Faculty Bibliography

PURPOSE OF REVIEW/OBJECTIVE:The aim of this study was to highlight recent progress in xenotransplantation and discuss the remaining obstacles/steps before the FDA is likely to approve a clinical trial. RECENT FINDINGS/RESULTS:Long-term survival of life-supporting xenografts in preclinical models has led to discussion of clinical trials of xenotransplantation. The reports of clinical cardiac xenotransplant based on compassionate use FDA approval and renal xenotransplants to brain-dead humans have led to further considerations of clinical trials. Discussions between the transplant community and the FDA have established critical next steps before a clinical trial of xenotransplants is likely to be approved. These steps include testing the clinical immunosuppression protocol and the organ from a genetically modified source animal in nonhuman primates with reproducible survival of at least 6 months. In addition, appropriate viral surveillance protocols and confirmation that the xenografts support appropriate human physiology are likely to be critical elements for FDA-approval. Finally, further studies in the human decedent model are likely to provide critical information about human immune and physiologic responses to xenografts. SUMMARY/CONCLUSIONS:This review highlights the current progress in nonhuman primate models and recent reports of human xenotransplantation. It also describes the remaining hurdles and currently understood FDA requirements that remain to be achieved before a clinical trial of xenotransplantation can be approved.

INTRODUCTION/BACKGROUND:Living donor liver transplantation (LDLT) is a promising option for mitigating the deceased donor organ shortage and reducing waitlist mortality. Despite excellent outcomes and data supporting expanding candidate indications for LDLT, broader uptake throughout the United States has yet to occur. METHODS:In response to this, the American Society of Transplantation hosted a virtual consensus conference (October 18-19, 2021), bringing together relevant experts with the aim of identifying barriers to broader implementation and making recommendations regarding strategies to address these barriers. In this report, we summarize the findings relevant to the selection and engagement of both the LDLT candidate and living donor. Utilizing a modified Delphi approach, barrier and strategy statements were developed, refined, and voted on for overall barrier importance and potential impact and feasibility of the strategy to address said barrier. RESULTS:Barriers identified fell into three general categories: 1) awareness, acceptance, and engagement across patients (potential candidates and donors), providers, and institutions, 2) data gaps and lack of standardization in candidate and donor selection, and 3) data gaps regarding post-living liver donation outcomes and resource needs. CONCLUSIONS:Strategies to address barriers included efforts toward education and engagement across populations, rigorous and collaborative research, and institutional commitment and resources.

PURPOSE OF REVIEW/OBJECTIVE:The greatest challenge facing end-stage kidney disease (ESKD) patients is the scarcity of transplantable organs. Advances in genetic engineering that mitigate xenogeneic immune responses have made transplantation across species a potentially viable solution to this unmet need. Preclinical studies and recent reports of pig-to-human decedent renal xenotransplantation signify that clinical trials are on the horizon. Here, we review the physiologic differences between porcine and human kidneys that could impede xenograft survival. Topics addressed include porcine renin and sodium handling, xenograft water handling, calcium, phosphate and acid-base balance, responses to porcine erythropoietin and xenograft growth. RECENT FINDINGS/RESULTS:Studies in nonhuman primates (NHPs) have demonstrated that genetically modified pig kidneys can survive for an extended period when transplanted into baboons. In recent studies conducted by our group and others, hyperacute rejection did not occur in pig kidneys lacking the α1,3Gal epitope transplanted into brain-dead human recipients. These experimental trials did not study potential clinical abnormalities arising from idiosyncratic xenograft responses to human physiologic stimuli due to the brief duration of observation this model entails. SUMMARY/CONCLUSIONS:Progress in biotechnology is heralding an era of xenotransplantation. We highlight the physiologic considerations for xenogeneic grafts to succeed.

