Evaluation of electrophoretic immunoblotting for the detection of antibodies against the bovine leukosis virus in cattle

Six antigen preparations of bovine leukemia virus, including affinity-purified glycoprotein gp51, gradient-purified fetal lamb kidney-bovine leukemia virus antigen, and four crude antigens, were used in combination with several groups of cattle sera, for the evaluation of electrophoretic immunoblotting as a serological test method. Sera (89) from cattle naturally-infected with bovine leukosis virus, a panel of reference sera from infected and uninfected cattle (18), and serial bleedings from experimentally-infected cows (4) were used. Major differences between the six antigen preparations were observed in their reactivity with the various sera. The immunological variabilities of these antigens were confirmed further by their reactions with a gp51-specific monoclonal antibody. The known immunodominant gp51 failed as a reliable indicator for the serological status of the sera in blots when compared to the results on the same sera, two gp51-specific ELISAs and the agar gel immunodiffusion test were used as reference tests. There was a lack of staining of gp51 antigen by many sera, probably due to the labile nature of the gp51 molecule. On the other hand, non-specific staining in the gp51 region appeared with high frequency in some antigens. Antibody staining of the internal viral protein p24 correlated well with the results of the three reference tests. Other bands stained infrequently and were of no diagnostic value.

An effective peptide vaccine to eliminate bovine leukaemia virus (BLV) infected cells in carrier sheep

Protective effects of the gp51 of bovine leukaemia virus (BLV) expressed by a recombinant baculovirus (rgp51) and synthetic multiple antigenic peptides (MAP) of T-helper, T-cytotoxic, and B-cell epitopes of gp51 were investigated against BLV challenge. Two and three sheep were immunized with rgp51 and a mixture of peptides with Freund's complete adjuvant, respectively. BLV was detected from all the immunized sheep at 2 weeks and showed peak levels at 4 weeks after the challenge. However, in two sheep immunized with the mixed peptides, the titer of BLV gradually decreased and one sheep eliminated BLV completely at 28 weeks after the challenge. These two sheep showed higher lymphocyte proliferative responses against the immunized peptides than the other sheep. One of the sheep also showed the specific cytotoxic lymphocyte activity against the BLV gp51-expressing target in vitro. These results suggest the possibility of the peptide vaccine for elimination of BLV in carrier animals in vivo.

Predominant p53 mutations in enzootic bovine leukemic cell lines

The role of the p53 tumor suppressor gene in bovine lymphosarcomas, a fragment of about 100 bp corresponding to approximately 97% of the open reading frame of the p53 gene was first amplified from single-strand cDNA originated from calf thymus by polymerase chain reaction PCR) and sequenced to obtain the bovine wild-type p53 gene. At the amino acid level, the omologies of the bovine p53 gene with the human, mouse, chicken and cat p53 genes were 0.9%, 72.8%, 52.7% and 82.3%, respectively. Moreover, eight bovine leukemic cells lines were studied for alterations in the p53 gene. These lines showed no significant somatic alterations in southern blot analysis, and expressed 2.5 kb p53-specific transcripts in Northern blot analysis. In mutation analysis using the reverse transcriptase-PCR technique, we detected three missense point mutations in four of these bovine leukemic cell lines. These mutations occurred in the 'hotspots' of the p53 gene. Thus p53 mutations predominantly occur in BLV-transformed cell lines and seem to be necessary for development of enzootic bovine leukosis (EBL).

Viral status and antibody response in cattle inoculated with recombinant bovine leukemia virus-vaccinia virus vaccines after challenge exposure with bovine leukemia virus-infected lymphocytes

OBJECTIVE

To determine the bovine leukemia virus (BLV) antibody response and infectivity status in BLV-vaccinated cattle after challenge exposure with BLV-infected lymphocytes.

DESIGN

Steers were inoculated with vaccinia virus constructs expressing the gag, pol, and env genes of the BLV or the env gene only of the BLV genome, then challenge exposed with BLV-infected lymphocytes. The steers' BLV antibody and infectivity status was monitored.

