Construction of infectious feline foamy virus genomes: cat antisera do not cross-neutralize feline foamy virus chimera with serotype-specific Env sequences

Comparison of a Genotype 1 and a Genotype 2 Macaque Foamy Virus env Gene Indicates Distinct Infectivity and Cell-Cell Fusion but Similar Tropism and Restriction of Cell Entry by Interferon-Induced Transmembrane Proteins

Foamy viruses (FVs) are naturally found in many different animals and also in primates with the notable exception of humans, but zoonotic infections are common. In several species, two different envelope (env) gene sequence clades or genotypes exist. We constructed a simian FV (SFV) clone containing a reporter gene cassette. In this background, we compared the env genes of the SFVmmu-DPZ9524 (genotype 1) and of the SFVmmu_R289hybAGM (genotype 2) isolates. SFVmmu_R289hybAGM env-driven infection was largely resistant to neutralization by SFVmmu-DPZ9524-neutralizing sera. While SFVmmu_R289hybAGM env consistently effected higher infectivity and cell-cell fusion, we found no differences in the cell tropism conferred by either env across a range of different cells. Infection by both viruses was weakly and non-significantly enhanced by simultaneous knockout of interferon-induced transmembrane proteins (IFITMs) 1, 2, and 3 in A549 cells, irrespective of prior interferon stimulation. Infection was modestly reduced by recombinant overexpression of IFITM3, suggesting that the SFV entry step might be weakly restricted by IFITM3 under some conditions. Overall, our results suggest that the different env gene clades in macaque foamy viruses induce genotype-specific neutralizing antibodies without exhibiting overt differences in cell tropism, but individual env genes may differ significantly with regard to fitness.

Plasma antibodies from humans infected with zoonotic simian foamy virus do not inhibit cell-to-cell transmission of the virus despite binding to the surface of infected cells

Zoonotic simian foamy viruses (SFV) establish lifelong infection in their human hosts. Despite repeated transmission of SFV from nonhuman primates to humans, neither transmission between human hosts nor severe clinical manifestations have been reported. We aim to study the immune responses elicited by chronic infection with this retrovirus and previously reported that SFV-infected individuals generate potent neutralizing antibodies that block cell infection by viral particles. Here, we assessed whether human plasma antibodies block SFV cell-to-cell transmission and present the first description of cell-to-cell spreading of zoonotic gorilla SFV. We set-up a microtitration assay to quantify the ability of plasma samples from 20 Central African individuals infected with gorilla SFV and 9 uninfected controls to block cell-associated transmission of zoonotic gorilla SFV strains. We used flow-based cell cytometry and fluorescence microscopy to study envelope protein (Env) localization and the capacity of plasma antibodies to bind to infected cells. We visualized the cell-to-cell spread of SFV by real-time live imaging of a GFP-expressing prototype foamy virus (CI-PFV) strain. None of the samples neutralized cell-associated SFV infection, despite the inhibition of cell-free virus. We detected gorilla SFV Env in the perinuclear region, cytoplasmic vesicles and at the cell surface. We found that plasma antibodies bind to Env located at the surface of cells infected with primary gorilla SFV strains. Extracellular labeling of SFV proteins by human plasma samples showed patchy staining at the base of the cell and dense continuous staining at the cell apex, as well as staining in the intercellular connections that formed when previously connected cells separated from each other. In conclusion, SFV-specific antibodies from infected humans do not block cell-to-cell transmission, at least in vitro, despite their capacity to bind to the surface of infected cells.

Trial registration: Clinical trial registration: www.clinicaltrials.gov, https://clinicaltrials.gov/ct2/show/NCT03225794/.

Foamy viruses are the oldest known retroviruses and have been mostly described to be nonpathogenic in their natural animal hosts. Simian foamy viruses (SFVs) can be transmitted to humans, in whom they establish persistent infection, as have the simian viruses that led to the emergence of two major human pathogens, human immunodeficiency virus type 1 (HIV-1) and human T lymphotropic virus type 1 (HTLV-1). Such cross-species transmission of SFV is ongoing in many parts of the world where humans have contact with nonhuman primates. We previously showed high titers of neutralizing antibodies in the plasma of most SFV-infected individuals. These antiviral antibodies can inhibit cell-free virus entry. However, SFV efficiently spread from one cell to another. Here, we demonstrate that plasma antibodies do not block such cell-to-cell transmission, despite their capacity to bind to the surface of infected cells. In addition, we document for the first time the cell-to-cell spread of primary zoonotic gorilla SFV.

Genetic and biological characterization of feline foamy virus isolated from a leopard cat (Prionailurus bengalensis) in Vietnam

Foamy viruses have been isolated from various mammals and show long-term co-speciation with their hosts. However, the frequent inter-species transmission of feline foamy viruses (FFVs) from domestic cats to wild cats across genera has been reported. Because infectious molecular clones of FFVs derived from wild cats have not been available, whether there are specific characteristics enabling FFVs to adapt to the new host species is still unknown. Here, we obtained the complete genome sequences of two FFV isolates (strains NV138 and SV201) from leopard cats (Prionailurus bengalensis) in Vietnam and constructed an infectious molecular clone, named pLC960, from strain NV138. The growth kinetics of the virus derived from pLC960 were comparable to those of other FFVs derived from domestic cats. Phylogenetic analysis revealed that these two FFVs from leopard cats are clustered in the same clade as FFVs from domestic cats in Vietnam. Comparisons of the amino acid sequences of Env and Bet proteins showed more than 97% identity among samples and no specific amino acid substitutions between FFVs from domestic cats and ones from leopard cats. These results indicate the absence of genetic constraint of FFVs for interspecies transmission from domestic cats to leopard cats.

Identification of Feline Foamy Virus-derived MicroRNAs

MicroRNAs (miRNAs) classified as non-coding RNAs regulate various metabolic systems and viral life cycles. To date, numerous DNA viruses, many of which are members of the herpesvirus family, and a relatively small number of RNA viruses, including retroviruses, have been reported to encode and express miRNAs in infected cells. A few retroviruses have been shown to express miRNAs, and foamy viruses (FVs) were initially predicted by computational analyses to possess miRNA-coding regions. Subsequent studies on simian and bovine FVs confirmed the presence of functional and biologically active miRNA expression cassettes. We herein identified feline FV-derived miRNAs using a small RNA deep sequencing ana­lysis. We confirmed their repressive functions on gene expression by dual-luciferase reporter assays. We found that the seed sequences of the miRNAs identified in the present study were conserved among all previously reported FFV isolates. These results suggest that FFV-derived miRNAs play a pivotal role in FFV infection.

