Identification of receptors for pig endogenous retrovirus

Epigenetic regulation on the 5'-proximal CpG island of human porcine endogenous retrovirus subgroup A receptor 2/GPR172B

Porcine endogenous retroviruses (PERVs) have been considered one of the major risks of xenotransplantation from pigs to humans. PERV-A efficiently utilizes human PERV-A receptor 2 (HuPAR-2)/GPR172B to infect human cells; however, there has been no study on the regulation mechanisms of HuPAR-2/GPR172B expression. In this study, we examined the expression of HuPAR-2/GPR172B from the standpoint of epigenetic regulation and discussed the risks of PERV-A infection in xenotransplantation. Quantitative real-time RT-PCR revealed that HuPAR-2 mRNA was preferentially expressed in placental tissue, whereas it was highly suppressed in BeWo cells (a human choriocarcinoma cell line) and HEK293 cells. A CpG island containing the HuPAR-2 transcription starting site was identified by in silico analysis. The DNA methylation ratio (the relative quantity of methylcytosine to total cytosine) and histone modification (H3K9me3) levels in the CpG island measured by bisulfite genomic sequencing and ChIP assay, respectively, were inversely correlated with the mRNA levels. Both HuPAR-2 mRNA and HuPAR-2 protein were up-regulated in HEK293 cells by inhibiting DNA methylation and histone deacetylation. Additionally, promoter/enhancer activities within the CpG island were suppressed by in vitro DNA methylation. Our results demonstrated that epigenetic modification regulates HuPAR-2 expression.

The rapid production of high-titer porcine endogenous retrovirus(PERV)-B env pseudotype and construction of an EGFP-expressing replication competent PERV-A vector

Porcine endogenous retroviruses (PERVs) present a unique concern associated with xenotransplantation because they have been shown to infect certain human cells in vitro and it is also difficult to generate herds of pigs free of PERVs. A simple system for the production of high-titer MoMLV-PERV pseudotypes is reported; an EGFP-expressing replication-competent molecular clone that allows direct measurement of titer was also constructed. To improve the MLV-based retroviral vector system, a 2.1-kb PERV-B env product was amplified from PK-15 genomic DNA and cloned into the pCL-Eco retroviral vector. The titer of lacZ (PERV-B) from the 293 cells was about 1.0×10(4) CFU/ml. In contrast, the titer of lacZ (PERV-B) from a conventional murine retroviral vector (split genome) was found to be 1.2×10(2) CFU/ml when the PERV-B env expression vector was transfected into TELCeB6 cells, which harbor MFGnlslacZ and the gag-pol-expressing vector. In addition, an infectious PERV-A clone containing enhanced GFP (EGFP) by using a PCR-based method was developed. This EGFP-expressing PERV-A-IRES-EGFP molecular clone was found to be stable genetically on transfection in 293 cells.

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

Background

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

Principal Findings

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

Conclusions

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

[Receptors for animal retroviruses]

Diseases caused by animal retroviruses have been recognized since 19th century in veterinary field. Most livestock and companion animals have own retroviruses. To disclose the receptors for these retroviruses will be useful for understanding retroviral pathogenesis, developments of anti-retroviral drugs and vectors for human and animal gene therapies. Of retroviruses in veterinary field, receptors for the following viruses have been identified; equine infectious anemia virus, feline immunodeficiency virus, feline leukemia virus subgroups A, B, C, and T, Jaagsiekte sheep retrovirus, enzootic nasal tumor virus, avian leukosis virus subgroups A, B, C, D, E, and J, reticuloendotheliosis virus, RD-114 virus (a feline endogenous retrovirus), and porcine endogenous retrovirus subgroup A. Primate lentiviruses require two molecules (CD4 and chemokine receptors such as CXCR4) as receptors. Likewise, feline immunodeficiency virus also requires two molecules, i.e., CD134 (an activation marker of CD4 T cells) and CXCR4 in infection. Gammaretroviruses utilize multi-spanning transmembrane proteins, most of which are transporters of amino acids, vitamins and inorganic ions. Betaretroviruses and alpharetroviruses utilize transmembrane and/or GPI-anchored proteins as receptors. In this review, I overviewed receptors for animal retroviruses in veterinary field.

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.

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.

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.

Single-round selection yields a unique retroviral envelope utilizing GPR172A as its host receptor

The recognition by a viral envelope of its cognate host-cell receptor is the initial critical step in defining the viral host-range and tissue specificity. This study combines a single-round of selection of a random envelope library with a parallel cDNA screen for receptor function to identify a distinct retroviral envelope/receptor pair. The 11-aa targeting domain of the modified feline leukemia virus envelope consists of a constrained peptide. Critical to the binding of the constrained peptide envelope to its cellular receptor are a pair of internal cysteines and an essential Trp required for maintenance of titers >10(5) lacZ staining units per milliliter. The receptor used for viral entry is the human GPR172A protein, a G-protein-coupled receptor isolated from osteosarcoma cells. The ability to generate unique envelopes capable of using tissue- or disease-specific receptors marks an advance in the development of efficient gene-therapy vectors.

