Long-term absence of porcine endogenous retrovirus infection in chronically immunosuppressed patients after treatment with the porcine cell-based Academic Medical Center bioartificial liver

Quality assurance in corneal transplants: Donor cornea assessment and oversight

The cornea is the most frequently transplanted human tissue, and corneal transplantation represents the most successful allogeneic transplant worldwide. In order to obtain good surgical outcome and visual rehabilitation and to ensure the safety of the recipient, accurate screening of donors and donor tissues is necessary throughout the process. This mitigates the risks of transmission to the recipient, including infectious diseases and environmental contaminants, and ensures high optical and functional quality of the tissues. The process can be divided into 3 stages: (1) donor evaluation and selection before tissue harvest performed by the retrieval team, (2) tissue analysis during the storage phase conducted by the eye bank technicians after the retrieval, and, (3) tissue quality checks undertaken by the surgeons in the operating room before transplantation. Although process improvements over the years have greatly enhanced safety, quality, and outcome of the corneal transplants, a lack of standardization between centers during certain phases of the process still remains, and may impact on the quality and number of transplanted corneas. Here we detail the donor screening process for the retrieval teams, eye bank operators. and ophthalmic surgeons and examine the limitations associated with each of these stages.

Pig-to-human heart xenotransplantation in two recently deceased human recipients

Genetically modified xenografts are one of the most promising solutions to the discrepancy between the numbers of available human organs for transplantation and potential recipients. To date, a porcine heart has been implanted into only one human recipient. Here, using 10-gene-edited pigs, we transplanted porcine hearts into two brain-dead human recipients and monitored xenograft function, hemodynamics and systemic responses over the course of 66 hours. Although both xenografts demonstrated excellent cardiac function immediately after transplantation and continued to function for the duration of the study, cardiac function declined postoperatively in one case, attributed to a size mismatch between the donor pig and the recipient. For both hearts, we confirmed transgene expression and found no evidence of cellular or antibody-mediated rejection, as assessed using histology, flow cytometry and a cytotoxic crossmatch assay. Moreover, we found no evidence of zoonotic transmission from the donor pigs to the human recipients. While substantial additional work will be needed to advance this technology to human trials, these results indicate that pig-to-human heart xenotransplantation can be performed successfully without hyperacute rejection or zoonosis.

Preclinical Experience of the Mayo Spheroid Reservoir Bioartificial Liver (SRBAL) in Management of Acute Liver Failure

The Spheroid Reservoir Bioartificial Liver (SRBAL) is an innovative treatment option for acute liver failure (ALF). This extracorporeal support device, which provides detoxification and other liver functions using high-density culture of porcine hepatocyte spheroids, has been reported in three randomized large animal studies. A meta-analysis of these three preclinical studies was performed to establish efficacy of SRBAL treatment in terms of survival benefit and neuroprotective effect. The studies included two hepatotoxic drug models of ALF (D-galactosamine, α-amanitin/lipopolysaccharide) or a liver resection model (85% hepatectomy) in pigs or monkeys. The SRBAL treatment was started in three different settings starting at 12 h, 24 h or 48 h after induction of ALF; comparisons were made with two similar control groups in each model. SRBAL therapy was associated with significant survival and neuroprotective benefits in all three animal models of ALF. The benefits of therapy were dose dependent with the most effective configuration of SRBAL being continuous treatment of 24 h duration and dose of 200 g of porcine hepatic spheroids. Future clinical testing of SRBAL in patients with ALF appears warranted.

Long-term efficacy and safety of porcine islet macrobeads in nonimmunosuppressed diabetic cynomolgus macaques

Although human islet transplantation has proven to provide clinical benefits, especially the near complete amelioration of hypoglycemia, the supply of human islets is limited and insufficient to meet the needs of all people that could benefit from islet transplantation. Porcine islets, secreting insulin nearly identical to that of human insulin, have been proposed as a viable supply of unlimited islets. Further, encapsulation of the porcine islets has been shown to reduce or eliminate the use of immunosuppressive therapy that would be required to prevent rejection of the foreign islet tissue. The goal of the current study was to determine the long-term safety and efficacy of agarose encapsulated porcine islets (macrobeads) in diabetic cynomolgus macaques, in a study emulating a proposed IND trial in which daily exogenous insulin therapy would be reduced by 50% with no loss of glucose regulation. Four of six animals implanted with macrobeads demonstrated ≥ 30% reduction in insulin requirements in year 1 of follow-up. Animals were followed for 2, 3.5, and 7.4 years with no serious adverse events, mortality or evidence of pathogen transmission. This study supports the continued pursuit of encapsulated porcine islet therapy as a promising treatment option for diabetes mellitus.

