No evidence of antibody to human foamy virus in widespread human populations

Horizontal transmission of endemic viruses among rhesus macaques (Macaca mulatta): Implications for human cytomegalovirus vaccine/challenge design

INTRODUCTION

Rhesus macaques are natural hosts to multiple viruses including rhesus cytomegalovirus (RhCMV), rhesus rhadinovirus (RRV), and Simian Foamy Virus (SFV). While viral infections are ubiquitous, viral transmissions to uninfected animals are incompletely defined. Management procedures of macaque colonies include cohorts that are Specific Pathogen Free (SPF). Greater understanding of viral transmission would augment SPF protocols. Moreover, vaccine/challenge studies of human viruses would be enhanced by leveraging transmission of macaque viruses to recapitulate expected challenges of human vaccine trials.

MATERIALS AND METHODS

This study characterizes viral transmissions to uninfected animals following inadvertent introduction of RhCMV/RRV/SFV-infected adults to a cohort of uninfected juveniles. Following co-housing with virus-positive adults, juveniles were serially evaluated for viral infection.

RESULTS

Horizontal viral transmission was rapid and absolute, reaching 100% penetrance between 19 and 78 weeks.

CONCLUSIONS

This study provides insights into viral natural histories with implications for colony management and modeling vaccine-mediated immune protection studies.

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.

The ecology of primate retroviruses - an assessment of 12 years of retroviral studies in the Taï national park area, Côte d׳Ivoire

The existence and genetic make-up of most primate retroviruses was revealed by studies of bushmeat and fecal samples from unhabituated primate communities. For these, detailed data on intra- and within-species contact rates are generally missing, which makes identification of factors influencing transmission a challenging task. Here we present an assessment of 12 years of research on primate retroviruses in the Taï National Park area, Côte d'Ivoire. We discuss insights gained into the prevalence, within- and cross-species transmission of primate retroviruses (including towards local human populations) and the importance of virus-host interactions in determining cross-species transmission risk. Finally we discuss how retroviruses ecology and evolution may change in a shifting environment and identify avenues for future research.

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.

Human RNA "rumor" viruses: the search for novel human retroviruses in chronic disease

Retroviruses are an important group of pathogens that cause a variety of diseases in humans and animals. Four human retroviruses are currently known, including human immunodeficiency virus type 1, which causes AIDS, and human T-lymphotropic virus type 1, which causes cancer and inflammatory disease. For many years, there have been sporadic reports of additional human retroviral infections, particularly in cancer and other chronic diseases. Unfortunately, many of these putative viruses remain unproven and controversial, and some retrovirologists have dismissed them as merely "human rumor viruses." Work in this field was last reviewed in depth in 1984, and since then, the molecular techniques available for identifying and characterizing retroviruses have improved enormously in sensitivity. The advent of PCR in particular has dramatically enhanced our ability to detect novel viral sequences in human tissues. However, DNA amplification techniques have also increased the potential for false-positive detection due to contamination. In addition, the presence of many families of human endogenous retroviruses (HERVs) within our DNA can obstruct attempts to identify and validate novel human retroviruses. Here, we aim to bring together the data on "novel" retroviral infections in humans by critically examining the evidence for those putative viruses that have been linked with disease and the likelihood that they represent genuine human infections. We provide a background to the field and a discussion of potential confounding factors along with some technical guidelines. In addition, some of the difficulties associated with obtaining formal proof of causation for common or ubiquitous agents such as HERVs are discussed.

Clinical and virological characterization of persistent human infection with simian foamy viruses

Persons occupationally exposed to nonhuman primates (NHPs) can be persistently infected with simian foamy virus (SFV). The clinical significance and person-to-person transmissibility of zoonotic SFV infection is unclear. Seven SFV-infected men responded to annual structured interviews and provided whole blood, oral, and urogenital specimens for study. Wives were tested for SFV infection. Proviral DNA was consistently detected by PCR in PBMCs of infected men and inconsistently in oral or urogenital samples. SFV was infrequently cultured from their PBMCs and throat swabs. Despite this and a long period of intimate exposure (median 20 years), wives were SFV negative. Most participants reported nonspecific symptoms and diseases common to aging. However, one of two persons with mild thrombocytopenia had clinically asymptomatic nonprogressive, monoclonal natural killer cell lymphocytosis of unclear relationship to SFV. All participants worked with NHPs before 1988 using mucocutaneous protection inconsistently; 57% described percutaneous injuries involving the infecting NHP species. SFV likely transmits to humans through both percutaneous and mucocutaneous exposures to NHP body fluids. Limited follow-up has not identified SFV-associated illness and secondary transmission among humans.

