Experimental cross-transmissions of bovine leukemia virus (BLV) between several animal species

No evidence of bovine leukemia virus proviral DNA and antibodies in human specimens from Japan

Background

The potential risk and association of bovine leukemia virus (BLV) with human remains controversial as it has been reported to be both positive and negative in human breast cancer and blood samples. Therefore, establishing the presence of BLV in comprehensive human clinical samples in different geographical locations is essential.

Result

In this study, we examined the presence of BLV proviral DNA in human blood and breast cancer tissue specimens from Japan. PCR analysis of BLV provirus in 97 Japanese human blood samples and 23 breast cancer tissues showed negative result for all samples tested using long-fragment PCR and highly-sensitive short-fragment PCR amplification. No IgG and IgM antibodies were detected in any of the 97 human serum samples using BLV gp51 and p24 indirect ELISA test. Western blot analysis also showed negative result for IgG and IgM antibodies in all tested human serum samples.

Conclusion

Our results indicate that Japanese human specimens including 97 human blood, 23 breast cancer tissues, and 97 serum samples were negative for BLV.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12977-022-00592-6.

Animal models of acute lymphoblastic leukemia: Recapitulating the human disease to evaluate drug efficacy and discover therapeutic targets

Acute lymphoblastic leukemia (ALL) is a malignant hematologic tumor with highly aggressive characteristics, which is prone to relapse, has a poor prognosis and few clinically effective drugs. It is meaningful to gain a better understanding of its pathogenesis in order to discover and evaluate potential therapeutic drugs and new treatment targets. The goal of developing novel targeted drugs and treatment methods is to increase complete remission, reduce toxicity and morbidity, and that is also the most important prerequisite for modern leukemia treatment. However, the process of new drugs from research and development to clinical application is long and difficult. Many promising drugs were rejected by the USFoodandDrugAdministration(FDA) due to serious adverse drug reactions (ADR) in clinical phase I trials. Animal models provide us with an excellent tool to understand the complex pathological mechanisms of human diseases, to evaluate the potential of new targeted drugs and therapeutic approaches to treat ALL in vivo and, more importantly, to assess the potential ADR they may have on healthy organs. In this article we review ALL animal models' progression, their roles in revealing the pathogenesis of ALL and drug development. Additionally, we mainly focus on the mouse models, especially xenotransplantation and transgenic models that more closely reproduce the human phenotype. In conclusion, we summarize the advantages and limitations of each model, thereby facilitating further understanding the etiology of ALL, and eventually contributing to the effective management of the disease.

Evidence of bovine leukemia virus circulating in sheep and buffaloes in Colombia: insights into multispecies infection

Bovine leukemia virus (BLV) is the causative agent of leukemia/lymphoma in cattle. However, previous evidence has shown its presence in other species of livestock as well as in humans, suggesting that other species can be accidental hosts of the virus. In viral infections, receptors that are common to different animal species are proposed to be involved in cross-species infections. For BLV, AP3D1 has been proposed to be its receptor, and this protein is conserved in most mammalian species. In Colombia, BLV has been reported in cattle with high prevalence rates, but there has been no evidence of BLV infections in other animal species. In this study, we tested for the virus in sheep (n = 44) and buffaloes (n = 61) from different regions of Colombia by nested PCR, using peripheral blood samples collected from the animals. BLV was found in 25.7% of the animals tested (12 buffaloes and 15 sheep), and the results were confirmed by Sanger sequencing. In addition, to gain more information about the capacity of the virus to infect these species, the predicted interactions of AP3D1 of sheep and buffaloes with the BLV-gp51 protein were analyzed in silico. Conserved amino acids in the binding domains of the proteins were identified. The detection of BLV in sheep and buffaloes suggests circulation of the virus in multiple species, which could be involved in dissemination of the virus in mixed livestock production settings. Due to the presence of the virus in multiple species and the high prevalence rates observed, integrated prevention and control strategies in the livestock industry should be considered to decrease the spread of BLV.

