CD1-restricted adaptive immune responses to Mycobacteria in human group 1 CD1 transgenic mice

Lipid and small-molecule display by CD1 and MR1

The antigen-presenting molecules CD1 and MHC class I-related protein (MR1) display lipids and small molecules to T cells. The antigen display platforms in the four CD1 proteins are laterally asymmetrical, so that the T cell receptor (TCR)-binding surfaces are comprised of roofs and portals, rather than the long grooves seen in the MHC antigen-presenting molecules. TCRs can bind CD1 proteins with left-sided or right-sided footprints, creating unexpected modes of antigen recognition. The use of tetramers of human CD1a, CD1b, CD1c or MR1 proteins now allows detailed analysis of the human T cell repertoire, which has revealed new invariant TCRs that bind CD1b molecules and are different from those that define natural killer T cells and mucosal-associated invariant T cells.

Coevolution of T-cell receptors with MHC and non-MHC ligands

The structure and amino acid diversity of the T-cell receptor (TCR), similar in nature to that of Fab portions of antibodies, would suggest that these proteins have a nearly infinite capacity to recognize antigen. Yet all currently defined native T cells expressing an α and β chain in their TCR can only sense antigen when presented in the context of a major histocompatibility complex (MHC) molecule. This MHC molecule can be one of many that exist in vertebrates, presenting small peptide fragments, lipid molecules, or small molecule metabolites. Here we review the pattern of TCR recognition of MHC molecules throughout a broad sampling of species and T-cell lineages and also touch upon T cells that do not appear to require MHC presentation for their surveillance function. We review the diversity of MHC molecules and information on the corresponding T-cell lineages identified in divergent species. We also discuss TCRs with structural domains unlike that of conventional TCRs of mouse and human. By presenting this broad view of TCR sequence, structure, domain organization, and function, we seek to explore how this receptor has evolved across time and been selected for alternative antigen-recognition capabilities in divergent lineages.

Regulation of Lipid Specific and Vitamin Specific Non-MHC Restricted T Cells by Antigen Presenting Cells and Their Therapeutic Potentials

Since initial reports, more than 25 years ago, that T cells recognize lipids in the context on non-polymorphic CD1 molecules, our understanding of antigen presentation to non-peptide-specific T cell populations has deepened. It is now clear that αβ T cells bearing semi-invariant T cell receptor, as well as subsets of γδ T cells, recognize a variety of self and non-self lipids and contribute to shaping immune responses via cross talk with dendritic cells and B cells. Furthermore, it has been demonstrated that small molecules derived from the microbial riboflavin biosynthetic pathway (vitamin B2) bind monomorphic MR1 molecules and activate mucosal-associated invariant T cells, another population of semi-invariant T cells. Novel insights in the biological relevance of non-peptide-specific T cells have emerged with the development of tetrameric CD1 and MR1 molecules, which has allowed accurate enumeration and functional analysis of CD1- and MR1-restricted T cells in humans and discovery of novel populations of semi-invariant T cells. The phenotype and function of non-peptide-specific T cells will be discussed in the context of the known distribution of CD1 and MR1 molecules by different subsets of antigen-presenting cells at steady state and following infection. Concurrent modulation of CD1 transcription and lipid biosynthetic pathways upon TLR stimulation, coupled with efficient lipid antigen processing, result in the increased cell surface expression of antigenic CD1-lipid complexes. Similarly, MR1 expression is almost undetectable in resting APC and it is upregulated following bacterial infection, likely due to stabilization of MR1 molecules by microbial antigens. The tight regulation of CD1 and MR1 expression at steady state and during infection may represent an important mechanism to limit autoreactivity, while promoting T cell responses to foreign antigens.

Role of Group 1 CD1-Restricted T Cells in Infectious Disease

The evolutionarily conserved CD1 family of antigen-presenting molecules presents lipid antigens rather than peptide antigens to T cells. CD1 molecules, unlike classical MHC molecules, display limited polymorphism, making CD1-restricted lipid antigens attractive vaccine targets that could be recognized in a genetically diverse human population. Group 1 CD1 (CD1a, CD1b, and CD1c)-restricted T cells have been implicated to play critical roles in a variety of autoimmune and infectious diseases. In this review, we summarize current knowledge and recent discoveries on the development of group 1 CD1-restricted T cells and their function in different infection models. In particular, we focus on (1) newly identified microbial and self-lipid antigens, (2) kinetics, phenotype, and unique properties of group 1 CD1-restricted T cells during infection, and (3) the similarities of group 1 CD1-restricted T cells to the closely related group 2 CD1-restricted T cells.

