Transcriptional activation of bovine leukemia virus in blood cells from experimentally infected, asymptomatic sheep with latent infections

Role of the cellular factor CTCF in the regulation of bovine leukemia virus latency and three-dimensional chromatin organization

Bovine leukemia virus (BLV)-induced tumoral development is a multifactorial phenomenon that remains incompletely understood. Here, we highlight the critical role of the cellular CCCTC-binding factor (CTCF) both in the regulation of BLV transcriptional activities and in the deregulation of the three-dimensional (3D) chromatin architecture surrounding the BLV integration site. We demonstrated the in vivo recruitment of CTCF to three conserved CTCF binding motifs along the provirus. Next, we showed that CTCF localized to regions of transitions in the histone modifications profile along the BLV genome and that it is implicated in the repression of the 5′Long Terminal Repeat (LTR) promoter activity, thereby contributing to viral latency, while favoring the 3′LTR promoter activity. Finally, we demonstrated that BLV integration deregulated the host cellular 3D chromatin organization through the formation of viral/host chromatin loops. Altogether, our results highlight CTCF as a new critical effector of BLV transcriptional regulation and BLV-induced physiopathology.

The contribution of bovines to human health against viral infections

In the last 40 years, novel viruses have evolved at a much faster pace than other pathogens. Viral diseases pose a significant threat to public health around the world. Bovines have a longstanding history of significant contributions to human nutrition, agricultural, industrial purposes, medical research, drug and vaccine development, and livelihood. The life cycle, genomic structures, viral proteins, and pathophysiology of bovine viruses studied in vitro paved the way for understanding the human counterparts. Calf model has been used for testing vaccines against RSV, papillomavirus vaccines and anti-HCV agents were principally developed after using the BPV and BVDV model, respectively. Some bovine viruses-based vaccines (BPIV-3 and bovine rotaviruses) were successfully developed, clinically tried, and commercially produced. Cows, immunized with HIV envelope glycoprotein, produced effective broadly neutralizing antibodies in their serum and colostrum against HIV. Here, we have summarized a few examples of human viral infections for which the use of bovines has contributed to the acquisition of new knowledge to improve human health against viral infections covering the convergence between some human and bovine viruses and using bovines as disease models. Additionally, the production of vaccines and drugs, bovine-based products were covered, and the precautions in dealing with bovines and bovine-based materials.

Analysis of Nucleotide Sequence of Tax, miRNA and LTR of Bovine Leukemia Virus in Cattle with Different Levels of Persistent Lymphocytosis in Russia

Bovine Leukemia Virus (BLV) is the etiological agent of enzootic bovine leucosis (EBL), a lymphoproliferative disease of the bovine species. In BLV-infected cells, the long terminal repeat (LTR), the viral Tax protein and viral miRNAs promote viral and cell proliferation as well as tumorigenesis. Although their respective roles are decisive in BLV biology, little is known about the genetic sequence variation of these parts of the BLV genome and their impact on disease outcome. Therefore, the objective of this study was to assess the relationship between disease progression and sequence variation of the BLV Tax, miRNA and LTR regions in infected animals displaying either low or high levels of persistent lymphocytosis (PL). A statistically significant association was observed between the A(+187)C polymorphism in the downstream activator sequence (DAS) region in LTR (p-value = 0.00737) and high lymphocytosis. Our study also showed that the mutation A(−4)G in the CAP site occurred in 70% of isolates with low PL and was not found in the high PL group. Conversely, the mutations G(−133)A/C in CRE2 (46.7%), C(+160)T in DAS (30%) and A(310)del in BLV-mir-B4-5p, A(357)G in BLV-mir-B4-3p, A(462)G in BLV-mir-B5-5p, and GA(497–498)AG in BLV-mir-B5-3p (26.5%) were often seen in isolates with high PL and did not occur in the low PL group. In conclusion, we found several significant polymorphisms among BLV genomic sequences in Russia that would explain a progression towards higher or lower lymphoproliferation. The data presented in this article enabled the classification between two different genotypes; however, clear association between genotypes and the PL development was not found.

Development of an in situ hybridization assay using an AS1 probe for detection of bovine leukemia virus in BLV-induced lymphoma tissues

Enzootic bovine leukosis (EBL) is a malignant B cell lymphoma caused by infection with bovine leukemia virus (BLV). Histopathological examination is commonly used for diagnosis of the disease, but observation of lymphoma alone does not confirm EBL because cattle may be affected by sporadic forms of lymphoma that are not associated with BLV. Detection of BLV in tumor cells can be definitive evidence of EBL, but currently, there is no technique available for such a purpose. In this study, we focused on a viral non-coding RNA, AS1, and developed a novel in situ hybridization assay for the detection of BLV from formalin-fixed paraffin-embedded (FFPE) tissues. RNA-seq analysis revealed that all examined B lymphocytes derived from clinical EBL abundantly expressed AS1 RNA, indicating a possible target for detection. The in situ hybridization assay using an AS1 probe clearly detected AS1 RNA in fetal lamb kidney cells persistently infected with BLV. The utility of this assay in clinical samples was assessed using three EBL-derived lymph node specimens and one BLV-negative specimen, and AS1 RNA was detected specifically in the EBL-derived tissues. These results suggest that AS1 RNA is a useful target for the detection of BLV from FFPE specimens of tumor tissues. This technique is expected to become a powerful tool for EBL diagnosis.

Dairy cattle with bovine leukaemia virus RNA show significantly increased leukocyte counts

Infection with bovine leukaemia virus (BLV), a retrovirus, causes dysfunction of the immune system and can have a marked economic impact on dairy industries due to decreased milk production and reduced lifespan in affected dairy cattle. The presence of proviral DNA has been the major diagnostic indicator of BLV infection. However in the course of BLV infection, the viral genome can be dormant, without detectable gene expression, resulting in limited impact on infected animals. At present, there is limited knowledge regarding haematological indices in dairy cattle that could indicate activation of the BLV genome and suggest reactivated BLV infection. In this study, BLV infection and BLV genome reactivation were evaluated based on the presence of BLV DNA and BLV env gene transcripts, respectively. BLV RNA transcription was confirmed. Among 93 whole blood samples obtained from asymptomatic dairy cattle, the prevalence of BLV proviral DNA and transcripts was 93.5% (n = 87/93) and 83.9% (n = 78/93), respectively. Between groups with and without BLV, the mean counts of white blood cells and lymphocytes in whole blood were significantly associated with the presence of BLV RNA (P < 0.05), but not with BLV proviral DNA. These results shed light on the activation status of the BLV genome and should be taken into account when evaluating the possible impact of BLV on cattle.

Bovine leukemia virus pre-miRNA genes' polymorphism

MicroRNAs (miRNAs) are small non-coding RNAs with a wide distribution in nature among the living things. They play a key role both in normal signaling pathways and in pathological ones. Bovine leukemia virus (BLV) is an oncogenic retrovirus of Deltaretrovirus genus causing persistent infection in its natural hosts - cattle, zebu and water buffalo with diverse clinical manifestations through the defeat of B-lymphocytes (B-cells). Ten BLV encoded miRNAs (further miRs-B) transcribed from five different pre-miRNA (further pre-miR-B) genes are abundantly detected in BLV infected B-cells. Here we report about several alleles of each of pre-miRs-B' genes, some of which have a highly significant association with an increase or a decrease of the number of leukocytes (WBCs - white blood cells) in BLV-infected cows.

