CD1a-, b-, and c-restricted TCRs recognize both self and foreign antigens

Insights into the CD1 lipidome

CD1 isoforms are MHC class I-like molecules that present lipid-antigens to T cells and have been associated with a variety of immune responses. The lipid repertoire bound and presented by the four CD1 isoforms may be influenced by factors such as the cellular lipidome, subcellular microenvironment, and the properties of the binding pocket. In this study, by shotgun mass spectrometry, we performed a comprehensive lipidomic analysis of soluble CD1 molecules. We identified 1040 lipids, of which 293 were present in all isoforms. Comparative analysis revealed that the isoforms bind almost any cellular lipid.CD1a and CD1c closely mirrored the cellular lipidome, while CD1b and CD1d showed a preference for sphingolipids. Each CD1 isoform was found to have unique lipid species, suggesting some distinct roles in lipid presentation and immune responses. These findings contribute to our understanding of the role of CD1 system in immunity and could have implications for the development of lipid-based therapeutics.

Fat and Protein Combat Triggers Immunological Weapons of Innate and Adaptive Immune Systems to Launch Neuroinflammation in Parkinson's Disease

Parkinson's disease (PD) is the second-most common neurodegenerative disease in the world, affecting up to 10 million people. This disease mainly happens due to the loss of dopaminergic neurons accountable for memory and motor function. Partial glucocerebrosidase enzyme deficiency and the resultant excess accumulation of glycosphingolipids and alpha-synuclein (α-syn) aggregation have been linked to predominant risk factors that lead to neurodegeneration and memory and motor defects in PD, with known and unknown causes. An increasing body of evidence uncovers the role of several other lipids and their association with α-syn aggregation, which activates the innate and adaptive immune system and sparks brain inflammation in PD. Here, we review the emerging role of a number of lipids, i.e., triglyceride (TG), diglycerides (DG), glycerophosphoethanolamines (GPE), polyunsaturated fatty acids (PUFA), sphingolipids, gangliosides, glycerophospholipids (GPL), and cholesterols, and their connection with α-syn aggregation as well as the induction of innate and adaptive immune reactions that trigger neuroinflammation in PD.

CD1-Restricted T Cells During Persistent Virus Infections: "Sympathy for the Devil"

Some of the clinically most important viruses persist in the human host after acute infection. In this situation, the host immune system and the viral pathogen attempt to establish an equilibrium. At best, overt disease is avoided. This attempt may fail, however, resulting in eventual loss of viral control or inadequate immune regulation. Consequently, direct virus-induced tissue damage or immunopathology may occur. The cluster of differentiation 1 (CD1) family of non-classical major histocompatibility complex class I molecules are known to present hydrophobic, primarily lipid antigens. There is ample evidence that both CD1-dependent and CD1-independent mechanisms activate CD1-restricted T cells during persistent virus infections. Sophisticated viral mechanisms subvert these immune responses and help the pathogens to avoid clearance from the host organism. CD1-restricted T cells are not only crucial for the antiviral host defense but may also contribute to tissue damage. This review highlights the two edged role of CD1-restricted T cells in persistent virus infections and summarizes the viral immune evasion mechanisms that target these fascinating immune cells.

A Subset of Human Autoreactive CD1c-Restricted T Cells Preferentially Expresses TRBV4-1+ TCRs

