Our immune system is our first line of defense against nasty pathogens and, thanks to some remarkable biological weapons, it saves us again and again from disease. Among the footsoldiers on this front line are the neutrophils. These common white blood cells can deploy a toxin-laden net made of their own DNA that acts like a spider's web to trap and kill invaders like bacteria and yeast. Now, a new study reveals how cancer cells hijack these DNA nets - potentially using them to cut holes in tissue so they can migrate through the body.
In the journal Science Translational Medicine, researchers from Cold Spring Harbor Laboratory (CSHL) in New York and the Dana-Farber Cancer Institute in Boston, Massachusetts, describe how they used electron microscopy to observe cancer cells hijacking DNA nets, and how they experimented with ways to overcome this process in mice.
Neutrophils are a group of white blood cells that are among the first to confront potential enemies. First, they try to gobble them up and digest them. However, some pathogens can get around this, so neutrophils have another weapon at their disposal. They can deploy toxin-studded nets - appropriately named neutrophil extracellular traps, or NETs - to trap and kill the enemy.
The NET is ejected from an activated neutrophil when the neutrophil detects a threat. It is a lattice made of the white blood cell's own DNA and studded with tiny toxic enzymes that chemically attack and digest the trapped pathogen.
Led by Associate Prof. Mikala Egeblad, the CSHL researchers used live-imaging technology to look at how cancer cells hijack NETs to promote their ability to spread. They used a mouse model of triple-negative breast cancer - a cancer known to be most aggressive in humans, and notorious for spreading and relapsing. The authors note in their paper:
"Using intravital imaging, we observed NET-like structures around metastatic 4T1 cancer cells that had reached the lungs of mice. We also found NETs in clinical samples of triple-negative human breast cancer."
Although they did not uncover the precise mechanisms involved, the researchers believe the makeup of the NETs helps cancer cells migrate through tissue by literally eating through the protein-scaffolding that forms the tissue structure. They suggest the NETs create small holes and crevices for the cancer cells, thus helping to promote new colonies away from the primary tumor.
"The remarkable thing we witnessed in live imaging was the ability of cancer cells to induce nearby neutrophils to eject their NETs even when no infection or invader was present. Our experiments showed that the NETs, in such situations, can promote metastasis."
Prof. Mikala Egeblad
How to stop NETs helping cancer cells
The team also investigated the question of how to stop NETs helping cancer cells. An obvious dilemma is if you stop the process altogether then you could cause enormous harm, as the body relies on neutrophils deploying their NETs to keep it safe.
For some clues on how to proceed, the team looked at a treatment used in patients with cystic fibrosis, a disease that prevents the lungs from clearing infections. The lungs of cystic fibrosis patients are burdened with too many NETs from neutrophils trying to fight the infection.
Cystic fibrosis patients can inhale a drug that includes a DNA-dissolving enzyme called DNase to eliminate the NETs.
Up to this point, the work was being done in Prof. Egeblad's lab at CSHL. The next challenge was how to develop a mechanism that might be used in cancer patients to deliver something like DNase into the affected tissue. Prof. Egeblad describes what happened next:
"We are incredibly lucky to have the help of Dr. Michael Goldberg and his team at the Dana-Farber Cancer Institute. We were explaining our discovery at a conference and mentioned that we were lacking a way to get DNase to work inside tissues and Dr. Goldberg said he might be able to help."
Dr. Goldberg and his team had found a stable way to deliver enzymes such as DNase by attaching them to nanoparticles that can be injected into humans. The researchers saw this as promising because as the DNase would be on the outside of the nanoparticle, its cutting action could begin on contact with the DNA NETs.
Tests of the DNase-coated nanoparticles in a mouse model of triple-negative breast cancer showed they markedly reduced - and in some animals even prevented - the cancer spreading to the lungs, the most common destination for metastases in this animal model.
For their experiments, the team used the same formulation of DNase that is already approved for use in the United States as a treatment for cystic fibrosis. This should make it easier to proceed to drug development for breast cancer.
Questioning cancer treatments that boost neutrophils
Prof. Egeblad points out there is still a lot of work to do. For instance, finding out which breast cancer patients are most likely to benefit from such a treatment, and when would be the best time to treat them.
The study also raises questions about current cancer treatments. For example, many patients receiving chemotherapy are also given a neutrophil-boosting growth factor called G-CSF. This is to compensate for the fact chemotherapy quickly destroys white blood cells, leaving patients vulnerable to potentially lethal infections.
Because of what they have discovered about the effect of neutrophil NETs on helping cancer to spread, Prof. Egeblad says it will be important to evaluate if giving patients G-CSF to boost neutrophils "may actually be dangerous in some cases."
The team is already investigating this problem, as they proceed with further work on optimizing DNase treatment to reduce neutrophil NETs when the risk of cancer spread is high.
In the following CSHL video, Prof. Egeblad outlines the work and results of the study, and what needs to happen next.