Pig-to-baboon lung xenotransplantation: Extended survival with targeted genetic modifications and pharmacologic treatments

Future directions for xenotransplantation in lungs

PURPOSE OF REVIEW

Advancements in preclinical xenotransplant studies have opened doors for clinical heart and kidney xenotransplantation. This review assesses recent progress in lung xenotransplantation research and its potential clinical implications.

RECENT FINDINGS

The efficacy of the humanized von Willebrand factor in reducing platelet sequestration in ex-vivo and in-vivo lung xenotransplant models was showcased. Combining human tissue factor pathway inhibitor and CD47 expression with selectin and integrin inhibition delayed neutrophil and platelet sequestration. Enhanced expression of human complement regulatory proteins and thrombomodulin in genetically engineered pig lungs improved graft survival by reducing platelet activation and modulating coagulation disruptions. Knocking out the CMAH gene decreased antibody-mediated inflammation and coagulation activation, enhancing compatibility for human transplantation. Furthermore, CMAH gene knockout in pigs attenuated sialoadhesin-dependent binding of human erythrocytes to porcine macrophages, mitigating erythrocyte sequestration and anemia. Meanwhile, in-vivo experiments demonstrated extended survival of xenografts for up to 31 days with multiple genetic modifications and comprehensive treatment strategies.

SUMMARY

Experiments have uncovered vital insights for successful xenotransplantation, driving further research into immunosuppressive therapy and genetically modified pigs. This will ultimately pave the way for clinical trials designed to improve outcomes for patients with end-stage lung disease.

Extended survival of 9- and 10-gene-edited pig heart xenografts with ischemia minimization and CD154 costimulation blockade-based immunosuppression

BACKGROUND

Xenotransplantation has made significant advances recently using pigs genetically engineered to remove carbohydrate antigens, either alone or with addition of various human complement, coagulation, and anti-inflammatory ''transgenes''. Here we evaluated results associated with gene-edited (GE) pig hearts transplanted in baboons using an established costimulation-based immunosuppressive regimen and a cold-perfused graft preservation technique.

METHODS

Eight baboons received heterotopic abdominal heart transplants from 3-GE (GalKO.β4GalNT2KO.hCD55, n = 3), 9-GE (GalKO.β4GalNT2KO.GHRKO.hCD46.hCD55. TBM.EPCR.hCD47. HO-1, n = 3) or 10-G (9-GE+CMAHKO, n = 2) pigs using Steen's cold continuous perfusion for ischemia minimization. Immunosuppression (IS) included induction with anti-thymocyte globulin and αCD20, ongoing αCD154, MMF, and tapered corticosteroid.

RESULTS

All three 3-GE grafts functioned well initially, but failed within 5 days. One 9-GE graft was lost intraoperatively due to a technical issue and another was lost at POD 13 due to antibody mediated rejection (AMR) in a baboon with a strongly positive pre-operative cross-match. One 10-GE heart failed at POD113 with combined cellular and antibody mediated rejection. One 9-GE and one 10-GE hearts had preserved graft function with normal myocardium on protocol biopsies, but exhibited slowly progressive graft hypertrophy until elective necropsy at POD393 and 243 respectively. Elevated levels of IL-6, MCP-1, C-reactive protein, and human thrombomodulin were variably associated with conditioning, the transplant procedure, and clinically significant postoperative events.

CONCLUSION

Relative to reference genetics without thrombo-regulatory and anti-inflammatory gene expression, 9- or 10-GE pig hearts exhibit promising performance in the context of a clinically applicable regimen including ischemia minimization and αCD154-based IS, justifying further evaluation in an orthotopic model.

Physiological basis for xenotransplantation from genetically modified pigs to humans

The collective efforts of scientists over multiple decades have led to advancements in molecular and cellular biology-based technologies including genetic engineering and animal cloning that are now being harnessed to enhance the suitability of pig organs for xenotransplantation into humans. Using organs sourced from pigs with multiple gene deletions and human transgene insertions, investigators have overcome formidable immunological and physiological barriers in pig-to-nonhuman primate (NHP) xenotransplantation and achieved prolonged pig xenograft survival. These studies informed the design of Revivicor's (Revivicor Inc, Blacksburg, VA) genetically engineered pigs with 10 genetic modifications (10 GE) (including the inactivation of 4 endogenous porcine genes and insertion of 6 human transgenes), whose hearts and kidneys have now been studied in preclinical human xenotransplantation models with brain-dead recipients. Additionally, the first two clinical cases of pig-to-human heart xenotransplantation were recently performed with hearts from this 10 GE pig at the University of Maryland. Although this review focuses on xenotransplantation of hearts and kidneys, multiple organs, tissues, and cell types from genetically engineered pigs will provide much-needed therapeutic interventions in the future.

