Humoral immune response of experimentally infected sheep defines two early periods of bovine leukemia virus replication

Development of a novel monoclonal antibody-based competitive ELISA for antibody detection against bovine leukemia virus

Infection with bovine leukemia virus (BLV) leads to enzootic bovine leukosis, the most prevalent neoplastic disease in cattle. Due to the lack of commercially available vaccines, reliable eradication of the disease can be achieved through the testing and elimination of BLV antibody-positive animals. In this study, we developed a novel competitive ELISA (cELISA) to detect antibodies against BLV capsid protein p24. Recombinant p24 protein expressed by Escherichia coli, in combination with the monoclonal antibody 2G11 exhibiting exceptional performance, was used for the establishment of the cELISA. Receiver-operating characteristic curve analysis showed that the sensitivity and specificity of the assay were 98.85 % and 98.13 %, respectively. Furthermore, the established cELISA was specific for detecting BLV-specific antibodies, without cross-reactivity to antisera for six other bovine viruses. Significantly, experimental infection of cattle and sheep with BLV revealed that the cELISA accurately monitors seroconversion. In a performance evaluation, the established cELISA displayed a high agreement with Western blotting and the commercial BLV gp51 cELISA kit in the detection of 242 clinical samples, respectively. In conclusion, the novel p24 cELISA exhibited the potential to be a reliable and efficient diagnostic tool for BLV serological detection with a broad application prospect.

Correlation between the Biodistribution of Bovine Leukemia Virus in the Organs and the Proviral Load in the Peripheral Blood during Early Stages of Experimentally Infected Cattle

Bovine leukemia virus (BLV) is the etiological agent of enzootic bovine leukosis. However, the propagation and distribution of BLV after primary infection still need to be fully elucidated. Here, we experimentally infected seven cattle with BLV and analyzed the BLV proviral load (PVL) in the blood and various organs. BLV was first detected in the blood of the cattle after one week, and the blood PVL increased for three weeks after infection. The PVL was maintained at a high level in five cattle, while it decreased to a low or medium level in two cattle. BLV was distributed in various organs, such as the heart, lung, liver, kidney, abomasum, and thymus, and, notably, in the spleen and lymph nodes. In cattle with a high blood PVL, BLV was detected in organs other than the spleen and lymph nodes, whereas in those with a low blood PVL, BLV was only detected in the spleen and lymph nodes. The amount of BLV in the organs was comparable to that in the blood. Our findings point to the possibility of estimating the distribution of BLV provirus in organs, lymph nodes, and body fluids by measuring the blood PVL, as it was positively correlated with the biodistribution of BLV provirus in the body of BLV infection during early stages.

Development of a new recombinant p24 ELISA system for diagnosis of bovine leukemia virus in serum and milk

Bovine leukemia virus (BLV) is a retrovirus that causes enzootic bovine leucosis. Here, we designed a p24 enzyme-linked immunosorbent assay (ELISA) to detect antibodies specific for BLV capsid protein p24 (encoded by the gag gene) in bovine serum samples. The p24 gene was inserted into an Escherichia coli expression system, and recombinant proteins (GST-p24, p24, and His-p24) were purified. His-p24 was the most suitable antigen for using in the ELISA. The cut-off point was calculated from a receiver operating characteristic curve derived from a set of 582 field samples that previously tested positive or negative by BLV-CoCoMo-qPCR-2, which detects BLV provirus. The new p24 ELISA showed almost the same specificity and sensitivity as a commercial gp51 ELISA kit when used to test field serum samples, and allowed monitoring of p24 antibodies in raw milk and whey. Comparing the results for the p24 ELISA and gp51 ELISA revealed that p24 antibodies were detected earlier than gp51 antibodies in three out of eight calves experimentally infected with BLV, indicating improved detection without diminishing BLV serodiagnosis. Thus, the p24 ELISA is a robust and reliable assay for detecting BLV antibodies in serum or milk, making it is a useful tool for large-scale BLV screening.

