Xenotransplantation ( xenos- from the Greek meaning “foreign”), is the transplantation of living cells , tissues or organs from one species to another.  Such cells, tissues or organs are called xenografts or xenotransplants . It is contrasted with allotransplantation (from other individual of the same species), syngeneic transplantation or isotransplantation (grafts transplanted between two genetically identical individuals of the same species) and autotransplantation (from one part of the body to another in the same person).
Xenotransplantation of human tumor cells in immunocompromised mice is a research technique frequently used in pre-clinical oncology research.
Human xenotransplantation offers a potential treatment for end-stage organ failure , a significant health problem in parts of the industrialized world. It also raises many new medical, legal and ethical issues.  A continuing concern is that many animals, such as pigs, have shorter lifespan than humans, meaning that their tissues age at a quicker rate. Disease transmission ( xenozoonosis ) and permanent alteration to the genetic code of animals are also causes for concern. A few successful cases of xenotransplantation are published. 
It is not uncommon for patients and physicians to use the term “allograft”, but it is not useful for all-purpose research (or-to-human) or xenograft (animal-to-human), but it is helpful scientifically scientific literature ) to maintain the more precise distinction in use .
The first serious attempts at xenotransplantation (then called heterotransplantation) appeared in the scientific literature in 1905, when slices of rabbit kidney were transplanted into a child with renal insufficiency.  In the first two decades of the 20th century, several subsequent efforts were made by the United States of America, pigs and primates were published. 
Scientific interest in xenotransplantation has been described when the immunological basis of the organ rejection process has been described. The next waves of studies on the topic with the discovery of immunosuppressive drugs . Even more studies followed Dr. Joseph Murray’s first successful kidney transplantation in 1954 and scientists, facing the ethical issues of organ donation for the first time, accelerated their effort in looking for alternatives to human organs. 
In 1963, doctors at Tulane University attempted chimpanzee-to-human kidney transplants in six people who were near death; after this and several subsequent unsuccessful attempts to use primates and the development of a cadaver organ procuring program, which is in xenotransplantation for kidney failure dissipated. 
An American infant girl known as ” Baby Fae ” with hypoplastic left heart syndrome was the first recipient of a xenotransplantation, when she received a baby in 1984. The procedure was performed by Leonard L. Bailey at Loma Linda University Medical Center in Loma Linda, California . Fae died 23 days later due to a humoral-based graft rejection thought to be caused by an ABO blood type mismatch, considered unavoidable due to the rarity of type O baboons. The graft was meant to be temporary, but unfortunately a suitable allograft replacement could not be found in time. 
Xenotransplantation of human tumor cells in immunocompromised mice is a research technique frequently used in oncology research.  It is used to predict the sensitivity of the transplanted tumor to various cancer treatments; Several companies offer this service, including the Jackson Laboratory  and Altogen Labs . 
Human organs have been transplanted into human beings with a powerful research technique for studying human biology without harming human patients. This technique has also been proposed as an alternative source of human organs for future transplantation into human patients.  For example, researchers from the Ganogen Research Institute, transplanted human fetal kidneys into rats. 
A worldwide shortage of organs for clinical implantation causes 20-35% of patients who need replacement.  Certain procedures, some of qui are being white Investigated in early clinical trials, aim to use cells or tissue from other species to treat life-threatening and debilitating Illnesses Such As Cancer , diabetes , liver failure and Parkinson’s disease . If vitrification can be perfected, it could allow for long-term storage of xenogenic cells, tissues and organs so they would be more readily available for transplant.
Xenotransplants could save thousands of patients waiting for donated organs. The animal organ, probably from a pig or a human body, has a certain role to play in the treatment of a patient’s immune system. They have re-emerged because of the lack of available organs and the constant battle to keep immune systems from rejecting allotransplants. Xenotransplants are therefore a more effective alternative.   
Xenotransplantation is also a valuable tool used in research laboratories to study developmental biology .  It seeks to be a mutable architecture, which remains open for perpetual modification and enhancement following the navigational impulse of militant ethical reasoning. 
Patient derived tumor xenografts in animals can be used to test treatments. 
