Mouse model to study the replication of primate foamy viruses

Feline foamy virus-based vectors: advantages of an authentic animal model

New-generation retroviral vectors have potential applications in vaccination and gene therapy. Foamy viruses are particularly interesting as vectors, because they are not associated to any disease. Vector research is mainly based on primate foamy viruses (PFV), but cats are an alternative animal model, due to their smaller size and the existence of a cognate feline foamy virus (FFV). The potential of replication-competent (RC) FFV vectors for vaccination and replication-deficient (RD) FFV-based vectors for gene delivery purposes has been studied over the past years. In this review, the key achievements and functional evaluation of the existing vectors from in vitro cell culture systems to out-bred cats will be described. The data presented here demonstrate the broad application spectrum of FFV-based vectors, especially in pathogen-specific prophylactic and therapeutic vaccination using RD vectors in cats and in classical gene delivery. In the cat-based system, FFV-based vectors provide an advantageous platform to evaluate and optimize the applicability, efficacy and safety of foamy virus (FV) vectors, especially the understudied aspect of FV cell and organ tropism.

Simian foamy virus infection in humans: prevalence and management

Simian foamy viruses (SFVs) are highly prevalent in all nonhuman primate species and can infect humans following occupational and non-occupational exposure to infected animals and their tissues, blood or body fluids. Virus transmission results in a stable, persistent infection that seems to be latent. SFV infections are thus far nonpathogenic, with no evidence of adverse clinical outcome in their natural nonhuman primate hosts or by experimental injection in animals and upon cross-species transmission in humans. Since the emergence of pathogenic viruses from nonpathogenic viruses upon cross-species infection is well-documented for several retroviruses, it is prudent to take necessary precautions to deter SFV infections in humans. These steps will help prevent the emergence of a novel pathogen and reduce the risk of transmission of another potential pathogenic human retrovirus.

Experimental therapy of allogeneic solid tumors induced in athymic mice with suicide gene-transducing replication-competent foamy virus vectors

A replication competent foamy virus derived retroviral vector expressing suicide genes has been constructed and characterized in vitro. Here we used vectors expressing the purine nucleoside phosphorylase (FOV-7/pnp), the nitroreductase (FOV-7/ntr), or the thymidine kinase (FOV-7/tk) suicide gene in an in vivo athymic (nude) mice/human glioblastoma tumor model. Gliomas were induced by subcutanous injection of U87 tumor cells. The virus vector was injected when the tumor became visible. Mice with vector virus-injected tumors were treated with the respective prodrug. The treatment resulted in significant inhibition of tumor growth. Surprisingly, in mice with vector virus-injected tumors without prodrug treatment a similar suppression of tumor growth was observed. In 65% (pnp vector), 75% (ntr vector) and 37% (tk vector) of these mice the tumors stopped growing or vanished and the animals remained tumor free for the 25 weeks of the experiment, whereas all mice of the control groups had to be killed because of the tumor growth. In control experiments, the suppression of tumor growth could also be observed when wild-type foamy virus was injected instead of the suicide gene-transducing vectors. Similar results were obtained using the nude mice/G59 human glioblastoma tumor model. In conclusion, the experiments demonstrate an oncolytic activity of foamy virus replication in a nude-mice glioblastoma xenograft tumor model. The analysis of vector virus DNA by PCR revealed that the vector persisted in different organs of the animals irrespective of the use of a prodrug or the elimination of a tumor.

Foamy virus integration

It had been suggested that during integration of spumaretroviruses (foamy viruses) the right (U5) end of the cDNA is processed, while the left (U3) remains uncleaved. We confirmed this hypothesis by sequencing two-long terminal repeat (LTR) circle junctions of unintegrated DNA. Based on an infectious foamy virus molecular clone, a set of constructs harboring mutations at the 5' end of the U3 region in the 3' LTR was analyzed for particle export, reverse transcription, and replication. Following transient transfection some mutants were severely impaired in generating infectious virus, while others replicated almost like the wild type. The replication competence of the mutants was unrelated to the cleavability of the newly created U3 end. This became obvious with two mutants both belonging to the high-titer type. One mutant containing a dinucleotide artificially transferred from the right to the left end was trimmed upon integration, while another one with an unrelated dinucleotide in that place was not. The latter construct in particular showed that the canonical TG motif at the beginning of the provirus is not essential for foamy virus integration.