Seroprevalence of feline foamy virus in domestic cats in Poland

Introduction

Feline foamy virus (FFVfca) is widespread and its prevalence in naturally infected domestic cats ranges between 30% and 80% worldwide. The infection is persistent, with a sustained antibody response in FFVfca-positive cats; however to date, no defined disease or clinical symptoms have been proved to be associated with it. The goal of the presented study was to determine the prevalence of FFVfca infection in domestic cats in Poland.

Material and Methods

A total of 223 serum samples collected from domestic cats were tested with a glutathione S-transferase capture ELISA test to detect antibodies specific to capsid (Gag), accessory (Bet) and envelope (Env) FFVfca antigens. A Western blot test was used to confirm the ELISA results.

Results

The cut-off value for the Gag antigen was established by calculation and evaluation with the immunoblotting assay. The cut-off values for Bet and Env were calculated from the reactivity of Gag-negative samples. The sera of 99 cats (44%) showed reactivity to Gag, those of 80 did so (35.9 %) to Bet, while only 56 samples (25%) were reactive to Env. Only 51 (22.9%) sera were positive for all antigens. The main diagnostic antigen was selected to be Gag. A statistically significant association was found between FFVfca status and the age of the cat.

Conclusions

This study proved the high seroprevalence of FFVfca in domestic cats in Poland for the first time and confirmed that adult cats are at higher FFVfca infection risk than preadult cats. Its results correspond to those reported from other countries.

First report on the prevalence and genetic relatedness of Feline Foamy Virus (FFV) from Turkish domestic cats

Feline Foamy Virus (FFV) is an important retroviral agent affecting domestic cats in Turkey that has been studied less intensively than Feline Immunodeficiency Virus (FIV) and Feline Leukemia Virus (FeLV). Accordingly, we aimed to investigate the presence and prevalence of FFV among domestic cats by molecular techniques. PCR was used to amplify the gag-pol gene overlap in order to detect the presence of FFV. The gene encoding bet, an important accessory gene, was also characterized. Molecular characteristics were analyzed and phylogenetic trees were constructed. We determined the positivity rate as 10% in all samples (20/200) based on the gag-pol test. The phylogenetic analysis indicated that the Turkish FFV sequences form a separate cluster among other isolates in the constructed maximum likelihood (ML) tree. bet-based products were obtained for two samples (1%; 2/200) that were also positive for gag-pol. These bet gene sequences confirm the presence of a separate cluster for the Turkish FFV isolates. The results suggest that FFV is prevalent and widespread in Turkish domestic cats. Additionally, these new FFV sequences represent the first FFV sequences from Turkey to be submitted to GenBank. This study paves the way for studies on the pathogenicity of FFV.

Feline foamy virus seroprevalence and demographic risk factors in stray domestic cat populations in Colorado, Southern California and Florida, USA

Objectives

Our study aim was to document the seroprevalence and associated risk factors of feline foamy virus (FFV) infection in domestic cat populations presented to animal shelters located in Southern California, Colorado and Florida, USA.

Methods

We used a glutathione S-transferase capture ELISA targeting the FFV Gag antigen to screen domestic cat serum collected from cats with unknown owners at eight different animal shelters from Colorado (n = 105, three shelters), Southern California (n = 172, three shelters) and Florida (n = 31, two shelters). χ² statistics determined location effect on seroprevalence. Bayesian generalized linear models were used to explore age and sex as potential risk factors for infection.

Results

FFV seroprevalence was 64.0% across all locations. Seroprevalence by location was as follows: Southern California 75.0%, Colorado 52.4% and Florida 41.9%, with Southern California’s seroprevalence being significantly higher. Age had a significant effect on model fit for all locations, with adults having a higher probability of being infected. In Colorado, sex also had a significant effect on model fit, with males having a higher probability of being infected.

Conclusions and relevance

We have documented that FFV is extremely common in stray domestic cat populations across varied geographic and ecological niches throughout the USA. Adult cats are at a higher FFV infection risk than young cats. FFV has been associated with a higher risk of other retroviral infections and has been implicated in several chronic diseases of cats. Additional epidemiological and clinical studies are warranted to investigate the potential impacts of FFV on domestic cat health.

An Immunodominant and Conserved B-Cell Epitope in the Envelope of Simian Foamy Virus Recognized by Humans Infected with Zoonotic Strains from Apes

Cross-species transmission of simian foamy viruses (SFVs) from nonhuman primates (NHPs) to humans is currently ongoing. These zoonotic retroviruses establish lifelong persistent infection in their human hosts. SFV are apparently nonpathogenic in vivo, with ubiquitous in vitro tropism. Here, we aimed to identify envelope B-cell epitopes that are recognized following a zoonotic SFV infection. We screened a library of 169 peptides covering the external portion of the envelope from the prototype foamy virus (SFVpsc_huHSRV.13) for recognition by samples from 52 Central African hunters (16 uninfected and 36 infected with chimpanzee, gorilla, or Cercopithecus SFV). We demonstrate the specific recognition of peptide N₉₆-V₁₁₀ located in the leader peptide, gp18^LP Forty-three variant peptides with truncations, alanine substitutions, or amino acid changes found in other SFV species were tested. We mapped the epitope between positions 98 and 108 and defined six amino acids essential for recognition. Most plasma samples from SFV-infected humans cross-reacted with sequences from apes and Old World monkey SFV species. The magnitude of binding to peptide N₉₆-V₁₁₀ was significantly higher for samples of individuals infected with a chimpanzee or gorilla SFV than those infected with a Cercopithecus SFV. In conclusion, we have been the first to define an immunodominant B-cell epitope recognized by humans following zoonotic SFV infection.IMPORTANCE Foamy viruses are the oldest known retroviruses and have been mostly described to be nonpathogenic in their natural animal hosts. SFVs can be transmitted to humans, in whom they establish persistent infection, like the simian lenti- and deltaviruses that led to the emergence of two major human pathogens, human immunodeficiency virus type 1 and human T-lymphotropic virus type 1. This is the first identification of an SFV-specific B-cell epitope recognized by human plasma samples. The immunodominant epitope lies in gp18^LP, probably at the base of the envelope trimers. The NHP species the most genetically related to humans transmitted SFV strains that induced the strongest antibody responses. Importantly, this epitope is well conserved across SFV species that infect African and Asian NHPs.

