Risk Assessment of Bovine Major Histocompatibility Complex Class II DRB3 Alleles for Perinatal Transmission of Bovine Leukemia Virus

Genetic factors underlying host resistance to Rhipicephalus microplus tick infestation in Braford cattle: a systems biology perspective

Approximately 80% of the world's cattle are raised in regions with a high risk of tick-borne diseases, resulting in significant economic losses due to parasitism by Rhipicephalus (Boophilus) microplus. However, the lack of a systemic biology approach hampers a comprehensive understanding of tick-host interactions that mediate tick resistance phenotypes. Here, we conducted a genome-wide association study (GWAS) of 2933 Braford cattle and found 340 single-nucleotide polymorphisms (SNPs) associated with tick counts. Gene expression analyses were performed on skin samples obtained from previously tick-exposed heifers with extremely high or low estimated breeding values for R. microplus counts. Evaluations were performed both before and after artificial infestation with ticks. Differentially expressed genes were found within 1-Mb windows centered at significant SNPs from GWAS. A total of 330 genes were related to the breakdown of homeostasis that was induced by larval attachment to bovine skin. Enrichment analysis pointed to a key role of proteolysis and signal transduction via JAK/STAT, NFKB and WNT/beta catenin signaling pathways. Integrative analysis on matrixEQTL revealed two cis-eQTLs and four significant SNPs in the genes peptidyl arginine deiminase type IV (PADI4) and LOC11449251. The integration of genomic data from QTL maps and transcriptome analyses has identified a set of twelve key genes that show significant associations with tick loads. These genes could be key candidates to improve the accuracy of genomic predictions for tick resistance in Braford cattle.

The Global Epidemiology of Bovine Leukemia Virus: Current Trends and Future Implications

Bovine leukemia virus (BLV) is a retrovirus that causes enzootic bovine leucosis (EBL), which is the most significant neoplastic disease in cattle. Although EBL has been successfully eradicated in most European countries, infections continue to rise in Argentina, Brazil, Canada, Japan, and the United States. BLV imposes a substantial economic burden on the cattle industry, particularly in dairy farming, as it leads to a decline in animal production performance and increases the risk of disease. Moreover, trade restrictions on diseased animals and products between countries and regions further exacerbate the problem. Recent studies have also identified fragments of BLV nucleic acid in human breast cancer tissues, raising concerns for public health. Due to the absence of an effective vaccine, controlling the disease is challenging. Therefore, it is crucial to accurately detect and diagnose BLV at an early stage to control its spread and minimize economic losses. This review provides a comprehensive examination of BLV, encompassing its genomic structure, epidemiology, modes of transmission, clinical symptoms, detection methods, hazards, and control strategies. The aim is to provide strategic information for future BLV research.

BoLA-DRB3 Polymorphism Associated with Bovine Leukemia Virus Infection and Proviral Load in Holstein Cattle in Egypt

Bovine leukemia virus (BLV) is the etiological agent of enzootic bovine leukosis, the most prevalent neoplastic disease of cattle worldwide. The immune response to BLV and disease susceptibility and resistance in cattle are strongly correlated with the bovine leukocyte antigen (BoLA)-DRB3 allelic polymorphism. BLV infection continues to spread in Egypt, in part because the relationships between BLV infection, proviral load in Egypt, and BoLA-DRB3 polymorphism are unknown. Here, we identified 18 previously reported alleles in 121 Holstein cows using a polymerase chain reaction sequence-based typing method. Furthermore, BoLA-DRB3 gene polymorphisms in these animals were investigated for their influence on viral infection. BoLA-DRB3*015:01 and BoLA-DRB3*010:01 were identified as susceptible and resistant alleles, respectively, for BLV infection in the tested Holsteins. In addition, BoLA-DRB3*012:01 was associated with low PVL in previous reports but high PVL in Holstein cattle in Egypt. This study is the first to demonstrate that the BoLA-DRB3 polymorphism confers resistance and susceptibility to PVL and infections of BLV in Holstein cattle in Egypt. Our results can be useful for the disease control and eradication of BLV through genetic selection.

