Toll-like Receptor Signaling Promotes Development and Function of Sensory Neurons Required for a C. elegans Pathogen-Avoidance Behavior

Toll-like receptors (TLRs) play critical roles in innate immunity in many animal species. The sole TLR of C. elegans-TOL-1-is required for a pathogen-avoidance behavior, yet how it promotes this behavior is unknown. We show that for pathogen avoidance TOL-1 signaling is required in the chemosensory BAG neurons, where it regulates gene expression and is necessary for their chemosensory function. Genetic studies revealed that TOL-1 acts together with many conserved components of TLR signaling. BAG neurons are activated by carbon dioxide (CO2), and we found that this modality is required for pathogen avoidance. TLR signaling can therefore mediate host responses to microbes through an unexpected mechanism: by promoting the development and function of chemosensory neurons that surveil the metabolic activity of environmental microbes.