In humans, a substantial portion of T cells recognize lipids presented by the monomorphic CD1 proteins. Recent studies have revealed the molecular basis of mycobacterial lipid recognition by CD1c-restricted T cells. Subsets of CD1c-restricted T cells recognize self-lipids in addition to foreign lipids, which may have implications in human diseases involving autoimmunity and malignancy. However, the molecular identity of these self-reactive T cells remains largely elusive. In this study, using a novel CD1c⁺ artificial APC (aAPC)-based system, we isolated human CD1c-restricted autoreactive T cells and characterized them at the molecular level. By using the human cell line K562, which is deficient in MHC class I/II and CD1 expression, we generated an aAPC expressing CD1c as the sole Ag-presenting molecule. When stimulated with this CD1c⁺ aAPC presenting endogenous lipids, a subpopulation of primary CD4⁺ T cells from multiple donors was consistently activated, as measured by CD154 upregulation and cytokine production in a CD1c-specific manner. These activated CD4⁺ T cells preferentially expressed TRBV4-1⁺ TCRs. Clonotypic analyses of the reconstituted TRBV4-1⁺ TCR genes confirmed CD1c-restricted autoreactivity of this repertoire, and the strength of CD1c reactivity was influenced by the diversity of CDR3β sequences. Finally, alanine scanning of CDR1 and CDR2 sequences of TRBV4-1 revealed two unique residues, Arg³⁰ and Tyr⁵¹, as critical in conferring CD1c-restricted autoreactivity, thus elucidating the molecular basis of the observed V gene bias. These data provide new insights into the molecular identity of human autoreactive CD1c-restricted T cells.

CD1b-autoreactive T cells contribute to hyperlipidemia-induced skin inflammation in mice

A large proportion of human T cells are autoreactive to group 1 CD1 proteins, which include CD1a, CD1b, and CD1c. However, the physiological role of the CD1 proteins remains poorly defined. Here, we have generated a double-transgenic mouse model that expresses human CD1b and CD1c molecules (hCD1Tg) as well as a CD1b-autoreactive TCR (HJ1Tg) in the ApoE-deficient background (hCD1Tg HJ1Tg Apoe-/- mice) to determine the role of CD1-autoreactive T cells in hyperlipidemia-associated inflammatory diseases. We found that hCD1Tg HJ1Tg Apoe-/- mice spontaneously developed psoriasiform skin inflammation characterized by T cell and neutrophil infiltration and a Th17-biased cytokine response. Anti-IL-17A treatment ameliorated skin inflammation in vivo. Additionally, phospholipids and cholesterol preferentially accumulated in diseased skin and these autoantigens directly activated CD1b-autoreactive HJ1 T cells. Furthermore, hyperlipidemic serum enhanced IL-6 secretion by CD1b+ DCs and increased IL-17A production by HJ1 T cells. In psoriatic patients, the frequency of CD1b-autoreactive T cells was increased compared with that in healthy controls. Thus, this study has demonstrated the pathogenic role of CD1b-autoreactive T cells under hyperlipidemic conditions in a mouse model of spontaneous skin inflammation. As a large proportion of psoriatic patients are dyslipidemic, this finding is of clinical significance and indicates that self-lipid-reactive T cells might serve as a possible link between hyperlipidemia and psoriasis.

CD1b-autoreactive T cells recognize phospholipid antigens and contribute to antitumor immunity against a CD1b+ T cell lymphoma

Adoptive immunotherapy for cancer treatment is an emerging field of study. Till now, several tumor-derived, peptide-specific T cell responses have been harnessed for treating cancers. However, the contribution of lipid-specific T cells in tumor immunity has been understudied. CD1 molecules, which present self- and foreign lipid antigens to T cells, are divided into group 1 (CD1a, CD1b, and CD1c) and group 2 (CD1d). Although the role of CD1d-restricted natural killer T cells (NKT) in several tumor models has been well established, the contribution of group 1 CD1-restricted T cells in tumor immunity remains obscure due to the lack of group 1 CD1 expression in mice. In this study, we used a double transgenic mouse model expressing human group 1 CD1 molecules (hCD1Tg) and a CD1b-restricted, self-lipid reactive T cell receptor (HJ1Tg) to study the potential role of group 1 CD1-restricted autoreactive T cells in antitumor response. We found that HJ1 T cells recognized phospholipids and responded more potently to lipid extracted from tumor cells than the equivalent amount of lipids extracted from normal cells. Additionally, the autoreactivity of HJ1 T cells was enhanced upon treatment with various intracellular toll-like receptor (TLR) agonists, including CpG oligodeoxynucleotides (ODN), R848, and poly (I:C). Interestingly, the adoptive transfer of HJ1 T cells conferred protection against the CD1b-transfected murine T cell lymphoma (RMA-S/CD1b) and CpG ODN enhanced the antitumor effect. Thus, this study, for the first time, demonstrates the antitumor potential of CD1b-autoreactive T cells and their potential use in adoptive immunotherapy.