Association of haplotypes for SNPs in the LTR regions of bovine leukemia virus with hematological indices of cattle

Molecular typing of BLV samples isolated from Holsteinized Russian Black Pied cattle was carried out, and various cytofluorometric and morphological blood indices were examined. We performed the total count of white blood cells (WBC), lymphocyte (lymf), granulocyte (gran), monocyte (mon), red blood cell (RBC), hemoglobin (HGB), hematocrit (HTC), mean corpuscular volume (MCV), mean corpuscular hemoglobin (MCH), mean corpuscular hemoglobin concentration (MCHC), red blood cell distribution width (RDW), platelet count (PLT), mean platelet volume (MPV), platelet distribution width (PDW), and platelet crit count (PCT). The LTR-region of BLV was haplotyped. Only viruses of haplotypes I (0.33±0.03) and III (0.67±0.03) of the eight possible were detected. The ratio of hematologically sick, healthy, and suspected carriers of BLV of haplotypes I and II was comparable with the results of other researchers. The numbers of leukocytes, erythrocytes and platelets in the blood of carriers of haplotype III exceeded the corresponding parameters of cattle affected by the virus of haplotype I. It is interesting to note that the difference in the hemolytic status of animals was manifested not only by the concentration of leukocytes as direct immune agents but also by the count of erythrocytes and platelets, which are not directly involved in the immune response. The number of particles of haplotype III of the BLV circulating in the blood of infected individuals exceeded that of the carriers of haplotype I. In this connection, an assumption was made about the evolutionary advantage of the more virulent haplotype III. However, the results of our own research in conjunction with the data of other scientists indicate that the high virulence of individual virus strains is a consequence of the tendency to implement the maximum possible intensity of the synthesis of virus particles but not of the high damaging effect alone. It is shown that high lethality is evolutionarily disadvantageous for viruses, since the extinction of the carrier as a biological species is fraught with the disappearance of the virus itself.

Study of horn flies as vectors of bovine leukemia virus

Bovine leukemia virus (BLV) is the agent responsible for enzootic bovine leukosis, the most common neoplastic disease in cattle. The horn fly, a major hematophagous pest of cattle, is able to transmit different diseases in cattle. However, its implication in BLV transmission under a natural environment is still discussed. The objectives of this work were to determine the presence of BLV in horn flies (by sequencing) and to evaluate the ability of horn flies to transmit BLV to cattle (through an experimental assay under a natural environment). To demonstrate the presence of BLV in the flies, 40 horn flies were collected from a BLV-positive cow with a sweep net and 10 pools with four horn-fly mouthparts each were prepared. The presence of BLV was determined by nested polymerase chain reaction and sequencing. To demonstrate BLV transmission, other 40 flies were collected from the same BLV-positive cow with a sweep net. Eight homogenates containing five horn-fly mouthparts each were prepared and injected to eight cows of different breeds, and blood samples were collected every 21 days. Then, to evaluate the ability of horn flies to transmit BLV to grazing cattle under natural conditions, both infected and uninfected cattle from the experimental transmission assay were kept together in the same paddock with more than 200 horn flies per animal for 120 days. Blood samples were collected every 20 days and the number of flies was determined. The sequencing results confirmed the presence of the provirus in horn flies. The results also confirmed that BLV transmission is a possible event, at least experimentally. However, the role of horn flies as vectors of BLV under a natural grazing system is still discussed.

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

Bovine leukemia virus (BLV) is a retrovirus that affects cattle causing a lymphoproliferative disease. BLV infection has been associated with misbalance of the immune response causing a higher incidence of other infections. Mastitis is one of the most important conditions that affect milk production in cattle. The aim of this study was to stably infect a bovine mammary epithelial cell line (MAC-T). MAC-T cell line was successfully infected with BLV and the infection was confirmed by nested PCR, qPCR, immunocytochemistry, western blot and transmission electron microscopy. This is the first report of a bovine mammary epithelial cell line stably infected with BLV. This new cell line could be used as an in vitro model to study the effect of BLV on the immune response in the mammary gland and the relationship with other agents causing mastitis.

How Does HTLV-1 Undergo Oncogene-Dependent Replication Despite a Strong Immune Response?

In 1987, Mitsuaki Yoshida proposed the following model (Yoshida and Seiki, 1987): “... T-cells activated through the endogenous p40x would express viral antigens including the envelope glycoproteins which are exposed on the cell surface. These glycoproteins are targets of host immune surveillance, as is evidenced by the cytotoxic effects of anti-envelope antibodies or patient sera. Eventually all cells expressing the viral antigens, that is, all cells driven by the p40x would be rejected by the host. Only those cells that did not express the viral antigens would survive. Later, these antigen-negative infected cells would begin again to express viral antigens, including p40x, thus entering into the second cycle of cell propagation. These cycles would be repeated in so-called healthy virus carriers for 20 or 30 years or longer....” Three decades later, accumulated experimental facts particularly on intermittent viral transcription and regulation by the host immune response appear to prove that Yoshida was right. This Hypothesis and Theory summarizes the evidences that support this paradigm.

MicroRNAs Encoded by Bovine Leukemia Virus (BLV) Are Associated with Reduced Expression of B Cell Transcriptional Regulators in Dairy Cattle Naturally Infected with BLV

Bovine leukemia virus (BLV) is estimated to infect over 83% of dairy herds and over 40% of all dairy cows in the United States. While, BLV only causes leukemia in a small proportion of animals, research indicates that BLV+ cattle exhibit reduced milk production and longevity that is distinct from lymphoma development. It is hypothesized that BLV negatively affects production by interfering with cattle immunity and increasing the risk of secondary infections. In particular, BLV+ cows demonstrate reduced circulating levels of both antigen-specific and total IgM. This study investigated possible mechanisms by which BLV could interfere with the production of IgM in naturally infected cattle. Specifically, total plasma IgM and the expression of genes IGJ, BLIMP1, BCL6, and PAX5 in circulating IgM+ B cells were measured in 15 naturally infected BLV+ and 15 BLV− cows. In addition, BLV proviral load (PVL) (a relative measurement of BLV provirus integrated into host DNA) and the relative expression of BLV TAX and 5 BLV microRNAs (miRNAs) were characterized and correlated to the expression of selected endogenous genes. BLV+ cows exhibited lower total plasma IgM and lower expression of IGJ, BLIMP1, and BCL6. While, BLV TAX and BLV miRNAs failed to correlate with IGJ expression, both BLV TAX and BLV miRNAs exhibited negative associations with BLIMP1 and BCL6 gene expression. The results suggest a possible transcriptional pathway by which BLV interferes with IgM production in naturally infected cattle.

Characterization of new RNA polymerase III and RNA polymerase II transcriptional promoters in the Bovine Leukemia Virus genome

Bovine leukemia virus latency is a viral strategy used to escape from the host immune system and contribute to tumor development. However, a highly expressed BLV micro-RNA cluster has been reported, suggesting that the BLV silencing is not complete. Here, we demonstrate the in vivo recruitment of RNA polymerase III to the BLV miRNA cluster both in BLV-latently infected cell lines and in ovine BLV-infected primary cells, through a canonical type 2 RNAPIII promoter. Moreover, by RPC6-knockdown, we showed a direct functional link between RNAPIII transcription and BLV miRNAs expression. Furthermore, both the tumor- and the quiescent-related isoforms of RPC7 subunits were recruited to the miRNA cluster. We showed that the BLV miRNA cluster was enriched in positive epigenetic marks. Interestingly, we demonstrated the in vivo recruitment of RNAPII at the 3′LTR/host genomic junction, associated with positive epigenetic marks. Functionally, we showed that the BLV LTR exhibited a strong antisense promoter activity and identified cis-acting elements of an RNAPII-dependent promoter. Finally, we provided evidence for an in vivo collision between RNAPIII and RNAPII convergent transcriptions. Our results provide new insights into alternative ways used by BLV to counteract silencing of the viral 5′LTR promoter.

