Miniature swine as organ donors for man: Strategies for prevention of xenotransplant-associated infections

Improvement of islet function and survival by integration of perfluorodecalin into microcapsules in vivo and in vitro

Hypoxic injury of islets is a major obstacle for encapsulated islet transplantation into the peritoneal cavity. To improve oxygen delivery to encapsulated islets, we integrated 20% of the oxygen carrier material, perfluorodecalin (PFD), in alginate capsules mixed with islets (PFD-alginate). Integration of PFD clearly improved islet viability and decreased reactive oxygen species production compared to islets encapsulated with alginate only (alginate) and naked islets exposed to hypoxia in vitro. In PFD-alginate capsules, HIF-1α expression was minimal, and insulin expression was well maintained. Furthermore, the best islet function represented by glucose-stimulated insulin secretion was observed for the PFD-alginate capsules in hypoxic condition. For the in vivo study, the same number of naked islets and encapsulated islets (alginate and PFD-alginate) was transplanted into streptozotocin-induced diabetic mice. Nonfasting blood glucose levels and the area under the curve for glucose based on intraperitoneal glucose tolerance tests in the PFD-alginate group were lower than in the alginate group. The harvested islets stained positive for insulin in all groups, but the ratio of dead cell area was 4 times higher in the alginate group than in the PFD-alginate group. In conclusion, integration of PFD in alginate microcapsules improved islet function and survival by minimizing the hypoxic damage of islets after intraperitoneal transplantation.

Xenogeneic Cell Therapy: Current Progress and Future Developments in Porcine Cell Transplantation

The multitude of distinct cell types present in mature and developing tissues display unique physiologic characteristics. Cellular therapy is a novel technology with the promise of utilizing this diversity to treat a wide range of human degenerative diseases. Intractable diseases, disorders, and injuries are characterized by cell death or aberrant cellular function. Cell transplantation can replace diseased or lost tissue to provide restorative therapy for these conditions. The limited use of cell transplants as a basis for current therapy can, in part, be attributed to the lack of available human cells suitable for transplantation. This has prevented further realization of the promise of cell transplantation as a platform technology. Accordingly, cell-based therapies such as blood transfusions, for which the cells are readily available, are a standard part of current medical practice. Despite numerous attempts to expand primary human cells in tissue culture, current technological limitations of this approach in regard to proliferative capacity and maintenance of the differentiated phenotype has prevented their use for transplantation. Further, use of human stem cells for the derivation of specific cell types for transplantation is an area of future application with great potential, but hurdles remain in regard to deriving and sufficiently expanding these multi-potential cells. Thus, it appears that primary cells are at present a superior source for transplantation. This review focuses on pigs as a source of a variety of primary cells to advance cell therapy to the clinic and implement achievement of its full potential. We outline the advantages and disadvantages of xenogeneic cell therapy while underscoring the utility of transplantable porcine cells for the treatment of human disease. © 1998 Elsevier Science Inc.

The case for xenotransplantation

The availability of organs and cells from deceased humans for transplantation is not meeting the demand. Xenotransplantation, specifically the transplantation of organs and cells from genetically engineered pigs, could resolve this problem. Diabetic monkeys have remained normoglycemic and insulin-independent after pig islet transplantation for >one yr, and a pig heterotopic (non-life-supporting) heart transplant recently reached the one-yr milestone in a baboon. With these encouraging results, why is it that, with some notable exceptions, research into xenotransplantation has received relatively little support by industry, government funding agencies, and medical charitable foundations? Industry appears reluctant to support research that will take more than two to three yr to come to clinical trial, and the funding agencies appear to have been "distracted" by the current appeal of stem cell technology and regenerative medicine. It has only been the willingness of living donors to provide organs that has significantly increased the number of transplants being performed worldwide. These altruistic donations are not without risk of morbidity and even mortality to the donor. Although with the best of intentions, we are therefore traversing the Hippocratic Oath of doctors to "do no harm." This should be a stimulus to fund exploration of alternative approaches, including xenotransplantation.

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.

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.

Natural protection from zoonosis by alpha-gal epitopes on virus particles in xenotransmission

Clinical transplantation has become one of the preferred treatments for end-stage organ failure, and one of the novel approaches being pursued to overcome the limited supply of human organs involves the use of organs from other species. The pig appears to be a near ideal animal due to proximity to humans, domestication, and ability to procreate. The presence of Gal-alpha1,3-Gal residues on the surfaces of pig cells is a major immunological obstacle to xenotransplantation. Alpha1,3galactosyltransferase (alpha1,3GT) catalyzes the synthesis of Gal alpha 1-3Gal beta 1-4GlcNAc-R (alpha-gal epitope) on the glycoproteins and glycolipids of non-primate mammals, but this does not occur in humans. Moreover, the alpha-gal epitope causes hyperacute rejection of pig organs in humans, and thus, the elimination of this antigen from pig tissues is highly desirable. Recently, concerns have been raised that the risk of virus transmission from such pigs may be increased due to the absence of alpha-gal on their viral particles. In this study, transgenic cells expressing alpha1,3GT were selected using 1.25 mg/ml neomycin. The development of HeLa cells expressing alpha1,3GT now allows accurate studies to be conducted on the function of the alpha-gal epitope in xenotransmission. The expressions of alpha-gal epitopes on HeLa/alpha-gal cells were demonstrated by flow cytometry and confocal microscopy using cells stained with IB4-fluorescein isothiocyanate lectin. Vaccinia viruses propagated in HeLa/alpha-gal cells also expressed alpha-gal on their viral envelopes and were more sensitive to inactivation by human sera than vaccinia virus propagated in HeLa cells. Moreover, neutralization of vaccinia virus was inhibited in human serum by 10 mm ethylene glycol bis(beta-aminoethylether)tetraacetic acid (EDTA) treatment. Our data indicated that alpha-gal epitopes are one of the major barriers to zoonosis via xenotransmission.

