Detection and prevalence of serotypes of feline syncytial spumaviruses

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.

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.

Diagnostic Uncertainty and the Epidemiology of Feline Foamy Virus in Pumas (Puma concolor)

Feline foamy virus (FFV) is a contact-dependent retrovirus forming chronic, largely apathogenic, infections in domestic and wild felid populations worldwide. Given there is no current ‘gold standard’ diagnostic test for FFV, efforts to elucidate the ecology and epidemiology of the virus may be complicated by unknown sensitivity and specificity of diagnostic tests. Using Bayesian Latent Class Analysis, we estimated the sensitivity and specificity of the only two FFV diagnostic tests available—ELISA and qPCR—as well as the prevalence of FFV in a large cohort of pumas from Colorado. We evaluated the diagnostic agreement of ELISA and qPCR, and whether differences in their diagnostic accuracy impacted risk factor analyses for FFV infection. Our results suggest ELISA and qPCR did not have strong diagnostic agreement, despite FFV causing a persistent infection. While both tests had similar sensitivity, ELISA had higher specificity. ELISA, but not qPCR, identified age to be a significant risk factor, whereas neither qPCR nor ELISA identified sex to be a risk factor. This suggests FFV transmission in pumas may primarily be via non-antagonistic, social interactions between adult conspecifics. Our study highlights that combined use of qPCR and ELISA for FFV may enhance estimates of the true prevalence of FFV and epidemiological inferences.

Feline Foamy Virus Infection: Characterization of Experimental Infection and Prevalence of Natural Infection in Domestic Cats with and without Chronic Kidney Disease

Foamy viruses (FVs) are globally prevalent retroviruses that establish apparently apathogenic lifelong infections. Feline FV (FFV) has been isolated from domestic cats with concurrent diseases, including urinary syndromes. We experimentally infected five cats with FFV to study viral kinetics and tropism, peripheral blood mononuclear cell (PBMC) phenotype, urinary parameters, and histopathology. A persistent infection of primarily lymphoid tropism was detected with no evidence of immunological or hematologic perturbations. One cat with a significant negative correlation between lymphocytes and PBMC proviral load displayed an expanded FFV tissue tropism. Significantly increased blood urea nitrogen and ultrastructural kidney changes were noted in all experimentally infected cats, though chemistry parameters were not outside of normal ranges. Histopathological changes were observed in the brain, large intestine, and other tissues. In order to determine if there is an association of FFV with Chronic Kidney Disease, we additionally screened 125 Australian pet cats with and without CKD for FFV infection and found that FFV is highly prevalent in older cats, particularly in males with CKD, though this difference was not statistically significant compared to controls. Acute FFV infection was clinically silent, and while some measures indicated mild changes, there was no overt association of FFV infection with renal disease.

Isolation of an Equine Foamy Virus and Sero-Epidemiology of the Viral Infection in Horses in Japan

An equine foamy virus (EFV) was isolated for the first time in Japan from peripheral blood mononuclear cells of a broodmare that showed wobbler syndrome after surgery for intestinal volvulus and the isolate was designated as EFVeca_LM. Complete nucleotide sequences of EFVeca_LM were determined. Nucleotide sequence analysis of the long terminal repeat (LTR) region, gag, pol, env, tas, and bel2 genes revealed that EFVeca_LM and the EFV reference strain had 97.2% to 99.1% identities. For a sero-epidemiological survey, indirect immunofluorescent antibody tests were carried out using EFVeca_LM-infected cells as an antigen against 166 sera of horses in five farms collected in 2001 to 2002 and 293 sera of horses in eight farms collected in 2014 to 2016 in Hokkaido, Japan. All of the farms had EFV antibody-positive horses, and average positive rates were 24.6% in sera obtained in 2001 to 2002 and 25.6% in sera obtained in 2014 to 2016 from broodmare farms. The positive rate in a stallion farm (Farm A) in 2002 was 10.7%, and the positive rates in two stallion farms, Farms A and B, in 2015 were 40.9% and 13.3%, respectively. The results suggested that EFV infection is maintained widely in horses in Japan.

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.

