First update of the International Xenotransplantation Association consensus statement on conditions for undertaking clinical trials of porcine islet products in type 1 diabetes-Chapter 5: recipient monitoring and response plan for preventing disease trans

International Xenotransplantation Association (IXA) Position Paper on Infectious Disease Considerations in Xenotransplantation

Clinical xenotransplantation has the potential to address shortages of human organs for patients with end-stage organ failure. Advances in genetic engineering, immunosuppressive regimens, and infectious disease diagnostics have improved prospects for clinical xenotransplantation. Management of the infectious risks posed by clinical xenotransplantation requires biosecure breeding and validated methods for microbiological surveillance of source animals and recipients. Novel infection control protocols may complement biosafety requirements. Infectious risks in xenotransplantation include both known human pathogens common to immunosuppressed organ recipients and from porcine organisms or xenozoonoses for which the clinical manifestations are less well defined and for which microbial assays and therapies are more limited. Some pig-specific organisms do not infect human cells but have systemic manifestations when active within the xenograft. The human risk posed by porcine endogenous retroviruses (PERV) is uncertain. There are no documented transmissions of PERV in humans and swine are available with inactivated genomic PERV loci. Metagenomic sequencing will complement more traditional diagnostic tools in the detection of any unknown pathogens in xenotransplantation recipients. Such data are required for the development of protocols for donor and recipient microbiological surveillance, infection control, and antimicrobial therapies that will enhance the safety of clinical xenotransplantation.

Surveillance and prevention of infection in clinical xenotransplantation

SUMMARYXenotransplantation, the transplantation of living organs, tissues, or cells between species, carries the potential to address the global shortage of human organs for patients with end-stage organ failure. Recent advances in genetic engineering have improved prospects for clinical xenotransplantation by reducing immune and inflammatory responses to grafts, controlling coagulation on endothelial surfaces, and modifying viral risks, including the porcine endogenous retrovirus (PERV). Management of infectious risks posed by clinical xenotransplantation requires meticulous attention to the biosecure breeding and microbiological surveillance of source animals and recipients and consideration of novel infection control requirements. Infectious risks in xenotransplantation stem from both known human pathogens in immunosuppressed transplant recipients and from porcine organisms for which the clinical manifestations, microbial assays, and therapies are generally limited. Both known and unknown zoonoses may be transmitted from pigs to humans. Some pig-specific pathogens do not infect human cells but have systemic manifestations when active within the xenograft, including porcine cytomegalovirus/porcine roseolovirus (PCMV/PRV), which contributes to graft rejection and consumptive coagulopathy. The role of porcine endogenous retrovirus (PERV) in humans remains uncertain despite the absence of documented transmissions and the availability of swine with inactivated genomic PERV. New technologies, such as metagenomic sequencing and multi-omics approaches, will be essential for detection of novel infections and for understanding interactions between the xenograft, the host's immune system, and potential pathogens. These approaches will allow development of infection control protocols, pathogen surveillance requirements, and tailored antimicrobial therapies to enhance the safety and success of clinical xenotransplantation.

International Xenotransplantation Association (IXA) Position Paper on Infectious Disease Considerations in Xenotransplantation

Clinical xenotransplantation has the potential to address shortages of human organs for patients with end-stage organ failure. Advances in genetic engineering, immunosuppressive regimens, and infectious disease diagnostics have improved prospects for clinical xenotransplantation. Management of the infectious risks posed by clinical xenotransplantation requires biosecure breeding and validated methods for microbiological surveillance of source animals and recipients. Novel infection control protocols may complement biosafety requirements. Infectious risks in xenotransplantation include both known human pathogens common to immunosuppressed organ recipients and from porcine organisms or xenozoonoses for which the clinical manifestations are less well defined and for which microbial assays and therapies are more limited. Some pig-specific organisms do not infect human cells but have systemic manifestations when active within the xenograft. The human risk posed by porcine endogenous retroviruses (PERV) is uncertain. There are no documented transmissions of PERV in humans and swine are available with inactivated genomic PERV loci. Metagenomic sequencing will complement more traditional diagnostic tools in the detection of any unknown pathogens in xenotransplantation recipients. Such data are required for the development of protocols for donor and recipient microbiological surveillance, infection control, and antimicrobial therapies that will enhance the safety of clinical xenotransplantation.

