Genomic integration of bovine leukemia provirus: comparison of persistent lymphocytosis with lymph node tumor form of enzootic.

Abstract

Integration of bovine leukemia proviral DNA in the genome of infected cells was investigated in cattle affected by either the persistent lymphocytosis or the lymph node tumor form of enzootic bovine leukosis. In persistent lymphocytosis, proviral DNA was found to be integrated at a large number of genomic sites in one-fourth to one-third of circulating leukocytes. In the lymph node tumor form, in contrast, proviral DNA was found to be integrated at one or very few sites in the genomes of a larger fraction of both circulating leukocytes and lymph node tumor cells. Enzootic bovine leukosis (EBL) is a lymphoproliferative disease of cattle induced by an exogenous retrovirus, the bovine leukemia virus (BLV) (1-3), whose target cell is the B lymphocyte (4). Two forms of the disease are known: persistent lymphocytosis (PL), characterized by a permanent large number of peripheral lymphocytes (5-7), and a tumoral form, characterized by lymph node tumors. These two forms generally represent the early and the late stage of the disease, respectively. In some cases, however, these two pathological conditions do not affect the same cattle herds, suggesting that they may represent separate responses to BLV infection, both of them being under the control of genetic factors (8-10). In general, the fate of BLV-infected animals is variable and depends upon several factors, including age, genetic make-up, environmental factors, and immunological surveillance (see ref. 11 for a review).In the present work, we have compared proviral integration into the genomes of host cells of naturally infected animals that were in either the PL or the tumor stage of the disease. The main result obtained in these investigations was that proviral copies were found to be integrated at several sites in the genomes of one-fourth to one-third of the leukocytes of animals in PL, but at only one or very few sites in the genomes of a larger fraction of leukocytes or tumor cells of animals in the tumor stage of the disease. MATERIALS AND METHODSBovine tissues and cells were collected from seven animals with EBL. Four animals (nos. 928, 641, 2586, and 4) were in PL and three (nos. 15, 950, and 82) Liquid hybridization was performed as described (2). RESULTS Persistent lymphocytosisAs shown in Fig. 1A, BLV [32P]cDNA hybridized on a large number of EcoRI fragments from leukocyte DNA of animals in PL. Molecular weights of the fragments cover a wide range, varying between 5-6 X 106 and 10-12 X 106. In some samples, hybridization bands emerged over the continuous background, an indication that some integration sites were preferred. No fragment with Mr lower than 5 X 106 was apparent. The control, calf thymus DNA digest, showed a weak band at Mr 5 X 106, corresponding to ribosomal DNA (13,14). The integrated BLV proviruses examined so far (13) showed none or at the most one EcoRI restriction site.BLV [32P]cDNA also hybridized on two BamHI fragments, Mr 2.0 X 106 and 1.3 X 106, from the above DNAs. Weak hybridization bands were also observed in some cas...

Alterations in Humoral Immune Response to Bovine Leukaemia Virus Antigens in Cattle with Lymphoma

“…Rising antibody titres appear to be stimulated by virus persistence and replication. An increasing number of infected lymphocytes can eventually be detected by Southern blotting at 6 months to 1 year post-infection (Kettmann et al, 1980a). However, neither the spectrum of target cells nor the immunopathogenesis to the various chronic disease states are yet understood.…”

“…Expression of certain viral genes is believed to be necessary for transformation by these viruses; however, neither viral RNA nor viral proteins have been detected in cases of enzootic bovine lymphoma (EBL) (Kettmann et al, 1980b(Kettmann et al, , 1982. Southern blot analysis of infected, non-transformed lymphocytes has demonstrated a large number of provirus integration sites per cell (Kettmann et al, 1980a). The tumours are clonal and transformed cells contain relatively fewer (one to four) t Present address: Laboratory of Viral Carcinogenesis, Section of Genetics, NCI Frederick Cancer Research Facility.…”

Increased cell proliferation, but not reduced cell death, induces lymphocytosis in bovine leukemia virus-infected sheep

“…3 and 4). BLV and human T lymphotropic virus (HTLV-1) share many structural and functional homologies: both are exogenous to their host species (5,6), have similar genomic organizations (7), integrate into dispersed sites within the host genome (8,9), and are apparently silent in vivo, at least in the majority of detectable infected cells (10,11).…”

Even transcriptionally competent proviruses are silent in bovine leukemia virus-induced sheep tumor cells.

“…All BLV-induced tumors are clonal and contain at least a portion of a provirus (14)(15)(16) integrated at many sites in the host genome (17,18). All deleted proviral copies examined have shown preservation of the X region, stressing again its probable role in tumorigenesis.…”