Novel herpesviruses of Suidae: indicators for a second genogroup of artiodactyl gammaherpesviruses

Five novel herpesviruses were identified in suid species from Africa (common warthog, Phacochoerus africanus) and South-East Asia (bearded pig, Sus barbatus; babirusa, Babyrousa babyrussa) by detection and analysis of their DNA polymerase genes. Three of the novel species, P. africanus cytomegalovirus 1, P. africanus lymphotropic herpesvirus 1 (PafrLHV-1) and S. barbatus lymphotropic herpesvirus 1 (SbarLHV-1), were closely related to known beta- (porcine cytomegalovirus) and gammaherpesviruses [porcine lymphotropic herpesvirus (PLHV) 1 and 3] of domestic pigs. In contrast, two novel species, S. barbatus rhadinovirus 1 (SbarRHV-1) and Babyrousa babyrussa rhadinovirus 1 (BbabRHV-1), were more closely related to a ruminant gammaherpesvirus, bovine herpesvirus 4 (BoHV-4), than to the porcine gammaherpesviruses PLHV-1, -2, -3, PafrLHV-1 and SbarLHV-1. SbarRHV-1, BbabRHV-1 and BoHV-4 were therefore tentatively assigned to a novel genogroup of artiodactyl gammaherpesviruses. This latter genogroup may also contain an as yet undiscovered gammaherpesvirus of domestic pigs, thereby adding a concern to their use in xenotransplantation.

Control of microbial contamination during surgical harvest of pig renal xenografts

The pig has been identified as the most likely source of xenograft material for clinical use and studies are ongoing to overcome the immunological hurdles of pig-to-human transplantation. Attention is now being focussed on identifying and reducing the potential microbiological hazards associated with this technique. Studies have primarily addressed issues surrounding the production and health monitoring of xenograft source pigs and none have so far specifically evaluated the possible risks of microbial contamination during xenograft harvest. In this report, we evaluate the possible routes for contamination of a pig kidney xenograft during organ harvest and describe approaches to the control of these hazards, including the novel use of a custom designed airtight surgical canopy. A standard procedure for microbiological monitoring during xenograft harvest was devised and evaluated. This allowed the rapid identification and anti-microbial sensitivity testing of any isolated organisms. This would enable an early and appropriate pre-emptive treatment of infection because of transmission of pig micro-organisms.

Xenotransplantation: public health risks--patient vs. society in an emerging field

Xenotransplantation is a public health concern because it has the potential to infect human recipients with zoonotic and other infectious agents that are not endemic in human populations, thereby potentially introducing new infections to the human community. From this perspective, xenotransplantation clinical trials combine a potential benefit for individual recipients with a potential risk to the human community. However, the potential for benefit also extends beyond the individual recipient to society as a whole, a fact infrequently recognized in discussions of this topic. Further, diseases neither endemic in human communities nor recognized as classic zoonoses are introduced into humans periodically through routine interactions between human and nonhuman animals. Thus, xenotransplantation is one of multiple potential routes by which infectious agents of nonhuman origin may enter human ecosystems. The intentional and controlled nature of xenotransplantation exposures enables implementation of measures to minimize associated biohazards. Development of guidelines and implementation of regulatory oversight of xenotransplantation clinical trials in most nations where such research occurs has promoted a standard level of practice in the field and markedly reduced the risk of xenotransplant-introduced infection compared to the situation in 1995.

Molecular cloning and functional characterization of infectious PERV and development of diagnostic tests

Pigs are the donor animals of choice for xenotransplantation (XTx) and xenogeneic cell therapy measurements. Most known porcine pathogens can be controlled by conventional means like vaccination, medication or specific pathogen-free breeding conditions. As pigs have co-evolved very closely with humans for a few millennia it is not very likely that even asymptomatic pathogens have escaped attention. Porcine endogenous retroviruses (PERV) are different from conventional pathogens as they are chromosomally fixed in every cell of the animal, hence PERV cannot be easily controlled. While PERV show no phenotype in the porcine host, recent data demonstrate that some polytropic proviruses can be activated by external stimuli and that those can productively infect human cells in vitro. In evaluation of the retrovirological safety of XTx, we determined the number of replication-competent PERV to be limited and to exhibit a heterogeneous distribution, therefore suggesting that they could be removed by conventional breeding. The transcriptional regulation of some PERV due to repetitive elements in their long terminal repeats enables their adaptation to new host cells. The diagnostic tools available, based on immunological and polymerase chain reaction techniques, were shown to be sensitive in both the animal and in vitro, but must still show their potential in human XTx recipients, where they are confronted with very low antigen expression and the phenomenon of microchimerism.

