Red blood cells drive blood clot shrinkage, overturning old assumptions

Red blood cells, long thought to be passive bystanders in the formation of blood clots, actually play an active role in helping clots contract, according to a new study by researchers at the University of Pennsylvania.

“This discovery reshapes how we understand one of the body’s most vital processes,” says Rustem Litvinov, a senior researcher at the Perelman School of Medicine (PSOM) and co-author of the study. “It also opens the door to new strategies for studying and potentially treating clotting disorders that cause either excessive bleeding or dangerous clots, like those seen in strokes.”

The finding, published in Blood Advances, upends the long-standing idea that only platelets, the small cell fragments that initially plug wounds, drive clot contraction. Instead, the Penn team found that red blood cells themselves contribute to this crucial process of shrinking and stabilizing blood clots.

“Red blood cells have been studied since the 17th century,” says co-author Prashant Purohit, Professor of Mechanical Engineering and Applied Mechanics within Penn Engineering. “The surprising fact is that we’re still finding out new things about them in the 21st century.”

An unexpected finding

Until now, researchers believed that only platelets were responsible for clot contraction. These tiny cell fragments pull on rope-like strands of the protein fibrin to tighten and stabilize clots.

“Red blood cells were thought to be passive bystanders,” says co-author John Weisel, Professor of Cell and Developmental Biology within PSOM and an affiliate of the Bioengineering graduate group within Penn Engineering. “We thought they were just helping the clot to make a better seal.”

That assumption began to unravel when Weisel and Litvinov ran a test they expected to fail. They created blood clots without platelets. “We expected nothing to happen,” says Weisel. “Instead, the clots shrank by more than 20%.”

To double-check their results, the team repeated the experiment using regular blood treated with chemicals to block platelet activity. The clots still contracted. “That’s when we realized red blood cells must be doing more than just taking up space,” says Litvinov.

Modeling the mechanics of blood clots

To figure out how red blood cells were driving this unexpected behavior, the team turned to Purohit, a mechanical engineer by training.

An expert on soft materials like blood clots and gels, Purohit developed a mathematical model that suggested that red blood cells compact together primarily due to “osmotic depletion.”

This process also explains how particles in colloids—mixtures like paint, milk or muddy water—can gather and form clusters when the conditions around them change.

“Essentially, the proteins in the surrounding fluid create an imbalance in pressure that pushes red blood cells together,” says Purohit. “This attractive force causes them to pack more tightly, helping the clot contract even without platelets.”

How clotting works without platelets

As blood begins to clot, a web-like protein called fibrin forms a mesh that traps red blood cells and pulls them close together. “That packing sets the stage for osmotic depletion forces to take over,” says Purohit.

Once the red blood cells are packed tightly within the fibrin mesh, proteins in the surrounding fluid are squeezed out from the narrow spaces between the cells. This creates an imbalance: The concentration of proteins is higher outside the packed cells than between them, which results in a difference in “osmotic pressure.”

That pressure difference acts like a squeeze from the outside, pushing the red blood cells even closer together.

“This attraction causes the cells to aggregate and transfer mechanical forces to the fibrin network around them,” adds Purohit. “The result is a stronger, more compact clot, even without the action of platelets.”

Validating the model

Prior research suggested another possible explanation: bridging, in which the attraction between small molecules on the surface of red blood cells causes them to adhere.

“Our model suggested the bridging effect was real,” says Purohit, “but much smaller than the effect of osmotic depletion.”

To test the model, first author Alina Peshkova, now a postdoctoral researcher in Pharmacology within PSOM, performed a series of experiments on modified blood clots.

In the absence of the molecules that cause the bridging effect, the clots still contracted, but little contraction occurred in an environment designed to prevent osmotic depletion.

“We experimentally confirmed what the model predicted,” says Peshkova. “It’s an example of theory and practice coming together to support each other.”

Fighting clotting diseases and strokes

Gaining a better understanding of the role red blood cells play in the formation and maturation of clots could lead to new treatments for conditions like thrombocytopenia, in which low platelet counts can cause uncontrolled bleeding.

The findings could also shed light on how clots break into fragments that travel through the bloodstream and cause blockages—known as embolisms—that can trigger strokes.

“Ultimately, our model is going to be helpful in understanding, preventing and treating diseases related to clotting inside the bloodstream,” says Purohit.

Additional co-authors include Ekaterina K. Rednikova, Rafael R. Khismatullin and Vladimir R. Muzykantov of PSOM, and Oleg V. Kim of PSOM and Virginia Tech.