In recent years, researchers have made substantial progress in designing vaccine immunogens that prime naïve B cell precursors of HIV-specific bNAbs and initiate the stepwise process of maturing these responses.

Leo Stamatatos from the Fred Hutchinson Cancer Research Center shared data on the clinical testing of the germline targeting immunogen 426 c.Mod.Core nanoparticle, which was administered intramuscularly along with the adjuvants 3M-052-AF combined with aluminum hydroxide. This immunogen activates a range of B cell precursors of VRC01-class bNAbs, which target the critical CD4-binding site on HIV Env. In the HVTN 301 trial (NCT05471076), 48 volunteers from six sites across the U.S. received either full bolus or fractional doses17 of the 426 c.Mod.Core vaccine immunogen as a prime and full bolus as a boost. To date, 38 monoclonal antibodies induced by this vaccine have been isolated and characterized by biolayer interferometry (BLI), in vitro neutralization assays, and by Cryo-electron microscopy, which are revealing similarities in VRC-01 reactivity.

Bill Schief of The Scripps Research Institute, IAVI, and Moderna reported on a trio of trials (IAVI G001-NCT03547245, G002-NCT05001373, and G003-NCT05414786) involving another engineered germline targeting immunogen, the eOD-GT8 60-mer protein. This antigen is also designed to induce VRC01-class B cell precursors. In the G001 trial, the vaccine achieved a high response rate in 35 out of 36 participants (97% response rate)18. Only one individual in the trial did not generate detectable IgG B cells expressing VRC01 class B cell receptor precursors following two eOD-GT8 immunizations, since this individual lacked VRC01 permissive IGHV1-2 alleles (see section Understanding the role of genetic variation).

In the IAVI G002 and G003 trials, the eOD-GT8 60-mer immunogen was administered using Moderna’s mRNA platform. Initial observations indicate that the priming of VRC01-class B cell precursors was at least as effective with mRNA in the G002 vaccine recipients when compared to in the protein immunized G001 vaccine recipients. Standardized B-cell and serum antibody binding assays enabled direct comparison of the immunogenicity results of the two trials. Notably, the number of mutations in IGHV1-2-using VRC01-class monoclonal antibodies induced in G002 vaccine recipients was greater than in G001. These data support a role for mRNA vaccination to prime VRC01-class B cell precursors.

Rogier Sanders from Amsterdam University Medical Center presented data on another germline targeting B cell vaccine approach. This approach utilizes a native-like trimer immunogen, designated as BG505 SOSIP GT1.1, which is modified to bind to both VRC01-class and apex-specific B cell precursors. In infant macaques, after three immunizations, expanded VRC01-class B cells accumulated several mutations associated with VRC01-class bNAbs, suggesting that the antibodies elicited by BG505 SOSIP GT1.1 are substantially advanced on the path toward becoming bNAbs, according to Sanders19.

In addition to the efforts aimed at inducing VRC01-class (CD4-binding site) B cell precursors, other epitopes on the HIV envelope protein are being explored. Barton Haynes from Duke University, who leads the Consortium for HIV/AIDS Vaccine Development (CHAVD) funded by the NIAID, presented data from the HVTN 133 trial (NCT03934541), which tested an MPER peptide-liposome immunogen.

Although the HVTN 133 trial was stopped before the scheduled fourth and last immunization due to a case of anaphylaxis, Haynes shared insights from the analysis of B cell responses. The immunogen induced neutralizing-antibody responses capable of neutralizing some heterologous HIV strains. Although there were no substantial levels of circulating bnAbs, a monoclonal antibody isolated from a vaccinee neutralized 15% of tier-2 global HIV strains, and 35% of Clade B HIV strains, and represents an important benchmark for DMCT20,21. While a long interval is usually required for an HIV-1-induced bNAb B cell lineage to acquire the mutations required for breadth, in the HVTN 133, Haynes and colleagues demonstrated that MPER bNAb lineage was initiated after two immunizations in three out of five vaccinees, indicating affinity maturation to the target epitope.

These examples illustrate the importance of analyzing vaccine-induced antibody repertoires as deeply as possible. Participants of the workshop discussed the importance of sequencing as many immunogen-specific B cells as possible from each trial participant to assess the response following vaccination. Sequencing large numbers of antigen-specific paired heavy and light chains provides valuable information of antibody gene usage, clonality, and SHM. Schief noted that for germline-targeting immunogens, the effectiveness of the response can be directly evaluated from sequence information as the immunogens are intended to induce B cells with specific genetic features. Furthermore, sequencing longitudinal samples following priming and boosting provides information about the maturation of promising B cell lineages.

For the non-germline targeting approaches, sequencing alone does not provide sufficient information about epitope specificity and function. Efforts to measure serum neutralizing activity, antibody epitope mapping, including electron microscopy, polyclonal epitope mapping (EMPEM) studies22, and isolation and characterization of monoclonal antibodies (mAbs) are therefore needed. It was noted that these are critical complementary analyses also for germline-targeting approaches. A bottleneck in the evaluation of vaccine-induced responses is the ability to clone and express enough mAbs for functional and structural studies. All speakers emphasized the importance of thoroughly analyzing vaccine-induced antibody responses at depth, using both high-throughput sequencing and mAb isolation.