Identification and characterization of common B cell epitope in bovine leukemia virus via high-throughput peptide screening system in infected cattle

Background

Bovine leukemia virus (BLV) is the causative agent of enzootic bovine leukosis, the most common neoplastic disease of cattle. BLV is closely related to human T cell leukemia virus. B cell epitopes are important for the use of antibodies as therapeutic agents, the epitope-driven vaccine design, and immunological assays. A common B cell epitope for BLV has not yet been found due to individual differences in disease susceptibility.

Results

We used a peptide microarray with 156 synthetic 15-mer peptides covering the envelope glycoprotein gp51 and the Gag proteins p15, p24, and p12 to map B cell epitope and one B cell epitope, gp51p16, was recognized by all four cattle experimentally infected with BLV. A newly developed high-throughput peptide ELISA system revealed 590 (91.2 %) of 647 cattle naturally infected with BLV, carrying 25 different bovine leukocyte antigen class II DRB3 (BoLA-DRB3) alleles, responded to a 20-mer gp51p16-C peptide containing a C-terminal cysteine and gp51p16. Alanine mutation and comparison of the sequences at 17 amino acid positions within gp51p16-C revealed that R7, R9, F10, V16, and Y18 were the common binding sites to BLV antibodies, and two of these sites were found to be highly conserved. Transient expression in the cells of five infectious molecular clones of BLV with a single alanine mutation at five common antibody binding sites had no effect syncytia formation of the gp51 protein. In addition, the mutant proteins, R7A and R9A had no effect on the expression of gp51 protein; the gp51 protein expressions of F10A, V16A and Y18A were lower than that of the wild type protein.

Conclusions

This is the first report to identify a common B cell epitope in BLV by comprehensive screening of BLV-infected cattle with varied genetic backgrounds in BoLA-DRB3. Our results have important implications for disease control and diagnosis.

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.

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.

The influence of ovine MHC class II DRB1 alleles on immune response in bovine leukemia virus infection

We have reported previously that the alleles of the ovine leukocyte antigen (OLA)-DRB1 gene that encode the Arg-Lys (RK) motif and the Ser-Arg (SR) motif at positions beta70/71 of the OLA-DRbeta1 domain are associated with resistance and susceptibility, respectively, to development of bovine leukemia virus (BLV)-induced ovine lymphoma. Here, to investigate the different immune response in sheep that carried alleles associated with resistance and susceptible for 30 weeks after infection with BLV, we selected sheep that had the RK/RK or SR/SR genotype among the 52 sheep analyzed by polymerase chain reaction-restriction fragment length polymorphism and DNA sequencing of PCR product for the OLA-DRB1 exon 2 and infected them with BLV. Although the number of BLV-infected cells and virus titer had been maintaining low levels throughout the experimental period, the sheep with the RK/RK genotype could induce expansion of CD5- B-cells and rapid production of neutralizing antibody in the early phase of infection. The level of incorporation of [3H]thymidine by peripheral blood mononuclear cells from the sheep with RK/RK genotype gave a strong response to BLV virion antigen and synthetic antigenic peptides that corresponded to T-helper epitope of the BLV envelope glycoprotein gp51. In contrast, the sheep with SR/SR genotype showed a strong response to BLV virion antigen and synthetic antigenic peptides that corresponded to T-cytotoxic and B-cell epitopes. In such cases, the animals with the RK/RK strongly expressed IFN-gamma, the animals with SR/SR genotype strongly expressed IL-2. To determine the proliferating cells, we tried a blocking assay with monoclonal antibodies such as anti-CD4, -CD8 and -DR molecule. We found that these proliferating cells were MHC-restricted CD4+ T-cells.

