Xenotransplantation has entered a transformative era with the initiation of clinical trials involving genetically engineered pig organs in human recipients. This milestone represents the culmination of decades of rigorous research in preclinical models, which provided critical insights into the immunological barriers that drive xenograft rejection. Early studies demonstrated that conventional immunosuppressive regimens, although effective in allotransplantation, were insufficient to prevent the complex immune responses elicited by xenografts.1 These findings prompted the development of targeted immunosuppressive strategies, including costimulation blockades and complement inhibition, which significantly improved graft survival in nonhuman primates (NHPs).2,3 Parallel advances in gene-editing technologies have revolutionized the field by enabling precise modifications of the pig genome. Key genetic alterations, such as knockout of xenoantigens and insertion of human regulatory genes, have mitigated hyperacute and acute vascular rejection, historically the major obstacles to clinical application. These innovations laid the foundation for the decedent human experiments4 and the first human xenotransplantation attempts, which included cardiac,5 renal,6 and hepatic grafts from genetically engineered pigs. Preliminary outcomes demonstrated encouraging graft function and reasonable patient survival, underscoring the feasibility of this approach. However, these pioneering cases also revealed novel rejection mechanisms not previously observed in animal models.7 These include delayed antibody and complement-mediated injury and thrombotic microangiopathy, which appear to be exacerbated by the fragile clinical status of recipients undergoing these experimental procedures. Such findings highlight the complexity of translating xenotransplantation from controlled preclinical settings to critically ill human patients. Moving forward, addressing these newly identified immunopathological processes is imperative for the success of xenotransplantation as a routine clinical therapy. This will require a multipronged approach: refining genetic modifications, optimizing immunosuppressive protocols, and developing predictive biomarkers for early detection of rejection. Furthermore, ethical considerations, regulatory frameworks, and long-term safety monitoring must evolve in parallel with scientific progress to ensure responsible implementation. Although recent clinical experiences mark a historic advance, they also underscore the need for continued innovation and vigilance. Xenotransplantation holds immense promise for alleviating the global organ shortage, but its widespread adoption will depend on overcoming these emerging challenges through sustained interdisciplinary collaboration. Keiler et al8 identified a critical role for natural killer (NK) cells in endothelial cell activation and subsequent graft rejection in kidney xenotransplantation. The investigators compared 2 transplant models in which NK-cell activity was either blocked or left unblocked. Although attempts to rely solely on clinically applicable or USA Food and Drug Administration-approved immunosuppressive regimens proved insufficient to control xenoreactive responses, the addition of an NK cell–blocking agent (anti-interleukin IL-15) successfully overcame this limitation by prolonging the graft survival. The study employed rhesus macaques as recipients of kidneys from GGTA1KO/CD55TG pigs. Notably, Rhesus macaques have been shown to require less immunosuppression than baboons, reflecting differences in xenogeneic antigen responses between NHPs. The immunosuppressive regimen also differed from prior kidney xenotransplantation protocols: Thymoglobulin was replaced with anti-CD4 and anti-CD8 antibodies in the induction regimen to deplete T cells, tacrolimus was omitted, and maintenance therapy relied on belatacept, mycophenolate mofetil, and steroid taper. In the experimental group, adjuvant anti-IL-15 was added; the control group received the same regimen without anti-IL-15. Serial percutaneous biopsies monitored rejection, and in vitro assays (mixed lymphocyte reaction, ELISA, flow cytometry) confirmed the extent of immune suppression. The adjuvant anti-IL-15 reduced T-cell and cytotoxic NK-cell populations, significantly extending graft survival from 14 to 49.5 d. The treatment with anti-IL-15 did not prevent rejection; however, their repeated treatment converted the cytotoxic phenotype of NK cells to a noncytotoxic phenotype. These double-negative CD16−CD56− phenotypes were found in both peripheral blood and in necropsy tissue of the rejected animal. However, the role of these double-negative NK cells remains unclear and warrants further investigation. Although other studies have achieved more prolonged survival using costimulation blockade (anti-CD40/CD154), NK-cell activity in those experiments remained unchecked and perhaps inconsequential. Furthermore, Keiler et al used 2-gene-modified pigs, whereas more advanced gene-edited donor pigs are now available, which may influence outcomes. The authors had previously demonstrated the benefits of anti-CD40L, and combining this approach with anti-IL-15 might have yielded even greater and consistent survival. In this experiment, rejection in the anti-IL-15-treated group was mediated by T effector memory cells, predominantly CD4 cells, found both in the periphery and in necropsy samples. This indicates that long-term T-cell suppression is required, and the regimen used in this study was ineffective in controlling cell-mediated rejection. Other anticytokine agents, including anti-IL-6, antitumor necrosis factor-α, and anti-IL-2, have shown promise in modulating T-cell and inflammatory responses. Still, the high cost of NHP studies has limited the ability to isolate their individual effects. Nevertheless, based on innate immune responses observed in human xenotransplantation cases, suppression of NK cells and macrophages may prove pivotal in achieving persistent long-term xenograft survival in clinical settings.9,10 Finally, the study underscores the indispensable role of large animal models in elucidating the mechanisms of xenograft rejection. These models not only replicate key aspects of human xenotransplantation but also provide a platform for testing strategies to overcome rejection. Given the limited number of human xenotransplantation cases, NHP models remain essential, along with human cases, for elucidating the complex interplay of immune actors and for developing effective interventions.
Maaz Muhammad (Fri,) studied this question.