Darwin J. Prockop, M.D., Ph.D.: “It’s taken a long time for us to understand how the cells are actually doing their beneficial effects. Now I think we are onto it.”
Research and regenerative medicine—from paradigm shifts and ethics to scientific advances—was reviewed Monday during “Scientific Breakthroughs of the Year: Stem Cells and Regenerative Medicine.”
Two keynote addresses served as bookends for the session. Renowned researcher Darwin J. Prockop, M.D., Ph.D., opened the session with a keynote address about what researchers have learned about how stem cells act. Bioethics expert Arthur Caplan, Ph.D., closed the session with an address about ethical dilemmas in regenerative medicine. In other presentations, four researchers reviewed important recent discoveries.
Changing Paradigms
The recognition that mesenchymal stem cells (MSCs) do not always replace injured tissue but can respond to signals from injured tissue to reduce excessive inflammatory and immune reactions was addressed by Dr. Prockop in “Changing Paradigms for Therapy with Adult Stem/Progenitor Cells.”
“The new paradigm is that the cells go to injured tissues, they respond to signals from those injured tissues and begin to put out a series of paracrine factors that either decrease the damage by suppressing inflammation and immune reactions, or stimulate regeneration of progenerative cells in that tissue,” said Dr. Prockop, who is professor and director of the Institute for Regenerative Medicine at Texas A&M College of Medicine Health Science Center. “It’s taken a long time for us to understand how the cells are actually doing their beneficial effects. Now, I think we are onto it.”
Research using animal models has shown that the body reacts to injury as if the injury is always accompanied by micro-organisms, he said. An excessive reaction damages tissue, but MSCs turn off the excessive inflammatory response.
“Somehow, the cells are modulating the immune system in a way that is still a bit mysterious, but the beneficial effects are clearly there,” Dr. Prockop said. “We are at the stage now of really becoming aware of how the cells work, and, in some cases, we’ve found the proteins they produce, which themselves can be used for therapy. In some models, these proteins are having marvelous effects.”
The next step for researchers may be to choose disease targets that might be treated more effectively with MSCs. Asthma and fibrotic conditions of the lung appear to be among the most promising potential uses.
“The damage to the lung does not involve any micro-organism,” Dr. Prockop said of these two conditions. “Asthma is largely an excessive immune response and x-ray radiation is in part excessive inflammation to tissue damage, so these look like very prime areas right now.”
Ethical Dilemmas
While many in medicine and science focus on the benefits of regenerative medicine, those advances have raised some ethical dilemmas, which were addressed by Arthur Caplan, Ph.D., director of the Center for Bioethics at the University of Pennsylvania.
“Partly, my job is to alert people at this meeting that even though everyone is all excited about stem cells and regenerative medicine, there are concerns,” Dr. Caplan said, adding that there are five main objections:
The world will become aged, at a great cost to society;
Young people will not be able to advance because they will be blocked by older, less vital people;
It is not “natural to live beyond the four-score, four-year average lifespan;”
Enhancement medicine will follow regenerative medicine, which is a “slippery slope;” and
People may lose their personal identity, doing more harm than good.
“Some of these are not particularly powerful arguments,” Dr. Caplan said. “The idea that we are going to make decrepit old people is a bad idea, and I don’t think many people would sign up for that. You have a situation where you want to have vital aging.”
“You may have to do some social engineering, and you have to be fair and give younger people an opportunity, but I’m not sure that’s a principled reason why you should not try to repair damaged cells, replace heart cells or rebuild islet cells for diabetics,” he continued.
The most legitimate concern, Dr. Caplan said, is the effect of repairs to the brain.
“I think that does have some traction. If you repaired the brain, but did not keep people’s memories intact, you would have the problem of saving the body, but losing the person,” he explained.
The Importance of Creating iPS Cells
The second presentation, “The Generation of Lung-Disease Specific Cystic Fibrosis iPS Cells Free of Any Exogenous Reprogramming Transgenes,” reviewed the process of creating induced pluripotent stem cells (iPS) cells that could eventually be used to study cystic fibrosis, alpha 1-antitrypsin deficiency and other lung diseases.
Aba Somers, M.D., a pulmonary fellow at Boston University Medical Center, said the principle of developing lung-disease specific iPS stem cells has never been published before in humans.
“We are taking somatic cells, like skin biopsies, and reprogramming them with a lentivirus into cells that resemble embryonic stem cells,” Dr. Somers said. “What is unique about our study is that we have done it for patients with inheritable lung diseases, such as cystic fibrosis or alpha 1-antitrypsin.”
Inside the virus is a reprogramming vector that contains a special part of DNA so that after researchers reprogram these cells into iPS cells, they can remove the reprogramming vector.