CD1 and mycobacterial lipids activate human T cells

For decades, proteins were thought to be the sole or at least the dominant source of antigens for T cells. Studies in the 1990s demonstrated that CD1 proteins and mycobacterial lipids form specific targets of human αβ T cells. The molecular basis by which T-cell receptors (TCRs) recognize CD1-lipid complexes is now well understood. Many types of mycobacterial lipids function as antigens in the CD1 system, and new studies done with CD1 tetramers identify T-cell populations in the blood of tuberculosis patients. In human populations, a fundamental difference between the CD1 and major histocompatibility complex systems is that all humans express nearly identical CD1 proteins. Correspondingly, human CD1 responsive T cells show evidence of conserved TCRs. In addition to natural killer T cells and mucosal-associated invariant T (MAIT cells), conserved TCRs define other subsets of human T cells, including germline-encoded mycolyl-reactive (GEM) T cells. The simple immunogenetics of the CD1 system and new investigative tools to measure T-cell responses in humans now creates a situation in which known lipid antigens can be developed as immunodiagnostic and immunotherapeutic reagents for tuberculosis disease.

Bee venom processes human skin lipids for presentation by CD1a

Venoms frequently co-opt host immune responses, so study of their mode of action can provide insight into novel inflammatory pathways. Using bee and wasp venom responses as a model system, we investigated whether venoms contain CD1-presented antigens. Here, we show that venoms activate human T cells via CD1a proteins. Whereas CD1 proteins typically present lipids, chromatographic separation of venoms unexpectedly showed that stimulatory factors partition into protein-containing fractions. This finding was explained by demonstrating that bee venom-derived phospholipase A2 (PLA2) activates T cells through generation of small neoantigens, such as free fatty acids and lysophospholipids, from common phosphodiacylglycerides. Patient studies showed that injected PLA2 generates lysophospholipids within human skin in vivo, and polyclonal T cell responses are dependent on CD1a protein and PLA2. These findings support a previously unknown skin immune response based on T cell recognition of CD1a proteins and lipid neoantigen generated in vivo by phospholipases. The findings have implications for skin barrier sensing by T cells and mechanisms underlying phospholipase-dependent inflammatory skin disease.

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.

The T-Cell Response to Lipid Antigens of Mycobacterium tuberculosis

T-cells recognize lipid antigens presented by dedicated antigen-presenting molecules that belong to the CD1 family. This review discusses the structural properties of CD1 molecules, the nature of mycobacterial lipid antigens, and the phenotypic and functional properties of T-cells recognizing mycobacterial lipids. In humans, the five CD1 genes encode structurally similar glycoproteins that recycle in and thus survey different cellular endosomal compartments. The structure of the CD1-lipid-binding pockets, their mode of intracellular recycling and the type of CD1-expressing antigen-presenting cells all contribute to diversify lipid immunogenicity and presentation to T-cells. Mycobacteria produce a large variety of lipids, which form stable complexes with CD1 molecules and stimulate specific T-cells. The structures of antigenic lipids may be greatly different from each other and each lipid may induce unique T-cells capable of discriminating small lipid structural changes. The important functions of some lipid antigens within mycobacterial cells prevent the generation of negative mutants capable of escaping this type of immune response. T-cells specific for lipid antigens are stimulated in tuberculosis and exert protective functions. The mechanisms of antigen recognition, the type of effector functions and the mode of lipid-specific T-cell priming are discussed, emphasizing recent evidence of the roles of lipid-specific T-cells in tuberculosis.

