Inhibition of porcine endogenous retroviruses (PERVs) in primary porcine cells by RNA interference using lentiviral vectors

Xenotransplantate vom Schwein – ist das Ende des Organmangels in Sicht?

Unter „Xenotransplantation“ wird die Übertragung von funktionsfähigen Zellen, Geweben oder Organen zwischen verschiedenen Spezies verstanden, insbesondere von Schweinen auf den Menschen. In den meisten Industrieländern klafft eine große Lücke zwischen der Anzahl geeigneter Spenderorgane und der Anzahl benötigter Transplantate. Weltweit können nur etwa 10% des Organbedarfs durch Spenden gedeckt werden. Eine erfolgreiche Xenotransplantation könnte diesen Mangel mildern oder sogar weitgehend vermeiden. Das Schwein wird aus verschiedenen Erwägungen heraus als am besten geeignete Spenderspezies angesehen. Bei einer Übertragung porziner Organe auf Primaten treten verschiedene immunologisch bedingte Abstoßungsreaktionen auf, die das übertragene Organ innerhalb kurzer Zeit zerstören können, wie die HAR (hyperakute Abstoßung), die AVR (akute vaskuläre Abstoßung) und die spätere zelluläre Abstoßung. Diese Abstoßungsreaktionen müssen durch genetische Modifikationen im Schwein und eine geeignete immunsuppressive Behandlung des Empfängers kontrolliert werden. Dazu müssen Tiere mit mehrfachen genetischen Veränderungen produziert und im Hinblick auf ihre Eignung für eine erfolgreiche Xenotransplantation geprüft werden. Inzwischen können die HAR und auch die AVR durch Knockouts von antigenen Oberflächenepitopen (z. B. αGal [Galaktose-α1,3-Galaktose]) und transgene Expression humaner Gene mit antiinflammatorischer, antiapoptotischer oder antikoagulativer Wirkung zuverlässig kontrolliert werden. Nach orthotopen Transplantationen in nicht humane Primaten konnten inzwischen mit Schweineherzen Überlebensraten von bis zu 264 Tagen und mit porzinen Nieren von 435 Tagen erzielt werden. Eine Übertragung pathogener Erreger auf den Empfänger kann bei Einhaltung einschlägiger Hygienemaßnahmen ausgeschlossen werden. PERV (porzine endogene Retroviren) können durch RNA-(Ribonukleinsäure-)Interferenz oder Gen-Knockout ausgeschaltet werden. Sie stellen damit kein Übertragungsrisiko für den Empfänger mehr dar. Anfang 2022 wurde in Baltimore (USA) ein Schweineherz mit 10 genetischen Modifikationen auf einen Patienten mit schwerem Herzleiden übertragen, mit dem der Empfänger 2 Monate offenbar ohne größere Probleme lebte. Es wird erwartet, dass Xenotransplantate vom Schwein in absehbarer Zeit zur klinischen Anwendungsreife kommen werden. Dazu werden klinische Versuche zur systematischen Erfassung aller Auswirkungen solcher Transplantate auf den Patienten sowie geeignete rechtliche und finanzielle Rahmenbedingungen benötigt.

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:

Porcine Endogenous Retroviruses and Xenotransplantation, 2021

Porcine endogenous retroviruses (PERVs) are integrated in the genome of all pigs, and some of them are able to infect human cells. Therefore, PERVs pose a risk for xenotransplantation, the transplantation of pig cells, tissues, or organ to humans in order to alleviate the shortage of human donor organs. Up to 2021, a huge body of knowledge about PERVs has been accumulated regarding their biology, including replication, recombination, origin, host range, and immunosuppressive properties. Until now, no PERV transmission has been observed in clinical trials transplanting pig islet cells into diabetic humans, in preclinical trials transplanting pig cells and organs into nonhuman primates with remarkable long survival times of the transplant, and in infection experiments with several animal species. Nevertheless, in order to prevent virus transmission to the recipient, numerous strategies have been developed, including selection of PERV-C-free animals, RNA interference, antiviral drugs, vaccination, and genome editing. Furthermore, at present there are no more experimental approaches to evaluate the full risk until we move to the clinic.

Porcine genome engineering for xenotransplantation

The extreme shortage of human donor organs for treatment of patients with end-stage organ failures is well known. Xenotransplantation, which might provide unlimited organ supply, is a most promising strategy to solve this problem. Domestic pigs are regarded as ideal organ-source animals owing to similarity in anatomy, physiology and organ size to humans as well as high reproductive capacity and low maintenance cost. However, several barriers, which include immune rejection, inflammation and coagulative dysfunctions, as well as the cross-species transmission risk of porcine endogenous retrovirus, blocked the pig-to-human xenotransplantation. With the rapid development of genome engineering technologies and the potent immunosuppressive medications in recent years, these barriers could be eliminated through genetic modification of pig genome together with the administration of effective immunosuppressants. A number of candidate genes involved in the regulation of immune response, inflammation and coagulation have been explored to optimize porcine xenograft survival in non-human primate recipients. PERV inactivation in pigs has also been accomplished to firmly address the safety issue in pig-to-human xenotransplantation. Many encouraging preclinical milestones have been achieved with some organs surviving for years. Therefore, the clinical trials of some promising organs, such as islet, kidney and heart, are aimed to be launched in the near future.

