No evidence for infection of human cells with porcine endogenous retrovirus (PERV) after exposure to porcine fetal neuronal cells

Porcine endogenous retrovirus: classification, molecular structure, regulation, function, and potential risk in xenotransplantation

Xenotransplantation with porcine organs has been recognized as a promising solution to alleviate the shortage of organs for human transplantation. Porcine endogenous retrovirus (PERV), whose proviral DNAs are integrated in the genome of all pig breeds, is a main microbiological risk for xenotransplantation. Over the last decades, some advances on PERVs' studies have been achieved. Here, we reviewed the current progress of PERVs including the classification, molecular structure, regulation, function in immune system, and potential risk in xenotransplantation. We also discussed the problem of insufficient study on PERVs as well as the questions need to be answered in the future work.

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.

Genome-Wide PERV Inactivation in Pigs Using CRISPR/Cas9

The shortage of organs for transplantation is probably the biggest unmet medical need. A potential problem with the clinical use of porcine xenografts is the risk that porcine endogenous retroviruses (PERVs) could infect human cells. In the past, we determined the PERV copy number in the porcine kidney epithelial cell line PK15 and in primary fibroblasts. Using CRISPR-Cas9, we disrupted the catalytic center of pol, which is essential for virus replication. Next, we isolated cells in which 100% of the PERV elements had been inactivated. This method enables the possibility of eradicating PERVs in vitro for application to pig-to-human xenotransplantation. Here we describe the methodological bases of this work.

Establishment of gene-edited pigs expressing human blood-coagulation factor VII and albumin for bioartificial liver use

BACKGROUND AND AIM

Bioartificial livers (BALs) are considered as a solution to bridge patients with acute liver failure to liver transplantation or to assist in spontaneous recovery for patients with end-stage liver disease. Pig is the best donor of hepatocytes for BALs in clinical trials, because metabolic and detoxification function of its liver are close to human. However, using pig hepatocytes for BALs remains controversial for safety concern owing to nonhuman proteins secretion. Herein, we attempt to establish modified pigs expressing humanized liver proteins, blood-coagulation factor VII (F7), and albumin (ALB). These pigs should also be porcine endogenous retrovirus subtype C (PERV-C) free so that their ability of transmitting PERV to human could be diminished seriously.

METHODS

We devised both homology-dependent and independent knock-in approaches to insert a fusion of hF7 and hALB gene downstream the site of pig endogenous F7 promoter in pig fetal fibroblasts negative for PERV-C. The modified pigs were then generated through somatic cell nuclear transfer.

RESULTS

We obtained 14 and 10 cloned pigs by homology-dependent and independent approaches, respectively. Among them, 19 cloned pigs were with expected gene modification and 13 are alive to date. These modified pigs can successfully express hF7 and hALB in the liver and serum, and the expressed hF7 exhibits normal coagulation activity.

CONCLUSIONS

The gene-edited pigs expressing hF7 and hALB in the liver were generated successfully. We anticipate that our pigs could provide an alternative cell source for BALs as a promising treatment for patients with acute liver failure.

Detection of Pig Cells Harboring Porcine Endogenous Retroviruses in Non-Human Primate Bladder After Renal Xenotransplantation

Pigs are used as potential donor animals for xenotransplantation. However, porcine endogenous retrovirus (PERV), shown to infect both human and non-human primate (NHP) cells in vitro, presents a risk of transmission to humans in xenotransplantation. In this study, we analyzed PERV transmission in various organs after pig-to-NHP xenotransplantation. We utilized pig-to-NHP xenotransplant tissue samples obtained using two types of transgenic pigs from the National Institute of Animal Science (NIAS, Republic of Korea), and examined them for the existence of PERV genes in different organs via PCR and RT-PCR with specific primers. To determine PERV insertion into chromosomes, inverse PCR using PERV long terminal repeat (LTR) region-specific primers was conducted. The PERV gene was not detected in NHP organs in cardiac xenotransplantation but detected in NHP bladders in renal xenotransplantation. The insertion experiment confirmed that PERVs originate from porcine donor cells rather than integrated provirus in the NHP chromosome. We also demonstrate the presence of pig cells in the NHP bladder after renal xenotransplantation using specific-porcine mitochondrial DNA gene PCR. The PERV sequence was detected in the bladder of NHPs after renal xenotransplantation by porcine cell-microchimerism but did not integrate into the NHP chromosome.

Porcine to Human Heart Transplantation: Is Clinical Application Now Appropriate?

Cardiac xenotransplantation (CXTx) is a promising solution to the chronic shortage of donor hearts. Recent advancements in immune suppression have greatly improved the survival of heterotopic CXTx, now extended beyond 2 years, and life-supporting kidney XTx. Advances in donor genetic modification (B4GALNT2 and CMAH mutations) with proven Gal-deficient donors expressing human complement regulatory protein(s) have also accelerated, reducing donor pig organ antigenicity. These advances can now be combined and tested in life-supporting orthotopic preclinical studies in nonhuman primates and immunologically appropriate models confirming their efficacy and safety for a clinical CXTx program. Preclinical studies should also allow for organ rejection to develop xenospecific assays and therapies to reverse rejection. The complexity of future clinical CXTx presents a substantial and unique set of regulatory challenges which must be addressed to avoid delay; however, dependent on these prospective life-supporting preclinical studies in NHPs, it appears that the scientific path forward is well defined and the era of clinical CXTx is approaching.

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

PURPOSE OF REVIEW

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

RECENT FINDINGS

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

SUMMARY

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

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.

Genome-wide inactivation of porcine endogenous retroviruses (PERVs)

The shortage of organs for transplantation is a major barrier to the treatment of organ failure. Although porcine organs are considered promising, their use has been checked by concerns about the transmission of porcine endogenous retroviruses (PERVs) to humans. Here we describe the eradication of all PERVs in a porcine kidney epithelial cell line (PK15). We first determined the PK15 PERV copy number to be 62. Using CRISPR-Cas9, we disrupted all copies of the PERV pol gene and demonstrated a >1000-fold reduction in PERV transmission to human cells, using our engineered cells. Our study shows that CRISPR-Cas9 multiplexability can be as high as 62 and demonstrates the possibility that PERVs can be inactivated for clinical application of porcine-to-human xenotransplantation.

