Characterization of a foamy virus isolated from Cercopithecus aethiops lymphoblastoid cells

Genome Analysis and Replication Studies of the African Green Monkey Simian Foamy Virus Serotype 3 Strain FV2014

African green monkey (AGM) spumaretroviruses have been less well-studied than other simian foamy viruses (SFVs). We report the biological and genomic characterization of SFVcae_FV2014, which was the first foamy virus isolated from an African green monkey (AGM) and was found to be serotype 3. Infectivity studies in various cell lines from different species (mouse, dog, rhesus monkey, AGM, and human) indicated that like other SFVs, SFVcae_FV2014 had broad species and cell tropism, and in vitro cell culture infection resulted in cytopathic effect (CPE). In Mus dunni (a wild mouse fibroblast cell line), MDCK (Madin-Darby canine kidney cell line), FRhK-4 (a fetal rhesus kidney cell line), and MRC-5 (a human fetal lung cell line), SFVcae_FV2014 infection was productive resulting in CPE, and had delayed or similar replication kinetics compared with SFVmcy_FV21 and SFVmcy_FV34[RF], which are two Taiwanese macaque isolates, designated as serotypes 1 and 2, respectively. However, in Vero (AGM kidney cell line) and A549 (a human lung carcinoma cell line), the replication kinetics of SFVcae_FV2014 and the SFVmcy viruses were discordant: In Vero, SFVcae_FV2014 showed rapid replication kinetics and extensive CPE, and a persistent infection was seen in A549, with delayed, low CPE, which did not progress even upon extended culture (day 55). Nucleotide sequence analysis of the assembled SFVcae_FV2014 genome, obtained by high-throughput sequencing, indicated an overall 80–90% nucleotide sequence identity with SFVcae_LK3, the only available full-length genome sequence of an AGM SFV, and was distinct phylogenetically from other AGM spumaretroviruses, corroborating previous results based on analysis of partial env sequences. Our study confirmed that SFVcae_FV2014 and SFVcae_LK3 are genetically distinct AGM foamy virus (FV) isolates. Furthermore, comparative infectivity studies of SFVcae_FV2014 and SFVmcy isolates showed that although SFVs have a wide host range and cell tropism, regulation of virus replication is complex and depends on the virus strain and cell-specific factors.

Foamy virus zoonotic infections

BACKGROUND

Foamy viruses (FV) are ancient complex retroviruses that differ from orthoretroviruses such as human immunodeficiency virus (HIV) and murine leukemia virus (MLV) and comprise a distinct subfamily of retroviruses, the Spumaretrovirinae. FV are ubiquitous in their natural hosts, which include cows, cats, and nonhuman primates (NHP). FV are transmitted mainly through saliva and appear nonpathogenic by themselves, but they may increase morbidity of other pathogens in coinfections.

CONCLUSIONS

This review summarizes and discusses what is known about FV infection of natural hosts. It also emphasizes what is known about FV zoonotic infections A large number of studies have revealed that the FV of NHP, simian foamy viruses (SFV), are transmitted to humans who interact with infected NHP. SFV from a variety of NHP establish persistent infection in humans, while bovine foamy virus and feline foamy virus rarely or never do. The possibility of FV recombination and mutation leading to pathogenesis is considered. Since humans can be infected by SFV, a seemingly nonpathogenic virus, there is interest in using SFV vectors for human gene therapy. In this regard, detailed understanding of zoonotic SFV infection is highly relevant.

Origin, evolution and innate immune control of simian foamy viruses in humans

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Simian foamy viruses (SFV) are transmitted to humans after contact, mainly bites, with infected monkeys and apes.

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Contexts of transmission include mainly hunting activities and monkeys’ sympatry.

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In humans, active immune response probably explains SFV latency in blood and saliva.

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It is a model of restriction of retroviral emergence after cross-species transmission.

Most viral pathogens that have emerged in humans have originated from various animal species. Emergence is a multistep process involving an initial spill-over of the infectious agent into single individuals and its subsequent dissemination into the human population. Similar to simian immunodeficiency viruses and simian T lymphotropic viruses, simian foamy viruses (SFV) are retroviruses that are widespread among non-human primates and can be transmitted to humans, giving rise to a persistent infection, which seems to be controlled in the case of SFV. In this review, we present current data on the discovery, cross-species transmission, and molecular evolution of SFV in human populations initially infected and thus at risk for zoonotic emergence.