Pig kidney xenotransplantation: Progress toward clinical trials

Pig organ xenotransplantation offers a solution to the shortage of deceased human organs for transplantation. The pathobiological response to a pig xenograft is complex, involving antibody, complement, coagulation, inflammatory, and cellular responses. To overcome these barriers, genetic manipulation of the organ-source pigs has largely been directed to two major aims-(a) deletion of expression of the known carbohydrate xenoantigens against which humans have natural (preformed) antibodies, and (b) transgenic expression of human protective proteins, for example, complement- and coagulation-regulatory proteins. Conventional (FDA-approved) immunosuppressive therapy is unsuccessful in preventing an adaptive immune response to pig cells, but blockade of the CD40:CD154 costimulation pathway is successful. Survival of genetically engineered pig kidneys in immunosuppressed nonhuman primates can now be measured in months. Non-immunological aspects, for example, pig renal function, a hypovolemia syndrome, and rapid growth of the pig kidney after transplantation, are briefly discussed. We suggest that patients on the wait-list for a deceased human kidney graft who are unlikely to receive one due to long waiting times are those for whom kidney xenotransplantation might first be considered. The potential risk of infection, public attitudes to xenotransplantation, and ethical, regulatory, and financial aspects are briefly addressed.

How the COVID-19 pandemic may impact public support for clinical xenotransplantation in the United States?

Many patients who would undergo organ transplantation cannot proceed due to the inability of human organ donation to satisfy medical needs. Xenotransplantation has the potential to offer unlimited availability of pig organs for transplantation, and pig-to-non-human primate models have demonstrated outcomes that may soon justify clinical trials. However, one of the unique ethical challenges faced by xenotransplantation is that the risk of introducing potential zoonotic disease into the community must be weighed along with the benefit to the patient. While most experts believe that zoonosis is manageable, apprehension over disease transmission from animal donors to human recipients remains a frequent concern of many who are undecided or opposed to clinical xenotransplantation. The COVID-19 pandemic represents a scenario (rapid worldwide spread of a highly contagious novel zoonotic disease with no natural defense in humans) that would seem to justify apprehension, especially in the United States, which has largely avoided previous pandemic outbreaks. However, there are many differences between zoonosis found in the wild or after xenotransplantation that favor the safety of the latter. Still, these differences, as well as the benefits of xenotransplantation, are not widely understood outside of the field. We must therefore ask what impact the COVID-19 pandemic will have on attitudes toward xenotransplantation.

Regulation of porcine endogenous retrovirus by dual LTR1+2 (Long Terminal Region) miRNA in primary porcine kidney cells

Porcine endogenous retroviruses (PERVs) integrate into germline DNA as proviral genome that enables vertical transmission from parents to their offspring. The provirus usually survives as part of the host genome rather than as an infectious agent, but may become pathogenic if it crosses species barriers. Therefore, replication-competent PERV should be controlled through selective breeding or knockout technologies. Two microRNAs (miRNAs), dual LTR1 and LTR2, were selected to inhibit the expression of PERV in primary porcine kidney cells. The inhibition efficiency of the miRNAs was compared based on their inhibition of different PERV regions, specifically long terminal repeats (LTRs), gag, pol, and env. Gene expression was quantified using real-time polymerase chain reaction and the C-type reverse transcriptase (RT) activity was determined. The messenger RNA (mRNA) expression of the PERV LTR and env regions was determined in HeLa cells co-cultured with primary porcine kidney cells. The mRNA expression of the LTR, gag, pol, and env regions of PERV was dramatically inhibited by dual miRNA from 24 to 144 h after transfection, with the highest inhibition observed for the LTR and pol regions at 120 h. Additionally, the RT activity of PERV in the co-culture experiment of porcine and human cells was reduced by 84.4% at the sixth passage. The dual LTR 1+2 miRNA efficiently silences PERV in primary porcine kidney cells.

Introduction: The Present Status of Xenotransplantation Research

There is a well-known worldwide shortage of deceased human donor organs for clinical transplantation. The transplantation of organs from genetically engineered pigs may prove an alternative solution. In the past 5 years, there have been sequential advances that have significantly increased pig graft survival in nonhuman primates. This progress has been associated with (1) the availability of increasingly sophisticated genetically engineered pigs; (2) the introduction of novel immunosuppressive agents, particularly those that block the second T-cell signal (costimulation blockade); (3) a better understanding of the inflammatory response to pig xenografts; and (4) increasing experience in the management of nonhuman primates with pig organ or cell grafts. The range of investigations required in experimental studies has increased. The standard immunologic assays are still carried out, but increasingly investigations aimed toward other pathobiologic barriers (e.g., coagulation dysregulation and inflammation) have become more important in determining injury to the graft.Now that prolonged graft survival, extending to months or even years, is increasingly being obtained, the function of the grafts can be more reliably assessed. If the source pigs are bred and housed under biosecure isolation conditions, and weaned early from the sow, most microorganisms can be eradicated from the herd. The potential risk of porcine endogenous retrovirus (PERV) infection remains unknown, but is probably small. Attention is being directed toward the selection of patients for the first clinical trials of xenotransplantation.

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.

Justification of specific genetic modifications in pigs for clinical organ xenotransplantation

Xenotransplantation research has made considerable progress in recent years, largely through the increasing availability of pigs with multiple genetic modifications. We suggest that a pig with nine genetic modifications (ie, currently available) will provide organs (initially kidneys and hearts) that would function for a clinically valuable period of time, for example, >12 months, after transplantation into patients with end-stage organ failure. The national regulatory authorities, however, will likely require evidence, based on in vitro and/or in vivo experimental data, to justify the inclusion of each individual genetic modification in the pig. We provide data both from our own experience and that of others on the advantages of pigs in which (a) all three known carbohydrate xenoantigens have been deleted (triple-knockout pigs), (b) two human complement-regulatory proteins (CD46, CD55) and two human coagulation-regulatory proteins (thrombomodulin, endothelial cell protein C receptor) are expressed, (c) the anti-apoptotic and "anti-inflammatory" molecule, human hemeoxygenase-1 is expressed, and (d) human CD47 is expressed to suppress elements of the macrophage and T-cell responses. Although many alternative genetic modifications could be made to an organ-source pig, we suggest that the genetic manipulations we identify above will all contribute to the success of the initial clinical pig kidney or heart transplants, and that the beneficial contribution of each individual manipulation is supported by considerable experimental evidence.

