Bovine leukemia virus, an animal model for the study of intrastrain variability

Molecular Characterization of the env Gene of Bovine Leukemia Virus in Cattle from Pakistan with NGS-Based Evidence of Virus Heterogeneity

Characterization of the global genetic diversity of the bovine leukemia virus (BLV) is an ongoing international research effort. Up to now BLV sequences have been classified into eleven distinct genotypes. Although BLV genotyping and molecular analysis of field isolates were reported in many countries, there is no report describing BLV genotypes present in cattle from Pakistan. In this study we examined 27 env gene sequences from BLV-infected cattle coming from four farms located in Khyber Pakhtunkwa, Gilgit Baltisan and Punjab provinces. Phylogenetic analyses revealed the classification of Pakistani sequences into genotypes G1 and G6. The alignment with the FLK-BLV sequence revealed the presence of 45 mutations, namely, seven in genotype G1 and 33 in genotype G6. Five mutations were found in both, G1 and G6 genotypes. Twelve amino acid substitutions were found in the analyzed sequences, of which only one P264S was specific for sequences from Pakistan. Furthermore, a certain degree of nucleotide heterogeneity was identified by NGS. These results highlight the need for further study on the importance of genetic variability of BLV, especially in the context of its pathogenicity and potential effect on serological detection.

BLV: lessons on vaccine development

Vaccination against retroviruses is a challenge because of their ability to stably integrate into the host genome, undergo long-term latency in a proportion of infected cells and thereby escape immune response. Since clearance of the virus is almost impossible once infection is established, the primary goal is to achieve sterilizing immunity. Besides efficacy, safety is the major issue since vaccination has been associated with increased infection or reversion to pathogenicity. In this review, we discuss the different issues that we faced during the development of an efficient vaccine against bovine leukemia virus (BLV). We summarize the historical failures of inactivated vaccines, the efficacy and safety of a live-attenuated vaccine and the economical constraints of further industrial development.

Detection and molecular characterization of bovine leukemia virus in various regions of Iran

Purpose. Bovine leukemia virus (BLV) infects cattle worldwide, imposing an economic impact on the dairy cattle industry. The purpose of this study was to evaluate the molecular epidemiology of BLV in Iran.Methodology. Blood samples taken from 280 cows aged over 2 years old from 13 provinces of Iran were used for leukocyte count and blocking ELISA. Genomic DNA was extracted from the peripheral blood leukocytes of BLV-infected samples and fetal lamb kidney cells to perform PCR of partial env, rex and tax genes and long-terminal-repeat region. The PCR products were sequenced, the phylogenetic tree of each gene was constructed, and nucleotide and amino acid sequence pair distances were calculated.Results. The frequency of BLV infection was 32.8 % among animals and was 80 % among provinces. In BLV seropositive animals, the rate of persistent lymphocytosis was 36.9 %. The constructed phylogenetic trees showed the presence of two BLV genotypes (1 and 4) in Iranian strains. As previous studies, our results showed that the env gene was more variable than previously thought, the Rex protein could withstand more amino acid changes compared to the Tax protein, and no significant differences were observed in average changes of the nucleotide of these genes between clinical stages.Conclusions. Our data indicates an increase in the frequency of this infection in Iran. This is the first study report of the presence of BLV genotype 4 in Iranian farms. These findings may have an important role in the control and prevention of BLV infection in Iran and other countries.

Computational analysis of envelope glycoproteins from diverse geographical isolates of bovine leukemia virus identifies highly conserved peptide motifs

BACKGROUND

Bovine leukemia virus (BLV) is a deltaretrovirus infecting bovine B cells and causing enzootic bovine leucosis. The SU or surface subunit, gp51, of its envelope glycoprotein is involved in receptor recognition and virion attachment. It contains the major neutralizing and CD4+ and CD8+ T cell epitopes found in naturally infected animals. In this study, we aimed to determine global variation and conservation within gp51 in the context of developing an effective global BLV vaccine.

RESULTS

A total of 256 sequences extracted from the NCBI database and collected in different parts of the world, were studied to identify conserved segments along the env gene sequences that encode the gp51 protein. Using the MEME server and the conserved DNA Region module for analysis within DnaSP, we identified six conserved segments, referred to as A-F, and five semi-conserved segments, referred to as G-K. The amino acid conservation ranged from 98.8 to 99.8% in conserved segments A to F, while segments G to K had 89.6-95.2% conserved amino acid sequence. Selection analysis of individual segments revealed that residues of conserved segments had undergone purifying selection, whereas, particular residues in the semi-conserved segments are currently undergoing positive selection, specifically at amino acid positions 48 in segment K, 74 in segment G, 82 in segment I, 133 and 142 in segment J, and residue 291 in segment H. Each of the codons for these six residues contain the most highly variable nucleotides within their respective semi-conserved segments.

CONCLUSIONS

The data described here show that the consensus amino acid sequence constitutes a strong candidate from which a global vaccine can be derived for use in countries where eradication by culling is not economically feasible. The most conserved segments overlap with amino acids in known immunodeterminants, specifically in epitopes D-D', E-E', CD8+ T-cell epitopes, neutralizing domain 1 and CD4+ T-cell epitopes. Two of the segments reported here represent unique segments that do not overlap with previously identified antigenic determinants. We propose that evidence of positive selection in some residues of the semi-conserved segments suggests that their variation is involved in viral strategy to escape immune surveillance of the host.

Determinants of the Bovine Leukemia Virus Envelope Glycoproteins Involved in Infectivity, Replication and Pathogenesis

Interaction of viral envelope proteins with host cell membranes has been extensively investigated in a number of systems. However, the biological relevance of these interactions in vivo has been hampered by the absence of adequate animal models. Reverse genetics using the bovine leukemia virus (BLV) genome highlighted important functional domains of the envelope protein involved in the viral life cycle. For example, immunoreceptor tyrosine-based activation motifs (ITAM) of the envelope transmembrane protein (TM) are essential determinants of infection. Although cell fusion directed by the aminoterminal end of TM is postulated to be essential, some proviruses expressing fusion-deficient envelope proteins unexpectedly replicate at wild-type levels. Surprisingly also, a conserved N-linked glycosylation site of the extracellular envelope protein (SU) inhibits cell-to-cell transmission suggesting that infectious potential has been limited during evolution. In this review, we summarize the knowledge pertaining to the BLV envelope protein in the context of viral infection, replication and pathogenesis.

