Lipid droplets (LDs) are intracellular organelles that play vital roles in life-cycles of multiple viruses and have been shown to host multiple antiviral proteins, however, their role as a potential signaling platform in the enhancement of antiviral innate signaling pathways has not been explored. Recently, they have been demonstrated to accumulate in Dengue virus infected cells of the mosquito midgut, suggesting a role for LDs in development of mosquito immunity, however their role in the innate immune response in mammalian cells is uncharacterised.
To investigate the dynamics of LDs in mammalian cells, we utilised a range of epithelial and non-epithelial cell types, including both primary murine astrocytes, embryonic fibroblasts and primary human blood derived monocytes/macrophages. Both LD number and size was found to increase by up to 5-fold in response to viral mimics (dsRNA; Poly(I:C) and dsDNA; Poly(dA:dT)) as well as early Influenza A, and Zika virus infection in all cell types. LD induction, occurred as early as 2 hours post infection, was transient, and returned to basal levels by 72 hours post stimulation/infection. Upregulation of LDs was found to be type I interferon (IFN) independent, as the dynamics of induction remained unaltered in IFNAR-/- cells. LD induction was also absent following infection/stimulation with RNA viruses in cell lines lacking TLR3, indicating that LD Induction may also be independent of cytosolic RNA sensors. To evaluate the efficiency of the early innate immune response in cells with reduced LD content, we established a model of reduced cellular LDs. Lowered LD content did not impede the entry of Sendai virus nor dsRNA (Poly(I:C)) into cells, however significantly decreased the production of type I and III IFN, as well as the production of antiviral interferon stimulated genes (ISGs), in response to infection. Furthermore, enhancing the number of LDs with treatment of Oleic Acid prior to stimulation/infection significantly enhanced both type I and III IFN production as well as expression of downstream ISGs such as Viperin.
Here we show, for the first time, that LDs play vital roles in the magnitude of the early innate immune signaling, in particular the production of IFN following viral infection. Ongoing work is being performed to elucidate the host proteins involved in driving this phenomenon.