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1. REVIEWS CD1 antigen presentation: how it works Duarte C. Barral and Michael B. Brenner Abstract | The classic concept of self–non-self discrimination by the immune system focused on the recognition of fragments from proteins presented by classical MHC molecules. However, the discovery of MHC-class-I-like CD1 antigen-presentation molecules now explains how the immune system also recognizes the abundant and diverse universe of lipid- containing antigens. The CD1 molecules bind and present amphipathic lipid antigens for recognition by T-cell receptors. Here, we outline the recent advances in our understanding of how the processes of CD1 assembly, trafficking, lipid-antigen binding and T-cell activation are achieved and the new insights into how lipid antigens differentially elicit CD1-restricted innate and adaptive T-cell responses.Endocytic pathway The CD1 family of MHC-class-I-like glycoproteins strategies by which antigen-presenting cells (APCs)A trafficking pathway used by (CD1a, CD1b, CD1c, CD1d and CD1e) present both stimulate CD1-restricted T-cell responses and howall cells for the internalization foreign and self lipids as cognate antigens to T cells. these responses are linked to the types of lipid antigenof molecules from the plasma The CD1 proteins share sequence homology and (self versus foreign), the CD1 isoform (group 1 versusmembrane to lysosomes. overall domain structure with MHC class I molecules, group 2) and the responding T-cell population (clonallyCapping motifs being comprised of a heavy chain with three extracell specific versus population en masse). The mechanismsCarbohydrates that are ular domains that are non-covalently associated with by which CD1-restricted T cells affect a broad rangeattached to the branches of β2-microglobulin (β2m). On the basis of sequence analy- of immune responses can now be understood. For athe arabinan domain in sis, the CD1 isoforms can be classified into three groups: detailed overview on the roles of NKT cells in disease,lipoarabinomannan (LAM).In the case of Man-LAM, which group 1 comprises CD1a, CD1b and CD1c; group 2 the reader is referred to other excellent reviews on thisis found in Mycobacterium comprises CD1d; and group 3 comprises CD1e. All subject1,2.tuberculosis and other mammals studied so far have been found to express CD1pathogenic species of molecules. Interestingly, however, humans express all Diversity of lipids presented by CD1 moleculesmycobacteria, the CD1 isoforms (CD1a–CD1e), whereas muroid rodents The lipid antigens presented by CD1 molecules includecarbohydrates are mannosegroups. Ara-LAM is not express only CD1d. Group 1 CD1 molecules mainly a broad array of classes, ranging from foreign lipidscapped and it is found in present lipid antigens to clonally diverse T cells that that are unique to specific microorganisms to commonnon-pathogenic, fast growing mediate adaptive immunity to the vast range of micro- mammalian self lipids.strains of mycobacteria. bial lipid antigens. By contrast, CD1d (group 2) mol- ecules present lipid antigens to natural killer T (NKT) Foreign lipid antigens. Many of the unique lipids found cells, a subset of which, the invariant NKT (iNKT) cells, in Mycobacterium tuberculosis can be presented by CD1a, expresses an invariant T-cell receptor (TCR) α-chain, CD1b and CD1c to activate clonally diverse T cells. These responds rapidly en masse following antigen recognition include the abundant mycolates, such as the free fattyDivision of Rheumatology, and is a potent effector of innate immunity. acid mycolic acid, and mycolates with esterified glycans,Immunology and Allergy, CD1 proteins survey the endocytic pathway to intersect such as glucose monomycolate. Mycolic acid (BOX 1) wasDepartment of Medicine, and bind lipid antigens. Several molecules involved in the first CD1-presented lipid antigen to be identified3.Brigham and Women’sHospital, Harvard Medical lipid metabolism have recently been shown to also func- M. tuberculosis-derived lipoglycans (such as lipoarabinoSchool, 1 Jimmy Fund Way, tion in CD1-mediated lipid-antigen presentation, such mannan), which are composed of a phosphatidylinositolBoston, Massachusetts as saposins, which mediate the loading of lipids onto anchor, a macromolecular polysaccharide backbone and02115, USA. CD1 molecules in lysosomes. In this Review, we describe capping motifs, or their component phosphatidylinositolCorrespondence to M.B.B.e-mail: email@example.com. the pathway of CD1 antigen presentation, including the mannosides4, were found to activate CD1b-restrictedharvard.edu delivery, processing, loading and presentation of lipid T cells5. By contrast, Mycobacterium spp. lipids that aredoi:10.1038/nri2191 antigens to T cells. New insights now delineate the closely related to mannosyl β-1-phosphomycoketides arenature reviews | immunology volume 7 | december 2007 | 929 © 2007 Nature Publishing Group
REVIEWS Box 1 | Structures of microbial and self lipids presented by CD1 molecules Mycobacterial lipids (see figure, part a), such as sulpholipids from Mycobacterium tuberculosis, have a sulphate group on a trehalose moiety, which is acylated by two to four very long chain branched fatty acids. The diacylated sulpholipid has been shown to be recognized by CD1b-restricted T cells8. Lipopeptides, such as didehydroxymyco- bactin, an intermediate in the biosynthesis of the mycobacterial iron scavenger mycobactin siderophores, can also be presented by CD1a molecules124. Mycobacterial mycolates contain α-alkyl branched β-hydroxy fatty acids in which the main meromycolate chain (C20–C90) may contain R-substitutions such as the keto function group that is shown3. Diacylglycerols, such as the α-galactosyldiacylglycerol from the spirochete Borrelia burgdorferi, and the common mammalian phosphoglycerides, such as phosphatidylinositol (part b), can be presented by CD1d to stimulate invariant natural killer T (iNKT) cells11,18. Glycosphingolipids (part c) are among the most studied lipids presented by CD1 molecules. α-galactosylceramide is a synthetic antigen mimic that, in contrast to mammalian glycosphingolipids, contains an α-linkage between the 1′ carbon of the sugar and the sphingosine base125. A similar α-linkage glycosphingolipid (α-glucuronosylceramide) found in α-proteobacteria activates iNKT cells9,10. By contrast, abundant mammalian β-linked ceramides, such as sulphatide from myelin, and isoglobo-series glycosphingolipids, such as isoglobotrihexosylceramide, can be presented by CD1 molecules19,23. a O OH HO OH O NH O O O O O OSO3H NH O O Didehydroxymycobactin: Diacylated sulpholipid: M. tuberculosis M. tuberculosis HO OH OH O O O NH HN O O N HO O OH HOOC Keto mycolic acid: M. tuberculosis HO b O O O O HO OH O O HO –O O O O P OH O O O Phosphatidylinositol: self O OH α-Galactosyldiacylglycerol: B. burgdorferi HO OH HO OTrehalose c OA disaccharide formed by two HNglucose units. HN O OH OH OH OSiderophores O OLow molecular-weight OH OH OH HO OHcompounds that are secretedby numerous types of bacteria O HO Isoglobotrihexosylceramide: self HO α-Galactosylceramide: synthetic orand that have a high affinity for OH marine sponge Oiron and other metal ions.These molecules chelate metal HO OH Oions and carry them into the O OH Ocell through specific receptors. O HNThey are bacterial virulence OH HN HO OH OHfactors. O O O HO OHSphingosine O OH O OH OHAn amino alcohol that can be HO OH Sulphatide: self OHlinked to a fatty acid via the HOamino group to form the basic OSO3–structure of sphingolipids. α-Glucuronosylceramide: Sphingomonas spp. Nature Reviews | Immunology930 | december 2007 | volume 7 www.nature.com/reviews/immunol © 2007 Nature Publishing Group