Recent Advances in BLV Research

Different animal models have been proposed to investigate the mechanisms of Human T-lymphotropic Virus (HTLV)-induced pathogenesis: rats, transgenic and NOD-SCID/γcnull (NOG) mice, rabbits, squirrel monkeys, baboons and macaques. These systems indeed provide useful information but have intrinsic limitations such as lack of disease relevance, species specificity or inadequate immune response. Another strategy based on a comparative virology approach is to characterize a related pathogen and to speculate on possible shared mechanisms. In this perspective, bovine leukemia virus (BLV), another member of the deltaretrovirus genus, is evolutionary related to HTLV-1. BLV induces lymphoproliferative disorders in ruminants providing useful information on the mechanisms of viral persistence, genetic determinants of pathogenesis and potential novel therapies.

Sequencing and phylogenetic analysis of the gp51 gene from Korean bovine leukemia virus isolates

BACKGROUND

Bovine Leukemia virus (BLV) infection of cattle has been reported in Korea for more than three decades. However, to date, there have been few studies regarding Korean BLV since 1980s. Thus, the purpose of this study is to perform a diagnosis and molecular characterization of BLV strains circulating in Korea and to estimate genetic diversity of different genotypes of BLV.

METHOD

To investigate the distribution of BLV variants in the world and assess the evolutionary history of Korean BLV isolates, a comprehensive molecular analysis of the BLV env gp51 gene was conducted using recent worldwide BLV isolates. The isolates included 50 samples obtained from two cattle farms in southeastern Korea in 2014.

RESULTS

Sequence and phylogenetic analyses of partial 444-nt fragment sequences and complete gp51 sequences of BLV revealed eight distinct genotypes of BLV showing geographic distribution of the world. Most Korean BLV isolates were found to belong to genotype 1 which is a major genotype prevailed throughout the world, and only four isolates from one farm were classified as genotype 3 related to the US and Japan isolates. Analysis of amino acids of Korean BLV isolates showed several sequence substitutions in the leader peptide, conformational epitope, and neutralizing domain regions. The observations suggest the possibility of affecting on viral infectivity and formation.

CONCLUSION

Korean BLV isolates showed the close relationship to genotype 1 and 3. Further study to identify the diversity of BLV circulating in Korea is necessary with samples collected nationwide because this study is the first report of BLV genotype 3 being in circulation in Korea.

Endogenous viruses: Connecting recent and ancient viral evolution

The rapid rates of viral evolution allow us to reconstruct the recent history of viruses in great detail. This feature, however, also results in rapid erosion of evolutionary signal within viral molecular data, impeding studies of their deep history. Thus, the further back in time, the less accurate the inference becomes. Furthermore, reconstructing complex histories of transmission can be challenging, especially where extinct viral lineages are concerned. This problem has been partially solved by the discovery of viruses embedded in host genomes, known as endogenous viral elements (EVEs). Some of these endogenous viruses are derived from ancient relatives of extant viruses, allowing us to better examine ancient viral host range, geographical distribution and transmission routes. Moreover, our knowledge of viral evolutionary timescales and rate dynamics has also been greatly improved by their discovery, thereby bridging the gap between recent and ancient viral evolution.

The molecular characterization of bovine leukaemia virus isolates from Eastern Europe and Siberia and its impact on phylogeny

Recent studies have shown that bovine leukemia virus (BLV) sequences can be classified into seven distinct genotypes based on full gp51 sequence. This classification was based on available sequence data that mainly represented the BLV population that is circulating in cattle from the US and South America. In order to aid with a global perspective inclusion of data from Eastern Europe is required. In this study we examined 44 BLV isolates from different geographical regions of Poland, Belarus, Ukraine, and Russia. Phylogenetic analysis based on a 444bp fragment of env gene revealed that most of isolates belonged to genotypes 4 and 7. Furthermore, we confirmed the existence of a new genotype, genotype 8, which was highly supported by phylogenetic computations. A significant number of amino acid substitutions were found in the sequences of the studied Eastern European isolates, of which 71% have not been described previously. The substitutions encompassed mainly the C-part of the CD4+ epitope, zinc binding peptide region, CD8+ T cell epitope, and overlapping linear epitope E. These observations highlight the use of sequence data to both elucidate phylogenetic relationships and the potential effect on serological detection of geographically diverse isolates.

Identification of a new genotype of bovine leukemia virus

To investigate the degree of genetic variability of bovine leukemia virus (BLV) strains circulating in Croatia, 29 isolates from the six largest dairy farms were examined by PCR for a segment of the gp51 env gene, followed by DNA sequencing and phylogenetic analysis. The nucleotide sequences were compared with other previously characterized BLV strains from different geographical areas, comprising all seven known BLV genotypes. The Croatian sequences showed six to eight nucleotide substitutions: six silent substitutions and two amino acid changes. Four of those substitutions were within epitopes. In comparison to the sequences of other BLV genotypes, our isolates showed the closest relationship to genotype 1 isolates PL-3252 (FJ808585) and AL-148 (FJ808573) from Argentina. The degree of variation between our sequences and those of genotype 1 was 0.2- 4.6 %. In phylogenetic trees based on 400-nt and 519-nt sequences, all of the Croatian sequences clustered separately from the other sequences, revealing a new genotype.

Preventive and therapeutic strategies for bovine leukemia virus: lessons for HTLV

Bovine leukemia virus (BLV) is a retrovirus closely related to the human T-lymphotropic virus type 1 (HTLV-1). BLV is a major animal health problem worldwide causing important economic losses. A series of attempts were developed to reduce prevalence, chiefly by eradication of infected cattle, segregation of BLV-free animals and vaccination. Although having been instrumental in regions such as the EU, these strategies were unsuccessful elsewhere mainly due to economic costs, management restrictions and lack of an efficient vaccine. This review, which summarizes the different attempts previously developed to decrease seroprevalence of BLV, may be informative for management of HTLV-1 infection. We also propose a new approach based on competitive infection with virus deletants aiming at reducing proviral loads.

