Even Attenuated Bovine Leukemia Virus Proviruses Can Be Pathogenic in Sheep

Abstract

Based on a reverse genetics approach, we previously reported that bovine leukemia virus (BLV) mutants harboring deletions in the accessory R3 and G4 genes persist at very low proviral loads and are unable to induce leukemia or lymphoma in sheep, indicating that these R3 and G4 gene sequences are required for pathogenesis. We now show that lymphoma can occur, albeit infrequently (1 case of 20) and after extended periods of latency (7 years). Direct sequencing and reinfection experiments demonstrated that lymphomagenesis was not due to the reversion of the mutant to the wild type. Similar observations with another type of attenuated mutant impaired in the transmembrane protein (TM) YXXL signaling motifs were made. We conclude that the R3 and G4 genes and the TM YXXL motifs are not strictly required for pathogenesis but that their integrity contributes to disease frequency and latency.Viral persistence results from a dynamic interplay between the host immune defense and the replicative capacity of the virus. Bovine leukemia virus (BLV) replication occurs via two main processes: the infection of new cellular targets and the mitosis of the host cell (recently reviewed in references 7 and 11). Although these two mechanisms can theoretically occur simultaneously, the former operates mainly in the early stages of infection, i.e., the seroconversion period. In contrast, viral spread by mitotic cell division accounts for most of the proviral loads during the asymptomatic and leukemic phases (20). Amazingly, the leukemic clone that generates the tumor originates from a cell that was infected soon after seroconversion. It seems, therefore, that the premalignant cell clone and/or its progeny persist throughout the long latency period. However, the molecular mechanisms that govern viral persistence and spread are still largely unknown.In an attempt to define the viral determinants required for transformation, we first established a model system based on the infection of sheep with a cloned BLV provirus (36). Among a series of proviruses, the 344 strain recapitulated all aspects associated with a lymphoproliferative disease followed by the onset of leukemia, lymphoma, or lymphosarcoma. In order to discover the role of viral genes in the leukemogenesis process (which includes all events leading to leukemia, lymphoma, or lymphosarcoma), we used reverse genetics by introducing directed deletions, insertions, and point mutations into the 344 proviral clone (35). By this approach, we previously demonstrated that the R3 and G4 accessory genes are dispensable for infection but are required for the maintenance of high proviral loads (34). Furthermore, the G4 protein, but not R3, exhibits transforming potential in primary rat embryo fibroblasts when coexpressed with the Ha-ras oncogene (15). Indeed, the G4-and Ha-ras-double-positive cells form transformed foci in cell cultures and induce tumors in nude mice. Mechanistically, G4

A detailed molecular analysis of complete Bovine Leukemia Virus genomes isolated from B-cell lymphosarcomas

“…These sequences correspond to the third and second exons of R3 and G4, respectively, revealing that these sequences may not be essential for infectivity in vivo. Although previous studies have shown that deletions in R3/G4 interferes with the efficiency of BLV propagation and restricts pathogenesis [14,15,46,49], another study has shown that one out of 20 sheep infected with a R3/G4 mutant developed a lymphoma after 7.5 years of latency, suggesting that the deleted sequences may not be strictly required for pathogenesis [51]. Further studies will be needed to address the biological significance of these findings in studies using the cow as a model for BLV infection.…”

Bovine Leukemia Virus Gene Segment Detected in Human Breast Tissue

“…Several laboratory trials showed that Tax could transform cells in in vitro cultures and being oncogenic in animal models where cells can be transformed when Tax is combined with ha-ras oncogene to produce tumor cells in athymic mice and fibroblasts immortalization coming from rat embryo [24,25]; 2) Viral genome integration process close to highly active cellular genome segments associated with cell division [26]. This is how the Avian Sarcoma Leukosis Virus (ASLV) can integrate in many places of cellular genome, but when there is tumor in the host, the virus has been found upstream of the c-myc cellular gene, which codifies for the Myc proteins implicated in the cellular DNA synthesis control [26,27].…”

Stable infection of a bovine mammary epithelial cell line (MAC-T) with bovine leukemia virus (BLV)

“…Thus, this silent infection is spread worldwide with the exception of Europe, where it has been eradicated (Bartlett et al, 2014). Transmission occurs naturally and iatrogenically through the transfer of infected cells, mainly by blood or milk (Florins et al, 2007). Based on this assumption, the mammary epithelium could play an important role in viral transmission.…”