Human CD59 incorporation into porcine endogenous retrovirus particles: implications for the use of transgenic pigs for xenotransplantation

Relating GPI-Anchored Ly6 Proteins uPAR and CD59 to Viral Infection

The Ly6 (lymphocyte antigen-6)/uPAR (urokinase-type plasminogen activator receptor) superfamily protein is a group of molecules that share limited sequence homology but conserved three-fingered structures. Despite diverse cellular functions, such as in regulating host immunity, cell adhesion, and migration, the physiological roles of these factors in vivo remain poorly characterized. Notably, increasing research has focused on the interplays between Ly6/uPAR proteins and viral pathogens, the results of which have provided new insight into viral entry and virus-host interactions. While LY6E (lymphocyte antigen 6 family member E), one key member of the Ly6E/uPAR-family proteins, has been extensively studied, other members have not been well characterized. Here, we summarize current knowledge of Ly6/uPAR proteins related to viral infection, with a focus on uPAR and CD59. Our goal is to provide an up-to-date view of the Ly6/uPAR-family proteins and associated virus-host interaction and viral pathogenesis.

Porcine endogenous retroviruses in xenotransplantation--molecular aspects

In the context of the shortage of organs and other tissues for use in human transplantation, xenotransplantation procedures with material taken from pigs have come under increased consideration. However, there are unclear consequences of the potential transmission of porcine pathogens to humans. Of particular concern are porcine endogenous retroviruses (PERVs). Three subtypes of PERV have been identified, of which PERV-A and PERV-B have the ability to infect human cells in vitro. The PERV-C subtype does not show this ability but recombinant PERV-A/C forms have demonstrated infectivity in human cells. In view of the risk presented by these observations, the International Xenotransplantation Association recently indicated the existence of four strategies to prevent transmission of PERVs. This article focuses on the molecular aspects of PERV infection in xenotransplantation and reviews the techniques available for the detection of PERV DNA, RNA, reverse transcriptase activity and proteins, and anti-PERV antibodies to enable carrying out these recommendations. These methods could be used to evaluate the risk of PERV transmission in human recipients, enhance the effectiveness and reliability of monitoring procedures, and stimulate discussion on the development of improved, more sensitive methods for the detection of PERVs in the future.

No biological evidence of XMRV in blood or prostatic fluid from prostate cancer patients

Background

XMRV (xenotropic murine leukemia virus-related virus) was initially discovered in association with prostate cancer and later with chronic fatigue syndrome (CFS). Its association with CFS is now largely discredited, and current results support a laboratory origin for XMRV with no reproducible evidence for infection of humans. However, some results indicating the presence of XMRV in prostate cancer are difficult to attribute to sample contamination. Here we have sought biological evidence that might confirm the presence of XMRV in prostate cancer samples previously having tested positive.

Methods and Results

We have tested for infectious XMRV and neutralizing antibodies against XMRV in blood plasma from 29 subjects with prostate cancer, and for infectious XMRV in prostate secretions from another five prostate cancer subjects. Nine of these subjects had previously tested positive for XMRV by PCR or by virus assay. We did not detect XMRV or related retroviruses in any sample, and the neutralizing activities of the plasma samples were all very low, a result inconsistent with XMRV infection of the plasma donors.

Conclusions

We find no evidence for XMRV infection of any human subject tested, either by assay for infectious virus or for neutralizing antibodies. Our results are consistent with the majority of published studies on XMRV, which find that XMRV is not present in humans. The observed low to undetectable XMRV neutralization by human plasma indicates a lack of innate restriction of XMRV replication by soluble factors in human blood.

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.

Induction of ebolavirus cross-species immunity using retrovirus-like particles bearing the Ebola virus glycoprotein lacking the mucin-like domain

Background

The genus Ebolavirus includes five distinct viruses. Four of these viruses cause hemorrhagic fever in humans. Currently there are no licensed vaccines for any of them; however, several vaccines are under development. Ebola virus envelope glycoprotein (GP_1,2) is highly immunogenic, but antibodies frequently arise against its least conserved mucin-like domain (MLD). We hypothesized that immunization with MLD-deleted GP_1,2 (GPΔMLD) would induce cross-species immunity by making more conserved regions accessible to the immune system.