Potential animal organ donors
Since they are the closest to humans, non-human primates have been considered as a potential source for xenotransplantation to humans. Chimpanzees were originally considered the best option since they are of similar size, and they have good blood type compatibility with humans, which makes them potential candidates for xenotransfusions. However, since they are listed as endangered species, other potential donors have been sought. Baboons are more readily available, but impractical as potential donors. Problems include their smaller body size, the O (the universal donor), their long gestation period, and their typically small number of offspring. In addition, a major problem with the use of nonhuman primates is the increased risk of disease transmission, since they are so closely related to humans. 
Pigs are currently thought to be the best candidates for organ donation. The risk of cross-species disease is increased because of their increased phylogenetic distance from humans.  They are readily available, their organs are anatomically comparable in size, and new infectious agents are less likely to be in contact with humans through domestication for many generations.  Current experiments in xenotransplantation most often use pigs as the donor, and baboons as human models.
In the field of regenerative medicine, pancreatogenesis- or nephrogenesis-disabled pig embryos, unable to form a specific organ, allowable experimentation towards the in vivo generation of functional organs from xenogenic pluripotent stem cells in large animals through compensation for an empty developmental niche (blastocyst complementation).  Such experiments provide the basis for potential future application of transplantation to transplantable human organs from the patient’s own cells, using livestock animals, to increase the quality of life for those with end-stage organ failure.
Barriers and issues
To date, no xenotransplantation trials have been successful due to the many obstacles arising from the response of the recipient’s immune system . This response, which is generally more extreme than in allotransplantations, results in rejection of the xenograft, and can in some cases result in the immediate death of the recipient. There are several types of organ rejection, including these:
- Hyperacute rejection
- Acute vascular rejection
- Cellular rejection
- Chronic rejection
A rapid, violent, and hyperacute response comes as a result of antibodies present in the host organism. These antibodies are known as xenoractive natural antibodies (XNAs). 
This rapid and violent type of rejection occurs within minutes to the time of the transplant. It is mediated by the binding of XNAs (xenoreactive natural antibodies) to the donor endothelium, Causing activation of the human complement system , qui results in endothelial damage, inflammation, thrombosis and necrosis of the transplant. XNAs are first produced and started circulating in neonates, after colonization of the galactose moieties on their cell walls. Most of these antibodies are IgM class, but also include IgG , and IgA . 
The XNAs epitope targets an α-linked galactose moiety, Gal-α-1,3Gal (also called the α-Gal epitope), produced by the α-galactosyl transferase enzyme.  Most non-primate Contain this enzyme THUS, this epitope is present on the organ epithelium and is Perceived as a foreign antigen by primates, qui Lack the galactosyl transferase enzyme. In pig to primate xenotransplantation, XNAs recognize porcine glycoproteins of the integrin family. 
The binding of XNAs complementary complement activation through the classical complement pathway . Complement activation causes a cascade of events leading to: destruction of endothelial cells, platelet degranulation, inflammation, coagulation, fibrin deposition, and hemorrhage. The end result is thrombosis and necrosis of the xenograft. 
Overcoming hyperacute rejection
Since there are several obstacles to the success of xenografts,
Interruption of the complement cascade
- The recipient’s complement cascade can be inhibited by the use of cobra venom factor (which C3 depletes), soluble complement receptor type 1, anti-C5 antibodies, or C1 inhibitor (C1-INH). Disadvantages of this approach include the toxicity of cobra venom factor, and most importantly these treatments would deprive the individual of a functional complementary system. 
Transgenic organs (Genetically engineered pigs)
- 1,3 galactosyl transferase gene knockouts – These pigs are not responsible for the enzyme responsible for expression of the immunogenic gal-α-1,3Gal moiety (the α-Gal epitope). 
- Increased expression of H-transferase (α 1,2 fucosyltransferase), an enzyme that competes with galactosyl transferase. Experiments have shown this α-Gal reduction expression by 70%. 
- Expression of human complement regulators ( CD55 , CD46 , and CD59 ) to inhibit the complement cascade. 
- Plasmaphoresis, on humans to remove 1,3 galactosyltransferase, reduces the risk of CTL (CD8 T cells) activation.
