Faculty Bibliography

The first 2 living recipients of pig hearts died unexpectedly within 2 months, despite both recipients receiving what over 30 years of nonhuman primate (NHP) research would suggest were the optimal gene edits and immunosuppression to ensure success. These results prompt us to question how faithfully data from the NHP model translate into human outcomes. Before attempting any further heart xenotransplants in living humans, it is highly advisable to gain a more comprehensive understanding of why the promising preclinical NHP data did not accurately predict outcomes in humans. It is also unlikely that additional NHP data will provide more information that would de-risk a xenoheart clinical trial because these cases were based on the best practices from the most successful NHP results to date. Although imperfect, the decedent model offers a complementary avenue to determine appropriate treatment regimens to control the human immune response to xenografts and better understand the biologic differences between humans and NHP that could lead to such starkly contrasting outcomes. Herein, we explore the potential benefits and drawbacks of the decedent model and contrast it to the advantages and disadvantages of the extensive body of data generated in the NHP xenoheart transplantation model.

BACKGROUND AIMS/UNASSIGNED:In liver transplantation, cold preservation induces ischemia, resulting in significant reperfusion injury. Hypothermic Oxygenated Machine Perfusion (HMP-O2) has shown benefit compared to static cold storage (SCS) by limiting ischemia-reperfusion injury. This study reports outcomes using a novel portable HMP-O2 device in the first US randomized control trial. APPROACH RESULTS/UNASSIGNED:The PILOT™ trial (NCT03484455) was a multicenter, randomized, open-label, non-inferiority trial, with participants randomized to HMP-O2 or SCS. HMP-O2 livers were preserved using the Lifeport® Liver Transporter and Vasosol® perfusion solution. Primary outcome was early allograft dysfunction (EAD). Non-inferiority margin was 7.5%. From 4/3/19-7/12/22, 179 patients were randomized to HMP-O2 (n=90) or SCS (n=89). Per protocol cohort included 63 HMP-O2 and 73 SCS. EAD occurred in 11.1% HMP-O2 (N=7) and 16.4% SCS (N=12). The risk difference between HMP-O2 and SCS was -5.33% (one-sided 95% upper confidence limit of 5.81%), establishing noninferiority. Risk of graft failure as predicted by L-GrAFT7 was lower with HMP-O2 (median [IQR] 3.4% [2.4-6.5] vs. 4.5% [2.9-9.4], p=0.024). Primary nonfunction occurred in 2.2%, all SCS (n=3, p=0.10). Biliary strictures occurred in 16.4% SCS (n=12) and 6.3% (n=4) HMP-O2 (p=0.18). Non-anastomotic biliary strictures occurred only in SCS (n=4). CONCLUSIONS:HMP-O2 demonstrates safety and noninferior efficacy for liver graft preservation in comparison to SCS. EAD by L-GrAFT7 was lower in HMP-O2, suggesting improved early clinical function. Recipients of HMP-O2 livers also demonstrated a lower incidence PNF and biliary strictures, although this difference did not reach significance.

BACKGROUND:Cross-species immunological incompatibilities have hampered pig-to-human xenotransplantation, but porcine genome engineering recently enabled the first successful experiments. However, little is known about the immune response after the transplantation of pig kidneys to human recipients. We aimed to precisely characterise the early immune responses to the xenotransplantation using a multimodal deep phenotyping approach. METHODS:We did a complete phenotyping of two pig kidney xenografts transplanted to decedent humans. We used a multimodal strategy combining morphological evaluation, immunophenotyping (IgM, IgG, C4d, CD68, CD15, NKp46, CD3, CD20, and von Willebrand factor), gene expression profiling, and whole-transcriptome digital spatial profiling and cell deconvolution. Xenografts before implantation, wild-type pig kidney autografts, as well as wild-type, non-transplanted pig kidneys with and without ischaemia-reperfusion were used as controls. FINDINGS:cells. Both xenografts showed increased expression of genes biologically related to a humoral response, including monocyte and macrophage activation, natural killer cell burden, endothelial activation, complement activation, and T-cell development. Whole-transcriptome digital spatial profiling showed that antibody-mediated injury was mainly located in the glomeruli of the xenografts, with significant enrichment of transcripts associated with monocytes, macrophages, neutrophils, and natural killer cells. This phenotype was not observed in control pig kidney autografts or in ischaemia-reperfusion models. INTERPRETATION:Despite favourable short-term outcomes and absence of hyperacute injuries, our findings suggest that antibody-mediated rejection in pig-to-human kidney xenografts might be occurring. Our results suggest specific therapeutic targets towards the humoral arm of rejection to improve xenotransplantation results. FUNDING:OrganX and MSD Avenir.

Genetically modified xenografts are one of the most promising solutions to the discrepancy between the numbers of available human organs for transplantation and potential recipients. To date, a porcine heart has been implanted into only one human recipient. Here, using 10-gene-edited pigs, we transplanted porcine hearts into two brain-dead human recipients and monitored xenograft function, hemodynamics and systemic responses over the course of 66 hours. Although both xenografts demonstrated excellent cardiac function immediately after transplantation and continued to function for the duration of the study, cardiac function declined postoperatively in one case, attributed to a size mismatch between the donor pig and the recipient. For both hearts, we confirmed transgene expression and found no evidence of cellular or antibody-mediated rejection, as assessed using histology, flow cytometry and a cytotoxic crossmatch assay. Moreover, we found no evidence of zoonotic transmission from the donor pigs to the human recipients. While substantial additional work will be needed to advance this technology to human trials, these results indicate that pig-to-human heart xenotransplantation can be performed successfully without hyperacute rejection or zoonosis.

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