Background: Myeloid-derived suppressor cells (MDSCs) are a heterogeneous population of cells in tumor microenvironment, which suppress antitumor immunity. Expansion of various MDSC subpopulations is closely associated with poor clinical outcomes in cancer. Lysosomal acid lipase (LAL) is a key enzyme in the metabolic pathway of neutral lipids, whose deficiency (LAL-D) in mice induces the differentiation of myeloid lineage cells into MDSCs. These Lal -/- MDSCs not only suppress immune surveillance but also stimulate cancer cell proliferation and invasion. Understanding and elucidating the underlying mechanisms of MDSCs biogenesis will help to facilitate diagnosis/prognosis of cancer occurrence and prevent cancer growth and spreading. Methods: Single-cell RNA sequencing (scRNA-seq) was performed to distinguish intrinsic molecular and cellular differences between normal versus Lal -/- bone marrow-derived Ly6G+ myeloid populations in mice. In humans, LAL expression and metabolic pathways in various myeloid subsets of blood samples of patients with non-small cell lung cancer (NSCLC) were assessed by flow cytometry. The profiles of myeloid subsets were compared in patients with NSCLC before and after the treatment of programmed death-1 (PD-1) immunotherapy. Results: scRNA-seq of Lal -/- CD11b+Ly6G+ MDSCs identified two distinctive clusters with differential gene expression patterns and revealed a major metabolic shift towards glucose utilization and reactive oxygen species (ROS) overproduction. Blocking pyruvate dehydrogenase (PDH) in glycolysis reversed Lal -/- MDSCs' capabilities of immunosuppression and tumor growth stimulation and reduced ROS overproduction. In the blood samples of human patients with NSCLC, LAL expression was significantly decreased in CD13+/CD14+/CD15+/CD33+ myeloid cell subsets. Further analysis in the blood of patients with NSCLC revealed an expansion of CD13+/CD14+/CD15+ myeloid cell subsets, accompanied by upregulation of glucose-related and glutamine-related metabolic enzymes. Pharmacological inhibition of the LAL activity in the blood cells of healthy participants increased the numbers of CD13+ and CD14+ myeloid cell subsets. PD-1 checkpoint inhibitor treatment in patients with NSCLC reversed the increased number of CD13+ and CD14+ myeloid cell subsets and PDH levels in CD13+ myeloid cells. Conclusion: These results demonstrate that LAL and the associated expansion of MDSCs could serve as targets and biomarkers for anticancer immunotherapy in humans. Keywords: immunotherapy; myeloid-derived suppressor cells; tumor biomarkers.

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