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.

Immunological response in cynomolgus macaques to porcine α-1,3 galactosyltransferase knockout viable skin xenotransplants-A pre-clinical study

BACKGROUND

Allogeneic skin recovered from human deceased donors (HDD) has been a mainstay interim treatment for severe burns, but unfortunately risk of infectious disease and availability limitations exist. Genetically engineered ɑ-1,3 galactosyltransferase knockout (GalT-KO) porcine source animals for viable skin xenotransplants may provide a promising clinical alternative.

METHODS

Four cynomolgus macaque recipients received full-thickness surgical wounds to model the defects arising from excision of full-thickness burn injury and were treated with biologically active skin xenotransplants derived from GalT-KO, Designated Pathogen Free (DPF) miniature swine. Evaluations were conducted for safety, tolerability, and recipient immunological response.

RESULTS

All skin xenotransplants demonstrated prolonged survival, vascularity, and persistent dermal adhesion until the study endpoint at post-operative day 30. No adverse outcomes were observed during the study. Varying levels of epidermolysis coincided with histologic detection of CD4+ and CD8+ T cells, and other cellular infiltrates in the epidermis. Recipient sera IgM and IgG demonstrated significant antibody immune response to non-α-1,3-galactose porcine xenoantigens. Separately, specific wound healing mediators were quantified. Neither porcine cell migration nor PERV were detected in circulation or any visceral organs.

CONCLUSIONS

These results provide a detailed analysis of vital skin xenotransplants utilizing a non-human primate model to predict the anticipated immunological response of human patients. The lack of adverse rejection even in the presence of elevated Ig indicates this is a prospective therapeutic option. The findings reported here directly supported regulatory clearance for a first-in-man, Phase I xenotransplantation clinical trial.

End-stage liver failure: filling the treatment gap at the intensive care unit

End-stage liver failure is a condition of collapsing liver function with mortality rates up to 80. Liver transplantation is the only lifesaving therapy. There is an unmet need for therapy to extend the waiting time for liver transplantation or regeneration of the native liver. Here we review the state-of-the-art of non-cell based and cell-based artificial liver support systems, cell transplantation and plasma exchange, with the first therapy relying on detoxification, while the others aim to correct also other failing liver functions and/or modulate the immune response. Meta-analyses on the effect of non-cell based systems show contradictory outcomes for different types of albumin purification devices. For bioartificial livers proof of concept has been shown in animals with liver failure. However, large clinical trials with two different systems did not show a survival benefit. Two clinical trials with plasma exchange and one with transplantation of mesenchymal stem cells showed positive outcomes on survival. Detoxification therapies lack adequacy for most patients. Correction of additional liver functions, and also modulation of the immune system hold promise for future therapy of liver failure.

Bioartificial liver support systems for acute liver failure: A systematic review and meta-analysis of the clinical and preclinical literature

BACKGROUND

Acute liver failure (ALF) has a high mortality varying from 80% to 85% with rapid progress in multi-organ system failure. Bioartificial liver (BAL) support systems have the potential to provide temporary support to bridge patients with ALF to liver transplantation or spontaneous recovery. In the past decades, several BAL support systems have been conducted in clinical trials. More recently, concerns have been raised on the renovation of high-quality cell sources and configuration of BAL support systems to provide more benefits to ALF models in preclinical experiments.

AIM

To investigate the characteristics of studies about BAL support systems for ALF, and to evaluate their effects on mortality.

METHODS

Eligible clinical trials and preclinical experiments on large animals were identified on Cochrane Library, PubMed, and EMbase up to March 6, 2019. Two reviewers independently extracted the necessary information, including key BAL indicators, survival and indicating outcomes, and adverse events during treatment. Descriptive analysis was used to identify the characteristics of the included studies, and a meta-analysis including only randomized controlled trial (RCT) studies was done to calculate the overall effect of BAL on mortality among humans and large animals, respectively.

