Germline targeting is an HIV vaccine design strategy in which an engineered priming immunogen activates rare naive B cells that are precursors of a desired broadly neutralizing antibody (bnAb) class, followed by sequential heterologous boosts that shepherd the induced lineages toward bnAb development. The 2019 Science study by Steichen and colleagues established that N332-GT trimers can prime precursors of the V3-glycan-targeted, HCDR3-dominant bnAb BG18, but left open whether any defined boosting series could drive those lineages to neutralize circulating HIV, and whether the strategy works in humans. Subsequent work has now resolved the preclinical feasibility core: in 2026, a full N332-GT5 prime plus seven shepherding boosts elicited BG18-class antibodies in outbred rhesus macaques that broadly neutralize clinical HIV isolates, the first germline-targeting bnAb induction in non-transgenic animals. The surviving open problem is human translation: whether such a regimen can safely and reproducibly recruit BG18-class precursors in people — whose precursor frequencies and immunoglobulin genotypes vary widely — and mature them into serum responses that broadly neutralize circulating HIV-1. A Phase 1 trial containing N332-GT5 mRNA (NCT06694753, South Africa, begun December 2025) is testing the priming step in humans, with no results published yet.

Background

HIV-1's envelope glycoprotein spike (Env, a trimer of gp120/gp41 heterodimers) is so heavily glycosylated and sequence-variable that ordinary vaccination rarely elicits antibodies capable of neutralizing the world's circulating virus diversity. However, a minority of infected individuals eventually develop broadly neutralizing antibodies (bnAbs) — antibodies that neutralize a large fraction of global HIV isolates. One major bnAb target is the V3-glycan supersite: a patch of conserved gp120 protein surface (including the V3 loop) that is flanked by high-mannose N-linked glycans, most prominently the glycan at position N332. The human bnAb BG18 targets this site using an unusually long heavy-chain complementarity determining region 3 (HCDR3, the hypervariable loop encoded by V(D)J recombination that dominates antigen contact in many potent antibodies) that penetrates the glycan shield to reach the protein surface.

Naive B cells carrying the specific germline features needed to become BG18-like bnAbs are extremely rare (on the order of one in hundreds of thousands to millions of naive B cells, depending on the subclass), so native Env trimers do not detectably activate them. Germline targeting overcomes this: a priming immunogen is engineered (by removing obstructing glycans and mutating the epitope region) to bind those precursors with high affinity. Steichen and co-workers (Science, 2019) designed the N332-GT trimers, which primed BG18-class precursors — classified into type I, II, and III subclasses by genotype — in humanized knock-in mice (mice engineered to carry the human precursor antibody genes) and from human blood cells ex vivo, and proposed a strategy of sequential immunizations: engineered 'shepherding' boosts with progressively fewer design mutations and increasing native-like character, ending with wild-type (WT) Env trimer 'polishing' boosts, intended to guide affinity maturation (the progressive accumulation of somatic hypermutation in germinal centers — the lymphoid-tissue structures where B cells proliferate and diversify their receptors — that raises antibody affinity and can create bnAb activity) along the pathway to breadth rather than into off-pathway responses such as V1-loop-directed antibodies.

That study explicitly left the pipeline unproven: no boosting series had driven BG18-class lineages to bnAbs, and nothing had been tested in humans. Since then, the field has advanced stepwise. Clinical trials of a different germline-targeting prime, the eOD-GT8 60mer (an engineered Env outer-domain protein displayed on a 60-subunit self-assembling nanoparticle, targeting VRC01-class precursors of the CD4-binding-site bnAbs, which bind the gp120 site where HIV attaches to its CD4 receptor), showed that 97% of vaccinated humans develop the targeted precursors (IAVI G001, 2022), and an mRNA–lipid nanoparticle (mRNA-LNP) version — messenger RNA encoding the immunogen, delivered in lipid nanoparticles — with one heterologous boost increased mutation, affinity, and early neutralization in humans (IAVI G002/G003, 2025) — but neither induced mature bnAbs, and neither concerns the BG18/V3-glycan class. For the BG18 class specifically, N332-GT5 priming was demonstrated in outbred macaques (2024), two novel boost immunogens extended maturation in knock-in mice (2024), and adjuvant/delivery studies (escalating dose plus the saponin nanoparticle adjuvant SMNP, a particulate adjuvant built from plant-derived saponin immune stimulants, 2025) showed how rare-precursor priming can be made reproducible. In 2026, a complete N332-GT5 prime plus seven shepherding boosts (including multiclade WT-like Env trimers) elicited BG18-class bnAbs in outbred macaques, with isolated antibodies achieving up to 67% of the reference bnAb BG18's neutralization breadth on a 63-isolate panel of BG18-sensitive clinical HIV strains, and serum bnAb activity in 44% of animals — proof of principle in a non-transgenic animal, but still not in humans. A Phase 1 trial including N332-GT5 delivered as mRNA (NCT06694753, South Africa, started December 2025) is the first human test of the priming step; no results have been published.

