Is DNA replaced after organ donation?

Is DNA replaced after organ donation?

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If an organ from person A is transplanted to a new human body B, is it possible that we can detect A's DNA in B?

How long until the organ's DNA is replaced by B's DNA so that we are no longer able to detect any signature of A?

DNA will be never replaced (unless you are speaking about something, where DNA might be only trash, like in the case of blood transplantation or in the case when "organ" would be slowly replaced itself by host regenerative power and "organ" transplantation would be only something temporary, but then we are probably speaking about wider definition of organs).

For further evidence, you may look at cases of chimerism. That is, when young embryos fuse together and create single one with two different cell-lines, which later differentiate to create different organs.

Furthermore, rather more common but with less drastic effect can be somatic chimeras, that is when (usually) early, when embryo is formed only by small amount of cells, one of cell has a mutation, then later in life, this cell again can give rise to organ or several of them. In that case, those organs will carry this mutation while rest of the body will not. This can be potentially more common as this can happen in any single time during development and even after it (in fact, cancer cells are sort of product of that). More interesting cases here:

So, from simple fact that these cases exists and are well-spread, and basically means that several different cell-lines can live in the same body, the answer on your question, if organ DNA will be replaced after transplantation is no.

edit: In the case of whole cell replacement, they can be from both donor and host. However, there are a few problems:

Most of cells that we have are not totipotent, not even pluripotent or multipotent (see:, most of our cells are plain and stupid. This means that highly specialized cells of some organ would probably be replaced only by other highly-specialized cells of that organ (or their precursor cells that are kept for this solo reason, keep regenerativ ability of particular organ, as quite a lot of highly-specialized cells can't divide any more). However, there are a few documented cases with regnancy, where a few cells (embryogenic, thus with bigger "healing" power) migrated through placenta to mother's body and healed her diabetes. So theoretically this is possible, as well as host getting cancer from donor's organ. But again, reason why we are transplanting organs is because human body is not particularly great in regenerating itself and those organs won't grow. So theoretically possible, but not great concern (except for cancer).

Genetically Modified Pigs as Organ Donors for Xenotransplantation

The growing shortage of available organs is a major problem in transplantology. Thus, new and alternative sources of organs need to be found. One promising solution could be xenotransplantation, i.e., the use of animal cells, tissues and organs. The domestic pig is the optimum donor for such transplants. However, xenogeneic transplantation from pigs to humans involves high immune incompatibility and a complex rejection process. The rapid development of genetic engineering techniques enables genome modifications in pigs that reduce the cross-species immune barrier.


Transplanted vascularized organs shed passenger cells, normal constituents of whole organs, which migrate to recipient lymphoid tissues and produce microchimerism. These cells lysed by recipient cytotoxic cells release cellular organelles into the recipient circulation. In addition, warm and cold ischemia as well as immune rejection of the transplanted organ or tissue brings about destructive changes in the graft parenchymatous cells. Fragments of disintegrated cellular organelles are phagocytized by recipient scavenger cells located in lymph nodes, spleen and liver and digested. Some fragments are incorporated into dendritic cells (DC) and processed [ 1 ]. Donor DNA is present in the ingested cellular debris. In sex-mismatched male to female transplantation, the Y-chromosome [sex-determining region Y (Sry)] can be detected using specific primers. Donor DNA identified with this assay was found in blood cells after gut [ 2, 3 ], kidney [ 4-6 ], and liver [ 7 ] transplantation. A standard amount of DNA is used after extraction from recipient blood cells and tissue biopsies. The question arises as to whether the detected donor Y-Sry fragment is present in the surviving donor cells, the recipient macrophages phagocytizing rejected donor cells or recipient DCs internalizing donor-origin apoptotic bodies or cell fragments [ 8 ]. The knowledge of the fate of donor DNA distributed in passenger cells and in fragments of disrupted nuclei as well as the role of recipient cells internalizing donor DNA could give some insight into the mechanism of graft destruction and immunization or tolerance to donor antigens. Moreover, a futuristic question may be posed as to whether donor DNA fragments could enter the nuclei of recipient DCs and get incorporated into their genome, especially in individuals with viral infections.

In this study, we provide evidence that allogeneic organ transplantation is followed by ‘seeding’ of donor DNA from the rejecting graft cells and its internalization in recipient macrophages and DCs in lymphoid organs. Immunosuppression with cyclosporin A (CsA) and tacrolimus (FK506) prolonged retention of donor DNA in recipient tissues and DCs.

