Single-nucleus RNA sequencing and spatial transcriptomics reveal an immunosuppressive tumor microenvironment related to metastatic dissemination during pancreatic cancer liver metastasis

<b>Background:</b> Pancreatic ductal adenocarcinoma (PDAC) is a highly aggressive malignancy characterized by early liver metastasis and high mortality. The tumor microenvironment plays a pivotal role in tumor progression; however, the immune microenvironment's involvement in PDAC liver metastasis remains poorly understood. <b>Methods:</b> This study investigates cellular heterogeneity in primary tumor (PT) and liver metastasis (LM) tissues of PDAC using single-nucleus RNA sequencing and spatial transcriptomics. Intra-tumor heterogeneity and cell interactions were elucidated through deconvolution, intercellular signalling, pseudotime analysis, and immune infiltration profiling. The spatial distribution of immune cells was assessed by multiplexed immunofluorescence staining, and prognostic models were developed and validated through immunohistochemistry (IHC). Analyzing the regulatory role of CITED4 in the invasion and metastasis of pancreatic cancer cells through transwell assay and scratch wound healing assay. <b>Results:</b> A total of 62,326 cells were sequenced, with metastatic dissemination cells showing significant upregulation of epithelial-mesenchymal transition (EMT)-related genes during liver metastasis. Spatial transcriptomics revealed the enrichment of metastatic dissemination cells and FOXP3-related T_reg cells at the tumor front in PT tissues. In comparison to LM tissues, the tumor front in PT tissues fosters an immunosuppressive microenvironment through the accumulation of T_reg cells. Interaction analysis identified the SPP1 pathway as a key promoter of this immunosuppressive environment. Furthermore, prognostic models highlighted CITED4 as critical biomarkers in PDAC. Elevated CITED4 expression is correlated with liver metastasis and poor prognosis in patients with PDAC. siRNA-mediated knockdown of CITED4 suppresses the invasion and metastasis of pancreatic cancer cells. <b>Conclusions:</b> In summary, this study revealed that T_reg cell alterations, mediated by metastatic dissemination cells within the immune microenvironment, significantly contribute to PDAC liver metastasis, and that CITED4 enhances the metastatic potential of metastatic dissemination cells.