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

Molecular Characterization of Bovine Leukemia Virus with the Evidence of a New Genotype Circulating in Cattle from Kazakhstan

Bovine leukemia virus (BLV) is a retrovirus that causes enzootic bovine leukosis (EBL) and has worldwide distribution. Infections with BLV have been reported in cattle from Kazakhstan but the virus has not yet been thoroughly characterized. In this study, we detect and estimate the level of BLV proviral DNA by qPCR in DNA samples from 119 cattle naturally infected with BLV, from 18 farms located in four different geographical regions of Kazakhstan. Furthermore, we conducted the phylogenetic and molecular analysis of 41 BLV env-gp51 gene sequences from BLV infected cattle. Phylogenetic analysis showed the affiliation of sequences to two already known genotypes G4 and G7 and also to a new genotype, classified as genotype G12. In addition, a multivariate method was employed for analysis of the association between proviral load and different variables such as the geographical location of the herd, cattle breeds, age of animals, and the presence of particular BLV genotypes. In summary, the results of this study provide the first evidence on molecular characterization of BLV circulating in cattle from Kazakhstan.

Bovine Leukaemia Virus: Current Epidemiological Circumstance and Future Prospective

Bovine leukaemia virus (BLV) is a deltaretrovirus that is closely related to human T-cell leukaemia virus types 1 and 2 (HTLV-1 and -2). It causes enzootic bovine leukosis (EBL), which is the most important neoplastic disease in cattle. Most BLV-infected cattle are asymptomatic, which potentiates extremely high shedding rates of the virus in many cattle populations. Approximately 30% of them show persistent lymphocytosis that has various clinical outcomes; only a small proportion of animals (less than 5%) exhibit signs of EBL. BLV causes major economic losses in the cattle industry, especially in dairy farms. Direct costs are due to a decrease in animal productivity and in cow longevity; indirect costs are caused by restrictions that are placed on the import of animals and animal products from infected areas. Most European regions have implemented an efficient eradication programme, yet BLV prevalence remains high worldwide. Control of the disease is not feasible because there is no effective vaccine against it. Therefore, detection and early diagnosis of the disease are essential in order to diminish its spreading and the economic losses it causes. This review comprises an overview of bovine leukosis, which highlights the epidemiology of the disease, diagnostic tests that are used and effective control strategies.

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.

Phylogenetic Analysis of South African Bovine Leukaemia Virus (BLV) Isolates

Bovine leukaemia virus (BLV) causes chronic lymphoproliferative disorder and fatal lymphosarcoma in cattle, leading to significant economic losses in the beef and dairy industries. BLV is endemic globally and eleven genotypes have been identified. To date, only Zambian isolates have been genotyped from Africa. Although high BLV prevalence has been reported in South Africa, there has been no molecular characterisation of South African BLV isolates. To characterise BLV isolates in South Africa for the first time, we investigated the phylogenetic relationships and compared the genetic variability of eight South African BLV isolates with BLV isolates representing the eleven known genotypes from different geographical regions worldwide. Phylogenetic analyses based on full-length and partial env sequences as well as full-length gag sequences revealed that at least two genotypes, genotypes 1 (G1) and 4 (G4), are present in cattle in South Africa, which is consistent with studies from Zambia. However, our analysis revealed that the G1 South African isolate is more similar to other G1 isolates than the G1 Zambian isolates whereas, the G4 South African isolates are more divergent from other G4 isolates but closely related to the G4 Zambian isolate. Lastly, amino acid sequence alignment identified genotype-specific as well as novel amino acid substitutions in the South African isolates. The detection of two genotypes (G1 and G4) in southern Africa highlights the urgent need for disease management and the development of an efficacious vaccine against local strains.

PRMT5 Is Required for Bovine Leukemia Virus Infection In Vivo and Regulates BLV Gene Expression, Syncytium Formation, and Glycosylation In Vitro

Bovine leukemia virus (BLV) is the causative agent of enzootic bovine leukosis, which is the most common neoplastic disease of cattle and is closely related to human T-cell leukemia viruses. We investigated the role of a new host protein, PRMT5, in BLV infection. We found that PRMT5 is overexpressed only in BLV-infected cattle with a high proviral load, but not in those with a low proviral load. Furthermore, this upregulation continued to the lymphoma stage. PRMT5 expression was upregulated in response to experimental BLV infection; moreover, PRMT5 upregulation began in an early stage of BLV infection rather than after a long period of proviral latency. Second, siRNA-mediated PRMT5 knockdown enhanced BLV gene expression at the transcript and protein levels. Additionally, a selective small-molecule inhibitor of PRMT5 (CMP5) enhanced BLV gene expression. Interestingly, CMP5 treatment, but not siRNA knockdown, altered the gp51 glycosylation pattern and increased the molecular weight of gp51, thereby decreasing BLV-induced syncytium formation. This was supported by the observation that CMP5 treatment enhanced the formation of the complex type of N-glycan more than the high mannose type. In conclusion, PRMT5 overexpression is related to the development of BLV infection with a high proviral load and lymphoma stage and PRMT5 inhibition enhances BLV gene expression. This is the first study to investigate the role of PRMT5 in BLV infection in vivo and in vitro and to reveal a novel function for a small-molecule compound in BLV-gp51 glycosylation processing.