RNase H: specificity, mechanisms of action, and antiviral target

The Ribonuclease (RNase) H is one of the four enzymes encoded by all retroviruses, including HIV. Its main activity is the hydrolysis of the RNA moiety in RNA-DNA hybrids. The RNase H ribonuclease is essential in the retroviral life cycle, since it generates and removes primers needed by the Reverse Transcriptase (RT) for initiation of DNA synthesis. Retroviruses lacking RNase H activity are noninfectious. Despite its importance, RNase H is the only enzyme of HIV not yet targeted by antiretroviral therapy. Here, we describe functions and mechanisms of RNase H during the HIV life cycle and describe a cleavage assay, which is suitable to determine RNase H activity in samples of various kinds. In this assay, an artificial, fluorescence-labeled RNA-DNA hybrid is cleaved in vitro by an RT/RNase H enzyme. Cleavage products are analyzed by denaturing polyacrylamide gel electrophoresis (PAGE). This assay may be used to detect the RNase H, assess the effect of inhibitors, or even activators, of the RNase H, as we have described, as candidates for novel antiretroviral agents.

Evolution of foamy viruses: the most ancient of all retroviruses

Recent evidence indicates that foamy viruses (FVs) are the oldest retroviruses (RVs) that we know and coevolved with their hosts for several hundred million years. This coevolution may have contributed to the non-pathogenicity of FVs, an important factor in development of foamy viral vectors in gene therapy. However, various questions on the molecular evolution of FVs remain still unanswered. The analysis of the spectrum of animal species infected by exogenous FVs or harboring endogenous FV elements in their genome is pivotal. Furthermore, animal studies might reveal important issues, such as the identification of the FV in vivo target cells, which than require a detailed characterization, to resolve the molecular basis of the accuracy with which FVs copy their genome. The issues of the extent of FV viremia and of the nature of the virion genome (RNA vs. DNA) also need to be experimentally addressed.

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.

Impact of viral accessory proteins of SIVsmmPBj on early steps of infection of quiescent cells

Although lentiviruses like HIV-1 are able to infect non-dividing cells, particular resting cells such as non-stimulated primary peripheral blood mononuclear cells (PBMC) are resistant to infection. In contrast to other lentiviruses, SIVsmmPBj can replicate in non-stimulated PBMC. Moreover, SIVsmmPBj-derived, but not HIV-1-derived, replication-incompetent vectors enable gene transfer into G(0)-arrested human cell lines and primary human monocytes. Here, we demonstrate that transduction of G(0)-arrested cell lines by SIVsmmPBj-derived vectors is independent of the viral accessory proteins Vif, Vpx, Vpr, or Nef. In contrast, for the transduction of primary human monocytes, the Vpx protein proved to be essential. However, trans-complementation of HIV-1 vectors with SIVsmmPBj Vpx did not provide the property of gene transfer into monocytes. Taken together, these data indicate that Vpx is essential for the infection of primary monocytes by SIVsmmPBj. Additionally, further genome functions besides the accessory proteins are required for the particular capacity of SIVsmmPBj in transduction or infection events.

Stable Transduction of Primary Human Monocytes by Simian Lentiviral Vector PBj

Despite the ability to infect nonproliferating cells, current lentiviral vectors are inefficient at mediating gene transfer into quiescent primary human cells such as monocytes. Here, a replication-incompetent vector based on a molecular clone of simian immunodeficiency virus strain PBj (SIVsmmPBj1.9) was generated that, in contrast to lenti- and gamma-retroviral control vectors, enabled transfer of heterologous genes into human diploid fibroblasts and cell lines blocked in the G(0) phase of the cell cycle. Moreover, freshly isolated human monocytes refractory to HIV-1-derived vectors were efficiently transduced by the PBj vector independent of the viral Nef protein. Stable chromosomal integration of PBj-derived viral expression vectors was verified in transduced cells. The capability of the PBj vector to transduce quiescent cells such as unstimulated primary human monocytes is an important extension of human gene therapy perspectives.

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.