Potential offspring

As for the DNA in his sperm being altered, it's unlikely whether we will know if he could pass on the genes of his German donor.

That's because Chris, who is now in remission, had a vasectomy after his second child was born.

But experts say that passing on someone else's genes as a result of a transplant was impossible.

Dr Rezvani said: "There shouldn’t be any way for someone to father someone else’s child."

What is a chimera?

According to Greek mythology, a chimera was a fire-breathing creature that was composed of a goat, a lion, and a snake.

In humans, it's a person who has two different sets of DNA inside their body.

They can occur naturally, and usually have no symptoms, so most people don't know they have two complete sets of DNA, known as genomes.

Typically, one set of DNA is found in one region or organ, while the other can be predominant in other organs or tissues.

A DNA test result would be entirely different depending on where the sample was originally from - blood, saliva, fingernail clippings or hair, etc.

So, some chimeras may have different colour eyes or patches of different skin tones or sections of hair.

Chimerism cany can also happen after a bone marrow transplant - such as in Chris Long's case - often used as a treatment for blood cancers including leukaemia.

Bone marrow is the tissue inside the bones that is responsbile for making white blood cells, red blood cells and platelets.

Doctors use chemotherapy or radiation to destroy all the recipient's diseased bone marrow, then a donor's healthy marrow is put in its place.

And Dr Mehrdad Abedi, who treated Chris, said that it was probably his vasectomy that explained how his semen came to contain his donor's DNA.

For Chris's co-workers at the crime lab it's a different story - they're now looking into how this could affect criminal cases and forensic work.

Brittney Chilton, a criminalist at the sheriff's office, explained that cases of chimeras could be misleading for them.

There shouldn’t be any way for someone to father someone else’s child

Dr Andrew Rezvani Stanford University Medical Center

She began researching chimerism and found that in 2004, investigators in Alaska uploaded a DNA profile extracted from semen to a database.

It matched a potential suspect, but the man had been in prison at the time - and it turned out heɽ had a bone marrow transplant.

Research with chimeras has been going on for decades. Among previous experiments, scientists using different techniques were able to produce a mouse with a rat’s pancreas, and mice with livers almost completely composed of human cells. In the latter case, the liver was not human-sized – one of the reasons why scientists are now using pigs.

There are several areas of concern, not least worries about animal welfare and the possibility that viruses could jump from animals to humans. What’s more, some are concerned that human cells could be involved not only in forming the pancreas of the pig, but also other tissues, including the brain. That proposition gives rise to a host of conundrums around whether the pigs could become human-like in some way. “It would have to be a major contribution either overall or to specific parts of the brain before I would expect it to have any significant effect on pig behaviour,” said Robin Lovell-Badge, a geneticist at the Francis Crick Institute in London.

Clarifying the paradigm for the ethics of donation and transplantation: was 'dead' really so clear before organ donation?

Recent commentaries by Verheijde et al, Evans and Potts suggesting that donation after cardiac death practices routinely violate the dead donor rule are based on flawed presumptions. Cell biology, cardiopulmonary resuscitation, critical care life support technologies, donation and transplantation continue to inform concepts of life and death. The impact of oxygen deprivation to cells, organs and the brain is discussed in relation to death as a biological transition. In the face of advancing organ support and replacement technologies, the reversibility of cardiac arrest is now purely related to the context in which it occurs, in association to the availability and application of support systems to maintain oxygenated circulation. The 'complete and irreversible' lexicon commonly used in death discussions and legal statutes are ambiguous, indefinable and should be replaced by accurate terms. Criticism of controlled DCD on the basis of violating the dead donor rule, where autoresuscitation has not been described beyond 2 minutes, in which life support is withdrawn and CPR is not provided, is not valid. However, any post mortem intervention that re-establishes brain blood flow should be prohibited. In comparison to traditional practice, organ donation has forced the clarification of the diagnostic criteria for death and improved the rigour of the determinations.

Opinion: Ethical Considerations of “Three-Parent” Babies

John D. Loike and Nancy Reame
Dec 22, 2016

© JENNY NICHOLS, WELLCOME IMAGES I n 2015, the US Congress banned the use of gene-editing techniques, such as CRISPR, for the creation of genetically modified human embryos. President Obama also signed into law a policy that precludes modification of the human germline. An unresolved issue with Congress&rsquos ban and President Obama&rsquos law is whether they encompass mitochondrial replacement therapy (MRT), a procedure that allows women with mitochondrial disease to give birth to unaffected children by inserting a nucleus from one of her eggs into an enucleated egg from a woman with healthy mitochondria, followed by in vitro fertilization.