Bovine Leukemia Virus Small Noncoding RNAs Are Functional Elements That Regulate Replication and Contribute to Oncogenesis In Vivo

Retroviruses are not expected to encode miRNAs because of the potential problem of self-cleavage of their genomic RNAs. This assumption has recently been challenged by experiments showing that bovine leukemia virus (BLV) encodes miRNAs from intragenomic Pol III promoters. The BLV miRNAs are abundantly expressed in B-cell tumors in the absence of significant levels of genomic and subgenomic viral RNAs. Using deep RNA sequencing and functional reporter assays, we show that miRNAs mediate the expression of genes involved in cell signaling, cancer and immunity. We further demonstrate that BLV miRNAs are essential to induce B-cell tumors in an experimental model and to promote efficient viral replication in the natural host.

Differences in intermittent and continuous fecal shedding patterns between natural and experimental Mycobacterium avium subspecies paratuberculosis infections in cattle

The objective of this paper is to study shedding patterns of cows infected with Mycobacterium avium subsp. paratuberculosis (MAP). While multiple single farm studies of MAP dynamics were reported, there is not large scale meta-analysis of both natural and experimental infections. Large difference in shedding patterns between experimentally and naturally infected cows were observed. Experimental infections are thus probably driven by different pathological mechanisms. For further evaluations of shedding patterns only natural infections were used. Within such infections, the transition to high shedding was studied as a proxy to the development of a clinical disease. The majority of studied cows never developed high shedding levels. Those that do, typically never reduced their shedding level to low or no shedding. Cows that eventually became high shedders showed a pattern of continuous shedding. In contrast, cows with an intermittent shedding pattern had a low probability to ever become high shedders. In addition, cows that start shedding at a younger age (less than three years of age) have a lower hazard of becoming high shedders compared to cows starting to shed at an older age. These data suggest the presence of three categories of immune control. Cows that are intermittent shedders have the infection process under control (no progressive infection). Cows that start shedding persistently at a young age partially control the infection, but eventually will be high shedders (slow progressive infection), while cows that start shedding persistently at an older age cannot effectively control the infection and become high shedders rapidly.

Two state model for a constant disease hazard in paratuberculosis (and other bovine diseases)

Many diseases are characterized by a long and varying sub-clinical period. Two main mechanisms can explain such periods: a slow progress toward disease or a sudden transition from a healthy state to a disease state induced by internal or external events. We here survey epidemiological features of the amount of bacteria shed during Mycobacterium Avium Paratuberculosis (MAP) infection to test which of these two models, slow progression or sudden transition (or a combination of the two), better explains the transition from intermittent and low shedding to high shedding. Often, but not always, high shedding is associated with the occurrence of clinical signs. In the case of MAP, the clinical signs include diarrhea, low milk production, poor fertility and eventually emaciation and death. We propose a generic model containing bacterial growth, immune control and fluctuations. This proposed generic model can represent the two hypothesized types of transitions in different parameter regimes. The results show that the sudden transition model provides a simpler explanation of the data, but also suffers from some limitations. We discuss the different immunological mechanism that can explain and support the sudden transition model and the interpretation of each term in the studied model. These conclusions are applicable to a wide variety of diseases, and MAP serves as a good test case based on the large scale measurements of single cow longitudinal profiles in this disease.

Bovine leukemia virus: a major silent threat to proper immune responses in cattle

Bovine leukemia virus (BLV) infection is widespread in the US dairy industry and the majority of producers do not actively try to manage or reduce BLV incidence within their herds. However, BLV is estimated to cost the dairy industry hundreds of millions of dollars annually and this is likely a conservative estimate. BLV is not thought to cause animal distress or serious pathology unless infection progresses to leukemia or lymphoma. However, a wealth of research supports the notion that BLV infection causes widespread abnormal immune function. BLV infection can impact cells of both the innate and adaptive immune system and alter proper functioning of uninfected cells. Despite strong evidence of abnormal immune signaling and functioning, little research has investigated the large-scale effects of BLV infection on host immunity and resistance to other infectious diseases. This review focuses on mechanisms of immune suppression associated with BLV infection, specifically aberrant signaling, proliferation and apoptosis, and the implications of switching from BLV latency to activation. In addition, this review will highlight underdeveloped areas of research relating to BLV infection and how it causes immune suppression.

Detection and molecular characterization of bovine leukemia virus in Philippine cattle

Bovine leukemia virus (BLV) is the etiological agent of enzootic bovine leukosis, which is the most common neoplastic disease of cattle. BLV infects cattle worldwide, imposing a severe economic impact on the dairy cattle industry. However, there are no comprehensive studies on the distribution of BLV in the Philippines, and the genetic characteristics of Philippine BLV strains are unknown. Therefore, the aim of this study was to detect BLV infections in the Philippines and determined their genetic variability. Blood samples were obtained from 1116 cattle from different farms on five Philippine islands, and BLV provirus was detected by BLV-CoCoMo-qPCR-2 and nested PCR targeting BLV long terminal repeats. Out of 1116 samples, 108 (9.7 %) and 54 (4.8 %) were positive for BLV provirus, as determined by BLV-CoCoMo-qPCR-2 and nested PCR, respectively. Of the five islands, Luzon Island showed the highest prevalence of BLV infection (23.1 %). Partial env gp51 genes from 43 samples, which were positive for BLV provirus by both methods, were sequenced for phylogenetic analysis. Phylogenetic analysis based on a 423-bp fragment of the env gene revealed that Philippine BLV strains clustered into either genotype 1 or genotype 6. Substitutions were mainly found in antigenic determinants, such as the CD4(+) T-cell epitope, the CD8(+) T-cell epitope, the second neutralizing domain, B and E epitopes, and these substitutions varied according to genotype. This study provides comprehensive information regarding BLV infection levels in the Philippines and documents the presence of two BLV genotypes, genotypes 1 and 6, in this population.

Vaccination against δ-retroviruses: the bovine leukemia virus paradigm

Bovine leukemia virus (BLV) and human T-lymphotropic virus type 1 (HTLV-1) are closely related δ-retroviruses that induce hematological diseases. HTLV-1 infects about 15 million people worldwide, mainly in subtropical areas. HTLV-1 induces a wide spectrum of diseases (e.g., HTLV-associated myelopathy/tropical spastic paraparesis) and leukemia/lymphoma (adult T-cell leukemia). Bovine leukemia virus is a major pathogen of cattle, causing important economic losses due to a reduction in production, export limitations and lymphoma-associated death. In the absence of satisfactory treatment for these diseases and besides the prevention of transmission, the best option to reduce the prevalence of δ-retroviruses is vaccination. Here, we provide an overview of the different vaccination strategies in the BLV model and outline key parameters required for vaccine efficacy.