Polyclonal type II natural killer T cells require PLZF and SAP for their development and contribute to CpG-mediated antitumor response

CD1d-restricted natural killer T (NKT) cells are innate-like T cells with potent immunomodulatory function via rapid production of both Th1 and Th2 cytokines. NKT cells comprise well-characterized type I NKT cells, which can be detected by α-galactosylceramide-loaded CD1d tetramers, and less-studied type II NKT cells, which do not recognize α-galactosylceramide. Here we characterized type II NKT cells on a polyclonal level by using a Jα18-deficient IL-4 reporter mouse model. This model allows us to track type II NTK cells by the GFP(+)TCRβ(+) phenotype in the thymus and liver. We found type II NKT cells, like type I NKT cells, exhibit an activated phenotype and are dependent on the transcriptional regulator promyelocytic leukemia zinc finger (PLZF) and the adaptor molecule signaling lymphocyte activation molecule-associated protein (SAP) for their development. Type II NKT cells are potently activated by β-D-glucopyranosylceramide (β-GlcCer) but not sulfatide or phospholipids in a CD1d-dependent manner, with the stimulatory capacity of β-GlcCer influenced by acyl chain length. Compared with type I NKT cells, type II NKT cells produce lower levels of IFN-γ but comparable amounts of IL-13 in response to polyclonal T-cell receptor stimulation, suggesting they may play different roles in regulating immune responses. Furthermore, type II NKT cells can be activated by CpG oligodeoxynucletides to produce IFN-γ, but not IL-4 or IL-13. Importantly, CpG-activated type II NKT cells contribute to the antitumor effect of CpG in the B16 melanoma model. Taken together, our data reveal the characteristics of polyclonal type II NKT cells and their potential role in antitumor immunotherapy.

Biology of CD1- and MR1-restricted T cells

Over the past 15 years, investigators have shown that T lymphocytes can recognize not only peptides in the context of MHC class I and class II molecules but also foreign and self-lipids in association with the nonclassical MHC class I-like molecules, CD1 proteins. In this review, we describe the most recent events in the field, with particular emphasis on (a) structural and functional aspects of lipid presentation by CD1 molecules, (b) the development of CD1d-restricted invariant natural killer T (iNKT) cells and transcription factors required for their differentiation, (c) the ability of iNKT cells to modulate innate and adaptive immune responses through their cross talk with lymphoid and myeloid cells, and (d) MR1-restricted and group I (CD1a, CD1b, and CD1c)-restricted T cells.

Lipid antigens in immunity

Lipids are not only a central part of human metabolism but also play diverse and critical roles in the immune system. As such, they can act as ligands of lipid-activated nuclear receptors, control inflammatory signaling through bioactive lipids such as prostaglandins, leukotrienes, lipoxins, resolvins, and protectins, and modulate immunity as intracellular phospholipid- or sphingolipid-derived signaling mediators. In addition, lipids can serve as antigens and regulate immunity through the activation of lipid-reactive T cells, which is the topic of this review. We will provide an overview of the mechanisms of lipid antigen presentation, the biology of lipid-reactive T cells, and their contribution to immunity.

Lipid presentation by human CD1 molecules and the diverse T cell populations that respond to them

CD1 molecules bind and present lipid-based antigens to T cells. Humans express both Group 1 (CD1a, CD1b and CD1c) and Group 2 (CD1d) CD1 molecules with nonredundant functions in the human immune response. Studies of Group 1 CD1 molecules and the T cells that respond to them have lagged behind Group 2 due to the lack of a suitable model system. However, recent work has thrust the Group 1 CD1s into the limelight, revealing their importance in tissue surveillance and microbial defense. Here I review recent advances in Group 1 CD1 lipid presentation, the T cell populations that respond to them and the role of CD1 molecules in engagement of human γδ T cells.

The immune response in tuberculosis

There are 9 million cases of active tuberculosis reported annually; however, an estimated one-third of the world's population is infected with Mycobacterium tuberculosis and remains asymptomatic. Of these latent individuals, only 5-10% will develop active tuberculosis disease in their lifetime. CD4(+) T cells, as well as the cytokines IL-12, IFN-γ, and TNF, are critical in the control of Mycobacterium tuberculosis infection, but the host factors that determine why some individuals are protected from infection while others go on to develop disease are unclear. Genetic factors of the host and of the pathogen itself may be associated with an increased risk of patients developing active tuberculosis. This review aims to summarize what we know about the immune response in tuberculosis, in human disease, and in a range of experimental models, all of which are essential to advancing our mechanistic knowledge base of the host-pathogen interactions that influence disease outcome.

