The nucleotide sequence of the env gene and post-env region of bovine leukemia virus

Genetic Variability of Bovine Leukemia Virus: Evidence of Dual Infection, Recombination and Quasi-Species

We have characterized the intrahost genetic variation in the bovine leukemia virus (BLV) by examining 16 BLV isolates originating from the Western Siberia-Tyumen and South Ural-Chelyabinsk regions of Russia. Our research focused on determining the genetic composition of an 804 bp fragment of the BLV env gene, encoding for the entire gp51 protein. The results provide the first indication of the quasi-species genetic nature of BLV infection and its relevance for genome-level variation. Furthermore, this is the first phylogenetic evidence for the existence of a dual infection with BLV strains belonging to different genotypes within the same host: G4 and G7. We identified eight cases of recombination between these two BLV genotypes. The detection of quasi-species with cases of dual infection and recombination indicated a higher potential of BLV for genetic variability at the intra-host level than was previously considered.

Genotypes diversity of env gene of Bovine leukemia virus in Western Siberia

BACKGROUND

This study describes the biodiversity and properties of Bovine leukemia virus in Western Siberia. This paper explores the effect of different genotypes of the env gene of the cattle leukemia virus on hematological parameters of infected animals. The researchers focused on exploring the polymorphism of the env gene and, in doing so, discovered the new genotypes I_a and I_b, which differ from genotype I. Several hypotheses on the origin of the different genotypes in Siberia are discussed.

RESULTS

We obtained varying length of the restriction fragments for genotypes I_. Additionally using restrictase Hae III were received fragments was named genotype I_a, and genotype I_b. There are 2.57 ± 0.55% (20 out of 779) samples of genotype I_b which does not differ significantly from 1% (χ² = 2.46). Other genotypes were observed in the cattle of Siberia as wild type genotypes (their frequency varied from 17.84 to 32.73%). The maximum viral load was observed in animals with the II and IV viral genotypes (1000-1400 viral particles per 1000 healthy cells), and the minimum viral load was observed animals with genotype I_b (from 700 to 900 viral particles per 1000 healthy cells).

CONCLUSIONS

The probability of the direct introduction of genotype II from South America to Siberia is extremely small and it is more likely that the strain originated independently in an autonomous population with its distribution also occurring independently. A new variety of genotype I (I_b) was found, which can be both a neoplasm and a relict strain.

Transient expression of a bovine leukemia virus envelope glycoprotein in plants by a recombinant TBSV vector

Plants offer a unique combination of advantages for the production of valuable recombinant proteins in a relatively short time. For instance, a variety of diagnostic tests have been developed that use recombinant antigens expressed in plants. The envelope glycoprotein gp51 encoded by Bovine leukemia virus (BLV) is one of the essential subunits for viral infectivity. It was indicated that the recombinant gp51 (rgp51) of BLV сan be used as an synthetic alternative antigen useful in the diagnosis of BLV infection in cattle. Here we evaluate the potential for using a viral vector based on the genome of Tomato bushy stunt virus (TBSV) for the efficient expression of BLV envelope glycoprotein rgp51 in Nicotiana benthamiana plants. The codon-optimized gene encoding rgp51 was synthesized by the de novo DNA synthesis to replace the GFP gene in the TBSV-derived viral vector that was then delivered into 4-5 week old N. benthamiana plants by agroinfiltration. Expression of recombinant his-tagged rgp51 was verified by protein extraction followed by western blot procedures, and by purification using Ni²⁺-affinity chromatography. The molecular weight of this plant-expressed rgp51 ranged from 43 to 55 kDa and it was shown to be glycosylated. Important for potential use in diagnostic tests, purified rgp51 specifically reacted with BLV infected bovine sera while no reaction was observed with the negative serum samples.

Detection and genotyping of bovine leukemia virus in Mexican cattle

Bovine leukemia virus (BLV) was detected and genotyped in a population of 201 dairy cattle from central Mexico. Using a commercial indirect enzyme-linked immunosorbent assay (iELISA) kit, 118 polymerase chain reaction (PCR)-positive and BLV antibody-positive samples were identified; the concordance between tests was substantial. A phylogenetic study of 27 partial sequences of the env gene gp30 was performed. Four mutations were detected involving the PXXP motif in the cytoplasmic domain of the transmembrane protein. This study provided evidence of the efficacy of PCR for the detection of BLV and demonstrated the presence of genotype 1 BLV in Mexico.

Molecular characterization of bovine leukemia virus from Moldovan dairy cattle

Bovine leukemia virus (BLV) is the causative agent of enzootic bovine leukosis (EBL), a disease that has worldwide distribution. Whilst it has been eradicated in most of Western Europe and Scandinavia, it remains a problem in other regions, particularly Eastern Europe and South America. For this study, in 2013, 24 cattle from three farms in three regions of Moldova were screened by ELISA and nested PCR. Of these cattle, 14 which were PCR positive, and these were molecularly characterized based on the nucleotide sequence of the env gene and the deduced amino acid sequence of the encoded gp51 protein. Our results demonstrated a low level of genetic variability (0-2.9%) among BLV field strains from Moldova, in contrast to that observed for other retroviruses, including human immunodeficiency virus (HIV) (20-38%) Mason IL (Trudy vologod moloch Inst 146–164, 1970) and equine infectious anemia virus (EIAV) (~40%) Willems L et al (AIDS Res Hum Retroviruses 16(16):1787–1795, 2000), where the envelope gene exhibits high levels of variation Polat M et al (Retrovirology 13(1):4, 2016). Sequence comparisons and phylogenetic analysis revealed that BLV genotype 7 (G7) is predominant in Moldova and that the BLV population in Moldovan cattle is a mixture of at least three new sub-genotypes: G7D, G7E and G4C. Neutrality tests revealed that negative selection was the major force operating upon the 51-kDa BLV envelope surface glycoprotein subunit gp51, although one positively selected site within conformational epitope G was detected in the N-terminal part of gp51. Furthermore, two functional domains, linear epitope B and the zinc-binding domain, were found to have an elevated ratio of nonsynonymous to synonymous codon differences. Together, these data suggest that the evolutionary constraints on epitopes G and B and the zinc-binding domains of gp51 differ from those on the other domains, with a tendency towards formation of homogenous genetic groups, which is a common concept of global BLV diversification during virus transmission that may be associated with genetic drift.

