Efforts to generate vaccines that elicit broadly neutralizing antibodies have been thwarted by features of the HIV-1 envelope glycoprotein (Env) that limit access to conserved, neutralizing epitopes and difficulties in mimicking neutralizing structures in vaccine constructs. These challenges apply especially to the pretransmembrane region (PTM) of gp41, which is the target of approximately half of the broadly neutralizing monoclonals that have been identified (2F5, 4E10, Z13), and the coiled-coil domain of the gp41 prehairpin intermediate, which is the target of the broadly-active fusion inhibitor T20 (Enfuvirtide) and a recently described neutralizing antibody D5. Here we propose to create and evaluate rationally-designed gp41 immunogens that present PTM, coiled-coil, and other HIV mimotopes in stable, highly exposed, oligomeric structures for eliciting broadly neutralizing antibodies. These studies will further investigate the influence of the virion membrane and virus-like particle (VLP) structure on antibody induction, especially for PTM determinants, by comparing gp41 oligomers in the context of VLPs or soluble immunogens. The immunogens utilize a novel, oligomeric gp41 scaffold, in which the N- and C-heptad repeats (HR) of SIV serve as a trimerization domain and the transmembrane domain (TM) and cytoplasmic tail of HIV serve as targeting signals for incorporationg oligomers into VLPs. We have shown that this gp41 scaffold allows insertion of large regions of Env between the trimerizaton and TM domains, without impairing oligomer assembly or incorporation of the gp41 immunogen into VLPs. The gp41 scaffold also contains an alternative insertion site, between the N- and C-HR, which may also be used to provide a highly exposed, loop context for evaluating some epitopes. Specific aims include creating and evaluating the following Env determinants presented in the oligomeric gp41 scaffold in VLP and soluble formats: 1) PTM region containing the 2F5 and 4E10 epitopes;2) N-HR coiled-coil region 3) combination inserts involving PTM, coiled coil, and other HIV neutralizing mimotopes. Immunogens will be extensively characterized, evaluated as single immunogens, and directly compared with other immunogens in the program. Promising constructs will be further assessed in prime-boost combinations involving other vectors and immunogens in the program, with different formulations and adjuvants.