Well-designed immunogens are crucial to our rational approach to HIV vaccine design, but equally important is the optimization of immunization strategies.
CHAVD Mindset
Broadly Neutralizing Antibodies (bnAbs)
What are bnAbs?
They are antibodies able to neutralize (inactivate) a large proportion of global circulating HIV strains. They arise in some HIV-infected individuals, usually after several years of infection. Through exposure to many different viruses (immunogens), antibodies evolve to recognize a variety of shapes exhibited by distinct viral strains.
Long period of continuous antigen exposure, small percentage of individuals
Read MoreWe’re currently researching multiple bnAb sites rather than a single site for two major reasons. First, comprehensive neutralization coverage is more likely to be achieved by vaccine targeting of more than one site, so the chances of vaccine failure are reduced. Second, the basis for these vaccine regimens is tested in animal models before going into the clinic. Given the uncertainties involved in translation into humans, we consider it wise to target several sites with the ultimate goal of combining the corresponding immunogens to achieve the requisite breadth and potency.
Vaccine Design
The bnAb Challenge
The challenge, and essence of the bnAb approach to HIV vaccine design, is then to generate bnAbs in HIV- uninfected humans. We do not have the luxury of presenting many different immunogens over several years. In vaccination, we must present a manageable number of immunogens over a limited time period.
Relatively short period of antigen exposure, high percentage of individuals
The most likely format for a successful bnAb-based HIV vaccine is one that induces bnAbs to at least 2 sites by sequential administration of 3-4 immunogens per specificity. Ideally, the immunogens for different specificities would be combined in a single product (e.g., one protein), or if this isn’t possible, then co-administered.
Germline-Targeting (GT) Vaccine Strategy: Our overarching CHAVD goal is to develop a sequential vaccine regimen that induces human Abs to at least two of the bnAb target sites at protective levels. Each immunogen will serve either as a prime (germline-targeting or germline-agnostic) to initiate the response from naive B cell bnAb precursors, or will be part of a series of structure-guided Env immunogen boosts that can shepherd and polish the Ab responses along a pathway to bnAbs.
Read MoreAn effective HIV vaccine will need to consistently induce broadly neutralizing antibodies (bnAbs) against 2-3 different Env sites in most (>90%) of vaccine recipients because targeting of multiple sites is necessary to provide the best coverage against the diversity of virus strains. Use of Germline-Targeting (GT) priming immunogens to activate appropriate precursors, followed by structure-guided boost immunogens to shepherd the response, will enable B cells to mutate and produce bnAbs that are better at binding to HIV targets. Iterative animal studies and human clinical trials are required to determine the simplest vaccine regimen that consistently induces sufficient neutralization breadth and protection over time.
*Figure provided by the Schief Lab with some modifications made by the Burton Lab, The Scripps Research Institute.
PRIMARY GOAL
Develop a sequential vaccine regimen that induces sustained protective levels of broadly neutralizing antibodies in humans.
01.
Broadly Neutralizing Antibodies
Broadly neutralizing antibodies or bnAbs are antibodies that have the ability to neutralize multiple viral strains of HIV. bnAbs are very efficient due to the fact that they can successfully target vulnerability sites on the Env, and because humans themselves have the potential to generate potent HIV bnAbs.
02.
Sequential Vaccine Regimen
When we talk about focusing our research around a sequential vaccine regimen, we mean that the key aspect of a successful HIV vaccine lies in training the immune system with a set of different immunogens, as opposed to a classical vaccine protocol that involves multiple administrations of the same immunogen.
03.
Vaccine Delivery Approaches
Well-designed immunogens are crucial to our rational approach to HIV vaccine design, but equally important is the optimization of immunization strategies. We have demonstrated impressive improvements in nAb responses in NHPs by utilizing novel vaccine delivery approaches designed to enhance germinal center responses. We will further optimize these approaches using preclinical models and then proceed to validate their utility in human clinical trials.
TARGETS
Env vulnerability sites targeted by bnAbs
- V2-apex
- V3-glycan
- CD4 binding site
- Gp120-gp41 interface (fusion peptide)
- Membrane proximal external region (MPER)
HIV Env Trimer: Targets for Immunogens Entering GMP Manufacturing or Clinical Trials
Read MoreHIV Env trimers: For any immunization strategy to induce bnAbs, native-like trimer immunogens are likely necessary for the final boost(s) and as a platform for all immunogens in a sequence.
• V2-apex: Immunofocusing Env immunogen design is based on glycan deletions, either naturally occurring or engineered “glycan holes,” providing enhanced access to the desired epitope. A potential advantage of the immunofocusing prime approach is the breadth of responses that may be primed, which could be more diverse and robust than the more specific Germline Targeting strategy if successive boosting can mature those responses to bnAbs.