Cellular distribution of bovine leukemia virus proteins gp51SU, Pr72(env), and Pr66(gag-pro) in persistently infected cells

Monoclonal antibodies (mAbs) against bovine leukemia virus (BLV) mature proteins and precursors were used to map the localization of these proteins in persistently infected non-lymphocytic cell lines using immunofluorescence assay (IFA) and immuno-electron microscopy. IFA staining was observed in the basolateral surface of live FLK-BLV cells. When using a mAb against Pr66(gag-pro), mottled pinpoint fluorescence was seen in the cell surface of polarized cells, but no reaction was observed in cells undergoing mitosis. However, a mAb against Pr72(env) stained only mitotic cells and cellular fragments. Additionally, in these dividing cells, this envelope (Env) precursor polyprotein was not evenly distributed but concentrated predominantly in only one daughter cell. To the best of our knowledge, this observation has not been reported previously, either for BLV or for other retroviruses. The results of immunogold electron microscopy confirmed the specificity of the mAbs in the intracellular level. In infected cells, Pr72(env) and gp51SU were seen in proximity at the plasma membrane in incipient budding sites. Additionally, the mAb against Pr72(env) also reacted with Env precursor polyproteins in the mitochondria of BLV-bat(2) ultrathin sections. These mAbs may be used as a tool for mapping virus excretion sites in the cell surface of naturally or in vitro infected cells in the different stages of the cell cycle.

Bovine leukemia virus structural gene vectors are immunogenic and lack pathogenicity in a rabbit model

Infection with a replication-competent bovine leukemia virus structural gene vector (BLV SGV) is an innovative vaccination approach to prevent disease by complex retroviruses. Previously we developed BLV SGV that constitutively expresses BLV gag, pol, and env and related cis-acting sequences but lacks tax, rex, RIII, and GIV and most of the BLV long terminal repeat sequences, including the cis-acting Tax and Rex response elements. The novel SGV virus is replication competent and replicates a selectable vector to a titer similar to that of the parental BLV in cell culture. The overall goal of this study was to test the hypothesis that infection with BLV SGV is nonpathogenic in rabbits. BLV infection of rabbits by inoculation of cell-free BLV or cell-associated BLV typically causes an immunodeficiency-like syndrome and death by 1 year postinfection. We sought to evaluate whether in vivo transfection of BLV provirus recapitulates pathogenic BLV infection and to compare BLV and BLV SGV with respect to infection, immunogenicity, and clinical outcome. Three groups of rabbits were subjected to in vivo transfection with BLV, BLV SGV, or negative control DNA. The results of our 20-month study indicate that in vivo transfection of rabbits with BLV recapitulates the fatal BLV infection produced by cell-free or cell-associated BLV. The BLV-infected rabbits exhibited sudden onset of clinical decline and immunodeficiency-like symptoms that culminated in death. BLV and BLV SGV infected peripheral blood mononuclear cells and induced similar levels of seroconversion to BLV structural proteins. However, BLV SGV exhibited a reduced proviral load and did not trigger the immunodeficiency-like syndrome. These results are consistent with the hypothesis that BLV SGV is infectious and immunogenic and lacks BLV pathogenicity in rabbits, and they support the use of this modified proviral vector delivery system for vaccines against complex retroviruses like BLV.

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.

Cytokine transcription in lymph nodes of cattle in different stages of bovine leukemia virus infection

Bovine leukemia virus (BLV) is a transforming oncovirus that contains no oncogenes or preferred site of proviral integration. The role of cytokines in the disease process of BLV is potentially important due to the similarity of BLV with other retroviruses in which cytokines play a role, such as HTLV-I and -II. Mesenteric and supra-mammary lymph nodes were obtained from a panel of nine cattle. Three were non-infected controls, three were BLV-positive aleukemic (AL), and three were BLV-positive persistent lymphocytotic (PL). Mononuclear cells were perfused from the organs and total RNA extracted from either 1 x 10(8) unseparated cells or 1 x 10(7) purified CD4/CD8 T-cells. cDNA was generated and subjected to RT-PCR to analyze cytokine transcription during disease progression. cDNA levels were normalized using beta-actin PCR at sub-plateau cycle number, enabling a semi-quantitative assessment of cytokine gene transcripts. Using this approach, IL-2, IL-10 and IFN-gamma message was detected in the T-cell fractions of all of the BLV-infected animals, but not in the non-infected controls.