Immune modulation in xenotransplantation

The use of animals as donors of tissues and organs for xenotransplantations may help in meeting the increasing demand for organs for human transplantations. Clinical studies indicate that the domestic pig best satisfies the criteria of organ suitability for xenotransplantation. However, the considerable phylogenetic distance between humans and the pig causes tremendous immunological problems after transplantation, thus genetic modifications need to be introduced to the porcine genome, with the aim of reducing xenotransplant immunogenicity. Advances in genetic engineering have facilitated the incorporation of human genes regulating the complement into the porcine genome, knockout of the gene encoding the formation of the Gal antigen (α1,3-galactosyltransferase) or modification of surface proteins in donor cells. The next step is two-fold. Firstly, to inhibit processes of cell-mediated xenograft rejection, involving natural killer cells and macrophages. Secondly, to inhibit rejection caused by the incompatibility of proteins participating in the regulation of the coagulation system, which leads to a disruption of the equilibrium in pro- and anti-coagulant activity. Only a simultaneous incorporation of several gene constructs will make it possible to produce multitransgenic animals whose organs, when transplanted to human recipients, would be resistant to hyperacute and delayed xenograft rejection.

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.

Detection of nanolevel drug metabolites in an organotypic culture of primary human hepatocytes and porcine hepatocytes with special reference to a two-compartment model

The quantification of drug metabolites produced during drug metabolism is a growing concern for the pharmaceutical industry, regulatory agencies such as the US Food and Drug Administration, the European Medicines Agency, and others. As 70% of drugs are known reactive metabolites and have black box warnings, they are a major cause of drug-induced injury and lead to drug attrition in early or late clinical stages. According to a 2006 survey report of pharmaceutical companies, drug-induced liver injury was ranked first in terms of adverse events, and it remains the most common reason for restriction or withdrawal of a drug from the market by the Food and Drug Administration. Although there are many reasons underlying drug-induced liver injury, one of the most important is liver failure induced by drug metabolites. Generally, a drug produces metabolites that may bind to cellular molecules and trigger a toxicological effect, cause serious adverse drug reactions, or alter cellular functions. Experimental cellular models that attempt to qualify drug metabolites from cell cultures rely on human plasma and urine obtained from clinical trials and supernatant during early in vitro experiments. However, there is a lack of information about the quantification of drug metabolites inside human hepatocytes, where the drug is extensively metabolized. To overcome this limitation, we used the highly accepted, gold standard organotypic cellular model of primary human hepatocytes to investigate and quantify the parent drug, as well as drug metabolites inside human hepatocytes and outside human hepatocytes to evaluate the quantity of drug metabolites, which are assumed to have remained inside the primary human hepatocytes. We refer to this as a two-compartment model, where one compartment is supernatant compared with in vivo hepatic blood circulation, and the other is inside the hepatocyte cell compared with the inside of in vivo human liver. We detected the nanoconcentrations of all major metabolites (desmethyldiazepam, temazepam, and oxazepam) of the diazepam drug, both inside the cells (matrix) and outside the hepatocyte cells (supernatant) at different time points (primary human hepatocytes: 0, 1, 2, 4, 8, and 24 hours; primary porcine hepatocytes: 0, 1, 2, 5, and 24 hours) during biotransformation in an organotypic sandwich cellular model. Although it is difficult to detect tissue distribution of metabolites in humans, we strongly recommend testing in a two-compartment model of primary human hepatocytes, as nonhuman models may not reflect human metabolism. Preclinical drug screening assessment tests that use this two-compartment strategy may facilitate safer registration of new drug candidates.

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 released by a bioartificial liver infects primary human cells

BACKGROUND

Porcine endogenous retrovirus (PERV) remains a safety risk in pig-to-human xenotransplantation. There is no evidence of in vivo productive infection in humans because PERV is inactivated by human serum. However, PERV can infect human cell lines and human primary cells in vitro and inhibit human immune functions.

AIMS

We investigated the potential of primary porcine liver cells to transmit PERV to primary human cells in a bioreactor-based bioartificial liver (BAL).

METHODS

Primary human hepatocytes, endothelial cells and the human cell line HEK 293 were exposed to supernatants from BAL or from the porcine cell line PK-15. PERV polymerase-specific reverse-transcriptase polymerase chain reaction (RT-PCR) and PCR were used to investigate PERV transmission to human cells. An assay of RT activity was used to detect the presence of retrovirus in the supernatants of BAL, primary human hepatocytes and endothelial cells.

RESULTS

Primary human hepatocytes (hHep), endothelial cells and HEK 293 cells were reproducibly infected by PERV, originating from primary porcine liver cells within the BAL and from PK-15 cells. Infected cells were positive for PERV-specific DNA and RNA after 8-10 days on an average, and RT activity was detectable in the supernatants of infected hHep and HEK 293 cells.

CONCLUSION

A risk of PERV infection in human cells is documented in this study, indicating that short-term contact of primary porcine liver cell supernatants with primary human cells could result in PERV transmission.

Cell-based therapies for Parkinson's disease: past, present, and future

Parkinson's disease (PD) researchers have pioneered the use of cell-based therapies (CBTs) in the central nervous system. CBTs for PD were originally envisioned as a way to replace the dopaminergic nigral neurons lost with the disease. Several sources of catecholaminergic cells, including autografts of adrenal medulla and allografts or xenografts of mesencephalic fetal tissue, were successfully assessed in animal models, but their clinical translation has yielded poor results and much controversy. Recent breakthroughs on cell biology are helping to develop novel cell lines that could be used for regenerative medicine. Their future successful clinical application depends on identifying and solving the problems encountered in previous CBTs trials. In this review, we critically analyze past CBTs' clinical translation, the impact of the host in graft survival, and the role of preclinical studies and emerging new cell lines. We propose that the prediction of clinical results from preclinical studies requires experimental designs that allow blind data acquisition and statistical analysis, assessment of the therapy in models that parallel clinical conditions, looking for sources of complications or side effects, and limiting optimism bias when reporting outcomes.