A brief review of the current status of pig islet xenotransplantation

An estimated 1.5 million Americans suffer from Type I diabetes mellitus, and its incidence is increasing worldwide. Islet allotransplantation offers a treatment, but the availability of deceased human donor pancreases is limited. The transplantation of islets from gene-edited pigs, if successful, would resolve this problem. Pigs are now available in which the expression of the three known xenoantigens against which humans have natural (preformed) antibodies has been deleted, and in which several human 'protective' genes have been introduced. The transplantation of neonatal pig islets has some advantages over that of adult pig islets. Transplantation into the portal vein of the recipient results in loss of many islets from the instant blood-mediated inflammatory reaction (IBMIR) and so the search for an alternative site continues. The adaptive immune response can be largely suppressed by an immunosuppressive regimen based on blockade of the CD40/CD154 T cell co-stimulation pathway, whereas conventional therapy (e.g., based on tacrolimus) is less successful. We suggest that, despite the need for effective immunosuppressive therapy, the transplantation of 'free' islets will prove more successful than that of encapsulated islets. There are data to suggest that, in the absence of rejection, the function of pig islets, though less efficient than human islets, will be sufficient to maintain normoglycemia in diabetic recipients. Pig islets transplanted into immunosuppressed nonhuman primates have maintained normoglycemia for periods extending more than two years, illustrating the potential of this novel form of therapy.

The origin of porcine endogenous retroviruses (PERVs)

Porcine endogenous retroviruses (PERVs) are integrated in the genome of all pigs, and they produce viral particles that are able to infect human cells and therefore pose a special risk for xenotransplantation. In contrast to other pig microorganisms that also pose a risk, such as porcine cytomegalovirus and hepatitis E virus, PERVs cannot be eliminated from pigs by vaccines, antiviral drugs, early weaning, or embryo transfer. Since PERVs are relevant for xenotransplantation, their biology and origin are of great interest. Recent studies have shown that PERVs are the result of a transspecies transmission of precursor retroviruses from different animals and further evolution in the pig genome. PERVs acquired different long terminal repeats (LTRs), and recombination took place. In parallel, it has been shown that the activity of the LTRs and recombination in the envelope are important for the transmissibility and pathogenesis of PERVs. Transspecies transmission of retroviruses is common, a well-known example being the transmission of precursor retroviruses from non-human primates to humans, resulting in human immunodeficiency virus (HIV). Here, recent findings concerning the origin of PERVs, their LTRs, and recombination events that occurred during evolution are reviewed and compared with other findings regarding transspecies transmission of retroviruses.

Corneal xenotransplantation: Where are we standing?

The search for alternatives to allotransplants is driven by the shortage of corneal donors and is demanding because of the limitations of the alternatives. Indeed, current progress in genetically engineered (GE) pigs, the introduction of gene-editing technology by clustered regularly interspaced short palindromic repeats (CRISPR)-Cas9, and advanced immunosuppressants have made xenotransplantation a possible option for a human trial. Porcine corneal xenotransplantation is considered applicable because the eye is regarded as an immune-privileged site. Furthermore, recent non-human primate studies have shown long-term survival of porcine xenotransplants in keratoplasty. Herein, corneal immune privilege is briefly introduced, and xenogeneic reactions are compared with allogeneic reactions in corneal transplantation. This review describes the current knowledge on special issues of xenotransplantation, xenogeneic rejection mechanisms, current immunosuppressive regimens of corneal xenotransplantation, preclinical efficacy and safety data of corneal xenotransplantation, and updates of the regulatory framework to conduct a clinical trial on corneal xenotransplantation. We also discuss barriers that might prevent xenotransplantation from becoming common practice, such as ethical dilemmas, public concerns on xenotransplantation, and the possible risk of xenozoonosis. Given that the legal definition of decellularized porcine cornea (DPC) lies somewhere between a medical device and a xenotransplant, the preclinical efficacy and clinical trial data using DPC are included. The review finally provides perspectives on the current standpoint of corneal xenotransplantation in the fields of regenerative medicine.

Safety and function of a new pre-vascularized bioartificial pancreas in an allogeneic rat model

Cell encapsulation could overcome limitations of free islets transplantation but is currently limited by inefficient cells immune protection and hypoxia. As a response to these challenges, we tested in vitro and in vivo the safety and efficacy of a new macroencapsulation device named MailPan^®. Membranes of MailPan^® device were tested in vitro in static conditions. Its bio-integration and level of oxygenation was assessed after implantation in non-diabetic rats. Immune protection properties were also assessed in rat with injection in the device of allogeneic islets with incompatible Major Histocompatibility Complex. Finally, function was assessed in diabetic rats with a Beta cell line injected in MailPan^®. In vitro, membranes of the device showed high permeability to glucose, insulin, and rejected IgG. In rat, the device displayed good bio-integration, efficient vascularization, and satisfactory oxygenation (>5%), while positron emission tomography (PET)-scan and angiography also highlighted rapid exchanges between blood circulation and the MailPan^®. The device showed its immune protection properties by preventing formation, by the rat recipient, of antibodies against encapsulated allogenic islets. Injection of a rat beta cell line into the device normalized fasting glycemia of diabetic rat with retrieval of viable cell clusters after 2 months. These data suggest that MailPan^® constitutes a promising encapsulation device for widespread use of cell therapy for type 1 diabetes.

