**8. Promising vaccine strategies: general broadly neutralizing antibody vaccine**

With the intricate and complex immune response to HIV-1 infection, it is no surprise that multiple vaccine strategies exist. Some promising vaccine strategies include SOSIP trimers, eOD-GT8 60mer and gene therapy, HIVcons immunogens, and mosaic immunogens. All these strategies are discussed in detail in Sections 9–12.

A feasible long-term solution or "the holy grail of HIV-1 vaccine development" is a vaccine that induces the production of BnAbs [25]. The current belief is that a prophylactic vaccine must induce BnAbs with a wide neutralization breath and/or HIV-1-specific antibodies to mediate antibody-dependent cellular cytotoxicity or other effector functions [39].

Multiple different concepts of BnAb-based vaccines are being investigated, including germline-targeted and lineage-based designs as well as SOSIP trimers detailed in Section 13 [27, 39]. In an effort to produce a germline-based vaccine, there are ongoing studies following HIV-1 isolates in people with active HIV-1 infections and BnAb [39]. A germline-targeted vaccine assesses an unmutated common ancestor of a BnAb, then approximates the critical BnAb precursor features, and designs the immunogens based on the structural and immunological information [27]. The goal of this germline approach is to preferentially activate B cells that are BnAb precursors and eventually produce memory B cells capable of BnAb production [46].

A lineage-based vaccine is focused on the immunological pathways forming antibody lineages that generate BnAb [27]. Regardless of which design is used, both germline-based and lineage-based designs use the "reverse vaccinology" to drive the production of the BnAb [44, 47].

These concepts are further complicated by the strategy of prime-boost vaccination models, where a priming vector is administered followed by a series of protein boosts. For this strategy to lead to BnAb production, the prime would be a precursor naive B cell, and the boosts would then select for neutralizing members of antibody lineage [27, 47].

### **9. Promising vaccine strategies: env glycoprotein trimers/SOSIP trimers**

Another promising vaccine strategy that may result in BnAb production is immunogens based off the recombinant stabilized HIV envelope (Env) glycoprotein trimers, specifically the SOSIP trimers [47, 48]. SOSIP trimers are proteins that adopt a native-like confirmation and strongly approximate the features of HIV-1 virion [46]. Currently, the SOSIP trimers induce neutralizing antibodies against "relatively

neutralization resistant (tier 2) autologous viruses" but there are multiple studies suggesting their neutralization breadth [39, 47, 49], as well as improvements in their production [39, 50], stabilization [46], and immunogenicity as future vaccine candidates [51].

To study the neutralization breadth, one study was completed through computational modeling that assessed different combination and sequential vaccine series and then administered to groups of rabbits. While this experiment did not induce BnAb formation, it proved that the neutralizing antibody response can be cross-boosted when Env trimers from different clades were administered sequentially [49]. Further research is needed to identify a SOSIP trimer that induces BnAb production.

One study determined the adjuvants compatible with SOSIP trimers, since the adjuvant is a key component to boost the immune response against the vaccine antigens. This study found that different adjuvant classes did not interfere with the integrity of the SOSIP trimer, with the exception of aluminum sulfate formulations [51].

Additionally, to assess how feasible and scalable SOSIP production was, Dey et al. completed a trial that resulted 3.52 grams of fully purified trimers. The overall production was performed while abiding by current Good Manufacturing Practice guidelines. The quality of SOSIP composition was compared to samples generated from a research laboratory and reported to be the same in terms of antigenicity, disulfide bond patterns, and glycan composition. This trial also helped assess how stable the trimers were at different temperatures and storage conditions [50].