A recent National Academies of Sciences, Engineering, and Medicine report stated that MRT is ethical if conducted exclusively in male embryos and limited to women with mitochondrial diseases. And recent studies have shown that mitochondrial donation has broader clinical applications beyond treating mitochondrial diseases, specifically in relation to infertility problems typically.

Mitochondrial replacement therapy will probably emerge as an effective method to enable women with mitochondrial disease to have healthy children, among other possible medical benefits, and should not be banned because of presumed social or ethical complexities.

MRT does raise a number of social and ethical challenges, however. One of the most debated aspects of mitochondrial donation is its potential for enabling scientists to “play God.” But, humans have engaged in genetic modifications of plants and animals for thousands of years. Moreover, preimplantation genetic diagnosis (PGD), a genetic profiling technique for IVF-generated embryos, can be viewed as a form of genetic selection to alter the human population’s genome pool by eliminating embryos with various genetic diseases. We argue that MRT is akin to these long-standing practices.

Another scientific and legal point of contention is how the term “genetic intervention” is interpreted. Many ethicists restrict its use to the exclusive involvement of germline modification of nuclear DNA. We and others propose that any genetic modification of germ cells—whether in the DNA of the nucleus or the mitochondria—constitutes germline intervention because the donor is providing essential genetic information to the child. Similar to sperm or egg donation, we argue that MRT should be considered a form of genetic intervention and that the congressional and presidential bans be reconsidered.

Concerns have also been raised about the unintended consequences of “three-parent” children. A key consideration here is whether mitochondrial donation should be regulated in the same way as a conventional organ donation—in which case the recipient acquires all the rights and responsibilities of the donated organ and the donor has neither parental rights nor legal responsibility for any unforeseen genetic diseases that the recipient receives from the donation—or be treated as egg donation, which carries unique ethical and legal considerations. Because the genetic contribution from the mitochondrial DNA donor is essential and critical for the development of a healthy child, and because emerging studies show that several mitochondrial genes affect mental illness, we argue that the genetic contribution from mtDNA should not be considered irrelevant to the status of parenthood. But there are legal ways to navigate this ethical challenge, similar to the avenues for giving up parental rights in the case of adoption.

As is the case with adoption as well as with other third-party reproductive technologies, such as egg donation and gestational carriers, MRT also raises the question of whether the resulting offspring have a right to know their genetic parentage and lines of ancestry. This could compromise donor privacy. But again, there are now ways to deal with such challenges. Namely, as the price of gene sequencing decreases, every child can ultimately have his or her genetic information available without revealing the identity of the genetic contributors.

There are two other ethical concerns to MRT that are related to its high costs, which run between $25,000 and $50,000. The first is the fair distribution of medical resources to make this therapy available to all patients regardless of their financial capabilities. This concern can be circumvented, in part, by virtue of its potential eligibility as an approved treatment for infertility or for mitochondrial mutations, as many US states allow insurance providers to cover some costs of IVF for infertility. The second concern is that if the access to this technology is not fairly distributed, then economically disadvantaged women may be at risk of exploitation because of the excessive compensation paid to mitochondrial donors. This financial coercion of impoverished egg providers is already a global problem and serves as a cautionary warning for mitochondrial donation. There should be strict legal safeguards against such practices.

MRT will probably emerge as an effective method to enable women with mitochondrial disease to have healthy children, among other possible medical benefits, and should not be banned because of presumed social or ethical complexities. Given its potential for permanent alterations of DNA, this technology should not be viewed as equivalent to classical organ donation but rather treated with precautions in line with other germline interventions, such as egg or sperm donation, for which regulatory practices are already in place. Using these as a framework, governments and the scientific community should invest time and money into making MRT widely available to patients.

John D. Loike and Nancy Reame are faculty members at Columbia University College of Physicians and Surgeons.

Ten Frequently Asked Questions (FAQs) about Organ Donation

In an effort to demystify the organ and tissue donation process and encourage more Americans to become donors, the National Kidney Foundation provides answers to ten of the most often asked questions about organ and tissue donation.

