The gamma delta IEL effector API5 masks genetic susceptibility to Paneth cell death

Loss of Paneth cells and their antimicrobial granules compromisesthe intestinal epithelial barrier and is associated with Crohn's disease, a major type of inflammatory bowel disease(1-)(7). Non-classical lymphoid cells, broadly referred to as intraepithelial lymphocytes (IELs), intercalate the intestinal epithelium(8,9). This anatomical position has implicated them as first-line defenders in resistance to infections, but their role in inflammatory disease pathogenesis requires clarification. The identification of mediatorsthat coordinate crosstalk between specific IEL and epithelial subsets could provide insight into intestinal barrier mechanisms in health and disease. Here we showthat the subset of IELsthat express gamma and delta T cell receptor subunits (gamma delta IELs) promotesthe viability of Paneth cells deficient in the Crohn's disease susceptibility gene ATG16L1. Using an ex vivo lymphocyte-epithelium co-culture system, we identified apoptosis inhibitor 5 (API5) as a Paneth cell-protective factor secreted by gamma delta IELs. In the Atg16l1-mutant mouse model, viral infection induced a loss of Paneth cells and enhanced susceptibility to intestinal injury by inhibiting the secretion of APIS from gamma delta IELs. Therapeutic administration of recombinant API5 protected Paneth cells in vivo in mice and ex vivo in human organoids with the ATG16L1 risk allele. Thus, we identify API5 as a protective gamma delta IEL effector that masks genetic susceptibility to Paneth cell death.