ANIMALS

Fifteen 8- to 9-month-old Holstein steers previously determined to be BLV antibody and BLV negative.

PROCEDURE

1 month after second after inoculation, steers were challenge exposed with 10(6) BLV-infected lymphocytes from a highly infective BLV-positive cow. Serum and blood lymphocytes were obtained regularly for 6 months. The agar gel immunodiffusion assay, ELISA, and serum neutralization assay were used to detect BLV antibody in serum of steers. The sheep infectivity and syncytium-forming assays were used to determine the viral status of the steers.

RESULTS

Differences were seen in antibody responses between the BLV-vaccinated and non-BLV-vaccinated control groups. All cattle were susceptible to infection when challenge exposed with BLV-infected lymphocytes.

CONCLUSION

Despite the enhanced immune response in the BLV-vaccinated cattle after challenge exposure, none of the BLV-vaccinated cattle was protected from BLV infection.

CLINICAL RELEVANCE

Vaccination is not an effective way to protect cattle from BLV infection.

The prevalence of proviral bovine leukemia virus in peripheral blood mononuclear cells at two subclinical stages of infection

The bovine leukemia virus (BLV) is an oncogenic retrovirus that is associated with the development of persistent lymphocytosis (PL) and lymphoma in cattle. While B lymphocytes have been shown to be the primary cellular target of BLV, recent studies suggest that some T lymphocytes and monocytes may be infected by the virus. Because virally altered functions of monocytes and/or T cells could contribute to the development of lymphoproliferative disease, we sought to clarify the distribution of the BLV provirus in subpopulations of peripheral blood mononuclear cells in seropositive cows with and without PL. CD2+ T cells, monocytes, and CD5+ and CD5- B cells were sorted by flow cytometry and tested for the presence of BLV by single-cell PCR. We did not obtain convincing evidence that peripheral blood monocytes or T lymphocytes contain the BLV provirus in seropositive cows with or without PL. In seropositive cows without PL (n=14), BLV-infected CD5+ and CD5- B cells accounted for 9.2% +/- 19% and 0.1% +/- 1.8% of circulating B lymphocytes, respectively. In cows with PL (n=5), BLV-infected CD5+ and CD5- B cells accounted for 66% +/- 4.8% and 13.9% +/- 6.6% of circulating B lymphocytes, respectively. The increase in lymphocyte numbers in cows with PL was entirely attributable to the 45-fold and 99-fold expansions of infected CD5+ and CD5- B-cell populations, respectively. Our results demonstrate that B cells are the only mononuclear cells in peripheral blood that are significantly infected with BLV. On the basis of the absolute numbers of infected cells in seropositive, hematologically normal animals, there appear to be differences in susceptibility to viral spread in vivo that may be under the genetic control of the host.

Bovine leukaemia-virus infection in Costa Rica

The geographical distribution of bovine leukaemia-virus (BLV)-infected herds in Costa Rica and the isolation of BLV from cases of enzootic bovine leukosis are presented. It was found that BLV is prevalent throughout the country, affecting mostly dairy cattle (Bos taurus) located in the highlands around the central valley of Costa Rica. By contrast, beef cattle (Bos indicus) raised in the lowlands were affected to a lesser extent. One out of four isolates of BLV obtained from local cattle with lymphosarcomatous tumours was analysed and had similar physical, serological and biological characteristics to a reference strain of BLV isolated in the USA. However, different patterns of recognition of BLV-protein p24 were observed among naturally infected cattle using the western-blotting technique.