Isolation of bovine foamy virus in Japan

Bovine foamy virus (BFV) is endemic in many countries, but has not been reported in Japan. A syncytium-forming virus was isolated from peripheral blood leukocytes of clinically healthy cattle on a farm in Kanagawa prefecture during a periodic epidemiological survey of viral diseases. The isolate was propagated in primary fetal bovine muscle cells and subsequently passaged in Madin–Darby bovine kidney cells. Since the isolate appeared to be distinct from the viruses with syncytium-forming ability previously isolated in Japan, we attempted to identify it using genomic analyses and electron microscopy. A phylogenetic analysis revealed that the isolate belongs to the bovine foamy virus cluster and is highly similar to a BFV strain isolated in China. A sero-epidemiological survey was performed using agar gel immunodiffusion test with the isolated virus as the antigen, and five of the 57 cattle tested were found to be seropositive.

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.

Important role of N108 residue in binding of bovine foamy virus transactivator Tas to viral promoters

BACKGROUND

Bovine foamy virus (BFV) encodes the transactivator BTas, which enhances viral gene transcription by binding to the long terminal repeat promoter and the internal promoter. In this study, we investigated the different replication capacities of two similar BFV full-length DNA clones, pBS-BFV-Y and pBS-BFV-B.

RESULTS

Here, functional analysis of several chimeric clones revealed a major role for the C-terminal region of the viral genome in causing this difference. Furthermore, BTas-B, which is located in this C-terminal region, exhibited a 20-fold higher transactivation activity than BTas-Y. Sequence alignment showed that these two sequences differ only at amino acid 108, with BTas-B containing N108 and BTas-Y containing D108 at this position. Results of mutagenesis studies demonstrated that residue N108 is important for BTas binding to viral promoters. In addition, the N108D mutation in pBS-BFV-B reduced the viral replication capacity by about 1.5-fold.

CONCLUSIONS

Our results suggest that residue N108 is important for BTas binding to BFV promoters and has a major role in BFV replication. These findings not only advances our understanding of the transactivation mechanism of BTas, but they also highlight the importance of certain sequence polymorphisms in modulating the replication capacity of isolated BFV clones.

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.

Is feline foamy virus really apathogenic?

Feline foamy virus (FFV) is a retrovirus commonly found in cats. It is generally thought to be apathogenic, making it a suitable candidate as a gene therapy vector. However, there have been reports of association of FFV with chronic progressive arthritis and a cofactor effect with feline immunodeficiency virus. This study investigated experimental FFV infection and whether this was associated with signs of disease. Eight young specific pathogen free cats were inoculated intramuscularly with FFV. The cats were examined twice weekly and blood and pharyngeal samples were taken. Haematology, biochemistry and FFV quantitative polymerase chain reaction (qPCR) were performed. Tissue samples were also collected throughout the six month period. FFV was initially detected by qPCR in the blood within the first two weeks of infection and viraemia persisted throughout the study. Two peaks of viraemia were observed, at day 20 (80-170FFU/ml blood) and day 155 (332-415FFU/ml blood). FFV was also consistently detected in oropharyngeal samples after day 36. Anti-FFV IgG was detected in all cats by ELISA; antibody levels had an early peak around day 35 and then increased again following the second rise in circulating viral load. All cats remained clinically normal, except for one cat with an unrelated gingivitis. None of the cats developed pyrexia. The biochemical profile and blood cell counts remained within normal limits except for one cat with a persistent eosinophilia. Initial fluctuations in white cell counts settled within three weeks and did not deviate outside of the normal ranges. All tissue samples contained FFV DNA; lymphoreticular tissues, salivary gland and lung had the highest viral loads. Although there were no gross pathological lesions on post mortem examination, histologically a mild glomerulonephritis and a moderate interstitial pneumonia were observed in all cats. We conclude that during the six month period of infection, although cats appeared clinically normal, histopathological changes were observed in the lungs and kidneys. Further investigation of the significance of these changes is warranted before FFV is developed as a vector for gene delivery.

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.

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.

