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Chimeras rely on stem cells and their ability to differentiate into the necessary cells needed by the body. It is this feature of stem cells that allow scientist to culture tissue samples and eventually produce transplantable organs. The procuration of stem cells is the subject of a controversial argument as the methodologies of some variations raises several moral and ethical issues.
As the name suggests, this form of stem cells are derived from human embryos. Contrary to popular belief, these cells are not obtained from eggs fertilised inside a woman’s body; the embryos are usually donated for research purposes by In Vitro Fertilisation Clinics, with the consent of the donors. The embryos are then suspended in a culture medium ,mirroring similar conditions to that of a mother’s womb, allowing the embryo to divide into a mass of cells known as the blastocyst. The cells within the blastocyst are usually referred to as totipotent stem cells. It is here that the first ethical issue arises. The beginning of life is said to be conception or fertilisation therefore this method of obtaining stem cells can be considered as taking a life without its consent. (U.S. Department of Health and Human Services, 2016)
Another limitation of hESCs includes carcinogenic risk when the culture medium is altered in order to induce differentiation of stem cells to form specialised cells such as: heart cells, lungs cells, liver cells and nerve cells. If the wrong mix of proteins or hormones are added to the stem cells there’s a potential risk of mutation of DNA resulting in the production of cancerous or faulty cells. Conversely, hESCs are more accepted in the scientific community as the production of it can be done at lower cost with much more efficient differentiation and the cells produced are within a suitable HLA spectrum.  (Pappas, 2008)
It is possible however to bypass the ethical and moral issues that hESCs present, as these issues only arise if the cell is post-fertilisation. Therefore, if stem cells are extracted from an unfertilised egg, then arguably life which begins at conception or fertilisation, has not yet begun, making the use of the stem cells less controversial. However, the ethical implications have not been bypassed altogether, as it can still be argued that stem cells from unfertilised eggs do still have the potential to make a living individual. Parthenogenesis allows for the egg cell to be activated without the need for a sperm. Parthenogenetic embryos will develop to the blastocyst stage and so can serve as a source of embryonic stem cells. Parthenogenetic Embryonic Stem Cells (pESCs) have been shown to have the properties of self-renewal and the capacity to generate cell derivatives from the three germ layers, confirmed by contributions to chimeric animals (Department of Animal Science, Michigan State University, East Lansing, Michigan, USA, 2006)
The process behind iPSCs was a big medical breakthrough as it allowed somatic (body) cells to be reprogrammed into regenerative cells. The formation of iPSCs require the donor to undergo shave or punch biopsies, this procedure can be done under local anesthetic and is minimally invasive so the procuration of the adult cells poses no moral or ethical predicaments. The induction of pluripotency in adult somatic cells via proteins, will produce genetical and immune-histocompatibility matches thus, lowering the chance of rejection (if used for transplantation), this also reduces the need for the patient to take immunosuppressant which can result in a compromised immune response. But this form of stem cells comes with its disadvantages, as it is a new concept the cost of production is high. Therefore this process in its current state of development is economically viable for a large population size. Furthermore, the mechanisms behind how the reprogramming factors work are unknown, this presents the chances of mutagenesis, oncogene activation risk, and retroviral gene delivery (Pappas, 2008)
As of 2015, there are 36.7 million people living with HIV as per WHO and UNAIDS. (WHO, 2016). The field of vaccines for diseases such as Hepatitis-B and HIV (Human Immunodeficiency Virus) have taken a heavy toll in developing countries and have faced major failures. In the hopes of improving the current situation. Human-animal chimeras, developed with a 'humanized' immune system could be useful to study infectious diseases, including many neglected diseases. These would also serve as an important tool for the efficient testing of new vaccine candidates to streamline promising candidates for further trials in humans. (Bhan, et al., 2010).
Human hematopoietic stem cells, or in layman’s terms, bone marrow cells, have the unique capacity of engrafting, greatly expanding, and repopulating immunodeficient mice, with virtually all different types of human immune cells; as shown by the image above. Humanized mouse models are produced via transplantation of CD34+ stem cells and/or implantation of human tissue into immunodeficient mice. Depending on whether tissue or CD34+ cells are used and the strain of mouse, this results in mice which have a part or a complete human immune system. (Garcia, 2016) This xenografted mouse is then used as a disease model. This allows scientists to better understand the mechanisms behind the disease, which results in a more efficient treatment plan for those who suffer from. Hepatitis-B.
