The ability of stem cells to differentiate into a plethora of distinct adult cells has led to great excitement in both scientific literature and lay media. After years of research, however, tangible evidence for their promise remains elusive. Are stem cells a magic bullet for multiple spinal pathologies? Wellington Hsu (Evanston, USA) and Mark Erwin (Toronto, Canada) evaluate the science behind the hype.
One of the most exciting potential applications for stem cells is degenerative disc disease. Why would regeneration with stem cells be preferable to discectomy?
Wellington Hsu: Disc degeneration is probably the major clinical problem that spine surgeons see. Everybody has disc degeneration to varying levels and, when this causes low back pain, it can be debilitating. It also costs healthcare systems billions of dollars.
Despite having this problem, we really do not have an adequate solution for it.
The only surgical treatment for degenerative disc disease (DDD) is really to take the entire disc out and to either fuse or replace that segment. This is a relatively extreme and aggressive approach because we are not preserving the physiology of the disc or the biomechanics of that segment. If there were a solution that would provide the capacity to regenerate the disc—to restore what nature had intended—then in many authors’ opinions, the outcomes for these patients could be improved.
Mark Erwin: It is important to note that surgical treatment addresses the downstream effects of DDD including disc herniations, bony remodelling at the disc/vertebral interface in the form of osteochondral bars, as well as hypertrophic ligaments and facet disease. As such, surgical treatment can relieve compressed neural structures and can afford increased stability to spinal segments when necessary.
However, surgical treatments such as discectomy are not capable of addressing the fundamental problems of the degenerative process itself: loss of viable cells, degeneration of the extracellular matrix, and compromised biomechanical properties.
Progressive DDD results in the loss of extracellular matrix integrity within the intervertebral disc (IVD) nucleus pulposus (NP) that is accompanied by fissuring and tearing of the annulus fibrosus (AF). Another cardinal feature of DDD is the progressive loss of viable cells due to apoptosis, necrosis and autophagy, the sum total of which leads to a more acellular IVD NP with dysregulated homeostasis. As the IVD degenerates, there is a shift towards a pro-inflammatory and catabolic environment, leading to a positive feedback cycle of further degeneration and compromised IVD function.
An effective cell-based biologic therapy would need to mitigate the loss of viable cells and the pro-inflammatory/catabolic IVD milieu, and re-establish homeostatic regulation. Depending upon the degree of DDD, injectable agents might function in this way. The efficacy of such an approach, however, would be dependent upon native, non-senescent, functional cells that could in turn translate necessary proteins in order to positively impact the IVD environment.
In theory, cellular replacement could re-supply the IVD NP with de novo cells capable of integrating within the NP matrix and, via their secretory properties, suppress the pro-catabolic degenerative environment, synthesise new, healthy extracellular matrix proteins that could aid in the re-establishment of IVD homeostasis, and confer pro-survival effects upon resident NP cells.
Convincing data remains elusive
Stem cells may be the most appropriate type of cell for these purposes due to their inherent differentiation plasticity, anti-inflammatory properties and low immunogenicity. However convincing data supporting an “NP” destination phenotype remains elusive. A recent search of PubMed using “Stem cell therapy for degenerative disc disease” as a search term yielded 287 hits, demonstrating the keen interest in the scientific community.
Approximately 30 pre-clinical animal studies using stem cells to treat DDD have been published, with many demonstrating positive outcomes and others reporting no or worsened effects. Currently there are a number of human clinical trials underway using stem cells of various sources as potential treatments for DDD, however much remains to be determined in terms of the best choice of cell, route of administration, cell numbers, patient selection and more.
WH: Many different methods have been proposed to regenerate human discs. Stem cells seem to be a promising therapy, largely because this area is avascular. A lot of alternative treatments require a richly vascular environment; they need a lot of blood vessels to supply the body with the necessary components to regenerate tissue.
In this kind of biological environment, stem cells may have the potential to function because they can theoretically proliferate themselves. They can help produce a micro-environment without a vascular supply that may allow the body to regenerate that tissue.