Effectiveness of integrated bovine leukemia virus eradication strategies utilizing cattle carrying resistant and susceptible major histocompatibility complex class II DRB3 alleles

Bovine leukemia virus (BLV) has spread worldwide and causes serious problems in the cattle industry owing to the lack of effective treatments and vaccines. Bovine leukemia virus is transmitted via horizontal and vertical infection, and cattle with high BLV proviral load (PVL), which is a useful index for estimating disease progression and transmission risk, are considered major infectious sources within herds. The PVL strongly correlates with highly polymorphic bovine lymphocyte antigen (BoLA)-DRB3 alleles. The BoLA-DRB3*015:01 and *012:01 alleles are known susceptibility-associated markers related to high PVL, and cattle with susceptible alleles may be at a high risk of BLV transmission via direct contact with healthy cows. In contrast, the BoLA-DRB3*009:02 and *014:01:01 alleles comprise resistant markers associated with the development of low PVL, and cattle with resistant alleles may be low-risk spreaders for BLV transmission and disrupt the BLV transmission chain. However, whether polymorphisms in BoLA-DRB3 are useful for BLV eradication in farms remains unknown. Here, we conducted a validation trial of the integrated BLV eradication strategy to prevent new infection by resistant cattle and actively eliminate susceptible cattle in addition to conventional BLV eradication strategies to maximally reduce the BLV prevalence and PVL using a total of 342 cattle at 4 stall-barn farms in Japan from 2017 to 2019. First, we placed the resistant milking cattle between the BLV-positive and BLV-negative milking cattle in a stall barn for 3 yr. Interestingly, the resistant cattle proved to be an effective biological barrier to successfully block the new BLV infections in the stall-barn system among all 4 farms. Concomitantly, we actively eliminated cattle with high PVL, especially susceptible cattle. Indeed, 39 of the 60 susceptible cattle (65%), 76 of the 140 neutral cattle (54%), and 20 of the 41 resistant cattle (48.8%) were culled on 4 farms for 3 years. Consequently, BLV prevalence and mean PVL decreased in all 4 farms. In particular, one farm achieved BLV-free status in May 2020. By decreasing the number of BLV-positive animals, the revenue-enhancing effect was estimated to be ¥5,839,262 ($39,292.39) for the 4 farms over 3 yr. Our results suggest that an integrated BLV eradication program utilization of resistant cattle as a biological barrier and the preferential elimination of susceptible cattle are useful for BLV infection control.

Influence of BoLA-DRB3 Polymorphism and Bovine Leukemia Virus (BLV) Infection on Dairy Cattle Productivity

Enzootic bovine leukosis caused by the bovine leukemia virus (BLV) results in substantial damage to the livestock industry; however, we lack an effective cure or vaccine. BoLA-DRB3 polymorphism in BLV-infected cattle is associated with the proviral load (PVL), infectivity in the blood, development of lymphoma, and in utero infection of calves. Additionally, it is related to the PVL, infectivity, and anti-BLV antibody levels in milk. However, the effects of the BoLA-DRB3 allele and BLV infection on dairy cattle productivity remain poorly understood. Therefore, we investigated the effect of BLV infection and BoLA-DRB3 allele polymorphism on dairy cattle productivity in 147 Holstein dams raised on Japanese dairy farms. Our findings suggested that BLV infection significantly increased milk yield. Furthermore, the BoLA-DRB3 allele alone, and the combined effect of BLV infection and the BoLA-DRB3 allele had no effect. These results indicate that on-farm breeding and selection of resistant cattle, or the preferential elimination of susceptible cattle, does not affect dairy cattle productivity. Additionally, BLV infection is more likely to affect dairy cattle productivity than BoLA-DRB3 polymorphism.

