Elicitation of cross-neutralizing Abs (cNAbs) to inhibit entry of diverse clinical HIV isolates following Env vaccination remains a high priority to develop a broadly effective vaccine, but the elicitation of cNAbs against cross-conserved Env determinants has been elusive. However, very recently we accomplished an important initial step in this process. We primed rabbits with immunization of native flexibly linked (NFL) trimer-liposomes containing targeted N-glycan deletions proximal to the highly conserved CD4 binding site (CD4bs) to better activate B cell responses to this region. The engineered gaps in the N-glycan shield were gradually restored by heterologous sequential boosting. We elicited cross-neutralizing activity in the serum and purified IgG of selected animals and, subsequently, cloned two cNAbs. The cNAb most relevant to this proposal, E70, provides proof-of- principle for a fundamentally different approach. The CD4bs-directed E70 is relatively potent against multiple tier 2 isolates and, importantly, recognizes approximately 50% of N-glycan as part of its epitope as well as adjacent conserved polypeptide that is part of the CD4bs. (The 2nd mAb, 1C2, is a broadly neutralizing antibody (bNAb), neutralizing 80% of a 40 isolate panel and is directed to the gp41:gp120 interface.) The high-resolution cryoEM structure of the E70 epitope on the NFL native-like trimer provides critical preliminary data on how to train the immune system to recognize other N-glycans proximal to the conserved protein surface of the CD4bs to mediate cross-neutralization of tier 2 isolates. We call this approach ?N-glycan baiting?. For this approach, we will leave one selected N-glycan proximal to the CD4bs intact as ?the bait? while removing all other proximal N- glycans on the initial priming immunogens. The priming will be performed at 6 individual N-glycan sites that ring the conserved CD4bs protein surface. This allows a given cell receptor to recognize chimeric epitopes comprised of the ?anchor glycan? and surrounding conserved polypeptide. With boosting, all N-glycans are eventually restored in a step-wise manner, limiting angles of approach to a given N-glycan bait to regenerate the intact N- glycan shield while still driving a subset of B cells directed to the original anchor glycan and protein as a chimeric, non-self-epitope. This strategy fundamentally differs from our original approach in targeting the receptor binding site by full deletion of all proximal N-glycans. Accordingly, the major objective of this grant is to generate novel HIV trimeric Env immunogens by structure-based design, containing N-glycan deletions at the CD4bs but to retain individual ?anchor glycans?. We will first characterize the N-glycan-anchored priming immunogens by state-of-the-art biophysical methods (Aim 1). Next, we will determine immunogenicity in small animals in Aims 2 and 3, including neonates. Following targeting of the N-glycan shield in small animals, we will perform non- human primate (NHP) immunogenicity and challenge (Aim 4). If we are successful at N-glycan baiting at the CD4bs, we can apply this approach to other sites on the HIV Env, essentially turning the protective N-glycan shield against the virus as a target of neutralization as is often done by the remarkable HIV-elicited bNAbs.
In the new era of well-ordered HIV envelope glycoprotein trimers and elicitation of tier 2 neutralizing antibodies by these timers, innovative applications as immunogens may further efforts toward a broadly effective HIV vaccine. In that context, we propose to investigate a process called ?N-glycan baiting? to attack the HIV envelope glycoprotein carbohydrate shield using the well-ordered, cleavage-independent NFL trimers developed by our group. We will assess trimers devoid of several N-glycans adjacent to the conserved receptor binding site and arrayed at high- density on synthetic nanoparticles to determine the impact on the elicitation of neutralizing antibodies capacity in both neonatal and adult animals toward a protective HIV vaccine to advance human health.