FIP: a novel approach to vaccination. Proceedings from the 2nd International FCoV/FIP Symposium, Glasgow, 4-7 August 2002

Feline infectious peritonitis (FIP) is a fatal disease of cats. Early attempts at vaccination have been unsuccessful, some even serving to exacerbate the disease through antibody-dependent enhancement. Replication-incompetent feline foamy virus (FFV) transducing vectors are being developed as potential vaccine agents, into which immunogenic fragments of feline coronavirus (FCoV) proteins will be inserted. To use a recombinant viral vector to express FCoV proteins, the agent chosen should be apathogenic and replication incompetent within the host following gene delivery. Spumaviruses confer several advantages over the more traditionally explored retroviral vectors. Stable helper cell line clones have been established by transfection of CRFK cells with FFV tas and assessed using beta-galactosidase assays, PCR, immunofluorescence and western blotting. The generation of infectious virions using these cell lines has been investigated using tas-deleted FFV vectors containing the enhanced green fluorescent protein (eGFP) cassette.

Demonstration of feline foamy virus in experimentally infected cats by immunohistochemistry

Foamy viruses (FV) are complex retroviruses which are commonly isolated from cats, cattle and non-human primates. The infection is persistent and infected animals have a sustained antibody response. The role of FV in diseases remains unclear, in cats, a possible association with uncharacterized renal symptoms remains to be confirmed. To demonstrate feline FV (FFV) in tissues of experimentally infected cats three polyclonal monospecific antisera from rabbits against three different viral proteins, the structural Gag and the non-structural Bel 1 and Bet proteins were tested for their applicability in immunohistochemistry with paraffin sections. Only the Bet antiserum allowed detection of FFV-specific proteins, the antibodies against Gag and Bel 1 did not work even after pre-treatment of the slides with proteinase K or cooking in a pressure cooking pot. The Bet-reactive antibodies were detected using a commercial streptavidin kit and revealed Bet in the cytoplasm of cells from different lymphoid tissues like lymphnodes, tonsils, thymus and spleen. The method described opens new ways to explore the in vivo replication and tissue specificity of FFV and its possible role in disease.

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.

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.

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.

Genetic analyses of feline foamy virus isolates from domestic and wild feline species in geographically distinct areas

To know the genetic diversities and phylogenetic relationship among feline foamy virus (FeFV) isolates from domestic cats (Felis catus) and FeFV-related viruses from the Iriomote cats (Felis iriomotensis) and leopard cats (Felis bengalensis) in geographically distinct areas, we sequenced a partial gag-pol region of 17 strains and a partial env region of nine strains, and the U3 region of long terminal repeat of three strains of the viruses. FeFV-related viruses from the feral cats were quite similar to the FeFV from domestic cats in the sequenced regions. In the partial gag region, the identities of nucleotide sequences among the isolates were from 94 to 99%. In the partial env gene, the isolates were divided into two distinct genotypes (F17- and FUV-types) as reported by Winkler et al. (Virology 247 (1999) 144-151). More than 94% nucleotide identities were observed in the env region within a particular env genotype and about 75% nucleotide identities were noted between the two genotypes.

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.

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

Full-length genomes of the feline foamy virus (FFV or FeFV) isolate FUV were constructed. DNA clone pFeFV-7 stably directed the expression of infectious FFV progeny virus indistinguishable from wild-type, uncloned FFV isolate FUV. The env and bel 1 genes of pFeFV-7 were substituted for by corresponding sequences of the FFV serotype 951 since previous studies implicated a defined part of FFV Env protein as responsible for serotype-specific differences in serum neutralization (I. G. Winkler, R. M. Flügel, M. Löchelt, and R. L. P. Flower, 1998. Virology 247: 144-151). Recombinant virus derived from chimeric plasmid pFeFV-7/951 containing the hybrid env gene and the parental clone pFeFV-7 were used for neutralization studies. By means of a rapid titration assay for FFV infectivity, we show that progeny virus derived from plasmid pFeFV-7 was neutralized by FUV- but not by 951-specific antisera, whereas pFeFV-7/951-derived chimeric virus was neutralized by 951-specific antisera only. Both recombinant proviruses will be useful for repeated delivery of foreign genes for therapeutic gene applications into cats.

