The role of lipid droplets (LDs) in innate immune pathways remains relatively undescribed, however, recent research by us and others suggests that LDs may underpin the outcome of several signalling pathways.
We can now demonstrate that LDs are transiently upregulated as a host repose to multiple viral infections, both in vitro and in vivo, with this upregulation of LDs driving an increase in type I and III interferons. To understand if LDs enhance interferon responses through a bystander effect in virally infected cells, we generated GFP and mCherry labelled LD cell lines. A co-culture of these two cell lines revealed for the first, that LDs were able to cross between cells. Live-time microscopy trafficking analysis of individual LDs revealed that during viral infection, LD velocity and distance travelled increased significantly as early as 2 hrs post-infection, and remained upregulated for 72 hrs. As LDs are known to interact with other organelles to transfer lipids and proteins, we developed techniques to isolate pure LDs from cells to analyse their lipidome and proteome before and following viral infection. Lipidomic analysis revealed significant changes within the lipid profile of LDs during infection, with an upregulation of long-chain fatty acids, and bioactive lipid mediator precursors. Significant changes were also seen in the LD proteome, with 92 proteins upregulated 24 hrs post-infection, including multiple antiviral proteins, and members of the early innate antiviral signaling pathways, demonstrating for the first time that the LD may act as a signaling platform during an effective antiviral response.
We believe that LDs play vital roles in facilitating the magnitude of the early antiviral immune response, in particular, the production of IFN following viral infection. Here, we characterise the changing lipidome and proteome of LDs during an early antiviral response for the first time. This data represents a paradigm shift in our understanding of the molecular mechanisms which coordinate an effective antiviral response by implicating LDs as a critical signaling organelle.