Production of alpha 1,3-galactosyltransferase-deficient pigs

Research prospects for kidney xenotransplantation: a bibliometric analysis

BACKGROUND

Xenograft kidney transplantation has been receiving increasing attention. The purpose of this study is to use bibliometric analysis to identify papers in this research field and explore their current status and development trends.

METHODS

Using the data in the Web of Science core database from Clarivate Analytics as the object of study, we used 'TS = Kidney OR Renal AND xenotransplantation' as the search term to find all literature from 1980 to 2 November 2022.

RESULTS

In total, 1005 articles were included. The United States has the highest number of publications and has made significant contributions in this field. Harvard University was at the forefront of this study. Professor Cooper has published 114 articles in this field. Xenotransplantation has the largest number of relevant articles. Transplantation was the most cited journal. High-frequency keywords illustrated the current state of development and future trends in xenotransplantation. The use of transgenic pigs and the development of coordinated co-stimulatory blockers have greatly facilitated progress in xenotransplantation research. We found that 'co-stimulation blockade', 'xenograft survival', 'pluripotent stem cell', 'translational research', and 'genetic engineering' were likely to be the focus of attention in the coming years.

CONCLUSIONS

This study screened global publications related to xenogeneic kidney transplantation; analyzed their literature metrology characteristics; identified the most cited articles in the research field; understood the current situation, hot spots, and trends of global research; and provided future development directions for researchers and practitioners.

Current Status of Cardiac Xenotransplantation: Report of a Workshop of the German Heart Transplant Centers, Martinsried, March 3, 2023

This report comprises the contents of the presentations and following discussions of a workshop of the German Heart Transplant Centers in Martinsried, Germany on cardiac xenotransplantation. The production and current availability of genetically modified donor pigs, preservation techniques during organ harvesting, and immunosuppressive regimens in the recipient are described. Selection criteria for suitable patients and possible solutions to the problem of overgrowth of the xenotransplant are discussed. Obviously microbiological safety for the recipient and close contacts is essential, and ethical considerations to gain public acceptance for clinical applications are addressed. The first clinical trial will be regulated and supervised by the Paul-Ehrlich-Institute as the National Competent Authority for Germany, and the German Heart Transplant Centers agreed to cooperatively select the first patients for cardiac xenotransplantation.

The pig as an optimal animal model for cardiovascular research

Cardiovascular disease is a worldwide health problem and a leading cause of morbidity and mortality. Preclinical cardiovascular research using animals is needed to explore potential targets and therapeutic options. Compared with rodents, pigs have many advantages, with their anatomy, physiology, metabolism and immune system being more similar to humans. Here we present an overview of the available pig models for cardiovascular diseases, discuss their advantages over other models and propose the concept of standardized models to improve translation to the clinical setting and control research costs.

Global status of gene edited animals for agricultural applications

Gene editing (GnEd) involves using a site-directed nuclease to introduce a double-strand break (DSB) at a targeted location in the genome. A literature search was performed on the use of GnEd in animals for agricultural applications. Data was extracted from 212 peer-reviewed articles that described the production of at least one living animal employing GnEd technologies for agricultural purposes. The most common GnEd system reported was CRISPR/Cas9, and the most frequent type of edit was the unguided insertion or deletion resulting from the repair of the targeted DSB leading to a knock-out (KO) mutation. Animal groups included in the reviewed papers were ruminants (cattle, sheep, goats, n=63); monogastrics (pigs and rabbits, n=60); avian (chicken, duck, quail, n=17); aquatic (many species, n=65), and insects (honeybee, silkworm, n=7). Yield (32%), followed by reproduction (21%) and disease resistance (17%) were the most commonly targeted traits. Over half of the reviewed papers had Chinese first-authorship. Several countries, including Argentina, Australia, Brazil, Colombia and Japan, have adopted a regulatory policy that considers KO mutations introduced following GnEd DSB repair as akin to natural genetic variation, and therefore treat these GnEd animals analogously to those produced using conventional breeding. This approach has resulted in a non-GMO determination for a small number of GnEd food animal applications, including three species of GnEd KO fast-growing fish, (red sea bream, olive flounder and tiger pufferfish in Japan), KO fish and cattle in Argentina and Brazil, and porcine reproductive and respiratory syndrome (PRRS) virus disease-resistant KO pigs in Colombia.

Influence of relatively short-term culture on adult porcine islets for xenotransplantation

Porcine islet xenotransplantation is a promising therapy for severe diabetes mellitus. Maintenance of the quality and quantity of porcine islets is important for the success of this treatment. Here, we aimed to elucidate the influence of relatively short-term (14 days) culture on adult porcine islets isolated from three micro-minipigs (P111, P112 and P121). Morphological characteristics of islets changed little after 14 days of culture. The viability of cultured islets was also maintained at a high level (> 80%). Furthermore, cultured islets exhibited similar glucose-stimulated insulin secretion and insulin content at Day 14 were preserved comparing with Day 1, while the expressions of Ins, Gcg and Sst were attenuated at Day 14. Xenotransplantation using diabetic nude mice showed no normalization of blood glucose but increased levels of plasma porcine C-peptide after the transplantation of 14 day cultured porcine islets. Histological assessment revealed that relatively short-term cultured porcine islets were successfully engrafted 56 days following transplantation. These data show that relatively short-term culture did not impair the quality of adult porcine islets in regard to function, morphology, and viability. Prevention of impairment of gene correlated with endocrine hormone is warranted for further improvement.

Cellular dynamics in pig-to-human kidney xenotransplantation

BACKGROUND

Xenotransplantation of genetically engineered porcine organs has the potential to address the challenge of organ donor shortage. Two cases of porcine-to-human kidney xenotransplantation were performed, yet the physiological effects on the xenografts and the recipients' immune responses remain largely uncharacterized.

METHODS

We performed single-cell RNA sequencing (scRNA-seq) and longitudinal RNA-seq analyses of the porcine kidneys to dissect xenotransplantation-associated cellular dynamics and xenograft-recipient interactions. We additionally performed longitudinal scRNA-seq of the peripheral blood mononuclear cells (PBMCs) to detect recipient immune responses across time.

FINDINGS

Although no hyperacute rejection signals were detected, scRNA-seq analyses of the xenografts found evidence of endothelial cell and immune response activation, indicating early signs of antibody-mediated rejection. Tracing the cells' species origin, we found human immune cell infiltration in both xenografts. Human transcripts in the longitudinal bulk RNA-seq revealed that human immune cell infiltration and the activation of interferon-gamma-induced chemokine expression occurred by 12 and 48 h post-xenotransplantation, respectively. Concordantly, longitudinal scRNA-seq of PBMCs also revealed two phases of the recipients' immune responses at 12 and 48-53 h. Lastly, we observed global expression signatures of xenotransplantation-associated kidney tissue damage in the xenografts. Surprisingly, we detected a rapid increase of proliferative cells in both xenografts, indicating the activation of the porcine tissue repair program.

CONCLUSIONS

Longitudinal and single-cell transcriptomic analyses of porcine kidneys and the recipient's PBMCs revealed time-resolved cellular dynamics of xenograft-recipient interactions during xenotransplantation. These cues can be leveraged for designing gene edits and immunosuppression regimens to optimize xenotransplantation outcomes.

FUNDING

This work was supported by NIH RM1HG009491 and DP5OD033430.

