Competition between commensal protists shapes gut mucosal immunity in mice

ABSTRACT Intestinal protists are emerging as key modulators of host immunity and microbial ecology, yet their impact on mammalian hosts remains poorly defined. Here, we investigated the role of two distinct protists, the amoeba Entamoeba muris , and the parabasalid, Tritrichomonas , to determine how they shape gut immunity in vivo individually and together. Unlike the well-characterized inducer of type 2 immunity, Tritrichomonas , which activates the tuft cell–IL-25–ILC2 circuit in the small intestine, E. muris failed to elicit robust immune responses in the intestine or colon. However, the introduction of E. muris into mice naturally colonized by Tritrichomonas spp., or co-infection with E. muris and Tritrichomonas spp. reduced Tritrichomonas -induced type-2 response in the small intestine and Tritrichomonas- dependent immune activation in the colon. Our data suggest that E. muris may limit the abundance of Tritrichomonas spp., with reduced protist loads in the cecum specifically correlating with diminished tuft cell activation. We also identified sex-specific differences in the intestinal response to Tritrichomonas spp., which have not previously been reported. Taken together, these findings reveal that the presence of E. muris reduces Tritrichomonas -dependent activation of type 2 immunity in the small intestine, even when both protists can be detected in the cecum, without triggering overt inflammation. This work provides a framework for understanding how protists interact within the gut ecosystem and shape mucosal immunity in the absence of pathogenicity. IMPORTANCE Single-cell parasites called protists are common in mammalian intestinal tracts, yet their modulation of the host immune response and interactions with each other remain poorly defined. Here, we investigated the role of two protists, Entamoeba and Tritrichomonas , to determine how they shape gut immunity individually and together. Unlike the well-characterized inducer of type 2 immunity, Tritrichomonas , which activates the tuft cell circuit, Entamoeba failed to elicit a robust immune response. The introduction of Entamoeba into mice naturally colonized by Tritrichomonas , or co-infection with Entamoeba and Tritrichomonas, reduced the Tritrichomonas -induced immune response. Our data suggest that Entamoeba limits the abundance of Tritrichomonas , correlating with diminished tuft cell activation. We also identified sex-specific differences in the intestinal response to Tritrichomonas . These findings show that Entamoeba reduces Tritrichomonas -dependent activation of type 2 immunity without triggering much inflammation. It helps our understanding of how protists interact within the gut and shape immunity without disease.