CD1c tetramers detect ex vivo T cell responses to processed phosphomycoketide antigens

Diet-induced dyslipidemia enhances IFN-γ production in mycolic acid-specific T cells and affects mycobacterial control

Dyslipidemia, characterized by altered lipid profiles, influences host immune responses against infections, including Mycobacterium tuberculosis (Mtb). While the effects of dyslipidemia on conventional T cell responses are well documented, its impact on group 1-CD1 restricted T cells, a distinct subset of lipid antigen-specific unconventional T cells, during Mtb infection remains unclear. In this study, we developed a double-transgenic mouse model expressing human group 1 CD1 (hCD1Tg) and mycolic acid (MA)-specific CD1b-restricted T cell receptor (DN1Tg) in a Rag-deficient and low-density lipoprotein receptor-deficient background to investigate how diet-induced dyslipidemia affects the functionality of MA-specific T cells and their role in anti-Mtb immunity. We found that diet-induced dyslipidemia led to increased IFN-γ production by MA-specific T cells, which promoted mycobacterial clearance in vitro. Mechanistically, this enhanced IFN-γ production was associated with increased TCR signaling and enhanced glycolysis in DN1 T cells, rather than changes in antigen presentation by dendritic cells. However, dyslipidemia also increased apoptosis in DN1 T cells, which may have impaired their ability to control mycobacterial infection in vivo, resulting in reduced bacterial clearance. These findings highlight a complex interplay between diet-induced dyslipidemia and lipid antigen-specific T-cell responses in Mtb infection, providing insights for potential therapeutic strategies to mitigate dyslipidemia-induced changes in T-cell functions.

Establishment of CD1b-restricted immunity to lipid antigens in the pulmonary response to Mycobacterium tuberculosis infection

CD1 is an antigen-presenting glycoprotein homologous to MHC I; however, CD1 proteins present lipid rather than peptide antigens. CD1 proteins are well established to present lipid antigens of Mycobacterium tuberculosis (Mtb) to T cells, but understanding the role of CD1-restricted immunity in vivo in response to Mtb infection has been limited by the availability of animal models naturally expressing the CD1 proteins implicated in human response: CD1a, CD1b, and CD1c. Guinea pigs, in contrast to other rodent models, express four CD1b orthologs, and here we utilize the guinea pig to establish the kinetics of gene and protein expression of CD1b orthologs, as well as the Mtb lipid-antigen and CD1b-restricted immune response at the tissue level over the course of Mtb infection. Our results indicate transient upregulation of CD1b expression during the effector phase of adaptive immunity that wanes with disease chronicity. Gene expression indicates that the upregulation of CD1b is the result of transcriptional induction across all CD1b orthologs. We show high CD1b3 expression on B cells, and identify CD1b3 as the predominant CD1b ortholog in pulmonary granuloma lesions. We identify ex vivo cytotoxic activity directed against CD1b that parallels the kinetic changes in CD1b expression in Mtb-infected lungs and spleen. This study confirms that CD1b expression is modulated by Mtb infection in lung and spleen, leading to pulmonary and extrapulmonary CD1b-restricted immunity as a component of the antigen-specific response to Mtb infection.

A structural perspective of how T cell receptors recognize the CD1 family of lipid antigen-presenting molecules

The CD1 family of antigen-presenting molecules adopt a major histocompatibility complex class I (MHC-I) fold. Whereas MHC molecules present peptides, the CD1 family has evolved to bind self- and foreign-lipids. The CD1 family of antigen-presenting molecules comprises four members-CD1a, CD1b, CD1c, and CD1d-that differ in their architecture around the lipid-binding cleft, thereby enabling diverse lipids to be accommodated. These CD1-lipid complexes are recognized by T cell receptors (TCRs) expressed on T cells, either through dual recognition of CD1 and lipid or in a new model whereby the TCR directly contacts CD1, thereby triggering an immune response. Chemical syntheses of lipid antigens, and analogs thereof, have been crucial in understanding the underlying specificity of T cell-mediated lipid immunity. This review will focus on our current understanding of how TCRs interact with CD1-lipid complexes, highlighting how it can be fundamentally different from TCR-MHC-peptide corecognition.