Replication of enhancer-deficient amphotropic murine leukemia virus in human fibrosarcoma but not in primary human fibroblasts

Amphotropic murine leukemia virus (MLV) replicates in cells from various mammalian species including humans and is a potential contaminant in MLV vector preparations for human gene transfer studies. In general, MLV replication depends on the expression of viral genes under the control of 75 bp enhancer elements in the long terminal repeat. However, in specific human fibrosarcoma and lymphoma lines replication of amphotropic MLV is possible without these enhancers. Fibrosarcomas are malignant tumors of fibroblast origin. To test the replication potential of intact and enhancerless amphotropic MLV in untransformed cells, infection studies with these viruses were carried out in three types of primary human fibroblasts. Replication of amphotropic MLV is observed in two of three tested fibroblast strains. None of these primary human fibroblasts is permissive for enhancer-deficient MLV, suggesting that replication of this virus may be limited to transformed cells.

Genetic stability of foamy viruses: long-term study in an African green monkey population

The genetic variability of the envelope surface domain (SU) of simian foamy virus (FV) of African green monkeys was studied. To assess the interindividual diversity of FV, isolates were obtained from 19 animals living together in a monkey house. The monkeys had been imported from Kenya prior to being placed in long-term housing in the research institute. In addition, a simian FV isolate and proviral DNA were obtained from an animal caretaker infected in this setting. DNA of the complete SU (1779 to 1793 bp) was analyzed by PCR and sequencing. The sequences revealed four clusters with high homologies (>95%). Between the clusters, divergencies ranged from 3 to 25%. Obviously, the clusters reflect four different strains or subtypes of simian FV type 3 that were prevalent in the colony. In contrast to lentiviruses, hypervariable regions could not be detected in the FV SU. Furthermore, to analyze the intraindividual diversity of FV, we investigated the virus population within an individual monkey at a given time point and its evolution over 13 years. For this purpose, 22 proviral SU clones generated by PCR from one oral swab and seven isolates obtained from the same animal between 1982 and 1995 were examined. These sequences revealed exceptionally high homology rates (99.5 to 100%), and only a minimal genetic drift was recognized within the series of isolates. In conclusion, the low in vivo divergency of FV SU suggests that genetic variability is not important for the maintenance of FV persistence.

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.

Sensitive assays for isolation and detection of simian foamy retroviruses

Simian foamy viruses (SFVs) are highly prevalent in a variety of nonhuman primate species ranging from prosimians to apes. SFVs possess a broad host range, and human infections can occur by cross-species transfer (W. Heneine et al., Nat. Med. 4:403-407, 1998). Retrovirus screening of potential sources of infection, such as laboratory research animals and simian-derived biological products, could minimize human exposure to SFVs by reducing the risk of potential retrovirus infection in humans. We describe a variety of sensitive assays for SFV isolation and detection which were developed with a prototype strain of SFV serotype 2. The Mus dunni cell line (M. R. Lander and S. K. Chattopadhyay, J. Virol. 52:695-698, 1984) was found to be highly sensitive for SFV production on the basis of various general and specific retrovirus detection assays such as reverse transcriptase assay, transmission electron microscopy, immunofluorescence assay, and Western blotting. A highly sensitive PCR assay was developed on the basis of the sequences in primary SFV isolates obtained from pig-tailed macaques (Macaca nemestrina) and rhesus macaques (Macaca mulatta). Analysis of naturally occurring SFV infection in macaques indicated that analysis by a combination of assays, including both highly sensitive, specific assays and less sensitive, broadly reactive assays, is important for evaluation of retrovirus infection.

Gamma interferon is a major suppressive factor produced by activated human peripheral blood lymphocytes that is able to inhibit foamy virus-induced cytopathic effects

The activation of human peripheral blood lymphocytes by mitogens or by triggering the T-cell receptor with anti-CD3 antibodies leads to the production of a potent soluble inhibitory activity against foamy virus-induced cytopathic effects in vitro. The inhibitory activity acts in a species-specific manner. As a consequence, the isolation of foamy viruses from blood lymphocytes of infected humans is accelerated in a heterologous coculture system. Antibodies against gamma interferon (IFN-gamma) are able to suppress most of the inhibitory activity, suggesting that IFN-gamma is the dominant component.

Human fibroblasts are able to inhibit growth of human melanoma cells

The influence of normal cells on the growth and fate of transformed cells has been described repeatedly. This report deals with results obtained when human melanoma cells of two different lines were seeded in very small quantities onto contact-inhibited cultures of seven different primary human fibroblasts (originating from abortion material, cheloid, dermal biopsies and foreskin). The expected proliferation of melanoma colonies was seen in five fibroblast cultures. However, proliferation of the melanoma cells was inhibited and cells were even eliminated by two different foreskin fibroblasts. This shows growth inhibition and elimination of malignant cells by normal cells of the homologous human system.

