Recombinant Lloviu virus as a tool to study viral replication and host responses

Next generation sequencing has revealed the presence of numerous RNA viruses in animal reservoir hosts, including many closely related to known human pathogens. Despite their zoonotic potential, most of these viruses remain understudied due to not yet being cultured. While reverse genetic systems can facilitate virus rescue, this is often hindered by missing viral genome ends. A prime example is Lloviu virus (LLOV), an uncultured filovirus that is closely related to the highly pathogenic Ebola virus. Using minigenome systems, we complemented the missing LLOV genomic ends and identified cis-acting elements required for LLOV replication that were lacking in the published sequence. We leveraged these data to generate recombinant full-length LLOV clones and rescue infectious virus. Similar to other filoviruses, recombinant LLOV (rLLOV) forms filamentous virions and induces the formation of characteristic inclusions in the cytoplasm of the infected cells, as shown by electron microscopy. Known target cells of Ebola virus, including macrophages and hepatocytes, are permissive to rLLOV infection, suggesting that humans could be potential hosts. However, inflammatory responses in human macrophages, a hallmark of Ebola virus disease, are not induced by rLLOV. Additional tropism testing identified pneumocytes as capable of robust rLLOV and Ebola virus infection. We also used rLLOV to test antivirals targeting multiple facets of the replication cycle. Rescue of uncultured viruses of pathogenic concern represents a valuable tool in our arsenal for pandemic preparedness.

Due to increasing utilization of high-throughput sequencing technologies, RNA sequences of many unknown viruses have been discovered in bats and other animal species. Research on the pathogenic potential of these viruses is hampered by incomplete viral genome sequences and difficulties in isolating infectious virus from the animal hosts. One example of these potentially zoonotic pathogens is Lloviu virus (LLOV), a filovirus which is closely related to Ebola virus. Here we applied molecular virological approaches, including minigenome assays, to complement the incomplete LLOV genome ends with sequences from related viruses and identify cis-acting elements required for LLOV replication and transcription that were missing in the published LLOV sequence. The resulting full-length clones were used to generate infectious recombinant LLOV. We used this virus for electron microscopic analyses, infection studies in human cells, host response analysis, and antiviral drug testing. Our results provide new insights into the pathogenic potential of LLOV and delineate a roadmap for studying uncultured viruses.