The ability of a virus to productively replicate in a particular host is in part governed by the availability of host cell factors that support viral entry and replication1. The virus-host range can also be dictated by the capacity of a virus to counter cellular antiviral defenses that would otherwise impede viral replication2,3. It is the outcome of these complex virus-host interactions that ultimately decide whether a virus will be able to complete its life cycle in a particular host. Given the potentially pathogenic consequences for the host if viral replication ensues, it is critical to develop experimental strategies to further our understanding of the key virus-host interactions that may tip the balance between abortive and productive infections. Elucidating the molecular features of virus-host interplay will be instrumental in the development of new and alternative antiviral therapeutic strategies.
With the advent of RNA interference (RNAi)4,5 and genome-editing tools (e.g., CRISPR-Cas9, Zinc finger nucleases, TALENs)6,7, it has become experimentally feasible to alter the expression of cellular factors on genome-wide scales and explore the impact of these alterations on virus replication. Indeed, numerous RNAi and genome-editing-based screens have been conducted in invertebrate and vertebrate host cell types that have unveiled new facets of virus-host interactions8,9,10,11,12. These screens typically employ viruses encoding reporters, such as firefly luciferase (LUC) or fluorescent proteins (e.g., GFP, DsRed), that provide convenient means of quantitatively assessing viral gene expression as a readout for viral replication9,12. This strategy allows researchers to identify host factors that either promote or antagonize viral replication as evidenced by increases or decreases, respectively, in viral reporter signals9,12. However, in the vast majority of cases, these screens have been conducted using viruses that are well-adapted to the host cell type in which they are being studied. While this strategy can be important for understanding coevolutionary relationships between viral pathogens and their natural hosts, it does pose fundamental concerns regarding their use in uncovering host antiviral factors. In these cases, an enhancement in virus reporter signal upon RNAi knockdown is being looked for, or the inactivation of a cellular factor that normally impedes viral replication. First, if a virus is already able to robustly replicate in the host cell being examined under control conditions, the dynamic range of the screen (i.e., the ability to distinguish between background and enhanced viral reporter signals) may be limited. Second, this issue is further compounded by the situations in which the virus is well-adapted to the host cell and effective at countering host defense pathways that are being targeted in the screen.
Due to the above concerns regarding traditional virus-host interaction screening methods, we developed a new paradigm for studying virus-host interactions that exploit naturally abortive arbovirus infections in lepidopteran insect cells. This strategy derives from an observation that the well-studied human arbovirus, VSV, undergoes an abortive infection in cells derived from the gypsy moth (L. dispar)13. VSV is naturally transmitted by dipteran insects (i.e., sand flies) to mammalian hosts, and has been shown experimentally to infect a wide range of invertebrate and vertebrate hosts both in cell culture and in vivo14. The 11-kb negative-sense single-stranded RNA genome of VSV encodes five subgenomic mRNAs that are each translated into the proteins that make up the enveloped virion. However, VSV reverse genetic systems have allowed for the creation of replication-competent strains encoding LUC or fluorescent proteins, in addition to the five natural VSV gene products15,16,17. Because these reporter proteins are not incorporated into the VSV virion, they provide a convenient readout for VSV gene expression that occurs post-entry. Using VSV strains encoding GFP or LUC, we have previously shown that VSV gene expression is severely restricted upon the entry of LD652 cells and that VSV titers do not increase by 72 hours post-infection (hpi). In contrast, the coinfection of LD652 cells with VSV and the mammalian poxvirus, vaccinia virus (VACV), leads to logarithmic increases in both VSV gene expression and titers by this time point. VACV undergoes early gene expression, DNA replication, and late gene expression in LD652 cell infections, but the VACV replication cycle is ultimately abortive due to incomplete virion morphogenesis18. The large ~192-kb DNA genome of VACV encodes > 200 proteins, many of which display immunomodulatory properties that promote viral replication through the suppression of host immune responses19. Therefore, we hypothesized that the "rescue" of VSV replication in LD652 cells by VACV coinfection was likely mediated by VACV immunomodulators that inhibited L. dispar responses normally restricting VSV replication. In support of this, the treatment of LD652 cells with the host RNA polymerase II inhibitor actinomycin D also rescues VSV replication in LD652 cells, indicating that the transcription-dependent host responses block VSV replication post-entry13.
The above observations suggest that the naturally restrictive nature of LD652 cells to VSV infection may provide a relatively low background when screening for the conditions that enhance VSV-encoded reporter signals (i.e., those that inhibit host antiviral defenses). Here, we provide the methods for using fluorescence or LUC-based assays to screen for conditions that relieve VSV restriction in lepidopteran cells. First, we show how these assays can be used to identify virally encoded immunomodulatory factors that break VSV restriction during either coinfection experiments or through ectopic expression of candidate viral factors. As an example, we illustrate how we used these screening techniques to identify poxvirus-encoded A51R proteins as a new family of immunomodulatory factors that rescue VSV replication in the absence of other poxvirus factors13. Second, we illustrate how RNAi screening in restrictive VSV-LD652 cell infections can be used to directly identify eukaryotic host factors involved in arbovirus restriction13.