Functional and biological implications of clonotypic diversity among human donor-unrestricted T cells

T cells express a T-cell receptor (TCR) heterodimer that is the product of germline rearrangement and junctional editing resulting in immense clonotypic diversity. The generation of diverse TCR repertoires enables the recognition of pathogen-derived peptide antigens presented by polymorphic major histocompatibility complex (MHC) molecules. However, T cells also recognize nonpeptide antigens through nearly monomorphic antigen-presenting systems, such as cluster of differentiation 1 (CD1), MHC-related protein 1 (MR1) and butyrophilins (BTNs). This potential for shared immune responses across genetically diverse populations led to their designation as donor-unrestricted T cells (DURTs). As might be expected, some CD1-, MR1- and BTN-restricted T cells express a TCR that is conserved across unrelated individuals. However, several recent studies have reported unexpected diversity among DURT TCRs, and increasing evidence suggests that this diversity has functional consequences. Recent reports also challenge the dogma that immune cells are either innate or adaptive and suggest that DURT TCRs may act in both capacities. Here, we review this evidence and propose an expanded view of the role for clonotypic diversity among DURTs in humans, including new perspectives on how DURT TCRs may integrate their adaptive and innate immune functions.

CD1 lipidomes reveal lipid-binding motifs and size-based antigen-display mechanisms

The CD1 system binds lipid antigens for display to T cells. Here, we solved lipidomes for the four human CD1 antigen-presenting molecules, providing a map of self-lipid display. Answering a basic question, the detection of >2,000 CD1-lipid complexes demonstrates broad presentation of self-sphingolipids and phospholipids. Whereas peptide antigens are chemically processed, many lipids are presented in an unaltered form. However, each type of CD1 protein differentially edits the self-lipidome to show distinct capture motifs based on lipid length and chemical composition, suggesting general antigen display mechanisms. For CD1a and CD1d, lipid size matches the CD1 cleft volume. CD1c cleft size is more variable, and CD1b is the outlier, where ligands and clefts show an extreme size mismatch that is explained by uniformly seating two small lipids in one cleft. Furthermore, the list of compounds that comprise the integrated CD1 lipidome supports the ongoing discovery of lipid blockers and antigens for T cells.

CD1-mediated immune responses in mucosal tissues: molecular mechanisms underlying lipid antigen presentation system

The cluster of differentiation 1 (CD1) molecule differs from major histocompatibility complex class I and II because it presents glycolipid/lipid antigens. Moreover, the CD1-restricted T cells that recognize these self and foreign antigens participate in both innate and adaptive immune responses. CD1s are constitutively expressed by professional and nonprofessional antigen-presenting cells in mucosal tissues, namely, the skin, lung, and intestine. This suggests that CD1-reactive T cells are involved in the immune responses of these tissues. Indeed, evidence suggests that these cells play important roles in diverse diseases, such as inflammation, autoimmune disease, and infection. Recent studies elucidating the molecular mechanisms by which CD1 presents lipid antigens suggest that defects in these mechanisms could contribute to the activities of CD1-reactive T cells. Thus, improving our understanding of these mechanisms could lead to new and effective therapeutic approaches to CD1-associated diseases. In this review, we discuss the CD1-mediated antigen presentation system and its roles in mucosal tissue immunity.