Strategies of Bladder Reconstruction after Partial or Radical Cystectomy for Bladder Cancer

The standard strategy is to reconstruct bladder by use of bowel segments as material in bladder cancer with radical cystectomy clinically. Both natural derived and non natural derived materials are investigated in bladder reconstruction. Studies on mechanical bladder, bladder transplantation and bladder xenotransplantation are currently limited although heart and kidney transplantation or xenotransplantation are successful to a certain extent, and bone prostheses are applied in clinical contexts. Earlier limited number of studies associated with bladder xenograft from animals to humans were not particular promising in results. Although there have been investigations on pig to human cardiac xenotransplantation with CRISPR Cas9 gene editing, the CRISPR Cas technique is not yet widely researched in porcine bladder related gene editing for the potential of human bladder replacement for bladder cancer. The advancement of technologies such as gene editing, bioprinting and induced pluripotent stem cells allow further research into partial or whole bladder replacement strategies. Porcine bladder is suggested as a potential source material for bladder reconstruction due to its alikeness to human bladder. Challenges that exist with all these approaches need to be overcome. This paper aims to review gene editing technology such as the CRISPR Cas systems as tools in bladder reconstruction, bladder xenotransplantation and hybrid bladder with technologies of induced pluripotent stem cells and genome editing, bioprinting for bladder replacement for bladder reconstruction and to restore normal bladder control function after cystectomy for bladder cancer.

How should cardiac xenotransplantation be initiated in Japan?

The world's first clinical cardiac xenotransplantation, using a genetically engineered pig heart with 10 gene modifications, prolonged the life of a 57-year-old man with no other life-saving options, by 60 days. It is foreseeable that xenotransplantation will be introduced in clinical practice in the United States. However, little clinical or regulatory progress has been made in the field of xenotransplantation in Japan in recent years. Japan seems to be heading toward a "device lag", and the over-importation of medical devices and technology in the medical field is becoming problematic. In this review, we discuss the concept of pig-heart xenotransplantation, including the pathobiological aspects related to immune rejection, coagulation dysregulation, and detrimental heart overgrowth, as well as genetic modification strategies in pigs to prevent or minimize these problems. Moreover, we summarize the necessity for and current status of xenotransplantation worldwide, and future prospects in Japan, with the aim of initiating xenotransplantation in Japan using genetically modified pigs without a global delay. It is imperative that this study prompts the initiation of preclinical xenotransplantation research using non-human primates and leads to clinical studies.

Pigs in Transplantation Research and Their Potential as Sources of Organs in Clinical Xenotransplantation

The pig has long been used as a research animal and has now gained importance as a potential source of organs for clinical xenotransplantation. When an organ from a wild-type (i. e., genetically unmodified) pig is transplanted into an immunosuppressed nonhuman primate, a vigorous host immune response causes hyperacute rejection (within minutes or hours). This response has been largely overcome by 1) extensive gene editing of the organ-source pig and 2) the administration to the recipient of novel immunosuppressive therapy based on blockade of the CD40/CD154 T cell costimulation pathway. Gene editing has consisted of 1) deletion of expression of the 3 known carbohydrate xenoantigens against which humans have natural (preformed) antibodies and 2) the introduction of human 'protective' genes. The combination of gene editing and novel immunosuppressive therapy has extended life-supporting pig kidney graft survival to greater than 1 y and of pig heart survival to up to 9 mo. This review briefly describes the techniques of gene editing, the potential risks of transfer of porcine endogenous retroviruses with the organ, and the need for breeding and housing of donor pigs under biosecure conditions.

Xenotransplantation experiments in brain-dead human subjects-A critical appraisal

Brain-dead human subjects (decedents) were recently introduced as a potential preclinical experimental model in xenotransplantation. Brain death is associated with major pathophysiological changes, eg, structural injury and cell infiltration in vital organs, and major hormonal, metabolic, inflammatory, and hemodynamic changes. In 2 of the 3 initial experiments, the design of the experiments resulted in little or no new information becoming available. In the third, the experiment was unfortunately unsuccessful as neither of the 2 pig kidneys transplanted into the decedent functioned adequately. Failure may well have been associated with the effects of brain death, but an immune/inflammatory response to the xenograft could not be excluded. Subsequently, 2 further pig kidney transplants and 2 pig heart transplants have been carried out in human decedents, but again the data obtained do not add much to what is already known. In view of the profound changes that take place during and after brain death, it may prove difficult to determine whether graft failure or dysfunction results from the effects of brain death or from an immune/inflammatory response to the xenograft. A major concern is that, if the results are confusing, they may impact decisions relating to the introduction of clinical xenotransplantation.

Human ST3Gal II and ST6GalNAc IV genes increase human serum-mediated cytotoxicity to xenogeneic cells

Carbohydrate-antigens widely existed on glycoproteins and glycosphingolipids of all mammalian cells play a crucial role in self-defense and immunity. Xeno-reactive antibodies included in natural human sera play a protecting role in an acute phase-rejection of xenotransplantation. In this study, we investigated the effect of an alteration of glycosylation-pattern, caused by human sialyltransferases such as hST3Gal II or hST6GalNAc IV, on human serum mediated cytotoxicity in pig kidney PK15 cells. From LDH cytotoxicity assay, cytotoxicity to human serum was significantly increased in hST3Gal II and hST6GalNAc IV-transfected PK15 cells, as compared to the control. In the hST6Gal I-carrying cells, the cytotoxicity to human serum was rather decreased. Moreover, flow cytometry analysis revealed that an alteration of pig glycosylation-pattern by hST3Gal II or hST6GalNAc IV influences on a binding of human IgM or IgG, respectively, in pig kidney cells, regardless of Gal antigen alteration. Finally, we found that hST6GalNAc IV contributed to increase of terminal disialylated tetrasaccharide structure, disialyl T antigen, as evidenced by increase of the MAL II lectin binding capacity in the hST6GalNAc IV-transfected PK15 cells, compared with control. Therefore, our results suggest that carbohydrate antigens, such as disialyl T antigen, newly synthesized by the ST3Gal II- and ST6GalNAc IV are potentially believed to be new xeno-reactive elements.

Advances in Organ and Tissue Xenotransplantation

End-stage organ failure can result from various preexisting conditions and occurs in patients of all ages, and organ transplantation remains its only treatment. In recent years, extensive research has been done to explore the possibility of transplanting animal organs into humans, a process referred to as xenotransplantation. Due to their matching organ sizes and other anatomical and physiological similarities with humans, pigs are the preferred organ donor species. Organ rejection due to host immune response and possible interspecies infectious pathogen transmission have been the biggest hurdles to xenotransplantation's success. Use of genetically engineered pigs as tissue and organ donors for xenotransplantation has helped to address these hurdles. Although several preclinical trials have been conducted in nonhuman primates, some barriers still exist and demand further efforts. This review focuses on the recent advances and remaining challenges in organ and tissue xenotransplantation.

