C. elegans RIG-I-like receptor DRH-1 signals via CARDs to activate anti-viral immunity in intestinal cells

Upon sensing viral RNA, mammalian RIG-I-like receptors activate downstream signals using caspase activation and recruitment domains (CARDs), which ultimately promote transcriptional immune responses that have been well-studied. In contrast, the downstream signaling mechanisms for invertebrate RIG-I-like receptors are much less clear. For example, the Caenorhabditis elegans RIG-I-like receptor DRH-1 lacks annotated CARDs and upregulates the distinct output of RNA interference (RNAi). Here we found that, similar to mammal RIG-I-like receptors, DRH-1 signals through two tandem caspase activation and recruitment domains (2CARD) to induce a transcriptional immune response. Expression of DRH-1(2CARD) alone in the intestine was sufficient to induce immune gene expression, increase viral resistance, and promote thermotolerance, a phenotype previously associated with immune activation. We also found that DRH-1 is required in the intestine to induce immune gene expression, and we demonstrate subcellular colocalization of DRH-1 puncta with double-stranded RNA inside the cytoplasm of intestinal cells upon viral infection. Altogether, our results reveal mechanistic and spatial insights into anti-viral signaling in C. elegans, highlighting unexpected parallels in RIG-I-like receptor signaling between C. elegans and mammals. SignificanceViruses are ubiquitous pathogens that challenge diverse organisms, from bacteria to killer whales. While anti-viral defense has been well-studied in mammals, less is known about defense in invertebrates, including the roundworm Caenorhabditis elegans. Here we show that the C. elegans viral sensor DRH-1 shares similarities to a viral sensor in mammals called RIG-I. We find that DRH-1 has a signaling motif resembling the 2CARD motif, which is found in RIG-I and activates anti-viral immunity. We demonstrate that overexpression of DRH-1(2CARD) in C. elegans promotes resistance to viral infection, and that DRH-1 forms clusters inside intestinal cells during viral infection, similar to RIG-I in humans. Overall, these findings provide insights into C. elegans anti-viral immunity, highlighting similarities with mammalian anti-viral immunity.