Methods

To test this hypothesis, mice were immunized with retrovirus-like particles (retroVLPs) bearing Ebola virus GPΔMLD, DNA plasmids (plasmo-retroVLP) that can produce such retroVLPs in vivo, or plasmo-retroVLP followed by retroVLPs.

Results

Cross-species neutralizing antibody and GP_1,2-specific cellular immune responses were successfully induced.

Conclusion

Our findings suggest that GPΔMLD presented through retroVLPs may provide a strategy for development of a vaccine against multiple ebolaviruses. Similar vaccination strategies may be adopted for other viruses whose envelope proteins contain highly variable regions that may mask more conserved domains from the immune system.

No evidence of murine-like gammaretroviruses in CFS patients previously identified as XMRV-infected

Members of the gammaretroviruses--such as murine leukemia viruses (MLVs), most notably XMRV [xenotropic murine leukemia virus (X-MLV)-related virus--have been reported to be present in the blood of patients with chronic fatigue syndrome (CFS). We evaluated blood samples from 61 patients with CFS from a single clinical practice, 43 of whom had previously been identified as XMRV-positive. Our analysis included polymerase chain reaction and reverse transcription polymerase chain reaction procedures for detection of viral nucleic acids and assays for detection of infectious virus and virus-specific antibodies. We found no evidence of XMRV or other MLVs in these blood samples. In addition, we found that these gammaretroviruses were strongly (X-MLV) or partially (XMRV) susceptible to inactivation by sera from CFS patients and healthy controls, which suggested that establishment of a successful MLV infection in humans would be unlikely. Consistent with previous reports, we detected MLV sequences in commercial laboratory reagents. Our results indicate that previous evidence linking XMRV and MLVs to CFS is likely attributable to laboratory contamination.

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

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

Optimized method for computing (18)O/(16)O ratios of differentially stable-isotope labeled peptides in the context of postdigestion (18)O exchange/labeling

Differential (18)O/(16)O stable isotope labeling of peptides that relies on enzyme-catalyzed oxygen exchange at their carboxyl termini in the presence of H(2)(18)O has been widely used for relative quantitation of peptides/proteins. The role of tryptic proteolysis in bottom-up shotgun proteomics and low reagent costs have made trypsin-catalyzed (18)O postdigestion exchange a convenient and affordable stable isotope labeling approach. However, it is known that trypsin-catalyzed (18)O exchange at the carboxyl terminus is in many instances inhomogeneous/incomplete. The extent of the (18)O exchange/incorporation fluctuates from peptide to peptide mostly due to variable enzyme-substrate affinity. Thus, accurate calculation and interpretation of peptide ratios are analytically complicated and in some regard deficient. Therefore, a computational approach capable of improved measurement of actual (18)O incorporation for each differentially labeled peptide pair is needed. In this regard, we have developed an algorithmic method that relies on the trapezoidal rule to integrate peak intensities of all detected isotopic species across a particular peptide ion over the retention time, which fits the isotopic manifold to Poisson distributions. Optimal values for manifold fitting were calculated and then (18)O/(16)O ratios derived via evolutionary programming. The algorithm is tested using trypsin-catalyzed (18)O postdigestion exchange to differentially label bovine serum albumin (BSA) at a priori determined ratios. Both accuracy and precision are improved utilizing this rigorous mathematical approach. We further demonstrate the effectiveness of this method to accurately calculate (18)O/(16)O ratios in a large scale proteomic quantitation of detergent resistant membrane microdomains (DRMMs) isolated from cells expressing wild-type HIV-1 Gag and its nonmyristylated mutant.