Pig-to-non-human primate heart transplantation: The final step toward clinical xenotransplantation?

BACKGROUND

The demand for donated human hearts far exceeds the number available. Xenotransplantation of genetically modified porcine organs provides an alternative. In 2000, an Advisory Board of the International Society for Heart and Lung Transplantation set the benchmark for commencing clinical cardiac xenotransplantation as consistent 60% survival of non-human primates after life-supporting porcine heart transplantations. Recently, we reported the stepwise optimization of pig-to-baboon orthotopic cardiac xenotransplantation finally resulting in consistent success, with 4 recipients surviving 90 (n = 2), 182, and 195 days. Here, we report on 4 additional recipients, supporting the efficacy of our procedure.

RESULTS

The first 2 additional recipients succumbed to porcine cytomegalovirus (PCMV) infections on Days 15 and 27, respectively. In 2 further experiments, PCMV infections were successfully avoided, and 3-months survival was achieved. Throughout all the long-term experiments, heart, liver, and renal functions remained within normal ranges. Post-mortem cardiac diameters were slightly increased when compared with that at the time of transplantation but with no detrimental effect. There were no signs of thrombotic microangiopathy. The current regimen enabled the prolonged survival and function of orthotopic cardiac xenografts in altogether 6 of 8 baboons, of which 4 were now added. These results exceed the threshold set by the Advisory Board of the International Society for Heart and Lung Transplantation.

CONCLUSIONS

The results of our current and previous experimental cardiac xenotransplantations together fulfill for the first time the pre-clinical efficacy suggestions. PCMV-positive donor animals must be avoided.

A Comprehensive Strategy for Screening for Xenotransplantation-Relevant Viruses in a Second Isolated Population of Göttingen Minipigs

Xenotransplantation using pig tissues and organs is under development in order to alleviate the increasing shortage of human transplants. Since xenotransplantation may be associated with the transmission of porcine microorganisms to the human recipient, the donor pigs should be carefully analyzed, especially for the presence of potentially zoonotic viruses. Göttingen Minipigs (GöMP) are potential donors of islet cells for the treatment of diabetes. Despite the fact that all animals produced at Ellegaard Göttingen Minipigs A/S carry porcine endogenous retroviruses (PERVs) in their genome and that very few animals were infected with porcine cytomegalovirus (PCMV), hepatitis E virus (HEV) and porcine lymphotropic herpesvirus (PLHV), no transmission of these viruses was observed in a preclinical trial transplanting GöMP islet cells into cynomolgus monkeys. Using a new comprehensive strategy, we then analyzed an isolated subpopulation of Göttingen Minipigs which remained at the University of Göttingen. We concentrated on 11 xenotransplantation-relevant viruses and combined co-incubation assays with susceptible human target cells and molecular biological methods to evaluate the risk posed by PERV. All animals in Göttingen carry PERV-A, PERV-B, and PERV-C in their genome but they are not infected with PCMV, PLHV and HEV. The difference may be explained by selection of negative animals and/or de novo infection. The PERV copy number was established using ddPCR (93 copies) and a human-tropic PERV-A/C was found released from PBMCs of one animal with a high expression of PERV-C.

Xenotransplantation: Progress Along Paths Uncertain from Models to Application

For more than a century, transplantation of tissues and organs from animals into man, xenotransplantation, has been viewed as a potential way to treat disease. Ironically, interest in xenotransplantation was fueled especially by successful application of allotransplantation, that is, transplantation of human tissue and organs, as a treatment for a variety of diseases, especially organ failure because scarcity of human tissues limited allotransplantation to a fraction of those who could benefit. In principle, use of animals such as pigs as a source of transplants would allow transplantation to exert a vastly greater impact than allotransplantation on medicine and public health. However, biological barriers to xenotransplantation, including immunity of the recipient, incompatibility of biological systems, and transmission of novel infectious agents, are believed to exceed the barriers to allotransplantation and presently to hinder clinical applications. One way potentially to address the barriers to xenotransplantation is by genetic engineering animal sources. The last 2 decades have brought progressive advances in approaches that can be applied to genetic modification of large animals. Application of these approaches to genetic engineering of pigs has contributed to dramatic improvement in the outcome of experimental xenografts in nonhuman primates and have encouraged the development of a new type of xenograft, a reverse xenograft, in which human stem cells are introduced into pigs under conditions that support differentiation and expansion into functional tissues and potentially organs. These advances make it appropriate to consider the potential limitation of genetic engineering and of current models for advancing the clinical applications of xenotransplantation and reverse xenotransplantation.

