Spontaneous virus reactivation in cattle chronically infected with bovine leukemia virus

A complex network of transcription factors and epigenetic regulators involved in bovine leukemia virus transcriptional regulation

Bovine Leukemia Virus (BLV) is the etiological agent of enzootic bovine leukosis, a disease characterized by the neoplastic proliferation of B cells in cattle. While most European countries have introduced efficient eradication programs, BLV is still present worldwide and no treatment is available. A major feature of BLV infection is the viral latency, which enables the escape from the host immune system, the maintenance of a persistent infection and ultimately the tumoral development. BLV latency is a multifactorial phenomenon resulting in the silencing of viral genes due to genetic and epigenetic repressions of the viral promoter located in the 5' Long Terminal Repeat (5'LTR). However, viral miRNAs and antisense transcripts are expressed from two different proviral regions, respectively the miRNA cluster and the 3'LTR. These latter transcripts are expressed despite the viral latency affecting the 5'LTR and are increasingly considered to take part in tumoral development. In the present review, we provide a summary of the experimental evidence that has enabled to characterize the molecular mechanisms regulating each of the three BLV transcriptional units, either through cis-regulatory elements or through epigenetic modifications. Additionally, we describe the recently identified BLV miRNAs and antisense transcripts and their implications in BLV-induced tumorigenesis. Finally, we discuss the relevance of BLV as an experimental model for the closely related human T-lymphotropic virus HTLV-1.

Bovine leukemia virus encoded blv-miR-b4-3p microRNA is associated with reduced expression of anti-oncogenic gene in vivo

Bovine leukemia virus (BLV) is a retrovirus that causes malignant B-cell lymphoma in up to ten-percent of infected cattle. To date, the mechanisms of BLV linked to malignant transformation remain elusive. Although BLV-encoded miRNAs have been associated with the regulation of different genes involved in oncogenic pathways, this association has not been evaluated in cattle naturally infected with BLV. The objective of this study was to determine the relative expression of BLV-encoded miRNA blv-miR-b4-3p, the host analogous miRNA bo-miR-29a and a couple of potential target mRNAs (HBP-1 and PXDN, with anti-tumorigenic function in B-cells), in cattle naturally infected with BLV compared to uninfected animals (control group). We observed that PXDN was significantly downregulated in BLV-infected cattle (P = 0.03). Considering the similar expression of endogenous bo-miR-29a in both animal groups, the downregulation of PXDN in BLV-naturally infected cattle could be linked to blv-miR-b4-3p expression in these animals. Knowing that PXDN is involved in anti-tumoral pathways in B-cells, the results presented here suggest that blv-miR-b4-3p might be involved in BLV tumorigenesis during natural infection with BLV in cattle.

Potential Risk Factors Associated with Infection with Bovine Leukaemia Virus in Dairy and Beef Cattle in Taiwan

Bovine leukaemia virus (BLV), which is classified as a Deltaretrovirus, is the aetiologic agent of enzootic bovine leukosis (EBL), a chronic lymphoproliferative disorder with a worldwide distribution. EBL is widespread in dairy herds and causes a direct economic impact due to reduced milk production and the early culling of BLV-infected cattle. The BLV infection status in Taiwan remains largely unknown; a high prevalence of BLV in dairy cows was recently revealed. The present study further investigated BLV infections in beef cattle. Surprisingly, the prevalence of BLV proviral DNA was as low as 11.8% (23/195), which is significantly lower than that noted in dairy cows, which was 42.5% (102/240) (p < 0.001). Factors associated with BLV infections were subsequently investigated. Due to the differences in herd management, an analysis of risk factors for a BLV infection was independently conducted in these two sectors. Several factors associated with a BLV infection were identified. Age was significantly associated with BLV infection status in dairy cows (p < 0.001) but not in beef cattle. A high prevalence of BLV was observed in cattle >15.5 months old (57.8%) compared with those ≤15.5 months old (11.4%). Moreover, after stratification analysis, based on the critical age of 15.5 months, as determined by the receiver operating characteristic (ROC) curve, a significantly higher BLV prevalence was demonstrated in lactating dairy cows, cattle undergoing bull breeding, heifers at older ages, and those undergoing routine rectal palpation. Due to the high prevalence of BLV in Taiwan, the development of an effective control program, based on the identified risk factors, is important for interrupting the routes of BLV transmission within herds.

