Lysosomal Acid Lipase Deficiency Controls T- and B-Regulatory Cell Homeostasis in the Lymph Nodes of Mice with Human Cancer Xenotransplants

Utilization of proper preclinical models accelerates development of immunotherapeutics and the study of the interplay between human malignant cells and immune cells. Lysosomal acid lipase (LAL) is a critical lipid hydrolase that generates free fatty acids and cholesterol. Ablation of LAL suppresses immune rejection and allows growth of human lung cancer cells in lal^−/− mice. In the lal^−/− lymph nodes, the percentages of both T- and B-regulatory cells (Tregs and Bregs, respectively) are increased, with elevated expression of programmed death-ligand 1 and IL-10, and decreased expression of interferon-γ. Levels of enzymes in the glucose and glutamine metabolic pathways are elevated in Tregs and Bregs of the lal^−/− lymph nodes. Pharmacologic inhibitor of pyruvate dehydrogenase, which controls the transition from glycolysis to the citric acid cycle, effectively reduces Treg and Breg elevation in the lal^−/− lymph nodes. Blocking the mammalian target of rapamycin or reactivating peroxisome proliferator-activated receptor γ, an LAL downstream effector, reduces lal^−/− Treg and Breg elevation and PD-L1 expression in lal^−/− Tregs and Bregs, and improves human cancer cell rejection. Treatment with PD-L1 antibody also reduces Treg and Breg elevation in the lal^−/− lymph nodes and improves human cancer cell rejection. These observations conclude that LAL-regulated lipid metabolism is essential to maintain antitumor immunity.

Transplantable animal models, in which the relationship and interplay between malignant cells and immune cells can be studied, play a fundamental role in the study of oncoimmunology and development of therapeutic approaches to treat human cancer. Utilization of proper preclinical models accelerates development of immunotherapies and understanding of underlying mechanisms.^1,2 The host immune system determines the fate of invading cancer cells.³ In searching for appropriate immunosuppressive mouse models to study human cancer-derived xenografts, a genetic ablation mouse model (lal^−/−) of lysosomal acid lipase (LAL) was evaluated.⁴ LAL is a lipid metabolic enzyme catalyzing the hydrolysis of cholesteryl esters and triglycerides in the lysosome to generate free fatty acids and cholesterol.⁵ The hydrolyzed products are transported to the cytoplasm for either storage or utilization in membrane biogenesis, steroid hormone synthesis, and energy production. Although lal^−/− mice survive into adulthood, the metabolic defect of LAL deficiency leads to severe immunodeficiency, in which the lymphocyte levels are extremely low because of impaired development, maturation, and proliferation.⁶ The ratio of T-regulatory cells (Tregs) and myeloid-derived suppressive cells (MDSCs) is significantly increased, which suppresses T-cell function and directly stimulates tumor growth and invasion.6, 7, 8, 9, 10 More importantly, the compromised immunity accelerates growth and invasion of various murine tumor cells not only in syngeneic, but also allogeneic, lal^−/− mice.⁹ In this report, we show that immunodeficiency delays and reduces immune rejection of human cancer cells in lal^−/− mice. Herein, functional roles of Tregs and B-regulatory cells (Bregs) in the lal^−/− lymph nodes during failure of immune rejection, as well as the abnormal expression and function of PD-L1 in lal^−/− Tregs and Bregs in association with the metabolic switch toward glycolysis and glutamine utilization are systematically evaluated. Furthermore, the functional roles of mammalian target of rapamycin (mTOR) and peroxisome proliferator-activated receptor γ (PPARγ) nuclear receptor are characterized in Tregs and Bregs of the lal^−/− lymph nodes during human cancer cell rejection. We conclude that Tregs and Bregs in the lal^−/− lymph nodes play critical roles in suppression of immune rejection of human cancer growth, together with other cell types [eg, MDSCs and endothelial cells (ECs)], in lal^−/− mice. Therefore, the immunodeficient lal^−/− mouse model serves as a potential preclinical model that can be used for clinical human cancer studies.