Porcine endogenous retrovirus and other viruses in xenotransplantation

Envelope Recombination: A Major Driver in Shaping Retroviral Diversification and Evolution within the Host Genome

Endogenous retroviruses (ERVs) are integrated into host DNA as the result of ancient germ line infections, primarily by extinct exogenous retroviruses. Thus, vertebrates' genomes contain thousands of ERV copies, providing a "fossil" record for ancestral retroviral diversity and its evolution within the host genome. Like other retroviruses, the ERV proviral sequence consists of gag, pro, pol, and env genes flanked by long terminal repeats (LTRs). Particularly, the env gene encodes for the envelope proteins that initiate the infection process by binding to the host cellular receptor(s), causing membrane fusion. For this reason, a major element in understanding ERVs' evolutionary trajectory is the characterization of env changes over time. Most of the studies dedicated to ERVs' env have been aimed at finding an "actual" physiological or pathological function, while few of them have focused on how these genes were once acquired and modified within the host. Once acquired into the organism, genome ERVs undergo common cellular events, including recombination. Indeed, genome recombination plays a role in ERV evolutionary dynamics. Retroviral recombination events that might have been involved in env divergence include the acquisition of env genes from distantly related retroviruses, env swapping facilitating multiple cross-species transmission over millions of years, ectopic recombination between the homologous sequences present in different positions in the chromosomes, and template switching during transcriptional events. The occurrence of these recombinational events might have aided in shaping retroviral diversification and evolution until the present day. Hence, this review describes and discusses in detail the reported recombination events involving ERV env to provide the basis for further studies in the field.

Establishment of decellularized extracellular matrix scaffold derived from caprine pancreas as a novel alternative template over porcine pancreatic scaffold for prospective biomedical application

In this study, the caprine pancreas has been presented as an alternative to the porcine organ for pancreatic xenotransplantation with lesser risk factors. The obtained caprine pancreas underwent a systematic cycle of detergent perfusion for decellularization. It was perfused using anionic (0.5% w/v sodium dodecyl sulfate) as well as non-ionic (0.1% v/v triton X-100, t-octyl phenoxy polyethoxy ethanol) detergents and washed intermittently with 1XPBS supplemented with 0.1% v/v antibiotic and nucleases in a gravitation-driven set-up. After 48 h, a white decellularized pancreas was obtained, and its extracellular matrix (ECM) content was examined for scaffold-like properties. The ECM content was assessed for removal of cellular content, and nuclear material was evaluated with temporal H&E staining. Quantified DNA was found to be present in a negligible amount in the resultant decellularized pancreas tissue (DPT), thus prohibiting it from triggering any immunogenicity. Collagen and fibronectin were confirmed to be preserved upon trichrome and immunohistochemical staining, respectively. SEM and AFM images reveal interconnected collagen fibril networks in the DPT, confirming that collagen was unaffected. sGAG was visualized using Prussian blue staining and quantified with DMMB assay, where DPT has effectively retained this ECM component. Uniaxial tensile analysis revealed that DPT possesses better elasticity than NPT (native pancreatic tissue). Physical parameters like tensile strength, stiffness, biodegradation, and swelling index were retained in the DPT with negligible loss. The cytocompatibility analysis of DPT has shown no cytotoxic effect for up to 72 h on normal insulin-producing cells (MIN-6) and cancerous glioblastoma (LN229) cells in vitro. The scaffold was recellularized using isolated mouse islets, which have established in vitro cell proliferation for up to 9 days. The scaffold received at the end of the decellularization cycle was found to be non-toxic to the cells, retained biological and physical properties of the native ECM, suitable for recellularization, and can be used as a safer and better alternative as a transplantable organ from a xenogeneic source.

Virus Safety of Xenotransplantation

The practice of xenotransplantation using pig islet cells or organs is under development to alleviate the shortage of human donor islet cells or organs for the treatment of diabetes or organ failure. Multiple genetically modified pigs were generated to prevent rejection. Xenotransplantation may be associated with the transmission of potentially zoonotic porcine viruses. In order to prevent this, we developed highly sensitive PCR-based, immunologicals and other methods for the detection of numerous xenotransplantation-relevant viruses. These methods were used for the screening of donor pigs and xenotransplant recipients. Of special interest are the porcine endogenous retroviruses (PERVs) that are integrated in the genome of all pigs, which are able to infect human cells, and that cannot be eliminated by methods that other viruses can. We showed, using droplet digital PCR, that the number of PERV proviruses is different in different pigs (usually around 60). Furthermore, the copy number is different in different organs of a single pig, indicating that PERVs are active in the living animals. We showed that in the first clinical trials treating diabetic patients with pig islet cells, no porcine viruses were transmitted. However, in preclinical trials transplanting pig hearts orthotopically into baboons, porcine cytomegalovirus (PCMV), a porcine roseolovirus (PCMV/PRV), and porcine circovirus 3 (PCV3), but no PERVs, were transmitted. PCMV/PRV transmission resulted in a significant reduction of the survival time of the xenotransplant. PCMV/PRV was also transmitted in the first pig heart transplantation to a human patient and possibly contributed to the death of the patient. Transmission means that the virus was detected in the recipient, however it remains unclear whether it can infect primate cells, including human cells. We showed previously that PCMV/PRV can be eliminated from donor pigs by early weaning. PERVs were also not transmitted by inoculation of human cell-adapted PERV into small animals, rhesus monkey, baboons and cynomolgus monkeys, even when pharmaceutical immunosuppression was applied. Since PERVs were not transmitted in clinical, preclinical, or infection experiments, it remains unclear whether they should be inactivated in the pig genome by CRISPR/Cas. In summary, by using our sensitive methods, the safety of xenotransplantation can be ensured.

