

It would be another 20 years before oncolytic viruses resurfaced as an area of study in cancer research. When they did, they came back with a bang. In 1974, the Japanese physician Teruo Asada published a series of strikingly powerful photos of various patients’ tumors before and after treatment with mumps virus.4 These images highlighted the tremendous tumor-reducing power of oncolytic virotherapy and renewed the interest of cancer researchers around the world.

Finding the perfect virus

Viruses exist all around us: in the air, soil, and water, as well as in living organisms. While many human viruses are potent cancer killers, they are also associated with an unacceptable level of risk, as viruses have evolved a medley of tricks that make it difficult for the immune system to contain them. The perfect oncolytic virus strikes a balance between anticancer activity and safety, allowing the immune system to control and ultimately shut down the virus using antibodies, T cells, natural killer cells, and other immune helpers once the virus has performed its cancer-killing duties.

Every virus has a unique character, and selecting one that can safely kill cancer is difficult. Historically, a huge effort has gone into finding naturally effective oncolytic viruses. One of the first reports of a screen designed to identify nonhuman animal viruses with cancer-killing properties was published in 1963 by William Hammon of the University of Pittsburgh.5 Hammon analyzed 24 viruses from cats, cows, monkeys, horses, and pigs, and found six potential cancer killers that were safe in humans. This list was finally narrowed down to two: a bovine and an equine herpesvirus, both of which are still being studied today for their oncolytic properties. Since then, oncolytic viruses have been identified from such diverse virus families as adenovirus, herpesvirus, rhabdovirus, poxvirus, paramyxovirus, reovirus, togavirus, parvovirus, and picornavirus. Many of these viruses are being developed for clinical use, with no less than 50 oncolytic virus–based clinical trials currently underway, according to clinicaltrials.gov.

Here in David Stojdl’s lab at the Children’s Hospital of Eastern Ontario Research Institute in Ottawa, Canada, we have focused on finding oncolytic candidates within the rhabdovirus family of bullet-shaped, enveloped, single-stranded RNA viruses. After panning through biobanks of rhabdoviruses isolated from all over the world, we struck oncolytic gold with Maraba virus, which was first isolated from a male sandfly in 1983 by Yale University’s Robert Tesh during an expedition to the Amazon Basin in Brazil. The virus remained virtually unexplored until 2010, when we discovered that Maraba was a formidable cancer killer, chewing through cells of colon, breast, central nervous system, skin, lung, ovary, and prostate tumors.6 Since then, we have bolted on extra genetic hardware, and, with promising safety and efficacy results in mice and monkeys, Maraba virus is set to make its debut in cancer patients in Phase 1 clinical trials later this year.

Building a better virus

© THOM GRAVESIn addition to working with naturally existing oncolytic viruses, we can create new, more powerful oncolytic viruses through bioselection, a process that passes viruses through several rounds of replication in tumor tissue or exposes them to UV light to encourage the emergence of new, genetically reassorted isolates and mutants. Such derivative viruses often have features that are more desirable than those of their parents, such as spreading more efficiently through susceptible cells; replicating more quickly to produce bigger viral populations; or forming greater numbers of syncytia, in which an infected cell fuses with its neighbors to form a giant multinucleated cell brimming with viruses.

With the advent of simple yet powerful genetic recombination technology in the 1990s, it has become even easier to tweak the safety and efficacy profiles of oncolytic viruses. While many viruses show an inherent affinity for tumor over normal tissue, we can genetically engineer mutations that widen this differential by conferring greater replication dependence on common tumor features. In fact, two of the top clinical candidates currently in advanced-stage trials, Amgen’s T-VEC and SillaJen’s Pexa-Vec, contain mutations that lock in their tumor specificity by gutting the viruses’ ability to replicate in the absence of an attenuated interferon response. Our own Maraba virus clinical candidate contains protein mutations that not only improve tumor specificity by the same mechanism, but also boost replication rates in tumor cells and enhance cytotoxic effects.

The high-throughput genomics revolution has also enabled researchers to design oncolytic viruses from scratch. We can now mine entire viral genomes to sort out the positive and negative impacts of individual genes on viral infectivity, spread, persistence, immune engagement, and anticancer activity. Then we can use this information to draw up genetic blueprints of the perfect cancer killer. Here in the Stojdl lab, we are stripping oncolytic vaccinia virus down to its bare bones to create a powerfully focused therapy. Using high-throughput robotic screens, we are flipping through thousands of genes to identify the minimal skeleton that represents the ultimate oncolytic vaccinia virus. We’re also working to discover common genes expressed by cancer cells that hinder oncolytic virus activity. Once we have such disruptive targets in our sights, we can engineer oncolytic viruses carrying microRNA payloads to suppress them. Similar approaches have delivered liver-specific microRNAs to block the hepatotoxic replication of oncolytic adenovirus in normal liver cells7 and have introduced microRNAs that reinforce the tumor specificity of oncolytic viruses that would otherwise exert off-target effects in healthy tissues.8

Recruiting immune help

Oncolytic viruses not only kill cancer cells directly, they also help train the immune system to view cancer as a threat. At the heart of the immune system’s response to viruses is its biological recognition software, which gets reinforced, updated, and improved through years of pathogen exposure. Learning more about these processes has allowed us to create protective immunity firewalls using similar, but safe, viruses as vaccines.

The idea of using relatively benign, low-risk viruses to prevent or treat disease is not a modern one. Virus-based vaccines, which use natural or engineered infectious viruses studded with danger signals to train the immune system to recognize infectious threats, have successfully eradicated smallpox, and continue to control many otherwise debilitating childhood infections, including measles, polio, and varicella zoster (chickenpox) virus.