Co-Circulation of Bovine Leukemia Virus Haplotypes among Humans, Animals, and Food Products: New Insights of Its Zoonotic Potential

Bovine leukemia virus (BLV) is the causative agent of leukemia/lymphoma in cattle. It has been found in humans and cattle-derived food products. In humans, it is described as a potential risk factor for breast cancer development. However, the transmission path remains unclear. Here, a molecular epidemiology analysis was performed to identify signatures of genetic flux of BLV among humans, animals, and food products. Sequences obtained from these sources in Colombia were used (n = 183) and compared with reference sequences available in GenBank. Phylogenetic reconstruction was performed in IQ-TREE software with the maximum likelihood algorithm. Haplotype (hap) distribution among the population was carried out with a median-joining model in Network5.0. Recombination events were inferred using SplitsTree4 software. In the phylogenetic analysis, no specific branches were identified for the Colombian sequences or for the different sources. A total of 31 haps were found, with Hap 1, 4, 5 and 7 being shared among the three sources of the study. Reticulation events among the different sources were also detected during the recombination analysis. These results show new insights about the zoonotic potential of BLV, showing evidence of genetic flux between cattle and humans. Prevention and control strategies should be considered to avoid viral dissemination as part of the One Health program policies.

CAT1/SLC7A1 acts as a cellular receptor for bovine leukemia virus infection

Bovine leukemia virus (BLV) is the causative agent of enzootic bovine leukosis, the most common neoplastic disease of cattle, which is closely related to human T-cell leukemia viruses. BLV has spread worldwide and causes a serious problem for the cattle industry. The cellular receptor specifically binds with viral envelope glycoprotein (Env), and this attachment mediates cell fusion to lead virus entry. BLV Env reportedly binds to cationic amino acid transporter 1 (CAT1)/solute carrier family 7 member 1 (SLC7A1), but whether the CAT1/SLC7A1 is an actual receptor for BLV remains unknown. Here, we showed that CAT1 functioned as an infection receptor, interacting with BLV particles. Cells expressing undetectable CAT1 levels were resistant to BLV infection but became highly susceptible upon CAT1 overexpression. CAT1 exhibited specific binding to BLV particles on the cell surface and colocalized with the Env in endomembrane compartments and membrane. Knockdown of CAT1 in permissive cells significantly reduced binding to BLV particles and BLV infection. Expression of CAT1 from various species demonstrated no species specificity for BLV infection, implicating CAT1 as a functional BLV receptor responsible for its broad host range. These findings provide insights for BLV infection and for developing new strategies for treating BLV and preventing its spread.-Bai, L., Sato, H., Kubo, Y., Wada, S., Aida, Y. CAT1/SLC7A1 acts as a cellular receptor for bovine leukemia virus infection.

Experimental infection of sheep with Bovine leukemia virus (BLV): Minimum dose of BLV-FLK cells and cell-free BLV and neutralization activity of natural antibodies

Bovine leukemia virus (BLV) is an important cattle pathogen that causes major economic losses worldwide, especially in dairy farms. The use of animal models provides valuable insight into the pathogenesis of viral infections. Experimental infections of sheep have been conducted using blood from BLV-infected cattle, infectious BLV molecular clones or tumor-derived cells. The Fetal Lamb Kidney cell line, persistently infected with BLV (FLK-BLV), is one of the most commonly used long-term culture available for the permanent production of virus. FLK-BLV cells or the viral particles obtained from the cell-free culture supernatant could be used as a source of provirus or virus to experimentally infect sheep. In this report, we aimed to determine the minimum amount of FLK-BLV cells or cell-free supernatant containing BLV needed to produce infection in sheep. We also evaluated the amount of antibodies obtained from a naturally-infected cow required to neutralize this infection. We observed that both sheep experimentally inoculated with 5000 FLK-BLV cells became infected, as well as one of the sheep receiving 500 FLK-BLV cells. None of the animals inoculated with 50 FLK-BLV cells showed evidence of infection. The cell-free FLK-BLV supernatant proved to be infective in sheep up to a 1:1000 dilution. Specific BLV antibodies showed neutralizing activity as none of the sheep became infected. Conversely, the animals receiving a BLV-negative serum showed signs of BLV infection. These results contribute to the optimization of a sheep bioassay which could be useful to further characterize BLV infection.

