Every ten minutes, another name is added to the national organ transplant waiting list. Every day, seventeen people on that list die. As of 2024, more than 100,000 Americans are waiting for a lifesaving organ transplant, and the gap between supply and demand grows wider each year. The arithmetic is brutal: roughly 46,000 transplants are performed annually in the United States, but the need is more than double that figure.
For decades, scientists have looked across the species barrier for a solution. What if organs from animals could be engineered to work inside human bodies? The concept is called xenotransplantation — from the Greek xenos, meaning "foreign" — and it has been a dream of medicine since the 1960s. For most of that time, it remained exactly that: a dream, thwarted by the ferocious immune response that the human body mounts against animal tissue.
Then came CRISPR.
Between 2022 and 2025, a series of extraordinary surgeries thrust xenotransplantation from the theoretical into the clinical. Surgeons transplanted gene-edited pig hearts and kidneys into human patients, achieving survival times that would have been unthinkable a decade earlier. Behind these operations lay years of painstaking genetic engineering — dozens of edits to the pig genome, designed to make porcine organs invisible to the human immune system and free of infectious risk.
This is the story of how gene editing turned xenotransplantation from science fiction into surgical reality, and why pig organs may soon become a routine source of transplantable tissue for humans.
A kidney transplant procedure. Surgeons have now begun transplanting gene-edited pig kidneys into human patients, opening a new frontier in transplant medicine. Image: Wikimedia Commons / CC BY-SA 4.0
The Organ Shortage Crisis
The numbers tell a story of quiet, ongoing catastrophe. According to the Organ Procurement and Transplantation Network (OPTN), 104,954 candidates were on the national transplant waiting list as of January 2024. Kidneys account for the largest share — about 89,000 of those waiting need a kidney. Livers, hearts, and lungs make up most of the remainder.
The crisis is not evenly distributed. Black Americans are three to four times more likely to develop kidney failure than white Americans, but they wait significantly longer for transplants. Geographic disparities compound the problem: patients in some regions wait years longer than those in others for the same organ.
"We have been talking about the organ shortage for forty years, and it has only gotten worse," said Dr. Robert Montgomery, director of the NYU Langone Transplant Institute, in a 2024 interview with The New York Times. "We need a fundamentally new source of organs. Incremental improvements in donation are not going to solve this."
The consequences extend beyond the waiting list itself. Many patients die before they ever make it onto the list because they are deemed too sick or too old. The true unmet need for organ transplantation is far larger than official figures suggest. The National Kidney Foundation estimates that more than 800,000 Americans live with end-stage kidney disease, the vast majority managed through dialysis — a treatment that keeps patients alive but exacts a devastating toll on quality of life and costs the Medicare system roughly $50 billion per year.
Why Pigs?
Scientists considered many animal species before settling on pigs as the most promising organ source. Non-human primates — baboons, chimpanzees — were early candidates because of their close evolutionary relationship to humans. But primate organs are too small, primates breed slowly, and the ethical concerns around using our closest relatives proved insurmountable. In 1984, surgeon Leonard Bailey transplanted a baboon heart into a newborn infant known as "Baby Fae" at Loma Linda University Medical Center. The child survived 21 days before the organ was rejected. The case drew worldwide attention and intense criticism.
Pigs, by contrast, offer compelling practical advantages:
- Size match: Adult pig organs are roughly the same size as human organs. A pig heart weighs about 300 grams, comparable to a human heart.
- Physiological compatibility: Pig cardiovascular and renal physiology is remarkably similar to human physiology. Pig heart valves have been used in human cardiac surgery since the 1960s.
- Breeding efficiency: Pigs reproduce quickly, with large litters and short gestation periods, making it feasible to maintain herds of genetically engineered animals.
- Ethical framework: While raising pigs for organs raises its own ethical questions, society already accepts raising pigs for food on an enormous scale — more than 120 million pigs are slaughtered annually in the United States alone.