Expression-based analysis of genes related to single nucleotide polymorphism hits associated with bovine leukemia virus proviral load in Argentinean dairy cattle

In dairy cattle infected with bovine leukemia virus (BLV), the proviral load (PVL) level is directly related to the viral transmission from infected animals to their healthy herdmates. Two contrasting phenotypic groups can be identified when assessing PVL in peripheral blood of infected cows. A large number of reports point to bovine genetic variants (single nucleotide polymorphisms) as one of the key determinants underlying PVL level. However, biological mechanisms driving BLV PVL profiles and infection progression in cattle have not yet been elucidated. In this study, we evaluated whether a set of candidate genes affecting BLV PVL level according to whole genome association studies are differentially expressed in peripheral blood mononuclear cells derived from phenotypically contrasting groups of BLV-infected cows. During a 10-mo-long sampling scheme, 129 Holstein cows were phenotyped measuring anti-BLV antibody levels, PVL quantification, and white blood cell subpopulation counts. Finally, the expression of 8 genes (BOLA-DRB3, PRRC2A, ABT1, TNF, BAG6, BOLA-A, LY6G5B, and IER3) located within the bovine major histocompatibility complex region harboring whole genome association SNP hits was evaluated in 2 phenotypic groups: high PVL (n = 7) and low PVL (n = 8). The log₂ initial fluorescence value (N₀) transformed mean expression values for the ABT1 transcription factor were statistically different in high- and low-PVL groups, showing a higher expression of the ABT1 gene in low-PVL cows. The PRRC2A and IER3 genes had a significant positive (correlation coefficient = 0.61) and negative (correlation coefficient = -0.45) correlation with the lymphocyte counts, respectively. Additionally, the relationships between gene expression values and lymphocyte counts were modeled using linear regressions. Lymphocyte levels in infected cows were better explained (coefficient of determination = 0.56) when fitted a multiple linear regression model using both PRRC2A and IER3 expression values as independent variables. The present study showed evidence of differential gene expression between contrasting BLV infection phenotypes. These genes have not been previously related to BLV pathobiology. This valuable information represents a step forward in understanding the BLV biology and the immune response of naturally infected cows under a commercial milk production system. Efforts to elucidate biological mechanisms leading to BLV infection progression in cows are valuable for BLV control programs. Further studies integrating genotypic data, global transcriptome analysis, and BLV progression phenotypes are needed to better understand the BLV-host interaction.

Regulation of Expression and Latency in BLV and HTLV

Human T-lymphotrophic virus type 1 (HTLV-1) and Bovine leukemia virus (BLV) belong to the Deltaretrovirus genus. HTLV-1 is the etiologic agent of the highly aggressive and currently incurable cancer adult T-cell leukemia (ATL) and a neurological disease HTLV-1-associated myelopathy (HAM)/tropical spastic paraparesis (TSP). BLV causes neoplastic proliferation of B cells in cattle: enzootic bovine leucosis (EBL). Despite the severity of these conditions, infection by HTLV-1 and BLV appear in most cases clinically asymptomatic. These viruses can undergo latency in their hosts. The silencing of proviral gene expression and maintenance of latency are central for the establishment of persistent infection, as well as for pathogenesis in vivo. In this review, we will present the mechanisms that control proviral activation and retroviral latency in deltaretroviruses, in comparison with other exogenous retroviruses. The 5′ long terminal repeats (5′-LTRs) play a main role in controlling viral gene expression. While the regulation of transcription initiation is a major mechanism of silencing, we discuss topics that include (i) the epigenetic control of the provirus, (ii) the cis-elements present in the LTR, (iii) enhancers with cell-type specific regulatory functions, (iv) the role of virally-encoded transactivator proteins, (v) the role of repressors in transcription and silencing, (vi) the effect of hormonal signaling, (vii) implications of LTR variability on transcription and latency, and (viii) the regulatory role of non-coding RNAs. Finally, we discuss how a better understanding of these mechanisms may allow for the development of more effective treatments against Deltaretroviruses.

