To establish a diagnostic index for predicting enzootic bovine leukosis (EBL), proviral bovine leukemia virus (BLV) copies in whole blood, lymph nodes and spleen were examined by quantitative real-time PCR (qPCR). Cattle were divided into two groups, EBL and BLV-infected, based on meat inspection data. The number of BLV copies in all specimens of EBL cattle was significantly higher than those of BLV-infected cattle (p < 0.0001), and the number of BLV copies in the lymph nodes was particularly large. Over 70 % of the superficial cervical, medial iliac and jejunal lymph nodes from EBL cattle had more than 1,000 copies/10 ng DNA, whereas lymph nodes from BLV-infected cattle did not. These findings suggest that the cattle harboring more than 1,000 BLV copies may be diagnosed with EBL.

Bovine leukemia virus (BLV) is the causative agent of enzootic bovine leukosis (EBL). The incidence of EBL in Japan is increasing annually; and the cases of EBL in cattle younger than 2 years old has been reported. Therefore, it is vital to find a method to control BLV infection, especially in young calves. In this study, to evaluate the protective ability of colostral antibodies against BLV infection, as well as the potential for BLV infection mediated by colostrum/milk, we investigated temporal fluctuations in the anti-BLV antibody titer and BLV proviral load (PVL) in colostrum/milk and peripheral blood of six infected dams during lactation. The association between PVL and antibody titer in colostrum and peripheral blood was then investigated using samples from a further twenty-seven cattle. Antibody concentrations were measured with a Syncytium-induction Inhibition Assay using colostral/milk whey and serum. PVL in peripheral blood and colostrum was measured by real-time PCR. Colostral antibodies showed high inhibitory activity until day 3 of lactation. The antibody titer and PVL in peripheral blood showed lesser changes than those in colostrum/milk throughout lactation. The colostral antibody titer was significantly higher than the serum antibody titer in all samples, whereas the colostrum PVL was significantly lower than the blood PVL. The blood PVL showed a significant correlation with serum antibody titer, colostrum PVL, and colostral antibody titer. However, there were no major correlations between the serum and colostral antibody titers. This is the first report investigating the temporal changes in colostral antibody titer in terms of inhibiting BLV infection in vitro. The results of antibody detection by Syncytium-induction Inhibition Assay suggested that the protective activity of the colostral antibodies against BLV infection would be conferred by anti-BLV gp51 antibody. The high antibody titer of colostral whey suggests that colostral whey could be a potential source of antibodies with a low risk of infection in neonatal calves.

Bovine leukemia virus (BLV) is the causative agent of enzootic bovine leukosis (EBL), a malignant B cell lymphoma. However, the mechanisms of BLV-associated lymphomagenesis remain poorly understood. Here, after deep sequencing, we performed comparative analyses of B cell microRNAs (miRNAs) in cattle infected with BLV and those without BLV. In BLV-infected cattle, BLV-derived miRNAs (blv-miRNAs) accounted for 38% of all miRNAs in B cells. Four of these blv-miRNAs (blv-miR-B1-5p, blv-miR-B2-5p, blv-miR-B4-3p, and blv-miR-B5-5p) had highly significant positive correlations with BLV proviral load (PVL). The read counts of 90 host-derived miRNAs (bta-miRNAs) were significantly down-regulated in BLV-infected cattle compared to those in uninfected cattle. Only bta-miR-375 had a positive correlation with PVL in BLV-infected cattle and was highly expressed in the B cell lymphoma tissue of EBL cattle. There were a few bta-miRNAs that correlated with BLV tax/rex gene expression; however, BLV AS1 expression had a significant negative correlation with many of the down-regulated bta-miRNAs that are important for tumor development and/or tumor suppression. These results suggest that BLV promotes lymphomagenesis via AS1 and blv-miRNAs, rather than tax/rex, by down-regulating the expression of bta-miRNAs that have a tumor-suppressing function, and this downregulation is linked to increased PVL.

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