Colorectal Cancer-Derived Exosomes Impair CD4 ⁺ T Cell Function and Accelerate Cancer Progression via Macrophage Activation

<b><i>Background:</i></b> Exosomal programmed death ligand 1 (PD-L1), an exosomal membrane protein found in many tumor types, is consider to aid in regulation of the immune microenvironment. However, the functions and the mechanisms underlying the exosome-mediated regulation of the immune microenvironment in colorectal cancer (CRC) remain unknown. <b><i>Methods:</i></b> Western blotting was used to investigate the levels of exosomal PD-L1 in the peripheral blood of patients with CRC and healthy controls. A CRC mouse model was constructed by administering 10 mg/kg azoxymethane (AOM) and dextrane sodium sulfate (DSS) intraperitoneally. The mice were then administered the control or CRC-derived exosomes to examine the regulatory effect of the exosomes on macrophage infiltration and CRC development. <i>In vitro</i> studies, using a coculture system, and flow cytometry analysis were conducted to examine the relationship between the regulatory effect of CRC-derived exosomes on CD4⁺ T cells and tumor-associated macrophages. RNA-seq and reverse transcription-quantitative polymerase chain reaction assays were used to investigate the mechanisms underlying the regulatory effect of the CRC-derived exosomes on macrophage proliferation and the regulation of the immune microenvironment during CRC development. <b><i>Results:</i></b> In patients with CRC, higher levels of exosomal PD-L1 were associated with a more severe form of disease. The treatment of mice with AOM/DSS-induced CRC with CRC-derived exosomes resulted in high levels of macrophage proliferation, increased PD-L1 levels in macrophages, and accelerated CRC progression. Importantly, analysis of an <i>in vitro</i> coculture system and flow cytometry analysis showed that the CRC-derived exosomes transported PD-L1 into macrophages and impaired CD4⁺ T cell function. Preliminary data suggest that the NF-κb signaling pathway regulates the function of CRC-derived exosomal PD-L1-dependent macrophages. <b><i>Conclusion:</i></b> CRC-derived exosomes induce the proliferation of macrophages and increase their PD-L1 levels. They also impair CD4⁺ T cell function and promote CRC progression. Our findings reveal a novel exosomal PD-L1-mediated crosstalk between the CRC cells and immune cells in the CRC microenvironment.