Are human iNKT cells keeping tabs on lipidome perturbations triggered by oxidative stress in the blood?

The central paradigm of conventional MHC-restricted T cells is that they respond specifically to foreign peptides, while displaying tolerance to self-antigens. In contrast, it is now becoming clear that a number of innate-like T cell subsets-CD1-restricted T cells, Vγ9Vδ2 T cells, and MAIT cells-may operate by different rules: rather than focusing on the recognition of specific foreign antigens, these T cells all appear to respond to alterations to lipid-related pathways. By monitoring perturbations to the "lipidome," these T cells may be able to spring into action to deal with physiological situations that are of self as well as microbial origin. iNKT cells are a prime example of this type of lipidome-reactive T cell. As a result of their activation by self lyso-phospholipid species that are generated downstream of blood lipid oxidation, human iNKT cells in the vasculature may respond sensitively to a variety of oxidative stresses. Some of the cytokines produced by activated iNKT cells have angiogenic effects (e.g., GM-CSF, IL-8), whereas others (e.g., IFN-γ) are pro-inflammatory factors that can propagate vascular pathology by influencing the functions of macrophages and dendritic cells. Consistent with this, evidence is accumulating that iNKT cells contribute to atherosclerosis, which is one of the most common inflammatory pathologies, and one that is integrally related to characteristics of the lipidome.

Human autoreactive T cells recognize CD1b and phospholipids

In contrast with the common detection of T cells that recognize MHC, CD1a, CD1c, or CD1d proteins, CD1b autoreactive T cells have been difficult to isolate in humans. Here we report the development of polyvalent complexes of CD1b proteins and carbohydrate backbones (dextramers) and their use in identifying CD1b autoreactive T cells from human donors. Activation is mediated by αβ T-cell receptors (TCRs) binding to CD1b-phospholipid complexes, which is sufficient to activate autoreactive responses to CD1b-expressing cells. Using mass spectrometry and T-cell responses to scan through the major classes of phospholipids, we identified phosphatidylglycerol (PG) as the immunodominant lipid antigen. T cells did not discriminate the chemical differences that distinguish mammalian PG from bacterial PG. Whereas most models of T-cell recognition emphasize TCR discrimination of differing self and foreign structures, CD1b autoreactive T cells recognize lipids with dual self and foreign origin. PG is rare in the cellular membranes that carry CD1b proteins. However, bacteria and mitochondria are rich in PG, so these data point to a more general mechanism of immune detection of infection- or stress-associated lipids.

CD1A-positive cells and HSP60 (HSPD1) levels in keratoacanthoma and squamous cell carcinoma

CD1a is involved in presentation to the immune system of lipid antigen derived from tumor cells with subsequent T cell activation. Hsp60 is a molecular chaperone implicated in carcinogenesis by, for instance, modulating the immune reaction against the tumor. We have previously postulated a synergism between CD1a and Hsp60 as a key factor in the activation of an effective antitumor immune response in squamous epithelia. Keratoacantomas (KAs) are benign tumors that however can transform into squamous cell carcinomas (SCCs), but the reasons for this malignization are unknown. In a previous study, we found that CD1a-positive cells are significantly more numerous in KA than in SCC. In this study, we analyzed a series of KAs and SCCs by immunohistochemistry for CD1a and Hsp60. Our results show that the levels of both are significantly lower in KA than in SCC and support the hypothesis that KA may evolve towards SCC if there is a failure of the local modulation of the antitumor immune response. The data also show that immunohistochemistry for CD1a and Hsp60 can be of help in differential diagnosis between KAs and well-differentiated forms of SCC.

