Most of us have somewhere around a trillion tiny platelets zooming around our bloodstreams. Joseph Italiano, PhD, of Boston Children’s Hospital’s Vascular Biology Program, calls them the “Swiss Army knives of the blood.” In addition to their key role in clotting, platelets are important in immunity, wound healing, chemical delivery, blood vessel development and more.
At healthcare facilities, platelets are in constant demand for patients with blood diseases, or those receiving radiation or chemotherapy for cancer. But unlike other blood products, platelets can’t be stored for more than a few days. If there’s a snowstorm or other emergency preventing donors from giving platelets, a hospital can easily run out. So researchers have been trying to make platelets in a lab setting.
Two teams at Boston Children’s Hospital are tackling the problem in slightly different ways. Using a high-throughput approach, Thorsten Schlaeger, PhD, and colleagues in Boston Children’s Stem Cell Research Program have identified two chemical compounds that more than triple the output of platelets in the lab. Across the street, the Italiano lab is looking at the mechanics of platelet formation. Both labs are screening compounds for their effect on platelet maturation.
Pumping out platelets
To make platelets, many labs start with induced pluripotent stem cells (iPSCs). iPSCs are derived from adult cells, such as skin or blood cells, and can grow to become almost any cell in the body. Coaxing them down the developmental path that leads to megakaryocytes — platelets’ direct precursors — takes time, expensive ingredients and dozens of steps.
So instead, the Schlaeger team works with other platelet precursors called immortalized megakaryocyte progenitor cell lines (imMKCLs). Compared to iPSCs, it takes a lot less space to grow imMKCLs in large amounts. Moreover, scientists can replicate them indefinitely, freeze them for preservation and mature them into usable platelets in just a few days. Those are big advantages, given that each transfusion contains hundreds of billions of platelets.
Once the imMKCLs have produced megakaryocytes, the next challenge is getting them to make platelets. Current strategies only produce about 10 to 20 platelets per megakaryocyte, says Schlaeger, despite claims that each should be able to make more than 1,000. Schlaeger’s team has screened roughly 4,000 compounds, looking for one that, when added to the culture, will increase that number.
When imaged with an automated system, the round megakaryoctyes can be seen developing extensions with bead-like swellings that Schlaeger likens to ornaments on a Christmas tree. Each ornament represents a platelet getting assembled by the megakaryocyte. Two structurally related compounds dramatically increased platelet formation, the team found.
Moreover, platelets matured with these compounds showed a good ability to be activated, expanding and releasing important chemicals such as clotting factors. At the same time, most weren’t activated prematurely, a common concern with lab-grown platelets.
In partnership with the Japanese company Megakaryon, Schlaeger and his team hope to combine the factors they’ve identified with others to see if they can increase platelet quantities even more.
Joseph Italiano’s lab is taking a deep look at the biology of megakaryocyte development.
“Megakaryocytes were not able to be cultured until 1995,” notes Italiano. “One of my first tasks was catching them in the act of forming platelets.”
And catch them he did. The Italiano lab has found that microtubules — which provide scaffolding for cells and play a role in cell division — are key to platelet formation. By marking microtubules with green florescent proteins from jellyfish, Italiano was able to see them coiling and sliding past each other, forming loops that ultimately become new platelets.
Understanding microtubules’ role in megakaryocyte maturation allows scientists to better understand what’s going on at the structural level while developing lab-grown platelets. It could also lead to a therapeutic target for treating low platelet count, Italiano says.
Along with Jonathan Thon, PhD, Italiano co-founded the platelet production company Platelet Biogenesis (Cambridge, Mass.). The pair has leveraged their knowledge to create a microfluidic bioreactor that mimics the bone marrow environment where platelets naturally develop. The bioreactor allows proplatelets — platelets in development — to branch off megakaryocytes in a way that mirrors how they branch through the walls of blood vessels in the body.
Building a better platelet
“In the future,” Italiano says, “there might be the ability to create designer platelets.” Both labs are investigating this possibility. For example, lab-grown platelets could be genetically engineered to lack certain antigens that can cause side effects in patients, to withstand colder temperatures or even to deliver medicine.
Platelets serve as natural delivery vehicles, transporting chemical messengers throughout the body. As part of the immune system, they regularly visit tumors, which the body perceives as wounds. Italiano wants to take advantage of these attributes.
“One day, platelets could be loaded with specially designed cancer-targeting molecules,” he says.
One idea out of the Schlaeger lab is to engineer platelets to carry extra clotting factors. This would prevent the need for direct clotting factor regimens, which can cost millions of dollars a year.
While gene-edited platelets are still many years away from being widely available, ordinary lab-grown platelets might not be. According to Schlaeger, clinical trials of Megakaryon’s lab-matured platelets are expected to start this year.