Another disease model being used are primates, these are considered to be the most accurate as we share a common ancestor. Additionally, primates have the closest metabolic conditions to humans. When this model was injected with HIV-1 (via IV), HIV-2 (via vagina) and SIV (via rectum) the results were advantageous as they provided useful information for vaccine and therapeutic studies. However, the cost of producing this model is very high and raises many moral and ethical concerns; furthermore, despite having some genetic similarities, primates do have different cellular and molecular markers and the time and course of infection could vary.
Chimeras are also benefiting the treatment of Japanese encephalitis. This disease is a type of viral brain infection that’s spread through mosquito bites, commonly found in South-East Asia. Although there’s no cure for Japanese encephalitis, it can be prevented through vaccination, which is usually only available privately (NHS, 2016).
A recently developed vaccine, which is an animal-human chimera which is a mouse brain-derived, inactivated JE vaccine (MBV). In order to evaluate its efficacy case controlled studies were carried out. A randomized double-blinded study conducted in northern Thailand, using JE MBV produced in Thailand, yielded an overall effectiveness of 91%. Another trial in Taiwan revealed an effectiveness of approximately 85% when two or more doses were administered. The effectiveness of the JE vaccine in Northern Vietnam was 92.9% efficacious. (Marks, et al., 2012).
Another therapeutic use of animal-human chimeras is the development of drugs to aid in the treatment of known diseases.The drug called Rituximab, is a chimeric antibody which means it contains portions of both human and mouse antibodies mixed together. The drug was licensed in 1997 for the treatment of NHL (Non-Hodgkin’s lymphoma)-a form of cancer which causes B-cells to mutate and divide abnormally. The drug targets the CD20 receptor on B-cells as this receptor is located on the surface of the cell and it doesn’t mutate, move inside the cell or fall off in the life cycle of the B-cell. The drug contains the variable domain of the mouse antibody, the portion that specifically binds CD20, along with the constant domain of human antibody, the portion that recruits other components of the immune system to the target-the B-cells and so after it is administered, and a large number of tumour cells are immediately destroyed and eliminated from the body.
Rituximab is also used to treat advanced rheumatoid arthritis and it has also been part of anti-rejection treatments for kidney transplants (both involve B cells). The disadvantage only that the mouse antibody was unsuitable for direct use in humans and clinical trial results varied, likely due to the differing sizes of tumors between the patients, (Speaking of Research, 2017)
The demand for organ transplantation has rapidly increased all over the world due to the increased incidence of vital organ failure. However, the unavailability of adequate organs for transplantation procedures to meet this growing demand has resulted in a major organ crisis. In 2014, 429 patients died while on the waiting list for an organ transplant- that’s up to 3 patients a day. (Knapton, 2015). Currently, the government plan on changing the organ donation system to an opt out system, which hopes to promote organ donation and increase the availability of organs. The opt-out system presumes the donor’s consent unless the individual expresses a refusal to become a potential donor- allowing the donor to make a free choice (Abouna, 2008). As well as increasing obtainability of organs, it also increases the likelihood of more organs found within a suitable HLA spectrum. (Department of Health and Social Care and Cabinet Office, 2017).
But it can be argued that this system of obtaining organs is seen as unfair as majority of organ donors must be recently deceased (excluding kidney donors) therefore the longevity of one person’s life is at the cause of another’s death. (World Health Organisation, 2005) To prevent this choice being made, alternative solutions are being developed in order to aid the organ crisis-one of them being animal-human chimeras. Current research on stem cells have shown that they can differentiate into different cell types but cannot effectively produce usable tissues and organs as a culture medium cannot replicate the growth of an organ in a body. A recent breakthrough by the (Salk Institute of Biological Research, 2017) shows a pig-human chimera, which would be capable of making human organs.
