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Upon sensing viral RNA, mammalian RIG-I-like receptors (RLRs) 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 RLRs are much less clear. For example, theCaenorhabditis elegansRLR DRH-1 lacks annotated CARDs and up-regulates the distinct output of RNA interference. Here, we found that similar to mammal RLRs, DRH-1 signals through two tandem CARDs (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 inC. elegans. 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 antiviral signaling inC. elegans, highlighting unexpected parallels in RLR signaling betweenC. elegansand mammals. 

Abstract Orsay virus infection in the nematodeCaenorhabditis eleganspresents an opportunity to study host‐virus interactions in an easily culturable, whole‐animal host. Previously, a major limitation ofC. elegansas a model for studying antiviral immunity was the lack of viruses known to naturally infect the worm. With the 2011 discovery of the Orsay virus, a naturally occurring viral pathogen,C. eleganshas emerged as a compelling model for research on antiviral defense. From the perspective of the host, the genetic tractability ofC. elegansenables mechanistic studies of antiviral immunity while the transparency of this animal allows for the observation of subcellular processes in vivo. Preparing infective virus filtrate and performing infections can be achieved with relative ease in a laboratory setting. Moreover, several tools are available to measure the outcome of infection. Here, we describe workflows for generating infective virus filtrate, achieving reproducible infection ofC. elegans, and assessing the outcome of viral infection using molecular biology approaches and immunofluorescence. © 2024 The Authors. Current Protocols published by Wiley Periodicals LLC. Basic Protocol 1: Preparation of Orsay virus filtrate Support Protocol: SynchronizeC. elegansdevelopment by bleaching Basic Protocol 2: Orsay virus infection Basic Protocol 3: Quantification of Orsay virus RNA1/RNA2 transcript levels by qRT‐PCR Basic Protocol 4: Quantification of infection rate and fluorescence in situ hybridization (FISH) fluorescence intensity Basic Protocol 5: Immunofluorescent labeling of dsRNA in virus‐infected intestinal tissue 

Abstract Although the type‐I interferon (IFN‐I) response is considered vertebrate‐specific, recent findings about the Intracellular Pathogen Response (IPR) in nematodeCaenorhabditis elegansindicate that there are similarities between these two transcriptional immunological programs. The IPR is induced during infection with natural intracellular fungal and viral pathogens of the intestine and promotes resistance against these pathogens. Similarly, the IFN‐I response is induced by viruses and other intracellular pathogens and promotes resistance against infection. Whether the IPR and the IFN‐I response evolved in a divergent or convergent manner is an unanswered and exciting question, which could be addressed by further studies of immunity against intracellular pathogens inC. elegansand other simple host organisms. Here we highlight similar roles played by RIG‐I‐like receptors, purine metabolism enzymes, proteotoxic stressors, and transcription factors to induce the IPR and IFN‐I response, as well as the similar consequences of these defense programs on organismal development.