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.

Identification and functional characterization of a novel human and rat riboflavin transporter, RFT1

Absorption of riboflavin is mediated by transporter(s). However, a mammalian riboflavin transporter has yet to be identified. In the present study, the novel human and rat riboflavin transporters hRFT1 and rRFT1 were identified on the basis of our rat kidney mRNA expression database (Horiba N, Masuda S, Takeuchi A, Saito H, Okuda M, Inui K. Kidney Int 66: 29-45, 2004). hRFT1 and rRFT1 cDNAs have an open reading frame encoding 448- and 450-amino acid proteins, respectively, that exhibit 81.1% identity and 96.4% similarity to one another. In addition, an inactive splice variant of hRFT1, hRFT1sv, was also cloned. The hRFT1sv cDNA, which encodes a 167-amino acid protein, retains an intron between exons 2 and 3 of hRFT1. Real-time PCR revealed that the sum of hRFT1 and hRFT1sv mRNAs was expressed strongly in the placenta and small intestine and was detected in all tissues examined. In addition, hRFT1 and hRFT1sv were expressed in human embryonic kidney (HEK)-293 and Caco-2 cells. HEK-293 cells transfected with green fluorescent protein-tagged hRFT1 and rRFT1 exhibited a fluorescent signal in the plasma membrane. Overexpression of hRFT1 and rRFT1, but not hRFT1sv, increased the cellular accumulation of [(3)H]riboflavin. The transfection of small interfering RNA targeting both hRFT1 and hRFT1sv significantly decreased the uptake of [(3)H]riboflavin by HEK-293 and Caco-2 cells. Riboflavin transport is Na(+), potential, and pH independent. Kinetic analyses demonstrated that the Michaelis-Menten constants for the uptake by HEK-293 and Caco-2 cells were 28.1 and 63.7 nM, respectively. We propose that hRFT1 and rRFT1 are novel mammalian riboflavin transporters, which belong to a new mammalian riboflavin transporter family.

Recombinant porcine endogenous retroviruses (PERV-A/C): a new risk for xenotransplantation?

PERVs are integrated in the genome of all pigs. Some of them infect human cells and represent therefore a potential risk for xenotransplantation using pig cells or organs. Three replication-competent subtypes have been described, PERV-A, PERV-B and PERV-C. Whereas PERV-A and PERV-B are polytropic viruses and infect, among others, human cells, PERV-C is an ecotropic virus, infecting only pig cells. Recombinant PERV-A/C are able to infect human cells, they are characterised by high-titre replication and their proviruses have been found de novo integrated in the genome of somatic pig cells, but not in the germ line. This review compares recombinant PERVs with other recombinant retroviruses in order to evaluate their potential pathogenicity.

Identification of residues outside of the receptor binding domain that influence the infectivity and tropism of porcine endogenous retrovirus

Identification of determinants of human tropism of porcine endogenous retrovirus (PERV) is critical to understanding the risk of transmission of PERV to recipients of porcine xenotransplantation products. Previously, we showed that a chimeric envelope cDNA encoding the 360 N-terminal residues of the human-tropic PERV envelope class A (PERV-A) SU and the 130 C-terminal residues of the pig-tropic PERV-C SU and all of TM (PERV-A/C) showed a 100-fold decrease in infectivity titer on human cells (M. Gemeniano, O. Mpanju, D. R. Salomon, M. V. Eiden, and C. A. Wilson, Virology 346:108-117, 2006). To identify residues important for human cell infection, we performed site-directed mutagenesis on each of the nine residues, singly or in combination, that distinguish the C-terminal region of PERV-C from PERV-A. Of the nine amino acids, two single-amino-acid substitutions, Q374R and I412V, restored the infectivity of human cells to the chimeric PERV-A/C to a titer equivalent to that of PERV-A. In contrast, PERV-A/C mutant envelope Q439P resulted in undetectable infection of human cells and an approximately 1,000-fold decrease in control pig cells. Mutation of K441R rescued mutants that carried Q439P, suggesting an incompatibility between the proline residue at this position and the presence of KK in the proteolytic cleavage signal. We confirmed this incompatibility with vectors carrying PERV-A envelope mutant R462K that were also rendered noninfectious. Finally, tropism of vectors carrying PERV-C envelope mutants with only four amino acid changes in the C terminus of PERV-C envelope, NHRQ436YNRP plus K441R, was shifted to one similar to that of PERV-A. Our results show an important and previously unrecognized role for infectivity and tropism for residues at the C terminus of SU.