A conserved human T cell population targets mycobacterial antigens presented by CD1b

T cell receptors (TCRs) pair in millions of combinations to create complex and personally unique T cell repertoires. Using tetramers to analyze CD1b-reactive TCRs, we detected T cells with highly stereotyped TCR α chains present among genetically unrelated tuberculosis patients. These germline-encoded mycolyl-reactive (GEM) T cells were defined by CD4 expression and rearrangement of TRAV1-2 to TRAJ9 with few N-region additions. TCR analysis by high throughput sequencing, binding and crystallography showed linkage of TCR α sequence motifs to high affinity antigen recognition. Thus, the CD1-reactive TCR repertoire is composed of at least two compartments, high affinity GEM TCRs and more diverse TCRs with low affinity for CD1b-lipid complexes. These data demonstrate high inter-donor conservation of TCRs, which likely results from selection by a non-polymorphic antigen presenting molecule and an immunodominant antigen.

Therapeutic manipulation of natural killer (NK) T cells in autoimmunity: are we close to reality?

T cells reactive to lipids and restricted by major histocompatibility complex (MHC) class I-like molecules represent more than 15% of all lymphocytes in human blood. This heterogeneous population of innate cells includes the invariant natural killer T cells (iNK T), type II NK T cells, CD1a,b,c-restricted T cells and mucosal-associated invariant T (MAIT) cells. These populations are implicated in cancer, infection and autoimmunity. In this review, we focus on the role of these cells in autoimmunity. We summarize data obtained in humans and preclinical models of autoimmune diseases such as primary biliary cirrhosis, type 1 diabetes, multiple sclerosis, systemic lupus erythematosus, rheumatoid arthritis, psoriasis and atherosclerosis. We also discuss the promise of NK T cell manipulations: restoration of function, specific activation, depletion and the relevance of these treatments to human autoimmune diseases.

CD1a, CD1b, and CD1c in immunity against mycobacteria

The CD1 system is composed of five types of human CD1 proteins, CD1a, CD1b, CD1c, CD1d, and CD1e, and their mammalian orthologs. Each type of CD1 protein has a distinct antigen binding groove and shows differing patterns of expression within cells and in different tissues. Here we review the molecular mechanisms by which CD1a, CD1b, and CD1c capture distinct classes of self- and mycobacterial antigens. We discuss how CD1-restricted T cells participate in the immune response, emphasizing new evidence for mycobacterial recognition in vivo in human and non-human models.

Major T cell response to a mycolyl glycolipid is mediated by CD1c molecules in rhesus macaques

Human CD1b molecules contain a maze of hydrophobic pockets and a tunnel capable of accommodating the unusually long, branched acyl chain of mycolic acids, an essential fatty acid component of the cell wall of mycobacteria. It has been accepted that CD1b-bound mycolic acids constitute a scaffold for mycolate-containing (glyco)lipids stimulating CD1b-restricted T cells. Remarkable homology in amino acid sequence is observed between human and monkey CD1b molecules, and indeed, monkey CD1b molecules are able to bind glucose monomycolate (GMM), a glucosylated species of mycolic acids, and present it to specific human T cells in vitro. Nevertheless, we found, unexpectedly, that Mycobacterium bovis bacillus Calmette-Guerin (BCG)-vaccinated monkeys exhibited GMM-specific T cell responses that were restricted by CD1c rather than CD1b molecules. GMM-specific, CD1c-restricted T cells were detected in the circulation of all 4 rhesus macaque monkeys tested after but not before vaccination with BCG. The circulating GMM-specific T cells were detected broadly in both CD4(+) and CD8(+) cell populations, and upon antigenic stimulation, a majority of the GMM-specific T cells produced both gamma interferon (IFN-γ) and tumor necrosis factor alpha (TNF-α), two major host protective cytokines functioning against infection with mycobacteria. Furthermore, the GMM-specific T cells were able to extravasate and approach the site of infection where CD1c(+) cells accumulated. These observations indicate a previously inconceivable role for primate CD1c molecules in eliciting T cell responses to mycolate-containing antigens.