Incidence of Neoplasia in Pigs and Its Relevance to Clinical Organ Xenotransplantation

As clinical pig organ xenotransplantation draws closer, more attention is being paid to diseases that affect pigs and those that provide a potential risk to human recipients of pig organs. Neoplasia arising from the pig organ graft is one such concern. Various tumors and other neoplastic diseases are well known to show increased incidence in organ allotransplant recipients receiving immunosuppressive therapy. Whether this effect will prove to be the case after xenotransplantation has not yet been established. Malignant tumors in young pigs are rare, with lymphosarcoma, nephroblastoma, and melanoma being the most common. The combination of noninvasive techniques and intraoperative examination of the pig organ likely will readily confirm that a pig organ graft is tumor-free before xenotransplantation. Posttransplantion lymphoproliferative disorder (PTLD) is a concern after allotransplantation, but the incidence after solid organ allotransplantation is low when compared with hematopoietic cell allotransplantation (for example, bone marrow transplantation), unless immunosuppressive therapy is particularly intensive. Organ-source pigs used for clinical xenotransplantation will be bred and housed under designated pathogen-free conditions and will be free of the γ-herpesvirus that is a key factor in the development of PTLD in pigs. Therefore if a recipient of a pig xenograft develops PTLD, it will almost certainly be of recipient origin. The increasing availability of organs from pigs genetically-engineered to protect them from the human immune response likely will diminish the need for intensive immunosuppressive therapy. Considering the low incidence of malignant disease in young pigs, donor-derived malignancy is likely to be rare in patients who receive pig organ grafts. However, if the graft remains viable for many years, the incidence of graft malignancy may increase.

Perspectives on the Optimal Genetically Engineered Pig in 2018 for Initial Clinical Trials of Kidney or Heart Xenotransplantation

For a clinical trial today, what might realistically be the optimal pig among those currently available? Deletion of expression of the 3 pig carbohydrate antigens, against which humans have natural (preformed) antibodies (triple-knockout pigs), should form the basis of any clinical trial. However, because both complement and coagulation can be activated in the absence of antibody, the expression of human complement- and coagulation-regulatory proteins is likely to be important in protecting the graft further. Any genetic manipulation that might reduce inflammation of the graft, for example, expression of hemeoxygenase-1 or A20, may also be beneficial to the long-term survival of the graft. The transgene for human CD47 is likely to have a suppressive effect on monocyte/macrophage and T-cell activity. Furthermore, deletion of xenoantigen expression and expression of a human complement-regulatory protein are both associated with a reduced T-cell response. Although there are several other genetic manipulations that may reduce the T-cell response further, it seems likely that exogenous immunosuppressive therapy, particularly if it includes costimulation blockade, will be sufficient. We would therefore suggest that, with our present knowledge and capabilities, the optimal pig might be a triple-knockout pig that expressed 1 or more human complement-regulatory proteins, 1 or more human coagulation-regulatory proteins, a human anti-inflammatory transgene, and CD47. Absent or minimal antibody binding is important, but we suggest that the additional insertion of protective human transgenes will be beneficial, and may be essential.

Will Genetic Engineering Carry Xenotransplantation of Pig Islets to the Clinic?

PURPOSE OF REVIEW

Porcine islets represent a potentially attractive beta-cell source for xenotransplantation into patients with type 1 diabetes, who are not eligible to islet allo-transplantation due to a lack of suitable human donor organs. Recent progress in genetic engineering/gene editing of donor pigs provides new opportunities to overcome rejection of xeno-islets, to improve their engraftment and insulin secretion capacity, and to reduce the risk for transmission of porcine endogenous retroviruses. This review summarizes the current issues and progress in islet xenotransplantation with special emphasis on genetically modified/gene edited donor pigs.

RECENT FINDINGS

Attempts to overcome acute rejection of xeno-islets, especially after intraportal transplantation into the liver, include the genetic elimination of specific carbohydrate antigens such as αGal, Neu5Gc, and Sd(a) for which humans and-in part-non-human primates have natural antibodies that bind to these targets leading to activation of complement and coagulation. A complementary approach is the expression of one or more human complement regulatory proteins (hCD46, hCD55, hCD59). Transgenic attempts to overcome cellular rejection of islet xenotransplants include the expression of proteins that inhibit co-stimulation of T cells. Expression of glucagon-like peptide-1 and M3 muscarinic receptors has been shown to increase the insulin secretion of virally transduced porcine islets in vitro and it will be interesting to see the effects of these modifications in transgenic pigs and islet products derived from them. Genome-wide inactivation of porcine endogenous retrovirus (PERV) integrants by mutating their pol genes using CRISPR/Cas9 is a recent approach to reduce the risk for PERV transmission by xeno-islets. Genetic engineering/gene editing of xeno-islet donor pigs facilitated major progress towards clinical islet xenotransplantation. The required set of genetic modifications will depend on the source of islets (fetal/neonatal vs. adult), the mode of delivery (encapsulated vs. free), and the transplantation site.

Genetically Modified Pigs as Organ Donors for Xenotransplantation

The growing shortage of available organs is a major problem in transplantology. Thus, new and alternative sources of organs need to be found. One promising solution could be xenotransplantation, i.e., the use of animal cells, tissues and organs. The domestic pig is the optimum donor for such transplants. However, xenogeneic transplantation from pigs to humans involves high immune incompatibility and a complex rejection process. The rapid development of genetic engineering techniques enables genome modifications in pigs that reduce the cross-species immune barrier.