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.

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.

Reprogramming and transdifferentiation - two key processes for regenerative medicine

Regenerative medicine based on transplants obtained from donors or foetal and new-born mesenchymal stem cells, encounter important obstacles such as limited availability of organs, ethical issues and immune rejection. The growing demand for therapeutic methods for patients being treated after serious accidents, severe organ dysfunction and an increasing number of cancer surgeries, exceeds the possibilities of the therapies that are currently available. Reprogramming and transdifferentiation provide powerful bioengineering tools. Both procedures are based on the somatic differentiated cells, which are easily and unlimitedly available, like for example: fibroblasts. During the reprogramming procedure mature cells are converted into pluripotent cells - which are capable to differentiate into almost any kind of desired cells. Transdifferentiation directly converts differentiated cells of one type into another differentiated cells type. Both procedures allow to obtained patient's dedicated cells for therapeutic purpose in regenerative medicine. In combination with biomaterials, it is possible to obtain even whole anatomical structures. Those patient's dedicated structures may serve for them upon serious accidents with massive tissue damage but also upon cancer surgeries as a replacement of damaged organ. Detailed information about reprogramming and transdifferentiation procedures as well as the current state of the art are presented in our review.

Lung tissue bioengineering for chronic obstructive pulmonary disease: overcoming the need for lung transplantation from human donors

Introduction: Chronic obstructive pulmonary disease (COPD) affects more than 380 million people, causing more than 3 million deaths annually worldwide. Despite this enormous burden, currently available therapies are largely limited to symptom control. Lung transplant is considered for end-stage disease but is severely limited by the availability of human organs. Furthermore, the pre-transplant course is a complex orchestration of locating and harvesting suitable lungs, and the post-transplant course is complicated by rejection and infection. Lung tissue bioengineering has the potential to relieve the organ shortage and improve the post-transplant course by generating patient-specific lungs for transplant. Additionally, emerging progenitor cell therapies may facilitate in vivo regeneration of pulmonary tissue, obviating the need for transplant. Areas Covered: We review several lung tissue bioengineering approaches including the recellularization of decellularized scaffolds, 3D bioprinting, genetically-engineered xenotransplantation, blastocyst complementation, and direct therapy with progenitor cells. Articles were identified by searching relevant terms (see Key Words) in the PubMed database and selected for inclusion based on novelty and uniqueness of their approach. Expert Opinion: Lung tissue bioengineering research is in the early stages. Of the methods reviewed, only direct cell therapy has been investigated in humans. We anticipate a minimum of 5-10 years before human therapy will be feasible.

Rare and Emerging Viral Infections in the Transplant Population

Viral infections account for a large proportion of emerging infectious diseases, and the agents included in this group consist of recently identified viruses as well as previously identified viruses with an apparent increase in disease incidence. In transplant recipients, this group can include viruses with no recognized pathogenicity in immunocompetent patients and those that result in atypical or more severe disease presentations in the immunocompromised host. In this chapter, we begin by discussing viral diagnostics and techniques used for viral discovery, specifically as they apply to emerging and rare infections in this patient population. Focus then shifts to specific emerging and re-emerging viruses in the transplant population, including human T-cell leukemia virus 1, rabies, lymphocytic choriomeningitis virus, human bocavirus, parvovirus 4, measles, mumps, orf, and dengue. We have also included a brief discussion on emerging viruses and virus families with few or no reported cases in transplant recipients: monkeypox, nipah and hendra, chikungunya and other alphaviruses, hantavirus and the Bunyaviridae, and filoviruses. Finally, concerns regarding infectious disease complications in xenotransplantation and the reporting of rare viral infections are addressed. With the marked increase in the number of solid organ and hematopoietic stem cell transplants performed worldwide, we expect a corresponding rise in the reports of emerging viral infections in transplant hosts, both from known viruses and those yet to be identified.

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.

