We have previously demonstrated that lipid droplets (LDs) are upregulated as a host response to early infection of multiple viruses (IAV, ZIKV, DENV and HSV-1), driving effective type I /III interferon responses. We have also discovered that important antiviral proteins localise to LDs, positioning them as important signalling platforms during this response. It’s not known however if LDs may also contribute to the antiviral bystander effect seen during viral infection.
To explore this, stable cell lines fluorescently expressing the LD resident protein; ADRP in both mCherry and GFP were generated to assess LD transfer between cells. Co-culture of the two fluorescently tagged LD cell lines revealed transfer of LDs between cells occurs in vitro. Isolated fluorescent LDs from these cell lines were also very frequently taken up into the cell cytoplasm of naïve cells, using a dynamin-dependent mechanism of entry. To understand if LDs contribute to a bystander effect during viral infection, fluorescent LDs isolated from cells that had activated innate immune responses (via dsRNA viral mimic stimulation) were placed onto naïve cells prior to ZIKV infection. This treatment of LDs significantly increased type I and III interferon responses resulting in a 50% decrease in ZIKV replication. Utilising proteomics and super-resolution microscopy we identified multiple ESCRT pathway proteins known to be involved in the secretion of extracellular vesicles (EVs) present on LDs. Additionally, using cell lines fluorescently expressing GFP-labelled EVs, and mCherry-labelled LDs, we were able to demonstrate that LDs can also egress from cells packaged inside EVs in vitro, providing an alternate secretion mechanism.
We have shown that LDs play vital roles in facilitating the magnitude of the early antiviral immune response, in particular the production of interferon following viral infection. To our knowledge, this study is the first to examine the extracellular role of LDs, placing them as contributors to the antiviral state of bystander cells. This data represents a paradigm shift in our understanding of the molecular mechanisms coordinating an effective antiviral response by implicating LDs as a critical organelle in this response.