Immune effector mechanisms that confer protection against HIV acquisition remains poorly understood. Recent data from the Phase III RV144 trial of the prime/boost ALVAC+gp120 protein vaccine, which delivered an overall 31.2% reduction in virus acquisition, suggest potential protective effects of anti-gp120 antibody responses. Specifically, of the six primary immunological parameters evaluated in the RV144 case-control study, high IgG responses to the V2 loop of HIV envelope gp120 significantly correlate with protection from HIV infection. However, it is unclear if anti-V2 antibodies have direct anti-viral functions for blocking HIV infection. V2 and other variable loops of gp120 are thought to be undesirable targets for HIV vaccines, due to their highly variable sequences. While a closer scrutiny of V2 sequences has demonstrated a significant level of AA conservation including a conserved LDV/I motif, which is involved in binding 47 integrin, the gut homing receptor expressed on key CD4 T cell targets for mucosal transmission, anti-V2 antibodies have little or no neutralizing activities against many HIV-1 primary isolates when measured under conventional assay conditions and with target cells expressing no 47. We propose herein to utilize non-conventional in vitro and in vivo approaches to investigate the capacity of anti-V2 antibodies to block HIV transmission. We will first measure the ability of anti-V2 human monoclonal antibodies to block virus infection in vitro by varying incubation time, temperature, target CD4 T cells with or without active 47, cell-free and cell-to-cell transmission. Secondly, we will test human anti-V2 monoclonal antibodies in vivo for the ability to reduce HIV acquisition in passive transfer and virus challenge in humanized mice. Finally, we will evaluate the ability of polyclonal anti-V2 antibodies raised by vaccination to mediate virus blocking in vitro and in vivo. Our results will define the potential contributions of anti-V2 antibody response in preventing HIV acquisition, opening a new venue for future design of HIV vaccines.