Human PD-L1 overexpression decreases xenogeneic human T-cell immune responses towards porcine kidneys

Xenotransplantation offers a promising alternative to circumvent the lack of donated human organs available for transplantation. Different attempts to improve the survival of xenografts led to the generation of transgenic pigs expressing various combinations of human protective genes or knocked out for specific antigens. Currently, testing the efficiency of porcine organs carrying different genetic modifications in preventing xenogeneic immune responses completely relies on in vitro assays, humanized mouse models, or non-human primate transplantation models. However, these tests are often associated with major concerns due to reproducibility and generation of insufficient data as well as they raise ethical, logistical, and economic issues. In this study, we investigated the feasibility of specifically assessing the strength of human T-cell responses towards the kidneys of wild-type (WT) or transgenic pigs overexpressing human programmed death-1 ligand 1 (hPD-L1) during ex vivo kidney perfusion (EVKP). Human T cells were shown to adhere to the endothelium and transmigrate into WT and hPD-L1 kidneys. However, transcript levels of TNF-a and IFN-y as well as cytotoxic molecules such as granzyme B and perforin secreted by human T cells were significantly decreased in the tissue of hPD-L1 kidneys in comparison to WT kidneys. These results were confirmed via in vitro assays using renal endothelial cells (ECs) isolated from WT and hPD-L1 transgenic pigs. Both CD4+ and CD8+ T cells showed significantly lower proliferation rates after exposure to hPD-L1 porcine renal ECs in comparison to WT ECs. In addition, the secretion of pro-inflammatory cytokines was significantly reduced in cultures using hPD-L1 ECs in comparison to WT ECs. Remarkably, hPD-L1 EC survival was significantly increased in cytotoxic assays. This study demonstrates the feasibility of evaluating the human response of specific immune subsets such as human T cells towards the whole xenograft during EVKP. This may represent a robust strategy to assess the potency of different genetic modifications to prevent xenogeneic immune responses and thereby predict the risk of immune rejection of new genetically engineered xenografts.

Developments in kidney xenotransplantation

The search for kidney xenografts that are appropriate for patients with end-stage renal disease has been ongoing since the beginning of the last century. The major cause of xenograft loss is hyperacute and acute rejection, and this has almost been overcome via scientific progress. The success of two pre-clinical trials of α1,3-galactosyltransferase gene-knockout porcine kidneys in brain-dead patients in 2021 triggered research enthusiasm for kidney xenotransplantation. This minireview summarizes key issues from an immunological perspective: the discovery of key xenoantigens, investigations into key co-stimulatory signal inhibition, gene-editing technology, and immune tolerance induction. Further developments in immunology, particularly immunometabolism, might help promote the long-term outcomes of kidney xenografts.

Swine models in translational research and medicine

Swine are increasingly studied as animal models of human disease. The anatomy, size, longevity, physiology, immune system, and metabolism of swine are more like humans than traditional rodent models. In addition, the size of swine is preferred for surgical placement and testing of medical devices destined for humans. These features make swine useful for biomedical, pharmacological, and toxicological research. With recent advances in gene-editing technologies, genetic modifications can readily and efficiently be made in swine to study genetic disorders. In addition, gene-edited swine tissues are necessary for studies testing and validating xenotransplantation into humans to meet the critical shortfall of viable organs versus need. Underlying all of these biomedical applications, the knowledge of husbandry, background diseases and lesions, and biosecurity needs are important for productive, efficient, and reproducible research when using swine as a human disease model for basic research, preclinical testing, and translational studies.

A Comparison Between GalT-/-;hCD39;hCD55 and GalT-/-;hCD39;hCD46;hCD55;TBM Pig Kidneys Transplanted in Nonhuman Primates

Because there is a shortage of donor kidneys, researchers are exploring the possibility of using genetically modified pig kidneys for transplantation. Approaches involving knockout of carbohydrate genes or knockin of protective proteins have been attempted to determine the best gene modifications. In this study, we utilized GalT-/-;hCD39;hCD55 and GalT-/-;hCD39;hCD46;hCD55;thrombomodulin (TBM) pigs for transplantation in nonhuman primates (NHPs). The NHPs survived for 4 weeks after kidney transplantation (4 WAT) from the GalT-/-;hCD39;hCD55 pig and for 6 WAT from the GalT-/-;hCD39;hCD46;hCD55;TBM pig. However, messenger RNA (mRNA) sequencing and immunohistochemistry analysis revealed that the 6 WAT kidney exhibited more severe apoptosis, inflammation, loss of renal function, and renal fibrosis than the 4 WAT kidney. These results indicate that additional knockin of complement regulator (hCD46) and coagulation regulator (TBM) is not enough to prevent renal damage, suggesting that improved immune suppression is needed for more prolonged survival.

Initial investigation on the feasibility of porcine red blood cells from genetically modified pigs as an alternative to human red blood cells for transfusion

The decline in blood donation rates and the ongoing shortage of blood products pose significant challenges to medical societies. One potential solution is to use porcine red blood cells (pRBCs) from genetically modified pigs as an alternative to human red blood cells (hRBCs). However, adverse immunological reactions remain a significant obstacle to their use. This study aimed to evaluate the compatibility of diverse genetically modified pRBCs with human serum. We acquired human complement-competent serum, complement 7 (C7)-deficient serum, and hRBCs from all ABO blood types. Additionally, we used leftover clinical samples from health checkups for further evaluation. pRBCs were collected from wild-type (WT) and genetically modified pigs: triple knockout (TKO), quadruple KO (QKO), and TKO/hCD55.hCD39 knockin (hCD55.hCD39KI). The extent of C3 deposition on RBCs was measured using flow cytometry after incubation in C7-deficient serum diluted in Ca++-enriched or Ca++-depleted and Mg++-enriched buffers. The binding of immunoglobulin (Ig) M/IgG antibody to RBCs after incubation in ABO-type human serum was evaluated using flow cytometry. Naïve human serum- or sensitized monkey serum-mediated hemolysis was also evaluated. Phagocytosis was assessed by incubating labeled RBCs with the human monocytic cell line THP-1 and measurement by flow cytometry. All three genetic modifications significantly improved the compatibility of pRBCs with human serum relative to that of WT pRBCs. The extent of IgM/IgG binding to genetically modified pRBCs was lower than that of WT pRBCs and similar to that of O-type hRBCs. Total and alternative pathway complement activation in all three genetically modified pRBCs was significantly weaker than that in WT pRBCs and did not differ from that in O-type hRBCs. The extent of serum-mediated hemolysis and phagocytosis of these genetically modified pRBCs was low and similar to that of O-type hRBCs. Sensitized monkey serum-mediated hemolysis in QKO and TKO/hCD55.hCD39KI pRBCs was higher than in O-type hRBCs but lower than in TKO pRBCs. The elimination of porcine carbohydrate antigens in genetically modified pigs significantly enhanced pRBC compatibility with naïve human sera, which was comparable to that of O-type hRBCs. These findings provide valuable insights into the development of pRBCs as potential alternatives to hRBCs.

Update on the ethical, legal and technical challenges of translating xenotransplantation

This manuscript reports on a landmark symposium on the ethical, legal and technical challenges of xenotransplantation in the UK. King's College London, with endorsement from the British Transplantation Society (BTS), and the European Society of Organ Transplantation (ESOT), brought together a group of experts in xenotransplantation science, ethics and law to discuss the ethical, regulatory and technical challenges surrounding translating xenotransplantation into the clinical setting. The symposium was the first of its kind in the UK for 20 years. This paper summarises the content of the expert lectures showcasing the progress which has been made in xenotransplantation including-the history of xenotransplantation, advances in gene edited animals and progress towards clinical xenotransplantation. We then set out the ethical and legal issues still to be resolved. Finally, we report the themes of the roundtable discussion highlighting areas of consensus and controversy. While the detail of the legal discussion was directed towards the UK, the principles and summary reported here are intended to be applicable to any jurisdiction seeking to implement clinical xenotransplantation.