Mutation of a Single Envelope N-Linked Glycosylation Site Enhances the Pathogenicity of Bovine Leukemia Virus

Viruses have coevolved with their host to ensure efficient replication and transmission without inducing excessive pathogenicity that would indirectly impair their persistence. This is exemplified by the bovine leukemia virus (BLV) system in which lymphoproliferative disorders develop in ruminants after latency periods of several years. In principle, the equilibrium reached between the virus and its host could be disrupted by emergence of more pathogenic strains. Intriguingly but fortunately, such a hyperpathogenic BLV strain was never observed in the field or designed in vitro. In this study, we sought to understand the role of envelope N-linked glycosylation with the hypothesis that this posttranslational modification could either favor BLV infection by allowing viral entry or allow immune escape by using glycans as a shield. Using reverse genetics of an infectious molecular provirus, we identified a N-linked envelope glycosylation site (N230) that limits viral replication and pathogenicity. Indeed, mutation N230E unexpectedly leads to enhanced fusogenicity and protein stability.

IMPORTANCE

Infection by retroviruses requires the interaction of the viral envelope protein (SU) with a membrane-associated receptor allowing fusion and release of the viral genomic RNA into the cell. We show that N-linked glycosylation of the bovine leukemia virus (BLV) SU protein is, as expected, essential for cell infection in vitro. Consistently, mutation of all glycosylation sites of a BLV provirus destroys infectivity in vivo. However, single mutations do not significantly modify replication in vivo. Instead, a particular mutation at SU codon 230 increases replication and accelerates pathogenesis. This unexpected observation has important consequences in terms of disease control and managing.

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.

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.

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.

Genetic determinants of bovine leukemia virus pathogenesis

The understanding of HTLV-induced disease is hampered by the lack of a suitable animal model allowing the study of both viral replication and leukemogenesis in vivo. Although valuable information has been obtained in different species, such as rabbits, mice, rats, and monkeys, none of these systems was able to conciliate topics as different as viral infectivity, propagation within the host, and generation of leukemic cells. An alternate strategy is based on the understanding of diseases induced by viruses closely related to HTLV-1, like bovine leukemia virus (BLV). Both viruses indeed belong to the same subfamily of retroviruses, harbor a similar genomic organization, and infect and transform cells of the hematopoietic system. The main advantage of the BLV system is that it allows direct experimentation in two different species, cattle and sheep.

T-cell responses to highly conserved CD4 and CD8 epitopes on the outer membrane protein of bovine leukemia virus: relevance to vaccine development

Bovine leukemia virus (BLV) is a retrovirus that infects cattle and sheep and may provide a model for studying human leukemia. Cell-mediated immune mechanisms may play a major role in protection against BLV infection. We describe here for the first time the identification of proliferative (CD4) and cytotoxic T-lymphocyte (CD8) epitopes of the gp51 envelope (env) protein of BLV. This protein and a recombinant form expressed by a vaccinia virus construct have been shown to be potential vaccine candidates. A complete series of overlapping peptides, 20 amino acids in length, was prepared to identify epitopes from gp51. These peptides were tested for the ability to elicit peripheral blood lymphocyte proliferation and cytotoxic T-lymphocyte responses in infected and uninfected cattle and sheep. Peptides 51-70 and 61-80 produced a proliferative response in lymphocytes from only uninfected animals (both sheep and cattle), and this was shown by T-cell subset deletion to be a CD4-mediated response. Seven BLV-infected sheep did not show a response to either peptide. Cytotoxic T-lymphocyte activity, however, was associated only with peptides 121-140 and 131-150. In this case, the response was demonstrated to be CD8 dependent and was found only in BLV-infected animals (sheep). Knowledge of the location of these T-cell recognition domains will complement data available on B-cell epitopes in gp51 and may be useful in the design of a subunit vaccine.