Potent neutralizing antibodies in humans infected with zoonotic simian foamy viruses target conserved epitopes located in the dimorphic domain of the surface envelope protein

Human diseases of zoonotic origin are a major public health problem. Simian foamy viruses (SFVs) are complex retroviruses which are currently spilling over to humans. Replication-competent SFVs persist over the lifetime of their human hosts, without spreading to secondary hosts, suggesting the presence of efficient immune control. Accordingly, we aimed to perform an in-depth characterization of neutralizing antibodies raised by humans infected with a zoonotic SFV. We quantified the neutralizing capacity of plasma samples from 58 SFV-infected hunters against primary zoonotic gorilla and chimpanzee SFV strains, and laboratory-adapted chimpanzee SFV. The genotype of the strain infecting each hunter was identified by direct sequencing of the env gene amplified from the buffy coat with genotype-specific primers. Foamy virus vector particles (FVV) enveloped by wild-type and chimeric gorilla SFV were used to map the envelope region targeted by antibodies. Here, we showed high titers of neutralizing antibodies in the plasma of most SFV-infected individuals. Neutralizing antibodies target the dimorphic portion of the envelope protein surface domain. Epitopes recognized by neutralizing antibodies have been conserved during the cospeciation of SFV with their nonhuman primate host. Greater neutralization breadth in plasma samples of SFV-infected humans was statistically associated with smaller SFV-related hematological changes. The neutralization patterns provide evidence for persistent expression of viral proteins and a high prevalence of coinfection. In conclusion, neutralizing antibodies raised against zoonotic SFV target immunodominant and conserved epitopes located in the receptor binding domain. These properties support their potential role in restricting the spread of SFV in the human population.

The chromatin binding domain, including the QPQRYG motif, of feline foamy virus Gag is required for viral DNA integration and nuclear accumulation of Gag and the viral genome

The retroviral Gag protein, the major component of released particles, plays different roles in particle assembly, maturation or infection of new host cells. Here, we characterize the Gag chromatin binding site including the highly conserved QPQRYG motif of feline foamy virus, a member of the Spumaretrovirinae. Mutagenesis of critical residues in the chromatin binding site/QPQRYG motif almost completely abrogates viral DNA integration and reduces nuclear accumulation of Gag and viral DNA. Genome packaging, reverse transcription, particle release and uptake into new target cells are not affected. The integrity of the QPQRYG motif appears to be important for processes after cytosolic entry, likely influencing incoming virus capsids or disassembly intermediates but not Gag synthesized de novo in progeny virus-producing cells. According to our data, chromatin binding is a shared feature among foamy viruses but further work is needed to understand the mechanisms involved.

Replacement of feline foamy virus bet by feline immunodeficiency virus vif yields replicative virus with novel vaccine candidate potential

BACKGROUND

Hosts are able to restrict viral replication to contain virus spread before adaptive immunity is fully initiated. Many viruses have acquired genes directly counteracting intrinsic restriction mechanisms. This phenomenon has led to a co-evolutionary signature for both the virus and host which often provides a barrier against interspecies transmission events. Through different mechanisms of action, but with similar consequences, spumaviral feline foamy virus (FFV) Bet and lentiviral feline immunodeficiency virus (FIV) Vif counteract feline APOBEC3 (feA3) restriction factors that lead to hypermutation and degradation of retroviral DNA genomes. Here we examine the capacity of vif to substitute for bet function in a chimeric FFV to assess the transferability of anti-feA3 factors to allow viral replication.

RESULTS

We show that vif can replace bet to yield replication-competent chimeric foamy viruses. An in vitro selection screen revealed that an engineered Bet-Vif fusion protein yields suboptimal protection against feA3. After multiple passages through feA3-expressing cells, however, variants with optimized replication competence emerged. In these variants, Vif was expressed independently from an N-terminal Bet moiety and was stably maintained. Experimental infection of immunocompetent domestic cats with one of the functional chimeras resulted in seroconversion against the FFV backbone and the heterologous FIV Vif protein, but virus could not be detected unambiguously by PCR. Inoculation with chimeric virus followed by wild-type FFV revealed that repeated administration of FVs allowed superinfections with enhanced antiviral antibody production and detection of low level viral genomes, indicating that chimeric virus did not induce protective immunity against wild-type FFV.

CONCLUSIONS

Unrelated viral antagonists of feA3 cellular restriction factors can be exchanged in FFV, resulting in replication competence in vitro that was attenuated in vivo. Bet therefore may have additional functions other than A3 antagonism that are essential for successful in vivo replication. Immune reactivity was mounted against the heterologous Vif protein. We conclude that Vif-expressing FV vaccine vectors may be an attractive tool to prevent or modulate lentivirus infections with the potential option to induce immunity against additional lentivirus antigens.

Epitope Mapping of the Antibody Response Against the Envelope Proteins of the Feline Foamy Virus

Foamy viruses (FV) are retroviruses that infect several species without pathological signs, but induce substantial antibody responses in the infected host. In the case of feline FV (FFV), antibodies against Gag, Bet, and Env have been used to indicate infection; however, it is unclear whether the response to specific epitopes correlates with immunity. Here, we investigated the epitope specificity of antibodies targeting the Env protein using peptide microarrays. Sera from naturally and experimentally FFV-infected cats and pumas and from rats immunized with FFV Env expression plasmids were analyzed. An immunodominant epitope was identified in the Env leader protein (Elp), and a strong reactivity to two epitope clusters in the transmembrane (TM) subunit of Env was observed. Moreover, a short stretch of residues in the C-terminal part of the surface (SU) protein was found to be significantly associated with FFV serotype FUV-mediated neutralization. Taken together, our results add a new level of detail on the B cell epitope repertoire induced during FFV infection. Furthermore, our results provide a basis for current attempts to modify FV vectors to express and present vaccine epitopes for the directed induction of humoral immunity.

Mutagenesis of N-terminal residues of feline foamy virus Gag reveals entirely distinct functions during capsid formation, particle assembly, Gag processing and budding

BACKGROUND

Foamy viruses (FVs) of the Spumaretrovirinae subfamily are distinct retroviruses, with many features of their molecular biology and replication strategy clearly different from those of the Orthoretroviruses, such as human immunodeficiency, murine leukemia, and human T cell lymphotropic viruses. The FV Gag N-terminal region is responsible for capsid formation and particle budding via interaction with Env. However, the critical residues or motifs in this region and their functional interaction are currently ill-defined, especially in non-primate FVs.

