PROJECT SUMMARY In humans, neutralizing antibodies elicited by HIV-1 coevolve with viral Envs in distinctive patterns, in some cases acquiring substantial breadth. We found that primary HIV-1 Envs, when expressed by simian-human immunodeficiency viruses (SHIVs) in 22 rhesus macaques (RMs), elicited patterns of Env-Ab coevolution strikingly similar to those in humans (Science 371:eabd2638, 2021). This included conserved immunogenetic, structural and chemical solutions to epitope recognition and precise Env-amino acid substitutions, insertions and deletions leading to virus persistence. The structure of one rhesus antibody, capable of neutralizing 49% of a 208-strain panel, revealed a V2-apex mode of recognition like that of human bNAbs PGT145 and PCT64. We subsequently expanded this study to include 150 RMs infected by SHIVs bearing any of 15 different primary HIV-1 Envs; 24 (16%) of these animals developed bNAbs targeting conserved V2 apex, V3 glycan, CD4bs or fusion peptide epitopes. The V2 apex was the most common bNAb epitope targeted in RMs. We concluded that Env-Ab coevolution in RMs recapitulates developmental features of human bNAbs and may serve to guide and accelerate HIV-1 immunogen design for humans. From these preclinical data, we identified HIV-1 Q23.17 Env as the immunogen that most consistently elicited V2 apex bNAbs. Here, we propose to elucidate the Env-Ab coevolutionary pathways by which HIV-1 Q23.17 Env selectively primes, boosts and affinity-matures V2 apex bNAb responses and to translate these findings into an all-SOSIP Env trimer vaccine regimen consisting of a germline-targeted Q23.17 Env prime followed by boosts with lineage-designed Q23.17 Env “imunotypes” capable of affinity-maturing B cells to achieve breadth. Specific aims are: (i) to decipher molecular pathways of Env-Ab coevolution in SHIV.Q23.17 infected RMs that lead to the development of V2 apex bNAbs, including the identification of inferred germline bNAb precursors and lineage intermediates and corresponding Env immunotypes that bind to them; (ii) to use mammalian display saturation mutagenesis to generate Q23.17 Env variants that exhibit enhanced binding affinity to multiple rhesus germline V2 apex bNAb B cell precursors and to engineer these Envs as nanoparticle-delivered SOSIP trimers; (iii) to test the immunogenicity of germline- targeted and lineage-designed Q23.17 Env SOSIP trimers in V2 apex bNAb UCA knockin mice and outbred RMs and to advance the most promising combinations to a proof-of-concept preclinical vaccine trial in RMs; and (iv) to conduct an appropriately powered preclinical vaccine trial in 28 RMs to test the hypothesis that reverse- engineered, B lineage-designed Q23.17 SOSIP Env trimers can prime, boost and affinity mature V2 apex bNAb responses in RMs to an extent that is superior to conventional SOSIP Env immunogens and that protects RMs from heterologous virus challenge. The significance of these studies could be far-reaching: if we can d...