No transmission of porcine endogenous retroviruses (PERVs) in a long-term pig to rat xenotransplantation model and no infection of immunosuppressed rats

BACKGROUND

Xenotransplantation from pig to humans may be associated with the risk of transmission of porcine endogenous retroviruses (PERVs) that are present in the genome of all pigs and that infect human cells in vitro. However, it remains unclear whether PERVs infect transplant recipients in vivo and, if so, whether they are pathogenic. It is therefore essential to perform in vivo infection studies in animal models.

MATERIAL/METHODS

To study PERV transmission in rats, rat primary cells and cell lines were treated in vitro with virus from different sources. Based on the assumption that susceptible cell lineages not yet tested in vitro could be present in the animal, PERV was inoculated into naïve and immunosuppressed animals. To investigate PERV transmission in a long-term exposure experiment, sera from animals grafted with pig Langerhans islet cells were tested in a Western blot assay for antibodies against PERVs. The animals were treated with streptozotocin to induce diabetes and microencapsulated and non-microencapsulated pig islet cells were applied without immunosuppression.

RESULTS

No productive infection of a few selected rat primary cells or cell lines was observed in vitro. PERV-specific antibodies were found in none of the animals and no integration of PERV into rat cells of different organs was observed, indicating that infection had not occurred.

CONCLUSIONS

This report demonstrates a lack of infection of rats in vivo even during immunosuppression or long-term exposure (up to 460 days) to a functioning xenotransplant. This report also shows that rats possibly due to a low receptor concentration on their cells are not a suitable animal model to study PERV transmission in vivo.

Reverse transcriptase activity in patients with primary biliary cirrhosis and other autoimmune liver disorders

BACKGROUND

Patients with biliary disease make retroviral antibodies and the Human Betaretrovirus has been characterized in patients with primary biliary cirrhosis.

AIM

To screen patients with autoimmune liver disease for evidence of retroviral infection.

METHODS

Real-time reverse transcriptase polymerase chain reaction was used to detect Human Betaretrovirus, and a reverse transcriptase assay to measure reverse transcriptase activity in plasma.

RESULTS

Using reverse transcriptase polymerase chain reaction, 24% of primary biliary cirrhosis samples were positive for Human Betaretrovirus when compared to 13% with autoimmune hepatitis, 5% of other liver diseases and 3% of the non-liver disease control subjects. Reverse transcriptase activity was found in 73% of patients with autoimmune hepatitis, 42% with primary biliary cirrhosis, 22% of liver patients without viral or autoimmune disease and 7% of subjects without liver disease. In patients with autoimmune liver disease, detection of reverse transcriptase activity was related to higher ALT levels, whereas others stabilized on immunosuppressive therapy either preliver or postliver transplantation were less likely to be reverse transcriptase-positive.

CONCLUSIONS

Most patients with autoimmune hepatitis have detectable reverse transcriptase activity. Investigations will be required to assess whether this represents the expression of endogenous retroviruses and retrotransposable elements in inflamed tissue, or signifies the presence of exogenous retroviral infection.

Feline leukaemia virus (FeLV) and feline immunodeficiency virus infections in cats in the Pisa district of Tuscany, and attempts to control FeLV infection in a colony of domestic cats by vaccination

The seroprevalence of feline immunodeficiency virus (FIV) in 203 apparently healthy domestic cats living in the district of Pisa, central Italy, was 11.3 per cent, and the prevalence of feline leukaemia virus (FeLV) was 8.4 per cent. The prevalence of FIV depended significantly on the lifestyle and age of the cats; cats living outdoors were more likely to be FIV-positive than cats living indoors, and the proportion of FIV-positive cats increased with age. In contrast, there was no significant relationship between these variables and the prevalence of FeLV. There was no significant relationship between the cats' seropositivity for FIV and FeLV. The results of a five-year field study to control FeLV infection by vaccination in a colony of 30 domestic adult cats naturally exposed to the infection suggest that the vaccination was effective in FIV-negative cats, but failed to protect FIV-positive cats against FeLV.