REVIEWSPurified protein derivative presented by CD1c to human T-cell clones6. Among the and B. burgdorferi infection11,12. So, microbial lipids thatA protein from Mycobacterium other unique mycobacterial lipids are lipopeptides and are distinct to those derived from M. tuberculosis andtuberculosis that is used in the sulpholipids (BOX 1), which can be presented by CD1a that are presented by CD1a, CD1b and CD1c can betuberculin sensitivity test, and CD1b, respectively. Importantly, reactivity against presented by CD1d and can stimulate iNKT cells.which determines previousinteractions of the host with several of these lipids has been observed with increasedthe bacterium. A positive frequency in patients infected with M. tuberculosis or Mammalian self-lipid antigens. The first reportedtuberculin test is generally in healthy individuals who are positive for the purified examples of group-1-CD1-restricted human T cells13taken as an indication of protein derivative6–8, suggesting a role for these lipids in and CD1d-restricted mouse T cells14,15 were examplesprevious exposure to host defence against tuberculosis infection. Although all of self-reactive responses. It is widely assumed thatM. tuberculosis. of the mycobacterial lipids noted above are recognized by iNKT cells recognize self lipids during selection in theα-Galactosylceramide a clonal population of CD1a-, CD1b- and CD1c-restricted thymus16. Yet, it is also clear that the same iNKT cells(α-GalCer). A synthetic or T cells, so far, similar reactivity for individual iNKT cells bearing self-lipid–CD1d-reactive TCRs also recognizemarine-sponge-derived has not been observed. Instead, different microorganism- foreign-lipid antigens in the periphery1. Similarly,glycolipid containing anα-anomeric glycosidic linkage derived lipids of several chemical classes have been shown group-1-CD1-restricted human T cells are weakly selfof the galactose residue to the to activate iNKT cells. reactive and display stronger reactivity to foreign-lipidsphingosine base. This lipid, Studies of iNKT cells for many years focused on their antigens17. So, the combination of self- and foreign-and structurally related ones, activation by the synthetic mimic α-galactosylceramide antigen reactivity is likely to be an inherent featurepotently activates CD1d- (α-GalCer)(BOX 1). This raised the question of whether of CD1-restricted T cells, as it is for MHC-restrictedrestricted natural killer T cellsthat express the semi-invariant similar structures existed in microorganisms in nature. T cells.Vα14–Jα18 T-cell receptor in Although their occurrence among pathogens remains In cell-free assays, it was shown that CD1d-restrictedmice (and the Vα24–Jα18 unclear, non-pathogenic, soil and water α-proteobacteria T cells depend on loading of the CD1 molecules withequivalent receptor in have now been found to contain iNKT-cell stimulating mammalian self lipids to activate NKT-cell hybri-humans). α-linked sphingolipids9,10 (BOX 1). Instead of α-linked domas18. These studies revealed that small mammalian sphingolipids, another class of lipids, the diacylglycerols phosphoglycerides, such as phosphatidylinositol (BOX 1), (BOX 1) that are present in the spirochete Borrelia burg- phosphatidylglycerol and phosphatidylethanolamine, dorferi (the aetiological agent of lyme disease in humans), could stimulate various NKT-cell populations. One fea- were found to stimulate iNKT cells11. Further, the lipo- ture common to these self antigens was that they were phosphoglycan of the protozoan parasite Leishmania weak agonists compared with the potent α-GalCer donovani was found to bind CD1d and stimulate mimic. Several glycosphingolipids have also been iNKT cells in vitro12. Importantly, iNKT-cell-deficient shown to be presented by CD1 molecules (TABLE 1). mice showed increased susceptibility to L. donovani Sulphatide, a major component of myelin (BOX 1), was found to be a promiscuous self-lipid antigen that could be presented by CD1a, CD1b and CD1c and activate Table 1 | CD1-restricted lipid antigens clonally restricted human T cells19. Further, GM1 and Source Antigen CD1 isoform Refs related gangliosides were also found to be presented by CD1b, suggesting that self-lipid antigens may be Mycobacterium Mycolic acids CD1b 3 tuberculosis and other important targets for autoimmune T cells in condi- mycobacteria Glucose monomycolate CD1b 132 tions such as multiple sclerosis20,21. Glycosphingolipids Sulpholipid (diacylated CD1b 8 have also been reported to stimulate iNKT cells. The sulphoglycolipid) ganglioside GD3, which is markedly overexpressed Phosphatidylinositol mannosides CD1b, CD1d 5,133 in human melanomas, is a major target for ‘autoanti Mannosylated lipoarabinomannan CD1b 5 bodies’. The injection of mice with human melanoma cells or GD3-loaded dendritic cells (DCs) also stimu- Mannosyl-β1-phosphomycoketides CD1c 6,134 lates a CD1d-restricted NKT-cell response22. Recently, Didehydroxymycobactin CD1a 124 iGb3 (isoglobotrihexosylceramide), a member of the Sphingomonas spp. α-Glucuronosylceramide CD1d 9,10 globo/isoglobo-series glycosphingolipids (BOX 1), was found to activate iNKT cells in vitro10,23. However, the sug- Borrelia burgdorferi α-Galactosyldiacylglycerol CD1d 11 gestion that iGb3, as a single glycosphingolipid, is the Leishmania donovani Lipophosphoglycan CD1d 12 critical self-lipid antigen controlling both iNKT-cell Mammalian (self) Phosphatidylinositol CD1d 18 development in the thymus and activation in the periph- Phosphatidylglycerol CD1d 18 ery in vivo has been challenged by the failure to detect iGb3 in mouse or human thymi using sensitive high- Phosphatidylethanolamine CD1d 18 performance liquid chromatography methods24 and by GM1 CD1b 20,21 the finding that iGb3-synthase-deficient mice, which GD3 CD1d 22 lack the entire family of isoglobo-series glycosphingo lipids, have normal numbers of iNKT cells and iNKT- Sulphatide CD1a, CD1b, 19 CD1c cell development25. So, it seems likely that there may be several self-lipid antigens that collectively provide a Isoglobotrihexosylceramide CD1d 10,23 range of ligands for positive and negative selection of Synthetic or marine α-Galactosylceramide CD1d 125 NKT cells in the thymus and the activation of NKT cells sponge in the periphery.nature reviews | immunology volume 7 | december 2007 | 931 © 2007 Nature Publishing Group