Up-regulation of IL-2 receptor alpha and MHC class II expression on lymphocyte subpopulations from bovine leukemia virus infected lymphocytotic cows

Infection with bovine leukemia virus (BLV) leads to a persistent lymphocytosis (PL) characterized by a marked increase in circulating B lymphocytes that express the orthologue of CD5. To gain insight into the factors accounting for lymphocytosis, experiments were conducted to determine the functional activation status of lymphocytes from BLV seronegative and BLV infected aleukemic cows with PL. Stimulation with the B lymphocyte mitogen Staphylococcus aureus Cowan strain I (SAC), recombinant human interleukin-2 (rIL-2), or pokeweed mitogen (PWM), a T lymphocyte-dependent B lymphocyte mitogen, revealed differences in the pattern of expression of IL-2 receptor alpha (IL-2R alpha) and major histocompatibility (MHC) class II molecules on B and T lymphocytes from uninfected and BLV infected PL cows. rIL-2 induced expression of IL-R alpha on B lymphocytes from PL cows but not B lymphocytes from BLV seronegative cows. SAC alone, or in combination with rIL-2, had no effect on B lymphocytes from BLV seronegative cows. However, rIL-2 alone or in combination with SAC induced expression of IL-2R alpha on B lymphocytes from PL cows. PWM stimulated expression of IL-2R alpha on bovine B lymphocytes regardless of BLV status, and induced a significantly higher level of expression on B lymphocytes from PL cows. Mitogens and rIL-2 had a similar stimulatory effect on induction of IL-2R alpha expression on CD4 T lymphocytes regardless of BLV status. Only PWM induced expression of IL-2R alpha on bovine CD8 T lymphocytes and induced a significantly higher level of expression on this T lymphocyte subset from PL cows. Examination of freshly isolated B lymphocytes from PL cows revealed increased spontaneous expression of the MHC class II molecule compared to B lymphocytes from control cows. None of the culture conditions examined induced MHC-II expression on CD4 and CD8 T lymphocytes from BLV seronegative cows. In contrast, SAC+IL-2 and PWM induced MHC-II expression on CD4 and CD8 T lymphocytes from BLV infected PL cows, resulting in a significantly greater proportions of these lymphocyte subsets expressing this molecule compared to CD4 and CD8 T lymphocytes from control cows. The data indicate that infection with BLV affects the response of B and T lymphocytes to signals of activation, up-regulating the expression of surface molecules involved in both direct contact and cytokine-mediated T lymphocyte-dependent B lymphocyte activation.

Chemiluminescence ELISA for the detection of antibodies to bovine leukaemia virus antigens

A chemiluminescence enzyme-linked immunosorbent assay (ELISA) for the detection of antibodies to bovine leukaemia virus antigens (BLV) has been developed. The possibility of using an enhanced chemiluminescence reaction for the determination of adsorbed immunoperoxidase conjugates was studied in this work. The intensity of chemiluminescence depends on both the concentration of reagents and experimental conditions used. The efficiency of the assay is determined by the formation of an immobilized antigen monolayer. A relationship between the quantity of the protein added and adsorbed has been shown. The optimal time and temperature for the antigen-antibody incubation steps have been estimated for each system (3 h at 37 degrees C was chosen as a standard incubation time). A linear dependence of the chemiluminescence intensity and optical density on the concentration of antibodies to the BLV antigens was observed. The detection limit of antibodies in the chemiluminescence ELISA is 2-3 times lower than that in the spectrophotometric one. The results obtained indicate the possibility of using both methods.

Bovine leukaemia virus: rapid detection of proviral DNA by nested PCR in blood and organs of experimentally infected calves

The early stage of bovine leukaemia virus (BLV) infection was studied in experimentally infected calves in order to assess the diagnostic applicability of a double polymerase chain reaction (PCR). In addition, the kinetics of infection and virus distribution were evaluated. To simulate the natural route of virus transmission, the calves were infected by transferring two different infectious doses of whole blood from a BLV infected cow. The establishment of infection was determined by the double PCR and syncytia formation assay and by indirect serological methods including indirect ELISA, gp51/p24 ELISA, agar gel immunodiffusion (AGID) and Western blotting. BLV antibodies were first detected in ELISA on post infection (p.i.) day 26. Close agreement was found between the results of the various indirect methods. BLV infection was first detected in peripheral blood lymphocytes (PBL) by the PCR on p.i. day 7. No animal became seropositive to BLV prior to direct detection of BLV infection by the PCR. At slaughter, urine and saliva specimens as well as various organs were collected from the calves and tested by the double PCR. Several of the organs yielded positive results: e.g. spleen, uterus, liver, kidney, abomasum, and lymph nodes. Nine out of eleven spleen suspensions were positive by the PCR, including the spleen from one calf, which otherwise remained negative in all tests throughout the experiment. This phenomenon indicates that an animal may be infected without detectable levels of BLV proviral DNA in PBLs and without circulating antibodies, further emphasizing the diagnostic importance of the PCR. The findings indicate that the PCR is the most rapid method for the early detection of BLV infection in cattle and a valuable tool for studying the tropism of the virus.

