Efficient reverse genetics reveals genetic determinants of budding and fusogenic differences between Nipah and Hendra viruses and enables real-time monitoring of viral spread in small animal models of henipavirus infection

Nipah virus (NiV) and Hendra virus (HeV) are closely related henipaviruses of the Paramyxovirinae. Spillover from their fruit bat reservoirs can cause severe disease in humans and livestock. Despite their high sequence similarity, NiV and HeV exhibit apparent differences in receptor and tissue tropism, envelope-mediated fusogenicity, replicative fitness, and other pathophysiologic manifestations. To investigate the molecular basis for these differences, we first established a highly efficient reverse genetics system that increased rescue titers by ≥3 log units, which offset the difficulty of generating multiple recombinants under constraining biosafety level 4 (BSL-4) conditions. We then replaced, singly and in combination, the matrix (M), fusion (F), and attachment glycoprotein (G) genes in mCherry-expressing recombinant NiV (rNiV) with their HeV counterparts. These chimeric but isogenic rNiVs replicated well in primary human endothelial and neuronal cells, indicating efficient heterotypic complementation. The determinants of budding efficiency, fusogenicity, and replicative fitness were dissociable: HeV-M budded more efficiently than NiV-M, accounting for the higher replicative titers of HeV-M-bearing chimeras at early times, while the enhanced fusogenicity of NiV-G-bearing chimeras did not correlate with increased replicative fitness. Furthermore, to facilitate spatiotemporal studies on henipavirus pathogenesis, we generated a firefly luciferase-expressing NiV and monitored virus replication and spread in infected interferon alpha/beta receptor knockout mice via bioluminescence imaging. While intraperitoneal inoculation resulted in neuroinvasion following systemic spread and replication in the respiratory tract, intranasal inoculation resulted in confined spread to regions corresponding to olfactory bulbs and salivary glands before subsequent neuroinvasion. This optimized henipavirus reverse genetics system will facilitate future investigations into the growing numbers of novel henipaviruslike viruses.