Differences in cellular function and viral protein expression between IgMhigh and IgMlow B-cells in bovine leukemia virus-infected cattle

Bovine leukemia virus (BLV) induces abnormal B-cell proliferation and B-cell lymphoma in cattle, where the BLV provirus is integrated into the host genome. BLV-infected B-cells rarely express viral proteins in vivo, but short-term cultivation augments BLV expression in some, but not all, BLV-infected B-cells. This observation suggests that two subsets, i.e. BLV-silencing cells and BLV-expressing cells, are present among BLV-infected B-cells, although the mechanisms of viral expression have not been determined. In this study, we examined B-cell markers and viral antigen expression in B-cells from BLV-infected cattle to identify markers that may discriminate BLV-expressing cells from BLV-silencing cells. The proportions of IgM(high) B-cells were increased in blood lymphocytes from BLV-infected cattle. IgM(high) B-cells mainly expressed BLV antigens, whereas IgM(low) B-cells did not, although the provirus load was equivalent in both subsets. Several parameters were investigated in these two subsets to characterize their cellular behaviour. Real-time PCR and microarray analyses detected higher expression levels of some proto-oncogenes (e.g. Maf, Jun and Fos) in IgM(low) B-cells than those in IgM(high) B-cells. Moreover, lymphoma cells obtained from the lymph nodes of 14 BLV-infected cattle contained IgM(low) or IgM(-) B-cells but no IgM(high) B-cells. To our knowledge, this is the first study to demonstrate that IgM(high) B-cells mainly comprise BLV-expressing cells, whereas IgM(low) B-cells comprise a high proportion of BLV-silencing B-cells in BLV-infected cattle.

Mechanisms of pathogenesis induced by bovine leukemia virus as a model for human T-cell leukemia virus

Bovine leukemia virus (BLV) and human T-cell leukemia virus type 1 (HTLV-1) make up a unique retrovirus family. Both viruses induce chronic lymphoproliferative diseases with BLV affecting the B-cell lineage and HTLV-1 affecting the T-cell lineage. The pathologies of BLV- and HTLV-induced infections are notably similar, with an absence of chronic viraemia and a long latency period. These viruses encode at least two regulatory proteins, namely, Tax and Rex, in the pX region located between the env gene and the 3′ long terminal repeat. The Tax protein is a key contributor to the oncogenic potential of the virus, and is also the key protein involved in viral replication. However, BLV infection is not sufficient for leukemogenesis, and additional events such as gene mutations must take place. In this review, we first summarize the similarities between the two viruses in terms of genomic organization, virology, and pathology. We then describe the current knowledge of the BLV model, which may also be relevant for the understanding of leukemogenesis caused by HTLV-1. In addition, we address our improved understanding of Tax functions through the newly identified BLV Tax mutants, which have a substitution between amino acids 240 and 265.

Presence of Gumprecht shadows (smudge cells) in bovine leukemia virus-positive cattle

Enzootic Bovine Leukosis is a chronic disease caused by the bovine leukemia virus (BLV). Smudge cells, also known as Gumprecht shadows, are not simple artifacts of slide preparation, but ragged lymphoid cells found mainly in peripheral blood smears from human patients with chronic lymphocytic leukemia. In this study, we report the presence of Gumprecht shadows in peripheral blood from BLV-positive cattle.

Estimation of bovine leukemia virus (BLV) proviral load harbored by lymphocyte subpopulations in BLV-infected cattle at the subclinical stage of enzootic bovine leucosis using BLV-CoCoMo-qPCR

Background

Bovine leukemia virus (BLV) is associated with enzootic bovine leukosis (EBL), which is the most common neoplastic disease of cattle. BLV infection may remain clinically silent at the aleukemic (AL) stage, cause persistent lymphocytosis (PL), or, more rarely, B cell lymphoma. BLV has been identified in B cells, CD2⁺ T cells, CD3⁺ T cells, CD4⁺ T cells, CD8⁺ T cells, γ/δ T cells, monocytes, and granulocytes in infected cattle that do not have tumors, although the most consistently infected cell is the CD5⁺ B cell. The mechanism by which BLV causes uncontrolled CD5⁺ B cell proliferation is unknown. Recently, we developed a new quantitative real-time polymerase chain reaction (PCR) method, BLV-CoCoMo-qPCR, which enabled us to demonstrate that the proviral load correlates not only with BLV infection, as assessed by syncytium formation, but also with BLV disease progression. The present study reports the distribution of BLV provirus in peripheral blood mononuclear cell subpopulations isolated from BLV-infected cows at the subclinical stage of EBL as examined by cell sorting and BLV-CoCoMo-qPCR.

Results

Phenotypic characterization of five BLV-infected but clinically normal cattle with a proviral load of > 100 copies per 1 × 10⁵ cells identified a high percentage of CD5⁺ IgM⁺ cells (but not CD5^- IgM⁺ B cells, CD4⁺ T cells, or CD8⁺T cells). These lymphocyte subpopulations were purified from three out of five cattle by cell sorting or using magnetic beads, and the BLV proviral load was estimated using BLV-CoCoMo-qPCR. The CD5⁺ IgM⁺ B cell population in all animals harbored a higher BLV proviral load than the other cell populations. The copy number of proviruses infecting CD5^- IgM⁺ B cells, CD4⁺ cells, and CD8⁺ T cells (per 1 ml of blood) was 1/34 to 1/4, 1/22 to 1/3, and 1/31 to 1/3, respectively, compared with that in CD5⁺ IgM⁺ B cells. Moreover, the BLV provirus remained integrated into the genomic DNA of CD5⁺ IgM⁺ B cells, CD5^- IgM⁺ B cells, CD4⁺ T cells, and CD8⁺ T cells, even in BLV-infected cattle with a proviral load of <100 copies per 10⁵ cells.

Conclusions

The results of the recent study showed that, although CD5⁺ IgM⁺ B cells were the main cell type targeted in BLV-infected but clinically normal cattle, CD5^- IgM⁺ B cells, CD4⁺ cells, and CD8⁺ T cells were infected to a greater extent than previously thought.

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.

Animal models on HTLV-1 and related viruses: what did we learn?

Retroviruses are associated with a wide variety of diseases, including immunological, neurological disorders, and different forms of cancer. Among retroviruses, Oncovirinae regroup according to their genetic structure and sequence, several related viruses such as human T-cell lymphotropic viruses types 1 and 2 (HTLV-1 and HTLV-2), simian T cell lymphotropic viruses types 1 and 2 (STLV-1 and STLV-2), and bovine leukemia virus (BLV). As in many diseases, animal models provide a useful tool for the studies of pathogenesis, treatment, and prevention. In the current review, an overview on different animal models used in the study of these viruses will be provided. A specific attention will be given to the HTLV-1 virus which is the causative agent of adult T-cell leukemia/lymphoma (ATL) but also of a number of inflammatory diseases regrouping the HTLV-associated myelopathy/tropical spastic paraparesis (HAM/TSP), infective dermatitis and some lung inflammatory diseases. Among these models, rabbits, monkeys but also rats provide an excellent in vivo tool for early HTLV-1 viral infection and transmission as well as the induced host immune response against the virus. But ideally, mice remain the most efficient method of studying human afflictions. Genetically altered mice including both transgenic and knockout mice, offer important models to test the role of specific viral and host genes in the development of HTLV-1-associated leukemia. The development of different strains of immunodeficient mice strains (SCID, NOD, and NOG SCID mice) provide a useful and rapid tool of humanized and xenografted mice models, to test new drugs and targeted therapy against HTLV-1-associated leukemia, to identify leukemia stem cells candidates but also to study the innate immunity mediated by the virus. All together, these animal models have revolutionized the biology of retroviruses, their manipulation of host genes and more importantly the potential ways to either prevent their infection or to treat their associated diseases.