Simian foamy virus isolated from an accidentally infected human individual

Evidence for natural foamy virus (FV) infections in humans is still lacking. However, accidental infections of humans with simian FV have been demonstrated by serology and PCR, but all previous attempts to recover infectious virus in such cases have failed. Here we describe the isolation of a simian FV from peripheral blood mononuclear cells (PBMC) of a healthy animal caretaker, who acquired the virus 20 years ago from an African green monkey (AGM) bite. Properties of the human isolate such as host range in cell cultures including human PBMC and ability to induce neutralizing antibodies in the primate host proved to be similar to those of FV obtained from AGM. The genomic sequence of the isolate was found to be virtually identical to the proviral sequence present in the host lymphocytes and related to AGM isolates but distinct from those of all FV isolates handled in the laboratory. For successful virus isolation, it was essential to stimulate the host lymphocytes by phytohemagglutinin and interleukin-2 for 2 weeks prior to cocultivation with permissive cells. In contrast to the situation found in FV-infected monkeys, virus isolation from the saliva of the animal caretaker was not possible, and no evidence for FV transmission to family contacts was obtained. We conclude that, in contrast to active infection in monkeys, FV persists in a state of latency following accidental infection of humans.

Long terminal repeat U3 length polymorphism of human foamy virus

Size determination of the long terminal repeat (LTR) of an early (1985) and a more recent (1993) passage of wild-type human foamy virus (HFV) revealed that the virus has undergone substantial deletions in the U3 region upon replication in tissue culture. Two LTR deletion variants (HSRV1 and 2) have been characterized in the past and used to construct molecular clones which are replication competent in cell culture. We now report the molecular cloning, sequencing, and biological characterization of an HFV genome with full-length LTR (pHFV2). Sequence analysis revealed that the deletions in HSRV1 and 2 are nonrandom and probably occurred by misalignment during reverse transcription. The comparative analysis of HFV2 and the variant with the largest U3 deletion, HSRV2, revealed a differential ability to replicate in human cell cultures. While HSRV2 replicated faster in diploid human fibroblasts, cells which have been used extensively for amplification of HFV in the past, replication of HFV2 was faster in a lymphoblastoid cell line. Reporter gene assays indicated that the cell-type specific ability of the LTRs to respond to the viral transcriptional transactivator may be a likely, reason for the different growth properties of both viruses and for the occurrence of the HFV U3 deletions. In foamy virus-infected chimpanzees only the full-length type of LTR was observed; however, the HSRV1 deletion variant was detected as the dominating virus in an accidentally HFV-infected human.

A sorting motif localizes the foamy virus glycoprotein to the endoplasmic reticulum

We recently identified an endoplasmic reticulum (ER) retrieval signal-the dilysine motif-in the glycoproteins of all five foamy viruses (FVs) for which sequences were available (P. A. Goepfert, G. Wang, and M. J. Mulligan, Cell 82:543-544, 1995). In the present study, expression of recombinant human FV (HFV) glycoprotein and analyses of oligosaccharide modifications and precursor cleavage indicated that the protein was localized to the ER. HFV glycoproteins encoding seven different dilysine motif mutations were then expressed. The results indicated that disruptions of the dilysine motif resulted in higher levels of forward transport of the HFV glycoprotein from the ER through the Golgi apparatus to the plasma membrane. We conclude that the dilysine motif is responsible for ER sorting of the FV glycoprotein. Signal-mediated ER localization has not previously been described for a retroviral glycoprotein.

Analysis of west African hunters for foamy virus infections

Foamy viruses are a genus of complex retroviruses that infect a wide variety of mammals. However, a clear association with any disease process has yet to be proven for these viruses. A higher human seroprevalence was reported in African populations, perhaps due to exposure to simian foamy viruses (SFV) endemic in primates. However, the earlier serologic surveys were not confirmed by studies employing nucleic acid amplification. Foamy virus infections of humans clearly do occur as rare zoonoses among primate center or laboratory workers exposed to captive primates or their blood. We sought to detect foamy virus infections in a cohort of humans also presumed to be exposed to SFV, i.e., West African hunters. We constructed recombinant vaccinia viruses that expressed human foamy virus (HFV) Gag or Env polyproteins in mammalian cells. The sera from 17 monkey hunters or several controls were tested in radioimmunoprecipitation assays (RIPAs) against the recombinant HFV proteins. Chimpanzee sera or HFV-positive human sera immunoprecipitated gp130, the HFV Env precursor, as well as p74, the HFV Gag polyprotein. None of the hunters' sera recognized both of these recombinant proteins. We then employed a nested polymerase chain reaction (PCR) analysis of the hunters' DNA but also failed to detect foamy virus infections. Therefore, by utilizing a recombinant RIPA and a nested PCR assay, we have not identified foamy virus infections occurring naturally in hunters exposed to wild monkeys in West Africa.