Porcine endogenous retrovirus (PERV) and its transmission characteristics: a study of the New Zealand designated pathogen-free herd

Previously a strategy for monitoring of pigs intended for cell transplantation was developed and successfully applied to several representative herds in New Zealand. A designated pathogen-free (DPF) herd has been chosen as a good candidate for xenotransplantation. This herd has previously tested free of infectious agents relevant to xenotransplantation and we present here an in depth study of porcine endogenous retrovirus (PERV) transmission. A panel of assays that describes the constraints for the transmission of PERV has been suggested. It includes a) infectivity test in coculture of DPF pig primary cells with both human and pig target cell lines; b) RT activity in supernatant of stimulated primary cells from DPF pigs; c) viral load in donor's blood plasma; d) PERV proviral copy number in DPF pig genome; e) PERV class C prevalence in the herd and its recombination potential. There was no evidence of PERV transmission from DPF pig tissue to either pig or human cells. Additionally, there was no evidence of PERV RNA present in pig blood plasma. PERV copy number differs in individual pigs from as low as 3 copies to 30 copies and the presence of PERV-C varied between animals and breeds. In all DPF pigs tested, a specific locus for PERV-C potentially associated with the recombination of PERV in miniature swine was absent. Presented data on the PERV transmission allows us to classify the DPF potential donors as "null" or noninfectious pigs.

Cellular xenotransplantation

PURPOSE OF REVIEW

Expectations are high on cellular therapy. Being fundamental to elucidate organogenesis, it is unlikely that embryonic stem cells will be used for clinical purposes. Postembryonic stage, developing cells are, therefore, the front-runner for regenerative medicine. In addition to autologous cells, both allogeneic and xenogeneic cells are hypothetical candidates to treat specific diseases. This review summarizes the current knowledge on immunological and functional aspects of xeno(allo)-cellular transplantation for cardiomyopathy, diabetes, liver failure, neural diseases, and bone regeneration.

RECENT FINDINGS

Xenocellular transplantation is promising for tissue repair in immunologically privileged sites such as the central nervous system or nonvascularized tissues in which no or moderate immunosuppression is required. In vascularized organs, major immune responses are present when cells are transplanted without additional conditioning. Positive results from encapsulation methods that protect cells from the immune system should further stimulate preclinical research. Also, conditioning immunosuppression could be used to circumvent the initial immune response. Transgenic pigs cells are probably the best xenogeneic substitute for human application, although basic research on innate and noninnate immunity toward pig cells is still required.

SUMMARY

In several fields of medicine, cellular xenotransplantation is slowly emerging as a potential therapeutic tool.

Toward development and production of human T cells in swine for potential use in adoptive T cell immunotherapy

Immunotherapy and vaccination for cancer or infection are generally approached by administration of antigen or stimulation of antigen-presenting cells or both. These measures may fail if the treated individual lacks T cells specific for the immunogen(s). We tested another strategy-the generation of new T cells from hematopoietic stem cells that might be used for adoptive immunotherapy. To test this concept, we introduced T cell-depleted human bone marrow cells into fetal swine and tested the swine for human T cells at various times after birth. Human T cells were detected in the thymus and blood of the treated swine. These cells were generated de novo as they contained human T cell receptor excision circles not present in the T cell-depleted bone marrow. The human T cells were highly diverse and included novel specificities capable of responding to antigen presented by human antigen-presenting cells. Our findings constitute a first step in a new promising approach to immunotherapy in which tumor- or virus-specific T cell clones lacking in an individual might be generated in a surrogate host from hematopoietic stem cells of the individual to be treated.

Stem cells for enhancing recovery after stroke: a review

The potential application for stem cell therapy is vast, and development for use in ischaemic stroke is still in its infancy. Access to stem cells for research is contentious; however, stem cells are obtainable from both animal and human. Despite a limited understanding of their mechanisms of action, clinical trials assessing stem cells in human stroke have been performed. Trials are also underway evaluating haematopoietic precursors mobilised with granulocyte-colony stimulating factor, an approach offering an autologous means of administrating stem cells for therapeutic purposes. This review summarises current knowledge in regard to stem cells and their potential for helping improve recovery after stroke.

Identification of two distinct structural regions in a human porcine endogenous retrovirus receptor, HuPAR2, contributing to function for viral entry

BACKGROUND

Of the three subclasses of Porcine Endogenous Retrovirus (PERV), PERV-A is able to infect human cells via one of two receptors, HuPAR1 or HuPAR2. Characterizing the structure-function relationships of the two HuPAR receptors in PERV-A binding and entry is important in understanding receptor-mediated gammaretroviral entry and contributes to evaluating the risk of zoonosis in xenotransplantation.

RESULTS

Chimeras of the non-permissive murine PAR and the permissive HuPAR2, which scanned the entire molecule, revealed that the first 135 amino acids of HuPAR2 are critical for PERV-A entry. Within this critical region, eighteen single residue differences exist. Site-directed mutagenesis used to map single residues confirmed the previously identified L109 as a binding and infectivity determinant. In addition, we identified seven residues contributing to the efficiency of PERV-A entry without affecting envelope binding, located in multiple predicted structural motifs (intracellular, extracellular and transmembrane). We also show that expression of HuPAR2 in a non-permissive cell line results in an average 11-fold higher infectivity titer for PERV-A compared to equal expression of HuPAR1, although PERV-A envelope binding is similar. Chimeras between HuPAR-1 and -2 revealed that the region spanning amino acids 152-285 is responsible for the increase of HuPAR2. Fine mapping of this region revealed that the increased receptor function required the full sequence rather than one or more specific residues.

