Foreign Lipid Research Articles

Use of isotropically tumbling bicelles to measure curvature induced by membrane components.

Isotropically tumbling discoidal bicelles are a useful biophysical tool for the study of lipids and proteins by NMR, dynamic light scattering, and small-angle X-ray scattering. Isotropically tumbling bicelles present a low-curvature central region, typically enriched with DMPC in the lamellar state, and a highly curved detergent rim, typically composed of DHPC. In this report, we study the impact of the partitioning and induced curvature of a few molecules of a foreign lipid on the bicelle size, structure, and curvature. Previous approaches for studying curvature have focused on macroscopic and bulk properties of membrane curvature. In the approach presented here, we show that the conical shape of the DOPE lipid and the inverted-conical shape of the DPC lipid induce measurable curvature changes in the bicelle size. Bicelles with an average of 1.8 molecules of DOPE have marked increases in the size of bicelles, consistent with negative membrane curvature in the central region of the bicelle. With bicelle curvature models, radii of curvature on the order of -100 Å and below are measured, with a greater degree of curvature observed in the more pliable Lα state above the phase-transition temperature of DMPC. Bicelles with an average of 1.8 molecules of DPC are reduced in size, consistent with positive membrane curvature in the rim, and at higher temperatures, DPC is distributed in the central region to form mixed-micelle structures. We use translational and rotational diffusion measurements by NMR, size-exclusion chromatography, and structural models to quantitate changes in bicelle size, curvature, and lipid dynamics.

The effect of intracellular trafficking of CD1d on the formation of TCR repertoire of NKT cells.

CD1 molecules belong to non-polymorphic MHC class I-like proteins and present lipid antigens to T cells. Five different CD1 genes (CD1a-e) have been identified and classified into two groups. Group 1 include CD1a-c and present pathogenic lipid antigens to αβ T cells reminiscence of peptide antigen presentation by MHC-I molecules. CD1d is the only member of Group 2 and presents foreign and self lipid antigens to a specialized subset of αβ T cells, NKT cells. NKT cells are involved in diverse immune responses through prompt and massive production of cytokines. CD1d-dependent NKT cells are categorized upon the usage of their T cell receptors. A major subtype of NKT cells (type I) is invariant NKT cells which utilize invariant Vα14-Jα18 TCR alpha chain in mouse. The remaining NKT cells (type II) utilize diverse TCR alpha chains. Engineered CD1d molecules with modified intracellular trafficking produce either type I or type II NKT cell-defects suggesting the lipid antigens for each subtypes of NKT cells are processed/generated in different intracellular compartments. Since the usage of TCR by a T cell is the result of antigen-driven selection, the intracellular metabolic pathways of lipid antigen are a key in forming the functional NKT cell repertoire. [BMB Reports 2014; 47(5): 241-248]

The CD1 size problem: lipid antigens, ligands, and scaffolds

Whereas research on CD1d has emphasized a few glycosyl ceramides, the broader family of four human CD1 antigen-presenting molecules binds hundreds of distinct self-lipids. Individual lipid types bind within CD1 grooves in different ways, such that they partially fill the groove, match the groove volume, or protrude substantially from the groove. These differing modes of binding can now be connected to differing immunological functions, as individual lipids can act as stimulatory antigens, inhibitory ligands, or space-filling scaffolds. Because each type of CD1 protein folds to produce antigen-binding grooves with differing sizes and shapes, CD1a, CD1b, CD1c, CD1d, and CD1e have distinct mechanisms of capturing self-lipids and exchanging them for foreign lipids. The size discrepancy between endogeneous lipids and groove volume is most pronounced for CD1b. Recent studies show that the large CD1b cavity can simultaneously bind two self-lipids, the antigen, and its scaffold lipid, which can be exchanged for one large bacterial lipid. In this review, we will highlight recent studies showing how cells regulate lipid antigen loading and the roles CD1 groove structures have in control of the presentation of chemically diverse lipids to T cells.