Exogenous porcine viruses

Porcine organs, cells and tissues provide a viable source of transplants in humans, though there is some concern of public health risk from adaptation of swine infectious agents in humans. Limited information is available on the public health risk of many exogenous swine viruses, and reliable and rapid diagnostic tests are available for only a few of these. The ability of several porcine viruses to cause transplacental fetal infection (parvoviruses, circoviruses, and arteriviruses), emergence or recognition of several new porcine viruses during the last two decades (porcine circovirus, arterivirus, paramyxoviruses, herpesviruses, and porcine respiratory coronavirus) and the immunosuppressed state of the transplant recipients increases the xenozoonoses risk of humans to porcine viruses through transplantation. Much of this risk can be eliminated with vigilance and sustained monitoring along with a better understanding of pathogenesis and development of better diagnostic tests. In this review we present information on selected exogenous viruses, highlighting their characteristics, pathogenesis of viral infections in swine, methods for their detection, and the potential xenozoonoses risk they present. Emphasis has been given in this review to swine influenza virus, paramyxovirus (Nipah virus, Menagle virus, LaPiedad paramyxovirus, porcine paramyxovirus), arterivirus (porcine reproductive and respiratory syndrome virus) and circovirus as either they represent new swine viruses or present the greatest risk. We have also presented information on porcine parvovirus, Japanese encephalitis virus, encephalomyocarditis virus, herpesviruses (pseudorabies virus, porcine lymphotropic herpesvirus, porcine cytomegalovirus), coronaviruses (TGEV, PRCV, HEV, PEDV) and adenovirus. The potential of swine viruses to infect humans needs to be assessed in vitro and in vivo and rapid and more reliable diagnostic methods need to be developed to assure safe supply of porcine tissues and cells for xenotransplantation.

Analyses of prevalence and polymorphisms of six replication-competent and chromosomally assigned porcine endogenous retroviruses in individual pigs and pig subspecies

As porcine endogenous retroviruses (PERV) productively infect human cells in vitro, they pose a serious risk in xenotransplantation and xenogeneic cell therapies. We have analyzed the prevalence of six well-characterized full-length PERV, five of them being replication-competent and four of them being chromosomally assigned (J. Virol. 75 (2001) 5465; J. Virol. 76 (2002) 2714). These analyses revealed a heterogeneous distribution of PERV among individuals and, as no PERV is present in every pig, it seems feasible to generate pigs free of functional PERV by conventional breeding. Conversely, as PERV are polymorphic, single proviruses may have escaped detection and this kind of assay must be performed for every herd used in xenotransplantation or xenogeneic cell therapies. In addition, specific proviruses show internal point mutations which significantly affect their replicational capacities. As there are two different types of PERV LTR structures showing varying levels of transcriptional capacity (J. Virol. 75 (2001) 6933), an analysis of 21 distinct chromosomal locations revealed that PERV which harbor highly active LTRs with repeat elements in U3 are dominant.

Approaching virus safety in xenotransplantation: a search for unrecognized herpesviruses in pigs

The identification of porcine viruses so far unrecognized is required to minimize virus-related risks associated with xenotransplantation. We used a pan-herpes consensus polymerase chain reaction assay to search for unrecognized porcine species of the Herpesviridae. The assay targets conserved regions of the herpesvirus DNA polymerase (DPOL) gene, using primers that were modified to diminish the assay's recognition capacity for the highly prevalent porcine lymphotropic herpesviruses 1, 2 and 3 (PLHV-1, -2, -3), without substantially lowering the universal detection capacity of the assay. Analysis of 495 porcine blood and tissue samples from 294 animals, including 35 samples from 20 immunosuppressed pigs, resulted in the amplification of 128 herpesviral DPOL sequences. Sequence analysis attributed 127 of the amplimers to the known porcine herpesviruses (PLHV-1, -2, -3; porcine cytomegalovirus; pseudorabiesvirus). In none of the pig samples analyzed here, evidence was obtained for the presence of additional novel porcine herpesvirus species. Therefore we conclude that pigs bred for the purpose of xenotransplantation pose a negligible risk of transmitting presently unrecognized herpesviruses to organ recipients.

Monitoring for potentially xenozoonotic viruses in New Zealand pigs

Shortage of human donor organs for transplantation has prompted evaluation of animals as an alternative donor source. Pigs are the most acceptable candidate animals but issues of xenozoonozes remain. Despite careful monitoring of high-health-status (HHS) pigs, there is still a risk that their tissues may carry infectious agents. Furthermore, pathogens which are significant in xenotransplantation are not necessarily those of veterinary importance. The detection of these potentially transmissible infectious agents may require the development and application of new surveillance technologies. We present data on monitoring for five potentially xenotic viruses in New Zealand pig herds, namely pig cytomegalovirus (PCMV), pig lymphotropic herpesvirus (PLHV), encephalomyocarditis virus (EMCV), pigcircovirus (PCV), and hepatitis E virus (HEV). These five viruses are either potentially oncogenic, establish persistent infection, or are known to be zoonotic. This study has expanded significantly the information on porcine viruses in New Zealand. Using this information, it is now possible to complete protocols for monitoring pig herds and tissues prior to their use in xenotransplantation. The study resulted in selection of a possible source herd for swine-to-human islet transplantation.