Diverse antigen presentation by the Group 1 CD1 molecule, CD1c

CD1 molecules are Major Histocompatibility Complex (MHC) class I-like proteins that present diverse lipid antigens to T cells. Most of our understanding of CD1 lipid presentation and T cell recognition has come from study of the invariant Natural Killer T cell recognition of CD1d. However, in addition to CD1d, humans possess three additional CD1 molecules: CD1a, CD1b and CD1c, referred to as the Group 1 CD1s. The lack of an appropriate murine molecule to probe the function and disease relevance of these molecules has hindered understanding their precise immunological role, despite their pivotal role in human immunity. In this perspective, we discuss the progress of functional and molecular studies of CD1c. CD1c has been shown to specifically present lipids from Mycobacterium tuberculosis and other related pathogenic mycobacteria. αβ T cells reactive to these lipids presented in the context of CD1c have been characterized and upon stimulation secrete IFN-γ, an important cytokine in tuberculosis disease clearance. Other ligands characterized for CD1c include PI and PC, a lipopeptide with a dodecameric peptide moiety and sulfatides. These structurally and chemically diverse ligands suggest that CD1c has the capacity to present a wide repertoire of antigens to reactive T cells. Indeed, a substantial percentage (∼2%) of the circulating αβ T cell population is reactive to CD1c presenting endogenous antigens, suggesting that this particular Group 1 molecule may play an important role in the human immune response.

Lysosomal-associated transmembrane protein 5 (LAPTM5) is a molecular partner of CD1e

The CD1e protein participates in the presentation of lipid antigens in dendritic cells. Its transmembrane precursor is transported to lysosomes where it is cleaved into an active soluble form. In the presence of bafilomycin, which inhibits vacuolar ATPase and consequently the acidification of endosomal compartments, CD1e associates with a 27 kD protein. In this work, we identified this molecular partner as LAPTM5. The latter protein and CD1e colocalize in trans-Golgi and late endosomal compartments. The quantity of LAPTM5/CD1e complexes increases when the cells are treated with bafilomycin, probably due to the protection of LAPTM5 from lysosomal proteases. Moreover, we could demonstrate that LAPTM5/CD1e association occurs under physiological conditions. Although LAPTM5 was previously shown to act as a platform recruiting ubiquitin ligases and facilitating the transport of receptors to lysosomes, we found no evidence that LATPM5 controls either CD1e ubiquitination or the generation of soluble lysosomal CD1e proteins. Notwithstanding these last observations, the interaction of LAPTM5 with CD1e and their colocalization in antigen processing compartments both suggest that LAPTM5 might influence the role of CD1e in the presentation of lipid antigens.

RLR-mediated production of interferon-β by a human dendritic cell subset and its role in virus-specific immunity

Cytosolic RIG-I-like helicases (RLR) are PRRs involved in type I IFN production and antiviral immunity. This study focuses to the comparison of the expression, function, and signaling cascades associated to RLR in the previously identified CD14(-)DC-SIGN(+)PPARγ(low)CD1a(+) and CD14(low)DC-SIGN(+)PPARγ(high)CD1a(-) human moDC subsets. Our results revealed that the expression of RLR genes and proteins as well as the activity of the coupled signaling pathways are significantly higher in the CD1a(+) subset than in its phenotypically and functionally distinct counterpart. Specific activation of RLR in moDCs by poly(I:C) or influenza virus was shown to induce the secretion of IFN-β via IRF3, whereas induction of proinflammatory cytokine responses were predominantly controlled by TLR3. The requirement of RLR-mediated signaling in CD1a(+) moDCs for priming naïve CD8(+) T lymphocytes and inducing influenza virus-specific cellular immune responses was confirmed by RIG-I/MDA5 silencing, which abrogated these functions. Our results demonstrate the subset-specific activation of RLR and the underlying mechanisms behind its cytokine secretion profile and identify CD1a(+) moDCs as an inflammatory subset with specialized functional activities. We also provide evidence that this migratory DC subset can be detected in human tonsil and reactive LNs.

Natural killer T cells in lipoprotein metabolism and atherosclerosis

Cells of both the innate and adaptive immune system participate in the development of atherosclerosis, a chronic inflammatory disorder of medium and large arteries. Natural killer T (NKT) cells express surface markers characteristic of natural killer cells and conventional T cells and bridge the innate and adaptive immune systems. The development and activation of NKT cells is dependent upon CD1d, a MHC-class I-type molecule that presents lipids, especially glycolipids to the T cell receptors on NKT cells. There are two classes of NKT cells; invariant NKT cells that express a semi-invariant T cell receptor and variant NKT cells. This review summarises studies in murine models in which the effect of the activation, overexpression or deletion of NKT cells or only invariant NKT cells on atherosclerosis has been examined.

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

Group 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 (CD44(hi)CD69(+)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.