CONCLUSION

HuPAR2 has two distinct structural regions. In one region, a single residue determines binding; however, in both regions, multiple residues influence receptor function for PERV-A entry.

Absence of replication of porcine endogenous retrovirus and porcine lymphotropic herpesvirus type 1 with prolonged pig cell microchimerism after pig-to-baboon xenotransplantation

Porcine endogenous retrovirus (PERV), porcine cytomegalovirus (PCMV), and porcine lymphotropic herpesvirus (PLHV) are common porcine viruses that may be activated with immunosuppression for xenotransplantation. Studies of viral replication or transmission are possible due to prolonged survival of xenografts in baboon recipients from human decay-accelerating factor transgenic or alpha-1,3-galactosyltransferase gene knockout miniature swine. Ten baboons underwent xenotransplantation with transgenic pig organs. Graft survival was 32 to 179 days. Recipient serial samples of peripheral blood mononuclear cells (PBMC) and plasma were analyzed for PCMV, PERV, and PLHV-1 nucleic acids and viral replication using quantitative PCR assays. The PBMC contained PERV proviral DNA in 10 animals, PLHV-1 DNA in 6, and PCMV in 2. PERV RNA was not detected in any PBMC or serum samples. Plasma PLHV-1 DNA was detected in one animal. Pig cell microchimerism (pig major histocompatibility complex class I and pig mitochondrial cytochrome c oxidase subunit II sequences) was present in all recipients with detectable PERV or PLHV-1 (85.5%). Productive infection of PERV or PLHV-1 could not be demonstrated. The PLHV-1 viral load did not increase in serum over time, despite prolonged graft survival and pig cell microchimerism. There was no association of viral loads with the nature of exogenous immune suppression. In conclusion, PERV provirus and PLHV-1 DNA were detected in baboons following porcine xenotransplantation. Viral detection appeared to be due to persistent pig cell microchimerism. There was no evidence of productive infection in recipient baboons for up to 6 months of xenograft function.

APOBEC3 proteins and porcine endogenous retroviruses

Xenotransplantation of porcine cells, tissues, and organs offers a solution to overcome the shortage of human donor materials. In addition to the immunological and physiological barriers, the existence of numerous porcine microorganisms including viruses poses a risk for xenozoonosis. Three classes of functional gamma-type porcine endogenous retroviruses (PERV) have been identified, whereby functional polytropic PERV-A and PERV-B infect human embryonic kidney (HEK 293) and other cell lines in vitro. In the course of risk assessment for xenotransplantation the capacity of human cells to counteract PERV infections should be analyzed. Primates and other mammals display different means of protection against viral infections. APOBEC3 proteins which are cytidine deaminases and a part of the intrinsic immunity mediate potent activity against a wide range of retroviruses including murine leukemia viruses (MLV). As PERV and MLV belong to the same genus, we raised the question as to whether PERV is affected by APOBEC3 proteins. Initial data indicate that human and porcine cytidine deaminases inhibit PERV replication, thereby possibly reducing the risk for infection of human cells by PERV as a consequence of pig-to-human xenotransplantation.

Reappraisal of biosafety risks posed by PERVs in xenotransplantation

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

Somatic cell nuclear transfer in pigs: recent achievements and future possibilities

During the past 6 years, considerable advancement has been achieved in experimental embryology of pigs. This process was mainly generated by the rapidly increasing need for transgenic pigs for biomedical research purposes, both for future xenotransplantation to replace damaged human organs or tissues, and for creating authentic animal models for human diseases to study aetiology, pathogenesis and possible therapy. Theoretically, among various possibilities, an established somatic cell nuclear transfer system with genetically engineered donor cells seems to be an efficient and reliable approach to achieve this goal. However, as the result of unfortunate coincidence of known and unknown factors, porcine embryology had been a handicapped branch of reproductive research in domestic animals and a very intensive and focused research was required to eliminate or minimise this handicap. This review summarises recent achievements both in the background technologies (maturation, activation, embryo culture) and the actual performance of the nuclear replacement. Recent simplified methods for in vivo development after embryo transfer are also discussed. Finally, several fields of potential application for human medical purposes are discussed. The authors conclude that although in this early phase of research no direct evidence can be provided about the practical use of transgenic pigs produced by somatic cell nuclear transfer as organ donors or disease models, the future chances even in medium term are good, and at least proportional with the efforts and sums that are invested into this research area worldwide.

Biological and Biomaterial Approaches for Improved Islet Transplantation

Islet transplantation may be used to treat type I diabetes. Despite tremendous progress in islet isolation, culture, and preservation, the clinical use of this modality of treatment is limited due to post-transplantation challenges to the islets such as the failure to revascularize and immune destruction of the islet graft. In addition, the need for lifelong strong immunosuppressing agents restricts the use of this option to a limited subset of patients, which is further restricted by the unmet need for large numbers of islets. Inadequate islet supply issues are being addressed by regeneration therapy and xenotransplantation. Various strategies are being tried to prevent beta-cell death, including immunoisolation using semipermeable biocompatible polymeric capsules and induction of immune tolerance. Genetic modification of islets promises to complement all these strategies toward the success of islet transplantation. Furthermore, synergistic application of more than one strategy is required for improving the success of islet transplantation. This review will critically address various insights developed in each individual strategy and for multipronged approaches, which will be helpful in achieving better outcomes.