The production of transgenic pigs for potential use in clinical xenotransplantation: baseline clinical pathology and organ size studies

This report describes the results of hematology, serum biochemistry, growth, and organ weight studies undertaken on pigs from nine cohorts of qualified pathogen free (QPF) pigs reared within a high welfare bioexclusion facility as potential organ source animals. Confirmation of the high health status of the pigs was given through total leukocyte counts and serum globulin concentrations that fell below the expected reference range for conventional pigs. The calculated mean growth rate for QPF pigs was found to exceed target rates set for optimum genotype commercial pig herds. Body weights of QPF pigs were compared with kidney, heart and liver weights at necropsy.

The production of transgenic pigs for potential use in clinical xenotransplantation: microbiological evaluation

Debate over the infection hazards of pig-to-human xenotransplantation has focused mainly on the porcine endogenous retroviruses (PERV). However, hazards of exogenous infectious agents possibly associated with the xenograft have also been evaluated (Xenotransplantation 2000; 7: 143). We report the results of a health monitoring program demonstrating the exclusion of more than 80 potential pathogens from nine cohorts of pigs reared in a high welfare bioexclusion facility as potential xenograft source animals. A dynamic bacterial flora of pigs reared under barrier conditions was characterized, emphasizing the significance of monitoring for multiresistant antimicrobial sensitivity patterns. Evidence was found for exclusion of two commonly residual exogenous viruses, porcine cytomegalovirus and porcine lymphotropic herpesviruses, among a proportion of the cohorts tested. Finally, there was histopathological evidence for low grade pneumonitis among sentinel pigs, likely to have been associated with the use of quaternary ammonium disinfectants during the production process, indicating a need for review of toxicology data for disinfectant agents used in such bioexclusion systems. Intensive health monitoring programs, based upon regularly updated recommendations from the microbiological research community, will enable significant reductions in the potential hazards associated with pig-to-human xenotransplantation.

Characterization of chromosomally assigned replication-competent gamma porcine endogenous retroviruses derived from a large white pig and expression in human cells

Vertically transmitted endogenous retroviruses pose an infectious risk in the course of pig-to-human transplantation of cells, tissues, and organs. Two classes of polytropic type C porcine endogenous retroviruses (PERV) productively infect human cells in vitro. The cloning and characterization of replication-competent PERV-B sequences from infected human cells (F. Czauderna, N. Fischer, K. Boller, R. Kurth, and R. R. Tönjes, J. Virol. 74:4028-4038, 2000) as well as the cloning of functional PERV-A and -B sequences from porcine cell line PK15 (U. Krach, N. Fischer, F. Czauderna, and R. R. Tönjes, J. Virol. 75:5465-5472, 2001) have been previously described. Here we report the isolation of four full-length proviral sequences from a porcine bacterial artificial chromosome (BAC) library that comprises chromosomally assigned PERV. Clones Bac-PERV-A(130A12) and Bac-PERV-A(151B10) map to pig chromosome 1 and demonstrate close homology to PK15-PERV-A(58) in env and to PERV-MSL in long terminal repeat (LTR), gag, and pro/pol sequences. Clone Bac-PERV-A(463H12) is located on pig chromosome 3 and demonstrates close homology to PK15-PERV-A(58) in env and to 293-PERV-B(43) in LTR, gag, and pro/pol (Czauderna et al.; R. R. Tönjes, F. Czauderna, N. Fischer, U. Krach, K. Boller, P. Chardon, C. Rogel-Gailard, M. Niebert, G. Scheef, A. Werner, and R. Kurth, Transplant Proc. 32:1158-1161, 2000). Clone Bac-PERV-B(192B9) is located on pig chromosome 7 in the swine leukocyte antigen region and is highly homologous with but distinct from the previously described functional clone 293-PERV-B(43) and bears the number of repeats initially observed in the LTRs of clone 293-PERV-A(42) (Czauderna et al.; Krach et al.). Clones Bac-PERV-A(130A12), Bac-PERV-A(151B10), and Bac-PERV-A(463H12) were replication competent upon transfection into susceptible 293 and HeLa cells. Bac-PERV-B(192B9), however, bears two stop codons in pro/pol preventing this clone from being replication competent in some individual pigs, but initial screenings indicate that this provirus might be intact in others. The data suggest that the porcine genome harbors a limited number of infectious PERV sequences, allowing for specific screening in different pig breeds.

Human cytomegalovirus productively infects porcine endothelial cells in vitro

BACKGROUND

The possibility that human cytomegalovirus (HCMV) may infect porcine endothelial cells (ECs) was investigated. This may be relevant during xenotransplantation of porcine cells or organs into human recipients.

METHODS

HCMV was inoculated into low-passage porcine ECs. Replication of virus was detected by development of characteristic cytopathogenic effect. Appearance of immediate early, early, and late antigens was studied by immunocytochemical staining. Infectious virus was detected in human fibroblast cells. Presence of HCMV RNA was studied by Northern Blot and reverse transcriptase polymerase chain reaction.

RESULTS

All parameters indicated that a fresh clinical HCMV isolate productively infects porcine ECs. The same cells do not fully support replication of the laboratory strain Ad 169.