Non-primate foamy viruses

Foamy viruses (PFVs), also called spumaviruses, are complex retroviruses inducing a characteristic cytopathic effect in cell culture, leading rapidly to cell lysis. These viruses have been isolated mostly in non-human primates, but three non primate PFVs were characterized, namely the bovine foamy virus, the feline foamy virus and more recently the equine foamy virus. In their hosts, PFVs seem to be apathogenic, mirroring an efficient control of virus replication in vivo. Comparing the biology of the different virus isolates will certainly help to unravel the biology of these retroviruses.

Replication of primate foamy viruses in natural and experimental hosts

Foamy viruses (FVs) are common apathogenic retroviruses readily spread by horizontal transmission in nonhuman primate and some other mammalian host populations. Primate FV infections have been known for half a century, i.e., 15 years before the definition of retroviruses and another 15 years before the detection of primate immune deficiency viruses. The emerging interest in human retroviruses included primate FV, and although the role of human hosts for FV was greatly overestimated temporarily, enthusiastic researchers compiled invaluable data on molecular biology and classic as well as molecular epidemiology of these viruses. It has been shown that lytic FV infection in a wide range of cell cultures is in great contrast to the silent state of the infection in animals. Once transmitted by saliva via biting, FVs reside in all tissues as DNA copies, but their replication is untraceable except in oral submucosal cells, which are thought to supply the virus for transmission. FVs have not definitely been associated with any disease, regardless of viral phylogenetic differences. Various primate and nonprimate species have been used for studies on the natural carrier state and primary infection. Experimental infections have mostly proven to be inefficient in primates as well as lower laboratory animals. However, investigation of the immune response in FV-infected animals has only partly explained the control of FV replication in the animal host. Thus, the biological role of FV remains an enigma to be resolved in the future.

Foamy virus vectors

Gene therapy is a promising novel treatment for a variety of human diseases. Successful application of gene therapy requires the availability of vehicles with the ability to efficiently deliver and express genes. Viral vectors are efficient means of transferring a gene of interest into target cells. Current available vehicles for gene transfer are either inefficient or potentially unsafe for human gene therapy applications. Foamy viruses offer a fresh alternative vector system for gene transfer with the potential to overcome the concerns of the current vectors. Foamy viruses are nonpathogenic and have a broad host range with the ability to infect various types of cells from different species. Foamy virus replication is distinct and may provide an edge for foamy virus vector usage over other retroviral vectors. These features offer the foamy vectors unique opportunities to deliver several genes into a number of different cell types in vivo safely and efficiently. The principal problems for the design of foamy virus vectors have been solved, and several foamy virus vectors that efficiently transduce a variety of cell types are available. This chapter reviews specific features of foamy virus vector systems and recent advances in the development and use of these vectors.

Replication-competent hybrids between murine leukemia virus and foamy virus

Replication-competent chimeric retroviruses constructed of members of the two subfamilies of Retroviridae, orthoretroviruses and spumaretroviruses, specifically murine leukemia viruses (MuLV) bearing hybrid MuLV-foamy virus (FV) envelope (env) genes, were characterized. All viruses had the cytoplasmic tail of the MuLV transmembrane protein. In ESL-1, a truncated MuLV leader peptide (LP) was fused to the complete extracellular portion of FV Env, and ESL-2 to -4 contained the complete MuLV-LP followed by N-terminally truncated FV Env decreasing in size. ESL-1 to -4 had an extended host cell range compared to MuLV, induced a cytopathology reminiscent of FVs, and exhibited an ultrastructure that combined the features of the condensed core of MuLV with the prominent surface knobs of FVs. Replication of ESL-2 to -4 resulted in the acquisition of a stop codon at the N terminus of the chimeric Env proteins. This mutation rendered the MuLV-LP nonfunctional and indicated that the truncated FV-LP was sufficient to direct Env synthesis into the secretory pathway. Compared to the parental viruses, the chimeras replicated with only moderate cell-free titers.