A newly cloned pig dolichyl-phosphate mannosyl-transferase for preventing the transmission of porcine endogenous retrovirus to human cells

Porcine endogenous retrovirus (PERV) is a major problem associated with successful clinical xenotransplantation. In our previous study, reducing the high mannose type of N-glycan content proved to be very effective in downregulating PERV infectivity. In this study, dolichyl-phosphate mannosyltransferase (D-P-M), an enzyme related to the early stages of N-linked sugar synthesis was studied. The pig cDNA of the encoding D-P-M was newly isolated. The RNA interference (siRNA) for the D-P-M was applied and transfected to PEC(Z)/PB cells, a pig endothelial cell line with the Lac Z gene and PERV-B, to reduce the levels of high mannose type N-glycans. Compared with the mock line, the temporary PEC(Z)/PB lines showed a decreased mRNA expression for pig D-P-M, and each line then showed a clear destruction of PERV infectivity to human cells in the Lac Z pseudotype assay. The PEC(Z)/PB was next transfected with pSXGH-siRNA, H1-RNA gene promoter. The established PEC(Z)/PB clones with pSXGH-siRNA clearly led to the downregulation of PERV infectivity, as evidenced by the decreased levels of the mRNA for pig D-P-M. Reducing D-P-M enzyme activity represents a potentially useful approach to address the problem of PERV infections in clinical xenotransplantations.

Differential human serum-mediated neutralization of PERV released from pig cells transfected with variants of hDAF

BACKGROUND

Expression of complement regulatory proteins (CRP) on pig endothelial cells (PEC) is an effective means of avoiding induction of hyperacute rejection by human sera. However, pig endogenous retrovirus (PERV) from PEC transfected with CRP may acquire resistance to human sera. This study investigated a form of transfected CRP that is easily expressed on PERV particles.

METHODS

The PEC line was transfected with the Lac Z gene and PERV-B to investigate PERV infectivity using a Lac Z pseudo-type assay. The cDNAs of several modified DAF (CD55) were then transfected into the PEC(Lac Z)/P-B lines using lipofection. DAF expression was verified by FACS analysis. Complement-dependent PEC lysis was tested to verify the complement regulatory function of the expressed DAF. HEK293 cells were incubated with PEC culture supernatants with or without human sera. The inoculated 293 cells were histochemically stained and Lac Z-positive blue foci were counted. The rate of reduction in Lac Z-positive cells resulting from the addition of human serum was then calculated. In addition, to assess the localization of the expressed DAF, flotation sucrose density analysis was performed.

RESULTS

While PERV released from PEC expressing delta-short consensus repeat 2 (delta-SCR2) DAF (lacking CRP function) showed no change in resistance to human serum compared to control cells, PERV from cells expressing delta-SCR1 DAF (with CRP function) showed a significant increase in resistance. The DAF-blocking antibody assay indicated that PERV from the DAF transfectants expressed DAF molecules on the surface of the retrovirus. While delta-SCR1 DAF (PI-anchor form) significantly inhibited the reduction of Lac Z-positive cells by human serum, the reduction of Lac Z-positive cells by human serum was less inhibited in the case of transmembrane (TM)-types of DAF-HLA-G, modified influenza hemagglutinin (HA) and MCP (delta-CYT form). However, the reduction in each TM-type DAF was slightly less than that observed in naive and mock cells. The flotation sucrose density analysis of these transfectants indicated that the PI-anchor form of DAF is a raft-associated protein, and most TM-types of DAF are non-raft proteins.

CONCLUSION

Induction of resistance to human serum in PERV, depends on the form of the CRP tail. The CRP/TM hybrid that does not associate with lipid rafts, is a suitable form of CRP for gene transduction.