REVIEWSa b O α-GalCer C16 spacer lipid HN OH OH O O α1 OH Phe70 OH α-GalCer variant HO HO Cys12α2 A F c O O HO HO HO β2-microglobulin O HO Glucose monomycolate (C60) OH Phe70 α3 Val12 F A T C Figure 1 | Crystal structures of CD1b and mouse CD1d loaded with lipids. a | A ribbon diagram of the mouse CD1d crystal structure loaded with a synthetic variant of α-galactosylceramide (α-GalCer) that contains a shorter fatty acid Nature Reviews | Immunology chain (the α-GalCer variant is shown in stick representation with the carbon backbone in yellow and oxygen atoms in red)130. A C16 spacer lipid was found to fill the empty space in the A′ channel of CD1d, which would be occupied by the longer fatty acid of the non-variant α-GalCer. b | Surface representation of the mouse CD1d antigen-binding groove loaded with the same lipids as in a. The A′ and F′ hydrophobic channels are shown, as well as two amino-acid residues that form the A′ pole (Cys12 and Phe70) and divert the channel containing the lipid alkyl chain around them. The short fatty acid chain of the lipid inserts into the A′ channel, whereas the sphingosine chain is inserted into the F’ channel. c | Surface representation of CD1b loaded with the C60 species of glucose monomycolate47. The long fatty acid chain sequentially traverses the A′, T′ and F′ superchannel and the α-branched chain inserts into the C′ channel. The diagrams were generated using CCP4MG software131. Structure of CD1 proteins F′ and the A′ channels. There are similar channels in Similar to MHC class I molecules, CD1 heavy chains CD1a, with the A′ channel having a fixed length, in which consist of α1 and α2 domains that form the antigen- a closed terminus limits the length of the alkyl chains binding region, contained within two antiparallel that can be accommodated, like a ‘molecular ruler’, to α-helical structures that are situated on a β-pleated approximately 18–23 carbon atoms 27. Interestingly, sheet (FIG. 1a). The α1 and α2 antigen-binding region the F′ channel of CD1a can bind either an alkyl chainLyme diseaseA disease caused by the is linked to an immunoglobulin-like α3 domain, which or a peptide, which enables the molecule to bind andbacterium Borrelia burgdorferi is attached to the membrane by a transmembrane seg- present antigens to T cells that have one or two alkylor other Borrelia spp. that is ment, followed by a short cytoplasmic tail. However, chains, such as the lipopeptide didehydroxymycobac-transmitted to humans via the unlike MHC class I molecules, CD1 proteins bind alkyl tin and sulphatide, respectively27,28. By contrast, CD1bbites of infected blacklegged chains in hydrophobic channels that reside beneath the can accommodate longer alkyl chains and containsticks. Symptoms can includeskin rash, fever, fatigue, surface of CD1 molecules, whereas the hydrophilic head four channels: the A′, F′, C′ and T′ channels29 (FIG. 1c).headache, muscle pain, stiff groups of the lipid antigens protrude where the hydro- In CD1b, the A′ and F′ channels are connected via theneck and swelling of the knee phobic channels open to the membrane distal surface T′ tunnel, which forms a superchannel that can accom-and other large joints. Most of the CD1 molecule (FIG. 1a). These head moieties are modate multiple alkyl chains or a single alkyl chaincases can be successfullytreated with antibiotics. stabilized by hydrogen bonds, which also contribute of up to 60 carbons. Moreover, besides the surface to the correct positioning of the lipid antigens. CD1 region where the A′ and F′ channels converge, CD1bGlobo/isoglobo-series proteins have deeper and more voluminous antigen- has an extra portal where the C′ channel contacts theglycosphingolipids binding compartments than MHC class I molecules. molecular surface. This allows egress of the alkyl chainOne arm of the glycosphingo- Moreover, the different architecture of the antigen- under the α2 helix of CD1b such that chains longerlipid family, which ischaracterized by an α-linked binding compartment of different CD1 isoforms allows than 16 carbon atoms might extend out of this chan-galactose sugar in the third them to bind distinct lipid antigens. The antigen- nel. Together, these features enable the CD1b groove tosugar position. Globotrihexo- binding groove of mouse CD1d can be divided into two accommodate very long fatty acid chains and, poten-sylceramide (Gb3) has an channels: the A′ channel and the F′ channel, classified tially, lipids with three alkyl chains29.α1–4 linked galactose sugar,whereas isoglobotrihexosyl by comparison with the pockets of the MHC class I Recently, the crystal structure of the TCR–α-GalCer–ceramide (iGb3) has an α1–3 peptide-binding groove26 (FIG. 1b). A narrow entrance CD1d ternary complex was elucidated30. As predicted,linked galactose sugar. into the groove is found at the junction between the the TCR of the NKT cell contacts the protruding head932 | december 2007 | volume 7 www.nature.com/reviews/immunol © 2007 Nature Publishing Group
REVIEWS group of α-GalCer and the CD1 α-helices. Interestingly, to MHC class I molecules, CD1d also associates with the the footprint of the NKT-cell TCR on the surface of the thiol oxidoreductase ERp57, which is involved in the α-GalCer–CD1d complex is substantially different from formation of disulphide bonds33–35 (FIG. 2a). In general, the footprint of MHC-class-I-restricted TCRs, being CD1 proteins depend on the association with β2m for parallel to the long axis of the CD1d binding groove their exit from the ER and for trafficking through the and positioned in the extreme end of CD1d, above the secretory pathway to the cell surface31,36,37, although CD1d F′ pocket; and with the invariant TCR α-chain regions can also be expressed at the cell surface independently CDR1 (complementarity-determining region 1) and of β2m and might still be functional38–41. CDR3 dominating the recognition of the antigen. Loading of endogenous lipids onto CD1 in the ER. CD1 trafficking in the secretory pathway Studies in which natural ligands have been eluted from Assembly of CD1 molecules. Newly synthesized CD1 CD1 have revealed that CD1b and CD1d associate with molecules have signal sequences for translocation into endogenous lipids, including phosphatidylinositol and the lumen of the endoplasmic reticulum (ER). Following glycosylphosphatidylinositols42–44. The use of native synthesis, they rapidly become glycosylated by the mass spectrometry to study lipid–CD1 complexes addition of N-linked oligosaccharides. This enables the revealed that CD1b was associated with phosphati- newly synthesized heavy chains to bind the ER chaper- dylcholine45. Another study that used a recombinant ones calnexin and calreticulin, which have specificity for form of CD1d containing a KDEL ER-retention signal monoglucosylated immature glycans31–33 (FIG. 2a). Similar showed that phosphatidylinositol associated with the Plasma membrane c Clathrin-coated pit AP2 d b Trans-Golgi network Sorting endosome e Golgi AP3 Endocytic recycling compartment Peptide Ii ER MHC MHC class II class I Self lipid f ERp57Signal sequencesShort peptide sequences Nucleus ainvolved in the post- β2mtranslational targeting ofproteins. In particular, the Calnexin Calreticulinendoplasmic reticulum (ER)signal sequence is recognized CD1 (heavy chain) MHC class II compartment or lysosomeby the signal-recognitionpeptide after synthesis of thesignal and the protein is Figure 2 | Intracellular trafficking of CD1 molecules. a | CD1 heavy chains are assembled in the endoplasmicco-translationally inserted into Nature Reviews | Immunology reticulum (ER), where they bind the chaperones calnexin, calreticulin and ERp57. They also bind β2-microglobulin (β2m)the ER lumen. The signal non-covalently in the ER. b | CD1 molecules then follow the secretory route through the Golgi apparatus to the plasmasequence is normally removed membrane. MHC class I and II molecules also assemble in the ER and follow a similar route, with MHC class II moleculesby a signal peptidase. (in complex with invariant chain (Ii)) being diverted from the trans-Golgi network to the endosomes. c | CD1 molecules are internalized in clathrin-coated pits via the interaction of the adaptor complex AP2 with tyrosine-based sorting motifsSecretory pathway present in the cytoplasmic tails of CD1. From the sorting endosome, CD1 molecules can follow two main routes. d | CD1A trafficking pathway from theendoplasmic reticulum to molecules such as CD1a and CD1c can follow the slow recycling pathway, back to the plasma membrane, through thethe plasma membrane that is endocytic recycling compartment. e | CD1 molecules such as CD1b and mouse CD1d can traffic to late endosomal andtaken by newly synthesized lysosomal compartments via the interaction of AP3 also with tyrosine-based motifs contained in the cytoplasmic tails ofmolecules that are destined these CD1 molecules. f | CD1 and MHC class II molecules recycle from lysosomal compartments to the plasma membrane.to be secreted. During their trafficking, CD1 molecules are thought to be loaded with a lipid molecule.nature reviews | immunology volume 7 | december 2007 | 933 © 2007 Nature Publishing Group