Background: Pediatric transplant candidates have historically been disadvantaged on the transplant waitlist, with nearly half of pediatric deceased donor organs allocated to adult recipients (Hsu, Gastroenterology, 2017), and allocation pediatric end-stage liver disease (PELD) scores that underestimate children's expected 3-month mortality compared to that of adult patients (Chang, JAMA Pediatrics, 2018). Disparities in organ distribution prompted revision of the liver allocation policy in 2020 from donation services areas (DSA) to a series of distance-based concentric circles called acuity circles (AC) before being offered nationally (US GAO, 2022), which was designed to minimize geographic inequity in liver transplant. Prior to implementation of the new liver allocation policy, analysis using the Liver Simulated Allocation Model projected that AC allocation would decrease disparities for pediatric liver transplant candidates and recipients by increasing transplants and decreasing waitlist mortality (Mogul, Transplantation, 2020). In this study, we evaluate differences in pediatric whole liver transplants performed before and after the implementation of acuity circle liver allocation policy.
Study Design: We evaluated patient characteristics, adjusted MELD/PELD at time of transplant, calculated donor age at time of transplant among pediatric whole liver transplant recipients in low versus high-volume pediatric liver transplant centers performed before and after implementation of AC-based liver allocation policy using the Scientific Registry of Transplant Recipients.
Result(s): Before and after the implementation of ACs, differences in pediatric liver transplants by age group (<2 years, 2-5 years old, 5-12 years old, and 12-18 years old) remained significantly different between low and high-volume pediatric transplant centers. Under DSA allocation policy, the median MELD/PELD at transplant was 37.0 (IQR 30.0-41.0) in low-volume centers and 40.0 (IQR 30.0-41.0) in high-volume centers. After the implementation of acuity circles, median MELD/PELD at transplant decreased to 35.0 (IQR 21.0-41.0) in low-volume centers and 35.0 (IQR 25.0-41.0) in high-volume centers. Finally, donor age at time of transplant increased from 8.0 (IQR 2.00-18.0) to 13.5 (IQR 4.5-21.0) at low-volume centers, and from 3.0 (IQR 1.0-14.0) to 4.0 (IQR 1.0-14.0) at high-volume centers before and after the implementation of ACs.
Conclusion(s): The change from DSAs to ACs in allocation policy and the shift from regional to national review boards have affected the characteristics of organ recipients, adjusted MELD/PELD at time of transplant, and donor age at time of transplant differentially between whole liver transplant recipients at low-and high-volume pediatric liver transplant centers

To assess the impact of technical variant grafts (TVG) (including living donor [LD] and deceased donor split/partial grafts) on waitlist (WL) and transplant outcomes for pediatric liver transplant (LT) candidates, we performed a retrospective analysis of OPTN data on first-time LT or liver-kidney pediatric candidates listed at centers that performed >10 LT during the study period, 2004-2020. Center variance was plotted for LT volume, TVG usage, and survival. A composite center metric of TVG usage and WL mortality was developed to demonstrate existing variation and potential for improvement. 64 centers performed 7842 LT; 657 children died on the WL. Proportions of WL mortality by center ranged from 0-31% and TVG usage from 0-76%. Higher TVG usage, from deceased or LD, independently or in combination, significantly correlated with lower WL mortality. In multivariable analyses, death from listing was significantly lower with increased center TVG usage (HR 0.611, CI [0.40-0.92]) and LT volume (HR 0.995, CI [0.99-1.0]). Recipients of living donor transplants (HR 0.637, CI [0.51-0.79]) had significantly increased survival from transplant compared with other graft types, and recipients of deceased donor technical variant grafts (HR 1.066, CI [0.93-1.22]) had statistically similar outcomes compared to whole graft recipients. Increased TVG utilization may decrease WL mortality in the U.S. Policy and training to increase TVG usage, availability and expertise is critical.

BACKGROUND:Xenografts from genetically modified pigs have become one of the most promising solutions to the dearth of human organs available for transplantation. The challenge in this model has been hyperacute rejection. To avoid this, pigs have been bred with a knockout of the alpha-1,3-galactosyltransferase gene and with subcapsular autologous thymic tissue. METHODS:We transplanted kidneys from these genetically modified pigs into two brain-dead human recipients whose circulatory and respiratory activity was maintained on ventilators for the duration of the study. We performed serial biopsies and monitored the urine output and kinetic estimated glomerular filtration rate (eGFR) to assess renal function and xenograft rejection. RESULTS:in Recipient 2. In both recipients, the creatinine level, which had been at a steady state, decreased after implantation of the xenograft, from 1.97 to 0.82 mg per deciliter in Recipient 1 and from 1.10 to 0.57 mg per deciliter in Recipient 2. The transplanted kidneys remained pink and well-perfused, continuing to make urine throughout the study. Biopsies that were performed at 6, 24, 48, and 54 hours revealed no signs of hyperacute or antibody-mediated rejection. Hourly urine output with the xenograft was more than double the output with the native kidneys. CONCLUSIONS:Genetically modified kidney xenografts from pigs remained viable and functioning in brain-dead human recipients for 54 hours, without signs of hyperacute rejection. (Funded by Lung Biotechnology.).