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.

A detailed molecular analysis of complete bovine leukemia virus genomes isolated from B-cell lymphosarcomas

It is widely accepted that the majority of cancers result from multiple cellular events leading to malignancy after a prolonged period of clinical latency, and that the immune system plays a critical role in the control of cancer progression. Bovine leukemia virus (BLV) is an oncogenic member of the Retroviridae family. Complete genomic sequences of BLV strains isolated from peripheral blood mononuclear cells (PBMC) from cattle have been previously reported. However, a detailed characterization of the complete genome of BLV strains directly isolated from bovine tumors is much needed in order to contribute to the understanding of the mechanisms of leukemogenesis induced by BLV in cattle. In this study, we performed a molecular characterization of BLV complete genomes from bovine B-cell lymphosarcoma isolates. A nucleotide substitution was found in the glucocorticoid response element (GRE) site of the 5' long terminal repeat (5'LTR) of the BLV isolates. All amino acid substitutions in Tax previously found to be related to stimulate high transcriptional activity of 5'LTR were not found in these studies. Amino acid substitutions were found in the nucleocapsid, gp51 and G4 proteins. Premature stop-codons in R3 were observed. Few mutations or amino acid substitutions may be needed to allow BLV provirus to achieve silencing. Substitutions that favor suppression of viral expression in malignant B cells might be a strategy to circumvent effective immune attack.

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.

Characterisation of env and gag gene fragments of bovine leukemia viruses (BLVs) from cattle in Turkey

The aim of this work was to investigate the molecular characteristics of bovine leukemia viruses (BLVs) in Turkey. The variability of env and gag fragments of BLVs was examined using DNA from blood samples obtained for sequence analysis of BLVs in four cattle herds from three different geographical areas in Turkey. The env gene sequences were highly similar to those of Brasilian, Argentine, and Japanese BLV strains, while gag genes from Turkish BLV isolates showed greatest similarity to those of Iranian isolates. This paper is the first report on the partial characterisation of env and gag genetic fragments of BLVs from Turkey.

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.

Intersubunit disulfide isomerization controls membrane fusion of human T-cell leukemia virus Env

Human T-cell leukemia virus (HTLV-1) Env carries a typical disulfide isomerization motif, C(225)XXC, in the C-terminal domain SU. Here we have tested whether this motif is used for isomerization of the intersubunit disulfide of Env and whether this rearrangement is required for membrane fusion. We introduced the C225A and C228A mutations into Env and found that the former but not the latter mutant matured into covalently linked SU-TM complexes in transfected cells. Next, we constructed a secreted Env ectodomain and showed that it underwent incubation-dependent intersubunit disulfide isomerization on target cells. However, the rearrangement was blocked by the C225A mutation, suggesting that C(225) carried the isomerization-active thiol. Still, it was possible to reduce the intersubunit disulfide of the native C225A ectodomain mutant with dithiothreitol (DTT). The importance of the CXXC-mediated disulfide isomerization for infection was studied using murine leukemia virus vectors pseudotyped with wild-type or C225A HTLV-1 Env. We found that the mutant Env blocked infection, but this could be rescued with DTT. The fusion activity was tested in a fusion-from-within assay using a coculture of rat XC target and transfected BHK-21 effector cells. We found that the mutation blocked polykaryon formation, but this could be reversed with DTT. Similar DTT-reversible inhibition of infection and fusion was observed when a membrane-impermeable alkylator was present during the infection/fusion incubation. We conclude that the fusion activity of HTLV-1 Env is controlled by an SU CXXC-mediated isomerization of the intersubunit disulfide. Thus, this extends the applicability of the isomerization model from gammaretroviruses to deltaretroviruses.

Natural genetic variations in bovine leukemia virus envelope gene: Possible effects of selection and escape

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. Here we report the addition of 28 new sequences to the current literature of 16 full-length BLV envelope gene sequences. The phylogenetic clustering, genotyping, and geographic distribution of BLV env variations corresponded in most cases. Most natural variations are mapped to the surface of the proposed conformational models of BLV gp51 N-terminus and gp30 external domain, overlapping with or adjacent to immunogenic epitopes. Analyses for evidence of possible selection pressures suggest the BLV env is under stringent negative selection overall, while strong positive selection is indicated for immunogenic epitope G. Natural env deletions bounded by similar flanking sequences were observed in multiple isolates and would result in truncated signal peptides, missing gp51, and aberrant coding frames for other proteins.

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.

