Lysosomes in the immunometabolic reprogramming of immune cells in atherosclerosis

Abstract

Lysosomes have a central role in the disposal of extracellular and intracellular cargo and also function as metabolic sensors and signalling platforms in the immunometabolic reprogramming of macrophages and other immune cells in atherosclerosis. Lysosomes can rapidly sense the presence of nutrients within immune cells, thereby switching from catabolism of extracellular material to the recycling of intracellular cargo. Such a fine-tuned degradative response supports the generation of metabolic building blocks through effectors such as mTORC1 or TFEB. By coupling nutrients to downstream signalling and metabolism, lysosomes serve as a crucial hub for cellular function in innate and adaptive immune cells. Lysosomal dysfunction is now recognized to be a hallmark of atherogenesis. Perturbations in nutrient-sensing and signalling have profound effects on the capacity of immune cells to handle cholesterol, perform phagocytosis and efferocytosis, and limit the activation of the inflammasome and other inflammatory pathways. Strategies to improve lysosomal function hold promise as novel modulators of the immunoinflammatory response associated with atherosclerosis. In this Review, we describe the crosstalk between lysosomal biology and immune cell function and polarization, with a particular focus on cellular immunometabolic reprogramming in the context of atherosclerosis.

Fig. 1 |. Lysosomes orchestrate macrophage metabolic reprogramming.

Macrophage plasticity can be broadly exemplified by the characteristics of the two most extreme subsets of the macrophage phenotype spectrum: namely pro-inflammatory or anti-inflammatory (pro-resolving) macrophages. The energy metabolism of these macrophage subsets (including lysosomal dynamics) is adapted to their function. a, Pro-inflammatory macrophages preferentially have anabolic glycolytic metabolism with lactate production from pyruvate coupled with the presence of fragmented mitochondria (produced by fission process in which mitochondria divide into separate organelles), the production of mitochondrial reactive oxygen species (ROS) and accumulation of α-ketoglutarate (αKG) and arginine in the context of disrupted tricarboxylic acid, all of which can contribute to metabolic reprogramming. This metabolic configuration is driven by activation of mechanistic target of rapamycin complex 1 (mTORC1), which is anchored to the lysosomal membrane by the accessory proteins neutral amino acid transporter 9 (SLC38A9) and lysosomal cholesterol signalling protein (LYCHOS). mTORC1 activation results in the sequestration in the cytoplasm of transcription factor EB (TFEB) via its phosphorylation and the inhibition of lysosomal functions such as autophagy (including mitophagy and lipophagy) and lipid degradation of cholesteryl esters (CE) and triglycerides (TG) by lysosomal acid lipase (LIPA). b, Anti-inflammatory macrophages rely on cellular oxidative phosphorylation, in which fused mitochondria have a key role. Fused mitochondria contribute to ATP generation from fatty acid oxidation (FAO) that is sustained by the lysosomal hydrolysis of lipid droplet-derived free cholesterol (FC) and fatty acids (FA) from CE and TG, respectively, catalysed by LIPA. Accordingly, anti-inflammatory macrophages have AMP-activated protein kinase (AMPK) activation and lipophagy driven by mTORC2.

Fig. 2 |. Lysosomes govern the immunometabolic phenotype of lymphocytes.