Acute vascular rejection
Also known as delayed xenoactive rejection, this type of rejection occurs in discordant xenografts within 2 to 3 days, if hyperacute rejection is prevented. The process is much more complex than hyperacute rejection and is currently not completely understood. Acute vascular rejection requires de novo protein synthesis and is driven by interactions between the graft endothelial cells and host antibodies, macrophages, and platelets. Infectious infiltration of macrophages and natural killer cells (with small numbers of T cells ), intravascular thrombosis, and fibrinoid necrosis of vessel walls. 
Binding of the previously mentioned XNAs to the endothelium donor leads to the activation of host macrophages and the endothelium itself. The endothelium activation is considered type II since gene induction and protein synthesis are involved. The binding of XNAs ultimately leads to the development of a procoagulant state, the secretion of inflammatory cytokines and chemokines , and the expression of leukocyte adhesion molecules such as E-selectin , intercellular adhesion molecule-1 ( ICAM-1 ), and vascular cell. adhesion molecule-1 ( VCAM-1 ). 
This response is further enhanced by the use of binding agents and the control of coagulation and inflammatory responses. However, due to molecular incompatibilities between the molecules of the donor species and recipient (such as porcine major histocompatibility complex molecules and human natural killer cells), this may not occur. 
Overcoming acute vascular rejection
Due to its complexity, the use of immunosuppressive drugs along with a wide array of approaches to prevent acute vascular rejection, and include:
- Administering a synthetic thrombin inhibitor to modulate thrombogenesis
- Depletion of anti-galactose antibodies (XNAs) by such techniques as immunoadsorption, to prevent endothelial cell activation
- Inhibiting activation of macrophages (stimulated by CD4 + T cells) and NK cells (stimulated by the release of IL-2). Thus, the role of MHC molecules and T cell responses in activation would be reassessed for each species combo. 
If hyperacute and acute vascular rejection are possible, which is the survival of the xenograft despite the presence of circulating XNAs. The graft is given a break from humoral rejection  when the complement is interrupted, and their function is changed, or there is a change in the expression of surface antigens on the graft. This allows the expression of protective effects, which in the case of injury, such as heme oxygenase-1 (an enzyme that catalyzes the degradation of heme). 
Rejection of the xenograft in hyperactute and acute vascular rejection is due to the response of the humoral immune system , since the response is elicited by the XNAs. Cellular rejection is based on cellular immunity , and is mediated by:
- Natural killer cells, which accumulate in the xenograft; and
- T-lymphocytes – which are activated by MHC molecules through both direct and indirect xenorecognition.
In direct xenorecognition, antigen presenting cells from the xenograft present receptor peptides to CD4 + T cells via xenogeneic MHC class II molecules, resulting in the production of interleukin 2 (IL-2). Indirect xenorecognition involves the presentation of antigen presenting cells to CD4 + T cells. Antigens of phagocytosed graft cells can also be presented by the MHC molecules to CD8 + T cells.  
The strength of cellular rejection in xenografts remains uncertain, but it is expected to be stronger in allografts due to differences in peptides among different animals. This leads to more antigens, and is therefore recognized as foreign, thus eliciting a greater indirect xenogenic response. 
Overcoming cellular rejection
Proposed strategy to avoid cellular rejection is to induce donor non-responsiveness using hematopoietic chimerism. Donor stem cells are introduced into the bone marrow of the recipient, where they coexist with the recipient’s stem cells. The bone marrow stem cells give rise to cells of hematopoietic lineages, through the process of hematopoiesis . Lymphoid progenitor cells are created by this process and move to the thymus where negative selection eliminates T cells found to be reactive to self. The existence of donor stem cells in the recipient’s bone marrow causes donor reactive T cells to be considered self and undergoes apoptosis . 