Reactivation of a complex retrovirus is controlled by a molecular switch and is inhibited by a viral protein

Spumaviruses, commonly called foamy viruses (FV), are complex retroviruses that establish lifelong persistent infections without any accompanying pathologies. In tissue culture, cells can be either lytically or latently infected, depending on cell type. Regulation of FV replication is controlled by two promoters: the LTR and a second promoter within the env gene termed the internal promoter (IP). The IP directs expression of the transcriptional activator, Tas, and a second accessory protein, Bet, whose function has been elusive. In this study, we report that expression of exogenous Tas is sufficient to initiate a switch from latent to lytic replication. We also show that treatment with the phorbol ester phorbol 12-myristate 13-acetate (PMA) can lead to an increase in transcription from the IP, and that Bet protein expression abrogates this effect. Finally, we demonstrate that Bet expression severely limits the ability of PMA to activate transcription of latent FV genomes, and that replication of a Bet(-) virus is more easily activated than wild-type FV. Taken together, these data suggest that viral transcription is regulated by a sensitive switch, and that Bet functions as a negative regulator of basal IP activity.

Construction and functional characterization of feline foamy virus-based retroviral vectors

Replication-competent feline foamy or spuma virus (FFV) vectors were constructed and functionally tested. The unmodified FFV vector genome expressed by the strong human cytomegalovirus immediate early promoter encodes FFV particles that were replication-competent in cell cultures. Virus derived from the cloned FFV DNA replicated and persisted in experimentally infected cats similar to the FFV isolate FUV. A FFV vector partially deleted in the noncoding area of the U3 region was used to transduce the gene for the green fluorescent protein (Gfp) into cell cultures. Gfp was expressed either by an internal ribosomal entry site (IRES) or as C-terminal fusion protein linked to Bet that was recently shown to be essential for FFV replication. Whereas the genetic stability of the IRES-Gfp construct was comparably low, the Bet-Gfp fusion protein was detectable upon serial cell-free vector passages. However, genetic rearrangements also occurred leading to the concomitant loss of marker gene expression.

Improved primate foamy virus vectors and packaging constructs

Foamy virus (FV) vectors that have minimal cis-acting sequences and are devoid of residual viral gene expression were constructed and analyzed by using a packaging system based on transient cotransfection of vector and different packaging plasmids. Previous studies indicated (i) that FV gag gene expression requires the presence of the R region of the long terminal repeat and (ii) that RNA from packaging constructs is efficiently incorporated into vector particles. Mutants with changes in major 5' splice donor (SD) site located in the R region identified this sequence element as responsible for regulating gag gene expression by an unidentified mechanism. Replacement of the FV 5' SD with heterologous splice sites enabled expression of the gag and pol genes. The incorporation of nonvector RNA into vector particles could be reduced to barely detectable levels with constructs in which the human immunodeficiency virus 5' SD or an unrelated intron sequence was substituted for the FV 5' untranslated region and in which gag expression and pol expression were separated on two different plasmids. By this strategy, efficient vector transfer was achieved with constructs that have minimal genetic overlap.

