‘Microvasculature on a chip’ may improve understanding of blood clots, therapy response
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ORLANDO — A new microvascular-on-a-chip assay has allowed researchers to grow blood clots and track inflammatory thrombi with the goal of better targeting antithrombotic and anticoagulant therapies, according to results of a study presented at ASH Annual Meeting and Exposition.
“This allowed us to test out several drugs,” Yongzhi Qiu, PhD, instructor of pediatrics in the Aflac Cancer & Blood Disorders Center at Emory University School of Medicine, told Healio. “One of the things we found is that enoxaparin is not enough [in terms of treatment] for when it is given as [preventative treatment] for thrombo-inflammatory conditions.”
Quick resolution of thrombi to restore blood flow and avoid ischemic injury is key in treatment of thrombosis. However, the step-by-step process of how thromboses resolve has yet to be fully understood, largely due to a lack of technology.
Because thromboses generally take days or weeks to fully resolve, existing in vitro and in vivo systems cannot monitor the progress.
Qiu and colleagues developed a microfluidic system that assesses endothelial dysfunction and permeability in response to proinflammatory signals over months.
The researchers sought to leverage the system to assess how microvascular thromboses resolve over a long period of time, particularly under thrombo-inflammatory conditions in which patients are consistently at risk. The system also allows researchers to monitor the functioning of antithrombotic drugs and anticoagulants in the setting of existing inflammatory thrombi.
Researchers demonstrated the system — which researchers described as a perfusable vascularized thrombus resolution assay — accurately mimics the biophysical microenvironment of the microvasculature, including microvessel size, geometry, wall shear stress and shear gradients. It also models the biological microenvironment, including the perfusion of whole blood, endothelial cells activated with inflammatory mediators and vascular permeability. This allows researchers to study how these factors interact during the formation and resolution of microvascular thrombi over long time periods.
Using this assay, researchers discovered that the exposure of the microvasculature to
tumor necrosis factor-alpha induces von Willebrand factor multimers that deposit onto the inflamed endothelium at splits of the smallest vessels, where wall shear stress gradients exist. The deposited multimers then cause platelets to accumulate in the split within minutes. This is accompanied by the formation of gradual fibrin.
Neutrophils attach to the inflamed endothelium at a relatively later stage, usually in areas with lower wall shear stress, and combine with platelets and the growing fibrin mesh.
Platelets are mostly undetectable the first day after thrombosis, whereas neutrophils and fibrin continue as the clotting begins to resolve. This blocks flow and prevents endothelial barrier function recovery.
Using this assay, researchers were also able to monitor commonly used anticoagulants, including enoxaparin, and how they resolve clotting.
Results showed that, when given as prophylaxis in thrombo-inflammatory conditions, enoxaparin prevents fibrin formation but does not weaken platelet-neutrophil collection.
Enoxaparin also decreases endothelial dysfunction and restores barrier function.
“In the future, we are really going to look at the mechanism and underlying causes for the formation of thrombi,” Qiu said. “Particularly, we are looking into the interaction between von Willebrand factor and platelets and also platelet activation. We will also look at more ways of how thrombosis is relieved and will test more drugs that treat this.” – by John DeRosier
Reference:
Qiu Y, et al. Abstract 441. Presented at: ASH Annual Meeting and Exposition; Dec. 7-10, 2019; Orlando.
Disclosures: NIH funded this study. Qiu reports no relevant financial disclosures. Please see the abstract for all other authors’ relevant financial disclosures.
Perspective
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Maria T. DeSancho, MD, MSc
The understanding of the step-by-step process of how a blood clot resolves over time remains limited due to the lack of sufficient technologies.
There have been some previous models in mice and in vitro, but what’s different about this device is that it mimics small blood vessels and visualizes the formation and dissolution of a clot. Qiu and colleagues created this “microvascular-on-a-chip” assay with features that recapitulates what happens in human circulation.
When the wall of the blood vessel is damaged, the researchers tested how long it takes to clot and for the clot to resolve. They also tested a blood thinner and a clot buster, which that is used to dissolve blood clots.
This device helps us understand the lifetime of a clot from its formation until its resolution and how are the players in the process of clotting including platelets, von Willebrand factor, neutrophils and fibrin interact with each other. The length of this process varies depending on whether the inflammation that damages the blood vessel is mild or severe, with a longer time to resolution with severe inflammation.
This represents a new opportunity to teach us about clotting. My patients ask me all the time how long it is going to take for their blood clot to resolve, so if we can get a better understanding with this technique, it will really help me to explain how a clot forms and resolves with the use of different treatments.
Before this goes forward, however, I do think this microvascular-on-chip experiment should be reproduced in another laboratory to validate the experiment. During the conference, some experts asked the researchers whether the system accounts for cells that support the lining of the blood vessels, which I agree is an important question to answer going forward with additional experiments.
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