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.

In vivo study of genetically simplified bovine leukemia virus derivatives that lack tax and rex

Genetically simplified derivatives of complex retroviruses that replicate in animal models are useful tools to study the role of the complex regulatory genes in virus infection and pathogenesis and were proposed as a novel approach toward the development of vaccines against complex retroviruses. Previously we developed genetically simple derivatives of bovine leukemia virus (BLV) that can replicate in tissue culture independently of the BLV regulatory proteins, Tax and Rex, and the RIII and GIV open reading frames (K. Boris-Lawrie and H. M. Temin, J. Virol. 69:1920-1924, 1995). These derivatives are encoded on novel, hybrid retrovirus genomes that contain transcriptional control sequences of a simple retrovirus and gag-pol or env genes of the complex BLV. The first-generation simple BLV derivatives replicate as complementary viruses (coviruses) by using separate gag-pol or env genomes, and therefore virus spread is limited to cells that are infected with both covirus genomes. Here we describe a second-generation simple BLV derivative that is encoded on a single hybrid genome. We show the virus to be replication competent by successive passage on D17 target cells and by analysis of viral RNA and proteins in the infected cells. Furthermore, we evaluate the immunogenicity and infectivity of the simple BLV derivatives in a BLV animal model. Small groups of rats were injected either with virus-producing cells or with proviral DNA. Western immunoblot analysis revealed that antibodies against the major viral antigenic determinants are induced in response to either method of introduction and that seroconversion is sustained in most of the rats for at least 6 months (the duration of the study). The magnitudes of the antiviral responses were similar in rats infected with the first-generation simple BLV coviruses, the second-generation replication-competent derivative, or wild-type BLV. Wild-type BLV typically infects peripheral blood mononuclear cells (PBMC), and the simple BLV derivatives were also found to infect PBMC as demonstrated by PCR amplification of proviral sequences and reverse transcriptase PCR amplification of viral RNA in treated rats. These results establish that simple BLV derivatives lacking tax and rex are infectious and immunogenic in rats. These viruses will be useful tools in comparative studies with BLV to evaluate the role of tax and rex in maintenance of virus load and in disease outcome.

Peripheral blood mononuclear cells from sheep infected with a variant of bovine leukemia virus synthesize envelope glycoproteins but fail to induce syncytia in culture

Peripheral blood mononuclear cells (PBMCs) infected with the oncogenic retrovirus bovine leukemia virus (BLV) produce virus when cultured briefly. BLV can be transmitted in cocultures to adherent susceptible cells, which become infected, express viral proteins, and fuse into multinucleated syncytia several days later. PBMCs from 3 of 10 BLV-infected sheep displayed a lifelong deficiency in induction of syncytium formation among indicator cells in culture, although large numbers of PBMCs synthesized viral transcripts or capsid protein. Since the infected, syncytium-deficient PBMCs were > or = 97% B cells, the deficiency could not be attributed to altered host cell tropism. The syncytium-deficient phenotype was recapitulated in newly infected sheep, demonstrating that this property is regulated by the viral genotype. The alteration in the BLV genome delayed but did not prohibit the establishment of BLV infection in vivo. Envelope glycoproteins were synthesized in syncytium-deficient PBMCs, translocated to the cell surface, and incorporated into virions. However, monoclonal antibodies specific for the BLV surface glycoprotein did not stain fixed PBMCs of the syncytium-deficient phenotype. Moreover, an animal with syncytium-deficient PBMCs had lower titers of neutralizing antibodies throughout the first 5 years of infection than an animal with similar numbers of infected PBMCs of the syncytium-inducing phenotype. The syncytium-deficient variant productively infected indicator cells at greatly reduced efficiency, showing that the alteration affects an early step in viral entry or replication. These results suggest that the alteration maps in the env gene or in a gene whose product affects the maturation or conformation, and consequently the function, of the envelope protein complex.

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.

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.

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.