Mycolic acid-specific T cells protect against Mycobacterium tuberculosis infection in a humanized transgenic mouse model

Group 1 CD1 molecules, CD1a, CD1b and CD1c, present lipid antigens from Mycobacterium tuberculosis (Mtb) to T cells. Mtb lipid-specific group 1 CD1-restricted T cells have been detected in Mtb-infected individuals. However, their role in protective immunity against Mtb remains unclear due to the absence of group 1 CD1 expression in mice. To overcome the challenge, we generated mice that expressed human group 1 CD1 molecules (hCD1Tg) and a CD1b-restricted, mycolic-acid specific TCR (DN1Tg). Using DN1Tg/hCD1Tg mice, we found that activation of DN1 T cells was initiated in the mediastinal lymph nodes and showed faster kinetics compared to Mtb Ag85B-specific CD4⁺ T cells after aerosol infection with Mtb. Additionally, activated DN1 T cells exhibited polyfunctional characteristics, accumulated in lung granulomas, and protected against Mtb infection. Therefore, our findings highlight the vaccination potential of targeting group 1 CD1-restricted lipid-specific T cells against Mtb infection.

DOI:http://dx.doi.org/10.7554/eLife.08525.001

Most cases of tuberculosis are caused by a bacterium called Mycobacterium tuberculosis, which is believed to have infected one third of the world’s population. Most of these infections are dormant and don’t cause any symptoms. However, active infections can be deadly if left untreated and often require six months of treatment with multiple antibiotics. One reason why these infections are so difficult to treat is because the M. tuberculosis cell walls contain fatty molecules known as mycolic acids, which make the bacteria less susceptible to antibiotics. These molecules also help the bacteria to subvert and then hide from the immune system.

The prevalence of the disease and the increasing problem of antibiotic resistance have spurred the search for an effective vaccine against tuberculosis. While most efforts have focused on using protein fragments in tuberculosis vaccines, some evidence suggests that human immune cells can recognize fatty molecules such as mycolic acids and that these cells could help manage and control M. tuberculosis infections. However, it has been difficult to determine whether these immune cells genuinely play a protective role against the disease because most vaccine research uses mouse models and mice do not have an equivalent of these immune cells.

Now, Zhao et al. have engineered a “humanized” mouse model that produces the fatty molecule-specific immune cells, and show that these mice do respond to the presence of mycolic acids. Infecting the genetically engineered mice with M. tuberculosis revealed that the fatty molecule-specific immune cells were quickly activated within lymph nodes at the center of the chest. These cells later accumulated at sites in the lung where the bacteria reside, and ultimately protected against M. tuberculosis infection. The results show that these specific immune cells can counteract M. tuberculosis, and highlight the potential of using mycolic acids to generate an effective vaccine that provides protection against tuberculosis.

DOI:http://dx.doi.org/10.7554/eLife.08525.002

Group 1 CD1-restricted T cells and the pathophysiological implications of self-lipid antigen recognition

T cell responses are generally regarded as specific for protein-derived peptide antigens. This is based on the molecular paradigm dictated by the T cell receptor (TCR) recognition of peptide-major histocompatibility complexs, which provides the molecular bases of the specificity and restriction of the T cell responses. An increasing number of findings in the last 20 years have challenged this paradigm, by showing the existence of T cells specific for lipid antigens presented by CD1 molecules. CD1-restricted T cells have been proven to be frequent components of the immune system and to recognize exogenous lipids, derived from pathogenic bacteria, as well as cell-endogenous self-lipids. This represents a young and exciting area of research in immunology with intriguing biological bases and a potential direct impact on human health.

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.

All hands on DE(T)C: Epithelial-resident γδ T cells respond to tissue injury

Immunology has traditionally focused on the lymphocytes circulating among primary lymphoid organs while the large reservoir of tissue-resident T cells have received relatively less attention. In epithelia, these populations are comprised of significant, and sometimes exclusive, subsets of γδ T cells that are highly specialized in promoting tissue homeostasis. As the epithelial layers of the skin and gut are permanently exposed to the environment, they are continually subject to injury and therefore require highly efficient repair processes to maintain barrier functions. Here, we review the role of γδ T cells in promoting wound healing, a critical and complex process occurring in the skin and other barrier sites.

