Evolution and pathogenesis of Usutu virus, an emerging arbovirus

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Project Summary West Nile virus (WNV) and Usutu virus (USUV) are closely-related mosquito-borne viruses that cause neuroinvasive disease in humans. WNV emerged from African and Europe into the U.S. in 1999, and it is now the most common mosquito-borne disease in the continental U.S.; however, no vaccines or therapeutics are available. USUV is emerging in Europe, where it has been introduced at least three times from Africa by migratory birds, and human disease cases are increasing. The viral factors that lead to increases in flavivirus pathogenesis include naturally-occurring viral genetic mutations and cross-reactive antibodies from co- circulating heterologous flaviviruses. We have found that African and European strains of USUV differ drastically in the level of disease generated in a wild-type mouse model, with differences in viremia of up to 100-fold between USUV strains. We have also found that human WNV convalescent sera partially cross- neutralizes USUV in vitro. The long-term goal of this project is to understand the effect of viral evolution and antibody cross-reactivity on flavivirus emergence and disease. The objectives of this study are to identify the genetic mutations in USUV that dictate differences in pathogenesis and to determine whether WNV cross- reactive antibodies alter USUV disease. The hypothesis is that viral mutations acquired during emergence increase USUV pathogenesis and that heterologous WNV antibodies decrease USUV pathogenesis. Two specific aims will address this hypothesis: 1) Identify viral genetic determinants of USUV disease; and 2) Determine the impact of WNV antibodies on USUV disease. In the first aim, a reverse genetics system that we recently developed will be used to identify mutations in USUV that alter viremia and disease in mice. In the second aim, the effect of IgG purified from human WNV convalescent sera on USUV disease will be evaluated in vivo. The research proposed here is innovative because it investigates significant differences in pathogenesis caused by contemporary strains of an emerging, neglected virus using a novel reverse genetics system. Upon successful completion of the proposed research, the anticipated contribution of this work will be the identification of viral factors that affect USUV pathogenesis and host immune correlates of USUV disease. This contribution is expected to be significant because it will lead to the ability to develop therapeutics and vaccines to reduce flavivirus transmission and disease.