Evidence for bovine leukemia virus in mammary epithelial cells of infected cows

BACKGROUND

Bovine leukemia virus (BLV), a retrovirus, usually causes a subclinical infection of dairy and beef cattle, but in < 1% of infected cattle a B cell lymphoma may develop after several years of infection. BLV is transmitted horizontally among cattle, and infected animals have anti-BLV titers. Expression of BLV antigen, however, is silent in peripheral blood lymphocytes in vivo. The tropism of BLV has been assumed to be limited to B lymphocytes, because no other cell type has been found to harbor the virus in vivo. Since retrovirus-like particles had been identified in milk, and infection can be transmitted by milk, we decided to investigate whether BLV was in mammary epithelial cells.

EXPERIMENTAL DESIGN

Pure cultures of mammary epithelial cells were established from cells shed into the milk of 28 cows. BLV was searched in these cultures by immunocytochemistry and the polymerase chain reaction, the specificity of the latter confirmed by Southern blot hybridization and DNA sequencing. BLV was searched immunocytochemically in mammary tissue sections from 12 cows.

RESULTS

Antigenic and/or molecular evidence of BLV was found in the cultured cells of 20 cows. Antigenic evidence of BLV was found in tissue sections from 10 cows, indicating virus expression in vivo. Simultaneous detection of BLV p24 and cytokeratins, localized antigen expression to the mammary epithelial cells.

CONCLUSIONS

These indications of BLV in mammary epithelial cells in vivo suggest that BLV is capable of infecting and expressing antigen in glandular epithelium in vivo and has a broader tissue tropism than was previously thought. They raise the question of how persistent mammary infection by BLV may influence the course of bovine lymphoma, and what effect the virus may have on the mammary gland.

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.

Importance of primer selection in the application of PCR technology to the diagnosis of bovine leukemia virus

The polymerase chain reaction (PCR) was used to detect bovine leukemia virus in bovine blood samples. When applied to leucocytes extracted from the blood samples, the standard method of DNA extraction gave good correlation with agar gel immunodiffusion, but a method in which 5 microliters of blood was the starting material was unreliable. Selection of the primers was important, and differences in results were observed when the PCR method was applied to blood samples from different geographic areas. The sensitivity varied from 50% to 90%, depending on the primer set applied to the gag gene of proviral nucleic acid. This variation was based on geographic origin of the cattle, suggesting an influence of viral strain. In some areas, more than 1 primer may needed to optimize results.

A field evaluation of the polymerase chain reaction procedure for the detection of bovine leukaemia virus proviral DNA in cattle

A polymerase chain reaction (PCR) procedure that detects proviral bovine leukaemia virus (BLV) in peripheral blood mononuclear cell DNA was evaluated. Blood samples from all animals (164) in a commercial dairy herd with a 30% prevalence of BLV infection, and from 194 animals from BLV free herds were tested. The absence of any positive PCR results in animals from BLV free herds confirmed the specificity of the assay. Initial testing of the infected herd using a single amplification PCR (SA-PCR), detected BLV infection in 62 of 72 adult animals that were seropositive by the agar gel immunodiffusion (AGID) test and in one persistently seronegative cow. Infection in this cow was confirmed by sheep bioassay. Subsequent testing of the SA-PCR negative, seropositive animals using a double amplification PCR (DA-PCR) detected proviral BLV in eight of nine animals that were available for retesting. The PCR assay was also able to distinguish BLV infected calves from uninfected calves that were serologically positive because of the presence of colostral antibody. Lymphocytes from all seropositive animals were cultured for determination of BLV antigen expression. Cultures from 37 of 62 SA-PCR positive animals produced detectable quantities of viral antigens. However, antigen expression was not detected in cultures from seropositive animals that were negative in the SA-PCR. In addition, in experimental transmission tests, inoculation of more than 10(6) lymphocytes from these cows was required for sheep to become seropositive to BLV.(ABSTRACT TRUNCATED AT 250 WORDS)