Past, present, and future of CRISPR genome editing technologies

Genome editing has been a transformative force in the life sciences and human medicine, offering unprecedented opportunities to dissect complex biological processes and treat the underlying causes of many genetic diseases. CRISPR-based technologies, with their remarkable efficiency and easy programmability, stand at the forefront of this revolution. In this Review, we discuss the current state of CRISPR gene editing technologies in both research and therapy, highlighting limitations that constrain them and the technological innovations that have been developed in recent years to address them. Additionally, we examine and summarize the current landscape of gene editing applications in the context of human health and therapeutics. Finally, we outline potential future developments that could shape gene editing technologies and their applications in the coming years.

Current Status and Trends in Lung Transplant Research Funded by the National Natural Science Foundation of China

OBJECTIVES

This study aimed to analyze research projects on lung transplant funded by the National Natural Science Foundation of China from 1986 to 2022 and to provide a scientific reference for lung transplant research.

MATERIALS AND METHODS

We identified research hotspots and frontiers in the field of lung transplant research using CiteSpace visualization.

RESULTS

From 1986 to 2022, the National Natural Science Foundation of China funded 93 projects related to lung transplant, with an average of 2.51 projects and ¥0.94 million annually. The National Natural Science Foundation of China funded 30 institutions across 20 provinces, with general and youth science foundation projects comprising 45.16% and 41.93% of the total projects, respectively. The main categories of disciplines included H0113 respiratory intervention, tracheal reconstruction, and lung transplantation; H1105 organ transplantation and transplant immunization; and H0109 acute lung injury and acute respiratory distress syndrome. The research hotspots mainly included ischemia-reperfusion injury, gene regulation, obliterative bronchiolitis, rejection reaction, T cells, and stem cells. The 6 main research clusters were ischemia-reperfusion injury, immune tolerance, obliterative bronchiolitis, stem cells, pulmonary fibrosis, and rejection reaction. The main key word bursts in the past 5 years were "vein endothelial" and "ex vivo lung perfusion."

CONCLUSIONS

In the past 37 years, National Natural Science Foundation of China-funded projects have significantly advanced the clinical application and basic research of lung transplantation. However, compared with developed countries and other solidorgan transplantations, several problems still require attention and improvements in lung transplant research in China.

Lung Xenotransplantation

Although efforts have been made to expand the pool of donor lung allografts for human lung transplantation, a shortage remains. Lung xenotransplantation has been proposed as an alternative approach, but lung xenotransplantation in humans has not yet been reported. In addition, significant biological and ethical barriers will have to be addressed before clinical trials can be undertaken. However, significant progress has been made toward addressing biological incompatibilities that present a barrier, and recent advances in genetic engineering tools promise to accelerate further progress.

Increased human complement pathway regulatory protein gene dose is associated with increased endothelial expression and prolonged survival during ex-vivo perfusion of GTKO pig lungs with human blood

INTRODUCTION

Expression of human complement pathway regulatory proteins (hCPRP's) such as CD46 or CD55 has been associated with improved survival of pig organ xenografts in multiple different models. Here we evaluate the hypothesis that an increased human CD46 gene dose, through homozygosity or additional expression of a second hCPRP, is associated with increased protein expression and with improved protection from injury when GTKO lung xenografts are perfused with human blood.

METHODS

Twenty three GTKO lungs heterozygous for human CD46 (GTKO.heteroCD46), 10 lungs homozygous for hCD46 (GTKO.homoCD46), and six GTKO.homoCD46 lungs also heterozygous for hCD55 (GTKO.homoCD46.hCD55) were perfused with human blood for up to 4 h in an ex vivo circuit.

RESULTS

Relative to GTKO.heteroCD46 (152 min, range 5-240; 6/23 surviving at 4 h), survival was significantly improved for GTKO.homoCD46 (>240 min, range 45-240, p = .034; 7/10 surviving at 4 h) or GTKO.homoCD46.hCD55 lungs (>240 min, p = .001; 6/6 surviving at 4 h). Homozygosity was associated with increased capillary expression of hCD46 (p < .0001). Increased hCD46 expression was associated with significantly prolonged lung survival (p = .048),) but surprisingly not with reduction in measured complement factor C3a. Hematocrit, monocyte count, and pulmonary vascular resistance were not significantly altered in association with increased hCD46 gene dose or protein expression.

