NKT cells: T lymphocytes with innate effector functions
Introduction
It is now well established that some T lymphocytes can react with lipid and glycolipid antigens rather than with peptides [1]. This antigen-specificity is characteristic of T lymphocytes that are restricted by the MHC class I-related antigen-presenting molecule CD1d [1]. CD1d-restricted T cells express phenotypic markers that are typical of conventional T cells and natural killer (NK) cells, and these cells are therefore referred to as natural killer T (NKT) cells. Two subtypes of CD1d-restricted NKT cells have been identified [2] (Figure 1). Type I NKT cells, also called invariant NKT (iNKT) cells, have a highly restricted T-cell receptor (TCR) repertoire and express TCR Vα14–Jα18 and Vβ8.2, -7 or -2 chains in mice and homologous TCR Vα24–Jα18 and Vβ11 chains in humans. Type II NKT cells, which are also referred to as non-iNKT cells, express a more diverse TCR repertoire.
Most studies using NKT cells have focused on iNKT cells. All iNKT cells from mice and humans react with the marine sponge-derived glycolipid α-galactosylceramide (α-GalCer; Figure 1a); consequently, staining with fluorescently labeled α-GalCer-loaded CD1d molecules is a reliable way to identify these cells [2]. A hallmark of iNKT cells is their capacity to rapidly (within hours) produce a mixture of T helper type 1 (Th1) and Th2 cytokines upon TCR engagement [1, 3, 4]. Stimulation of iNKT cells in this manner also results in the activation of a variety of other cell types, including dendritic cells (DCs), NK cells, B cells and conventional T cells. Consequently, iNKT cell activation can strongly impact adaptive immune responses and can influence a wide array of host defenses and pathologies. However, the mechanisms of iNKT cell activation and immune modulation remain poorly understood. Because of their potent immunomodulatory properties, iNKT cells hold great promise as targets for the development of vaccine adjuvants and immunotherapies. Preclinical studies have demonstrated that the iNKT cell ligand α-GalCer and related glycolipids exhibit therapeutic activities against several tumors and a variety of autoimmune and inflammatory diseases, but mechanisms have remained poorly understood [1, 3, 4].
In this review, I will highlight progress made during the past two years in understanding the specificity, mechanisms of activation, effector functions and therapeutic properties of iNKT cells and, where available, non-iNKT cells.
Section snippets
Physiological NKT cell antigens
Recent studies have provided much insight into the physiological antigens that can activate NKT cells. Although reactivity of iNKT cells to α-GalCer was demonstrated several years ago, this reagent was derived from a marine sponge and, therefore, has been thought to be a mimic of a natural endogenous or exogenous ligand. Reactivity of iNKT cells with lipids derived from certain autologous cells, tumors, microbes and pollen has been documented [5], but this reactivity appears to be restricted to
CD1d-restricted antigen-presentation mechanisms
Studies over the past several years have provided evidence for remarkable relationships between the CD1d antigen-presentation pathway and lipid metabolism [13, 14] (Figure 2). CD1d molecules are synthesized in the endoplasmic reticulum, where they initially assemble with cellular phosphatidylinositol (PtdIns) and/or PtdIns-glycans, which are thought to stabilize the hydrophobic CD1d-binding pocket and facilitate CD1d transport to the cell surface [15]. A few years ago, it was demonstrated that
Microbial subversion of CD1d-restricted antigen presentation
Because NKT cells play a crucial role in protective immune responses against a variety of microbial pathogens, it is not surprising that pathogens, in particular viruses that establish latency, have devised ways to interfere with the CD1d antigen-presentation pathway. Several elegant mechanisms have been identified [18] (Figure 2). The modulator of immune recognition (MIR) proteins of Kaposi sarcoma-associated herpesvirus promote the downregulation of cell surface CD1d expression [19•]. MIR1
Mechanisms of NKT cell activation
CD1d is expressed on a variety of cell types, including CD4+CD8+ thymocytes, hepatocytes, B cells, macrophages and DCs, but the identity of the physiological APCs that are responsible for iNKT cell activation in vivo has remained unclear. Two groups of investigators recently demonstrated that DCs are crucially important for activating iNKT cells in vivo following α-GalCer treatment of mice [26•, 27•]. Presentation of α-GalCer by DCs resulted in profound Th1 and Th2 cytokine production and NK
NKT cell development, effector differentiation and homeostasis