Lack of association between amino acid sequences of the bovine leukemia virus envelope and varying stages of infection in dairy cattle

We collected 724 blood samples from dairy cattle from six Mexican states, and tested them for the presence of antibodies against BLV using a commercial ELISA test. Our study groups consisted of 32 samples: 12 asymptomatic cows, 12 cows with lymphocytosis and 8 samples of tumor tissue of the abomasum and heart of cattle with lymphoma. We designed three pairs of primers to amplify the complete BLV env gene, and obtained a fragment of 1548 nucleotides in length with the sequenced products. According to the phylogenetic tree we constructed to identify the viral genotype, 96.87 % of the sequences grouped into genotype 1, while a single sample from a cow with lymphocytosis (3.13 %) was associated with genotype 3 sequences. The similarity between the Mexican BLV sequences ranged from 0.985-1.00. In addition, the proportion of non-synonymous and synonymous mutations indicated negative selection. We did not identify any conserved residues in the viral protein sequences that could be related to BLV infection stage in cattle. Proviral quantification was performed using quantitative polymerase chain reaction, and we used Mood´s median test as statistical analysis. We found no significant association between proviral load and phase of infection. The sequences showed high similarity without any association between BLV surface glycoprotein and the different infection stages, nor differences in the proviral load. BLV genotype 1 was identified as prevalent in the studied samples, and for the first time in Mexico, we identified BLV genotype 3 in cattle.

In silico and in vitro analysis of boAP3d1 protein interaction with bovine leukaemia virus gp51

The envelope glycoprotein 51 (gp51) is essential for bovine leukaemia virus (BLV) entry to bovine B-lymphocytes. Although the bovine adaptor protein 3 complex subunit delta-1 (boAP3D1) has been proposed as the potential receptor, the specific ligand-receptor interaction has not yet been completely defined and boAP3D1 receptor and gp51 3D structures have not been determined. This study was thus aimed at a functional annotation of boAP3D1 cellular adaptor protein and BLV gp51 and, proposing a reliable model for gp51-AP3D1 interaction using bioinformatics tools. The boAP3D1 receptor interaction patterns were calculated based on models of boAP3D1 receptor and gp51 complexes’ 3D structures, which were constructed using homology techniques and data-driven docking strategy. The results showed that the participation of 6 key amino acids (aa) on gp51 (Asn170, Trp127, His115, Ala97, Ser98 and Glu128) and 4 aa on AP3D1 (Lys925, Asp807, Asp695 and Arg800) was highly probable in the interaction between gp51 and BLVR domains. Three gp51 recombinant peptides were expressed and purified to validate these results: the complete domain (rgp51), the N-terminal portion (rNgp51) and the C-terminal fragment (rCgp51); and binding assays to Madin-Darby bovine kidney (MDBK) cells were then carried out with each recombinant. It was found that rNgp51 preferentially bound to MDBK cells, suggesting this domain’s functional role during invasion. The rNgp51-MDBK cell interaction was sensitive to trypsin (98% reduction) and chymotrypsin treatment (80% reduction). These results highlighted that the N-terminal portion of gp51 interacted in vitro with the AP3D1 receptor and provides a plausible in silico interaction model.

Restricted viral cDNA synthesis in cell lines that fail to support productive infection by bovine leukemia virus

Bovine leukemia virus (BLV) is the causative agent of enzootic bovine leucosis, which results in significant economic losses on many affected farms. BLV infects a wide range of animals as well as cell lines derived from various mammalian species and organs; however, studies show that only some cell lines support sustained production of viral progeny. The differences between cells that produce viral progeny and those that do not are unclear. The aim of this study was to identify the steps of BLV replication that are associated with the capacity of a cell to support a productive infection. Eleven cell lines derived from various species were categorized into two groups, those that produce BLV progeny and those that do not, and the efficiency of viral attachment was compared. In addition, viral entry and reverse transcription were compared for two BLV-producing cell lines and three non-producing cell lines. BLV attached to and entered all of the tested cells. However, synthesis of viral DNA was inhibited in all three non-virus-producing cell lines, suggesting that BLV production was blocked either prior to or at the stage of reverse transcription. These results increase our understanding of the BLV life cycle and should enable better control over the spread of BLV.