CONCLUSION

Genetic engineering approaches designed to augment hCPRP activity - increasing the expression of hCD46 through homozygosity or co-expressing hCD55 with hCD46 - were associated with prolonged GTKO lung xenograft survival. Increased expression of hCD46 was associated with reduced coagulation cascade activation, but did not further reduce complement activation relative to lungs with relatively low CD46 expression. We conclude that coagulation pathway dysregulation contributes to injury in GTKO pig lung xenografts perfused with human blood, and that the survival advantage for lungs with increased hCPRP expression is likely attributable to improved endothelial thromboregulation.

Antibody-mediated rejection in xenotransplantation: Can it be prevented or reversed?

Antibody-mediated rejection (AMR) is the commonest cause of failure of a pig graft after transplantation into an immunosuppressed nonhuman primate (NHP). The incidence of AMR compared to acute cellular rejection is much higher in xenotransplantation (46% vs. 7%) than in allotransplantation (3% vs. 63%) in NHPs. Although AMR in an allograft can often be reversed, to our knowledge there is no report of its successful reversal in a pig xenograft. As there is less experience in preventing or reversing AMR in models of xenotransplantation, the results of studies in patients with allografts provide more information. These include (i) depletion or neutralization of serum anti-donor antibodies, (ii) inhibition of complement activation, (iii) therapies targeting B or plasma cells, and (iv) anti-inflammatory therapy. Depletion or neutralization of anti-pig antibody, for example, by plasmapheresis, is effective in depleting antibodies, but they recover within days. IgG-degrading enzymes do not deplete IgM. Despite the expression of human complement-regulatory proteins on the pig graft, inhibition of systemic complement activation may be necessary, particularly if AMR is to be reversed. Potential therapies include (i) inhibition of complement activation (e.g., by IVIg, C1 INH, or an anti-C5 antibody), but some complement inhibitors are not effective in NHPs, for example, eculizumab. Possible B cell-targeted therapies include (i) B cell depletion, (ii) plasma cell depletion, (iii) modulation of B cell activation, and (iv) enhancing the generation of regulatory B and/or T cells. Among anti-inflammatory agents, anti-IL6R mAb and TNF blockers are increasingly being tested in xenotransplantation models, but with no definitive evidence that they reverse AMR. Increasing attention should be directed toward testing combinations of the above therapies. We suggest that treatment with a systemic complement inhibitor is likely to be most effective, possibly combined with anti-inflammatory agents (if these are not already being administered). Ultimately, it may require further genetic engineering of the organ-source pig to resolve the problem entirely, for example, knockout or knockdown of SLA, and/or expression of PD-L1, HLA E, and/or HLA-G.

Future of Lung Transplantation: Xenotransplantation and Bioengineering Lungs

Xenotransplantation promises to alleviate the issue of donor organ shortages and to decrease waiting times for transplantation. Recent advances in genetic engineering have allowed for the creation of pigs with up to 16 genetic modifications. Several combinations of genetic modifications have been associated with extended graft survival and life-supporting function in experimental heart and kidney xenotransplants. Lung xenotransplantation carries specific challenges related to the large surface area of the lung vascular bed, its innate immune system's intrinsic hyperreactivity to perceived 'danger', and its anatomic vulnerability to airway flooding after even localized loss of alveolocapillary barrier function. This article discusses the current status of lung xenotransplantation, and challenges related to immunology, physiology, anatomy, and infection. Tissue engineering as a feasible alternative to develop a viable lung replacement solution is discussed.

Genetic modifications designed for xenotransplantation attenuate sialoadhesin-dependent binding of human erythrocytes to porcine macrophages

The phenomenon of diminishing hematocrit after in vivo liver and lung xenotransplantation and during ex vivo liver xenoperfusion has largely been attributed to action by resident liver porcine macrophages, which bind and destroy human erythrocytes. Porcine sialoadhesin (siglec-1) was implicated previously in this interaction. This study examines the effect of porcine genetic modifications, including knockout of the CMAH gene responsible for expression of Neu5Gc sialic acid, on the adhesion of human red blood cells (RBCs) to porcine macrophages. Wild-type (WT) porcine macrophages and macrophages from several strains of genetically engineered pigs, including CMAH gene knockout and several human transgenes (TKO+hTg), were incubated with human RBCs and "rosettes" (≥3 erythrocytes bound to one macrophage) were quantified by microscopy. Our results show that TKO+hTg genetic modifications significantly reduced rosette formation. The monoclonal antibody 1F1, which blocks porcine sialoadhesin, significantly reduced rosette formation by WT and TKO+hTg macrophages compared with an isotype control antibody. Further, desialation of human RBCs with neuraminidase before addition to WT or TKO+hTg macrophages resulted in near-complete abrogation of rosette formation, to a level not significantly different from porcine RBC rosette formation on porcine macrophages. These observations are consistent with rosette formation being mediated by binding of sialic acid on human RBCs to sialoadhesin on porcine macrophages. In conclusion, the data predict that TKO+hTg genetic modifications, coupled with targeting of porcine sialoadhesin by the 1F1 mAb, will attenuate erythrocyte sequestration and anemia during ex vivo xenoperfusion and following in vivo liver, lung, and potentially other organ xenotransplantation.