Dynamics of the Antigen-binding Grooves in CD1 Proteins: REVERSIBLE HYDROPHOBIC COLLAPSE IN THE LIPID-FREE STATE

CD1 proteins mediate the presentation of endogenous and foreign lipids on the cell surface for recognition by T cell receptors. To sample a diverse antigen pool, CD1 proteins are repeatedly internalized and recycled, assisted, in some cases, by lipid transfer proteins such as saposins. The specificity of each CD1 isoform is, therefore, conferred in part by its intracellular pathway but also by distinct structural features of the antigen-binding domain. Crystal structures of CD1-lipid complexes reveal hydrophobic grooves and pockets within these binding domains that appear to be specialized for different lipids. However, the mechanism of lipid loading and release remains to be characterized. Here we gain insights into this mechanism through a meta-analysis of the five human CD1 isoforms, in the lipid-bound and lipid-free states, using all-atom molecular dynamics simulations. Strikingly, for isoforms CD1b through CD1e, our simulations show the near-complete collapse of the hydrophobic cavities in the absence of the antigen. This event results from the spontaneous closure of the binding domain entrance, flanked by two α-helices. Accordingly, we show that the anatomy of the binding cavities is restored if these α-helices are repositioned extrinsically, suggesting that helper proteins encountered during recycling facilitate lipid exchange allosterically. By contrast, we show that the binding cavity of CD1a is largely preserved in the unliganded state because of persistent electrostatic interactions that keep the portal α-helices at a constant separation. The robustness of this binding groove is consistent with the observation that lipid exchange in CD1a is not dependent on cellular internalization.

Implication of vascular endothelial cell-reactive invariant NKT cells in pathogenesis of small vessel vasculitis in rats

iNKT cells are innate T lymphocytes recognizing endogenous and foreign lipid antigens presented in the MHC-like molecule CD1d. The semi-invariant iNKT cell TCR can detect certain bacterial and parasitic lipids and drive iNKT cell responses. How iNKT cells respond to fungi, however, is unknown. We found that CD1d-deficient mice, which lack iNKT cells, poorly control infection with the fungal pathogen Aspergillus fumigatus. Furthermore, A. fumigatus rapidly activates iNKT cells in vivo and in vitro in the presence of APCs. Surprisingly, despite a requirement for CD1d recognition, the antifungal iNKT cell response does not require fungal lipids. Instead, Dectin-1- and MyD88-mediated responses to β-1,3 glucans, major fungal cell-wall polysaccharides, trigger IL-12 production by APCs that drives self-reactive iNKT cells to secrete IFN-γ. Innate recognition of β-1,3 glucans also drives iNKT cell responses against Candida, Histoplasma, and Alternaria, suggesting that this mechanism may broadly define the basis for antifungal iNKT cell responses.

Proper Fatty Acid Composition Rather than an Ionizable Lipid Amine Is Required for Full Transport Function of Lactose Permease from Escherichia coli

Energy-dependent uphill transport but not energy-independent downhill transport by lactose permease (LacY) is impaired when expressed in Escherichia coli cells or reconstituted in liposomes lacking phosphatidylethanolamine (PE) and containing only anionic phospholipids. The absence of PE results in inversion of the N-terminal half and misfolding of periplasmic domain P7, which are required for uphill transport of substrates. Replacement of PE in vitro by lipids with no net charge (phosphatidylcholine (PC), monoglucosyl diacylglycerol (GlcDAG), or diglucosyl diacylglycerol (GlcGlcDAG)) supported wild type transmembrane topology of the N-terminal half of LacY. The restoration of uphill transport in vitro was dependent on LacY native topology and proper folding of P7. Support of uphill transport by net neutral lipids in vitro (PE > PC ≫ GlcDAG ≠ GlcGlcDAG provided that PE or PC contained one saturated fatty acid) paralleled the results observed previously in vivo (PE = PC > GlcDAG ≠ GlcGlcDAG). Therefore, a free amino group is not required for uphill transport as previously concluded based on the lack of in vitro uphill transport when fully unsaturated PC replaced E. coli-derived PE. A close correlation was observed in vivo and in vitro between the ability of LacY to carry out uphill transport, the native conformation of P7, and the lipid headgroup and fatty acid composition. Therefore, the headgroup and the fatty acid composition of lipids are important for defining LacY topological organization and catalytically important structural features, further illustrating the direct role of lipids, independent of other cellular factors, in defining membrane protein structure/function.

Invariant natural killer T cells: an innate activation scheme linked to diverse effector functions

Invariant natural killer T (iNKT) cells exist in a 'poised effector' state, which enables them to rapidly produce cytokines following activation. Using a nearly monospecific T cell receptor, they recognize self and foreign lipid antigens presented by CD1d in a conserved manner, but their activation can catalyse a spectrum of polarized immune responses. In this Review, we discuss recent advances in our understanding of the innate-like mechanisms underlying iNKT cell activation and describe how lipid antigens, the inflammatory milieu and interactions with other immune cell subsets regulate the functions of iNKT cells in health and disease.