Functional hierarchy of two L domains in porcine endogenous retrovirus (PERV) that influence release and infectivity

The porcine endogenous retrovirus (PERV) Gag protein contains two late (L) domain motifs, PPPY and P(F/S)AP. Using viral release assays we demonstrate that PPPY is the dominant L domain involved in PERV release. PFAP represents a novel retroviral L domain variant and is defined by abnormal viral assembly phenotypes visualized by electron microscopy and attenuation of early PERV release as measured by viral genomes. PSAP is functionally dominant over PFAP in early PERV release. PSAP virions are 3.5-fold more infectious in vitro by TCID(50) and in vivo results in more RNA positive tissues and higher levels of proviral DNA using our human PERV-A receptor (HuPAR-2) transgenic mouse model [Martina, Y., Marcucci, K.T., Cherqui, S., Szabo, A., Drysdale, T., Srinivisan, U., Wilson, C.A., Patience, C., Salomon, D.R., 2006. Mice transgenic for a human porcine endogenous retrovirus receptor are susceptible to productive viral infection. J. Virol. 80 (7), 3135-3146]. The functional hierarchies displayed by PERV L domains, demonstrates that L domain selection in viral evolution exists to promote efficient viral assembly, release and infectivity in the virus-host context.

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.

Beta-cell replacement and regeneration: Strategies of cell-based therapy for type 1 diabetes mellitus

Pancreatic islet transplantation has demonstrated that long-term insulin independence may be achieved in patients suffering from diabetes mellitus type 1. However, because of limited availability of islet tissue, new sources of insulin producing cells that are responsive to glucose are required. Development of pancreatic beta-cell lines from rodent or human origin has progressed slowly in recent years. Current experiments for ex vivo expansion of beta cells and in vitro differentiation of embryonic and adult stem cells into insulin producing beta-cell phenotypes led to promising results. Nevertheless, the cells generated to date lack important characteristics of mature beta cells and generally display reduced insulin secretion and loss of proliferative capacity. Therefore, much better understanding of the mechanisms that regulate expansion and differentiation of stem/progenitor cells is necessary. Here, we review recent advances in the identification of potential cellular sources, and the development of strategies to regenerate or fabricate insulin producing and glucose sensing cells that might enable future cell-based therapies of diabetes mellitus type 1.

Differential resistance to cell entry by porcine endogenous retrovirus subgroup A in rodent species

Background

The risk of zoonotic infection by porcine endogenous retroviruses (PERV) has been highlighted in the context of pig-to-human xenotransplantation. The use of receptors for cell entry often determines the host range of retroviruses. A human-tropic PERV subgroup, PERV-A, can enter human cells through either of two homologous multitransmembrane proteins, huPAR-1 and huPAR-2. Here, we characterised human PARs and their homologues in the PERV-A resistant rodent species, mouse and rat (muPAR and ratPAR, respectively).

Results

Upon exogenous expression in PERV-A resistant cells, human and rat PARs, but not muPAR, conferred PERV-A sensitivity. Exogenously expressed ratPAR binds PERV-A Env and allows PERV-A infection with equivalent efficiency to that of huPAR-1. Endogenous ratPAR expression in rat cell lines appeared to be too low for PERV-A infection. In contrast, the presence of Pro at position 109 in muPAR was identified to be the determinant for PERV-A resistance. Pro109. was shown to be located in the second extracellular loop (ECL2) and affected PERV-A Env binding to PAR molecules.

Conclusion

The basis of resistance to PERV-A infection in two rodent species is different. Identification of a single a.a. mutation in muPAR, which is responsible for mouse cell resistance to PERV-A highlighted the importance of ECL-2 for the viral receptor function.

Transspecies transmission of the endogenous koala retrovirus

The koala retrovirus (KoRV) is a gammaretrovirus closely related to the gibbon ape leukemia virus and induces leukemias and immune deficiencies associated with opportunistic infections, such as chlamydiosis. Here we characterize a KoRV newly isolated from an animal in a German zoo and show infection of human and rat cell lines in vitro and of rats in vivo, using immunological and PCR methods for virus detection. The KoRV transmembrane envelope protein (p15E) was cloned and expressed, and p15E-specific neutralizing antibodies able to prevent virus infection in vitro were developed. Finally, evidence for immunosuppressive properties of the KoRV was obtained.