A novel self-lipid antigen targets human T cells against CD1c(+) leukemias

T cells that recognize self-lipids presented by CD1c are frequent in the peripheral blood of healthy individuals and kill transformed hematopoietic cells, but little is known about their antigen specificity and potential antileukemia effects. We report that CD1c self-reactive T cells recognize a novel class of self-lipids, identified as methyl-lysophosphatidic acids (mLPAs), which are accumulated in leukemia cells. Primary acute myeloid and B cell acute leukemia blasts express CD1 molecules. mLPA-specific T cells efficiently kill CD1c(+) acute leukemia cells, poorly recognize nontransformed CD1c-expressing cells, and protect immunodeficient mice against CD1c(+) human leukemia cells. The identification of immunogenic self-lipid antigens accumulated in leukemia cells and the observed leukemia control by lipid-specific T cells in vivo provide a new conceptual framework for leukemia immune surveillance and possible immunotherapy.

TCR bias and affinity define two compartments of the CD1b-glycolipid-specific T Cell repertoire

Current views emphasize TCR diversity as a key feature that differentiates the group 1 (CD1a, CD1b, CD1c) and group 2 (CD1d) CD1 systems. Whereas TCR sequence motifs define CD1d-reactive NKT cells, the available data do not allow a TCR-based organization of the group 1 CD1 repertoire. The observed TCR diversity might result from donor-to-donor differences in TCR repertoire, as seen for MHC-restricted T cells. Alternatively, diversity might result from differing CD1 isoforms, Ags, and methods used to identify TCRs. Using CD1b tetramers to isolate clones recognizing the same glycolipid, we identified a previously unknown pattern of V gene usage (TRAV17, TRBV4-1) among unrelated human subjects. These TCRs are distinct from those present on NKT cells and germline-encoded mycolyl lipid-reactive T cells. Instead, they resemble the TCR of LDN5, one of the first known CD1b-reactive clones that was previously thought to illustrate the diversity of the TCR repertoire. Interdonor TCR conservation was observed in vitro and ex vivo, identifying LDN5-like T cells as a distinct T cell type. These data support TCR-based organization of the CD1b repertoire, which consists of at least two compartments that differ in TCR sequence motifs, affinity, and coreceptor expression.

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.

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.

Effects of HIV-1-induced CD1c and CD1d modulation and endogenous lipid presentation on CD1c-restricted T-cell activation

Background

It has been shown that human immunodeficiency virus (HIV)-1 infection induces the production of endogenous lipids required for effective viral production, and the cluster of differentiation (CD)1 molecule CD1d is downregulated by HIV-1 infection. However, the role of endogenous lipid presentation and the implications of CD1 downregulation by HIV-1 infection have not yet been characterized.

Results

In this study, we observed downregulation of both CD1c and CD1d expression through a Vpu-dependent and Nef-independent mechanism, and the concomitant HIV-1-induced production of host cholesterol decreased the extent of CD1c and CD1d modulation. While the modest downregulation of CD1c by HIV-1 infection decreased the ability of CD1c-restricted T cells to respond and secrete interferon-γ, the cholesterol upregulation in the same cells by HIV-1 infection appears to limit the downregulation of CD1c.

Conclusions

The two conflicting HIV-1-mediated changes in CD1c expression appear to minimize the modulation of CD1c expression, thus leading the host to maintain a CD1c-restricted T-cell response against HIV-1.

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.

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.

Analysis of the CD1 antigen presenting system in humanized SCID mice

CD1 molecules are glycoproteins that present lipids and glycolipids for recognition by T cells. CD1-dependent immune activation has been implicated in a wide range of immune responses, however, our understanding of the role of this pathway in human disease remains limited because of species differences between humans and other mammals: whereas humans express five different CD1 gene products (CD1a, CD1b, CD1c, CD1d, and CD1e), muroid rodents express only one CD1 isoform (CD1d). Here we report that immune deficient mice engrafted with human fetal thymus, liver, and CD34(+) hematopoietic stem cells develop a functional human CD1 compartment. CD1a, b, c, and d isoforms were highly expressed by human thymocytes, and CD1a(+) cells with a dendritic morphology were present in the thymic medulla. CD1(+) cells were also detected in spleen, liver, and lungs. APCs from spleen and liver were capable of presenting bacterial glycolipids to human CD1-restricted T cells. ELISpot analyses of splenocytes demonstrated the presence of CD1-reactive IFN-γ producing cells. CD1d tetramer staining directly identified human iNKT cells in spleen and liver samples from engrafted mice, and injection of the glycolipid antigen α-GalCer resulted in rapid elevation of human IFN-γ and IL-4 levels in the blood indicating that the human iNKT cells are biologically active in vivo. Together, these results demonstrate that the human CD1 system is present and functionally competent in this humanized mouse model. Thus, this system provides a new opportunity to study the role of CD1-related immune activation in infections to human-specific pathogens.