Application of the Luminescence Syncytium Induction Assay to Identify Chemical Compounds That Inhibit Bovine Leukemia Virus Replication

Bovine leukemia virus (BLV) infection causes endemic bovine leukemia and lymphoma, resulting in lower carcass weight and reduced milk production by the infected cattle, leading to economic losses. Without effective measures for treatment and prevention, high rates of BLV infection can cause problems worldwide. BLV research is limited by the lack of a model system to assay infection. To overcome this, we previously developed the luminescence syncytium induction assay (LuSIA), a highly sensitive and objectively quantifiable method for visualizing BLV infectivity. In this study, we applied LuSIA for the high-throughput screening of drugs that could inhibit BLV infection. We screened 625 compounds from a chemical library using LuSIA and identified two that markedly inhibited BLV replication. We then tested the chemical derivatives of those two compounds and identified BSI-625 and -679 as potent inhibitors of BLV replication with low cytotoxicity. Interestingly, BSI-625 and -679 appeared to inhibit different steps of the BLV lifecycle. Thus, LuSIA was applied to successfully identify inhibitors of BLV replication and may be useful for the development of anti-BLV drugs.

Anti-BLV antibodies in whey correlate with bovine leukemia virus disease progression and BoLA-DRB3 polymorphism

Introduction

Bovine leukemia virus (BLV) belongs to the family Retroviridae and is a causative agent for enzootic bovine leucosis, the most common neoplastic disease affecting cattle worldwide. BLV proviral load (PVL) is associated with disease progression and transmission risk but requires blood collection and quantitative PCR testing. Anti-BLV antibodies in whey have been used as a diagnostic tool for BLV infection; however, quantitative utilization has not been fully investigated. Furthermore, bovine leukocyte antigen (BoLA)-DRB3 is a polymorphic gene associated with BLV infectivity and PVL, but its effect on anti-BLV antibody levels in whey from BLV infected dams is unknown. Therefore, we aimed to investigate whether it is possible to correctly predict PVL in the blood and milk based on the amount of anti-BLV antibodies in milk, and whether the BoLA-DRB3 alleles associate with the amount of anti-BLV antibodies in milk.

Methods

We examined whey from 442 dams from 11 different dairy farms located in 6 prefectures in Japan, including susceptible dams carrying at least one BoLA-DRB3^* 012:01 or ^* 015:01 allele related with high PVL, resistant dams carrying at least one BoLA-DRB3 ^* 002:01, ^* 009:02, or ^* 014:01:01 allele related with low PVL, and neutral dams carrying other alleles.

Results

First, our results provided compelling evidence that anti-BLV antibody levels in whey were positively correlated with the anti-BLV antibody levels in serum and with BLV PVL in blood and milk, indicating the possibility of estimating BLV PVL in blood and milk by measuring anti-BLV antibody levels in whey. Thus, our results showed that antibody titers in milk might be effective for estimating BLV transmission risk and disease progression in the field. Second, we demonstrated that anti-BLV antibody levels in whey from BLV resistant dams were significantly lower than those from susceptible and neutral dams.

Discussion

This is the first report suggesting that the BoLA-DRB3 polymorphism affects anti-BLV antibody levels in whey from BLV-infected dams. Taken together, our results suggested that anti-BLV antibody levels in whey, measured by enzyme-linked immunosorbent assay, may be a useful marker to diagnose the risk of BLV infection and estimate PVL in blood and milk.

BoLA-DRB3 Polymorphism Controls Proviral Load and Infectivity of Bovine Leukemia Virus (BLV) in Milk

Bovine leukemia virus (BLV), which causes enzootic bovine leukosis, is transmitted to calves through the milk of BLV-infected dams. Bovine leukocyte antigen (BoLA)-DRB3 is a polymorphic gene associated with BLV infectivity and proviral load (PVL). However, the effect of BoLA-DRB3 polymorphism on the infectivity and PVL of milk from BLV-infected dams remains unknown. This study examined milk from 259 BLV-infected dams, including susceptible dams carrying at least one BoLA-DRB3*012:01 or *015:01 allele with high PVL, resistant dams carrying at least one BoLA-DRB3*002:01, *009:02, or *014:01:01 allele with low PVL, and neutral dams carrying other alleles. The detection rate of BLV provirus and PVL were significantly higher in milk from susceptible dams than in that from resistant dams. This result was confirmed in a three-year follow-up study in which milk from susceptible dams showed a higher BLV provirus detection rate over a longer period than that from resistant dams. The visualization of infectivity of milk cells using a luminescence syncytium induction assay showed that the infectious risk of milk from BLV-infected dams was markedly high for susceptible dams compared to resistant ones. This is the first report confirming that BoLA-DRB3 polymorphism affects the PVL and infectivity of milk from BLV-infected dams.