Two surface-exposed elements of the B30.2/SPRY domain as potency determinants of N-tropic murine leukemia virus restriction by human TRIM5alpha

Human TRIM5alpha (TRIM5alpha(hu)) potently restricts N-tropic (N-MLV), but not B-tropic, murine leukemia virus in a manner dependent upon residue 110 of the viral capsid. Rhesus monkey TRIM5alpha (TRIM5alpha(rh)) inhibits N-MLV only weakly. The study of human-monkey TRIM5alpha chimerae revealed that both the v1 and v3 variable regions of the B30.2/SPRY domain contain potency determinants for N-MLV restriction. These variable regions are predicted to be surface-exposed elements on one face of the B30.2 domain. Acidic residues in v3 complement basic residue 110 of the N-MLV capsid. The results support recognition of the retroviral capsid by the TRIM5alpha B30.2 domain.

Pre-screening of miniature swine may reduce the risk of transmitting human tropic recombinant porcine endogenous retroviruses

BACKGROUND

It has been reported that peripheral blood mononuclear cells from miniature swine are capable of transmitting human tropic porcine endogenous retrovirus (PERV) recombinants to both human and pig cells. It has been suggested that these recombinants are exogenous and/or driven by one or more critical loci present in the pig genome.

METHODS AND RESULTS

Genomic analysis of a miniature swine capable of transmitting human tropic replication competent (HTRC) recombinant PERV-A/C identified a PERV-C provirus in a region with homology to sequences located on chromosome 7. In "null" swine, incapable of in vitro transmission of PERV to human or pig cells, amplification using specific primers revealed that only two of five animals retained this locus in comparison to a total of five out of five transmitters (recombinant PERV-A/C transmission to both human and pig cells) and seven out of seven non-transmitters (replication of non-recombinant PERV in pig cells only).

CONCLUSION

These data suggest that further analysis of these loci may provide a genetic basis for identifying pigs that are less likely to transmit human tropic PERV and would, therefore, be more suitable as source animals for human xenotransplantation.

Inhibition of TGF-beta with neutralizing antibodies prevents radiation-induced acceleration of metastatic cancer progression

We investigated whether TGF-beta induced by anticancer therapies accelerates tumor progression. Using the MMTV/PyVmT transgenic model of metastatic breast cancer, we show that administration of ionizing radiation or doxorubicin caused increased circulating levels of TGF-beta1 as well as increased circulating tumor cells and lung metastases. These effects were abrogated by administration of a neutralizing pan-TGF-beta antibody. Circulating polyomavirus middle T antigen-expressing tumor cells did not grow ex vivo in the presence of the TGF-beta antibody, suggesting autocrine TGF-beta is a survival signal in these cells. Radiation failed to enhance lung metastases in mice bearing tumors that lack the type II TGF-beta receptor, suggesting that the increase in metastases was due, at least in part, to a direct effect of TGF-beta on the cancer cells. These data implicate TGF-beta induced by anticancer therapy as a pro-metastatic signal in tumor cells and provide a rationale for the simultaneous use of these therapies in combination with TGF-beta inhibitors.

Immune control and prevention of chronic Friend retrovirus infection

T cells are critical to control acute infection of a host with retroviruses but they are usually unable to prevent the development of chronic infections. This review summarizes studies from the Friend virus mouse model that reveal some of the mechanisms by which T cells control chronic retroviral infection, and also reveal why these responses ultimately fail to fully eradicate infection. Also summarized are findings from vaccine studies demonstrating the immunological requirements for the prevention of chronic retroviral infection. The implications of these findings for chronic infections in humans are discussed.

Vaccination against the feline leukaemia virus: outcome and response categories and long-term follow-up

Feline leukaemia virus (FeLV) is a pathogen inducing fatal disease in cats worldwide. By applying sensitive molecular assays, efficacious commonly used FeLV vaccines that protect cats from antigenaemia were found not to prevent proviral integration and minimal viral replication after challenge. Nonetheless, vaccines protected cats from FeLV-associated disease and prolonged life expectancy. The spectrum of host response categories was refined by investigating plasma viral RNA loads. All cats initially fought similar virus loads, although subsequently loads were associated with infection outcomes. Persistence of plasma viral RNA was moderately associated with reactivation of FeLV infection. In conclusion, sensitive molecular assays are important tools for reviewing pathogenesis of FeLV infection.