  1. Are organ and tissue transplants experimental?
    Medication and medical advances have resulted in transplant surgeries today that are very successful, in fact as high as 95 percent. The transplantation of vital organs has become routine surgical operation and is no longer experimental.
  2. How are organs and tissues for transplantation obtained?
    Many organ and tissues are donated by individuals at the time of their death. Others are donated by living donors.
  3. How are organs from deceased donors distributed?
    Generally, donated organs are matched with individuals on an organ waiting list. Matching is based on a variety of factors including blood and tissue types, medical need, length of time on the waiting list and weight of donor and recipient.
  4. Who can become an organ or tissue donor?
    People of all different ages are able to donate. It is essential that anyone who wants to be a donor expresses this wish to others in the family. For more information about becoming an organ and tissue donor, click here.
  5. Do I have to register as an organ and tissue donor with any hospital or national registry?
    There are different ways to identify yourself as an organ donor.

For deceased donation: if an online registry is available in your state, you can sign up for that. (You can also designate your wishes on your driver's license or sign a donor card, but the online registry is the best method to use). It is extremely important to discuss your decision with your loved ones, because they will be asked to sign a consent form at the time of the donation.

For more information about organ and tissue donation, visit the A to Z Guide and the National Kidney Foundation's webpages on Organ Donation and Transplantation.

Could we clone our organs to be used in a transplant?

­How would you like a clone of yourself stowed away somewhere in case you need a new heart or liver, like a spare tire in the trunk of a car? That, in a nutshell, was the plot of the 2005 high-dollar, low-attendance sci-fi movie, "The Island." Hollywood heartthrobs Scarlett Johansson and Ewan McGregor play dual roles portraying the rich and famous -- and their genetically identical clones. In an appropriate Orwellian twist, doctors must murder the "spare" clones in order to harvest needed body parts.

Chances are, "The Island" isn't a glimpse into the future. Nevertheless, it brings up a relevant point about the potential uses for human reproductive cloning. Organ transplants are difficult undertakings for two major reasons. First, you have to find a donor, and second, there's no guarantee that your body will accept the new organ. Statistically, organ demand far outweighs current supply. According to the Organ Procurement and Transplantation Network, 28,356 Americans received organ transplants in 2007 -- around 78 percent of those came from deceased people. Yet as of August 2008, more than 99,000 people in the United States were on the national waiting list for organs [source: OPTN].

­What if you could eliminate the wait time and risky odds with traditional organ transplants by creating custom, cloned organs from your own cells that your body would recognize? Cloning advocates have touted this type of science as therapeutic cloning. This is different from reproductive cloning since therapeutic cloning deals with embryos only, not human babies carried to term.

Embryos contain pluripotent embryonic stem cells, meaning that they can differentiate into more than 200 types of cells. Scientists extract these stem cells when embryos are in the blastocyst phase, the stage when an embryo contains around 150 cells. The stem cells come from the interior of the blastocyst. In November 2007, scientists successfully cloned monkey embryos for the pur­pose of removing stem cells -- this is the closest we've ever come to performing the same procedure in humans. But removing the stem cells effectively destroys the embryo. Many people within and beyond the scientific community disagree with this practice of cloning that terminates embryos, sparking a continued debate about the bioethics of embryonic stem cell research.

­Controversy aside, how would cloned organ transplants work? If you wanted to keep living, doctors obviously couldn't remove your heart and clone a new one, presto-chango. Cloning yourself in order to use the clone's organs wouldn't fly either. Here's where stem cells come in, along with recent scientific breakthroughs that sidestep cloning altogether.

How Organ Cloning Could Work

In order to understand how organ cloning might work, let's first talk about cloning itself. The most common method of therapeutic and reproductive cloning is somatic cell nuclear transfer (SCNT). SCNT involves removing the nucleus from a donor egg, and replacing it with the DNA from the organism meant to be cloned. Scientists could potentially clone organs with SCNT by cloning embryos, extracting the stem cells from the blastocyst, and stimulating the stem cells to differentiate into the desired organ. Coaxing a human stem cell to become a liver, for instance, will require further research. Scientists can reverse engineer cell differentiation processes to understand what chemical or physical signals stem cells receive to properly differentiate. However, that genetic information isn't known for all of the more than 200 types of body cells [source: The National Academies].

Research into human therapeutic cloning has largely come to a halt in the United States [source: Singer]. Aside from bioethical issues, there's a lack of available human eggs for research. Laws and ethical regulations from the National Academy of Sciences and the International Society for Stem Cell Research prohibit monetary compensation for females who donate their eggs for embryonic stem cell research. Coupled with the newness of the science and the potential risks involved with egg donation, stem cell researchers have been hard pressed to find donors. And given the low rate of success with embryonic cloning in general, researchers need an abundance of eggs if they hope to achieve progress. To compensate for the human egg scarcity, Ian Wilmut, who cloned Dolly the sheep, has suggested injecting human DNA into animal eggs instead [source: Singer].

