The question of adult stem cells differentiating into needed tissue is not based on opinion but on over 2 decades of solid peer reviewed research at international academic institutions. To state there is controversy regarding the ability of adult stem cells to therapeutically augment natural healing processes, is like saying there is controversy whether red blood cells transport oxygen.
There is no question as to the scientific rationale of therapeutic benefits. The only question is the extent of the benefits and the selection of patients for undergoing stem cell therapy.
In order to understand these issues, we must first begin by understand a little bit about stem cells. We will begin with the bone marrow stem cell.
1. What is the bone marrow stem cell?
Traditionally, the bone marrow was viewed as the source of all blood cells, being responsible for production of trillions of cells per hour. Essentially one type of cell in the bone marrow, called the hematopoietic (“hematopoietic” means “blood making”) stem cells, possesses the unique ability to make copies of itself, but also, depending on the needs of the body, to make other blood cell types as well. At a molecular level we know that the human bone marrow hematopoietic stem cells expresses the markers CD34 and does not express markers of lineage commitment such as HLA-DR, CD38, CD11, CD31, etc . This essentially means that bone marrow hematopoietic stem cells can be isolated and studied as a discrete entity.
The bone marrow hematopoietic stem cell adapts to the needs of the body and accordingly produces specific cell types when the body needs them. For example, when a person goes up to live on the mountains, the person’s body needs more red blood cells than usual since there is less oxygen at high altitudes. In order to compensate for this, the kidney starts to produce more erythropoietin (a hormone that travels throughout the body), which instructs the hematopoietic stem cell to produce more blood cells . The same occurs in situations of infections in which the body needs more white blood cells (such as neutrophils) to protect itself against the external pathogens. In response to various molecular signals (G-CSF, GM-CSF) generated by the immune system of the person, the bone marrow hematopoietic stem cell starts to generate more white blood cells .
In addition to containing the cells that make blood, the bone marrow contains cells called “stromal cells” that support the hematopoietic stem cells. Essentially, stromal cells are comprised of various actual cell populations The stroma includes adipocytes, osteoblasts, and mesenchymal stem cells . The primary function of mesenchymal stem cells in the bone marrow is to control proliferation of the hematopoietic stem cell, through provided growth factor support. This is why certain scientists are using mesenchymal stem cells to accelerate blood formation after administration of hematopoietic stem cells in patients .
Bottom line: There are two main types of stem cells found in the bone marrow: 1) hematopoietic stem cells that make blood and 2) mesenchymal stem cells that provide support for the hematopoietic stem cell.
2. Bone Marrow Transplantation: The first application of stem cell therapy.
Stem cell therapy is not new. Ever since the discovery of the hematopoietic stem cell by Drs Till and McCulloch in the 1960s, the use of these cells for transplantation into patients with defective bone marrow was envisioned. The first hematopoietic stem cell transplant, or bone marrow transplant, was performed in 1956 by Dr. E. Donnall Thomas using bone marrow cells isolated from an identical twin donor for a recipient who had leukemia. The idea was that if the patient was irradiated with high doses, then the radiation would kill all of the leukemia cells. Unfortunately, the radiation would also destroy the healthy bone marrow stem cells. So the idea was to utilize donor bone marrow to replenish the recipient with healthy hematopoietic stem cells. Dr. Thomas, along with Joseph E. Murray, won the Nobel Prize in 1990 for this discovery. The usefulness of transplanting the hematopoietic system was not limited to leukemias, in 1968, bone marrow transplantation was performed successfully on a patient with a genetic mutation called severe combined immunodeficiency. In this disease the bone marrow stem cells are deficient in ability to generate T and B cells, as a result the patient is immune compromised and is forced to live in a sterile environment. The administration of healthy bone marrow cells resulted in the child being able to function normally as a result of a non-mutated hematopoietic stem cell that is capable of making T and B cells.
Since the initial bone marrow transplant procedure was developed, several hundred thousands procedures have been performed, literally giving a new lease on life to many patients whose diseases were previously considered lethal. Variations on the theme of bone marrow transplantation have also been performed in order to increase efficacy. For example, patients with some leukemias are known to have a higher probability of relapse (leukemia coming back) after the transplant. In order to fight off the relapse, clinicians have started infusing lymphocytes from the donor as a type of cellular therapy. These Donor Lymphocyte Infusions are currently part of the accepted medical practice for treatment of post-transplant relapsed CML. Additional modifications to the transplant procedure have included the use of G-CSF mobilized donor stem cells. Instead of performing puncture of the iliac crest for bone marrow aspiration, a gentler procedure that was developed involved “telling the bone marrow stem cells” to leave the bone marrow and enter systemic circulation, by the administration of the drug G-CSF. The stem cells are collected from the blood using a procedure called leukopheresis . Another advancement in the area of hematopoietic stem cell transplantation has been the use of cord blood as a source of blood-forming stem cells. Cord blood stem cells express higher regenerative potential on a per-cell basis compared to bone marrow stem cells. Although cord blood stem cells are generally safer in the sense that they do not evoke graft versus host (a side effect of transplantation) with the same severity as bone marrow stem cells, cord blood stem cells are available in small numbers and therefore their use in adults is still limited to certain patient subgroups .
