The work could lead to laboratory production of blood cells for transfusions and to clinical innovations in bone marrow transplants and gene therapy
Scientists at Jefferson Medical College have found a way to isolate hard-to-find hematopoietic stem cells. The researchers, in identifying a chemical beacon – a protein marker – on the cell, believe the new work will lead to laboratory production of all types of blood cells for transfusions and innovative approaches for bone marrow transplants and gene therapy.
"The hematopoietic stem cell has been considered the elusive Holy Grail of hematology and immunology," says Cesare Peschle, M.D., professor of microbiology and immunology at Thomas Jefferson University, who led the work. "Now it has been found and captured by identifying the first specific and functional stem cell marker."
Dr. Peschle, who is also Chairman of the Department of Hematology-Oncology at Istituto Superiore di Sanita in Rome, and his co-workers at Jefferson’s Kimmel Cancer Center, and in Italy, Germany, and at the University of Nevada report their work September 3 in the journal Science.
Hematopoietic stem cells, created by bone marrow, have the two unique abilities: to develop into any kind of blood cell and to self-renew by generating new daughter stem cells. Yet they are very rare, making up only 1 in 100,000 marrow cells. They have been notoriously difficult to distinguish from the blood’s other progenitor cells, which are further along in the differentiation process.
"We have for the first time a marker – KDR – which seems to be specific for the hematopoietic stem cell as compared to other primitive hematopoietic cells," Dr. Peschle says.
"The use of the stem cell is very important and broad," Dr. Peschle says. "Having the stem cell purified, we have the possibility finally of characterizing it at a functional, phenotypic and molecular level." Scientists can also learn to manipulate the cell in the laboratory, he says, and "will induce the stem cell to do in the laboratory what it does in the body – self-renew and differentiate to generate a huge number of red blood cells, white blood cells and platelets." Scientists then can generate in the laboratory the circulating blood cells required for blood transfusions, which currently are obtained by transfusions from normal donors.
For years, researchers have had tremendous difficulty distinguishing between two types of precursor blood cells: Hematopoietic progenitor and stem cells. Progenitor cells are immature cells that can differentiate into red blood cells, white blood cells and platelets. They are used to restore patients’ blood and immune systems after high-dose chemotherapy or radiation for cancer. Stem cells are earlier cells which have the unique capacity to self-perpetuate. They generate progenitor cells and blood cells throughout life. Progenitors have no self-renewal capacity – they only give rise to more differentiated precursor cells.
Scientists can isolate undifferentiated progenitor cells using a marker on the cell known as CD34. This methodology was pioneered by Dr. Peschle and co-workers in a report in Science in 1990. But identifying hematopoietic stem cells has been more difficult, explains Dr. Peschle. Hematopoietic progenitor cells are rare – between .5 and 1 percent of bone marrow cells are progenitors carrying CD34. Yet, stem cells are even less frequent, perhaps .1 percent of CD34 cells, or 1 stem cell in 100,000 marrow cells.
The problem was that there was no specific marker on the cell surface of stem cells comparable to CD34 on progenitor cells. "Once you have a marker protein for hematopoietic stem cells," Dr. Peschle explains, "you can theoretically raise antibodies against the marker, and then you can separate stem cells from other cell populations."
They focused on KDR, a protein that functions as a receptor for the vascular endothelial growth factor, or VEGF. KDR is expressed on endothelial cells. The scientists found that it is also expressed at low levels on CD34-positive progenitor cells. In embryonic life, primitive hematopoietic cells are made in close contact with KDR-positive endothelial cells. They argued that KDR may represent a marker for hematopoietic stem cells after birth.
They found an antibody that recognized the KDR receptor and which could isolate the KDR-expressing cells from the other progenitors in the CD34-positive progenitor population. Those cells with KDR comprised all of the hematopoietic stem cells and no progenitor cells. In contrast, the great majority of CD34 cells lacking KDR contained no stem cells and all progenitors.
Now, Dr. Peschle says, he and his co-workers have precisely evaluated the capability of KDR-positive stem cells to repopulate the bone marrow with blood cells in transplanted animals. They can also determine the exact frequency of stem cells in the KDR-positive population.
Dr. Peschle believes that the ability to eventually harness the hematopoietic stem cell in both the laboratory and clinic will someday help alleviate blood shortages for transfusions and to develop innovative approaches to bone marrow transplants (such as in solid tumor patients, by purging the transplanted stem cells from tumor cells) and gene therapy. In AIDS patients, for example, anti-HIV sequences could be inserted in isolated stem cells.
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