Conformational dynamics of the HIV-1 envelope glycoprotein from CRF01_AE is associated with susceptibility to antibody-dependent cellular cytotoxicity

The HIV-1 envelope glycoprotein (Env) is expressed at the surface of infected cells and, as such, can be targeted by non-neutralizing antibodies (nnAbs) that mediate antibody-dependent cellular cytotoxicity (ADCC). Previous single-molecule Förster resonance energy transfer (smFRET) studies demonstrated that Envs from clinical isolates predominantly adopt a “closed” conformation (State 1), which is resistant to nnAbs. After interacting with the cellular receptor CD4, the conformational equilibrium of Env shifts toward States 2 and 3, exposing the coreceptor-binding site (CoRBS) and permitting targeting by CD4-induced (CD4i) antibodies. We showed that the binding of anti-CoRBS Abs enables the engagement of other nnAbs that target the cluster A epitopes on Env. Anti-cluster A nnAbs stabilize an asymmetric Env conformation, State 2A, and have potent ADCC activity. CRF01_AE strains were suggested to be intrinsically susceptible to ADCC mediated by nnAbs. This may be due to the presence of a histidine at position 375, known to shift Env toward more “open” conformations. In this work, through adaptation of an established smFRET imaging approach, we report that native, unliganded CRF01_AE HIV-1 Envs frequently sample the State 2A conformation. This is in striking contrast with Envs from clades A and B, for example HIV-1_JR-FL, which do not transition to State 2A in the absence of ligands. These findings inform on the conformational dynamics of CRF01_AE Env, which are relevant for structure-based design of both synthetic inhibitors of receptor binding and enhancers of ADCC as therapeutic alternatives. IMPORTANCE A concerning increase in infections with HIV-1 from CRF01_AE has occurred globally and regionally in recent years, especially in Southeast Asia. Despite the advances made in understanding HIV-1 envelope glycoprotein (Env) conformational dynamics, the knowledge about Env from CRF01_AE HIV-1 is limited. Here, we demonstrate that the unliganded CRF01_AE Env readily samples an “open” conformation (State 2A), which is susceptible to antibody-dependent cellular cytotoxicity (ADCC). This is in contrast with the subtypes previously studied from HIV-1 group M that rely on anti-cluster A antibodies to adopt State 2A. These findings are relevant for the structure-based design of novel synthetic inhibitors of CD4 binding and enhancers of ADCC for the elimination of infected cells.