Stem Cell Surprise: Blood cells form liver, nerve cells

A person’s blood could someday provide replacement cells for that individual’s damaged brain or liver, a provocative study suggests. Human blood contains so-called stem cells that can be transformed outside the body into a variety of cell types, according to the report. This unexpected, and accidental, discovery may add a new element to the politicized debate over whether stem cells that persist in adults can match the therapeutic potential of stem cells derived from human embryos.

The possible new source of adult stem cells came to light when a coworker became ill and couldn’t attend to petri dishes containing human blood cells called monocytes, says Eliezer Huberman of Argonne (Ill.) National Laboratory. In the body, these white blood cells migrate into tissues and mature into specialized immune cells, such as macrophages. Huberman’s team has been studying the cellular signals behind this maturation.

Left without nutrients, some of the unattended monocytes morphed into cells that didn’t look like immune cells, Huberman’s team noticed. Following up on this chance observation, the researchers identified a subgroup of monocytes that they could convert into various kinds of cells.

For example, exposing these select monocytes to a chemical called nerve growth factor transformed about 90 percent of them into cells with the shape and projections characteristic of nerve cells. These cells also contained typical nerve cell proteins and enzymes, the researchers report in an upcoming Proceedings of the National Academy of Sciences. Using different growth factors, Huberman and his colleagues found that they could also readily switch the isolated monocytes into what appear to be liver cells, blood vessel cells, and immune cells unrelated to monocytes or macrophages.

The researchers speculate that the versatile monocytes they’ve identified help the human body repair itself. “I think the function of these stem cells is to go to organs that are damaged,” says Huberman.

Some stem cell researchers, however, remain unpersuaded by Huberman’s data. “It is hard to know if this is just a curious artifact of cell culture or reflects cells with actual therapeutic potential,” cautions George Daley of the Whitehead Institute for Biomedical Research in Cambridge, Mass.

Researchers must take the various cells derived from the blood monocytes and inject them into animals to confirm that the cells have assumed new functions, says Daley. Such experiments are next on the agenda, says Huberman.

Until recently, researchers had generally assumed that most adult tissues either have no stem cells or harbor ones that can reproduce only the tissue in which they reside.

Yet some experiments have suggested that bone marrow, already well known as the source of all blood cells, contains certain cells that can form nerve, muscle, and other types of nonblood cells (SN: 6/22/02, p. 390: Available to subscribers at Stem Cell Gain: Bone marrow cells seem to have what it takes).

Huberman notes that the monocyte population his group identified differs from those multipurpose bone marrow stem cells. “Our cells have the typical monocyte-macrophage markers,” he says. And Huberman points out that it will be much easier to harvest the blood cells from a person than to surgically collect bone marrow.

In the past, opponents of research on human embryos have used promising findings from experiments with adult stem cells to argue against work with embryonic stem cells.

Abandoning that line of research would be premature, according to Huberman. “It’s very difficult to predict which system will give a better payoff. Both embryonic and adult stem cells should be pursued,” he says.

Huberman’s team and Argonne National Laboratory have submitted patent applications on the identification and medical use of the monocyte stem cells.

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