- Controlled environment: Pigs can be raised in biosecure, pathogen-free facilities, reducing infectious risk.
"The pig is nature's gift to the transplant surgeon," Dr. David Cooper, a xenotransplantation pioneer at the University of Alabama at Birmingham, has said. Cooper, who has worked on cross-species transplantation since the 1980s, helped establish the scientific framework for pig-to-human organ transfer.
Three Barriers to Xenotransplantation
Despite their advantages, pig organs face three formidable biological barriers when placed inside a human body. Understanding these barriers is essential to appreciating why CRISPR has been so transformative.
Barrier 1: Hyperacute Rejection
The most immediate threat is hyperacute rejection — a violent immune response that occurs within minutes of transplantation. The culprit is a sugar molecule called alpha-gal (galactose-alpha-1,3-galactose) that coats the surface of pig cells. Humans lost the gene that produces alpha-gal during evolution but retain preformed antibodies against it. The moment human blood contacts pig tissue, these antibodies bind to alpha-gal, activate the complement cascade, and trigger massive inflammation that destroys the organ within hours.
Two additional sugar antigens — Neu5Gc (produced by the CMAH gene) and the SDa blood group antigen (produced by the B4GALNT2 gene) — provoke similar, if somewhat less explosive, immune reactions. Together, these three carbohydrate antigens represent the first and most critical barrier to xenotransplantation.
Barrier 2: Porcine Endogenous Retroviruses (PERVs)
Embedded within the pig genome are remnants of ancient viral infections — porcine endogenous retroviruses, or PERVs. These viral sequences are stitched into the DNA of every pig cell and can, under certain conditions, produce active viral particles. In laboratory experiments, PERVs have been shown to infect human cells in culture.
The fear was that transplanting a pig organ into an immunosuppressed human patient could unleash these dormant viruses, potentially causing disease in the recipient or even sparking a new epidemic. While no PERV transmission to humans has been documented in clinical settings, regulators and scientists agreed that the risk needed to be eliminated rather than merely monitored.
Barrier 3: Coagulation Incompatibility
Even if the immune system does not immediately destroy a pig organ, molecular incompatibilities between pig and human coagulation systems can cause clotting disorders. Pig thrombomodulin, a key anticoagulant protein on the surface of blood vessel cells, does not interact efficiently with human thrombin. The result is dysregulated coagulation — blood clots form inside the transplanted organ, destroying it over days or weeks.
Additional incompatibilities in the complement regulatory system and in cellular signaling between pig endothelial cells and human immune cells contribute to chronic rejection and organ failure. These subtler barriers proved to be the hardest to overcome.
Xenotransplantation research has required overcoming multiple biological barriers, including immune rejection, viral risk, and coagulation incompatibility. Image: Wikimedia Commons / CC BY-SA 4.0
CRISPR Rewrites the Pig Genome
Before CRISPR, genetic engineering in pigs was agonizingly slow. Creating a single gene knockout could take years of breeding. The idea of making dozens of precise edits to the pig genome was not merely difficult — it was practically impossible.
CRISPR-Cas9 changed the calculus entirely. The technology allowed scientists to make multiple targeted cuts in the pig genome simultaneously, deleting unwanted genes and inserting beneficial human genes in a single generation. The result was a new class of genetically engineered pigs designed from the ground up for organ donation.
eGenesis: The 69-Edit Pig
The most ambitious genetic engineering program belongs to eGenesis, a Cambridge, Massachusetts, biotechnology company co-founded by George Church, the legendary Harvard geneticist, and Luhan Yang, a former postdoc in his lab. eGenesis has created pigs carrying an unprecedented 69 individual genomic edits — the most extensively engineered mammals ever produced.
These edits fall into three categories:
Knockout of pig antigens (3 edits): The genes responsible for producing alpha-gal (GGTA1), Neu5Gc (CMAH), and the SDa antigen (B4GALNT2) are all deleted. Without these sugar molecules on their cell surfaces, pig organs no longer trigger hyperacute rejection.