A naive-like population of human CD1d-restricted T cells expressing intermediate levels of promyelocytic leukemia zinc finger

Rare CD1d-α-galactosylceramide-specific T cells that do not express the invariant Vα24 chain of human NKT cells were recently identified after expansion in vitro with the lipid Ag, but their phenotype and frequency in vivo and lineage relationship with NKT cells could not be elucidated. By using a CD1d tetramer-based method to enrich these cells from fresh peripheral blood, we demonstrated their naive-like CD62L(high)CD45RO(-)CD4(+) phenotype and relatively high frequency of ∼10(-5) in several healthy individuals. Notably, these cells expressed the NKT lineage-specific transcription promyelocytic leukemia zinc finger (PLZF), indicating a developmental relationship with NKT cells and ruling out the possibility that they were conventional MHC-restricted T cells cross-reacting against CD1d-α-galactosylceramide. Although PLZF is known to direct the effector program of NKT cells, we show in this study that the naive-like cells expressed it at a significantly lower amount than NKT cells. Further, we present mouse studies demonstrating a sharp PLZF expression threshold requirement for induction of the effector phenotype. These findings directly demonstrate in vivo the existence of naive-like CD1d-restricted human T cells marked by intermediate levels of PLZF.

Immunologic mapping of glycomes: implications for cancer diagnosis and therapy

Cancer associated glycoconjugates are important biomarkers, as exemplified by globo-H, CA125, CA15.3 and CA27.29. However, the exact chemical structures of many such biomarkers remain unknown because of technological limitations. In this article, we propose the "immunologic mapping" of cancer glycomes based on specific immune recognition of glycan structures, which can be hypothesized theoretically, produced chemically, and examined biologically by immuno-assays. Immunologic mapping of glycans not only provides a unique perspective on cancer glycomes, but also may lead to the invention of powerful reagents for diagnosis and therapy.

Lipoarabinomannan and related glycoconjugates: structure, biogenesis and role in Mycobacterium tuberculosis physiology and host-pathogen interaction

Approximately one third of the world's population is infected with Mycobacterium tuberculosis, the causative agent of tuberculosis. This bacterium has an unusual lipid-rich cell wall containing a vast repertoire of antigens, providing a hydrophobic impermeable barrier against chemical drugs, thus representing an attractive target for vaccine and drug development. Apart from the mycolyl–arabinogalactan–peptidoglycan complex, mycobacteria possess several immunomodulatory constituents, notably lipomannan and lipoarabinomannan. The availability of whole-genome sequences of M. tuberculosis and related bacilli over the past decade has led to the identification and functional characterization of various enzymes and the potential drug targets involved in the biosynthesis of these glycoconjugates. Both lipomannan and lipoarabinomannan possess highly variable chemical structures, which interact with different receptors of the immune system during host–pathogen interactions, such as Toll-like receptors-2 and C-type lectins. Recently, the availability of mutants defective in the synthesis of these glycoconjugates in mycobacteria and the closely related bacterium, Corynebacterium glutamicum, has paved the way for host–pathogen interaction studies, as well as, providing attenuated strains of mycobacteria for the development of new vaccine candidates. This review provides a comprehensive account of the structure, biosynthesis and immunomodulatory properties of these important glycoconjugates.

A mycolic acid-specific CD1-restricted T cell population contributes to acute and memory immune responses in human tuberculosis infection

Current tuberculosis (TB) vaccine strategies are largely aimed at activating conventional T cell responses to mycobacterial protein antigens. However, the lipid-rich cell wall of Mycobacterium tuberculosis (M. tuberculosis) is essential for pathogenicity and provides targets for unconventional T cell recognition. Group 1 CD1-restricted T cells recognize mycobacterial lipids, but their function in human TB is unclear and their ability to establish memory is unknown. Here, we characterized T cells specific for mycolic acid (MA), the predominant mycobacterial cell wall lipid and key virulence factor, in patients with active TB infection. MA-specific T cells were predominant in TB patients at diagnosis, but were absent in uninfected bacillus Calmette-Guérin-vaccinated (BCG-vaccinated) controls. These T cells were CD1b restricted, detectable in blood and disease sites, produced both IFN-γ and IL-2, and exhibited effector and central memory phenotypes. MA-specific responses contracted markedly with declining pathogen burden and, in patients followed longitudinally, exhibited recall expansion upon antigen reencounter in vitro long after successful treatment, indicative of lipid-specific immunological memory. T cell recognition of MA is therefore a significant component of the acute adaptive and memory immune response in TB, suggesting that mycobacterial lipids may be promising targets for improved TB vaccines.

