Emerging viral pathogens present a critical threat to U.S.health and economy. Nipah (NiV) and Hendra (HeV) viruses are members of the newly defined Henipavirus genus of the Paramyxoviridae. Nipah virus (NiV) is an emergent paramyxovirus that causes fatal encephalitis in up to 70% of infected patients, and there is increasing evidence of human-to-human transmission. NiV is designated a priority pathogen in the NIAID Biodefense Research Agenda, and could be a devastating agent of agrobioterrorism if used against the pig farming industry. Endothelial syncytia is a pathognomonic feature of NiV infections, and is mediated by the fusion (F) and attachment (G) envelope glycoproteins. Identification of the NiV receptor will shed light on the pathobiology of NiV infection, and spur the rational development of effective therapeutics. In our preliminary results, we show that ephrinB2, the membrane bound ligand for the ephB class of receptor tyrosine kinases (RTKs), specifically bound to the attachment (G) glycoprotein of NiV.Soluble Fc-fusion proteins of ephrinB2 but not ephrinBI effectively blocked NiV fusion and entry. Transfection of ephrinB2 into non-permissive cells rendered them permissive for NiV fusion and entry. EphrinB2 is expressed on endothelial cells and neurons, consistent with the known cellular tropism for NiV. Significantly, NiV envelope mediated infection of microvascular endothelial cells, and primary cortical rat neurons, was inhibited by soluble ephrinB2, but not the related ephrinBI protein. Cumulatively, our data show that ephrinB2 is a functional receptor for NiV.We also show that ephrinB3, a related protein, can serve as an alternative receptor;differential usage of ephrinB2 versus B3 may explain the variant pathogenic profiles observed between NiV and HeV. Identifying the NiV receptor opens the door for a more comprehensive analysis of the NiV envelope-receptor interactions. We propose the following Specific Aims to increase our understanding of NiV pathobiology, and facilitate the development of anti-NiV vaccines and therapeutics. They are: (1) Identify cognate domains and/or residues in ephrinB2/B3 that mediate interactions with NiV-G and HeV-G, (2) Characterize the minimal receptor binding domain in NiV-G and HeV-G, and identify the critical residues involved in ephrin receptor interactions, (3) Use small molecule antagonists that block NiV- G's interaction with ephrinB2 to probe the receptor binding site in NiV-G and HeV-G, and (4) Investigate the properties of novel rabbit monoclonal antibodies against NiV-G. Public Health Relevance: Nipah and Hendra viruses are designated priority pathogens, are deadly, and can be devasting agents of bioterrorism and agroterrorism (devastation of the live-stock industry). Identifying the virus receptor allows us to better study how the virus gets into cells. These investigations are crucial to the development of effective anti- Nipah vaccines and therapeutics.