Exploring the molecular mechanisms underlying the inflammation–cancer transformation in Hashimoto thyroiditis using single-cell transcriptomics

Hashimoto thyroiditis (HT) is closely associated with the development of papillary thyroid carcinoma (PTC); however, the tumor immune microenvironment and the molecular mechanisms underlying the ʻinflammation-to-cancer’ transition in HT-PTC coexistence remain poorly understood. In this study, we integrated single-cell RNA sequencing (scRNA-seq) data from HT-associated and PTC tissues, annotating 11 distinct cell subtypes, including endothelial cells, red blood cells, fibroblasts, follicular epithelial cells (FECs), macrophages, monocytes, T cells, NK cells, B cells, plasma cells, and mast cells. Malignant epithelial cells were identified through copy number variation (CNV) analysis, followed by differential gene expression screening and functional enrichment analysis. Cell-cell communication analysis was employed to delineate intercellular interactions, and key findings were validated using a lipopolysaccharide (LPS)-stimulated Transwell co-culture model incorporating TPC-1 cells, Nthy-ori 3-1 cells, and mast cells. We found that IL1B was significantly upregulated in malignant FECs, and differentially expressed genes were enriched in immune-related processes including antigen presentation and lymphocyte activation. Notably, IL1B- FECs specifically communicated with mast cells via the FN1/CD44 signaling axis. In vitro experiments further confirmed that, under inflammatory conditions, mast cells secreted IL-8, which activated the PI3K/AKT pathway and promoted malignant phenotypes. Collectively, these findings suggest that IL1B⁻ FECs recruit mast cells through the FN1/CD44 axis, and mast cell-derived IL-8 subsequently activates the PI3K/AKT pathway to drive the transformation from HT to PTC, providing novel mechanistic insights into the “inflammation-to-cancer” transition.