Evaluation of immunogenicity and efficacy of combined DNA and adjuvanted protein vaccination in a human immunodeficiency virus type 1/murine leukemia virus pseudotype challenge model

A DNA plasmid encoding human immunodeficiency virus type 1 (HIV-1) env, nef and tat genes was used in mice in a prime-boost immunization regimen with the corresponding recombinant proteins. The genetic immunogen was delivered with a gene gun and the proteins were injected intramuscularly together with the adjuvant AS02A. Immunizations were followed by experimental challenge with pseudotyped HIV-1 subtype A or B virus. In an initial experiment in which animals were challenged four weeks after the final immunization, all single modality and prime-boost vaccinations resulted in a significant level of protection as compared to control animals. There was a trend for DNA-alone immunization yielding the highest protection. In a subsequent study, a late challenge was performed 19 weeks after the final immunization. All groups having received the DNA vaccine, either alone or in combination with adjuvanted protein, exhibited strong protection against HIV replication. The subtype-specific protection against the experimental HIV challenge was significantly stronger than the cross-protection. Cellular and humoral immune responses were assessed during immunization and after challenge, but without clear correlation to protection against HIV replication. The data suggest that either DNA or protein antigens alone provide partial protection against an HIV-1/MuLV challenge and that DNA immunization is essential for achieving very high levels of efficacy in this murine HIV-1 challenge model. While prime-boost combinations were more immunogenic than DNA alone, they did not appear to provide any further enhancement over DNA vaccine mediated efficacy. The DNA immunogen might prime low levels of CD8+ T cells responsible for virus clearance or possibly a yet unidentified mechanism of protection.

Isolation of an active Lv1 gene from cattle indicates that tripartite motif protein-mediated innate immunity to retroviral infection is widespread among mammals

Lv1/TRIM5alpha (tripartite motif 5alpha) has recently emerged as an important factor influencing species-specific permissivity to retroviral infection in a range of primates, including humans. Old World monkey TRIM5alpha blocks human immunodeficiency virus type 1 (HIV-1) infectivity, and the human and New World monkey TRIM5alpha proteins are inactive against HIV-1 but active against divergent murine (N-tropic murine leukemia virus [MLV-N]) and simian (simian immunodeficiency virus from rhesus macaque [SIVmac]) retroviruses, respectively. Here we demonstrate antiviral activity of the first nonprimate TRIM protein, from cattle, active against divergent retroviruses, including HIV-1. The number of closely related human TRIM sequences makes assignment of the bovine sequence as a TRIM5alpha ortholog uncertain, and we therefore refer to it as bovine Lv1. Bovine Lv1 is closely related to primate TRIM5alpha proteins in the N-terminal RING and B-box 2 domains but significantly less homologous in the C-terminal B30.2 domain, particularly in the region shown to influence antiviral specificity. Intriguingly, some viruses restricted by bovine Lv1, including HIV-1 and MLV-N, are unable to synthesize viral DNA by reverse transcription, whereas restricted HIV-2 makes normal amounts of DNA. The data support the conclusion that TRIM protein-mediated restriction of retroviral infection is a more common attribute of mammals than previously appreciated.

Human APOBEC3 proteins, retrovirus restriction, and HIV drug resistance

Over 40 million people worldwide currently have HIV/AIDS. Many antiretroviral drugs have proven effective, but drug-resistant HIV variants frequently emerge to thwart treatment efforts. Reverse transcription errors undoubtedly contribute to drug resistance, but additional significant sources of viral genetic variation are debatable. The human APOBEC3F and APOBEC3G proteins can potently inhibit retrovirus infection by a mechanism that involves retroviral cDNA cytosine deamination. Here we review the current knowledge on the mechanism of APOBEC3-dependent retrovirus restriction and discuss whether this innate host-defense system actively contributes to HIV genetic variation.

Retrovirus-induced oxidative stress with neuroimmunodegeneration is suppressed by antioxidant treatment with a refined monosodium alpha-luminol (Galavit)

Oxidative stress is involved in many human neuroimmunodegenerative diseases, including human immunodeficiency virus disease/AIDS. The retrovirus ts1, a mutant of Moloney murine leukemia virus, causes oxidative stress and progressive neuro- and immunopathology in mice infected soon after birth. These pathological changes include spongiform neurodegeneration, astrogliosis, thymic atrophy, and T-cell depletion. Astrocytes and thymocytes are directly infected and killed by ts1. Neurons are not infected, but they also die, most likely as an indirect result of local glial infection. Cytopathic effects of ts1 infection in cultured astrocytes are associated with accumulation of the viral envelope precursor protein gPr80env in the endoplasmic reticulum (ER), which triggers ER stress and oxidative stress. We have reported (i) that activation of the Nrf2 transcription factor and upregulation of antioxidative defenses occurs in astrocytes infected with ts1 in vitro and (ii) that some ts1-infected astrocytes survive infection by mobilization of these pathways. Here, we show that treatment with a refined monosodium alpha-luminol (Galavit; GVT) suppresses oxidative stress and Nrf2 activation in cultured ts1-infected astrocytes. GVT treatment also inhibits the development of spongiform encephalopathy and gliosis in the central nervous system (CNS) in ts1-infected mice, preserves normal cytoarchitecture in the thymus, and delays paralysis, thymic atrophy, wasting, and death. GVT treatment of infected mice reduces ts1-induced oxidative stress, cell death, and pathogenesis in both the CNS and thymus of treated animals. These studies suggest that oxidative stress mediates ts1-induced neurodegeneration and T-cell loss.

