Cell therapy involves transplanting cells to restore tissue or organ function. The objective is to offer the patient long-term treatment through a single injection of therapeutic cells. These cells are obtained from pluripotent stem cells (able to differentiated into all cell types) or multipotent cells (able to yield a limited number of cell types) taken from the patient him/herself or a donor. Numerous approaches to cell therapy are currently in development. Moreover, some have already been validated.
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Understanding the issues surrounding cell therapy
Different sorts of stem cells
Several sorts of stem cells are used to yield differentiated and functional cells suited to cell therapy. These different cell types nonetheless share two properties: they have the power to self-renew indefinitely, offering an unlimited supply of material, and can yield several cell types.
Pluripotent stem cells
Pluripotent stem cells can yield all types of cells of the body. This may concern:
- embryonic stem cells taken from embryos having developed for 5 to 7 days,
- induced pluripotent stem cells (iPSC) taken from adults and reprogrammed into pluripotent cells by genetic engineering.
Researchers now know how to obtain the differentiation of pluripotent cells into several cell types, such as retinal or skin cells. Each cell type is obtained owing to a cocktail of specific differentiation and growth factors, the fruit of complex and time-consuming development. This essential cocktail has not yet been discovered for certain cell types, such as skeletal muscle cells.
Multipotent stem cells
Cell therapy can also be performed using multipotent stem cells which are able to differentiate into a limited number of cell types.
Mesenchymal stem cells, found in the whole body, in adipose tissue, bone marrow, tissue supporting the organs, and also in bone, cartilage and muscle, etc., are the most widely used. These stem cells are particularly easy to extract from adipose tissue or bone marrow. These can generate cartilaginous cells (chondrocytes), bone cells (osteoblasts), fat cells (adipocytes), muscle fibers (myocytes), cardiomyocytes, etc. They also secrete growth factors conducive to the surrounding cells, and are sometimes exclusively used for this property. They also produce anti-inflammatory factors which cause local immunosuppression and promote immune regulatory cell function. These properties limit local inflammation and, a priori, offer protection against graft rejection.
Other multipotent cells may be used in cell therapy, such as skin stem cells. The latter have been used since the 1980s to reconstruct the different layers of the epidermis in severe burns victims. Eye stem cells, taken from the limbus (on the periphery of the corneum), are able to repair corneal lesions. Lastly, hematopoietic stem cells taken from bone marrow are the source of all blood cells: in the event of blood cancer, these can be used to rebuild healthy blood cell supplies in the patient, after destroying diseased cells by chemotherapy. This procedure has been performed since the 1970s.
Cord blood, another valuable source of stem cells
Umbilical cord blood contains hematopoietic stem cells naïve in terms of immunity and therefore very well tolerated following transplantation. Cord blood is used to treat malignant blood diseases, such as leukemia or lymphomas, and also genetic diseases, such as Fanconi anemia. It offers a serious alternative to bone marrow transplantation if a compatible donor cannot be found. However, the number of therapeutic cells recovered per cord is limited.
Storage of placental blood is only authorized in France to treat other patients, anonymously and free of charge. The French Placental Blood Network (RFSP) coordinates the collection and storage of cord blood, via a network of partner maternity units covering more than a quarter of births in France. Women agreeing to donate this blood product when their child is born, do so in an altruistic manner to help patients not known to them, suffering from fatal blood diseases. Mothers can give their consent from the fourth month of pregnancy, if they are eligible. Collection takes place within a few minutes of delivery, when the umbilical cord has just been cut and the placenta is still in the uterus. The blood is then frozen and stored in a bank for subsequent use.
Obtaining therapeutic cells
The indication for cell therapy usually defines the choice of stem cells to be used. Hence, embryonic stem cells differentiate spontaneously into retinal cells and are therefore particularly suited to the development of treatments for diseases affecting this organ. Mesenchymal cells, which are able to yield cartilage cells, are the more spontaneous choice for the treatment of osteoarthritis.
Pharmaceutical companies hold a number of stem cell lines and work with the teams aiming to develop cell therapies. The difficulty lies in designing the culture medium able to guide the stem cells to the desired cell type with a high degree of homogeneity, guaranteeing stability after implantation. A single cell which has remained undifferentiated would renew itself indefinitely in the recipient patient's body, with the risk of causing cancer.
Once the appropriate culture medium has been obtained, the laboratories should adapt their procedure to Good Manufacturing Practice guidelines (GMP) and in line with storage guidelines, so as to yield "clinical-grade" therapeutic cells. This is the indispensable condition in order for these cells to be approved by the health authorities and for clinical trials to be conducted in humans.
In certain indications, stem cells produced for cell therapy may be genetically modified by gene therapy.
