Microbial-Dependent Recruitment of Immature Myeloid Cells Promotes Intestinal Regeneration

The intestinal epithelium functions both in nutrient absorption and as a barrier, separating the luminal contents from a network of vascular, fibroblastic, and immune cells underneath. After injury to the intestine, multiple cell populations cooperate to drive regeneration of the mucosal barrier, including lymphatic endothelial cells (LECs). A population of granulocytic immature myeloid cells (IMCs), marked by Hdc, participate in regeneration of multiple organs such as the colon and central nervous system, and their contribution to intestinal regeneration was investigated.

By using male and female histidine decarboxylase (Hdc) green fluorescent reporter (GFP) mice, we investigated the role of Hdc⁺ IMCs in intestinal regeneration after exposure to 12 Gy whole-body irradiation. The movement of IMCs was analyzed using flow cytometry and immunostaining. Ablation of Hdc⁺ cells using the Hdc^CreERT2 tamoxifen-inducible recombinase Cre system, conditional knockout of Prostaglandin-endoperoxidase synthase 2 (Ptgs2) in Hdc⁺ cells using Hdc^Cre; Ptgs2 floxed mice, and visualization of LECs using Prox1^tdTomato mice also was performed. The role of microbial signals was investigated by knocking down mice gut microbiomes using antibiotic cocktail gavages.

We found that Hdc⁺ IMCs infiltrate the injured intestine after irradiation injury and promote epithelial regeneration in part by modulating LEC activity. Hdc⁺ IMCs express Ptgs2 (encoding cyclooxygenase-2/COX-2), and enables them to produce prostaglandin E₂. Prostaglandin E₂ acts on the prostaglandin E₂ receptor 4 receptor (EP4) on LECs to promote lymphangiogenesis and induce the expression of proregenerative factors including R-spondin 3. Depletion of gut microbes leads to reduced intestinal regeneration by impaired recruitment of IMCs.

Altogether, our results unveil a critical role for IMCs in intestinal repair by modulating LEC activity and implicate gut microbes as mediators of intestinal regeneration.