Like conventional T cells, NKT cells develop in the thymus [36]. CD1d is required for positive selection of iNKT cells, which is mediated by double-positive (DP) thymocytes rather than by thymic epithelial cells. As hexb-deficient mice are defective in iNKT cell maturation, it has been suggested that iGb3 functions as an endogenous iNKT cell agonist that mediates positive selection of these cells [6]. However, because mice that carry a diverse range of mutations that affect glycolipid
NKT cell functions
iNKT cells have been implicated in immune responses against infectious agents, tumors and tissue grafts and in regulating a variety of autoimmune and inflammatory diseases [1, 3, 4, 29]. Recent studies have further revealed a role of iNKT cells in regulating hematopoiesis, during both steady-state conditions and conditions of activation (i.e. α-GalCer treatment) [58]. Consequently, iNKT cell-deficient mice displayed impaired hematopoiesis. These effects of iNKT cells correlated with their
Functions of distinct NKT cell subsets and subtypes
Two main populations of iNKT cells have been identified: CD4+ and CD4−CD8− (double negative [DN]) iNKT cells. However, functional differences between these distinct subsets of iNKT cells have remained unclear [73]. A recent article compared the antitumor activities of CD4+ and DN iNKT cells from different organs in mice [74•]. It was found that the DN subset of iNKT cells in the liver had superior antitumor activities to the CD4+ subset from liver and both the CD4+ and DN subsets from thymus
Exploiting the effector functions of NKT cells for immunotherapy
There has been continued progress in developing iNKT cell-based adjuvants and immunotherapies of cancer, autoimmune and inflammatory diseases [4, 29, 77]. However, preclinical studies have also revealed that the therapeutic efficacy of α-GalCer is influenced by a wide variety of parameters, including the dose, frequency, timing and route of treatment, as well as the genetic background, sex and age of the animals and the particular animal model that is analyzed.
In mice, significant side-effects
Conclusions
NKT cells are now recognized as bridging the innate and adaptive immune systems. Although important progress has been made regarding the specificity and effector functions of these cells, much remains to be learned. Important areas of future research include: the role of iGb3 and other endogenous glycolipids in NKT cell development and function; the specificity and function of distinct lipid-binding proteins in CD1d-restricted antigen presentation; the relevance of the relationship between
Update
Two recent studies [86•, 87•] have called the physiological role of iGb3 as an endogenous iNKT cell selecting ligand into question. These new findings re-open the search for endogenous iNKT cell antigens.
References and recommended reading
Papers of particular interest, published within the period of review, have been highlighted as:
• of special interest
•• of outstanding interest
Acknowledgements
I apologize to colleagues whose work I did not cite due to space constraints or omission. I thank members of my laboratory and many collaborators, including Drs. Sebastian Joyce, Yasu Koezuka, Mitchell Kronenberg, Amy Major, Fu-Dong Shi, Ram Singh, Derya Unutmaz, and Chyung-Ru Wang for helpful discussions. I also thank Dr. Sebastian Joyce for critical reading of the manuscript. Work in my laboratory has been supported by grants from the National Institutes of Health (HL68744, AI50953 and
References (87)
- et al.
Apolipoprotein-mediated pathways of lipid antigen presentation
Nature
(2005) - et al.
Viral evasion of antigen presentation: not just for peptides anymore
Nat Immunol
(2006) - et al.
Regulation of CD1d expression and function by a herpesvirus infection
J Clin Invest
(2005) - et al.
Distinct roles of dendritic cells and B cells in Va14Ja18 natural T cell activation in vivo
J Immunol
(2005) - et al.
Expansion and long-range differentiation of the NKT cell lineage in mice expressing CD1d exclusively on cortical thymocytes
J Exp Med
(2005) - et al.
Defective NKT cell development in mice and humans lacking the adapter SAP, the X-linked lymphoproliferative syndrome gene product
J Exp Med
(2005) - et al.
T-bet concomitantly controls migration, survival, and effector functions during the development of Vα14i NKT cells
Blood
(2006) - et al.
CD4+CD25+ Tregs and NKT cells: regulators regulating regulators
Trends Immunol
(2006) - et al.
CD1-reactive natural killer T cells are required for development of systemic tolerance through an immune-privileged site
J Exp Med
(1999) - et al.
Cooperation of invariant NKT cells and CD4+CD25+ regulatory T cells in the prevention of autoimmune myasthenia
J Immunol
(2005)