Gene therapy for chronic granulomatous disease

Chronic granulomatous disease (CGD) is a congenital immune deficiency that is a promising therapeutic target for gene replacement into haematopoietic stem cells (HSCs). CGD results from mutations in any one of four genes encoding subunits of the superoxide-generating NADPH oxidase of phagocytes. Life-threatening, recurrent bacterial and fungal infections, as well as inflammatory granulomas, are the hallmarks of the disease. NADPH oxidase activity can be reconstituted by retroviral- or lentiviral-mediated gene transfer to human CGD marrow in vitro and in xenograft transplant models. Gene transfer studies in knockout mouse models that resemble the human disease suggest that correction of oxidase activity in a minority of phagocytes will be of clinical benefit. Phase I clinical studies in unconditioned CGD patients showed transient expression of small numbers of gene-corrected neutrophils. Areas of research at present include efforts to enhance gene transfer rates into repopulating HSCs using vectors that transduce quiescent cells, and to increase the engraftment of genetically corrected HSCs using non-myeloablative conditioning and drug resistance genes for selection.

Absence of human T-cell lymphotropic virus type I and human foamy virus in thymoma

The cause of thymoma, a rare malignancy of thymic epithelial cells, is unknown. Recent studies have reported the detection of DNA from human T-cell lymphotropic virus type I (HTLV-I) and human foamy virus (HFV) in small numbers of thymoma tumours, suggesting an aetiologic role for these retroviruses. In the present study, we evaluated 21 US thymoma patients and 20 patients with other cancers for evidence of infection with these viruses. We used the polymerase chain reaction to attempt to amplify viral DNA from tumour tissues, using primers from the pol and tax (HTLV-I) and gag and bel1 (HFV) regions. In these experiments, we did not detect HTLV-I or HFV DNA sequences in any thymoma or control tissues, despite adequate sensitivity of our assays (one HTLV-I copy per 25 000 cells, one HFV copy per 7500 cells). Additionally, none of 14 thymoma patients evaluated serologically for HTLV I/II infection was positive by enzyme-linked immunoassay (ELISA), while five (36%) had indeterminate Western blot reactivity. In comparison, one of 20 US blood donors was HTLV-I/II ELISA positive, and nine (45%) donors, including the ELISA-positive donor, had indeterminate Western blot reactivity. Western blot patterns varied across individuals and consisted mostly of weak reactivity. In conclusion, we did not find evidence for the presence of HTLV-I or HFV in US thymoma patients.

Frequent simian foamy virus infection in persons occupationally exposed to nonhuman primates

The recognition that AIDS originated as a zoonosis heightens public health concerns associated with human infection by simian retroviruses endemic in nonhuman primates (NHPs). These retroviruses include simian immunodeficiency virus (SIV), simian T-cell lymphotropic virus (STLV), simian type D retrovirus (SRV), and simian foamy virus (SFV). Although occasional infection with SIV, SRV, or SFV in persons occupationally exposed to NHPs has been reported, the characteristics and significance of these zoonotic infections are not fully defined. Surveillance for simian retroviruses at three research centers and two zoos identified no SIV, SRV, or STLV infection in 187 participants. However, 10 of 187 persons (5.3%) tested positive for SFV antibodies by Western blot (WB) analysis. Eight of the 10 were males, and 3 of the 10 worked at zoos. SFV integrase gene (int) and gag sequences were PCR amplified from the peripheral blood lymphocytes available from 9 of the 10 persons. Phylogenetic analysis showed SFV infection originating from chimpanzees (n = 8) and baboons (n = 1). SFV seropositivity for periods of 8 to 26 years (median, 22 years) was documented for six workers for whom archived serum samples were available, demonstrating long-standing SFV infection. All 10 persons reported general good health, and secondary transmission of SFV was not observed in three wives available for WB and PCR testing. Additional phylogenetic analysis of int and gag sequences provided the first direct evidence identifying the source chimpanzees of the SFV infection in two workers. This study documents more frequent infection with SFV than with other simian retroviruses in persons working with NHPs and provides important information on the natural history and species origin of these infections. Our data highlight the importance of studies to better define the public health implications of zoonotic SFV infections.

