CCL18^＋tumor-associated macrophages induce immune escape by expressing PD-L1 and TGF-β

Objective To explore the role of CC chemokine ligand-18 (CCL18) positive tumor-associated macrophages in immune escape of gastric cancer and their impact on the tumor microenvironment.

Methods Three gastric cancer patient cohorts were included in Zhongshan Hospital, Fudan University (449 cases in the radical surgery cohort, 46 cases in the palliative treatment cohort, and 48 cases in the flow cytometry analysis cohort), and one group of patients from The Cancer Genome Atlas (TCGA) database were included. The CCL18⁺ tumor-associated macrophage gene signature was obtained from the TCGA database, and differential gene expression in the TCGA cohort was analyzed based on this signature. Immunofluorescence was used to assess the infiltration of CCL18⁺ tumor-associated macrophages in tissue microarrays of patients from two treatment cohorts. The association between CCL18⁺ tumor-associated macrophages and clinical outcomes as well as related gene expression was examined. Using single-cell suspensions derived from tumor tissues, flow cytometry was employed to evaluate the immunophenotype of CCL18⁺ tumor-associated macrophages; the potential therapeutic effects of blocking CCL18 in gastric cancer were assessed through in vitro culture.

Results In curative surgery cohort, patients with high infiltration of CCL18^＋tumor-associated macrophages had shorter overall survival and disease-free survival compared to those with low infiltration (P＜0.01). In palliative cohort, patients with high infiltration of CCL18^＋tumor-associated macrophages exhibited shorter progression-free survival compared to those with low infiltration (P＝0.015). The proportion of exhausted CD8^＋T cells was significantly increased in the cancer tissues with high infiltration of CCL18^＋tumor-associated macrophages compared to those with low infiltration (P＜0.001). Tumors with high CCL18^＋tumor-associated macrophages gene signature expressed significantly higher level of programmed death-ligand 1 (PD-L1) and transforming growth factor-β (TGF-β）than those with low CCL18^＋tumor-associated macrophages gene signature in TCGA queue (P＜0.001). The expression levels of PD-L1 and TGF-β were higher in CCL18^＋tumor-associated macrophages than those in CCL18^－tumor-associated macrophages (P＜0.05). After CCL18 intervention, the proportions of cytotoxic CD8⁺ T cells and apoptotic epithelial cells (Annexin Ⅴ^＋) were increased (P＜0.05), while the proportion of proliferating epithelial cells (Ki67^＋) was decreased (P＜0.001).

Conclusion CCL18^＋tumor-associated macrophages infiltration is associated with prognoses in patients with gastric cancer, and might promote an immunosuppressive microenvironment in gastric cancer by expressing PD-L1 and TGF-β, and blockade of CCL18 might be a new direction for gastric cancer treatment.