Competition between commensal protists shapes gut mucosal immunity in mice

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 <i>Entamoeba muris</i>, and the parabasalid, <i>Tritrichomonas</i>, to determine how they shape gut immunity <i>in vivo</i> individually and together. Unlike the well-characterized inducer of type 2 immunity, <i>Tritrichomonas</i>, which activates the tuft cell-IL-25-ILC2 circuit in the small intestine, <i>E. muris</i> failed to elicit robust immune responses in the intestine or colon. However, the introduction of <i>E. muris</i> into mice naturally colonized by <i>Tritrichomonas</i> spp., or co-infection with <i>E. muris</i> and <i>Tritrichomonas</i> spp. reduced <i>Tritrichomonas</i>-induced type-2 response in the small intestine and <i>Tritrichomonas-</i>dependent immune activation in the colon. Our data suggest that <i>E. muris</i> may limit the abundance of <i>Tritrichomonas</i> 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 <i>Tritrichomonas</i> spp., which have not previously been reported. Taken together, these findings reveal that the presence of <i>E. muris</i> reduces <i>Tritrichomonas</i>-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.<h4>Importance</h4>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, <i>Entamoeba</i> and <i>Tritrichomonas</i>, to determine how they shape gut immunity individually and together. Unlike the well-characterized inducer of type 2 immunity, <i>Tritrichomonas</i>, which activates the tuft cell circuit, <i>Entamoeba</i> failed to elicit a robust immune response. The introduction of <i>Entamoeba</i> into mice naturally colonized by <i>Tritrichomonas</i>, or co-infection with Entamoeba and Tritrichomonas, reduced the <i>Tritrichomonas</i>-induced immune response. Our data suggest that <i>Entamoeba</i> limits the abundance of <i>Tritrichomonas</i>, correlating with diminished tuft cell activation. We also identified sex-specific differences in the intestinal response to <i>Tritrichomonas</i>. These findings show that <i>Entamoeba</i> reduces <i>Tritrichomonas</i>-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.