Cell-type-specific regulation of the two foamy virus promoters

The foamy virus (FV) genome contains two promoters, the canonical long terminal repeat (LTR) promoter, containing three consensus AP-1 binding sites, and an internal promoter (IP) within the env gene. We investigated the regulation of the two promoters in lytic and persistent infections and found that in the presence of a constitutive source of the viral transactivator protein Tas, transactivation of the LTR promoter and that of the IP differ. In lytic infections, both the LTR promoter and the IP are efficiently transactivated by Tas, while in persistent infections, the IP is efficiently transactivated by Tas, but the LTR promoter is not. Analysis of proteins expressed from the LTR promoter and the IP during infection indicated that IP transcription is more robust than that of the LTR promoter in persistently infected cells, while the opposite is true for lytically infected cells. Coculture experiments also showed that LTR promoter transcription is greatest in cells which support lytic replication. Replacement of much of the LTR promoter with the IP leads to increased viral replication in persistent but not lytic infections. We also found that the induction of persistently infected cells with phorbol 12-myristate 13-acetate (PMA) greatly enhanced viral replication and transcription from the SFVcpz(hu) (new name for human FV) LTR promoter. However, mutation of three consensus AP-1 binding sites in the FV LTR promoter did not affect viral replication in lytically or persistently infected cells, nor did the same mutations affect LTR promoter transactivation by Tas in PMA-treated cells. Our data indicate that differential regulation of transcription is important in the outcome of FV infection but is unlikely to depend on AP-1.

Historical perspective of foamy virus epidemiology and infection

Foamy viruses (FV) are complex retroviruses which are widespread in many species. Despite being discovered over 40 years ago, FV are among the least well characterized retroviruses. The replication of these viruses is different in many interesting respects from that of all other retroviruses. Infection of natural hosts by FV leads to a lifelong persistent infection, without any evidence of pathology. A large number of studies have looked at the prevalence of primate foamy viruses in the human population. Many of these studies have suggested that FV infections are prevalent in some human populations and are associated with specific diseases. More recent data, using more rigorous criteria for the presence of viruses, have not confirmed these studies. Thus, while FV are ubiquitous in all nonhuman primates, they are only acquired as rare zoonotic infections in humans. In this communication, we briefly discuss the current status of FV research and review the history of FV epidemiology, as well as the lack of pathogenicity in natural, experimental, and zoonotic infections.

Primate foamy virus Pol proteins are imported into the nucleus

Mouse monoclonal antibodies (MAbs) that specifically detect the 127 kDa Pol precursor and the 85 kDa reverse transcriptase/RNase H (RT/RN) or pr127 and the 40 kDa integrase (IN) in immunoblot and immunofluorescence assays (IFA) were used to investigate the subcellular localization of primate foamy virus (PFV) proteins. IFA of cells infected with PFV using the anti-Pol MAbs and rabbit anti-capsid (Gag) serum revealed that both the Gag and Pol proteins are transported into the nucleus. Transfection of cells with eukaryotic expression constructs for pr127(Pol), p85(RT/RN) and p40(IN) served to show Gag-independent subcellular localization of Pol proteins. Interestingly, not only the Pol precursor and IN molecules were found to be localized to the nucleus, but also the RT/RN subdomain. It is therefore suggested that PFV cores bear at least three separate nuclear localization signals, one in Gag and two in Pol. The latter appear to be localized to the two Pol subdomains.

Why aren't foamy viruses pathogenic?

Foamy viruses are complex retroviruses that lead to either highly cytopathic or persistent infections in vitro, but to non-pathogenic lifelong infections in naturally or accidentally infected hosts. Factors that could contribute to these benign persistent infections include regulated transcription from the two viral promoters, the functions of the Bet accessory protein and the host immune response.

Persistent zoonotic infection of a human with simian foamy virus in the absence of an intact orf-2 accessory gene

Although foamy viruses (FVs) are endemic among nonhuman primates, FV infection among humans is rare. Recently, simian foamy virus (SFV) infection was reported in 4 of 231 individuals occupationally exposed to primates (1.8%). Secondary transmission to spouses has not been seen, suggesting that while FV is readily zoonotic, humans may represent dead-end hosts. Among different simian species, SFV demonstrates significant sequence diversity within the U3 region of the long terminal repeat (LTR) and 3' accessory open reading frames (ORFs). To examine if persistent human SFV infection and apparent lack of secondary transmission are associated with genetic adaptations in FV regulatory regions, we conducted sequence analysis of the LTR, internal promoter, ORF-1, and ORF-2 on a tissue culture isolate and peripheral blood mononuclear cell samples from a human infected with SFV of African green monkey origin (SFV-3). Compared to the prototype SFV-3 sequence, the LTR, internal promoter, and FV transactivator (ORF-1) showed sequence conservation, suggesting that FV zoonosis is not dependent on host-specific adaptation to these transcriptionally important regions. However, ORF-2 contains a number of deleterious mutations predicted to result in premature termination of protein synthesis. ORF-2 codes in part for the 60-kDa Bet fusion protein, proposed to be involved in the establishment of persistent cellular SFV infections. These results suggest that persistent human infection by SFV and reduced transmissibility may be influenced by the absence of a functional ORF-2.