REVIEWS recombinant CD1d, which supports the idea that self proteins into clathrin-coated pits56. Using surface plasmon lipids are loaded into CD1 molecules during their resonance, the cytoplasmic tails of CD1b, CD1c and assembly in the ER43,44. Such ER-loaded self lipids might mouse CD1d were shown to bind AP2 or one of its be antigenic or alternatively they might function as subunits, and immunoelectron microscopy studies of chaperones to facilitate the assembly and to stabilize CD1b and CD1c showed that these molecules promi- CD1 molecules in the ER. Consistent with this, no nently localize to clathrin-coated pits and vesicles53,57–59. CD1b crystal structures have been obtained without Accordingly, a marked reduction in CD1b and CD1d a lipid or other molecule filling the entire antigen- internalization, and consequent accumulation on the binding groove of the molecule, suggesting that the cell surface, occurred when the respective cytoplasmic hydrophobic groove is stable only when occupied by tails of these molecules were deleted, demonstrating a bound lipid45–47. the importance of these domains for internalization of the proteins from the plasma membrane53,54,57,59–61.Chylomicrons Role for MTP in CD1 antigen presentation. Micro- In contrast to the other isoforms, CD1a does not containLarge lipoprotein particlesprimarily composed of somal triglyceride transfer protein (MTP; also known any sorting motifs in its cytoplasmic tail, and yet it is alsotriglycerides, secreted by the as MTTP) is an ER-resident lipid-transfer protein that internalized from the plasma membrane into endosomalintestine into the lymphatic is required for the proper assembly and secretion of compartments by an unknown mechanism.system and degraded by apolipoprotein-B-containing lipoproteins, namelylipoprotein lipase. very low-density lipoproteins (VLDLs) by the liver and CD1 trafficking through the endocytic pathway. AfterEndoglycosidase-H chylomicrons by the intestine48,49. Interestingly, the inhi- internalization into the early or sorting endosomes, theresistance bition of MTP in APCs causes a defect in the presenta- different CD1 isoforms follow different trafficking path-Endoglycosidase H cleaves tion of lipid antigens by CD1d, and mice lacking MTP ways. CD1a and CD1c, but not CD1b, co-localize withhigh-mannose oligosaccharides are also deficient in CD1d antigen presentation50,51. a dominant-negative form of the small GTPase ARF6from N-linked glycoproteins.The acquisition of resistance to Although the precise mechanism of action of MTP (ARF6-T27N) in the endocytic recycling compartment,this enzyme is related to the has not been delineated, MTP was shown to transfer indicating that CD1a and CD1c follow the slow recyclingprocessing of the high a phospholipid (phosphatidylethanolamine) to mouse pathway back to the plasma membrane58,62 (FIG. 2d). Indeed,mannose into complex CD1d in vitro51, and therefore could have a role in the CD1a was shown to co-localize with RAB11, anotheroligosaccharides in the medial loading of CD1d with endogenous lipids that are marker for the endocytic recycling compartment, andGolgi apparatus and thereforeindicates progression through important both for NKT-cell selection in the thymus to recycle back to the plasma membrane from an intrathe secretory pathway. and antigen presentation by APCs in the periphery. cellular pool63. Unexpectedly, MTP deficiency was also shown to Some of the tyrosine-based sorting motifs, suchInvariant chain inhibit the recycling of mouse CD1d from lysosomes as those in CD1b and mouse CD1d molecules, can(Ii). A non-polymorphicmolecule that associates with to the cell surface, suggesting possible effects of MTP bind both AP2, which mediates CD1 internalizationMHC class II proteins. By that are distal from its location in the ER52. from the plasma membrane, and AP3, which divertsoccupying the antigen-binding these CD1 molecules from the early recycling path-cleft, the invariant chain CD1 trafficking from the ER to the plasma membrane. way to late endosomes and lysosomes 59,64–66 (FIG. 2e).stabilizes newly synthesized Studies of the time-course of acquisition of endoglyco- Accordingly, AP3-deficient cells have increased cell-MHC class II molecules in theendoplasmic reticulum and sidase-H resistance of CD1b and the appearance of CD1b at surface expression of CD1b, with an accumulation ofdirects the mature molecules the cell surface suggest that, following assembly in the ER, molecules in the early recycling pathway and a nearto compartments in which this isoform follows the secretory pathway through the absence in lysosomes 64. The functional importancebinding with antigenic peptides Golgi directly to the plasma membrane53 (FIG. 2b). CD1d of AP3-dependent CD1b trafficking is highlightedoccurs. has also been observed to associate with MHC class II by the defects displayed by the AP3-deficient cells inSurface plasmon resonance molecules and the invariant chain (Ii), which can direct the CD1b-restricted presentation of a microbial-lipidA technique used to measure CD1 complexes with these proteins from the trans-Golgi antigen, such as glucose monomycolate64. In the case ofmolecular interactions by network to endosomal compartments without first reach- mouse CD1d, AP3-deficient mice show a similar defectobserving how much of an ing the plasma membrane54,55. However, the functional in CD1d localization to lysosomes and CD1d-mediatedinput molecule (for example,a protein) is bound to a chip significance of this alternative CD1 trafficking pathway antigen presentation, which is reflected in a significantimmobilized with another is not known. reduction in the number of NKT cells in these mice65,66.molecule. The amount of Interestingly, unlike CD1b and mouse CD1d, thebound input is directly CD1 trafficking in the endocytic pathway tyrosine-based motifs of CD1c and CD1d (human) areproportional to the changein the light reflected off the CD1 internalization from the plasma membrane. The not capable of binding AP3, as assessed using yeast two-immobilized chip, which is internalization of CD1 from the plasma membrane is hybrid assays64. As a result, AP3-deficient human cellsspecifically measured. This essential for its ability to sample antigens in the endo- do not display any defects in the presentation of CD1c-technique can be used to cytic system. After