Inhibition of Porcine Endogenous Retrovirus by Multi-Targeting Micro RNA Against Long Terminal Region

BACKGROUND

There might be much benefit in xenotransplantation, however, the risk of infections across species barriers remains, especially porcine endogenous retrovirus (PERV). To date, many attempts have been made to knock down active PERVs by inhibitory RNA (RNAi) and micro RNA (miRNA), which target different genes of PERV. There are a few studies that have explored whether targeting promoter regions of PERV could exert an inhibition effect.

METHODS

miRNAs were automatically selected based on an online program BLOCK-iT RNAi Designer. The inhibition efficiency between miRNAs was compared based on their inhibition of different PERV genes: long terminal repeats (LTR), gag, and pol. Both relative quantitative real-time polymerase chain reaction (PCR) and C-type reverse transcriptase activity were performed.

RESULTS

The results demonstrated that miRNA targeting the LTR region degraded the target sequence, and simultaneously inhibited the mRNA expression of both gag and pol genes of PERV. The LTR1, LTR2, and dual LTR1 + LTR2 miRNA inhibited 76.2%, 22%, and 76.8% of gag gene expression, respectively. Similarly, the miRNA was found to knock down the pol gene expression of 69.8%, 25.5%, and 77.7% for single targeting miRNA (LTR1 and LTR2) and multi-targeting miRNA (LTR1 + LTR2), respectively. A stable PK15 clone constitutively expressed dual LTR1 + LTR2 miRNA and exhibited higher inhibitory up to 82.8% and 92.7% of the expressions of the gag and pol genes, respectively. Also, the result of co-cultivation of dual LTR1 + LTR2 miRNA transfected PK15 cell with a human cell line inhibited expression of LTR, gag, and pol genes of PERV.

CONCLUSIONS

In conclusion, this study suggested that the LTR might be an alternative target for gene silencing of PERV, and that multi-targeting miRNA had better inhibitory effect than single- targeting miRNA. In an in vitro model, the presence of miRNA was able to reduce PERV infectivity in a human cell line.

Why was PERV not transmitted during preclinical and clinical xenotransplantation trials and after inoculation of animals?

Porcine endogenous retroviruses (PERVs) are present in the genome of all pigs, they infect certain human cells and therefore pose a special risk for xenotransplantation using pig cells, tissues and organs. Xenotransplantation is being developed in order to alleviate the reduced availability of human organs. Despite the fact that PERVs are able to infect certain human cells and cells from other species, transmission of PERVs has not been observed when animals (including non-human primates) were inoculated with PERV preparations or during preclinical xenotransplantations. The data indicate that PERVs were not transmitted because they were not released from the transplant or were inhibited by intracellular restriction factors and innate immunity in the recipient. In a single study in guinea pigs, a transient PERV infection and anti-PERV antibodies were described, indicating that in this case at least, the immune system may also have been involved.

Regulation of Clinical Xenotransplantation-Time for a Reappraisal

The continual critical shortage of organs and cells from deceased human donors has stimulated research in the field of cross-species transplantation (xenotransplantation), with the pig selected as the most suitable potential source of organs. Since the US Food and Drug Administration concluded a comprehensive review of xenotransplantation in 2003, considerable progress has been made in the experimental laboratory to improve cell and organ xenograft survival in several pig-to-nonhuman primate systems that offer the best available models to predict clinical outcomes. Survival of heart, kidney, and islet grafts in nonhuman primates is now being measured in months or even years. The potential risks associated with xenotransplantation, for example, the transfer of an infectious microorganism, that were highlighted in the 2003 Food and Drug Administration guidance and subsequent World Health Organization consensus documents have been carefully studied and shown to be either less likely than previously thought or readily manageable by donor selection or recipient management strategies. In this context, we suggest that the national regulatory authorities worldwide should re-examine their guidelines and regulations regarding xenotransplantation, so as to better enable design and conduct of safe and informative clinical trials of cell and organ xenotransplantation when and as supported by the preclinical data. We identify specific topics that we suggest require reconsideration.

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.

The role of genetically engineered pigs in xenotransplantation research

There is a critical shortage in the number of deceased human organs that become available for the purposes of clinical transplantation. This problem might be resolved by the transplantation of organs from pigs genetically engineered to protect them from the human immune response. The pathobiological barriers to successful pig organ transplantation in primates include activation of the innate and adaptive immune systems, coagulation dysregulation and inflammation. Genetic engineering of the pig as an organ source has increased the survival of the transplanted pig heart, kidney, islet and corneal graft in non-human primates (NHPs) from minutes to months or occasionally years. Genetic engineering may also contribute to any physiological barriers that might be identified, as well as to reducing the risks of transfer of a potentially infectious micro-organism with the organ. There are now an estimated 40 or more genetic alterations that have been carried out in pigs, with some pigs expressing five or six manipulations. With the new technology now available, it will become increasingly common for a pig to express even more genetic manipulations, and these could be tested in the pig-to-NHP models to assess their efficacy and benefit. It is therefore likely that clinical trials of pig kidney, heart and islet transplantation will become feasible in the near future.

Review on porcine endogenous retrovirus detection assays--impact on quality and safety of xenotransplants

Xenotransplantation of porcine organs, tissues, and cells inherits a risk for xenozoonotic infections. Viable tissues and cells intended for transplantation have to be considered as potentially contaminated non-sterile products. The demands on microbial testing, based on the regulatory requirements, are often challenging due to a restricted shelf life or the complexity of the product itself. In Europe, the regulatory framework for xenogeneic cell therapy is based on the advanced therapy medicinal products (ATMP) regulation (2007), the EMA CHMP Guideline on xenogeneic cell-based medicinal products (2009), as well as the WHO and Council of Europe recommendations. In the USA, FDA guidance for industry (2003) regulates the use of xenotransplants. To comply with the regulations, validated test methods need to be established that reveal the microbial status of a transplant within its given shelf life, complemented by strictly defined action alert limits and supported by breeding in specific pathogen-free (SPF) facilities. In this review, we focus on assays for the detection of the porcine endogenous retroviruses PERV-A/-B/-C, which exhibit highly polymorphic proviral loci in pig genomes. PERVs are transmitted vertically and cannot be completely eliminated by breeding or gene knock out technology. PERVs entail a public health concern that will persist even if no evidence of PERV infection of xenotransplant recipients in vivo has been revealed yet. Nevertheless, infectious risks must be minimized by full assessment of pigs as donors by combining different molecular screening assays for sensitive and specific detection as well as a functional analysis of the infectivity of PERV including an adequate monitoring of recipients.