Characterization of porcine endogenous retrovirus particles released by the CRISPR/Cas9 inactivated cell line PK15 clone 15

The infection of human transplant recipients by porcine endogenous retrovirus (PERV) is a safety issue for xenotransplantation (XTx). CRISPR/Cas9 technology has enabled the generation of pigs free of functional PERVs, and the susceptibility of these animals to reinfection by PERVs remains unclear. To assess virological safety, we characterized a cell line in which PERVs have been inactivated by CRISPR/Cas9 (PK15 clone 15) for its susceptibility to infectious PERV. First, basal expression of PERV pol, the porcine PERV-A receptor (POPAR), and reverse transcriptase (RT) activity of PERV were determined. PK15 clone 15 cells were inoculated with PERV and monitored post infection for virus expression and RT activity. Particles were visualized by electron microscopy. Our data show that PK15 clone 15 cells still produce viral proteins that assemble to produce impaired viral particles. These virions have an irregular morphology that diverges from that of mature wild type. The particles are no longer infectious when tested in a downstream infection assay using supernatants of PK15 clone 15 cells to infect susceptible swine testis-IOWA (ST-IOWA) cells. The expression of POPAR was quantified to exclude the possibility that lack of susceptibility to reinfection, for PERV-A, is caused by absence of viral host receptor(s). PK15 and PK15 clone 15 cells do, in fact, express POPAR equally. PERV RT inactivation mediated by CRISPR/Cas9 does not compromise virus assembly but affects virion structure and proviral integration. The constitutive virion production seems to maintain cellular resistance to superinfection and possibly indicates a protective side effect of this specific CRISPR/Cas9 mediated RT inactivation.

Overcoming Immunobiological Barriers Against Porcine Islet Xenografts: What Should Be Done?

Porcine islets might represent an ideal solution to the severe shortage of living donor islets available for transplantation and thus have great potential for the treatment of diabetes. Although tremendous progress has been achieved through recent experiments, the immune response remains a major obstacle. This review first describes the 3 major pathways of rejection: hyperacute rejection mediated by preformed natural antibodies and complement, instant blood-mediated inflammatory reactions, and acute cell-mediated rejection. Furthermore, this review examines immune-related strategies, including major advances, which have been shown to extend the life and/or function of porcine islets in vitro and in vivo: (1) genetic modification to make porcine islets more compatible with the recipient, (2) optimization of the newly defined biological agents that have been shown to promote long-term survival of xenografts in nonhuman primates, and (3) development of novel immunoisolation technologies that maintain the long-term survival of islet xenografts without the use of systemic immunosuppressive drugs. Finally, the clinical application of porcine islet transplantation is presented. Even though less clinical information is available, experimental data indicate that porcine islet xenografts are likely to become a standard treatment for patients with type 1 diabetes in the future.

Pathogen elimination and prevention within a regulated, Designated Pathogen Free, closed pig herd for long-term breeding and production of xenotransplantation materials

BACKGROUND

We established a Source Animal (barrier) Facility (SAF) for generating designated pathogen-free (DPF) pigs to serve as donors of viable organs, tissues, or cells for xenotransplantation into clinical patients. This facility was populated with caesarian derived, colostrum deprived (CDCD) piglets, from sows of conventional-specific (or specified) pathogen-free (SPF) health status in six cohorts over a 10-month period. In all cases, CDCD piglets fulfilled DPF status including negativity for porcine circovirus (PCV), a particularly environmentally robust and difficult to inactivate virus which at the time of SAF population was epidemic in the US commercial swine production industry. Two outbreaks of PCV infection were subsequently detected during sentinel testing. The first occurred several weeks after PCV-negative animals were moved under quarantine from the nursery into an animal holding room. The apparent origin of PCV was newly installed stainless steel penning, which was not sufficiently degreased thereby protecting viral particles from disinfection. The second outbreak was apparently transmitted via employee activities in the Caesarian-section suite adjacent to the barrier facility. In both cases, PCV was contained in the animal holding room where it was diagnosed making a complete facility depopulation-repopulation unnecessary.

METHOD

Infectious PCV was eliminated during both outbreaks by the following: euthanizing infected animals, disposing of all removable items from the affected animal holding room, extensive cleaning with detergents and degreasing agents, sterilization of equipment and rooms with chlorine dioxide, vaporized hydrogen peroxide, and potassium peroxymonosulfate, and for the second outbreak also glutaraldehyde/quaternary ammonium. Impact on other barrier animals throughout the process was monitored by frequent PCV diagnostic testing.

RESULT

After close monitoring for 6 months indicating PCV absence from all rooms and animals, herd animals were removed from quarantine status.

CONCLUSION

Ten years after PCV clearance following the second outbreak, due to strict adherence to biosecurity protocols and based on ongoing sentinel diagnostic monitoring (currently monthly), the herd remains DPF including PCV negative.