Viral expression directs the fate of B cells in bovine leukemia virus-infected sheep

The host immune response is believed to tightly control viral replication of deltaretroviruses such as human T-lymphotropic virus type 1 (HTLV-1) and bovine leukemia virus (BLV). However, this assumption has not been definitely proven in vivo. In order to further evaluate the importance of the immune response in the BLV model, we studied the fate of cells in which viral expression was transiently induced. Using a dual fluorochrome labeling approach, we showed that ex vivo induction of viral expression induces higher death rates of B cells in vivo. Furthermore, cyclosporine treatment of these animals indicated that an efficient immune response is required to control virus-expressing cells.

Chromatin disruption in the promoter of bovine leukemia virus during transcriptional activation

Bovine leukemia virus expression relies on its chromatin organization after integration into the host cell genome. Proviral latency, which results from transcriptional repression in vivo, represents a viral strategy to escape the host immune system and likely allows for tumor progression. Here, we discriminated two types of latency: an easily reactivable latent state of the YR2 provirus and a ‘locked’ latent state of the L267 provirus. The defective YR2 provirus was characterized by the presence of nuclease hypersensitive sites at the U3/R junction and in the R/U5 region of the 5′-long terminal repeat (5′-LTR), whereas the L267 provirus displayed a closed chromatin configuration at the U3/R junction. Reactivation of viral expression in YR2 cells by the phorbol 12-myristate 13-acetate (PMA) plus ionomycin combination was accompanied by a rapid but transient chromatin remodeling in the 5′-LTR, leading to an increased PU.1 and USF-1/USF-2 recruitment in vivo sustained by PMA/ionomycin-mediated USF phosphorylation. In contrast, viral expression was not reactivated by PMA/ionomycin in L267 cells, because the 5′-LTR U3/R region remained inaccessible to nucleases and hypermethylated at CpG dinucleotides. Remarkably, we elucidated the BLV 5′-LTR chromatin organization in PBMCs isolated from BLV-infected cows, thereby depicting the virus hiding in vivo in its natural host.

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

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

BLV-CoCoMo-qPCR: Quantitation of bovine leukemia virus proviral load using the CoCoMo algorithm

Background

Bovine leukemia virus (BLV) is closely related to human T-cell leukemia virus (HTLV) and is the etiological agent of enzootic bovine leukosis, a disease characterized by a highly extended course that often involves persistent lymphocytosis and culminates in B-cell lymphomas. BLV provirus remains integrated in cellular genomes, even in the absence of detectable BLV antibodies. Therefore, to understand the mechanism of BLV-induced leukemogenesis and carry out the selection of BLV-infected animals, a detailed evaluation of changes in proviral load throughout the course of disease in BLV-infected cattle is required. The aim of this study was to develop a new quantitative real-time polymerase chain reaction (PCR) method using Coordination of Common Motifs (CoCoMo) primers to measure the proviral load of known and novel BLV variants in clinical animals.

Results

Degenerate primers were designed from 52 individual BLV long terminal repeat (LTR) sequences identified from 356 BLV sequences in GenBank using the CoCoMo algorithm, which has been developed specifically for the detection of multiple virus species. Among 72 primer sets from 49 candidate primers, the most specific primer set was selected for detection of BLV LTR by melting curve analysis after real-time PCR amplification. An internal BLV TaqMan probe was used to enhance the specificity and sensitivity of the assay, and a parallel amplification of a single-copy host gene (the bovine leukocyte antigen DRA gene) was used to normalize genomic DNA. The assay is highly specific, sensitive, quantitative and reproducible, and was able to detect BLV in a number of samples that were negative using the previously developed nested PCR assay. The assay was also highly effective in detecting BLV in cattle from a range of international locations. Finally, this assay enabled us to demonstrate that proviral load correlates not only with BLV infection capacity as assessed by syncytium formation, but also with BLV disease progression.

Conclusions

Using our newly developed BLV-CoCoMo-qPCR assay, we were able to detect a wide range of mutated BLV viruses. CoCoMo algorithm may be a useful tool to design degenerate primers for quantification of proviral load for other retroviruses including HTLV and human immunodeficiency virus type 1.

DNA cytosine methylation in the bovine leukemia virus promoter is associated with latency in a lymphoma-derived B-cell line: potential involvement of direct inhibition of cAMP-responsive element (CRE)-binding protein/CRE modulator/activation transcription factor binding

Bovine leukemia virus (BLV) proviral latency represents a viral strategy to escape the host immune system and allow tumor development. Besides the previously demonstrated role of histone deacetylation in the epigenetic repression of BLV expression, we showed here that BLV promoter activity was induced by several DNA methylation inhibitors (such as 5-aza-2'-deoxycytidine) and that overexpressed DNMT1 and DNMT3A, but not DNMT3B, down-regulated BLV promoter activity. Importantly, cytosine hypermethylation in the 5'-long terminal repeat (LTR) U3 and R regions was associated with true latency in the lymphoma-derived B-cell line L267 but not with defective latency in YR2 cells. Moreover, the virus-encoded transactivator Tax(BLV) decreased DNA methyltransferase expression levels, which could explain the lower level of cytosine methylation observed in the L267(LTaxSN) 5'-LTR compared with the L267 5'-LTR. Interestingly, DNA methylation inhibitors and Tax(BLV) synergistically activated BLV promoter transcriptional activity in a cAMP-responsive element (CRE)-dependent manner. Mechanistically, methylation at the -154 or -129 CpG position (relative to the transcription start site) impaired in vitro binding of CRE-binding protein (CREB) transcription factors to their respective CRE sites. Methylation at -129 CpG alone was sufficient to decrease BLV promoter-driven reporter gene expression by 2-fold. We demonstrated in vivo the recruitment of CREB/CRE modulator (CREM) and to a lesser extent activating transcription factor-1 (ATF-1) to the hypomethylated CRE region of the YR2 5'-LTR, whereas we detected no CREB/CREM/ATF recruitment to the hypermethylated corresponding region in the L267 cells. Altogether, these findings suggest that site-specific DNA methylation of the BLV promoter represses viral transcription by directly inhibiting transcription factor binding, thereby contributing to true proviral latency.

Transcriptional regulation of the bovine leukemia virus promoter by the cyclic AMP-response element modulator tau isoform

Bovine leukemia virus (BLV) expression is controlled at the transcriptional level through three Tax(BLV)-responsive elements (TxREs) responsive to the viral transactivator Tax(BLV). The cAMP-responsive element (CRE)-binding protein (CREB) has been shown to interact with CRE-like sequences present in the middle of each of these TxREs and to play critical transcriptional roles in both basal and Tax(BLV)-transactivated BLV promoter activity. In this study, we have investigated the potential involvement of the cAMP-response element modulator (CREM) in BLV transcriptional regulation, and we have demonstrated that CREM proteins were expressed in BLV-infected cells and bound to the three BLV TxREs in vitro. Chromatin immunoprecipitation assays using BLV-infected cell lines demonstrated in the context of chromatin that CREM proteins were recruited to the BLV promoter TxRE region in vivo. Functional studies, in the absence of Tax(BLV), indicated that ectopic CREMtau protein had a CRE-dependent stimulatory effect on BLV promoter transcriptional activity. Cross-link of the B-cell receptor potentiated CREMtau transactivation of the viral promoter. Further experiments supported the notion that this potentiation involved CREMtau Ser-117 phosphorylation and recruitment of CBP/p300 to the BLV promoter. Although CREB and Tax(BLV) synergistically transactivated the BLV promoter, CREMtau repressed this Tax(BLV)/CREB synergism, suggesting that a modulation of the level of Tax(BLV) transactivation through opposite actions of CREB and CREMtau could facilitate immune escape and allow tumor development.

