摘要Objective:The heterogeneity of epithelial cells and their interaction with the immune microenvironment play crucial roles in tumor progression,but the underlying mechanisms remain unclear.Methods:We analyzed single-cell transcriptomic data from normal and tumor tissues to characterize epithelial cells and their microenvironment.Key genes were identified and used,via survival analysis and multiple machine learning methods,to construct a prognostic model termed the Epithelial Signature(EpiSig).We further validated,through a series of experiments,the critical immunological roles of the key genes incorporated into the EpiSig model.Results:Tumor tissues showed a marked increase in epithelial cells,a reduction in natural killer(NK)/T cells,and cell co-occurrence patterns distinct from normal tissues.We identified differentially expressed genes in tumor epithelial cells and integrated multiple machine-learning algorithms to construct the EpiSig model.This model effectively stratified patient prognosis,with the high-EpiSig group exhibiting significantly worse survival;receiver operator characteristic curve(ROC)and principal component analysis(PCA)analyses further supported its accuracy and robustness.Immune analyses indicated lower immune cell infiltration,decreased human leukocyte antigen(HLA)expression,and elevated programmed cell death ligand 1/programmed cell death protein 1(PD-L1/PD-1)in the high-EpiSig group,reflecting a more pronounced immunosuppressive microenvironment.The core gene KRT18 correlated strongly with EpiSig scores(R=0.65)and promoted lung adenocarcinoma(LUAD)cell proliferation in functional assays.Further,low KRT18 tumors showed higher CD4+/CD8+T cell and CD20+B cell infiltration;in a mouse LLC subcutaneous tumor model,both Krt18 knockdown and PD-1 blockade suppressed tumor growth,with greater efficacy in combination.Mechanistically,KRT18 activated NF-kB/p65 to upregulate PD-L1,promoting immune exclusion and impairing T-cell effector function.Conclusions:This study highlights the close relationship between epithelial cell heterogeneity and immune microenvironment alterations in tumors,and presents the EpiSig as a robust tool for prognostic prediction.KRT18 may serve as a promising therapeutic target in LUAD.