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

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.

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.

gammadelta(+) T-Lp6phocyte cytotoxicity against envelope-expressing target cells is unique to the alymphocytic state of bovine leukemia virus infection in the natural host

Bovine leukemia virus (BLV) is a complex B-lymphotrophic retrovirus of cattle and the causative agent of enzootic bovine leukosis. Serum antibody in infected animals does not correlate with protection from disease, yet only some animals develop severe disease. While a cytotoxic T-lymphocyte response may be responsible for directing BLV pathogenesis, this possibility has been left largely unexplored, in part since the lack of readily established cytotoxic target cells in cattle has hampered such studies. Using long-term naturally infected alymphocytic (AL) cattle, we have established the existence of cytotoxic T-lymphocyte response against BLV envelope proteins (Env; gp51/gp30). In vitro-expanded peripheral blood mononuclear (PBM) cell effector populations consisted mainly of gammadelta(+) (>40%), CD4(+) (>35%), and CD8(+) (>10%) T lymphocytes. Specific lysis of autologous fibroblasts infected with recombinant vaccinia virus (rVV) delivering the BLV env gene ranged from 30 to 65%. Depletion studies indicated that gammadelta(+) and not CD8(+) T cells were responsible for the cytotoxicity against autologous rVVenv-expressing fibroblasts. Additionally, cultured effector cells lysed rVVenv-expressing autologous fibroblasts and rVVenv-expressing xenogeneic targets similarly, suggesting a lack of genetic restricted killing. Restimulation of effector populations increased the proportion of gammadelta(+) T cells and concomitantly Env-specific cytolysis. Interestingly, culture of cells from BLV-negative or persistently lymphocytic cattle failed to elicit such cytotoxic responses or increase in gammadelta(+) T-cell numbers. These results imply that cytotoxic gammadelta(+) T lymphocytes from only AL cattle recognize BLV Env without a requirement for classical major histocompatibility complex interactions. It is known that gammadelta(+) T lymphocytes are diverse and numerous in cattle, and here we show that they may serve a surveillance role during natural BLV infection.

Analysis by sodium dodecyl sulfate polyacrylamide gel electrophoresis and Western blot of nonspecific and specific viral proteins frequently detected in different antigen preparations of bovine leukemia virus

Bovine leukemia virus (BLV) infection in cattle is seldom manifested clinically, and is routinely diagnosed by serologic tests such as enzyme-linked immunosorbent assay or Western blot (WB). Because of the difficulty in interpreting WB results, the aim of the present study was to determine which of the bands observed in WB were specifically produced by BLV and which corresponded to nonspecific proteins, either derived from medium components or of a cellular nature. Five different BLV antigen preparations from 2 cell lines (FLK-BLV and BLV-bat2) frequently used for the production of BLV antigen were compared. The protein profiles of these antigen preparations were analyzed using sodium dodecyl sulfate polyacrylamide gel electrophoresis and WB. Fetal calf serum, required for cellular growth and important in induction of viral transcription in vitro, was identified as a source of irrelevant proteins. In this study, 15 nonspecific protein bands in the growth medium were observed. These bands interfered with the interpretation of results. A nonspecific protein (25 kD) that was highly reactive in cell lysate preparation from BLV-bat2 was also detected. The unequivocal identification of protein bands, both specific and nonspecific, seen in WB is important not for understanding the protein profile of antigen preparations but also for determining if an animal is BLV positive or negative.

Comparison of four tests to evaluate the reactivity of rabbit sera against envelope or Gag-related proteins of bovine leukemia virus (BLV)

Bovine leukemia virus (BLV) has a long latency period during which animals are inapparently infected, may spread the disease, and are only detected by serological techniques or by the most cumbersome molecular biology techniques. We have compared techniques for detecting either total antibodies (ELISA), anti-p24 and Gag-related proteins (Western blot), or anti-gp51 (agar gel immunodiffusion, AGID, and syncytia inhibition, SI) in rabbits inoculated experimentally with inocula of variable immunogenicity. The two tests to detect antibodies to gp51 correlated well in sera clearly positive or clearly negative by either one, but correlation was poor in the intermediate groups. All sera positive by AGID were also positive by ELISA, but results did not agree in sera negative by AGID, ELISA proving to be more sensitive. Western blot was a good technique for detecting antibodies against Gag-related proteins. However, no band was identified to clearly correspond to anti-Env-related proteins. As for other retroviruses, testing of animals for infection with BLV should include the detection of antibodies anti-Gag and anti-Env proteins.