Combined polymerase chain reaction-hybridization microplate assay used to detect bovine leukemia virus and Salmonella

Here we describe the use of an assay that integrates the polymerase chain reaction (PCR) with hybridization of the amplified product for detection in the same microwell. Traditional PCR requires transportation of the amplified product to another system for characterization of samples. Transportation means time-consuming manipulation and risk of contaminating the laboratory with amplified product. Integration of amplification and specific product detection greatly reduces sample manipulations and the risk of contamination. We used the assay for detection of bovine leukemia virus and Salmonella. The results were identical with those produced by two traditional PCR methods. This assay could easily be adapted for other organisms, simply by using other primers and probes.

Immunopathologic study and characterization of the phenotype of transformed cells in sheep with bovine leukemia virus-induced lymphosarcoma

We used monoclonal antibodies and immunohistologic examination of lymph nodes, to elucidate the pathogenesis of lymphosarcoma induced by infection with bovine leukemia virus (BLV). The superficial cervical lymph nodes from 3 BLV-infected but apparently healthy sheep and 5 sheep with full-blown lymphosarcoma were examined. We also investigated the integration of bovine leukemia provirus by use of Southern blotting. In lymph nodes from sheep lacking clinical signs of infection, in which the provirus had been integrated at multiple sites in the genome, many large hypertrophic follicles were observed in the cortex. These follicles had germinal centers consisting of CD4+T cells and B cells that expressed surface IgM (sIgM) and major histocompatibility complex (MHC) class-II antigens, but not B cell-specific B2 molecule. The percentage of CD4+T cells in the cortex was significantly (P < 0.05) higher than that of the controls and sheep with lymphosarcoma. In all sheep with lymphosarcoma, the lymph nodes were completely destroyed by proliferating neoplastic cells, and in addition, small atrophic follicles, which consisted of normal B-cell marker-positive cells, were seen near the trabecula and the subcapsule. In these instances, neoplastic cells appeared to be a monoclonal population derived from a single CD5- B-cell lineage and to be classified as 2 types, CD5-CD4-CD8-B2+MHC class-II+sIgM+ and CD5-CD4-CD8-B2+MHC class-II+sIgM-. Moreover, CD8+T cells infiltrated diffusely throughout the tumorous lymph nodes apart from the atrophic follicles, and CD4+ cells were observed around atrophic follicles. Both types of T cells were small-size, normal lymphocytes with round and noncleaved nuclei, and were apparently non-neoplastic cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Comparison of serum, milk and urine as samples in an enzyme immunoassay for bovine leukaemia virus infection

Serum, milk and urine specimens were taken from 15 bovine leukaemia virus (BLV)-positive and 20 BLV-negative cattle which had been determined previously to be infected or not by the use of a monoclonal enzyme-linked immunosorbent assay (ELISA). An ELISA was performed on the samples for the detection of IgG1 antibodies to the BLV surface glycoprotein, gp 51. The three types of samples had parallel optical density (OD) values apart from three urine samples which, although accepted as negative for anti-BLV antibodies, had numerically higher ODS than those of control BLV-negative animals. Therefore, detection of IgG1 antibodies against BLV in the urine of naturally infected animals could be an indication for the use of urine for diagnosis of BLV infection.