CONCLUSION

Our results may indicate the possibility of cross-species infectivity of HCMV to porcine cells.

Infection in xenotransplantation

Advances in transplantation immunology have enhanced the possibility of xenotransplantation as a therapeutic option for end-stage organ failure. The potential spread of animal-derived pathogens to the recipient and to the general population, termed "xenosis," is a potential complication of interspecies transplantation. Recognition of such novel infections may be complicated by infections due to altered microbiologic behavior and clinical symptomatology of these organisms, particularly in the immunocompromised xenograft recipient. Particular concern exists over the activation of latent viruses, including retroviruses, from xenograft tissues. Based on experience with human allogeneic transplantation, those pathogens considered most likely to cause human disease can be excluded prospectively from herds of animals developed for organ donation. Research is needed into the activation and behavior of retroviruses and other potential pathogens in xenotransplantation. Xenotransplantation may also provide unique opportunities not only for the care of patients with organ failure, but in the therapy of individuals with chronic infections to which the xenograft may be resistant. Clinical protocols must be developed so as to enhance the safety of the recipient and of the community-at-large.

Comparison of replication-competent molecular clones of porcine endogenous retrovirus class A and class B derived from pig and human cells

Vertically transmitted endogenous retroviruses pose an infectious risk in the course of pig-to-human transplantation of cells, tissues, and organs. Two classes of polytropic type C porcine endogenous retroviruses (PERV) which are infectious for human cells in vitro are known. Recently, we described the cloning and characterization of replication-competent PERV-B sequences from productively infected human cells (F. Czauderna, N. Fischer, K. Boller, R. Kurth, and R. R. Tönjes, J. Virol. 74:4028-4038, 2000). Here, we report the isolation of infectious molecular PERV-A and PERV-B clones from pig cells and compare these proviruses with clones derived from infected human 293 cells. In addition to clone PERV-A(42) derived from 293 cells, four "native" full-length proviral PERV sequences derived from a genomic library of the porcine cell line PK15 were isolated. Three identical class A clones, designated PK15-PERV-A(42), PK15-PERV-A(45), and PK15-PERV-A(58), and one class B clone, PK15-PERV-B(213), were characterized. PK15-PERV-B(213) is highly homologous but distinct from the previously described clone PERV-B(43). PK15-PERV-A(58) demonstrates close homology to PERV-A(42) in env and to PERV-C in long terminal repeat, gag, and pro/pol sequences. All three PERV clones described here were replication competent upon infection of susceptible cell lines. The findings suggest that the pig genome harbors a limited number of infectious PERV-A and -B sequences.

Xenogeneic transplantation

This review summarizes the clinical history and rationale for xenotransplantation; recent progress in understanding the physiologic, immunologic, and infectious obstacles to the procedure's success; and some of the strategies being pursued to overcome these obstacles. The problems of xenotransplantation are complex, and a combination of approaches is required. The earliest and most striking immunologic obstacle, that of hyperacute rejection, appears to be the closest to being solved. This phenomenon depends on the binding of natural antibody to the vascular endothelium, fixation of complement by that antibody, and finally, activation of the endothelium and initiation of coagulation. Therefore, these three pathways have been targeted as sites for intervention in the process. The mechanisms responsible for the next immunologic barrier, that of delayed xenograft/acute vascular rejection, remain to be fully elucidated. They probably also involve multiple pathways, including antibody and/or immune cell binding and endothelial cell activation. The final immunologic barrier, that of the cellular immune response, involves mechanisms that are similar to those involved in allograft rejection. However, the strength of the cellular immune response to xenografts is so great that it is unlikely to be controlled by the types of nonspecific immunosuppression used routinely to prevent allograft rejection. For this reason, it may be essential to induce specific immunologic unresponsiveness to at least some of the most antigenic xenogeneic molecules.

Posttransplantation lymphoproliferative disease in miniature swine after allogeneic hematopoietic cell transplantation: similarity to human PTLD and association with a porcine gammaherpesvirus

Posttransplantation lymphoproliferative disease (PTLD) is a major complication of current clinical transplantation regimens. The lack of a reproducible large-animal model of PTLD has limited progress in understanding the pathogenesis of and in developing therapy for this clinically important disease. This study found a high incidence of PTLD in miniature swine undergoing allogeneic hematopoietic stem cell transplantation and characterized this disease in swine. Two days before allogeneic peripheral blood stem cell transplantation, miniature swine were conditioned with thymic irradiation and in vivo T-cell depletion. Animals received cyclosporine daily beginning 1 day before transplantation and continuing for 30 to 60 days. Flow cytometry and histologic examination were performed to determine the cell type involved in lymphoproliferation. Polymerase chain reaction was developed to detect and determine the level of porcine gammaherpesvirus in involved lymph node tissue. PTLD in swine is morphologically and histologically similar to that observed in human allograft recipients. Nine of 21 animals developed a B-cell lymphoproliferation involving peripheral blood (9 of 9), tonsils, and lymph nodes (7 of 9) from 21 to 48 days after transplantation. Six of 9 animals died of PTLD and 3 of 9 recovered after reduction of immunosuppression. A novel porcine gammaherpesvirus was identified in involved tissues. Miniature swine provide a genetically defined large-animal model of PTLD with many characteristics similar to human PTLD. The availability of this reproducible large-animal model of PTLD may facilitate the development and testing of diagnostic and therapeutic approaches for prevention or treatment of PTLD in the clinical setting.