RESULTS

Mutagenesis of N-terminal Gag residues of feline FV (FFV) reveals key residues essential for either capsid assembly and/or viral budding via interaction with the FFV Env leader protein (Elp). In an in vitro Gag-Elp interaction screen, Gag mutations abolishing particle assembly also interfered with Elp binding, indicating that Gag assembly is a prerequisite for this highly specific interaction. Gradient sedimentation analyses of cytosolic proteins indicate that wild-type Gag is mostly assembled into virus capsids. Moreover, proteolytic processing of Gag correlates with capsid assembly and is mostly, if not completely, independent from particle budding. In addition, Gag processing correlates with the presence of packaging-competent FFV genomic RNA suggesting that Pol encapsidation via genomic RNA is a prerequisite for Gag processing. Though an appended heterogeneous myristoylation signal rescues Gag particle budding of mutants unable to form capsids or defective in interacting with Elp, it fails to generate infectious particles that co-package Pol, as evidenced by a lack of Gag processing.

CONCLUSIONS

Changes in proteolytic Gag processing, intracellular capsid assembly, particle budding and infectivity of defined N-terminal Gag mutants highlight their essential, distinct and only partially overlapping roles during viral assembly and budding. Discussion of these findings will be based on a recent model developed for Gag-Elp interactions in prototype FV.

A new sensitive indicator cell line reveals cross-transactivation of the viral LTR by gorilla and chimpanzee simian foamy viruses

The majority of currently identified simian foamy virus (SFV)-infected Cameroonian and Gabonese individuals harbor SFV from the gorilla lineage. We constructed an indicator cell line for the quantification of gorilla SFVs, in which the U3 sequence of a gorilla SFV directs the expression of the β-galactosidase protein. The gorilla foamy virus activated β-galactosidase (GFAB) cells efficiently quantified two zoonotic primary gorilla isolates and SFVs from three chimpanzee subspecies. Primary gorilla SFVs replicated more slowly and at lower levels than primary chimpanzee SFVs. Analysis of previously described motifs of Tas proteins and U3 LTRs involved in viral gene synthesis revealed conservation of such motifs in Tas proteins from gorilla and chimpanzee SFVs, but little sequence homology in the LTR regions previously shown to interact with viral and cellular factors.

Cocirculation of Two env Molecular Variants, of Possible Recombinant Origin, in Gorilla and Chimpanzee Simian Foamy Virus Strains from Central Africa

Simian foamy virus (SFV) is a ubiquitous retrovirus in nonhuman primates (NHPs) that can be transmitted to humans, mostly through severe bites. In the past few years, our laboratory has identified more than 50 hunters from central Africa infected with zoonotic SFVs. Analysis of the complete sequences of five SFVs obtained from these individuals revealed that env was the most variable gene. Furthermore, recombinant SFV strains, some of which involve sequences in the env gene, were recently identified. Here, we investigated the variability of the env genes of zoonotic SFV strains and searched for possible recombinants. We sequenced the complete env gene or its surface glycoprotein region (SU) from DNA amplified from the blood of (i) a series of 40 individuals from Cameroon or Gabon infected with a gorilla or chimpanzee foamy virus (FV) strain and (ii) 1 gorilla and 3 infected chimpanzees living in the same areas as these hunters. Phylogenetic analyses revealed the existence of two env variants among both the gorilla and chimpanzee FV strains that were present in zoonotic and NHP strains. These variants differ greatly (>30% variability) in a 753-bp-long region located in the receptor-binding domain of SU, whereas the rest of the gene is very conserved. Although the organizations of the Env protein sequences are similar, the potential glycosylation patterns differ between variants. Analysis of recombination suggests that the variants emerged through recombination between different strains, although all parental strains could not be identified.

IMPORTANCE

SFV infection in humans is a great example of a zoonotic retroviral infection that has not spread among human populations, in contrast to human immunodeficiency viruses (HIVs) and human T-lymphotropic viruses (HTLVs). Recombination was a major mechanism leading to the emergence of HIV. Here, we show that two SFV molecular envelope gene variants circulate among ape populations in Central Africa and that both can be transmitted to humans. These variants differ greatly in the SU region that corresponds to the part of the Env protein in contact with the environment. These variants may have emerged through recombination between SFV strains infecting different NHP species.

Replication-Competent Foamy Virus Vaccine Vectors as Novel Epitope Scaffolds for Immunotherapy

The use of whole viruses as antigen scaffolds is a recent development in vaccination that improves immunogenicity without the need for additional adjuvants. Previous studies highlighted the potential of foamy viruses (FVs) in prophylactic vaccination and gene therapy. Replication-competent FVs can trigger immune signaling and integrate into the host genome, resulting in persistent antigen expression and a robust immune response. Here, we explored feline foamy virus (FFV) proteins as scaffolds for therapeutic B and T cell epitope delivery in vitro. Infection- and cancer-related B and T cell epitopes were grafted into FFV Gag, Env, or Bet by residue replacement, either at sites of high local sequence homology between the epitope and the host protein or in regions known to tolerate sequence alterations. Modified proviruses were evaluated in vitro for protein steady state levels, particle release, and virus titer in permissive cells. Modification of Gag and Env was mostly detrimental to their function. As anticipated, modification of Bet had no impact on virion release and affected virus titers of only some recombinants. Further evaluation of Bet as an epitope carrier was performed using T cell epitopes from the model antigen chicken ovalbumin (OVA), human tyrosinase-related protein 2 (TRP-2), and oncoprotein E7 of human papillomavirus type 16 (HPV16E7). Transfection of murine cells with constructs encoding Bet-epitope chimeric proteins led to efficient MHC-I-restricted epitope presentation as confirmed by interferon-gamma enzyme-linked immunospot assays using epitope-specific cytotoxic T lymphocyte (CTL) lines. FFV infection-mediated transduction of cells with epitope-carrying Bet also induced T-cell responses, albeit with reduced efficacy, in a process independent from the presence of free peptides. We show that primate FV Bet is also a promising T cell epitope carrier for clinical translation. The data demonstrate the utility of replication-competent and -attenuated FVs as antigen carriers in immunotherapy.

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.

Construction and characterisation of replicating foamy viral vectors expressing HIV-1 epitopes recognised by broadly neutralising antibodies

With the aim to develop a replicating vector system for the delivery of HIV-1 antigens on the basis of an apathogenic foamy virus we recently showed that immunisation with purified recombinant hybrid antigens composed of the feline foamy virus Bet protein and parts of the transmembrane envelope protein of HIV-1 induced antibodies with an epitope specificity identical to that of the broadly neutralising antibody 2F5 (Mühle et al., Immunol Res., 2013, 56:61-72). Here we set out to further improve the HIV-1 inserts consisting of the membrane proximal external region (MPER) and the fusion peptide proximal region (FPPR) by stepwise shortening distinct linker residues between both domains. In a subset of these antigens, enhanced recognition by 2F5 and 4E10 was observed, indicating that a specific positioning of FPPR and MPER domains is critical for improved antibody binding. Introduction of these optimised inserts as well as of the MPER domain alone into the feline foamy virus backbone was compatible with virus replication, giving viral titres similar to wild-type virus after extended passaging. Most importantly, expression of the HIV-1 transgenes in infected feline CRFK cells remained stable in three out of four constructs and was detectable after serial passages for several weeks. These data encourage further testing of these vectors in vivo, which may allow insights into the necessity of affinity maturation for the induction of broadly reactive HIV-1 antibodies.

