Anti-HIV response elicited in rabbits by anti-adiotype monoclonal antibodies mimicking the CD4-binding site on GP120

The anti-human immunodeficiency virus type 1 (HIV-1) immune responseis erroneously focused on highly variable regions of HIV-1 surface glycoproteins not fundamental to viral replication. However,crucial conserved portions (as the CD4-binding site - CD4bs - on gp120) exist but are not exposed to the immune system. Classical antigen-basedvaccine approaches are not able of stimulating antibodiesagainst these regions. In this study we describe the generation of two anti-idiotype (AI) murine antibodies recognizing anti-CD4bs IgG purified from long-term non progressor (LTNP) patients. The mAbs were shown to react with b12, the most potent anti-CD4bs human mAb, and to elicit in rabbits anti-HIV-1 antibodies confirming themas CD4bs mimotopes. The same mAbs were also used to detect the presence of anti-CD4bs antibody subpopulations in the serum of infectedpatients. To obtain the murine mAbs with AI features, anti-CD4bs IgG were purified from LTNP through several purification steps. The purified IgG were then used as Fab fragments to immunize mice for hybridoma generation. Hybridomas P1 and P2 only reacting in ELISA with the anti-CD4bs IgG, and not with standard IgG, were used to immunize rabbits. CD4bs-mimicking P1 and P2 antibodies recognized the idiotype of the widely neutralizing anti-CD4bs b12 human mAb, and inhibited its binding to gp120. P1 and P2-immunized rabbit sera showed a strong anti-gp120 titer associated to a neutralizing activity in a pseudovirusassay (HXB2 strain). In particular, 3/5 rabbits in the P1 group and 1/5 in the P2 group showed an 80% neutralization at dilutions ranging from 1:20 to 1:150. In the case of two rabbits an 80% neutralizationwas also shown against virions bearing glycoproteins from a different strain (MN). This proof-of-concept study describes the first epitope-based vaccinal approach performed recurring to CD4bs-mimicking AI molecules. Although comparison of different immunization protocols will be necessaryto improve the strength and the breadth of the neutralizing response, these data document that immunogens designed on the idiotypeof broadly neutralizing Abs are feasible and could help in the design of future anti-HIV strategies.