Bovine leukemia virus can be classified into seven genotypes: evidence for the existence of two novel clades

Previous studies have classified the env sequences of bovine leukemia virus (BLV) provirus from different locations worldwide into between two and four genetic groupings. These different studies gave unique names to the identified groups and no study has yet integrated all the available sequences. Thus, we hypothesized that many of the different groups previously identified actually correspond to a limited group of genotypes that are unevenly distributed worldwide. To examine this hypothesis, we sequenced the env gene from 28 BLV field strains and compared these sequences to 46 env sequences that represent all the genetic groupings already identified. By using phylogenetic analyses, we recovered six clades, or genotypes, that we have called genotypes 1, 2, 3, 4, 5 and 6. Genotypes 1–5 have counterparts among the sequence groupings identified previously. One env sequence did not cluster with any of the others and was highly divergent when compared with the six genotypes identified here. Thus, an extra genotype, which we named 7, may exist. Similarity comparisons were highly congruent with phylogenetic analyses. Furthermore, our analyses confirmed the existence of geographical clusters.

Early and transient reverse transcription during primary deltaretroviral infection of sheep

Background

Intraindividual genetic variability plays a central role in deltaretrovirus replication and associated leukemogenesis in animals as in humans. To date, the replication of these viruses has only been investigated during the chronic phase of the infection when they mainly spread through the clonal expansion of their host cells, vary through a somatic mutation process without evidence for reverse transcriptase (RT)-associated substitution. Primary infection of a new organism necessary involves allogenic cell infection and thus reverse transcription.

Results

Here we demonstrate that the primary experimental bovine leukemia virus (BLV) infection of sheep displays an early and intense burst of horizontal replicative dissemination of the virus generating frequent RT-associated substitutions that account for 69% of the in vivo BLV genetic variability during the first 8 months of the infection. During this period, evidence has been found of a cell-to-cell passage of a mutated sequence and of a sequence having undergone both RT-associated and somatic mutations. The detection of RT-dependent proviral substitution was restricted to a narrow window encompassing the first 250 days following seroconversion.

Conclusion

In contrast to lentiviruses, deltaretroviruses display two time-dependent mechanisms of genetic variation that parallel their two-step nature of replication in vivo. We propose that the early and transient RT-based horizontal replication helps the virus escape the first wave of host immune response whereas somatic-dependent genetic variability during persistent clonal expansion helps infected clones escape the persistent and intense immune pressure that characterizes the chronic phase of deltaretrovirus infection.

Complete suppression of viral gene expression is associated with the onset and progression of lymphoid malignancy: observations in Bovine Leukemia Virus-infected sheep

Background

During malignant progression, tumor cells need to acquire novel characteristics that lead to uncontrolled growth and reduced immunogenicity. In the Bovine Leukemia Virus-induced ovine leukemia model, silencing of viral gene expression has been proposed as a mechanism leading to immune evasion. However, whether proviral expression in tumors is completely suppressed in vivo was not conclusively demonstrated. Therefore, we studied viral expression in two selected experimentally-infected sheep, the virus or the disease of which had features that made it possible to distinguish tumor cells from their nontransformed counterparts.

Results

In the first animal, we observed the emergence of a genetically modified provirus simultaneously with leukemia onset. We found a Tax-mutated (Tax_K303) replication-deficient provirus in the malignant B-cell clone while functional provirus (Tax_E303) had been consistently monitored over the 17-month aleukemic period. In the second case, both non-transformed and transformed BLV-infected cells were present at the same time, but at distinct sites. While there was potentially-active provirus in the non-leukemic blood B-cell population, as demonstrated by ex-vivo culture and injection into naïve sheep, virus expression was completely suppressed in the malignant B-cells isolated from the lymphoid tumors despite the absence of genetic alterations in the proviral genome. These observations suggest that silencing of viral genes, including the oncoprotein Tax, is associated with tumor onset.

Conclusion

Our findings suggest that silencing is critical for tumor progression and identify two distinct mechanisms-genetic and epigenetic-involved in the complete suppression of virus and Tax expression. We demonstrate that, in contrast to systems that require sustained oncogene expression, the major viral transforming protein Tax can be turned-off without reversing the transformed phenotype. We propose that suppression of viral gene expression is a contributory factor in the impairment of immune surveillance and the uncontrolled proliferation of the BLV-infected tumor cell.

Molecular characterization of the env gene from Brazilian field isolates of Bovine Leukemia Virus

Molecular characterization of Bovine leukemia virus (BLV) isolates from Brazil using the env gene sequences revealed a high conservation of this gene. In most cases the substitutions corresponded to silent transitions. In addition, cystein residues, potential glycosylation sites, neutralization domains and other critical residues involved with the envelope structural domains and viral infectivity were conserved. Most of the substitutions found in the aminoacid sequences of the gp51 protein were localized in the G and H epitopes. Using the SIFT software, it was predicted that they should not alter the protein functions. Phylogenetic analyses showed that partial or complete env gene sequences grouped in three or four phylogenetic clusters, respectively. The sequences from the Brazilian isolates had similar mutation rates as compared to samples from other countries, and belonged to at least two phylogenetic clusters.

Suppression of viral gene expression in bovine leukemia virus-associated B-cell malignancy: interplay of epigenetic modifications leading to chromatin with a repressive histone code

Ovine leukemia/lymphoma resulting from bovine leukemia virus infection of sheep offers a large animal model for studying mechanisms underlying leukemogenesis. Silencing of viral information including Tax, the major contributor to the oncogenic potential of the virus, is critical if not mandatory for tumor progression. In this study, we have identified epigenetic mechanisms that govern the complete suppression of viral expression, using a lymphoma-derived B-cell clone carrying a silent provirus. Silencing was not relieved by injection of the malignant B cells into sheep. However, exogenous expression of Tax or treatment with either the DNA methyltransferase inhibitor 5'azacytidine or the histone deacetylase (HDAC) inhibitor trichostatin A rescued viral expression, as demonstrated by in vivo infectivity trials. Comparing silent and reactivated provirus, we found mechanistic connections between chromatin conformation and tumor-associated transcriptional repression. Silencing is associated with DNA methylation and decreased accessibility of promoter sequences. HDAC1 and the transcriptional corepressor mSin3A are associated with the inactive but not the reactivated promoter. Silencing correlates with a repressed chromatin structure marked by histone H3 and H4 hypoacetylation, a loss of methylation at H3 lysine 4, and an increase of H3 lysine 9 methylation. These observations point to the critical role of epigenetic mechanisms in tumor-specific virus/oncogene silencing, a potential strategy to evade immune response and favor the propagation of the transformed cell.