Cloning and functional characterization of a gamma-hydroxybutyrate receptor identified in the human brain

Two parent clones of a gamma-hydroxybutyrate (GHB) receptor, C12K32 and GHBh1, were isolated from a human frontal cortex cDNA library. The two clones differ by a deleted cytosine in C12K32. CHO cells transfected with either C12K32 or GHBh1 responded positively to submicromolar GHB stimulation. However, unlike C12K32, GHBh1 desensitizes rapidly on application of low concentrations of GHB. GHB receptor properties were then studied on C12K32 expressed in CHO cells. C12K32 bound GHB with a Kd of 114 nM and has no affinity for GABA or glutamate. GHB and NCS-382 displaced [3H]GHB with an IC50 of 53 +/- 8 and 120 +/- 18 nM, respectively. In patch-clamp experiments, GHB induced a dose-dependent response with an EC50 of 130 nM. This response was antagonized by NCS-382, was not reproduced by GABA, and was sensitive to the addition of GTP-gamma-S in the recording pipette. CHO cells transfected with C12K32 exhibited GTPgamma-35S binding with an EC50 of 462 nM for GHB and an IC50 of 2.9 microM for NCS-382. The present data led to the conclusion that both C12K32 and GHBh1 are two closely related isoforms of a human GHB receptor, GHBh1, that is described in the databank as the GPCR 172A.

Isolation and characterization of an infectious replication-competent molecular clone of ecotropic porcine endogenous retrovirus class C

Xenotransplantation of pig organs is complicated by the existence of polytropic replication-competent porcine endogenous retroviruses (PERV) capable of infecting human cells. The potential for recombination between ecotropic PERV-C and human-tropic PERV-A and PERV-B adds another level of infectious risk. Proviral PERV-C were characterized in MAX-T cells derived from d/d haplotype miniature swine. Three proviruses were cloned from a genomic library. Clone PERV-C(1312) generated infectious particles after transfection into porcine ST-IOWA cells. Electron microscopy revealed the same morphologies of virions in MAX-T cells and in ST-IOWA cells infected with cell-free PERV-C(1312) particles, indicating that MAX-T cells harbor one functional PERV-C provirus.

A novel strategy for preventing PERV transmission to human cells by remodeling the viral envelope glycoprotein

BACKGROUND

Porcine endogenous retrovirus (PERV) released from pig cells is a main problem associated with clinical xenotransplantation. In a previous study, we demonstrated that the high mannose type of N-glycan of the envelope glycoprotein is closely related to PERV infectivity with respect to human cells. In this study, we addressed the effects of reducing the high mannose type of N-glycan on PERV infectivity.

METHODS

Pig endothelial cells (PEC) were transduced with the LacZ gene by a pseudotype infection to produce PEC(Z). The PEC(Z)s were then further infected with PERV subtype B (PERV-B) to produce PEC(Z)/PB. The PEC(Z)/PBs were next transfected with the alpha 1,2 mannosidase Ib (Man Ib), N-acetylglucosaminyltransferase I (GnT-I) or alpha-mannosidase II (Man II) gene in order to reduce the levels of high mannose type of N-glycan. HEK293 cells were inoculated with the PERV in each of the culture supernatants. The inoculated cells were histochemically stained and the LacZ-positive cells were counted.

RESULTS

In experiment I, PERV transmission from the PEC(Z)/PB with GnT-I or Man II to HEK 293 cells was significantly reduced in comparison with control PEC(Z)/PB, while the PEC(Z)/PB with Man Ib was not. However, in experiment II, PERV transmission from the PEC(Z)/PB with ManIb to HEK 293 cells was also significantly reduced in comparison with control PEC(Z)/PB.

CONCLUSION

The transfection of these genes to pig cells is effective in reducing the susceptibility of human cells to PERV infection. The results suggest that this represents a potentially useful strategy for further decreasing the likelihood of PERV infections.

Mice transgenic for a human porcine endogenous retrovirus receptor are susceptible to productive viral infection

Porcine endogenous retrovirus (PERV) is considered one of the major risks in xenotransplantation. No valid animal model has been established to evaluate the risks associated with PERV transmission to human patients by pig tissue xenotransplantation or to study the potential pathogenesis associated with PERV infection. In previous work we isolated two genes encoding functional human PERV receptors and proved that introduction of these into mouse fibroblasts allowed the normally nonpermissive mouse cells to become productively infected (T. A. Ericsson, Y. Takeuchi, C. Templin, G. Quinn, S. F. Farhadian, J. C. Wood, B. A. Oldmixon, K. M. Suling, J. K. Ishii, Y. Kitagawa, T. Miyazawa, D. R. Salomon, R. A. Weiss, and C. Patience, Proc. Natl. Acad. Sci. USA 100:6759-6764, 2003). In the present study we created mice transgenic for human PERV-A receptor 2 (HuPAR-2). After inoculation of transgenic animals with infectious PERV supernatants, viral DNA and RNA were detected at multiple time points, indicating productive replication. This establishes the role of HuPAR-2 in PERV infection in vivo; in addition, these transgenic mice represent a new model for determining the risk of PERV transmission and potential pathogenesis. These mice also create a unique opportunity to study the immune response to PERV infection and test potential therapeutic or preventative modalities.

