Abstract
The intestinal epithelium forms a vital barrier between luminal microbes and the underlying mucosal immune system. Epithelial barrier function is maintained by continuous renewal of the epithelium and is pivotal for gut homeostasis. Breaching of the barrier causes mobilization of immune cells to promote epithelial restitution. However, it is not known whether microbes at the luminal surface of a healthy epithelial barrier influence immune cell mobilization to modulate tissue homeostasis. Using a mouse colonic mucosal explant model, we demonstrate that close proximity of luminal microbes to a healthy, intact epithelium results in rapid mucus secretion and movement of Ly6C+7/4+ monocytes closer to epithelial stem cells. These early events are driven by the epithelial MyD88-signaling pathway and result in increased crypt cell proliferation and intestinal stem cell number. Over time, stem cell number and monocyte–crypt stem cell juxtapositioning return to homeostatic levels observed in vivo. We also demonstrate that reduced numbers of tissue Ly6C+ monocytes can suppress Lgr5EGFP+ stem cell expression in vivo and abrogate the response to luminal microbes ex vivo. The functional link between monocyte recruitment and increased crypt cell proliferation was further confirmed using a crypt–monocyte coculture model. This work demonstrates that the healthy gut epithelium mediates communication between luminal bacteria and monocytes, and monocytes can modulate crypt stem cell number and promote crypt cell proliferation to help maintain gut homeostasis.
Highlights
Morphometric analysis showed that 4 h of E. coli, muramyl dipeptide (MDP), or LPS luminal stimulation resulted in an increase in the number of nuclei/crypt axis (Fig. 1D) and an increase in crypt length (Fig. 1E)
Basal membrane exposure of the colonic epithelium to LPS or MDP in colonic crypt culture had no effect on BrdU incorporation (Fig. 2C), which suggested that the increased proliferation in response to MDP or LPS in mucosal explant culture was not due to a direct effect on the epithelial basal membrane
The majority of fluorescent mCherry E. coli were located on top of the mucus layer, and none was detected beneath the surface epithelium (Fig. 2D)
Summary
Recruitment of Ly6C+7/4+ cells to the surface epithelium and the epithelial stem cell niche following luminal stimulation with LPS, MDP, or E. coli Following a 1-h luminal stimulation with LPS, there was an increase in the total number of 7/4+ cells in the mucosa and an increase in the numbers both at the crypt base/stem cell niche and surface crypt epithelium (Fig. 3) compared with control-treated mucosal explants.
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