We have long been aware of the astounding sequence variation in the HIV-1 envelope glycoprotein (Env) gene, but the structural and functional implications of this diversity are only beginning to be grasped. Structural variation in Env impacts its interactions with all key drivers of viral fitness and replication, and this is not captured by variation in sequence. These differences underlie viral phenotypic traits such as neutralization sensitivity, tropism, infectivity, and transmissibility. While recent studies have provided detailed structural information for trimeric Env ectodomain from a range of viral isolates, the structures represent a static, Platonic ideal of the Env assembly. Under native conditions, HIV Env is a highly dynamic fusion protein complex that can flicker between antigenically and functionally distinct conformational states. Biophysical studies from our group and others are providing the first structure-based indications that the propensity for Env to undergo large-scale dynamic movements is highly isolate-specific in nature. These changes directly impact a given Env's interactions with biological factors in the host. Here we will apply innovative structural analytical approaches, including structural mass spectrometry and cryo-EM, to characterize Env diversity and to identify the consequences of structural variation and dynamics on antibody binding and antigen presentation by the dendritic cell displayed lectin, DC-SIGN. We have developed an effective approach for purifying the native-like SOSIP trimers from highly divergent HIV-1 isolates and have demonstrated in preliminary studies that trimers even from neutralization resistant primary isolates exhibit significant differences in stability, local structural dynamics of bNAb epitopes, large-scale conformational breathing, and structure. We will characterize structural dynamic profiles of the Envs from across the neutralization Tier spectrum and from the Global Panel of resistant viruses, which was composed to be highly representative of circulating neutralization phenotypes. We will test whether variants that produced favorable immune responses (breadth and potency) exhibit structural dynamic traits. And we will assess the effect of dynamics on antibody and DC-SIGN reactivity for diverse viral isolates. An understanding of variation in Env structure and function will provide a foundation for understanding differences in HIV pathogenicity and viral fitness, and a better understanding of how the immune system grapples with this extremely variable antigen. Progress in these areas will inform our understanding of HIV evolution, and may help guide development of Env-based vaccines and immunogens.
In its role as the sole virally encoded antigen on the exterior of HIV-1, the Env glycoprotein is the target for neutralizing antibodies; it also dictates how the virus interacts with CD4 receptor and chemokine coreceptors, as well as host lectins on the surface of antigen presenting cells, serum proteins found in the blood, and antimicrobial peptides such as defensins in the mucosal mileu. Despite recent advances in mapping its architecture, we fundamentally lack an understanding of Env structural variation among diverse isolates, which is a defining trait of HIV that makes it a profound challenge for targeting by the immune system and for vaccine development. The primary goals of the proposed studies are to advance our understanding of the structural basis for Env antigenicity and immunogenicity by applying powerful structural analysis approaches to characterize Env structural diversity, andto determine the consequences of structural dynamic variation on antibody binding and interaction of Env with antigen presenting proteins such as lectins.