The porcine virome and xenotransplantation

The composition of the porcine virome includes viruses that infect pig cells, ancient virus-derived elements including endogenous retroviruses inserted in the pig chromosomes, and bacteriophages that infect a broad array of bacteria that inhabit pigs. Viruses infecting pigs, among them viruses also infecting human cells, as well as porcine endogenous retroviruses (PERVs) are of importance when evaluating the virus safety of xenotransplantation. Bacteriophages associated with bacteria mainly in the gut are not relevant in this context. Xenotransplantation using pig cells, tissues or organs is under development in order to alleviate the shortage of human transplants. Here for the first time published data describing the viromes in different pigs and their relevance for the virus safety of xenotransplantation is analysed. In conclusion, the analysis of the porcine virome has resulted in numerous new viruses being described, although their impact on xenotransplantation is unclear. Most importantly, viruses with known or suspected zoonotic potential were often not detected by next generation sequencing, but were revealed by more sensitive methods.

Long-term safety from transmission of porcine endogenous retrovirus after pig-to-non-human primate corneal transplantation

BACKGROUND

The risk of xenozoonosis mainly by porcine endogenous retrovirus (PERV) has been considered as one of the main hurdles in xenotransplantation and therefore should be elucidated prior to the clinical use of porcine corneal grafts. Accordingly, an investigation was performed to analyze the infectivity of PERVs from porcine keratocytes to human cells, and the long-term risk of transmission of PERVs was determined using pig-to-non-human primate (NHP) corneal transplantation models.

METHODS

The infectivity of PERVs from the SNU miniature pig keratocytes was investigated by coculture with a human embryonic kidney cell line. Twenty-two rhesus macaques underwent xenocorneal transplantation as follows: (i) group 1 (n=4): anterior lamellar keratoplasty (LKP) with freshly preserved porcine corneas, (ii) group 2 (n=5): anterior LKP with decellularized porcine corneas followed by penetrating keratoplasty (PKP) with allografts, (iii) group 3 (n=3): PKP under steroid-based immunosuppression, (iv) group 4 (n=4): PKP under anti-CD154 antibody-based immunosuppression, (v) group 5 (n=4): deep anterior LKP with freshly preserved porcine corneas under anti-CD40 antibody-based immunosuppression, and (vi) group 6 (n=2): PKP under anti-CD40 antibody-based immunosuppression. Postoperative blood samples were serially collected, and tissue samples were obtained from thirteen different organs at the end of each experiment. The existence of PERV DNA and RNA was investigated using PCR and RT-PCR.

RESULTS

Using two independent in vitro infectivity tests, neither PERV pol nor pig mitochondrial cytochrome oxidase II was detected after 41 and 92 days of coculture, respectively. After xenocorneal transplantation, a total of 257 serial peripheral blood mononuclear cell samples, 34 serial plasma samples, and 282 tissue samples were obtained from the NHP recipients up to 1176 days post-transplantation. No PERV transmission was evident in any samples.

CONCLUSIONS

Within the limits of this study, there is no evidence to support any risk of PERV transmission from porcine corneal tissues to NHP recipients, despite the existence of PERV-expressing cells in porcine corneas.

Bioengineering Hearts: Simple yet Complex

PURPOSE OF REVIEW

In this review, we focus on the multiple advancements in the field of cardiovascular regenerative medicine and the state-of-the art of building a heart. An organ is comprised of cells, but cells alone do not comprise an organ. We summarize the components needed, the hurdles, and likely translational steps defining the opportunities for discovery.

RECENT FINDINGS

The therapies being developed in regenerative medicine aim not only to repair, but also to regenerate or replace ailing tissues and organs. The first generation of cardiac regenerative medicine was gene therapy. The past decade has focused primarily on cell therapy, particularly for repair after ischemic injury with mixed results. Although cell therapy is promising, it will likely never reverse end-stage heart failure; and thus, the unmet need is, and will remain, for organs. Scientists have now tissue engineering and regenerative medicine concepts to invent alternative therapies for a wide spectrum of diseases encompassing cardiovascular, respiratory, gastrointestinal, hepatic, renal, musculoskeletal, ocular, and neurodegenerative disorders. Current studies focus on potential scaffolds and applying concepts and techniques learned with testbeds to building human sized organs. Special focus has been given to scaffold sources, cells types and sources, and cell integration with scaffolds. The complexity arises in combining them to yield an organ.

SUMMARY

Regenerative medicine has emerged as one of the most promising fields of translational research and has the potential to minimize both the need for, and increase the availability of, donor organs. The field is characterized by its integration of biology, physical sciences, and engineering. The proper integration of these fields could lead to off-the-shelf bioartificial organs that are suitable for transplantation. Building a heart will necessarily require a scaffold that can provide cardiac function. We believe that the advent of decellularization methods provides complex, unique, and natural scaffold sources. Ultimately, cell biology and tissue engineering will need to synergize with scaffold biology, finding cell sources and reproducible ways to expand their numbers is an unmet need. But tissue engineering is moving toward whole organ synthesis at an unparalleled pace.

