1C). that form and expose the co-receptor binding region of gp120 (Kwong et al., 1998;Salzwedel et al., 2000;Wyatt and Sodroski, 1998;Xiang et al., 2002). The V3 region of gp120 is critical for co-receptor recognition and determines which co-receptor, CCR5 or CXCR4, is used for viral entry (Cormier and Dragic, 2002;Huang et al., 2005;Suphaphiphat et al., 2003). Hence, while relatively variable in linear sequence, the V3 region has some level of functional and structural conservation (Cardozo et al., 2007;Haynes and Montefiori, 2006;Huang et al., 2005;Rosen et al., 2005;Sharon et al., 2003). During HIV-1 infection, antibodies to the V3 loop are common (Broliden et Chloroquine Phosphate al., 1992;Gorny et al., 2006;Haynes and Montefiori, 2006;Krachmarov et al., 2001;Kraft et al., 2007;Pantophlet and Burton, 2006;Profy et al., 1990;Schreiber et al., 1994;Spenlehauer et al., 1998;Wu et al., 1995;Zolla-Pazner, 2004). However, the V3 region appears to play a limited role in the neutralization of most primary virus isolates (Binley et al., 2004;Burton et al., 2004;Lusso et al., 2005;Stamatos et al., 1998;Vancott et al., 1995). Early vaccine studies using V3 peptides as immunogens showed a highly type-specific neutralizing antibody (NAb) response to V3 (Javaherian et al., 1990), while more recent studies of V3 mAbs and immune sera suggest that the V3 NAb response can be more broadly reactive (Derby et al., 2007;Haynes et al., 2006;Moore et al., 1995b;Wu et al., 2006;Yang et al., 2004;Zolla-Pazner, 2005). Human anti-V3 mAbs from both subtype B and non-subtype B infected individuals can neutralize a subset of subtype B and non-subtype B primary virus strains. Interestingly, the breadth and potency of this neutralizations is maximized when the V3 region is built into an unmasked V3 sensitive Env such as on virus SF162 (Binley et al., 2004;Gorny et al., 2006;Krachmarov et al., 2006;Li et al., 2005;Moore et al., 1995a;Pantophlet et al., 2007;Patel, Hoffman, and Swanstrom, 2008;Zolla-Pazner et al., 2008). These data, along with recently described atomic level structures of V3 mAb liganded to cognate peptides, confirm that there are conserved motifs within the V3 region (Cardozo et al., 2007;Huang et al., 2005;Sharon et al., 2003;Stanfield et al., 2004;Stanfield et al., 2006). These findings are consistent with our understanding that the V3 region is displayed to varying degrees in the context of the quaternary structure of the native viral spike of individual strains of HIV-1. However after Rabbit Polyclonal to AKAP8 binding to the CD4 receptor, conformational changes in Env result in exposure of specific regions previously inaccessible to antibody. CD4 binding significantly enhances gp120 binding by mAb 17b, which recognizes the co-receptor binding site (Decker et al., 2005;Hoffman et al., 1999;Salzwedel et al., 2000;Sullivan et al., 1998a;Sullivan et al., 1998b;Xiang et al., 2002). Similarly, the V3 loop appears to be accessible to antibody when gp120 is in a CD4-bound state (Krachmarov et Chloroquine Phosphate al., 2006;Lusso et al., 2005;Mbah Chloroquine Phosphate et al., 2001;Potts et al., 1993;Sullivan et al., 1998b). We therefore postulated that the limited breadth of neutralization by recently isolated broadly reactive V3 mAbs, and by V3-directed vaccine sera, may be due to poor epitope accessibility rather than antigenic diversity of the V3 region (Bou-Habib et al., 1994;Krachmarov et al., 2005;Stamatos et al., 1998;Vancott et al., 1995). To study the breadth and potency of anti-V3 antibody mediated virus neutralization, we used recently established reference panels of 12 acute subtype B and 12 acute subtype C Env-pseudoviruses (Li et al., 2005;Li et al., 2006). We evaluated three well-characterized anti-V3 mAbs and five guinea pig (GP) vaccine-induced immune sera known to contain high levels of anti-V3 antibodies. The mAbs and immune sera were assayed in the presence of varying concentrations of sCD4 to test if the resulting conformational change in Env would lead to exposure and recognition of the V3 loop. One anti-V3 mAb, 447-52D, is known Chloroquine Phosphate to react with the GPGR sequence at the tip of the V3 loop which is conserved among most subtype B isolates (Gorny et al., 1992;Zolla-Pazner et al., 2004). The other two mAbs, 2219 and 3074, were derived from subtype B and the circulating recombinant form (CRF) 02_AG infected individuals respectively, and were selected.