Role of norovirus capsid dynamics in adaptive immune evasion - Project Summary/Abstract Human noroviruses are responsible for almost a fifth of all cases of gastroenteritis worldwide. Containment of disease is difficult since as few as ten virions are sufficient to infect a normal adult. Noroviruses evolve continuously with new strains arising every 2-4 years that cause worldwide epidemics. Emergence of new strains can increase the number of cases by 50%. In the US alone, there are more than 2 million outpatient clinic visits, ~100,000 hospitalizations, and ~900 deaths (mostly among adults 65 and older) annually. Globally, there are ~200 million cases among children less than 5 years old, leading to ~50,000 deaths every year. However, no vaccines or antivirals are approved to limit infections. Efforts to develop an effective vaccine have been hindered by a lack of detailed structural information about antibody binding and the mechanisms of antibody escape. Understanding these processes has been difficult with human noroviruses because of the lack of a tissue culture system that supports the generation of a cell culture-derived virus stock and small animal model. To this end, we will be using the highly tractable mouse norovirus system where we have a highly efficient cell culture system, infectious clone, and natural mouse model. We have recently shown, for the first-time, that a virus (i.e., mouse norovirus, MNV) can use host metabolites to switch between two ‘faces’: one recognized by antibodies from recovered animals (apo) and one activated for infection. In conditions found in systemic circulation, the protruding domain (P domain) of the apo form floats above the shell by ~16Å and the loops at the tip are splayed apart. It is this conformation to which the antibody response is made. Once the virus enters the alimentary canal, low pH, bile salts, and metal ions individually and synergistically activate the virus whereby the P domain rotates and contracts onto the shell, and the antibody binding epitope at the tip of the P domain closes. This blocks antibody recognition while enhancing receptor binding. Essentially, each time MNV infects the host, the antibody response is naïve for that conformation. Importantly, we propose that this could explain why the antibody response to norovirus infection in both mice and humans appears to be notoriously short-lived. The goal of this proposal is to test our immune evasion model by creating mutant forms of MNV locked into either the apo or activated states and observing how those changes affect the pathogenesis and immune response in the host. These studies will detail the activation process at the molecular level, elucidate the reason why caliciviruses evolved such mobile P domains, and greatly impact the development of norovirus vaccines and therapeutics. Finally, subsequent to our publications, similar host metabolite immune evasion was observed with COVID-19. Therefore, our results will increase awareness of possible similar features in other virus families and may become paradigms in the future.
