ABSTRACT
This review highlights groundbreaking progress from July to December 2024, including developments in gene‐edited pigs, cellular therapies, organ preservation, and transplantation techniques. Recent advancements, particularly in gene‐editing technologies and immunosuppressive protocols, have brought the field closer to clinical application. While significant ethical, immunological, and societal challenges remain, this progress underscores the immense potential of xenotransplantation to revolutionize transplantation medicine.
Keywords: clinical trials, federal drug administration, genetic modifications, heart xenotransplantation, immunosuppression, kidney xenotransplantation, pig‐to‐human, public perception
1. Studies in Human
In the final months of 2024, we witnessed the third application of clinical pig kidney xenotransplantation when an Alabama woman successfully underwent a xenotransplantation at New York University Langone Health (media sources). At the time of this writing, the patient is reportedly doing well clinically, and the pig xenograft is functioning as expected. This exciting development bookends the growing knowledge highlighted in this review—ongoing evidence that the xenotransplantation of genetically engineered pig organs can be a clinical reality.
2. Recipient Immune Response
As more information is obtained regarding preclinical and clinical xenotransplantation studies, we can focus attention on the most pertinent mechanisms causing xenograft failure. Single‐cell and longitudinal transcriptomic analyses of human cellular immune responses to pig kidney xenografts highlight early antibody‐mediated rejection, immune infiltration, and porcine tissue repair programs as key areas of focus to optimize outcomes [1]. The endothelial glycocalyx, the sugar‐rich coating that lines endothelial cells, is also one of those potential targets. Studies in pig‐to‐baboon cardiac xenotransplantation show that changes in glycocalyx components, including hyaluronan and syndecan‐1, are similar to or less pronounced than in human settings. These findings suggest that preserving the glycocalyx could improve xenograft survival and may offer a novel therapeutic avenue for enhancing graft survival [2].
Multi‐omic profiling of two human decedents receiving cardiac xenografts from 10‐GE pigs revealed heterogeneity in immune and metabolic responses between the recipients. One recipient exhibited pronounced T‐cell and natural killer cell activation, ischemia‐reperfusion injury, and systemic physiological dysfunction, attributed to suboptimal immunosuppression. In contrast, the second recipient experienced minimal immune activation and superior graft adaptation, emphasizing the critical role of individualized immunosuppressive regimens in achieving successful outcomes [3, 4]. These findings further confirm the potential for 10‐GE pigs to avoid immediate xenograft hyperacute rejection while highlighting the necessity for tailored therapeutic approaches to optimize long‐term graft viability. Additionally, this study provides a potential framework for how the integration of advanced molecular analyses into xenotransplantation research could refine immunosuppressive protocols and address patient‐specific variables, thereby advancing the field toward clinical implementation [5].
3. Recipient Selection
As it does for allotransplantation, recipient selection and appropriate screening protocol will no doubt play a critical role in mitigating the risks of xenograft hyperacute rejection (HAR). A recent retrospective analysis of pig‐to‐baboon kidney xenotransplantation cases revealed that pretransplant detectable preformed natural antibodies, particularly high IgG, were independently associated with HAR. These findings led to the development of a two‐step screening algorithm that integrates complement‐dependent cytotoxicity and antibody‐binding assays to predict and manage rejection risks [6].
Highly sensitized recipients pose a unique challenge for xenotransplantation as they theoretically carry preformed antibodies against the major histocompatibility complex (MHC) that could cross‐react with the pig MHC. A recent study demonstrated that kidneys from genetically modified pigs, specifically GGTA1 knockout pigs expressing human CD55 transgenes (1KO.1TG), resulted in accelerated xenograft rejection in allo‐sensitized rhesus macaques compared to historical controls. However, the addition of seven human transgenes (3KO.7TG) and disruption of the CMAH and B4GALNT2 enzymes significantly prolonged graft survival. The addition of these modifications was thought to lead to a dampened humoral immune response, reduced AMR, and improved xenograft viability [7].