“The whole idea is to use these cells eventually for clinical trials,” Dr. Somers said. “Leaving that reprogramming vector in would be dangerous if we would want to use them for future clinical trials. We are able to remove that reprogramming vector to produce clinical-grade iPS cells.”
The next step in the process would be gene correction, he said.
A Path to Modulating Mucous
Modulating the Notch pathway in the airway epithelium suppresses the formation of mucous cells, which are often associated with airway diseases such as cystic fibrosis, asthma and bronchitis.
“We found a developmental signal in the embryo, a Notch signal that results in the formation of excess mucous cells and inhibits the formation of ciliated cells in the airway epithelium,” said Jayaraj Rajagopal, M.D., an investigator at the Center for Regenerative Medicine at Massachusetts General Hospital, who presented “Modulating Stem Cells Using Development Pathways to Identify Novel Therapies for Lung Disease: The Notch Pathway Blocks Immune-Mediated Airway Mucous Metaplasia.”
The aberrant distribution of cell types discovered in the embryo is very similar to a dominant pathology in airways disease—mucous metaplasia. This suggests that if a Notch signaling pathway could be blocked at the epithelial developmental level, then the number of mucous cells, and therefore mucous, would be decreased, he said.
“If we can modulate developmental molecules that we think are important for the behavior of stem cells in airway epithelium, then we might be able to modulate aberrantly behaving cells in many respiratory diseases,” Dr. Rajagopal said. “It’s an example of how we can add a developmental paradigm to our scientific armamentarium.”
Promoting Vascular Remodeling
The successful use of a receptor tyrosine kinase inhibitor, imatinib mesylate, to inhibit and reverse pulmonary arterial vascular remodeling in chronic hypoxia-induced pulmonary hypertensive disease was discussed in the presentation of “Imatinib Attenuates and Reverses Bone-Marrow-Derived Progenitor Cell Contribution to Hypoxia-Induced Pulmonary Arterial Hypertension and Remodeling.”
In the study, investigators wanted to determine if imatinib mesylate, which has anti-platelet-derived growth factor receptor antagonist properties, also inhibited the accumulation of stem cell factor receptor positive (SCF+) c-Kit+ cells in the remodeled pulmonary arteries. SCF+ c-Kit+ cells are a generally accepted marker for bone marrow-derived progenitor cells and are considered to be predominantly from a bone marrow-derived circulating source, said Joseph T. Crossno Jr., M.D., Ph.D., assistant professor of medicine, University of Colorado Denver, School of Medicine.
There have been numerous reports of circulating progenitor cell involvement and contribution to both reparative and pathogenic vascular remodeling in pulmonary hypertensive disease models. In addition, imatinib mesylate is an inhibitor of c-Kit, he said.
By using a bone marrow-transplanted chimeric mouse model and inducing pulmonary hypertension using hyperbaric hypoxia, imatinib inhibited and reversed previously established pulmonary arterial remodeling, even in the presence of persistent hypoxia, Dr. Crossno said.
“What we found was that while it did decrease the inflammatory cell influx, most of the c-Kit-positive cells were actually resident from a vessel wall source, not a circulating source,” he said.
“The importance of this study is that there should be a two-pronged approach toward treatment of pulmonary hypertensive vascular disease. One is basically inhibiting vasoconstriction,” Dr. Crossno said. “However, what has become more recognized is that in addition to inhibiting vasoconstriction, if you could also inhibit or reverse the vascular remodeling and the vessel-wall thickening, then perhaps you would have a better clinical outcome in patients with pulmonary hypertension. Thus, you need a two-pronged approach—anti-vasoconstriction as well as anti-proliferative or anti-remodeling therapy to produce a better long-term clinical outcome.”
Using MSCs as a Therapy in Lung Injury
Jae-Woo Lee, M.D., associate professor in the department of anesthesiology at the University of California, San Francisco, discussed research into the use of allogenic human mesenchymal stem cells as a form of therapy in lung injury.
The presentation “Intravenous Allogenic Human Mesenchymal Stem Cells Home to the Site of Injury and Restore Alveolar Fluid Clearance to a Normal Level in an Ex Vivo Perfused Human Lung Injured by E. Coli Endotoxin” reviewed the development of an acute lung injury model in an ex vivo perfused human lung preparation from donor lungs that are not used for transplantation.
“The data demonstrate that these allogenic mesenchymal stem cells can actually improve some function after acute lung injury, such as alveolar fluid clearance, which is the ability of the lungs to clear edema fluid,” Dr. Lee said.
“We also suggest that these allogenic mesenchymal stem cells, given intravenously, will home to the site of injury. For any further clinical trial, these cells may be more appropriate to give intravenously,” he said.
He added that this research follows similar work done by Michael Matthay, M.D., a professor of medicine and anesthesia at the University of California, San Francisco.