Non-simian foamy viruses: molecular virology, tropism and prevalence and zoonotic/interspecies transmission

Within the field of retrovirus, our knowledge of foamy viruses (FV) is still limited. Their unique replication strategy and mechanism of viral persistency needs further research to gain understanding of the virus-host interactions, especially in the light of the recent findings suggesting their ancient origin and long co-evolution with their nonhuman hosts. Unquestionably, the most studied member is the primate/prototype foamy virus (PFV) which was originally isolated from a human (designated as human foamy virus, HFV), but later identified as chimpanzee origin; phylogenetic analysis clearly places it among other Old World primates. Additionally, the study of non-simian animal FVs can contribute to a deeper understanding of FV-host interactions and development of other animal models. The review aims at highlighting areas of special interest regarding the structure, biology, virus-host interactions and interspecies transmission potential of primate as well as non-primate foamy viruses for gaining new insights into FV biology.

Identification of the feline foamy virus Bet domain essential for APOBEC3 counteraction

Background

APOBEC3 (A3) proteins restrict viral replication by cytidine deamination of viral DNA genomes and impairing reverse transcription and integration. To escape this restriction, lentiviruses have evolved the viral infectivity factor (Vif), which binds A3 proteins and targets them for proteolytic degradation. In contrast, foamy viruses (FVs) encode Bet proteins that allow replication in the presence of A3, apparently by A3 binding and/or sequestration, thus preventing A3 packaging into virions and subsequent restriction. Due to a long-lasting FV-host coevolution, Bet proteins mainly counteract restriction by A3s from their cognate or highly related host species.

Results

Through bioinformatics, we identified conserved motifs in Bet, all localized in the bel2 exon. In line with the localization of these conserved motifs within bel2, this part of feline FV (FFV) Bet has been shown to be essential for feline A3 (feA3) inactivation and feA3 protein binding. To study the function of the Bet motifs in detail, we analyzed the ability of targeted deletion, substitution, and chimeric FFV-PFV (prototype FV) Bet mutants to physically bind and/or inactivate feA3. Binding of Bet to feA3Z2b is sensitive to mutations in the first three conserved motifs and N- and C-terminal deletions and substitutions across almost the complete bel2 coding sequence. In contrast, the Bel1 (also designated Tas) domain of Bet is dispensable for basal feA3Z2b inactivation and binding but mainly increases the steady state level of Bet. Studies with PFV Bel1 and full-length FFV Bel2 chimeras confirmed the importance of Bel2 for A3 inactivation indicating that Bel1 is dispensable for basal feA3Z2b inactivation and binding but increases Bet stability. Moreover, the bel1/tas exon may be required for expression of a fully functional Bet protein from a spliced transcript.

Conclusions

We show that the Bel2 domain of FV Bet is essential for the inactivation of APOBEC3 cytidine deaminase restriction factors. The Bel1/Tas domain increases protein stability and can be exchanged by related sequence. Since feA3 binding and inactivation by Bet are highly correlated, the data support the view that FV Bet prevents A3-mediated restriction of viral replication by creating strong complexes with these proteins.

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.

Identification of recombination in the envelope gene of simian foamy virus serotype 2 isolated from Macaca cyclopis

The full-length sequence of simian foamy virus serotype 2 (SFVmcy-2), isolated from a Taiwanese macaque, was determined. SFVmcy-2 was highly related to SFV serotype 1 (SFVmcy-1), an isolate from the same species, except in the putative receptor binding domain (RBD) in env, which contained novel sequences related to SFV serotype 3 (SFVagm-3), isolated from an African green monkey. The results identify a potential region of neutralization in SFVs and demonstrate recombination between genetically divergent foamy viruses.

Immunisation with foamy virus Bet fusion proteins as novel strategy for HIV-1 epitope delivery

The induction of 2F5- and 4E10-like antibodies broadly neutralising HIV-1 and targeting the membrane external proximal region (MPER) of the transmembrane envelope protein gp41 would be a major advancement for the development of a preventive HIV-1 vaccine, but successful attempts remain rare. Recent studies demonstrated that broadly reactive antibodies develop relatively late during infection and after intensive affinity maturation. Therefore, a prolonged antigen delivery might be beneficial to induce them. Replicating foamy viruses which are characterised by apathogenic but persistent infection could represent suitable carrier viruses for this purpose. In order to develop such a system, we modified the accessory foamy virus Bet protein to contain the MPER of gp41, or the MPER linked to the stabilising fusion peptide proximal region of gp41 and analysed here the antigenic and immunogenic properties of such hybrid proteins. The antigens, expressed and purified to homogeneity, were recognised by the monoclonal antibodies 2F5 and 4E10 with nanomolar affinities and induced high levels of antibodies specific to gp41 after immunisation of rats. The antisera also bound to virus particles attached to infected cells, and peptide-based epitope mapping showed that they recognised the 2F5 epitope. Although no HIV-1 neutralising activity was observed, the presented data demonstrate that using the foamy virus Bet for HIV-1 epitope delivery is successfully applicable. Together with the attractive potential for sustained antigen expression after transfer to replicating virus, these results should therefore provide a first basis for the development of chimeric foamy viruses as novel HIV-1 vaccine vectors.

Similar patterns of infection with bovine foamy virus in experimentally inoculated calves and sheep

Foamy viruses (FVs) are the least known retroviruses commonly found in primates, cats, horses, and cattle. Although FVs are considered apathogenic, simian and feline FVs have been shown to be associated with some transient health abnormalities in animal models. Currently, data regarding the course of infection with bovine FV (BFV) are not available. In this study, we conducted experimental infections of natural (cattle) and heterologous (sheep) hosts with the BFV(100) isolate and monitored infection patterns in both hosts during the early phase postinoculation as well as after long-term infection. Four calves and six sheep inoculated with BFV(100) showed no signs of pathology but developed persistent infection, as confirmed by virus rescue, consistent detection of BFV-specific antibodies, and presence of viral DNA. In both hosts, antibodies against BFV Gag and Bet appeared early after infection and persisted at high and stable levels while seroreactivity toward Env was consistently detectable only in BFV-infected sheep. Interestingly, the BFV proviral DNA load was highest in lung, spleen, and liver and moderate in leukocytes, while salivary glands contained either low or undetectable DNA loads in calves or sheep, respectively. Additionally, comparison of partial BFV sequences from inoculum and infected animals demonstrated very limited changes after long-term infection in the heterologous host, clearly less than those found in BFV field isolates. The persistence of BFV infection in both hosts suggests full replication competence of the BFV(100) isolate with no requirement of genetic adaptation for productive replication in the authentic and even in a heterologous host.