Naturally acquired simian retrovirus infections in central African hunters

BACKGROUND

Hunting and butchering of wild non-human primates infected with simian immunodeficiency virus (SIV) is thought to have sparked the HIV pandemic. Although SIV and other primate retroviruses infect laboratory workers and zoo workers, zoonotic retrovirus transmission has not been documented in natural settings. We investigated zoonotic infection in individuals living in central Africa.

METHODS

We obtained behavioural data, plasma samples, and peripheral blood lymphocytes from individuals living in rural villages in Cameroon. We did serological testing, PCR, and sequence analysis to obtain evidence of retrovirus infection.

FINDINGS

Zoonotic infections with simian foamy virus (SFV), a retrovirus endemic in most Old World primates, were identified in people living in central African forests who reported direct contact with blood and body fluids of wild non-human primates. Ten (1%) of 1099 individuals had antibodies to SFV. Sequence analysis from these individuals revealed three geographically-independent human SFV infections, each of which was acquired from a distinct non-human primate lineage: De Brazza's guenon (Cercopithecus neglectus), mandrill (Mandrillus sphinx), and gorilla (Gorilla gorilla), two of which (De Brazza's guenon and mandrill) are naturally infected with SIV.

INTERPRETATION

Our findings show that retroviruses are actively crossing into human populations, and demonstrate that people in central Africa are currently infected with SFV. Contact with non-human primates, such as happens during hunting and butchering, can play a part in the emergence of human retroviruses and the reduction of primate bushmeat hunting has the potential to decrease the frequency of disease emergence.

Transduction of long-term and mobilized peripheral blood-derived NOD/SCID repopulating cells by foamy virus vectors

Foamy virus (FV) vectors are a promising gene delivery system for use in hematopoietic stem cell gene therapy. Previous FV vector marking studies in the NOD/SCID xenotransplantation model used umbilical cord blood (UCB)-derived SCID repopulating cells (SRCs) that were assayed 5-10 weeks posttransplantation. We now report efficient FV vector transduction (>65%) of UCB-derived primitive, long-term SRCs engrafted for 18 weeks. In addition, we evaluated gene transfer into mobilized peripheral blood (MPB)-derived SRCs by improved, deleted FV vectors containing minimal cis-acting sequences and packaged by split helper constructs that would be appropriate for use in clinical trials. When used at a multiplicity of infection of 1 in a 10-hr transduction protocol, these improved vectors transduced 34% of engrafted MPB-derived SRCs.

Transient foamy virus vector production by adenovirus vectors

The genome of the prototype foamy virus (PFV) has been introduced into an adenoviral/PFV hybrid vector and tested for stable in vitro gene transfer. Three different adenoviruses are used to encode: (i) the PFV structural genes gag and pol (Ad-GagPolDeltaPacI); (ii) the PFV structural gene env (Ad-Env); and (iii) the PFV vector genome (Ad-MD9) encoding the transgene (the enhanced green fluorescent protein (eGFP) gene). Following cotransduction by the three adenoviruses, the target cells become transient PFV vector-producing cells, resulting in the in situ release of recombinant PFV at a titre of up to 10(3) vector particles/ml, which can then infect surrounding cells, leading to stable integration of the expression cassette. Stable eGFP expression, observed for up to 60 days (11 passages) in cells transduced with all three adenoviral vectors, was shown by PCR to be the result of PFV integration. In contrast, cells transduced with only the adenovirus encoding the PFV vector genome showed a marked decrease in eGFP expression by passage 2 (16 days post-transduction) and did not contain integrated PFV vector. In short, this paper describes the production of a hybrid vector capable of high in vitro transduction and stable transgene expression using adenovirus and PFV vectors.

Potential of zoonotic transmission of non-primate foamy viruses to humans

The zoonotic introduction of an animal pathogen into the human population and the subsequent extension or alteration of its host range leading to the successful maintenance of the corresponding pathogen by human‐to‐human transmission pose a serious risk for world‐wide health care. Such a scenario occurred for instance by the introduction of simian immunodeficiency viruses into the human population resulting in the human immunodeficiency viruses (HIV) and the subsequent AIDS pandemic or the proposed recent host range switch of the SARS coronavirus from a presently unknown animal species to humans. The occurrence of zoonotic transmissions of animal viruses to humans is a permanent threat to human health and is even increased by changes in the human lifestyle. In this review, the potential of the zoonotic transmission of bovine, feline and equine foamy retroviruses will be discussed in the light of well‐documented cases of zoonotic transmissions of different simian foamy viruses to humans.