RESULTS

Of the 30 selected studies, 18 were clinical trials and 12 were preclinical experiments. The meta-analysis result suggested that BAL might reduce mortality in ALF in large animals, probably due to the recent improvement of BAL, including the type, cell source, cell mass, and bioreactor, but seemed ineffective for humans [BAL vs control: relative risk (95% confidence interval), 0.27 (0.12-0.62) for animals and 0.72 (0.48-1.08) for humans]. Liver and renal functions, hematologic and coagulative parameters, encephalopathy index, and neurological indicators seemed to improve after BAL, with neither meaningful adverse events nor porcine endogenous retrovirus infection.

CONCLUSION

BAL may reduce the mortality of ALF by bridging the gap between preclinical experiments and clinical trials. Clinical trials using improved BAL must be designed scientifically and conducted in the future to provide evidence for transformation.

Analysis of Protein Expression in Human Cells Cocultured with Porcine Peripheral Blood Mononuclear Cells

OBJECTIVE

Porcine endogenous retroviruses (PERV) involved in pig to human xenotransplantation have raised great concerns because of their ubiquitous nature in pigs and their ability of infecting human cells in vitro. Although no significant cytopathic effect attributed to PERV was evident on PERV-infected human embryonic kidney 293 (HEK293) cells, we did proteomic analysis to investigate the differences of protein profile in order to further characterize the effect of PERV infection.

METHODS

HEK293 cells were cocultured with porcine peripheral blood mononuclear cells (PBMCs). Protein profiles of PERV-infected and -noninfected HEK293 cells were analyzed by two-dimensional gel electrophoresis (2-DE). Protein spots with at least 1.5-fold alteration were identified by high-definition mass spectrometry (HDMS) analysis. Then real-time RT-PCR and Western blotting were performed to validate the proteomic results.

RESULTS

Differential analysis of PERV-infected and -noninfected HEK293 cells by 2-DE revealed ten differentially regulated proteins. The proteins identified by HDMS were involved in various cellular pathways including signal transduction, cell apoptosis, and protein synthesis.

CONCLUSION

The results of this study revealed differentially expressed proteins in HEK293 cells cocultured with porcine PBMCs and implied that these changes were probably induced by PERV infection. These results provide clues and potential links to understanding the molecular effect of the infection by human-tropic PERV.

Porcine Endogenous Retrovirus (PERV) - Molecular Structure and Replication Strategy in the Context of Retroviral Infection Risk of Human Cells

The xenotransplantation of porcine tissues may help overcome the shortage of human organs for transplantation. However, there are some concerns about recipient safety because the risk of porcine endogenous retrovirus (PERV) transmission to human cells remains unknown. Although, to date, no PERV infections have been noted in vivo, the possibility of such infections has been confirmed in vitro. Better understanding of the structure and replication cycle of PERVs is a prerequisite for determining the risk of infection and planning PERV-detection strategies. This review presents the current state of knowledge about the structure and replication cycle of PERVs in the context of retroviral infection risk.

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.

Three cysteine residues of SLC52A1, a receptor for the porcine endogenous retrovirus-A (PERV-A), play a critical role in cell surface expression and infectivity

Porcine endogenous retrovirus-A (PERV-A), a gammaretrovirus, infects human cells in vitro, thus raising the potential risk of cross-species transmission in xenotransplantation. Two members of the solute carrier family 52 (SLC52A1 and SLC52A2) are PERV-A receptors. Site-directed mutagenesis of the cDNA encoding SLC52A1 identified that only one of two putative glycosylation signals is occupied by glycans. In addition, we showed that glycosylation of SLC52A1 is not necessary for PERV-A receptor function. We also identified that at a minimum, three cysteine residues are sufficient for SLC52A1 cell surface expression. Mutation of cysteine at position 365 and either of the two cysteine residues in the C-terminal tail at positions 442 or 446 reduced SLC52A1 surface expression and PERV-A infection suggesting that these residues may contribute to overall structural stability and receptor function. Understanding interactions between PERV-A and its cellular receptor may provide novel strategies to prevent zoonotic infection in the setting of xenotransplantation.

[Renal transplantation in 2046: Future and perspectives]

OBJECTIVES

To report major findings that may build the future of kidney transplantation.