Productive persistent infection of hematopoietic cells by human foamy virus

Human foamy virus can establish persistent infections in human hematopoietic cell lines, such as H92.1.7 (erythroblastoid cells), Jurkat (CD4+ T cells), and U937 (myeloid-monocytic cells). The infection is characterized by constant production of infectious viruses (for > 2 1/2 years) with no cytopathic effects on the host cells. Electron microscopy of the infected cells showed a viral morphology similar to that observed for particles produced after acute infection. We have detected, in addition to the full-length form of bel1, a previously described deletion in the bel1 gene of the proviral DNA in these cells. RNA containing this 301-bp deletion, which mapped to the splice donor and acceptor sites of the intron of the bet gene, was also found in encapsidated virion RNA. However, the presence of this defective provirus harboring the deletion in bel1 does not prevent productive persistence in these chronically infected cells, since the virus titer does not decrease during cultivation.

Cell tropism of the simian foamy virus type 1 (SFV-1)

Several cell lines representing different species and cell types were tested for simian foamy virus type 1 (SFV-1) infection. SFV-1 infections were monitored by polymerase chain reaction, reverse transcriptase, cytopathology, and immunofluorescent assays. All cells tested were permissive for SFV-1, demonstrating that SFV-1 has a broad host range with respect to species and cell types. Infected fibroblasts, epithelial cells, and neural cells all showed extensive cytopathology that is characteristic of foamy virus infection. No cytopathology was induced in T cell-derived Jurkats and Hut-78 cell lines. The cytopathic effects in B and macrophage originated cells were delayed by several days. Cytopathology in these cell lines was not as dramatic as seen in the infected fibroblast and epithelial cells. The reverse transcriptase values in the SFV-1 infected lymphoid and macrophage cell lines were severalfold lower than that of the fibroblasts and epithelial cells. Therefore, SFV-1 appears to establish a low level persistent infection in lymphoid and macrophage cell lines.

Markers of foamy virus infections in monkeys, apes, and accidentally infected humans: appropriate testing fails to confirm suspected foamy virus prevalence in humans

Foamy viruses (FVs) persist in healthy individuals of various mammalian species, including nonhuman primates. Laboratory markers of FV infection are (1) virus in throat epithelium or peripheral blood lymphocytes (PBLs), (2) proviral DNA sequences in PBLs and various solid organs, and (3) antibodies reactive to viral antigens on Western blots, in radioimmunoprecipitation tests, and in immunofluorescence assays. Using PCR and serological tests, we readily detected FV markers in naturally infected African green monkeys, rhesus monkeys, and chimpanzees, as well as in accidentally infected humans. Transmission of simian foamy viruses to humans (by bite or inadvertent laboratory infection) leads to viral markers, without affecting the recipient. Reports on FV-associated clinical disorders (e.g., thyroid or neurological) have remained controversial. In this study we failed to detect, by PCR, viral sequences in the samples from 223 patients, including 16 HIV-infected Africans, 46 Graves' disease patients, and 28 patients with the de Quervain's thyroiditis. Evaluation of 2688 sera from suspected high-risk areas (e.g., Central and East Africa, or high-risk groups such as HIV-infected individuals and patients with AIDS, thyroid, and neurological disorders) did not reveal FV-specific antibodies in a single case. Previously reported FV seroprevalence in various populations has never been verified by appropriate confirmatory tests. The strain of "human foamy virus" has remained a unique isolate. In conclusion, FVs are unlikely--at present--to circulate in human populations.