Insertion of human transgenes (7 edits): Seven human genes are added to the pig genome to help the organ evade the human immune system and regulate coagulation. These include human CD46, CD55, and CD59 (which inhibit the complement cascade), human thrombomodulin (THBD, which regulates clotting), human EPCR (endothelial protein C receptor), and human CD47 (a "don't eat me" signal that prevents macrophages from attacking the organ).
Inactivation of PERVs (59 edits): In a remarkable 2015 paper published in Science, Church and Yang demonstrated that CRISPR could simultaneously inactivate all 62 copies (later refined to 59 in their production line) of porcine endogenous retroviruses in the pig genome. This was a technical tour de force — no one had previously used CRISPR to make so many edits in a single cell line. The PERV-free pigs eliminated the viral transmission risk that had haunted xenotransplantation for decades.
"We were told it couldn't be done — that you couldn't make that many edits without destroying the genome," George Church told STAT News in 2023. "But the pigs are healthy. They're normal pigs, except that their organs are designed for humans."
Revivicor, a subsidiary of United Therapeutics, pursued a different but overlapping strategy, producing pigs with ten genetic modifications — three knockouts and seven human gene insertions — without addressing PERVs. Their GalSafe pigs received FDA approval in 2020 for potential use as a source of human therapeutics, the first intentional genomic alteration in an animal approved for both food and medical use.
The Surgeries That Changed Everything
David Bennett and the First Pig Heart (2022)
On January 7, 2022, surgeons at the University of Maryland Medical Center performed the first transplant of a genetically modified pig heart into a living human patient. David Bennett Sr., a 57-year-old handyman from Maryland, was dying of heart failure and had been rejected from conventional transplant lists because of his poor health and history of non-compliance with medical protocols.
The surgery, led by Dr. Bartley Griffith, used a heart from a Revivicor pig carrying ten genetic modifications. Bennett survived for two months — 60 days — with the pig heart supporting his circulation.
"It was either die or do this transplant," Bennett said the day before his surgery. "I want to live. I know it's a shot in the dark, but it's my last choice."
Bennett's death on March 8, 2022, was initially attributed to a combination of factors, but subsequent analysis revealed a critical finding: the pig heart was infected with porcine cytomegalovirus (pCMV), a herpesvirus that was not detected by standard screening tests before transplantation. The virus reactivated after transplant and likely contributed to the organ's failure.
"The presence of the pig virus was almost certainly a contributing factor," said Dr. Muhammad Mohiuddin, who led the University of Maryland xenotransplantation program. "It taught us a lesson about how rigorous our screening protocols need to be."
The Bennett case was simultaneously a breakthrough and a cautionary tale. It proved that a pig heart could sustain human life for weeks, but it underscored the importance of pathogen-free animal husbandry and comprehensive viral screening.
Rick Slayman: The First Pig Kidney in a Living Patient (2024)
On March 16, 2024, surgeons at Massachusetts General Hospital performed the first transplant of a gene-edited pig kidney into a living human patient. Richard "Rick" Slayman, a 62-year-old man from Weymouth, Massachusetts, had been on dialysis after his previous human kidney transplant failed.
The kidney came from an eGenesis pig carrying 69 genetic edits. The surgery was led by Dr. Tatsuo Kawai, and the transplant was performed under the FDA's compassionate use pathway, which allows unapproved treatments for patients with no other options.
The pig kidney began producing urine almost immediately after the blood supply was connected — a sign that it was functioning. Slayman was discharged from the hospital two weeks after surgery with good kidney function.
"I saw the pig kidney transplant not only as a way to help me, but as a way to give hope to the thousands of people who need a transplant to survive," Slayman said in a statement released by Massachusetts General Hospital.
Tragically, Slayman died on May 11, 2024, approximately two months after the transplant. Massachusetts General Hospital stated that there was "no indication that the cause of death was a result of the recent transplant." Slayman had multiple serious comorbidities, including heart disease and diabetes, that had contributed to his kidney failure in the first place.