Effects of plasma and serum on the in vitro expression of bovine leukemia virus

BACKGROUND

Bovine leukemia virus (BLV) is difficult to detect in freshly isolated peripheral blood mononuclear cells (PBMCs) from infected cattle. The lack of viral expression has been attributed to a plasma blocking factor capable of inhibiting the in vitro expression of the BLV p24 antigen.

EXPERIMENTAL DESIGN

The purpose of this study was to investigate the effects of plasma and serum on the in vitro synthesis of the p24 antigen within short-term cultures of PBMCs from BLV-seropositive cows. BLV p24 antigen was detected in the cell lysate using an antigen capture enzyme-linked immunosorbent assay.

RESULTS

Antigen synthesis was suppressed in PBMCs cultured in plasma from both seropositive and seronegative cows. Serum from infected cows suppressed p24 expression, but to a lesser extent than any of the plasmas. Addition of phytohemagglutinin-P to the cultures resulted in the detection of more p24 in PBMCs cultured in serum, but had no effect on p24 expression within cells cultured in plasma. The reduction in p24 was not related to lower cell viability, nor was it influenced by the effects of the anticoagulant citrate-phosphate dextrose on calcium-dependent lymphocyte activation. No significant difference was observed in the potency of the p24 inhibition between cells grown in autologous and homologous serum, or between cells grown in autologous and homologous plasma from seropositive cows.

CONCLUSIONS

Blocking activity is present in the plasma of seronegative cows, and in the plasma and serum of BLV-seropositive cows. This activity may be important in regulating viral latency and provirus expression in vivo.

Mapping of B-neutralizing and T-helper cell epitopes on the bovine leukemia virus external glycoprotein gp51

A battery of 19 synthetic peptides was used to characterize efficient neutralizing and helper T-cell epitopes on the bovine leukemia virus (BLV) external envelope glycoprotein gp51. Four of the antipeptide antisera raised in rabbits inhibited the formation of BLV-induced syncytia; these antisera are directed against peptides 64-73, 98-117, and 177-192. Only antisera directed against the 177-192 region also neutralized vesicular stomatitis virus-BLV pseudotypes. This study clearly demonstrates that neutralizing properties can be observed with antibodies raised to regions undescribed so far and included in both the amino-terminal and central parts of the antigen. In addition, some helper T-cell determinants were defined from gp51-immunized mice and from BLV-infected cattle. Although none of the peptides tested behaved as a universal helper T-cell epitope, peptide 98-117 stimulated T-cell proliferation from BALB/c mice and from three infected cows, while peptide 169-188 strongly stimulated T-cell proliferation from one infected cow. Further experiments performed with three peptides overlapping the 169-188 region (177-192, 179-192, 181-192) demonstrated the particular relevance of residue(s) P-177 and/or D-178 in the helper T-cell epitope. These data should assist in the design of an efficient subunit vaccine against BLV infection that contains peptides possessing both B-neutralizing and helper T-cell determinants.

Detection of bovine leukemia virus proviral DNA in individual cells

We have developed a method of analyzing individual cells to detect proviral DNA of the bovine leukemia virus (BLV) using flow cytometry and PCR. Individual cells of the BL3* cell line, which contain multiple integrated copies of the BLV provirus, and the uninfected cell line BL3(0), were sorted into wells of a 96-well plate. Following cell lysis, portions of the BLV envelope (ENV) and cellular prolactin (PRL) genes were amplified simultaneously using PCR. Viral and cellular products of first-round PCR were amplified separately in a second round of PCR using "heminested" primers. Separation of the PCR products by polyacrylamide gel electrophoresis yielded distinct fragments of the predicted sizes. The operational sensitivity of this method for the detection of virus was > 90% when testing single infected cells. In addition, we were able to reliably amplify DNA from a single BL3* cell among as many as 10(5) BL3(0) cells and established that the sensitivity for detecting a single infected cell among 20, 100, or 1000 uninfected cells was at least 90%. Estimates of low percentages of infected cells were obtained by applying probability theory to results of experiments conducted on wells containing more than one cell. Using these methods, B lymphocytes obtained from the peripheral blood of BLV-infected cattle were tested for proviral DNA. BLV ENV was identified in 76.9 +/- 4.9% of single B cells tested from a seropositive animal with persistent lymphocytosis (PL), but in only 0.033 +/- 0.009% of B cells from another seropositive cow without PL.(ABSTRACT TRUNCATED AT 250 WORDS)