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.

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.

CD1a-autoreactive T cells are a normal component of the human αβ T cell repertoire

CD1 activates T cells, but the function and size of the possible human T cell repertoires that recognize each of the CD1 antigen-presenting molecules remain unknown. Using an experimental system that bypasses major histocompatibility complex (MHC) restriction and the requirement for defined antigens, we show that polyclonal T cells responded at higher rates to cells expressing CD1a than to those expressing CD1b, CD1c or CD1d. Unlike the repertoire of invariant natural killer T (NKT) cells, the CD1a-autoreactive repertoire contained diverse T cell antigen receptors (TCRs). Functionally, many CD1a-autoreactive T cells homed to skin, where they produced interleukin 22 (IL-22) in response to CD1a on Langerhans cells. The strong and frequent responses among genetically diverse donors define CD1a-autoreactive cells as a normal part of the human T cell repertoire and CD1a as a target of the T(H)22 subset of helper T cells.

Autoreactive natural killer T cells: promoting immune protection and immune tolerance through varied interactions with myeloid antigen-presenting cells

Natural killer T (NKT) cells are innate T lymphocytes that are restricted by CD1d antigen-presenting molecules and recognize lipids and glycolipids as antigens. NKT cells have attracted attention for their potent immunoregulatory effects. Like other types of regulatory lymphocytes, a high proportion of NKT cells appear to be autoreactive to self antigens. Thus, as myeloid antigen-presenting cells (APCs) such as monocytes, dendritic cells (DCs) and myeloid-derived suppressor cells (MDSCs) constitutively express CD1d, NKT cells are able to interact with these APCs not only during times of immune activation but also in immunologically quiescent periods. The interactions of NKT cells with myeloid APCs can have either pro-inflammatory or tolerizing outcomes, and a central question is how the ensuing response is determined. Here we bring together published results from a variety of model systems to highlight three critical factors that influence the outcome of the NKT-APC interaction: (i) the strength of the antigenic signal delivered to the NKT cell, as determined by antigen abundance and/or T-cell receptor (TCR) affinity; (ii) the presence or absence of cytokines that costimulate NKT cells [e.g. interleukin (IL)-12, IL-18 and interferon (IFN)-alpha]; (iii) APC intrinsic factors such as differentiation state (e.g. monocyte versus DC) and Toll-like receptor (TLR) stimulation. Together with recent findings that demonstrate new links between NKT cell activation and endogenous lipid metabolism, these results outline a picture in which the functions of NKT cells are closely attuned to the existing biological context. Thus, NKT cells may actively promote tolerance until a critical level of danger signals arises, at which point they switch to activating pro-inflammatory immune responses.

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

Group 1 CD1 (CD1a, CD1b, and CD1c)–restricted T cells recognize mycobacterial lipid antigens and are found at higher frequencies in Mycobacterium tuberculosis (Mtb)–infected individuals. However, their role and dynamics during infection remain unknown because of the lack of a suitable small animal model. We have generated human group 1 CD1 transgenic (hCD1Tg) mice that express all three human group 1 CD1 isoforms and support the development of group 1 CD1–restricted T cells with diverse T cell receptor usage. Both mycobacterial infection and immunization with Mtb lipids elicit group 1 CD1–restricted Mtb lipid–specific T cell responses in hCD1Tg mice. In contrast to CD1d-restricted NKT cells, which rapidly respond to initial stimulation but exhibit anergy upon reexposure, group 1 CD1–restricted T cells exhibit delayed primary responses and more rapid secondary responses, similar to conventional T cells. Collectively, our data demonstrate that group 1 CD1–restricted T cells participate in adaptive immune responses upon mycobacterial infection and could serve as targets for the development of novel Mtb vaccines.