A novel role of CD1c in regulating CD1d-mediated NKT cell recognition by competitive binding to Ig-like transcript 4.

Humans express four MHC-like CD1 molecules CD1a, b, c and d that are capable of presenting a wide variety of self or foreign lipid antigens to T cells. Much progress has been made in elucidating the function of CD1d-restricted NKT cells in both innate and adaptive immune responses. However, knowledge of the other CD1 molecules is less well defined in terms of lipid presentation and immune regulation. We have previously shown that immunoglobulin-like transcript 4 (ILT4) binds to CD1d and inhibits its recognition by NKT cells. In this study, we show that CD1c can also interact specifically with ILT4 with a higher affinity than that of CD1d. Furthermore, changes in CD1c expression seem to modulate CD1d function; up-regulation of CD1c enhances NKT recognition of CD1d and down-regulation reduces CD1d recognition. We propose that CD1c can act as a sink for the inhibitory receptor ILT4: when CD1c is up-regulated, ILT4 is recruited to CD1c, thus reducing the inhibitory effect of ILT4 on CD1d recognition. Consequently, CD1c could be a potential target for modulating NKT activity. NKT, CD1d, CD1c, ILT4, antigen presentation.

Lipid‐protein interactions as a determinant of the function and topogenesis of membrane proteins

LacY (lactose permease) expressed in Escherichia coli lacking PE (phosphatidylethanolamine) and containing only anionic phospholipids exhibits downhill but not uphill transport and is topologically misassembled. Substitution in vivo of PE by foreign lipids (including phosphatidylcholine (PC)) with no net charge supports native LacY topology and uphill transport. Here, we further investigate the effects of phospholipid head group and fatty acid composition on topology and function of LacY reconstituted into proteoliposomes. Anionic phospholipids alone do not support uphill transport or native topology. Addition of E. coli derived PE or PC supports both uphill transport and native topology. Synthetic PE or PC containing only unsaturated fatty acids supports proper topology but not uphill transport. PE or PC with one saturated fatty acid supports uphill transport. These results show that LacY structure and function are sensitive to both the polar head group and acyl group composition of phospholipids. Our results demonstrate that membrane protein organization and function are determined through direct interactions of the protein with its lipid environment. Supported by NIH grant R37‐GM20478.

Accumulation of Pro-Cancer Cytokines in the Plasma Fraction of Stored Packed Red Cells

IntroductionPerioperative blood transfusion has been linked to decreased survival in pancreatic cancer; however, the exact causal mechanism has not been elucidated. Allogeneic transfusions are known to expose patients to foreign cells and lipid mediators. We hypothesize that stored packed red cells (pRBCs) contain pro-cancer cytokines that augment tumor progression. We analyzed the plasma fraction of stored pRBCs for pro-cancer cytokines and evaluated the affect of both storage time and leukocyte reduction on these mediators. MethodsChemiarray™ analysis for pro-cancer cytokines was performed on the acellular plasma fraction of stored leukocyte-reduced (LR) and non-leukocyte-reduced (NLR) pRBCs at day 1 (D.1–fresh) and day 42 (D.42–outdate) of storage. Elevated expression of monocyte chemotactic protein-1 (MCP-1), regulated on activation, normal T cell expressed and secreted (RANTES), angiogenin, tumor necrosis factor-alpha (TNF-α), epidermal growth factor (EGF), and platelet-derived growth factor (PDGF) was found. Specific enzyme-linked immunosorbent assay was performed for each of these factors in LR and NLR blood at D.1, day 28, and D.42. Data were analyzed by ANOVA. A p value ≤0.05 was considered significant; N ≥ 4 per group. Migration assays were performed using inhibitors of EGF (gefitinib) and PDGF (imatinib) on murine pancreatic adenocarcinoma cells (Pan02) exposed to D.1 and D.42 LR and NLR plasma. Proliferation assays were performed on Pan02 cells to test the inhibition of PDGF. ResultsMCP-1 levels increased with storage time in LR blood, 86.3±6.3 pg/ml at D.1 vs. 121.2±6.1 pg/ml at D.42 (p = 0.007), and NLR blood, 78.2±7.3 pg/ml at D.1 vs. 647.8±220.7 pg/ml at D.42 (p = 0.02). RANTES levels are lower in LR compared to NLR stored blood, 3.0±1.9 vs. 15.8±0.7 pg/ml at D.42 (p < 0.001), but similar in D.1 blood, 13.8±1.8 pg/ml in LR vs. 12.0±1.6 pg/ml in NLR. Angiogenin levels were different between LR and NLR blood, 0 pg/ml (undetectable) vs. 44.2±3.7 pg/ml (p < 0.001). Storage time did not affect concentration. TNF-α levels were not different between LR and NLR blood, and there was no storage time effect on concentration. EGF and PDGF levels increased with storage time in NLR blood only, 216.4±3.8 pg/ml at D.1 vs. 1,436.4±238.6 pg/ml at D.42 for EGF (p = 0.001), and 61.6±6.0 pg/ml at D.1 vs. 76.5±1.7 pg/ml at D.42 (p = 0.003) for PDGF. Inhibition of EGF reduced migration in Pan02 cells treated with D.42 NLR blood, 245.9±11.2 vs. 164.6±10.6 cells/hpf (p < 0.001). Inhibition of PDGF had no effect on Pan02 migration and reduced cell proliferation in cells treated with D.42 NLR, 181.1 ± 1.5% over control vs. 157.5 ± 2.1% (p < 0.001). ConclusionPro-cancer cytokines that can augment tumor progression were identified in pRBCs. Some of these factors are present in fresh blood. The soluble factors identified herein may represent possible therapeutic targets to offset negative effects of transfusion. These data stress the need for efforts in cancer patients to reduce transfusion requirements if needed.

