Hot topic: Bovine leukemia virus (BLV)-infected cows with low proviral load are not a source of infection for BLV-free cattle.

The bovine leukemia virus (BLV) causes leukemia or lymphoma in cattle. Although most BLV-infected animals do not develop the disease, they maintain the transmission chain of BLV at the herd level. As a feasible approach to control the virus, selection of cattle carrying the BoLA-DRB3*0902 allele has been proposed, as this allele is strongly associated with a BLV infection profile or the low proviral load (LPL) phenotype. To test whether these cattle affect the BLV transmission chain under natural conditions, selected BLV-infected LPL-BoLA-DRB3*0902 heterozygous cows were incorporated into a BLV-negative dairy herd. An average ratio of 5.4 (range 4.17-6.37) BLV-negative cows per BLV-infected cow was maintained during the 20mo of the experiment, and no BLV-negative cattle became infected. The BLV incidence rate in this herd was thus zero, whereas BLV incidence rates in different local herds varied from 0.06 to 0.17 cases per 100 cattle-days. This finding strongly suggests that LPL-BoLA-DRB3*0902 cattle disrupted the BLV-transmission chain in the study period.

Partial molecular characterization of different proviral strains of bovine leukemia virus.

Bovine leukemia virus (BLV)-infected cattle were classified by their proviral load into low and high proviral load profiles (LPL and HPL, respectively). Blood from these animals was used to infect sheep to obtain multiple identical copies of integrated provirus. An env fragment of BLV was amplified from all infected sheep and sequenced. The sequences that were obtained were compared to already published BLV genome sequence, resulting in three clusters. Mutations could not be attributed to the passage of provirus from cattle to sheep and subsequent amplification and sequencing. The description of two different proviral load profiles, the association of the BoLA-DRB3.2 0902 allele with the LPL profile, the availability of complete BLV sequences, and the comparison of a variable region of the env gene from carefully characterized cattle are still not enough to explain the presence of animals in every herd that are resistant to BLV dissemination.

The complete genomic sequence of an in vivo low replicating BLV strain.

DNA was extracted from lamb lymphocytes that were infected in vivo with a BLV strain after inoculation with the peripheral blood mononuclear cells from a persistently sero-indeterminate, low viral load, BLV-infected Holstein cow (No. 41) from Argentina. The DNA was PCR amplified with a series of overlapping primers encompassing the entire BLV proviral DNA. The amplified BLV ARG 41 DNA was cloned, sequenced, and compared phylogenetically to other BLV sequences including an in vivo high replicating strain (BLV ARG 38) from the same herd in Argentina. Characterization of BLV ARG 41's deduced proteins and its relationship to other members of the PTLV/BLV genus of retroviruses are discussed.

Determination of proviral load in bovine leukemia virus-infected cattle with and without lymphocytosis.

OBJECTIVE: To determine proviral load in bovine leukemia virus (BLV)-infected cattle with and without persistent lymphocytosis to assess the potential of transmitting the virus. ANIMALS: Cattle in 6 dairy herds. PROCEDURES: Blood samples from infected cows were evaluated 3 times at 6-month intervals for determination of proviral load via PCR assay, serologic results via ELISA, and hematologic status via differential cell counts. RESULTS: Infected cattle were classified into lymphocytotic and nonlymphocytotic groups. Lymphocytotic cattle consistently had > 100,000 copies of integrated provirus/mug of DNA (ie, high proviral load) in peripheral blood leukocytes. Titers of antibodies against BLVgp51 and BLVp24 indicated a strong immune response. Nonlymphocytotic cattle comprised 2 subgroups: a group with high proviral load and strong immune response, and a group with a weaker immune response, mostly against BLVp24, and a proviral load of < 100 copies/microg of DNA (ie, low proviral load). CONCLUSIONS AND CLINICAL RELEVANCE: Results emphasized the importance of characterizing nonlymphocytotic BLV-infected cattle during eradication programs. The risk of transmitting BLV infection from nonlymphocytotic cattle may differ depending on the proviral load. Nonlymphocytotic cattle with high proviral load could be efficient transmitters (as efficient as lymphocytotic cattle), whereas nonlymphocytotic cattle with low proviral load could be inefficient transmitters under standard husbandry conditions. Because most cattle with low proviral load do not develop anti-BLVp24 antibodies, it appears that lack of an anti-BLVp24 antibody response may be a good marker of this condition.

Host soluble factors that regulate the synthesis of the major core protein of the bovine leukemia virus (BLV) in a naturally infected neoplastic B-cell line.

Bovine leukemia virus (BLV) is a B-cell tropic Deltaretrovirus that induces a lifelong infection and causes a fatal lymphosarcoma in less than 10% of the infected cattle. BLV is usually present in its host in a transcriptional repressed state but becomes de-repressed a few hours after the infected lymphocytes are cultured in vitro. In the present study we have examined the effect of soluble host factors and various substances on the synthesis of the major BLV protein (p24) in a permanent culture (cell line NBC-10) of neoplastic B-lymphocytes derived from BLV-infected cattle. Certain batches of fetal calf serum (FCS) and bovine platelet lysates (PLy) induced a rapid and drastic increase of the synthesis of BLVp24 in the NBC-10 cells. Neutralization experiments with specific antibodies demonstrated that the transforming growth factor-beta (TGF-beta) was responsible for the stimulatory activity of FCS and PLy on the synthesis of BLVp24 in the NBC-10 cells. Recombinant TGF-beta also stimulated the synthesis of BLVp24 in cultures of peripheral blood mononuclear cells (PBMCs) obtained from BLV-infected cattle. Mitogens, phorbol-myristate-acetate and prostaglandin E(2), previously shown to stimulate the expression of BLV in cultures of PBMC, did not induce the synthesis of BLVp24 in cultures of NBC-10 cells. Plasma, serum and milk from BLV-negative cattle inhibited the synthesis of BLVp24 induced by FCS, PLy or TGF-beta in the NBC-10 cells. The blocking activity was found in the whey and the beta-casein fractions of bovine milk. The relevance of these findings with regard to the previously reported plasma factor (PBB) with blocking activity on the expression of BLV in short-term PBMC cultures is discussed. Based on the information obtained in the present study we have standardized a reproducible and rapid assay system for the identification of factors that regulate the synthesis of BLVp24 in naturally infected neoplastic B cells.