trafficking to the plasma membrane, mediated lipid antigens. The importance of CD1 traf-calculate single-molecule CD1 molecules appear to follow a dominant, pre- ficking through the endocytic pathway for lipid loadingaffinities, as well as bindingon and off rates. sumably constitutive, pathway of internalization into is highlighted by studies that examined the function of endosomes (FIG. 2c). This clathrin-dependent pathway CD1 molecules that do not internalize properly fromSlow recycling pathway is common to proteins that contain well character- the plasma membrane owing to mutations in the cyto-One arm of the early recycling ized tyrosine-based sorting motifs of the YXXφ type plasmic tail. Indeed, mutant mouse CD1d and CD1bpathway that involvestrafficking through early (where Y is a tyrosine, X is any amino acid and φ is a molecules that lack the cytoplasmic tail are defectiveendosomes and the endocytic bulky hydrophobic residue), which bind the adaptor in the presentation of lipid antigens to iNKT cells andrecycling compartment. protein complex 2 (AP2) and allow sorting of cargo T cells, respectively60,61,67.934 | december 2007 | volume 7 www.nature.com/reviews/immunol © 2007 Nature Publishing Group
REVIEWS So, once internalized from the plasma membrane, and CD1 trafficking are differentially regulated during CD1a molecules traffic mainly through the endocytic DC maturation. recycling compartment, whereas CD1b and mouse CD1d molecules traffic mainly through late endosomes Lipid trafficking and lysosomes. These distinct trafficking routes taken As described, CD1 molecules can survey different cell by the different CD1 isoforms presumably allow each ular compartments by virtue of differential endosomal isoform to survey distinct intracellular compartments trafficking. Collectively, CD1 molecules ensure a broad for lipid antigens. Of the CD1 isoforms, CD1c appears survey of endocytic compartments for endogenous and to have the most promiscuous localization, as it can be exogenous lipid antigens, which are ultimately presented found in both early and late endocytic compartments58. to T cells. Therefore, CD1 molecules must intersect rele vant lipid antigens in endocytic compartments for lipid CD1 recycling from endosomes to the plasma membrane. loading to occur. The sequence of antigen loading onto CD1 molecules probably involves exchanging self lipids that are bound to Delivery of lipid antigens to APCs. Cells continuously CD1 during assembly in the ER or delivery to the plasma take up lipids for their metabolic needs. Most of these membrane with other self or foreign lipids that are are transported in the blood in complex with apoli- encountered within endosomes. Once this has occurred, poproteins, forming lipid-transport particles76. One CD1 molecules must return to the plasma membrane mechanism of uptake of lipid-transport particles is the for presentation of the antigens to T cells. For CD1a LDL receptor (LDLR)-dependent internalization of molecules, return to the plasma membrane can readily the VLDL particles into endosomes. The LDLR binds be accounted for because of their extensive localization apolipoprotein E (apoE) on VLDL particles and deliv- in the endocytic recycling compartment. Like other cell- ers it to the endocytic system 77 (FIG. 3b). The low pH surface molecules that are internalized into the endocytic in endosomes facilitates the release of bound apolipo- recycling compartment from the plasma membrane, after proteins from the receptor. The LDLR is then recycled a period of retention, they correspondingly recycle back to the plasma membrane, whereas the lipoproteins to the plasma membrane. In recent years, it has become are delivered to late endosomes and lysosomes for well recognized that recycling from late endosomes degradation78. Interestingly, apoE can bind exogenous and lysosomes back to the plasma membrane is also a CD1 lipid antigens and dramatically enhances the functionally significant trafficking route for molecules CD1d-dependent presentation of a lipid antigen that that are not degraded in these compartments, such as requires lysosomal processing79. It has also been shown MHC class II molecules68,69 (FIG. 2f). Although the route that APCs secrete large amounts of apoE, suggesting followed by CD1 molecules from lysosomes to the a mechanism for how APCs sample the extracellular plasma membrane has not been clearly delineated, this milieu for lipid antigens by secreting and taking up pathway is the target for immune evasion by herpes apoE-bound lipids from their local environment79. simplex virus 1 (HSV1), as it was shown that this virus Other families of receptors may also be involved in can inhibit the recycling of CD1d to the plasma mem- the uptake of lipid antigens by APCs. Scavenger recep- brane, with consequent accumulation of the protein in tors, including SR-A, SR-BI, LOX1 (lectin-type oxidized lysosomes70. LDL receptor 1; also known as OLR1) and CD36, bind modified forms of LDL, such as acetylated LDL and CD1 trafficking in mature DCs. The recycling of oxidized LDL, and also apoptotic cells80, and may there- molecules from lysosomes to the plasma membrane fore be involved in the uptake of exogenous lipids from is influenced by the maturation of DCs. Strikingly, infected cells (FIG. 3b). MHC class II molecules are rapidly delivered from C-type lectins bind carbohydrate moieties via their lysosomes to the plasma membrane within hours carbohydrate-recognition domains (CRDs) in a cal- of receiving DC maturation signals71. This is one of cium-dependent manner81 (FIG. 3b). Most C-type lectinsPinocytosis the several changes DCs undergo when exposed to bind mannosylated moieties, which are present inA type of endocytosis in which maturation stimuli, which also include decreased several CD1 lipid antigens. Indeed, lipoarabinoman-fluid is taken up by cells. pinocytosis and increased half-life of MHC class II nan has been shown to be internalized and delivered molecules on the cell surface71–74. By contrast, it is not to late endosomal and lysosomal compartments by theC-type lectinsReceptor proteins that bind clear whether the recycling of CD1 molecules shows macrophage mannose receptor82. Another C-type lectin,carbohydrates in a calcium- similar changes in response to DC maturation. Indeed, langerin, is expressed by Langerhans cells and localizes todependent manner. The cell-surface expression of CD1a, CD1b, CD1c and Birbeck granules, an endosomal recycling compartmentbinding activity of C-type CD1d only shows minor changes on DC maturation, in which CD1a is also found83. The failure of APCs tolectins is based on the and the presentation of glucose monomycolate is not present CD1a-dependent lipid antigens when langerinstructure of the carbohydrate-recognition domain, which is affected by DC maturation, whereas the presentation is blocked by antibodies suggests that langerin might behighly conserved between of an MHC-class-II-presented peptide is markedly involved in the uptake of glycolipid antigens84.members of this family. enhanced in mature DCs74. Another difference between Finally, the uptake of particulate material or the CD1 and MHC class II molecules is the continued active pathogens themselves by phagocytosis is also an impor-Langerhans cellsA type of dendritic cell that is internalization of CD1b after DC maturation, whereas tant route of delivery of exogenous and microbial lipidresident in the epidermal layer the internalization of MHC class II molecules is strongly antigens into the endocytic system, where CD1 isoformsof the skin. reduced in mature DCs72,75. Therefore, MHC class II can bind them (FIG. 3b).nature reviews | immunology volume 7 | december 2007 | 935 © 2007 Nature Publishing Group