Nevertheless, advancements in therapeutic cloning have been made in animal studies. In March 2008, researchers removed skin cells from mice with Parkinson's disease to test a way to use stem cells as an effective treatment. They inserted the DNA from those skin cells into enucleated eggs (eggs with the nuclei removed) and created cloned mice embryos, via SCNT [source: ScienceDaily]. After extracting stem cells from the cloned embryos, the researchers developed autologous dopamine neurons from them, which are the nerve cells affected by Parkinson's. After implanting the new neurons into the mice, the test animals exhibited signs of recovery [source: ­ScienceDaily].

Xenotransplantation, or transplanting animal organs into humans, has also been examined as a potential source for organ transplants. But if our bodies sometimes reject transplanted organs from other humans, how would they react to animal organs? In 2002, University of Missouri scientists cloned pigs that lack one of two genes called GATA1, which are primarily responsible for inducing that rejection response in humans [source: CNN]. Though primates would make more genetically suitable candidates for xenotransplantation, pigs are the best alternative until monkey cloning is a more viable option [source: Human Genome Project].

Future stem cell development for growing replacement organs may not even require cloning. In February 2008, a group of scientists at the University of California, Los Angeles derived stem cells from adult human skin cells. They were able to do so by controlling four regulator genes that influence cell differentiation [source: ScienceDaily]. By reprogramming the cells to act as stem cells, the altered skin cells became pluripotent and were called induced pluripotent stem cells. A few months later, Dutch researchers extracted adult stem cells from cellular material left over from open heart surgeries [source: ScienceDaily]. They used those stem cells to grow heart muscle cells, without the use of embryonic stem cells or cloning [source: ScienceDaily].

­Because of the ethical gray areas surrounding embryonic stem cell research, people have reacted more positively to alternative methods like the ones described above. In theory, we should be able to eventually grow new organs from stem cells. But the technological advances discussed above indicate that cloning might not be necessary to harness those valuable cells.


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 chronic kidney disease. [8] In the first two decades of the 20th century, several subsequent efforts to use organs from lambs, pigs, and primates were published. [8]

Scientific interest in xenotransplantation declined when the immunological basis of the organ rejection process was described. The next waves of studies on the topic came with the discovery of immunosuppressive drugs. Even more studies followed Dr. Joseph Murray's first successful renal transplantation in 1954 and scientists, facing the ethical questions of organ donation for the first time, accelerated their effort in looking for alternatives to human organs. [8]

In 1963, doctors at Tulane University attempted chimpanzee-to-human renal transplantations in six people who were near death after this and several subsequent unsuccessful attempts to use primates as organ donors and the development of a working cadaver organ procuring program, interest in xenotransplantation for kidney failure dissipated. [8]

An American infant girl known as "Baby Fae" with hypoplastic left heart syndrome was the first infant recipient of a xenotransplantation, when she received a baboon heart in 1984. The procedure was performed by Leonard Lee Bailey at Loma Linda University Medical Center in Loma Linda, California. Fae died 21 days later due to a humoral-based graft rejection thought to be caused mainly 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. While the procedure itself did not advance the progress on xenotransplantation, it did shed a light on the insufficient amount of organs for infants. The story grew so big that it made such an impact that the crisis of infant organ shortage improved for that time. [9] [10]

Xenotransplantation of human tumor cells into immunocompromised mice is a research technique frequently used in oncology research. [11] It is used to predict the sensitivity of the transplanted tumor to various cancer treatments several companies offer this service, including the Jackson Laboratory. [12]

Human organs have been transplanted into animals as 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. [13] For example, researchers from the Ganogen Research Institute transplanted human fetal kidneys into rats which demonstrated life supporting function and growth. [14]

A worldwide shortage of organs for clinical implantation causes about 20–35% of patients who need replacement organs to die on the waiting list. [15] Certain procedures, some of which are being investigated in early clinical trials, aim to use cells or tissues 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 that they would be more readily available for transplant. [ citation needed ]

Xenotransplants could save thousands of patients waiting for donated organs. The animal organ, probably from a pig or baboon could be genetically altered with human genes to trick a patient's immune system into accepting it as a part of its own body. [ citation needed ] They have re-emerged because of the lack of organs available and the constant battle to keep immune systems from rejecting allotransplants. Xenotransplants are thus potentially a more effective alternative. [16] [17] [18]

Xenotransplantation also is and has been a valuable tool used in research laboratories to study developmental biology. [19]

Patient derived tumor xenografts in animals can be used to test treatments. [4]