Identification of heterologous Torque Teno Viruses in humans and swine

Torque Teno Viruses (TTVs) are ubiquitous viruses which are highly prevalent in several mammalian species. Human TTV’s are epidemiologically associated with several human disease conditions such as respiratory illnesses, auto-immune disorders and hepatitis. Recently it was found that swine TTV’s (TTSuVs) can act as primary pathogens. The common occurrence of TTVs as environmental contaminants and the increasing interest in the use of swine organs for xenotransplantation lend importance to the question of whether TTV’s can cross-infect across species. In this study, we examined human and swine sera by swine or human TTV-specific PCRs, to determine whether swine TTVs (TTSuV) DNA can be detected in humans and vice versa. Surprisingly, both human and TTSuV DNA were present in a majority of the samples tested. Transfection of human PBMC’s with TTSuV1 genomic DNA resulted in productive viral infection which was sustained for the three serial passages tested. Lymphoproliferative responses in infected human PBMCs were diminished when compared to the controls. Furthermore, mild to moderate antibody responses against the TTSuV1 ORF2 protein was detected in 16 of the 40 human sera by ELISA. Therefore, these study findings provide initial and fundamental evidence for possible cross-species transmission of TTVs.

Lack of strong anti-viral immune gene stimulation in Torque Teno Sus Virus1 infected macrophage cells

While recent findings suggest that swine TTVs (TTSuVs) can act as primary or co-infecting pathogens, very little is known about viral immunity. To determine whether TTSuVs downregulate key host immune responses to facilitate their own survival, a swine macrophage cell line, 3D4/31, was used to over-express recombinant TTSuV1 viral particles or the ORF3 protein. Immune gene expression profiles were assessed by a quantitative PCR panel consisting of 22 immune genes, in cell samples collected at 6, 12, 24 and 48h post-transfection. Despite the upregulation of IFN-β and TLR9, interferon stimulated innate genes and pro-inflammatory genes were not upregulated in virally infected cells. The adaptive immune genes, IL-4 and IL-13, were significantly downregulated at 6h post-transfection. The ORF3 protein did not appear do not have a major immuno-suppressive effect, nor did it stimulate anti-viral immunity. Data from this study warrants further investigation into the mechanisms of TTV related immuno-pathogenesis.

An Evolutionarily Young Polar Bear (Ursus maritimus) Endogenous Retrovirus Identified from Next Generation Sequence Data

Transcriptome analysis of polar bear (Ursus maritimus) tissues identified sequences with similarity to Porcine Endogenous Retroviruses (PERV). Based on these sequences, four proviral copies and 15 solo long terminal repeats (LTRs) of a newly described endogenous retrovirus were characterized from the polar bear draft genome sequence. Closely related sequences were identified by PCR analysis of brown bear (Ursus arctos) and black bear (Ursus americanus) but were absent in non-Ursinae bear species. The virus was therefore designated UrsusERV. Two distinct groups of LTRs were observed including a recombinant ERV that contained one LTR belonging to each group indicating that genomic invasions by at least two UrsusERV variants have recently occurred. Age estimates based on proviral LTR divergence and conservation of integration sites among ursids suggest the viral group is only a few million years old. The youngest provirus was polar bear specific, had intact open reading frames (ORFs) and could potentially encode functional proteins. Phylogenetic analyses of UrsusERV consensus protein sequences suggest that it is part of a pig, gibbon and koala retrovirus clade. The young age estimates and lineage specificity of the virus suggests UrsusERV is a recent cross species transmission from an unknown reservoir and places the viral group among the youngest of ERVs identified in mammals.

A review of cellularization strategies for tissue engineering of whole organs

With the advent of whole organ decellularization, extracellular matrix scaffolds suitable for organ engineering were generated from numerous tissues, including the heart, lung, liver, kidney, and pancreas, for use as alternatives to traditional organ transplantation. Biomedical researchers now face the challenge of adequately and efficiently recellularizing these organ scaffolds. Herein, an overview of whole organ decellularization and a thorough review of the current literature for whole organ recellularization are presented. The cell types, delivery methods, and bioreactors employed for recellularization are discussed along with commercial and clinical considerations, such as immunogenicity, biocompatibility, and Food and Drug Administartion regulation.

Xenotransplantation of Cells, Tissues, Organs and the German Research Foundation Transregio Collaborative Research Centre 127

Human organ transplantation is the therapy of choice for end-stage organ failure. However, the demand for organs far exceeds the donation rate, and many patients die while waiting for a donor. Clinical xenotransplantation using discordant species, particularly pigs, offers a possible solution to this critical shortfall. Xenotransplantation can also increase the availability of cells, such as neurons, and tissues such as cornea, insulin producing pancreatic islets and heart valves. However, the immunological barriers and biochemical disparities between pigs and primates (human) lead to rejection reactions despite the use of common immunosuppressive drugs. These result in graft vessel destruction, haemorrhage, oedema, thrombus formation, and transplant loss. Our consortium is pursuing a broad range of strategies to overcome these obstacles. These include genetic modification of the donor animals to knock out genes responsible for xenoreactive surface epitopes and to express multiple xenoprotective molecules such as the human complement regulators CD46, 55, 59, thrombomodulin and others. We are using (new) drugs including complement inhibitors (e.g. to inhibit C3 binding), anti-CD20, 40, 40L, and also employing physical protection methods such as macro-encapsulation of pancreatic islets. Regarding safety, a major objective is to assure that possible infections are not transmitted to recipients. While the aims are ambitious, recent successes in preclinical studies suggest that xenotransplantation is soon to become a clinical reality.

