Mapping of B-neutralizing and T-helper cell epitopes on the bovine leukemia virus external glycoprotein gp51.

A battery of 19 synthetic peptides was used to characterize efficient neutralizing and helper T-cell epitopes on the bovine leukemia virus (BLV) external envelope glycoprotein gp51. Four of the antipeptide antisera raised in rabbits inhibited the formation of BLV-induced syncytia; these antisera are directed against peptides 64-73, 98-117, and 177-192. Only antisera directed against the 177-192 region also neutralized vesicular stomatitis virus-BLV pseudotypes. This study clearly demonstrates that neutralizing properties can be observed with antibodies raised to regions undescribed so far and included in both the amino-terminal and central parts of the antigen. In addition, some helper T-cell determinants were defined from gp51-immunized mice and from BLV-infected cattle. Although none of the peptides tested behaved as a universal helper T-cell epitope, peptide 98-117 stimulated T-cell proliferation from BALB/c mice and from three infected cows, while peptide 169-188 strongly stimulated T-cell proliferation from one infected cow. Further experiments performed with three peptides overlapping the 169-188 region (177-192, 179-192, 181-192) demonstrated the particular relevance of residue(s) P-177 and/or D-178 in the helper T-cell epitope. These data should assist in the design of an efficient subunit vaccine against BLV infection that contains peptides possessing both B-neutralizing and helper T-cell determinants.

In vivo infection of sheep by bovine leukemia virus mutants.

Direct inoculation of a cloned bovine leukemia virus (BLV) provirus into sheep has allowed study of the viral infectivity of genetic mutants in vivo. Three BLV variants cloned from BLV-induced tumors and 12 in vitro-modified proviruses were isolated and analyzed for viral expression in cell culture. The proviruses were then inoculated into sheep in order to assess viral infectivity in vivo. Of three variants cloned from BLV-induced tumors (344, 395, and 1345), one (344) was found infectious in vivo. This particular provirus was used to engineer 12 BLV mutants. A hybrid between the 5' region of the complete but noninfectious provirus 395 and the 3' end of mutant 344 was infectious in vivo, suggesting that the tax/rex sequences were altered in virus 395. As expected, several regions of the BLV genome appeared to be essential for viral infection: the protease, pol, and env genes. Even discrete modifications in the fusion peptide located at the NH2 end of the transmembrane gp30 glycoprotein destroyed the infectious potential. In contrast, mutations and deletions in the X3 region present between the env gene and the 3' tax/rex region did not interfere with viral infection in vivo. This region of unknown function could thus be used to introduce foreign sequences. A BLV recombinant carrying a ribozyme directed against the tax/rex sequences was still infectious in vivo. Cotransfection of two noninfectious mutants carrying deletions led to infection in two of four independent injections, the infectious virus being then a recombinant between the two deletants. The experimental approach described here should help to gain insight into essential mechanisms such as in vivo viral replication, cooperation between deletants for viral infectivity, and viral superinfections. The gene products in the X3 and X4 region which are dispensable for in vivo infection could be involved in leukemogenesis, and thus proviruses deleted in these sequences could constitute the basis for a live attenuated vaccine.

Enhancement of bovine leukemia virus-induced syncytia formation by di- and tripeptides.

Short hydrophobic peptides were previously shown to inhibit infectivity of para- and orthomyxoviruses. We tested the ability of a series of di- and tripeptides to interfere with cell fusion induced by bovine leukemia virus (BLV). Peptides containing a hydrophobic contribution and/or a positive net charge strongly enhanced syncytia formation induced by BLV on CC81 indicator cells. The size of the multinucleated cells was strongly increased (up to 10-fold) in the presence of the enhancer peptides whereas no effect was observed on the indicator cells in the absence of BLV. The peptides thus amplified the fusion process initiated by BLV envelope glycoproteins. The effect was dose-dependent at concentrations ranging from 20 to 640 microM and did not result from an increased expression of BLV proteins. The peptides did not compete with anti-gp51 monoclonal antibodies for the recognition of eight well-defined epitopes of gp51. We consequently hypothesize that the enhancer peptides interact with the membrane of BLV-producing cells and/or indicator cells and propose a model based on molecular modeling.

The 19-27 amino acid segment of gp51 adopts an amphiphilic structure and plays a key role in the fusion events induced by bovine leukemia virus.

Previous results indicate that the external glycoprotein gp51 of bovine leukemia virus plays an important role in the process of cell fusion induced by bovine leukemia virus (Bruck, C., Mathot, S., Portetelle, D., Berte, C., Franssen, J. D., Herion, P., and Burny, A. (1982) Virology 122, 342-352; Vonèche, V., Portetelle., D., Kettmann, R., Willems, L., Limbach, K., Paoletti, E., Ruysschaert, J. M., Burny, A., and Brasseur, R. (1992) Proc. Natl. Acad. Sci. U. S. A. 89, 3810-3814) and suggest that a region encompassing residues 23 and 25 of gp51 is involved in this process (Portetelle, D., Couez, D., Bruck, C., Kettmann, R., Mammerickx, M., Van der Maaten, M., Brasseur, R., and Burny, A. (1989) Virology 169, 27-33; Mamoun, R., Morisson, M., Rebeyrotte, N., Busetta, B., Couez, D., Kettmann, R., Hospital, M., and Guillemain, B. (1990) J. Virol. 64, 4180-4188). X-ray diffraction studies performed on envelope glycoproteins of influenza virus indicate that the NH2-terminal part of the external glycoprotein lies very close to the fusion peptide. The same overall structure seems to exist in human immunodeficiency virus as suggested by site-directed mutagenesis followed by syncytia induction assays. Our theoretical studies indicate that a segment expanding between residues 19 and 27 of gp51 probably adopts an amphipathic beta-strand structure. We hypothesize that the amphipathic 19-27 structure of gp51 plays an important role in the process of membrane fusion by interacting with the fusion peptide or with another region of gp30. Mutational analysis disrupting the amphipathy of the 19-27 region strongly altered the fusogenic capacity of the gp51-gp30 complex.

Fusogenic segments of bovine leukemia virus and simian immunodeficiency virus are interchangeable and mediate fusion by means of oblique insertion in the lipid bilayer of their target cells.

Modified bovine leukemia virus (BLV) glycoproteins were expressed by using vaccinia virus recombinants, and their fusogenic capacities were examined by a syncytia-formation assay. This analysis indicates that (i) both BLV envelope glycoproteins gp51 and gp30 are necessary for cell fusion; (ii) insertion of the N-terminal segment of gp30 (fusion peptide) into the lipid bilayer in an oblique orientation, as predicted by computer conformational analysis, results in fusogenic capacities higher than insertion in a perpendicular or parallel orientation; and (iii) replacement of the BLV fusion peptide with its simian immunodeficiency virus counterpart does not modify the fusogenic capacity of the BLV glycoprotein.