UcTCRdb: An unconventional T cell receptor sequence database with online analysis functions

Unlike conventional major histocompatibility complex (MHC) class I and II molecules reactive T cells, the unconventional T cell subpopulations recognize various non-polymorphic antigen-presenting molecules and are typically characterized by simplified patterns of T cell receptors (TCRs), rapid effector responses and ‘public’ antigen specificities. Dissecting the recognition patterns of the non-MHC antigens by unconventional TCRs can help us further our understanding of the unconventional T cell immunity. The small size and irregularities of the released unconventional TCR sequences are far from high-quality to support systemic analysis of unconventional TCR repertoire. Here we present UcTCRdb, a database that contains 669,900 unconventional TCRs collected from 34 corresponding studies in humans, mice, and cattle. In UcTCRdb, users can interactively browse TCR features of different unconventional T cell subsets in different species, search and download sequences under different conditions. Additionally, basic and advanced online TCR analysis tools have been integrated into the database, which will facilitate the study of unconventional TCR patterns for users with different backgrounds. UcTCRdb is freely available at http://uctcrdb.cn/.

Rational design of a hydrolysis-resistant mycobacterial phosphoglycolipid antigen presented by CD1c to T cells

Whereas proteolytic cleavage is crucial for peptide presentation by classical major histocompatibility complex (MHC) proteins to T cells, glycolipids presented by CD1 molecules are typically presented in an unmodified form. However, the mycobacterial lipid antigen mannosyl-β1-phosphomycoketide (MPM) may be processed through hydrolysis in antigen presenting cells, forming mannose and phosphomycoketide (PM). To further test the hypothesis that some lipid antigens are processed, and to generate antigens that lead to defined epitopes for future tuberculosis vaccines or diagnostic tests, we aimed to create hydrolysis-resistant MPM variants that retain their antigenicity. Here, we designed and tested three different, versatile synthetic strategies to chemically stabilize MPM analogs. Crystallographic studies of CD1c complexes with these three new MPM analogs showed anchoring of the lipid tail and phosphate group that is highly comparable to nature-identical MPM, with considerable conformational flexibility for the mannose head group. MPM-3, a difluoromethylene-modified version of MPM that is resistant to hydrolysis, showed altered recognition by cells, but not by CD1c proteins, supporting the cellular antigen processing hypothesis. Furthermore, the synthetic analogs elicited T cell responses that were cross-reactive with nature-identical MPM, fulfilling important requirements for future clinical use.

Chemical Synthesis of Cell Wall Constituents of Mycobacterium tuberculosis

The pathogen Mycobacterium tuberculosis (Mtb), causing tuberculosis disease, features an extraordinary thick cell envelope, rich in Mtb-specific lipids, glycolipids, and glycans. These cell wall components are often directly involved in host–pathogen interaction and recognition, intracellular survival, and virulence. For decades, these mycobacterial natural products have been of great interest for immunology and synthetic chemistry alike, due to their complex molecular structure and the biological functions arising from it. The synthesis of many of these constituents has been achieved and aided the elucidation of their function by utilizing the synthetic material to study Mtb immunology. This review summarizes the synthetic efforts of a quarter century of total synthesis and highlights how the synthesis layed the foundation for immunological studies as well as drove the field of organic synthesis and catalysis to efficiently access these complex natural products.

CD36 family members are TCR-independent ligands for CD1 antigen-presenting molecules

CD1c presents lipid-based antigens to CD1c-restricted T cells, which are thought to be a major component of the human T cell pool. However, the study of CD1c-restricted T cells is hampered by the presence of an abundantly expressed, non-T cell receptor (TCR) ligand for CD1c on blood cells, confounding analysis of TCR-mediated CD1c tetramer staining. Here, we identified the CD36 family (CD36, SR-B1, and LIMP-2) as ligands for CD1c, CD1b, and CD1d proteins and showed that CD36 is the receptor responsible for non-TCR-mediated CD1c tetramer staining of blood cells. Moreover, CD36 blockade clarified tetramer-based identification of CD1c-restricted T cells and improved identification of CD1b- and CD1d-restricted T cells. We used this technique to characterize CD1c-restricted T cells ex vivo and showed diverse phenotypic features, TCR repertoire, and antigen-specific subsets. Accordingly, this work will enable further studies into the biology of CD1 and human CD1-restricted T cells.

