PROJECT SUMMARY The human intestine is densely innervated, and it is becoming increasingly clear that the enteric nervous system plays a key role in maintaining immune homeostasis. However, disentangling the contribution of individual sensory neurons in the regulation of intestinal inflammation has been challenging, owing to the marked complexity of both neural-immune interactions and the microbial communities in the intestine. We made a surprising discovery that olfactory receptor development is linked to innate immune regulation in the intestine. From a forward genetic screen for regulators of host immune defenses in the nematode C. elegans, we found that loss-of-function mutations in the olfactory neuron gene olrn-1 caused aberrant immune hyperactivation in the intestinal epithelium. During nematode development, olrn-1 is required to program the expression of neuronal receptors in one particular type of sensory neuron, the amphid wing C (AWC) neurons, which allows odor discrimination and chemotaxis. During nematode development, low activity of OLRN-1 induces p38 signaling within AWC neurons themselves to promote olfactory receptor expression. We demonstrated that low OLRN-1 activity also de-represses the p38 PMK-1 innate immune pathway in the intestine to promote immune effector transcription, increased clearance of an intestinal pathogen, and resistance to bacterial infection. However, unchecked immune activation in olrn-1 mutants is toxic to nematode development, a physiological process that our laboratory, and others, have previously shown is particularly vulnerable to perturbations in intestinal immune homeostasis. These data suggest that OLRN-1-expressing sensory neurons in C. elegans optimize the degree of p38 PMK-1 immune activation, which is essential both to handle challenges from bacterial pathogens and to prevent the deleterious consequences of unchecked immune activation. However, it is not known how OLRN-1 in AWC sensory neurons communicates with the p38 PMK-1 pathway in the intestine to promote immune homeostasis. The central hypothesis of this proposal is that AWC olfactory neurons promote immune homeostasis through the neuroendocrine-mediated suppression of the intestinal p38 PMK-1 innate immune pathway. In support of our central hypothesis, the forward genetic screen for novel immune regulators, which uncovered olrn-1 mutants, also identified a loss-of-function mutant allele in an intestinal neuropeptide G protein-coupled receptor. We propose that this receptor and a cognate neuropeptide hormone act downstream of AWC sensory neurons to regulate anti-pathogen responses in C. elegans. These studies will characterize the genetic mechanisms that link sensory neuron activity to the regulation of immunity in the intestinal epithelium. We expect that these insights will reveal ancestral strategies of healthy immune control and provide fundamental insights into intestinal immune homeostasis that may be directly applicable to mammalia...