Project Summary Parasitic worms constitute a major global public health burden, with approximately one-fifth of the human population harboring one or more species within their gastrointestinal tract. We discovered that rare chemosensory epithelial cells called tuft cells sense parasites and initiate a type 2 immune response in the small intestine. Subsequent investigations into tuft cells at other barrier tissues throughout the body showed these cells respond to various microbial and environmental triggers. However, despite the massive density of microbes in the colon, the activation of tuft cells and their influence on type 2 immunity in the large intestine remains poorly understood. Several studies demonstrated key differences between tuft cells in the small intestine and colon, including distinct receptor profiles and divergent differentiation requirements. Recently, we found that commensal tritrichomonad protists in the microbiome are far more diverse than previously appreciated and stimulate colonic tuft cells and type 2 immunity independent of the tuft cell agonist, succinate. In Aim 1, we will examine the impact of tritrichomonad-tuft cell interactions on colonic immunity and determine the mechanisms colonic tuft cells use to initiate inflammation within the large intestine. Aim 2 will identify the novel tuft cell receptor(s) activated by tritrichomonads in the colon. Aim 3 will investigate the impact of colonic tuft cells on barrier function, epithelial protein secretion, and mucosal resistance to enteric pathogens. The results of this study will provide new insights into the microbial surveillance mechanisms of tuft cells in the colon and their influence on mucosal immunity. This knowledge will also reveal the potential for commensal protists to protect from enteric infections and provide a basis for using colonic tuft cells to treat inflammatory disorders in the large intestine.