WASHINGTON - Congressional staff members received a briefing last month on research into chimeras, organisms with at least two different genetic cell types that hold promise in the field of regenerative medicine.
In this approach, human stem cells are inserted into fetal animals to develop into new cells with both human characteristics and those of the host animal, researcher Esmail Zanjani said. He primarily has employed stem cells from adult bone marrow, including his own, and also has used cord blood stem cells, hematopoietic cells derived from federally approved embryonic stem cell lines and mesenchymal stem cells.
These animal-human chimeras "are proving to be extremely useful," Zanjani said, because of the "variety of ways" in which they might be functional. For example, they can be used to study the activity of human stem cells or for generating human blood or organs. In the latter case, he said, chimeras can generate cells for organ repair or even whole organs for transplant, and those can be generic or patient-specific.
The animal-human intersection isn't unheard of - porcine and bovine parts have long been employed in repairing human organs.
In his research, conducted at the University of Nevada in Reno, Zanjani employs fetal sheep as hosts for human stem cells. In essence, the animals serve as production factories, creating the potential for "an unlimited supply of cells and organs," he said, "with availability on demand."
Zanjani's principal success to date has centered on generating livers by inserting bone marrow stem cells into the sheep fetuses. Published peer-reviewed papers demonstrated that his transplantation of those cells into fetal sheep led to the formation of significant numbers of long-lasting, functional human liver cells, with some animals exhibiting levels as high as 20 percent of human liver cells nearly a year after transplantation.
It's that fetal environment, he told BioWorld Today, that allows the stem cells "to begin to express themselves to a much greater degree."
They are inserted 55 days into the animal's gestation period, when the fetus remains immunologically na ve, into its peritoneal cavity. From there the cells enter the lymphatic system and then the bloodstream over the next four or five hours. "In the process," Zanjani said, "I think the cells get coated with some of the fetus' proteins and become more palatable to the animal, and they get distributed rather nicely to all of the organs."
Functional livers can be produced within a month or two, because in that period, the cells have developed into livers genetically rooted in sheep and humans. Without immunosuppression, the sheep liver cells presumably will be rejected after human transplantation, while the autologous human liver cells would regenerate an entirely new organ for the patient.
Zanjani thinks the cells "piggyback" on normal migration patterns inherent in the fetal sheep, essentially taking advantage of a paved development path en route to different organs. "And once there, they fall under the influence of that particular organ," he said. "If they're the kind of cells that respond to those influences, they begin to make that particular kind of material."
Zanjani said the bone marrow stem cells tend to be heterogeneous, so while "they look very much alike, one is different from the others." Resulting liver cells, besides being used in producing new organs, also might be useful for in vitro testing of drug toxicity.
Besides his success with livers, Zanjani said his research has led to hearts and pancreases with 10 percent human cells, as well as "really high levels" in lungs, skin and the gut.
His published papers also describe the formation of blood cells after transplanting bone marrow stem cells into fetal sheep. Zanjani has managed to coax blood cells from neural stem cells, though the blood cells showed some characteristics that led him to conclude that neural stem cell-derived blood cells are "inherently different" from that of true blood stem cell-derived blood cells.
Amy Wagers, an assistant professor at Harvard Medical School who has studied just how versatile hematopoietic stem cells are, is skeptical of Zanjani's claims that it's blood-forming stem cells that make the liver cells in his model.
"There's definitely an alternative explanation, which is that there's separate progenitors for blood and liver engraftment. [Zanjani] acknowledges that himself in the paper," she told BioWorld Today. Wagers has published work demonstrating that hematopoietic stem cells either do not turn into brain cells, or do so only at vanishingly low frequency. (See BioWorld Today, June 1, 2005.)
"It's possible that there's a different mechanism at work when you engraft in utero," she said. "But that this is true transdifferentiation has not been demonstrated" - for that, instead of large numbers of mixed cells, it would be necessary to engraft a single cell and demonstrate both blood and liver progenitors, or retrospectively tease apart the lineage of engrafted cells by marking them.
Whether what he observes is technically speaking true transdifferentiation, Zanjani said fusion isn't part of the equation, at least not in his research. Fusion of cells from different cell types sometimes will make it falsely look like a stem cell has taken root and produced daughter cells. But Zanjani said that, "We don't know why, but in the sheep model that we use, fusion is not a major mechanism. We actually don't see any fusion products at all, which is really a remarkable thing."
In contrast, chimeric research in fetal pigs conducted at another institute produced fused cells. But pigs have a far shorter gestation period than sheep, Zanjani said, making them more difficult for in utero transplants because the timing isn't as precise. Acknowledging that he's yet to answer the lack of fused cells in fetal sheep - "We spend more of our time trying to prove fusion in this model than nonfusion" - he attributes it to being a function of their natural development. For example, in fetal liver, there are very few binucleated cells, he said, whereas they are more common in adult livers. "So it's possible that during the fetal development of these organs, fusion is not a real mechanism involved, and therefore we are taking advantage of that."
He believes early human studies aren't that far off, envisioning such a test in the next several years. A researcher would need the blessing of a local institutional review board, as well as a well-informed patient. Wider use would necessitate FDA oversight.
Such a leap into the clinical setting would certainly raise safety concerns, given the potential for animal viruses to enter humans. But Zanjani said that the use of farm animals such as sheep, which "have been part of our sustainability for millennia," would be of benefit. He called the safety matter "an approachable problem."
The event was put on by the Joint Steering Committee for Public Policy for the Congressional Biomedical Caucus to highlight the fruits of research sponsored by the National Institutes of Health and advocate for better NIH funding. Zanjani himself has had NIH support throughout his entire 40-year career as a researcher.