HIV-1 infection begins with the fusion of viral and target cell membranes, which is mediated by the viral envelope glycoprotein upon engagement with host cellular receptors. The HIV-1 envelope glycoprotein undergoes large structural rearrangements during viral entry. There are at least three distinct conformational states of the HIV-1 envelope protein: the prefusion, prehairpin intermediate, and postfusion conformations. Each conformation presents distinct antigenic surfaces to the immune system. In this proposal, we seek to assess the impact of HIV-1 envelope conformation on immunogenicity. We hypothesize that different functional conformers of the HIV-1 envelope protein elicit distinct profiles of antibody responses. A deeper understanding of the relationship between HIV-1 envelope structure and immunogenicity will likely facilitate future structure-based immunogen design strategies at a level of sophistication and structural detail that has not previously been possible. To explore this hypothesis, we propose the following three Specific Aims: 1. We will assess the immunogenicity of the three principal conformational states of gp41 corresponding to the prefusion, prehairpin intermediate, and postfusion conformations in guinea pigs;2. We will investigate the immunogenicity of uncleaved, precursor state (gp140)3 trimers and cleaved (gp120/gp41ecto)3 trimer complexes in guinea pigs;and, 3. We will determine the immunogenicity and protective efficacy of the optimal HIV-1 envelope immunogens in rhesus monkeys. An HIV-1 envelope immunogen with the ability to induce broadly reactive NAbs will likely be an essential component for any successful AIDS vaccine. Given the recent disappointing results from the Merck rAd5 vaccine trials aimed at eliciting T cell responses, the need to identify envelope immunogens that can elicit broadly reactive neutralizing antibodies has never been greater. In this proposal, we will assess the impact of HIV-1 envelope conformation on the profile of elicited neutralizing antibody responses.