Analysis of the Day 70 and Day 98 sera showed that while the gp140 and gp140(-) constructs induced similar binding antibody responses against gp120 and gp41, gp140(-) induced a stronger response to the V1, V2 and V3-deleted gp120 Core (Fig. induced significantly improved neutralizing antibodies to several HIV-1 pseudoviruses, compared to gp140 lacking a trimerization motif. However, modest differences in both binding specificity and neutralizing antibody responses were observed among the various immunogens. Keywords: Vaccine, HIV-1, Envelope, trimerization motifs, ATCase, T4 fibritin, GCN, CD4, monoclonal antibody, immunization, neutralizing antibody Introduction The ongoing HIV-1 epidemic has resulted in 2.7 million new infections and 2.1 million AIDS deaths in 2007 for a total of 33 million people living with HIV/AIDS (UNAIDS, 2008). Prophylactic measures against HIV-1 are clearly required, and the discovery of vaccine candidates that elicit broadly neutralizing antibody responses against HIV-1 is an important goal (Karlsson Hedestam et al., 2008; Montefiori et al., 2007; Walker and Burton, 2008). Some insights for vaccine research can be gained from a better understanding of the antibody response to HIV-1 during natural infection. Most HIV-infected individuals develop neutralizing antibodies with limited cross-reactivity (Hu and Stamatatos, 2007; Karlsson Hedestam et al., 2008; Richman et al., 2003), a result that is likely due to the predominance of autologous neutralization determinants in envelope (Env) glycoproteins. However, the existence of some patients who go on to develop broadly neutralizing immune sera suggests that such antibodies can be elicited under certain conditions (Binley et al., 2008; Carotenuto et al., 1998; Li et al., 2007; Pilgrim et al., 1997; Sather et al., 2009). Several monoclonal antibodies (mAbs) capable of neutralizing a broad range of primary isolates from various subtypes have been isolated from HIV-1 infected individuals. These demonstrate that conserved neutralization epitopes are present in Env (Binley et al., 2004; Calarese et al., 2003; Saphire et al., 2001; Sharon et al., 2003; Zwick et al., 2001). Studies with non-human primates have further demonstrated that broadly neutralizing antibodies can protect from infection TNFRSF8 (Baba et al., 2000; Hessell et al., 2009; Hofmann-Lehmann et al., 2001; Mascola, 2003; Nishimura et al., 2002; Shibata et al., 1999). Although it is not known what magnitude and breadth of neutralization will confer some measure of protection against HIV-1 infection, it is clear that current Env immunogens elicit antibodies that neutralize only a small fraction of circulating isolates (Montefiori et al., 2007). The HIV-1 Env complex on the surface of infectious viral particles is the target of neutralizing antibodies. This complex is formed by a trimer of heterodimers of the gp120 and gp41 subunits of the Env transmembrane protein Fruquintinib (Dey et al., 2008; Hu and Stamatatos, 2007; Karlsson Hedestam et al., 2008; Poignard et al., 2003; Poignard et al., 2001). The Env structures on the viral membrane are apparently heterogeneous (Dey et al., 2008; Moore et al., 2006; Poignard et al., 2003; Sougrat et al., 2007). Only complexes formed by intact gp120-gp41 trimers are thought to be functional and capable of mediating HIV-1 entry into target cells (Moore et al., 2006; Poignard et al., 2003; Poignard et al., 2001). Non-functional forms of Env could serve as decoys to prevent the host immune system from recognizing Fruquintinib conserved neutralization epitopes (Dey et al., 2008; Moore et al., 2006; Poignard et al., 2003). Recent cryo-electron tomographic studies on SIV and HIV-1 particles have provided further evidence that the viral Env forms three-lobed trimeric structures containing the membrane-distal gp120, with considerable heterogeneity (Liu et al., 2008; Sougrat et al., 2007; Zanetti et al., 2006; Zhu et al., Fruquintinib 2006). Clinical testing of several vaccines based on gp120 has shown a lack of protective efficacy (Pitisuttithum et al., 2006). Some investigators have proposed that soluble trimers of the gp120-gp41 Env ectodomain (i.e., lacking the transmembrane and cytoplasmic domains) might better preserve or mimic the structure of functional Env complexes (Burton et al., 2004; Schulke et al., 2002; Srivastava et al., 2003b; Yang et al., 2002; Zhang et al., 2007). The inclusion of the gp41 ectodomain sequences is also likely to be a positive factor in immunogenicity (Grundner et al., 2004) since gp41 contains helical regions that often harbor T-helper epitopes,. The Env ectodomain contains all of the known neutralization epitopes. This gp140 polypeptide, if cleaved at the gp120-gp41 junction, will typically result in free gp120 subunits along with a thermodynamically favored 6-helix bundle formed by the gp41 moiety. Mutating the furin cleavage site to inhibit processing of gp140 creates Fruquintinib a polypeptide [referred to.