Endostatin induces normalization of blood vessels in colorectal cancer and promotes infiltration of CD8+ T cells to improve anti-PD-L1 immunotherapy

INTRODUCTION: The purpose of this study was to evaluate recombinant human endostatin (rHE)-induced normalization of the tumor vasculature in colorectal cancer (CRC) and to evaluate the therapeutic effects of combined treatment with rHE and a programmed death ligand-1 (PD-L1) inhibitor. METHODS: A mouse subcutaneous tumorigenesis model was established to evaluate the antitumor effects of endostatin combined with a PD-L1 inhibitor on CRC. Intravoxel incoherent motion diffusion-weighted magnetic resonance imaging (IVIM-DW MRI) was used to evaluate changes in the intratumor microcirculation in response to combined treatment with endostatin and a PD-L1 inhibitor. The infiltration density and function of CD8+ T cells in tumors were evaluated using flow cytometry. Finally, clinical specimens were used to evaluate the expression area of tumor vascular pericytes and CD8+ T cells in tumor tissues. RESULTS: The antitumor effects of endostatin combined with a PD-L1 inhibitor were significantly greater than those of endostatin or a PD-L1 inhibitor alone. On the ninth day of intervention, the endostatin group showed significantly higher pseudo diffusion parameter (D*) and microvascular volume fraction (F) values in tumors than those in the control group or PD-L1 group. After 27 days of intervention, the endostatin groups showed significantly lower levels of vascular endothelial growth factor (VEGF) and transforming growth factor (TGF)-β than those in the control group. Treatment of CD8+ T cells with endostatin for 24 h did not alter the expression levels of markers of reduced T-cell activity. However, endostatin reversed the VEGF-mediated inhibition of the secretion of interferon (IFN)-γ from T cells. The results in CRC clinical samples showed that treatment with endostatin induced significantly higher infiltration of CD8+ T cells compared with treatment that did not include endostatin. Furthermore, the expression area of pericytes was significantly positively related to the infiltration density of CD8+ T cells and overall survival time. CONCLUSION: Endostatin improved the antitumor effects of PD-L1 inhibitors on CRC, significantly increased the activity of CD8+ T cells, and synergistically improved the tumor treatment effect of the two inhibitors.