3D Image of a tumor

Working from within, modified germs expose cancer cells to an attack from the immune system

Parvaiz Yousuf

During a war, spies behind enemy lines often signal into the sky to direct aircraft to fire their way. Scientists have done something similar, using bacterial Trojans in tumor cells to attract attack from the immune system.

Researchers have long known that certain bacteria take up residence in tumors. Roger Geiger and his team at the Institute for Research in Biomedicine just turned them effectively into spies.

The tumor world

Before we wade in, we ought to understand tumors a little better. Well, a tumor is an abnormal mass of cells. If they don’t spread beyond the local area, they are called benign; if they do run loose, they are termed malignant. The area surrounding the tumor is the tumor microenvironment (let’s call it the TME). This consists of immune cells, a complex network of proteins and other molecules that hold up the tissues, signaling molecules, blood vessels, etc.

Cancer cells interact a lot with their environment, leaking chemical substances that help the tumor to grow while resisting the body’s immune system.

How the tumor weakens the immune response
A protein produced by the tumor inhibits immune cell response. Image courtesy the Geiger Lab

Tumors produce a molecule, (now called PD-L1 or programmed death ligand 1), which goes to the surface of the attacking immune cell and effectively switches it off. It does this by plugging itself to the PD-1 (programmed death 1), a protein found on a T-cell. PD-1 slows down T-cell activity, thus diluting the immune system response.

One molecule that acts as kryptonite for tumors, though, is L-arginine, an amino acid (an amino acid is one of the 20 building blocks for proteins we see in humans). It is found in many foods, including red meat, fish, whole grains, poultry, dairy products, beans, etc. However, there are some problems, One, there is too little L-arginine around the tumor to do sufficient damage. And two, T-cells, a type of immune cell that do the damage, cannot easily enter the tumor environment and kill the tumor cells.

An inside job

In this study, bacteria were biologically tweaked to enter tumor cells and increase L-arginine concentrations, enhancing immune response.

“L-arginine does not kill tumor cells directly, but rather enhances T cell activity within tumors,” Geiger, the study’s principal investigator, told Truly Curious.

As mentioned, T-cells identify and kill tumor cells when they can. But before Geiger’s work, there was no way to increase the number of T-cell in tumors. So Geiger got the right amino acid for the job.

“We had been studying L-arginine for a long time, so we were aware of its effect within the TME,” he said. Scientists had observed that increasing L-arginine encourages the accumulation of those warrior T-cells, there was no straightforward approach for them to do that in the lab.

Furthermore, he said, it was hard to increase just T-cell number in tumors because the tumor environment resisted the entry of immune cells.

Finally, a friend came up with the suggestion that they use a Trojan.

“This is how the idea of using bacteria to increase L-arginine concentrations came to our mind,” Geiger said. By infecting tumor cells with bacteria that produced more L-arginine, the cells became select targets for T-cells.

There is a big advantage in getting the L-arginine produced locally.

E.Coli makes L-arginine
Modified E. coli converts ammonia, a natural waste, into L-arginine, which evokes a strong immune response. Image courtesy the Geiger Lab

To get L-arginine into a cancer patient weighing 75 kgs (150 lbs), a doctor needs to inject 150 grams of L-arginine daily. That is not practical, as Geiger pointed out.

Even when directly injected into the tumor, thus keeping the dose concentrated, “the L-arginine rapidly diffuses out of the tumors, affecting the treatment,” he said. Having bacteria produce the amino acid locally ensures there is a consistently high level of L-arginine within the tumor environment.

Hobbling the defenses

Bacteria have long been used to kill tumor cells. Previous studies have described how genetically modified bacteria invading mouse cancer cells can accelerate the immune response against tumors.

Most bacteria have no problem making a home in tumors. The researchers just manipulated them so that they could use the cancer cell’s resources to produce new proteins it would never have otherwise.

In this case, Geiger’s team engineered E.coli to convert ammonia, a natural toxic waste product, into L-arginine.

A solution containing the modified E. coli was injected into the tumors. The bacteria went forth and multiplied, the colonies forming within 72 hours of injection. Three days after that, the tumor environment was crowded with T-cell numbers hard at work destroying the cancer cells.

To double the anti-tumor activity, besides the bacteria that produced the L-arginine, another therapy was incorporated. Called the anti-programmed death-ligand 1 (anti-PD-L1) immune therapy, this type of immunotherapy does not kill cancer cells directly but blocks the PD-L1 pathway we had earlier described as protecting malignant cells from the immune system.

Anti-PD-1 therapy hobbles tumor defense system
Anti-PD-1 therapy reduces the tumor’s ability to weaken the immune response. Image courtesy the Geiger Lab

The combination of the Anti-PD-L1 therapy and L-arginine-producing bacteria significantly cut tumor growth in 74 percent of cases.

“One of the best things about the discovery is that a long-term memory is formed [in the immune system],” Geiger said. So, after 90 days, when the researchers injected the mice with cancer cells, no new tumors formed. The immune system was ready this time round.

“It will only work with tumors where there is already an immune response mediated by T-cells,” Dr. Geiger pointed out. “The bacteria do not kill the tumor cells; they don’t cause a de novo [new] response. However, it shows that bacteria can alter the TME.”

From the lab to the ward

The success of any treatment depends on how long does it take for a viable version to reach the common masses. In principle, what was done in the lab can be replicated in a factory.

“I don’t exactly know how expensive it would be, but I hope it’s not too much,” said Geiger, rather vaguely, given that he’s not really a manufacturer. But the team has to consider the parameters that can affect future results. For example, tumors can vary in size, affecting the ease of injecting the modified bacteria. While large tumors are quickly colonized, smaller ones are not.

“In a mouse model, we saw all the tumors were inhabited,” Geiger said. “If [injected] directly into a vein (intravenously), only certain size tumors – of the 100 square mm size – were colonized.”

In addition, it is not clear if it works with all cancers or only those that can get the body’s defences up.

“I cannot comment whether it will work for the majority of tumors, but the tumor must be immunogenic [able to initiate an immune response],” Geiger said. It makes sense to remember that while we are all mammals, the immune systems of mice and humans have evolved to deal with quite different threats than mice. So it is necessary to check for any adverse reactions occurring within the human body. “We hope to get the same results in humans, too,” Geiger said.

Researchers Gaia Antonini, Fernando Canale, and Roger Geiger
Roger Geiger (right) with fellow researchers Fernando Canale (center) and Gaia Antonini (left). Image courtesy Geiger Lab

Short answer: No, the world is not quite ready for ninja bacteria beating back cancer. It may take more studies to determine the safety and efficacy of the technique. Whether or not all immunogenic tumors show a similar response is another potential area of study.

But, hey, it looks like a good thing, and we may yet be heading towards a great new world of cancer-fighting synthetic biology.

Parvaiz Yousuf

Parvaiz Yousuf is a writer who also doubles up as a researcher. He has publications on cancer biology and biochemistry and has an abiding interest in ornithology.

Edited by Catarina Nunes. With over five years’ experience in medical communications, Catarina holds a BSc in biomedical sciences and an MSc in neurosciences. She is also involved in translational and clinical research

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