Understanding xenotransplantation risks from nonhuman primate retroviruses

Significant progress in making animal-to-human transplantation a viable adjunct to human organ donation will require a greater understanding of the intricacies of immunologic rejection. Recent success in generating cloned knockout piglets increases the possibility that xenotransplantation may find its way into the clinics. Nonhuman primates' organs have been used for human transplants in the past and there is reason to believe that if ethical considerations and inherent problems with supply were overcome, their close genetic proximity to humans would lessen complications of rejection. Unfortunately, nonhuman primates harbor several pathogens known to be infectious in humans and the potential of other viral infections has precluded further use of monkeys in this setting. Baboons are generally considered the nonhuman primate species of choice yet this species carries several retroviruses considered a threat to humans in transplantation. Both known and potentially undiscovered retroviruses pose an important risk that is the focus of this review.

Screening for simian foamy virus infection by using a combined antigen Western blot assay: evidence for a wide distribution among Old World primates and identification of four new divergent viruses

Simian foamy viruses (SFVs) belong to a genetically and antigenically diverse class of retroviruses that naturally infect a wide range of nonhuman primates (NHPs) and can also be transmitted to humans occupationally exposed to NHPs. Current serologic detection of SFV infection requires separate Western blot (WB) testing by using two different SFV antigens [SFV(AGM) (African green monkey) and SFV(CPZ) (chimpanzee)]. However, this method is labor intensive and validation is limited to only small numbers of NHPs. To facilitate serologic SFV testing, we developed a WB assay that combines antigens from both SFV(AGM) and SFV(CPZ). The combined-antigen WB (CA-WB) assay was validated with 145 serum samples from 129 NHPs (32 African and Asian species) and 16 humans, all with known SFV infection status determined by PCR. Concordant CA-WB results were obtained for all 145 PCR-positive or -negative primate and human specimens, giving the assay a 100% sensitivity and specificity. In addition, no reactivity was observed in sera from persons positive for human immunodeficiency virus or human T cell lymphotropic virus (HIV/HTLV) (n = 25) or HIV/HTLV-negative U.S. blood donors (n = 100). Using the CA-WB assay, we screened 360 sera from 43 Old World primate species and found an SFV prevalence of about 68% in both African and Asian primates. We also isolated SFV from the blood of four seropositive primates (Allenopithecus nigroviridis, Trachypithecus françoisi, Hylobates pileatus, and H. leucogenys) not previously known to be infected with SFV. Phylogenetic analysis of integrase sequences from these isolates confirmed that all four SFVs represent new, distinct, and highly divergent lineages. These results demonstrate the ability of the CA-WB assay to detect infection in a large number of NHP species, including previously uncharacterized infections with divergent SFVs.

Comparison of three retroviral vector systems for transduction of nonobese diabetic/severe combined immunodeficiency mice repopulating human CD34+ cord blood cells

The use of recombinant vectors based on wild-type viruses that are absent in humans and are not associated with any disease in their natural animal hosts or in accidentally infected humans would add an additional level of safety for human somatic gene therapy approaches. These criteria are fulfilled by foamy viruses (FVs), a family of complex retroviruses whose members are widely found among mammals and are apathogenic in all hosts. Here, we show by comparison of identically designed vector constructs that recombinant retroviral vectors based on FVs were as efficient as lentiviral vectors in transducing nonobese diabetic/severe combined immunodeficiency (NOD/SCID) mice repopulating human CD34(+) cord blood (CB) cells. The FV vector was able to achieve gene transfer levels up to 84% of engrafted human cells in a short overnight transduction protocol. In contrast, without prestimulation of the target cells, a human immunodeficiency virus type 1 (HIV-1)-based lentiviral vector pseudotyped with gibbon ape leukemia virus envelope (GALV Env) was nearly as inefficient as murine leukemia virus (MLV)-based oncoretroviral vectors in transducing NOD/SCID repopulating cells. The same HIV vector pseudotyped with the vesicular stomatitis virus glycoprotein G (VSV-G) achieved high marking efficiency. Clonality analysis of bone marrow samples showed oligoclonal hematopoiesis with single to multiple insertions per cell, both for FV and HIV vectors. These data demonstrate that vectors based on FVs warrant further investigation and development for medical use.