Mechanisms of leukemogenesis induced by bovine leukemia virus: prospects for novel anti-retroviral therapies in human

In 1871, the observation of yellowish nodules in the enlarged spleen of a cow was considered to be the first reported case of bovine leukemia. The etiological agent of this lymphoproliferative disease, bovine leukemia virus (BLV), belongs to the deltaretrovirus genus which also includes the related human T-lymphotropic virus type 1 (HTLV-1). This review summarizes current knowledge of this viral system, which is important as a model for leukemogenesis. Recently, the BLV model has also cast light onto novel prospects for therapies of HTLV induced diseases, for which no satisfactory treatment exists so far.

Tax & rex: overlapping genes of the Deltaretrovirus group

Bovine leukemia virus and human T-cell leukemia viruses I and II, members of the Deltaretrovirus group, have two regulatory genes, tax and rex, that are coded in overlapping reading frames. We found that sequence variations in the rex gene of each virus result in amino acid differences significantly more often than variations in the tax gene. For all three viruses the highest ratio of non-synonymous to synonymous changes was found in the rex gene. In the overlapping regions of tax and rex, the second codon position of Rex corresponds to the third codon position of Tax. Nucleotide C was present in all genes of the three viruses at the highest frequency and this bias was most pronounced in the rex gene. More specifically we found that the C bias and nucleotide variation is greatest at the second codon position of Rex and the third codon position of Tax in the area of tax/rex overlap. Changes in the second codon position of Rex always resulted in amino acid change whereas changes in the third codon position of Tax resulted in amino acid changes less than a third of the time. Analysis of the amino acid frequencies in both proteins shows that there is a disproportionately large percentage of the amino acids alanine, proline, serine and threonine (the four amino acids whose second codon position is C) in Rex. These findings led us to hypothesize that the Rex protein can withstand more amino acid changes than can the Tax protein suggesting that the Tax protein experiences higher evolutionary constraints and is the more conserved of the two proteins.

Evaluation of a new antibody-based enzyme-linked immunosorbent assay for the detection of bovine leukemia virus infection in dairy cattle

The objective of this study was to validate a new blocking enzyme-linked immunosorbent assay (ELISA) (designated M108 for milk and S108 for serum samples) for detecting bovine leukemia virus (BLV) infection in dairy cattle. Milk, serum, and ethylenediaminetetraacetic acid-blood samples were collected from 524 adult Holstein cows originating from 6 dairy herds in Central Argentina. The M108 and S108 were compared with agar gel immunodiffusion (AGID), polymerase chain reaction and a commercial ELISA. Because there is currently no reference test capable of serving as a gold standard, the test sensitivity (SE) and specificity (SP) were evaluated by the use of a latent class model. Statistical inference was performed by classical maximum likelihood and by Bayesian techniques. The maximum-likelihood analysis was performed assuming conditional independence of tests, whereas the Bayesian approach allowed for conditional dependence. No clear conclusion could be drawn about conditional dependence of tests. Results with maximum likelihood (under conditional independence) and posterior Bayes (under conditional dependence) were practically the same. Conservative estimates of SE and SP (with 95% confidence intervals) for M108 were 98.6 (96.7; 99.6) and 96.7 (92.9; 98.8) and for S108 99.5 (98.2; 99.9) and 95.4 (90.9; 98.1), respectively. The ELISA 108 using either milk or serum to detect BLV-infected animals had comparable SE and SP with the official AGID and a commercial ELISA test, which are currently the most widely accepted tests for the serological diagnosis of BLV infection. Therefore, ELISA 108 can be used as an alternative test in monitoring and control programs.

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.

tax and rex Sequences of bovine leukaemia virus from globally diverse isolates: rex amino acid sequence more variable than tax

Bovine leukaemia virus (BLV) is an important agricultural problem with high costs to the dairy industry. Here, we examine the variation of the tax and rex genes of BLV. The tax and rex genes share 420 bases and have overlapping reading frames. The tax gene encodes a protein that functions as a transactivator of the BLV promoter, is required for viral replication, acts on cellular promoters, and is responsible for oncogenesis. The rex facilitates the export of viral mRNAs from the nucleus and regulates transcription. We have sequenced five new isolates of the tax/rex gene. We examined the five new and three previously published tax/rex DNA and predicted amino acid sequences of BLV isolates from cattle in representative regions worldwide. The highest variation among nucleic acid sequences for tax and rex was 7% and 5%, respectively; among predicted amino acid sequences for Tax and Rex, 9% and 11%, respectively. Significantly more nucleotide changes resulted in predicted amino acid changes in the rex gene than in the tax gene (P < or = 0.0006). This variability is higher than previously reported for any region of the viral genome. This research may also have implications for the development of Tax-based vaccines.

Establishment of a new bovine leukosis virus producing cell line

Due to the prevalence of different bovine leukosis virus (BLV) species in the cattle population in Europe, problems may arise in the serological diagnosis of BLV infections. In addition, earlier investigations demonstrated that contamination of the BLV antigen-producing cell culture systems by bovine viral diarrhea virus (BVDV) may give rise to misinterpretation of serological test results after BVDV vaccination of cattle. By co-cultivation of peripheral leukocytes of a BLV-infected cow with a permanent sheep kidney cell line, a new BLV-producing cell line named PO714 was established. This line carries a BLV provirus of the Belgian species and has been tested to be free of a variety of possibly contaminating viruses and mycoplasms. Investigations of a panel of well-characterised sera by agar gel immunodiffusion (AGID) and capture ELISA (cELISA) tests using antigen prepared from this new cell line in comparison with antigen of the well-known cell line FLK/BLV yielded comparable results. False positive results caused by BVDV cross-reactions could be eliminated when tests were carried out with antigen derived from the new cell line.