Phylogenesis and genetic complexity of the nonhuman primate retroviridae

The three known groups of nonhuman primate retroviruses (simian immunodeficiency virus, simian T cell lymphotropic/leukemic virus type I, and simian foamy virus) are thought to have equivalent human counterparts. This is clearly the case with human immunodeficiency virus types 1 and 2, the causative agents of acquired immunodeficiency syndrome, and with human T cell lymphotropic/leukemia virus type I (HTLV-I), which causes T cell leukemia and a progressive form of myelopathy (tropical spastic paraparesis/HTLV-I-associated myelopathy), and HTLV-II. However, the presence of spumaviruses (foamy viruses) in humans remains uncertain. Data accumulated in the last 5 years suggest the possibility that the human retroviruses are indeed the result of transmission of simian retroviruses to humans. In this article we attempt to parallel the genetic features of the simian retroviridae with their human counterparts and argue for the possibility of horizontal transmission of these viruses from monkeys to humans.

Nuclear localization of foamy virus Gag precursor protein

All foamy viruses give rise to a strong nuclear staining when infected cells are reacted with sera from infected hosts. This nuclear fluorescence distinguishes foamy viruses from all other retroviruses. The experiments reported here indicate that the foamy virus Gag precursor protein is transiently located in the nuclei of infected cells and this is the likely reason for the typical foamy virus nuclear fluorescence. By using the vaccinia virus expression system, a conserved basic sequence motif in the nucleocapsid domain of foamy virus Gag proteins was identified to be responsible for the nuclear transport of the gag precursor molecule. This motif was also found to be able to direct a heterologous protein, the Gag protein of human immunodeficiency virus, into the nucleus.

Absence of foamy virus DNA in Graves' disease

A report on the high prevalence of foamy virus DNA in lymphocytes from French patients with Graves' disease prompted us to investigate a similar cohort of 41 German patients. Using PCR amplification and Southern blot hybridization, we detected foamy virus DNA only in lymphocytes of two accidentally infected humans and five naturally infected monkeys, as well as in DNA samples from four Graves' disease patients investigated in the French study. However, we failed to detect foamy virus DNA in peripheral blood lymphocytes from any of the 41 Graves' disease patients of the German cohort. Thus, a causative role of foamy viruses in this thyroid disease is highly improbable.

Functional analysis of human foamy virus accessory reading frames

Foamy viruses belong to the retroviruses which possess a complex genome structure. The human foamy virus (HFV) isolate bears three open reading frames (the so-called bel genes) in the 3' region of the genome which have been reported to give rise to possibly six different proteins via alternative splicing (W. Muranyi and R. M. Flügel, J. Virol. 65:727-735, 1991). In order to analyze the requirements of these proteins for HFV replication in vitro, we constructed a set of single and combinatory bel gene mutants of an infectious molecular clone of HFV. The mutant which lacked the transacting activator, bel-1, was found to be replication incompetent. All other mutants replicated equally well and gave rise to comparable titers of infectious cell-free virus. When HFV proviruses were put under the control of a heterologous promoter (simian virus 40), none of the accessory gene products was found to be required for expression of structural (gag) proteins. There was no evidence for a posttranscriptional regulatory protein that is present in other complex retroviruses.

Transient immunosuppressive effect induced in rabbits and mice by the human spumaretrovirus prototype HFV (human foamy virus)

Spumaviruses (foamy viruses) constitute one of the three retroviral genera isolated from man. Although spumaviruses have not been clearly linked to a given pathology in humans and other infected species, it is well established that they lead in vivo to chronic infections without detectable viral expression. We thought it of interest to investigate certain aspects of the pathology induced in laboratory animals by human foamy virus (HFV). In this work, we demonstrate that HFV infection of rabbits and mice gives rise to a transient immunosuppressive effect, as evaluated in vitro by lymphocyte transformation tests. This phenomenon occurs shortly after viral inoculation, at around 4-5 days, and regresses within thirty days.

The foamy virus family: molecular biology, epidemiology and neuropathology

The family of foamy viruses designates a group of retroviruses which share a specific morphology and provoke characteristic cytopathic effects in cultured cells. Like HTLV and HIV, foamy viruses are complex viruses encoding a number of ancillary genes in addition to gag, pol and env, including a transcriptional transactivator. Foamy viruses are endemic in various primate species, and human foamy viruses (HFV) have been isolated from patients with various neoplastic and degenerative diseases. Despite a growing body of knowledge on the biology of foamy viruses, it has not yet been possible to identify a disease specifically caused by foamy virus infection. After reviewing the epidemiology and molecular biology of the various animal foamy viruses, this article focuses on the pathogenic properties of HFV in transgenic mouse systems. HFV transgenes exhibit a striking neurotropism and elicit a progressive degenerative disease of the central nervous system and striated muscle. Similarly to patients with HIV-associated encephalopathy, HFV transgenic mice develop accumulations of syncytial giant cells in their brains. The relevance of these findings for human neuropathology is discussed.