Simian foamy virus infection in a blood donor

BACKGROUND

Infections with simian foamy virus (SFV) are widely prevalent in nonhuman primates. SFV infection was confirmed in a worker, occupationally exposed to nonhuman primates, who donated blood after the retrospectively documented date of infection. Human-to-human transmission of SFV through transfusion and its pathogenicity have not been studied.

STUDY DESIGN AND METHODS

Recipients of blood from this donor were identified and blood samples from such recipients were tested for SFV infection by Western blot and PCR assay.

RESULTS

One recipient of RBCs and another recipient of FFP had died; retroviral infections were not implicated. One platelet recipient could not be tested. Recipients of RBCs (two), a WBC-reduced RBC unit (one), and a platelet unit (one) tested SFV-negative 19 months to 7 years after transfusion. Tested recipients had transfusions 3 to 35 days after blood donation. Samples of one lot of albumin and three lots of plasma protein fraction (manufactured from recovered plasma from two donations) tested negative both for antibodies and for viral RNA.

CONCLUSION

SFV transmission through transfusion was not identified among four recipients of cellular blood components from one SFV-infected donor. Derivatives containing plasma from that donor tested negative for SFV.

Transduction of human NOD/SCID-repopulating cells with both lymphoid and myeloid potential by foamy virus vectors

The efficiency of gene transfer into human hematopoietic stem cells by oncoretroviral vectors is too low for effective gene therapy of most hematologic diseases. Retroviral vectors based on the nonpathogenic foamy viruses (FV) are an alternative gene-transfer system. In this study, human umbilical cord blood CD34(+) cells were transduced with FV vectors by a single 10-h exposure to vector stocks and then injected into sublethally irradiated nonobese diabetic-severe combined immunodeficiency (NOD/SCID) mice. At 5-7 weeks after transplantation, high transgene expression rates were observed in engrafted human hematopoietic cells, including over 60% of clonogenic progenitors. Significant transgene silencing did not occur. We developed an approach for expanding human cell populations derived from transplanted mice to show that multiple SCID repopulating cells (SRCs) had been transduced, including some that were capable of both lymphoid and myeloid differentiation. These findings demonstrate for the first time that human pluripotent (lympho-myeloid) hematopoietic stem cells repopulate NOD/SCID mice and can be efficiently transduced by FV vectors.

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.

Gene transfer into murine hematopoietic stem cells with helper-free foamy virus vectors

Gene transfer into hematopoietic stem cells (HSCs) is an ideal treatment strategy for many genetic and hematologic diseases. However, progress has been limited by the low HSC transduction rates obtained with retroviral vectors based on murine leukemia viruses. This study examined the potential of vectors derived from the nonpathogenic human foamy virus (HFV) to transduce human CD34(+) cells and murine HSCs. More than 80% of human hematopoietic progenitors present in CD34(+) cell preparations derived from cord blood were transduced by a single overnight exposure to HFV vector stocks. Mice that received transduced bone marrow cells expressed the vector-encoded transgene long term in all major hematopoietic cell lineages and in over 50% of cells in some animals. Secondary bone marrow transplants and integration site analysis confirmed that gene transfer occurred at the stem cell level. Transgene silencing was not observed. Thus vectors based on foamy viruses represent a promising approach for HSC gene therapy. (Blood. 2001;98:604-609)

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.

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.

Complex effects of deletions in the 5' untranslated region of primate foamy virus on viral gene expression and RNA packaging

Due to various advantageous features there is current interest in retroviral vectors derived from primate foamy viruses (PFVs). Two PFV cis-acting sequences have been mapped in the 5' region of the RNA (pre-)genome and in the 3' pol genomic region. In order to genetically separate PFV packaging constructs from vector constructs, we investigated the effect of deletions in the 5' untranslated region (UTR) of PFV packaging constructs and vectors on gene expression and RNA incorporation into viral particles. Our results indicate that the 5' UTR serves different previously unknown functions. First, the R region of the long terminal repeat was found to be required for PFV gag gene expression. This regulation of gene expression appeared to be mainly posttranscriptional. Second, constructs with sequence deletions between the R region and the gag gene start codon packaged as much viral mRNA into particles as the undeleted construct, and RNA from such a 5'-UTR-deleted packaging construct was copackaged into vector-virus particles, together with vector RNA which was preferentially packaged. Finally, in the U5 region a sequence was identified that was required to allow cleavage of the Gag precursor protein by the pol gene-encoded protease, suggesting a role of RNA in PFV particle formation. Taken together, the results indicate that complex interactions of the viral RNA, capsid, and polymerase proteins take place during PFV particle formation and that a clear separation of PFV vector and packaging construct sequences may be difficult to achieve.

