Identification of a novel overlapping sequential E epitope (E′) on the bovine leukaemia virus SU glycoprotein and analysis of immunological data

Bovine leukaemia virus (BLV), an oncogenic C-type retrovirus, is the causative agent of enzootic bovine leucosis. Binding of BLV to its cellular receptor is mediated by the surface envelope glycoprotein subunit (SU). Previous studies have identified eight different epitopes (A through H) on the BLV SU. In this study, a new sequential epitope was identified using the monoclonal antibody 2G7 (MAb 2G7) on the C-terminal region of the BLV SU. To localise and refine the map of this epitope, a series of deleted forms in the C and N-terminal ends of the glycoprotein were made and synthesised in baculovirus and Escherichia coli expression systems. The synthetic proteins were analysed both in Western blot and MAb-capture ELISA assays. MAb 2G7 recognised a stretch of 11 amino acids, named epitope E′, corresponding to residues 189-SDWVPSVRSWA-199 (comprising the 33 amino acids signal peptide) overlapping with the E epitope of the SU. The data obtained by Enzyme-Linked Immunosorbent Assay (ELISA) revealed that the E′ epitope was hidden on whole BLV particles and that the variation in reactivity between epitope E′ and MAb 2G7 depends on the glycosylation state of SU. Similarly, the analysis of immunological data evidenced that the failure of interaction between the MAb anti-DD′ and its epitope was also due to a steric hindrance of the glycosylation. Finally, the ELISA assay analysis performed with the deleted and mutated forms of rSU evidenced that the conformational epitopes F, G and H lied into in the 34-173 amino-acids residues of N-terminal region of SU.