Sequence polymorphisms in the long terminal repeat of bovine leukemia virus: evidence for selection pressures in regulatory sequences

Bovine leukemia virus (BLV) is an oncogenic virus widespread in cattle. It belongs to the genus Deltaretrovirus of the family Retroviridae along with human and simian T-lymphotropic viruses. The BLV transcriptional promoter is located in the proviral 5' long terminal repeat (LTR), composed of U3, R, and U5 regions. BLV LTR contains multiple cis-acting elements important for promoter activity, a short coding sequence (encoding the NH(2) terminus of the G4 regulatory protein), and non-regulatory/non-coding regions. Variation in coding sequences of BLV structural proteins has been studied extensively, but little work has been done on sequence variability of non-coding regions, mostly located in LTR. Here, we report the first study on the natural diversity of the BLV LTR, using viral isolates from 52 cattle in several different areas worldwide. Nucleotide variations from the consensus sequence were observed in most isolates and clustered phylogenetically, corresponding to the geographic distribution of donor cattle. Overall, regulatory regions were significantly more conserved than non-regulatory regions in the BLV LTR, as well as in LTR sub-regions (U3, R, and U5). Evidence of selection pressures in BLV LTR suggests that selection occurs not only in coding sequences, but may also involve regulatory sequences.

Molecular analysis of a 444 bp fragment of the bovine leukaemia virus gp51 env gene reveals a high frequency of non-silent point mutations and suggests the presence of two subgroups of BLV in Chile

With the aim of achieve a better understanding of the epidemiology and distribution of bovine leukaemia virus (BLV) infection in Chile, we assessed the suitability of using DNA isolated from the leukocyte fraction of bulk milk samples to carry out PCR-RFLP and DNA sequence analysis. The env fragment of BLV was successfully amplified from 33 serologically positive bulk milk samples collected from different geographical areas in the south of Chile. Restriction analysis allowed to classify 17 isolates within the Australian subgroup and 16 within the Belgium subgroup. DNA sequence and multiple alignment analysis of eight Chilean isolates showed a significantly higher frequency of single and double nucleotide substitutions. Most of these mutations were non-silent, resulting in changes at the protein level in several important epitopes of gp51. The Chilean sequences and 59 BLV env sequences available at GenBank, were subjected to a phylogenetic analysis, resulting in four different clusters. The groups identified were not related to those previously defined by restriction analysis. Chilean isolates were included in two different clusters and were genetically not related to isolates collected from neighbouring countries. Considering our results we can conclude: (i) bulk milk samples are suitable to identify the presence of BLV allowing epidemiological and genetic studies to be conducted on large geographical areas; (ii) at least four different genetic groups of BLV were identified by phylogenetic analysis, with Chilean isolates included in two different sub clusters.

Sensitive and specific detection of proviral bovine leukemia virus by 5' Taq nuclease PCR using a 3' minor groove binder fluorogenic probe

Sensitive assays are required to detect proviral bovine leukemia virus (BLV) in donor cattle used for the in vivo preparation of Australian tick fever vaccines. 5' Taq nuclease assays using 3' minor groove binder DNA probes (TaqManMGB) were developed and compared to conventional PCR assays for sensitive detection of Australian BLV. Seven beef and dairy herds were screened using DNA prepared by a variety of protocols to evaluate these tests. Comparative sensitivities of PCR tests were determined by testing log(10) dilutions of plasmids with inserted BLV sequences. Animals were also screened by the BLV standard agar-gel immunodiffusion test (AGID) and commercial enzyme linked immunosorbent assays (ELISA) for antibodies, and an ELISA for detecting viral antigens expressed (VAE) in lymphocyte cultures. The TaqMan MGB assay based on the pol region was the most sensitive and specific for the detection of BLV. This is the first report of a sensitive BLV 5' Taq nuclease assay.

Provirus variants of bovine leukemia virus in naturally infected cattle from Argentina and Japan

A serologic subgroup of bovine leukemia virus (BLV) has not been identified, whereas genetic diversity among BLVs has been reported by polymerase chain reaction (PCR)-restriction fragment length polymorphism (RFLP). To investigate the distribution of BLV provirus variants, 42 isolates from Argentina and Japan were examined by nested PCR for a segment of the env gene, followed by DNA sequencing. The nucleotide sequences were compared with other previously characterized BLV variants from different geographical areas (Belgium, France, Italy, North America, Australia, Japan and Argentina). The majority of analyzed segments had a tendency for nucleotide substitution without changing the amino acid. The constructed phylogenetic tree showed the relations and differences between proviruses and within each one. Most of the samples in Argentina formed one cluster. The samples in Japan, except one, also formed one cluster and some of them showed high homology with the isolates from Australia and the USA. Considering the sequence analysis of env PCR products of all Japanese and Argentine samples and comparing them with the other previously isolated sequences, the variation was up to 3.5% and was characterized geographically in each area.

