P-D8 New insights on the human anti-HIV-1 Env antibody-mediated cell cytotoxicity (ADCC) against HIV-1 virus: Allosteric regulation of FcRs binding upon antigen engagement

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HIV-1 vaccine field is rapidly evolving and a deeper knowledge of the mechanisms of defense against HIV-1 infection is needed. In this regard, several studies highlighted the relevance of the antibody-mediated cell cytotoxicity (ADCC) in the context of HIV-1 infection, linking Fc-effector functions to protection against HIV-1 acquisition. The current model for ADCC activation is based on the concept that antibodies, bound to their respective antigens on the surface of HIV-1 sensitized cells, form aggregates which engage the FcRs, activating the effector cells. Our aim is to understand the molecular basis of ADCC and to identify the critical factors that lead to the triggering of the cytotoxicity against HIV-1 virus. Utilizing multiple approaches, such as ELISA, FCS (Fluorescence Correlation Spectroscopy), H/DX MS (Hydrogen/Deuterium Exchange Mass Spectroscopy) and crystallography, we studied the very first step of ADCC activation: the monovalent binding of viral antigen. Here, we demonstrate an allosteric regulation in anti-HIV gp120 Cluster A mAbs resulting from immune complex (IC) formation with a monomeric gp120-CD4 chimera antigen. We established that IC formation dramatically increases the efficiency of Ab interaction to low affinity FcRs compared to free IgG, impacting, in turn, the activation of the cytotoxicity against HIV-1 positive targets. In conclusion, we believe that monomeric antigen-antibody IC formation might be the very first step of HIV-1-specific ADCC triggering that likely precedes the IgG aggregation required for Fc receptors binding and in turn, effector cell activation. This might be a mechanism that enables the fine tuning of Fc-effector functions in vaccine regimens or HIV-1 passive treatments.

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