Epidemiology, geographical distribution, and economic consequences of swine zoonoses: a narrative review

We sought to review the epidemiology, international geographical distribution, and economic consequences of selected swine zoonoses. We performed literature searches in two stages. First, we identified the zoonotic pathogens associated with swine. Second, we identified specific swine-associated zoonotic pathogen reports for those pathogens from January 1980 to October 2012. Swine-associated emerging diseases were more prevalent in the countries of North America, South America, and Europe. Multiple factors were associated with the increase of swine zoonoses in humans including: the density of pigs, poor water sources and environmental conditions for swine husbandry, the transmissibility of the pathogen, occupational exposure to pigs, poor human sanitation, and personal hygiene. Swine zoonoses often lead to severe economic consequences related to the threat of novel pathogens to humans, drop in public demand for pork, forced culling of swine herds, and international trade sanctions. Due to the complexity of swine-associated pathogen ecology, designing effective interventions for early detection of disease, their prevention, and mitigation requires an interdisciplinary collaborative “One Health” approach from veterinarians, environmental and public health professionals, and the swine industry.

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.

Role of DNA methylation in expression and transmission of porcine endogenous retroviruses

Porcine endogenous retroviruses (PERV) represent a major safety concern in pig-to-human xenotransplantation. To date, no PERV infection of a xenograft recipient has been recorded; however, PERVs are transmissible to human cells in vitro. Some recombinants of the A and C PERV subgroups are particularly efficient in infection and replication in human cells. Transcription of PERVs has been described in most pig cells, but their sequence and insertion polymorphism in the pig genome impede identification of transcriptionally active or silenced proviral copies. Furthermore, little is known about the epigenetic regulation of PERV transcription. Here, we report on the transcriptional suppression of PERV by DNA methylation in vitro and describe heavy methylation in the majority of PERV 5' long terminal repeats (LTR) in porcine tissues. In contrast, we have detected sparsely methylated or nonmethylated proviruses in the porcine PK15 cells, which express human cell-tropic PERVs. We also demonstrate the resistance of PERV DNA methylation to inhibitors of methylation and deacetylation. Finally, we show that the high permissiveness of various human cell lines to PERV infection coincides with the inability to efficiently silence the PERV proviruses by 5'LTR methylation. In conclusion, we suggest that DNA methylation is involved in PERV regulation, and that only a minor fraction of proviruses are responsible for the PERV RNA expression and porcine cell infectivity.

Susceptibility of human liver cells to porcine endogenous retrovirus

OBJECTIVES

The risk of porcine endogenous retrovirus infection is a major barrier for pig-to-human xenotransplant. Porcine endogenous retrovirus, present in porcine cells, can infect many human and nonhuman primate cells in vitro, but there is no evidence available about in vitro infection of human liver cells. We investigated the susceptibility of different human liver cells to porcine endogenous retrovirus.

MATERIALS AND METHODS

The supernatant from a porcine kidney cell line was added to human liver cells, including a normal hepatocyte cell line (HL-7702 cells), primary hepatocytes (Phh cells), and a liver stellate cell line (Lx-2 cells), and to human embryonic kidney cells as a reference control. Expression of the porcine endogenous retrovirus antigen p15E in the human cells was evaluated with polymerase chain reaction, reverse transcription-polymerase chain reaction, and Western blot.

RESULTS

The porcine endogenous retrovirus antigen p15E was not expressed in any human liver cells (HL-7702, Phh, or Lx-2 cells) that had been exposed to supernatants from porcine kidney cell lines. Porcine endogenous retrovirus-specific fragments were amplified in human kidney cells.

CONCLUSIONS

Human liver cells tested were not susceptible to infection by porcine endogenous retrovirus. Therefore, not all human cells are susceptible to porcine endogenous retrovirus.

Current status of islet encapsulation

Cell encapsulation is a method of encasing cells in a semipermeable matrix that provides a permeable gradient for the passage of oxygen and nutrients, but effectively blocks immune-regulating cells from reaching the graft, preventing rejection. This concept has been described as early as the 1930s, but it has exhibited substantial achievements over the last decade. Several advances in encapsulation engineering, chemical purification, applications, and cell viability promise to make this a revolutionary technology. Several obstacles still need to be overcome before this process becomes a reality, including developing a reliable source of islets or insulin-producing cells, determining the ideal biomaterial to promote graft function, reducing the host response to the encapsulation device, and ultimately a streamlined, scaled-up process for industry to be able to efficiently and safely produce encapsulated cells for clinical use. This article provides a comprehensive review of cell encapsulation of islets for the treatment of type 1 diabetes, including a historical perspective, current research findings, and future studies.

