Flow cytometric analysis of neutrophil populations present in orthotopic KCKO cancers

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 anti-cancer CD8 T cell cytotoxicity. The strong suppressive
function was successfully recapitulated by incubating naïve 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 upregulated 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.