by Elie J. Diner
I love a good underdog story, and I'm pretty sure I'm not alone. Just look at the popularity of sports movies like Rudy, Rocky, Hoosiers and this blogger's favorite, the Mighty Ducks. What is it about these stories that capture our collective imaginations? There is something alluring about watching a group of seemingly average individuals achieve greatness against the odds. And as biologists who think about natural selection on a daily basis, where the little guy typically gets eaten by the bigger guy, it is fun to see the roles reversed.
Following the State of the Union Address by President Obama and his declaration "to cure cancer once and for all," a recent post on this blog pondered the following: How can microbes contribute to this effort of finding a cure for cancer? This seems to be a true underdog situation considering the size difference between a bacterial cell (~1 µm3) and a mammalian cancer cell (~2,000 µm3). Given how many of these cancerous cells could make up a tumor, this head-to-head battle seems to have a predetermined winner; the little guys don't stand a chance. But as we know from the prototypical underdog, David, and his foe Goliath, size is not always the crucial factor. And as readers of this blog are aware, bacteria have quite a few tricks up their microscopic sleeves. For now, I will leave other microbes (viruses and single-celled eukaryotes) out of this discussion, but do not be fooled, even the algae can contribute to the cancer fighting cause. Below, I divide up our examples of cancer fighting bacteria into first, their use as a cancer vaccine and second, the ability of some bacterial species to "sniff" out the most elusive of tumors.
Getting by with a little help from my inflammatory friends
During the course of infecting an unlucky human, some pathogenic bacteria evade detection by the mammalian immune system, responsible for recognizing and eliminating anything foreign. In doing so, these pathogens setup their home in their favorite tissue and can make our lives pretty unpleasant. Most bacteria are not so crafty and tend to present a plethora of molecules (often called PAMPs) that can send the immune system into red alert. This alert is often literal, leading to increased blood flow (reddening), swelling of the infected tissue, a general increase in body temperature (the hallmarks of inflammation). Plus, tissues become infiltrated by immune cells that release inflammatory signals called cytokines, important actors in the elimination of all things bacterial. So, what if this dramatic immune response could be harnessed and focused on the elimination of cancerous cells instead of bacteria?
There has been significant effort in recent years on this front, spawning a new field called cancer immunotherapy. This has proven difficult, as the immune system goes to great lengths to prevent the recognition of itself, and cancerous cells contain many of the same recognition elements as normal cells. At best, this makes cancer cells only weakly immunogenic.
Enter William Coley (Figure 1), a practicing bone surgeon in the mid-1800s and one of the first to discover that one could harness the immune response and direct it at cancer. Devastated by the loss of his first patient, Coley scoured old medical records, looking for patients who had serendipitously survived sarcomas (say it three times fast!). Coley found what he was looking for in a patient that had been operated on repeatedly in an attempt to remove a sarcoma occupying his cheek. As you may imagine, 19th century surgical technique was not as aseptic as it is today, and the patient came down with a Streptococcus pyogenes infection following one of his many surgeries. Being a pre-antibiotic era, there was not much to do for the patient. Yet, following a sharp fever, not only did the patient recover from the bacterial infection but his sarcoma disappeared. Coley was struck by this miraculous recovery and he tracked down the patient to confirm the story, noting the patient was still tumor free. Coley began treating patients with inoperable sarcomas using live S. pyogenes, injected at the tumor site. This was a risky treatment plan on Coley's part, yet he discovered that fever was a critical step in tumor regression. After further refining his methods, Coley began using a cocktail of heat-killed S. pyogenes and Serratia marcescens to induce fever more reproducibly and lessen the risk of causing an active bacterial infection. This vaccine came to be known as 'Coley's toxin' (a bit of a misnomer that stemmed from the idea that a specific bacterial toxin caused tumor regression) and through an injection regimen he developed, not only cured sarcomas, but also melanomas, carcinomas, lymphomas, and myelomas.
