--Paneth cell niche function is critical for intestinal stem cells in radiation injury but dispensable in GVHD

Paneth cells (PCs) produce WNT3 and EGF in the small intestine and are thought to contribute to the intestinal stem cell (ISC) niche, which is a target of graft vs. host disease (GVHD). However, PC deletion models have failed to demonstrate substantial consequences for ISCs or other epithelial pathology. The physiologic role of PC niche function has thus remained opaque, and the significance of this function after bone marrow transplantation (BMT) is poorly understood. Using Lysozyme1-DTR mice to ablate PCs, we found stable WNT3 expression in vivo, and crypts from PC-ablated mice retained organoid-forming capacity, further reflecting persistent WNT3 production. However, PC ablation reduced EGF expression and increased intestinal radiation injury. Irradiated PC-deficient mice exhibited attenuated epithelial regeneration and exacerbated ISC and crypt loss, which were restored by exogenous EGF administration, thus indicating EGF-dependent PC contributions in radiation injury. In contrast, after allogeneic BMT, PC ablation and the associated EGF reduction had no substantial impact on intestinal pathology or systemic GVHD. WNT3 levels were comparable between PC-intact and PC-deficient BMT recipients, and other secretory cells emerged as potential alternative WNT3 sources. Furthermore, human duodenal biopsies from transplant patients demonstrated persistence of goblet and enteroendocrine cells in clinical GVHD, despite PC reductions, resulting in overall stable secretory lineage frequencies. These findings highlight an essential role for PC niche function as a nonredundant source of EGF in radiation injury. However, this function is dispensable in experimental GVHD. PC reductions in GVHD may thus reflect injury severity more than they contribute to it.