Quantitative analysis of porcine endogenous retroviruses in different organs of transgenic pigs generated for xenotransplantation

The pig appears to be the most promising animal donor of organs for use in human recipients. Among several types of pathogens found in pigs, one of the greatest problems is presented by porcine endogenous retroviruses (PERVs). Screening of the source pig herd for PERVs should include analysis of both PERV DNA and RNA. Therefore, the present study focuses on quantitative analysis of PERVs in different organs such as the skin, heart, muscle, and liver and blood of transgenic pigs generated for xenotransplantation. Transgenic pigs were developed to express the human α-galactosidase, the human α-1,2-fucosyltransferase gene, or both genetic modifications of the genome (Lipinski et al., Medycyna Wet 66:316-322, 2010; Lipinski et al., Ann Anim Sci 12:349-356, 2012; Wieczorek et al., Medycyna Wet 67:462-466, 2011). The copy numbers of PERV DNA and RNA were evaluated using real-time Q-PCR and QRT-PCR, respectively. Comparative analysis of all PERV subtypes revealed the following relationships: PERV A > PERV B > PERV C. PERV A and B were found in all samples, whereas PERV C was detected in 47 % of the tested animals. The lowest level of PERV DNA was shown in the muscles for PERV A and B and in blood samples for PERV C. The lowest level of PERV A RNA was found in the skin, whereas those of PERV B and C RNA were found in liver specimens. Quantitative analysis revealed differences in the copy number of PERV subtypes between various organs of transgenic pigs generated for xenotransplantation. Our data support the idea that careful pig selection for organ donation with low PERV copy number may limit the risk of retrovirus transmission to the human recipients.

Suggestions for the diagnosis and elimination of hepatitis E virus in pigs used for xenotransplantation

The hepatitis E virus (HEV) is considered a zoonotic pathogen. In xenotransplantation, given the high prevalence of HEV infection in pigs, the risk of zoonotic transmission from a porcine source is considered high. Currently no clear data are available on how to diagnose and eliminate HEV in herds used for medical purposes and the importance of viral infection at the stage of harvest. In this study, several groups of animals currently used for medical purposes were found RNA positive in both serum and faeces for HEV genotype 3. In addition, viraemia was found in animals up to 3.6 yr of age, which is much longer than originally expected. Herd transmission rates appeared to be significantly lower in animals kept under minimal barrier conditions, compared with those observed for commercial animals, and as expected, segregation of animals at an early age prevented spread of infection. This study makes suggestions to ensure appropriate detection and eradication of HEV from a donor herd to be used for xenotransplantation purposes.

Carbohydrate antigens

PURPOSE OF REVIEW

To summarize the current knowledge of carbohydrate antigens as related to xenotransplantation. The emphasis is on non-Gal carbohydrate antigens identified in many institutes. In addition, several topics such as glycosyltransferase-transgenic pigs, innate cell receptors and porcine endogenous retrovirus (PERV) will be discussed.

RECENT FINDINGS

Studies related to iGb3 and neoantigens after knocking out GalT (GGTA1) were reviewed. Available data do not support the conclusion that GalT-KO remains iGb3 and/or that neoantigens are produced in the pigs. Concerning non-Gal antigen, in addition to the Hanganutziu-Deicher (H-D) antigen (NeuGc), Forrsman antigen, Galα1-3Lew(x), α-linked or β-linked GalNAc, β3 linked Gal, NeuAc, such as Neu5Acα2-3Galβ1-3GlcNAc, and Sid blood group (Sd(a))-like antigens are candidates. However, to date some of these remain controversial and others need further study to completely identify them. Regarding the H-D antigen, different from the α-Gal, it has a complicated expression system, but has cytotoxic effects toward pig cells. To modify other carbohydrate antigen apart from α-Gal, only the overexpression of GnT-III appears to have an effect on the suppression of the N-linked sugar of non-Gal antigen. Concerning innate cell receptors related to carbohydrates (ligands), the focus turned from natural killer (NK) receptor to others, such as monocytes. Finally, PERV contains a ligand with an N-linked sugar. Modification of the glycosylation pattern appears to be associated with regulating PERV infectivity.

SUMMARY

A considerable amount of data related to carbohydrate antigens is now available. At the same time, however, discrepancies between studies complicate this issue. Further studies will be needed to completely understand this complicated area of interest.

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.

Organ transplantation: historical perspective and current practice

Over the course of the last century, organ transplantation has overcome major technical limitations to become the success it is today. The breakthroughs include developing techniques for vascular anastomoses, managing the immune response (initially by avoiding it with the use of identical twins and subsequently controlling it with chemical immunosuppressants), and devising preservation solutions that enable prolonged periods of ex vivo storage while preserving function. One challenge that has remained from the outset is to overcome the shortage of suitable donor organs. The results of organ transplantation continue to improve, both as a consequence of the above innovations and the improvements in peri- and postoperative management. This review describes some of the achievements and challenges of organ transplantation.