Physiologic and immunologic hurdles to xenotransplantation

The major problem in the field of renal transplantation is currently the shortage of available kidneys. However, the use of animals as a source of kidneys, i.e., xenotransplantation, is increasingly being viewed as a potential solution to this problem. One preeminent hurdle to xenotransplantation is the immune response of the recipient against the graft; other hurdles include the physiologic limitations of the transplant, infection, and ethical considerations. This review summarizes what is currently known regarding the obstacles to xenotransplantation and some potential solutions to those problems.

Cardiac xenotransplantation: clinical experience and future direction

The shortage of human organs has focused research on finding an animal source of replacement organs. The immunological barriers to xenotransplantation are now more clearly defined, allowing retrospective interpretation of past clinical experience in humans. Due to physiological compatibilities as well as ethical and infectious considerations, pigs have now emerged as the most likely source of future xenografts. The introduction of transgenic pigs expressing human complement regulatory proteins and new immunosuppressive regimens have shown early promise in the laboratory, although further advancements are needed to advance to clinical trials. Additional clarification of infectious risks and patient strategies are remaining obstacles to application in the clinical arena.

An approach to the control of disease transmission in pig-to-human xenotransplantation

Although several major immunologic hurdles need to be overcome, the pig is currently considered the most likely source animal of cells, tissues and organs for transplantation into humans. Concerns have been raised with regard to the potential for the transfer of infectious agents with the transplanted organ to the human recipient. This risk is perceived to be increased as it is likely that the patient will be iatrogenically immunocompromised and the organ-source pig may be genetically engineered in such a way to render its organs particularly susceptible to infection with human viruses. Furthermore, the risk may not be restricted to the recipient, but may have consequences for the health of others in the community. The identification of porcine endogenous retroviruses and of hitherto unknown viruses have given rise to the most concern. We document here the agents we believe should be excluded from the organ-source pigs. We discuss the likelihood of achieving this aim and outline the potential means by which it may best be achieved.

Establishment and characterization of molecular clones of porcine endogenous retroviruses replicating on human cells

The use of pig xenografts is being considered to alleviate the shortage of allogeneic organs for transplantation. In addition to the problems overcoming immunological and physiological barriers, the existence of numerous porcine microorganisms poses the risk of initiating a xenozoonosis. Recently, different classes of type C porcine endogenous retoviruses (PERV) which are infectious for human cells in vitro have been partially described. We therefore examined whether completely intact proviruses exist that produce infectious and replication-competent virions. Several proviral PERV sequences were cloned and characterized. One molecular PERV class B clone, PERV-B(43), generated infectious particles after transfection into human 293 cells. A second clone, PERV-B(33), which was highly homologous to PERV-B(43), showed a G-to-A mutation in the first start codon (Met to Ile) of the env gene, preventing this provirus from replicating. However, a genetic recombinant, PERV-B(33)/ATG, carrying a restored env start codon, became infectious and could be serially passaged on 293 cells similar to virus clone PERV-B(43). PERV protein expression was detected 24 to 48 h posttransfection (p. t.) using cross-reacting antiserum, and reverse transcriptase activity was found at 12 to 14 days p.t. The transcriptional start and stop sites as well as the splice donor and splice acceptor sites of PERV mRNA were mapped, yielding a subgenomic env transcript of 3. 1 kb. PERV-B(33) and PERV-B(43) differ in the number of copies of a 39-bp segment in the U3 region of the long terminal repeat. Strategies to identify and to specifically suppress or eliminate those proviruses from the pig genome might help in the production of PERV-free animals.

Xenotransplantation

BACKGROUND

The success of clinical transplantation has led to a large discrepancy between donor organ availability and demand; considerable pressure exists to develop an alternative source of organs. The use of animal organs for donation is a possible solution that is not yet clinically applicable.

METHODS AND RESULTS

A literature review was performed based on a Medline search to find articles on xenotransplantation. Keywords included hyperacute, acute vascular, xenograft rejection combined with concordant and discordant. Additional references cited in these articles from journals not included in Medline were obtained from the British Library. Limited information on unpublished, preliminary work has been included from sources known to the authors, based on their research work in the field. One hundred and forty-six references and four personal communications have been included in this review article.

CONCLUSION

A greater understanding of the pathogenesis of xenograft rejection is developing rapidly. Strategies to abrogate hyperacute rejection have proved successful, but control of antibody-driven acute vascular rejection has not yet been achieved. The safety and viability of xenotransplantation as a therapeutic modality are still unproven.