Molecular and functional interactions of cat APOBEC3 and feline foamy and immunodeficiency virus proteins: different ways to counteract host-encoded restriction

Defined host-encoded feline APOBEC3 (feA3) cytidine deaminases efficiently restrict the replication and spread of exogenous retroviruses like Feline Immunodeficiency Virus (FIV) and Feline Foamy Virus (FFV) which developed different feA3 counter-acting strategies. Here we characterize the molecular interaction of FFV proteins with the diverse feA3 proteins. The FFV accessory protein Bet is the virus-encoded defense factor which is shown here to bind all feA3 proteins independent of whether they restrict FFV, a feature shared with FIV Vif that induces degradation of all feA3s including those that do not inactivate FIV. In contrast, only some feA3 proteins bind to FFV Gag, a pattern that in part reflects the restriction pattern detected. Additionally, one-domain feA3 proteins can homo- and hetero-dimerize in vitro, but a trans-dominant phenotype of any of the low-activity feA3 forms on FFV restriction by one of the highly-active feA3Z2 proteins was not detectable.

Immunological properties of the transmembrane envelope protein of the feline foamy virus and its use for serological screening

The transmembrane envelope (TM) proteins of retroviruses are used as antigen in diagnostic immunoassays and they represent a conserved target for neutralizing antibodies. To analyze the situation in infections with the feline foamy virus (FFV), its recombinant TM protein was produced and used for ELISA and Western blot analyses. Screening sera from 404 German cats showed that 39% reacted against the TM protein, the same infection rate was determined using the Gag protein. Epitope mapping showed antibodies against the membrane proximal external region (MPER) of the TM protein in the sera from infected cats, but attempts to induce neutralizing antibodies by immunization with the recombinant TM protein failed. This is the first report demonstrating that the TM protein of the FFV is highly immunogenic and valuable for serological screening. Similar to HIV-1, but in contrast to different gammaretroviruses, immunization with the TM protein of FFV did not induce neutralizing antibodies.

Importance of the major splice donor and redefinition of cis-acting sequences of gutless feline foamy virus vectors

Foamy virus vectors are potent alternatives to lenti- and gamma-retroviral vectors for gene therapy. To construct and optimize gutless feline foamy virus (FFV) replication-deficient (RD) vectors, viral elements essential for optimal efficient marker gene transduction were characterized and fine-mapped and packaging clones constructed. For these purposes, new Gag and Pol expression clones which allow efficient expression of packaging proteins and vectors carrying deletions in coding and non-coding regions of the genome were constructed and functionally evaluated. These studies demonstrate that the 5' major splice donor (5' SD) is indispensable for RD vectors while defined mutations introduced to inactivate the gag start codon improve transgene delivery efficiency. Based on these findings, new gutless FFV vectors were generated yielding un-concentrated vector titers above 10(5) transducing units (TU)/ml. By minimizing the second cis-acting sequence in the pol gene, only 3.8 kb viral sequences are maintained in the novel gutless FFV RD vectors.

Construction and characterization of a full-length infectious simian T-cell lymphotropic virus type 3 molecular clone

Together with their simian T-cell lymphotropic virus (STLV) equivalent, human T-cell lymphotropic virus type 1 (HTLV-1), HTLV-2, and HTLV-3 form the primate T-cell lymphotropic virus (PTLV) group. Over the years, understanding the biology and pathogenesis of HTLV-1 and HTLV-2 has been widely improved by the creation of molecular clones. In contrast, so far, PTLV-3 experimental studies have been restricted to the overexpression of the tax gene using reporter assays. We have therefore decided to construct an STLV-3 molecular clone. We generated a full-length STLV-3 proviral clone (8,891 bp) by PCR amplification of overlapping fragments. This STLV-3 molecular clone was then transfected into 293T cells. Reverse transcriptase PCR experiments followed by sequence analysis of the amplified products allowed us to establish that both gag and tax/rex mRNAs were transcribed. Western blotting further demonstrated the presence of the STLV-3 p24gag protein in the cell culture supernatant from transfected cells. Transient transfection of 293T cells and of 293T-long terminal repeat-green fluorescent protein cells with the STLV-3 clone promoted syncytium formation, a hallmark of PTLV Env expression, as well as the appearance of fluorescent cells, also demonstrating that the Tax3 protein was expressed. Virus particles were visible by electron microscopy. These particles are infectious, as demonstrated by our cell-free-infection experiments with purified virions. All together, our data demonstrate that the STLV-3 molecular clone is functional and infectious. This clone will give us a unique opportunity to study in vitro the different pX transcripts and the putative presence of antisense transcripts and to evaluate the PTLV-3 pathogenicity in vivo.

Construction and characterization of efficient, stable and safe replication-deficient foamy virus vectors

As serious side effects affected recent virus-mediated gene transfer studies, novel vectors with improved safety profiles are urgently needed. In the present study, replication-deficient retroviral vectors based on feline foamy virus (FFV) were constructed and analyzed. The novel FFV vectors are devoid of almost the complete env gene plus the internal promoter - accessory bel gene cassette including the gene for the viral transcriptional transactivator Bel1/Tas. In these Bel1/Tas-independent vectors, expression of the lacZ (beta-galactosidase) marker gene is directed by the heterologous, constitutively active human ubiquitin C promoter (ubi). Env-transcomplemented vectors have un-concentrated titers of more than 10(5) transducing units/ml. The vectors allow efficient transduction of a broad array of diverse target cells, which can be increased by repeated vector exposure. However, the number of lacZ marker gene expressing cells decreased slightly upon serial passages of the transduced cells. Vectors carrying a self-inactivating (SIN) deletion of the TATA box and most parts of the viral promoter were not rescued by wt FFV whereas those with the intact or a partially deleted promoter were readily reactivated. This finding indicates that the viral promoters are in fact non-functional, pointing to a highly advantageous safety profile of these new FFV-ubi-lacZ-SIN vectors.

