Listeria, a bacterium that is often cited in food recalls, is one of the leading causes of death from foodborne illness in the U.S. It might be deadly in some cases, but a group of scientists believes that listeria can actually save lives by fighting cancer.
Soon Laguna Biotherapeutics, a startup founded by UC Berkeley professor of molecular and cell biology and of plant and microbial biology Daniel Portnoy three years ago, will seek Food and Drug Administration clearance to evaluate the listeria therapy in children with leukemia. If it goes forward, the research would include only leukemia patients who received unmatched bone marrow transplants to start.
Portnoy has been researching ways to turn listeria into a healer for decades. Bacteria and fungus have long been used to treat disease – the American Chemical Society notes that “ancient Egyptians had the practice of applying a poultice of moldy bread to infected wounds.”
When bacteriologist Alexander Fleming discovered that mold prevented the growth of harmful bacteria in 1928, it ushered in a new era of antibiotic medicines starting with Fleming’s discovery, penicillin. Per the U.S. Department of Agriculture, antibiotics are “compounds produced by bacteria and fungi which are capable of killing, or inhibiting, competing microbial species.”
What is listeria?
Listeria monocytogenes is described by the U.S. Centers for Disease Control and Prevention as a “facultatively anaerobic, rod-shaped, gram-positive bacterium.” It can cause gastrointestinal disease and fever and infections occasionally spread from the intestines to cause deadly sepsis or meningitis.
In the U.S., the annual incidence of laboratory-confirmed listeriosis in the United States is about 0.24 cases per 100,000 population, per the CDC. Though approximately 800 laboratory-confirmed cases are reported annually to CDC’s National Notifiable Disease Surveillance System, the centers said approximately 1,250 cases probably occur annually across the nation. Of those in the U.S. who develop the listeria infection, known as listeriosis, about 172 die each year.
“Nearly everyone with listeriosis is hospitalized. The case-fatality rate is about 20%,” said the CDC. “Nearly 25% of pregnancy-associated cases result in fetal loss or death of the newborn.”
Typically, bacteria are engulfed by scavenger cells called phagocytes after infections. There, they are captured by an organelle called a phagosome that digests them and other invaders. However, UC Berkeley explained that Portnoy showed around 40 years ago that listeria can escape the phagosome.
After breaking free, the bacteria can then “set up shop in the cell interior, hiding out from the host’s immune system until they reproduce and spread to infect new cells,” the university said.
Even as it hides out in the cell, interior, listeria triggers the body’s adaptive immune system to make cytotoxic T cells, or CD8 T cells. They can kill the listeria-infected cells.
How could listeria fight cancer?
That adaptive immune response is at the heart of Portnoy’s research. In the 2000s, he partnered with a company called Aduro Biotech to develop a cancer treatment that used engineered listeria to provoke responses and to target tumors. First, he had to make the bacteria less dangerous.
Portnoy engineered a new version of listeria called LADD (Listeria attenuated double deleted) by deleting two genes that allow the bacteria to exit cells and spread. Listeria usually hijacks a protein called actin in the cell’s cytoskeleton to “construct finger-like protrusions, which are internalized by neighboring cells,” UC Berkeley said.
“We found that a strain that was unable to nucleate actin will still get into the cytosol of cells, still grow and induce a potent immune response, but since it doesn’t spread, it’s a thousandfold less virulent,” Portnoy explained.
Aduro paired LADD with a cancer antigen and used it to treat nearly 1,000 patients with pancreatic cancer and mesothelioma. Unfortunately, the treatment didn’t work as well in humans as it did in mice. For example, it “failed to mount a robust cytotoxic T cell response,” and Aduro halted the trials.
Portnoy still had hope for a listeria cancer treatment, though. He was inspired by an observation from his colleagues at Aduro – that listeria triggered T cells of adaptive immune system and the innate immune system.
A new approach
While LADD was safer than typical listeria, Portnoy wanted to improve it. So, he removed two more genes. Now, the QUAIL (quadruple attenuated intracellular Listeria) therapy lacks two enzymes that basically keep the bacteria locked inside cells. This is the therapy developed for Laguna Bio.
“We said, ‘Oh my gosh, this strain fits the criteria that we were looking for’ – a mutant of listeria that could grow inside of cells but not outside of cells,” Portnoy explained. “We have a strain that can’t grow in blood, it can’t grow in the intestine, it doesn’t grow in the gallbladder – these are all extracellular sites for growth – but it grows inside of cells. So that’s the new safer strain, QUAIL. We’re very excited about that.”
Last year, Portnoy and his Berkeley and Laguna Bio collaborators published details about the therapy’s success in mice in the mBio journal. They also reported in another study posted last year on the BioRxiv preprint server that listeria can also be engineered mucosal-associated invariant T cell, or MAIT, something that could be harnessed to “defend against infections and possibly cancer,” UC Berkeley said. QUAIL could also possibly rev up gamma T cells and other immune responds that could help cancer patients.
“Because of its inability to grow outside of cells, QUAIL, unlike LADD, cannot grow on the ports and implants often used to treat cancer patients,” the university added.
What makes QUAIL stand out compared to other immune system-stimulating cancer therapies is its activation of the innate immune system rather than just the adaptive immune system.
“The issue is that tumors are a suppressive environment, and so the immune system isn’t really even working,” said Portnoy. “There are lots of attempts to try to reawaken the immune system, such as using checkpoint inhibitors, which were originally developed at UC Berkeley. The idea is somewhat similar with listeria: listeria itself is seen as foreign and induces an innate immune response, which allows the body to overcome the suppression.”
Laguna Bio CEO Jonathan Kotula said that Portnoy’s research demonstrates that listeria generates a “really unique” immune system response. In its initial trials with pediatric leukemia patients, the company plans to use QUAIL to “elicit a gamma delta T cell response,” in hopes that those cells will fight infection, rejection and recurrence by directly killing leukemic cells “in a patient where the T cells of the adaptive immune system have been suppressed to prevent rejection of the transplant.”
Sepcifically, “Stanford University Medical Center doctors hope that the engineered listeria will boost gamma delta T cells in pediatric patients and help them stave off graft-versus-host disease, fight potentially deadly infections that take advantage of a transplant patient’s compromised immune system and prevent the cancer from returning,” UC Berkeley said.
If the trials are successful, Kotula said the company hopes the therapy can be used for other diseases as well, from cancers to tuberculosis.