Stable infection of a bovine mammary epithelial cell line (MAC-T) with bovine leukemia virus (BLV)

Altered apoptosis and proliferation in milk cells and PBMc from BLV-infected bovines with different proviral loads: Possible role of the BCL-2 family proteins, TNF-alpha, and receptors

Bovines infected by bovine leukemia virus (BLV) are characterized by presenting low proviral load (LPL) or high proviral load (HPL). It is reported that animals with HPL in peripheral blood mononuclear cells (PBMCs) present a decrease in apoptosis, an increase in viability and the proliferation rate, while animals that maintain an LPL have an intrinsic ability to control the infection, presenting an increased apoptosis rate of their PBMCs. However, there is little information on the effect of BLV on these mechanisms when the virus infects somatic milk cells (SC). This study investigates the mechanisms underlying apoptosis in milk and blood from BLV-infected animals with HPL and LPL. Relative levels of mRNA of tumor necrosis factor-α (TNF-α), TNF receptor 1 (TNF-RI), TNF receptor 2 (TNF-RII), anti-apoptotic B-cell lymphoma 2 protein (Bcl-2), and pro-apoptotic Bcl-2-like protein 4 (Bax) were measured in SC and PBMCs using quantitative reverse transcription-polymerase chain reaction (RT-qPCR) assay. A significant decrease in the expression of TNF-α in SC from HPL animals vs non-infected bovines was observed, but the infection in SC with BLV did not show a modulation on the expression of TNF receptors. A significant increase in TNF-RI expression in PBMCs from HPL bovines compared to LPL bovines was observed. No significant differences in PBMCs between HPL and LPL compared to non-infected animals concerning TNF-α, TNF-RI, and TNF-RII expression were found. There was a significant increase of both Bcl-2 and Bax in SC from LPL compared to non-infected bovines, but the Bcl-2/Bax ratio showed an anti-apoptotic profile in LPL and HPL bovines compared to non-infected ones. Reduced mRNA expression levels of Bax were determined in the PBMCs from HPL compared to LPL subjects. In contrast, BLV-infected bovines did not differ significantly in the mRNA expression of Bax compared to non-infected bovines. Our data suggest that the increased mRNA expression of Bax corresponds to the late lactation state of bovine evaluated and the exacerbated increase of mRNA expression of Bcl-2 may be one of the mechanisms for the negative apoptosis regulation in the mammary gland induced by BLV infection. These results provide new insights into the mechanism of mammary cell death in HPL and LPL BLV-infected bovine mammary gland cells during lactation.

BLV-miR-B1-5p Promotes Staphylococcus aureus Adhesion to Mammary Epithelial Cells by Targeting MUC1

Bovine leukemia virus (BLV) is widely prevalent worldwide and can persistently infect mammary epithelial cells in dairy cows, leading to reduced cellular antimicrobial capacity. BLV-encoded microRNAs (BLV-miRNAs) can modify host genes and promote BLV replication. We previously showed that BLV-miR-B1-5p significantly promoted Staphylococcus aureus (S. aureus) adhesion to bovine mammary epithelial (MAC-T) cells; however, the pathway responsible for this effect remained unclear. This study aims to examine how BLV-miR-B1-5p promotes S. aureus adhesion to MAC-T cells via miRNA target gene prediction and validation. Target site prediction showed that BLV-miR-B1-5p could target the mucin family gene mucin 1 (MUC1). Real-time polymerase chain reaction, immunofluorescence, and dual luciferase reporter assay further confirmed that BLV-miR-B1-5p could target and inhibit the expression of MUC1 in bovine MAC-T cells while interfering with the expression of MUC1 promoted S. aureus adhesion to MAC-T cells. These results indicate that BLV-miR-B1-5p promotes S. aureus adhesion to mammary epithelial cells by targeting MUC1.

In vitro Susceptibility of Human Cell Lines Infection by Bovine Leukemia Virus

Evidence of the presence of bovine leukemia virus (BLV) in human beings and its association with breast cancer has been published in the literature, proposing it as a zoonotic infection. However, not enough evidence exists about transmission pathways nor biological mechanisms in human beings. This study was aimed at gathering experimental evidence about susceptibility of human cell lines to BLV infection. Malignant and non-malignant human cell lines were co-cultured with BLV-infected FLK cells using a cell-to-cell model of infection. Infected human cell lines were harvested and cultured for 3 to 6 months to determine stability of infection. BLV detection was performed through liquid-phase PCR and visualized through in situ PCR. Seven out of nine cell lines were susceptible to BLV infection as determined by at least one positive liquid-phase PCR result in the 3-month culture period. iSLK and MCF7 cell lines were able to produce a stable infection throughout the 3-month period, with both cytoplasmic and/or nuclear BLV-DNA visualized by IS-PCR. Our results support experimental evidence of BLV infection in humans by demonstrating the susceptibility of human cells to BLV infection, supporting the hypothesis of a natural transmission from cattle to humans.