High affinity 3H-phenylalanine uptake by brush border membrane vesicles from whole larvae of Aedes aegypti (AaBBMVw)

Brush border membrane vesicles from whole Aedes aegypti larvae (AaBBMVw) are confirmed to be valid preparations for membrane transport studies. The Abdul-Rauf and Ellar method was used to isolate AaBBMVw that were frozen, stored for several months, transported to a distant site, thawed and used to study Na+-coupled, 3H-labeled, phenylalanine (Phe) uptake. The affinity for all components of the uptake was very high with half maximal values in the sub-micromolar range. By contrast a K0.5Phe of 0.2mM and a K0.5Na of 26mM were calculated from Phe-induced electrical currents in Xenopus oocytes that were heterologously expressing the Anopheles gambiae symporter (co-transporter), AgNAT8, in a buffer with 98mM Na+. What accounts for the >1000-fold discrepancy in affinity for substrates between the BBMV and oocyte experiments? Is it because Ae. aegypti were used to isolate BBMVw whereas An. gambiae were used to transfect oocytes? More likely, it is because BBMVw were exposed to [Na+] in the micromolar range with the transporter(s) being surrounded by native lipids. By contrast, the oocyte measurements were made at [Na+] 100,000 times higher with AgNAT8 surrounded by foreign frog lipids. The results show that AaBBMVw are osmotically sealed; the time-course has a Na+-induced overshoot, the pH optimum is ∼7 and the K0.5 values for Phe and Na+ are very low. The transport is virtually unchanged when Na+ is replaced by K+ or Li+ but decreased by Rb+. This approach to resolving discrepancies between electrical data on solute transporters such as AgNAT8 that are over-expressed in oocytes and flux data on corresponding transporters that are highly expressed in native membrane vesicles, may serve as a model for similar studies that add membrane biochemistry to molecular biology in efforts to identify targets for the development of new methods to control disease-vector mosquitoes.

Innate Recognition of Cell Wall β-Glucans Drives Invariant Natural Killer T Cell Responses against Fungi

iNKT cells are innate T lymphocytes recognizing endogenous and foreign lipid antigens presented in the MHC-like molecule CD1d. The semi-invariant iNKT cell TCR can detect certain bacterial and parasitic lipids and drive iNKT cell responses. How iNKT cells respond to fungi, however, is unknown. We found that CD1d-deficient mice, which lack iNKT cells, poorly control infection with the fungal pathogen Aspergillus fumigatus. Furthermore, A. fumigatus rapidly activates iNKT cells in vivo and in vitro in the presence of APCs. Surprisingly, despite a requirement for CD1d recognition, the antifungal iNKT cell response does not require fungal lipids. Instead, Dectin-1- and MyD88-mediated responses to β-1,3 glucans, major fungal cell-wall polysaccharides, trigger IL-12 production by APCs that drives self-reactive iNKT cells to secrete IFN-γ. Innate recognition of β-1,3 glucans also drives iNKT cell responses against Candida, Histoplasma, and Alternaria, suggesting that this mechanism may broadly define the basis for antifungal iNKT cell responses.

