If the dog is man’s best friend, it seems that pigs may soon be the best allies of cardiac surgeons and their patients awaiting a transplant. This porcine heart xenotransplantation project pushes to the limit the saying that “nothing is wasted from a pig.” The demand for organs is progressively increasing in a population with higher expectations for both quality and quantity of life, paralleled by an almost epidemic rise in advanced heart failure. Although Spain leads in organ donation and heart transplantation (as demonstrated in our blog analysis of the 2022 Spanish heart transplant registry), the supply-demand imbalance remains. Various alternatives have been proposed to compensate for this disparity:

• Acceptance of older and hepatitis C-positive donors, topics we have discussed in our blog.
• The implantation of intracorporeal ventricular assist devices (VADs) as a bridge to candidacy or to reduce mortality on the waiting list. However, in some cases, conversion to destination therapy is the least harmful option when an organ does not arrive or an indication is lost. The REGALAD registry on long-term VAD use in Spain, as well as updates on current status and advances in VAD technology, have also been covered in recent blog posts.
• Controlled circulatory death donation, which has shown promising growth. However, in our country, this has not succeeded in increasing the number of transplants. Despite demonstrating good outcomes that may even surpass those from brain death, it seems that many controlled circulatory death donors in centers where this is practiced are the same candidates as those for brain death. Thus, it merely advances the process and limits organ deterioration in potential donors. This practice is primarily confined to transplant centers, with organs from peripheral centers primarily resulting from brain death. Expanding ECMO-supported portable systems for controlled circulatory death in other centers may enhance organ availability. Although we have yet to analyze articles on controlled circulatory death donation in Spain in our blog, we have discussed the situation in the United States through commentary in 2023 and 2024.

In June 2022, we witnessed one of the milestones in medical history, the first genetically modified porcine heart xenotransplantation performed by the University of Maryland team in Baltimore. Although the procedure was marred by complications, including intraoperative aortic dissection that required correction, the patient survived for two months, ultimately dying from an unexplained condition suggestive of early rejection. Unfortunately, this truncated hope received little attention in the media or on social networks, raising concerns about the direction of our society. Such news remains almost anecdotal, failing to compare to the publicity LIFE magazine gave Barnard’s milestone.

This was not the first xenotransplantation attempt; Hardy at the University of Mississippi had already performed one using a chimpanzee heart in 1964, though the patient died two hours later. The official explanation for what appeared to be a hyperacute rejection was that the organ was too small to sustain human cardiac output. After a long period without further attempts, the above-mentioned procedure was followed by two new implants with analogous methodology in June and July 2022 by the New York University team on brain-dead patients who had previously donated their bodies to science. Both procedures were successful for 72 hours, with adequate functioning of the implanted hearts, though the experiment was limited in duration for ethical reasons. The Maryland team performed a second human transplant in September 2023, with the patient dying four days later, likely due to rejection. Both candidates, aged 45 and 58, had been deemed ineligible for transplants due to comorbidities.

While xenotransplantation has been discussed, aside from sourcing from an animal rather than a human, what implications does this entail and why has it taken 60 years to achieve, even with limited success? Both humans and many anthropoid primates (such as baboons used in pre-human experiences) develop immunity to antigens present in porcine tissues. This is highly relevant for the construction of valvular prostheses, where these antigens are destroyed by decellularization and aldehyde solutions. However, this is not feasible in a living organ, leading to an antigen-antibody reaction that results in hyperacute rejection. Without delving too deeply, it is worth mentioning the alpha-Gal, Sd, and Neu5Gc antigens. Humans naturally produce antibodies against the latter two due to dietary contact. To ensure biocompatibility, genetically modified donor pigs are triple knock-out, meaning they do not express any of these three antigens. Although this may seem sufficient, two more hurdles remain for achieving biocompatibility:

• The first relates to immune damage unrelated to antigen-antibody complexes, such as ischemia-reperfusion injury. While we know little about the human heart’s behavior in this situation, in porcine hearts, the expression of human CD46, CD55, and CD59 genes—regulators of complement activation absent in pigs—is promoted, limiting human complement damage to animal tissues.
• The second hurdle involves porcine vascular bed dysregulation in response to human blood’s hemostatic function, as porcine endothelium does not interact or fulfill the antithrombotic role as in humans. In fact, the porcine model for hemostasis is suboptimal, showing a procoagulant state compared to humans. This facilitates hemostasis after tissue injury, but what is valid in a pig’s body is not for a porcine heart in a human body. This procoagulant endothelium state would lead to obstructive/thrombotic microangiopathy of the graft, compensated by enhancing the expression of anticoagulants like thrombomodulin and endothelial protein C receptor factor in genetically modified pigs.