Porcine endogenous retrovirus copy number in different pig breeds is not related to genetic diversity

The risk of zoonoses is a major obstacle to xenotransplantation. Porcine endogenous retrovirus (PERV) poses a potential risk of zoonotic infection, and its control is a prerequisite for the development of clinical xenotransplantation. The copy number of PERV varies among different breeds, and it has been suggested that the PERV integrations number is increased by inbreeding. The purpose of this study was (i) to examine the copy number of PERV in different Spanish pig breeds, Spanish wild boar and commercial cross-bred pigs from five different farms and genetic background (CCP1-CCP5) and (ii) to investigate the correlation between PERV copy number and the genetic background of the pigs in order to improve the selection of pigs for xenotransplantation. PERV copy number was determined by quantitative, real-time polymerase chain reactions. Thirty-four microsatellite markers were genotyped to describe the genetic diversity within populations (observed and expected heterozygosities, Ho and He, respectively) and the inbreeding coefficient (F). Pearson's correlation coefficient was used to determine the relationship between PERV copy number and Ho, He and F. The copy number of PERV among different pig breeds was estimated to range between three (CCP1) and 43 copies (Iberian Pig). Statistical differences were found among the studied populations concerning PERV copy number. No correlation was found between the PERV copy number and the heterozygosity (calculated at an individual level or at a population level) or the inbreeding coefficient of each population. Our data suggest that pigs inbreeding does not increase PERV copy number and support the idea that careful selection of pigs for organ donation with reduced PERV copy number will minimize the risk of retrovirus transmission to the human receptor.

Potential zoonotic infection of porcine endogenous retrovirus in xenotransplantation

Porcine endogenous retrovirus (PERV) is considered the major biosafety issue in xenotransplantation. Several techniques have been employed for the analysis of the PERV status in the animal donor and for the assessment of PERV transmission/infection in the xenograft recipient. In this chapter, methods to assess the expression of PERV and the potential for PERV transmission from a donor animal are described in addition to the identification of relevant loci within the porcine genome.PERV detection can be carried out using several techniques of which quantitative polymerase chain reaction (PCR) and RT-PCR are the most sensitive. However, other procedures can be employed such as detection of reverse transcriptase activity (i.e. viral replication) in the sample or immunostaining of the infected cells using an anti-PERV antibody. The PERV transmission assay has been described to identify the transmission phenotype of the pig donor, and subsequent risk from a donor. This assay can, therefore, direct the selection of the most suitable animal. Finally, it is important to determine the presence of critical PERV loci involved in transmission in the pig genome and compare between different animals. One of the methods for the analysis of these PERV integration sites is described.

Microbial safety in xenotransplantation

PURPOSE OF REVIEW

As clinical trials are in progress involving porcine islet cell transplantation, microbial safety remains a key issue. Therefore, in the context of pig-to-human xenotransplantation, we provide an overview of the recent progress in the studies of relevant viruses including well known problematic viruses, such as herpesviruses and porcine endogenous retroviruses (PERV) in addition to some emerging issues regarding other pathogens.

RECENT FINDINGS

The ability of herpesvirus to infect across species barriers is probably underestimated and requires monitoring and control of both xenograft donors and recipients for latent infection. Exclusion from donors and recipient monitoring for other exogenous pathogens including newly identified Parvovirus-4 are warranted. The availability of the swine whole genome sequence may help to characterize and select donor animals with less PERV infectivity. Rigorous PERV monitoring in both clinical and preclinical xenotransplantation experiments must be included in clinical protocols.

SUMMARY

A wide range of pathogens, both viruses and bacteria, pose potential safety problems in xenotransplantation, highlighting the importance of prescreening of the donor animals, and careful monitoring and follow-up of the patients.

Torque teno sus virus (TTSuV) in cell cultures and trypsin

Torque teno sus virus (TTSuV), a member of the family Anelloviridae, is a single-stranded, circular DNA virus, widely distributed in swine populations. Presently, two TTSuV genogroups are recognized: Torque teno sus virus 1 (TTSuV1) and Torque teno sus virus 2 (TTSuV2). TTSuV genomes have been found in commercial vaccines for swine, enzyme preparations and other drugs containing components of porcine origin. However, no studies have been made looking for TTSuV in cell cultures. In the present study, a search for TTSuV genomes was carried out in cell culture lineages, in sera used as supplement for cell culture media as well as in trypsin used for cell disaggregation. DNA obtained from twenty-five cell lineages (ten from cultures in routine multiplication and fifteen from frozen ampoules), nine samples of sera used in cell culture media and five batches of trypsin were examined for the presence of TTSuV DNA. Fifteen cell lineages, originated from thirteen different species contained amplifiable TTSuV genomes, including an ampoule with a cell lineage frozen in 1985. Three cell lineages of swine origin were co-infected with both TTSuV1 and TTSuV2. One batch of trypsin contained two distinct TTSuV1 plus one TTSuV2 genome, suggesting that this might have been the source of contamination, as supported by phylogenetic analyses of sequenced amplicons. Samples of fetal bovine and calf sera used in cell culture media did not contain amplifiable TTSuV DNA. This is the first report on the presence of TTSuV as contaminants in cell lineages. In addition, detection of the viral genome in an ampoule frozen in 1985 provides evidence that TTSuV contamination is not a recent event. These findings highlight the risks of TTSuV contamination in cell cultures, what may be source for contamination of biological products or compromise results of studies involving in vitro multiplied cells.

