Expression of a cDNA clone corresponding to the long open reading frame (XBL-I) of the bovine leukemia virus

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.

In vivo leukocyte tropism of bovine leukemia virus in sheep and cattle

Bovine leukemia virus (BLV), an oncovirus related to human T-cell leukemia virus type I, causes a B-cell lymphoproliferative syndrome in cattle, leading to an inversion of the T-cell/B-cell ratio and, more rarely, to a B-cell lymphosarcoma. Sheep are highly sensitive to BLV experimental infection and develop B-cell pathologies similar to those in cattle in 90% of the cases. BLV tropism for B cells has been well documented, but the infection of other cell populations may also be involved in the BLV-induced lymphoproliferative syndrome. We thus looked for BLV provirus in other leukocyte populations in sheep and cattle by using PCR. We found that while B cells harbor the highest proviral load, CD8+ T cells, monocytes, and granulocytes, but not CD4+ T cells, also bear BLV provirus. As previously described, we found that persistent lymphocytosis in cows is characterized by an expansion of the CD5+ B-cell subpopulation but we did not confirm this observation in sheep in which the expanded B-cell population expressed the CD11b marker. Nevertheless, BLV could be detected both in bovine CD5+ and CD5- B cells and in sheep CD11b+ and CD11b- B cells, indicating that the restricted BLV tropism for a specific B-cell subpopulation cannot explain its expansion encountered in BLV infection. Altogether, this work shows that BLV tropism in leukocytes is wider than previously thought. These results lead the way to further studies of cellular interactions among B cells and other leukocytes that may intervene in the development of the lymphoproliferative syndrome induced by BLV infection.

Identification of alternatively spliced mRNAs encoding potential new regulatory proteins in cattle infected with bovine leukemia virus

The polymerase chain reaction was used to detect and characterize low-abundance bovine leukemia virus (BLV) mRNAs. In infected cattle we could detect spliced mRNA with a splice pattern consistent with a Tax/Rex mRNA, as well as at least four alternatively spliced RNAs. Two of the alternatively spliced mRNAs encoded hitherto unrecognized BLV proteins, designated RIII and GIV. The Tax/Rex and alternatively spliced mRNAs could be detected at their highest levels in BLV-infected cell cultures; the next highest levels were found in samples from calves experimentally infected at 6 weeks postinoculation. Alternatively spliced mRNAs were also expressed, albeit at lower levels, in naturally infected animals; they were detected by a nested polymerase chain reaction. Interestingly, the GIV mRNA was specifically detected in naturally infected cows with persistent lymphocytosis and in two of five calves at 6 months after experimental infection with BLV. Furthermore, the calf with the strongest signal for GIV had the highest lymphocyte counts. These data may suggest a correlation between expression of the GIV product and development of persistent lymphocytosis. Some of the donor and acceptor sites in the alternatively spliced mRNAs were highly unusual. The biological mechanisms and significance of such a choice of unexpected splice sites are currently unknown.

Fusogenic segments of bovine leukemia virus and simian immunodeficiency virus are interchangeable and mediate fusion by means of oblique insertion in the lipid bilayer of their target cells

Modified bovine leukemia virus (BLV) glycoproteins were expressed by using vaccinia virus recombinants, and their fusogenic capacities were examined by a syncytia-formation assay. This analysis indicates that (i) both BLV envelope glycoproteins gp51 and gp30 are necessary for cell fusion; (ii) insertion of the N-terminal segment of gp30 (fusion peptide) into the lipid bilayer in an oblique orientation, as predicted by computer conformational analysis, results in fusogenic capacities higher than insertion in a perpendicular or parallel orientation; and (iii) replacement of the BLV fusion peptide with its simian immunodeficiency virus counterpart does not modify the fusogenic capacity of the BLV glycoprotein.

