The Miracle of Xenotransplants: Are We Ready for Prime Time?

Xenotransplantation represents a groundbreaking advancement in medical science, offering a beacon of hope for countless individuals suffering from end-stage kidney disease (ESKD). For history enthusiasts, the first successful clinical xenotransplant in ancient legend was performed by Daedalus, who grafted bird feathers onto his arms to escape by flight from Crete to Athens. Fast forward to the present era, xenotransplantation is now rapidly moving from the bench to the bedside. In 2021, porcine kidneys were successfully transplanted to two brain-dead recipients. To overcome the challenge of hyperacute rejection, pigs were inbred with the knockout of the alpha-1,3-galactosyltransferase gene and subcapsular autologous thymic tissue.[1] Pigs have short gestation, can be bred in large numbers, and their organs resemble the size and function of humans, making them the perfect xeno-donor. Advances in cloning, gene editing, and infection control have paved the way for the breakthrough in human xenotransplantation.[2] Now, let's behold the historic debut of the first live xenotransplant. In January 2022, a 57-year-old male, with end-stage heart failure and on venoarterial extracorporeal membrane oxygenation support, received the world's first genetically modified pig heart at the University of Maryland School of Medicine.[3] The patient had normal cardiac function with no obvious signs of acute rejection for nearly 7 weeks after the surgery; however, the sudden onset of cardiac failure led to his demise 2 months after the xenotransplant. Endomyocardial biopsy revealed widespread endothelial injury suggestive of antibody-mediated rejection and reactivation of porcine Cytomegalovirus/porcine Roseolovirus in the xenograft. Mycophenolate mofetil, which is an integral component of maintenance immunosuppression, was stopped from Day 20 onward in view of pancytopenia, and the recipient also received intravenous immunoglobulin (IVIG). It was postulated that the reduction of immunosuppression, IVIG, and porcine-derived viruses could have triggered the inflammatory cascade and xenograft rejection. Another attempt at cardiac xenotransplant happened in September 2023, when a 58-year-old man underwent this experimental procedure under a "compassionate use" pathway at the University of Maryland. Unfortunately, this recipient also died 6 weeks posttransplant due to cardiac rejection.[4] Both the cardiac xenotransplant recipients received standard immunosuppressive drugs and an experimental one. While the first received KPL-404 (a monoclonal antibody that binds CD40 and interferes with CD40–CD40L interaction), the second recipient received tegoprubart (anti-CD 154 monoclonal antibody). The detailed posttransplant clinical course has not yet been reported in the literature, and we know much less about the factors that might have contributed to his demise. However, such disappointments are to be anticipated when pioneering a completely new form of therapy. What could have compromised the cardiac xenotransplantation outcome? There are several hypotheses that can be used to address this. Ensuring the selection of a healthy recipient is crucial for achieving success. A healthy recipient is more likely to have a stronger immune system and overall better physiological condition, which can significantly reduce the risk of organ rejection and infections. Both the cardiac xenotransplant recipients had extremely poor myocardial function with left ventricular ejection fractions of 11%–12%.[5] For a number of reasons, neither was deemed suitable for allotransplantation. The very low levels of immunoglobulins in Mr. Bennett's blood stimulated his medical advisors to administer IVIG, which likely contained anti-pig antibodies and may have contributed to the development of the antibody-mediated rejection.[6] It is also suggested that perhaps clinical pig xenotransplantation should, at present, best be directed toward kidney transplantation. If the kidney fails or there are other complications, the pig kidney can be excised, all immunosuppressive therapy can be discontinued, and the patient can return to chronic dialysis. However, there can be no "Plan B" in pig heart transplant – if the heart fails, the patient will die.[7] Now, let's dive into the world of kidney xenotransplantation. The world's first successful transplant of a genetically edited pig (porcine) kidney into a 62-year-old man living with ESKD was performed at Massachusetts General Hospital in March 2024.[8] The recipient, who was a known diabetic/hypertensive, had previously undergone a deceased donor kidney transplant. His first allograft had failed, with the result of the patient being on dialysis with multiple vascular access-related issues. Clustered regularly interspaced short palindromic repeats (CRISPR) technology was used to edit 69 genes in the donor pig to inactivate viral genes and "humanize" the pig. The recipient, in addition, received induction immunosuppression with anti-CD40 L-monoclonal antibody (Tegoprubart), and Ravulizumab, a humanized monoclonal antibody that specifically binds to the human terminal complement protein C5.[9] Following that, very recently, a second kidney xenotransplant was performed on April 12, 2024, where a 54-year-old woman already on a left ventricular assist device received a thymus and kidney from a gene-edited pig.[10] However, the graft kidney was removed 47 days after the transplant. The complete details and posts operative course are not known yet. In this issue Sharma et al. have delved deeply into the current state of xenotransplantation, highlighting its promise as well as the hurdles that must be overcome. As they point out, one of the primary obstacles is achieving immunological compatibility, besides the risk of xenozoonosis and ethical concerns.[11] What is essential for xenotransplantation gene-edited pigs expressing human complement regulatory proteins (CD55/CD46/CD59) to inhibit complement activation and the inactivation of pigs carbohydrates (aGal)/porcine endogenous retroviruses. Modern gene editing techniques such as CRISPR allow inserting/deleting genes to interfere with complement activation/cellular immune response/coagulation/inflammation and apoptosis. Prevention and treatment of humoral rejection in this field will likely require a combination of safe complement inhibition (Eculizumab/C1 inhibitor/C5a receptor antagonist) and potent immunosuppressive drugs such as thymoglobulin/rituximab and anti-CD40/CD154 therapy for costimulatory blockage. This pig–human kidney transplant raised many hopes, but still, we are far from fixing organ shortages. It is worthwhile to note that early phase 1 clinical trials with anti-CD 40 in rheumatological diseases raised safety concerns and were terminated early due to the increased thromboembolic risk.[10] Utilizing an agent not yet sanctioned by the Food and Drug Administration can pose subtle yet considerable risks. However, an anti-CD154mAb-based immunosuppressive regimen is known to be more effective than an anti-CD40mAb-based regimen with less thrombogenic potential.[12] Third, pig vasopressin is more potent than human vasopressin, and porcine renin is unable to efficiently cleave human angiotensinogen, suggesting the renin–angiotensin–aldosterone system may be impaired after kidney xenotransplantation. These differences pose the risk of physiological derangement after xenotransplant including hypovolemia, hypotension, and electrolyte abnormalities.[13] Several barriers still exist before xenotransplantation can be considered a clinically available therapy for patients with end-stage organ failure.[14] Lingering questions include the recipient's functional status, optimal gene editing techniques, retroviral elimination of xeno donor, immunosuppressive regimen to support the patient posttransplantation, and the best regulatory approvals. Failures aside, the lessons gained from the experiences are priceless, and each setback will become a stepping stone that will guide us toward improvement in the field. And so, although xenotransplantation offers a promising avenue to alleviate the organ shortage crisis, it requires further advancements and rigorous research to address the immunological and safety challenges. With continued dedication to overcoming these hurdles, xenotransplantation could 1 day become a routine and life-saving procedure, transforming the landscape of organ transplantation.