Science and technology of farm animal cloning: state of the art

Details of the first mammal born after nuclear transfer cloning were published by Steen Malte Willadsen in 1986. In spite of its enormous scientific significance, this discovery failed to trigger much public concern, possibly because the donor cells were derived from pre-implantation stage embryos. The major breakthrough in terms of public recognition has happened when Ian Wilmut et al. [Wilmut, I., Schnieke, A.E., McWhir, J., Kind, A.J., Campbell, K.H., 1997. Viable offspring derived from fetal és adult mammalian cells. Nature 385, 810-813] described the successful application of almost exactly the same method, but using the nuclei of somatic cells from an adult mammal, to create Dolly the sheep. It has become theoretically possible to produce an unlimited number of genetic replicates from an adult animal or a post-implantation foetus. Since 1997 a number of different species including pigs, goats, horses, cats, etc. have been cloned with the somatic cell nuclear transfer technique. Although the technology still has relatively low success rates and there seems to be substantial problems with the welfare of some of the cloned animals, cloning is used both within basic research and the biomedical sector. The next step seems to be to implement cloning in the agricultural production system and several animals have been developed in this direction. This article reviews the current state of the art of farm animal cloning from a scientific and technological perspective, describes the animal welfare problems and critically assess different applications of farm animal cloning. The scope is confined to animal biotechnologies in which the use of cell nuclear transfer is an essential part and extends to both biomedical and agricultural applications of farm animal cloning. These applications include the production of genetically identical animals for research purposes, and also the creation of genetically modified animals. In the agricultural sector, cloning can be used as a tool within farm animal breeding. We do not intend to give an exhaustive review of the all the literature available; instead we pinpoint issues and events pivotal to the development of current farm animal cloning practices and their possible applications.

Xenotransplantation of porcine neonatal islets of Langerhans and Sertoli cells: a 4-year study

OBJECTIVE

Porcine islets of Langerhans for xenotransplantation into humans have been proposed as a solution to the shortage of human donors. Rejection is one of the main constraints. This study presents the results of a clinical trial using a novel method for transplanting and immunoprotecting porcine islets in type 1 diabetic patients.

DESIGN

A 4-year follow up of a clinical trial involving 12 patients, with no immunosuppressive drugs at any point. Eleven age matched untransplanted diabetics served as controls.

METHODS

We have developed a procedure for protecting neonatal porcine islets by combining them with Sertoli cells and placing them in a novel subcutaneous autologous collagen-covered device.

RESULTS

In the patients in the treatment group, no complications arose and no porcine endogenous retrovirus infection was detected. Half of the patients showed a significant reduction in insulin requirements compared with both their pre transplant levels and controls, and this reduction was maintained for up to 4 years. Two patients became insulin-independent for several months. Porcine insulin was detected in three patients' sera following glucose stimulation up to 4 years post transplant. Three years post transplant, one of four devices was removed from four patients, and the presence of insulin-positive cells in the transplant was demonstrated by immunohistology in all 4 patients.

CONCLUSIONS

Long-term cell survival with concurrent positive effects on metabolic control are possible by this technique.

Immune problems in central nervous system cell therapy

Transplantation of cells and tissues to the mammalian brain and CNS has revived the interest in the immunological status of brain and its response to grafted tissue. The previously held view that the brain was an absolute "immunologically privileged site" allowing indefinite survival without rejection of grafts of cells has proven to be wrong. Thus, the brain should be regarded as a site where immune responses can occur, albeit in a modified form, and under certain circumstances these are as vigorous as those seen in other peripheral sites. Clinical cell transplant trials have now been performed in Parkinson's disease, Huntington's disease, demyelinating diseases, retinal disorders, stroke, epilepsy, and even deafness, and normally are designed as cell replacement strategies, although implantation of genetically modified cells for supplementation of growth factors has also been tried. In addition, some disorders of the CNS for which cell therapies are being considered have an immunological basis, such as multiple sclerosis, which further complicates the situation. Embryonic neural tissue allografted into the CNS of animals and patients with neurodegenerative conditions survives, makes and receives synapses, and ameliorates behavioral deficits. The use of aborted human tissue is logistically and ethically complicated, which has lead to the search for alternative sources of cells, including xenogeneic tissue, genetically modified cells, and stem cells, all of which can and will induce some level of immune reaction. We review some of the immunological factors involved in transplantation of cells to CNS.

Cell therapy for stroke

Increasing experimental evidence suggests that cell transplantation can enhance recovery from stroke in animal models of focal cerebral ischemia. Clinical trials have been investigating the effects of a human immortalized neuronal cell line and porcine fetal neurons in stroke victims with persistent and stable deficits. Preclinical studies are focusing on the effects of human stem cells from various sources including brain, bone marrow, umbilical cord, and adipose tissue. This review presents an overview of preclinical and clinical studies on cell therapy for stroke. We emphasize the current, limited knowledge about the biology of implant sources and discuss special conditions in stroke that will impact the potential success of neurotransplantation in clinical trials.

Pseudotyping of porcine endogenous retrovirus by xenotropic murine leukemia virus in a pig islet xenotransplantation model

The potential of porcine endogenous retrovirus (PERV) as a human pathogen, particularly as a public health risk, is a major concern for xenotransplantation. In vitroPERV transmission to human cells is well established. Evidence from human/pig hematopoietic chimeras in immunodeficient mice suggests PERV transmission from pig to human cells in vivo. However, recently Yang et al. demonstrated in such a model that PERV-C, a nonhuman-tropic class, could be transmitted via pseudotyping by xenotropic murine leukemia virus (X-MLV). We developed a mouse pig islet xenotransplant model, where pig and human cells are located in physically separate compartments, to directly assess PERV transmission from a functional pig xenograft. X-MLV efficiently pseudotypes all three classes of PERV, including PERV-A and -B that are known to productively infect human cell lines and PERV-C that is normally not infectious for human cells. Pseudotyping also extends PERV's natural tropism to nonpermissive, nonhuman primate cells. X-MLV is activated locally by the surgical procedure involved in the tissue transplants. Thus, the presence and activation of endogenous X-MLV in immunodeficient mice limits the clinical significance of previous reports of in vivo PERV transmission from pig tissues to human cells.

Neurotransplantation of Fetal Porcine Cells in Patients with Basal Ganglia Infarcts: A Preliminary Safety and Feasibility Study

BACKGROUND

Cell transplantation is safe in animal models and enhances recovery from stroke in rats.

