Neuroimmune Regulation in Caenorhabditis elegans: The Roles of the Amphid Sensory Neurons and Olfaction in Host Immune Responses

Abstract

The ability of hosts to rapidly sense invading pathogens and mount an appropriate response is paramount for host survival. Pathogens, pervasive in the natural world, present a perpetual threat to host organisms. Examining how hosts like the model organism Caenorhabditis elegans navigate their pathogen-laden environments offers valuable insights into host-pathogen interactions and fundamental host defence mechanisms. The free-living, bacterivorous nematode C. elegans forages for food around decaying vegetation, upon which it encounters a myriad of natural pathogens, including Gram-positive bacteria, Gram-negative bacteria, fungi, microsporidia, and viruses. To overcome such encounters, the worm is only equipped with an innate immune system comprising evolutionarily conserved signalling cascades. However, unlike higher-order animals, the worm lacks specialised immune cells and most conventional pathogen recognition receptors (PRRs). Nevertheless, it is well-established that the worm recognises pathogens and mounts largely pathogen-specific responses, inspiring researchers to search for non-canonical PRRs in C. elegans.

With increasing evidence suggesting immunomodulatory roles for neuronal components, we decided to investigate the roles of the amphid sensory neurons in immune regulation. The amphid sensilla, comprising twelve pairs of neurons in the head, forms the largest chemosensory organ in the worm. These neurons perform critical functions such as thermosensation, chemosensation, osmosensation, olfaction, pheromone-sensation, and lifespan modulation. The ciliary projections of eight of the twelve amphid neuron pairs are directly exposed to the environment and are involved in sensing environmental cues, making them the primary site of sensory perception and integration. Utilising a genetic ablation approach using split caspases, we genetically ablated individual amphid neuron pairs and subjected these neuron-ablated worms to infection with three major classes of pathogens, namely—a Gram-negative bacterium Pseudomonas aeruginosa PA14, a Gram-positive bacterium Enterococcus faecalis OG1RF, and a pathogenic yeast Cryptococcus neoformans H99α. We looked at the survival of these worms on pathogenic lawns as a proxy for immune responses. We demonstrate that some amphid neurons broadly suppress host survival and the colonisation of all tested pathogens, whereas others differentially regulate host immunity. Interestingly, we also identified neurons that played pathogen-specific roles. For example, the chemosensory neurons ASG and ASK broadly suppress immune responses against all tested pathogens, the nociceptive neuron ASH was essential for survival on all pathogens, and the olfactory neuron AWA suppressed immune responses against E. faecalis while promoting survival against C. neoformans. Interestingly, we identified significant roles for odour sensory neurons in host immunity and survival on pathogens.

Finally, we examined the roles of olfaction in pathogen recognition and immune modulation. In C. elegans, the AWA and AWC olfactory neurons detect odours that are attractive, while the AWB neurons detect repellents. The olfactory system in C. elegans has been primarily believed to be involved in foraging by sensing food cues. However, a recent report from our lab demonstrated an alternate role for the olfactory neurons, in particular, the AWB neurons. This neuron pair senses 1-undecene, an odour produced by P. aeruginosa PA14, and elicits a flight and fight response. Thus, 1-undecene acts as a non-canonical pathogen-associated molecular pattern (PAMP) and its cognate receptor in the AWB acts as a non-canonical PRR. Given that this neuron is also involved in detecting repellent odours produced by other pathogenic bacteria, we hypothesised that AWB, through its array of odour receptors, might act as a PRR and thus modulate immune responses in the worm. To test this, we used a genetic ablation line of AWB neurons and subjected them to infection with an array of pathogens. These neuron-ablated worms were more susceptible to infection than their wild-type counterparts, suggesting critical roles for the AWB neuron during infection. Further, using histamine-mediated silencing, we show that the AWB neuronal function is required early on during infection. Moreover, we demonstrate that the mere activation of this neuron through specific pathogen-associated odour cues primes host survival during infection with an array of pathogens. Through transcriptomics and computational tools, we identified a set of immunity-related candidate genes regulated by the neuron. Interestingly, a majority of these genes encoded detoxification enzymes. Furthermore, using gene knockdown studies, we validated the roles of the identified genes in regulating host immunity and survival during infection. In particular, the knockdown of ugt-18, one of the hits identified through transcriptomics, rendered the population half-life (TD50) to 20% of that of the wild-type. UDP-glucuronosyl transferases (UGTs) are detoxification enzymes that glycosylate non-polar, xenobiotic toxins, facilitating their excretion and further catalysis and detoxification by other detoxification enzymes. In this study, we show that ugt-18 is involved in the detoxification of phenazines, a secondary metabolite of P. aeruginosa, and is regulated by the AWB neuron. Furthermore, we show that ugt-18 also plays vital roles during infection with other pathogens, even those that do not produce phenazines, suggesting a promiscuous role for ugt-18 in detoxification and survival on pathogens.

In conclusion, we show that the amphid sensilla of C. elegans has differential roles during infection with different classes of pathogens. Notably, our screen identified five neuronal pairs as broad suppressors of immunity and three pairs as promotors of immunity against the three major classes of pathogens. Furthermore, we demonstrate that the AWB odour sensory neurons prime immune responses upon activation through pathogenic odours, thus elucidating hitherto unknown roles for olfaction in pathogen sensation and immune modulation.