Conclusion: Stem cell therapy in the sense of hematopoietic stem cell transplantation, has been occurring since 1956 for treatment of disturbances of the blood making components of patients. These disturbances can be caused by genetic abnormalities (ie severe combined immunodeficiency, sickle cell anemia, etc) or induced as a side effect of treatment (ie dose radiation for clearing leukemia). The hematopoietic-reconstituting stem cell therapy should not be confused with stem cell therapy for regenerative medicine. In regenerative medicine the stem cells are administered in absence of the destruction of the recipient’s bone marrow hematopoietic compartment.
3. Bone Marrow and Regeneration: The Bone Marrow Cells Can Become Different Cells besides Just Blood Cells
As we described above, transfer of bone marrow stem cells has been performed for decades. Scientists have wondered, if the bone marrow stem cell possesses the potential to differentiate into all the different types of blood cells, maybe it can also differentiate into other cells as well. This process was originally termed “transdifferentiation”. The first report of transdifferentiation to appear in the major medical literature was a paper by Orlic et al. , in which mouse bone marrow derived stem cells were injected into mice that were given an experimental heart attack. The interesting thing about this experiment was that the bone marrow stem cells used were labeled to glow green. The donor animals were genetically engineered to express the green fluorescent protein (GFP) gene throughout their bodies. This essentially means that all cells derived from the GFP donor mice were green. Additionally, the experimenters purified the mouse equivalent of the human CD34 bone marrow hematopoietic stem cell. The molecular markers used where positivity for stem cell antigen (SCA-1) and negativity for the lineage markers (lin negative). Following induction of a heart attack by ligation of one of the coronary arteries, the researchers implanted the cells in the area of infarct. The mice which received implanted hematopoietic stem cells, but not control cells, had increased pumping ability of the heart and decreased levels of heart damage. Most interestingly, when mice where sacrificed, green cells were observed throughout the area of damage.
This paper served as a strong indication in animals that bone marrow derived cells are capable of differentiating into heart tissue and helping to repair injury. Eventually scientists started finding that bone marrow stem cells can differentiate into other tissues. For example, human bone marrow derived CD34 cells have been demonstrated to differentiate into cells expressing liver markers, and can actually generate human liver proteins when injected into animals .
The concept of bone marrow cells differentiating into cells other than hematopoietic cells has subsequently been demonstrated in numerous laboratories for many types of tissues. The table below summarizes some of the tissues. We will include both hematopoietic and mesenchymal stem cells in the analysis since both are found in the bone marrow. It is important to make a note that some researchers believe embryonic stem cells are the only cell types capable of differentiation into different tissues. While it is true that embryonic stem cells indeed are “totipotent”, these cells are very far from clinical use. Injection of embryonic stem cells into animals causes a type of aggressive tumor called “teratomas” , and furthermore, the differentiation of these cells is difficult to control. In other words if you want to generate livers cells from embryonic stem cells, and you add liver-induction factors, such as hepatocyte growth factor, some cells become liver, but other cells still become neurons, kidney cells, and skin cells, thus until science advances, adult stem cells still seem to be superior for clinical applications.
4. Importance of Bone Marrow Resident Stem Cells for Natural Repair of Tissue Injury
The examples above, as well as the published literature, demonstrate in an unequivocal manner that bone marrow derived cells have a certain amount of plasticity, or ability or ability to differentiate into non-hematopoietic tissues.
The main idea being that bone marrow can differentiate into a variety of tissues. Now why would the bone marrow have this ability? One theory is that the bone marrow acts as a reserve of “regenerative cells” that are important in healing the body as the body ages. The ability to regenerate is most potently seen in the salamander, which can regenerate whole limbs, based on potent stem cell activity. Although humans do not have regenerative activity that potent, there are still evidences that bone marrow stem cells do in fact contribute to regeneration. First we will provide an in vivo example of bone marrow “transdifferentiating” into other tissues in the human, and then we will provide examples of bone marrow helping in healing processes.