Episodic occurrence of antibodies against the bovine leukemia virus Rex protein during the course of infection in sheep

Infection by bovine leukemia virus (BLV) is characterized by a long clinical latency after which some individuals develop B-cell tumors. The contributions of the viral regulatory proteins Tax and Rex during clinical latency and disease are incompletely understood. To learn about Rex expression in the host, we used a sensitive immunoprecipitation assay to detect Rex antibodies throughout the course of BLV infection in sheep. Sixty percent of the infected animals produced Rex antibodies in intermittent episodes. This pattern differed markedly from that of antibodies to virion structural proteins, which were maintained in all animals throughout infection. Only one of two animals that developed tumors had detectable Rex antibodies at the time, although the other had previously demonstrated an especially strong Rex antibody response. We examined the Rex response in the context of BLV infection by comparing it with the frequency of circulating mononuclear blood cells that could transcribe BLV RNA or produce infectious virus. Episodes of Rex antibody occurrence followed some but not all increases in the number of BLV-transcribing cells. Since the appearance of circulating antibodies requires that the intracellular Rex protein be available to serve as antigen, the episodic pattern of occurrence of Rex antibodies could result from intermittent killing by virus-specific cytotoxic cells. Fluctuations in titer that were observed during some episodes of Rex response could be due to antibody retention by antigen present in lymphoid tissue.

Synthesis of functional bovine leukemia virus (BLV) p34tax protein by recombinant baculoviruses

The bovine leukemia virus (BLV) p34tax (also called tat, p34, XLOR gene product) is a 34-kDa polypeptide encoded in the 3'-terminal region of the virus. This protein is responsible for positive transcriptional trans-activation of promoter elements located within the BLV long-terminal repeat. We introduced the protein-coding region of BLV p34tax into the genome of the baculovirus Autographa californica nuclear polyhedrosis virus. After infection of the insect Spodoptera frugiperda (SF9) cell line, this recombinant strain of baculovirus produced approximately 100 to 150 mg of p34tax per 2 X 10(9) cells. This protein, when introduced into mammalian fibroblasts by using a cell-to-cell fusion technique, functionally trans-activated the BLV long-terminal repeat. Analysis of 32P-labeled proteins of SF9 cells expressing BLV tax by two-dimensional gel electrophoresis indicated that the BLV p34tax was phosphorylated.

Expression of the bovine leukemia virus transactivator protein p34 by a recombinant vaccinia virus

In order to characterize the bovine leukemia virus transactivator protein, a recombinant vaccinia virus (v-LOR) containing the BLV post-envelope long open reading frame was constructed. v-LOR was shown to encode a functional protein able to transactivate the BLV long terminal repeat-directed gene expression in the infected cells. The encountered level of transactivation was about one third of that measured in BLV-infected fetal lamb kidney cell lysates.

Structural and functional characterization of mutants of the bovine leukemia virus transactivator protein p34

Mutants of the bovine leukemia virus (BLV) transactivator protein (tat, tax, p34, the XLOR gene product) were constructed by site-directed deletions, in-phase linker insertions, or fragment replacements (swapping) among BLV variants. The mutant constructs were transfected into cos cells and transiently expressed. Western blot analysis using a mixture of monoclonal antibodies to wild-type p34 revealed the presence of mutated XLOR gene products in all the mutants tested. The transactivating activity of 11 tax mutants containing site-directed deletions and in-phase linker insertions was completely abolished. Only the swapping mutant tested, a hybrid between two BLV variants, transactivated LTR-directed gene expression at wild-type levels. These data illustrate the narrow range of structural variations that allow full activity of the BLV tax product and suggest that the present molecular structure of the transactivator protein results from heavy evolutionary constraints.

Even transcriptionally competent proviruses are silent in bovine leukemia virus-induced sheep tumor cells

To investigate the role of proviral integration and expression in cellular transformation induced by bovine leukemia virus (BLV), three BLV-induced tumors harboring a single proviral copy were selected upon restriction and hybridization analysis. Tumors 344 and 395 were shown to contain a full-size proviral copy, whereas in tumor 1345 the provirus appeared to be heavily deleted. RNA gel blot hybridization with an antisense RNA probe showed no transcription of the viral sequences in the fresh tumors or in sheep tumor cells growing in vitro. The proviruses were cloned and transfected in mammalian cell lines. Transient-expression experiments revealed that the complete proviruses were still able to express the trans-activating protein (Tat) as well as structural proteins, demonstrating that the nonexpression of a provirus in a tumor cell does not necessarily imply a structural alteration of the viral information. In contrast, sequence analysis of the provirus with a large deletion and transient-expression assays proved that this truncated provirus, isolated from a tumor, was unable to code for viral proteins. These data indicate that expression of viral genes, including tat, is not required for the maintenance of the transformed state.