Dileucine and YXXL motifs in the cytoplasmic tail of the bovine leukemia virus transmembrane envelope protein affect protein expression on the cell surface

Several retroviruses downmodulate the cell surface expression of envelope (Env) proteins through peptide sequences located in the cytoplasmic tail of the transmembrane (TM) subunit. We investigated whether cell surface expression of a chimeric protein containing the cytoplasmic domain of the TM protein (CTM) of bovine leukemia virus (BLV) was regulated by two membrane-proximal dileucine motifs or by tyrosine Y487 or Y498 in YXXL motifs. A chimeric protein composed of the extracellular and membrane-spanning portions of human CD8-alpha plus a wild-type (wt) BLV CTM was detectable on the surface of only 40% of the cells in which it was transiently expressed. Replacement of either dileucine pair with alanines increased the level of surface display of chimeric proteins. Nearly all cells became surface positive when both dileucine motifs were altered simultaneously and when either an N-terminal segment containing both dileucine motifs or a C-terminal segment containing all YXXL motifs was deleted. In contrast, replacement of Y487 or Y498 with alanine or phenylalanine enabled only small increases in surface display compared with wt levels. Chimeric proteins had similar stabilities but were downmodulated from the cell surface at three different rates. Point mutants segregated into each of the three groups of proteins categorized according to these different rates. Interestingly, Y487 mutants were downmodulated less efficiently than Y498 mutants, which behaved like wt. CD8-CTM chimeric proteins were phosphorylated on serine residues, but the native BLV Env protein was not phosphorylated either in transfected cells or in a lymphoid cell line constitutively producing BLV. Thus, both dileucine and YXXL motifs within the BLV CTM contribute to downmodulation of a protein containing this domain. Interactions with other proteins may influence surface exposure of Env protein complexes in virus-infected cells, assisting in viral evasion of adaptive immunity.

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.

Envelope proteins containing single amino acid substitutions support a structural model of the receptor-binding domain of bovine leukemia virus surface protein

Functional domains of the strikingly conserved envelope (Env) glycoproteins of bovine leukemia virus (BLV) and its close relative, human T-cell leukemia virus type 1 (HTLV-1), are still being defined. We have used BLV Env protein variants to gain insights into the structure and function of this important determinant of viral infectivity. Each of 23 different single amino acid variants found in cDNA clones of env transcripts present after short-term culture of peripheral blood mononuclear cells from BLV-infected sheep was expressed in COS-1 cells and tested for the ability to mediate cell fusion and to be cleaved to surface (SU) and transmembrane (TM) protein subunits. Of 11 Env variants that failed to induce syncytia or did so poorly, 7 contained changes in amino acids identical or chemically conserved in the HTLV-1 Env protein. These seven included the four variants that showed aberrant proteolytic cleavage and poor cell surface expression, underscoring their importance for Env structure. Ten of 12 variants that retained wild-type syncytium-inducing ability clustered in the N-terminal half of BLV SU, which forms the putative receptor-binding domain (RBD). Several variants in the RBD showed evidence of subtle misfolding, as judged by reduced binding to monoclonal antibodies recognizing conformational epitopes F, G, and H formed by the N terminus of SU. We modeled the BLV RBD by aligning putative structural elements with known elements of the ecotropic Friend murine leukemia virus RBD monomer. All the variant RBD residues but one are exposed on the surface of this BLV model. These variants as well as function-altering, antibody-reactive residues defined by other investigators group on one face of the molecular model. They are strikingly absent from the opposite face, implying that it is likely to face inward in Env complexes. This surface might interact with the C-terminal domain of SU or with an adjacent monomer in the Env oligomer. This location suggests an orientation for the monomer of ecotropic Friend murine leukemia virus RBD.

Partial sequencing of env gene of bovine leukaemia virus from Brazilian samples and phylogenetic analysis

Analysis of the partial bovine leukaemia virus (BLV) env gp51 gene sequences obtained from three BLV strains isolated in three different regions of Brazil was carried out. The Brazilian BLV env gp51 sequences were compared with seven other corresponding sequences of BLV strains isolated in different countries and with consensus sequence as well. The obtained data point on qualitative and quantitative differences among the analysed strains as far as the occurrence of single point mutations is concerned. Two Brazilian strains show significantly higher mutation rate than other analysed strains. Amino acid analysis did not show, however, any substantial changes of the primary protein structure coded by well conserved region of BLV env gp51 gene. Based on the obtained data, the putative dendogram image of possible phylogenetic relations among the studied BLV strains is presented as well.

Genetic heterogeneity among bovine leukemia virus genotypes and its relation to humoral responses in hosts

The existence of bovine leukemia virus (BLV) genotypes was investigated by restriction fragment length polymorphism (RFLP) analysis using bovine peripheral blood leukocytes collected from different geographical areas of Japan. For this purpose a nested polymerase chain reaction (PCR) for a 444 bp fragment of the envelope (env) gene was used because it was previously reported that this region might be responsible for the serological status in the host. The PCR products from 60 samples of BLV-infected cells were digested with endonucleases BamH I, Bgl I, Bcl I, Hae III and Pvu II. RFLP analysis demonstrated that there were six different genotypes of BLV present among cattle in Japan. In some herds PCR-positive animals were infected with only one genotype, but in other herds a few genotypes were found. One genotype was dominant throughout infected cattle and it was also detected in neoplastic cells from three of four animals with lymphosarcoma and three cell lines persistently infected with BLV. Production of antibodies to BLV in each cattle was surveyed by agar gel immunodiffusion and indirect hemagglutination tests, and the results were compared with those obtained from PCR. No genotype related to decreased immunoreactivity was detected. The difference in anti-viral immune responses of each animal appears to be related to the infection stage and other host factors, not to genetic heterogeneity of the envelope gene.

