Science / Medicine : Mouse Cell Able to Spawn Entire Blood Supply and an Immune System

Culminating 30 years of research, scientists reported last week that they have isolated a rare mouse cell that can spawn an entire blood supply and immune system and could make human bone marrow transplants safer and more readily available.

For the first time, researchers purified a type of cell known as a “stem cell” from mouse bone marrow and are now on their way to isolating human stem cells, the researchers wrote in the journal Science.

“This is an important benchmark in the effort to broaden and improve bone marrow transplantation’s applications in medicine,” said Dr. Irving Weissman, professor of pathology and of cancer biology at Stanford University School of Medicine. “And it should help accelerate the introduction of gene therapy to medicine.”

Bone marrow is where blood cells are made. Marrow transplants--used to treat some forms of leukemia, blood and immunological disorders and radiation and post-chemotherapy sickness--provide a fresh source of blood and immune cells to a patient whose own marrow is damaged or diseased.

Only one in about 2,000 bone marrow cells is a stem cell, which provides the ultimate source for constant production and replacement of all of the body’s short-lived blood and immune cells, including red blood cells, platelets and lymphocytes.

Until now, scientists have been unable to isolate stem cells because of difficulty in distinguishing them from other marrow cells, Weissman said.

Using new methods, the Stanford scientists painstakingly eliminated other kinds of cells from the bone marrow, step by step.

Finally, they were left with a small batch of cells that still could completely reconstitute the blood and immune systems of a mouse whose own bone marrow had been destroyed by radiation, Weissman explained.

“We concluded these were the purified stem cells,” sought by scientists since the 1950s, he said.

The scientists found that whereas 200,000 to 300,00 bone marrow cells must be injected into a “lethally irradiated” mouse to reconstruct its blood and immune systems, it took only 80 to 100 stem cells to produce the same effect.

In bone marrow transplants, a patient’s damaged marrow can either be replaced by injecting new, healthy marrow from a donor or by reimplanting some of the patient’s own marrow from which diseased cells have been removed.

Unless donor and patient have closely matching tissue types, transplants of donated marrow tend to fail because white blood cells in the donated marrow attack the recipient’s tissues, causing the potentially deadly “graft-versus-host disease.” Injection of purified donor stem cells might prevent this disease, Weissman said.

In addition, the cells would eliminate the need for a matching donor, making bone marrow transplants much more widely available, he said.

The cells could also make safer autologous bone marrow transplants--used to treat some leukemia patients--in which some of the patient’s own bone marrow is removed, cleansed of cancer cells and reimplanted.

If even a few cancer cells remain in the reimplanted marrow, the tumors could recur--a problem that might be overcome if the stem cells were separated from the remaining cancerous cells and reimplanted, Weissman said.

Pure stem cells also could eventually lead to gene therapy--replacing defective genes with good ones--for inherited blood disorders, he said.