Macrophages infected with bovine leukaemia virus (BLV) induce humoral response in rabbits

BLV is a lymphotropic retrovirus which infects mainly B-cells. However, the possible infection of cells of the monocyte/macrophage lineage (M/M) might explain some aspects of the disease such as latency or disease progression. We infected sheep M/M with BLV either by culturing M/M with supernatant containing virus, or coculturing M/M with persistently infected cell lines. These BLV-infected M/M were inoculated into rabbits and the serological response was followed for two years. ELISA results using adsorbed sera showed a persistent production of specific antibodies from as early as the first week post inoculation. Two tests were used to detect the response against envelope glycoprotein gp51: Agar gel immunodiffusion (AGID) and a virus neutralization test read as syncytia inhibition (SI). Sera were positive by AGID after the second or third inoculation. Neutralizing titres (SI) were higher than those seen in control rabbits inoculated with persistently infected cell lines, suggesting that the virus may be expressed better in M/M. Gag-related proteins were analyzed by Western Blot (WB). Sera from rabbits inoculated with BLV-infected M/M recognized as many viral proteins as sera from BLV immunized control rabbits or infected cows, and this profile did not change with repeated inoculations. All these results suggest that BLV may infect M/M, where viral proteins are actively expressed to the point that they induce a humoral immune response in animals, and that animals get persistently infected.

Polyclonal bovine sera but not virus-neutralizing monoclonal antibodies block bovine leukemia virus (BLV) gp51 binding to recombinant BLV receptor BLVRcp1

Bovine leukemia virus (BLV), a transactivating lymphotropic retrovirus, is the etiologic agent of enzootic lymphosarcoma or leukemia in cattle. Sera from BLV-infected animals possess high BLV-neutralizing antibody titres. The availability of the recombinant BLV receptor candidate, BLVRcp1, allowed us to determine a mechanism of virus neutralization by polyclonal sera and monoclonal antibodies (MAbs). Bovine sera from animals naturally infected with BLV blocked gp51 binding to recombinant BLVRcp1. In contrast, virus-neutralizing MAbs specific for gp51 F, G, and H epitopes did not prevent gp51-receptor attachment. Furthermore, gp51 neutralization epitopes F, G, and H were accessible to antibodies following gp51 attachment to BLVRcp1. This finding implies that virus neutralization by MAbs to defined BLV gp51 epitopes can occur subsequent to virus engagement of the receptor while polyclonal sera can specifically block virus attachment to the receptor. In conclusion, these data suggest that cell infection by BLV is a multistep process requiring receptor binding (inhibited by polyclonal sera) followed by a second, postbinding event(s) at the cell membrane (inhibited by anti-gp51 MAbs).

Detrimental effect of bovine leukemia virus (BLV) on the immunological state of cattle

Bovine leukemia virus (BLV) is a retrovirus which seems to affect both the humoral and the cellular immune response. Cows affected by enzootic bovine leukemia (EBL) showed a reduction of IgM-producing cells in the spleen and lymph nodes. Experimentally infected calves had lower levels of secretory IgM and a decrease in T lymphocytes in the peripheral blood. The reduction in the amount of T cells was noticed mainly in cells bearing the CD4 markers. BLV-infected animals showed diminished responsiveness to newly encountered antigens. Cows naturally infected by BLV produced Igs with impaired structural or biological reactivity. The primary immune response was shown to be deficient in BLV-infected cows following vaccination with synthetic antigen. A marked shift in the proportion of PBL, especially of the CD5+ subset, was noticed. Peripheral blood mononuclear cells from BLV-infected cows secrete elevated levels of certain cytokines and contain increased levels of cytokine mRNA. High levels of cytokines are also found in the sera of BLV-infected cows compared to non-infected animals. A correlation was found between BLV infection and lack of spontaneous recovery from Trichophyton verrucosum infection. Moreover, some studies ascertained a significant association between the herd BLV infection status and disease incidence. The culling rate was higher and milk production lower in BLV-infected vs. BLV-free herds. It seems that BLV infection affects the immune system of a cow to such an extent that it ceases to be productive enough to be kept and, in most cases, the animal is culled before any symptoms of illness associated with persistent immunodeficiency become apparent.

