Rescue of HIV-1 Broad Neutralizing Antibody-Expressing B Cells in 2F5 VH × VL Knockin Mice Reveals Multiple Tolerance Controls

The HIV-1 broadly neutralizing Ab (bnAb) 2F5 has been shown to be poly-/self-reactive in vitro, and we previously demonstrated that targeted expression of its VDJ rearrangement alone was sufficient to trigger a profound B cell developmental blockade in 2F5 VH knockin (KI) mice, consistent with central deletion of 2F5 H chain-expressing B cells. In this study, we generate a strain expressing the entire 2F5 bnAb specificity, 2F5 VH × VL KI mice, and find an even higher degree of tolerance control than observed in the 2F5 VH KI strain. Although B cell development was severely impaired in 2F5 VH × VL KI animals, we demonstrate rescue of their B cells when cultured in IL-7/BAFF. Intriguingly, even under these conditions, most rescued B cell hybridomas produced mAbs that lacked HIV-1 Envelope (Env) reactivity due to editing of the 2F5 L chain, and the majority of rescued B cells retained an anergic phenotype. Thus, when clonal deletion is circumvented, κ editing and anergy are additional safeguards preventing 2F5 VH/VL expression by immature/transitional B cells. Importantly, 7% of rescued B cells retained 2F5 VH/VL expression and secreted Env-specific mAbs with HIV-1–neutralizing activity. This partial rescue was further corroborated in vivo, as reflected by the anergic phenotype of most rescued B cells in 2F5 VH × VL KI × Eμ-Bcl-2 transgenic mice and significant (yet modest) enrichment of Env-specific B cells and serum Igs. The rescued 2F5 mAb-producing B cell clones in this study are the first examples, to our knowledge, of in vivo-derived bone marrow precursors specifying HIV-1 bnAbs and provide a starting point for design of strategies aimed at rescuing such B cells.