Exposing HIV-1 Env: Implications for therapeutic strategies.

The human immunodeficiency virus (HIV-1) envelope glycoprotein trimer (Env) is exposed on the surfaces of both virions and infected cells. Thus, Env is the principal target for neutralizing antibodies and antibodies able to mediate antibodydependent cellular cytotoxicity (ADCC). The HIV-1 Env is a flexible molecule known to exist in at least three different conformational states: states 1, 2 and 3. Before interacting with the primary receptor, CD4, Env preferentially adopts a compact, "closed" conformation (state 1) that is largely antibody-resistant. The CD4 binding "opens" Env increasing the vulnerability of infected cells to ADCC mediated by non-neutralizing antibodies, as these easily-elicited antibodies preferentially recognize epitopes exposed in the open conformational states (states 2/3). These antibodies include the anti-coreceptor binding site and the anti-cluster A families of antibodies that, in combination with small CD4-mimetic compounds, stabilize a new asymmetric Env conformation (state 2A) that is vulnerable to ADCC. Approaches aimed at stabilizing this "open" conformation represent new interventional approaches to fight HIV-1 infection.