Problem Statement

Can a germline-targeting sequential vaccination regimen — an N332-GT-based Env trimer priming immunogen, delivered as recombinant protein or as mRNA encoding it, followed by a defined series of heterologous shepherding boosts progressing toward wild-type-like Env trimers — induce BG18-class (V3-glycan-supersite-targeted, HCDR3-dominant) broadly neutralizing antibody responses in humans? Concretely, this comprises: (1) safe priming that reproducibly recruits BG18-class precursor B cells in human vaccine recipients, across the natural variability of naive-precursor frequency and immunoglobulin germline-gene genotype among individuals and populations; (2) boosting that steers those lineages through somatic hypermutation and affinity maturation toward, rather than away from, broad neutralization; and (3) production of serum antibodies or vaccine-induced monoclonal lineages that neutralize a substantial fraction of circulating wild-type HIV-1 clinical isolates (for scale, the 2026 preclinical study measured breadth as the fraction of a 63-isolate panel of BG18-sensitive clinical strains neutralized, relative to the reference bnAb BG18). An affirmative answer requires clinical demonstration, with B-cell-receptor sequencing evidence of induced BG18-class lineages and standardized multi-clade neutralization assays of sera and isolated antibodies, that recruitment and maturation to breadth are reproducible across the natural variability of naive-precursor frequency and immunoglobulin germline-gene genotype among individuals and populations scoped in (1). Because no human data bearing on (1)-(3) are yet published, the answer is a reasoned judgement on each of (1)-(3) from the available preclinical and human evidence: for each, whether it is supported, refuted, or still indeterminate, and what that settles. The clinical demonstration specified above would confirm an affirmative answer but is not something the answer must itself supply; and because the affirmative case requires all three parts, a refutation of any one of (1)-(3) answers the overall question in the negative. If none of (1)-(3) is refuted and no clinical demonstration is available, the overall answer is indeterminate: state which of (1)-(3) is decisive for the overall question and what evidence would settle it.

References

  1. Steichen JM, Lin Y-C, Havenar-Daughton C, et al. (2019) A generalized HIV vaccine design strategy for priming of broadly neutralizing antibody responses. Science 366:eaax4380. doi:10.1126/science.aax4380, https://doi.org/10.1126/science.aax4380
  2. Leggat DJ, Cohen KW, Willis JR, et al. (2022) Vaccination induces HIV broadly neutralizing antibody precursors in humans. Science 378:eadd6502. doi:10.1126/science.add6502, https://doi.org/10.1126/science.add6502
  3. Steichen JM, Phung I, Salcedo E, et al. (2024) Vaccine priming of rare HIV broadly neutralizing antibody precursors in nonhuman primates. Science 384:eadj8321. doi:10.1126/science.adj8321, https://doi.org/10.1126/science.adj8321
  4. Xie Z, Lin YC, Steichen JM, et al. (2024) mRNA-LNP HIV-1 trimer boosters elicit precursors to broad neutralizing antibodies. Science 384:eadk0582. doi:10.1126/science.adk0582, https://doi.org/10.1126/science.adk0582
  5. Madden PJ, Marina-Zárate E, Rodrigues KA, et al. (2025) Diverse priming outcomes under conditions of very rare precursor B cells. Immunity 58:997-1014.e11. doi:10.1016/j.immuni.2025.03.003, https://doi.org/10.1016/j.immuni.2025.03.003
  6. Willis JR, Prabhakaran M, Muthui M, et al. (2025) Vaccination with mRNA-encoded nanoparticles drives early maturation of HIV bnAb precursors in humans. Science 389:eadr8382. doi:10.1126/science.adr8382, https://doi.org/10.1126/science.adr8382
  7. Steichen JM, Madden PJ, Flynn CT, et al. (2026) Vaccination elicits HIV broadly neutralizing antibodies in primates. Nature 656:723-733. doi:10.1038/s41586-026-10837-5, https://doi.org/10.1038/s41586-026-10837-5
  8. Skelly AN, Gristick H, Li H, et al. (2026) Induction of broadly neutralizing HIV antibodies by a two-step mechanism informs vaccine design. Science 392:eaec6396. doi:10.1126/science.aec6396, https://doi.org/10.1126/science.aec6396
  9. Marchitto L, Wagh K, Roark RS, et al. (2026) Enhanced B cell priming induces broadly neutralizing HIV-1 apex antibodies. Nature 656:712-722. doi:10.1038/s41586-026-10838-4, https://doi.org/10.1038/s41586-026-10838-4
  10. ClinicalTrials.gov (2025) NCT06694753: A Phase 1, Placebo-controlled, Blinded, Dose-escalation Study to Evaluate the Safety and Immunogenicity of mRNAs Encoding HIV Immunogens (eOD-GT8 60mer, Core-g28v2 60mer, N332-GT5 gp151) in Adult Participants Without HIV and in Overall Good Health in South Africa. https://clinicaltrials.gov/study/NCT06694753