The research began by creating an interspecies chimera consisting of a rat and mouse. They used a gene editing technology known as CRISPR (Clustered Regularly Interspaced Short Palindromic Repeats) to “turn off” the gene that makes the pancreas. They then inserted rat iPSCs which contained a pancreas gene into the mouse embryo. The result, when implanted into surrogate mouse mothers, was a fully developed mouse with a growing rat pancreas.
This concept was then mirrored using pigs’ embryos and human stem cells; as pigs have similar organ sizes and developmental timescales as humans. Although this experiment had to be halted at 4 weeks of development due to ethical issues and the lack of consent- as the experiment was designed to prove it was possible, not to produce a human organ-we can safely assume that, if the development of the pig was allowed to continue, the pig would have a whole human organ inside it.
Theoretically, this concept can then be implemented, producing specific human organs, eliminating the wait for a human donor and reducing the risk of organ rejection.
Scientific research is not always accepted as they require the use of controversial methods to obtain the necessary results. The methodologies behind creating chimeras have ethical and moral dilemmas primarily due to the use of animals. There is a large emphasis on animal welfare, although the use of animals as chimeras or in general medical research is considered very valuable as they help the medical community to better under the effects of treatments (drugs or otherwise) on living organisms. The matter still finds itself to be the subject of a very heated debate; as those opposing the use of animals – animal rights extremists and anti-vivisectionist groups-believe that animal experimentation is unnecessary and cruel regardless of its benefits ergo the opposition want total abolition of animal research and if the majority supports this view then there will be severe consequences for scientific research. (Festing & Wilkinson, 2007)
On the other hand, the UK has gone further than most countries in regards to the ethical framework by introducing the Animals (Scientific Procedure) Act 1986 which regulates the use of animal research. Along with this, there is more and more public awareness as polls run by Ipsos MORI state that in 2005 64% of the population agreed with the use of animals in research if the research objectives are important and the animals experience minimal suffering and all alternatives are considered. (Department for Business & Freeman, 2014)
Another bioethical view that must be considered is `whether we treat the chimeras as animals or human?’ this arises as some chimeras require the altering of cognitive capacities. The chimeras are to be used to develop a better understanding of diseases such as Parkinson’s and Dementia which affect 850 000 people every year (Anon., 2014) Unfortunately, the research is very slow due to moral views as some people regard this form experimentation a violation of human dignity and the order of nature as well as, the initial disagreement of using chimeras in the first place. (Hermerén, 2015)
Opportunely, there is some support for the use of animal-human chimeras as previous medical techniques that are widely accepted today allow the use of porcine, bovine and equine biological heart valves are implanted in those with cardiac valve dysfunction. Moreover, insulin extracted from porcine pancreas is routinely used with those with diabetes. And so, the prospect of a pig carrying a pancreas or liver of human origin should be justifiable. (Bourret, et al., 2016)
A lesser conventional view is the alternatives to chimeras, these methods do not require the use of animals to carry out medical research, which hopefully, should eliminate bioethical arguments. The issue that arises with this is the efficiency and viability of the results. The alternatives to chimeras include cell cultures, human tissues and computer models. Almost all cell types can be recreated in laboratory conditions and these can be coaxed to grow into 3D structures- miniature organs. Cell cultures have also been used to create `organs-on-chips’ which can be used to study disease mechanisms, as well as, drug metabolism. This form biotechnology has already managed to mimic the heart, lungs and kidneys. The goal is to be able to this for all organ systems. The idea is already aided in the development in the production of vaccines, and drug testing on top of aided research in the study of cancers, sepsis and AIDS.
Human tissues can be donated by both healthy and diseased volunteers through surgeries such as biopsies, cosmetic surgery and transplants or via post mortem- such as brain tissue from a patient with Multiple Sclerosis to help better understand a large variety of diseases furthermore the tissues can make more effective models than through chimeras as they will contain only human DNA thus providing a more relevant way of studying human biology.
Finally, computer models can be used to create virtual experiments based on existing information. Models of the musculoskeletal systems, heart, lungs etc. already exist. Inopportunely, this method isn’t as effective as testing in vivo as the concept is very theoretical. (Anon., n.d.)
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