Elicitation of broadly neutralizing HIV-1 antibodies by guiding the immune responses using primary and secondary immunogens

Abstract

Background: Broadly neutralizing HIV-1 monoclonal antibodies (bnmAbs) are potential key components of protective immunity. Such antibodies have not been induced by any vaccine immunogens so far. HIV-1 envelop proteins lack detectable binding to putative germline form of bnmAbs, which may partly explain the poor performance of envelop-based vaccine. We hypothesized that B-cell-lineage immunogens targeting the germline B cell receptors of bnmAbs may help prime the immune system and guide the immune response. Methods: Using b12 as a model antibody, we screened various cDNA libraries against b12 germline and intermediate antibodies. Isolated (poly) peptides were either synthesized or expressed as human Fc fusion proteins. Rabbit immunization was carried out by priming with the (poly) peptides and boosting with SF162 gp140 trimer. Immunization with SF162 trimer alone was used as a control. Rabbit IgGs were tested by competition ELISA with mature IgG1 b12, and by TZM-bl neutralization assay. Results: Prime with b12 germline antibody specific peptides or peptide-Fc fusions followed by boost with envelop trimer induced rabbit antibodies that competed with mature IgG1 b12, while immunization with SF162 envelop trimer alone did not induce b12-like antibodies. The estimated EC50s were 675nM and 133.33nM for peptide and peptide-Fc primed rabbit IgGs, respectively. We are currently testing purified rabbit IgGs with a large panel of HIV-1 isolates in TZM-bl neutralization assay. Preliminary result indicated that immune rabbit IgGs did not neutralize four tier 2 HIV-1 isolates. Conclusion: Our results indicate that ‘‘two immunogens’’ (primary and secondary) strategy can direct the immune response and elicit antibodies that mimic the target bnmAb(s) in binding, but eliciting antibodies that mimic the target bnmAb(s) in neutralization requires further investigation. Boost with HIV-1 envelop trimer may distract the targeted immune responses. Our study suggests that non-HIV-derived (poly) peptides may be used as primary immunogens to guide HIV-1-specific immune responses.