Microbiosis in lung allotransplantation and xenotransplantation: State of the art and future perspective

The respiratory tract is known to harbor a microbial community including bacteria, viruses, and fungi. New techniques contribute enormously to the identification of unknown or culture-independent species and reveal the interaction of the community with the host immune system. The existing respiratory microbiome and substantial equilibrium of the transplanted microbiome from donor lung grafts provide an extreme bloom of dynamic changes in the microenvironment in lung transplantation (LT) recipients. Dysbiosis in grafts are not only related to the modified microbial components but also involve the kinetics of the host-graft "talk," which signifies the destination of graft allograft injury, acute rejection, infection, and chronic allograft dysfunction development in short- and long-term survival. Microbiome-derived factors may contribute to lung xenograft survival when using genetically multimodified pig-derived organs. Here, we review the most advanced knowledge of the dynamics and resilience of microbial communities in transplanted lungs with various pretransplant indications. Conceptual and analytical points of view have been illustrated along the time series, gaining insight into the microbiome and lung grafts. Future endeavors on precise tools, sophisticated models, and novel targeted regimens are needed to improve the long-term survival in these patients.

Pig heart and lung xenotransplantation: Present status

The recent pig heart transplant in a patient at the University of Maryland Medical Center has stimulated renewed interest in the xenotransplantation of organs from genetically engineered pigs. The barriers to the use of pigs as sources of organs have largely been overcome by 2 approaches - (1) the deletion of expression of the three known pig carbohydrate xenoantigens against which humans have preformed antibodies, and (2) the transgenic introduction of human 'protective' proteins, such as complement-regulatory proteins. These gene modifications, coupled with immunosuppressive therapy based on blockade of the CD40/CD154 costimulation pathway, have resulted in survival of baboons with life-supporting pig heart grafts for almost 9 months. The initial clinical success at the University of Maryland reinforces encouraging preclinical results. It suggests that pig hearts are likely to provide an effective bridge to an allotransplant, but their utility for destination therapy remains uncertain. Because of additional complex immunobiological problems, the same approach has been less successful in preclinical lung xenograft transplantation, where survival is still measured in days or weeks. The first formal clinical trials of pig heart transplantation may include patients who do not have access to an allotransplant, those with contraindications for mechanical circulatory support, those in need of retransplantation or with a high level of panel-reactive antibodies. Infants with complex congenital heart disease, should also be considered.

Genetically modified immunomodulatory cell-based biomaterials in tissue regeneration and engineering

To date, the wide application of cell-based biomaterials in tissue engineering and regeneration is remarkably hampered by immune rejection. Reducing the immunogenicity of cell-based biomaterials has become the latest direction in biomaterial research. Recently, genetically modified cell-based biomaterials with immunomodulatory genes have become a feasible solution to the immunogenicity problem. In this review, recent advances and future challenges of genetically modified immunomodulatory cell-based biomaterials are elaborated, including fabrication approaches, mechanisms of common immunomodulatory genes, application and, more importantly, current preclinical and clinical advances. The fabrication approaches can be categorized into commonly used (e.g., virus transfection) and newly developed approaches. The immunomodulatory mechanisms of representative genes involve complicated cell signaling pathways and metabolic activities. Wide application in curing multiple end-term diseases and replacing lifelong immunosuppressive therapy in multiple cell and organ transplantation models is demonstrated. Most significantly, practices of genetically modified organ transplantation have been conducted on brain-dead human decedent and even on living patients after a series of experiments on nonhuman primates. Nevertheless, uncertain biosecurity, nonspecific effects and overlooked personalization of current genetically modified immunomodulatory cell-based biomaterials are shortcomings that remain to be overcome.

Pig-to-human kidney transplantation using brain-dead donors as recipients: One giant leap, or only one small step for transplantkind?