The Maryland group has continued research and collaborates with other teams, such as the German team responsible for the work analyzed here. This document summarizes experiences shared during a xenotransplantation workshop. Among the advances and future prospects, the following can be highlighted:

• Preservation of organs under low ischemic aggression, as porcine hearts are more sensitive than human ones. They advocate for preservation under perfusion at 8°C, with hyperoncotic, cardioplegic, oxygenated blood solution enriched with hormones and nutrients, minimizing time to implantation. This solution was used for the first case in Maryland.
• Development of custom immunotherapies, given that the antigenic profile of donors is more controlled, based on selective CD40 blockade with monoclonal antibodies, cortisone, and antiCD20 (rituximab). This pharmacological protocol limits renal damage and reduces the need for serum level monitoring compared to agents used for human-to-human organs.
• Control of organ growth: Donor pigs at the time of sacrifice are significantly young, reaching 200-300 kg with hearts of 1 kg, overly large for an adult human. This disproportion posed a serious challenge in the animal model, where recipients were baboons weighing barely 20 kg. As has occurred with kidney transplantation, the organ would continue to grow, leading to disproportion in the human recipient if long-term survival is achieved. Projects are underway to develop genetically modified pigs of breeds that reach 70-90 kg in adulthood to address this issue.
• Selection of recipients more compatible with porcine donors. Beyond the three previously mentioned antigens that are genetically modified in pigs, other antigenic reactions may occur or humans may naturally have immunity against other uncontrolled antigens. Identifying sensitized human receptors could improve compatibility. After the second human implant, a rejection component was suspected.
• Microbiological control: Essential to limit zoonosis transmission to humans. The most concerning agents are hepatitis E virus, porcine cytomegalovirus or roseolovirus. The last two viruses resemble human herpesviruses and were transmitted to the first human recipient, potentially contributing to the fatal outcome. Strict pig testing and rigorous rearing control from birth are proposed to prevent transmission, avoiding nursing by sows, a primary transmission source. Porcine endogenous retroviruses (PERV) have three subtypes (A-B-C). While it is possible to breed pigs free from PERV-C, types A and B can infect human cells, although their significance remains uncertain.

The report provides various reflections on the ethical principles of the procedure and the legal framework governing it. Ethically, a well-developed and experience-backed technique justifies its use in patients who have been rejected for heart transplantation, aiming to achieve outcomes comparable to those of previous alternatives, such as mechanical circulatory support. For patients not eligible for support or transplant, it could be an option given their grim prognosis. With improved survival, it could even be considered for patients awaiting a human organ, especially in regions with limited availability. Some countries, such as Japan, have a scarce organ availability due to restrictive death definitions, only recognizing cardiorespiratory death. Legally, the EU has a regulatory framework for advanced therapy medicinal products that includes animals and human recipients, though it appears to be designed primarily for devices or animal products rather than whole organs. Consequently, the European Medicines Agency (EMA) has set up a commission to address this legal gap.

COMMENTARY:

This work provides an update on a fascinating yet controversial topic. Filled with a pioneering spirit reminiscent of the early days of cardiac transplantation, its acceptability in today’s healthcare context is driven by an international project’s scale, aimed at “manufacturing” organs for a growing number of patients with dire prognoses. Centers worldwide are joining the Maryland initiative, sharing knowledge toward a common goal. Amidst the appearance of a chimera lies a vast body of serious, groundbreaking research. We hope it comes to fruition within our professional lifetimes. Though our country may not participate in this project, we can at least take pride in our saying, “we love everything about pigs,” and now, even their hearts.

REFERENCE:

Schmoeckel M, Längin M, Reichart B, Abicht JM, Bender M, Michel S, et al. Current Status of Cardiac Xenotransplantation: Report of a Workshop of the German Heart Transplant Centers, Martinsried, March 3, 2023. Thorac Cardiovasc Surg. 2024 Jun;72(4):273-284. doi: 10.1055/a-2235-8854.