Project Summary/Abstract HIV research efforts are strongly focused on developing vaccines that can elicit highly protective antibody responses. There is evidence from human and nonhuman vaccine studies that both neutralizing and non- neutralizing antibody activities contribute to protection against acquisition. Importantly, the only phase III vaccine efficacy trial to show a signal of protection in human volunteers demonstrated that antibody dependent cell- mediated cytotoxicity (ADCC) was associated with reduced acquisition, compelling us to know more about this Fc-mediated antiviral function. We previously recovered a unique collection of more than 300 monoclonal antibodies from 6 recently HIV-1 infected individuals from Zambia and Rwanda and found that a subset of these antibodies mediated ADCC against target cells coated with the envelope gp120 protein from the autologous, transmitted/founder (T/F) variant. This activity was associated with antibodies that had shorter CDRH3 regions and lower neutralization activity against the T/F envelope pseudovirus than antibodies lacking ADCC activity. Modeling and competition studies suggested that some of the ADCC-mediating antibodies recognize novel epitopes. Here, we will test these monoclonal antibodies for ADCC against target cells infected with the authentic T/F variant created by molecular cloning, and compare the results with ADCC against target cells coated with the T/F gp120 protein and neutralization of the T/F Env, the latter being the hallmark of antibody binding to the functional, virion-associated envelope spike. By using a large number of autologous T/F envelope-antibody combinations, where the envelope presents the exact epitope or a very close approximation to that which the antibody was selected against, and two widely used ADCC assays, our studies have the potential to reveal previously unappreciated mechanistic features of ADCC and identify novel epitopes that can be further developed and explored for vaccines.