Comparative gene expression profiling of pig-derived iPSC-like cells: Effects of induced pluripotency on expression of porcine endogenous retrovirus (PERV)

BACKGROUND

Porcine induced pluripotent stem cells (piPSCs) offer an alternative strategy in xenotransplantation (XTx). As human endogenous retroviruses (HERV), particularly HERV-K, are highly expressed in natural human stem cells, we compared the expression of porcine endogenous retroviruses (PERV) and retrotransposon LINE-1 (L1) open reading frames 1 and 2 (pORF1 and pORF2) in different piPSC-like cell lines with their progenitors (porcine fetal fibroblasts, pFF).

METHODS

Cells reprogrammed via Sleeping Beauty-transposed transcription factors were cultured and analyzed on a custom-designed microarray representing the reference pig genome. Data were complemented by qRT-PCR and reverse transcriptase (RT) assay.

RESULTS

The expression profiles revealed that 8515 of 26 967 targets were differentially expressed. A total of 4443 targets showed log₂ expression ratio >1, and 4072 targets showed log₂ expression ratio less than -1 with 0.05 P-value threshold. Approximately ten percent of the targets showed highly significant expression ratios with log₂ ≥4 or ≤-4. Besides this general switch in cellular gene expression that was accompanied by an altered morphology, expression of both PERV and L1 pORF1/pORF2 was significantly enhanced. piPSC-like cells revealed a 10-fold to 100-fold higher transcription of the viral PERV-A and PERV-B envelope genes (env), viral protease/polymerase (prt/pol), and L1 elements. No functional retrovirus could be detected under these conditions.

CONCLUSION

Epigenetic reprogramming has functional impact on retrotransposons. Thus, the induction of pig-derived pluripotent cells influences their PERV expression profile. Data emphasize the necessity to focus on animals, which show non-functional endogenous viral background to ensure virological safety.

Xenoantigen Deletion and Chemical Immunosuppression Can Prolong Renal Xenograft Survival

OBJECTIVE

Xenotransplantation using pig organs could end the donor organ shortage for transplantation, but humans have xenoreactive antibodies that cause early graft rejection. Genome editing can eliminate xenoantigens in donor pigs to minimize the impact of these xenoantibodies. Here we determine whether an improved cross-match and chemical immunosuppression could result in prolonged kidney xenograft survival in a pig-to-rhesus preclinical model.

METHODS

Double xenoantigen (Gal and Sda) knockout (DKO) pigs were created using CRISPR/Cas. Serum from rhesus monkeys (n = 43) was cross-matched with cells from the DKO pigs. Kidneys from the DKO pigs were transplanted into rhesus monkeys (n = 6) that had the least reactive cross-matches. The rhesus recipients were immunosuppressed with anti-CD4 and anti-CD8 T-cell depletion, anti-CD154, mycophenolic acid, and steroids.

RESULTS

Rhesus antibody binding to DKO cells is reduced, but all still have positive CDC and flow cross-match. Three grafts were rejected early at 5, 6, and 6 days. Longer survival was achieved in recipients with survival to 35, 100, and 435 days. Each of the 3 early graft losses was secondary to IgM antibody-mediated rejection. The 435-day graft loss occurred secondary to IgG antibody-mediated rejection.

CONCLUSIONS

Reducing xenoantigens in donor pigs and chemical immunosuppression can be used to achieve prolonged renal xenograft survival in a preclinical model, suggesting that if a negative cross-match can be obtained for humans then prolonged survival could be achieved.

β Cell Replacement Therapy: The Next 10 Years

β cell replacement with either pancreas or islet transplantation has progressed immensely over the last decades with current 1- and 5-year insulin independence rates of approximately 85% and 50%, respectively. Recent advances are largely attributed to improvements in immunosuppressive regimen, donor selection, and surgical technique. However, both strategies are compromised by a scarce donor source. Xenotransplantation offers a potential solution by providing a theoretically unlimited supply of islets, but clinical application has been limited by concerns for a potent immune response against xenogeneic tissue. β cell clusters derived from embryonic or induced pluripotent stem cells represent another promising unlimited source of insulin producing cells, but clinical application is pending further advances in the function of the β cell like clusters. Exciting developments and rapid progress in all areas of β cell replacement prompted a lively debate by members of the young investigator committee of the International Pancreas and Islet Transplant Association at the 15th International Pancreas and Islet Transplant Association Congress in Melbourne and at the 26th international congress of The Transplant Society in Hong Kong. This international group of young investigators debated which modality of β cell replacement would predominate the landscape in 10 years, and their arguments are summarized here.

