Newswise – Preventing the sticky, web-like substance that can form in blood vessels after stroke could protect the brain and lead to better patient outcomes, studies in mice suggest. The research was reported in the Clinical Investigation Journal and led by scientists at the University of Utah Health.
Stroke is one of the leading causes of disability in the United States. The most common form, ischemic stroke, occurs when a clot blocking a vessel prevents blood flow to the brain. Prompt treatment to break up or remove the clot can restore blood flow and limit brain damage.
But according to the new study, the returning blood carries cells that could cause further damage. Brain damage can worsen even after a clot is cleared when immune cells in the blood release sticky webs, called neutrophil extracellular traps (NETs), that further clog vessels.
“These nets can clog the vessels [in the brain] by trapping other cells and reducing the amount of blood flow, causing further brain damage,” says the study’s lead author, Robert Campbell, Ph.D.., Assistant Research Professor of Internal Medicine. “NET markers were correlated with poorer stroke outcomes in the patients we saw here at the University of Utah.”
Campbell and his colleagues found that they could prevent these effects in a mouse model of stroke by treating the animals with an NET-blocking compound.
“We desperately need new treatments for ischemic stroke,” says Jennifer Majersik, MD., professor of neurology and stroke specialist at U of U Health and co-author of the study. “At most, only 30% of patients are eligible for currently available standard therapies to open a blocked blood vessel.” She added that taking a fresh look at what causes and can prevent brain damage in stroke patients is an important step towards better therapies.
A useful defense also causes harm
NETs are part of the immune system’s defense against pathogens. White blood cells called neutrophils can release webs, made up of long chains of DNA and bits of toxic protein, to trap viruses and bacteria, but these can also damage the body’s own tissues. Evidence is mounting that NETs contribute to the development of dangerous blood clots in a range of conditions including sepsis, a life-threatening inflammatory disease, and COVID-19. Campbell and his colleagues wondered if they might also be involved in a stroke.
The team first looked for signs of NETs in the brains of stroke patients. They examined post-mortem tissue samples that had been stored in the National Institutes of Health’s NeuroBioBank and found neutrophils, a type of immune cell, that lurk in areas affected by stroke. Some of the cells appeared to have been actively producing NETs at the time of the patient’s death. Inside blood vessels, NETs served as traps, where cells passing through stopped and accumulated in a new blockage.
“They are able to trap many other cells and potentially reduce the amount of blood flow that enters the tissues and the amount of oxygen that enters the brain,” says Campbell. “They are very small and fragile, but they can do a lot of damage.”
When the research team analyzed blood samples from patients being treated for ischemic stroke at U of U Health, they found more evidence that NETs could have negative consequences for stroke patients. Proteins in the blood indicated that NET production was elevated compared to the blood of healthy individuals. Additionally, functional impairments caused by stroke patients were most severe in those whose blood tests at the time of hospital admission indicated the greatest production of NET.
These observations prompted Campbell and postdoctoral researcher Frederik Denorme, Ph.D., the study’s lead author, to return to the lab to understand how NETs formed after stroke. To simulate a stroke in mice, they temporarily blocked a blood vessel supplying the brain, then allowed blood flow to resume. Within 24 hours of the procedure, elevated levels of NETs were detectable in both brain and blood. The increased production of NET by neutrophils appears to be triggered by another type of blood cell, platelets.
“We really think that when blood flow is initially restored in the brain is when you get platelet activation and those platelets then go on to activate neutrophils,” Campbell says. The result is vessel-clogging NETs, obstructed blood flow, and brain damage beyond that caused by the original clot.
Blocking NETs protects the brain
Although the role of NETs in stroke is troubling, it also suggests an opportunity for intervention. Campbell and colleagues experimented with administering nNIF to mice, an NET-blocking peptide discovered by a U of U Health neonatologist Christian Con Yost, MD They found that if they prevented NET formation by administering nNIF soon after a simulated stroke, they could protect the brain. Mice that received this treatment suffered less brain damage from stroke than animals that did not receive nNIF, showing better neurological and motor function weeks after the event.
Encouragingly, the treatment improved outcomes not only in healthy young mice, but also in older mice and mice with diabetes. This is important because diabetes and other conditions that become more common with age can make the brain’s vascular system less healthy and more resistant to treatment.
Cambell says the nNIF peptide, which is currently being explored as a potential treatment for sepsis, could also help limit the damage strokes do to patients. The peptide cannot stop NETs once they have formed, but the findings in mice suggest that there may be a window of opportunity in which to intervene and prevent the formation of toxic webs. Further research is needed to further evaluate the effects of nNIF in animal models of stroke before determining whether it is appropriate for clinical trials in this setting.
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Additional study co-authors include Irina Portier, John L. Rustad, Mark J. Cody, Claudia V. De Araujo, Chieko Hoki, Matthew D. Alexander, and Ramesh Grandhi of U of U Health and Mitchell R. Dyer and Matthew D. Neal of the University of Pittsburgh.
The research was published under the title “Neutrophil Extracellular Traps Regulate Ischemic Stroke Brain Injury” and was supported by the National Institutes of Health and the American Heart Association.