Developing an effective HIV-1 vaccine remains a major global health priority. Broadly neutralizing antibodies (bnAbs) that protect against HIV-1 infection cannot be elicited by vaccination. A reason for this is that bnAbs have unusual features that while critical for breadth development, are problematic for vaccine induction. Confounding this, information regarding which features are problematic for vaccination is lacking. Accordingly, we created a series of knock-in (KI) mice expressing precursors of prototype bnAbs, allowing the study of their in vivo maturation. Such studies have helped in identifying candidate bnAbs more tractable for vaccines to elicit. One such promising lead is the V2 apex-directed set of bnAbs, CH01-04. We recently identified an HIV Envelope (Env) stabilized trimer vaccine regimen that reproducibly elicits heterologous tier 2 nAb responses in CH01 precursor (UCA) ?HC only? KI mice, the most potent and consistent serum breadth elicited to date in a semi-polyclonal model. However, in fully polyclonal models, even single epitope immunogens fail to elicit and expand relevant clones if infrequent in the nave repertoire. Indeed, bnAb responses like those elicited in our KI mice fail to develop in animal models with fully polyclonal repertoires, because naive precursor frequencies are far too low for even simple antigen, let alone Env, which induces many competing clones to off-target epitopes. However, suboptimal yet detectable responses do develop when CH01 precursors are introduced at comparable limiting frequencies in chimeric KI mice, suggesting that devising methods to clonally expand above such thresholds may be an effective approach. Thus, we hypothesize that the rate-limiting step to CH01-type response generation is their precursor frequencies being prohibitively low for current Env-based regimens. The corollary of this posit is that expanding them above activation thresholds would be transformative, but will require adding a pre-priming step, prior to existing Env immunization. Our main objective is to screen various rationally selected/designed non-HIV (or atypical HIV) ?pre-primogens?, for the ability to expand a larger ?proxy? pool of CH01-type precursors via a novel approach we term ?priming by proxy?, a concept based on tricking the immune system into eliciting functionally-independent yet structurally-convergent precursors bearing Ab- combining sites amenable for both bnAb maturation and function.
In Aim 1, we will genetically determine the minimal number of convergent precursors (?CH01 proxies?) that permit Env regimens to induce V2 apex- directed breadth.
In Aim 2, we will test novel ?pre-primogens? for their ability to pre-expand limiting numbers of CH01 proxies, while at the same time, test the breadth-inducing potential of various novel Envs (or other priming immunogens) engineered to more specifically target them. Finally, in Aim 3, we will determine ?CH01 proxy? frequencies in polyclonal, human Ig TrianniTM mice before and after expansion with candidate pre- primogen/Env combinations. These studies will inform on how to potentiate subdominant vaccine responses, particularly those to other occluded Env sites (or pathogens), needing atypical Ab-combining regions.
An effective HIV vaccine is needed to stem the AIDS pandemic. Because of the remarkable viral diversity of HIV-1's glycoprotein Envelope, a major ongoing challenge in developing such a vaccine is inducing antibodies that can neutralize a broad array of HIV-1 strains (bnAbs). A major hurdle is that most sites in the HIV-1 Envelope (to which bnAbs are directed) are hidden, and thus require unusual antibody structures that originate from rare B-cells. The goal of this application is to evaluate a novel strategy we have termed ?priming by proxy?, that exploits redundancy in the immune system, and thus aims at coaxing it into expanding such rare clones at levels that allow current vaccine regimens designed to ?fine-tune? them towards developing broad neutralizing function. These studies, even if partially successful, will be relevant for learning how to deal with other viral pathogens whose genetic diversity and/or epitope inaccessibility requires inducing antibodies bearing similar, unusual features.