Towana Looney: Kidney Transplant at NYU (2024)
In April 2024, surgeons at NYU Langone Health transplanted a gene-edited pig kidney into Towana Looney, a 54-year-old woman from Alabama who had been on dialysis for years. The surgery was led by Dr. Robert Montgomery, who had previously conducted pig kidney transplants in brain-dead patients.
Looney's case used a pig kidney with a different modification profile, and her survival extended further than Slayman's, providing additional clinical data on how gene-edited pig organs perform in living humans over time. The NYU team monitored her closely for signs of rejection, infection, and organ function, generating data that would help shape future clinical trials.
Dr. Montgomery described the moment the kidney began working: "When we connected the blood vessels and you could see the kidney turn pink and start making urine, there was this collective holding of breath in the operating room, and then it happened. It was remarkable."
Lisa Pisano: Pig Kidney and Mechanical Heart (2024)
In an even more complex case, NYU Langone surgeons transplanted a pig kidney into Lisa Pisano, a 54-year-old woman from New Jersey who also received a mechanical heart pump (left ventricular assist device). The dual procedure — combining a pig kidney with mechanical cardiac support — represented the most technically challenging xenotransplant attempted.
Pisano's case illustrated both the potential and the limitations of current xenotransplantation. The pig kidney functioned initially but eventually needed to be removed due to complications. Her case highlighted the challenges of managing immunosuppression in patients with multiple organ systems in failure.
Brain-Dead Donor Studies
Before the living patient surgeries, several teams conducted critical experiments using brain-dead individuals maintained on ventilators — patients who had been declared brain-dead and whose families consented to the research.
In September and November 2021, Dr. Montgomery's team at NYU Langone attached gene-edited pig kidneys to brain-dead patients and observed them functioning outside the body. In January 2022, surgeons at the University of Alabama at Birmingham transplanted pig kidneys into a brain-dead man and observed function for over 74 hours. These studies provided essential proof-of-concept data and helped convince the FDA to authorize compassionate use in living patients.
Immunological Challenges Beyond the First Barrier
While genetic engineering has largely solved hyperacute rejection, longer-term immunological challenges remain formidable. Patients who receive pig organs require intensive immunosuppressive therapy — often more aggressive than what is used for human-to-human transplants.
The immune system attacks foreign tissue through multiple mechanisms:
- Antibody-mediated rejection: Even with the three main sugar antigens removed, human immune systems can develop antibodies against other pig proteins over time.
- Cellular rejection: T cells can recognize pig cells as foreign through pathways that are not addressed by current genetic modifications.
- Chronic inflammation: Low-grade immune activation can cause fibrosis and gradual organ deterioration over months and years.
Researchers are exploring additional genetic modifications to address these challenges. Some teams are investigating the deletion of swine leukocyte antigen (SLA) genes — the pig equivalent of human HLA — to reduce T cell recognition. Others are inserting additional human immune-modulatory genes, such as PD-L1 and HLA-E, to create a more comprehensive cloak of invisibility.
"We have overcome the first mountain — hyperacute rejection — but there is a whole mountain range ahead of us," said Dr. Joseph Tector, a xenotransplantation researcher at the University of Miami. "Each patient we treat teaches us something new about how the human immune system responds to pig tissue over time."
Ethical Debates
Xenotransplantation raises a constellation of ethical questions that extend beyond the science.
Animal Rights and Welfare
Raising pigs specifically to harvest their organs troubles many ethicists and animal welfare advocates. These are not ordinary farm pigs — they are bred and housed in sterile, biosecure facilities, often in isolation to prevent pathogen exposure. Their lives are entirely instrumentalized for human benefit.
Proponents counter that the moral calculus is straightforward: millions of pigs are already killed for food, and using genetically engineered pigs to save human lives represents a higher-order use. Others argue that the comparison is flawed — that creating animals with the sole purpose of organ extraction represents a different moral category than raising animals for nutrition.