MATERIAL AND METHODS

Relevant publications were identified through Medline (http://www.ncbi.nlm.nih.gov) and Embase (http://www.embase.com) database from 1960 to 2016 using the following keywords, in association, "bio-engineering; heterotransplantation; immunomodulation; kidney; regenerative medicine; xenotransplantation". Articles were selected according to methods, language of publication and relevance. A total of 5621 articles were identified including 2264 for xenotransplantation, 1058 for regenerative medicine and 2299 for immunomodulation; after careful selection, 86 publications were eligible for our review.

RESULTS

Despite genetic constructs, xenotransplantation faces the inevitable obstacle of species barrier. Uncertainty regarding xenograft acceptance by recipients as well as ethical considerations due to the debatable utilization of animal lives, are major limits for its future. Regenerative medicine and tridimensional bioprinting allow successful implantation of organs. Bioengineering, using decellularized tissue matrices or synthetic scaffold, seeded with pluripotent cells and assembled using bioreactors, provide exciting results but remain far for reconstituting renal complexity and vascular patency. Immune tolerance may be achieved through a tough initial T-cell depletion or a combined haplo-identical bone marrow transplant leading to lymphohematopoietic chimerism.

CONCLUSION

Current researches aim to increase the pool of organs available for transplantation (xenotransplants and bio-artificial kidneys) and to increase allograft survival through the induction of immune tolerance. Reported results suggest the onset of a thrilling new era for renal transplantation providing end-stage renal disease-patients with an improved survival and quality of life.

New Phase of Growth for Xenogeneic-Based Bioartificial Organs

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

No transmission of porcine endogenous retrovirus in an acute liver failure model treated by a novel hybrid bioartificial liver containing porcine hepatocytes

BACKGROUND

A novel hybrid bioartificial liver (HBAL) was constructed using an anionic resin adsorption column and a multi-layer flat-plate bioreactor containing porcine hepatocytes co-cultured with bone marrow mesenchymal stem cells (MSCs). This study aimed to evaluate the microbiological safety of the HBAL by detecting the transmission of porcine endogenous retroviruses (PERVs) into canines with acute liver failure (ALF) undergoing HBAL.

METHODS

Eight dogs with ALF received a 6-hour HBAL treatment on the first day after the modeling by D-galactosamine administration. The plasma in the HBAL and the whole blood in the dogs were collected for PERV detection at regular intervals until one year later when the dogs were sacrificed to retrieve the tissues of several organs for immunohistochemistry and Western blotting for the investigation of PERV capsid protein gag p30 in the tissue. Furthermore, HEK293 cells were incubated to determine the in vitro infectivity.

RESULTS

PERV RNA and reverse transcriptase activity were observed in the plasma of circuit 3, suggesting that PERV particles released in circuit 3. No positive PERV RNA and reverse transcriptase activity were detected in other plasma. No HEK293 cells were infected by the plasma in vitro. In addition, all PERV-related analyses in peripheral blood mononuclear cells and tissues were negative.

CONCLUSION

No transmission of PERVs into ALF canines suggested a reliable microbiological safety of HBAL based on porcine hepatocytes.

Screening pigs for xenotransplantation: expression of porcine endogenous retroviruses in transgenic pig skin

Pigs seem to be the answer to worldwide organ donor shortage. Porcine skin may also be applied as a dressing for severe burns. Genetic modifications of donor animals enable reduction of immune response, which prolongs xenograft survival as temporary biological dressing and allows achieving resistance against xenograft rejection. The risk posed by porcine endogenous retroviruses (PERVs) cannot be eliminated by breeding animals under specific-pathogen-free conditions and so all recipients of porcine graft will be exposed to PERVs. Therefore our study has been focused on the assessment of PERV DNA and mRNA level in skin samples of transgenic pigs generated for xenotransplantation. Porcine skin fragments were obtained from 3- to 6-month-old non-transgenic and transgenic Polish Landrace pigs. Transgenic pigs were produced by pronuclear DNA microinjection and were developed to express the human α-galactosidase and the human α-1,2-fucosyltransferase gene. The copy numbers of PERV DNA and RNA were evaluated using real-time Q-PCR and QRT-PCR. Comparative analysis of all PERV subtypes revealed that PERV-A is the main subtype of PERVs in analyzed skin samples. There was no significantly different copy number of PERV-A, PERV-B and PERV-C between non-transgenic pigs, pigs with the human α-galactosidase and pigs expressing the human α-1,2-fucosyltransferase gene, except of PERV-C DNA. It brings the conclusion, that transgenesis process exerts no influence on PERVs transinfection. That is another step forward in the development of pig skin xenografts as burn wounds dressing.

