Efficient colonic mucosal wound repair requires Trem2 signaling

The colonic epithelial lining undergoes constant replacement, driven by epithelial stem cells in crypts of Lieberkühn. Stem cells lost because of damage or disease can be replaced by adjacent crypts that undergo fission. The close proximity of an extraordinary number of luminal microbes creates a challenge for this repair process; infection must be prevented while immune system activation and epithelial stem cell genetic damage must be minimized. To understand the factors that modulate crypt/stem cell replacement in the mouse colon, we developed an in vivo acute injury system analogous to punch biopsy of the skin. In contrast to epidermal stem cells, colonic epithelial progenitors did not migrate over the wound bed. Instead, their proliferative expansion was confined to crypts adjacent to wound beds and was delayed to the latter phase of healing. This increased epithelial proliferation was coincident with the infiltration of Trem2 expressing macrophages and increased expression of IL-4 and IL-13 in the wound bed. Interestingly, Trem2^-/- mice displayed slow and incomplete wound healing of colonic mucosal injuries. We found the latter phase of healing in Trem2^-/- mice showed a diminished burst of epithelial proliferation, increased expression of IFN-γ and TNF-α, diminished expression of IL-4 and IL-13, and increased markers of classical macrophage activation. Ablation of these cytokines in injured WT and Trem2 ^-/- mice demonstrated that their expression ultimately determined the rate and nature of wound healing. These studies show that Trem2 signaling is an important pathway to promote healing of wounds in the colon where stem cell replacement is necessary.