Identification and characterization of a PU.1/Spi-B binding site in the bovine leukemia virus long terminal repeat

Bovine leukemia virus (BLV) is a B-lymphotropic oncogenic retrovirus whose transcriptional promoter is located in the viral 5' long terminal repeat (LTR). To date, no B-lymphocyte-specific cis-regulatory element has been identified in this region. Since ETS proteins are known to regulate transcription of numerous retroviruses, we searched for the presence in the BLV promoter region of binding sites for PU.1/Spi-1, a B-cell- and macrophage-specific ETS family member. In this report, nucleotide sequence analysis of the viral LTR identified a PUbox located at -95/-84 bp. We demonstrated by gel shift and supershift assays that PU.1 and the related Ets transcription factor Spi-B interacted specifically with this PUbox. A 2-bp mutation (GGAA-->CCAA) within this motif abrogated PU.1/Spi-B binding. This mutation caused a marked decrease in LTR-driven basal gene expression in transient transfection assays of B-lymphoid cell lines, but did not impair the responsiveness of the BLV promoter to the virus-encoded transactivator Tax(BLV). Moreover, ectopically expressed PU.1 and Spi-B proteins transactivated the BLV promoter in a PUbox-dependent manner. Taken together, our results provide the first demonstration of regulation of the BLV promoter by two B-cell-specific Ets transcription factors, PU.1 and Spi-B. The PU.1/Spi-B binding site identified here could play an important role in BLV replication and B-lymphoid tropism.

Increased cell proliferation, but not reduced cell death, induces lymphocytosis in bovine leukemia virus-infected sheep

Lymphocyte homeostasis is the result of a critical balance between cell proliferation and death. Disruption of this subtle equilibrium can lead to the onset of leukemia, an increase in the number of lymphocytes being potentially due to both of these parameters. The relative importance of cell proliferation vs. apoptosis during pathogenesis induced by the primate T cell lymphotropic viruses and bovine leukemia virus (BLV) has been difficult to assess because of conflicting data from a range of in vitro and ex vivo experimental systems. Here, we aim to resolve this issue by measuring the rates of cell proliferation and death in the BLV-ovine system, an animal model of human T lymphotropic virus (HTLV-1). We use a method based on the i.v. injection of 5-bromodeoxyuridine into BLV-infected sheep. We show that B lymphocytes in BLV(+) asymptomatic sheep proliferate significantly faster than in uninfected controls (average proliferation rate: 0.020 per day vs. 0.011 per day). In contrast, the rates of cell death were not significantly different between aleukemic BLV-infected and control sheep (average death rate 0.089 per day vs. 0.094 per day, respectively). We conclude that the increase in the number of B cells during BLV-induced lymphocytosis results from higher proliferation rates but is not due to a significant decrease in apoptosis, in contrast to data from in vitro (ex vivo) experiments. The imbalance created by the net increase in proliferation in the absence of compensating cell death reveals a complex mechanism of feedback regulation controlling homeostasis in the blood compartment.

Delayed emergence of bovine leukemia virus after vaccination with a protective cytotoxic T cell-based vaccine

In a previous study eight MHC class I-matched sheep were vaccinated with a minimal cytotoxic T lymphocyte (CTL) peptide epitope vaccine and were challenged with the retrovirus, bovine leukemia virus (BLV). Half the vaccinated animals remained PCR negative after challenge, whereas the remaining half and the placebo group became PCR positive within 4 weeks postchallenge (Hislop AD, Good MF, Mateo L, Gardner J, Gatei MH, Daniel RCW, Meyers BV, Lavin MF, and Suhrbier A: Nat Med 1998;4:1193). Here we show that neither epitope mutations nor processing differences explained why half the peptide-vaccinated animals failed to resist the BLV challenge. However, in these animals the development of BLV-induced lymphosarcomas was significantly delayed compared with the placebo group, suggesting a role for CTLs in preventing retrovirus-induced cancers. Importantly, two of the initially protected animals become PCR positive after approximately 1.5 years, indicating extended suppression but not elimination of challenge virus by vaccine-induced CTLs. The late emergence of virus could not be explained by epitope escape mutations or the loss of memory CTL responses. We speculate that high levels of effector CTL may be needed to protect animals from a postchallenge viremia and maintenance of such effector CTLs, rather than memory CTLs, may be required to prevent subsequent emergence of virus from latent pools.

Evaluation of virus excretion by cells persistently infected with the bovine leukaemia virus (BLV) using monoclonal antibodies

BACKGROUND

bovine leukaemia virus (BLV) is the causative agent of enzootic bovine leukaemia. Studies in vitro usually require the use of infected cell lines, mostly to produce antigen. Two of the most widely used cell lines are FLK-BLV and BLV-bat2.

OBJECTIVE

the dynamics of the excretion of BLV proteins and whole virus by the persistently BLV-infected cell lines mentioned above was studied using an indirect ELISA in combination with eight monoclonal antibodies (mAbs) and cow and rabbit serum.

STUDY DESIGN

tissue culture flasks were seeded with different concentrations of cells (13000-67000 cells per cm2, corresponding to 1-5 million cells per 75 cm2 flask) and were studied for 20 days. Samples (1.5 ml) were removed every 24 h and the presence of BLV proteins was determined using an indirect ELISA assay in which the antigen reaction with the monoclonal antibodies was evidenced by peroxidase labeled anti-mouse immunoglobulins.

RESULTS

cell line FLK-BLV produced a complete monolayer as early as 4 days after passage, 3 days earlier than BLV-bat2. Using mAbs, the amount of viral proteins in the supernatant showed a cyclic pattern, with two evident peaks at days ca. 8 and 16. These peaks occurred even in the absence of passage or medium change, which causes depletion of essential nutrients and acidity. In comparison to polyclonal serum, mAbs gave more clear and defined values and are useful for determining the dynamics of viral production.

CONCLUSION

when aiming for high viral yield, BLV should be harvested between days 6 and 8 after passage, when viral shedding is at its maximum. These results are very useful for preparing antigen for monoclonal antibody production, or for techniques such as ELISA or Western blot.

Comparative study of PCR as a direct assay and ELISA and AGID as indirect assays for the detection of bovine leukaemia virus

The choice of a diagnostic method depends on the characteristics of the herd to be analysed. Two herds with different prevalences of enzootic bovine leukaemia were chosen to study the concordance between agar gel immunodiffusion (AGID), enzyme-linked immunosorbent assay (ELISA) and polymerase chain reaction (PCR) methods. PCR, an increasingly used virological method, was performed with four sets of primers, amplifying different genomic regions (env, pol and tax), from DNA extracted either from peripheral blood monocytes (PBMCs) or milk leucocytes. The highest percentage of positive animals was obtained using PCR performed with DNA extracted from PBMCs using primers which amplified either env or pol, followed by PCR using PBMCs and primers which hybridized with tax, then ELISA using serum and finally AGID. The results of PCR were more consistent with PBMCs than when milk leucocytes were used.