METHODS

We studied the safety and feasibility of fetal porcine transplantation in 5 patients with basal ganglia infarcts and stable neurological deficits. To prevent rejection, cells were pretreated with an anti-MHC1 antibody and no immunosuppressive drugs were given to the patients.

RESULTS

The first 3 patients had no adverse cell, procedure, or imaging-defined effects. The fourth patient had temporary worsening of motor deficits 3 weeks after transplantation, and the fifth patient developed seizures 1 week after transplantation. MRI in both patients demonstrated areas of enhancement remote from the transplant site, which resolved on subsequent imaging. Two patients showed improvement in speech, language, and/or motor impairments over several months and persisted at 4 years. The study was terminated by the FDA after the inclusion of 5 patients.

CONCLUSION

This is the first report on the transplantation of nontumor cells in ischemic stroke patients.

History of xenotransplantation

The present historical review reports the clinical experiences of transplantations from animal to human. The first transplantation attempts were made without any knowledge of the species barrier. The pioneers of xenotransplantation realized xenotransfusions as early as the 16th century, then cell and tissue xenotransplantations in the 19th century. At the beginning of the 20th century, xenotransplantation of testicles became the latest craze. At the same time, and later in the 1960s, organ xenotransplantations were attempted, with disappointing results. Mathieu Jaboulay, Serge Voronoff, Keith Reemtsma, James Hardy, Denton Cooley, Thomas Starzl, Christiaan Barnard and Leonard Bailey were among the pionneers of xenotransplantation. Recent trials concerned above all tissue and cell xenotransplantations. Nowadays, with encapsulation, transgenesis, and cloning, great advances have been made for controlling xenograft rejection, but ethical questions linked to the risk of infections have become a major pre-occupation within the scientific community and the general population.

Monitoring for presence of potentially xenotic viruses in recipients of pig islet xenotransplantation

This study represents a long-term follow-up of human patients receiving pig islet xenotransplantation. Eighteen patients had been monitored for up to 9 years for potentially xenotic pig viruses: pig endogenous retrovirus, pig cytomegalovirus, pig lymphotropic herpesvirus, and pig circovirus type 2. No evidence of viral infection was found.

Evidence and consequence of porcine endogenous retrovirus recombination

The genetic nature and biological effects of recombination between porcine endogenous retroviruses (PERV) were studied. An infectious molecular clone was generated from a high-titer, human-tropic PERV isolate, PERV-A 14/220 (B. A. Oldmixon, et al. J. Virol. 76:3045-3048, 2002; T. A. Ericsson et al. Proc. Natl. Acad. Sci. USA 100:6759-6764, 2003). To analyze this sequence and 15 available full-length PERV nucleotide sequences, we developed a sequence comparison program, LOHA(TM) to calculate local sequence homology between two sequences. This analysis determined that PERV-A 14/220 arose by homologous recombination of a PERV-C genome replacing an 850-bp region around the pol-env junction with that of a PERV-A sequence. This 850-bp PERV-A sequence encompasses the env receptor binding domain, thereby conferring a wide host range including human cells. In addition, we determined that multiple regions derived from PERV-C are responsible for the increased infectious titer of PERV-A 14/220. Thus, a single recombination event may be a fast and effective way to generate high-titer, potentially harmful PERV. Further, local homology and phylogenetic analyses between 16 full-length sequences revealed evidence for other recombination events in the past that give rise to other PERV genomes that possess the PERV-A, but not the PERV-B, env gene. These results indicate that PERV-A env is more prone to recombination with heterogeneous backbone genomes than PERV-B env. Such recombination events that generate more active PERV-A appear to occur in pigs rather frequently, which increases the potential risk of zoonotic PERV transmission. In this context, pigs lacking non-human-tropic PERV-C would be more suitable as donor animals for clinical xenotransplantation.

Prevalent human coxsackie B-5 virus infects porcine islet cells primarily using the coxsackie-adenovirus receptor

BACKGROUND

We have previously demonstrated that transplanting porcine encephalomyocarditis virus (EMCV)-infected porcine islet cells (PICs) results in transmission of the virus to recipient mice, which is manifested by acute fatal infection within 5 to 8 days. Here, we determined PIC susceptibility to a related and highly prevalent human picornavirus, coxsackie B-5 virus (CVB-5).

METHODS

PICs were inoculated with CVB-5 in vitro for up to 96 hours and infectivity, level of virus replication, and cellular function determined. Subsequently, monoclonal and polyclonal antibody blocking experiments were used to investigate the receptor CVB-5 uses to enter PICs, and the ability of CVB-5-infected islets to reverse diabetes analyzed in mice.

RESULTS

Adult pig islets inoculated with CVB-5 in vitro showed a typical picornaviral replication cycle with a 2-h lag phase followed by a 4-h exponential phase during which the virus titer increased by 4 logs. However, CVB-5 was less cytolytic to PICs than EMCV, resulting in a persistent productive infection lasting for up to 96 h, with minimal evidence of cell lysis. Double immunostaining confirmed the presence of CVB-5 antigens in insulin-producing islets. Infection of PICs in the presence of antibodies against human coxsackie-adenovirus receptor (CAR) resulted in near complete blockage in production of infectious virus particles whereas blocking with anti-porcine decay-accelerating factor (DAF, also called CD55) or anti-porcine membrane cofactor protein (MCP, also called CD46) only slightly decreased the number of infectious CVB-5 particles produced. Immunofluoresence staining showed CAR and MCP expression on the islet surface, but not DAF. Transplanting CVB-5-infected PICs into diabetic C57BL/6 mice resulted in reversal of diabetes.

CONCLUSION

Although PICs are susceptible to human CVB-5, the infection does not appear to affect xenograft function in vitro or in vivo in the short term.