REVIEWS Pathogen ApoE Plasma b membrane Scavenger receptor Phagocytosis C-type lectin Clathrin c CD1e LDLR Clathrin-coated pit ER MHC class II compartment or lysosome ? a Saposin B MTP Microbial CD1 lipid Nucleus Self lipid Figure 3 | Lipid loading and exchange in the endocytic pathway. a | Self lipids (shown in orange) from the endoplasmic reticulum (ER) are loaded onto CD1 molecules. In the case of CD1d, this process is facilitated by a poorly Nature Reviews | Immunology characterized mechanism that involves microsomal triglyceride transfer protein (MTP). b | Four possible mechanisms for the uptake of foreign lipid antigens are shown: clathrin-dependent internalization of apolipoprotein E (apoE)–lipid complexes bound to the low-density lipoprotein receptor (LDLR); phagocytosis of particulate material or whole pathogens; C-type lectins, which can bind mannose residues on glycolipids; and internalization through scavenger receptors, which can bind modified forms of LDL and apoptotic cells. c | The exchange of endogenous lipids, loaded in the ER or the secretory pathway, by foreign lipids or different endogenous lipid antigens (in blue), takes place in endocytic compartments, such as lysosomes. Several accessory molecules, such as saposins and CD1e, have been implicated in the loading of lipids in these compartments. In the case of saposin B, the protein probably binds lipids, extracts them from membranes and transfers them onto CD1d molecules. Intracellular trafficking of lipids. Our understand- Lipid exchange in the endocytic pathway ing of intracellular lipid trafficking is very incomplete. Several lines of evidence indicate that microbial and Nevertheless, it is now appreciated that lipids are distrib- self antigens may be predominantly loaded onto CD1 uted differently among compartments and, consequently, molecules in the late endosomal or lysosomal compart- different compartments of the endocytic pathway have ments. This would explain why the steady state distribu- distinct lipid compositions. For example, early endo- tion of many CD1 molecules (such as CD1b and mouse somes have been shown to be enriched in PtdIns3P CD1d) is mainly localized to these compartments. The (phosphatidylinositol-3-phosphate) and cholesterol, many examples of defects in presentation that occur whereas late endosomes are enriched in PtdIns(3,5)P2 when CD1 molecules fail to be internalized from the (phosphatidylinositol-3,5-bisphosphate) and bis(mono- plasma membrane and/or delivered to late endosomes acylglycero)phosphate (also known as lysobisphosphatidic and lysosomes are consistent with the acquisition of lipid acid)85,86. Furthermore, it has been shown that some lipids antigens in endosomal compartments. Some self lipids traffic intracellularly according to their biophysical prop- are loaded onto CD1 molecules in the ER, which sug- erties. Lipid analogues that contain long and saturated gests that the nascent CD1 molecules internalized from lipid tails traffic to late endocytic compartments, whereas the plasma membrane contain bound lipids that can be lipids with short or unsaturated tails, and the same head exchanged for others in the endocytic compartments, group, follow the endocytic recycling pathway87. Indeed, such as late endosomes and lysosomes. Given that the the long-chain version of the CD1b-restricted antigen lipids in endocytic compartments are probably inserted glucose monomycolate (containing 80 carbons) prefer- into endosomal membranes, these lipid antigens must be entially accumulates in late endocytic compartments, transferred from the internal or limiting membrane of an when compared with a shorter version (containing 32 endosome into the CD1 antigen-binding groove. This carbons)88. The stereochemistry of the lipid has also been process is facilitated by accessory molecules that have shown to influence lipid internalization and trafficking89. been recently identified. Given that lipids are sorted into different compartments and traffic according to their biophysical properties, it has Saposins. Saposins are membrane-perturbing sphingo been proposed that CD1 trafficking evolved to sample the lipid activator proteins that were previously charac- compartments to which the lipid antigens traffic90. terized for their role in the lysosomal degradation of936 | december 2007 | volume 7 www.nature.com/reviews/immunol © 2007 Nature Publishing Group
REVIEWS glycosphingolipids. Although not enzymatically active, of lipid antigens onto CD1, in lysosomes, as recently deficiencies in these glycoproteins lead to sphingolipid- shown for Niemann-Pick type C2 protein98. storage disorders by failing to activate the enzymatic deg- radation of glycosphingolipids by glycosidic enzymes91 CD1e. CD1e is the only CD1 isoform that is not expressed (BOX 2). Saposins are generated from a single precursor on the cell surface of DCs99,100. In immature DCs, CD1e protein, prosaposin, that yields saposins A–D upon its is mainly localized in the Golgi apparatus, whereas in endosomal proteolytic cleavage91. Prosaposin-deficient mature DCs it is nearly exclusively detected in late endo- fibroblasts were found to have impaired presentation of somes and lysosomes, where it is cleaved into a functional CD1b- and mouse CD1d-restricted exogenous antigens soluble form99–101. It has been suggested that CD1e could in vitro92,93. Interestingly, the CD1b presentation defect be involved in antigen processing as it was found that it could be rescued with the addition of saposin C but facilitates the α-mannosidase-dependent processing of not other saposins92. Such selectivity for saposin–CD1 PIM6 into PIM2 (Ref. 101). combinations is further supported by the finding that In the case of peptide loading onto MHC class II mol- presentation of α-GalCer by CD1d is preferentially ecules, an accessory protein, HLA-DM (known as H-2M restored by saposin B 94. In addition, prosaposin- in mice) facilitates the displacement of class-II-associated deficient mice exhibit defects in CD1d-mediated antigen invariant chain peptide (CLIP) from the peptide-binding presentation to NKT cells, as well as in the thymic devel- groove to allow new peptides to bind to MHC class II opment of NKT cells95. Saposins are known to mobilize molecules102,103. So far, no specific mechanism for displac- sphingolipids from donor membranes and transfer ing the ER-bound lipids to favour loading of other lipids them to acceptor membranes (FIG. 3c). Indeed, saposin B onto CD1 molecules has been identified. Future studies was shown to transfer GM1, phosphatidylinositol and will be needed to fully understand the specific molecules phosphatidylcholine between membranes96,97. The exact and topologies of lipid-antigen loading onto CD1 mol- mechanism by which saposins mediate the transfer of ecules. Interestingly, it is clear that some lipids may load lipid antigens onto CD1 is not known, but it was found onto CD1 molecules on the cell surface88,104 or in early by co-immunoprecipitation that saposin C and, to a endosomes, where saposins have not been detected. lesser