Properties of human foamy virus relevant to its development as a vector for gene therapy

The Spumaviridae (foamy viruses) are increasingly being considered as potential vectors for gene therapy, yet little has been documented of their basic cell biology. This study demonstrates that human foamy virus (HFV) has a broad tropism and that the receptor for HFV is expressed not only on many mammalian, but on avian and reptilian cells. Receptor interference assays using an envelope-expressing cell line and a vesicular stomatitis virus/HFV pseudotype virus demonstrate that the cellular receptor is common to all primate members of the genus. The majority of foamy virus particles assemble and remain sequestered intracellularly. A rapid and quantitative method of assaying foamy virus infectivity by reverse transcriptase activity facilitates the use of classical protocols to increase infectious virus titres in vitro to > or = 10(6) TCID/ml.

Sites of simian foamy virus persistence in naturally infected African green monkeys: latent provirus is ubiquitous, whereas viral replication is restricted to the oral mucosa

Foamy viruses (FV), retroviruses of the genus Spumavirus, are able to infect a wide variety of animal species and replicate in nearly all types of cultured cells. To identify the cells targeted by FV in the natural host and define the sites of viral replication, multiple organs of four African green monkeys naturally infected with simian FV type 3 were investigated for the presence of FV proviral DNA and viral transcripts. All organs contained significant amounts of FV proviral DNA. In addition to proviruses containing the complete transactivator gene taf, proviral genomes carrying a specific 295-bp deletion in the taf gene were detected in all monkeys. As in the case of human foamy virus the deletion leads to the formation of the bet gene that is regarded to be instrumental in the regulation of viral persistence. FV RNA was detected by RT-PCR and in situ hybridization only in the oral mucosa of one monkey. No other samples contained detectable levels of viral transcripts. Histopathological changes were not observed in any of the tissue samples analyzed. Our results show that the natural history of FV is characterized by latent infection in all organs of the host and by minimal levels of harmless viral replication in the oral mucosa. The broad host cell range in vivo further encourages the development of FV-derived vectors for therapeutic gene delivery.

Characterization of a human foamy virus 170-kilodalton Env-Bet fusion protein generated by alternative splicing

Primate foamy viruses (FVs) express, in addition to the 130-kDa envelope protein, a 170-kDa glycoprotein, which reacts with antisera specific for the envelope and Bel proteins. We determined the exact nature of this 170-kDa glycoprotein by using the molecularly cloned human FV (HFV). Radioimmunoprecipitation analysis of 293T cells transfected with appropriate expression constructs by using antisera specific for the HFV Env, Bel1, and Bel2 proteins, as well as reverse transcription-PCR analysis of HFV-infected cells, demonstrated that this protein is an Env-Bet fusion protein that is secreted into the supernatant. However, it is only loosely associated, or not associated, with viral particles. gp170 is generated by an alternatively spliced Env mRNA using a splice donor and splice acceptor pair localized within the env open reading frame (ORF), which is normally used to generate Bell and Bet transcripts derived from the internal promoter within the env ORF. gp170 is expressed at a level 30 to 50% of the Env precursor gp130. However, it alone does not confer infectivity to HFV particles, because capsids derived from proviruses expressing only the gp170 were not released into the supernatant. In contrast, viruses derived from proviral clones deficient in gp170 expression showed similar in vitro infectivity and replication kinetics to wild-type virus. Furthermore, both types of viruses were inactivated to a similar extent by neutralizing sera, indicating that shedding of gp170 probably does not affect the humoral immune response in the infected host.