Feline foamy virus-mediated marker gene transfer: identification of essential genetic elements and influence of truncated and chimeric proteins

Retroviral vectors derived from foamy or spumaretroviruses are considered promising tools for targeted gene delivery and vaccination purposes. In order to fully exploit this potential, we identified essential cis-acting sequences on the feline foamy virus (FFV) genome by constructing and analyzing a series of FFV-based replication-deficient vector genomes. Cis-acting sequences essentially required for marker gene transfer were found to be localized at two sites on the FFV genome: (i) in the 5'-untranslated region and close to the gag ATG and (ii) in the central part of the pol gene. The presence of two cis-acting sequences and their relative location on the FFV genome are similar but not identical to the functionally corresponding elements described for simian and primate foamy viruses.

The antiretroviral activity of APOBEC3 is inhibited by the foamy virus accessory Bet protein

Genome hypermutation of different orthoretroviruses by cellular cytidine deaminases of the APOBEC3 family during reverse transcription has recently been observed. Lentiviruses like HIV-1 have acquired proteins preventing genome editing in the newly infected cell. Here we show that feline foamy virus (FFV), a typical member of the foamy retrovirus subfamily Spumaretrovirinae, is also refractory to genome deamination. APOBEC3-like FFV genome editing in APOBEC3-positive feline CRFK cells only occurs when the accessory FFV Bet protein is functionally inactivated. Editing of bet-deficient FFV genomes is paralleled by a strong decrease in FFV titer. In contrast to lentiviruses, cytidine deamination already takes place in APOBEC3-positive FFV-producing cells, because edited proviral DNA genomes are consistently present in released particles. By cloning the feline APOBEC3 orthologue, we found that its homology to the second domain of human APOBEC3F is 48%. Expression of feline APOBEC3 in APOBEC3-negative human 293T cells reproduced the effects seen in homologous CRFK cells: Bet-deficient FFV displayed severely reduced titers, high-level genome editing, reduced particle release, and suppressed Gag processing. Although WT Bet efficiently preserved FFV infectivity and genome integrity, it sustained particle release and Gag processing only when fe3 was moderately expressed. Similar to lentiviral Vif proteins, FFV Bet specifically bound feline APOBEC3. In particles from Bet-deficient FFV, feline APOBEC3 was clearly present, whereas its foamy viral antagonist Bet was undetectable in purified WT particles. This is the first report that, in addition to lentiviruses, the foamy viruses also developed APOBEC3-counter-acting proteins.

Characterization of Env antigenicity of feline foamy virus (FeFV) using FeFV-infected cat sera and a monoclonal antibody

To characterize neutralizing antigenicity in relation to env genotypes of feline foamy virus (FeFV), serological analyses were performed using FeFV-infected cat sera and several field isolates including two env genotypes (F17- and FUV-types). Since three cats from which FeFV were isolated were found to have undetectable titers of virus neutralization (VN) antibodies, even to the homologous virus, VN antibodies were further examined with complement supplementation as an enhancement factor. With the presence of complement, the VN titers of FeFV-infected cat sera increased drastically. Although most of serum samples neutralized strains of either env genotype, sera sampled from two cats neutralized all the strains examined at similar titers, suggesting that superinfection with both env genotypes of FeFV might have occurred in the two cats. Further, we produced a monoclonal antibody (mAb) specifically neutralizing FeFV strains of FUV-type. The mAb was shown to have higher affinity to an epitope on Env of FUV-type than that of F17-type by immunoprecipitation assay. This study supplies basic information important for studies on FeFV vector development as well as on the relationship between the virus and the host immune response.

Furin-mediated cleavage of the feline foamy virus Env leader protein

The molecular biology of spuma or foamy retroviruses is different from that of the other members of the Retroviridae. Among the distinguishing features, the N-terminal domain of the foamy virus Env glycoprotein, the 16-kDa Env leader protein Elp, is a component of released, infectious virions and is required for particle budding. The transmembrane protein Elp specifically interacts with N-terminal Gag sequences during morphogenesis. In this study, we investigate the mechanism of Elp release from the Env precursor protein. By a combination of genetic, biochemical, and biophysical methods, we show that the feline foamy virus (FFV) Elp is released by a cellular furin-like protease, most likely furin itself, generating an Elp protein consisting of 127 amino acid residues. The cleavage site fully conforms to the rules for an optimal furin site. Proteolytic processing at the furin cleavage site is required for full infectivity of FFV. However, utilization of other furin proteases and/or cleavage at a suboptimal signal peptidase cleavage site can partially rescue virus viability. In addition, we show that FFV Elp carries an N-linked oligosaccharide that is not conserved among the known foamy viruses.

Kinetics and characteristics of replication-competent revertants derived from self-inactivating foamy virus vectors

In this study, self-inactivating (SIN) retroviral vectors based on feline foamy virus (FFV) were constructed and analysed. The FFV SIN vectors were devoid of the core FFV long terminal repeat promoter plus upstream sequences but contained all structural and regulatory genes. This design allowed sensitive detection of replication-competent revertants (RCRs). The FFV SIN vectors efficiently transduced the green fluorescence protein into recipient cells. However, RCRs appeared after serial passages of transduced cells. In all RCR clones analysed, parts of the heterologous cytomegalovirus immediate early promoter, originally driving expression of the FFV vector genome, were taken up to restore the deleted SIN promoter function required for replication competence. The RCRs were strongly reduced in replication capacity compared with the parental replication-competent vectors containing the FFV promoter. In all RCR genomes analysed, the uptake of the heterologous promoter was accompanied by deletion of almost the complete marker gene. Although the RCRs described in this study may not have the capacity to spread in humans and animals, they may pose a theoretical risk, for instance during transduction of haematopoietic stem cells. Thus, FV-based SIN vectors require additional genetic modifications in order to avoid RCRs.

Application of chimeric feline foamy virus-based retroviral vectors for the induction of antiviral immunity in cats

In order to define the potential and applicability of replication-competent foamy virus-based vaccine vectors, recombinant feline foamy virus (FFV) vectors encoding defined segments of the feline calicivirus (FCV) capsid protein E domain were constructed. In cell cultures, these FFV-FCV vectors efficiently transduced and expressed a hybrid fusion protein consisting of the essential FFV Bet protein and the attached FCV E domains. The stability of the vectors in vitro was inversely correlated to the size of the heterologous insert. The deletion of a part of the FFV U3 sequence in these FFV-FCV vectors did not interfere with replication and titer in cell cultures but increased the genetic stability of the hybrid vectors. Selected chimeric vectors were injected into immunocompetent cats and persisted in the transduced host concomitant with a strong and specific humoral immune response against vector components. In a substantial number of cats, antibodies directed against the FCV E domain were induced by the FFV-FCV vectors, but no FCV-neutralizing activities were detectable in vitro. When the vaccinated cats were challenged with a high-titer FCV dose, sterile immunity was not induced by any of the hybrid FFV-FCV vectors. However, the FFV-FCV vector with a truncated U3 region of the long terminal repeat promoter significantly reduced the duration of FCV shedding after challenge and suppressed the appearance of FCV-specific ulcers. Possible mechanisms contributing to the partial protection will be discussed.

