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Abstract
Purpose: Interleukin-13 (IL-13) is a known mediator of radiation-induced lung injury (RILI). IL-13Rα2 has an accepted role in antagonizing IL-13 signaling by acting as a decoy receptor. We sought to understand the role of IL-13Rα2 in the progression of RILI.

Methods and materials: Mice deficient in IL-13Rα2 (Ra2 KO) and wild-type (WT) mice were exposed to thoracic irradiation (IR) in 5 daily fractions of 6 Gy and followed for survival (n > 15 per group) and tissue collection (n > 5 per group). Collagen accumulation in the lung was evaluated with Masson's trichrome staining and hydroxyproline content. Gene expression was evaluated by RNA sequencing. Expression of IL-13Rα2 and macrophage markers in murine lung and human lung tissue (n = 63) was assessed with immunohistochemistry. The role of IL-13Rα2 in IL-13-mediated macrophage polarization was determined in primary macrophage cultures from Ra2 KO mice and after RNA silencing of a human monocyte cell line (THP-1).

Results: Membrane-bound IL-13Rα2 expression in murine lung was increased after IR and localized to macrophages. Irradiated Ra2 KO mice exhibited reduced sensitivity to thoracic IR compared with WT mice as measured by median survival (19 vs. 21 weeks, P < .05), histology, hydroxyproline content, transforming growth factor-β expression, and macrophage accumulation. Gene sets linked to cytokine signaling and macrophage recruitment were enriched in irradiated WT compared with Ra2 KO lung tissue. IL-13-mediated expression of CCL2 and M2 markers was reduced in murine and human macrophages deficient in IL-13Rα2. Increased expression of in IL-13Rα2 and co-localization with CD163 was confirmed in irradiated fibrotic human lung.

Conclusions: IL-13Rα2 is predominantly expressed in macrophages within irradiated lung and plays a crucial role in CCL2 expression, macrophage polarization, and transforming growth factor-β expression in response to IL-13. These studies demonstrate an unexpected profibrotic role of IL-13Rα2 in RILI and suggest that strategies targeting IL-13Rα2 may ameliorate chronic inflammation and fibrosis.

Affiliations

Seokjoo Kwon 1, Eun Joo Chung 1, Santwana Kc 1, Ayla O White 1, Su I Chung 1, Jason A Horton 2, Hong Shik Yun 1, Heesu Ahn 1, Uma Shankavaram 1, Joon-Yong Chung 3, Joon Seon Song 4, Deborah E Citrin 5
1Radiation Oncology Branch, Center for Cancer Research; National Cancer Institute, National Institute of Health, Bethesda, Maryland.
2Department of Orthopedic Surgery, Upstate Medical University, Syracuse, New York.
3Molecular Imaging Branch, Center for Cancer Research, National Cancer Institute, National Institutes of Health, Bethesda, Maryland.
4Department of Pathology, Asan Medical Center, University of Ulsan College of Medicine, Seoul, Republic of Korea.
5Radiation Oncology Branch, Center for Cancer Research; National Cancer Institute, National Institute of Health, Bethesda, Maryland. Electronic address: citrind@mail.nih.gov.