T lymphocyte responses of sheep to bovine leukaemia virus infection

Sheep were experimentally infected with bovine leukaemia virus (BLV) by inoculation of peripheral blood lymphocytes (PBL) from BLV infected sheep. Monoclonal antibodies were used to monitor changes in lymphocyte subpopulations in the first few weeks after inoculation. The polymerase chain reaction (PCR) detected BLV DNA in PBL of infected sheep 11-15 days after inoculation, that is, before antibodies to viral structural proteins were detected at 15-39 days post-inoculation. A rise in the number of both B and T lymphocytes coincided with detection of infection by PCR. At this time, an increase in the number of circulation CD8+ lymphocytes resulted in a low CD4: CD8 ratio. It appears that in BLV infection there is a host specific cell-mediated immune response to infected lymphocytes rather than a general immune response to foreign antigens. This response, which is characterized by an increase in the number of circulating CD8+ lymphocytes, precedes seroconversion. There is considerable variation between animals in this cytotoxic T lymphocyte response.

Early detection of bovine leukosis virus DNA in infected sheep using the polymerase chain reaction

The early diagnosis of bovine leukosis virus (BLV) infection, the aetiological agent in enzootic bovine leukosis, is important for the implementation of control measures. BLV infection is currently assessed by the detection of circulating antibodies against the viral envelope protein, gp51. However, this approach has shortcomings in the time taken to detect anti-BLV antibodies (three to four weeks after infection), and in the failure to detect antibodies in some animals. Clearly a technique such as the polymerase chain reaction (PCR), which directly detects the presence of viral DNA, has advantages over methods designed to measure host antibodies. The use of PCR for the detection of proviral DNA in an affected DNA sample with as little as 10(-5) micrograms of host DNA using agarose gel electrophoresis followed by ethidium bromide staining is described here. It was possible to improve the sensitivity of this assay by using hybridisation analysis with a BLV gene probe. PCR used in combination with hybridisation analysis will provide a sensitive diagnostic assay to detect BLV when antibody tests give weakly positive or equivocal results.

Suppression of immunological responses in rabbits experimentally infected with bovine leukemia virus

Ten 2- to 4-month-old rabbits were inoculated subcutaneously with bovine leukemia virus (BLV)-infected bovine or sheep cells. By 6 weeks after inoculation all ten rabbits had converted to BLV antibody-positive, and BLV or BLV antigen was detected in lymphocytes from most of the rabbits tested, although there were few antigen-producing cells. Three rabbits showed continuous respiratory symptoms after infection and one died with pneumonia. Humoral immune responses against mouse serum were significantly suppressed in BLV-infected rabbits compared with non-infected control rabbits. The lymphocyte blastogenesis response was also suppressed in BLV-infected rabbits. At the time of necropsy, six rabbits showed pulmonary lesions; however, none of the BLV-infected rabbits had tumors during an observation period of over 1 year.