The infectivity and host range of the ecotropic porcine endogenous retrovirus, PERV-C, is modulated by residues in the C-terminal region of its surface envelope protein

Endogenous retroviral genetic material serves as a reservoir for the generation of retroviral pathogens by recombination between activated endogenous or exogenous infectious agents. Some porcine tissues actively express infectious porcine endogenous retroviruses (PERVs). Of the three classes of PERV characterized to date, two, PERV-A and B, are capable of infecting human cells in vitro, whereas PERV-C cannot. Here, we demonstrate that the PERV-C envelope surface protein (SU) when disassociated from its C-terminus binds human cells. Further, we show that PERV-C binding to human cells is not inhibited in 293 cells productively infected with PERV-A, confirming that the molecule PERV-C interacts with on human cells is distinct from that used by PERV-A. Moreover, we demonstrate that the envelope region encompassing the proline-rich region is required for binding to cells in addition to the putative variable region A (VRA) and B (VRB). The region in the C-terminus of the SU that alters the binding and infectivity properties of PERV-C differs by only nine residues from the analogous region of PERV-A. Caution may be warranted even when a xenotransplantation product is from source pigs that do not express human-tropic viruses, as minimal mutations within PERV-C combined with selection in a human recipient could render PERV-C infectious in humans.

Human endogenous retroviruses: from infectious elements to human genes

Mammalian genomes contain a heavy load (42% in humans) of retroelements, which are mobile sequences requiring reverse transcription for their replicative transposition. A significant proportion of these elements is of retroviral origin, with thousands of sequences resembling the integrated form of infectious retroviruses, with two LTRs bordering internal regions homologous to the gag, prt, pol, and env genes. These elements, named endogenous retroviruses (ERVs), are most probably the proviral remnants of ancestral germ-line infections by active retroviruses, which have thereafter been transmitted in a Mendelian manner. The complete sequencing of the human genome now allows a comprehensive survey of human ERVs (HERVs), which can be grouped according to sequence homologies into approximately 80 distinct families, each containing a few to several hundred elements. As reviewed here, strong similarities between HERVs and present-day retroviruses can be inferred from phylogenetic analyses on the reverse transcriptase (RT) domain of the pol gene or the transmembrane subunit (TM) of the env gene, which disclose interspersion of both classes of elements and suggest a common history and shared ancestors. Similarities are also observed at the functional levels, since despite the fact that most HERVs have accumulated mutations, deletions, and/or truncations, several elements still possess some of the functions of retroviruses, with evidence for viral-like particle formation, and occurrence of envelope proteins allowing cell-cell fusion and even conferring infectivity to pseudotypes. Along this line, a genomewide screening for human retroviral genes with coding capacity has revealed 16 fully coding envelope genes. These genes are transcribed in several healthy tissues including the placenta, three of them at a very high level. Besides their impact in modelling the genome, HERVs thus appear to contain still active genes, which most probably have been subverted by the host for its benefit and should be considered as bona fide human genes. Some of their characteristic features and possible physiological roles, as well as potential pathological effects inherited from their retroviral ancestors are also reviewed.

HTLV-1 tropism and envelope receptor

The identification of CD4 as the primary receptor for HIV followed shortly after the discovery of the virus, but the HTLV receptor remained long elusive, until its recent identification as the GLUT1 glucose transporter. In the present review, we describe the status of the literature that surrounded this discovery as well as the in vitro and in vivo observations that led to the identification of GLUT1. Also, we will explore a few tracks to conciliate the in vitro and in vivo data on HTLV-1 tropism within the context of the HTLV literature that has accumulated over the past 25 years. A close examination of these data leads us to conclude that the preferential detection of HTLV-1 in CD4+ T lymphocyte subsets in vivo, even in the absence of leukemia, is not likely to be directly related to envelope receptor interactions, but rather to an array of postentry selection bottlenecks in vivo. Furthermore, we propose that infection of other hematopoietic and nonhematopoietic cells is likely to take place during the lifetime of an individual, with a burst early during the infection.