Search for retrovirus in the chronic fatigue syndrome

AIM

To examine peripheral blood and skeletal muscle from patients with chronic fatigue syndrome for exogenous retrovirus.

METHODS

Blood samples from 30 patients and muscle biopsy specimens of 15 patients were examined for retroviral sequences by DNA extraction, polymerase chain reaction (PCR), and Southern blotting hybridisation. Sera were examined for human foamy virus by western immunoblotting and indirect immunofluorescence techniques.

RESULTS

No differences between the patient and control populations was found for any of the PCR primer sets used (gag, pol, env, and tax regions of HTLV I/II). An endogenous gag band was observed in both the patient and control groups. All sera were negative for antibody to human foamy virus.

CONCLUSION

The results indicate that there is no evidence of retroviral involvement in the chronic fatigue syndrome.

Human spumavirus antibodies in sera from African patients

Serum samples collected from patients with a wide variety of diseases from African and other countries were tested for antibodies to the human spumaretrovirus (HSRV). A spumaviral env-specific ELISA was employed as screening test. Out of 3020 human sera screened, 106 were found to be positive (3.2%). While the majority of patients' sera from Europe (1581) were negative, 26 were positive (1.6%). Sera from healthy adult blood donors (609), from patients with multiple sclerosis (48), Graves' disease (45), and chronic fatigue syndrome (41) were negative or showed a very low prevalence for spumaviral env antibodies. A higher percentage of seropositives (6.3%) were found among 1338 African patients from Tanzania, Kenya, and Gabon. Out of 1180 patients from Tanzania, 708 suffered from tumors, 75 from AIDS, and 128 had gynecological problems; 51 of the Tanzanian patients were HSRV seropositive (4.3%). A particularly high percentage of 16.6% seropositives were identified among nasopharyngeal carcinoma patients (NPC) from Kenya and Tanzania consistent with results reported 10 years ago. However, 20 nasopharyngeal carcinoma patients from Malaysia were HSRV-seronegative. In selected cases, sera from seropositive individuals were reacted with proteins from HSRV-infected cells in vitro. HSRV env- and gag-specific antibodies were specifically detected by these sera in Western blots. The results indicate spumavirus infections in human patients with various diseases at a relatively low prevalence worldwide; in African patients, however, the prevalence of spumavirus infections is markedly higher.

Generation of macaque B lymphoblastoid cell lines with simian Epstein-Barr-like viruses: transformation procedure, characterization of the cell lines and occurrence of simian foamy virus

Two simian Epstein-Barr-like viruses, a rhesus Epstein-Barr virus and Herpesvirus papio, were used to transform B cells from rhesus or cynomolgus macaques. The resulting cell lines exhibited predominantly a B lymphocyte phenotype and expressed Epstein-Barr virus antigens. The majority of B lymphoblastoid cell lines from macaques, which were seropositive for simian foamy virus, developed giant cells in culture. The cytopathic agent was identified as a foamy virus and was transmissible to human embryonal fibroblasts. Treatment of cell cultures with AZT abolished giant cell formation.

Mechanism of action of regulatory proteins encoded by complex retroviruses

Complex retroviruses are distinguished by their ability to control the expression of their gene products through the action of virally encoded regulatory proteins. These viral gene products modulate both the quantity and the quality of viral gene expression through regulation at both the transcriptional and posttranscriptional levels. The most intensely studied retroviral regulatory proteins, termed Tat and Rev, are encoded by the prototypic complex retrovirus human immunodeficiency virus type 1. However, considerable information also exists on regulatory proteins encoded by human T-cell leukemia virus type I, as well as several other human and animal complex retroviruses. In general, these data demonstrate that retrovirally encoded transcriptional trans-activators can exert a similar effect by several very different mechanisms. In contrast, posttranscriptional regulation of retroviral gene expression appears to occur via a single pathway that is probably dependent on the recruitment of a highly conserved cellular cofactor. These two shared regulatory pathways are proposed to be critical to the ability of complex retroviruses to establish chronic infections in the face of an ongoing host immune response.