Position Paper of the Ethics Committee of the International Xenotransplantation Association

Xenotransplantation (XTx) provides a potential solution to the shortage of human organs and tissues, and has several advantages over other possible solutions to this problem. However, a number of scientific and ethical barriers exist, and need to be addressed in order to advance the field of XTx in a manner that optimizes its potential to benefit society and minimizes its risk. Some of the most pressing ethical issues are discussed, and the position of the Ethics Committee of the International Xenotransplantation Association is presented.

Mouse retrovirus mediates porcine endogenous retrovirus transmission into human cells in long-term human-porcine chimeric mice

Porcine endogenous retrovirus (PERV) is a potential pathogen in clinical xenotransplantation; transmission of PERV in vivo has been suggested in murine xenotransplantation models. We analyzed the transmission of PERV to human cells in vivo using a model in which immunodeficient NOD/SCID transgenic mice were transplanted with porcine and human lymphohematopoietic tissues. Our results demonstrate, we believe for the first time, that human and pig cells can coexist long-term (up to 25 weeks) without direct PERV infection of human cells. Despite the transplantation of porcine cells that did not produce human-tropic PERV, human cells from the chimeric mice were frequently found to contain PERV sequences. However, this transmission was due to the pseudotyping of PERV-C (a virus without human tropism) by xenotropic murine leukemia virus, rather than to de novo generation of human-tropic PERV. Thus, pseudotyping might account for the PERV transmission previously observed in mice. The absence of direct human cell infection following long-term in vivo coexistence with large numbers of porcine cells provides encouragement regarding the potential safety of using pigs that do not produce human-tropic PERV as source animals for transplantation to humans.

No transmission of porcine endogenous retrovirus after transplantation of adult porcine islets into diabetic nude mice and immunosuppressed rats

BACKGROUND

The aim of this study was to investigate whether transmission of porcine endogenous retrovirus (PERV) occurs in a model of diabetes reversal by the xenotransplantation of adult porcine islets (APIs) into immunoincompetent diabetic rodents.

METHODS

Black-6 nu/nu mice and Lewis rats were immunosuppressed with cyclosporin A (CsA) and FTY 720, and rendered diabetic with streptozotocin. Purified APIs were transplanted into the renal subcapsular space; 5,000 islet equivalents (IEQs) were used in the nude mice (n = 4) and 40,000 IEQs in the rats (n = 4). The nude mice were sacrificed at 75 days after transplantation. In order to confirm chronic xenograft function, the graft-bearing kidney was removed prior to sacrifice. The rats were followed until xenograft rejection, at which time they were sacrificed. Immediately after sacrifice, tissue samples (liver, spleen, and small intestine) were taken for analysis. Quantitative polymerase chain reaction (PCR) was used to assess evidence of PERV transmission, and porcine cell chimerism.

RESULTS

All animals became normoglycemic within 48 h of transplantation. The nude mice remained normoglycemic during the 75-day study period, with removal of the graft-bearing kidney resulting in prompt hyperglycemia. The rats remained normoglycemic until xenograft rejection, which occurred at 66 +/- 28 days. Despite the evidence of porcine cell microchimerism in recipients, real-time PCR detected no evidence of PERV transmission in any of the tissue specimens tested.

CONCLUSIONS

There was no evidence of PERV transmission following transplantation of pig islets into diabetic nude mice and immunosuppressed rats.

Porcine endogenous retrovirus transmission characteristics of galactose alpha1-3 galactose-deficient pig cells

Galactose alpha1-3 galactose (Gal) trisaccharides are present on the surface of wild-type pig cells, as well as on viruses particles produced from such cells. The recognition of Gal sugars by natural anti-Gal antibodies (NAb) in human and Old World primate serum can cause the lysis of the particles via complement-dependent mechanisms and has therefore been proposed as an important antiviral mechanism. Recently, pigs have been generated that possess disrupted galactosyl-transferase (GGTA1) genes. The cells of these pigs do not express Gal sugars on their surface, i.e., are Gal null. Concerns have been raised that the risk of virus transmission from such pigs may be increased due to the absence of the Gal sugars. We investigated the sensitivity of porcine endogenous retrovirus (PERV) produced from Gal-null and Gal-positive pig cells to inactivation by purified NAb and human serum. PERV produced in Gal-null pig cells was resistant to inactivation by either NAb or human serum. In contrast, although Gal-positive PERV particles were sensitive to inactivation by NAb and human serum, they required markedly higher concentrations of NAb for inactivation compared to the Gal-positive cells from which they were produced. Complete inactivation of Gal-positive PERV particles was not achievable despite the use of high levels of NAb, indicating that NAb-mediated inactivation of cell-free PERV particles is an inefficient process.

Reduced sensitivity to human serum inactivation of enveloped viruses produced by pig cells transgenic for human CD55 or deficient for the galactosyl-alpha(1-3) galactosyl epitope

Complement activation mediated by the major xenogeneic epitope in the pig, galactosyl-alpha(1-3) galactosyl sugar structure (alpha-Gal), and human natural antibodies could cause hyperacute rejection (HAR) in pig-to-human xenotransplantation. The same reaction on viruses bearing alpha-Gal may serve as a barrier to zoonotic infection. Expressing human complement regulatory proteins or knocking out alpha-Gal epitopes in pig in order to overcome HAR may therefore pose an increased risk in xenotransplantation with regard to zoonosis. We investigated whether amphotropic murine leukemia virus, porcine endogenous retrovirus, and vesicular stomatitis virus (VSV) budding from primary transgenic pig aortic endothelial (TgPAE) cells expressing human CD55 (hCD55 or hDAF) was protected from human-complement-mediated inactivation. VSV propagated through the ST-IOWA pig cell line, in which alpha-galactosyl-transferase genes were disrupted (Gal null), was also tested for sensitivity to human complement. The TgPAE cells were positive for hCD55, and all pig cells except the Gal-null ST-IOWA expressed alpha-Gal epitopes. Through antibody binding, we were able to demonstrate the incorporation of hCD55 onto VSV particles. Viruses harvested from TgPAE cells were relatively resistant to complement-mediated inactivation by the three sources of human sera tested. Additionally, VSV from Gal-null pig cells was resistant to human complement inactivation. Such protection of enveloped viruses may increase the risk of zoonosis from pigs genetically modified for pig-to-human xenotransplantation.

