Development of a novel VRC01-class germline targeting immunogen derived from anti-idiotypic antibodies


BACKGROUND: Numerous broadly neutralizing antibodies (bNAbs), which exhibit remarkable breadth and potency in their ability to neutralize HIV-1, have been isolated from HIV-1-infected individuals. VRC01-class antibodies are among the most broad and potent bNAbs known and they were isolated from at least 10 different donors. They bind the CD4-binding site on Env and use a unique combination of VH1-2*02 VH gene paired with light chains expressing rare 5 amino acid long CDRL3 domains. Precursor VRC01-class bNAbs display no reactivity to recombinant Env, which lead to the development of germline-targeting immunogens to engage and stimulate naïve VRC01-class precursor B cells. However, these immunogens also present off-target epitopes that could hinder the maturation of VRC01-class bNAbs.
METHODS: As an alternative to Env-derived germline-targeting immunogens, we developed and characterized a panel of monoclonal anti-idiotypic antibodies (ai-mAbs) that can target and potentially activate putative VRC01-class precursors with high affinity.
RESULTS: In B cell sorting experiments, none of the ai-mAbs were able to reliably engage VRC01-class precursor B cells. By integrating ai-mAb binding, structural and B cell sorting analyses, we engineered a bispecific molecule (iv4/iv9) derived from two ai-mAbs (iv4 and iv9), from which the iv9 arm is specific for VRC01-class heavy chains and the iv4 arm for VRC01-class light chains. Compared to the parental ai-mAbs, iv4/iv9 could bind ex vivo B cell receptors comprised of a germline VRC01-class heavy or light chain, but it preferentially crosslinked and activated B cells expressing both VRC01-class heavy and light chains. Using a murine adoptive transfer model, we observed that when used as an immunogen, the bispecific ai-mAb was more efficient at engaging and expanding putative VRC01-class precursor B cells in vivo than either iv4 or iv9 Fabs.
CONCLUSIONS: Our results are relevant not only to the development of an HIV-1 vaccine aimed at eliciting VRC01-class antibodies, but to general effort to activate specific B cell lineages that produce protective antibodies, and further suggest that ai-mAbs-derived immunogens may have general utility as germline targeting immunogens against diverse B cell targets.