Researchers from Stanford University have shown that genetic differences in mitochondria found in cells made from pluripotent stem cells that were originally derived from cloned embryos can prompt rejection by the immune system of the host animal from which they were made, at least in mice.
According to a study in mice by researchers at the Stanford University School of Medicine and colleagues in Germany, England and at MIT, cells and tissues in mice made from cloned embryos are rejected by the body because of a previously unknown immune response to the cell’s mitochondria. These findings reveal a likely hurdle if such therapies are ever used in humans.
Regenerative therapies that utilize stem cells have the potential to repair organs, replace dead or dying tissues, and treat severe diseases. Many stem cell scientists think that pluripotent stem cells, which can differentiate into any kind of cell in the body, show the most promise for regenerative medical applications in the clinic. One method for deriving pluripotent stem cells that have the same genetic composition as that of the patients is called somatic cell nuclear transfer (SCNT) or cloning. This method takes the nucleus of an adult cell and injects it into an egg cell from which the nucleus has been removed.
SCNT can potentially make pluripotent stem cells that can repair a patient’s body. “One attraction of SCNT has always been that the genetic identity of the new pluripotent cell would be the same as the patient’s, since the transplanted nucleus carries the patient’s DNA,” said cardiothoracic surgeon Sonja Schrepfer, MD, PhD, who was the co-senior author of the study, which was published online Nov. 20 in Cell Stem Cell.
“The hope has been that this would eliminate the problem of the patient’s immune system attacking the pluripotent cells as foreign tissue, which is a problem with most organs and tissues when they are transplanted from one patient to another,” added Schrepfer, a visiting scholar at Stanford’s Cardiovascular Institute, and Heisenberg Professor of the German Research Foundation at the University Heart Center in Hamburg, and at the German Center for Cardiovascular Research.
Several years ago, Stanford University professor of pathology and developmental biology, Irving Weissman, MD, chaired a National Academies panel on SCNT cells. At this time, he raised the possibility that the immune system of a patient who received the cells derived from stem cells made from cloned embryos might still generate an immune response against proteins from the cells’ mitochondria. Mitochondria are the energy factories for cells, and they have their own genetic system (a DNA chromosome, protein-making structures called ribosomes, and enzymes for expressing and replicating DNA). This reaction could occur because cells created through SCNT contain mitochondria from the egg donor and not from the patient, and therefore could still appear as foreign tissue to the recipient’s immune system.
There were other indications that Weisman was probably correct. An experiment that was published in 2002 by William Rideout in the laboratory of Rudolf Jaenisch at the Whitehead Institute for Biological Research in the journal Cell derived embryonic stem cells from cloned mouse embryos and then differentiated those embryonic stem cells into bone marrow-based blood making stem cells. These blood making stem cells were then used to reconstitute the bone marrow of a mouse that had a mutation that prevented their bone marrow from forming normal types of disease-fighting white blood cells. However, even though the recipient mouse was genetically identical to the embryonic stem cells that had been used to derived the blood-making stem cells, the immune systems of the recipient mouse still rejected the implanted cells after a time. Weissman, however, was not able to directly test this claim himself at that time. Weissman directs the Stanford Institute for Stem Cell Biology and Regenerative Medicine, and now, in collaboration with Schrepfer and her colleagues, he was able to test this hypothesis.
“There was a thought that because the mitochondria were on the inside of the cell, they would not be exposed to the host’s immune system,” Schrepfer said. “We found out that this was not the case.”
Schrepfer, who heads the Transplant and Stem Cell Immunobiology Laboratory in Hamburg, used cells that were created by transferring the nuclei of adult mouse cells into enucleated eggs cells from genetically different mice. When transplanted back into the nucleus donor strain, the cells were rejected although there were only two single nucleotide substitutions in the mitochondrial DNA of these SCNT-derived cells compared to that of the nucleus donor. “We were surprised to find that just two small differences in the mitochondrial DNA was enough to cause an immune reaction,” she said.
“We didn’t do the experiment in humans, but we assume the same sort of reaction could occur,” Schrepfer added.
Until recently, researchers were able to perform SCNT in many species, but not in humans. However, scientists at the Oregon Health and Science University announced the successful derivation of human embryonic stem cells from cloned, human embryos. This reignited interest in eventually using SCNT for human therapies. Although many stem cell researchers are focused on a different method of creating pluripotent stem cells, called induced pluripotent stem cells, some believe that there are some applications for which SCNT-derived pluripotent cells are better suited.
The immunological reactions reported in the new paper will be a consideration if clinicians ever use SCNT-derived stem cells in human therapy, but Weissman thinks that such reactions should not prevent their use. “This research informs us of the margin of safety that would be required if, in the distant future, we need to use SCNT to create pluripotent cells to produce the tissue stem cells to treat someone,” he said. “In that case, clinicians would likely be able to handle the immunological reaction using the immunosuppression methods that are currently available.” I find such a statement somewhat cavalier given that the nature of the immunological rejection might be robust enough to endanger the patient regardless of the anti-rejection drugs that are used.
In the future, scientists might also lessen the immune reaction by using eggs from someone who is genetically similar to the recipient, such as a mother or sister, Schrepfer added. Except that now you have added the dangers of egg retrieval to this treatment regimen, which not only greatly jacks up the price of this type of treatment, but now endangers another person just to treat this one patient. Add to that the fact that you are making a cloned human embryo (a very young person) for the sole purpose of dismembering it, and now you have added a degree of barbarism to this treatment as well.
So if we some SCNT-based treatments for patients we have an added danger for the patient (immunological rejection), danger for the egg donor, the homicide of the young embryo, and a prohibitively expensive procedure that no insurance company in their right mind would fund. I say we abandon this mode of treatment for the morally-bankrupt option that it is and pursue more ethical ways of treating patients.