UNLABELLED: Stage IA gastric adenocarcinoma, characterized by foci of intramucosal signet ring cells (SRC), is found in nearly all asymptomatic patients with germline pathogenic CDH1 variants and hereditary diffuse gastric cancer syndrome (HDGC). The molecular steps involved in initiating malignant transformation and promoting SRC dormancy in HDGC are unknown. Here, whole-exome bulk RNA sequencing (RNA-seq) of SRCs and adjacent non-SRC epithelium (NEP) was performed on laser-capture microdissected (LCM) regions of interest found in risk-reducing total gastrectomy specimens from patients with HDGC (Clinicaltrials.gov ID: NCT03030404). In total, 20 patients (6 male, 14 female) with confirmed HDGC were identified. Analysis of differentially expressed genes (DEG) demonstrated upregulation of certain individual EMT and proliferation genes. However, no oncogenic pathways were found to be upregulated in SRCs. Rather, SRC regions had significant enrichment in pathways involved in T-cell signaling. CIBERSORTx predicted significant increases in the presence of regulatory T cells (Treg) specific to SRC regions. IHC confirmed an increase in FOXP3+ cells in SRC foci, as well as elevations in CD4+ T cells and HLA-DR staining. In summary, the tumor immune microenvironment is microscopically inseparable from stage IA gastric SRCs using a granular isolation technique. An elevation in CD4+ T cells within SRC regions correlates with clinically observed SRC dormancy, while Treg upregulation represents a potential immune escape mechanism. IMPLICATIONS: Characterization of the tumor-immune microenvironment in HDGC underscores the potential for the immune system to shape the transcriptional profile of the earliest tumors, which suggests immune-directed therapy as a potential cancer interception strategy in diffuse-type gastric cancer.