Background
HVTN 303, a trial of HIV-1 fusion peptide (FP)-directed vaccine, was designed to prime FP-binding antibodies with neutralizing potential. An 8-amino acid peptide (FP8) was conjugated to the heavy chain of tetanus toxoid to create a protein conjugate (FP8-rTTHC) that was given alone or in combination with Trimer 4571 (a stabilized version of Clade A BG505 SOSIP), adjuvanted with Adjuplex (20%). Due to multiple grade 3 adverse events, vaccinations were permanently discontinued in March 2023. 13 participants received one dose of Trimer 4571 (200 mg); 13 participants received one dose of FP (200 mg) and five received a 2nd dose at month 1; 9 participants received one vaccination with FP plus Trimer 4571 (each 200 mg), and 6 participants received a 2nd dose at month 1.
Methods
8 of 11 participants who received 2 doses (4 FP only/4 FP + Trimer 4571) were selected for B cell analysis. Antigen-specific B cells 2 weeks after vaccination were sorted by flow cytometry, and their B cell receptors (BCR) were sequenced. Paired variable heavy (VH) and variable light (VL) chains were used to generate monoclonal antibodies (mAbs) and were tested for binding affinity, FP fingerprint analysis, and selected mAbs underwent cryo-EM. Epitope angle of approach and detailed epitope-paratope structural analyses were performed. The BCR sequences, genetic profile, HCDR3 length, and somatic hypermutation (SHM) were assessed.
Results
7 of 8 participants had B cells that bound both FP8 & Trimer 4571; 6 had BCR sequences from the VH3 family (VH3-23, VH3-30, VH3-30-3) – a gene group used by FP broadly neutralizing mAbs ACS202, PGT151, and PGT152. Approximately 500 mAbs were generated from the BCRs, with 17 identified as high-binders and selected for more detailed epitope characterization. Although the mAbs did not induce neutralization, cryo-EM analysis showed FP-epitope binding through the trimer glycan shield with varying angles of approach. One example is mAb A1, the VH of which penetrates past the epitope canonical glycan N88 and engages multiple gp41 residues, forming 11 hydrogen bonds and 2 salt bridges.
Conclusions
FP-directed lineages, primarily using VH3 genes, were induced in most participants who received two doses of FP8-rTTHC alone or in combination with Trimer 4571. Observed binding to the intact trimer FP epitope with notable SHM and affinity suggests boosting vaccination may be able to mature FP mAb lineages with the potential for broad neutralization capacity.
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