Oncolytic virotherapy extends the idea of virus-mediated immune training to the treatment of cancer. Since a tumor is essentially a large mass of overgrown “self” tissue, the immune system has trouble breaching its own strict barriers that prevent potentially lethal autoimmune reactions. Oncolytic viruses can unmask the tumor to reveal a mutated, dangerous version of self, and train the immune system to attack it. After infecting cancer cells, oncolytic viruses lyse them open, spilling a soup of inflammatory molecules, cancer proteins, and pathogenic debris. Self proteins that get mixed up in this dangerous broth become temporary immune system targets until this inflammation gets resolved. The immune system then mops up and safely neutralizes any remaining virus, and remembers these dangerous targets, virus and cancer alike.

Such oncolytic virus therapies can be improved even further by morphing oncolytic viruses into bona fide cancer vaccines. To do this, oncolytic viruses are coated with fragments of cancer cell proteins that mask their own molecular outfit, causing the immune response to focus on the cancer proteins rather than on the virus itself. Researchers within the Cancer Vaccine Collaborative, from both the Krankenhaus Nordwest in Frankfurt, Germany, and Roswell Park Memorial Institute in Buffalo, New York, have successfully used a mix of oncolytic and nononcolytic virus vaccines coated with the common cancer antigen NY-ESO-1 to train melanoma patients’ immune cells to react against this key tumor target.9 Those patients receiving a vaccinia vaccine “prime” followed by a fowlpox vaccine “boost” lived longer, and those that had the highest number of immune cells reacting against the NY-ESO-1 cancer target did the best.

Oncolytic viruses can also initiate a general immune system reboot, particularly in the wake of cancer surgery. Following major surgical procedures, the body shuts down all nonessential systems—including the immune system—to divert energy into the healing process, providing the perfect environment for any small scraps of overlooked malignant tissue to take root, grow, and spread. Oncolytic viruses can overcome these immune defects not only by supplying a potent immune wake-up call through the presence of virus molecular signatures, but also by directly infecting immune cells and causing their maturation to restore functionality. While the exact mechanisms behind this effect remain somewhat elusive, Rebecca Auer’s group at the Ottawa Hospital Research Institute in Canada has successfully used both oncolytic Maraba and vaccinia viruses in mice undergoing tumor surgery to kick-start defective immune cells, restore their ability to secrete cytotoxic molecules and antitumor cytokines, and prevent cancer relapse.10

Getting the immune system on the right side of the battle against cancer can greatly impact the treatment of metastatic dis­ease.

Getting the immune system on the right side of the battle against cancer can greatly impact the treatment of metastatic disease. Immune cells can access almost any part of the body, and those that have been newly trained by an oncolytic virus infection can be sent out on seek-and-destroy missions to fight distant tumors that never come into direct contact with the virus itself. For example, in mice bearing identical tumors on their left and right legs, injecting the left tumor with oncolytic virus caused tumors on both legs to regress, even though virus couldn’t be found in the uninjected right leg.11 In humans, a similar effect was seen in metastatic melanoma patients receiving Amgen’s T-VEC therapy, where injections of oncolytic herpesvirus generated tumor regression both in directly infected tumors and in multiple noninfected tumor pockets.12

Doubling up on immunotherapy

Classic immunotherapies are designed to modulate the immune system by activating stimulatory pathways or deactivating inhibitory pathways. Provenge was the first immunotherapy to make it onto the market, receiving approval from the US Food and Drug Administration (FDA) as a prostate cancer treatment in mid-2010. The therapy delivers dendritic cells loaded with multiple copies of the prostate-specific antigen prostatic acid phosphatase (PAP) and an immune-maturing cytokine to patients to stimulate antitumor immune responses. The FDA and the European Medicines Agency have now approved the first immune-activating monoclonal antibody (mAb), ipilimumab, to activate T cells in melanoma patients, and several experimental immunotherapies are also becoming more mainstream. Steve Rosenberg’s group at the NIH, for example, is working on isolating immune cells from patient tumors, activating and expanding them in a dish, and reinfusing them back into patients to cure metastatic melanoma.13 Carl June’s group at the University of Pennsylvania is engineering immune cells to recognize cancer targets, an approach that is curing patients with liquid tumors, such as B cell lymphoma.14 (See “Deploying the Body’s Army,” The Scientist, April 2014.)

Oncolytic viruses are themselves considered immunotherapies and need a functional immune system to achieve their full anticancer potential. Pairing oncolytic viruses with classic immunotherapies, both of which are proving safe and effective, has great potential for amplified responses and improved cancer treatment outcomes. Exciting results coming out of the Memorial Sloan Kettering Cancer Center in New York City show that pairing oncolytic virotherapy with monoclonal antibody therapy is better at controlling metastatic melanoma than either treatment alone,11 and mathematical models continue to yield insights into how new oncolytic virus-immunotherapy pairings can improve anticancer outcomes.15

It seems increasingly clear that the long-term solution for treating cancer lies with the immune system. If immune responses can be retrained to recognize tumors in the same way that they recognize infectious agents, we will undoubtedly enter a golden age of cancer therapy. Oncolytic viruses combine infectious immune stimulation with tumor targeting to overhaul the immune system and activate multiple anticancer pathways, and several oncolytic virus combination therapies are generating long-term remissions in cancer patients.

Intriguingly, viruses are not the only infectious agents that can effectively orchestrate an immune assault on cancer. Bacteria are also effective couriers that can not only sensitize tumors to the effects of oncolytic viruses, but can also drive anticancer immune responses in their own right. Such approaches to cancer treatment solidify the idea that pathogens can be incredible tools for retraining and reinvigorating the immune system to defeat cancer.



Stephanie Swift is a scientist and science writer in the lab of David Stojdl at the Children’s Hospital of Eastern Ontario Research Institute in Ottawa, Canada. She blogs about science news at mmmbitesizescience.com.