Identification and molecular characterization of PERV gamma1 long terminal repeats

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

Reappraisal of biosafety risks posed by PERVs in xenotransplantation

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

The perspectives for porcine-to-human xenografts

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

Analysis of pig-to-human porcine endogenous retrovirus transmission in a triple-species kidney xenotransplantation model

Clinical pig-to-human xenotransplantation might be associated with the risk of transmission of xenozoonoses, especially porcine endogenous retroviruses (PERVs). We have established a pig-to-humanised-cynomolgus monkey xenotransplantation model allowing the analysis of potential PERV-transmission from normal or transgenic porcine organs to human vascular tissue. Pig-to-human kidney xenotransplantation was performed in cynomolgus monkeys. An interposition graft constructed from a human saphena vein replaced the porcine kidney vein. After graft rejection and/or death of the recipient (survival 2, 4, 6, 13, 16, 19 days), the human interposition grafts were removed. Human endothelial cells (huECs) were isolated from the interposition grafts and cultivated in vitro. Explanted human vascular tissue, isolated huECs, plasma and serum samples of the graft recipients were characterised by flow cytometry and immunohistochemistry and screened for indications of PERV transmission by quantitative polymerase chain reaction (PCR), reverse transcriptase-polymerase chain reaction (RT-PCR) and RT assay. PERV-specific immune response of recipients was analysed by Western blot. No evidence of PERV infection or PERV-specific immune response was detected.

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.

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.

ACTIVATION OF PRIMARY PORCINE ENDOTHELIAL CELLS INDUCES RELEASE OF PORCINE ENDOGENOUS RETROVIRUSES

BACKGROUND

Endothelial cells form the interface between the porcine graft and the recipient and frequently become activated after xenotransplantation. To evaluate the safety of xenotransplantation further, we assessed the effect of cellular activation on the expression and release of porcine endogenous retroviruses from primary endothelial cells isolated from transgenic and nontransgenic pigs.

METHODS

Primary porcine endothelial cells, cultured from pigs transgenic for human decay accelerating factor, were treated with human tumor necrosis factor-alpha, porcine interferon-gamma, or lipopolysaccharide. The release of porcine endogenous retroviruses into the supernatant was monitored at 24-hr intervals (up to 72 hr) by polymerase chain reaction-based reverse transcriptase (PBRT) assay. Activated and unactivated endothelial cells were co-cultured with human cells to investigate the capacity of any virus released from the porcine cells to infect human cells.

RESULTS

Virus was not detected in supernatants from quiescent cells by PBRT analysis. The number of viral particles released from endothelial cells was 10 to 5 x 10 viral particles/mL after cellular activation with tumor necrosis factor-alpha, interferon-gamma, or lipopolysaccharide, as shown by PBRT analysis. In contrast, in vitro infection of human cells was observed with unactivated endothelial cells only and was not observed in co-cultures with the activated porcine cells.

CONCLUSIONS

Cytokine treatment of primary porcine endothelial cells results in an increase in the release of virus into the supernatant, but the observed increase in viral titer was not mirrored by an increase in infectivity toward human cells.

Xenografts are an achievable breakthrough

The objective of this communication is to show that pig-to-human organ transplantation could be feasible through genetic engineering. By introducing into donor pigs several different tolerance promoting genetic modifications there can be a synergistic effect to produce extended tolerance for xenografted organs in human recipients. Nuclear-transfer cloning allows production of pigs with knockout mutations in the galactose-alpha-1,3-galactosyl transferase gene, in principle eliminating hyperacute rejection. Once hyperacute rejection is circumvented, long-term tolerance of xenografted organs should be possible through a combination of transgenic immunomodulating molecule, bone marrow chimerism and short to intermediate term use of immunosuppressive drugs. If immunomodulating transgenes are deleterious during pig development, inducible cre-recombinase excision of stop codons provides a means to delay expression of such transgenes until after transplantation. Zoonotic diseases can be circumvented via pathogen-free colonies and additional knockout mutations to disable porcine endogenous retrovirus and prion disease. Thus, there is now a technical and theoretical framework for serious efforts at cross-species transplantation.

