Chronic inflammatory milieu in the tumor microenvironment (TME) leads to the recruitment and differentiation of myeloid-derived suppressor cells (MDSCs). Polymorphonuclear (PMN)-MDSCs, which are phenotypically and morphologically defined as a subset of neutrophils, cause major immune suppression in the TME, posing a significant challenge in the development of effective immunotherapies. Despite recent advances in our understanding of PMN-MDSC functions, the mechanism that gives rise to immunosuppressive neutrophils within the TME remains elusive. Both in vivo and in vitro, newly recruited neutrophils into the tumor sites remained activated and highly motile for several days and developed immunosuppressive phenotypes, as indicated by increased arginase 1 (Arg1) and dcTrail-R1 expression and suppressed anticancer CD8 T cell cytotoxicity. The strong suppressive function was successfully recapitulated by incubating naive neutrophils with cancer cell culture supernatant in vitro. Cancer metabolite secretome analyses of the culture supernatant revealed that both murine and human cancers released lipid mediators to induce the differentiation of immunosuppressive neutrophils. Liquid chromatography–mass spectrometry (LC-MS) lipidomic analysis identified platelet-activation factor (PAF; 1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine) as a common tumor-derived lipid mediator that induces neutrophil differentiation. Lysophosphatidylcholine acyltransferase 2 (LPCAT2), the PAF biosynthetic enzyme, is up-regulated in human pancreatic ductal adenocarcinoma (PDAC) and shows an unfavorable correlation with patient survival across multiple cancer types. Our study identifies PAF as a lipid-driven mechanism of MDSC differentiation in the TME, providing a potential target for cancer immunotherapy.

Background/Objectives: Pancreatic ductal adenocarcinoma (PDAC), expecting to be the second leading cause of cancer deaths by 2030, resists immune checkpoint therapies due to its immunosuppressive tumor microenvironment (TME). Leukemia inhibitory factor (LIF) is a key target in PDAC, promoting stemness, epithelial–mesenchymal transition (EMT), and therapy resistance. Phase 1 clinical trials showed anti-LIF therapy is safe but with limited efficacy, suggesting better outcomes when combined with chemotherapy, radiotherapy, or immunotherapy. Methods: We assessed the combination of chemotherapy (gemcitabine/nab-paclitaxel) and dual blockade of LIF and PD-L1 on tumor growth and survival in orthotopic and spontaneous PDAC models. Flow cytometry and scRNA-seq were utilized to monitor the antitumor immune response. The role of key immune cells was further confirmed by depleting these immune cells, including CD4, CD8, or inflammatory monocytes. Results: Sequential treatment with chemotherapy (gemcitabine/nab-paclitaxel) and dual blockade of LIF and PD-L1 significantly improved antitumor efficacy compared to monotherapy or dual combinations of these therapies. This chemo/anti-LIF/anti-PD-L1 approach reduced EMT in tumor cells and enhanced the antitumor immune response, primarily through CD8 T cells, as depleting CD8 cells largely abrogated the effect of treatment. This combination therapy also shifted macrophages and dendritic cells towards an antitumor phenotype. Conclusions: The combination of chemotherapy, anti-LIF, and anti-PD-L1 not only targeted tumor cells but also augmented the anti-tumor immune response. These findings strongly support advancing chemo/anti-LIF/anti-PD-L1 combination therapy to clinical trials in PDAC.