Genetic Engineering of Donor Pig for the First Human Cardiac Xenotransplantation: Combatting Rejection, Coagulopathy, Inflammation, and Excessive Growth

PURPOSE OF REVIEW

The first successful pig to human cardiac xenotransplantation in January 2022 represented a major step forward in the fields of heart failure, immunology, and applied genetic engineering, using a 10-gene edited (GE) pig. This review summarizes the evolution of preclinical modelling data which informed the use of each of the 10 genes modified in the 10-GE pig: GGTA1, Β4GalNT2, CMAH, CD46, CD55, TBM, EPCR, CD47, HO-1, and growth hormone receptor.

RECENT FINDINGS

The translation of the 10-GE pig from preclinical modelling to clinical compassionate xenotransplant use was the culmination of decades of research combating rejection, coagulopathy, inflammation, and excessive xenograft growth. Understanding these 10 genes with a view to their combinatorial effects will be useful in anticipated xenotransplant clinical trials.

Accelerated Burn Healing in a Mouse Experimental Model Using α-Gal Nanoparticles

Macrophages play a pivotal role in the process of healing burns. One of the major risks in the course of burn healing, in the absence of regenerating epidermis, is infections, which greatly contribute to morbidity and mortality in such patients. Therefore, it is widely agreed that accelerating the recruitment of macrophages into burns may contribute to faster regeneration of the epidermis, thus decreasing the risk of infections. This review describes a unique method for the rapid recruitment of macrophages into burns and the activation of these macrophages to mediate accelerated regrowth of the epidermis and healing of burns. The method is based on the application of bio-degradable "α-gal" nanoparticles to burns. These nanoparticles present multiple α-gal epitopes (Galα1-3Galβ1-4GlcNAc-R), which bind the abundant natural anti-Gal antibody that constitutes ~1% of immunoglobulins in humans. Anti-Gal/α-gal nanoparticle interaction activates the complement system, resulting in localized production of the complement cleavage peptides C5a and C3a, which are highly effective chemotactic factors for monocyte-derived macrophages. The macrophages recruited into the α-gal nanoparticle-treated burns are activated following interaction between the Fc portion of anti-Gal coating the nanoparticles and the multiple Fc receptors on macrophage cell membranes. The activated macrophages secrete a variety of cytokines/growth factors that accelerate the regrowth of the epidermis and regeneration of the injured skin, thereby cutting the healing time by half. Studies on the healing of thermal injuries in the skin of anti-Gal-producing mice demonstrated a much faster recruitment of macrophages into burns treated with α-gal nanoparticles than in control burns treated with saline and healing of the burns within 6 days, whereas healing of control burns took ~12 days. α-Gal nanoparticles are non-toxic and do not cause chronic granulomas. These findings suggest that α-gal nanoparticles treatment may harness anti-Gal for inducing similar accelerated burn healing effects also in humans.

Shooting for the moon: Genome editing for pig heart xenotransplantation

Gene-edited pigs could eventually provide organs that are safely and effectively protected from the human immune response without exogenous immunosuppression.

Genome editing technology has revolutionized heart xenotransplantation and made transplantation of bioengineered pig hearts into humans a possibility. This first clinical application resulted from a tremendous amount of research. Dramatic early attempts of clinical cardiac xenotransplantation during the last century paved the way to modern xenotransplantation using bioengineered pig hearts. It appears that such genome-edited hearts will be most suitable for neonates and infants because of their immature immune system. The bioengineered pig heart may also be used as a bridge to human heart transplantation, avoiding the risk of thromboembolic events of durable ventricular assist devises in these young children. It is also intriguing to think that bioengineered hearts using pigs as a host may result in a new source of donor hearts that would not evoke the human immune response and minimize, if not eliminate, the need for immunosuppression.

It this issue of the Journal, a group of experts led by Dr Cooper, whose personal work spans over 50 years of heart transplantation research, outline the current state of the genome editing of bioengineered hearts and discuss the prospects of clinical application.

Revolutionizing Heart Transplantation: A Multidisciplinary Approach to Xenotransplantation, Immunosuppression, Regenerative Medicine, Artificial Intelligence, and Economic Sustainability

Heart transplantation (HTx) stands as a life-saving intervention for patients with end-stage heart disease, but the field is fraught with numerous challenges that span from the scarcity of donor organs to long-term complications arising from immunosuppressive therapies. This comprehensive review article offers an in-depth exploration of the multifaceted aspects of HTx. The review covers groundbreaking advancements in xenotransplantation, enabled by cutting-edge genetic engineering techniques, and the promising role of stem cell therapies, particularly porcine mesenchymal stem cells, in cardiac regeneration. It also delves into the evolution and limitations of immunosuppressive therapies and the revolutionary potential of artificial intelligence (AI) and machine learning (ML) in enhancing donor-recipient matching and predicting patient outcomes. Economic considerations, especially in the context of rising healthcare costs, are examined to assess the sustainability of these advancements. The article further discusses the significant improvements in patient outcomes over the years, while highlighting persisting challenges, such as graft failure, rejection, and infection. It underscores the importance of experience and specialized training, evidenced by the presence of an institutional learning curve. The review concludes by advocating for a multifaceted, collaborative approach involving clinicians, researchers, and policymakers to overcome existing challenges. Through coordinated efforts that consider medical, ethical, and economic factors, the field of HTx is poised for further evolution, offering renewed hope for improved patient care and outcomes.

Future prospects for the clinical transfusion of pig red blood cells

Transfusion of allogeneic human red blood cell (hRBCs) is limited by supply and compatibility between individual donors and recipients. In situations where the blood supply is constrained or when no compatible RBCs are available, patients suffer. As a result, alternatives to hRBCs that complement existing RBC transfusion strategies are needed. Pig RBCs (pRBCs) could provide an alternative because of their abundant supply, and functional similarities to hRBCs. The ability to genetically modify pigs to limit pRBC immunogenicity and augment expression of human 'protective' proteins has provided major boosts to this research and opens up new therapeutic avenues. Although deletion of expression of xenoantigens has been achieved in genetically-engineered pigs, novel genetic methods are needed to introduce human 'protective' transgenes into pRBCs at the high levels required to prevent hemolysis and extend RBC survival in vivo. This review addresses recent progress and examines future prospects for clinical xenogeneic pRBC transfusion.

Graft rejection in paediatric congenital heart disease

Congenital heart disease (CHD) affects around 1.35 million neonates worldwide per annum, and surgical repair is necessary in approximately 25% of cases. Xenografts, usually of bovine or porcine origin, are often used for the surgical reconstruction. These xenografts elicit an immune response due to significant immunological incompatibilities between host and donor. Current techniques to dampen the initial hyperacute rejection response involve aldehyde fixation to crosslink xenoantigens, such as galactose-α1,3-galactose and N-glycolylneuraminic acid. While this temporarily masks the epitopes, aldehyde fixation is a suboptimal solution, degrading over time, resulting in cytotoxicity and rejection. The immune response to foreign tissue eventually leads to chronic inflammation and subsequent graft failure, necessitating reintervention to replace the defective bioprosthetic. Decellularisation to remove immunoincompatible material has been suggested as an alternative to fixation and may prove a superior solution. However, incomplete decellularisation poses a significant challenge, causing a substantial immune rejection response and subsequent graft rejection. This review discusses commercially available grafts used in surgical paediatric CHD intervention, looking specifically at bovine jugular vein conduits as a substitute to cryopreserved homografts, as well as decellularised alternatives to the aldehyde-fixed graft. Mechanisms of biological prosthesis rejection are explored, including the signalling cascades of the innate and adaptive immune response. Lastly, emerging strategies of intervention are examined, including the use of tissue from genetically modified pigs, enhanced crosslinking and decellularisation techniques, and augmentation of grafts through in vitro recellularisation or functionalisation with human surface proteins.