Chronic rejection is slow and progressive, and usually occurs in transplants that survive the initial rejection phases. Scientists are still unclear how to avoid chronic rejection, exactly how it works. Nonetheless, it is known that XNAs and the complementary system are not involved.  Fibrosis in the xenograft occurs as a result of immune reactions, cytokines (which stimulate fibroblasts), or healing (following cellular necrosis in acute rejection). Perhaps the major cause of chronic rejection is arteriosclerosis. Lymphocytes, which have been previously activated by antigens in the vessel wall of the graft, activate macrophages to secrete smooth muscle growth factors. This results in a build up of smooth muscle cells on the vessel walls, the hardening and narrowing of vessels within the graft. Chronic rejection leads to pathologic changes of the organ, and is why transplants must be replaced after so many years.  It is also more likely to be aggressive in xenotransplants as opposed to allotransplants. 
Successful efforts have been made to create knockout mice without α1,3GT; The resulting reduction in the highly immunogenic αGal epitope has resulted in the occurrence of hyperacute rejection, as well as dysregulated coagulation, also known as coagulopathy . 
Different organ xenotransplants result in different responses in clotting. For example, transplants in a higher degree of coagulopathy , or odd clotting, than cardiac transplants, resulting in severe thrombocytopenia , causing death within a few days due to bleeding.  An alternate clotting disorder, thrombosisThe protein anticoagulant system may be initiated by preexisting antibodies. Due to this effect, porcine donors must be extensively screened before transplantation. Studies have shown that some porcine transplant cells are able to induce human tissue factor expression, thus stimulating platelet and monocyte aggregation around the organ xenotransplanted, causing severe clotting.  Additionally, spontaneous platelet accumulation can be caused by contact with pig von Willebrand factor. 
Just as the α1,3G epitope is a major problem in xenotransplantation, so too is dysregulated coagulation because of concern. Transgenic pigs that can control for variable coagulant activity based on the specific organ transplanted would make xenotransplantation a more readily available solution for the 70,000 patients per year who do not receive a human donation of the organ or tissue they need. 
Extensive research is required to determine whether animals can replace the physiological functions of human organs. Many issues include:
- Size – Differences in the size range of potential recipients of xenotransplants.
- Longevity – The lifespan of most people is roughly 15 years old, currently it is
- Hormone and protein differences – Some proteins will be molecularly incompatible, which could cause malfunction of important regulatory processes. These differences also make the prospect of hepatic xenotransplantation less promising, since the liver plays an important role in the production of so many proteins. 
- Environment – For example, pig hearts work in a different anatomical site and under different hydrostatic pressure than in humans. 
- Temperature – The body temperature of pigs is 39 ° C (2 ° C above the average human body temperature). Implications of this difference, if any, on the activity of important enzymes are currently unknown. 
Xenozoonosis, also known as zoonosis or xenosis, is the transmission of infectious agents between species via xenograft. Animal to human infection is normally rare, but has occurred in the past. An example of such is avian influenza , when an influenza virus was passed from birds to humans.  Xenotransplantation may increase the chance of disease transmission for 3 reasons:
- Implantation breaches the physical barrier that normally helps prevent disease transmission,
- The recipient of the transplant will be severely immunosuppressed; and
- Human complement regulators (CD46, CD55, and CD59) have been shown to be useful for the protection of viruses and viruses. 
Examples of viruses carried by pigs include porcine herpesvirus, rotavirus, parvovirus, and circovirus. Porcine herpesviruses and rotaviruses can be eliminated from the donor pool by screening, however others (such as parvovirus and circovirus) may contaminate food and footwear then re-infect the herd. Thus, pigs to be used as organ donors must be housed under strict regulations and screened regularly for microbes and pathogens. Unknown viruses, as well as those not harmful in the animal, may also pose risks.Of particular concern are PERVS (porcine endogenous retroviruses), vertically transmitted microbes that embed in swine genomes. The risks with xenosis are twofold, but not only could the individual become infected, but a novel infection could be an epidemic in the human population. Because of this risk, the FDA has suggested that recipients of xenotransplants will be closely monitored for the remainder of their lives, and quarantined if they show signs of xenosis. 
Baboons and pigs carry myriad transmittable agents that are harmless in their natural host, but extremely toxic and deadly in humans. HIV is an example of a disease believed to have jumped from monkeys to humans. Researchers also do not know if an outbreak of infectious diseases could occur and if they could contain the outbreak even though they have measures for control. Another obstacle facing xenotransplants is that of the body’s rejection of foreign objects by its immune system. These antigens (foreign objects) are often treated with powerful immunosuppressive drugs that could, in turn, make the patient vulnerable to other infections and actually aid the disease. This is the reason the organs would have to be altered to fit the patients’ DNA (histocompatibility).