Absence of interaction between porcine endogenous retrovirus and porcine cytomegalovirus in pig-to-baboon renal xenotransplantation in vivo

BACKGROUND

Studies of xenotransplantation from swine have identified porcine viruses as potential barriers to clinical trials. The biology of these viruses has not been extensively investigated in the in vivo xeno-environment. Enhancement of viral gene expression by viral and cellular factors acting in trans has been demonstrated for certain viruses, including bidirectional interactions between human herpesviruses and endogenous (HERV) and exogenous (HIV) retroviruses. Both porcine cytomegalovirus (PCMV) and porcine endogenous retrovirus (PERV) infections have been identified in xenografts from swine. PERV receptors exist on human cells with productive infection in vitro in permissive human target cell lines. PCMV is largely species-specific with infection restricted to the xenograft in pig-to-baboon transplants. It is unknown whether coinfection by PCMV affects the replication of PERV within xenograft tissues which might have implications for the risk of retroviral infection in the human host.

METHODS

A series of 11 functioning, life-supporting pig-to-baboon kidney xenografts from PERV-positive miniature swine were studied with and without PCMV co-infection. Frozen biopsy samples were analyzed using quantitative, real-time PCR with internal controls.

RESULTS

PERV replication was not altered in the presence of PCMV coinfection (P = .70). The absence of variation with coinfection was confirmed when PERV quantitation was expressed relative to simultaneous cellular GAPDH levels with or without PCMV coinfection (P = .59).

CONCLUSIONS

PCMV coinfection does not alter the replication of PERV in life-supporting renal xenotransplantation in vivo in baboons.

Long-term safety outcome of systemic immunosuppression in pig-to-nonhuman primate corneal xenotransplantation

BACKGROUND

Safety concerns exist for corneal recipients under immunosuppression. We report long-term safety results of porcine corneal xenotransplantation under immunosuppression in nonhuman primates.

METHODS

Systemic monitoring data from 49 Chinese rhesus macaques that received pig corneal transplant between 2009 and 2018 were retrospectively reviewed. The recipients were divided into 4 groups depending on the systemic immunosuppressants used: (a) conventional steroid group; costimulation blockade groups ([b] anti-CD154 antibody, [c] anti-CD40 antibody); and (d) commercially available immunosuppressants (anti-CD20 antibody, tacrolimus, basiliximab) group. We compared results of general condition monitoring; hematologic, biochemical, and electrolyte tests; and Rhesus Cytomegalovirus infection monitoring.

RESULTS

All recipients recovered from early weight loss. White blood cell counts significantly decreased at 6 months in the steroid and anti-CD154 groups. Abnormal liver and kidney function and electrolyte imbalance were not observed in all groups. The mean value of Rhesus Cytomegalovirus DNA copies was consistently lower than 200 copies/mL, and antibody titers did not change over time in all groups. Tacrolimus-associated thrombotic microangiopathy was developed in one case, which resolved after discontinuation of tacrolimus. In 2017, a simian varicella virus outbreak led to clinical signs in 5 that received immunosuppressive therapies, of which 3 died.

CONCLUSION

Costimulatory blockade-based and anti-CD20 antibody/tacrolimus-based immunosuppressive therapies seem to be comparably safe with steroid therapy in nonhuman primates receiving corneal xenotransplantation, as they did not reactivate Rhesus Cytomegalovirus and maintained manageable systemic status. Although reactivation is rare, antiviral prophylaxis for simian varicella virus should be considered in immunocompromised hosts.

Xenotransplantation: past, present, and future

PURPOSE OF REVIEW

To review the progress in the field of xenotransplantation with special attention to most recent encouraging findings which will eventually bring xenotransplantation to the clinic in the near future.

RECENT FINDINGS

Starting from early 2000, with the introduction of galactose-α1,3-galactose (Gal)-knockout pigs, prolonged survival especially in heart and kidney xenotransplantation was recorded. However, remaining antibody barriers to non-Gal antigens continue to be the hurdle to overcome. The production of genetically engineered pigs was difficult requiring prolonged time. However, advances in gene editing, such as zinc finger nucleases, transcription activator-like effector nucleases, and most recently clustered regularly interspaced short palindromic repeats (CRISPR) technology made the production of genetically engineered pigs easier and available to more researchers. Today, the survival of pig-to-nonhuman primate heterotopic heart, kidney, and islet xenotransplantation reached more than 900, more than 400, and more than 600 days, respectively. The availability of multiple-gene pigs (five or six genetic modifications) and/or newer costimulation blockade agents significantly contributed to this success. Now, the field is getting ready for clinical trials with an international consensus.

SUMMARY

Clinical trials in cellular or solid organ xenotransplantation are getting closer with convincing preclinical data from many centers. The next decade will show us new achievements and additional barriers in clinical xenotransplantation.

Gene Editing, Gene Therapy, and Cell Xenotransplantation: Cell Transplantation Across Species

PURPOSE OF REVIEW

Cell xenotransplantation has the potential to provide a safe, ethically acceptable, unlimited source for cell replacement therapies. This review focuses on genetic modification strategies aimed to overcome remaining hurdles standing in the way of clinical porcine islet transplantation and to develop neural cell xenotransplantation.

