The overall strategy of Research Focus #1 is the development of immunogens and immunization strategies that induce broadly protective antibody responses to HIV. This is based on characterizing and understanding broadly protective monoclonal antibodies and antibody responses and harnessing the information gained to develop immunogens and immunization strategies to induce broadly protective responses. There is a strong focus on broadly neutralizing antibodies as these have been shown to be protective in robust nonhuman primate (NHP) models, but some non- neutralizing antibodies may have protective qualities that might be exploited in vaccine discovery. Immunogen testing in animal models will lead to optimal immunogens and immunization strategies being evaluated in small-scale human trials.
We believe that the recent identification of a larger group of broadly neutralizing monoclonal antibodies (bnMAbs), progress towards determining the structure of Env proteins and the Env spike, advances in the design of scaffolded immunogens are all guiding us towards success in developing a protective vaccine. Furthermore, improved technologies to evaluate immune responses, such as deep sequencing of antibodies, B-cell receptor sequencing and single cell methods to isolate antibodies, are all highly promising developments.
1. Fully define the antibodies and epitopes associated with broad neutralization of HIV. Neutralizing antibodies are the best correlate of protection for many viral vaccines and, for HIV specific neutralizing antibodies provide robust protection against mucosal challenge in the macaque models. Therefore, a major goal of HIV vaccine research should be the discovery of immunogens and immunization strategies that can elicit broadlyneutralizing antibodies, given high sequence variation in HIV Env. As part of this discovery effort, it is important to fully map the landscape of antibody recognition of the HIV Env spike (the sole target of neutralizing antibodies) using the larger numbers of potent broadly neutralizing monoclonal antibodies recently available. A full complement of broadly neutralizing epitopes targets can then be exploited for generating immunogens with optimal precision.
The broadly neutralizing monoclonal antibodies are, in themselves, valuable tools for guiding vaccine discovery. These antibodies often have unusual characteristics. As we accumulate information on a large panel of antibodies, the requirements for broad neutralization will become clearer. This effort could be assisted by site-directed mutagenesis studies on the antibodies to determine those residues crucial for neutralization.
2. Determine the broadly neutralizing monoclonal antibodies that provide the best protection. Clearly, the property of antibodies that interests most in terms of vaccine discovery is protection from acquiring infection. Neutralization is simply a property that can be readily measured in laboratory experiments. Because collecting this data would require large-scale human passive immunization trials, a representative animal model would be valuable. Titration of the newer broadly neutralizing monoclonal antibodies in an appropriate animal challenge model will better define the relationship between protection and both neutralization and antibody specificity.
3. Determine the contribution of extra-neutralizing mechanisms of anti-viral antibody activity. There are suggestions from a number of studies that extra-neutralizing mechanisms may provide protective effects. Given the difficulties of generating immunogens that elicit broadly neutralizing antibodies, the possibility of protection against HIV by non-neutralizing antibodies needs further investigation.
4. Explain how and why a subset of HIV-infected individuals makes potent broadly neutralizing antibodies responses. Although the field has not yet described immunogens or immunization strategies capable of eliciting broadly neutralizing antibodies responses, natural HIV infection does generate such responses in percentage of individuals. The sera of these donors can be diluted by hundreds of fold and they are still able to neutralize many different isolates of HIV, a property that one would like to induce through vaccination. Investigating how these donors evolve such responses could provide vital clues for vaccine discovery and a number of strategies will be pursued to understand the critical factors that lead to the development of such responses. Furthermore, in-depth study of humoral and cellular immune responses in the same subjects will help define the link between these responses.
5. Determine immunogens and immunization strategies that favor the elicitation of HIV broadly neutralizing antibody responses in model systems. Although classically small animal models, such as mice and rabbits, are used as “gatekeepers” to determine whether to proceed with the immunogen in humans, this approach may be not be appropriate for HIV immunogens because of the complex antibody maturation. We must take advantage of our knowledge to generate high-throughput model systems that evaluate immunogens and immunization strategies for their ability to elicit broadly neutralizing antibody responses prior to testing in humans. An example of such a strategy is to create knock-in mice that carry germline forms of the different human monoclonal antibodies and screen Env antigens induction of murine B cell. It is anticipated that iterative cycles of vaccination followed by analysis of B and CD4+ T cell responses to guide re-design of immunogens and immunization strategies, will be needed to generate optimal vaccines.
6. Development of strategies to focus antibody responses on conserved HIV Env epitopes. A recurring theme of HIV Env immunization in soluble protein or particulate form is the generation of Abs that are either non-neutralizing or that neutralize in an isolate-specific fashion. A number of groups have explored potential strategies to focus Ab responses on conserved regions of Env and to dampen responses to non-desirable. Although these strategies have been unsuccessful so far in eliciting bnAb responses, they have provided us with an armamentarium of approaches that can be drawn upon to augment more promising immunogens and immunization regimes.
7. Design, engineer and produce a pure stable Env preparation that mimics the antigenic profile of the functional Env spike. A number of strategies to generate a pure stable Env preparation that mimics the antigenic profile of the functional Env spike are ongoing. Using co-crystallization and methodologies like cryo-electron microscopy and cryo-elctron tomography, resolution technologies may eventually provide enough molecular detail to allow the rational design of stable trimmers.
8. Increase the durability of Ab responses to Env to the level observed for successful human vaccines. One goal of HIV vaccine research must be to increase the durability of Env antibody responses by testing a variety of novel direct approaches. Studies on the preferential induction of Tfh cells by vaccination will likely contribute to immunization strategies giving more durable Env responses.