High density of CD68+ tumor-associated macrophages predicts a poor prognosis in gastric cancer mediated by IL-6 expression

The aim of the present study was to explore the potential role of cluster of differentiation CD68+ tumor-associated macrophages (TAMs) induced by interleukin (IL)-6 in the progression of gastric cancer (GC) and patient prognosis. The expression levels of IL-6 and CD68 were detected by immunohistochemical staining in 60 samples of tumor and non-tumor gastric tissues. CD14+ monocytes were isolated from peripheral blood mononuclear cells and stimulated with macrophage colony stimulation factor (M-CSF) and IL-6, and the expression levels of IL-10, IL-12, vascular endothelial growth factor (VEGF)-C and transforming growth factor (TGF)-β were measured by reverse transcription polymerase chain reaction and ELISA. The GC MGC-803 cell line was co-cultured with monocytes stimulated by M-CSF and IL-6 and the invasion ability of the MGC-803 was evaluated by Transwell analysis. The levels of STAT3, P-STAT3 and interferon-regulatory factor 4 (IRF4) in the monocytes stimulated by M-CSF and IL-6 were detected by western blotting. The results demonstrated that the frequencies of IL-6+ macrophages (Mφs) and CD68+ Mφs were significantly higher in tumor regions compared with the corresponding non-tumor regions of GC tissues. Kaplan-Meier analysis revealed that the densities of tumor-infiltrating CD68+ or IL-6+ Mφs were inversely associated with the overall survival rates of the patients. In vitro, the expression levels of IL-10, VEGF-C and TGF-β significantly increased in CD14+ monocytes subsequent to M-CSF and IL-6 stimulation. The invasion abilities of MGC-803 were increased by the monocytes stimulated with M-CSF and IL-6. The levels of STAT3, P-STAT3 and IRF4 proteins increased in the monocytes stimulated by M-CSF and IL-6. In conclusion, the results from the present study suggest that a high density of CD68+ TAMs predicts a poor prognosis in GC. IL-6 may polarize the Mφs and promote tumor invasion through the IL-6/STAT3/IRF4 signaling pathway.