Defibrinated bovine plasma inhibits retroviral transcription by blocking p52 activation of the NFkappaB element in the long terminal repeat

Bovine leukemia virus (BLV) induces a persistent but latent infection in cattle. Viral latency is invoked by a protein known as plasma blocking factor (PBF) that is found in both bovine and human plasma. We report here on pathways that mediate latency in the presence of PBF. Reporter-gene constructs driven by the promoters of 6 retroviruses were used to measure the production of chloramphenicol acetyl transferase (CAT) in cell lines cultured with or without defibrinated bovine plasma. Plasma inhibited CAT production only in constructs containing an NFkappaB-binding element proximal to the initiation site (BLV, human immunodeficiency virus, and human T-cell leukemia virus). The promoters of Bovine immunodeficiency virus, Feline immunodeficiency virus, or Feline leukemia virus were not inhibited in the presence of bovine plasma. Using gel mobility shift assays, we demonstrated that activation of viral transcription upon stimulation with phorbol esters and ionomycin was mediated through the NFkappaB element and that this was abrogated in the presence of plasma. Furthermore, analysis of individual NFkappaB proteins in nuclear extracts of mononuclear cells or Jurkat cells showed that all 5 members of the NFkappaB family were upregulated in response to stimulation, but only p52 was significantly downregulated in the presence of bovine plasma. Thus, we infer that plasma effects are mediated through interference with either p52 translocation to the nucleus or p52 synthesis.

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.

Peripheral blood B-cell death compensates for excessive proliferation in lymphoid tissues and maintains homeostasis in bovine leukemia virus-infected sheep

The size of a lymphocyte population is primarily determined by a dynamic equilibrium between cell proliferation and death. Hence, lymphocyte recirculation between the peripheral blood and lymphoid tissues is a key determinant in the maintenance of cell homeostasis. Insights into these mechanisms can be gathered from large-animal models, where lymphatic cannulation from individual lymph nodes is possible. In this study, we assessed in vivo lymphocyte trafficking in bovine leukemia virus (BLV)-infected sheep. With a carboxyfluorescein diacetate succinimidyl ester labeling technique, we demonstrate that the dynamics of lymphocyte recirculation is unaltered but that accelerated proliferation in the lymphoid tissues is compensated for by increased death in the peripheral blood cell population. Lymphocyte homeostasis is thus maintained by biphasic kinetics in two distinct tissues, emphasizing a very dynamic process during BLV infection.

Dissemination of bovine leukemia virus-infected cells from a newly infected sheep lymph node

To investigate the early establishment of bovine leukemia virus (BLV) infection, we injected BLV-infected or mock-infected allogeneic cells into the shoulder of sheep in which an efferent lymphatic duct of the draining prescapular lymph node had been cannulated. Rare mononuclear cells acting as centers of BLV infection in culture were present within 4 to 6 days in efferent lymph and within 6 to 10 days in blood. Soon after BLV injection, immunoglobulin M+ (IgM+) and CD8+ cells increased in efferent lymph and oscillated reciprocally in frequency. CD8+ blasts increased on days 4 to 6, when infectious centers increased 100-fold in lymph. On days 6 and 7, both lymph and blood were enriched with CD8+ cells that were labeled late on day 5 with an intravenous pulse of 5-bromo-2'-deoxyuridine (BrdU). Lymph, but not blood, was enriched with BrdU+ B cells on day 7. Capsid-specific antibodies became detectable in efferent lymph on days 6 to 8 and surface glycoprotein-specific antibodies on day 9, preceding their detection in serum by 9 to 14 days. Systemic dissemination of BLV-infected cells was thus accompanied by an increase in proliferating CD8+ cells and the onset of BLV-specific antibodies in lymph. Infectious centers reached maximum frequencies of 0.2% in lymph by days 11 to 13, and then their frequencies increased by 5- to 40-fold in blood cells, suggesting that many infected blood cells do not recirculate back into lymph. Beginning on days 10 to 13, a subpopulation of B cells having high levels of surface IgM increased sharply in peripheral blood. Such cells were not present in lymph. After a day 16 pulse of BrdU, recently proliferated cells that stained intensely for surface IgM appeared in blood within 15 h. Predominantly B lymphocytes contained the viral capsid protein when lymph and blood cells were cultured briefly to allow BLV expression. However, both early in lymph and later in blood, BrdU+ B cells greatly exceeded productively infected cells, indicating that new BLV infections stimulate proliferation of two different populations of B cells.

Intestinal Shiga toxin-producing Escherichia coli bacteria mitigate bovine leukemia virus infection in experimentally infected sheep

Ruminants often carry gastrointestinal Shiga toxin (Stx)-producing Escherichia coli (STEC). Stxs belong to a large family of ribosome-inactivating proteins (RIPs), found in many plants and some bacteria. Plant RIPs, secreted into extracellular spaces, limit the spread of viruses through plant tissues by penetrating and killing virally infected cells. Previously, we showed Stx activity against bovine leukemia virus (BLV)-infected cells in vitro and hypothesized that STEC bacteria have antiviral activity in ruminant hosts. Here, we investigated the impact of STEC on the initial phases of BLV infection in sheep. Sheep were treated with biweekly oral doses of E. coli O157:H7 (an STEC) or an isogenic stx mutant strain. A different group of sheep were similarly treated with five naturally occurring ovine STEC isolates or stx-negative E. coli. Intestinal STEC bacteria were enumerated and identified by standard fecal culture and DNA hybridization. Oral STEC treatment did not always result in carriage of STEC, although many animals consistently presented with >10(4) CFU/g feces. BLV viremia was assessed by spontaneous lymphocyte proliferation (SLP) in cultures of blood mononuclear cells and by syncytium formation in cocultures of the same with F-81 indicator cells. SLP was lower (P < 0.05) and syncytia were fewer (P < 0.05) in STEC-treated sheep than in untreated sheep. Both lower SLP and fewer syncytia positively correlated with fecal STEC numbers. Average weight gain post-BLV challenge was higher in STEC-treated sheep than in untreated sheep (P < 0.05). These results support the hypothesis that in ruminants, intestinal STEC bacteria have antiviral activity and mitigate BLV-induced disease.