Pathogenicity of molecularly cloned bovine leukemia virus

To delineate the mechanisms of bovine leukemia virus (BLV) pathogenesis, four full-length BLV clones, 1, 8, 9, and 13, derived from the transformed cell line FLK-BLV and a clone construct, pBLV913, were introduced into bovine spleen cells by microinjection. Microinjected cells exhibited cytopathic effects and produced BLV p24 and gp51 antigens and infectious virus. The construct, pBLV913, was selected for infection of two sheep by inoculation of microinjected cells. After 15 months, peripheral blood mononuclear cells from these sheep served as inocula for the transfer of infection to four additional sheep. All six infected sheep seroconverted to BLV and had detectable BLV DNA in peripheral blood mononuclear cells after amplification by polymerase chain reaction. Four of the six sheep developed altered B/T-lymphocyte ratios between 33 and 53 months postinfection. One sheep died of unrelated causes, and one remained hematologically normal. Two of the affected sheep developed B lymphocytosis comparable to that observed in animals inoculated with peripheral blood mononuclear cells from BLV-infected cattle. This expanded B-lymphocyte population was characterized by elevated expression of B-cell surface markers, spontaneous blastogenesis, virus expression in vitro, and increased, polyclonally integrated provirus. One of these two sheep developed lymphocytic leukemia-lymphoma at 57 months postinfection. Leukemic cells had the same phenotype and harbored a single, monoclonally integrated provirus but produced no virus after in vitro cultivation. The range in clinical response to in vivo infection with cloned BLV suggests an important role for host immune response in the progression of virus replication and pathogenesis.

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.

Bovine leukemia virus gene expression in vivo

The in vivo transcriptional status of bovine leukemia virus was assessed at three stages of infection during the progression of the disease: aleukemic stage, persistent lymphocytosis, and leukemia/lymphosarcoma. Bovine leukemia virus transcripts could be amplified from total or cytoplasmic enriched lymphocyte RNA by reverse transcription polymerase chain reaction in cells from all but a few aleukemic animals. With primer pairs diagnostic for differentially spliced transcripts (full length-genomic, envelope, tax/rex, and alternatively spliced), a trend toward exclusion of both full-length and envelope RNAs, with retention of the tax/rex message, appears as leukemia/lymphosarcoma develops.

Activation of bovine leukemia virus transcription in lymphocytes from infected sheep: rapid transition through early to late gene expression

Bovine leukemia virus (BLV) expression is mostly silent in peripheral blood mononuclear cells (PBMCs) of infected animals. However, when infected cells are cultured, they are stimulated to produce virus. We studied viral transcription in PBMCs taken from BLV-infected sheep because the pattern of transcriptional activation in these cells should closely mimic activation of virus expression within mononuclear cells in vivo. BLV transcription was activated as early as 30 min after PBMCs were cultured. Expression was characterized by early and late stages, each distinguished by a unique pattern of cytoplasmic RNAs. In early expression, cytoplasmic viral RNA was exclusively the doubly spliced tax/rex transcript, although all transcripts were present in the nucleus. Early expression gave way rapidly to late expression, in which all viral transcripts accumulated in the cytoplasm. The polyclonal B-cell activator lipopolysaccharide increased the amount of viral RNA by at least twofold but did not alter the pattern of transcription. The transition from early to late expression required new protein synthesis and was blocked by the inhibitor cycloheximide. This requirement reflects the essential role of the viral Rex protein in the transition, but synthesis of cellular factors may be required as well. These results provide the first demonstration of staged viral expression in lymphocytes naturally infected by either BLV or the closely related human T-cell leukemia virus (HTLV) and validate the model of BLV and HTLV gene expression that previously was derived from transfection experiments performed mainly in nonlymphoid cells.

Transcription of bovine leukemia virus in peripheral blood cells obtained during early infection in vivo

Bovine leukemia virus (BLV) is transcriptionally silent in most circulating peripheral blood mononuclear cells (PBMCs) of animals with well-established infections. Using PBMCs from a newly infected sheep, we asked whether viral transcription proceeded differently during the initial months of infection, when the prevalence of BLV-infected cells and the host's immunological response change markedly. Shortly after being injected with BLV, the animal displayed a characteristic, transient increase in PBMCs that transcribed BLV when cultured. Even when transcriptionally competent PBMCs were most prevalent (1.2%), only rare cells in the circulation (1 in 50,000) contained enough BLV transcripts to be identified readily by in situ hybridization. However, at one point several weeks later, some PBMCs appeared to contain small amounts of BLV RNA as soon as they had been purified from blood. Throughout this period, BLV-transcribing PBMCs greatly outnumbered virus-producing cells, which were counted using a new infectious centers assay. Its viscous medium reduced cell to cell contact among PBMCs, enabling increased detection of BLV-producing cells at a time when virus-specific killer cells might be active. Early infection was polyclonal, and most infected PBMCs transcribed BLV upon being cultured. By 2 months after infection, provirus-containing cells were as abundant as they had been earlier, but few cells transcribed BLV. These results suggest that BLV-infected cells are more easily stimulated to transcribe the provirus and produce infectious virus during the early months of a new infection.