Transient increases of blood mononuclear cells that could express bovine leukemia virus early after experimental infection of sheep

To investigate the early spread of bovine leukemia virus (BLV) infection in vivo, we enumerated infected mononuclear cells that could express the BLV genome in vitro as they appeared in the peripheral blood of lambs newly injected with the virus. Cells that transcribed viral RNA within a few hours of isolation and cells that produced infectious virus in culture were first detected in very small numbers. Soon afterward, cells that expressed BLV transiently increased to represent 0.2 to 1.5% of the mononuclear cells. The increases occurred within leukocyte populations of normal size and cellular composition. Then, throughout the rest of the first 8 months, sharply reduced numbers of cells transcribed BLV or produced virus. All the infected animals tested by in situ hybridization displayed increased numbers of cells that transcribed BLV RNA, but only two-thirds had large increases of cells that produced infectious BLV in culture. In addition, BLV-transcribing cells exceeded virus-producing cells at most times after infection. These results demonstrate that transient increases of circulating, expression-competent cells characterize the first 3 to 4 months of BLV infection and that the extent of BLV genome expression by cultured mononuclear cells can differ among animals.

Lymphocyte subpopulations in sheep with lymphosarcoma resulting from experimental infection with bovine leukaemia virus

Sheep were experimentally infected with bovine leukaemia virus (BLV) and developed leukaemia and lymphosarcoma 30-88 weeks later. Ten sheep with lymphosarcoma were necropsied and lymphocyte subpopulations were evaluated in peripheral blood lymphocytes (PBL) and lymphocyte suspensions prepared from a range of lymph nodes, tumours and spleen. The leukaemic phase of BLV infection was characterized by an increase in the number of circulating B lymphocytes. The number of T lymphocytes was also increased with the CD8+ subpopulation proliferating at a much greater rate than the CD4+ subpopulation. In PBL the CD4:CD8 ratio fell rapidly as leukaemia developed, being 1.15 (+/- 0.18) 5-8 weeks before necropsy and 0.38 (+/- 0.09) at necropsy. During this period the number of B lymphocytes increased from 11.2 (+/- 0.7) to 379.4 (+/- 85.8) x 10(9)/L. CD4:CD8 ratios were also low in all lymph nodes and spleens of leukaemic sheep at necropsy. Most of the cells in solid tumours were B lymphocytes but a small population of T lymphocytes with a low CD4:CD8 ratio was identified.

Observations on blood leucocytes and lymphocyte subsets in sheep infected with bovine leukaemia virus: a progressive study

Haematological parameters and reactivity of lymphocyte antigens to monoclonal antibodies were studied over a 10-month period in sheep experimentally infected with bovine leukaemia virus (BLV). BLV-inoculated animals seroconverted within 1 month and showed a significant lymphocytosis 2-6 weeks after infection. Control animals inoculated with BLV-free lymphocytes showed a stronger and more immediate neutrophil response than those inoculated with BLV-positive lymphocytes. One month after infection, BLV-inoculated sheep showed a relative increase of cells bearing antigens T4, T6, T8 and T19, and 10 months into the trial, MHC II lymphocytes increased, T6 remained elevated, but T4 helper cells were significantly decreased in number. Lymphoma tissue showed the presence of T8 cells, and lymph nodes from seroconverted sheep had areas of concentrated T4 staining cells. These results demonstrate responses in cellular immune mechanisms to infection with BLV.

Experimental infection of sheep with bovine leukemia virus: infectivity of blood, nasal and saliva secretions

This study was designed to determine the relative infectivity of lymphocytes and secretions from BLV-infected cattle with and without persistent lymphocytosis (BLV+PL+ and BLV+PL-). Ninety-seven sheep of mixed sex and age were assembled into 21 experimental groups. The recipient sheep were inoculated intravenously with serial dilutions of whole blood, saliva or nasal secretions from BLV+PL+ and BLV+PL- donor cows. Between 200 to 20,000 cells from single and mixed BLV+PL+ or single and mixed BLV+PL- donor cattle were used for inoculation. A very small number of BLV-infected lymphocytes (200 cells) was sufficient to induce BLV infection in sheep inoculated with diluted whole blood from BLV+PL+ cattle. The inoculation of whole blood (containing up to 20,000 lymphocyte cells) from BLV+PL- cattle did not induce BLV infection in recipient sheep. Saliva and nasal secretions also failed to bring about BLV transmission.