Induction of expression of bovine leukemia virus (BLV) in blood taken from BLV-infected cows without removal of plasma

The expression of bovine leukemia virus (BLV) is blocked at the transcriptional level during the so-called latency period. However, when peripheral blood mononuclear cells and B lymphocytes are isolated from BLV-infected animals and incubated in the presence of activating reagents, such as phorbol ester, the expression of BLV is markedly enhanced. Such "reactivation" is thought to play a crucial role in the spread of BLV from infected to uninfected cattle. In the present study, we found that the expression of BLV in samples of whole blood from BLV-infected cattle was activated immediately upon incubation at 37 degrees C and that such activation did not require the addition of any exogenous factors except for anticoagulants or the removal of blood cells from plasma. The expression of BLV was repressed by an inhibitor of protein kinase C (PKC), namely, H-7, and by a membrane-permeable chelator of Ca2+ ions, BAPTA/AM. We also found that several isotypes of PKC were translocated immediately from the cytoplasm to the membrane fraction upon incubation of whole blood at 37 degrees C. Our data suggest that the actual collection of blood and pathways that involve PKC and Ca2+ might play important roles in the reactivation of expression of BLV in blood from infected cattle.

Valproate activates bovine leukemia virus gene expression, triggers apoptosis, and induces leukemia/lymphoma regression in vivo

Leukemogenic viruses like human T-lymphotropic virus and bovine leukemia virus (BLV) presumably persist in the host partly by latent integration of the provirus in a fraction of infected cells, leading to accumulative increase in the outgrowth of transformed cells. Furthermore, viral infection also correlates with a blockade of the apoptotic mechanisms concomitant with an apparent latency of the host cell. Conceptually, induction of viral or cellular gene expression could thus also be used as a therapeutic strategy against retroviral-associated leukemia. Here, we provide evidence that valproate, an inhibitor of deacetylases, activates BLV gene expression in transient transfection experiments and in short-term cultures of primary B-lymphocytes. In vivo, valproate injection into newly BLV-inoculated sheep did not abrogate primary infection. However, valproate treatment, in the absence of any other cytotoxic drug, was efficient for leukemia/lymphoma therapy in the sheep model leading to decreased lymphocyte numbers (respectively from 25.6, 35.7, and 46.5 x 10(3) cells per mm3 to 1.0, 10.6, and 24.3 x 10(3) cells per mm3 in three leukemic sheep) and tumor regression (from >700 cm3 to undetectable). The concept of a therapy that targets the expression of viral and cellular genes might be a promising treatment of adult T cell leukemia or tropical spastic paraparesis/human T-lymphotropic virus-associated myelopathy, diseases for which no satisfactory treatment exists so far.

Involvement of bovine leukemia virus in induction and inhibition of apoptosis

In a previous study, we identified an interesting mutant form of the Tax protein of bovine leukemia virus (BLV), designated D247G, that has an enhanced capacity to transactivate the long terminal repeat (LTR) of BLV and the cellular proto-oncogene c-fos when compared with wild-type Tax (wt-Tax). We demonstrate here that an infectious strain of BLV containing the mutant D247G form of Tax also differs in its capacity to modulate cell survival both positively and negatively. When peripheral blood mononuclear cells (PBMCs) infected with wild-type or mutant BLV are cultured ex vivo with staurosporine, an agent known to induce a mitochondrial caspase cascade pathway regulating apoptosis, the rate of apoptosis is reduced to a greater extent in cells infected with mutant BLV than wild-type BLV, consistent with previous observations in cultures without staurosporine. The increase in survival was associated with an increase in expression of mRNA of bcl-xl but not bcl-2 and bax ex vivo. In contrast, when a tissue culture-adapted cell line, 293T, was transiently transfected with either wild-type or mutant BLV, apoptosis was induced. The increase in the rate of apoptosis was higher in cells transfected with mutant BLV. The same difference was noted in cells transiently transfected with wild-type and mutant D247G Tax, suggesting that the observed positive and negative modulation of cell survival is attributed to the functional characteristics of mutant D247G Tax.

In vivo transcription of bovine leukemia virus and bovine immunodeficiency-like virus

Cellular tropism and transcription of bovine leukemia virus (BLV) and bovine immunodeficiency-like virus (BIV) were investigated using peripheral blood mononuclear cells (PBMC) collected from a cow infected with both viruses. Each PBMC subset, purified by magnetic cell sorting, was subjected to PCR and RT-PCR for detection of their integrated proviruses and transcript mRNAs. Both BLV and BIV genomes were detected by nested PCR in CD3(+), CD4(+), CD8(+) and gammadelta T cells, B cells and monocytes. However, BLV tax transcription was only detected in B cells, and only B cells also formed BLV syncytia in CC81 cells. On the other hand, BIV transcript was detected in each subpopulation of PBMC. These results indicated that BLV can infect T cells and monocytes as well as B cells, but can be expressed by transcription only in B cells. In contrast, BIV can express its transcripts in all infected cells.

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

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

Latency of viral expression in vivo is not related to CpG methylation in the U3 region and part of the R region of the long terminal repeat of bovine leukemia virus

Bovine leukemia virus (BLV) is silent in most cells detectable in vivo, and the repression of its expression allows BLV to evade the host's immune response. In this study, we examined whether CpG methylation of DNA might be involved in the regulation of the expression of BLV in vivo. To investigate the effects of CpG methylation on the activity of the long terminal repeat (LTR) of BLV, we measured the transactivation activity of this region after treatment with the CpG methyltransferase SssI by using a luciferase reporter system. The activity of methylated LTR was significantly lower than that of nonmethylated LTR. Therefore, we examined the extent of CpG methylation of the U3 region and part of the R region of the LTR in BLV-infected cattle and in experimentally BLV-infected sheep at various clinical stages by the bisulfite genomic sequencing method. We detected no or minimal CpG methylation at all stages examined in cattle and sheep, and our results indicate that CpG methylation probably does not participate in the silencing of BLV in vivo.

A mutant form of the tax protein of bovine leukemia virus (BLV), with enhanced transactivation activity, increases expression and propagation of BLV in vitro but not in vivo

In a previous study, we identified an interesting mutant form of the Tax protein of bovine leukemia virus (BLV), designated D247G. This mutant protein strongly transactivated the long terminal repeat of BLV and was also able to transactivate the cellular proto-oncogene c-fos. This finding suggested that BLV that encode the mutant protein might propagate and induce lymphoma more efficiently than wild-type BLV. To characterize the effects of the strong transactivation activity of the mutant Tax protein, we constructed an infectious molecular clone of BLV that encoded D247G and examined the replication and propagation of the virus in vitro and in vivo. Cultured cells were transfected with the wild-type and mutant BLV, and then levels of viral proteins and particles and the propagation of viruses were compared. As expected, in vitro, mutant BLV produced more viral proteins and particles and was transmitted very effectively. We injected the wild-type and mutant BLV into sheep, which are easily infected with BLV, and monitored the proportion of BLV-positive cells in the blood and the expression of BLV RNA for 28 weeks. By contrast to the results of our analyses in vitro, we found no significant difference in the viral load or the expression of viral RNA between sheep inoculated with wild-type or mutant BLV. Our observations indicate that the mutant D247G Tax protein does not enhance the expansion of BLV and that there might be a dominant mechanism for regulation of the expression of BLV in vivo.