Induction of PERV antigen in porcine peripheral blood mononuclear cells by human herpesvirus 1

BACKGROUND

Xenotransplantation represents one of alternative candidates for allotransplantation due to the chronic shortage of suitable human tissues; however, many obstacles remain. Expression and release of endogenous retroviral antigens by porcine cells after transplantation may evoke adverse immune responses in human subjects. Here, we examined whether human herpesvirus 1 (HHV-1) could induce the production of porcine endogenous retrovirus (PERV) antigens in porcine peripheral blood mononuclear cells (PBMCs).

METHODS

Porcine PBMCs were infected with HHV-1 and examined for the production of PERV Gag protein and HHV-1 using antigen-capture ELISA and quantitative real-time polymerase chain reaction (PCR), respectively.

RESULTS

HHV-1 infection resulted in a 1.7- to 33.2-fold induction of PERV Gag relative to mock infection controls, compared to a 2.9- to 12.9-fold induction following treatment with PMA. Expression of PERV Gag was detected in porcine PBMCs and PK-15 cells after HHV-1 infection by double immunofluorescence staining of PERV Gag and HHV-1 antigen. The viability of HHV-1-infected porcine PBMCs was significantly lower than that of mock-infected cells. The HHV-1 level in the culture supernatant increased 5.2-fold relative to controls 24-h post-infection, indicative of active replication within these cells; decreased levels of HHV-1 were detected 72-h post-infection.

CONCLUSIONS

These results suggest that HHV-1 may be capable of infecting transplanted porcine cells, resulting in strong direct induction of PERV antigen.

Xenozoonoses

Immunological and technical advances have led to tremendous increases in the number of people potentially able to benefit from allotransplantation. Ironically, it is the success of the field that has led to a renewed interest in xenotransplantation during the past several decades. To a large part, this has occurred because of the great scarcity of human organ and tissue donors. However, it has expanded to include the use of cells from animals into humans such as porcine islet cells for diabetes or extracorporeal perfusion of human blood through animal organs or cells. Similar to allotransplantation, issues regarding transmission of infections from the graft to the human recipient were brought up for consideration with these procedures in the 1990s (Michaels and Simmons, 1994; Chapman et al., 1995; Hammel et al., 1998; Fishman et al., 1998). A risk for infection exists with the use of any biologic agent regardless of whether it is from a human or an animal source. Accordingly, transmission of infections from human organs, tissues, or cells is a well-recognized cause of disease after allotransplantation (Ison and Grossi, 2013; Green and Michaels, 2012). As the human graft shortage continues, newer cellular therapies are explored. Thus, attention continues to be given to the potential use of xenogeneic organs, tissues, or cells for human maladies through xenotransplantation. The potential for novel zoonotic infections to emerge because of xenotransplantation (xenozoonoses or xenosis) led to a debate on whether the field should be permitted to progress. This chapter reviews the issues of xenotransplantation related to infections from animals to humans. Lessons learned from infections with prior nonhuman primate xenotransplantation and human allotransplantation are used to help inform about risks with newer xenogeneic procedures. In addition, information on known zoonoses is reviewed to better develop constructs to decrease the hazard of infection with these novel procedures.