Cell therapy, a "one shot" treatment
A single treatment at an apparently reasonable cost: this seems to be the pattern with cell therapy. The production costs of therapeutic cells will decline as the processes become automated. From 750,000 thawed embryonic stem cells, it is now possible to create a bank of 325 million keratinocytes (the main cell constituting the epidermis) in four months, bearing in mind that approximately 500,000 cells are required to treat a single patient. This limit should increase ten-fold in the next few years. At present, the cost of a cell therapy product is estimated at between EUR 10,000 and 20,000. Furthermore, unlike standard medications, a single dose is sufficient to treat the patient.
Donor-recipient compatibility issue
Stem cells used for cell therapy may be taken from the patients themselves. They are then described as autologous and the therapeutic cells will be perfectly tolerated by the patients in terms of immunity. Autologous cells can be used when multipotent stem cells or iPSC are necessary. The downside of this solution is that it extend the time to initiate treatment compared with ready-to-use therapeutic cells obtained from banks.
When therapeutic stem cells are taken from someone other than the patient, they are then described as allogeneic. Their use can give rise to immune tolerance problems: the donor's cells might be perceived by the patient's immune system as foreign bodies which must be destroyed. Graft rejection may then theoretically take place. In the event of bone marrow transplantation, for instance, the recipient patient should receive immunosuppressant therapy to avoid rejection.
As regards the use of allogeneic iPSC, researchers are currently preparing for this problem by creating cell banks marked according to their immune profile (HLA), so as to select therapeutic cells compatible with the recipient patient's profile. This is a major undertaking, implemented through international partnerships and coordinated by the GAIT (Global Alliance for iPSC Therapy). Inserm is a participant in this project.
With embryonic stem cells, the donor-recipient compatibility issue is less acute: these cells appear to be weakly immunogenic and their use only a priori requires temporary immunosuppressant therapy. Nevertheless, this crucial point is being closely monitored in ongoing clinical trials. If the immunogenicity of these cells is greater than expected and requires long-term or, indeed, life-long immunosuppressant therapy, this could call into question their benefit in less serious indications.
Mesenchymal stem cells, widely used in current trials, weakly express HLA markers and, in addition, secrete immunosuppressant factors which limit immune reactions against the graft. No exogenous immunosuppressant therapy is therefore necessary when allogeneic mesenchymal stem cells are used. However, before implantation, researchers ensure that the recipient patient does not express any antibodies against the donor's HLA system.
Challenges of cell therapy – clinical trials
Ongoing clinical trials on embryonic stem cells
A US biotech firm (Ocata Therapeutics) uses human embryonic stem cells differentiated into retinal cells to treat AMD and differentiated into retinal pigment epithelial cells to treat Stargardt's disease. In both cases, phase I and II trials are currently in progress to evaluate the safety and therapeutic effect of this approach. The first results are modest, but positive. Another trial is currently being planned in AMD, led by The London Project to Cure Blindness in partnership with a pharmaceutical company (Pfizer). The concept is the same: to develop retinal cells from embryonic stem cells to inject them into patients aged over 50, suffering from reduced visual acuity.
On the Génopole d’Evry campus, researchers from the I-Stem laboratory (Inserm Unit 861) are working in close collaboration with the Institute of Vision (Inserm Unit 968) and AFM-Téléthon on other cell therapy applications, based on the use of human embryonic stem cells. This laboratory is notably developing the use of human embryonic stem cells differentiated into keratinocytes in the treatment of skin ulcers associated with a genetic disease, sickle-cell anemia. Preclinical research currently in progress aims to verify the biodistribution of injected cells and the absence of a tumorigenic risk.
In the field of cardiology, a team from Hôpital Européen Georges Pompidou (Inserm Unit 970) carried out transplantation of cardiac cells, derived from human embryonic stem cells, in October 2014, according to a process developed by researchers at Hôpital Saint-Louis (Inserm Unit 1160). Ten weeks later, the 68-year-old female patient, suffering from severe heart failure, showed a clear improvement in her condition, without any apparent complications.
Another disease targeted by this type of approach: type 1 diabetes mellitus. Another US biotech firm (ViaCyte) is planning a clinical trial based on the use of insulin-producing pancreatic cells obtained from embryonic stem cells. The cells to be transplanted are encapsulated in a sophisticated disk: this device enables diffusion of insulin and glucose, but protects the graft from a host immune reaction. The preclinical results are promising. The objective is to restore long-term insulin function in patients.
Ongoing clinical trials on iPSC
iPSC are not widely used in cell therapy, owing to the reprogramming step, the safety of which is questionable. Although clinical trials conducted with embryonic stem cells prove conclusive, particularly in terms of immune tolerance, there is little chance that iPSC will be used more extensively in the future. If, however, embryonic stem cells are ultimately more immunogenic than expected, the use of autologous iPSC is likely to expand rapidly.
