SUMMARY Broadly HIV-1 neutralizing VRC01 class antibodies target the CD4-binding site of Env. They are derived from VH1–2*02 antibody heavy chains paired with rare light chains expressing 5-amino acid-long CDRL3s. They have been isolated from infected subjects but have not yet been elicited by immunization. Env-derived immunogens capable of binding the germline forms of VRC01 B cell receptors on naive B cells have been designed and evaluated in knockin mice. However, the elicited antibodies cannot bypass glycans present on the conserved position N276 of Env, which restricts access to the CD4-binding site. Efforts to guide the appropriate maturation of these antibodies by sequential immunization have not yet been successful. Here, we report on a two-step immunization scheme that leads to the maturation of VRC01-like antibodies capable of accommodating the N276 glycan and displaying autologous tier 2 neutralizing activities. Our results are relevant to clinical trials aiming to elicit VRC01 antibodies.

Type III secretion systems are integral to the pathogenesis of many Gram-negative bacterial pathogens. A hallmark of these secretion systems is that they deliver effector proteins vectorially into the targeted host cell via a translocation pore.

Broadly HIV-1–neutralizing VRC01-class antibodies bind the CD4-binding site of Env and contain V H 1-2*02–derived heavy chains paired with light chains expressing five–amino acid–long CDRL3s. Their unmutated germline forms do not recognize HIV-1 Env, and their lack of elicitation in human clinical trials could be due to the absence of activation of the corresponding naïve B cells by the vaccine immunogens. To address this point, we examined Env-specific B cell receptor sequences from participants in the HVTN 100 clinical trial. Of all the sequences analyzed, only one displayed homology to VRC01-class antibodies, but the corresponding antibody (FH1) recognized the C1C2 gp120 domain. For FH1 to switch epitope recognition to the CD4-binding site, alterations in the CDRH3 and CDRL3 were necessary. Only germ line–targeting Env immunogens efficiently activated VRC01 B cells, even in the presence of FH1 B cells. Our findings support the use of these immunogens to activate VRC01 B cells in humans.