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.

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.

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.

Cells expressing the human foamy virus (HFV) accessory Bet protein are resistant to productive HFV superinfection

Bet is a foamy virus (FV) accessory protein not required for virus replication. The function of Bet is not understood. We report on the generation of cell lines stably expressing the HFV Bet protein. In Bet+ cells, HFV replication was reduced by approximately 3-4 orders of magnitude compared with control cells. The HFV Bet-expressing cells only partially resisted infection by the distantly related feline FV (FFV). Pseudotyping experiments, using murine retroviral vectors with an HFV envelope, revealed that the resistance was not due to downregulation of the unknown HFV receptor. In transfection experiments, using proviral reporter gene constructs and infectious proviruses, no significant differences were detected between Bet+ and control cells. In infection experiments, HFV vectors expressing an indicator gene under control of the HFV promoters showed no activity in Bet+ cells. The results are best compatible with the hypothesis that the main block to productive superinfection of Bet+ cells occurs at an early stage of replication between virus entry and provirus establishment. We suggest that inhibition of provirus integration by Bet protein may serve a distinct function in the unique foamy virus replication cycle.

Identification of a human population infected with simian foamy viruses

Studying the transmission of simian retroviruses to humans can help define the importance of these infections to public health. We identified a substantial prevalence (4/231, 1.8%) of infection with simian foamy viruses (SFV) among humans occupationally exposed to nonhuman primates. Evidence of SFV infection included seropositivity, proviral DNA detection and isolation of foamy virus. The infecting SFV originated from an African green monkey (one person) and baboons (three people). These infections have not as yet resulted in either disease or sexual transmission, and may represent benign endpoint infections.

Gene transfer using replication-defective human foamy virus vectors

Replication-defective vectors based on an infectious molecular clone of human foamy virus (HFV) were constructed by deletion and replacement of the accessory genes with expression cassettes for puromycin-resistance and beta-glucouronidase. Cell lines which produced in excess of 10(5) helper virus-free transducing units/ml were generated by trans-complementation of the replication defect using a BHK-21-derived cell line expressing the Bel-1 transactivator. Vectors based on the HFV genome may provide useful alternatives to existing retroviral vectors.

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.

A novel exogenous retrovirus sequence identified in humans

A 932-bp retrovirus sequence was cloned by reverse transcriptase PCR from salivary gland tissue of a patient with Sjögren's syndrome. The sequence is related to that of type B and type D retroviruses and was present in a sucrose density gradient fraction corresponding to that of an enveloped retrovirus particle. Sequences amplified from tissues of eight individuals with or without Sjögren's syndrome had over 90% similarity and were present at a level of less than one copy per 10(3) cells. The sequence was not detectable in human genomic DNA by PCR or by Southern hybridization. These data indicate that the sequence represents an infectiously acquired genome, provisionally called human retrovirus 5.

Identification of sites that act together to direct dimerization of human foamy virus RNA in vitro

Retroviral particles contain two molecules of genomic RNA, which are noncovalently linked near their 5' ends in a region called the dimer linkage structure (DLS). By using complementary DNA oligonucleotides and deletion mutants to impair RNA dimerization of the human foamy virus (HFV), three sites, designated SI, SII, and SIII, were found within a 159-nucleotide RNA fragment of HFV that are involved in dimerization in vitro. SI overlaps the primer-binding site; and SII contains the palindromic sequence, UCCCUAGGGA, the disruption of which impairs dimer formation; and SIII extends into the gag gene. The first two sites are highly conserved in the other primate foamy viruses, SFV-1, SFV-3, and SFVcpz, whereas the third appears to be shared only by HFV and SFVcpz. RNA of HFV and SFV-3 could form heterodimers, indicating that both viruses dimerize by similar mechanisms. On testing thermal stability, dimers of the 159-nucleotide fragment dissociated between 40 and 70 degrees, with half of the dimers dissociating at 55 degrees. Since the splice donor site of HFV is located at position 51 of viral RNA, the DLS is part of the genomic RNA exclusively.