Herd management practices associated with bovine leukemia virus incidence rate in Michigan dairy farms

The objective of this study was to identify associations between herd management practices and the incidence rate of bovine leukemia virus (BLV) infections in Michigan dairy herds. Previous management risk factor studies were of antibody prevalence rather than the rate of recent infections. Milk samples were collected from cohorts of cows on 112 Michigan dairy herds and tested for BLV using an antibody capture ELISA (n = 3849 cows). Cows were subsequently followed for an average of 21 months. Cows negative for anti-BLV antibodies and still present in their respective herds were retested by the same antibody capture ELISA to estimate within-herd incidence rates. The overall crude incidence rate was 1.46 infections per 100 cow-months at risk for the 1314 retested cows in 107 herds. The average within-herd incidence rate was 2.28 infections per 100 cow-months (range: 0 to 9.76 infections per 100 cow-months). A negative binomial regression model was used to identify herd management practices associated with the within-herd incidence rate. Results of the final multivariable model identified higher herd prevalence, milking frequency, needle reuse, as well as housing post-parturient cows separately, to be associated with increased incidence rate. Utilization of sand bedding for the lactating herd was found to be associated with decreased incidence rates. Results of this study suggest potential routes of BLV transmission which should be further investigated as disease control targets in ongoing control programs.

Examination of the fecal microbiota in dairy cows infected with bovine leukemia virus

Infection of cattle by bovine leukemia virus (BLV) causes significant economic losses in terms of milk and meat production in many countries. Because the gut microbiota may be altered by immunomodulation resulting from viral infections, we hypothesized that latent BLV infection would change the gut (i.e., rumen and hindgut) microbiota of infected cattle. In this study, we compared the gut microbiota of 22 uninfected and 29 BLV-infected Holstein-Friesian cows kept on the same farm, by 16S rRNA amplicon sequence analysis of fecal samples. First, we found that the fecal microbial diversity of BLV-infected cows differed slightly from that of uninfected cows. According to differential abundance analysis, some bacterial taxa associated with ruminal fermentation, such as Lachnospiraceae and Veillonellaceae families, were enriched in the fecal microbiota of uninfected cows. Second, the virus propagation ability of BLV strains was examined in vitro, and the correlation of the fecal microbiota with this virus propagation ability was analyzed. Higher virus propagation was shown to lead to less diversity in the microbiota. Differential abundance analysis showed that one bacterial taxon of genus Sanguibacteroides was negatively correlated with the virus propagation ability of BLV strains. Considering these results, BLV infection was speculated to decrease energy production efficiency in the cows via modification of rumen and hindgut microbiota, which partly relies on the virus propagation ability of BLV strains. This may explain the secondary negative effects of BLV infections such as increased susceptibility to other infections and decreased lifetime milk production and reproductive efficiency.

Detection of Bovine Leukemia Virus RNA in Blood Samples of Naturally Infected Dairy Cattle

The viral expression in vivo, in bovine leukemia virus (BLV)-infected cattle, is considered to be restricted to extremely low levels, and the mitosis of infected B lymphocytes is regarded as the main mode of virus persistence within the infected host. In this study, the presence of BLV RNA in whole blood from seven asymptomatic cows naturally infected with BLV during one year, including a complete milking cycle and two delivery time points, was investigated by nested-PCR using the oligonucleotides complementary to the tax and pol gene. BLV RNA was detected in four cows at different time points, especially in high blood proviral load cows and around delivery time. This study describes for the first time the detection of free BLV RNA in blood from BLV-infected asymptomatic cows. The results obtained suggest the occurrence of persistent low-level expression of the tax and pol genes that could be a result of viral reactivation, within the asymptomatic period. This finding may be important in the pathogenesis of BLV infection, associated with the delivery period.