Since they are the closest relatives to humans, non-human primates were first considered as a potential organ source for xenotransplantation to humans. Chimpanzees were originally considered the best option since their organs are of similar size, and they have good blood type compatibility with humans, which makes them potential candidates for xenotransfusions. However, since chimpanzees are listed as an endangered species, other potential donors were sought. Baboons are more readily available, but impractical as potential donors. Problems include their smaller body size, the infrequency of blood group 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. [20]

Pigs (Sus scrofa domesticus) are currently thought to be the best candidates for organ donation. The risk of cross-species disease transmission is decreased because of their increased phylogenetic distance from humans. [1] Pigs have relatively short gestation periods, large litters, and are easy to breed making them readily available. [21] They are inexpensive and easy to maintain in pathogen-free facilities, and current gene editing tools are adapted to pigs to combat rejection and potential zoonoses. [21] Pig organs are anatomically comparable in size, and new infectious agents are less likely since they have been in close contact with humans through domestication for many generations. [22] Treatments sourced from pigs have proven to be successful such as porcine-derived insulin for patients with diabetes mellitus. [23] Increasingly, genetically engineered pigs are becoming the norm, which raises moral qualms, but also increases the success rate of the transplant. [24] 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, allow experimentation toward the in vivo generation of functional organs from xenogenic pluripotent stem cells in large animals via compensation for an empty developmental niche (blastocyst complementation). [25] Such experiments provide the basis for potential future application of blastocyst complementation to generate transplantable human organs from the patient's own cells, using livestock animals, to increase quality of life for those with end-stage organ failure.

Immunologic barriers Edit

To date, no xenotransplantation trials have been entirely successful due to the many obstacles arising from the response of the recipient's immune system. "Xenozoonoses" are one of the biggest threats to rejections, as they are xenogenetic infections. The introduction of these microorganisms are a big issue that lead to the fatal infections and then rejection of the organs. [26] This response, which is generally more extreme than in allotransplantations, ultimately results in rejection of the xenograft, and can in some cases result in the immediate death of the recipient. There are several types of rejection organ xenografts are faced with, these include hyperacute rejection, acute vascular rejection, cellular rejection, and chronic rejection.

A rapid, violent, and hyperacute response comes as a result of antibodies present in the host organism. These antibodies are known as xenoreactive natural antibodies (XNAs). [1]

Hyperacute rejection Edit

This rapid and violent type of rejection occurs within minutes to hours from 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, which results in endothelial damage, inflammation, thrombosis and necrosis of the transplant. XNAs are first produced and begin circulating in the blood in neonates, after colonization of the bowel by bacteria with galactose moieties on their cell walls. Most of these antibodies are the IgM class, but also include IgG, and IgA. [22]

The epitope XNAs target is an α-linked galactose moiety, Gal-α-1,3Gal (also called the α-Gal epitope), produced by the enzyme α-galactosyl transferase. [27] Most non-primates contain this enzyme thus, this epitope is present on the organ epithelium and is perceived as a foreign antigen by primates, which lack the galactosyl transferase enzyme. In pig to primate xenotransplantation, XNAs recognize porcine glycoproteins of the integrin family. [22]

The binding of XNAs initiate 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. [22]

Overcoming hyperacute rejection Edit

Since hyperacute rejection presents such a barrier to the success of xenografts, several strategies to overcome it are under investigation:

Interruption of the complement cascade

  • The recipient's complement cascade can be inhibited through the use of cobra venom factor (which depletes C3), 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 complement system. [1]

Transgenic organs (Genetically engineered pigs)

  • 1,3 galactosyl transferase gene knockouts – These pigs don't contain the gene that codes for the enzyme responsible for expression of the immunogeneic gal-α-1,3Gal moiety (the α-Gal epitope). [28]
  • Increased expression of H-transferase (α 1,2 fucosyltransferase), an enzyme that competes with galactosyl transferase. Experiments have shown this reduces α-Gal expression by 70%. [29]
  • Expression of human complement regulators (CD55, CD46, and CD59) to inhibit the complement cascade. [30]
  • Plasmaphoresis, on humans to remove 1,3 galactosyltransferase, reduces the risk of activation of effector cells such as CTL (CD8 T cells), complement pathway activation and delayed type hypersensitivity (DTH).