Detection and molecular characterisation of feline foamy virus serotypes in naturally infected cats

The characterisation of two distinct feline foamy virus sequence groupings in the external surface portion of the viral env gene are reported. Although amino acid identities in the Gag nucleocapsid, Pol protease, and Env transmembrane domains were greater than 92% in the 12 proviral sequences examined, two distinct sequence groups were observed in the Env surface (SU) protein. Only 57% amino acid identity was observed in the Env SU between the two groups designated FUV7-like or 951-like, while within these groups > 97% identity was found. Isolates FUV7 and 951 represent two serogroups previously characterised by Flower et al. (1985). A 100% correlation was found among FeFV seroreactivity, virus isolation, and detection of viral DNA in feline leucocytes using a single round of PCR amplification. Serum neutralisation assay using autologous virus, as well as isolates 951 and FUV7, revealed that viruses with FUV7-like sequences were in a single neutralisation group and viruses with 951-like sequences were in a single neutralisation group. Based on these results, group-specific PCRs were developed, using the same sense primer with an antisense primer specific for each group. Using he PCR, no evidence of superinfection of any cat with virus from both serogroups was detected.

Characterization of the genome of feline foamy virus and its proteins shows distinct features different from those of primate spumaviruses

The genome of the feline foamy virus (FeFV) isolate FUV was characterized by molecular cloning and nucleotide sequence analysis of subgenomic proviral DNA. The overall genetic organization of FeFV and protein sequence comparisons of different FeFV genes with their counterparts from other known foamy viruses confirm that FeFV is a complex foamy virus. However, significant differences exist when FeFV is compared with primate foamy viruses. The FeFV Gag protein is smaller than that of the primate spumaviruses, mainly due to additional MA/CA sequences characteristic of the primate viruses only. Gag protein sequence motifs of the NC domain of primate foamy viruses assumed to be involved in genome encapsidation are not conserved in FeFV. FeFV Gag and Pol proteins were detected with monospecific antisera directed against Gag and Pol domains of the human foamy virus and with antisera from naturally infected cats. Proteolytic processing of the FeFV Gag precursor was incomplete, whereas more efficient proteolytic cleavage of the pre125Pro-Pol protein was observed. The active center of the FeFV protease contains a Gln that replaces an invariant Gly residue at this position in other retroviral proteases. Functional studies on FeFV gene expression directed by the promoter of the long terminal repeat showed that FeFV gene expression was strongly activated by the Bell/Tas transactivator protein. The FeFV Bell/Tas transactivator is about one-third smaller than its counterpart of primate spumaviruses. This difference is also reflected by a limited sequence similarity and only a moderate conservation of structural motifs of the different foamy virus transactivators analyzed.

A rapid streptavidin-capture ELISA specific for the detection of antibodies to feline foamy virus

We report a simple procedure for the rapid development of an ELISA with the potential for wide application to any defined protein antigen. The procedure involves the expression of protein encoded by a PCR product, using a commercially available T-vector that adds a biotin tag, and a single step purification by affinity for streptavidin for direct use in ELISA. In our experiments, a recombinant protein from the nucleocapsid domain of the feline foamy virus gag gene was expressed as a fusion protein with a biotin tag and then applied directly to streptavidin-coated ELISA wells. An extract from a clone with the insert in antisense orientation was used as a control. Non-specific reactions with antigen extracts from both sense and antisense clones were observed in 6 of the 376 (1.6%) sera tested. Antibody to feline foamy virus, which forms a stable persistent infection in cats, was detected in 107 of 201 (53%) Australian cats, but none of 175 sera from veterinarians. There was a 100% correlation between FeFV antibody detected by ELISA, immunoblot, serum neutralisation and virus isolation, confirming that this test is sensitive and specific.