extent, saposin D interact directly with CD1b and that saposin A and lipid-loaded CD1d can form Functions of CD1-restricted T cells a complex, as analysed by western blotting after native Characteristics of group-1-CD1-restricted T cells. Group- isoelectric focusing 92,95. These studies suggest that 1-CD1-restricted T cells specific for microbial-lipid anti- saposins may directly bind lipid antigens, extract them gens have highly diverse TCR α- and β-chains, which from endosomal membranes and then transfer the lipids are indistinguishable from comparable peptide-specific while bound to CD1. However, this mechanism would TCRs105. CD1a-, CD1b- and CD1c-restricted T cells require that saposins be capable of binding every lipid were initially identified among the CD4–CD8– double- that CD1 molecules present. Although this is conceiv- negative T-cell subset but have now been recognized to able, it is likely that some saposins have a more general be even more common in the CD4+ and CD8+ αβ TCR role as membrane disruptors that facilitate access to T-cell pool3,7,106–109. Most microbe-specific group-1-Class-II-associated invariant lipids or destabilize lipid topologies in membranes, CD1-restricted T cells appear to be T helper 1 (TH1)-likechain peptide rendering them more available for loading onto CD1 cells7,107–109. The clonal heterogeneity of group-1-CD1-(CLIP). A fragment of invariant molecules without directly transferring them (BOX 2). restricted T cells and their expansion in infection suggestschain that occupies the MHC Other molecules might also be involved in the loading that they provide adaptive immunity to microbial-lipidclass II antigen-binding grooveand prevents loading ofantigenic peptides. Box 2 | SaposinsMetachromatic The saposins (saposin A to saposin D) are small, non-enzymatic proteins that localize to lysosomes and facilitate theleukodystrophyAn inherited lipid storage hydrolysis of different glycosphingolipids in these compartments. For example, saposin B ‘activates’ the degradation ofdisorder that affects the growth sulphatide by arylsulphatase A, and saposin C promotes the degradation of glucosylceramide by glucosylceramide-β-and/or development of the glucosidase. Consequently, defects in these saposins lead to the accumulation of the referred substrates in lysosomesmyelin sheet. Although several and result in a variant form of metachromatic leukodystrophy and in a juvenile variant of Gaucher’s disease, respectively.cell types can be affected, the Saposin C and saposin D bind and destabilize phospholipid membranes in a pH-dependent manner126. The neutralizationpathology is essentially of the highly negatively charged surface of saposin C, by protonation of acidic residues upon a decrease in endosomalassociated with the nervous pH, is thought to facilitate the interaction with membranes, as the negatively charged surface might hinder the insertionsystem. This disease has into the apolar interior of membranes or cause electrostatic repulsion from the negatively charged groups of theinfantile, juvenile and adult membrane127. This destabilizes the membrane bilayer architecture and may make glycolipid substrates more accessibleforms. to hydrolases. By contrast, saposin B binds poorly to phospholipid membranes and does not destabilize themGaucher’s disease significantly, even at low pH126. Saposin B forms a dimer in solution and contains a large hydrophobic cavity where lipidsA heterogeneous genetic lipid can bind128. Indeed, a phospholipid was shown to co-purify and crystallize with saposin B and could be modelled in thestorage disorder. The most dimer structure, in which the acyl chains are buried inside the hydrophobic cavity of saposin B, whereas the hydrophiliccommon form, type 1, is head group is exposed to the solvent in a manner similar to lipid-loaded CD1 molecules. So, saposin B can extract andcharacterized by anaemia, low transfer sphingolipids and phospholipids between membranes and does so more efficiently at low pH97,129. Therefore, thisplatelet counts, enlarged liver saposin is considered to be a lipid ‘solubilizer’ that forms a complex with the lipid after extracting it from the membrane,and spleen and skeletal whereas saposin C is considered to be a membrane disruptor, increasing exposure of lipid head groups to enzymes. Bothdisorders and may involve lung of these mechanisms could facilitate the transfer of lipids from lysosomal membranes to CD1 molecules.and kidney impairment.nature reviews | immunology volume 7 | december 2007 | 937 © 2007 Nature Publishing Group
REVIEWS antigens, analogous to the adaptive immunity of MHC- the innate phase of the immune response, it is also clear restricted T cells against microbial-protein antigens that their early activation can provide help to B cells, (TABLE 2). influencing antibody production122, as well as enhancing MHC-restricted T-cell responses to peptide antigens123. Innate-like function of iNKT cells. CD1d-restricted iNKT cells comprise a large pool of up to several percent Activation of CD1-restricted T cells by microorganisms. of all T cells that respond rapidly and display the fea- The strategy for the activation of group-1-CD1-restricted tures of innate immune cells (TABLE 2). Resting NKT cells T cells by microbial antigens appears to be similar to that have a memory or partially activated phenotype and used for the recognition of microbial peptides presented respond rapidly following TCR stimulation to produce by classical MHC molecules. A clonally restricted T-cell cytokines, such as interferon-γ (IFNγ) and interleukin-4 response occurs that is rigorously determined by recog- (IL-4), and to become cytotoxic T lymphocytes (CTLs). nition of the lipid-antigen–CD1 complex by the TCR Although all iNKT cells use nearly the same TCR (FIG. 4a). As discussed earlier, microbial lipids presented α-chain sequence, they include both CD4 + and as cognate antigens that activate iNKT cells have also CD4– subsets that are distinct in terms of various func- been recently identified. Sphingolipids in α-proteobacte- tions. For example, human CD8+ and double-negative ria, diacylglycerols in spirochetes and lipophosphoglycans iNKT cells are more likely to be cytotoxic and produce in Leishmania spp. parasites stimulate iNKT cells and are TH1-type cytokines, whereas CD4+ iNKT cells are more recognized directly by the invariant Vα14–Jα18 (mouse) likely to produce both TH1-type and TH2-type cytokines or Vα24–Jα18 (human) TCRs (FIG. 4b) . Currently, after stimulation110,111, and these subsets also have some- α-GalCer-loaded CD1d tetramers are considered the what different patterns of chemokine and chemokine gold standard for identifying iNKT cells. By compari- receptor expression112,113. Moreover, the CD4– subset of son, tetramers of CD1d loaded with Sphingomonas spp. NKT cells has the main capacity for tumour rejection in α-glucuronosylceramide and B. burgdorferi α‑galactosyl several mouse models114. diacylglycerol stain 25% and 12% of iNKT cells, respec- Once activated, iNKT cells rapidly stimulate DCs, tively9–11. These subpopulations of iNKT cells are activated macrophages and NK cells and recruit neutrophils to en masse by such antigens. expand the immediate innate immune response and So far, no cognate lipid antigens that are recognized by have an impact on the subsequent adaptive T-cell and the iNKT-cell TCR have been found in the main