Lymphocytes show a distinct dependency on lysosome function depending on their activation and polarization state. a, Activated cytotoxic CD8⁺ T cells show an upregulation of cationic transporters (CAT), large-neutral amino acid (AA) transporters (LAT) and LDL receptor (LDLR), which fuel the flux of AA (arginine and α-ketoglutarate (αKG)) and free cholesterol (FC) to the lysosome, where they promote lysosomal interaction between neutral amino acid transporter 9 (SCL38A9) and mechanistic target of rapamycin complex 1 (mTORC1). This adaptation metabolically sustains T cell proliferation and the cytotoxic response. b, Regulatory T (T_reg) cells maintain their forkhead box protein P3 (FOXP3)-driven immunosuppressive phenotype by restraining mTORC1 activation and enabling autophagy. Autophagy-related proteins (ATGs), such as ATG5 and ATG7, are crucial for the initiation of autophagy in T_reg cells. c, By contrast, in memory T cells, a ‘futile cycle’ in which fatty acids (FA) synthesised de novo following glycolysis undergo degradation through FA oxidation (FAO) to support long-lasting ATP production. The lysosome has a central role in this process, contributing to the hydrolysis of triglycerides (TG) from lipid droplets by lysosomal acid lipase (LIPA). d, In B cells, antigen presentation occurs through lysosomal uptake and degradation of antigens that are recognized as not-self and the loading of a small sequence of the original antigen (the peptide) onto major histocompatibility complex class II (MHCII) molecules. Lysosomes participate in antigen processing and presentation by clustering in close proximity to the immunological synapse, the region of the cell membrane that is enriched in immune receptors and where peptides are loaded onto MHCII and presented to CD4⁺ T cells. BCR, B cell receptor; CE, cholesteryl esters.

Fig. 3 |. Lysosomes in macrophage-promoted inflammation or inflammation resolution in atherosclerosis.

Lipid deposition and immune cell enrichment within the arterial wall intima are the hallmarks of atherosclerosis. Macrophages are the main immune cell subsets that accumulate in atherosclerotic plaques, where they can become foam cells or act as efferocytes. a, Foam cells are characterized by impaired lysosomal degradative function that results in lipid accumulation due to reduced lysosomal acid lipase (LIPA) activity. Defective autophagy leads to endoplasmic reticulum (ER) stress, activation of the unfolded protein response (UPR) and mitochondrial stress associated with ROS production and mitochondrial DNA (mtDNA)-induced DNA-sensor responses, such as activation of the AIM2 inflammasome. Continuous uptake of oxidized LDL (oxLDL), together with increased Toll-like receptor (TLR) signalling can lead to cholesterol crystal formation and lysosomal membrane permeabilization, which promotes leakage of lysosomal proteases, such as cathepsins. A downstream consequence of cathepsin leakage is the activation of cell death pathways, including those involving the NLRP3 inflammasome and gasdermin D (GSDMD). b, After engulfment of apoptotic bodies, efferocytes engage in the canonical autophagic pathway or in the non-canonical pathway, known as LC3-associated phagocytosis (LAP). In the canonical pathway, elements of the autophagic machinery lead to the formation of the phagolysosome. In LAP, LC3 is conjugated to the phagosomal membranes to generate the LAPosome. This process leads to fusion of the LAPosome with the lysosomal membrane via the SNARE complex and to the release of their content into the lysosomal lumen. With the upregulation of LIPA, lipids present in apoptotic cells are hydrolysed. In parallel, fatty acids (FA) are used to generate ATP through mitochondrial FA oxidation (FAO), whereas free cholesterol (FC) is exported by phospholipid-transporting ATPase ABCA1 transporters. Glucose and glutamine are converted to glutamate by glutaminase 1 (GLS1) and enter the tricarboxylic acid (TCA) cycle, leading to ATP production. This process, together with arginine-derived putrescine from the urea cycle, contributes to actin polymerization and perpetuates efferocytosis. BAX, apoptosis regulator BAX; BID, BH3-interacting domain death agonist; CatG, cathepsin G; CD36, platelet glycoprotein 4; CE, cholesteryl esters; DAMP, damage-associated molecular pattern; DBL2, guanine nucleotide exchange factor; ELMO, engulfment and cell motility proteins; IRF3, interferon regulatory factor 3; MLKL, mixed lineage kinase domain-like protein; NF-κB, nuclear factor-κB; NOX, NADPH oxidase; RAC1, RAS-related C3 botulinum toxin substrate 1; RIPK, serine/threonine-protein kinase RIPK; SLC2A1, solute carrier family 2, facilitated glucose transporter member 1; STING, Stimulator of interferon genes protein; TG, triglycerides; V-ATPase, V-type proton ATPase.