Replication of feline syncytial virus in feline T-lymphoblastoid cells and induction of apoptosis in the cells

Feline syncytial virus (FSV) was isolated from feline peripheral blood mononuclear cells of FSV-seropositive cats. When the susceptibility of feline T-lymphocytes to FSV was examined using three strains of FSV, FSV antigens were detected in the FSV-infected T-lymphoblastoid cells. Further, a diversity of biological properties, including replication kinetics and syncytia formation, was noted among the strains, and condensation of chromatin and the fragmentation of cellular DNA were observed in the infected cells. From these data, we conclude that FSV is lymphotropic and can induce apoptosis in the lymphocytes.

Characterization of the spliced pol transcript of feline foamy virus: the splice acceptor site of the pol transcript is located in gag of foamy viruses

Foamy viruses, or spumaviruses, are distinct members of the Retroviridae. Here we have characterized the long terminal repeat of the feline, or cat, foamy virus by determining the locations of the transcriptional start site and the poly(A) addition site. The splice donor and splice acceptor sites of the subgenomic mRNA responsible for Pro-Pol protein expression were identified by nucleotide sequencing of the corresponding cDNAs. The leader exon of the feline foamy virus is 57 nucleotides long. The splice acceptor of the subgenomic pol mRNA was found to be located in gag. The location of the splice acceptor of the human foamy virus pol mRNA was confirmed to map in gag. The pol splice acceptor site in gag of the cat foamy virus is located further downstream than that of human foamy virus.

Viral infections of the feline urinary tract

The exact cause of hematuria, dysuria, and urethral obstruction remains unknown in a large percentage of naturally occurring cases of feline lower urinary tract disease (FLUTD). One attractive hypothesis implicates viruses as the cause of some idiopathic forms of FLUTD; supporting this hypothesis is the fact that a gamma herpesvirus, a calicivirus, and a retrovirus have been isolated from urine and tissues obtained from cats with this type of disease. Although the clinical course and laboratory findings of cats with idiopathic FLUTD are suggestive of an infectious cause, the question of whether viruses have a pathologic role in some forms of naturally acquired FLUTDs has not been completely answered.

Latent infection with feline syncytial virus of cell cultures prepared from the kidneys of new-born kittens

Feline syncytial virus infection was detected in 4 out of 30 secondary or tertiary cultures of kidney cells from new-born kittens. Such infection was not detected in any primary cell culture. The syncytia-forming virus was readily transmissible by infected cells. One strain was transmissible to heterologous (bovine and canine) secondary cells, but did not infect MDBK and PK-15 cell lines. Based on biochemical, untrastructural and serological characteristics, the virus was designated a member of the subfamily Spumavirinae of the family Retroviridae. As endogenous infection of primary cell cultures may not be detectable by cytopathic effect, it is suggested that only well-controlled secondary cat cells or cell lines should be used in work with feline viruses.

Feline leukemia/sarcoma viruses and immunodeficiency

This chapter discusses the structure feline leukemia virus (FeLV) and pathogenesis of lymphomas and leukemias BY FeLV. FeLV is quite similar to the better-studied murine leukemia viruses in structure and genetic map. The virus particles bud from cytoplasmic membranes into either extracellular spaces or into vacuoles. FeLV has long been considered a typical noncytopathogenic, longlatency leukemia virus based on its behavior in fibroblasts in vitro . Recent evidence suggests that its in vivo behavior in critical target hemolymphatic tissues is as likely to be cytopathic as transforming. The type of FeLV-related disease that occurs and the disease-free interval probably are influenced by viral envelope proteins and glycoproteins and the consequences of proviral integration. FeLV subgroup specificity apparently determines when and what type of disease will occur. The ecotropic FeLV-A is the most frequent subgroup found in pet cats and is transmitted contagiously. Immunosuppression is the most frequent and the most devastating manifestation of FeLV viremia in clinical and experimental studies. It seems that multiple cell types and multiple processes are involved in the development of feline retrovirus-induced immunosuppression. Although no solid evidence is available for the malfunctioning of cat T helper cells because of the paucity of T-cell specific markers, the circumstantial evidence provided thus far indicates an impaired T helper function in FeLV-infected cats similar to that observed in humans infected with HIV. Studies on the pathogenesis of FeLV-induced immunosuppression might provide a valuable mode for a better understanding and means of control of human AIDS.