Researchers generated blood stem cells and progenitor cells (shown) from human pluripotent stem cells.
Blood is made up of many different types of cells; from the red blood cells that carry oxygen to the various kinds of white blood cells that fight infection. A single blood stem cell can create all these cells and regenerate itself – or self-renew. Blood stem cells are found in the bone marrow where they produce a lifetime supply of blood cells. It is the transfer of these cells in bone marrow transplants that provides crucial, long-lasting blood production.
For over twenty years, researchers have been trying to create blood stem cells in the lab.
If they could create the stem cells from a patient's own cells, it could overcome problems with donor matching and immune rejection in bone marrow transplants. Lab-grown blood stem cells also hold great potential for cell therapies, looking for new, more efficient drugs (drug screening) and further research.
Using the knowledge gained from studying the blood stem cells development, two labs - using two different methods - have succeeded in making blood stem cells. Both studies were published in Nature, Sugimura et al (2017) from the Daley lab in Boston Children’s Hospital, Massachusetts and Lis et al (2017) from the Rafii lab in Weill Cornell Medical College, New York City.
During embryo development, there are different 'waves' of blood formation – or haematopoiesis. The first blood stem cells develop from special 'haemogenic' - or blood forming - endothelial cells, which line the inner wall of a blood vessel called the dorsal aorta. The blood stem cells will eventually move to the fetal liver, and finally, to their home in the bone marrow. Precise signals which cause the change from endothelial cell to blood stem cells are unknown but are being widely researched.
In the first study, Sugimura et al started with human induced pluripotent stem (iPS) cells. These cells have the potential to become any type of cell in the body - if given the right signals. Scientists used a cocktail of chemicals called ‘cytokines’ to instruct the iPS cells to first turn into haemogenic endothelial cells, similar to those that produce the very first blood stem cells in the embryo. Researchers identified 26 ‘transcription factors’ associated with blood stem cells. Transcription factors are proteins that bind DNA to influence gene expression.The authors inserted the transcription factors in various combinations into the haemogenic endothelial cells they made before, to see if they could trigger their transformation into blood stem cells. Using this method, the authors narrowed the down recipe to seven transcription factors which produced immature blood stem cells.
In the second study, Lis et al isolated endothelial cells from the lining of blood vessels in adult mice, growing them on a supportive layer of cells. Four transcription factors were inserted into the endothelial cells, two of which were also used by Sugimura et al, which resulted in the production of immature blood stem cells within 28 days.
Blood stem cells created by both studies needed further environmental signals in order to fully mature into cells that could potentially be used in patient transplantation. Sugimura et al. transplanted their cells into the bone marrow of adult mice, while Lis et al grew their cells on cells found in the human umbilical cord.
Both Sugimura and Lis injected these stem cells into mice with a damaged blood system and showed they produced mature blood cells. Cells from these animals were re-transplanted into other animals to demonstrate their ability to self-renew. Both studies noted that, while the lab-grown blood stem cells were very similar to those found in the body, they were not identical.
Though these results are promising, there are a number of limitations which need to be addressed before these cells can be considered for clinical use.
Sugimura et al produced human blood stem cells which were tested in mice, whereas Lis et al produced mouse blood stem cells which were also tested in mice. Lis and colleagues will need to show that this method can work using human cells. They do, however, have evidence from a previous study using human tissue suggesting the approach may be successful.
When considering creating patient-derived cells, there are two crucial factors; the starting material and the efficiency- or how to get the most cells as simply and quickly as possible. Though Sugimura et al use a longer method, the iPS cells can be made from adult skin cells and other easy to access tissue. Cells from the blood vessel lining used by Lis et al are harder to harvest and harder to keep alive in the lab. However, by bypassing the iPS stage entirely, the Lis approach is more efficient and requires fewer modifications of cells. One advantage of the Lis et al method is that the blood stem cells were fully matured in a petri dish and transplanted directly into a mouse, just as a person would receive a bone marrow transplant. Conversely, the Sugimura et al cells required that the blood cells first mature in a mouse.
There are also risks of cancer development involved when manipulating cells in the lab. Both approaches used viruses to introduce the transcription factor genes into the cells. Researchers couldn't control where in the DNA the genes were inserted, as this happens randomly. The expression of the transcription factors was then turned on by adding a particular drug. Using these viruses risks the accidental activation of cancer-causing genes or the transcription factors they introduced may still be expressed, even without the drug. Both teams are working on ways to supply the transcription factors in a safer way. This is very important as the transcription factors used in the studies have been associated with leukaemia. Lis et al found no evidence of cancer in mice for many months after transplantation. However, more extensive research will be required to confirm safety for both methods.
These findings are exciting but it will take a long time before patients can benefit from these treatments.