The region between amino acids 245 and 265 of the bovine leukemia virus (BLV) tax protein restricts transactivation not only via the BLV enhancer but also via other retrovirus enhancers

Bovine leukemia virus (BLV) is associated with enzootic bovine leukosis and is closely related to human T-cell leukemia virus type 1 (HTLV-1). The Tax protein of BLV acts through the 5' long terminal repeat (LTR) of BLV and activates the transcription of BLV. In this study, we amplified tax genes from BLV-infected cattle using PCR. We cloned the genes and monitored the transcriptional activities of the products. Seven independent mutant Tax proteins, with at least one amino acid substitution between residues 240 and 265, exhibited a markedly stronger ability to stimulate the viral LTR-directed transcription than the wild-type Tax protein. Analysis of chimeric Tax proteins derived from wild-type and mutant Tax proteins clearly demonstrated that a single substitution between residue 240 and 265 might be critical for the higher activities of the Tax mutant proteins. Furthermore, it appeared that transient expression of a Tax mutant protein was better able to increase the production of viral proteins and particles from a defective recombinant proviral clone of BLV than was wild-type Tax. Analysis of mutations within the U3 region of the LTR revealed that a cyclic AMP-responsive element in Tax-responsive element 2 might be sufficient for the enhanced activation mediated by the mutant proteins. In addition to the LTR of BLV, other viral enhancers, such as the enhancers of HTLV-1 and of mouse mammary tumor virus, which cannot be activated by wild-type BLV Tax protein, were activated by a Tax mutant protein. Our observations suggest that the transactivation activity and target sequence specificity of BLV Tax might be limited or negatively regulated by the region of the protein between amino acids 240 and 265.

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

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

In vivo rescue of a silent tax-deficient bovine leukemia virus from a tumor-derived ovine B-cell line by recombination with a retrovirally transduced wild-type tax gene

The lack of bovine leukemia virus (BLV) expression is a consistent finding in freshly isolated ovine tumor cells and in the B-cell lines derived from these tumors. In order to gain further insight into the mechanisms of BLV silencing in these tumors, we have used the YR2 B-cell line, which was derived from the leukemic cells of a BLV-infected sheep. This cell line contains a single, monoclonally integrated, silent provirus, which cannot be reactivated either by stimulation in vitro or by in vivo injection of the tumor cells or cloned proviral DNA in sheep. Sequence analysis of the tax gene from the YR2 cell line identified two G-to-A transitions (G7924 to A7924 and G8149 to A8149) that result in E-to-K amino acid changes at positions 228 and 303 in the Tax protein. Following retroviral vector-mediated transfer of a wild-type tax gene into YR2 cells, we showed that BLV mRNA, viral proteins, and virions were produced, demonstrating that the cellular factors required for virus expression were present in the original YR2 cell line. Injection of this transduced YR2 cell line in sheep led to the rescue of replication-competent BLV proviruses. The integrated competent proviruses exhibited unique chimeric tax genes, which arose from homologous recombination between the transduced wild-type tax and the YR2-derived tax sequences. Furthermore, in one of these functional recombinant proviruses, only the A8149-to-G8149 reversion was present, providing clear evidence that the defect underlying the silent phenotype in YR2 cells results from a single C-terminal E303-to-K303 amino acid substitution in the BLV Tax protein. Our observations suggest that a single strategically located mutation in tax provides a mechanism for BLV inactivation in B-cell tumors.

Provirus variants of the bovine leukemia virus and their relation to the serological status of naturally infected cattle

Infection of cattle with the bovine leukemia virus (BLV) results in a strong permanent antibody response to the BLV antigens some weeks after infection. However, cattle may carry provirus and not have detectable antibody titers. To prove the occurrence of different BLV provirus variants in German cattle and to study the influence of special BLV variants on the immunoreaction, a 444-bp fragment of the env gene of 35 naturally BLV infected animals was analyzed. Seven different groups of BLV provirus variants were found on the basis of restriction fragment length polymorphism. Three BLV provirus variant groups and five additionally sequenced BLV isolates showed a high similarity to BLV provirus isolates from other geographical areas. The variation in nucleotide sequence of the five BLV isolates compared with nine previously sequenced BLV isolates ranged up to 5. 3%. While BLV provirus variant groups A, C, D, E, F, and G were clearly related to agar-gel immunodiffusion test (AGID)- and enzyme-linked immunosorbent assay (ELISA)-positive animals, BLV provirus variant group B was solely found in permanent AGID- and ELISA-negative or in transient ELISA-positive animals. Altogether, these results indicate that special BLV provirus variants may be responsible for atypical forms of BLV infection in cattle.

Fusion of bovine leukemia virus with target cells monitored by R18 fluorescence and PCR assays

PCR and R18 fluorescence dequenching assays have been combined to monitor the kinetics of fusion of bovine leukemia virus with target cells (CC81, OVK, or Raji). Antibodies raised against gp51 allow us to demonstrate that not only the hydrophobic N-terminal domain of the transmembrane glycoprotein gp30 but also specific domains of gp51 (amino acids 39 to 103) are involved in bovine leukemia virus-cell fusion.