Effect of DNA repair protein Rad18 on viral infection

Host factors belonging to the DNA repair machineries are assumed to aid retroviruses in the obligatory step of integration. Here we describe the effect of DNA repair molecule Rad18, a component of the post-replication repair pathway, on viral infection. Contrary to our expectations, cells lacking Rad18 were consistently more permissive to viral transduction as compared to Rad18(+/+) controls. Remarkably, such susceptibility was integration independent, since retroviruses devoid of integration activity also showed enhancement of the initial steps of infection. Moreover, the elevated sensitivity of the Rad18(-/-) cells was also observed with adenovirus. These data indicate that Rad18 suppresses viral infection in a non-specific fashion, probably by targeting incoming DNA. Furthermore, considering data published recently, it appears that the interactions between DNA repair components with incoming viruses, often result in inhibition of the infection rather than cooperation toward its establishment.

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.

Adaptive evolution and antiviral activity of the conserved mammalian cytidine deaminase APOBEC3H

The APOBEC3 genes encode cytidine deaminases that act as components of an intrinsic immune defense that have potent activity against a variety of retroelements. This family of genes has undergone a rapid expansion from one or two genes in nonprimate mammals to at least seven members in primates. Here we describe the evolution and function of an uncharacterized antiviral effector, APOBEC3H, which represents the most evolutionarily divergent APOBEC3 gene found in primates. We found that APOBEC3H has undergone significant adaptive evolution in primates. Consistent with our previous findings implicating adaptively evolving APOBEC3 genes as antiviral effectors, APOBEC3H from Old World monkeys (OWMs) has efficient antiviral activity against primate lentiviruses, is sensitive to inactivation by the simian immunodeficiency virus Vif protein, and is capable of hypermutating retroviral genomes. In contrast, human APOBEC3H is inherently poorly expressed in primate cells and is ineffective at inhibiting retroviral replication. Both OWM and human APOBEC3H proteins can be expressed in bacteria, where they display significant DNA mutator activity. Thus, humans have retained an APOBEC3H gene that encodes a functional, but poorly expressed, cytidine deaminase with no apparent antiviral activity. The consequences of the lack of antiviral activity of human APOBEC3H are likely to be relevant to the current-day abilities of humans to combat retroviral challenges.

Cytolytic CD8+T Cells Directed against a Cryptic Epitope Derived from a Retroviral Alternative Reading Frame Confer Disease Protection

Cytolytic CD8(+) T cells (CTL) are key to the immune response that controls virus infections and mediates disease protection. The ability of CTL to induce apoptosis of infected cells and/or limit viral replication is determined by recognition of processed viral peptide epitopes on the surface of the target cell. An understudied source of MHC class I-presented peptides is the aptly named "cryptic epitopes," defined by their nontraditional methods of generation, including derivation from alternative reading frames (ARFs). Although ARF-encoded epitopes have now been documented in a few systems, their potential functional relevance in vivo has been debated. In this study, we demonstrate the physiological significance of an ARF-derived CTL epitope in a retrovirus-induced disease model. We show that disease-susceptible CD8-deficient mice reconstituted with CTL specific for the retroviral ARF-derived SYNTGRFPPL epitope controlled an infection by the LP-BM5 retrovirus isolate, evidently at the level of viral clearance, resulting in protection of these mice from disease. These data indicate that ARF-derived epitopes are indeed relevant inducers of the immune system and demonstrate the importance of atypically generated peptides as functional Ag with a physiologic role in disease protection.