The evolved functions of CD1 during infection

CD1 proteins display lipid antigens to T cell receptors. Studies using CD1d tetramers and CD1d-deficient mice provide important insight into the immunological functions of invariant NK T cells (iNKT) during viral and bacterial infections. However, the mouse CD1 locus is atypical because it encodes only CD1d, whereas most mammalian species have retained many CD1 genes. Viewed from the perspective that CD1 is a diverse gene family that activates several of classes of T cells, new insights into lipid loading and infection response are emerging.

Human marginal zone B cells

Human marginal zone (MZ) B cells are, in a sense, a new entity. Although they share many properties with their mouse counterpart, they also display striking differences, such as the capacity to recirculate and the presence of somatic mutations in their B cell receptor. These differences are the reason they are often not considered a separate, rodent-like B cell lineage, but rather are considered IgM memory B cells. We review here our present knowledge concerning this subset and the arguments in favor of the proposition that humans have evolved for their MZ B cell compartment a separate B cell population that develops and diversifies its Ig receptor during ontogeny outside T-dependent or T-independent immune responses.

CD1c bypasses lysosomes to present a lipopeptide antigen with 12 amino acids

The recent discovery of dideoxymycobactin (DDM) as a ligand for CD1a demonstrates how a nonribosomal lipopeptide antigen is presented to T cells. DDM contains an unusual acylation motif and a peptide sequence present only in mycobacteria, but its discovery raises the possibility that ribosomally produced viral or mammalian proteins that commonly undergo lipidation might also function as antigens. To test this, we measured T cell responses to synthetic acylpeptides that mimic lipoproteins produced by cells and viruses. CD1c presented an N-acyl glycine dodecamer peptide (lipo-12) to human T cells, and the response was specific for the acyl linkage as well as the peptide length and sequence. Thus, CD1c represents the second member of the CD1 family to present lipopeptides. lipo-12 was efficiently recognized when presented by intact cells, and unlike DDM, it was inactivated by proteases and augmented by protease inhibitors. Although lysosomes often promote antigen presentation by CD1, rerouting CD1c to lysosomes by mutating CD1 tail sequences caused reduction in lipo-12 presentation. Thus, although certain antigens require antigen processing in lysosomes, others are destroyed there, providing a hypothesis for the evolutionary conservation of large CD1 families containing isoforms that survey early endosomal pathways.

Human CD1 dimeric proteins as indispensable tools for research on CD1-binding lipids and CD1-restricted T cells

Antigen presenting molecules play an important role in both innate and adoptive immune responses by priming and activating T cells. Among them, CD1 molecules have been identified to present both exogenous and endogenous lipid antigens to CD1-restricted T cells. The involvement of CD1-restricted T cells in autoimmune diseases and in defense against infectious diseases, however, remains largely unknown. Identifying novel antigenic lipids that bind to CD1 molecules and understanding the role of CD1-restricted T cells should lead to the successful development of vaccines, because the lipids can be used as antigens and also as adjuvants. In this paper, we have constructed functional recombinant human CD1 dimeric proteins and established a competitive ELISA assay to measure the lipid binding to CD1 molecules using the CD1 dimers. By using the competitive ELISA assay, we were able to show that the lipid extracts from murine malaria parasites can actually be loaded onto CD1 molecules. In addition, we have demonstrated that artificial antigen-presenting cells, which consist of magnetic beads coated with CD1d dimer and anti-CD28 antibody, stimulated and expanded human invariant NKT cells as efficiently as autologous immature DCs. A set of the tools presented in the current study should be valuable for screening various CD1 molecule-binding lipid antigens and for isolating CD1-restricted T cells.