Characterization of a monoclonal antibody specific to the Gag protein of porcine endogenous retrovirus and its application in detecting the virus infection

The porcine endogenous retrovirus (PERV) has drawn extensive attention recently, due to the widespread use of biomaterials of porcine origin in organ transplantation. This virus is present in all pig strains and has been demonstrated to be capable of infecting human cells in vitro. Therefore, it is imperative to develop a highly sensitive and specific immunoassay for clinical surveillance in patients receiving xenotransplantation. We describe here the generation of a monoclonal antibody (mAb) named A-11 specifically against the Gag protein of PERV. The mAb was found to be able to detect PERV produced from cultured cells. No cross-reaction with Gag proteins of murine leukemia virus (MuLV) and human immunodeficiency virus-1/2 was observed indicating that it is highly specific to PERV. The mAb was characterized as IgG2b subtype and kappa light chain. The region recognized by the mAb A-11 was localized to amino acid 293-336 on the Gag protein, and a synthetic peptide corresponding to amino acid 313-322 effectively competed the binding of the mAb with recombinant Gag proteins. Both immunocytochemistry and flow cytometry showed that the antibody is suitable for detection of PERV infection. By using the assays, we found that PERV-infected cells primarily of epithelial origin, with the highest infection rate in 293 followed by HEp-2 cells. In summary, the A-11 mAb will be useful for the development of quantitative and qualitative immunoassays for monitoring PERV infection in xenotransplantation patients and individuals who have close contact with pigs.

Porcine Endogenous Retroviruses PERV-A and PERV-B Infect neither Mouse Cells in vitro nor SCID Mice in vivo

OBJECTIVE

Porcine endogenous retroviruses (PERVs) pose a risk for xenotransplantations using pig materials as they are present in the genome of all pigs and are able to infect human cells in vitro. Until recently, transmission of PERVs in vivo was only described in severe combined immunodeficient (SCID) and nude mice inoculated with PERV-producing cells. However, in this series of experiments microchimerism could not be excluded. To overcome this problem, the risk of PERV infection was addressed in a similar way but using cell-free inoculation of mouse cells in vitro and SCID mice in vivo.

METHODS

Mouse cell lines and primary cells were incubated in vitro with PERV-A, with a recombinant PERV-A/C and with PERV-B. Provirus integration was assessed by PCR. Reverse transcriptase activity was measured in the cell supernatants. SCID mice were inoculated in vivo with cell-free virus at high titers.

RESULTS

None of the mouse cell lines and primary cells could be infected by PERV and no provirus integration was observed in different organs of the inoculated SCID mice.

CONCLUSION

The data indicate that PERV-A, PERV-A/C and PERV-B could not infect different mouse cells. These data correlate with the recent finding that mouse cells lack a functional receptor for PERV-A. Although the receptor for PERV-B is still unknown, these data suggest that previously reported PERV transmissions to SCID and nude mice in vivo might be due to microchimerism or pseudotyping with murine viruses and indicate that normal mice are an inappropriate model for the study of PERV infection and pathogenesis.

Cell-binding properties of the envelope proteins of porcine endogenous retroviruses

To examine the binding properties of the envelope glycoproteins of porcine endogenous retrovirus subgroups A and B (PERV-A and PERV-B), we produced two forms of soluble envelope proteins, termed Env-ST and Env-SU, using a baculovirus expression system. Env-ST and Env-SU encompass one-third of the N-terminal and the entire surface unit (SU) of the envelope protein, respectively. Using these proteins, binding assays were performed in various mammalian cell lines. The binding properties of the Env-STs that contain the putative receptor binding domain (RBD) did not correlate with the susceptibility to the pseudotype viruses having PERV envelopes, whereas those of the Env-SUs correlated fairly well. These results suggested that the Env-SUs but not Env-STs interacted with their receptors in various cell lines. Interestingly, PERV-A Env-SU did not bind to a mink cell line (Mv1-Lu cells) that is highly susceptible to the PERV-A pseudotype virus. In addition, PERV-B Env-SU did not interfere with the PERV-B pseudotype virus on Mv1-Lu cells. These results suggest the existence of a cognate receptor-independent entry pathway as demonstrated in an immunodeficiency-inducing variant of feline leukemia virus FeLV.

Porcine cell microchimerism but lack of productive porcine endogenous retrovirus (PERV) infection in naive and humanized SCID-beige mice treated with porcine peripheral blood mononuclear cells

