Institute researcher discovers “Holy Grail” of human blood stem cell expansion in culture

The technique should open up vast capabilities in both basic science and clinical practice, the researchers say

May 17, 2023

Institute researcher Hiro Nakauchi, MD, PhD and his colleagues have found what has been called the “holy grail” of blood stem cell research: a way to expand the number of human blood stem cells in the lab. The discovery opens the door to new research and new therapies for diseases that are currently untreatable. 

Their work is published in the journal Nature. Nakauchi shares co-authorship with Satoshi Yamazaki, PhD, an associate professor of stem cell biology at the University of Tokyo. Former Stanford postdoctoral scholar Adam Wilkinson, PhD, is one of the lead authors.

Blood stem cells (called hematopoietic stem cells or HSC) give rise to all the various blood and immune cells in the body. But HSCs are incredibly rare, and hard to isolate in large numbers. Only about 1 out of 30,000 cells in the bone marrow is a blood stem cell.  This makes it difficult to obtain sufficient number of cells to do research.

Bone marrow transplantation, in which HSCs and other blood products are transplanted from one patient to another, is a well-established treatment for advanced leukemia, but opportunities to treat these cancers, as well as many other disorders, are limited by the difficulty of isolating HSCs in large volumes. Historically, HSCs can only be isolated from living organisms. Once they are out of the body, researchers have not been able to get HSCs to multiply in culture to create more HSCs. 

“Clinically, the rarity of HSCs and the difficulty of isolating many of them has been an issue for over 50 years,” Nakauchi said. 

Nakauchi is well aware of this issue, having been personally involved in trying to expand samples of HSCs ever since he first isolated purified mouse HSCs in the 1980s. “I thought it would be very easy to expand HSCs,” Nakauchi says. “I tried many different ways of going about it, but it never worked.” 

Nakauchi and his colleagues had a breakthrough in recent years when they discovered how to expand mouse HSCs in culture, a result that they published in 2019. “Over the years I tried adding many different cytokines”-- molecules that prompt various cell activities—"but none of them worked.” 

The trick turned out to be not adding things to the culture medium, but taking things away. “The difficulty was that even very slight impurities in the culture medium, even very small amounts of inflammatory cytokines in the culture medium, would push the stem cells to differentiate into other kinds of cells,” Nakauchi said. As they had when culturing mouse HSCs, the researchers decided not to use common culture media that may be contaminated with various compounds. Instead, they used polyvinyl alcohol, a synthetic chemical that can be obtained in pure, uncontaminated form.

But the protocol that the researchers had successfully used to expand mouse HSCs didn’t work well with human blood stem cells. To understand why, they analyzed the molecular pathways connected to the signal that prompts the stem cells to divide. They found that a certain molecular mechanism called PI3K phosphorylation was not as active in human cells as in mouse cells. Artificially boosting the activity of that pathway turned out to be the key to getting better replication of the human HSCs.

Now that Nakauchi and his colleagues have found out how to grow a few human blood stem cells into many, there are new possibilities for medical therapies. Currently, people with acute leukemia can only undergo stem cell transplantation if they can find a donor who has a matching immunological profile. While Caucasian patients have an 80 percent chance of finding a donor among bone marrow registries, many minorities have a much lower chance. African Americans have only about a 20 percent chance of finding a match, for instance.

Cord blood registries are much more representative of the population at large, but leukemia patients have not been able to make use of them because the samples are small and contain so few blood stem cells. “If we can grow those few cells into many, cord blood samples could be the source for blood stem cell transplantation, including for those patients who are immunologically hard to match,” Nakauchi says. In addition, because the HSCs from cord blood are so young and immature, the immunological match does not have to be as exact as it does for the stem cells taken from adult donors.

Currently, blood stem cell transplantation most often requires a dangerous, pre-transplant preparation procedure involving high doses of chemotherapy or radiation. This wipes out the existing blood stem cells, making room for the transplanted stem cells to engraft. Researchers have discovered, however, that if very large quantities of HSC are transplanted, they can take up residence in the patient’s body without the need for removing the patient’s own HSCs first. Institute researcher Matthew Porteus, MD, PhD, for instance, is interested in potentially making use of this effect to more safely transplant a blood stem cell that has been genetically modified to provide a cure for young patients with sickle cell disease. 

The ability to create large numbers of HSC in the lab should also be a boon for basic researchers, who will now be able to obtain the materials they need for larger, more in depth studies of blood stem function.

With such wide applicability in medicine, the interest in this research has been strong. “We are getting inquiries from around the world about or results,” Nakauchi said.