Critical Role of the mTOR Pathway in Development and Function of Myeloid-Derived Suppressor Cells in lal−/− Mice

Lysosomal acid lipase (LAL) is essential for the hydrolysis of cholesteryl esters and triglycerides to generate cholesterol and free fatty acids in cellular lysosomes. Ablation of the lal gene (lal−/−) systemically increased expansion of cluster of differentiation molecule 11b (CD11b), lymphocyte antigen 6G (Ly6G) myeloid-derived suppressor cells (MDSCs) that caused myeloproliferative neoplasms in mice. Study of lal−/− bone marrow Ly6G+ MDSCs via transcriptional profiling showed increases in mammalian target of rapamycin (mTOR) signaling pathway transcripts. Injection of mTOR pharmacologic inhibitors into lal−/− mice significantly reduced bone marrow myelopoiesis and systemic CD11b+Ly6G+ cell expansion. Rapamycin treatment of lal−/− mice stimulated a shift from immature CD11b+Ly6G+ cells to CD11b+ single-positive cells in marrow and tissues and partially reversed the increased cell proliferation, decreased apoptosis, increased ATP synthesis, and increased cell cycling of bone marrow CD11b+Ly6G+ cells obtained from lal−/− mice. Pharmacologic and siRNA suppression of mTOR, regulatory-associated protein of mTOR, rapamycin-insensitive companion of mTOR, and Akt1 function corrected CD11b+Ly6G+ cell in lal−/− mice development from Lin− progenitor cells and reversed the immune suppression on T-cell proliferation and function in association with decreased reactive oxygen species production, and recovery from impairment of mitochondrial membrane potential compared with control mutant cells. These results indicate a crucial role of LAL-regulated mTOR signaling in the production and function of CD11b+Ly6G+ cells. The mTOR pathway may serve as a novel target to modulate the emergence of MDSCs in those pathophysiologic states in which these cells play an immunosuppressive role.