Xenotransplantation: Bridging Organ Shortages with Pioneering Advances

As of September 2023, a staggering 96,000 patients were awaiting kidney transplants (1), underscoring a significant demand-supply gap. Cross-species transplantation (xenotransplantation) has witnessed multiple attempts throughout the past century, encompassing organs like the skin, liver, heart, and kidney. In the 1960s, Keith Reemtsma, MD, and his team embarked on a groundbreaking journey, performing 13 chimpanzee-to-human kidney xenotransplants (2) with varying degrees of success, attributed to limited immunosuppression availability and infectious complications. In 2022, the promising pig-to-human heart transplant was unfortunately curtailed at 2 months, presumably due to antibody-mediated rejection and activation of latent porcine cytomegalovirus (PCMV) and porcine roseolovirus infections (3).

Previous experiments involving pig-to-non-human primate transplants unveiled the presence of the galactose (Gal) oligosaccharide antigen on pig vascular endothelium, which is not expressed in humans. Genetic alterations were subsequently introduced to eliminate this porcine antigen, thus averting hyperacute rejections. Further genetic modifications targeting complement-mediated cytotoxicity and thrombosis were introduced. These genetically engineered pigs—endowed with 10 genetic (10-GE) modifications, including the insertion of human complement inhibitor and anticoagulant genes, along with deletion of pig Gal antigen and pig growth hormone receptor genes—now serve as subjects for xenotransplantation studies. Importantly, these 10-GE pigs do not express red blood cell antigens and are universal donors with respect to blood types.

A significant milestone was marked by Locke and colleagues (4) in 2022, who reported the first clinical-grade porcine-to-human xenotransplantation involving a 10-GE pig. The induction immunosuppression regimen included anti-thymocyte globulin, methylprednisolone, and rituximab, followed by maintenance with tacrolimus and mycophenolate mofetil. Despite the xenografts exhibiting urine production, renal function and creatinine clearance remained suboptimal, and the study was terminated at 77 hours due to severe coagulopathy and recipient exsanguination. Protocol biopsies taken on the first day revealed thrombotic microangiopathy (TMA), whereas day 3 biopsies showcased extensive cortical necrosis, with C4d remaining negative.

Montgomery et al. (5) undertook a series of two porcine-to-human xenotransplants using Gal knockout pigs. They transplanted a thymic autograft from the pig beneath the kidney capsule (thymokidney) to mitigate host T cell-mediated immune responses. Both recipients, closely monitored for 54 hours, demonstrated improved creatinine levels and glomerular filtration rates alongside brisk diuresis. However, post-explant phenotypic analysis (6) revealed microvascular inflammation, increased expression of genes linked to antibody-mediated rejection, and serological evidence of circulating donor-specific antibodies in both recipients. The absence of C4d hinted at alternative biological rejection pathways apart from complement cascade activation.

In a groundbreaking development in late 2023, Locke et al. (7) performed another porcine-to-human kidney xenotransplant using 10-GE pigs. This time, the procedure was done 24 hours after eculizumab administration, combined with conventional induction and maintenance immunosuppression. The recipient was monitored for 7 days; the xenografts initially produced 37 L of urine on day 1, eventually concentrating to 5 L by day 3, accompanied by improved creatinine clearance. Serial biopsies during this period did not show any signs of rejection or TMA.

Xenotransplantation is not without challenges and limitations. Concerns over PCMV and endogenous retrovirus transmission to humans can be managed through stringent pig housing and regular donor and recipient testing. Ethical considerations, encompassing informed consent, animal rights, and religious beliefs, also loom over xenotransplants. Despite favorable short-term outcomes, antibody-mediated rejection remains a possibility, necessitating the exploration of therapies targeting humoral immunity. A comprehensive evaluation of adaptive immune responses will hinge on long-term studies involving deceased human recipients.

In conclusion, xenotransplantation holds promise as a solution to the organ-shortage crisis, but it requires ongoing research, technological advancements, and careful consideration of ethical and safety concerns to become a widely accepted and sustainable medical practice.