As we discussed above, bone marrow transplantation is performed for hematological disorders, one of which is leukemia. Now if a female receives a male bone marrow transplant, one would imagine that the circulating blood cells of the female have the Y chromosome, which is correct. What is more interesting is whether tissues in the female’s body actually start to express the Y chromosome. In a very interesting study , researchers performed autopsy on female patients who had received male bone marrow transplants. When they examined the heart tissue, the female heart tissue contained cardiac muscles that had the Y-chromosome !! This conclusively demonstrates that in normal situations bone marrow derived cells differentiate to become different parts of the body. Similar findings were also reported in liver, pancreas, and kidney.
Now what about in situations of heart attacks? One would imagine that during a heart attack, bone marrow derived stem cells would migrate into the heart. If this is indeed the case, then taking blood samples of patients after heart attacks should demonstrate increases in stem cells. Indeed numerous studies have shown this to be the case. Below is an example from one study .
A similar argument can be made in stroke. Interestingly, in stroke, there has been reported an association between higher degree of mobilization and improved recovery as assessed by the NIHSS score .
From the examples presented above, as well as numerous other publications, endogenous bone marrow stem cells have been demonstrated to play a regenerative role in humans. The next question is whether the administration of bone marrow stem cell therapy can actually induce a clinical effect?
5. Clinical Examples of Bone Marrow Stem Cell Therapy for Regeneration
Bone marrow stem cell therapy for regenerative (non-hematopoietic) purposes originally started with Japanese research when bone marrow cells were injected into the heart muscle of patients undergoing bypass surgery. The idea was that the injected bone marrow cells will stimulate production of new blood vessels and thereby increase oxygenation to the heart . The procedure, although highly invasive, was associated with no treatment related adverse effects and 3 out of the 5 patients had increased blood vessel production as assessed radiologically, as well as improved cardiac function. This first demonstration in 2001, was repeated by numerous investigators. In 2003, the study was repeated using CD133 purified bone marrow stem cells and published in the prestigious journal Lancet , reporting positive results. Subsequently numerous studies have been conducted in the area of cardiology demonstrating that administration of a patient’s own bone marrow is associated with positive outcome. For example, there are published pictures of myocardial activity before and after stem cell therapy from a clinical study .
Convincingly, statistically significant improvements in left ventricular ejection fraction have been observed in double blind trials. For example there is published from a 200 patient trial .
In addition to the heart, bone marrow stem cells have been used for numerous other indications clinically. One interesting indication is critical limb ischemia, which involves occlusion of blood flow to the lower limbs and is associated with need for amputation. Below is a representative angiogram from a patient before and after bone marrow stem cell therapy as part of a double blind trial . Additionally, in the same trial, clinical endpoints such as ankle brachial index (how much blood flows to the leg), pain-free walking, and transcutanous oxygen where all increased in a statistically significant manner.
Bone marrow stem cells have also been demonstrated to be effective in regeneration of other damaged/degenerated organs. In a clinical trial with 9 patients suffering from liver failure , the administration of their own bone marrow cells intravenously elicited restoration in albumin production as seen in the figure below. Each line in the figure represents one patient. As well as overall decrease in clinical severity of disease (Child Pugh Score).
6. Bone Marrow Derived Mesenchymal Stem Cells
The clinical examples above were aimed at fresh, non-cultured bone marrow derived stem cells. Since it is difficult to enforce patents on medical procedures, numerous companies have developed “universal donor” off the shelf, stem cells that have been cultured in vitro and can be sold as “medicines”. Of the companies activity in this area (Neuronyx, www.neuronyx.com in Phase I clinical trials, Pluristem, www.pluristem.com in late preclinical, and Osiris Therapeutics, www.osiris.com, 2 phase IIIs, several phase II trials), Osiris therapeutics has been the clear-cut leader, anticipated to have a product on the US market in the near future. Below is a summary data of Osiris’s bone marrow mesenchymal stem cells used to treat Crohn’s Disease. Survival figures in patients treated with mesenchymal stem cells for a lethal inflammatory disease called are well published in the case of GVHD .
7. Conclusion: Bone Marrow Stem Cell Therapy…Enhancing the Body’s Regenerative Potential
Dr. Thomas would be proud. From the initial bone marrow “cell therapy” transplant in 1956, the use of bone marrow stem cells has expanded to cover almost any indication one could imagine. One only has to look at the www.clinicaltrials.gov database to see that clinical applications of bone marrow cells are being tested in conditions ranging from heart disease, to autoimmunity, to neurological conditions.
The wide applicability of bone marrow stem cells to so many diseases comes from the fact that stem cell therapy is only an augmentation of the natural regenerative processes. It is established that injured or damaged tissue releases distinct factors that “call in” bone marrow stem cells. By administering the cells in high concentrations intravenously, or locally, the bone marrow stem cell therapist is only helping the body to do what it is trying to do…to heal itself.
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