Absence of replication-competent human-tropic porcine endogenous retroviruses in the germ line DNA of inbred miniature Swine

The potential transmission of porcine endogenous retroviruses (PERVs) has raised concern in the development of porcine xenotransplantation products. Our previous studies have resulted in the identification of animals within a research herd of inbred miniature swine that lack the capacity to transmit PERV to human cells in vitro. In contrast, other animals were capable of PERV transmission. The PERVs that were transmitted to human cells are recombinants between PERV-A and PERV-C in the post-VRA region of the envelope (B. A. Oldmixon, J. C. Wood, T. A. Ericsson, C. A. Wilson, M. E. White-Scharf, G. Andersson, J. L. Greenstein, H. J. Schuurman, and C. Patience, J. Virol. 76:3045-3048, 2002); these viruses we term PERV-A/C. This observation prompted us to determine whether these human-tropic replication-competent (HTRC) PERV-A/C recombinants were present in the genomic DNA of these miniature swine. Genomic DNA libraries were generated from one miniature swine that transmitted HTRC PERV as well as from one miniature swine that did not transmit HTRC PERV. HTRC PERV-A/C proviruses were not identified in the germ line DNAs of these pigs by using genomic mapping. Similarly, although PERV-A loci were identified in both libraries that possessed long env open reading frames, the Env proteins encoded by these loci were nonfunctional according to pseudotype assays. In the absence of a germ line source for HTRC PERV, further studies are warranted to assess the mechanisms by which HTRC PERV can be generated. Once identified, it may prove possible to generate animals with further reduced potential to produce HTRC PERV.

Identification of exogenous forms of human-tropic porcine endogenous retrovirus in miniature Swine

The replication of porcine endogenous retrovirus subgroup A (PERV-A) and PERV-B in certain human cell lines indicates that PERV may pose an infectious risk in clinical xenotransplantation. We have previously reported that human-tropic PERVs isolated from infected human cells following cocultivation with miniature swine peripheral blood mononuclear cells (PBMC) are recombinants of PERV-A with PERV-C. Here, we report that these recombinants are exogenous viruses in miniature swine; i.e., they are not present in the germ line DNA. These viruses were invariably present in miniature swine that transmitted PERV to human cells and were also identified in some miniature swine that lacked this ability. These data, together with the demonstration of the absence of both replication-competent PERV-A and recombinant PERV-A/C loci in the genome of miniature swine (L. Scobie, S. Taylor, J. C. Wood, K. M. Suling, G. Quinn, C. Patience, H.-J. Schuurman, and D. E. Onions, J. Virol. 78:2502-2509, 2004), indicate that exogenous PERV is the principal source of human-tropic virus in these animals. Interestingly, strong expression of PERV-C in PBMC correlated with an ability of the PBMC to transmit PERV-A/C recombinants in vitro, indicating that PERV-C may be an important factor affecting the production of human-tropic PERV. In light of these observations, the safety of clinical xenotransplantation from miniature swine will be most enhanced by the utilization of source animals that do not transmit PERV to either human or porcine cells. Such animals were identified within the miniature swine herd and may further enhance the safety of clinical xenotransplantation.

Intracerebral cytokine profiles in adult rats grafted with neural tissue of different immunological disparity

To understand graft rejection in cell based therapies for brain repair we have quantified IL-1beta, IL-2, IL-4, IL-10, IL-12p40, IFN-gamma and TNF-alpha mRNA levels using real-time PCR, at days 4, 14, and 42 post-transplantation, in rats engrafted with syngeneic, allogeneic, concordant and discordant xenogeneic neural tissues. In addition, in the discordant xenografts immunohistochemistry and in situ hybridization were applied to detect local expression of IFN-gamma, TNF-alpha, IL-10 and TGF-beta. Allografts remained non-rejected but expressed IL-1beta, TNF-alpha and IL-4 transcripts but not IL-12p40 and IFN-gamma. Xenografts demonstrated distinct cytokine profiles that differed from syngeneic and allogeneic grafts. Non-rejected discordant xenografts contained higher levels of TNF-alpha transcripts and lower levels of IL-2 transcripts than the rejected ones at day 42. Discordant xenografts displayed a stronger and earlier expression of IL-1beta and TNF-alpha, followed by T-helper 1 and T-helper 2 associated cytokine expression. The number of cells expressing mRNA encoding TNF-alpha and TGF-beta was significantly increased over time in the discordant group. In conclusion, the immunological disparity of the implanted tissue explains survival rates and is associated with different cytokine profiles. In allografts, a chronic inflammatory reaction was detected and in xenogeneic grafts a delayed hypersensitivity like reaction may be involved in rejection.

Genotyping of porcine endogenous retroviruses from a family of miniature swine

The identification of animals in an inbred miniature swine herd that consistently fail to produce replication- competent humantropic porcine endogenous retrovirus (PERV) has prompted studies on the biology of PERV in transmitter and nontransmitter animals. We analyzed PERV RNA transcript profiles in a family of inbred miniature swine (SLA(d/d) haplotype) in which individual members differed in their capacity to generate humantropic and ecotropic (i.e., pigtropic) virus. We identified unique HaeIII and HpaII gag restriction fragment length polymorphism (RFLP) profiles resulting from single nucleotide polymorphisms in blood cells; these were found only in animals that produced humantropic PERV. These HaeIII and HpaII gag RFLP profiles proved to be components of humantropic PERV as they were transmitted to 293 human target cells in vitro. The humantropic HaeIII and HpaII gag RFLP genotypes in the family of study were not present in other miniature swine in the herd that produced humantropic PERV, indicating that these RFLP profiles relate specifically to this family's lineage.