Pigs are considered a suitable source of cells and organs for xenotransplantation. All known strains of pigs contain porcine endogenous retrovirus (PERV) and PERV released by porcine cells may infect human cells in vitro and severe-combined immunodeficient (SCID) mice in vivo. Humanized SCID (hu-SCID) mice develop immune response to porcine antigens. Here we investigated PERV transmission in humanized SCID-beige mice using porcine peripheral blood mononuclear cells (PBMC) as the donor tissue (and the source of PERV). Mice were infused in the peritoneal cavity with 1.5-3.0 x 10(7) unfractionated human PBMC. Unfractionated porcine PBMC (1.5-3.0 x 10(7) cell/mouse) were infused to the mice simultaneously with human PBMC or 3 weeks after human PBMC infusion. The treated mice were monitored for weight and skin changes, donor cell chimerism, anti-pig antibodies and PERV transmission. All humanized mice tested 5-12 weeks after human PBMC transplantation were macrochimeric (up to 40% of cells in blood) for human cells, where 99% of the human cells were T-lymphocytes. Although human B lymphocytes were very rare in the blood of humanized mice at that point, the mice were positive for human anti-pig natural antibodies. The control SCID-beige mice or mice treated with porcine PBMC alone were negative for anti-porcine antibodies. Approximately 70% of the humanized mice treated with porcine PBMC were also microchimeric for porcine cells. Although some tissue samples of these mice were positive for PERV DNA in the absence of porcine DNA indicating PERV infection, the infection was non-productive as PERV transcripts were not detectable in those tissues. PERV infection of human and mouse cells in vitro by co-culturing with porcine PBMC was also non-productive. Humanized SCID-beige mice suffered weight loss and occasional minor skin changes due to graft vs. host disease caused by human PBMC but none of the mice showed observable effect attributable to the apparent PERV infection alone.

Early and late HHV-6 gene transcripts in multiple sclerosis lesions and normal appearing white matter

Multiple sclerosis is an inflammatory demyelinating disease of the CNS, the aetiology of which is believed to have both genetic and environmental components. We have investigated one of the candidate viruses for the environmental component of multiple sclerosis, the neurotropic human herpesvirus 6 (HHV-6). Utilizing fluorescent in situ hybridization (FISH) techniques, we have examined human post-mortem tissues for the presence of immediate early and late viral gene expression in multiple sclerosis patient normal appearing white matter (NAWM), lesional tissue and normal control brain samples. HHV-6 gene transcription was detected in all tissue samples and was restricted to oligodendrocytes, as determined by double mRNA FISH analysis. Quantitative analysis of viral mRNA expression indicated that both NAWM and lesional multiple sclerosis samples exhibited significantly higher levels of HHV-6 expression compared with the normal control samples. Lesional samples exhibited the highest levels of viral gene expression, with NAWM exhibiting an intermediate level between lesional and control tissues. Immunofluorescence against early and late HHV-6 proteins verified active translation of HHV-6 viral mRNA in oligodendrocytes. Southern blot analysis of nested polymerase chain reactions using extracted genomic DNA and cDNA confirmed the presence of the HHV-6 genome in all individuals, with the active expression profile mirroring the FISH results. The frequent high level of HHV-6 infection in multiple sclerosis samples suggests a possible role in pathogenesis.

Determinants of high titer in recombinant porcine endogenous retroviruses

Porcine endogenous retroviruses (PERVs) pose a potential stumbling block for therapeutic xenotransplantation, with the greatest threat coming from viruses generated by recombination between members of the PERV subgroup A (PERV-A) and PERV-C families (PERV-A/C recombinants). PERV-A and PERV-B have been shown to infect human cells in culture, albeit with low titers. PERV-C has a more restricted host range and cannot infect human cells. A recombinant PERV-A/C virus (PERV-A14/220) contains the PERV-A sequence between the end of pol and the middle of the SU region in env. The remaining sequence is derived from PERV-C. PERV-A14/220 is approximately 500-fold more infectious than PERV-A. To determine the molecular basis for the increased infectivity of PERV-A14/220, we have made a series of vector constructs. The primary determinant for the enhanced replicative potential of the recombinant virus appeared to be the env gene. Using a series of chimeric env genes, we could identify two determinants of high infectivity; one was an isoleucine to valine substitution at position 140 between variable regions A and B, and the other lies within the proline rich region. Taken together, these results show that the novel juxtaposition of env gene sequences enhanced the infectivity of PERV-A14/220 for human cells, perhaps by stabilization of the envelope glycoprotein or increased receptor binding.

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.

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.

Porcine endogenous retroviruses infect cells lacking cognate receptors by an alternative pathway: implications for retrovirus evolution and xenotransplantation