The BLV-induced leukemia--lymphosarcoma complex in sheep

Sheep are highly susceptible to BLV infection and can be infected via several different means (routes). In all inoculated animals, specific anti-BLV antibodies can be demonstrated 1 to 3 months post-inoculation (p.i.). Between 10 and 13 months p.i., a moderate but persistent lymphocytosis (PL) may be detected in about 50% of the infected animals. This hematological disorder may be, but is not necessarily, associated with the development of a lymphosarcoma and can (might) be interpreted as a true lymphoid leukemia. According to findings revealed by immunolabelling and mitogen stimulation of peripheral blood lymphocytes, BLV-induced PL appears to be a B-cell disorder. Induced lymphosarcoma appears in about 40% of infected sheep during the 6 years p.i. It too is of B-lymphocyte lineage. In vitro studies demonstrate that BLV antigen is expressed exclusively in B-lymphocytes. Yet, BLV expression is greatly stimulated in whole lymphocyte culture by the addition of T-cell mitogen. This same phenomenon occurs when the supernatant of stimulated T-lymphocyte cultures is added to isolated BLV-infected B-lymphocytes. This observation supports the hypothesis that, as is the case with other retroviruses such as HIV, BLV is able to use the regular activation machinery of the immune system for its own replication and transmission. It seems, therefore, that the leukemia-lymphoma complex in sheep may serve as an accurate experimental model for the study of the biological properties of retroviruses.

Natural mode of horizontal transmission of bovine leukemia virus (BLV): the potential role of tabanids (Tabanus spp.)

In order to evaluate the potential role of hematophagous insects in the natural spreading of bovine leukemia virus (BLV) infection in cattle, a 2-year survey was carried out involving sequential serological tests on 3328 cattle in three different areas. A parallel entomological study was run over the same period, using continuous trapping, in order to determine both the density and variations of horsefly (Tabanus spp.) populations in the close vicinity of the herds. After statistical analysis, this space-time study showed that: (1) There is a significant positive geographical correlation between the rate of incidence of BLV infection and the density of the horsefly population. (2) Seasonal variations in the incidence rate exist; the highest rates are generally observed during summer (from July of September), and the lowest during winter, spring and early summer (from November to mid-July). (3) There is a time link between the rate of sero-conversion and the variations in activity of the horsefly population. All these data combined would appear to indicate that tabanids play a considerable role in the spread of BLV under natural conditions.

Lymphosarcoma development in sheep experimentally infected with bovine leukaemia virus

Twelve sheep were experimentally infected with a phytohemagglutinin (PHA) treated short term culture of lymphocytes from a cow naturally infected with BLV at the PL stage. Five of 12 (42%) BLV infected sheep had histologically confirmed lymphosarcoma 10-16 months after infection. The PBL's were increased to leukemic levels 3-21 weeks before death due to lymphoblastic leukemia. Lymphocyte proliferation and appearance of immature lymphocytes and lymphoblastic cells in the blood were a characteristic feature of tumour development following inoculation with an Australian strain of BLV. In contrast to a number of previous studies the peripheral lymph nodes of all infected sheep were clinically normal throughout the experimental period but at death gross tumours were evident in the mesentric lymph nodes and the heart in all cases. All the other lymph nodes, liver, spleen, kidney and lung were histologically infiltrated with lymphoid tumour cells. Gross tumours were present in the abomasum (1 out of 5) in the urinary tract (2 out of 5) and in the uterus (1 out of 2). The majority of the tumour cells isolated from the various tissues were centroblastic demonstrating that the malignant leukemia in experimentally infected sheep was of a multicentric centroblastic type. The central nervous system was not involved in any case.