Informed Consent
The first xenotransplant recipients were patients who had exhausted all other options. Critics have raised questions about whether truly informed consent is possible when a patient is facing death and the experimental treatment is their only alternative. The concept of "desperate consent" — agreeing to a radical procedure because the alternative is dying — complicates traditional informed consent frameworks.
Equity and Access
If xenotransplantation succeeds commercially, who will benefit? The technology is being developed primarily by well-funded American biotechnology companies. Will pig organs be available to patients in sub-Saharan Africa, South Asia, and other regions where organ shortages are most severe? Or will this become another life-saving technology available only to the wealthy and well-connected?
Infectious Disease Risk
Despite the inactivation of PERVs, the possibility of novel zoonotic infections — diseases jumping from animals to humans — remains a concern. The COVID-19 pandemic heightened awareness of the risks posed by close contact between human and animal biology. Regulators will need to maintain vigilant surveillance programs for any patient who receives a pig organ.
The FDA Regulatory Pathway
The FDA has been cautiously supportive of xenotransplantation research. The first pig organ transplants in living patients were authorized under the compassionate use (expanded access) pathway, which allows unapproved treatments for seriously ill patients on a case-by-case basis.
For xenotransplantation to become widely available, companies must conduct formal clinical trials and demonstrate safety and efficacy to the FDA's satisfaction. In 2024, eGenesis announced plans to file an Investigational New Drug (IND) application to begin a Phase 1 clinical trial of its pig kidneys in patients with end-stage renal disease.
The FDA's regulatory framework for xenotransplantation was originally established in the 1990s but has been updated to reflect advances in genetic engineering. Key regulatory requirements include:
- Animal source standards: Pigs must be raised in designated pathogen-free facilities with rigorous health monitoring.
- Comprehensive screening: Organs must be tested for a wide range of known and potential pathogens before transplantation.
- Lifelong monitoring: Recipients of xenotransplants must be monitored for the rest of their lives for signs of zoonotic infection.
- Archive samples: Biological samples from source animals and recipients must be archived indefinitely to allow retrospective analysis if problems emerge.
Dr. Peter Marks, director of the FDA's Center for Biologics Evaluation and Research, said in 2024 that the agency is "committed to working with sponsors to advance xenotransplantation as safely and expeditiously as possible. The unmet medical need is enormous."
The Commercial Timeline
The path from compassionate use to commercial availability is long but increasingly well-defined. Based on public statements from companies and regulatory experts, the timeline looks roughly as follows:
- 2024-2025: Compassionate use cases in individual patients; IND applications filed.
- 2025-2027: Phase 1 clinical trials begin, focusing on safety and initial efficacy in small patient groups.
- 2027-2030: Phase 2/3 trials expand to larger patient populations, generating the data needed for regulatory approval.
- 2030-2032: Potential FDA approval and commercial launch of pig kidneys for transplantation.
Several companies are racing toward this goal:
eGenesis (Cambridge, MA): Founded by George Church and Luhan Yang. Their 69-edit pigs represent the most comprehensively engineered animals for xenotransplantation. The company raised $125 million in Series D funding in 2023 and has partnerships with academic medical centers for clinical studies.
United Therapeutics / Revivicor (Silver Spring, MD): Led by CEO Martine Rothblatt, United Therapeutics has invested heavily in xenotransplantation infrastructure, including a pig production facility in Virginia capable of housing genetically engineered pigs in GMP (Good Manufacturing Practice) conditions. Their ten-gene-modified pigs were used in the David Bennett pig heart surgery and in brain-dead donor studies at NYU.
Makana Therapeutics (Indianapolis, IN): A newer entrant focusing on pig kidneys with a proprietary genetic modification strategy that includes knocking out SLA class I genes.
Industry analysts project that the pig organ market could be worth $6 billion to $10 billion annually if xenotransplantation achieves regulatory approval for kidneys alone. Adding hearts and livers could push the total addressable market significantly higher.