An outbreak of enzootic bovine leukosis in upper Egypt: clinical, laboratory and molecular-epidemiological studies

In 1989, 220 Holstein Friesian cattle (212 heifers and eight bulls) were imported from Minnesota, USA, to form a closed dairy herd in Arab El-Aoumar, Assiut, Upper Egypt. In November 1996, some abnormal signs such as loss of weight, decreased milk yield, external lymphadenopathy and decreased appetite were observed on this farm. Serological screening by enzyme-linked immunosorbent assay revealed a seroprevalence of antibodies directed against bovine leukaemia virus (BLV) of 37.7% in cattle under 2 years old and of 72.8% in animals more than 2 years old. Diagnosis was confirmed by the detection of BLV proviral DNA using polymerase chain reaction with primers amplifying a fragment of the env gene. Out of 21 tested leucocyte fractions from individual animals, 15 were positive showing a BLV-specific amplicon of 444 base pairs. Analysis of the amplicons for restriction fragment length polymorphisms and DNA sequencing results allowed the isolates to be typed. Since this was the first recorded case of enzootic bovine leukosis in Upper Egypt, strict quarantine measures were adopted and all serologically positive animals in the herd were culled.

Role of the proline-rich motif of bovine leukemia virus transmembrane protein gp30 in viral load and pathogenicity in sheep

The cytoplasmic tail of bovine leukemia virus (BLV) transmembrane protein gp30 has multiple amino acid motifs that mimic those present in signaling proteins associated with B-cell and T-cell receptors. The proline-rich motif of gp30, PX(2)PX(4-5)P, is analogous to the recognition site of Src homology 3 (SH3) domains of signaling molecules. Using site-directed mutagenesis of an infectious molecular clone of BLV, point mutations were introduced which changed three of the prolines of the motif to alanines. The influence of these mutations on the pathogenicity of BLV was studied in sheep which received either (i) plasmid DNA with provirus containing proline-to-alanine mutations (pppBLV), (ii) plasmid DNA with wild-type provirus (wtBLV), or (iii) transfection reagent alone. Although all of the BLV-injected animals seroconverted at approximately the same time, viral loads at later time points were high in five of five of the wtBLV group and two of five of the pppBLV group but low in three of five of the pppBLV group, as determined by semiquantitative PCR. Viral expression was lower in the pppBLV-transfected sheep, as measured by p24 antigen enzyme-linked immunosorbent assay in cultured cells, and serologic titers were lower. Thirty-one months after transfection, four of four wtBLV-transfected sheep had died of leukemia and lymphoma, and all five of the pppBLV-transfected sheep were clinically healthy and had normal peripheral blood lymphocyte counts. These data indicate that the proline-rich motif of gp30 is not required for viral infectivity but is important for high viral load in vivo, suggesting that SH3-mediated gp30 interactions are critical for viral pathogenesis following infection. Absence of interactions with the proline-rich motif may prevent or delay tumorigenesis in sheep.

A nucleotide deletion causing a translational stop in the protease reading frame of bovine leukaemia virus (BLV) results in modified protein expression and loss of infectivity

Bovine leukaemia virus (BLV) is an oncogenic retrovirus that causes B-cell lymphocytosis and in the terminal stage of the disease lymphosarcoma. The comparison of the previously published BLV provirus sequence from Belgium, Australia and Japan showed that the protease gene (prt) of the Australian and the Japanese isolate contain a nucleotide deletion when compared to the Belgian isolate. Because all these proviruses were isolated from tumour tissue, the prt gene of functionally active and infectious proviruses from peripheral blood leucocytes (PBLs) of BLV-infected cattle and from BLV-infected fetal lamb kidney cells were sequenced. The only variations between these sequences and the Belgian isolate consist of nucleotide substitutions. The delection of one nucleotide of the prt gene of the Japanese and the Australian BLV tumour isolate caused a changed reading frame and a premature translational stop. It was shown that the Japanese provirus is non-infectious in transfected cell culture and in injected sheep. To analyse the impact of the prt mutation on viral protein expression and infectivity, the prt region of the Japanese provirus was exchanged with the prt region from the Belgian provirus. The resulting pBLVprtbelg was infectious in transfected cells and enabled the expression of gag and gag-precursor proteins. One sheep was injected with this mutated provirus and became positive in BLV-PCR, but no seroconversion was developed. The prt mutation of the Japanese tumour isolates was shown to be responsible for the loss of infectivity and changed viral expression. These results and the occurrence of this mutation in only two isolates from lymphosarcoma indicate a possible relation between the prt mutation and the induction of cell transformation.

The SU and TM envelope protein subunits of bovine leukemia virus are linked by disulfide bonds, both in cells and in virions

After the polyprotein precursor of retroviral envelope proteins is proteolytically cleaved, the surface (SU) and transmembrane (TM) subunits remain associated with each other by noncovalent interactions or by disulfide bonds. Disulfide linkages confer a relatively stable association between the SU and TM envelope protein subunits of Rous sarcoma virus and murine leukemia virus. In contrast, the noncovalent association between SU and TM of human immunodeficiency virus leads to significant shedding of SU from the surface of infected cells. The SU and TM proteins of bovine leukemia virus (BLV) initially were reported to be disulfide linked but later were concluded not to be, since TM is often lost during purification of SU protein. Here, we show that SU and TM of BLV do, indeed, associate through disulfide bonds, whether the envelope proteins are overexpressed in transfected cells, are produced in virus-infected cells, or are present in newly produced virions.

