CSF1 is expressed by the intestinal epithelial cells to regulate Mφ macrophages and maintain epithelial homeostasis and is downregulated in neonates with necrotizing enterocolitis

Colony stimulating factor 1 (CSF1) is generally expressed by immune cells in response to pro-inflammatory stimuli. The CSF1 receptor (CSFR) is activated by CSF1, and plays a key role in macrophage homeostasis. Furthermore, the CSF1R⁺ macrophages maintain homeostasis in the intestinal epithelium. The aim of this study was to explore the functions of CSF1-expressing and CSF1R⁺ macrophages in necrotizing enterocolitis (NEC), which commonly affects the ileum of neonates.

In-situ CSF1 expression in the intestines of neonates with NEC or intestinal atresia (n = 4 each) was detected by immunofluorescence staining. The CSF1 levels in the intestinal crypt-derived organoid cultures were measured by ELISA. Peripheral blood monocyte-derived Mφ macrophages were co-cultured with the organoids and stimulated with lipopolysaccharide (LPS) to mimic the inflamed state of the ileum in NEC patients.

CSF1 was expressed in the intestinal epithelial cells of the fetal and neonatal samples, but suppressed in the NEC samples. Furthermore, CSF1 expression was downregulated in the intestinal crypt-derived organoids by LPS. CSF1R⁺ macrophages were detected near the intestinal crypts in the non-inflamed intestines but were absent in tissues obtained from pediatric NEC patients. Peripheral blood monocyte-derived macrophages promoted intestinal organoid proliferation in vitro following CSF1 stimulation. Finally, low concentrations of LPS slightly enhanced the proliferation of organoids co-cultured with the macrophages, whereas higher doses had a significant inhibitory effect.

Intestinal epithelial cells express CSF1 to regulate the resident macrophages, maintain epithelial homeostasis, and resist infection. The abundant CSF1R⁺ macrophages in the fetal intestine may overexpress TNF-α upon activation of the TLR4/NF-κB pathway, resulting in epithelial damage and NEC induction.