The overall goal of my project is to provide a detailed understanding of the human humoral response to norovirus (NoV) infection and to define the molecular and structural basis for inhibition of NoV by human antibodies. NoV is the leading cause of sporadic and epidemic gastroenteritis in humans. The virus is transmitted easily in closed crowded settings like nursing homes, schools, hospitals, and cruise ships. NoV research has been hindered by the lack of an in vitro cell-culture system. There are currently no vaccines, therapeutics, or prophylactics available to prevent or treat NoV infection. Vaccine design also has been difficult due to the antigenic variation within and between NoV genogroups. To design a vaccine that elicits broad protective immunity, we must have a solid understanding of the NoV-mediated human antibody response to infection and antigenic sites recognized by these antibodies, so a critical area of HuNoV research has become the identification of type specific and cross-genotype and genogroup epitopes. The general hypothesis of this study is that inter- and intragenogroup I and II cross-reactive broadly blocking human antibodies exist and that the blocking function of such antibodies is principally mediated by IgA isotype molecules. The approach to my project includes high-efficiency isolation of human mAbs from patients previously infected with GI and GII strains of human NoV. I then will use this panel of human antibodies to determine if antibody isotype influences binding to NoV virus-like particles (VLPs) and blocking of binding of VLPs to host attachment factors. Finally, using site-directed mutagenesis and three-dimensional structural analysis, I will map immunoreactive epitopes on current circulating strains of NoV and determine whether cross-reactive broadly blocking epitopes exist. This information will be valuable to the NoV field, because it can inform the development of epitope-based vaccines and monitoring of response to experimental NoV vaccines of all kinds.