Peripheral blood mononuclear cells from sheep infected with a variant of bovine leukemia virus synthesize envelope glycoproteins but fail to induce syncytia in culture

Peripheral blood mononuclear cells (PBMCs) infected with the oncogenic retrovirus bovine leukemia virus (BLV) produce virus when cultured briefly. BLV can be transmitted in cocultures to adherent susceptible cells, which become infected, express viral proteins, and fuse into multinucleated syncytia several days later. PBMCs from 3 of 10 BLV-infected sheep displayed a lifelong deficiency in induction of syncytium formation among indicator cells in culture, although large numbers of PBMCs synthesized viral transcripts or capsid protein. Since the infected, syncytium-deficient PBMCs were > or = 97% B cells, the deficiency could not be attributed to altered host cell tropism. The syncytium-deficient phenotype was recapitulated in newly infected sheep, demonstrating that this property is regulated by the viral genotype. The alteration in the BLV genome delayed but did not prohibit the establishment of BLV infection in vivo. Envelope glycoproteins were synthesized in syncytium-deficient PBMCs, translocated to the cell surface, and incorporated into virions. However, monoclonal antibodies specific for the BLV surface glycoprotein did not stain fixed PBMCs of the syncytium-deficient phenotype. Moreover, an animal with syncytium-deficient PBMCs had lower titers of neutralizing antibodies throughout the first 5 years of infection than an animal with similar numbers of infected PBMCs of the syncytium-inducing phenotype. The syncytium-deficient variant productively infected indicator cells at greatly reduced efficiency, showing that the alteration affects an early step in viral entry or replication. These results suggest that the alteration maps in the env gene or in a gene whose product affects the maturation or conformation, and consequently the function, of the envelope protein complex.

Forward mutation rate of human immunodeficiency virus type 1 in a T lymphoid cell line

An in vivo assay was previously developed for detecting forward mutations in human immunodeficiency virus type 1 (HIV-1) in a single cycle of replication. This system uses the lacZalpha peptide gene as a reporter for mutations, and allows for the rates and types of mutations that occur to be determined. The forward mutation rate for HIV-1 in HeLa cells was found to be 3 x 10(-5) mutations per target base pair per cycle. To test whether the mutation rate was influenced by cell type, the mutation rate of HIV-1 in CEM-A cells, a T lymphoid cell line, was determined. The mutation rate of HIV-1 reverse transcription in CEM-A cells was found to be 4 x 10(-5) mutations per target base pair per cycle. The number and types of mutations observed were similar to that in HeLa cells. Specifically, base substitution mutations predominated, and G-to-A transition mutations were the most common base substitution. G-to-A hypermutants were also characterized. The difference in HIV-1 mutation rate between HeLa and CEM-A cells was not significant, indicating that the accuracy of HIV-1 reverse transcription is comparable in both the HeLa and CEM-A cell lines.

Lower in vivo mutation rate of human immunodeficiency virus type 1 than that predicted from the fidelity of purified reverse transcriptase

The level of genetic variation of human immunodeficiency virus type 1 (HIV-1), a member of the lentivirus genus of the Retroviridae family, is high relative to that of retroviruses in some other genera. The high error rates of purified HIV-1 reverse transcriptase in cell-free systems suggest an explanation for this high genetic variation. To test whether the in vivo rate of mutation during reverse transcription of HIV-1 is as high as predicted by cell-free studies, and therefore higher than that rates of mutation of retroviruses in other genera, we developed an in vivo assay for detecting forward mutations in HIV-1, using the lacZ alpha peptide gene as a reporter for mutations. This system allows the rates and types of mutations that occur during a single cycle of replication to be studied. We found that the forward mutation rate for HIV-1 was 3.4 x 10(-5) mutations per bp per cycle. Base substitution mutations predominated; G-to-A transition mutations were the most common base substitution. The in vivo mutation rates for HIV-1 are three and seven times higher than those previously reported for two other retroviruses, spleen necrosis virus and bovine leukemia virus, respectively. In contrast, our calculated in vivo mutation rate for HIV-1 is about 20-fold lower than the error rate of purified HIV-1 reverse transcriptase, with the same target sequence. This finding indicates that HIV-1 reverse transcription in vivo is not as error prone as predicted from the fidelity of purified reverse transcriptase in cell-free studies. Our data suggest that the fidelity of purified HIV-1 reverse transcriptase may not accurately reflect the level of genetic variation in a natural infection.

Lack of LTR and ENV genetic variation during bovine leukemia virus-induced leukemogenesis

Genetic variation of the Bovine Leukemia Virus (BLV) appears to be limited in vitro and during the latent phase of the disease. However, cells in tumors often harbor deleted proviruses that are defective for expression. In order to gain insight into the involvement of viral genetic variation during pathogenesis, the BLV LTR and the env proviral sequences were analyzed in tumor tissues. A sheep (M230) was injected with the cloned BLV provirus 344 and became persistently infected with circulating lymphocytes reaching 345,000/mm3. After 11 months, this infected sheep developed leukemia-lymphoma. DNA was extracted from peripheral blood leukocytes at the time of tumor development and the LTR and the env gene were amplified, using the polymerase chain reaction procedure, cloned, and sequenced. Twenty independent LTR and twenty env clones were analyzed. It appeared that the in vivo mutation rate in the env gene was 0.043% (eight mutations including seven transitions out of 18,300 bp). Five point mutations (all transitions) were identified in the LTR, corresponding to 0.041% modifications (four mutations out of 9740 bp). These mutation rate values (0.043 and 0.041) were close to those due to the Taq DNA polymerase errors (0.030%). Altogether, these data demonstrate the lack of genetic variation in the LTR and the env gene during this case of BLV-induced pathogenesis in vivo. They confirm that the defectiveness of some BLV proviruses in vivo, thus, is not a mandatory step in the leukemogenic process.

Attenuation of bovine leukemia virus by deletion of R3 and G4 open reading frames

Complex oncoviruses contain, in addition to the classical retroviral genes (gag, pol, and env), a region (X) located between the envelope sequences and the 3' long terminal repeat. The X region contains two genes, tax and rex, whose protein products are involved in transcriptional and posttranscriptional regulation of viral expression. In addition to these activators, the bovine leukemia virus (BLV) and the human T-cell leukemia virus (HTLV) contain alternative open reading frames (R3 and G4 for BLV; p30, p13, and p12 for HTLV). As a virus/animal model for HTLV-induced leukemogenesis, BLV provirus can be injected intradermally into sheep, where it induced B-lymphocyte transformation. Deletion of the R3 and G4 sequences from an infectious and tumorigenic BLV provirus greatly impaired the in vivo propagation of the viruses as demonstrated by DNA polymerase chain reaction, RNA blots, structural-protein ELISA, and immunofluorescence analysis. Our results show that the alternative open reading frames are required for maintaining high virus loads during the course of persistent infection in vivo. Thus, R3 and G4 are candidates for antiviral drug development. Furthermore, viruses with a deletion in these sequences should be tested as live attenuated vaccines.

