Gram-Negative Microflora Dysbiosis Facilitates Tumor Progression and Immune Evasion by Activating the CCL3/CCL5–CCR1–MAPK–PD-L1 Pathway in Esophageal Squamous Cell Carcinoma

Abstract Gram-negative (G−) microflora dysbiosis occurs in multiple digestive tumors and is found to be the dominant microflora in the esophageal squamous cell carcinoma (ESCC) microenvironment. The continuous stimulation of G− bacterium metabolites may cause tumorigenesis and reshape the microimmune environment in ESCC. However, the mechanism of G− bacilli causing immune evasion in ESCC remains underexplored. We identified CC chemokine receptor 1 (CCR1) as a tumor-indicating gene in ESCC. Interestingly, expression levels of CCR1 and PD-L1 were mutually upregulated after G− bacilli metabolite lipopolysaccharide stimulation. First, we found that CCR1 high expression levels were associated with poor overall survival in ESCC. Importantly, we found that high levels of CCR1 expression upregulated PD-L1 expression by activating MAPK phosphorylation in ESCC and induced tumor malignant behavior. Finally, we found that T-cell exhaustion and cytotoxicity suppression were associated with CCR1 expression in ESCC, which were decreased after CCR1 inhibiting. Our work identifies CCR1 as a potential immune check point regulator of PD-L1 and may cause T-cell exhaustion and cytotoxicity suppression in ESCC microenvironment and highlights the potential value of CCR1 as a therapeutic target of immunotherapy. Implications: The esophageal microbial environment and its metabolites significantly affect the outcome of immunotherapy for ESCC.