Gram- B-cell responses. There is a striking reciprocal stimulation negative and Gram-positive bacterial pathogens that are axis between DCs and NKT cells, such that activation of prominent in human disease, and viruses do not encode iNKT cells by DCs, using either α-GalCer or CD1- enzymatic machinery for lipid synthesis. In the search for restricted self reactivity, results in upregulation of CD40 a mechanism by which a wider range of microorganisms ligand expression and IFNγ secretion by the NKT cells, might be able to stimulate iNKT cells, even in the absence which in turn stimulates DC maturation115–117. The cycle of specific cognate foreign lipid antigens, an alternative is self-amplifying as IL-12 production by the DCs in this strategy was found that depends on cytokines (such as IL- context further activates the iNKT cells118. The secretion 12) in addition to TCR stimulation by CD1d-presented of IFNγ by iNKT cells activates macrophages to enhance self antigens118 (FIG. 4c). Indeed, the stimulation by an array their capacity for killing intracellular microorganisms119,120. of Gram-negative bacteria (such as Salmonella typhimu- Therefore, NKT cells contribute to the innate phase of rium) or Gram-positive bacteria (such as Staphylococcus immunity to microorganisms through their role at the aureus) cultured with myeloid DCs could be blocked by centre of a broad immune response (TABLE 2). In fact, the either CD1d-specific or IL-12-specific monoclonal anti- full innate immune response to the Toll-like receptor (TLR) bodies in vitro and in vivo. So, TLR-stimulated cytokines agonist lipopolysaccharide is blunted in NKT-cell-deficient from APCs can drive iNKT cells interacting with the same mice121. Although iNKT cells are mainly associated with or nearby APC, enabling them to respond to virtually any Table 2 | Characteristics of CD1-restricted T cells Group-1-CD1-restricted CD1d-restricted iNKT cells CD1d-restricted T cells diverse NKT cells Antigens Microbial and self lipids Microbial and self lipids UnknownToll-like receptor T-cell population Clonally diverse Canonical TCRα but polyclonal Clonally diverse(TLR). A family of pattern-recognition receptors that TCR TCRα: diverse; TCRβ: diverse TCRα: invariant Vα14 or Vα24 and Jα18; TCRα: diverse;recognize conserved molecules TCRβ: limited Vβ repertoire with diverse TCRβ:diversefrom pathogens, such as CDR3lipopolysaccharide or Precursor One per thousands, unique <1% of T cells in humans; 2–50% of T cells Unknownendotoxin, initiating innate frequency specificity for single antigen in mice; pool of cells that respondsimmune responses. en masse to a single antigenSpirochetes Memmory Yes No UnknownPhylum of flagellated helical- Immunity Adaptive, slow Innate-like, rapid (hours to few days) Unknownshaped, Gram-negativebacteria. CDR3, complementarity-determining region 3; iNKT cell, invariant natural killer T cell; TCR, T-cell receptor.938 | december 2007 | volume 7 www.nature.com/reviews/immunol © 2007 Nature Publishing Group
REVIEWS a Cognate microbial-antigen b Cognate microbial-antigen c Cytokine-driven self-antigen activation of T cells restricted activation of CD1d-restricted activation of CD1d-restricted by CD1a, CD1b or CD1c iNKT cells iNKT cells Clonally unique T cells iNKT cells TCR Vα14(Vα24)–Jα18 TCR CD1a, Lipid Plasma TLR CD1b IL-12 or CD1c membrane CD1d CD1d Antigen- Microorganism presenting cell Phagosome Self-lipid antigen Saposin Microbial-lipid Lysosome antigen Figure 4 | Strategies for activation mechanisms of CD1-restricted T cells. a | Activation of CD1a-, CD1b- and CD1c- restricted T cells. Distinct microbial lipid antigens presented by CD1a, CD1b and CD1c are recognized by low Immunology Nature Reviews | precursor frequency T cells bearing individual diverse αβ T-cell receptors (TCRs). b | Cognate microbial-antigen activation of invariant natural killer T (iNKT) cells. Certain bacteria contain lipid antigens recognized by the Vα14–Jα18 (mouse) or Vα24–Jα18 (human) invariant TCR that stimulate a large pool of iNKT cells en masse. c | Cytokine-driven self-antigen activation of iNKT cells. Even in the absence of a cognate microbial lipid antigen for the iNKT-cell TCR, most microorganisms can activate iNKT cells by stimulating antigen-presenting cells to produce interleukin-12 (IL-12), which in combination with the self-lipid antigens presented by CD1d can stimulate potent iNKT-cell responses. This allows rapid activation of a large pool of iNKT cells without direct microbial lipid antigen recognition by the TCR. microorganism that can stimulate the APC accordingly118. they circulate through various endocytic compartments That this cytokine-dependent mechanism is depend- before returning to the plasma membrane bearing lipid ent on TLR engagement of the APC was confirmed in antigens. CD1 molecules first bind lipid antigens in studies showing that S. typhimurium-exposed wild-type the ER and then may exchange these for other self- or bone-marrow-derived DCs, but not cells deficient in microbial-lipid antigens that are encountered along the the TLR-adaptor proteins MyD88 (myeloid differen- endocytic pathway. The ER loading may involve MTP, tiation primary-response gene 88) or TRIF (Toll/IL-1- and ER-loaded lipids may be exchanged in lysosomes receptor (TIR)-domain-containing adaptor protein by saposins. Foreign lipid antigens may be delivered to inducing IFNβ), were able to stimulate iNKT cells APCs in phagocytosed microorganisms or in lipoprotein in vitro10. So, there are two major mechanisms for the particles. Lipid-antigen–CD1 complexes expressed on the activation of iNKT cells, either via cognate microbial cell surface then can activate both adaptive and innate-like antigens (for those organisms that have them) or via self lymphocyte responses. Group-1-CD1-restricted T cells antigens and APC-derived cytokines (for organisms with appear to function as clonally diverse adaptive immune potent TLR agonists)118,121 (FIG. 4). cells that expand following infection. By contrast, CD1d- restricted iNKT cells are innate-like lymphocytes that Concluding remarks respond within hours after activation as a pool of charged The structure of CD1 molecules enables the hydrocarbon cells that are potent sources of cytokines and elicit broad chains of lipids of a wide variety of types to embed them- responses. Recent studies of lipid antigens provide insight selves in the hydrophobic channels of the CD1 antigen- into this surprising range of T-cell responses, which are binding groove, below the surface of the molecule. The closely linked to the distinct CD1 isoforms and the invari- TCR then recognizes the surface-exposed polar moieties ant and diverse T-cell responses they elicit. Even more of the lipids along with CD1 surface determinants, in a remarkably, the ability of iNKT cells to be activated by manner similar to that of peptide recognition by MHC self antigens and APC-derived cytokines in the absence molecules. To survey the intracellular compartments of of cognate foreign antigen recognition appears an entirely APCs for lipid antigens, CD1 molecules traffic from the unexpected mechanism of T-cell response that has no ER to the cell surface and then back into the cell, where parallel among MHC-restricted responses.nature reviews | immunology volume 7 | december 2007 | 939 © 2007 Nature Publishing Group
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