Assessment of bovine leukemia virus transcripts in vivo

Reverse transcriptase PCR (RT-PCR) consistently detected bovine leukemia virus transcripts in fresh cells, and competitive RT-PCR enumerated these transcripts. The detection of transcripts in limited numbers of tumor cells indicated that expression occurs in a minority of cells. The data suggest that individual cells contain hundreds of copies of the tax/rex transcript in vivo.

A rapid and sensitive diagnosis of bovine leukaemia virus infection using the nested shuttle polymerase chain reaction

Bovine leukaemia virus (BLV) is the causative agent of enzootic bovine leukosis (EBL). In Argentina, where a program to eradicate EBL has been introduced, sensitive and reliable diagnosis has attained high priority. Although the importance of the agar gel immunodiffusion test remains unchanged for routine work, an additional diagnostic technique is necessary to confirm cases of sera with equivocal results or of calves carrying maternal antibodies.Utilizing a nested shuttle polymerase chain reaction, the proviral DNA was detected from cows experimentally infected with as little as 5 ml of whole blood from BLV seropositive cows that were nonetheless normal in haematological terms. It proved to be a very sensitive technique, since it rapidly revealed the presence of the provirus, frequently at 2 weeks postinoculation and using a two-round procedure of nested PCR taking only 3 hours. Additionally, the primers used flanked a portion of the viral genome often employed to differentiate BLV type applying BamHI digestion. It is concluded that this method might offer a highly promising diagnostic tool for BLV infection.

Characterization of the internal promoter of human T-cell leukemia virus type I

The HTLV-I provirus contains two different promoters: the classical retroviral promoter in the 5' long terminal repeat (LTR) and our previously identified second promoter in the pol gene just upstream of the ATG codon of the tax gene. Here, we demonstrated that the internal promoter expresses the gene for Tax but not Rex. As the deletion of upstream of the transcriptional initiation site (nt 5130) caused down-regulation of the promoter activity, we termed the region HTLV-I internal regulatory element (HIRE). We found a cellular sequence-specific DNA binding protein which binds to HIRE. Furthermore, we demonstrated that the 3' LTR regulates Tax expression from the internal promoter. These findings may shed light on a novel mechanism for gene expression in complex retroviruses of the HTLV family.

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.

Localization of the labile disulfide bond between SU and TM of the murine leukemia virus envelope protein complex to a highly conserved CWLC motif in SU that resembles the active-site sequence of thiol-disulfide exchange enzymes

Previous studies have indicated that the surface (SU) and transmembrane (TM) subunits of the envelope protein (Env) of murine leukemia viruses (MuLVs) are joined by a labile disulfide bond that can be stabilized by treatment of virions with thiol-specific reagents. In the present study this observation was extended to the Envs of additional classes of MuLV, and the cysteines of SU involved in this linkage were mapped by proteolytic fragmentation analyses to the CWLC sequence present at the beginning of the C-terminal domain of SU. This sequence is highly conserved across a broad range of distantly related retroviruses and resembles the CXXC motif present at the active site of thiol-disulfide exchange enzymes. A model is proposed in which rearrangements of the SU-TM intersubunit disulfide linkage, mediated by the CWLC sequence, play roles in the assembly and function of the Env complex.

A novel human T-leukemia virus type 1 cell-to-cell transmission assay permits definition of SU glycoprotein amino acids important for infectivity

Human T-leukemia virus type 1 (HTLV-1) envelope glycoproteins play a major role in viral transmission, which in the case of this virus occurs almost exclusively via cell-to-cell contact. Until very recently, the lack of an HTLV-1 infectivity assay precluded the determination of the HTLV-1 protein domains required for infectivity. Here, we describe an assay which allows the quantitative evaluation of HTLV-1 cell-to-cell transmission in a single round of infection. Using this assay, we demonstrate that in this system, cell-to-cell transmission is at least 100 times more efficient than transmission with free viral particles. We have examined 46 surface (SU) glycoprotein mutants in order to define the amino acids of the HTLV-1 SU glycoprotein required for full infectivity. We demonstrate that these amino acids are distributed along the entire length of the SU glycoprotein, including the N-terminus and C-terminus regions, which have not been previously defined as being important for HTLV-1 glycoprotein function. For most of the mutated glycoproteins, the capacity to mediate cell-to-cell transmission is correlated with the ability to induce formation of syncytia. This result indicates that the fusion capacity is the main factor responsible for infectivity mediated by the HTLV-1 SU envelope glycoprotein, as is the case for other retroviral glycoproteins. However, other factors must also intervene, since two of the mutated glycoproteins were correctly fusogenic but could not mediate cell-to-cell transmission. Existence of this phenotype shows that capacity for fusion is not sufficient to confer infectivity, even in cell-to-cell transmission, and could suggest that postfusion events involve the SU.