• V3-glycan: An initial Germline Targeting immunogen for this region was designed and established via proof-of-principle experiments. Then, through iterative display immunogen selection, we’ve obtained a V3-glycan GT prime Env trimer, N332-GT5, that retains high affinity and binds appropriate antibodies selectively.
• CD4 binding site: eOD-GT8 60mer, as our lead candidate, has completed GMP manufacturing and is in human Phase 1 clinical trial evaluation. Other immunogen designs for boosting and polishing the response are currently under development.
• Gp120-gp41 interface (fusion peptide): Vaccine targets have been identified based on analysis of human bnAbs that bind to this region. Several lead candidates have emerged in pre-clinical testing and further optimization is currently underway.
• Membrane proximal external region (MPER): Germline Targeting immunogens for this region have been designed and appear promising, but further preclinical evaluation is needed.
HIV ENV TRIMER
Protein structure that houses the machinery which enables HIV to infect cells - it is decorated with sugars and difficult for antibodies to recognize.
Three features make HIV Env trimer an evasive machine that can escape nAbs - a dense glycan shield, rapid sequence variation of the protein surface and significant structural flexibility.
90
glycans per molecule on surface
5
5 key vulnerability sites
Rotating HIV Envelope (Env) Trimer: Env houses the machinery that allows HIV to infect cells; it is the sole target of bnAbs.
Read MoreAt the heart of the difficulty in eliciting bnAbs is the nature of Env, which is a compact protein structure highly decorated with sugars (purple) and difficult for Abs to recognize. We have determined the molecular structure of Env and have defined the sites on Env recognized by bnAbs in molecular detail. These studies have facilitated the precise design of our vaccine candidates.
*Video provided by the Schief Lab, The Scripps Research Institute.
ROLE OF BNABS
The critical importance of bnAbs
Support for bnAbs in HIV vaccine protection comes from experiments conducted by us and other research groups. They show that bnAbs, administered passively or through vaccination, provide sterilizing immunity against a robust, clinically relevant virus challenge in appropriate animal models.
Generalized Representation of Passive Protection Experiments in Nonhuman Primates.
Read MoreAnimals are first treated with protective bnAb antibodies, generally over a series of levels to determine protective dose, or a control antibody. These animals then are exposed to infectious virus by a route that represents human exposures. The protective benefit of the passive bnAbs antibodies is evident by the number of animals that remain uninfected or the number of virus exposures needed to achieve infection, as compared to control animals.
*Figure provided by the Burton Lab and Sal Butera, The Scripps Research Institute.
When compared to immunization with corresponding soluble proteins, immunization with proteins expressed on particles is shown to result in enhanced antibody responses.
CHAVD Mindset
NATIVE-LIKE TRIMER IMMUNOGENS
Significant features and characteristics
- Native-like structure similar to the prefusion spike
- Good expression yield of purified, natively glycosylated trimer
- Native-like antigenicity similar to the prefusion spike
- Successful protein expression and retained structural integrity with different HIV strain sequences
- Trimer surfaces and epitopes can be modified and customized
Evolution of SOSIP Trimers: BG505.664.
Left. First images of BG505 SOSIP.664 taken in 2012 showing for the first time a native, prefusion Env trimer. Right. High resolution cryoEM reconstruction (2019) of the BG505 SOSIP.664 demo run material for the ongoing human clinical trial.
*Figures provided by the Ward Lab, The Scripps Research Institute.
NATIVE-LIKE TRIMER IMMUNOGENS
NFL stabilization and bnAbs
Colored spheres represent mutated residues that stabilize the native-like NFL trimers — BG505 Trimer Derived residues transferred to other Env sequences, helix-breaking glycine and proline substitutions in gp41 and substitutions in the highly dynamic V3 and fusion peptide regions of the Env. Together, these alterations provide a general means to generate well-ordered trimers from multiple Envs.
Design and immunogenicity of the HIV-1 envelope glycoprotein (Env) near-native NFL (Native Flexibly Linked) trimer.
Read MoreLeft. Ribbon representation of the 16055 NFL TD CC+ trimer crystal structure (PDB ID 5UM8) with the gp120 (light blue) and gp41 (light brown) subunits. Right. Composite image of the 16055 NFL TD CC+ trimer with the two vaccine-elicited rabbit bNAbs, E70 and 1C2, targeting the CD4 binding site and the gp120-gp41 interface, respectively, superimposed on the electron microscopy density of an HIV envelope glycoprotein spike.
*Figures provided by the Wyatt Lab, The Scripps Research Institute.