PGLYRP-1 Confers Immune Evasive Properties to Pancreactic Cancer Stem Cells [2015]

The crosstalk between tumor and immune cells has important implications for cancer treatment, in particular for immunotherapies. Pancreatic ductal adenocarcinoma (PDAC) generally displays a chemoresistant phenotype and is refractory to immune-based therapies. Highly tumorigenic and stem-like cells known as cancer stem cells (CSCs) within PDAC tumors have been demonstrated to contribute to its aggressiveness, metastatic capacity and chemoresistance. Here, we show that CSCs could be also relevant to establish an immunosuppresive environment. Using genetically engineered mouse models (GEMMs) of PDAC we isolated a triple positive (EpCAM+, Sca-1+ and CD133+) population of tumor cells, functionally validated these cells as bona fide CSCs and probed their transcriptional profile by microarray analysis. In addition to stem pathways, signatures related to tumor immune evasion, immune modulation, invasiveness, and innate immune response were enriched in this population. Peptidoglycan recognition protein 1 (Pglyrp1), an innate immune response anti-bacterial protein expressed primarily by neutrophils, was highly expressed in this triple-positive population and in CSC-enriched spheres from human cell lines. Interestingly, modulation of Pglyrp1 expression in murine PDAC primary cultures affected their immune evasive properties, specifically their ability to escape macrophage- and T-cell-mediated killing in vitro and to establish tumors in immunocompetent hosts. Pglyrp1 expression in tumor cells seemed to interfere with TNFα-mediated killing by T cells and to promote activated T-cell cytotoxicity. Importantly, these results could be validated in CSCs from human patient-derived xenografts, and PGLYRP1 protein levels were also significantly higher in PDAC patient samples versus healthy controls. Thus, our study demonstrates a hierarchical organization of cancer cells in a murine model of PDAC, validating the CSC concept, and identifies Pglyrp1 as a putative PDAC biomarker that confers immune privilege properties to CSCs which could have important therapeutic implications. Tumor cells from mouse pancreatic tumors were isolated using enzymatic digestion with collagenase P and trypsin. Cells were grown in RPMI medium with 10%fetal bovine serum and 50units/ml penicillin/streptomycin. Epithelial clones were picked and expanded. For adherent conditions, cells were grown for 4 days in RPMI medium with 10%fetal bovine serum and 50units/ml penicillin/streptomycin in regular culture dishes. For sphere conditions, cells were grown for 4 days in ultra-low attachment plates using serum-free RPMI supplemented with B27 1:50, 20 ng/mL bFGF, L-Glutamine and 50 units/ml penicillin/streptomycin.