Inhibition of porcine endogenous retrovirus in PK15 cell line by efficient multitargeting RNA interference

To effectively suppress porcine endogenous retroviruses (PERV)s, RNAi technique was utilized. RNAi is the up-to-date skill for gene knockdown which simultaneously multitargets both gag and pol genes critical for replication of PERVs. Previously, two of the most effective siRNAs (gag2, pol2) were found to reduce the expression of PERVs. Concurrent treatment of these two siRNAs (gag2+pol2) showed knockdown efficiency of up to 88% compared to negative control. However, despite the high initial knockdown efficiency 48 h after transfection caused by siRNA, it may only be a transient effect of suppressing PERVs. The multitargeting vector was designed, containing both gag and pol genes and making use of POL II miR Expression Vector, which allowed for persistent and multiple targeting. This is the latest shRNA system technique expressing and targeting like miRNA. Through antibiotics resistance characteristics utilizing this vector, miRNA-transfected PK15 cells (gag2-pol2) were selected during 10 days. An 88.1% reduction in the level of mRNA expression was found. In addition, we performed RT-activity analysis and fluorescence in situ hybridization assay, and it demonstrated the highest knockdown efficiency in multitargeting (gag2+pol2) miRNA group. Therefore, according to the results above, gene knockdown system (siRNA and shRNA) through multitargeting strategy could effectively inhibit PERVs.

Knockdown of porcine endogenous retroviruses by RNA interference in Chinese experimental miniature pig fibroblasts

BACKGROUND

The clinical application of porcine-derived xenotransplants is limited by the potential risk of infection due to the presence of porcine endogenous retrovirus (PERV) in tissues, organs, and cells. The establishment of pig fibroblasts with low PERV expression and without PERV-C can provide a nuclear donor to generate a safer transgenic pig.

METHODS

In this study, we obtained Chinese Experimental Miniature Pig fibroblasts (CEMPF) with low expression of PERV and none of PERV-C. We designed small interfering RNA (siRNA) expressed as short hairpin RNAs (shRNA) based on the highly conserved gag and pol regions of PERV and screened for the most effective siRNA to inhibit PERV expression. The selected shRNA-pol3 fragment was introduced into the CEMPF to obtain an engineered CEMPF stably expressing shRNA-pol3.

RESULTS

The PERV mRNA expression level in the engineered CEMPF was only 7.9% of that observed in fibroblasts from wild-type CEMPF, PERV P15E protein expression was significantly reduced. HEK293 cells cocultured with the supernate of the engineered CEMPF showed no PERV infection.

CONCLUSIONS

Engineered CEMPF, which possess no risk of PERV-A/C infection, can serve as a nuclear donor to generate xenograft donor pigs.

Porcine CD74 is involved in the inflammatory response activated by nuclear factor kappa B during porcine circovirus type 2 (PCV-2) infection

Human CD74 induces a signalling cascade that results in the activation of nuclear factor kappa B (NF-κB); however, porcine CD74 has not been widely studied. In this study, we show that porcine CD74 is mainly expressed in cells of the macrophage lineage and can be induced by lipopolysaccharide (LPS), polyinosinic acid-polycytidylic acid [Poly(I:C)], and infection with porcine circovirus type 2 (PCV2) in vitro. In addition, we confirmed that porcine CD74 can activate NF-κB by promoting IκBα degradation and nuclear translocation of p65. Furthermore, the transcription of NF-κB-regulated genes [Interleukin-6 (IL-6), Interleukin-8 (IL-8), and COX-2] was upregulated in response to the overexpression of porcine CD74. In general, porcine CD74 significantly enhanced the inflammatory response by regulating the NF-κB signalling pathway during PCV2 infection, which suggests that porcine CD74 may be implicated in the pathogenesis of PCV2 infection.

Long-term effects of PERV-specific RNA interference in transgenic pigs

BACKGROUND

Porcine endogenous retroviruses (PERVs) represent a risk of xenotransplantation using porcine cells, tissues, or organs, as they are integrated in the porcine genome and have been shown to be able to infect human cells in vitro. To increase viral safety by RNA interference, transgenic pigs expressing a PERV-specific small hairpin (sh)RNA targeted to a highly conserved sequence in the pol gene (pol2) were generated in which expression of PERVs was reduced (Xenotransplantation, 15, 2008, 38). However, it remains to be shown how long expression of the shRNA and the RNA interference is effective in reducing PERV expression.

METHODS

To analyze the long-term duration of RNA interference, expression of the PERV-specific pol2 shRNA and inhibition of PERV expression was studied repeatedly in fibroblasts and peripheral blood mononuclear cells (PBMCs) of transgenic pigs over a period of 3 yr, when animals were sacrificed and expression was studied in different organs. Expression of the PERV-specific shRNA was measured using a newly developed real-time PCR, and expression of PERV was measured using a PERV-specific real-time PCR.

RESULTS

Over a period of 3 yr, PERV-specific shRNA and green fluorescent protein (GFP) as reporter of the vector system were consistently expressed in transgenic animals. PERV expression was significantly reduced during the entire period. Levels of PERV and shRNA expression were different in the various organs. PERV expression was highest in the spleen and the lungs and lowest in liver and heart. However, in all organs of the transgenic pigs, PERV expression was inhibited compared with the vector control animals.