Familial transmission and minimal sequence variability of human T-lymphotropic virus type I (HTLV-I) in Zaire

Our group previously reported a strong familial clustering of HTLV-I-associated myelopathy/tropical spastic paraparesis (HAM/TSP) in Zaire, suggesting a familial transmission of the virus together with the presence of cofactors. In the present study among 84 relatives of 16 HTLV-I-positive or HAM/TSP index cases, we found that all 15 seropositive children had a seropositive mother and that all 15 children with a seropositive father but a seronegative mother were seronegative. Lymphocytes of 17 relatives from 2 families with a familial HTLV-I-associated neuropathy were tested in 2 polymerase chain reaction (PCR) assays amplifying pol and tax/rex gene fragments. The 10 seropositive individuals were PCR positive for HTLV-I and the 7 seronegatives were negative in both PCR assays. The PCR results showed no evidence for a long lag period between infection with HTLV-I and seroconversion. The HTLV-I long terminal repeat (LTR) of these 10 individuals, related in the first to the fourth degree, was amplified and sequenced. Identical sequences were found within the families except for one woman infected with two variants, one being the familial strain and the other a mutated one with a single nucleotide substitution in the 755 sequenced nucleotides of the LTR region. The family strain and the mutant were both present in two samples taken 1 year apart. Together, the HTLV-I serology, PCR, and sequencing results point toward mother-to-child transmission as the main mode of HTLV-I infection in this population. Comparison of the LTR sequences of the two families with other HTLV-I strains from different geographical regions shows that the Zairean HTLV-I strains form a separate cluster.(ABSTRACT TRUNCATED AT 250 WORDS)

Lower mutation rate of bovine leukemia virus relative to that of spleen necrosis virus

Genetic variation of the more complex retroviruses in the human T-cell leukemia virus/bovine leukemia virus (HTLV/BLV) group is less than in some other retroviral genera. To test whether reverse transcription of HTLV/BLV group members is less error prone than that of members of other groups, we developed an assay for detecting forward mutations in BLV, similar to that developed for the simpler spleen necrosis virus (SNV). We used this system to study the rates and types of mutations that occur during a single replication cycle. We found that BLV reverse transcription is approximately two and one-half times less error prone than SNV reverse transcription (4.8 x 10(-6) versus 1.2 x 10(-5) mutation per bp per cycle, respectively). The relative numbers of all types of observed mutations (that is, base pair substitutions, frameshifts, deletions, and deletions with insertions) were similar for BLV and SNV.

Pathogenicity of molecularly cloned bovine leukemia virus

To delineate the mechanisms of bovine leukemia virus (BLV) pathogenesis, four full-length BLV clones, 1, 8, 9, and 13, derived from the transformed cell line FLK-BLV and a clone construct, pBLV913, were introduced into bovine spleen cells by microinjection. Microinjected cells exhibited cytopathic effects and produced BLV p24 and gp51 antigens and infectious virus. The construct, pBLV913, was selected for infection of two sheep by inoculation of microinjected cells. After 15 months, peripheral blood mononuclear cells from these sheep served as inocula for the transfer of infection to four additional sheep. All six infected sheep seroconverted to BLV and had detectable BLV DNA in peripheral blood mononuclear cells after amplification by polymerase chain reaction. Four of the six sheep developed altered B/T-lymphocyte ratios between 33 and 53 months postinfection. One sheep died of unrelated causes, and one remained hematologically normal. Two of the affected sheep developed B lymphocytosis comparable to that observed in animals inoculated with peripheral blood mononuclear cells from BLV-infected cattle. This expanded B-lymphocyte population was characterized by elevated expression of B-cell surface markers, spontaneous blastogenesis, virus expression in vitro, and increased, polyclonally integrated provirus. One of these two sheep developed lymphocytic leukemia-lymphoma at 57 months postinfection. Leukemic cells had the same phenotype and harbored a single, monoclonally integrated provirus but produced no virus after in vitro cultivation. The range in clinical response to in vivo infection with cloned BLV suggests an important role for host immune response in the progression of virus replication and pathogenesis.

In vivo infection of sheep by bovine leukemia virus mutants

Direct inoculation of a cloned bovine leukemia virus (BLV) provirus into sheep has allowed study of the viral infectivity of genetic mutants in vivo. Three BLV variants cloned from BLV-induced tumors and 12 in vitro-modified proviruses were isolated and analyzed for viral expression in cell culture. The proviruses were then inoculated into sheep in order to assess viral infectivity in vivo. Of three variants cloned from BLV-induced tumors (344, 395, and 1345), one (344) was found infectious in vivo. This particular provirus was used to engineer 12 BLV mutants. A hybrid between the 5' region of the complete but noninfectious provirus 395 and the 3' end of mutant 344 was infectious in vivo, suggesting that the tax/rex sequences were altered in virus 395. As expected, several regions of the BLV genome appeared to be essential for viral infection: the protease, pol, and env genes. Even discrete modifications in the fusion peptide located at the NH2 end of the transmembrane gp30 glycoprotein destroyed the infectious potential. In contrast, mutations and deletions in the X3 region present between the env gene and the 3' tax/rex region did not interfere with viral infection in vivo. This region of unknown function could thus be used to introduce foreign sequences. A BLV recombinant carrying a ribozyme directed against the tax/rex sequences was still infectious in vivo. Cotransfection of two noninfectious mutants carrying deletions led to infection in two of four independent injections, the infectious virus being then a recombinant between the two deletants. The experimental approach described here should help to gain insight into essential mechanisms such as in vivo viral replication, cooperation between deletants for viral infectivity, and viral superinfections. The gene products in the X3 and X4 region which are dispensable for in vivo infection could be involved in leukemogenesis, and thus proviruses deleted in these sequences could constitute the basis for a live attenuated vaccine.