Virus triggering blood clots to form in the blood system

Arterial wall cells offer insight into coronavirus’ rampage from head to toe

Peter Libby, M.D., never imagined nearly 40 years ago that his research would contribute to the current understanding of how the novel coronavirus targets and ravages the blood vessels in people with COVID-19.

In the 1980s, Libby, then an assistant professor of medicine at Tufts University School of Medicine, was the first to discover that arterial wall cells—called endothelial cells—can produce and respond to small proteins that promote inflammation in cardiovascular disease. At the time, though, inflammation held the attention of few in cardiology, and the doctor’s groundbreaking research caused barely a stir.

Today, not only has Libby’s discovery been repeatedly validated and widely accepted in cardiovascular research, but it is now part of a school of thought that connects endothelial cells to SARS-CoV-2 viral infection, inflammation, leaky vessels and blood clots in severe cases of COVID-19.

“The idea of COVID-19 as an endothelial disease paints a unifying picture that shows the many ways the disease presents itself, and it gives us new insights about therapies that can be vigorously tested in clinical trials,” said Libby, now a preventative cardiologist at Brigham and Women’s Hospital and a professor of medicine at Harvard Medical School in Boston.

Libby is one of several NHLBI-funded researchers making the case that blood vessel injury is central to what some are calling a fatal “third phase” of the disease. And for Libby, endothelial cells are at the center of this evolving story.

In healthy people, endothelial cells help prevent and stop bleeding, inhibit clotting, regulate blood pressure, and prevent inflammation. Forming a single cell layer that lines blood vessels, they serve as gatekeepers that allow small molecules and even whole cells to pass in and out of the vessels. When infection, invasion, or injury occurs, they sound the alarm to immune cells and clotting factors to combat invaders or repair tissue.

COVID-19 is a disease caused by a virus called SARS-CoV-2. Some people with COVID-19 develop abnormal blood clots, including lots of tiny clots in the smallest blood vessels.

But when endothelial cells encounter the coronavirus, things can turn ugly. In a Sept. 3 commentary in the European Heart Journal, Libby and a colleague described what might be happening. They believe that once the virus enters the lungs, it invades the cells in the air sacs that transfer oxygen to the blood. This activates the endothelial cells that are lined up like cobblestones to protect the tiny blood vessels surrounding the sacs. Together with immune cells, the endothelial cells release a host of signaling molecules, including interleukins, which damage the very barrier they created around the air sacs. As a result, protein rich fluid accumulates in the air sacs, causing low blood oxygen, a hallmark of acute respiratory distress syndrome (ARDS), which Libby said is the reason why many people with severe COVID-19 can’t breathe.

While Libby and his colleagues scramble to learn more, physician-scientists at Yale have been scratching their heads, trying to understand why COVID-19 is causing all kinds of blood clotting events–-stroke, deep vein thrombosis, even “COVID toes,” an itchy, red-purple rash on the toes of people with the disease.

To help glean what might be going on, Alfred I. Lee, M.D., Ph.D., a hematologist, partnered with his fellow associate professor of medicine, cardiologist Hyung J. Chun, M.D., to perform blood tests in the lab to study blood vessel or endothelial injury, since none existed for clinical use. Together with other Yale colleagues, they examined the blood of 68 patients with COVID-19, 48 of whom were critically ill and being treated in the intensive unit (ICU) and 20 of whom were receiving care in a non-ICU hospital unit.

Their findings, Lee said, caught the attention of medical professionals at Yale. “When we presented the data, everybody got very excited because we saw high levels of several proteins that pointed to endothelial cell injury, activation, and dysfunction in all hospitalized patients, not just those in the ICU,” Lee said.

These proteins, Lee added, help place endothelial cells at the heart of critical illness. Both clotting and inflammation are essential defenses, but if COVID-19 sends these protein levels through the roof, they could make these natural defenses spiral out of control, leading to overactive inflammatory cells. At the same time they could also activate platelets and trigger uncontrolled clotting throughout the body, blocking the blood supply to vital organs.

While this was invaluable information, the research revealed something else equally invaluable—and surprising—about one of the proteins, called thrombomodulin. “We found that thrombomodulin had a strong correlation with clinical outcomes,” Chun said. “Patients who died ended up having higher levels of thrombomodulin than those who survived.” Although their findings, published in The Lancet Hematology, require further validation, they suggest that measuring levels of thrombomodulin—just one of a number of markers for endothelial dysfunction—could help doctors on the front lines manage patients and possibly identify who is more likely to progress towards critical illness and possibly death.

Two images of platelets (in green) from a healthy volunteer (left) and COVID-19 patient (left).
Platelets (in green) from COVID-19 patients (right) are hyperreactive compared to platelets from healthy donors (left) based on spreading assays performed on a fibrinogen matrix. Credit: Bhanu Kanth Manne, Ph.D./University of Utah.

Researchers elsewhere have been taking on yet another aspect of the blood clotting mystery—the extensive vascular inflammation caused by the coronavirus. This inflammation can significantly alter the function of the blood platelets on a molecular level, according to a study in the journal Blood. “This alteration makes the platelets hyperactive and aggregate faster to form dangerous and potentially deadly clots,” said Robert A. Campbell, Ph.D., assistant professor in the department of internal medicine at the University of Utah in Salt Lake City and senior author of the study. “This could explain why we are seeing increased numbers of blood clots in patients with COVID-19.”

Matthew T. Rondina, M.D., who is also a professor of internal medicine at the University of Utah and a study co-author, said that these alterations of platelets could have implications for COVID-19 “long haulers” who have yet to fully recover from the disease weeks or even months after symptoms first arose.

“From our studies of sepsis, we know that there are secondary complications like cardiovascular disease, stroke, cancer, further inflammatory diseases,” he said. “It could be that these molecular changes permanently alter the platelets’ functions, which would have long-term consequences on those with COVID-19. And this is concerning.”

Researchers say this combination of activated endothelial and immune cells and platelets can culminate in a final, destructive phase of inflammation called a cytokine storm that can cause multiorgan failure and other devastating clinical complications. If this idea proves true, then there are a number of existing drugs that could help stop the critical illness in its tracks. And clinical trials are now underway to determine which drugs have the therapeutic potential to do just that.

Clinical trials like the NIH-funded ACTIV-4 Antithrombotics will answer important questions about which blood thinners can treat adults diagnosed with COVID-19. Libby cautions that blood thinners could elevate the bleeding risk in many COVID-19 patients with underlying medical problems such as diabetes, obesity, or high blood pressure.

Colchicine is another drug being tested in a clinical trial—called COLCORONA—that is partly-funded by NHLBI. Colchicine, which exerts a generalized anti-inflammatory effect also holds promise in the treatment of patients with COVID-19, but also come with its own set of risks, Libby said. “Any kind of anti-inflammatory therapy is going to be a double-edged sword,” he added, because they impair the body’s natural defenses that fight an infection. “For the patient who is in the end-stage of COVID-19, who is intubated and on a respirator,” Libby said, “they’re sitting ducks for secondary infections. We have to be very careful about intervening in defenses that protect us against disease.

The kinds of studies that he and others have done over the years may seem dated, Libby noted, but they have provided the evidence behind these rigorous and controlled clinical trials. “We have a big job ahead of us,” he said, “but these types of ongoing collaborative efforts are the way forward and could give doctors a compass that help steer them to the best therapies for each individual.”