Exogenous control of the expression of Group I CD1 molecules competent for presentation of microbial nonpeptide antigens to human T lymphocytes

Group I CD1 (CD1a, CD1b, and CD1c) glycoproteins expressed on immature and mature dendritic cells present nonpeptide antigens (i.e., lipid or glycolipid molecules mainly of microbial origin) to T cells. Cytotoxic CD1-restricted T lymphocytes recognizing mycobacterial lipid antigens were found in tuberculosis patients. However, thanks to a complex interplay between mycobacteria and CD1 system, M. tuberculosis possesses a successful tactic based, at least in part, on CD1 downregulation to evade CD1-dependent immunity. On the ground of these findings, it is reasonable to hypothesize that modulation of CD1 protein expression by chemical, biological, or infectious agents could influence host's immune reactivity against M. tuberculosis-associated lipids, possibly affecting antitubercular resistance. This scenario prompted us to perform a detailed analysis of the literature concerning the effect of external agents on Group I CD1 expression in order to obtain valuable information on the possible strategies to be adopted for driving properly CD1-dependent immune functions in human pathology and in particular, in human tuberculosis.

Targeting iNKT cells for the treatment of sickle cell disease

Sickle cell disease (SCD) causes widely disseminated vaso-occlusive episodes. Building on evidence implicating invariant NKT (iNKT) cells in the pathogenesis of ischemia/reperfusion injury, recent studies demonstrate that blockade of iNKT cell activation in mice with SCD reduces pulmonary inflammation and injury. In patients with SCD, iNKT cells in blood are increased in absolute number and activated in comparison to healthy controls. iNKT cell activation is reduced by agonists of adenosine 2A receptors (A(2A)Rs) such as the clinically approved coronary vasodilator, regadenoson. An ongoing multi-center, dose-finding and safety trial of infused regadenoson, has been initiated and is providing preliminary data about its safety and efficacy to treat SCD. Very high accumulation of adenosine may have deleterious effects in SCD through activation of adenosine 2B receptors that are insensitive to regadenoson. Future possible therapeutic approaches for treating SCD include selective A(2B)R antagonists and antibodies that deplete iNKT cells.

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.

High-frequency and adaptive-like dynamics of human CD1 self-reactive T cells

CD1 molecules present lipid antigens to T cells. An intriguing subset of human T cells recognize CD1-expressing cells without deliberately added lipids. Frequency, subset distribution, clonal composition, naïve-to-memory dynamic transition of these CD1 self-reactive T cells remain largely unknown. By screening libraries of T-cell clones, generated from CD4(+) or CD4(-) CD8(-) double negative (DN) T cells sorted from the same donors, and by limiting dilution analysis, we find that the frequency of CD1 self-reactive T cells is unexpectedly high in both T-cell subsets, in the range of 1/10-1/300 circulating T cells. These T cells predominantly recognize CD1a and CD1c and express diverse TCRs. Frequency comparisons of T-cell clones from sorted naïve and memory compartments of umbilical cord and adult blood show that CD1 self-reactive T cells are naïve at birth and undergo an age-dependent increase in the memory compartment, suggesting a naïve/memory adaptive-like population dynamics. CD1 self-reactive clones exhibit mostly Th1 and Th0 functional activities, depending on the subset and on the CD1 isotype restriction. These findings unveil the unanticipated relevance of self-lipid T-cell response in humans and clarify the basic parameters of the lipid-specific T-cell physiology.

Cellular response to mycobacteria: balancing protection and pathology

There has been a recent increase in our understanding of T cell responses during mycobacterial infection; however, we have not yet identified the protective mechanisms capable of mediating vaccine-induced protection in the lung. Novel approaches have allowed the determination of the kinetics and location of naïve T cell activation, as well as the factors that affect of antigen-specific T cell responses, and the balance between protective and immunopathological consequences during the chronic stages of infection. With an urgent need for new and more efficient vaccination strategies, the integration of these data will result in improved vaccine strategies.

Natural killer T cell recognition of lipid antigens

Natural killer T cells recognize lipid antigens in the context of CD1 molecules. Recent publications show that this mode of recognition differs substantially from that of classic T-cell receptor peptide-major histocompatibility complex interaction.