Acute vascular rejection Edit

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. The response is characterized by an inflammatory infiltrate of mostly macrophages and natural killer cells (with small numbers of T cells), intravascular thrombosis, and fibrinoid necrosis of vessel walls. [27]

Binding of the previously mentioned XNAs to the donor endothelium leads to the activation of host macrophages as well as 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, as well as expression of leukocyte adhesion molecules such as E-selectin, intercellular adhesion molecule-1 (ICAM-1), and vascular cell adhesion molecule-1 (VCAM-1). [22]

This response is further perpetuated as normally binding between regulatory proteins and their ligands aid in 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. [27]

Overcoming acute vascular rejection Edit

Due to its complexity, the use of immunosuppressive drugs along with a wide array of approaches are necessary to prevent acute vascular rejection, and include administering a synthetic thrombin inhibitor to modulate thrombogenesis, depletion of anti-galactose antibodies (XNAs) by techniques such as immunoadsorption, to prevent endothelial cell activation, and 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 have to be reassessed for each species combo. [27]

Accommodation Edit

If hyperacute and acute vascular rejection are avoided accommodation is possible, which is the survival of the xenograft despite the presence of circulating XNAs. The graft is given a break from humoral rejection [31] when the complement cascade is interrupted, circulating antibodies are removed, or their function is changed, or there is a change in the expression of surface antigens on the graft. This allows the xenograft to up-regulate and express protective genes, which aid in resistance to injury, such as heme oxygenase-1 (an enzyme that catalyzes the degradation of heme). [22]

Cellular rejection Edit

Rejection of the xenograft in hyperacute 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 that accumulate in and damage 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 peptides to recipient CD4 + T cells via xenogeneic MHC class II molecules, resulting in the production of interleukin 2 (IL-2). Indirect xenorecognition involves the presentation of antigens from the xenograft by recipient antigen presenting cells to CD4 + T cells. Antigens of phagocytosed graft cells can also be presented by the host's class I MHC molecules to CD8 + T cells. [1] [32]

The strength of cellular rejection in xenografts remains uncertain, however, it is expected to be stronger than in allografts due to differences in peptides among different animals. This leads to more antigens potentially recognized as foreign, thus eliciting a greater indirect xenogenic response. [1]

Overcoming cellular rejection Edit

A proposed strategy to avoid cellular rejection is to induce donor non-responsiveness using hematopoietic chimerism. [ citation needed ] 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 all 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 undergo apoptosis. [1]

Chronic rejection Edit

Chronic rejection is slow and progressive, and usually occurs in transplants that survive the initial rejection phases. [27] Scientists are still unclear how chronic rejection exactly works, research in this area is difficult since xenografts rarely survive past the initial acute rejection phases. Nonetheless, it is known that XNAs and the complement system are not primarily involved. [27] 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 were 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, causing 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. [32] It is also anticipated that chronic rejection will be more aggressive in xenotransplants as opposed to allotransplants. [33]

Dysregulated coagulation Edit

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 reduction of the occurrence of hyperacute rejection, but has not eliminated other barriers to xenotransplantation such as dysregulated coagulation, also known as coagulopathy. [34]

Different organ xenotransplants result in different responses in clotting. For example, kidney transplants result in a higher degree of coagulopathy, or impaired clotting, than cardiac transplants, whereas liver xenografts result in severe thrombocytopenia, causing recipient death within a few days due to bleeding. [34] An alternate clotting disorder, thrombosis, may be initiated by preexisting antibodies that affect the protein C anticoagulant system. Due to this effect, porcine donors must be extensively screened before transplantation. Studies have also shown that some porcine transplant cells are able to induce human tissue factor expression, thus stimulating platelet and monocyte aggregation around the xenotransplanted organ, causing severe clotting. [35] Additionally, spontaneous platelet accumulation may be caused by contact with pig von Willebrand factor. [35]

Just as the α1,3G epitope is a major problem in xenotransplantation, so too is dysregulated coagulation a cause 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. [35]

Physiology Edit

Extensive research is required to determine whether animal organs can replace the physiological functions of human organs. Many issues include size – differences in organ size limit the range of potential recipients of xenotransplants longevity – The lifespan of most pigs is roughly 15 years, currently it is unknown whether or not a xenograft may be able to last longer than that 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 [1] environment – for example, pig hearts work in a different anatomical site and under different hydrostatic pressure than in humans [27] 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. [1]

Xenozoonosis Edit

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 the avian influenza, when an influenza A virus was passed from birds to humans. [36] Xenotransplantation may increase the chance of disease transmission for 3 reasons: (1) implantation breaches the physical barrier that normally helps to prevent disease transmission, (2) the recipient of the transplant will be severely immunosuppressed, and (3) human complement regulators (CD46, CD55, and CD59) expressed in transgenic pigs have been shown to serve as virus receptors, and may also help to protect viruses from attack by the complement system. [37]