Human and Pig Pluripotent Stem Cells: From Cellular Products to Organogenesis and Beyond

Pluripotent stem cells (PSCs) are important for studying development and hold great promise in regenerative medicine due to their ability to differentiate into various cell types. In this review, we comprehensively discuss the potential applications of both human and pig PSCs and provide an overview of the current progress and challenges in this field. In addition to exploring the therapeutic uses of PSC-derived cellular products, we also shed light on their significance in the study of interspecies chimeras, which has led to the creation of transplantable human or humanized pig organs. Moreover, we emphasize the importance of pig PSCs as an ideal cell source for genetic engineering, facilitating the development of genetically modified pigs for pig-to-human xenotransplantation. Despite the achievements that have been made, further investigations and refinement of PSC technologies are necessary to unlock their full potential in regenerative medicine and effectively address critical healthcare challenges.

Perspectives in Genome-Editing Techniques for Livestock

Genome editing of farm animals has undeniable practical applications. It helps to improve production traits, enhances the economic value of livestock, and increases disease resistance. Gene-modified animals are also used for biomedical research and drug production and demonstrate the potential to be used as xenograft donors for humans. The recent discovery of site-specific nucleases that allow precision genome editing of a single-cell embryo (or embryonic stem cells) and the development of new embryological delivery manipulations have revolutionized the transgenesis field. These relatively new approaches have already proven to be efficient and reliable for genome engineering and have wide potential for use in agriculture. A number of advanced methodologies have been tested in laboratory models and might be considered for application in livestock animals. At the same time, these methods must meet the requirements of safety, efficiency and availability of their application for a wide range of farm animals. This review aims at covering a brief history of livestock animal genome engineering and outlines possible future directions to design optimal and cost-effective tools for transgenesis in farm species.

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.

CD40-CD40L Blockade: Update on Novel Investigational Therapeutics for Transplantation

Effective immune responses require antigen presentation by major histocompatibility complexes with cognate T-cell receptor and antigen-independent costimulatory signaling for T-cell activation, proliferation, and differentiation. Among several costimulatory signals, CD40-CD40L is of special interest to the transplantation community because it plays a vital role in controlling or regulating humoral and cellular immunity. Blockade of this pathway has demonstrated inhibition of donor-reactive T-cell responses and prolonged the survival of transplanted organs. Several anti-CD154 and anti-CD40 antibodies have been used in the transplantation model and demonstrated the potential of extending allograft and xenograft rejection-free survival. The wide use of anti-CD154 antibodies was hampered because of thromboembolic complications in transplant recipients. These antibodies have been modified to overcome the thromboembolic complications by altering the antibody binding fragment (Fab) and Fc (fragment, crystallizable) receptor region for therapeutic purposes. Here, we review recent preclinical advances to target the CD40-CD40L pair in transplantation.

Antibody production and tolerance to the α-gal epitope as models for understanding and preventing the immune response to incompatible ABO carbohydrate antigens and for α-gal therapies

This review describes the significance of the α-gal epitope (Galα-3Galβ1-4GlcNAc-R) as the core of human blood-group A and B antigens (A and B antigens), determines in mouse models the principles underlying the immune response to these antigens, and suggests future strategies for the induction of immune tolerance to incompatible A and B antigens in human allografts. Carbohydrate antigens, such as ABO antigens and the α-gal epitope, differ from protein antigens in that they do not interact with T cells, but B cells interacting with them require T-cell help for their activation. The α-gal epitope is the core of both A and B antigens and is the ligand of the natural anti-Gal antibody, which is abundant in all humans. In A and O individuals, anti-Gal clones (called anti-Gal/B) comprise >85% of the so-called anti-B activity and bind to the B antigen in facets that do not include fucose-linked α1-2 to the core α-gal. As many as 1% of B cells are anti-Gal B cells. Activation of quiescent anti-Gal B cells upon exposure to α-gal epitopes on xenografts and some protozoa can increase the titer of anti-Gal by 100-fold. α1,3-Galactosyltransferase knockout (GT-KO) mice lack α-gal epitopes and can produce anti-Gal. These mice simulate human recipients of ABO-incompatible human allografts. Exposure for 2-4 weeks of naïve and memory mouse anti-Gal B cells to α-gal epitopes in the heterotopically grafted wild-type (WT) mouse heart results in the elimination of these cells and immune tolerance to this epitope. Shorter exposures of 7 days of anti-Gal B cells to α-gal epitopes in the WT heart result in the production of accommodating anti-Gal antibodies that bind to α-gal epitopes but do not lyse cells or reject the graft. Tolerance to α-gal epitopes due to the elimination of naïve and memory anti-Gal B cells can be further induced by 2 weeks in vivo exposure to WT lymphocytes or autologous lymphocytes engineered to present α-gal epitopes by transduction of the α1,3-galactosyltransferase gene. These mouse studies suggest that autologous human lymphocytes similarly engineered to present the A or B antigen may induce corresponding tolerance in recipients of ABO-incompatible allografts. The review further summarizes experimental works demonstrating the efficacy of α-gal therapies in amplifying anti-viral and anti-tumor immune-protection and regeneration of injured tissues.

Perioperative implications of patients with alpha gal allergies

Alpha Gal Syndrome (AGS) is an emerging immune response to mammalian products (MP) containing the oligosaccharide galactose-α-1,3 galactose (α-Gal) which includes meats and inactive ingredients in certain medications. This becomes clinically important in the perioperative realm as MPs are commonly found in the operating room, and pre- and post-operative settings, and can trigger responses as severe as anaphylaxis. In this review, authors discuss the epidemiology, diagnosis and treatment of AGS reactions. Additionally, strategies are explored in order to screen and prevent exposure to MP with a multidisciplinary approach. While this emerging allergy is still not fully understood, it is of paramount importance that all anesthesia providers recognize the implications of MP exposure in AGS patients and ultimately prevent harm in this highly vulnerable population.

Historical Review and Future of Cardiac Xenotransplantation

Along with the development of immunosuppressive drugs, major advances on xenotransplantation were achieved by understanding the immunobiology of xenograft rejection. Most importantly, three predominant carbohydrate antigens on porcine endothelial cells were key elements provoking hyperacute rejection: α1,3-galactose, SDa blood group antigen, and N-glycolylneuraminic acid. Preformed antibodies binding to the porcine major xenoantigen causes complement activation and endothelial cell activation, leading to xenograft injury and intravascular thrombosis. Recent advances in genetic engineering enabled knock-outs of these major xenoantigens, thus producing xenografts with less hyperacute rejection rates. Another milestone in the history of xenotransplantation was the development of co-stimulation blockaded strategy. Unlike allotransplantation, xenotransplantation requires blockade of CD40-CD40L pathway to prevent T-cell dependent B-cell activation and antibody production. In 2010s, advanced genetic engineering of xenograft by inducing the expression of multiple human transgenes became available. So-called 'multi-gene' xenografts expressing human transgenes such as thrombomodulin and endothelial protein C receptor were introduced, which resulted in the reduction of thrombotic events and improvement of xenograft survival. Still, there are many limitations to clinical translation of cardiac xenotransplantation. Along with technical challenges, zoonotic infection and physiological discordances are major obstacles. Social barriers including healthcare costs also need to be addressed. Although there are several remaining obstacles to overcome, xenotransplantation would surely become the novel option for millions of patients with end-stage heart failure who have limited options to traditional therapeutics.

