Abstract
BACKGROUND: The tumour microenvironment primarily constitutes macrophages in the form of an immunosuppressive M2 phenotype, which promotes tumour growth. Thus, the development of methodologies to rewire M2-like tumour-associated macrophages (TAMs) into the M1 phenotype, which inhibits tumour growth, might be a critical advancement in cancer immunotherapy research. METHODS: The expressions of IL-33 and indicators related to macrophage polarization in oesophageal squamous cell carcinoma (ESCC) tissues and peripheral blood mononuclear cell (PBMC)-derived macrophages were determined. Inhibition of ornithine decarboxylase (ODC) with small interfering RNA was used to analyse the phenotype of macrophage polarization and polyamine secretory signals. CCK-8, wound-healing and Transwell assays were used to detect the proliferation and migration of ECA109 cells in vitro. The tumour xenograft assay in nude mice was used to examine the role of IL-33 in ESCC development in vivo. RESULTS: This study showed the substantially elevated IL-33 expression in ESCC tissues compared with the normal tissues. Additionally, enhanced infiltration of M2-like macrophages into the ESCC tumour tissue was also observed. We observed a strong correlation between the IL-33 levels and the infiltration of M2-like macrophages in ESCC tumours locally. Mechanistically, IL-33 induces M2-like macrophage polarization by activating ODC, a key enzyme that catalyses the synthesis of polyamines. Inhibition of ODC suppressed M2-like macrophage polarization. Finally, in vivo, we confirmed that IL-33 promotes tumour progression. CONCLUSIONS: This study revealed an oncogenic role of IL-33 by actively inducing M2-like macrophage differentiation; thus, contributing to the formation of an immunosuppressive ESCC tumour microenvironment. Thus, IL-33 could act as a novel target for cancer immunotherapies.