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. [37] Of particular concern are PERVS (porcine endogenous retroviruses), vertically transmitted microbes that embed in swine genomes. The risks with xenosis are twofold, as not only could the individual become infected, but a novel infection could initiate an epidemic in the human population. Because of this risk, the FDA has suggested any recipients of xenotransplants shall be closely monitored for the remainder of their life, and quarantined if they show signs of xenosis. [38]

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. [39] 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. [40]

Porcine endogenous retroviruses Edit

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. [33] 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. [22] PERVS were originally discovered as retrovirus particles released from cultured porcine kidney cells. [41] Most breeds of swine harbor approximately 50 PERV genomes in their DNA. [42] 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. [43] 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. [42] [44] To date no experimental xenotransplantations have demonstrated PERV transmission, yet this does not mean PERV infections in humans are impossible. [37] Pig cells have been engineered to inactivate all 62 PERVs in the genome using CRISPR Cas9 genome editing technology, [45] and eliminated infection from the pig to human cells in culture. [46] [47] [48]

Ethics Edit

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. [49] In the 1960s, many organs came from the chimpanzees, and were transferred into people that were deathly ill, and in turn, did not live much longer afterwards. [50] Modern scientific supporters of xenotransplantation argue that the potential benefits to society outweigh the risks, making pursuing xenotransplantation the moral choice. [51] None of the major religions object to the use of genetically modified pig organs for life-saving transplantation. [52] Religions such as Buddhism and Jainism, however, have long espoused non-violence towards all living creatures. [24] 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 [update] . [53]

History of Xenotransplantation in Ethics Edit

At the beginning of the 20th century when studies in Xenotransplantation were just beginning, few questioned the morality of it, turning to animals as a "natural" alternative to allografts. [54] While satirical plays mocked Xenografters such as Serge Voronoff, and some images showing emotionally distraught primates appeared - who Voronoff had deprived of their testicles - no serious attempts were yet made to question the science based on animal rights concerns. [54] Xenotransplantation was not taken seriously, at least in France, during the first half of the 20th century. [54]

With the Baby Fae incident of 1984 as the impetus, animal rights activists began to protest, gathering media attention and proving that some people felt that it was unethical and a violation of the animal's own rights to use its organs to preserve a sick human's life. [54] Treating animals as mere tools for the slaughter on demand by human will would lead to a world they would not prefer. [54] Supporters of the transplant pushed back, claiming that saving a human life justifies the sacrifice of an animal one. [54] Most animal rights activists found the use of primate organs more reprehensible than those of, for example, pigs. [54] As Peter Singer et al. have expressed, many primates exhibit greater social structure, communication skills, and affection than mentally deficient humans and human infants. [55] Despite this, it is considerably unlikely that animal suffering will provide sufficient impetus for regulators to prevent xenotransplantation. [24]

Informed consent of patient Edit

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. [56] 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. [57] 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. [58] 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. In 2007, xenotransplantation was banned under ethical grounds in all countries but Argentina, Russia and New Zealand. Since then, the practice has only been carried out to treatment for diabetes type 1 to serve as a substitute for insulin injections.

The application of the four bioethics principal is found to be everywhere because it is now standardized in the moral conducts of a laboratory. [59] The four principles emphasize on the informed consent, the Hippocratic Oath to do no harm, apply one's skill to help others, and protecting the rights of others to quality care. [60]

Problems with xenotransplantation is that even though it has future medical benefits, it also has the serious risk of introducing and spreading the infectious diseases, into the human population. [61] There have been guidelines that have been drafted by the government that have the purpose of forming the foundation of infectious disease surveillance. [61] In the United Kingdom, the guideline that were introduced state that first, "the periodic provision of bodily samples that would then be archived for epidemiological purposes" second, "post-mortem analysis in case of death, the storage of samples post-mortem, and the disclosure of this agreement to their family" third, "refrain from donating blood, tissue or organs" fourth, "the use of barrier contraception when engaging in sexual intercourse" fifth, keep both name and current address on register and to notify the relevant health authorities when moving abroad" and lastly "divulge confidential information, including one's status as a xenotransplantation recipient to researchers, all health care professionals from whom one seeks professional services, and close contacts such as current and future sexual partners." [62] With these guidelines in place the patient has to abide to these rules until either their lifetime or until the government determines that there is no need for public health safe guards. [62]

Xenotransplantation guidelines in the United States Edit

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. [63]