Pig kidney xenotransplantation is increasingly regarded as a realistic solution to the current shortage of human organ donors for patients with end-stage organ failure. Recently, the news of three pig-to-human transplantation cases has awakened public interest. Notably, the case by the Alabama team reported detailed and important findings for the xenotransplantation field. Using a genetically modified pig, two porcine kidneys were transplanted into a brain-dead recipient. They applied several approaches established in the preclinical NHP study, including gene-edited pig kidney graft and preoperative laboratory inspection such as crossmatching and infection screening. The pig-to-human kidney xenotransplantation had no unexpected events during surgery or evidence of hyperacute rejection. Unfortunately, the grafts did not work appropriately, and the study had to be terminated due to the decompensation of the recipient. While this study demonstrated the outstanding achievement in this research area, it also revealed remaining gaps to move xenotransplantation to the clinic. While brain-dead human recipients could reinforce the compatibility achievements of gene-edited pigs in NHP, their pro-inflammatory and pro-coagulant environment, in combination with short-duration of experiments will limit the assessment of kidney function, infection and rejection risk post-transplant, in particular antibody-mediated rejection. The use of successful immunosuppressive protocols of non-human primates xenotransplant experiments including anti-CD154 antibody will be critical to maximize the success in the first in-human trials.

Commentary on achievement of first life-saving xenoheart transplant

Two months of post-transplant survival were achieved in an end-stage heart disease patient with a porcine heart from a pig whose genome had been modified in 10 of its genes. This first-ever life-saving cardiac xenotransplantation was the result of decades of work and close coordination by researchers in genetic engineering, animal cloning, immunology, ex vivo organ perfusion, and thoracic surgery.

Effects of human TFPI and CD47 expression and selectin and integrin inhibition during GalTKO.hCD46 pig lung perfusion with human blood

BACKGROUND

Loss of barrier function when GalTKO.hCD46 porcine lungs are perfused with human blood is associated with coagulation pathway dysregulation, innate immune system activation, and rapid sequestration of human formed blood elements. Here, we evaluate whether genetic expression of human tissue factor pathway inhibitor (hTFPI) and human CD47 (hCD47), alone or with combined selectin and integrin adhesion pathway inhibitors, delays GalTKO.hCD46 porcine lung injury or modulates neutrophil and platelet sequestration.

METHODS

In a well-established paired ex vivo lung perfusion model, GalTKO.hCD46.hTFPI.hCD47 transgenic porcine lungs (hTFPI.hCD47, n = 7) were compared to GalTKO.hCD46 lungs (reference, n = 5). All lung donor pigs were treated with a thromboxane synthase inhibitor, anti-histamine, and anti-GPIb integrin-blocking Fab, and were pre-treated with Desmopressin. In both genotypes, one lung of each pair was additionally treated with PSGL-1 and GMI-1271 (P- and E-selectin) and IB4 (CD11b/18 integrin) adhesion inhibitors (n = 6 hTFPI.hCD47, n = 3 reference).

RESULTS

All except for two reference lungs did not fail within 480 min when experiments were electively terminated. Selectin and integrin adhesion inhibitors moderately attenuated initial pulmonary vascular resistance (PVR) elevation in hTFPI.hCD47 lungs. Neutrophil sequestration was significantly delayed during the early time points following reperfusion and terminal platelet activation was attenuated in association with lungs expressing hTFPI.hCD47, but additional adhesion pathway inhibitors did not show further effects with either lung genotype.

CONCLUSION

Expression of hTFPI.hCD47 on porcine lung may be useful as part of an integrated strategy to prevent neutrophil adhesion and platelet activation that are associated with xenograft injury. Additionally, targeting canonical selectin and integrin adhesion pathways reduced PVR elevation associated with hTFPI.hCD47 expression, but did not significantly attenuate neutrophil or platelet sequestration. We conclude that other adhesive mechanisms mediate the residual sequestration of human formed blood elements to pig endothelium that occurs even in the context of the multiple genetic modifications and drug treatments tested here.

Human erythrocyte fragmentation during ex-vivo pig organ perfusion

Platelet sequestration is a common process during organ reperfusion after transplantation. However, instead of lower platelet counts, when using traditional hemocytometers and light microscopy, we observed physiologically implausible platelet counts in the course of ex-vivo lung and liver xenograft organ perfusion studies. We employed conventional flow cytometry (FC) and imaging FC (AMINS ImageStream X) to investigate the findings and found platelet-sized fragments in the circulation that are mainly derived from red blood cell membranes. We speculate that this erythrocyte fragmentation contributes to anemia during in-vivo organ xenotransplant.