In 2005, the Australian National Health and Medical Research Council (NHMRC) declared an eighteen-year moratorium on all animal-to-human transplantation, concluding that the risks of transmission of animal viruses to patients and the wider community had not been resolved. This was repealed in 2009 after an NHMRC review stated “… the risks, if appropriately regulated, are minimal and acceptable given the potential benefits.”, citing international developments on the management and regulation of xenotransplantation by the World Health Organisation and the European Medicines Agency.
Porcine endogenous retroviruses
Endogenous retroviruses are remnants of ancient viral infections, found in the genomes of most, if not all, mammalian species. Integrated into the chromosomal DNA, they are vertically transferred through inheritance. Due to the many deletions and mutations they accumulate over time, they usually are not infectious in the host species, however the virus may become infectious in another species. PERVS were originally discovered as retrovirus particles released from cultured porcine kidney cells. Most breeds of swine harbor approximately 50 PERV genomes in their DNA. Although it is likely that most of these are defective, some may be able to produce infectious viruses so every proviral genome must be sequenced to identify which ones pose a threat. In addition, through complementation and genetic recombination, two defective PERV genomes could give rise to an infectious virus. There are three subgroups of infectious PERVs (PERV-A, PERV-B, and PERV-C). Experiments have shown that PERV-A and PERV-B can infect human cells in culture. To date no experimental xenotransplantations have demonstrated PERV transmission, yet this does not mean PERV infections in humans are impossible. Pig cells have been engineered to inactivate all 62 PERVs in the genome using CRISPR Cas9 genome editing technology, and eliminated infection from the pig to human cells in culture.
Xenografts have been a controversial procedure since they were first attempted. Many, including animal rights groups, strongly oppose killing animals to harvest their organs for human use. None of the major religions object to the use of genetically modified pig organs for life-saving transplantation. The prohibition of the consumption of pig does pose problems in Jewish and Islamic communities. In general, the use of pig and cow tissue in humans has been met with little resistance, save some religious beliefs and a few philosophical objections. Experimentation without consent doctrines are now followed, which was not the case in the past, which may lead to new religious guidelines to further medical research on pronounced ecumenical guidelines. The “Common Rule” is the United States bio-ethics mandate as of 2011.
Informed consent of patient
Autonomy and informed consent are important when considering the future uses of xenotransplantation. A patient undergoing xenotransplantation should be fully aware of the procedure and should have no outside force influencing their choice. The patient should understand the risks and benefits of such a transplantation. However, it has been suggested that friends and family members should also give consent, because the repercussions of transplantation are high, with the potential of diseases and viruses crossing over to humans from the transplantation. Close contacts are at risk for such infections. Monitoring of close relations may also be required to ensure that xenozoonosis is not occurring. The question then becomes: does the autonomy of the patient become limited based on the willingness or unwillingness of friends and family to give consent, and are the principles of confidentiality broken?
The safety of public health is a factor to be considered. If there is any risk to the public at all for an outbreak from transplantation there must be procedures in place to protect the public. Not only does the recipient of the transplantation have to understand the risks and benefits, but society must also understand and consent to such an agreement.
The Ethics Committee of the International Xenotransplantation Association points out one major ethical issue is the societal response to such a procedure. The assumption is that the recipient of the transplantation will be asked to undergo lifelong monitoring, which would deny the recipient the ability to terminate the monitoring at any time, which is in direct opposition of the Declaration of Helsinki and the US Code of Federal Regulations.
Xenotransplantion guidelines in the USA
The Food and Drug Administration (FDA) has also stated that if a transplantation takes place the recipient must undergo monitoring for the rest of that recipient’s lifetime and waive their right to withdraw. The reason for requiring lifelong monitoring is due to the risk of acute infections that may occur. The FDA suggests that a passive screening program should be implemented and should extend for the life of the recipient.