Tumor Endothelial Cells Induce <scp>M2</scp> Macrophage Polarization via <scp>IL</scp> ‐4, Enhancing Tumor Growth in Hepatocellular Carcinoma

Tumor-associated macrophages (TAMs) and tumor endothelial cells (TECs) are important components in tumor microenvironments. This study examined the interaction between TECs and TAMs in hepatocellular carcinoma (HCC). The aim of this study is to clarify the mechanism of immune suppression and cancer progression mediated by the M2 polarization of TAMs. TECs were isolated from subcutaneous HCC tumors using murine BNL-T cells, and normal endothelial cells (NECs) were isolated from murine liver. M0 macrophages were obtained from bone marrow after incubation with M-CSF. Conditioned medium from TECs (TEC CM) or NECs (NEC CM) was added to M0 macrophages, and the polarization to M2 macrophages was assessed. The percentage of iNOS^-CD206⁺ cells was increased in the TEC CM group than in the NEC CM group. The subcutaneous tumor model with co-injection of BNL-T and TECs or NECs was applied, and the ratio of M2 macrophages (CD206⁺iNOS^-CD45⁺CD11b⁺F4/80⁺ cells) was elevated in the BNL-T + TEC group. IL-4 expression in TECs was upregulated compared to NECs, and the secretion of IL-4 was increased in the TEC CM compared to the NEC CM. After IL-4 down-regulation in TECs, the ratio of iNOS^-CD206⁺ cells decreased. The double immunofluorescent staining of CD31 and CD163 was performed in human HCC tissue. The number of CD31⁺ endothelial cells correlated positively with CD163⁺ TAMs. The high CD163/CD31 group showed poor prognosis in overall and disease-free survival. TECs induce M2 macrophage polarization through IL-4 secretion, leading to immune suppression and cancer progression.