A PHQ motif near the amino termini of gammaretroviral envelope glycoprotein surface (SU) subunits is important for infectivity but not for incorporation into virions or binding to cognate receptors. The H residue of this motif is most critical, with all substitutions we tested being inactive. Interestingly, porcine endogenous retroviruses (PERVs) of all three host-range groups, A, B, and C, lack full PHQ motifs, but most members have an H residue at position 10. H10A PERV mutants are noninfectious but were efficiently transactivated by adding to the assays a PHQ-containing SU or receptor-binding subdomain (RBD) derived from a gibbon ape leukemia virus (GALV). A requirement of this transactivation was a functional GALV receptor on the cells. In contrast to this heterologous transactivation, PERV RBDs and SUs were inactive in all tested cells, including porcine ST-IOWA cells. Surprisingly, transactivation by GALV RBD enabled wild-type or H10A mutant PERVs of all three host-range groups to efficiently infect cells from humans and rodents that lack functional PERV receptors and it substantially enhanced infectivities of wild-type PERVs, even for cells with PERV receptors. Thus, PERVs can suboptimally infect cells that contain cognate receptors or they can employ a transactivation pathway to more efficiently infect all cells. This ability to infect cells lacking cognate receptors was previously demonstrated only for nontransmissible variant gammaretroviruses with recombinant and mutant envelope glycoproteins. We conclude that some endogenously inherited mammalian retroviruses also have a receptor-independent means for overcoming host-range and interference barriers, implying a need for caution in xenotransplantation, especially of porcine tissues.

Xenotransplantation: infectious risk revisited

Xenotransplantation is a possible solution for the shortage of tissues for human transplantation. Multiple hurdles exist to clinical xenotransplantation, including immunologic barriers, metabolic differences between pigs--the source species most commonly considered--and humans, and ethical concerns. Since clinical trials were first proposed almost 10 years ago, the degree of risk for infection transmitted from the xenograft donor to the recipient has been extensively investigated. A number of potential viral pathogens have been identified including porcine endogenous retrovirus (PERV), porcine cytomegalovirus (PCMV), and porcine lymphotropic herpesvirus (PLHV). Sensitive diagnostic assays have been developed for each virus. Human-tropic PERV are exogenous recombinants between PERV-A and PERV-C sequences and are present in only a subset of swine. Porcine cytomegalovirus can be excluded from herds of source animals by early weaning of piglets. In contrast, the risks associated with PLHV remain undefined. Microbiologic studies and assays for potential xenogeneic pathogens have furthered understanding of risks associated with xenotransplantation. Thus far, clinical xenotransplantation of pig tissues has not resulted in transmission of viral infection to humans; significant risks for disease transmission from swine to humans have not been confirmed. If immunologic hurdles can be overcome, it is reasonable to initiate carefully monitored clinical trials.

Phylogenomics and bioinformatics of SARS-CoV

Tracing the history of molecular changes in coronaviruses using phylogenetic methods can provide powerful insights into the patterns of modification to sequences that underlie alteration to selective pressure and molecular function in the SARS-CoV (severe acute respiratory syndrome coronavirus) genome. The topology and branch lengths of the phylogenetic relationships among the family Coronaviridae, including SARS-CoV, have been estimated using the replicase polyprotein. The spike protein fragments S1 (involved in receptor-binding) and S2 (involved in membrane fusion) have been found to have different mutation rates. Fragment S1 can be further divided into two regions (S1A, which comprises approximately the first 400 nucleotides, and S1B, comprising the next 280) that also show different rates of mutation. The phylogeny presented on the basis of S1B shows that SARS-CoV is closely related to MHV (murine hepatitis virus), which is known to bind the murine receptor CEACAM1. The predicted structure, accessibility and mutation rate of the S1B region is also presented. Because anti-SARS drugs based on S2 heptads have short half-lives and are difficult to manufacture, our findings suggest that the S1B region might be of interest for anti-SARS drug discovery.

Porcine endogenous retroviral nucleic acid in peripheral tissues is associated with migration of porcine cells post islet transplant

Porcine islets represent an alternative source of insulin-producing tissue, however, porcine endogenous retrovirus (PERV) remains a concern. In this study, SCID mice were transplanted with nonencapsulated (non-EC), microencapsulated (EC) or macroencapsulated (in a TheraCyte trade mark device) neonatal porcine islets (NPIs), and peripheral tissues were screened for presence of viral DNA and mRNA. To understand the role of an intact immune system in PERV incidence, mice with established NPI grafts were reconstituted with splenocytes. Peripheral tissues were screened for PERV and porcine DNA using PCR. Tissues with positive DNA were analyzed for PERV mRNA using RT-PCR. No significant difference was observed between non-EC and EC transplants regarding presence of PERV or porcine-specific DNA or mRNA. In reconstituted animals, little PERV or porcine DNA, and no PERV mRNA was detected. No PERV or porcine-specific DNA was observed in animals implanted with a TheraCyte trade mark device. In conclusion, an intact immune system significantly lowered the presence of PERV. Microencapsulation of islets did not alter PERV presence, however, macroencapsulation in the TheraCyte device did. Lower PERV incidence coincided with lower porcine DNA in peripheral tissues, linking the presence of PERV to migration of porcine cells.

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.