The combined use of alphavirus replicons and pseudoinfectious particles for the discovery of antivirals derived from natural products.

Alphaviruses are a prominent class of reemergent pathogens due to their globally expanding ranges, potential for lethality, and possible use as bioweapons. The absence of effective treatments for alphaviruses highlights the need for innovative strategies to identify antiviral agents. Primary screens that use noninfectious self-replicating RNAs, termed replicons, have been used to identify potential antiviral compounds for alphaviruses. Only inhibitors of viral genome replication, however, will be identified using replicons, which excludes many other druggable steps in the viral life cycle. To address this limitation, we developed a western equine encephalitis virus pseudoinfectious particle system that reproduces several crucial viral life cycle steps in addition to genome replication. We used this system to screen a library containing ~26,000 extracts derived from marine microbes, and we identified multiple bacterial strains that produce compounds with potential antiviral activity. We subsequently used pseudoinfectious particle and replicon assays in parallel to counterscreen candidate extracts, and followed antiviral activity during biochemical fractionation and purification to differentiate between inhibitors of viral entry and genome replication. This novel process led to the isolation of a known alphavirus entry inhibitor, bafilomycin, thereby validating the approach for the screening and identification of potential antiviral compounds.