Eosinophil depletion suppresses radiation-induced small intestinal fibrosis.

Radiation-induced intestinal fibrosis (RIF) is a serious complication after abdominal radiotherapy for pelvic tumor or peritoneal metastasis. Herein, we show that RIF is mediated by eosinophil interactions with α-smooth muscle actin-positive (α-SMA+) stromal cells. Abdominal irradiation caused RIF especially in the submucosa (SM) of the small intestine, which was associated with the excessive accumulation of eosinophils in both human and mouse. Eosinophil-deficient mice showed markedly ameliorated RIF, suggesting the importance of eosinophils. After abdominal irradiation, chronic crypt cell death caused elevation of extracellular adenosine triphosphate, which in turn activated expression of C-C motif chemokine 11 (CCL11) by pericryptal α-SMA+ cells in the SM to attract eosinophils in mice. Inhibition of C-C chemokine receptor 3 (CCR3) by genetic deficiency or neutralizing antibody (Ab) treatment suppressed eosinophil accumulation in the SM after irradiation in mice, suggesting a critical role of the CCL11/CCR3 axis in the eosinophil recruitment. Activated α-SMA+ cells also expressed granulocyte-macrophage colony-stimulating factor (GM-CSF) to activate eosinophils. Transforming growth factor-ß1 from GM-CSF-stimulated eosinophils promoted collagen expression by α-SMA+ cells. In translational studies, treatment with a newly developed interleukin-5 receptor α-targeting Ab, analogous to the human agent benralizumab, depleted intestinal eosinophils and suppressed RIF in mice. Collectively, we identified eosinophils as a crucial factor in the pathogenesis of RIF and showed potential therapeutic strategies for RIF by targeting eosinophils.

Blockade of TLR3 protects mice from lethal radiation-induced gastrointestinal syndrome.

High-dose ionizing radiation induces severe DNA damage in the epithelial stem cells in small intestinal crypts and causes gastrointestinal syndrome (GIS). Although the tumour suppressor p53 is a primary factor inducing death of crypt cells with DNA damage, its essential role in maintaining genome stability means inhibiting p53 to prevent GIS is not a viable strategy. Here we show that the innate immune receptor Toll-like receptor 3 (TLR3) is critical for the pathogenesis of GIS. Tlr3(-/-) mice show substantial resistance to GIS owing to significantly reduced radiation-induced crypt cell death. Despite showing reduced crypt cell death, p53-dependent crypt cell death is not impaired in Tlr3(-/-) mice. p53-dependent crypt cell death causes leakage of cellular RNA, which induces extensive cell death via TLR3. An inhibitor of TLR3-RNA binding ameliorates GIS by reducing crypt cell death. Thus, we propose blocking TLR3 activation as a novel approach to treat GIS.