Detection of porcine endogenous retrovirus in xenotransplantation

Xenotransplantation can provide a virtually limitless supply of cells, tissues and organs for a variety of therapeutic procedures. Cells and tissues for use in human transplantation procedures could be supplied using material taken from pigs. However, there is a potential risk of transmission of porcine infectious agents, including porcine endogenous retroviruses (PERVs), to a novel human host, with as yet unknown consequences. Three subtypes of PERV have been identified, of which both PERV-A and PERV-B have the ability to infect human cells in vitro. The third subtype, PERV-C, does not show this ability. Recombinant PERV-A/C forms have demonstrated infectivity in human cell culture. Monitoring in xenotransplantation should comprise screening of the source pig herd (PERV-A and PERV-B level expression assessment, PERV-C detection) and screening of recipients (differentiation between PERV transmission and chimerism). The detection of PERVs includes analyses of both DNA and RNA (PCR and RT-PCR), quantitative determination of the level of PERV nucleic acids (real-time PCR and real-time RT-PCR), assessment of reverse transcriptase (RT) activity (RT assays) and viral and recipient protein detection (immunological methods). In summary, all available methods should be used in monitoring of PERVs in xenotransplantation, and caution should be exercised at all stages of monitoring. Such monitoring has enormous significance for eliminating the possibility of transmission of PERV infection, thus contributing to higher levels of safety in xenotransplantation.

No evidence of viral transmission following long-term implantation of agarose encapsulated porcine islets in diabetic dogs

We have previously described the use of a double coated agarose-agarose porcine islet macrobead for the treatment of type I diabetes mellitus. In the current study, the long-term viral safety of macrobead implantation into pancreatectomized diabetic dogs treated with pravastatin (n = 3) was assessed while 2 dogs served as nonimplanted controls. A more gradual return to preimplant insulin requirements occurred after a 2nd implant procedure (days 148, 189, and >652) when compared to a first macrobead implantation (days 9, 21, and 21) in all macrobead implanted animals. In all three implanted dogs, porcine C-peptide was detected in the blood for at least 10 days following the first implant and for at least 26 days following the second implant. C-peptide was also present in the peritoneal fluid of all three implanted dogs at 6 months after 2nd implant and in 2 of 3 dogs at necropsy. Prescreening results of islet macrobeads and culture media prior to transplantation were negative for 13 viruses. No evidence of PERV or other viral transmission was found throughout the study. This study demonstrates that the long-term (2.4 years) implantation of agarose-agarose encapsulated porcine islets is a safe procedure in a large animal model of type I diabetes mellitus.

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.

Long-term IgG response to porcine Neu5Gc antigens without transmission of PERV in burn patients treated with porcine skin xenografts

Acellular materials of xenogenic origin are used worldwide as xenografts, and phase I trials of viable pig pancreatic islets are currently being performed. However, limited information is available on transmission of porcine endogenous retrovirus (PERV) after xenotransplantation and on the long-term immune response of recipients to xenoantigens. We analyzed the blood of burn patients who had received living pig-skin dressings for up to 8 wk for the presence of PERV as well as for the level and nature of their long term (maximum, 34 y) immune response against pig Ags. Although no evidence of PERV genomic material or anti-PERV Ab response was found, we observed a moderate increase in anti-αGal Abs and a high and sustained anti-non-αGal IgG response in those patients. Abs against the nonhuman sialic acid Neu5Gc constituted the anti-non-αGal response with the recognition pattern on a sialoglycan array differing from that of burn patients treated without pig skin. These data suggest that anti-Neu5Gc Abs represent a barrier for long-term acceptance of porcine xenografts. Because anti-Neu5Gc Abs can promote chronic inflammation, the long-term safety of living and acellular pig tissue implants in recipients warrants further evaluation.

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.

Advances in cell sources of hepatocytes for bioartificial liver

BACKGROUND

Orthotopic liver transplantation (OLT) is the most effective therapy for liver failure. However, OLT is severely limited by the shortage of liver donors. Bioartificial liver (BAL) shows great potential as an alternative therapy for liver failure. In recent years, progress has been made in BAL regarding genetically engineered cell lines, immortalized human hepatocytes, methods for preserving the phenotype of primary human hepatocytes, and other functional hepatocytes derived from stem cells.

DATA SOURCES

A systematic search of PubMed and ISI Web of Science was performed to identify relevant studies in English language literature using the key words such as liver failure, bioartificial liver, hepatocyte, stem cells, differentiation, and immortalization. More than 200 articles related to the cell sources of hepatocyte in BAL were systematically reviewed.