An autologous treatment could also reduce some of the concerns with infection, cancer and other complications that could arise from stem cell therapy.
By what mechanisms could stem cells work to help regenerate discs?
WH: There are really two main mechanisms by which we see stem cells working—not only for disc regeneration, but also with other applications.
So, the first would be the fact that the cells proliferate and then they differentiate into the type of cell that would be required to produce tissue. In this case that would be a chondrogenic or cartilage-type cell that you want the stem cells to evolve to, and then, once this happens, they can produce the cartilage/disc tissue. This is a similar type of mechanism that has been proposed for bone regeneration as well.
The other—which has received more attention recently—is that the cells create a micro-environment that allows the body to regenerate the disc tissue itself. So, what I mean by this is that the cells that are delivered no longer become the cartilage or the disc-producing cell. Instead they remain stem cells. These cells then secrete growth factors and matrices, and the building blocks that the body can use to regenerate the tissue in the area.
This mechanism appears to be more feasible, largely because there are many steps that are required for a stem cell to become a primary cell in a foreign environment. By providing the building blocks necessary, these stem cells can ultimately lead to tissue regeneration.
Do you think that there is promising evidence in the research to back up this second mechanism?
WH: Right now we have a number of ongoing clinical trials in the USA looking at stem cells to regenerate discs. So, if these trials are showing safety and efficacy and improvement in patients with back pain, then yeah—I think we will have this treatment available to humans soon.
As it applies to the evidence now, there is much basic and clinical work that is ongoing to delineate the pertinent pathways. We hope that future evidence will help us to determine that.
How far away do you think we are from clinical applications of stem cells for regeneration?
WH: There is a product currently in phase 3 clinical trials at the US Food and Drug Administration (FDA) called Mesoblast.1 If the results from that study are favourable, then we could have stem cells available for the treatment of degenerative disc disease very soon. To my knowledge, of all cellular based products, this trial is the closest to being completed that may provide the spine community the evidence needed.
But, if that trial fails to show any significant improvement compared to placebo, then it could be five, ten or fifteen years before we see another product come about. But, I believe we are close to seeing commercially available products for disc regeneration, because of the progress of the products that have been studied in clinical trials.
ME: Although there are a number of human clinical trials currently underway, stem cell therapy as a treatment for human DDD must be considered experimental. The fate of transplanted mesenchymal stem cells (such as those of bone marrow origin) post-transplant is largely unknown. Do they survive? If so, for how long? What is their post-transplant differentiation phenotype? There is no consensus with respect to the number of cells to be transplanted.
Mechanisms of action remain controversial
One Mesoblast trial2 has investigated the efficacy of transplanting 6 or 18×106 cells/disc, and another European trial has transplanted up to 23×106 cells/disc. Other trials havetransplanted up to 30×106 cells/disc. Initial Mesoblast results suggest that the 6×106 dose was preferable, but clearly these are orders of magnitude greater than the numbers of cells that normally reside within a human disc. Normal levels vary according to degree of DDD, but number in the hundreds of thousands; a far cry from 20×106 cells!
Furthermore, the mechanism(s) of action remain controversial.
Do the cells actually integrate within the IVD NP milieu? Do they secrete anti-degenerative molecules and act upon remaining viable cells? Could such paracrine mechanisms have any effect in the case of senescent cells? How long do transplanted cells survive (if they do) and is it sufficient to transplant cells alone?
Additionally, in the more advanced degenerative disc, it is considered that degeneration/calcification of the endplates compromises diffusion into and out of the IVD NP. In this situation, how will the cells survive within such a hostile environment? And, which patient is the ideal candidate for transplantation?
Although many questions remain to be answered, there is considerable attraction to the notion of cellular replacement therapy.
Spotlight on notochordal cells
By Mark Erwin
Our lab has been investigating notochordal cells for over a decade. These remarkable cells are the building blocks of the intervertebral disc and are well known to persist within the discs of animals that are resistant to developing DDD, such as non-chondrodystrophic pigs and rabbits. These cells also richly populate the IVD NP in humans, but are lost by late childhood/early adolescence with their loss interestingly associated with early onset of DDD.
