Project Summary In this project, the Jardetzky and Crowe laboratories will collaborate to investigate the neutralizing antibody response to respiratory syncytial virus (RSV) and human metapneumovirus (HMPV), using complementary structural, biochemical and immunological approaches. RSV and HMPV are respiratory viruses that cause widespread morbidity within the human population second only to influenza virus. RSV is the most common cause of bronchiolitis and pneumonia in infants worldwide. The virus also impacts the elderly and others with weakened immune systems. HMPV was identified in 2001 and has been causing respiratory illnesses in the human population for over 50 years. Similar to RSV, HMPV infections are associated with a significant burden of hospitalizations and clinic visits in young children and in the elderly. Despite limited antigenic variation, RSV and HMPV cause repeated infections throughout life and are associated with a lack of induction of durable immunity. A clear explanation for this ability of the viruses to reinfect previously exposed individuals is still lacking, although multiple hypotheses have been developed. The RSV and HMPV fusion (F) proteins are the major targets of neutralizing antibodies (nAbs), and nAbs targeting the prefusion conformation of the RSV F protein have been shown to be the most potent inhibitors of viral entry. However, our understanding of the antibody response to HMPV is much more limited. In this proposal, the Jardetzky and Crowe laboratories will address outstanding issues in RSV and HMPV antibody immunity by studying monoclonal and repertoire- based B cell responses to both viruses, conducting functional and structural mapping of antibody epitopes and studying the sequence determinants of HMPV F conformational stabilization. !