Characterization of the DNA polymerase loci of the novel porcine lymphotropic herpesviruses 1 and 2 in domestic and feral pigs

Two novel porcine gammaherpesviruses, porcine lymphotropic herpesviruses 1 and 2 (PLHV-1 and -2), have been detected by amplification of short DNA polymerase (DPOL) sequences from blood and spleen of domestic pigs while searching for unknown herpesviruses in pigs as possible risk factors in xenotransplantation. In the present study, the DPOL genes of the two viruses and the open reading frames (ORFs) that follow in the downstream direction were amplified by PCR-based genome walking from adaptor-ligated restriction fragment libraries of porcine spleen samples. The sequences determined for the two PLHVs exhibited a very low G+C content (37 mol%) and a marked suppression of the CpG dinucleotide frequency. The DPOL proteins encoded were 95% identical and showed a close relationship (60% identity) to the DPOL protein of a ruminant gammaherpesvirus, alcelaphine herpesvirus 1 (AlHV-1). This was confirmed by phylogenetic analyses of the conserved regions of the two PLHV DPOL proteins. The PLHV ORFs downstream of DPOL exhibited 83% identity to each other and >>50% similarity to ORF A5, the position equivalent of AlHV-1. From these data, the PLHVs can be firmly classified to the subfamily Gammaherpesvirinae: To find a natural reservoir for the PLHVs, organs of feral pigs were screened with five different PCR assays, targetting either the DPOL gene or 3'-flanking sequences. In all samples, PLHV sequences were detected that originated predominantly from PLHV-2, suggesting the possibility of virus transfer between feral and domestic pig populations.

Xenotransplantation

Xenotransplantation is a potentially promising but exceedingly complex issue. It is critical that this subject be discussed within and outside the transplant community. The need for an expanded organ supply is urgent, but the scientific and ethical positions are complex. This article reviews the current status of xenotransplantation, including the potential benefits and risks; discusses multiple ethical issues; and makes recommendations for the transplant coordinator.

Evaluation of graft-host response for various tissue sources and animal models

The efficacy of pancreatic islet transplants in correcting hyperglycemia and slowing the progression of complications in diabetics has been confirmed by many experimental and clinical studies. Unfortunately, the availability of human islets is extremely limited and, therefore, treatment of large numbers of human diabetic patients will almost certainly require either the use of islets harvested from animals (xenografts) or the use of insulin-secreting genetically modified cells of either human or animal origin. There is currently no effective regimen which will allow long-term survival of xenogeneic islets from widely unrelated donor-recipient combinations, such as pig-to-rodent, pig-to-dog, or pig-to-primate. There is considerable interest in the development of immunoisolation techniques for protection of donor islets. However, most materials used in immunoisolation devices are relatively bio-incompatible. Poly-L-lysine-alginate microcapsules are biocompatible and provide an optimal geometry for transmembrane diffusion of insulin and nutrients. Microcapsules allow long-term survival of xenogeneic islets in diabetic rodents or dogs with induced diabetes. However, mice and rats with spontaneous diabetes destroy encapsulated islet grafts within 2 to 3 weeks. Biopsies reveal large numbers of macrophages, immunoglobulins and limited numbers of helper and cytotoxic T-cells in the peri-microcapsule environment of the peritoneal cavity. Cytokines have been identified in peritoneal fluid from mice with islet grafts and may play a role in encapsulated islet destruction. Targeted immunomodulation by treatment of recipients with either anti-helper T-cell antibodies, or fusion proteins which block costimulatory interactions between antigen presenting cells and host T-cells have demonstrated synergy in significant prolongation of encapsulated islet xenograft survival in NOD mice with spontaneous diabetes. Technical improvements in microcapsule design also have contributed to prolonged graft survival. "Double-wall" microencapsulation provides a more durable microcapsule and islet pretreatment prior to encapsulation reduces the frequency of defective capsules with islets entrapped in the membrane. Long-term durability of encapsulated islet grafts remains a concern and further improvements in microcapsule design are a prerequisite to clinical trials.

Transfer of porcine endogenous retrovirus across hollow fiber membranes: significance to a bioartificial liver

BACKGROUND

A porcine endogenous retrovirus (PERV) capable of infecting human cells has been identified. This study was designed to determine whether hollow fiber membranes, such as those used in a bioartificial liver, block the transfer of PERV.

METHODS

Three hollow fiber cartridges (HFCs) were studied in duplicate: cellulose fibers with 70 kD nominal molecular weight cut-off (MWCO), polysulfone fibers with 400 kD MWCO, and mixed cellulose fibers with 200 nm porosity. PK15 cells (porcine kidney cell line), known to produce PERV, were grown in the intraluminal compartment of HFCs fiber cartridges. Samples of medium were collected from both intraluminal and extraluminal compartments of the HFCs fiber cartridge during 14 days of culture. Samples were screened for PERV using reverse transcription polymerase chain reaction. All positive samples were tested for PERV infectivity in human 293 cells.

RESULTS

PERV was detected in all samples from the intraluminal space and all intraluminal samples seemed to infect 293 cells. All extraluminal samples from the fibers of 200 nm porosity tested positive for PERV. Detection of PERV in the extraluminal space was delayed by fibers of 400 kD MWCO and 70 kD MWCO until at least day 3 and day 7, respectively, after inoculation of PK15 cells. Positive extraluminal samples from fibers of 400 kD MWCO and 70 kD MWCO did not infect 293 cells.

CONCLUSION

Pore size, membrane composition, and duration of exposure influenced the transfer of PERV across HFCs. Some HFCs decrease the risk of viral exposure to patients during bioartificial liver therapy.

Endogenous retroviruses: a potential problem for xenotransplantation?

