Human parainfluenza virus (PIV) and metapneumovirus (MPV) are common causes of lower respiratory tract infections in infants and young children, and major causes of respiratory illness in immune compromised adults and the elderly. Unfortunately, there are no therapy or vaccine for either viruses, and only supportive medical care is available. Interestingly, PIV and MPV share many pathological and clinical manifestations as Respiratory Syncytial Virus (RSV). Indeed, neither PIV nor MPV are known to cause viremia in the blood of infected patients, indicating both infections are strictly localized in the airways similar to RSV that shed progeny viruses exclusively from the apical surface of infected cells. Such unique pathophysiology implies progeny viruses must traverse airway mucus (AM) before spreading to neighboring cells. This in turn motivated us to develop a pathogen-specific antiviral that could physically limit the spread of the infections within the airways. We recently discovered a novel Ab effector function in mucus – trapping individual pathogens in mucus based on carefully-tuned affinity between IgG-Fc and mucins – and developed a platform for enhancing mAb function at mucosal surfaces. We hypothesize that “muco-trapping” mAb delivered to the airways can directly intervene with the viral life cycle by intercepting and trapping shed progeny viruses in AM, rapidly eliminate trapped viruses from the airways by natural mucociliary clearance, and enable effective therapy in vivo. In support of this strategy, we engineered mAbs that potently trap RSV in AM, and showed that nebulized delivery of “muco-trapping” mAb to RSV-infected neonatal lambs beginning on Day 3 post infection effectively reduced the infectious viral load in lung tissues to non-detectible levels within 3 days, with viral RNA in bronchial tissues reduced by 11-fold compared to vehicle control. These results motivated us to explore whether a similar approach may be effective in treating PIV and MPV infections. In support of this application, we have engineered antibodies with picomolar affinity to diverse strains of both viruses, and demonstrated that we could effectively trap viruses in human AM and limit spread of infection in vitro in well-differentiated human airway epithelium (WD-HAE) grown at the air-liquid interface. In this proposal, we will continue our work with affinity maturation using mammalian and yeast display to produce high affinity mAb that broadly bind and neutralize diverse strains of PIV and MPV (Aim 1). We will validate whether these mAbs can trap PIV and MPV in fresh undiluted human AM, and whether they can inhibit the spread of pre-established PIV and MPV infections in WD-HAE cultures (Aim 2). We will then advance the lead mAb for both viruses for evaluation in a hamster nasal infection model (Aim 3). By enabling enhanced mAb function in mucus secretions, we expect we will help pave the way for improved, molecularly-targeted therapies and prophylaxis...