Association between BoLA-DRB3 polymorphism and bovine leukemia virus proviral load in Vietnamese Holstein Friesian cattle

Bovine leukemia virus (BLV) is the causative agent of enzootic bovine leukosis. Polymorphism in bovine leukocyte antigen (BoLA)-DRB3 allele can influence the host immune response to pathogens, including BLV. However, association between specific BoLA-DRB3 alleles and BLV proviral load (PVL), which is a useful index for estimating disease progression and transmission risk, in Vietnamese cattle are unknown. Here, association study of BoLA-DRB3 allele frequency between cattle with high or low PVL demonstrated BoLA-DRB3*12:01 associates with high PVL in Vietnamese Holstein-Friesian (HF) crossbred cattle. This is the first study to demonstrate that BoLA-DRB3 polymorphism confers susceptibility to BLV high PVL in HF crossbred kept in Vietnam. Our results may be useful in disease control and eradiation for BLV through genetic selection. This article is protected by copyright. All rights reserved.

A pooled testing system to rapidly identify cattle carrying the elite controller BoLA-DRB3*009:02 haplotype against bovine leukemia virus infection

As genetically resistant individuals, the “elite controllers” (ECs) of human immunodeficiency virus infection have been focused on as the keys to developing further functional treatments in medicine. In the livestock production field, identifying the ECs of bovine leukemia virus (BLV) infection in cattle is desired to stop BLV transmission chains on farms. Cattle carrying the bovine leukocyte antigen (BoLA)‐DRB3*009:02 allele (DRB3*009:02) have a strong possibility of being BLV ECs. Most of cattle carrying this allele maintain undetectable BLV proviral loads and do not shed virus even when infected. BLV ECs can act as transmission barriers when placed between uninfected and infected cattle in a barn. To identify cattle carrying DRB3*009:02 in large populations more easily, we developed a pooled testing system. It employs a highly sensitive, specific real‐time PCR assay and TaqMan MGB probes (DRB3*009:02‐TaqMan assay). Using this system, we determined the percentage of DRB3*009:02‐carrying cattle on Kyushu Island, Japan. Our pooled testing system detected cattle carrying the DRB3*009:02 allele from a DNA pool containing one DRB3*009:02‐positive animal and 29 cattle with other alleles. Its capacity is sufficient for herd‐level screening for DRB3*009:02‐carrying cattle. The DRB3*009:02‐TaqMan assay showed high‐discriminative sensitivity and specificity toward DRB3*009:02, making it suitable for identifying DRB3*009:02‐carrying cattle in post‐screening tests on individuals. We determined that the percentage of DRB3*009:02‐carrying cattle in Kyushu Island was 10.56%. With its ease of use and reliable detection, this new method strengthens the laboratory typing for DRB3*009:02‐carrying cattle. Thus, our findings support the use of BLV ECs in the field.

Kinetic Study of BLV Infectivity in BLV Susceptible and Resistant Cattle in Japan from 2017 to 2019

Bovine leukemia virus (BLV) is the causative agent of enzootic bovine leukosis. Polymorphism in bovine lymphocyte antigen (BoLA)-DRB3 alleles is related to susceptibility to BLV proviral load (PVL), which is a useful index for estimating disease progression and transmission risk. However, whether differential BoLA-DRB3 affects BLV infectivity remains unknown. In a three-year follow-up investigation using a luminescence syncytium induction assay for evaluating BLV infectivity, we visualized and evaluated the kinetics of BLV infectivity in cattle with susceptible, resistant and neutral BoLA-DRB3 alleles which were selected from 179 cattle. Susceptible cattle showed stronger BLV infectivity than both resistant and neutral cattle. The order of intensity of BLV infectivity was as follows: susceptible cattle > neutral cattle > resistant cattle. BLV infectivity showed strong positive correlation with PVL at each testing point. BLV-infected susceptible cattle were found to be at higher risk of horizontal transmission, as they had strong infectivity and high PVL, whereas BLV-infected resistant cattle were low risk of BLV transmission owing to weak BLV infection and low PVL. Thus, this is the first study to demonstrate that the BoLA-DRB3 polymorphism is associated with BLV infection.