RECENT FINDINGS

In addition to previously described genetic modifications aimed to mitigate hyperacute rejection, instant blood-mediated inflammatory reaction, and cell-mediated rejection, new data showing the possibility of increasing porcine islet insulin secretion by transgenesis is an interesting addition to the array of genetically modified pigs available for xenotransplantation. Moreover, combining multiple modifications is possible today thanks to new, improved genomic editing tools.

SUMMARY

Genetic modification of large animals, pigs in particular, has come a long way during the last decade. These modifications can help minimize immunological and physiological incompatibilities between porcine and human cells, thus allowing for better tolerance and function of xenocells.

Level of acceptance of islet cell and kidney xenotransplants by personnel of hospitals with and without experience in clinical xenotransplantation

BACKGROUND

Recently, significant progress in both safety and efficacy has been achieved in the field of xenotransplantation, as exemplified by results from the first clinical trials of porcine islet transplantation. It would be of interest to learn whether the attitude of the clinical staff involved in such trials changes as the trials are carried out in their own hospital.

METHODS

One hundred and four clinical staff members from the Eva Peron Hospital of San Martin (Buenos Aires, Argentina) where clinical trials of islet xenotransplantation have been performed and 92 similar staff members from the Diego Thompson Hospital (Buenos Aires, Argentina) where no such xenotransplantation has been carried out participated in the study. Data were collected anonymously using questionnaires.

RESULTS

Statistically significant differences between the acceptance of xenotransplantation by clinical personnel in a hospital that had carried out clinical xenotransplantation trials were observed when compared with the acceptance of a similar staff from the hospital that had not carried out such trials.

CONCLUSION

This study shows that involvement in clinical xenotransplantation trials significantly changes the attitude of the clinical staff towards this technology and suggests that better information given to the society may increase acceptance of the xenotransplantation.

Pig-to-Primate Islet Xenotransplantation: Past, Present, and Future

Islet allotransplantation results in increasing success in treating type 1 diabetes, but the shortage of deceased human donor pancreata limits progress. Islet xenotransplantation, using pigs as a source of islets, is a promising approach to overcome this limitation. The greatest obstacle is the primate immune/inflammatory response to the porcine (pig) islets, which may take the form of rapid early graft rejection (the instant blood-mediated inflammatory reaction) or T-cell-mediated rejection. These problems are being resolved by the genetic engineering of the source pigs combined with improved immunosuppressive therapy. The results of pig-to-diabetic nonhuman primate islet xenotransplantation are steadily improving, with insulin independence being achieved for periods >1 year. An alternative approach is to isolate islets within a micro- or macroencapsulation device aimed at protecting them from the human recipient's immune response. Clinical trials using this approach are currently underway. This review focuses on the major aspects of pig-to-primate islet xenotransplantation and its potential for treatment of type 1 diabetes.

A comprehensive microbiological safety approach for agarose encapsulated porcine islets intended for clinical trials

BACKGROUND

The use of porcine islets to replace insulin-producing islet β-cells, destroyed during the diabetogenic disease process, presents distinct challenges if this option is to become a therapeutic reality for the treatment of type 1 diabetes. These challenges include a thorough evaluation of the microbiological safety of the islets. In this study, we describe a robust porcine islet-screening program that provides a high level of confidence in the microbiological safety of porcine islets suitable for clinical trials.

METHODS

A four-checkpoint program systematically screens the donor herd (Large White - Yorkshire × Landrace F1 hybrid animals), individual sentinel and pancreas donor animals and, critically, the islet macrobeads themselves. Molecular assays screen for more than 30 known viruses, while electron microscopy and in vitro studies are employed to screen for potential new or divergent (emergent) viruses.

RESULTS

Of 1207 monthly samples taken from random animals over a 2-year period, only a single positive result for Transmissible gastroenteritis virus was observed, demonstrating the high level of biosecurity maintained in the source herd. Given the lack of clinical signs, positive antibody titers for Porcine reproductive and respiratory syndrome virus, Porcine parvovirus, and Influenza A confirm the efficacy of the herd vaccination program. Porcine respiratory coronavirus was found to be present in the herd, as expected for domestic swine. Tissue homogenate samples from six sentinel and 11 donor animals, over the same 2-year period, were negative for the presence of viruses when co-cultured with six different cell lines from four species. The absence of adventitious viruses in separate islet macrobead preparations produced from 12 individual pancreas donor animals was confirmed using validated molecular (n = 32 viruses), in vitro culture (cells from four species), and transmission electron microscopy assays (200 cell profiles per donor animal) over the same 2-year period. There has been no evidence of viral transmission following the implantation of these same encapsulated and functional porcine islets into non-immunosuppressed diabetic cynomolgus macaques for up to 4 years. Isolated peripheral blood mononuclear cells from all time points were negative for PCV (Type 2), PLHV, PRRSV, PCMV, and PERV-A, PERV-B, and PERV-C by PCR analysis in all six recipient animals.