CONCLUSIONS

Transgenic pigs expressing PERV-specific shRNA maintained their specific RNA interference long term, suggesting that PERV expression in the xenotransplants will be suppressed over extended periods of time.

Infection barriers to successful xenotransplantation focusing on porcine endogenous retroviruses

Xenotransplantation may be a solution to overcome the shortage of organs for the treatment of patients with organ failure, but it may be associated with the transmission of porcine microorganisms and the development of xenozoonoses. Whereas most microorganisms may be eliminated by pathogen-free breeding of the donor animals, porcine endogenous retroviruses (PERVs) cannot be eliminated, since these are integrated into the genomes of all pigs. Human-tropic PERV-A and -B are present in all pigs and are able to infect human cells. Infection of ecotropic PERV-C is limited to pig cells. PERVs may adapt to host cells by varying the number of LTR-binding transcription factor binding sites. Like all retroviruses, they may induce tumors and/or immunodeficiencies. To date, all experimental, preclinical, and clinical xenotransplantations using pig cells, tissues, and organs have not shown transmission of PERV. Highly sensitive and specific methods have been developed to analyze the PERV status of donor pigs and to monitor recipients for PERV infection. Strategies have been developed to prevent PERV transmission, including selection of PERV-C-negative, low-producer pigs, generation of an effective vaccine, selection of effective antiretrovirals, and generation of animals transgenic for a PERV-specific short hairpin RNA inhibiting PERV expression by RNA interference.

Current development of bioreactors for extracorporeal bioartificial liver (Review)

The research and development of extracorporeal bioartificial liver is gaining pace in recent years with the introduction of a myriad of optimally designed bioreactors with the ability to maintain long-term viability and liver-specific functions of hepatocytes. The design considerations for bioartificial liver are not trivial; it needs to consider factors such as the types of cell to be cultured in the bioreactor, the bioreactor configuration, the magnitude of fluid-induced shear stress, nutrients' supply, and wastes' removal, and other relevant issues before the bioreactor is ready for testing. This review discusses the exciting development of bioartificial liver devices, particularly the various types of cell used in current reactor designs, the state-of-the-art culturing and cryopreservation techniques, and the comparison among many today's bioreactor configurations. This review will also discuss in depth the importance of maintaining optimal mass transfer of nutrients and oxygen partial pressure in the bioreactor system. Finally, this review will discuss the commercially available bioreactors that are currently undergoing preclinical and clinical trials.

Restriction of porcine endogenous retrovirus by porcine APOBEC3 cytidine deaminases

Xenotransplantation of porcine cells, tissues, and organs shows promise to surmount the shortage of human donor materials. Among the barriers to pig-to-human xenotransplantation are porcine endogenous retroviruses (PERV) since functional representatives of the two polytropic classes, PERV-A and PERV-B, are able to infect human embryonic kidney cells in vitro, suggesting that a xenozoonosis in vivo could occur. To assess the capacity of human and porcine cells to counteract PERV infections, we analyzed human and porcine APOBEC3 (A3) proteins. This multigene family of cytidine deaminases contributes to the cellular intrinsic immunity and act as potent inhibitors of retroviruses and retrotransposons. Our data show that the porcine A3 gene locus on chromosome 5 consists of the two single-domain genes A3Z2 and A3Z3. The evolutionary relationships of the A3Z3 genes reflect the evolutionary history of mammals. The two A3 genes encode at least four different mRNAs: A3Z2, A3Z3, A3Z2-Z3, and A3Z2-Z3 splice variant A (SVA). Porcine and human A3s have been tested toward their antiretroviral activity against PERV and murine leukemia virus (MuLV) using novel single-round reporter viruses. The porcine A3Z2, A3Z3 and A3Z2-Z3 were packaged into PERV particles and inhibited PERV replication in a dose-dependent manner. The antiretroviral effect correlated with editing by the porcine A3s with a trinucleotide preference for 5' TGC for A3Z2 and A3Z2-Z3 and 5' CAC for A3Z3. These results strongly imply that human and porcine A3s could inhibit PERV replication in vivo, thereby reducing the risk of infection of human cells by PERV in the context of pig-to-human xenotransplantation.

The immense potential of xenotransplantation in surgery

There is a limited availability of deceased human organs and cells for the purposes of clinical transplantation. Genetically-engineered pigs may provide an alternative source. Although several immune barriers need to be overcome, considerable progress has been made in experimental models in recent years, largely through the increasing availability of pigs with new genetic modifications. Pig heterotopic heart graft survival in nonhuman primates has extended for 8 months, with orthotopic grafts supporting life for almost 2 months. Life-supporting kidney transplants have functioned for almost 3 months. The current barriers are related to coagulation dysfunction between pig and primate that results in thrombotic microangiopathy and/or a consumptive coagulopathy, which may in part be related to molecular incompatibilities in the coagulation systems of pigs and primates. Current efforts are concentrated on genetically-modifying the organ- or islet-source pigs by the introduction of 'anticoagulant' or 'anti-thrombotic' genes to provide protection from the recipient coagulation cascade and platelet activation. Progress with pig islet xenotransplantation has been particularly encouraging with complete control of glycemia in diabetic monkeys extending in one case for >12 months. Other areas where experimental data suggest the possibility of early clinical trials are corneal xenotransplantation and pig neuronal cell xenotransplantation, for example, in patients with Parkinson's disease. With the speed of advances in genetic engineering increasing steadily, it is almost certain that the remaining problems will be overcome within the foreseeable future, and clinical allotransplantation will eventually become of historical interest only.