Lymphocyte subpopulations in sheep during the early stage of experimental infection with bovine leukaemia virus

Sheep were experimentally infected with bovine leukaemia virus (BLV) by the inoculation of PBL from leukaemic sheep. Antibodies to viral structural proteins were detected at from 2 to 6 weeks after inoculation. At seroconversion, all sheep had a marked increase in the number of circulating lymphocytes, due essentially to an increase in the number of B cells. The number of circulating B cells then decreased but remained higher than pre-infection levels. A second increase in this population preceded the development of a B cell lymphoblastic leukaemia. Generalized lymphosarcoma was diagnosed at necropsy of all sheep. Variation between individual sheep in the time from infection to the development of tumours allowed two clearly delineated groups of nine sheep to be compared. A study of changes in the B cell and T cell populations during the first 16 weeks of infection suggested that the initial response to infection influences the subsequent rate of leukaemogenesis. At seroconversion the number of circulating B cells was significantly higher in group 1 (10.16 +/- 1.51 X 10(9)/l) than in group 2 (6.47 +/- 2.76 X 10(9)/l). Group 1 sheep became leukaemic at 20-50 weeks after infection, whereas group 2 sheep did not do so until 70-95 weeks after infection.

Bovine leukaemia: facts and hypotheses derived from the study of an infectious cancer

Bovine leukaemia virus (BLV) is the etiological agent of chronic lymphatic leukaemia/lymphoma in cows, sheep and goats. Infection without neoplastic transformation was also obtained in pigs, rhesus monkeys, chimpanzees, rabbits and observed in capybaras and water-buffaloes. Structurally and functionally, BLV is a relative of human T lymphotropic viruses 1 and 2 (HTLV-I and HTLV-II) In humans, HTLV-I induces a T-cell leukaemia and its type 2 counterpart has been found in dermatopathic lymphadenopathy, hairy T-cell leukaemia and prolymphocytic leukaemia cases. At variance with HTLV-I, BLV has not been associated with neurological diseases of the degenerative type. Bovine leukaemia virus, HTLV-I and HTLV-II show clearcut sequence homologies. The pathology of the BLV-induced disease, most notably the absence of chronic viraemia, a long latency period and lack of preferred proviral integration sites in tumours, is similar to that of adult T-cell leukaemia/lymphoma induced by HTLV-I. The most striking feature of these three naturally transmitted leukaemia viruses is the X region located between the env gene and the long terminal repeat (LTR) sequence. The X region contains several overlapping long open reading frames. One of them, designated XBL-I, encodes a trans-activator function capable of increasing the level of gene expression directed by BLV-LTR and most probably is involved in "genetic instability" of BLV-infected cells of the B cell lineage. The "genetic instability" renders the infected cell susceptible to move, along a number of stages, towards full malignancy. Little is known about these events and their causes; we present some theoretical possibilities. Bovine leukaemia virus infection has a worldwide distribution. In temperate climates, the virus spreads mostly via iatrogenic transfer of infected lymphocytes. In warm climates and in areas heavily populated by haematophagous insects, there are indications of insect-borne propagation of the virus.

Bovine leukemia: facts and hypotheses derived from the study of an infectious cancer

Bovine leukemia virus is the etiological agent of a chronic lymphatic leukemia/lymphoma in cows, sheep, and goats. Infection without neoplastic transformation also was obtained in pigs, rhesus monkeys, chimpanzees, and rabbits, and was observed in capybaras and water buffaloes. Structurally and functionally, BLV is a relative of the human T lymphotropic viruses (HTLV-I and HTLV-II). HTLV-I induces in humans a T cell leukemia, and its type II counterpart has been found in dermatopathic lymphadenopathy, hairy T cell leukemia and prolymphocytic leukemia cases. At variance with HTLV-I, BLV has not been associated with neurological diseases of the degenerative type. BLV, HTLV-I, and HTLV-II show clearcut sequence homologies. The pathology of the BLV-induced disease, most notably, the absence of chronic viremia, a long latency period, and a lack of preferred proviral integration sites in tumors, is similar to that of adult T cell leukemia/lymphoma induced by HTLV-I. The most striking feature of the three naturally transmitted leukemia viruses is the X region located between the env gene and the LTR sequence. The X region contains several overlapping long open reading frames. One of them designated XBL-I encodes a trans-activator function capable of increasing the level of gene expression directed by BLV-LTR and most probably involved in "genetic instability" of BLV-infected cells of the B cell lineage. The genetic instability puts the cell into a context of fragility and ready to move along a number of stages towards full malignancy. Little is known about these events and their causes; we have presented some theoretical possibilities. BLV infection has a worldwide distribution. In temperate climates the virus spreads mostly via iatrogenic transfer of infected lymphocytes. In warm climates and in areas heavily populated by hematophageous insects, there are indications of insect-born propagation of the virus.