4. Immunosuppression
A recent study demonstrated that conventional, FDA‐approved immunosuppression regimens can achieve long‐term xenograft survival in pig‐to‐NHP kidney transplantation. Using therapeutic tacrolimus levels alongside other clinically available agents, stable graft function was maintained for a median of 154.5 days, compared to early rejection in recipients with subtherapeutic levels [8]. Another study found similar success in pig‐to‐baboon heart xenotransplantation with 9‐GE and 10‐GE pig hearts, under clinically relevant immunosuppressive regimens and ischemia minimization. While 3‐GE hearts failed early, 9‐GE and 10‐GE hearts achieved prolonged graft survival up to POD 393 [9].
Further progress has also been made in mitigating inflammation and coagulation disorders, both of which are major contributors to xenograft dysfunction. Preclinical pig‐to‐baboon cardiac xenotransplantation models have demonstrated that a combination of anti‐inflammatory drugs, including C1 esterase inhibitors, IL‐6 receptor antagonists, TNF‐alpha inhibitors, and IL‐1 receptor antagonists, effectively controls inflammatory responses. Importantly, cytokine analyses showed perioperative increases in IL‐8 and stabilization of other inflammatory markers, emphasizing the feasibility of managing these complications in clinical settings. These findings underscore the importance of integrating both genetic engineering and tailored pharmacological regimens to address immune and inflammatory challenges in xenotransplantation [10].
Finally, a potential method to minimize immunosuppression and induce tolerance in recipients includes the utilization of thymokidneys. A novel anterior retroperitoneal approach was proposed to facilitate easier thymus tissue implantation, minimize surgical complications, reduce intra‐abdominal adhesions, and simplify graft procurement [11].
5. Organ Preservation
A multicenter clinical trial of xenotransplantation will require patient enrollment at multiple academic institutions—raising a question regarding how to best preserve the xenograft prior to transplantation. Recent studies on kidney xenografts have demonstrated a potential detrimental impact of ischemia‐reperfusion injury (IRI) associated with cold storage, particularly across xenogeneic barriers. Hypothermic machine perfusion was shown to significantly reduce IRI and decrease the incidence of early graft loss, emphasizing a possible need for advanced preservation techniques in clinical xenotransplantation settings [12].
6. Infectious Disease Concerns
The potential for zoonotic infections remains a critical barrier to the clinical application of xenotransplantation. Xenotransplantation poses unique zoonotic risks, necessitating rigorous screening protocols for both donor pigs and recipients to prevent porcine virus transmission. There is an obvious need for international collaboration to standardize infection control measures, particularly in regions with varying distributions of swine pathogens [13]. One avenue is the selection of donor pigs with a favorable immune profile. Recent studies have shown that Göttingen minipigs exhibit minimal pre‐existing antibodies against key AAV serotypes, including complete absence of antibodies against AAV9 in all animals tested. This low immunogenic profile makes them an ideal preclinical model for gene therapy safety studies and as donor animals for xenotransplantation. Their reduced risk of virus‐related complications enhances their value in advancing both gene therapy and xenotransplantation research [14].
7. Ethical and Regulatory Challenges
The ethical framework of xenotransplantation continues to evolve in response to societal reactions and public discourse surrounding early cases of xenotransplantation. These discussions highlighted tensions between scientific progress and societal ethical standards, underscoring the need for proactive public engagement to build trust and address normative judgments that influence societal acceptance and regulatory approaches to emerging biotechnologies [15]. The progression toward clinical xenotransplantation trials necessitates a patient selection process grounded in medical need, capacity to benefit, patient choice, and compliance to achieve ethically sound and scientifically valuable outcomes [16, 17].
A recent UK symposium, endorsed by the British Transplantation Society and the European Society of Organ Transplantation, highlighted advancements in gene‐edited animal models and addressed key ethical, regulatory, and technical challenges in translating xenotransplantation into clinical practice. While focused on the United Kingdom, the discussions provided globally applicable insights into unresolved legal and ethical issues [18].