CONCLUSION

The four-checkpoint program is a robust and reliable method for characterization of the microbiological safety of encapsulated porcine islets intended for clinical trials.

No PERV transmission during a clinical trial of pig islet cell transplantation

Xenotransplantation of pig islet cells is a promising alternative for the treatment of diabetes with insulin and may help to prevent numerous late complications such as blindness and amputation. First encouraging results using porcine islets have been reported in preclinical animal models as well in the first clinical trial in New Zealand. The goal of this manuscript is to examine the biological safety of a second trial performed in Argentina, specifically in regards to the transmission of porcine endogenous retroviruses (PERVs) using improved detection methods As in the first trial encapsulated islet cells from the well-characterised Auckland Island pigs were used. The animals were not genetically modified. The islet cells were transplanted in eight human recipients using a modified clinical protocol. Sera taken at different time points after transplantation (up to 55 weeks) were screened for the presence of antibodies against PERV proteins by Western blot analysis using viral antigens from highly purified virus particles. Positive sera obtained by immunization with recombinant PERV proteins were used as control sera. In none of the patients antibodies against PERV were detected, indicating the absence of infection. In parallel at different time points (up to 113 weeks) white blood cells (WBC) have been tested for PERV DNA, and WBC and plasma for PERV RNA by real-time RT-PCR. All tests were negative. In addition, using primers detecting pig mitochondrial cytochrome oxidase (COX) gene, patients were screened for microchimerism. In summary, the data are further evidence for the safety of pig islet cell transplantation.

New Phase of Growth for Xenogeneic-Based Bioartificial Organs

In this article, we examine the advanced clinical development of bioartificial organs and describe the challenges to implementing such systems into patient care. The case for bioartificial organs is evident: they are meant to reduce patient morbidity and mortality caused by the persistent shortage of organs available for allotransplantation. The widespread introduction and adoption of bioengineered organs, incorporating cells and tissues derived from either human or animal sources, would help address this shortage. Despite the decades of development, the variety of organs studied and bioengineered, and continuous progress in the field, only two bioengineered systems are currently commercially available: Apligraf® and Dermagraft® are both approved by the FDA to treat diabetic foot ulcers, and Apligraf® is approved to treat venous leg ulcers. Currently, no products based on xenotransplantation have been approved by the FDA. Risk factors include immunological barriers and the potential infectivity of porcine endogenous retrovirus (PERV), which is unique to xenotransplantation. Recent breakthroughs in gene editing may, however, mitigate risks related to PERV. Because of its primary role in interrupting progress in xenotransplantation, we present a risk assessment for PERV infection, and conclude that the formerly high risk has been reduced to a moderate level. Advances in gene editing, and more broadly in the field, may make it more likely than ever before that bioartificial organs will alleviate the suffering of patients with organ failure.

First update of the International Xenotransplantation Association consensus statement on conditions for undertaking clinical trials of porcine islet products in type 1 diabetes--Executive summary

The International Xenotransplantation Association has updated its original "Consensus Statement on Conditions for Undertaking Clinical Trials of Porcine Islet Products in Type 1 Diabetes," which was published in Xenotransplantation in 2009. This update is timely and important in light of scientific progress and changes in the regulatory framework pertinent to islet xenotransplantation. Except for the chapter on "informed consent," which has remained relevant in its 2009 version, all other chapters included in the initial consensus statement have been revised for inclusion in this update. These chapters will not provide complete revisions of the original chapters; rather, they restate the key points made in 2009, emphasize new and under-appreciated topics not fully addressed in 2009, suggest relevant revisions, and communicate opinions that complement the consensus opinion. Chapter 1 provides an update on national regulatory frameworks addressing xenotransplantation. Chapter 2 a, previously Chapter 2, suggests several important revisions regarding the generation of suitable source pigs from the perspective of the prevention of xenozoonoses. The newly added Chapter 2b discusses conditions for the use of genetically modified source pigs in clinical islet xenotransplantation. Chapter 3 reviews porcine islet product manufacturing and release testing. Chapter 4 revisits the critically important topic of preclinical efficacy and safety data required to justify a clinical trial. The main achievements in the field of transmission of all porcine microorganisms, the rationale for more proportionate recipient monitoring, and response plans are reviewed in Chapter 5. Patient selection criteria and circumstances where trials of islet xenotransplantation would be both medically and ethically justified are examined in Chapter 6 in the context of recent advances in available and emerging alternative therapies for serious and potentially life-threatening complications of diabetes. It is hoped that this first update of the International Xenotransplantation Association porcine islet transplant consensus statement will assist the islet xenotransplant scientific community, sponsors, regulators, and other stakeholders actively involved in the clinical translation of islet xenotransplantation.