Interaction with Cellular CD4 Exposes HIV-1 Envelope Epitopes Targeted by Antibody-Dependent Cell-Mediated Cytotoxicity

Abstract

Anti-HIV-1 envelope glycoprotein (Env) antibodies without broadly neutralizing activity correlated with protection in the RV144 clinical trial, stimulating interest in other protective mechanisms involving antibodies, such as antibody-dependent cellmediated cytotoxicity (ADCC). Env epitopes targeted by many antibodies effective at mediating ADCC are poorly exposed on the unliganded Env trimer. Here we investigated the mechanism of exposure of ADCC epitopes on Env and showed that binding of Env and CD4 within the same HIV-1-infected cell effectively exposes these epitopes. Env capacity to transit to the CD4-bound conformation is required for ADCC epitope exposure. Importantly, cell surface CD4 downregulation by Nef and Vpu accessory proteins and Vpu-mediated BST-2 antagonism modulate exposure of ADCC-mediating epitopes and reduce the susceptibility of infected cells to this effector function in vitro. Significantly, Env conformational changes induced by cell surface CD4 are conserved among Env from HIV-1 and HIV-2/SIVmac lineages. Altogether, our observations describe a highly conserved mechanism required to expose ADCC epitopes that might help explain the evolutionary advantage of downregulation of cell surface CD4 by the HIV-1 Vpu and Nef proteins. IMPORTANCEHIV-1 envelope epitopes targeted by many antibodies effective at mediating antibody-dependent cell-mediated cytotoxicity (ADCC) are poorly exposed on the unliganded envelope trimer. Here we investigated the mechanism of exposure of these epitopes and found that envelope interaction with the HIV-1 CD4 receptor is required to expose some of these epitopes. Moreover, our results suggest that HIV-1 CD4 downregulation might help avoid the killing of HIV-1-infected cells by this immune mechanism.

Influence of the Envelope gp120 Phe 43 Cavity on HIV-1 Sensitivity to Antibody-Dependent Cell-Mediated Cytotoxicity Responses

“…It has been shown that the replacement of serine 375 by a histidine or tryptophan residue fills the Phe 43 cavity and predisposes Env to assuming a slightly more CD4-bound-like conformation (9). Moreover, it has been shown that sampling of the CD4-bound conformation results in an enhanced exposure of epitopes recognized by anti-cluster A antibodies (14,34). To evaluate whether this enhanced recognition translates into enhanced susceptibility to ADCC, primary CD4 ϩ T cells from healthy HIV-1-negative donors were infected with HIV-1 NL4-3-GFP infectious molecular clones (IMCs) expressing wild-type (WT) strain ADA Env or mutant Env with a histidine (S375H) or a tryptophan (S375W) substitution at position 375.…”

“…In parallel, primary CD4 ϩ T cells were infected with SHIVs expressing clade C and D HIV-1 Env with the same substitutions in residue 375 (WT and S375H and S375W mutants). Since it is now well established that CD4 present on the cell surface affects Env conformation by forcing it to assume the CD4-bound conformation (13,14,21,22,35), we first evaluated the ability of all the IMCs to downregulate CD4. Figure 2 shows that all IMCs tested, independently of the residue present at position 375, efficiently downregulated CD4 from the cell surface ( Fig.…”

“…In agreement with the need for HIV-1 to avoid exposing Env in the CD4-bound conformation, it was recently shown that forcing Env to adopt this conformation with small CD4-mimetic compounds (CD4mcs) sensitizes HIV-1-infected cells to ADCC responses (18)(19)(20). Accordingly, we recently reported that the transition of Env to the CD4-bound conformation is required for efficient interactions with ADCC-mediating antibodies (14). We reported that the replacement of serine 375 by a tryptophan residue (S375W) enhanced the exposure of epitopes recognized by anti-cluster A antibodies, known to mediate potent ADCC responses (13,14,18,(20)(21)(22)(23)(24)(25)(26).…”

“…It has been shown that HIV-1 minimizes the exposure of this ADCC-susceptible Env conformation using a highly sophisticated strategy to keep Env on the surface of infected cells in the unbound "closed" conformation. HIV-1 accomplishes this through its accessory proteins Nef and Vpu, which decrease the overall amounts of Env (via Vpu-mediated BST-2 downregulation) and CD4 at the cell surface (13)(14)(15)(16). In addition, decreased amounts of Env at the cell surface due to efficient internalization also help the virus to avoid ADCC responses (17).…”

“…Accordingly, we recently reported that the transition of Env to the CD4-bound conformation is required for efficient interactions with ADCC-mediating antibodies (14). We reported that the replacement of serine 375 by a tryptophan residue (S375W) enhanced the exposure of epitopes recognized by anti-cluster A antibodies, known to mediate potent ADCC responses (13,14,18,(20)(21)(22)(23)(24)(25)(26). Thus, the transition of Env from the unbound to the CD4-bound conformation appears to be a prerequisite for the exposure of inner-domain ADCC epitopes.…”