[The isolation of the bovine leukemia provirus by the polymerase chain reaction with subsequent nonradioactive blot hybridization]

A multiprobe scheme for detecting provirus of bovine leukemia virus (BLV) in the peripheral blood of infected animals was developed. According to the scheme, a fragment of BLV X-gene was amplified by polymerase chain reaction and detected by blot hybridization with the biotinylated oligonucleotide probe complementary to the inner part of the generated fragment. 600 copies of provirus may be specifically detected in a sample containing 150,000 cells. In some cases, BLV was detected in blood samples negative if tested by the commercial double immunodiffusion test.

Bovine leukemia virus: purification and characterization of the aspartic protease

To develop efficient bovine leukemia virus (BLV) protease (PR) inhibitors, pure enzyme is required. For this, we have developed a two-step chromatographic nondenaturing purification protocol of PR from virions. As expected, the purified protein presents a molecular weight (14 kDa) and a NH2 terminal end fitting with previously reported data. The enzymatic activity of BLV PR was characterized using a synthetic peptide containing a potential cleavage site of the BLV gag-pro polypeptide precursor as substrate. The protease was most active at pH 6, 40 degrees, and high salt concentration (1-2 M NaCl or ammonium sulfate). In contrast, using a natural substrate such as a human T-cell leukemia virus recombinant gag precursor, BLV PR activity was higher at a low salt concentration (0.5 M NaCl). Besides, the use of different potentially cleavable molecules revealed that PR activity may be influenced by the substrate conformational structure around the cleavage site. Replacement of the two amino acids of a synthetic substrate cleavable site by a statin residue completely inhibited the enzymatic activity of the BLV PR.

Use of polymerase chain reaction to diagnose bovine leukemia virus infection in calves at birth

A specific polymerase chain reaction (PCR) assay was devised, allowing detection of 1 bovine leukemia virus (BLV)-infected cell in 10(4) bovine lymphocytes. The efficacy of field application of the developed method was verified by evaluating the rate of viral transmission to calves from infected cows, whether they have persistent lymphocytosis. With this objective, 43 calves were simultaneously tested at birth and at 6 months of age for viral antibodies in serum and for proviral DNA in lymphocytes. At birth, 36 calves were BLV-negative and 3 were BLV-positive by results of serologic and DNA-based assays. Conversely, results for 4 calves had lack of correlation between the diagnostic methods. In particular, 2 calves were DNA-positive and antibody-negative for BLV and 2 other calves had the opposite test results. At 6 months of age, when the immunologic pattern more closely reflects the status of calves' immune response, independent of maternal antibodies, all calves DNA-negative for BLV at birth (n = 38), were consistently PCR- and antibody-negative for BLV. On the contrary, the cattle DNA-positive for BLV at birth (n = 5), whether seropositive or not, were PCR- and antibody-positive for BLV, at the time of the second screening. Thus, these results indicate reliability of the PCR to diagnose perinatal BLV infection. Furthermore, the observation that all calves found to be infected at birth were born to BLV-positive cows with persistent lymphocytosis, indicates that the persistent lymphocytosis status of the cow may represent a factor associated with BLV infection in utero.

[Preparative isolation of the glycosylated protein and the major core protein of bovine leukemia virus]

A technique for purifying intact bovine leukemia virus (BLV) and a procedure for preparing gag protein (p24) and envelope glycoprotein (gp 51) from intact BLV are described. The preparation method employs solubilization of BLV by n-octyl-beta-D-Glycopyranoside and sodium desoxycholate with following ultracentrifugation in a linear 10-60% sucrose gradient. The isolated components preserve their antigenicity.