Use of milk samples and monoclonal antibodies directed against BLV-p24 to identify cattle infected with bovine leukemia virus (BLV)

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.

Herd management practices associated with bovine leukemia virus incidence rate in Michigan dairy farms

The objective of this study was to identify associations between herd management practices and the incidence rate of bovine leukemia virus (BLV) infections in Michigan dairy herds. Previous management risk factor studies were of antibody prevalence rather than the rate of recent infections. Milk samples were collected from cohorts of cows on 112 Michigan dairy herds and tested for BLV using an antibody capture ELISA (n = 3849 cows). Cows were subsequently followed for an average of 21 months. Cows negative for anti-BLV antibodies and still present in their respective herds were retested by the same antibody capture ELISA to estimate within-herd incidence rates. The overall crude incidence rate was 1.46 infections per 100 cow-months at risk for the 1314 retested cows in 107 herds. The average within-herd incidence rate was 2.28 infections per 100 cow-months (range: 0 to 9.76 infections per 100 cow-months). A negative binomial regression model was used to identify herd management practices associated with the within-herd incidence rate. Results of the final multivariable model identified higher herd prevalence, milking frequency, needle reuse, as well as housing post-parturient cows separately, to be associated with increased incidence rate. Utilization of sand bedding for the lactating herd was found to be associated with decreased incidence rates. Results of this study suggest potential routes of BLV transmission which should be further investigated as disease control targets in ongoing control programs.

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.

Evaluation of a bulk-milk ELISA test for the classification of herd-level bovine leukemia virus status

The results of a commercial bulk-milk enzyme-linked immunosorbent assay (ELISA) test for herd-level bovine leukemia virus (BLV) status were compared to results obtained from individual agar-gel immunodiffussion (AGID) testing on sampled cattle. A positive herd was defined as a herd having one or more AGID-positive animals. The estimated true herd status was based on the sensitivity and specificity of the AGID test and the number of cattle sampled per herd. Ninety-seven herds were used, with a mean of 13 cows sampled per herd. The AGID test indicated an apparent herd prevalence of 70.1%. After accounting for the number of cows sampled and the sensitivity and specificity of the AGID test, the estimated true herd prevalence of BLV was 52.3%. The ELISA test identified 79.4% of herds as positive for BLV, and had an apparent sensitivity and specificity of 0.97 and 0.62, respectively. However, after accounting for the sensitivity and specificity of the AGID test in individual animals, the specificity of the ELISA test was 0.44. The ELISA test was useful for identifying BLV-negative herds (i.e., ruling out the presence of BLV infection in test negative herds). With the moderately low specificity, herds identified as positive by the ELISA test would require further testing at the individual or herd level to definitively establish their BLV status.

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.

Ovine lentivirus antibody detection in serum, colostrum and milk using a recombinant transmembrane protein ELISA

An enzyme-linked immunosorbent assay (ELISA) was used to detect antibodies against ovine lentivirus (OLV) in serum, colostrum, and milk from naturally infected sheep. The assay used OLV recombinant transmembrane envelope protein (rTM) as a test antigen. Matched serum/colostrum and serum/milk samples were collected at 24h, 4 weeks (mid-lactation), and 8 weeks (weaning) post-lambing. Among 129 paired samples collected at 24 h post-lambing, there was overall test agreement (concordance) of 82.9% and a kappa value of 0.658 between serum and colostrum rTM ELISA results. Among 130 mid-lactation samples, the milk ELISA had 100% specificity and 64.9% sensitivity relative to the serum ELISA, there was concordance of 79.2%, and a kappa value of 0.602. At mid-lactation, the serum agar gel immunodiffusion test had a sensitivity of 0.390 and 0.560 relative to the serum and milk rTM ELISAs, respectively. Matched serum and milk rTM ELISA results at weaning were very similar to those at mid-lactation. Finally, increased occurrence and severity of subclinical mastitis at weaning was found in ELISA-seropositive compared with ELISA-seronegative ewes. Both subclinical mastitis and ewe OLV infection had a negative impact on lamb growth and weaning weights. Compared with blood, colostrum and milk are easier and less expensive to sample and store. These results suggest that rTM ELISA testing of colostrum and milk could be used to supplement serologic testing in OLV screening or eradication programs.

Two immunodominant regions revealed by monoclonal antibodies on the main structural protein p24 of bovine leukemia virus

Eleven different monoclonal antibodies (Mabs) directed against the main structural protein p24 of bovine leukemia virus (BLV) were prepared. All Mabs reacted with p24 in Western blot and in radioimmunoprecipitation. Competition antibody binding assays with the prepared Mabs distinguished three independent groups of Mabs. Two immunodominant regions (IDRs) of p24 BLV were defined by these Mabs. The Mabs were induced preferentially against two immunodominant regions on the native form of p24 BLV (BLVp24 IDR-1 and BLVp24 IDR-2). Mab of the third group was directed against a different immunogenic epitope of p24 BLV. A model of the IDRs based on the differences in the fine epitope specificity of Mabs defining these immunodominant regions is proposed.

Amplification and analysis of specific DNA and RNA sequences of bovine leukemia virus from infected cows by polymerase chain reaction

Bovine leukemia virus (BLV) is the etiologic agent of leukemia in cattle and is believed to cause decreases in milk productivity, fertility, and life span in infected cows. BLV is a type C retrovirus in the Oncovirinae subfamily. It is most closely related to human T-cell lymphoma/leukemia virus type I (HTLV-I) and type II (HTLV-II). Since the polymerase chain reaction (PCR) provides rapid and efficient amplification of DNA sequences, primers were designed to amplify regions of the polymerase (pol) and pX genes specific for BLV targets. These sets of primers consistently amplified as few as 10 copies of BLV DNA contained in a plasmid in the background of 1 microgram of either human or bovine chromosomal DNA. In addition, no amplification products were detected from cell lines infected with HTLV-I, HTLV-II, or human immunodeficiency virus type 1 or 2 by the BLV PCR systems. Samples of peripheral blood mononuclear cells from 18 cows, previously determined to be serologically positive or negative, were correctly identified in a blind study as containing proviral DNA by use of the BLV primers and probes. Cloning and sequencing of amplified products revealed finite sequence variations among a previously cloned BLV isolate, the wild-type virus, and the published genome. Reverse transcriptase-directed PCR with the primers for both BLV pol and BLV pX was performed on plasma from a BLV-infected cow and detected in vivo BLV RNA expression. In summary, we have developed a specific and sensitive assay using PCR for the detection and identification of BLV infections; this assay can now be applied to clinical and basic research questions in veterinary medicine.