Cells and Materials for Cardiac Repair and Regeneration

After more than 20 years following the introduction of regenerative medicine to address the problem of cardiac diseases, still questions arise as to the best cell types and materials to use to obtain effective clinical translation. Now that it is definitively clear that the heart does not have a consistent reservoir of stem cells that could give rise to new myocytes, and that there are cells that could contribute, at most, with their pro-angiogenic or immunomodulatory potential, there is fierce debate on what will emerge as the winning strategy. In this regard, new developments in somatic cells' reprogramming, material science and cell biophysics may be of help, not only for protecting the heart from the deleterious consequences of aging, ischemia and metabolic disorders, but also to boost an endogenous regeneration potential that seems to be lost in the adulthood of the human heart.

Milestones on the path to clinical pig organ xenotransplantation

Progress in pig organ xenotransplantation has been made largely through (1) genetic engineering of the organ-source pig to protect its tissues from the human innate immune response, and (2) development of an immunosuppressive regimen based on blockade of the CD40/CD154 costimulation pathway to prevent the adaptive immune response. In the 1980s, after transplantation into nonhuman primates (NHPs), wild-type (genetically unmodified) pig organs were rejected within minutes or hours. In the 1990s, organs from pigs expressing a human complement-regulatory protein (CD55) transplanted into NHPs receiving intensive conventional immunosuppressive therapy functioned for days or weeks. When costimulation blockade was introduced in 2000, the adaptive immune response was suppressed more readily. The identification of galactose-α1,3-galactose as the major antigen target for human and NHP anti-pig antibodies in 1991 allowed for deletion of expression of galactose-α1,3-galactose in 2003, extending pig graft survival for up to 6 months. Subsequent gene editing to overcome molecular incompatibilities between the pig and primate coagulation systems proved additionally beneficial. The identification of 2 further pig carbohydrate xenoantigens allowed the production of 'triple-knockout' pigs that are preferred for clinical organ transplantation. These combined advances enabled the first clinical pig heart transplant to be performed and opened the door to formal clinical trials.

Physiologic considerations of pig-to-human kidney xenotransplantation

PURPOSE OF REVIEW

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

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

Progress in biotechnology is heralding an era of xenotransplantation. We highlight the physiologic considerations for xenogeneic grafts to succeed.

Cardiac xenotransplantation: from concept to clinic

For many patients with terminal/advanced cardiac failure, heart transplantation is the most effective, durable treatment option, and offers the best prospects for a high quality of life. The number of potentially life-saving donated human organs is far fewer than the population who could benefit from a new heart, resulting in increasing numbers of patients awaiting replacement of their failing heart, high waitlist mortality, and frequent reliance on interim mechanical support for many of those deemed among the best candidates but who are deteriorating as they wait. Currently, mechanical assist devices supporting left ventricular or biventricular heart function are the only alternative to heart transplant that is in clinical use. Unfortunately, the complication rate with mechanical assistance remains high despite advances in device design and patient selection and management, and the quality of life of the patients even with good outcomes is only moderately improved. Cardiac xenotransplantation from genetically multi-modified (GM) organ-source pigs is an emerging new option as demonstrated by the consistent long-term success of heterotopic (non-life-supporting) abdominal and life-supporting orthotopic porcine heart transplantation in baboons, and by a recent 'compassionate use' transplant of the heart from a GM pig with 10 modifications into a terminally ill patient who survived for 2 months. In this review, we discuss pig heart xenotransplantation as a concept, including pathobiological aspects related to immune rejection, coagulation dysregulation, and detrimental overgrowth of the heart, as well as GM strategies in pigs to prevent or minimize these problems. Additional topics discussed include relevant results of heterotopic and orthotopic heart transplantation experiments in the pig-to-baboon model, microbiological and virologic safety concepts, and efficacy requirements for initiating formal clinical trials. An adequate regulatory and ethical framework as well as stringent criteria for the selection of patients will be critical for the safe clinical development of cardiac xenotransplantation, which we expect will be clinically tested during the next few years.

Xenotransplantation: Current Challenges and Emerging Solutions

To address the ongoing shortage of organs available for replacement, xenotransplantation of hearts, corneas, skin, and kidneys has been attempted. However, a major obstacle facing xenotransplants is rejection due to a cycle of immune reactions to the graft. Both adaptive and innate immune systems contribute to this cycle, in which natural killer cells, macrophages, and T-cells play a significant role. While advancements in the field of genetic editing can circumvent some of these obstacles, biomarkers to identify and predict xenograft rejection remain to be standardized. Several T-cell markers, such as CD3, CD4, and CD8, are useful in both the diagnosis and prediction of xenograft rejection. Furthermore, an increase in the levels of various circulating DNA markers and microRNAs is also predictive of xenograft rejection. In this review, we summarize recent findings on the advancements in xenotransplantation, with a focus on pig-to-human, the role of immunity in xenograft rejection, and its biomarkers.

Perpendicular implantation of porcine trachea extracellular matrix for enhanced xenogeneic scaffold surface epithelialization in a canine model

Objective

The availability of clinically applied medical materials in thoracic surgery remains insufficient, especially materials for treating tracheal defects. Herein, the potential of porcine extracellular matrix (P-ECM) as a new airway reconstruction material was explored by xenotransplanting it into a canine trachea.

Methods

P-ECM was first transplanted into the buttocks of Narc Beagle dogs (n = 3) and its overall immuno-induced effects were evaluated. Subsequently, nine dogs underwent surgery to create a tracheal defect that was 1 × 2 cm. In group A, the P-ECM was implanted parallel to the tracheal axis (n = 3), whereas in group B the P-ECM was implanted perpendicular to the tracheal axis (n = 6). The grafts were periodically observed by bronchoscopy and evaluated postoperatively at 1 and 3 months through macroscopic and microscopic examinations. Immunosuppressants were not administered. Statistical evaluation was performed for Bronchoscopic stenosis rate, graft epithelialization rate, shrinkage rate and ECM live-implantation rate.

Results

No sign of P-ECM rejection was observed after its implantation in the buttocks. Bronchoscopic findings showed no improvement concerning stenosis in group A until 3 months after surgery; epithelialization of the graft site was not evident, and the ECM site appeared scarred and faded. In contrast, stenosis gradually improved in group B, with continuous epithelium within the host tissues and P-ECM. Histologically, the graft site contracted longitudinally and no epithelialization was observed in group A, whereas full epithelialization was observed on the P-ECM in group B. No sign of cartilage regeneration was confirmed in both groups. No statistically significant differences were found in bronchoscopic stenosis rate, shrinkage rate and ECM live-implantation rate, but graft epithelialization rate showed a statistically significant difference (G-A; sporadic (25%) 3, vs. G-B; full covered (100%) 3; p = 0.047).