Differences in the lymphoproliferative response of cattle and sheep to bovine leucosis virus infection

Lymphoblastic leukaemia, preceded by a significantly increasing percentage of prolymphocytes in peripheral blood smears for from 12 to 68 weeks before death was a feature of sheep which developed lymphosarcoma following inoculation with the Australian strain of bovine leucosis virus (BLV). Lymphocytosis and/or the appearance of immature cells were a reliable predictor of tumour formation in sheep, but not in cattle. There was a terminal lymphoblastic leukaemia in only 43 of 84 cattle with lymphosarcoma. Differences in the morphological appearance and glycogen content of the leukaemic lymphoblasts of sheep and cattle were observed. In spite of these differences the high frequency of lymphocytosis and lymphosarcoma in experimentally infected sheep suggests that they could be a useful model for studying the pathological and immunological responses to BLV infection.

Epitopes of bovine leukemia virus glycoprotein gp51 recognized by sera of infected cattle and sheep

Sera of BLV-infected cattle and sheep are tested for their reactivity with different gp51 subregions by competition with radiolabeled monoclonal antibodies directed against 8 different gp51 epitopes. Sheep antisera are found to be very polyvalent, since they are able to displace the fixation of any of the mouse monoclonal antibodies to gp51. Bovine antisera do not display significant competition with monoclonal antibodies direct against 5 out of 8 gp51 epitopes. The bovine antibody response to gp51 is focused to a limited subregion of this molecule, bearing 3 epitopes (F, G and H) recognized by antibodies with virus-neutralizing activity. The differential reactivity of cattle and sheep antisera to BLV gp51 is discussed in relation to the pathology of BLV infection in these two species.

Structural and antigenic analysis of the nucleic acid-binding proteins of bovine and feline leukemia viruses

The nucleic acid-binding proteins of bovine leukemia virus (BLV) and feline leukemia virus (FeLV) were isolated in a high state of purity with chloroform-methanol extraction followed by reversed-phase liquid chromatography. Selective solubilization and purity of BLV p12 and FeLV p10 was confirmed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The compositions and molecular weights were determined by amino acid analysis. An abundance of lysine and arginine residues along with their size identifies both BLV p12 and FeLV p10 as small basic proteins similar to well-defined type C viral nucleoproteins. NH2-terminal degradation by the semiautomated Edman method provided the sequence of the first 40 amino acids for both proteins. The putative nucleic acid binding site found in several type C viral nucleoproteins was contained within this sequence, with the most homology centered around an eight-amino acid region involving seven identical residues and one substitution. Antisera were developed in rabbits, and specificity and titers were determined by electroblotting and immunoautoradiography. By this technique, an immunological cross-reaction was found between BLV p12 and FeLV p10. The shared antigenic determinant most likely exists in the highly conserved eight-amino acid region. Although this sequence is also highly conserved in the nucleic acid-binding proteins of murine leukemia viruses, the shared antigenic determinant is not found in these or any other type C viruses tested. It is suggested that substitution of arginine (BLV p12/FeLV p10) to lysine (murine leukemia virus p10) is sufficient to elicit a change in antibody specificity.