To overcome the shortage of suitable human donors for transplantation attention has recently turned to the possibility of using genetically modified pigs as a source of cells and organs. It has been suggested that such procedures might facilitate the introduction of novel retroviruses, normally resident in the pig germ line, into the human population (Stoye and Coffin, Nature Medicine 1: 1100, 1995). The consequences of such a transfer remain unclear; however, the demonstration that certain porcine cell lines express infectious retroviruses which can infect human cells (Patience et al., Nature Medicine 3: 282-286, 1997) emphasizes that there are grounds for practical concern. We have now cloned the envelope genes of the expressed viruses and are using these clones in studies of the interaction of the porcine viruses with their cellular receptors. We have also initiated studies of the inheritance and expression of human-tropic endogenous proviruses present in different pig populations. These studies reveal that at least two classes of human-tropic endogenous porcine retrovirus are widely distributed in pigs (Le Tissier et al., Nature 389: 681-681, 1997). The implications of our results for assessing the potential risk of retroviral transfer during xenotransplantation are discussed.

Definition of a production specification for xenotransplantation. A European perspective

In the absence of regulatory guidance on animal production for xenotransplantation and the need to conform to European, U.K. and international standards of animal health and welfare, Imutran set up a forum of experts in 1994 to assess the risks associated with the possible transmission of diseases from pig to man after transplantation of porcine tissue. This risk assessment set the specification for Qualified Pathogen-Free pig production and the animal testing required to demonstrate that the specification has been achieved. The invasive sampling required for detailed health monitoring of individual source animals is in conflict with the aim of producing healthy, normal animals that are not subject to undue stress, and produces predominantly retrospective data on a donor. Imutran has developed a sampling regime that qualifies age-class cohorts and the whole herd by the routine testing of representative sentinel animals. Imutran set up a pilot production unit in 1995 and this unit has remained populated beyond a 30-month period and has received in excess of 20 cohorts of animals by hysterectomy re-derivation. Rigorous testing of sentinels and noninvasive testing of individuals have demonstrated that this pilot unit has achieved the required specification on a routine basis in a system that has been designed to meet European ethical and animal welfare concerns, within the constraints of the strict U.K. animal research laws and an ISO 9001 quality environment.

Infection and xenotransplantation. Developing strategies to minimize risk

Infection in transplantation results from interaction between the level of immune suppression and the epidemiologic exposures of the recipient. "Xenosis," infection in xenotransplantation, may be increased beyond that of allotransplantation because: (1) the xenograft may serve as a permissive focus of infection for donor-derived organisms; (2) these organisms may be unknown or xenotropic; (3) microbiologic assays may be unavailable; (4) clinical syndromes due to such novel pathogens may not be recognized; (5) the necessary level of immune suppression may be greater than for allotransplantation; (6) donor-derived organisms may acquire new (e.g., genetic) characteristics in the human host; (7) the presence of immune suppression and the high, intrinsic rate of infection may mask the presence of xenosis; and (8) MHC-incompatibility may reduce the efficacy of the immune response within the xenograft. Because immunocompromised individuals are sentinels for infection by many types of novel infectious agents, and because there is some unknown level of risk that such pathogens will spread to the general population, microbiologic studies must be initiated in tandem with preclinical and clinical studies of xenotransplantation.

The risk of using baboons as transplant donors. Exogenous and endogenous viruses

African nonhuman primates harbor several exogenous and endogenous retroviruses which deserve further consideration in the transplant setting. In particular, simian foamy viruses (SFV), simian T-cell lymphotropic virus (STLV), baboon endogenous virus (BaEV), and simian endogenous retrovirus (SERV) are all carried by baboons and may be transmitted to humans by transplantation. We have found baboons to have high seroprevalence rates to both SFV and STLV, and molecular and serologic methods have been developed to detect such agents. In addition, current nonhuman primate breeding programs have thus far not focused on eliminating these viruses. In summary, the close genetic relationship with humans and number of persistent viral infections in baboons translates into a much greater infectious disease risk when compared to that of other domesticated species.

Host range and interference studies of three classes of pig endogenous retrovirus

Recent interest in the use of porcine organs, tissues, and cells for xenotransplantation to humans has highlighted the need to characterize the properties of pig endogenous retroviruses (PERVs). Analysis of a variety of pig cells allowed us to isolate and identify three classes of infectious type C endogenous retrovirus (PERV-A, PERV-B, and PERV-C) which have distinct env genes but have highly homologous sequences in the rest of the genome. To study the properties of these env genes, expression plasmids for the three env genes were constructed and used to generate retrovirus vectors bearing corresponding Env proteins. Host range analyses by the vector transduction assay showed that PERV-A and PERV-B Envs have wider host ranges, including several human cell lines, compared with PERV-C Env, which infected only two pig cell lines and one human cell line. All PERVs could infect pig cells, indicating that the PERVs have a potential to replicate in pig transplants in immunosuppressed patients. Receptors for PERV-A and PERV-B were present on cells of some other species, including mink, rat, mouse, and dog, suggesting that such species may provide useful model systems to study infection and pathogenicity of PERV. In contrast, no vector transduction was observed on nonhuman primate cell lines, casting doubt on the utility of nonhuman primates as models for PERV zoonosis. Interference studies showed that the three PERV strains use receptors distinct from each other and from a number of other type C mammalian retroviruses.