Evaluation of polymerase chain reaction (PCR) application in diagnosis of bovine leukaemia virus (BLV) infection in naturally infected cattle

The practical application of polymerase chain reaction (PCR) for the diagnosis of bovine leukaemia virus (BLV) infections in naturally infected cattle was evaluated. Compared to serological tests the PCR was definitely found to be a more sensitive method, yielding the highest number of positive results (10% more compared to enzyme-linked immunosorbent assay, (ELISA), and 17.7% more compared to agar-gel immunodiffusion, (AGID)). In testing cattle from herds with BLV incidence under 5%, out of 52 provirus positive cattle only 43 were correctly identified by ELISA. When compared to AGID only 37 of the 52 PCR positive animals were correctly identified. Of 18 cattle imported from the Slovak Republic and kept in a quarantine stable, four were found to be BLV provirus positive by PCR, while serological tests indicated one animal positive and three negative. Therefore, it is impossible to prevent the spread of the infection from one country to another by serological testing only. Moreover, it is feasible to identify animals with changing antibody titres correctly by PCR. Using PCR we were also able to distinguish BLV infected from uninfected calves that were serologically positive due to colostral antibodies. Higher sensitivity of BLV provirus detection by PCR was achieved using env gene rather than tax gene specific primers. Negative results by PCR in cases of positive serological reactions are still possible, as shown in case of one adult animal. These findings indicate that PCR is a highly sensitive method and might be successfully used and economically advantageous for different practical applications in detection of BLV infection in naturally infected cattle.

Molecular cloning of bovine mb-1 cDNA

Ig-alpha of the B-cell antigen receptor complex forms a heterodimeric structure with Ig-beta on the plasma membrane of B-lymphocytes and is apparently involved in signal transduction during the activation of B-cells. Bovine leukemia virus (BLV) is predominantly a B-cell tropic retrovirus, which induces persistent lymphocytosis and leukemia/lymphoma of B-cell lineage in cattle. To understand the mechanisms of proliferation and tumorigenesis of bovine B-cells that are associated with BLV infection, we investigated the B-cell antigen receptor complex, especially bovine mb-1 encoding the bovine Ig-alpha protein. We isolated a full-length bovine mb-1 cDNA clone encoding 223 amino acid residues. The deduced amino acid sequence of the bovine mb-1 showed extensive homology with those of human and murine mb-1. The cytoplasmic tail of the bovine mb-1 also contained a consensus motif (D/E-X7-D/E-X2-L/I-X7-Y-X2-L/I) that may interact with the SH2 domain of src-type kinase. Interestingly, a similar consensus sequence motif was found in the BLV gp30env, although the overall sequence similarity between bovine mb-1 and BLVgp30 was not significant. Furthermore, elevated levels of mb-1 transcript were detected in various bovine leukemia/lymphoma cell lines. These results indicated that the proliferation of B-cells associated with BLV-infection may be related to abnormal signal transduction through the B-cell antigen receptor complex.

Molecular characterization of a variant of proviral bovine leukaemia virus (BLV)

Southern-blot hybridization and partial sequencing of the pol and env genes were used to characterize BLV-integrated provirus of seropositive cattle from two dairy herds in northern Italy. Comparison of the data obtained with those of previously characterized BLV strains from other geographic areas (Australia, Belgium, Japan and USA) revealed the presence of a viral variant (BLV-12), which showed both conserved and unique features. Regarding the gp51 envelope glycoprotein, the BLV-12 variant showed: 1. A high extent of conservation, which included potential glycosylation sites and cysteine residues; 2. Three unique amino acid residues not present in any of the other BLV strains analysed; and 3. Some variability at the level of one (G) of the three (F, G and H) conformational epitopes, which is probably important in the process of infection. These results agree with the suggestion that the sequence variability of the gp51 glycoprotein preferentially involves structures whose change is thought to underlie the phenomenon of escape from immune surveillance.

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.

Mechanisms of Tax Regulation of Human T Cell Leukemia Virus Type I Gene Expression

During the last several years, the human T cell leukemia virus type I (HTLV-I) has become recognized as an important cause for public health concern throughout the world. HTLV-I is the causative agent of a variety of clinical diseases, including an aggressive lymphoproliferative disorder named adult T cell leukemia. HTLV-I induces pathogenicity in the infected host cell through the synthesis of a virally encoded protein called Tax. Expression of Tax is critical to the life cycle of the virus, as the protein greatly increases the efficiency of HTLV-I gene transcription and replication. Furthermore, Tax has been shown to deregulate the transcription of many cellular genes, leading to the hypothesis that the presence of Tax promotes unchecked growth in the HTLV-I-infected cell. The mechanism of Tax trans-activation of HTLV-I gene expression is not known. Tax does not bind directly to the Tax-responsive promoter elements of the virus, but appears to function through interaction with certain cellular DNA binding proteins, including activating transcription factor 2 and cAMP response element binding protein that recognize these sequences. This review summarizes some of the recent work in the field aimed at elucidating the mechanism of Tax trans-activation of HTLV-I gene expression. Copyright 1995 S. Karger AG, Basel

Identification of functional regions in the human T-cell leukemia virus type I SU glycoprotein

Single conservative and nonconservative amino acid substitutions were introduced into the gp45 external envelope protein (SU) of human T-cell leukemia virus type I (HTLV-I). The mutated amino acids were those identified as being conserved in HTLV-I, HTLV-II, and simian T-cell leukemia virus type I (but not in bovine leukemia virus). The mutated envelopes were tested for intracellular maturation and for function. Mutants with three major phenotypes could be defined: (i) 9 mutants with a wild-type phenotype, which included most of the conservative amino acid changes (five of seven) distributed throughout the SU protein; (ii) 8 mutants with affected intracellular maturation, 6 of which define a region in the central part of the SU protein essential for correct folding of the protein; and (iii) 13 mutants with normal intracellular maturation but impaired syncytium formation. These mutations likely affect the receptor binding step or postbinding events required for fusion. Five of these mutations are located between amino acids 75 and 101 of the SU protein, in the amino-terminal third of the molecule. The other mutations involve positions 170, 181, 195, 197, 208, 233, and 286, suggesting that two other domains, one central and one carboxy terminal, are involved in HTLV-I envelope functions.