Analysis of the molecular and regulatory properties of active porcine endogenous retrovirus gamma-1 long terminal repeats in kidney tissues of the NIH-Miniature pig

The pig genome contains the gamma 1 family of porcine endogenous retroviruses (PERVs), which are a major obstacle to the development of successful xenotransplantation from pig to human. Long terminal repeats (LTRs) found in PERVs are known to be essential elements for the control of the transcriptional activity of single virus by different transcription factors (TFs). To identify transcribed PERV LTR elements, RT-PCR and DNA sequencing analyses were performed. Twenty-nine actively transcribed LTR elements were identified in the kidney tissues of the NIH-Miniature pig. These elements were divided into two major groups (I and II), and four minor groups (I-1, I-2, I-3, and II-1), by the presence of insertion and deletion (INDEL) sequences. Group I elements showed strong transcriptional activity compared to group II elements. Four different LTR elements (PL1, PL2, PL3, and PL4) as representative of the groups were analyzed by using a transient transfection assay. The regulation of their promoter activity was investigated by treatment with M.SssI (CpG DNA methyltransferase) and garcinol (histone acetyltransferase inhibitor). The transcriptional activity of PERV LTR elements was significantly reduced by treatment with M.SssI. These data indicate that transcribed PERV LTR elements harbor sufficient promoter activity to regulate the transcription of a single virus, and the transcriptional activity of PERV LTRs may be controlled by DNA methylation events.

Retroviral restriction factors and infectious risk in xenotransplantation

The clinical application of xenotransplantation poses immunologic, ethical, and microbiologic challenges. Significant progress has been made in the investigation of each of these areas. Among concerns regarding infectious risks for human xenograft recipients is the identification in swine of infectious agents including porcine endogenous retroviruses (PERV) that are capable of replication in some human cell lines. PERV replication has, however, been difficult to demonstrate in primate-derived cell lines and in preclinical studies of non-human primates receiving porcine xenografts. Endogenous 'retroviral restriction factors' are intracellular proteins and components of the innate immune system that act at various steps in retroviral replication. Recent studies suggest that some of these factors may have applications in the management of endogenous retroviruses in xenotransplantation. The risks of PERV infection and the potential role of retroviral restriction factors in xenotransplantation are reviewed in detail.

Comparison of PERV genomic locations between Asian and European pigs

Xenotransplantation from pigs provides a possible solution to the shortage of human organs for allotransplantation. Porcine endogenous retroviruses (PERVs) are a possible obstacle to using porcine organs in addition to the immunological barriers. Three main types of PERVs (A, B and C) have been previously investigated in diverse pig breeds. To examine the copy numbers of PERVs and their genomic locations in the Korean native pig genome, we screened a BAC (Bacterial Artificial Chromosome) library with PERV-specific protease primers for initial recognition of PERV-positive clones and three sets of envelope-specific primers for the identification of PERV types. A total of 45 PERV-positive clones, nine PERV-A and 36 PERV-B, have been identified from the library screening and the BAC contigs were constructed using the primers designed from BAC end sequences (BESs). These primers were also used for SCH (Somatic Cell Hybrid) and RH (Radiation Hybrid) mapping of the PERV-positive clones. The results indicate that 45 PERV-positive BAC clones belong to nine contigs and a singleton. SCH and IMpRH (INRA-Minnesota Porcine Radiation Hybrid) mapping results indicated that there are at least eight separate PERV genomic locations, consisting of three PERV-A and five PERV-B. One contig could not be mapped, and two contigs are closely located on SSC7. Southern blotting indicates there may be up to 15 additional sites. Further investigation of these clones will contribute to a general strategy to generate PERV-free lines of pigs suitable for xenotransplantation.

Identification and molecular characterization of PERV gamma1 long terminal repeats

Porcine endogenous retroviruses (PERVs) gamma1 in the pig genome have the potential to act as harmful factors in xenotransplantation (pig-to-human). Long terminal repeats (LTRs) are known to be strong promoter elements that could control the transcription activity of PERV elements and the adjacent functional genes. To investigate the transcribed PERV gamma1 LTR elements in pig tissues, bioin-formatic and experimental approaches were conducted. Using RT-PCR amplification and sequencing approaches, 69 different transcribed LTR elements were identified. And 69 LTR elements could be divided into six groups (15 subgroups) by internal variation including tandem repeated sequences, insertion and deletion (INDEL). Remarkably, all internal variations were indentified in U3 region of LTR elements. Taken together, the identification and characterization of various PERV LTR transcripts allow us to extend our knowledge of PERV and its transcriptional study.

Progress and prospects: genetic engineering in xenotransplantation

In this review, we summarize the work published over the last 2 years using genetic modifications of animals in the field of xenotransplantation. Genetic engineering of the donor has become a powerful tool in xenotransplantation, both for the inactivation of one particular porcine gene and for the addition of human genes with the goal of overcoming xenogeneic barriers. We summarize the work relative to the knockout of the alpha1,3-galactosyltransferase gene, followed by genetic engineering aimed at reducing the humoral and cellular immune response, complement activation and coagulation. Finally, we report on the genetic modification of pigs to reduce porcine endogenous retrovirus infection risk in the xenogeneic context.

Strategies to enhance the safety profile of xenotransplantation: minimizing the risk of viral zoonoses

PURPOSE OF REVIEW

Pig-to-human xenotransplantation has taken steps closer to reality through advances in animal engineering to address immunological as well as microbial problems. The most highlighted problem in xenotransplantation safety has been the potential risk for zoonotic infection mediated by porcine endogenous retroviruses. Safety issues regarding viral zoonosis, particularly porcine endogenous retroviruses, are summarized and commented upon.

RECENT FINDINGS

Several molecular, transgenic strategies to provide safer transplant source animals with less porcine endogenous retrovirus infectivity have been developed. A genomics approach by selective breeding and porcine endogenous retrovirus loci knockout is at least theoretically possible. For other viruses, advances have been made in technologies for virus discovery and identification.