Conclusions

P-ECM can support full re-epithelialization without chondrocyte regeneration, with perpendicular implantation facilitating epithelialization of the ECM. Our results showed that our decellularized tracheal matrix holds clinical potential as a biological xenogeneic material for airway defect repair.

The respective relevance of sensitization to alloantigens and xenoantigens in pig organ xenotransplantation

BACKGROUND

Antibody-mediated rejection is a major cause of graft injury and contributes to failure of pig xenografts in nonhuman primates (NHPs). Most 'natural' or elicited antibodies found in humans and NHPs are directed against pig glycan antigens, but antibodies binding to swine leukocyte antigens (SLA) have also been detected. Of clinical importance is (i) whether the presence of high levels of antibodies directed towards human leukocyte antigens (HLA) (i.e., high panel-reactive antibodies) would be detrimental to the outcome of a pig organ xenograft; and (ii) whether, in the event of sensitization to pig antigens, a subsequent allotransplant would be at increased risk of graft failure due to elicited anti-pig antibodies that cross-react with human HLA or other antigens.

SUMMARY

A literature review of pig-to-primate studies indicates that relatively few highly-HLA-sensitized humans have antibodies that cross-react with pigs, predicting that most would not be at increased risk of rejecting an organ xenograft. Furthermore, the existing evidence indicates that sensitization to pig antigens will probably not elicit increased alloantibody titers; if so, 'bridging' with a pig organ could be carried out without increased risk of subsequent antibody-mediated allograft failure.

KEY MESSAGE

These issues have important implications for the design and conduct of clinical xenotransplantation trials.

Paleo-immunology of human anti-carbohydrate antibodies preventing primate extinctions

Two human natural anti-carbohydrate antibodies appeared in critical evolutionary events that brought primates and hominins to brink of extinction. The first is the anti-Gal antibody, produced in Old-World monkeys (OWM), apes and humans. It binds the carbohydrate-antigen 'α-gal epitope' (Galα1-3Galβ1-4GlcNAc-R) on carbohydrate-chains (glycans) synthesized by non-primate mammals, lemurs and New-World monkeys (NWM). The second is anti-N-glycolylneuraminic-acid (anti-Neu5Gc) antibody binding Neu5Gc on glycans synthesized by OWM, apes and most non-primate mammals. Ancestral OWM and apes synthesized α-gal epitopes and were eliminated ~20-30 million-years-ago (mya). Only few accidentally mutated offspring lacking α-gal epitopes, produced anti-Gal and survived. Hominin-populations living ~3 mya synthesized Neu5Gc and were eliminated, but few mutated offspring that accidently lost their ability to synthesize Neu5Gc, produced natural anti-Neu5Gc antibody. These hominins survived and ultimately evolved into present-day humans. It is argued that these two near-extinction events were likely to be the result of epidemics caused by highly virulent and lethal enveloped viruses that killed parental-populations. These viruses presented α-gal epitopes or Neu5Gc synthesized in host-cells of the parental-populations. Mutated offspring survived the epidemics because they were protected from the lethal virus by the natural anti-Gal or anti-Neu5Gc antibodies they produced due to loss of immune-tolerance to α-gal epitopes or to Neu5Gc, respectively.

Advances in Innate Immunity to Overcome Immune Rejection during Xenotransplantation

Transplantation is an effective approach for treating end-stage organ failure. There has been a long-standing interest in xenotransplantation as a means of increasing the number of available organs. In the past decade, there has been tremendous progress in xenotransplantation accelerated by the development of rapid gene-editing tools and immunosuppressive therapy. Recently, the heart and kidney from pigs were transplanted into the recipients, which suggests that xenotransplantation has entered a new era. The genetic discrepancy and molecular incompatibility between pigs and primates results in barriers to xenotransplantation. An increasing body of evidence suggests that innate immune responses play an important role in all aspects of the xenogeneic rejection. Simultaneously, the role of important cellular components like macrophages, natural killer (NK) cells, and neutrophils, suggests that the innate immune response in the xenogeneic rejection should not be underestimated. Here, we summarize the current knowledge about the innate immune system in xenotransplantation and highlight the key issues for future investigations. A better understanding of the innate immune responses in xenotransplantation may help to control the xenograft rejection and design optimal combination therapies.

Tissue-engineered grafts exploit axon-facilitated axon regeneration and pathway protection to enable recovery after 5-cm nerve defects in pigs

Functional restoration following major peripheral nerve injury (PNI) is challenging, given slow axon growth rates and eventual regenerative pathway degradation in the absence of axons. We are developing tissue-engineered nerve grafts (TENGs) to simultaneously "bridge" missing nerve segments and "babysit" regenerative capacity by providing living axons to guide host axons and maintain the distal pathway. TENGs were biofabricated using porcine neurons and "stretch-grown" axon tracts. TENG neurons survived and elicited axon-facilitated axon regeneration to accelerate regrowth across both short (1 cm) and long (5 cm) segmental nerve defects in pigs. TENG axons also closely interacted with host Schwann cells to maintain proregenerative capacity. TENGs drove regeneration across 5-cm defects in both motor and mixed motor-sensory nerves, resulting in dense axon regeneration and electrophysiological recovery at levels similar to autograft repairs. This approach of accelerating axon regeneration while maintaining the pathway for long-distance regeneration may achieve recovery after currently unrepairable PNIs.

Knock-out of N-glycolylneuraminic acid attenuates antibody-mediated rejection in xenogenically perfused porcine lungs

BACKGROUND

Antibody-mediated rejection has long been known to be one of the major organ failure mechanisms in xenotransplantation. In addition to the porcine α1,3-galactose (α1,3Gal) epitope, N-Glycolylneuraminic acid (Neu5Gc), a sialic acid, has been identified as an important porcine antigen against which most humans have pre-formed antibodies. Here we evaluate GalTKO.hCD46 lungs with an additional cytidine monophospho-N-acetylneuraminic acid hydroxylase (CMAH) gene knock-out (Neu5GcKO) in a xenogeneic ex vivo perfusion model METHODS: Eleven GalTKO.hCD46.Neu5GcKO pig lungs were perfused for up to 6 h with fresh heparinized human blood. Six of them were treated with histamine (H) blocker famotidine and 1-thromboxane synthase inhibitor Benzylimidazole (BIA) and five were left untreated. GalTKO.hCD46 lungs without Neu5GcKO (n = 18: eight untreated and 10 BIA+H treated) served as a reference. Functional parameters, blood, and tissue samples were collected at pre-defined time points throughout the perfusion RESULTS: All but one Neu5GcKO organs maintained adequate blood oxygenation and "survived" until elective termination at 6 h whereas two reference lungs failed before elective termination at 4 h. Human anti-Neu5Gc antibody serum levels decreased during the perfusion of GalTKO.hCD46 lungs by flow cytometry (∼40% IgM, 60% IgG), whereas antibody levels in Neu5GcKO lung perfusions did not fall (IgM p = .007; IgG p < .001). Thromboxane elaboration, thrombin generation, and histamine levels were significantly reduced with Neu5GcKO lungs compared to reference in the untreated groups (p = .007, .005, and .037, respectively); treatment with BIA+H masked these changes. Activation of platelets, measured as CD62P expression on circulating platelets, was lower in Neu5GcKO experiments compared to reference lungs (p = .023), whereas complement activation (as C3a rise in plasma) was not altered. MCP-1 and lactotransferin level elevations were blunted in Neu5GcKO lung perfusions (p = .007 and .032, respectively). Pulmonary vascular resistance (PVR) rise was significantly attenuated and delayed in untreated GalTKO.hCD46.Neu5GcKO lungs in comparison to the untreated GalTKO.hCD46 lungs (p = .003) CONCLUSION: Additional Neu5GcKO in GalTKO.hCD46 lungs significantly reduces parameters associated with antibody-mediated inflammation and activation of the coagulation cascade. Knock-out of the Neu5Gc sialic acid should be beneficial to reduce innate immune antigenicity of porcine lungs in future human recipients.