The processes governing the survival of notochordal cells are currently unknown—such as why they persist in the discs of some animals and not others (such as humans). Interestingly, cells displaying stemness markers are present within mature human IVD NP (as well as other animal species) even when the large, physaliferous notochordal cells are absent. This raises the question, do notochordal cells simply die over time or do they transdifferentiate into chondrocyte-like cells with maturity? Is there some as-of-yet unknown connection between notochordal cells and stem cells and if so what is it and what is the significance?
Our team and others have investigated the effects of molecules secreted by notochordal cells and the effects of these secreted molecules upon other NP cells. We have recently reported that notochordal cell conditioned medium (NCCM) suppresses NP cell death in humans,4 and have validated that canine NCCM injected into the degenerative disc mediates disc degeneration and confers a regenerative effect.
Furthermore, we recently identified the necessary and sufficient factors secreted by non-chondrodystrophic canine IVD NP-derived notochordal cells and have demonstrated that a single injection of recombinant forms of these factors into the degenerative disc can mediate DDD and confer a regenerative effect.5 I do not think that the use of notochordal cells themselves is of practical concern with respect to the treatment of DDD, however understanding their contribution to IVD homeostasis and being able to deliver the important factors that they secrete may well lead to a novel, molecular therapy to treat DDD.
An important next step would be to examine the use of recombinant versions of the important molecules secreted by notochordal cells in a suitable large animal model of DDD. Upon successful completion of such an expensive and complex trial comes the costly regulatory approval process leading to a human clinical trial. Depending upon a number of factors, this could be in the works within three to five years.
In addition to their biochemical/molecular role, some investigators hypothesise that notochordal cells may contribute to load-bearing. To this end we have determined that the notochordal cell-rich IVD NP displays superior biomechanical and biochemical properties as compared to notochordal deficient IVDs in a way that has similarities to the human condition,6 however much remains to be determined in this area. What is certain is that Mother Nature continues to jealously guard the precise role(s) played by these fascinating cells and their role in development, maturation, and maintenance of the IVD.
What other applications might there be for stem cells in spine?
ME: There are basically two areas where stem cells have been evaluated; spinal cord pathology and DDD. There is considerable interest in the use of stem cells to treat disorders such as amyotrophic lateral sclerosis (ALS/motor neurone disease) and spinal cord injury with some clinical trials completed or underway. Unfortunately, no significant breakthroughs have yet to occur although some safety studies have been completed.
WH: There is at least one ongoing spinal cord injury trial in the USA with the use of embryonic stem cells. This is a lot further away than the disc regeneration studies because we have to establish safety before efficacy. But, applying stem cells in an acutely-injured spinal cord may provide a reduction of secondary injury and may lead to better motor outcomes after treatment. These trials are in phase one, which means stem cell treatment for spinal cord injury is a long way off yet.
Many questions about the future of stem cells remain unanswered
ME: There are 15 spinal cord pathology studies listed on www.clinicaltrials.gov with only two studies completed, four recruiting, four status-unknown, one suspended, two active-not-recruiting, one withdrawn and one terminated. Although some preliminary information has been obtained, much remains to be determined with respect to the best method of stem cell delivery, source of stem cell, numbers of cells to be delivered and the optimal patient to receive such therapy. These considerations are common to all potential spine-related stem cell applications.
The “elephant in the room” questions, so to speak, require a reasonable understanding of what transplanted cells actually do, what they become, if and how they integrate within the extracellular matrix of the transplant site, and how long they survive once transplanted. Many of these questions remain unanswered.
WH: A promising arena for stem cell therapy would be bone formation, or spinal fusion. There are commercially-available products on the market that are cell-based and that are being used for that purpose.
Cells do have bone-forming potential, and we have more studies in that arena for spine surgery than we do for disc regeneration, since we already have products on the market.
What barriers are there to stem cell research in spine?