Invariant natural killer T cells recognize lipid self antigen induced by microbial danger signals

Invariant natural killer T cells (iNKT cells) play a prominent role during infection and other inflammatory processes, and these cells can be activated through their T cell receptors by microbial lipid antigens. However, increasing evidence shows that they are also activated in situations where no foreign lipid antigens are present, suggesting a role for lipid self-antigen. We now demonstrate that an abundant endogenous lipid, β-D-glucopyranosylceramide (β-GlcCer), is a potent iNKT cell self-antigen in mouse and human, and that its activity depends on N-acyl chain composition. Furthermore, β-GlcCer accumulates during infection and in response to Toll-like receptor agonists, contributing to iNKT cell activation. Thus, we propose that recognition of β-GlcCer by the invariant TCR translates innate danger signals into iNKT cell activation.

Autoreactive CD1b-restricted T cells: a new innate-like T-cell population that contributes to immunity against infection

AbstractGroup 1 CD1 (CD1a, -b, and -c) presents self and foreign lipid antigens to multiple T-cell subsets in humans. However, in the absence of a suitable animal model, the specific functions and developmental requirements of these T cells remain unknown. To study group 1 CD1-restricted T cells in vivo, we generated double transgenic mice (HJ1Tg/hCD1Tg) that express group 1 CD1 molecules in a similar pattern to that observed in humans (hCD1Tg) as well as a TCR derived from a CD1b-autoreactive T-cell line (HJ1Tg). Using this model, we found that similar to CD1d-restricted NKT cells, HJ1 T cells exhibit an activated phenotype (CD44hiCD69+CD122+) and a subset of HJ1 T cells expresses NK1.1 and is selected by CD1b-expressing hematopoietic cells. HJ1 T cells secrete proinflammatory cytokines in response to stimulation with CD1b-expressing dendritic cells derived from humans as well as hCD1Tg mice, suggesting that they recognize species conserved self-lipid antigen(s). Importantly, this basal autoreactivity is enhanced by TLR-mediated signaling and HJ1 T cells can be activated and confer protection against Listeria infection. Taken together, our data indicate that CD1b-autoreactive T cells, unlike mycobacterial lipid antigen-specific T cells, are innate-like T cells that may contribute to early anti-microbial host defense.

Identification of Self-lipids Presented by CD1c and CD1d Proteins

The CD1 family consists of five proteins that are related to the peptide-presenting MHC class I family. T cells can recognize the presentation of both foreign and self-derived lipids on four CD1 family members. The identities of the self-lipids capable of stimulating autoreactive T cell responses remain elusive or controversial. Here, we employed mass spectrometry to analyze the lipid content of highly purified CD1c and CD1d protein samples. We report the identification of 11 novel self-lipids presented by CD1c and nine by CD1d. Rigorous controls provide strong evidence that the identified lipids were specifically loaded into the lipid-binding site of the CD1 molecules. The diverse but distinct population of lipids identified from each CD1 family member implies each present a different subset of self-lipids, and the enrichment of particular motifs indicates that the lipids that are presented by CD1 family members could be predicted. Finally, our results imply the CD1 system surveys the endoplasmic reticulum, Golgi apparatus, and/or secretory compartments, in addition to its well characterized surveillance of the endocytic and lysosomal compartments.

Cutting Edge: Structural Basis for the Recognition of β-Linked Glycolipid Antigens by Invariant NKT Cells

Invariant NKT (iNKT) cells expressing a semi-invariant Vα14 TCR recognize self and foreign lipid Ags when presented by the nonclassical MHCI homolog CD1d. Whereas the majority of known iNKT cell Ags are characterized by the presence of a single α-linked sugar, mammalian self Ags are β-linked glycosphingolipids, posing the interesting question of how the semi-invariant TCR can bind to such structurally distinct ligands. In this study, we show that the mouse iNKT TCR recognizes the complex β-linked Ag isoglobotrihexosylceramide (iGb3; Galα1-3-Galβ1-4-Glcβ1-1Cer) by forcing the proximal β-linked sugar of the trisaccharide head group to adopt the typical binding orientation of α-linked glycolipids. The squashed iGb3 orientation is stabilized by several interactions between the trisaccharide and CD1d residues. Finally, the formation of novel contacts between the proximal and second sugar of iGb3 and CDR2α residues of the TCR suggests an expanded recognition logic that can possibly distinguish foreign Ags from self Ags.

