Abstract
BackgroundThe binding of HIV-1 Envelope glycoproteins (Env) to host receptor CD4 exposes vulnerable conserved epitopes within the co-receptor binding site (CoRBS) which are required for the engagement of either CCR5 or CXCR4 co-receptor to allow HIV-1 entry. Antibodies against this region have been implicated in the protection against HIV acquisition in non-human primate (NHP) challenge studies and found to act synergistically with antibodies of other specificities to deliver effective Fc-mediated effector function against HIV-1-infected cells. Here, we describe the structure and function of N12-i2, an antibody isolated from an HIV-1-infected individual, and show how the unique structural features of this antibody allow for its effective Env recognition and Fc-mediated effector function.ResultsN12-i2 binds within the CoRBS utilizing two adjacent sulfo-tyrosines (TYS) for binding, one of which binds to a previously unknown TYS binding pocket formed by gp120 residues of high sequence conservation among HIV-1 strains. Structural alignment with gp120 in complex with the co-receptor CCR5 indicates that the new pocket corresponds to TYS at position 15 of CCR5. In addition, structure-function analysis of N12-i2 and other CoRBS-specific antibodies indicates a link between modes of antibody binding within the CoRBS and Fc-mediated effector activities. The efficiency of antibody-dependent cellular cytotoxicity (ADCC) correlated with both the level of antibody binding and the mode of antibody attachment to the epitope region, specifically with the way the Fc region was oriented relative to the target cell surface. Antibodies with poor Fc access mediated the poorest ADCC whereas those with their Fc region readily accessible for interaction with effector cells mediated the most potent ADCC.ConclusionOur data identify a previously unknown binding site for TYS within the assembled CoRBS of the HIV-1 virus. In addition, our combined structural-modeling-functional analyses provide new insights into mechanisms of Fc-effector function of antibodies against HIV-1, in particular, how antibody binding to Env antigen affects the efficiency of ADCC response.
Highlights
The binding of HIV-1 Envelope glycoproteins (Env) to host receptor CD4 exposes vulnerable conserved epitopes within the co-receptor binding site (CoRBS) which are required for the engagement of either CCR5 or CXCR4 co-receptor to allow HIV-1 entry
In order to better understand the mechanisms governing Fceffector mechanism to the CoRBS epitopes, we examined if the fine epitope specificity and mode of binding of N12-i2 and other CoRBS-specific antibodies contribute to the effectiveness of Env engagement and Fc-mediated effector activities in the elimination of CEM-NKR cells sensitized with HIV-1 gp120 BaL or infected with the ADA virus
The bridging sheet is assembled within the exterior gp120 subunit upon CD4 binding to the HIV-1 Env trimer and consists of a four-stranded β-sheet formed by two strands from the outer domain and two strands from the inner domain with the latter forming the base for the variable loops 1 and 2 (V1V2 loop) [14, 43,44,45]
Summary
The binding of HIV-1 Envelope glycoproteins (Env) to host receptor CD4 exposes vulnerable conserved epitopes within the co-receptor binding site (CoRBS) which are required for the engagement of either CCR5 or CXCR4 co-receptor to allow HIV-1 entry Antibodies against this region have been implicated in the protection against HIV acquisition in non-human primate (NHP) challenge studies and found to act synergistically with antibodies of other specificities to deliver effective Fc-mediated effector function against HIV-1-infected cells. The transitional structures arising within the Env spike post-CD4 attachment in the HIV-1 entry process are referred to as “CD4-induced” (CD4i) and constitute effective targets for humoral immune responses [6,7,8,9,10] These CD4i targets include highly conserved epitopes that are located within/proximal to the assembled CoRBS [11,12,13,14] and epitopes elsewhere in the protein, including the C1/C2 region of gp120 [6,7,8, 15,16,17,18]. The structure of gp120 in complex with both CD4 and CCR5 has recently been solved by cryo-electron microscopy and revealed that CCR5 most likely acts to stabilize the gp120 in CD4-bound conformation and to moor the complex close to the cell surface [21]
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