A distinctive tumor compartment in pancreatic lobules defined by nascent stroma and classical tumor cell phenotype

Pancreatic ductal adenocarcinoma (PDAC) is a highly aggressive tumor type characterized by a particularly extensive stroma. While different types of cancer-associated fibroblasts (CAFs) in this desmoplastic stroma have been described, areas of early invasion and nascent stroma are understudied. Here, we identify a distinctive PDAC niche within the pancreatic lobules, a compartment dominated by pancreatic exocrine cells and slender stroma. Cellular interaction profiling using machine learning on whole slide images of human PDAC reveals that the tumor invasion front in the lobules is dominated by specific interactions of tumor cells and exocrine cells that have undergone acinar-to-ductal metaplasia (ADM). Multiplex protein and mRNA stains confirm that tumor growth in the lobules is closely linked to ADM in the lobules, and reveal stromal protein gradients from the gracile lobular stroma to the characteristic desmoplastic stroma. We identify nascent CAFs (nCAFs), co-expressing expressing nerve growth factor receptor (NGFR) and platelet-derived growth factor receptor alpha (PDGFRa) that are absent in the mature, desmoplastic stroma. Lobular invasion and nCAFs are intertwined with phenotypic changes of the cancer cells, such that tumor cells in lobules express classical subtype markers, while those embedded in the desmoplastic are on the basal end of the phenotypic continuum. In mice, the PDAC subtype - basal or classical - similarly depends on tissue location, suggesting microenvironmental factors rather than clonal selection as important drivers of tumor phenotype identity. Clinically, our results mandate factoring in tumor tissue location when calling PDAC subtypes. Biologically, they identify pancreatic lobules as a distinctive tissue niche associated with nascent stroma, and they suggest that lobular colonization by tumor cells is a significant route of PDAC progression.