Microsomal triglyceride transfer protein regulates endogenous and exogenous antigen presentation by group 1 CD1 molecules

Lipid antigens are presented to T cells by the non-polymorphic MHC class I-related CD1 molecules. Microsomal triglyceride transfer protein (MTP) is an endoplasmic reticulum (ER)-resident chaperone that has been shown to lipidate the group 2 CD1 molecule CD1d and thus to regulate its function. We now report that MTP also regulates the function of group 1 CD1 molecules CD1a, CD1b, and CD1c. Pharmacological inhibition of MTP in monocyte-derived dendritic cells and lymphoblastoid B cell lines transfected with group 1 CD1 resulted in a substantial decrease in endogenous self lipid antigen presentation to several CD1-restricted T cell lines. Silencing MTP expression in CD1c-transfected HeLa cells similarly resulted in decreased self reactivity. Unexpectedly, inhibition of ER-resident MTP, which was confirmed by confocal microscopy, also markedly decreased presentation of exogenous, endosomally loaded, mycobacterial lipid antigens by CD1a and CD1c to T cells. Thus, these studies indicate that MTP, despite its ER localization, regulates endogenous as well as exogenous lipid antigen presentation, and suggest a broad role for MTP in the regulation of CD1 antigen presentation.

CD1 antigen presentation: how it works

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.

CD1-restricted recognition of exogenous and self-lipid antigens by duodenal gammadelta+ T lymphocytes

Gammadelta T cells are present in the mucosal intestinal epithelia and secrete factors necessary to maintain tissue integrity. Ags recognized by these cells are poorly defined, although in mice non-classical MHC class I molecules have been implicated. Since MHC class I-like CD1 receptors are widely expressed at the surface of epithelial and dendritic intestinal cells and have the capacity to present lipid Ags to T cells, we hypothesized that these molecules might present autologous and/or exogenous phospholipids to intestinal gammadelta T lymphocytes. Intraepithelial T lymphocytes from normal human duodenal mucosal biopsies were cloned and exposed to natural and synthetic phospholipids using CD1a-, CD1b-, CD1c- or CD1d-transfected C1R lymphoblastoid or HeLa cell lines as APCs. Their cytolytic properties and regulatory cytokine secretion were also examined. Most clones obtained from duodenal mucosa (up to 70%) were TCRalphabeta+, and either CD4+ or CD8+, whereas 20% were CD4-CD8- (6 clones) or TCRgammadelta+ (12 clones). A relevant percentage (up to 66%) of TCRgammadelta+ but few (<5%) TCRalphabeta+ T cell clones responded to synthetic and/or natural phospholipids presented by CD1 molecules, as measured by both [(3)H]thymidine incorporation and IL-4 release assays. A Th1-like cytolytic and functional activity along with the ability to secrete regulatory cytokines was observed in most phospholipid-specific gammadelta T cell clones. Thus, a substantial percentage of TCRgammadelta+ but few TCRalphabeta+ from human duodenal mucosa recognize exogenous phospholipids in a CD1-restricted fashion. This adaptive response could contribute to mucosal homeostasis, but could also favor the emergence of inflammatory or allergic intestinal diseases.

TCR-mediated recognition of glycolipid CD1 complexes

Populations of unconventional T lymphocytes that express alpha beta T cell antigen receptors (TCRs) have been characterized, including T cells reactive to glycolipids presented by CD1 molecules. The CD1 molecules have a structure broadly similar to major histocompatibility complex (MHC) class I and class II proteins, but because the antigens CD 1 presents are so different from peptides, it is possible that glycolipid reactive TCRs have properties that distinguish them from TCRs expressed by conventional T cells. Consistent with this possibility, CD1-reactive T cells have an unrestrained pattern of co-receptor expression, as they include CD4+, CD8+, and double-negative cells. Furthermore, unlike peptide-reactive T cells, there are populations of glycolipid-reactive T cells with invariant alpha chain TCRs that are conserved across species. There are also glycolipid reactive populations with more variable TCRs, however, suggesting that it may be difficult to make categorical generalizations about glycolipid reactive TCRs. Among the glycolipid reactive TCRs, the invariant TCR expressed by CD1d reactive NKT cells has been by far the most thoroughly studied, and in this article we emphasize the unique features of this antigen recognition system, including repertoire formation, fine specificity, TCR affinity, and TCR structure.