Etude des fonctions de la protéine ASORF du virus de la leucémie bovine.

Abstract

The bovine leukemia virus (BLV) is an oncogenic retrovirus responsible for enzootic bovine leukosis (EBL), a disease affecting cattle. This retrovirus mainly infects B lymphocytes. Although the majority of infected animals remain asymptomatic, about one-third develop persistent lymphocytosis (PL), and 5 to 10% develop leukemia or lymphoma. Both strands of viral genomic DNA are transcribed from promoters located in the 5’ LTR and 3’ LTR. While sense transcription has been widely studied, this is not the case for antisense transcription which shows the expression of antisense RNAs (AS1 and AS2). Recently, a protein named ASORF, translated from the AS1 transcript, has been identified, but its functions remain to be elucidated. The aim of this work is to contribute to the study of the role(s) of the ASORF protein, in particular by using ovine models, infected either with the wild type virus or with the mutated virus unable to express ASORF. Several experimental approaches were implemented, including luciferase assays, qPCR, immunological tests (ELISA and LIPS) as well as flow cytometry. The results show that ASORF participates in maintaining a high proviral load without, however, preventing the establishment of the humoral response directed against the viral proteins gp51 and p24. Moreover, the detection of antibodies directed against ASORF demonstrates its immunogenic nature. No major difference in the distribution of B and T lymphocytes was observed between wild type and mutated virus. At the transcriptional level, ASORF increases the basal transcription of the LTR, potentially via a mechanism independent of CRE sites, without influencing the transactivation mediated by the transactivator protein Tax. Furthermore, no effect of ASORF has been demonstrated on the TGF-β, CREB, NF-κB and AP-1 signalling pathways. In conclusion, this work has highlighted the role of the ASORF protein in basal transcription and in the replication of the BLV virus.