Analysis of an Early Intermediate Antibody from a Broadly Neutralizing Antibody Lineage and Its Interaction with HIV Envelope

Worldwide, over 37 million people are currently infected with rapidly mutating human immunodeficiency virus (HIV). The virus and antibodies targeting the virus coevolve over the course of infection. The DH270 lineage of broadly neutralizing antibodies (bnAbs) was isolated from patient CH848, who was followed for approximately five years after initial infection with HIV. These antibodies target the V3-glycan supersite on HIV Env. The structures and binding modes of bnAbs from late in the lineage have been determined, but the binding modes of earlier intermediates and the unmutated common ancestor are unknown. Such information would guide the design of immunogens to elicit precursors of bnAb lineages. Towards this end, we sought to determine the structure of an early intermediate, IA4, and characterize its binding to the one autologous Env to which it was identified to bind. We obtained a 1.5 Å resolution X-ray crystal structure of unliganded IA4. A low-resolution model from negative stain electron microscopy (EM) data of IA4 in complex with this Env showed that Env is not fully saturated with antibody, potentially due to weak interactions. We therefore generated Env mutants that lack some glycosylation sites to increase binding. Kinetic data with these mutants suggest that they bind more tightly to the antibody, and so these constructs will be used for future EM studies, as well as modeling studies, to better understand how IA4 binds. This information will then allow us to better understand how the UCA binds Env and therefore what triggered this lineage. Knowledge of the evolution of bnAbs and the binding modes of bnAb precursors is crucial for the development an HIV vaccine to elicit bnAbs in uninfected patients to guard against infection