Of the, the 1088 MAb was the most potent, with an IC50 in the same order of magnitude as the positive control antibodies directed to the V3 domain name (1026) and the CD4 binding site (b12). first clinical trial to show that vaccination could prevent HIV contamination in humans. A recent RV144 correlates of protection study found that protection correlated with the presence of antibodies to the V2 domain name. It has been proposed that antibodies to the 47 binding site in the V2 domain name might prevent HIV-1 contamination by blocking the ability of virions to recognize 47 on activated T-cells. In this study we investigated the specificity of monoclonal antibodies (MAbs) to the V2 domain name of MN-rgp120 and examined the possibility that these antibodies could inhibit the binding of MN-rgp120 to the 47 Dichlorisone acetate integrin. Methodology/Principal Findings Nine MAbs to the V2 domain name were isolated from mice immunized with recombinant envelope proteins. The ability of these MAbs to inhibit HIV contamination, block the binding of gp120 to CD4, and block the binding of MN-rgp120 to the 47 integrin was measured. Mutational analysis showed that eight of the MAbs acknowledged two immunodominant clusters of amino acids (166C168 and 178C183) located at either end of the C strand within the four-strand anti-parallel sheet structure comprising the V1/V2 domain name. Conclusions/Significance These studies showed that this antigenic structure of the V2 domain name is exceedingly complex and that MAbs isolated from mice immunized with MN-rgp120 exhibited a high level of strain specificity compared to MAbs to the V2 domain name isolated from HIV-infected humans. We found that immunization with MN-rgp120 readily elicits antibodies to the V2 domain name and some of those were able to block the binding of MN-rgp120 to the 47 integrin. Introduction Recombinant gp120 from your MN strain of HIV-1 (MN-rgp120) has been investigated as a candidate HIV-1 vaccine to elicit protective antibody responses [1]C[4]. Two bivalent HIV-1 subunit vaccines, AIDSVAX B/B and AIDSVAX B/E, each Dichlorisone acetate made up of MN-rgp120, have been developed for use in North America and Thailand, respectively [5], [6]. Both vaccines have now been tested in large level Phase 3 trials alone, or in combination with vaccines such as vCP1521 designed to stimulate cellular immune responses [7]. In the VAX003 and VAX004 trials (1998C2003), immunization with these vaccines was ineffective in preventing new HIV-1 infections in cohorts of injection drug users (IDUs) and men who experienced sex with men (MSMs) [8], [9]. However, the RV144 clinical trial showed that a primary/boost immunization regimen, including priming immunizations with a recombinant canarypox vector vCP1521 followed by booster immunizations with AIDSVAX B/E, provided modest but significant protection from heterosexual HIV-1 transmission [7]. This trial provided the NS1 first evidence that vaccination can prevent HIV-1 contamination in humans. In order to establish a correlate of protection, there is renewed desire for defining the specificity of the antibody response to the vaccine immunogens, including MN-rgp120. Initial analysis of sera using the TZM-bl computer virus neutralization assay [10] failed to document a correlation between the level of computer virus neutralizing antibodies and protection Dichlorisone acetate [11], [12]. As a consequence, multiple investigators have begun to consider the possibility that antibodies might confer protection by means other than direct computer virus neutralization. In theory, non-neutralizing antibodies might confer protection by effectively lowering the probability of contamination from transmitted computer virus inocula. Several distinct mechanisms have been proposed by which non-neutralizing antibodies to the HIV-1 envelope protein might have a protective effect. These include inactivation of viruses or virus-infected cells by antibody-dependent cell-mediated computer virus inhibition [13]; aggregation of virions at mucosal surfaces, thus impairing computer virus transport across mucosal membranes [14]; or prevention of viruses from selectively targeting activated CD4+ T-cells [15]. Recently, antibodies that bind to the V2 domain name of the HIV-1 envelope protein gp120 have drawn considerable attention in connection with the RV144 trial [16]. The V2 domain name is known to play a major role in conformational masking, shielding epitopes located in other regions of the molecule from exposure to computer virus neutralizing antibodies [17]C[19]. Two recent studies have reported that potent neutralizing antibodies in sera from HIV-1 infected individuals are directed to the V2 domain name [20], [21]. Moreover, two extremely potent broadly neutralizing monoclonal antibodies (MAbs), PG9 and PG16, have been shown to target an epitope in the V2 domain name [22]. Finally, it has been demonstrated that Dichlorisone acetate this V2 domain name of gp120 can serve as a ligand for the T-cell associated integrin, 47 [23]. It has been proposed that 47 interactions play a major role in facilitating HIV-1 contamination by enabling HIV-1 virions to target activated CD4+ T-cells. It has been known for many years that contamination of activated CD4+ cells results in productive HIV-1 contamination, whereas contamination.