Sensitivity to human serum of gammaretroviruses produced from pig endothelial cells transduced with glycosyltransferase genes

Reduction of pig cell-surface alpha-galactosyl (Gal) epitope, Galalpha1, 3Galbeta1, 4GlcNAc-R, by the introduction of glycosyltransferase genes is effective in suppressing hyperacute rejection (HAR) in pig-to-human xenotransplantation. The transmission of porcine endogenous retroviruses (PERVs) has been recognized as a potential risk factor associated with xenotransplantation. In this study, effects of the introduction of glycosyltransferase genes to pig cells on the sensitivity of gammaretroviruses to human serum were investigated. Pig endothelial cells (PEC), PEC transduced with alpha1,2 fucosyltransferase (FT), alpha2,3 sialyltransferase (ST), or N-acetylglucosaminyltransferase III (GnT-III), and human embryonic kidney (HEK) 293 cells were transduced with the LacZ gene with the packaging signal of murine leukemia virus (MuLV) under the control of the long terminal repeat of MuLV by a pseudotype infection. Then, the cells were further infected with PERV subtype B (PERV-B) or feline leukemia virus subgroup B (FeLV-B). Culture supernatants of the infected cells were mixed with human serum (HS) and then inoculated to HEK293 cells. The inoculated cells were histochemically stained and lacZ-positive blue foci were counted. Glycosyltransferase activity, xenoantigenicity, and alpha-Gal epitope density in the cells were measured at the time of the infection experiments. PERV-B or FeLV-B particles from the parental PEC were efficiently neutralized by HS, while those from PEC transduced with alpha1,2FT, alpha2,3ST or GnT-III were less sensitive to HS. The transduced PEC exhibited high levels of activity of the introduced glycotransferases, and expressed fewer xenoantigens and cell-surface alpha-Gal epitopes. Our results suggest that gammaretroviruses including PERVs produced by transgenic pigs, that are generally modified to reduce the cell-surface alpha-Gal epitope to overcome the HAR in xenotransplantation, are less sensitive to HS.

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.

Xenotransplantation and pig endogenous retroviruses

Xenotransplantation, in particular transplantation of pig cells, tissues and organs into human patients, may alleviate the current shortage of suitable allografts available for human transplantation. This overview addresses the physiological, immunological and virological factors considered with regard to xenotransplantation. Among the issues reviewed are the merits of using pigs as xenograft source species, the compatibility of pig and human organ physiology and the immunological hindrances with regard to the various types of rejection and attempts at abrogating rejection. Advances in the prevention of pig organ rejection by creating genetically modified pigs that are more suited to the human microenvironment are also discussed. Finally, with regard to virology, possible zoonotic infections emanating from pigs are reviewed, with special emphasis on the pig endogenous retrovirus (PERV). An in depth account of PERV studies, comprising their discovery as well as recent knowledge of the virus, is given. To date, all retrospective studies on patients with pig xenografts have shown no evidence of PERV transmission, however, many factors make us interpret these results with caution. Although the lack of PERV infection in xenograft recipients up to now is encouraging, more basic research and controlled animal studies that mimic the pig to human xenotransplantation setting more closely are required for safety assessment.

Characterization of porcine endogenous retrovirus gamma pro-pol nucleotide sequences

Endogenous retroviral sequences in the pig genome (PERV) represent a potential infectious risk in xenotransplantation. All known infectious PERV have been asssigned to the PERV gamma1 family, consisting of the subfamilies A, B, and C. The aim of the study was the concise examination of PERV gamma by the analysis of the retroviral pro-pol sequences. The analysis of 52 pro-pol clones amplified in this study revealed eight PERV gamma families. In addition to four already-described families (gamma1, gamma4, gamma5, gamma6), four novel families (gamma7, gamma8, gamma9, gamma10) were identified. Quantitative analysis of the novel PERV gamma sequences in selected breeds revealed variations in the endogenous retroviral load. Open reading frames (ORF) in the amplified proviral fragment were only found for PERV gamma1. In addition, novel ORF-containing PERV gamma1 clones consisting of hybrid sequences were revealed. Sequence comparison from published full-length PERV gamma1 clones of the PERV subfamilies A, B, and C resulted in a lack of strict correlation of the classification of pro-pol and env. The results indicated the occurrence of causative recombination events between retroviral genomes. Thus, our study on PERV gamma provides new data for the evaluation and selection of pigs intended to be used in xenotransplantation.