Mutant murine leukemia virus Gag proteins lacking proline at the N-terminus of the capsid domain block infectivity in virions containing wild-type Gag

We have investigated the properties of murine leukemia virus Gag mutants in which the p12-CA cleavage site is altered. In one mutant, the cleavage is blocked; in the other, the conserved proline at the N-terminus of CA has been replaced with glycine. No infectivity was detected in either mutant. Mutant particles cannot synthesize full-length DNA upon infecting permissive cells. Particles composed of a mixture of wild-type and mutant proteins have severely impaired infectivity. These mixed particles are defective in their ability to synthesize DNA upon infection, but this defect is less severe than the loss of infectivity. Thus, proteins lacking the correct N-terminus of CA inhibit DNA synthesis and also interfere with formation or integration of a full-length, normal provirus. The results imply that CA proteins function as part of a large, highly organized structure in reverse transcription and apparently at a later step as well.

Neonate-primed CD8+ memory cells rival adult-primed memory cells in antigen-driven expansion and anti-viral protection

Immunizations early in life, when the host is most susceptible to infection, allow protective immunological memory to develop. Decreasing the dose of Cas-Br-E murine leukemia virus when priming neonatal mice results in adult-like, Type 1 protective responses, but the resulting memory cell populations are smaller than after adult priming. After secondary challenge, virus-specific CD8+ memory cell populations expand twice as much in neonate-primed mice as in adult-primed mice. We found that when equivalent numbers of virus-specific cells were transferred into virus-susceptible mice, protection from disease was similar whether donor, immune mice were primed as neonates or adults, and IL-4 did not alter in vivo virus-specific CD8+ memory cell effector function. Hence, neonate-primed CD8+ cells develop into memory cells that rival adult-primed cells in proliferation and effector function.

Peptide-induced immune protection of CD8+ T cell-deficient mice against Friend retrovirus-induced disease

CD8+ CTLs and virus-neutralizing antibodies have been associated with spontaneous and vaccine-induced immune control of retroviral infections. We previously showed that a single immunization with an env gene-encoded CD4+ T cell epitope protected mice against fatal Friend retrovirus infection. Here, we analyzed immune cell components required for the peptide-induced anti-retroviral protection. Mice lacking CD8+ T cells were nevertheless protected against Friend virus infection, while mice lacking B cells were not. Virus-producing cells both in the spleen and bone marrow decreased rapidly in their number and became undetectable by 4 weeks after infection in the majority of the peptide-immunized animals even in the absence of CD8+ T cells. In the vaccinated animals the production and class switching of virus-neutralizing and anti-leukemia cell antibodies were facilitated; however, virus-induced erythroid cell expansion was suppressed before neutralizing antibodies became detectable in the serum. Further, the numbers of virus-producing cells in the spleen and bone marrow in the early stage of the infection were smaller in the peptide-immunized than in unimmunized control mice in the absence of B cells. Thus, peptide immunization facilitates both early cellular and late humoral immune responses that lead to the effective control of the retrovirus-induced disease, but CD8+ T cells are not crucial for the elimination of virus-infected cells in the peptide-primed animals.

Human tripartite motif 5alpha domains responsible for retrovirus restriction activity and specificity

The tripartite motif 5alpha protein (TRIM5alpha) is one of several factors expressed by mammalian cells that inhibit retrovirus replication. Human TRIM5alpha (huTRIM5alpha) inhibits infection by N-tropic murine leukemia virus (N-MLV) but is inactive against human immunodeficiency virus type 1 (HIV-1). However, we show that replacement of a small segment in the carboxy-terminal B30.2/SPRY domain of huTRIM5alpha with its rhesus macaque counterpart (rhTRIM5alpha) endows it with the ability to potently inhibit HIV-1 infection. The B30.2/SPRY domain and an additional domain in huTRIM5alpha, comprising the amino-terminal RING and B-box components of the TRIM motif, are required for N-MLV restriction activity, while the intervening coiled-coil domain is necessary and sufficient for huTRIM5alpha multimerization. Truncated huTRIM5alpha proteins that lack either or both the N-terminal RING/B-Box or the C-terminal B30.2/SPRY domain form heteromultimers with full-length huTRIM5alpha and are dominant inhibitors of its N-MLV restricting activity, suggesting that homomultimerization of intact huTRIM5alpha monomers is necessary for N-MLV restriction. However, localization in large cytoplasmic bodies is not required for inhibition of N-MLV by huTRIM5alpha or for inhibition of HIV-1 by chimeric or rhTRIM5alpha.