POSTN⁺ CAF–SPP1⁺ macrophage crosstalk is associated with immune exclusion and poor prognosis in lung adenocarcinoma

BACKGROUND: Lung adenocarcinoma (LUAD) is a highly lethal malignancy in which the tumor microenvironment (TME) plays an important role in disease progression and therapeutic resistance. However, the spatial organization and molecular basis of stromal-immune interactions in LUAD remain incompletely understood. METHODS: We integrated bulk RNA sequencing, single-cell RNA sequencing, spatial transcriptomics, multiplex immunofluorescence, and in vitro functional assays to characterize TME heterogeneity and investigate mechanisms associated with stromal barrier formation and immune exclusion in LUAD. RESULTS: POSTN⁺ cancer-associated fibroblasts (CAFs) and SPP1⁺ macrophages were enriched in LUAD, showed marked spatial colocalization, and were associated with an immune-excluded spatial pattern characterized by limited effector T-cell infiltration. Cell-cell communication analysis identified SPP1-centered signaling from SPP1⁺ macrophages to POSTN⁺ CAFs, with CD44 emerging as a prominent receptor candidate and integrin-related receptor pairs suggested as additional candidate branches. In vitro experiments showed that TAM-like macrophage-conditioned medium activated CAF-like HFL-1 cells, increased POSTN, FN1, and COL1A1 expression, enhanced fibroblast adhesion and collagen gel contraction, and restricted primary CD8⁺ T-cell migration across CAF-like stromal barriers. These effects were attenuated by SPP1 neutralization or CD44 knockdown. In addition, CAF-derived C3 was implicated in a feedback loop that may reinforce the SPP1⁺/CD206⁺ macrophage-like phenotype. Pan-cancer analyses further showed that this stromal-myeloid program was conserved across multiple solid tumor types and was associated with poor overall survival, while exploratory cross-cancer immunotherapy analysis suggested a potential association with reduced response to immune checkpoint blockade. CONCLUSION: These findings further define a spatially organized POSTN⁺ CAF-SPP1⁺ macrophage niche that may contribute to stromal remodeling, CD8⁺ T-cell exclusion, and potentially reduced immunotherapy benefit in LUAD. The TAM-derived SPP1-CD44 axis and CAF-derived C3 feedback loop may provide a framework for biomarker development and therapeutic targeting in immune-excluded solid tumors.