Inhibition of histone deacetylases induces bovine leukemia virus expression in vitro and in vivo

Packaging into nucleosomes results in a global transcriptional repression as a consequence of exclusion of sequence-specific factors. This inhibition can be relieved by using inhibitors of histone deacetylases, acetylation being a major characteristic of transcriptionally active chromatin. Paradoxically, the expression of only approximately 2% of the total cellular genes is modulated by histone hyperacetylation. To unravel the potential role of this transcriptional control on BLV expression, we tested the effect of two highly specific inhibitors of deacetylases, trichostatin A (TSA) and trapoxin (TPX). Our results demonstrate that treatment with TSA efficiently enhanced long terminal repeat-directed gene expression of integrated reporter constructs in heterologous D17 stable cell lines. To further examine the biological relevance of these observations made in vitro, we analyzed ex vivo-isolated peripheral blood mononuclear cells (PBMCs) from bovine leukemia virus (BLV)-infected sheep. TSA deacetylase inhibitor induced a drastic increase in viral expression at levels comparable to those induced by treatment with phorbol-12-myristate 13-acetate and ionomycin, the most efficient activators of BLV expression known to date. TSA acted directly on BLV-infected B lymphocytes to increase viral expression and does not seem to require T-cell cooperation. Inhibition of deacetylation after treatment with TSA or TPX also significantly increased viral expression in PBMCs from cattle, the natural host for BLV. Together, our results show that BLV gene expression is, like that of a very small fraction of cellular genes, also regulated by deacetylation.

Suboptimal enhancer sequences are required for efficient bovine leukemia virus propagation in vivo: implications for viral latency

Repression of viral expression is a major strategy developed by retroviruses to escape from the host immune response. The absence of viral proteins (or derived peptides) at the surface of an infected cell does not permit the establishment of an efficient immune attack. Such a strategy appears to have been adopted by animal oncoviruses such as bovine leukemia virus (BLV) and human T-cell leukemia virus (HTLV). In BLV-infected animals, only a small fraction of the infected lymphocytes (between 1 in 5,000 and 1 in 50,000) express large amounts of viral proteins; the vast majority of the proviruses are repressed at the transcriptional level. Induction of BLV transcription involves the interaction of the virus-encoded Tax protein with the CREB/ATF factors; the resulting complex is able to interact with three 21-bp Tax-responsive elements (TxRE) located in the 5' long terminal repeat (5' LTR). These TxRE contain cyclic AMP-responsive elements (CRE), but, remarkably, the "TGACGTCA" consensus is never strictly conserved in any viral strain (e.g.,AGACGTCA, TGACGGCA, TGACCTCA). To assess the role of these suboptimal CREs, we introduced a perfect consensus sequence within the TxRE and showed by gel retardation assays that the binding efficiency of the CREB/ATF proteins was increased. However, trans-activation of a luciferase-based reporter by Tax was not affected in transient transfection assays. Still, in the absence of Tax, the basal promoter activity of the mutated LTR was increased as much as 20-fold. In contrast, mutation of other regulatory elements within the LTR (the E box, NF-kappa B, and glucocorticoid- or interferon-responsive sites [GRE or IRF]) did not induce a similar alteration of the basal transcription levels. To evaluate the biological relevance of these observations made in vitro, the mutations were introduced into an infectious BLV molecular clone. After injection into sheep, it appeared that all the recombinants were infectious in vivo and did not revert into a wild-type virus. All of them, except one, propagated at wild-type levels, indicating that viral spread was not affected by the mutation. The sole exception was the CRE mutant; proviral loads were drastically reduced in sheep infected with this type of virus. We conclude that a series of sites (NF-kappa B, IRF, GRE, and the E box) are not required for efficient viral spread in the sheep model, although mutation of some of these motifs might induce a minor phenotype during transient transfection assays in vitro. Remarkably, a provirus (pBLV-Delta 21-bp) harboring only two TxRE was infectious and propagated at wild-type levels. And, most importantly, reconstitution of a consensus CRE, within the 21-bp enhancers increases binding of CREB/ATF proteins but abrogates basal repression of LTR-directed transcription in vitro. Suboptimal CREs are, however, essential for efficient viral spread within infected sheep, although these sites are dispensable for infectivity. These results suggest an evolutionary selection of suboptimal CREs that repress viral expression with escape from the host immune response. These observations, which were obtained in an animal model for HTLV-1, are of interest for oncovirus-induced pathogenesis in humans.

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

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

Discordance between bovine leukemia virus tax immortalization in vitro and oncogenicity in vivo

Bovine leukemia virus (BLV) Tax protein, a transcriptional activator of viral expression, is essential for viral replication in vivo. Tax is believed to be involved in leukemogenesis because of its second function, immortalization of primary cells in vitro. These activities of Tax can be dissociated on the basis of point mutations within specific regions of the protein. For example, mutation of the phosphorylation sites at serines 106 and 293 abrogates immortalization potential in vitro but maintains transcriptional activity. This type of mutant is thus particularly useful for unraveling the role of Tax immortalization activity during leukemogenesis independently of viral replication. In this report, we describe the biological properties of BLV recombinant proviruses mutated in the Tax phosphorylation sites (BLVTax106+293). Titration of the proviral loads by semiquantitative PCR revealed that the BLV mutants propagated at wild-type levels in vivo. Furthermore, two animals (sheep 480 and 296) infected with BLVTax106+293 developed leukemia or lymphosarcoma after 16 and 36 months, respectively. These periods of time are within the normal range of latencies preceding the onset of pathogenesis induced by wild-type viruses. The phenotype of the mutant-infected cells was characteristic of a B lymphocyte (immunoglobulin M positive) expressing CD11b and CD5 (except at the final stage for the latter marker), a pattern that is typical of wild-type virus-infected target cells. Interestingly, the transformed B lymphocytes from sheep 480 also coexpressed the CD8 marker, a phenotype rarely observed in tumor biopsies from chronic lymphocytic leukemia patients. Finally, direct sequencing of the tax gene demonstrated that the leukemic cells did not harbor revertant proviruses. We conclude that viruses expressing a Tax mutant unable to transform primary cells in culture are still pathogenic in the sheep animal model. Our data thus provide a clear example of the discordant conclusions that can be drawn from in vitro immortalization assays and in vivo experiments. These observations could be of interest for other systems, such as the related human T-cell leukemia virus type 1, which currently lack animal models allowing the study of the leukemogenic process.

In vitro infection of cells of the monocytic/macrophage lineage with bovine leukaemia virus

The oncogenic retrovirus bovine leukaemia virus (BLV) primarily infects B cells. Most infected animals remain asymptomatic for long periods of time before an increase in circulating B cells or localized tumours can be observed. This long clinical latency period may be explained by cells of the monocyte/macrophage lineage (M/M) becoming infected and acting as a reservoir for the virus, as shown for other retroviruses (human immunodeficiency virus-1, feline immunodeficiency virus). M/M cells in different stages of differentiation (HL-60, THP-1, U-937, J774, BGM, PM2, primary macrophages of sheep and cows) were cultured with BLV produced by permanently infected donor cells (FLKBLV and BLV-bat(2)). Donor cells were inhibited from multiplying by either irradiation or treatment with mitomycin C. In other experiments, supernatant from donor cells containing virus was used. In co-culture with the donor cells, the less differentiated monocytic cells showed severe cellular changes such as differentiation, vacuolization, cell lysis and membrane blebbing; apoptosis was a frequent phenomenon. Budding and extracellular viruses were also observed. The more differentiated macrophage cells, although they showed less signs of infection by microscopy, had a complete BLV protein profile, as seen by Western blotting; bands corresponding to p24CA (Gag) and its precursors were clearly seen. In addition, gp51SU was identified by syncytia formation assays. It is concluded that M/M cells may be infected by BLV, the consequences of the infection differing according to the type of cell.

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.