Advance of genetically modified pigs in xeno-transplantation

As the standard of living improves, chronic diseases and end-stage organ failure have been a regular occurrence in human beings. Organ transplantation has become one of the hopes in the fight against chronic diseases and end-stage organ failure. However, organs available for transplantation are far from sufficient to meet the demand, leading to a major organ shortage crisis. To solve this problem, researchers have turned to pigs as their target since pigs have many advantages as xenograft donors. Pigs are considered the ideal organ donor for human xenotransplantation, but direct transplantation of porcine organs to humans faces many obstacles, such as hyperacute rejection, acute humoral xenograft rejection, coagulation dysregulation, inflammatory response, coagulation dysregulation, and endogenous porcine retroviral infection. Many transgenic strategies have been developed to overcome these obstacles. This review provides an overview of current advances in genetically modified pigs for xenotransplantation. Future genetic engineering-based delivery of safe and effective organs and tissues for xenotransplantation remains our goal.

Progress in xenotransplantation: overcoming immune barriers

A major limitation of organ allotransplantation is the insufficient supply of donor organs. Consequently, thousands of patients die every year while waiting for a transplant. Progress in xenotransplantation that has permitted pig organ graft survivals of years in non-human primates has led to renewed excitement about the potential of this approach to alleviate the organ shortage. In 2022, the first pig-to-human heart transplant was performed on a compassionate use basis, and xenotransplantation experiments using pig kidneys in deceased human recipients provided encouraging data. Many advances in xenotransplantation have resulted from improvements in the ability to genetically modify pigs using CRISPR-Cas9 and other methodologies. Gene editing has the capacity to generate pig organs that more closely resemble those of humans and are hence more physiologically compatible and less prone to rejection. Despite such modifications, immune responses to xenografts remain powerful and multi-faceted, involving innate immune components that do not attack allografts. Thus, the induction of innate and adaptive immune tolerance to prevent rejection while preserving the capacity of the immune system to protect the recipient and the graft from infection is desirable to enable clinical xenotransplantation.

α-Gal Nanoparticles Mediated Homing of Endogenous Stem Cells for Repair and Regeneration of External and Internal Injuries by Localized Complement Activation and Macrophage Recruitment

This review discusses a novel experimental approach for the regeneration of original tissue structure by recruitment of endogenous stem-cells to injured sites following administration of α-gal nanoparticles, which harness the natural anti-Gal antibody. Anti-Gal is produced in large amounts in all humans, and it binds the multiple α-gal epitopes (Galα1-3Galβ1-4GlcNAc-R) presented on α-gal nanoparticles. In situ binding of anti-Gal to α-gal nanoparticles activates the complement system and generates complement cleavage chemotactic-peptides that rapidly recruit macrophages. Macrophages reaching anti-Gal coated α-gal nanoparticles bind them via Fc/Fc receptor interaction and polarize into M2 pro-reparative macrophages. These macrophages secrete various cytokines that orchestrate regeneration of the injured tissue, including VEGF inducing neo-vascularization and cytokines directing homing of stem-cells to injury sites. Homing of stem-cells is also directed by interaction of complement cleavage peptides with their corresponding receptors on the stem-cells. Application of α-gal nanoparticles to skin wounds of anti-Gal producing mice results in decrease in healing time by half. Furthermore, α-gal nanoparticles treated wounds restore the normal structure of the injured skin without fibrosis or scar formation. Similarly, in a mouse model of occlusion/reperfusion myocardial-infarction, near complete regeneration after intramyocardial injection of α-gal nanoparticles was demonstrated, whereas hearts injected with saline display ~20% fibrosis and scar formation of the left ventricular wall. It is suggested that recruitment of stem-cells following anti-Gal/α-gal nanoparticles interaction in injured tissues may result in induction of localized regeneration facilitated by conducive microenvironments generated by pro-reparative macrophage secretions and “cues” provided by the extracellular matrix in the injury site.

CRISPR/Cas-edited pigs for personalized medicine: more than preclinical test-system

Novel CRISPR-Cas-based genome editing tools made it feasible to introduce a variety of precise genomic modifications in the pig genome, including introducing multiple edits simultaneously, inserting long DNA sequences into specifically targeted loci, and performing nucleotide transitions and transversions. Pigs serve as a vital agricultural resource and animal model in biomedical studies, given their advantages over the other models. Pigs share high similarities to humans regarding body/organ size, anatomy, physiology, and a metabolic profile. The pig genome can be modified to carry the same genetic mutations found in humans to replicate inherited diseases to provide preclinical trials of drugs. Moreover, CRISPR-based modification of pigs antigen profile makes it possible to offer porcine organs for xenotransplantation with minimal transplant rejection responses. This review summarizes recent advances in endonuclease-mediated genome editing tools and research progress of genome-edited pigs as personalized test-systems for preclinical trials and as donors of organs with human-fit antigen profile.

Graphical abstract:

Repair of bone defects in rhesus monkeys with α1,3-galactosyltransferase-knockout pig cancellous bone

Introduction: Since xenografts offer a wide range of incomparable advantages, they can be a better option than allografts but only if the possibility of immunological rejection can be eliminated. In this study, we investigated the ability of α1,3-galactosyltransferase (α1,3-GT) gene knockout (GTKO) pig cancellous bone to promote the repair of a femoral condyle bone defect and its influence on heterologous immune rejection.

Materials and methods: Cylindrical bone defects created in a rhesus monkey model were transplanted with GTKO bone, WT bone or left empty. For immunological evaluation, T lymphocyte subsets CD4⁺ and CD8⁺ in peripheral blood were assayed by flow cytometry, and the IL-2 and IFN-γ contents of peripheral blood serum were analyzed by ELISA at 2, 5, 7, 10, and 14 days post-surgery. Micro-CT scans and histological assessment were conducted at 4 and 8 weeks after implantation.

Results: Compared with WT-pig bone, the heterologous immunogenicity of GTKO-pig bone was reduced. The defect filled with fresh GTKO-pig bone was tightly integrated with the graft. Histological analysis showed that GTKO-pig cancellous bone showed better osseointegration and an appropriate rate of resorption. Osteoblast phenotype progression in the GTKO group was not affected, which revealed that GTKO-pig bone could not only fill and maintain the bone defect, but also promote new bone formation.

Conclusion: GTKO-pig cancellous bone decreased the ratio of CD4⁺ to CD8⁺ T cells and cytokines (IFN-γ and IL-2) to inhibit xenotransplant rejection. Moreover, GTKO group increased more bone formation by micro-CT analysis and osteoblastic markers (Runx2, OSX and OCN). Together, GTKO-pig cancellous bone showed better bone repair than WT-pig cancellous bone.