Interleukin 6 Signaling Blockade Exacerbates Acute and Late Injury From Focal Intestinal Irradiation

Abstract

To evaluate the acute changes in leukocyte populations after focal irradiation and to assess the role of interleukin 6 (IL-6) in acute and late radiation injury. Mice were surgically implanted with a radiopaque marker on the surface of the small intestine. Mice were then imaged with cone beam computed tomography to locate the marker and irradiated with 18Gy of 5×5mm collimated x-rays onto the marked intestine using the Small Animal Radiation Research Platform. Intestinal sections and blood were harvested 1, 3.5, 7, and 14days and 2months postirradiation (post-IR) for histology and complete blood count, respectively. Immune cell populations were assessed by immunofluorescence in the acute phase. Collagen deposition was assessed 2months post-IR. IL-6-/- intestinal sections were assessed post-IR for morphology, EdU, Ki67, and TUNEL in comparison to IL-6+/+ mice. Furthermore, a set of IL-6+/+ mice were treated with anti-IL-6R to assess the role of IL-6 in late intestinal injury. Intestinal radiation damage peaked 14days post-IR, and fibrosis had developed by 60days post-IR. There was a marked infiltration of immune cells into the irradiated intestine, with increased neutrophils, macrophages, B-cells, and CD4+ T cells maintained from 3.5 to 14days post-IR. CD8+ T cells were decreased from days 7 to 14 post-IR. Systemically, leukocytes were increased in the peripheral blood 14days post-IR with anemia being maintained from 14days to 2months. IL-6 was significantly increased in the serum post-IR. IL-6-/- mice demonstrated worsened intestinal injury acutely post-IR. Moreover, anti-IL-6R-treated mice presented with worsened intestinal fibrosis 2months post-IR. Focal irradiation of the intestine produced a significant increase in immune cells in the irradiated area and systemic inflammation and anemia. Blockade of IL-6 signaling was found to exacerbate acute intestinal injury and late intestinal injury after focal irradiation.