Comparing Alternatives: Artificial Organs, Stem Cells, and Bioprinting
Xenotransplantation is not the only approach to the organ shortage. Several competing technologies are in various stages of development.
Artificial Organs and Mechanical Devices
Mechanical hearts (left ventricular assist devices, or LVADs) and dialysis machines already serve as bridges to transplant or as destination therapy. However, these devices have significant limitations:
- LVADs require external power sources, carry infection risks at the driveline site, and cause blood clotting complications.
- Dialysis addresses only a fraction of normal kidney function and significantly impairs quality of life.
- A fully implantable artificial kidney remains in early development. The Kidney Project, a federally funded initiative, is developing a bioartificial kidney that combines silicon filters with living kidney cells, but clinical trials are still years away.
Stem Cell-Derived Organs
Researchers are working to grow functional organs from human stem cells — either induced pluripotent stem cells (iPSCs) or embryonic stem cells. In theory, this approach could produce immunologically matched organs that the body would accept without immunosuppression.
In practice, growing a full-sized, functional organ from stem cells remains enormously challenging. While scientists have created organoids — miniature, simplified versions of organs — scaling these up to transplantable size with proper vascularization, innervation, and architecture has not yet been achieved. Most experts estimate that transplantable stem cell-derived organs are at least 15 to 20 years away.
3D Bioprinting
Bioprinting uses modified 3D printers to deposit layers of living cells and biomaterials to create tissue structures. Companies like Organovo and CollPlant have made progress in printing liver tissue patches and corneal implants, but printing a full vascularized organ remains beyond current capabilities.
How Xenotransplantation Compares
Among these alternatives, xenotransplantation is the closest to clinical reality. The organs already exist — fully formed, properly vascularized, and the right size. The challenge is biological compatibility, not construction. This is why the field has attracted the most commercial investment and clinical attention.
"Xenotransplantation is the only near-term solution to the organ shortage that could operate at scale," said Dr. Jay Fishman, co-director of the MGH Transplant Center and an infectious disease specialist who consulted on the Rick Slayman case. "We should pursue all avenues, but if you ask me what could save the most lives in the next ten years, the answer is pig organs."
Heart surgery in a modern operating room. Xenotransplantation procedures require the same exacting surgical environments used for conventional organ transplants. Image: Wikimedia Commons / CC BY-SA 2.0
What Comes Next
The next several years will be decisive for xenotransplantation. Clinical trials will need to demonstrate not just short-term survival, but durable organ function measured in years, not weeks. Immunosuppression protocols will need to be refined to balance rejection prevention against infection risk. Manufacturing and supply chain systems will need to scale from individual compassionate-use cases to hundreds or thousands of organs per year.
Several key milestones to watch:
- Phase 1 trial results: The first formal clinical trial data will provide rigorous evidence about safety and efficacy in a controlled patient population.
- One-year survival: Demonstrating that a pig kidney or heart can function inside a human body for at least one year would represent a transformative milestone.
- Immunosuppression optimization: Developing immunosuppressive protocols specific to xenotransplantation — potentially including novel biologic agents — will be essential for long-term success.
- Pathogen screening standards: The pig cytomegalovirus finding in the Bennett case has driven the development of more comprehensive screening panels. These standards will need to be codified by regulatory agencies.
- Cost and reimbursement: Even if pig organs are approved, the economics must work. Producing genetically engineered pigs in biosecure facilities is expensive, and payers will need to see clear evidence of cost-effectiveness compared to dialysis or repeat human organ transplants.
The scientists, surgeons, and patients who have brought xenotransplantation to this point have accomplished something remarkable. They have shown that the species barrier, long considered impenetrable, can be crossed with the right combination of genetic engineering and surgical skill. The question is no longer whether pig organs can work in humans. The question is how well, for how long, and for how many.
For the 100,000 Americans on the transplant waiting list — and the millions more worldwide — the answer cannot come soon enough.
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