Broadly Neutralizing Antibody Epitopes on HIV-1 Particles are exposed after Virus Interaction with Host Cells

HIV-1 envelope glycoproteins (Env) are critical for infection and are key targets for vaccine development. Env proteins displayed on virions are conformationally diverse, comprising both functional and non-functional forms. These heterogeneous Env populations have important implications for neutralizing and non-neutralizing antibody elicitation and functions. This study aimed to interrogate the antigenic composition of Env on virions. Using a flow cytometry-based assay we show that only some epitopes including V2i, gp120-g41 interface, and gp41-MPER are accessible on HIV-1 particles, while V3, V2q, and select CD4bs epitopes are obscured for monoclonal antibody (mAb) binding. To investigate the mechanisms contributing to the masking of these epitopes we first asked whether time-dependent dynamics of Env can affect their exposure. Extending the time of virus-mAb interaction increased the binding of mAbs, epitopes of which were already accessible on virions but not those that are occluded. However, the occluded epitopes became accessible after conformational unmasking was induced by pre-binding of select mAbs, prompting us to test if similar conformational changes are required for these mAbs to exhibit their neutralization capability. We tested HIV-1 neutralization where virus-mAb mix was incubated or not incubated for one-hour prior to adding the target cells. Similar levels of neutralization were observed under both assay conditions, suggesting that interaction between virus and cells sensitizes the virions for neutralization via broadly-neutralizing antibodies (bNAbs). These findings provide insight into how bNAbs may gain access to these occluded epitopes to exert their neutralization effects. Further studies with larger virus and mAb panels are warranted.