Features of the Env leader protein and the N-terminal Gag domain of feline foamy virus important for virus morphogenesis

Previous studies have shown that foamy virus (FV) particle budding, especially the involvement of the viral env glycoprotein is different from that of other (ortho) retroviruses: the N-terminal Env leader protein Elp is a constituent of released FV particles. A defined sequence in Elp required for particle budding binds to the MA domain of Gag. To extend these findings, we show that feline FV Elp is a membrane-anchored protein with the N-terminus located inside the particle. Thus, the internal/cytoplasmic domain of Elp has the correct topology for interacting with Gag during budding. In addition to Elp, an Elp-related protein of about 9 kDa was shown to be virion associated and is probably generated by cellular signal peptidases. Besides the function of Elp binding, the N-terminal domain of Gag was shown to be required for proper localization of feline FV Gag to the cytoplasm and the perinuclear/nuclear region.

Establishment of a GFP-based indicator cell line to quantitate feline foamy virus

To quantitate infectious feline foamy virus (FeFV), Crandell feline kidney (CRFK) cells were transfected with the gfp gene under the control of the FeFV long terminal repeat (LTR) for establishing an indicator cell line named FFG cells. The FeFV activates promoter activity of the LTR to express green fluorescent protein (GFP) upon infection. The titers determined by GFP-positive FFG cells (GFP-based assay) were higher than those determined by the cytopathic effects-positive CRFK cells (CPE-based assay). The titers determined by the GFP-based assay reached a plateau at 3-4 days post infection (d.p.i.), while those by the CPE-based assay reached 6-8 d.p.i. When stock viruses of various FeFV strains were titrated by both assays, titers determined by both assays correlated well with each other. The results show that the GFP-based assay is simpler and more rapid and sensitive than the CPE-based assay. Using the GFP-based assay, we examined the in vitro host range of FeFV. It was found that FeFV can productively infect various cell lines derived from cats, dogs, chickens, a human and a bat.

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.

Isolation and sequencing of infectious clones of feline foamy virus and a human/feline foamy virus Env chimera

Full-length DNAs of the Coleman and S7801 strains (pSKY3.0, pSKY5.0) of infectious feline foamy viruses (FFVs) were cloned and sequenced. Parental viruses, designated SKY3.0 and SKY5.0, were secreted following transfection of Crandell feline kidney (CRFK) cells. Production of the rescued parental viruses was enhanced in the presence of trichostatin A. Amino acid sequence similarities between FFV and human foamy virus (HFV) are extremely low for the envelope protein and capsid antigen, as predicted from the two clones. However, a chimeric FFV clone was constructed with the HFV Env substituted for the FFV Env. The chimeric virus (HFFV, SKY4.0) was able to infect and replicate in CRFK cells as well as in peripheral blood mononuclear cells of cats in vivo. Consequently, the chimeric HFFV may be useful for the creation of FV vectors for gene transfer strategies.

The bet gene of feline foamy virus is required for virus replication

Foamy viruses (FV) are complex retroviruses with additional bel genes located between env and the 3' long-terminal repeat. The functions of the bel 2 and bet genes are unknown and both are dispensable for replication of the prototypic human foamy virus in cell cultures. We examined the function(s) of bel 2 and bet of the distantly related feline foamy virus (FFV) in the proviral context. Mutagenesis was used to alter the Bel 2 and Bet or to abrogate their expression. The Bel 2/Bet mutants showed a 1000-fold reduced viral titer in feline kidney cells; in human 293T cells, viral titer was only about 10-fold reduced compared to wild-type FFV. In both cell types, the Bel 2/Bet mutations resulted in a reduced release of FFV particles. The results indicate that FFV Bet is required for efficient virus replication. The functions of the Bel 2 and Bet proteins are discussed.

Specific interaction of a novel foamy virus Env leader protein with the N-terminal Gag domain

Cryoelectron micrographs of purified human foamy virus (HFV) and feline foamy virus (FFV) particles revealed distinct radial arrangements of Gag proteins. The capsids were surrounded by an internal Gag layer that in turn was surrounded by, and separated from, the viral membrane. The width of this layer was about 8 nm for HFV and 3.8 nm for FFV. This difference in width is assumed to reflect the different sizes of the HFV and FFV MA domains: the HFV MA domain is about 130 residues longer than that of FFV. The distances between the MA layer and the edge of the capsid were identical in different particle classes. In contrast, only particles with a distended envelope displayed an invariant, close spacing between the MA layer and the Env membrane which was absent in the majority of particles. This indicates a specific interaction between MA and Env at an unknown step of morphogenesis. This observation was supported by surface plasmon resonance studies. The purified N-terminal domain of FFV Gag specifically interacted with synthetic peptides and a defined protein domain derived from the N-terminal Env leader protein. The specificity of this interaction was demonstrated by using peptides varying in the conserved Trp residues that are known to be required for HFV budding. The interaction with Gag required residues within the novel virion-associated FFV Env leader protein of about 16.5 kDa.

Characterization of the humoral immune response and virus replication in cats experimentally infected with feline foamy virus

Cats were experimentally infected with cell culture-adapted feline foamy virus (FFV, spumaretrovirinae) isolate FUV. FFV was consistently recovered from peripheral blood leukocytes and throat samples of FFV-infected cats starting 2 to 3 weeks postinfection (p. i.), indicative of the establishment of persistent FFV infections. Viral persistence was established, even despite neutralizing antibodies that appeared early after infection. The humoral immune response toward FFV was quantitatively and qualitatively analyzed over time. FFV Gag-specific antibodies were first detected 2 weeks p. i. and increased further; reactivities to the other structural and nonstructural FFV proteins appeared slightly delayed. Reactivities against FFV Pol and Gag proteins were detectable by immunoblotting and radioimmunoprecipitation, whereas the latter techniques had to be employed for the unambiguous detection of FFV Env-, Bet-, and Bel 1-specific antibodies.