Suboptimal porcine endogenous retrovirus infection in non-human primate cells: implication for preclinical xenotransplantation

Background

Porcine endogenous retrovirus (PERV) poses a potential risk of zoonotic infection in xenotransplantation. Preclinical transplantation trials using non-human primates (NHP) as recipients of porcine xenografts present the opportunity to assess the zoonosis risk in vivo. However, PERV poorly infects NHP cells for unclear reasons and therefore NHP may represent a suboptimal animal model to assess the risk of PERV zoonoses. We investigated the mechanism responsible for the low efficiency of PERV-A infection in NHP cells.

Principal Findings

Two steps, cell entry and exit, were inefficient for the replication of high-titer, human-tropic A/C recombinant PERV. A restriction factor, tetherin, is likely to be responsible for the block to matured virion release, supported by the correlation between the levels of inhibition and tetherin expression. In rhesus macaque, cynomolgus macaque and baboon the main receptor for PERV entry, PERV-A receptor 1 (PAR-1), was found to be genetically deficient: PAR-1 genes in these species encode serine at amino acid 109 in place of the leucine in human PAR-1. This genetic defect inevitably impacts in vivo sensitivity to PERV infection of these species. In contrast, African green monkey (AGM) PAR-1 is functional, but PERV infection is still poor. Although the mechanism is unclear, tunicamycin treatment, which removes N-glycosylated sugar chains, increases PERV infection, suggesting a possible role for the glycosylation of the receptors.

Conclusions

Since cynomolgus macaque and baboon, species often used in pig-to-NHP xenotransplantation experiments, have a defective PAR-1, they hardly represent an ideal animal model to assess the risk of PERV transmission in xenotransplantation. Alternatively, NHP species, like AGM, whose both PARs are functional may represent a better model than baboon and cynomolgus macaque for PERV zoonosis in vivo studies.

Can human viruses infect porcine xenografts?

Xenotransplantation exposes the recipient to known and unknown pathogens of the donor pig (donor-derived xenosis). A major effort has been undertaken to minimize the risk of transmission from the donor using specialized breeding techniques. With the exception of endogenous retroviruses and porcine lymphotropic herpesvirus, exclusion of known pathogens was successful and has eliminated a majority of donor pathogens. In the recipient, enhanced replication of many pathogens will be stimulated by the immune responses induced by transplantation and by the immune suppression used to prevent graft rejection. Infection of the graft may occur with unpredictable consequences due to the cross-species situation. Infectivity may be decreased as entry or replication is altered by missing receptors or inability to use the cellular machinery. Replication of organisms in the xenograft and the inability of the human host to respond to human pathogens in the context of a xenograft infection due to immune suppression, or the presentation of such pathogens in the context of pig instead of human major histocompatibility complex (MHC) could impair control of such infections. Recent data suggest that some human herpesviruses infections, such as human cytomegalovirus, may infect porcine tissue and are associated with a pro-inflammatory phenotype. This review focuses on human or recipient-derived pathogens and their potential harmful role in xenograft infection.

Detection of a gammaretrovirus, XMRV, in the human population: open questions and implications for xenotransplantation

XMRV (xenotropic murine leukaemia virus-related virus) is a gammaretrovirus that has been detected in human patients with prostate carcinoma, chronic fatigue syndrome (CFS) and also in a small percentage of clinically healthy individuals. It is not yet clear whether the distribution of this virus is primarily limited to the USA or whether it is causally associated with human disease. If future investigations confirm a broad distribution of XMRV and its association with disease, this would have an impact on xenotransplantation of porcine tissues and organs. Xenotransplantation is currently being developed to compensate for the increasing shortage of human material for the treatment of tissue and organ failure but could result in the transmission of porcine pathogens. Maintenance of pathogen-free donor animals will dramatically reduce this risk, but some of the porcine endogenous retroviruses (PERVs) found in the genome of all pigs, can produce infectious virus and infect cultured human cells. PERVs are closely related to XMRV so it is critical to develop tests that discriminate between them. Since recombination can occur between viruses, and recombinants can exhibit synergism, recipients should be tested for XMRV before xenotransplantation.