8. Patient Selection and Equity
One of the most pressing ethical dilemmas in xenotransplantation is the selection of patients for clinical trials. Current recommendations prioritize critically ill patients who have exhausted all other therapeutic options. However, this approach has been criticized for its potential to produce unfavorable risk‐benefit ratios, particularly for patients with complex medical conditions. Furthermore, concerns about justice and beneficence arise when considering acutely ill patients who may struggle to provide fully informed consent [19].
Cardiac xenotransplantation is also actively being explored for pediatric patients—a population in which few therapeutic options exist at all. Successful xenotransplantation could represent a bridge to allotransplantation or a potential long‐term solution for this vulnerable population. Genetically modified pig hearts offer considerable promise in alleviating growing waitlists and reducing high mortality rates among pediatric patients [20]. However, the ethical challenges surrounding pediatric xenotransplantation are complex. Issues such as informed consent—particularly for minors—and the psychosocial impacts on patients and their families further complicate the discourse. Public perception adds another layer of difficulty, especially when determining whether adults or children should be prioritized in early clinical trials. Efforts to address these challenges include developing transparent patient selection frameworks prioritizing medical need, equitable access, and rigorous informed consent processes, particularly for vulnerable groups such as pediatric patients [21]. By integrating these considerations, the field can progress in a way that balances innovation with justice and beneficence
9. Public Perception
Public acceptance is a key determinant of the success of xenotransplantation. A recent survey of over 5000 individuals in the United States revealed that while 36% of respondents were open to experimental xenotransplants, concerns about the lack of outcome data and fear of complications persisted. Younger individuals, women, and racial minorities were less likely to support xenotransplantation, highlighting the need for targeted public education initiatives to address these disparities [22]. Finally, societal perceptions of justice and equity play a crucial role. The commercialization of genetically modified organs could exacerbate disparities in access to transplantation, particularly in low‐ and middle‐income countries where transplantation services are already limited [17]. Efforts to promote global equity in organ transplantation must be prioritized as xenotransplantation moves closer to clinical implementation.
10. Cellular Therapies
The application of xenotransplantation extends to cellular therapies, exemplified by recent advancements in subcutaneous islet transplantation for type‐1 diabetes mellitus (T1DM) [23]. This approach leverages controlled inflammation‐induced neovascularization to create a vascularized subcutaneous cavity, improving the survival and functionality of encapsulated islets. The technique uses a temporary nylon catheter to stimulate localized inflammation and promote neovascularization, enabling the implantation of alginate‐coated, islet‐seeded devices [24].
Preclinical studies in xenogeneic mouse and minipig models demonstrated sustained diabetes reversal, with recipients achieving normoglycemia without the need for systemic immunosuppression. Additionally, the approach allows for in situ replacement of encapsulation devices, providing a scalable and clinically promising solution to T1DM management. These findings highlight the potential of innovative strategies to overcome challenges in islet xenotransplantation, particularly immune rejection and limited oxygenation [25]. These innovative approaches complement advancements in solid organ transplantation, broadening the scope of xenotransplantation to chronic conditions like diabetes.
11. Animal Welfare
The use of genetically modified pigs for xenotransplantation has always raised moral concerns regarding animal welfare. To address these concerns, the concept of “genetic disenhancement,” modifying pigs to reduce their capacity for pain and suffering, has been theoretically proposed. While the specifics behind the concept are unclear, this could represent a future temporary ethical compromise until viable non‐animal alternatives are developed [26].
12. Conclusion
The advances in xenotransplantation from July to December 2024 reflect the field's rapid evolution and its potential to address the global organ shortage crisis. Scientific breakthroughs, particularly in genetic engineering and cellular therapies, have brought xenotransplantation closer to clinical reality but significant ethical, immunological, and societal challenges remain.
Conflicts of Interest
The authors declare no conflicts of interest.
Acknowledgments
The authors would like to thank Dr. David Cooper for his valuable assistance in reviewing this manuscript.
Funding: The authors received no specific funding for this work.
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