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.

Detection of bovine leukemia virus proviral DNA in individual cells

We have developed a method of analyzing individual cells to detect proviral DNA of the bovine leukemia virus (BLV) using flow cytometry and PCR. Individual cells of the BL3* cell line, which contain multiple integrated copies of the BLV provirus, and the uninfected cell line BL3(0), were sorted into wells of a 96-well plate. Following cell lysis, portions of the BLV envelope (ENV) and cellular prolactin (PRL) genes were amplified simultaneously using PCR. Viral and cellular products of first-round PCR were amplified separately in a second round of PCR using "heminested" primers. Separation of the PCR products by polyacrylamide gel electrophoresis yielded distinct fragments of the predicted sizes. The operational sensitivity of this method for the detection of virus was > 90% when testing single infected cells. In addition, we were able to reliably amplify DNA from a single BL3* cell among as many as 10(5) BL3(0) cells and established that the sensitivity for detecting a single infected cell among 20, 100, or 1000 uninfected cells was at least 90%. Estimates of low percentages of infected cells were obtained by applying probability theory to results of experiments conducted on wells containing more than one cell. Using these methods, B lymphocytes obtained from the peripheral blood of BLV-infected cattle were tested for proviral DNA. BLV ENV was identified in 76.9 +/- 4.9% of single B cells tested from a seropositive animal with persistent lymphocytosis (PL), but in only 0.033 +/- 0.009% of B cells from another seropositive cow without PL.(ABSTRACT TRUNCATED AT 250 WORDS)

gp51 of bovine leukemia virus gene expression in hamster cells

Recombinant pMMEx-bovine leukemia virus env gene DNA fragments were produced and expressed in eukaryotic cells. Clone C4, containing an SmaI-SmaI fragment of the gene coding for gp51, was co-transfected with pSV2neo DNA into Chinese hamster cells. About 800 geneticin-resistant cell clones were isolated and then morphologically and biologically characterized. The presence of gp51 encoding env gene fragments was detected in 17 of them by Southern blotting. The expression of gp51 gene in hamster cells was confirmed by Western blotting of their lysates with monoclonal antibodies (MAbs) directed against different epitopes of gp51 of bovine leukemia virus. The immunoreactivity of the expressed peptides with MAbs directed against neutralizing epitopes of gp51 of bovine leukemia virus was confirmed.

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

Intradermal injection of a cloned bovine leukemia virus (BLV) provirus (pV344) into sheep allowed direct evaluation of intrastrain variability. A sheep was injected with pV344 DNA mixed with DEAE-dextran and became persistently infected with BLV strain 344. After 18 months, DNA was extracted from peripheral blood leukocytes from a single 0.5-ml blood sample. The long terminal repeat (LTR) and the env gene were amplified by using the polymerase chain reaction, cloned, and sequenced. Nineteen independent LTR clones (0.6-kb inserts) and 16 env clones (1-kb inserts) were analyzed. The in vivo rate of nucleotide change was 0.009%/year (two mutations out of 14,464 bp in 1.5 years), corresponding to only one amino acid change in the env gene. Five point mutations (all transitions), corresponding to a modification rate of 0.034%/year (five mutations out of 9,709 bp in 1.5 years), were identified in the LTR. As a control for Taq DNA polymerase errors, the same procedure using pV344 plasmid DNA was carried out. Out of 9,944 bp sequenced, three point mutations were found (i.e., one misincorporation in 3,315 nucleotides). These data demonstrate the extremely low level (or absence) of intrastrain variability of BLV in vivo. Consequently, BLV persistence in the infected host does not seem to result from an escape mutant strategy, in sharp contrast with the high mutation rates observed in the lentivirus family. The lack of genetic variation supports the possibility of successful vaccine against BLV and probably against the related human T-cell leukemia viruses.

Identification of alternatively spliced mRNAs encoding potential new regulatory proteins in cattle infected with bovine leukemia virus

The polymerase chain reaction was used to detect and characterize low-abundance bovine leukemia virus (BLV) mRNAs. In infected cattle we could detect spliced mRNA with a splice pattern consistent with a Tax/Rex mRNA, as well as at least four alternatively spliced RNAs. Two of the alternatively spliced mRNAs encoded hitherto unrecognized BLV proteins, designated RIII and GIV. The Tax/Rex and alternatively spliced mRNAs could be detected at their highest levels in BLV-infected cell cultures; the next highest levels were found in samples from calves experimentally infected at 6 weeks postinoculation. Alternatively spliced mRNAs were also expressed, albeit at lower levels, in naturally infected animals; they were detected by a nested polymerase chain reaction. Interestingly, the GIV mRNA was specifically detected in naturally infected cows with persistent lymphocytosis and in two of five calves at 6 months after experimental infection with BLV. Furthermore, the calf with the strongest signal for GIV had the highest lymphocyte counts. These data may suggest a correlation between expression of the GIV product and development of persistent lymphocytosis. Some of the donor and acceptor sites in the alternatively spliced mRNAs were highly unusual. The biological mechanisms and significance of such a choice of unexpected splice sites are currently unknown.