University of Konstanz, Germany
The Department of Economic and Social Affairs of the United Nations has projected a world population of 9.2 billion by 2050. A quarter of these, that is 2.4 billion, will be over 60 years of age. It is to be expected that a major part of these 2.4 billion will need regenerative medicine, representing an enormous challenge, but also an enormous market.
With the advent of stem cells, particularly of human pluripotent stem cells (PSCs) such as embryonic stem cells (ESCs), nuclear transfer stem cells (NT-SCs) and induced pluripotent stem cells (iPSCs), as well as realistic culture expansion protocols, hopes for cures for degenerative diseases have soared. Compared to adult stem cells which grow sparsely or not at all in culture, PSCs are particularly attractive for regenerative medicine as they can be expanded in culture to sufficiently high quantities of cells, can be differentiated in culture into all cell types desired and thus alleviate the extreme shortage of human transplant material that has resulted in the death of many patients.
But while the quantity, purity and quality of stem cell-derived cells has improved significantly over the last decade, one major aspect has been neglected: the reaction of the patient’s immune system to such cells after transplantation . Embryonic stem cells are derived from supernumerary blastocysts (early embryonic stage) generated in large quantities around the globe during infertility treatments. Such blastocysts can be donated and cell lines that grow indefinitely can be generated. Such cells are however allogeneic for any given patient, i.e. not a genetic match, and would be rejected (destroyed) by the patient’s immune system. In order to circumvent mismatching, PSCs can also be generated by using human oocytes in which the DNA has been replaced with the patient’s own DNA. This feat was recently achieved when the first human cloned cell line was generated . Whether this approach will be used for patients in the future remains to be seen, as it is work intensive, time consuming and would entail a significant price tag. Moreover, such cells would still carry the mitochondrial DNA of the oocyte donor and could potentially elicit rejection by the patient . A faster approach to obtain patient-matched PSCs is through direct reprogramming of the patient’s somatic cells to pluripotency. While fast and cheap, this method induces, however, significant mutations in the generated cells and it is not clear at this point whether this caveat can be circumvented. Moreover, for most patients in need of regenerative medicine, iPSCs derived from their own cells may be of questionable desirability. By the time most people need a transplant, the cells of the body, in particular the skin cells, have been exposed to decades of noxious environmental influences leading to an accumulation of genetic and epigenetic changes. These changes may then be potentiated through the reprogramming process and extensive culturing of the cells, generating cells with a high potential for tumorigenesis.
In animal models a long list of degenerative diseases such as cardiovascular disease, diabetes, Parkinson’s disease, spinal cord injury and more have been cured. However, in most instances human cells were transplanted into mutant mouse strains incapable of an immune response. Only few studies reported transplantation into immune-competent primate models, more similar to humans than mutant rodents. In the human setting, more than 5 decades of solid organ and hematopoietic stem cell transplantation have taught us that immune reactions between patient and graft are a major problem, significantly restricting graft availability to any given patient. High polymorphism (differences) in major histocompatibility proteins (HLA genes) on the surface of almost all the cells in the body elicits an immune reaction by the immune system of any other individual but an identical twin. The more HLA genes matched between the patient and the donor, the less strong this response becomes and can be reduced to tolerable levels by immune suppressing drugs. However, matching to such a level is rare between patient and potential donors, which severely reduces the donor pool for any given patient. Many patients die on waiting lists for transplantation. Also, immune suppression, often to be taken life-long, carries major side effects such as increased risk for infections. Viral or fungal infections are particularly dangerous for the patient as no effective drugs truly exist. Often immune suppression has to be relaxed, potentiating chronic rejection which often can no longer be stopped, even by increasing immune suppression again, resulting in the loss of the graft sooner or later. Other major and frequent side effects include renal failure, osteoporosis, and the increased risk for developing cancer.
A priori, it has to be assumed that any cells from a non-identical sibling-donor would be rejected by the immune system of the patient. Adding to this is the potential to generate immunogenic graft cells by the expanded culture period required to generate sufficient and differentiated cells. It has been shown that during these highly artificial processes, cells may acquire surface markers recognized as foreign by the recipient. Thus, it cannot be excluded that even NT-SCs or hIPSCs could acquire immunogenicity during their generation in culture, and extended time in culture during the differentiation process.
Interestingly, it was found that human ESCs, in their pluripotent stage and up to a certain stage of differentiation, are not rejected. On the contrary, they seem to regulate the immune response by impacting directly on lymphocytes, attenuating their normal response to allogeneic cells. Murine ESCs also regulate the immune response, but with different mechanisms than the human cells. Response to differentiated cells is still not clear. In the murine setting, diverging results have been reported with reports showing rejection or no rejection. More differentiated human cells however, elicit a rejecting immune response when tested in vitro against allogeneic human lymphocytes. Whether undifferentiated iPSCs also have immune regulatory capabilities is not investigated in much detail at this stage. It is also not clear at this point, whether human iPSCs would be rejected or not after transplantation into patients. Conflicting results have been obtained with murine iPSCs transplanted into mice. One report has shown a rejection when murine iPSCs were injected into syngeneic mice. This may be due to minor histocompatibility genes which are not necessarily identical in syngeneic mice (identical in major histocompatible genes). Other reports have shown an absence of rejection in syngeneic mice. In a primate model, a recent report has shown that monkey iPSCs were not rejected in the monkeys from which the cells were derived. This result may be more predictive of the human clinical setting than murine studies. While transplantation of iPSCs into patients has not been attempted yet, the first transplantation of PSCs has taken place. Recently, human ESCs were differentiated to retinal pigment epithelial cells, the cells that degenerate during age-related macular degeneration (a leading cause of blindness in the over 60 years old) and transplanted into two patients with macular degeneration. Both patients did not show any rejection of the transplanted cells .
One other very interesting stem cell worth mentioning when discussing immune rejection after transplantation of cellular grafts, are mesenchymal stem cells (MSCs), a type of adult stem cells. These stem cells, best isolated out of bone marrow, fat tissue and umbilical cord, have a certain capacity to be expanded in culture and can thus yield cell quantities relevant for human transplantation. Several cell types can be generated from these cells, including bone, cartilage and tendon, muscle cells and (potentially) neurons. Moreover, these cells have a very interesting and well characterized immune phenotype in that they can regulate the immune system on several levels. These cells can thus be transplanted, without major rejection across compatibility barriers, into allogeneic patients. They can also be used as ‘immune therapy’ in that they can be co-injected to reduce rejection of other cells or organs. They may also be able to regulate autoimmune disease. Most importantly, MSCs can be expanded and banked easily, and as rejection is low, they could be potentially used for any given patient. Accordingly, several companies are already selling MSCs for transplantation purposes.
While there still remains a lot of progress to be achieved to enable wide spread use of stem cell grafts for regenerative purposes, current reports suggest that in the future transplantation of stem cells will be feasible and may provide grafts for a wide range of degenerative diseases.
About the Author
Dr. Suzanne Kadereit is a stem cell biologist who obtained her PhD at the Pasteur Institute in Paris. She worked on umbilical cord blood immune and stem cells in the US, prior to working with human embryonic stem cells in Singapore. While in Singapore she became founding head of the Singapore stem cell bank. She is now group leader of the stem cell group at the Chair for in vitro Toxicology and Biomedicine at the University of Konstanz, developing stem cell-based assay systems from human embryonic stem cells for disease modelling and toxicological studies.
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