ME: The notion of using stem cells to treat spinal maladies is not new, however the field has not made significant progress in large part due to the complexity of cellular replacement in general and, specifically, the challenges of addressing spinal pathologies.
WH: The patient population that suffers from disc degeneration can be a barrier to research. A lot of patients may have low back pain for reasons other than their degenerated discs.
If you do not have the right patient selection in these clinical trials, this can confound the data. If you accept somebody into a trial that has low back pain which is not really coming from their degenerative disc disease, but is, for example, coming from the surrounding soft tissues, then to regenerate the disc will not necessarily help that person. Because there is no single diagnostic test that can definitely locate the pain generator, patient selection is a significant barrier for a clinical trial to prove superiority.
In addition, the fact that these stem cells like to become many different types of cells—not just cartilage-producing cells but also bone-producing cells and muscle-producing cells—that is why they are called mesenchymal cells. Trying to block the body’s response, or this cell’s propensity to developing an alternative pathway, is perhaps more cumbersome and more difficult than leading a cell down the proper pathway. So I would say that, mechanistically, that is probably the second biggest barrier.
When do you think we might start seeing validated stem cell treatments used in clinical practice?
ME: With respect to the use of stem cells in clinical practice, the important word is “validation”. We are a long way away from a validated stem cell therapy for the treatment of DDD due to the reasons discussed earlier. This holds true for most potential stem cell therapies in spine and, although much has been learned, much more remains to be understood before any validation could be contemplated. Stem cell therapy must, at this point, remain experimental.
Barriers to stem cell transplants over and above the scientific hurdles that remain certainly must include regulatory concerns, since it is likely that the stem cells would require some kind of “manipulation”—that could include simply expanding the cell numbers. Risks of neoplasia and other adverse effects need to be thoroughly studied, as well as safety for each putative method of transplant.
Furthermore, these studies are tremendously expensive creating a financial hurdle that—in addition to regulatory challenges—raises the bar on entry into this exciting, but challenging arena.
What do you think the future holds for stem cells in spinal surgery? Will we use them commonly over the next decade?
WH: I certainly hope so. It is not like we just started studying stem cells. We have been investigating this arena for spinal surgery very intensively for the last 15–20 years, and there are still only limited applications at this point. I am hopeful that with more research, we will be able to better focus on a disease process in the delivery of stem cells to the proper patient.
I certainly do not think that stem cells are the panacea—that they could just be applied right away no matter what the condition. I do not foresee that happening. But, I do see a focused application of stem cells that will greatly benefit patients in the future. It will just take us a little while to get there because it is such a complex process.
“I have not failed, I’ve just found 10,000 ways that won’t work”
ME: There is considerable interest and clinical need for cellular replacement to treat spinal maladies such as spinal cord injury, DDD, ALS and others. However the rush to clinical use must be tempered with a sound scientific understanding and rationale. Therefore, it is vital to carefully evaluate potential novel therapeutic interventions.
The emergence of stem cell tourism and lack of regulation in these areas has seen the rise of very serious adverse events including neoplasia in the case of stem cell therapies for spinal cord injury. History is replete with examples of unfavourable outcomes in medicine that are a consequence of a rush to implement new treatments. To this end, Thomas Edison’s famous quote continues to be relevant today; “I have not failed, I’ve just found 10,000 ways that won’t work”.
Wellington Hsu is an orthopaedic surgeon at Northwestern University, Chicago, USA. Mark Erwin is a researcher at the University of Toronto, Toronto, Canada
1. https://clinicaltrials.gov (Identification No. NCT02412735)
2. https://clinicaltrials.gov (Identification No. NCT01290367)
3. Berkowitz, et al. New England Journal of Medicine 2016; 375:196-198
4. Mehrkens, et al. The Spine Journal 2017; 17(4): 579-588
5. Matta, et al. Scientific Reports 2017; 7: 45623 (doi: 10.1038/srep45623)
6. Erwin, et al. Arthritis Research & Therapy 2015; 17:240