RESULTS

Methods for preserving the phenotype of primary human hepatocytes have been successfully developed. Many genetically engineered cell lines and immortalized human hepatocytes have also been established. Among these cell lines, the incorporation of BAL with GS-HepG2 cells or alginate-encapsulated HepG2 cells could prolong the survival time and improve pathophysiological parameters in an animal model of liver failure. The cBAL111 cells were evaluated using the AMC-BAL bioreactor, which could eliminate ammonia and lidocaine, and produce albumin. Importantly, BAL loading with HepLi-4 cells could significantly improve the blood biochemical parameters, and prolong the survival time in pigs with liver failure. Other functional hepatocytes differentiated from stem cells, such as human liver progenitor cells, have been successfully achieved.

CONCLUSIONS

Aside from genetically modified liver cell lines and immortalized human hepatocytes, other functional hepatocytes derived from stem cells show great potential as cell sources for BAL. BAL with safe and effective liver cells may be achieved for clinical liver failure in the near future.

Microbiological safety of a novel bio-artificial liver support system based on porcine hepatocytes: a experimental study

Background

Our institute has developed a novel bio-artificial liver (BAL) support system, based on a multi-layer radial-flow bioreactor carrying porcine hepatocytes and mesenchymal stem cells. It has been shown that porcine hepatocytes are capable of carrying infectious porcine endogenous retroviruses (PERVs) into human cells, thus the microbiological safety of any such system must be confirmed before clinical trials can be performed. In this study, we focused on assessing the status of PERV infection in beagles treated with the novel BAL.

Methods

Five normal beagles were treated with the novel BAL for 6 hours. The study was conducted for 6 months, during which plasma was collected from the BAL and whole blood from the beagles at regular intervals. DNA and RNA in both the collected peripheral blood mononuclear cells (PBMCs) and plasma samples were extracted for conventional PCR and reverse transcriptase (RT)-PCR with PERV-specific primers and the porcine-specific primer Sus scrofa cytochrome B. Meanwhile, the RT activity and the in vitro infectivity of the plasma were measured.

Results

Positive PERV RNA and RT activity were detected only in the plasma samples taken from the third circuit of the BAL system. All other samples including PBMCs and other plasma samples were negative for PERV RNA, PERV DNA, and RT activity. In the in vitro infection experiment, no infection was found in HEK293 cells treated with plasma.

Conclusions

No infective PERV was detected in the experimental animals, thus the novel BAL had a reliable microbiological safety profile.

Factors Influencing the Transfer of Porcine Endogenous Retroviruses across the Membrane in Bioartificial Livers

Objectives:

to investigate the factors influencing the transfer of porcine endogenous retroviruses (PERVs) across the membrane in a new bioartificial liver (BAL).

Methods:

A new BAL containing 2 circuits was constructed using plasma component separators with membrane pore sizes of 10 nm, 20 nm, 30 nm, and 35 nm, or a plasma filter with a membrane pore size of 500 nm. Cocultured cells of porcine hepatocytes and mesenchymal stem cells or single porcine hepatocytes were incubated in the bioreactors, and the BAL worked for 72 hours, with supernatant samples in internal and external circuits collected every 12 hours. PERV RNA, reverse transcriptase (RT) activity, and in vitro infectivity of the supernatant were detected.

Results:

With the plasma filters, the results of PERV detection were the same in both circuits. With plasma component separators, PERV RNA was found in the external circuits, but no positive RT activity or HEK293 cell infection was found. The time at which the PERV RNA was first detected varied with the pore size of membrane; the larger the membrane pore size was, the earlier the RNA was detected. The PERV RNA level in the external circuits was reduced significantly compared with that in the internal circuits at any detecting time.

Conclusions:

The plasma component separators with membrane pore size ≤35 nm could significantly reduce the passage of infectious PERVs. And the membrane pore size, the treatment duration, and the viral level in the internal circuit were potential factors influencing the transfer of PERVs across the membrane in a BAL. In addition, a low risk of PERV transmission from porcine hepatocytes to human cells was found.

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.

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.

Xenotransplantation: an overview of the field

Xenotransplantation, the transplantation of cells, tissues, or organs between different species, has the potential to overcome the current shortage of human organs and tissues for transplantation. In the last decade, the progress made in the field is remarkable, suggesting that clinical xenotransplantation procedures, particularly those involving cells, may become a reality in the not-too-distant future. However, several hurdles remain, mainly immunological barriers, physiological discrepancies, and safety issues, making xenotransplantion a complex and multidisciplinary discipline.