Phenotypic Selection of Dairy Cattle Infected with Bovine Leukemia Virus Demonstrates Immunogenetic Resilience through NGS-Based Genotyping of BoLA MHC Class II Genes.

Chaelynne E Lohr,Paul C Bartlett,Andrew Borgman,Casey J Droscha,Laura A Pavliscak,Matelyn M Mason,Tasia M Taxis,Kelsey A Brigham,Kelly R B Sporer,Vickie J Ruggiero

doi:10.3390/pathogens11010104

Chaelynne E Lohr, Paul C Bartlett + Show 8 more

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https://doi.org/10.3390/pathogens11010104

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Abstract

Characterization of the bovine leukocyte antigen (BoLA) DRB3 gene has shown that specific alleles associate with susceptibility or resilience to the progression of bovine leukemia virus (BLV), measured by proviral load (PVL). Through surveillance of multi-farm BLV eradication field trials, we observed differential phenotypes within seropositive cows that persist from months to years. We sought to develop a multiplex next-generation sequencing workflow (NGS-SBT) capable of genotyping 384 samples per run to assess the relationship between BLV phenotype and two BoLA genes. We utilized longitudinal results from milk ELISA screening and subsequent blood collections on seropositive cows for PVL determination using a novel BLV proviral load multiplex qPCR assay to phenotype the cows. Repeated diagnostic observations defined two distinct phenotypes in our study population, ELISA-positive cows that do not harbor detectable levels of provirus and those who do have persistent proviral loads. In total, 565 cows from nine Midwest dairy farms were selected for NGS-SBT, with 558 cows: 168 BLV susceptible (ELISA-positive/PVL-positive) and 390 BLV resilient (ELISA-positive/PVL-negative) successfully genotyped. Three BoLA-DRB3 alleles, including one novel allele, were shown to associate with disease resilience, *009:02, *044:01, and *048:02 were found at rates of 97.5%, 86.5%, and 90.3%, respectively, within the phenotypically resilient population. Alternatively, DRB3*015:01 and *027:03, both known to associate with disease progression, were found at rates of 81.1% and 92.3%, respectively, within the susceptible population. This study helps solidify the immunogenetic relationship between BoLA-DRB3 alleles and BLV infection status of these two phenotypic groupings of US dairy cattle.

Highlights

Bovine leukemia virus (BLV) belongs to the deltaretrovirus genus within the Retroviridae family and is the cause of enzootic bovine leukosis (EBL), the most common neoplastic disease in dairy cattle
We have found that a proportion of cows are enzyme-linked immunosorbent assay (ELISA)-positive but do not have a detectable proviral load (PVL) and do not progress in disease status
We observed that 5–15% of ELISA-positive cows did not harbor detectable levels of provirus as determined by the SS1 quantitative PCR (qPCR) BLV PVL Assay

Summary

Introduction

Bovine leukemia virus (BLV) belongs to the deltaretrovirus genus within the Retroviridae family and is the cause of enzootic bovine leukosis (EBL), the most common neoplastic disease in dairy cattle. Most BLV-infected cattle are asymptomatic, yet 30% of ELISA-positive cattle develop persistent lymphocytosis (PL), defined as an increase in blood lymphocyte concentration caused by B-cell proliferation for at least three months [2]. A small number of cattle with PL (1–5%) develop B lymphocyte lymphoma; PL cattle have an increased probability of transmitting BLV to their herdmates, incurring a persistent economic burden to dairy producers [3,4]. Detection of BLV proviral DNA in whole blood is most often accomplished through quantitative PCR (qPCR). Our group defines proviral load (PVL) as the relative concentration of BLV proviral DNA to host DNA detected within a blood-derived genomic DNA extract (gDNA). We have found that a proportion of cows are ELISA-positive but do not have a detectable PVL and do not progress in disease status. One possible explanation for these differing phenotypes is the contribution of host genetics, within genomic loci directly tied to the humoral immune response

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