Novel Molecular Insights into Human Lipid-Mediated T Cell Immunity

T cells represent a critical arm of our immune defense against pathogens. Over the past two decades, considerable inroads have been made in understanding the fundamental principles underpinning the molecular presentation of peptide-based antigens by the Major Histocompatibility Complex molecules (MHC-I and II), and their molecular recognition by specialized subsets of T cells. However, some T cells can recognize lipid-based antigens presented by MHC-I-like molecules that belong to the Cluster of Differentiation 1 (CD1) family. Here, we will review the advances that have been made in the last five years to understand the molecular mechanisms orchestrating the presentation of novel endogenous and exogenous lipid-based antigens by the CD1 glycoproteins and their recognition by specific populations of CD1-reactive T cells.

T Cells Specific for a Mycobacterial Glycolipid Expand after Intravenous Bacillus Calmette-Guérin Vaccination

Intradermal vaccination with Mycobacterium bovis bacillus Calmette-Guérin (BCG) protects infants from disseminated tuberculosis, and i.v. BCG protects nonhuman primates (NHP) against pulmonary and extrapulmonary tuberculosis. In humans and NHP, protection is thought to be mediated by T cells, which typically recognize bacterial peptide Ags bound to MHC proteins. However, during vertebrate evolution, T cells acquired the capacity to recognize lipid Ags bound to CD1a, CD1b, and CD1c proteins expressed on APCs. It is unknown whether BCG induces T cell immunity to mycobacterial lipids and whether CD1-restricted T cells are resident in the lung. In this study, we developed and validated Macaca mulatta (Mamu) CD1b and CD1c tetramers to probe ex vivo phenotypes and functions of T cells specific for glucose monomycolate (GMM), an immunodominant mycobacterial lipid Ag. We discovered that CD1b and CD1c present GMM to T cells in both humans and NHP. We show that GMM-specific T cells are expanded in rhesus macaque blood 4 wk after i.v. BCG, which has been shown to protect NHP with near-sterilizing efficacy upon M. tuberculosis challenge. After vaccination, these T cells are detected at high frequency within bronchoalveolar fluid and express CD69 and CD103, markers associated with resident memory T cells. Thus, our data expand the repertoire of T cells known to be induced by whole cell mycobacterial vaccines, such as BCG, and show that lipid Ag-specific T cells are resident in the lungs, where they may contribute to protective immunity.

The role of donor-unrestricted T-cells, innate lymphoid cells, and NK cells in anti-mycobacterial immunity

Vaccination strategies against mycobacteria, focusing mostly on classical T‐ and B‐cells, have shown limited success, encouraging the addition of alternative targets. Classically restricted T‐cells recognize antigens presented via highly polymorphic HLA class Ia and class II molecules, while donor‐unrestricted T‐cells (DURTs), with few exceptions, recognize ligands via genetically conserved antigen presentation molecules. Consequently, DURTs can respond to the same ligands across diverse human populations. DURTs can be activated either through cognate TCR ligation or via bystander cytokine signaling. TCR‐driven antigen‐specific activation of DURTs occurs upon antigen presentation via non‐polymorphic molecules such as HLA‐E, CD1, MR1, and butyrophilin, leading to the activation of HLA‐E–restricted T‐cells, CD1‐restricted T‐cells, mucosal‐associated invariant T‐cells (MAITs), and TCRγδ T‐cells, respectively. NK cells and innate lymphoid cells (ILCs), which lack rearranged TCRs, are activated through other receptor‐triggering pathways, or can be engaged through bystander cytokines, produced, for example, by activated antigen‐specific T‐cells or phagocytes. NK cells can also develop trained immune memory and thus could represent cells of interest to mobilize by novel vaccines. In this review, we summarize the latest findings regarding the contributions of DURTs, NK cells, and ILCs in anti–M tuberculosis, M leprae, and non‐tuberculous mycobacterial immunity and explore possible ways in which they could be harnessed through vaccines and immunotherapies to improve protection against Mtb.

