Immunometabolic Activation of Invariant Natural Killer T Cells

Abstract

Invariant natural killer T (iNKT) cells are lipid-reactive T cells with profound immunomodulatory potential. They are unique in their restriction to lipid antigens presented in CD1d molecules, which underlies their role in lipid-driven disorders such as obesity and atherosclerosis. In this review, we discuss the contribution of iNKT cell activation to immunometabolic disease, metabolic programming of lipid antigen presentation, and immunometabolic activation of iNKT cells. First, we outline the role of iNKT cells in immunometabolic disease. Second, we discuss the effects of cellular metabolism on lipid antigen processing and presentation to iNKT cells. The synthesis and processing of glycolipids and other potential endogenous lipid antigens depends on metabolic demand and may steer iNKT cells toward adopting a Th1 or Th2 signature. Third, external signals such as toll-like receptor ligands, adipokines, and cytokines modulate antigen presentation and subsequent iNKT cell responses. Finally, we will discuss the relevance of metabolic programming of iNKT cells in human disease, focusing on their role in disorders such as obesity and atherosclerosis. The critical response to metabolic changes places iNKT cells at the helm of immunometabolic disease.

Keywords: AMPK; NKT; atherosclerosis; immunometabolism; mTOR; obesity; sphingolipid.

Figure 1

CD1d lipid loading at the crossroads of glycosphingolipid metabolism. In the ER–Golgi pathway that is similar for mouse and human, CD1d heavy chains assemble with β2M and chaperone lipids in the ER before transit to the cell surface. Alternatively, the Golgi complex produces GSL that are loaded onto CD1d by microsomal transfer protein, and may serve as endogenous lipid antigens. Ceramide precursors are transported to the Golgi by ceramide transfer protein (CERT). In the Golgi, UGCG and other glycosyltransferases convert ceramide into GSL. These GSL endproducts can be loaded onto CD1d, or transported to the plasma membrane in membrane-bound transport carriers. In the endolysosomal pathway, mouse CD1d1 may target the endosome and lysosome directly via interaction with sorting motifs adaptor protein 2 (AP2), which targets the endosomal compartment, and adaptor protein 3 (AP3), which targets the lysosomal compartment. Human CD1d is internalized via AP2 but cannot bind AP3. However, human CD1d can still be found in the lysosomal compartment. Possibly, ER-resident CD1d proteins gain access to the endolysosomal compartment via an auxiliary pathway, in conjunction with MHC class II-associated invariant chain (li). In the endolysosomal compartments, CD1d proteins are loaded with exogenous or endogenous lipid antigens, orchestrated by a variety of lipid transfer proteins including GM2 activator (GM2a), saposins A-D (Sap), and Niemann-Pick type C2 protein. Exogenous lipid antigens are delivered to the endosomal compartment via endocytosis of LDLR-associated glycolipids, MR-associated microbial lipids, and other scavenger receptors. Some exogenous lipids require processing into antigenic lipids before CD1d-loading, for example, through lipid hydrolases (Hy). Endogenous lipid antigens are delivered to the endolysosomal compartment via endocytosis of membrane-associated GSL, which can be loaded onto CD1d or degraded in lysosomes by glycohydrolases (Gly-Hy) and accessory proteins, before recycling to the ER (salvage pathway). Upon lipid antigen loading in the endolysosomal compartments, CD1d–lipid complexes recycle back to the cell surface for interaction with the invariant TCR on invariant natural killer T cells. Abbreviations: AP, adaptor protein; β2M, β2-microglobulin; ER, endoplasmic reticulum; Gly-Hy, glycohydrolase; GM2a, GM2 activator; GSL, glycosphingolipids; Hy, hydrolase; LDLR, LDL-receptor; Li, MHC class II-associated invariant chain; MR, mannose receptor; Sap, saposins; TCR, T-cell receptor; UGCG, UDP-glycose ceramide glucosyltransferase; VLDL, very-low-density lipoprotein.

Figure 2

Immunometabolic activation of invariant natural killer T (iNKT) cells. Schematic representation of metabolic re-programming of iNKT cell function in immunometabolic disease. Metabolic changes in the antigen-presenting cell may affect the GSL ligand pool or alter the availability of co-stimulatory molecules. AMPK versus mTOR are depicted as the main metabolic regulators of the cellular metabolic program, they can each inhibit the other. AMPK is activated in low glucose conditions and can be activated by adiponectin, the adipokine secreted by lean adipocytes. AMPK activation drives fatty acid oxidation and is associated with cellular longevity. In lean fat- or homeostatic conditions, the iNKT cell response is mainly Th2 skewed (IL-4), but changes to Th1 in metabolic disease (IFN-γ). Whether AMPK or mTOR activation can be linked directly to Th1 or Th2 skewing of the iNKT cell response has not yet been studied to our knowledge. In dyslipidemic conditions, the hallmark of metabolic disease, FFA may activate mTOR via TLR4 signaling. The TLR4-driven glycolysis observed in metabolic disease is reminiscent of glycolysis observed under normoxic conditions in cancer (the Warburg effect). mTOR blocks AMPK and in that manner removes at least one of the brakes on CD40 upregulation and IL-12 production, the co-stimulatory requirements for a Th1 iNKT cell response. In late stage metabolic disease, iNKT cells have been described as anergic, while simultaneously upregulating PD-1. Blocking the leptin receptor on iNKT cells may reverse this anergic state. Abbreviations: GSL, glycosphingolipid; FFA, free fatty acids.