Lipid–protein interactions as determinants of membrane protein structure and function

To determine how the lipid environment affects membrane protein structure and function, strains of Escherichia coli were developed in which normal phospholipid composition can be altered or foreign lipids can be introduced. The properties of LacY (lactose permease) were investigated as a function of lipid environment. Assembly of LacY in membranes lacking PE (phosphatidylethanolamine) results in misorientation of the N-terminal six-TM (transmembrane domain) helical bundle with loss of energy-dependent uphill transport and retention of energy-independent downhill transport. Post-assembly introduction of PE results in nearly native orientation of TMs and restoration of uphill transport. Foreign lipids with no net charge can substitute for PE in supporting native LacY topology, but restoration of uphill transport is dependent on native topology and the proper folding of a solvent-exposed domain. Increasing the positive charge density of the cytoplasmically exposed surface of LacY counters TM misorientation in the absence of neutral lipids, demonstrating that charge interactions between these domains and the surface of the membrane bilayer are determinants of TM orientation. Therefore membrane protein organization or reorganization is determined either during initial assembly or post-insertionally through direct interactions between the protein and the lipid environment, which affects the topogenic potency of opposing charged residues as topological signals independent of the translocon.

Innate and cytokine-driven signals, rather than microbial antigens, dominate in natural killer T cell activation during microbial infection

Invariant natural killer T cells (iNKT cells) are critical for host defense against a variety of microbial pathogens. However, the central question of how iNKT cells are activated by microbes has not been fully explained. The example of adaptive MHC-restricted T cells, studies using synthetic pharmacological α-galactosylceramides, and the recent discovery of microbial iNKT cell ligands have all suggested that recognition of foreign lipid antigens is the main driver for iNKT cell activation during infection. However, when we compared the role of microbial antigens versus innate cytokine-driven mechanisms, we found that iNKT cell interferon-γ production after in vitro stimulation or infection with diverse bacteria overwhelmingly depended on toll-like receptor-driven IL-12. Importantly, activation of iNKT cells in vivo during infection with Sphingomonas yanoikuyae or Streptococcus pneumoniae, pathogens which are known to express iNKT cell antigens and which require iNKT cells for effective protection, also predominantly depended on IL-12. Constitutive expression of high levels of IL-12 receptor by iNKT cells enabled instant IL-12-induced STAT4 activation, demonstrating that among T cells, iNKT cells are uniquely equipped for immediate, cytokine-driven activation. These findings reveal that innate and cytokine-driven signals, rather than cognate microbial antigen, dominate in iNKT cell activation during microbial infections.

Human NKT cells direct the differentiation of myeloid APCs that regulate T cell responses via expression of programmed cell death ligands

NKT cells are innate lymphocytes that can recognize self or foreign lipids presented by CD1d molecules. NKT cells have been shown to inhibit the development of autoimmunity in murine model systems, however, the pathways by which they foster immune tolerance remain poorly understood. Here we show that autoreactive human NKT cells stimulate monocytes to differentiate into myeloid APCs that have a regulatory phenotype characterized by poor conjugate formation with T cells. The NKT cell instructed myeloid APCs show elevated expression of the inhibitory ligand PD-L2, and blocking PD-L1 and PD-L2 during interactions of the APCs with T cells results in improved cluster formation and significantly increased T cell proliferative responses. The elevated expression of PD-L molecules on NKT-instructed APCs appears to result from exposure to extracellular ATP that is produced during NKT-monocyte interactions, and blocking purinergic signaling during monocyte differentiation results in APCs that form clusters with T cells and stimulate their proliferation. Finally, we show that human monocytes and NKT cells that are injected into immunodeficient mice co-localize together in spleen and liver, and after 3 days in vivo in the presence of NKT cells a fraction of the myeloid cells have upregulated markers associated with differentiation into professional APCs. These results suggest that autoreactive human NKT cells may promote tolerance by inducing the differentiation of regulatory myeloid APCs that limit T cell proliferation through expression of PD-L molecules.

Group 1 CD1‐restricted T cells take center stage

The recognition of lipid antigens by T cells is a complex and fascinating phenomenon. The MHC-like molecules of the CD1 family have evolved to present a wide variety of both self and foreign lipids for recognition by T-cell receptors. While much progress has been made in our understanding of the NKT cells that recognize lipids presented by CD1d molecules, our knowledge of the T-cell populations directed at the related group 1 CD1 molecules, i.e. CD1a, CD1b and CD1c, has lagged behind. In this issue of the European Journal of Immunology, a study identifies a surprisingly large portion of human peripheral blood T cells as being autoreactive to the group 1 CD1 proteins. This work and other recent developments highlight the presence of a substantial number of unconventional T cells as part of our normal T-cell repertoire. This interesting finding is discussed in details in this commentary.