S. but also by ADH-1 trifluoroacetate anti-V3 antibodies and antibodies directed against as yet unidentified Env regions, depending on the heterologous Env background. Our study indicates that common neutralization epitopes are differentially uncovered on diverse main HIV isolates and that the V1 loop contributes ADH-1 trifluoroacetate to this differential exposure. Therefore, the antibody responses elicited by soluble gp140 immunogens will have to overcome several unique obstacles in order to neutralize diverse main HIV isolates. The Env glycoprotein (Env) of human immunodeficiency computer virus (HIV) plays crucial roles in several steps of the viral life cycle, including its transmissibility, cellular tropism, and replication kinetics, and it is the target of both cell-mediated and antibody-mediated antiviral immune responses. Env is usually produced as a single, greatly glycosylated polypeptide that during intracellular processing is usually cleaved by cellular enzymes into two noncovalently associated subunits: ADH-1 trifluoroacetate a transmembrane subunit (gp41) and an extracellular subunit (gp120) (14). During processing, the Env oligomerizes into trimers of gp120/gp41 heterodimers (9, 23, 48), and it is this trimeric Env form that allows the viral lipid envelope to fuse with target cell plasma membranes expressing appropriate receptor molecules during the initial steps of contamination. Antibodies to almost every Env region have been isolated from infected patients (28). However, not every Env region around the virion-associated trimers is usually optimally available for antibody binding (5, 27, 31, 35, 39). In general, neutralizing antibodies (NAbs) bind to epitopes that are uncovered around the virion-associated Env trimer, although NAbs have also been explained that preferentially bind to their epitopes once Env attaches itself to cell surface CD4 and undergoes specific conformational changes (29, 42, 46). Most patients infected with HIV develop NAbs, and many monoclonal antibodies (MAbs) with neutralizing activity have been isolated from HIV-infected patients. However only a few of these MAbs display cross-neutralizing reactivity, i.e., can neutralize diverse HIV isolates (4, 8). These are the types of antibodies one would need to elicit during immunization, but thus far this goal has not been achieved (1-3, 12, 13, 15-17, 20, 22, 38, 40, 45). Since the target of NAbs is usually HIV Env, several variations of this viral antigen have been tested over the years as immunogens to elicit cross-reactive anti-HIV NAbs. Soluble monomeric gp120 immunogens are ineffective at eliciting such antibodies (18, 19, 24). Overall, soluble oligomeric forms of Env (comprising the gp120 subunit and the extracellular region of gp41, termed gp140s) are capable of eliciting cross-reactive NAbs but of limited breadth (1, 3, 12, 16, 17, 22, 45), although Rabbit Polyclonal to MMP15 (Cleaved-Tyr132) a recent study indicated that immunization with an Env protein (designated R2) derived from an HIV-infected subject who developed cross-reactive neutralizing antibody responses resulted in the elicitation of cross-neutralizing antibody responses against many heterologous viruses (47). Structural and antigenic studies of HIV Env and of certain broadly reactive NAbs provide valuable information on the presentation of neutralization epitopes on Env and on their interaction with NAbs. The ADH-1 trifluoroacetate immunogenic properties of HIV Env immunogens are not, however, predictable by using structural or antigenic studies. For instance, even though the SF162 Env contains epitopes recognized by many broadly reactive MAbs (such as IgG1b12, 2G12, 2F5, or 4E10) and the SF162 virus is susceptible to neutralization by such antibodies (33, 34, 37), immunization with SF162 Env-derived immunogens does not result in the generation of IgG1b12-, 2G12-, 2F5-, or 4E10-like antibodies (1, 7, 12). A large fraction of the homologous neutralizing antibodies elicited by SF162gp140 bind.