BoLA-DRB3 Polymorphism Controls Proviral Load and Infectivity of Bovine Leukemia Virus (BLV) in Milk

Bovine leukemia virus (BLV), which causes enzootic bovine leukosis, is transmitted to calves through the milk of BLV-infected dams. Bovine leukocyte antigen (BoLA)-DRB3 is a polymorphic gene associated with BLV infectivity and proviral load (PVL). However, the effect of BoLA-DRB3 polymorphism on the infectivity and PVL of milk from BLV-infected dams remains unknown. This study examined milk from 259 BLV-infected dams, including susceptible dams carrying at least one BoLA-DRB3*012:01 or *015:01 allele with high PVL, resistant dams carrying at least one BoLA-DRB3*002:01, *009:02, or *014:01:01 allele with low PVL, and neutral dams carrying other alleles. The detection rate of BLV provirus and PVL were significantly higher in milk from susceptible dams than in that from resistant dams. This result was confirmed in a three-year follow-up study in which milk from susceptible dams showed a higher BLV provirus detection rate over a longer period than that from resistant dams. The visualization of infectivity of milk cells using a luminescence syncytium induction assay showed that the infectious risk of milk from BLV-infected dams was markedly high for susceptible dams compared to resistant ones. This is the first report confirming that BoLA-DRB3 polymorphism affects the PVL and infectivity of milk from BLV-infected dams.

Specific antiviral effect of violaceoid E on bovine leukemia virus

Bovine leukemia virus (BLV) infection has spread worldwide causing significant economic losses in the livestock industry. In countries with a high prevalence of BLV, minimizing economic losses is challenging; thus, research into various countermeasures is important for improving BLV control. Because anti-BLV drugs have not been developed, the present study explored a promising chemical compound with anti-BLV activity. Initially, screening of a chemical compound library revealed that violaceoid E (vioE), which is isolated from fungus, showed antiviral activity. Further analysis demonstrated that the antiviral effect of vioE inhibited transcriptional activation of BLV. Cellular thermal shift assay and pulldown assays provided evidence for a direct interaction between vioE and the viral transactivator protein, Tax. These data indicate that interference with Tax-dependent transcription could be a novel target for development of anti-BLV drugs. Therefore, it is suggested that vioE is a novel antiviral compound against BLV.

Milk Macrophage Function in Bovine Leukemia Virus-Infected Dairy Cows

The implications of bovine leukemia virus (BLV) on innate and adaptive immune responses have been widely investigated; however, the effects of BLV on mammary gland immunity require further investigation. The present study investigated the viability, phagocytic capacity, and intracellular production of reactive oxygen and nitrogen species (RONS) by macrophages in milk samples from dairy cows naturally infected with BLV with or without persistent lymphocytosis (PL). No effect of BLV infection in the overall number of macrophages per milliliter and in the percentage of viable macrophages among overall milk viable cells was found. Furthermore, BLV-infected dairy cows had a higher frequency of viable milk macrophages, while healthy animals had a tendency toward a higher percentage of apoptotic milk macrophages. The percentage of milk macrophages that phagocytosed Staphylococcus aureus in seronegative animals was higher than that in BLV-infected dairy cows. No effect of BLV infection on the intracellular RONS production and the intensity of phagocytosis by milk macrophages was observed. Thus, this study provides new insights into the implications of BLV infections in the bovine mammary gland.

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.

Development of multipurpose recombinant reporter bovine leukemia virus

Bovine leukemia virus (BLV) is a global problem that results in significant economic losses to the livestock industry. We developed three virus strains by inserting the HiBiT reporter tag from NanoLuc luciferase (NLuc) into limited sites within BLV molecular clones. Initial analysis for site selection of the tag insertion revealed a permissible site immediately downstream of the viral envelope gene. Therefore, NLuc activity could be used to measure virus copy numbers in the supernatant and the levels of cell infection. Productivity and growth kinetics of the reporter virus were similar to those of the wild-type strain; therefore, the reporter virus can be used to characterize the replication of chimeric viruses as well as responses to the antiviral drug, amprenavir. Collectively, our results suggest that the BLV reporter virus with a HiBiT tag insertion is a highly versatile system for various purposes such as evaluating virus replication and antiviral drugs.

Effect of bovine leukemia virus (BLV) infection on bovine mammary epithelial cells RNA-seq transcriptome profile

Bovine leukemia virus (BLV) is a δ-retrovirus responsible for Enzootic Bovine Leukosis (EBL), a lymphoproliferative disease that affects cattle. The virus causes immune system deregulation, favoring the development of secondary infections. In that context, mastitis incidence is believed to be increased in BLV infected cattle. The aim of this study was to analyze the transcriptome profile of a BLV infected mammary epithelial cell line (MAC-T). Our results show that BLV infected MAC-T cells have an altered expression of IFN I signal pathway and genes involved in defense response to virus, as well as a collagen catabolic process and some protooncogenes and tumor suppressor genes. Our results provide evidence to better understand the effect of BLV on bovine mammary epithelial cell's immune response.

Effect of bovine leukemia virus on bovine mammary epithelial cells

Bovine leukemia virus (BLV) is a retrovirus that infects cattle and is associated with an increase in secondary infections. The objective of this study was to analyze the effect of BLV infection on cell viability, apoptosis and morphology of a bovine mammary epithelial cell line (MAC-T), as well as Toll like receptors (TLR) and cytokine mRNA expression. Our findings show that BLV infection causes late syncytium formation, a decrease in cell viability, downregulation of the anti-apoptotic gene Bcl-2, and an increase in TLR9 mRNA expression. Moreover, we analyzed how this stably infected cell line respond to the exposure to Staphylococcus aureus (S. aureus), a pathogen known to cause chronic mastitis. In the presence of S. aureus, MAC-T BLV cells had decreased viability and decreased Bcl-2 and TLR2 mRNA expression. The results suggest that mammary epithelial cells infected with BLV have altered the apoptotic and immune pathways, probably affecting their response to bacteria and favoring the development of mastitis.