From Pathogen to Treatment Basic Research on Listeria Bacteria Leads to Unique Cancer Therapy

Decades of basic research on the foodborne pathogen Listeria monocytogenes have culminated in an unexpected medical application: a novel cancer therapy designed to activate the body’s innate immune system.

After nearly 40 years of research on how Listeria bacteria manipulate our cells and battle our immune system to cause listeriosis, Daniel Portnoy and his colleagues have discovered a way to turn the bacteria into a potent booster of the immune system — and a potential weapon against cancer.

Three years ago, Portnoy cofounded a startup, Laguna Biotherapeutics, that worked with scientists in his University of California, Berkeley lab to eliminate the bacteria’s ability to cause disease while retaining its ability to rev up production of a type of immune system cell associated with increased survival in cancer patients. These so-called gamma delta T cells are general-purpose killers of cancer cells or any cell infected by a pathogen — bacteria, virus or fungus.

Laguna Bio will soon ask the FDA for clearance to evaluate the therapy in children with leukemia who have received unmatched bone marrow transplants. 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.

Portnoy and his colleagues foresee a broader application of this Listeria therapy, which is unique among cancer therapies in stimulating the body’s innate immune system to eliminate essentially any cell that puts out a distress signal indicating it’s been compromised. Today’s immunotherapies for cancer typically activate the “adaptive” immune system, boosting cells that recognize and kill cancer cells.

“The issue is that tumors are a suppressive environment, and so the immune system isn't really even working,” said Portnoy, a UC Berkeley professor of molecular and cell biology and of plant and microbial biology. “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.”

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Late last year, Portnoy and his Berkeley and Laguna Bio collaborators published details of the successful use of the attenuated Listeria therapy in mice in the journal mBio, a publication of the American Society for Microbiology. In another study posted last year on the Biorxiv preprint server, they reported that Listeria can also be engineered to boost another type of innate immune cell — mucosal-associated invariant T cell, or MAIT— that helps defend against infections and possibly cancer.

“What we’re doing is based on decades of literature, chief among them Dr. Portnoy’s work, showing that Listeria generates a really unique immune response,” said Laguna Bio CEO Jonathan Kotula. “We believe that if you want to generate a comprehensive immune response, you need to carefully orchestrate the entire immune system. And attenuated Listeria seems to be doing that.”

Escape from the Phagosome

Listeria monocytogenes is a foodborne pathogen that causes gastrointestinal disease and fever in some people but occasionally spreads from the intestines to cause deadly sepsis or meningitis. Researchers have documented how, after infection, the bacteria are engulfed by scavenger cells called phagocytes, where they are captured by an organelle called a phagosome that digests invaders. But Portnoy showed nearly 40 years ago that before that can happen, the bacteria escape the phagosome and set up shop in the cell interior, hiding out from the host’s immune system until they reproduce and spread to infect new cells.

Despite this immune evasion, Listeria triggers cytotoxic CD8 T cells. In the 2000s, Daniel Portnoy and Aduro Biotech used genetically weakened Listeria engineered with cancer antigens to stimulate anti-tumor immunity. To make the bacterium safer, two genes required for cell-to-cell spread were deleted, producing a strain that still entered cells and activated immunity but was far less virulent.

“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 said.

The resulting LADD strain was tested in nearly 1,000 patients but proved less effective in humans than in mice, leading Aduro to halt the trials in 2020. However, the studies revealed that Listeria also activates gamma delta T cells of the innate immune system, which can attack cancer cells and stimulate broader immune responses.

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Building on this insight, Portnoy’s team developed an even safer strain, QUAIL, by deleting two additional genes needed to synthesize riboflavin-derived cofactors. The bacterium can now grow only inside cells, not in blood or other extracellular sites.

“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 said. “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.”

A new study shows QUAIL is safe in mice and as potent as LADD, while avoiding risks such as growth on medical implants. Drawing on prior human data, researchers now plan early trials in pediatric leukemia to harness gamma delta T cells.

“Taking all that body of data that existed before from Aduro allowed us to go forward with this plan that I think is really unique in that it's informed by robust human data,” Kotula said.

If successful, the approach could extend to other cancers and even infectious diseases.

Original Article: Reprogramming Listeria monocytogenes flavin metabolism to improve its therapeutic safety profile and broaden innate T-cell activation; mBi; DOI:10.1128/mbio.03652-25

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