A clinical trial on iPSC is nonetheless in progress in Japan, in the treatment of wet AMD, the practically sole form of AMD in the country. Therapeutic cells are collected from patients (autologous cells), reprogrammed then redifferentiated into retinal cells, and, lastly, reinjected into the patients. Approximately ten or so patients will be treated in the context of this trial to evaluate the safety and feasibility of this approach.
In France, INGESTEM, a national infrastructure coordinated by Inserm and approved by the 2012-2019 Investments for the Future Programme, has brought together five pioneering research teams in the field of iPSC biology and tissue engineering. Their objective is to use cell reprogramming techniques to generate human disease and regenerative medicine models.
Ongoing clinical trials on mesenchymal stem cells
Over 350 clinical trials on cell therapy using mesenchymal cells are currently in progress worldwide. The therapeutic cells used in a third of these studies are autologous cells. The indications tested are extremely varied, owing to the ability of these cells to differentiate into different cell types and produce growth and immunosuppression factors. A number of trials focus on rheumatology (osteoarthritis, rheumatoid arthritis), muscle degeneration (myopathy), cardiology (stroke, myocardial infarction, ischemia of the lower limbs), diabetes, autoimmune disease (lupus), graft rejection, etc.
The ADIPOA trial is currently under way in the treatment of moderate to severe osteoarthritis at Montpellier University Hospital. It is being conducted on 18 patients who receive a single injection of mesenchymal stem cells directly into the joint. Three different cell doses are being tested. The first results evidence a response in 80% of subjects, with increased function and reduced pain nine months after the injection. A phase II trial is expected to be initiated by the end of 2015. It will include 150 patients divided into three groups: two groups each comprising 50 patients will receive a stem cell injection at different doses and one group comprising 50 control patients will not receive a stem cell injection.
Studies suggest that mesenchymal stem cells may promote the formation of new blood vessels, without, however, being able to differentiate into blood vessel cells. This effect is thought to be due to the production of growth factors which locally promote cell development. This property justifies research in the cardiovascular field, aiming to promote the growth of damaged tissue following myocardial infarction, stroke or peripheral arterial disease in the lower limbs. A number of early clinical trials are currently under way in these indications. A phase II study in ischemic stroke is thus being conducted at San Diego Hospital (United States), with allogeneic mesenchymal stem cells manufactured by the Stemedica firm. Another study is being conducted in Korea (at the Samsung Medical Center), with autologous mesenchymal stem cells.
A phase II trial is also being initiated at the Ottawa Hospital Research Institute (Canada), in multiple sclerosis. It aims to evaluate the benefit of the neuroprotective properties of autologous mesenchymal stem cells. Another phase II trial at Boston Hospital and the Mayo Clinic (Rochester, United States) is focusing on amyotrophic lateral sclerosis, again with autologous mesenchymal stem cells.
A national complementary expert report network
A national cell therapy platform based on the use of adult mesenchymal stem cells has been created at Montpellier University Hospital in France: ECELLFRANCE. It aims to harmonize and optimize the essential steps in the development of medicinal stem cells and regenerative medicine. It invites all academic or industrial teams to accelerate their R&D programs from validation of the project to phase I and II clinical trials.
"Validated" cell therapies
A number of cell therapy treatments have now been authorized by the health authorities:
- Skin stem cells have been used to reconstruct layers of the epidermis in the laboratory, to serve as skin grafts for severe burns victims since the 1970s.
- Hematopoietic stem cells (bone marrow transplantation) have been administered in the treatment of malignant blood diseases since the 1980s.
- Infusion of allogeneic mesenchymal stem cells is authorized in Canada to treat graft-versus-host disease in children (GvHD).
- Injection of allogeneic mesenchymal stem cells has been authorized in Korea for osteoarthritis since 2013.
- Holoclar is the first cell therapy product to have received marketing authorization in Europe (in February 2015). It is indicated for corneal lesions and burns. It is based on the collection of limbal stem cells (at the periphery of the corneum) from the patient, followed by ex vivo differentiation into corneal epithelial cells which are then reimplanted.
There are countless indications for cell therapy which have very real promise in numerous fields. This could concern clinical fields such as neurodegenerative diseases (Parkinson's or Alzheimer's disease) or muscle degeneration (Duchenne muscular dystrophy) if researchers are able to produce large quantities of different neuron sub-types and skeletal muscle cells. Dare we also envisage the possibility of producing blood cells, including platelets, in unlimited quantities to cover hospital blood needs? All hypotheses are now on the table.