How could a cancer panacea like this not be in use today!? Many doctors viewed Coley's results with skepticism and some were unable to reproduce them. This controversy and the growing popularity of radiation therapy at the turn of the century contributed to Coley's vaccine falling from favor. While there have been many modern day studies, the mechanism by which Coley's toxin leads to tumor regression is still not well explained. Many have tried to reproduce the anticancer response using a purified bacterial factor or cytokine to no avail.
Even so, interest in Coley's vaccine has seen a rebirth in recent years with the founding of a San Diego biotech company, attempting to improve Coley's vaccine and turn it into a commercially available product. The company, Decoy Biosystems, was recently featured as one of the top 12 San Diego biotech startups to watch in 2016 by Xconomy. So, perhaps there will be vindication for Coley and his cancer killing bacterial vaccine.
Mi casa es su casa
The home of a tumor can be a mess: chaotic blood flow can create regions lacking oxygen or a carbon source. Thus, cells within a tumor can die, spilling their contents into the extracellular milieu. Ultimately, this results in a jumbled mass of cancerous cells in various states of death, inactivity, or vigorous growth. This architecture also makes many tumors tough to reach by immune cells or small molecule anticancer drugs.
While all of these things are cons for the passive diffusion of a small molecule, they are pros for bacteria that can not only sense the presence of foodstuff, e.g., amino acids and sugars, but also swim chemotactically towards it. Such bacteria (e.g., Salmonella enterica serrotype typhimurium) can navigate the complex vasculature surrounding a tumor and penetrate deep within its tissue. Unlike the methods described by Coley, which required local injection of bacteria into a tumor, several groups have shown that many bacteria, including S. typhimurium (let me call it by its shortened name), can be administered systemically and will accumulate 1000-fold more in tumors than in normal tissues. Specifically, S. typhimurium chemotaxes towards serine, aspartate, and ribose. In addition, S. typhimurium auxotrophic for leucine and arginine has a greater specificity for localizing to many types of tumors. Readers of STC may have already been aware of the work of Abe Eisenstark (profiled here), who has worked for some time on the ability of Salmonella to localize to prostate tumors. Obligate anaerobic bacteria, like Clostridium or Bifidobacterium, also localize specifically to tumors when administered systemically, being that they grow specifically in the anaerobic environment deep within a tumor.
So, what can be done with these tumor-sniffing microbes? Several groups have shown that the natural ability of Clostridium and Salmonella to stimulate an immune response can lead to tumor regression. In addition, the ability to genetically engineer bacteria allows for the expression of therapeutic proteins at the tumor site by microbes that have infiltrated a tumor (Figure 2). This enables a whole new class of treatment options and the unique ability to deliver drugs specifically to a tumor.
Beware of the glowing urine
The ability of bacteria to localize to tumors not only allows them to deliver therapeutics, but also act as a sentinel for tumor detection. Recently, a liver cancer detection method was designed using the Escherichia coli Nissle 1917 strain, engineered to express high levels of LacZ (beta-galactosidase). This strain does not require local or systemic, but can be delivered orally. It accumulates in liver tumors or their derived metastases, for which methods of detection are severely lacking. Injection of a molecule called LuGal (conjugate of luciferin and galactose) intravenously allows it to pass through the liver and be cleaved by LacZ, expressed by E. coli Nissle that is growing specifically in liver tumors. Following cleavage, the released luciferin travels to the kidneys where it is cleared in the urine. A simple luminescence test of the urine then provides a sensitive diagnostic for the presence of liver tumors (Figure 3).
The recent push for a cure on cancer has been called a "moonshot," the goal of which is to have a therapeutic for the effective treatment of cancer as early as 2020. Given the cancer-fighting and cancer-detecting talents of the bacteria described here, it seems clear that these little critters could play a huge role in the cure for cancer. The role of the microbiota in tumorigenesis and even the efficacy of chemotherapeutic agents is currently emerging as another important aspect of bacteria and cancer. So, with bacteria on your side, this "moonshot" seems more like a slam dunk!
Elie is a postdoctoral fellow in the Romesberg lab at The Scripps Research Institute in La Jolla, CA. He is interested in synthetic biology, expansion of the genetic code and microbiology. In addition to science, he enjoys surfing, camping and spending time outside.