The HIV-1 Tat protein binds Env and enhances virus infectivity by affecting Env conformational dynamics and antigenicity

LB01 The HIV-1 Tat protein released by infected cells increases virus infectivity and modifies Env antigenicity by stabilizing the open conformations of Env Background: The HIV-1 Tat protein released by infected cells (extracellular Tat, eTat) exerts several actions increasing virus infectivity both at the portal of entry and in the acute and chronic phases of infection. In this context, we have previously shown that eTat binds Env forming a novel cell-entry complex (the Tat/Env complex), which enhances virus capture by dendritic cells as well as infection of CD4+ T cells through a transactivation-independent mechanism (P. Monini, PlosOne 2012). We now show that Tat enhances HIV-1 infectivity by altering the conformational dynamics of Env, thus affecting both CD4 recognition and Env antigenicity. Methods: Single cycle HIV-1 was incubated with the transactivation- silent mutant Tatcys22 and added to CD4+ T cells (A3.01, TZM-bl) in the presence/absence of soluble CD4 (sCD4) or the monoclonal antibody b12. The effects of recombinant Tat on Env conformational dynamics was determined by single-molecule fluorescence resonance energy transfer (smFRET). Surface plasmon resonance, Isothermal titration calorimetry, electron microscopy and hydrogen-deuterium exchange-mass spectrometry were used to study Tat binding to soluble, native-like BG505 and B41 SOSIP.664 gp140. Results: Tat binding to native Env on virus particles induces/ stabilizes the open conformation(s) of Env. Through this action, Tat enhances HIV-1 infectivity by favoring CD4 recognition. At the same time, this affects Env antigenicity as indicated by the increased neutralization potency of sCD4 and mAb b12 in the presence of Tat. Noteworthy, the ratio between enhanced infectivity and increased vulnerability to sCD4 and mAb b12 is shifted by Tat in favor of virus infection. Env opening by Tat was observed also with SOSIP Envs; however, stoichiometric concentrations of Tat lead to SOSIP gp140 collapse/dissociation and aggregation. Conclusions: These data highlight that Tat and the Tat/ Env complex are key targets for an effective vaccine against HIV. Further advancement of soluble native-like Envs is needed to develop immunogens based on the Tat/ Env complex; alternatively, mRNA vaccines assembling the native Tat/Env complex in the context of virus-like particles may represent a new avenue for a preventative vaccine. (Funding: B&MGF INV-037179)