SUMMARY

The consequences of possible zoonoses in xenotransplantation are largely unknown. Further work to identify and control potential zoonotic agents based on recent progress will improve the safety profile of xenotransplantation. Advances made should be subjected to cautious testing in well controlled, preclinical and clinical experiments.

Reappraisal of biosafety risks posed by PERVs in xenotransplantation

Donor materials of porcine origin could potentially provide an alternative source of cells, tissues or whole organs for transplantation to humans, but is hampered by the health risk posed by infection with porcine viruses. Although pigs can be bred in such a way that all known exogenous microorganisms are eliminated, this is not feasible for all endogenous pathogens, such as the porcine endogenous retroviruses (PERVs) which are present in the germline of pigs as proviruses. Upon transplantation, PERV proviruses would be transferred to the human recipient along with the xenograft. If xenotransplantation stimulates or facilitates replication of PERVs in the new hosts, a risk exists for adaptation of the virus to humans and subsequent spread of these viruses. In a worst-case scenario, this might result in the emergence of a new viral disease. Although the concerns for disease potential of PERVs are easing, only limited pre-clinical and clinical data are available. Small-scale, well-designed and carefully controlled clinical trials would provide more evidence on the safety of this approach and allow a better appreciation of the risks involved. It is therefore important to have a framework of protective measures and monitoring protocols in place to facilitate such initially small scale clinical trials. This framework will raise ethical and social considerations regarding acceptability.

The perspectives for porcine-to-human xenografts

The shortage of donated human organs for transplantation continues to be a life threatening problem for patients suffering from complete organ failure. Although this gap is increasing due to the demographic changes in aging Western populations, it is generally accepted that international trading in human organ is not an ethical solution. Alternatives to the use of human organs for transplantation must be developed and these alternatives include stem cell therapy, artificial organs and organs from other species, i.e. xenografts. For practical reasons but most importantly because of its physiological similarity with humans, the pig is generally accepted as the species of choice for xenotransplantation. Nevertheless, before porcine organs can be used in human xenotransplantation, it is necessary to make a series of precise genetic modifications to the porcine genome, including the addition of genes for factors which suppress the rejection of transplanted porcine tissues and the inactivation or removal of undesirable genes which can only be accomplished at this time by targeted recombination and somatic nuclear transfer. This review will give an insight into the advances in transgenic manipulation and cloning in pigs--in the context of porcine-to-human xenotransplantation.

The restriction of zoonotic PERV transmission by human APOBEC3G

The human APOBEC3G protein is an innate anti-viral factor that can dominantly inhibit the replication of some endogenous and exogenous retroviruses. The prospects of purposefully harnessing such an anti-viral defense are under investigation. Here, long-term co-culture experiments were used to show that porcine endogenous retrovirus (PERV) transmission from pig to human cells is reduced to nearly undetectable levels by expressing human APOBEC3G in virus-producing pig kidney cells. Inhibition occurred by a deamination-independent mechanism, likely after particle production but before the virus could immortalize by integration into human genomic DNA. PERV inhibition did not require the DNA cytosine deaminase activity of APOBEC3G and, correspondingly, APOBEC3G-attributable hypermutations were not detected. In contrast, over-expression of the sole endogenous APOBEC3 protein of pigs failed to interfere significantly with PERV transmission. Together, these data constitute the first proof-of-principle demonstration that APOBEC3 proteins can be used to fortify the innate anti-viral defenses of cells to prevent the zoonotic transmission of an endogenous retrovirus. These studies suggest that human APOBEC3G-transgenic pigs will provide safer, PERV-less xenotransplantation resources and that analogous cross-species APOBEC3-dependent restriction strategies may be useful for thwarting other endogenous as well as exogenous retrovirus infections.

Conversion from a calcineurin inhibitor-based immunosuppressive regimen to everolimus in renal transplant recipients: effect on renal function and proteinuria

New immunosuppressive agents are being actively researched to avoid complications of chronic allograft nephropathy (CAN), calcineurin inhibitor (CNI) nephrotoxicity, and posttransplantation cancer. The family of mTOR inhibitors offers a unique immunosuppressive opportunity to avoid CNI toxicity and reduce the incidence of malignancy. Nevertheless, increasing data have demonstrated that sirolimus (SRL), the first mTOR introduced in the treatment of solid organ transplant recipients, induces proteinuria, an adverse event that could produce deterioration of long-term renal function. In this short-term study of patients followed for 1 to 16 months, we examined changes in renal function and proteinuria among renal transplant recipients converted from a CNI-based regimen to an everolimus (EVL)-based one, a recently introduced mTOR inhibitor. Our data showed that renal function can be optimized after conversion to EVL by up to 42% in recipients showing CAN grade 1 or 2, or CNI nephrotoxicity. Importantly, patients who improved their creatinine clearance did not show increased proteinuria measured in a voided specimen as the ratio of urinary protein and creatinine concentration (P/C). These results, if confirmed with long-term follow-up and a larger number of patients, would allow us to consider EVL as a promising agent for maintenance immunosuppressive regimens in kidney transplantation.

Transgenic farm animals: an update

The first transgenic livestock species were reported in 1985. Since then microinjection of foreign DNA into pronuclei of zygotes has been the method of choice. It is now being replaced by more efficient protocols based on somatic nuclear transfer that also permit targeted genetic modifications. Lentiviral vectors and small interfering ribonucleic acid (siRNA) technology are also becoming important tools for transgenesis. In 2006 the European Medicines Agency (EMEA) gave green light for the commercialistion of the first recombinant protein produced in the milk of transgenic animals. Recombinant antithrombin III will be launched as ATryn for prophylactic treatment of patients with congenital antithrombin deficiency. This important milestone will boost the research activities in farm animal transgenesis. Recent developments in transgenic techniques of farm animals are discussed in this review.