ObjectiveTo explore the interaction between immune cell infiltration and extracellular matrix (ECM) in diffuse gastric cancer (DGC), and to identify novel diagnostic biomarkers and therapeutic targets. MethodsTranscriptomic data of DGC patients from The Cancer Genome Atlas (TCGA) database were analyzed to screen potential regulator factor of immune-related and ECM receptor-related signaling pathways. Differential expression of the identified regulator was assessed between the DGC tissues and the adjacent gastric tissues. Bioinformatics analysis was utilized to evaluate the relation between the regulator factor and immune cell infiltration and ECM, as well as prognosis. The clinical validation was performed using 90 paraffin-embedded DGC tissues and adjacent gastric tissues from the patients treated at The Lanzhou University Second Hospital (hereafter “our hospital”) from January 2017 to December 2019. The immunohistochemical staining was employed to examine the expression of regulator factor, followed by analysis of its association with immune cell infiltration, clinicopathologic features, and prognosis. Additionally, 10 paired DGC tissues and adjacent gastric tissues from the patients treated in our hospital in 2024 were collected for validation using real-time quantitative PCR to assess mRNA expression. The significance level was set at α=0.05. ResultsThe collagen type I alpha 1 chain (COL1A1), a potential regulator factor linked to immune and ECM receptor signaling pathways, was identified from the TCGA database. The COL1A1 was significantly overexpressed in the DGC tissues compared to the adjacent gastric tissues (P<0.001), and its high expression correlated with poorer prognosis [HR(95%CI)=2.98(1.21, 7.30), P=0.017]. The COL1A1 gene expression negatively correlated with CD8+ T cell enrichment score (CIBERSORT: r=−0.17, P<0.001; xCELL: r=−0.32, P<0.001) but positively correlated with M2 tumor-associated macrophage enrichment score (CIBERSORT: r=0.32, P<0.001; xCELL: r=0.24, P<0.001). The clinical validation confirmed that the COL1A1 protein and mRNA were both overexpressed in the DGC tissues (P<0.001). The patients with high COL1A1 protein expression had worse overall survival (P<0.001), and high expression (vs. low) was an independent risk factor for postoperative overall survival [HR(95%CI)=6.607(3.374, 12.940), P<0.001]. The COL1A1 protein expression positively correlated with CD163 (an M2 macrophage marker; r=0.76, P<0.001) and negatively with CD8+ (T cell marker, r=−0.84, P<0.001). ConclusionThis study demonstrates that COL1A1 is a potential therapeutic target for immune suppression and ECM interaction in DGC and a critical prognostic factor for long-term survival in patients with DGC.