The Bel-1 protein of human foamy virus activates human immunodeficiency virus type 1 gene expression via a novel DNA target site

The Bel-1 protein of human foamy virus can activate transcription directed by the long terminal repeat (LTR) promoter of human immunodeficiency virus type 1 (HIV-1). The target sequence for Bel-1 is shown to lie within the HIV-1 LTR U3 region but does not coincide with any previously described factor-binding site. Gene expression directed by an HIV-1 LTR lacking functional sites for the inducible cellular transcription factor NF-kappa B was activated over 100-fold by coexpression of Bel-1. These observations suggest that Bel-1 has the potential to significantly enhance the level of HIV-1 gene expression in cells dually infected with HIV-1 and human foamy virus.

Transacting transcriptional activation of human spumaretrovirus LTR in infected cells

The long terminal repeat (LTR) of the human spumaretrovirus (HSRV) was examined with respect to its ability to function as transcriptional promotor in virus-infected and uninfected cells. Transient transfections using a plasmid in which the 3' LTR of HSRV was coupled to the bacterial chloramphenicol acetyltransferase (cat) gene revealed that the level of HSRV LTR-directed cat gene expression was markedly increased in HSRV-infected cells compared to uninfected cells. Northern blot analysis of cat mRNA from transfected cultures suggests that transactivation of HSRV-directed gene expression occurs at the transcriptional level.

Infectious DNA of the human spumaretrovirus

An infectious molecular clone (pHSRV) of the human Spumaretrovirus (HSRV) was constructed using viral DNA and cDNA clones. The infectivity of pHSRV was proven by transfection of cell cultures and subsequent infection of susceptible cultures with cell free transfection derived virus. pHSRV derived virus produced foamy virus typical cytopathic effects in susceptible cultures. Infected cells could be stained specifically with foamy virus antisera by means of indirect immunofluorescence. Radioimmunoprecipitation revealed the presence of characteristic HSRV structural proteins in pHSRV infected cultures. By cotransfection of pHSRV and an indicator plasmid it was found that pHSRV is able to transactivate the viral LTR. Viral transcripts were found to be approximately 200 bases longer in pHSRV infected cultures compared to wildtype infected cultures. This difference is most likely due to an insertion of DNA of non-viral origin in the U3 region of the 3'LTR of the infectious clone.

Structural analysis of proviral DNA in simian foamy virus (LK-3)-infected cells

Proviral DNA of the T-cell lymphotropic simian foamy virus strain LK-3 was characterized. In infected cells, multiple copies of unintegrated linear duplex viral DNA of about 13 kbp length are present. Nuclease S1 treatment of the DNA generated two fragments of 6.5 and 6.0 kbp length that were cloned in phage and plasmid vectors. The proviral DNA contains a single-stranded gap of 109 nucleotides. A DNA fragment spanning the gap was cloned after completing the double strand by DNA synthesis in vitro. At the 3' end, the gap contains a polypurine tract (PPT) similar to the putative initiation site of retroviral plus strand DNA synthesis, suggesting discontinuous DNA synthesis. Further analysis of the genome architecture revealed LTRs of 1.7 kbp length. An additional 1.7 kbp DNA fragment was detected after nuclease S1 digestion of proviral DNA and probably represents trimmed intermediates of "strong-stop" DNA.

Heterogeneity of primate foamy virus genomes. Brief report

DNA of the T-lymphotropic simian foamy virus (SFV) LK-3 was cloned in a plasmid vector and used as a probe in comparative DNA:DNA hybridization studies with primate foamy viruses (SFV serotypes 1, 2, 3, 5, 6, 7, 8, fresh SFV isolates from 12 African green monkeys and one rhesus monkey, as well as human syncytium forming virus). All freshly isolated viruses seemed to be variants of SFV types 2 and 3. Especially LK-3 appeared to be closely related to SFV type 3. No hybridization of the LK-3 clone was observed to DNA sequences of human immunodeficiency virus (HIV-1).

Molecular cloning of the genome of human spumaretrovirus

DNA of human spumaretrovirus (HSRV) was cloned from both cDNA and from viral DNA into phage lambda and bacterial plasmid vectors. The recombinant plasmids harboring viral DNA were characterized by Southern blot hybridization and restriction mapping. Physical maps were constructed from cDNA and found to be colinear with the restriction maps obtained from viral DNA. The recombinant clones isolated contained viral DNA inserts which range in size from 2.2 kb to 15.4 kb. The recombinant clones allowed to construct a physical map of the complete HSRV provirus of 12.2 kb.