Synthetic mycobacterial diacyl trehaloses reveal differential recognition by human T cell receptors and the C-type lectin Mincle

The cell wall of Mycobacterium tuberculosis is composed of diverse glycolipids which potentially interact with the human immune system. To overcome difficulties in obtaining pure compounds from bacterial extracts, we recently synthesized three forms of mycobacterial diacyltrehalose (DAT) that differ in their fatty acid composition, DAT₁, DAT₂, and DAT₃. To study the potential recognition of DATs by human T cells, we treated the lipid-binding antigen presenting molecule CD1b with synthetic DATs and looked for T cells that bound the complex. DAT₁- and DAT₂-treated CD1b tetramers were recognized by T cells, but DAT₃-treated CD1b tetramers were not. A T cell line derived using CD1b-DAT₂ tetramers showed that there is no cross-reactivity between DATs in an IFN-γ release assay, suggesting that the chemical structure of the fatty acid at the 3-position determines recognition by T cells. In contrast with the lack of recognition of DAT₃ by human T cells, DAT_3, but not DAT₁ or DAT₂, activates Mincle. Thus, we show that the mycobacterial lipid DAT can be both an antigen for T cells and an agonist for the innate Mincle receptor, and that small chemical differences determine recognition by different parts of the immune system.

Human skin is colonized by T cells that recognize CD1a independently of lipid

CD1a-autoreactive T cells contribute to skin disease, but the identity of immunodominant self-lipid antigens and their mode of recognition are not yet solved. In most models, MHC and CD1 proteins serve as display platforms for smaller antigens. Here, we showed that CD1a tetramers without added antigen stained large T cell pools in every subject tested, accounting for approximately 1% of skin T cells. The mechanism of tetramer binding to T cells did not require any defined antigen. Binding occurred with approximately 100 lipid ligands carried by CD1a proteins, but could be tuned upward or downward with certain natural self-lipids. TCR recognition mapped to the outer A' roof of CD1a at sites remote from the antigen exit portal, explaining how TCRs can bind CD1a rather than carried lipids. Thus, a major antigenic target of CD1a T cell autoreactivity in vivo is CD1a itself. Based on their high frequency and prevalence among donors, we conclude that CD1a-specific, lipid-independent T cells are a normal component of the human skin T cell repertoire. Bypassing the need to select antigens and effector molecules, CD1a tetramers represent a simple method to track such CD1a-specific T cells from tissues and in any clinical disease.

Role of Group 1 CD1-Restricted T Cells in Host Defense and Inflammatory Diseases

Group 1 CD1-restricted T cells are members of the unconventional T cell family that recognize lipid antigens presented by CD1a, CD1b, and CD1c molecules. Although they developmentally mirror invariant natural killer T cells, they have diverse antigen specificity and functional capacity, with both anti-microbial and autoreactive targets. The role of group 1 CD1-restricted T cells has been best established in Mycobacterium tuberculosis (Mtb) infection in which a wide variety of lipid antigens have been identified and their ability to confer protection against Mtb infection in a CD1 transgenic mouse model has been shown. Group 1 CD1-restricted T cells have also been implicated in other infections, inflammatory conditions, and malignancies. In particular, autoreactive group 1 CD1-restricted T cells have been shown to play a role in several skin inflammatory conditions. The prevalence of group 1 CD1 autoreactive T cells in healthy individuals suggests the presence of regulatory mechanisms to suppress autoreactivity in homeostasis. The more recent use of group 1 CD1 tetramers and mouse models has allowed for better characterization of their phenotype, functional capacity, and underlying mechanisms of antigen-specific and autoreactive activation. These discoveries may pave the way for the development of novel vaccines and immunotherapies that target group 1 CD1-restricted T cells.

The thick waxy coat of mycobacteria, a protective layer against antibiotics and the host's immune system

Tuberculosis, caused by the pathogenic bacterium Mycobacterium tuberculosis (Mtb), is the leading cause of death from an infectious disease, with a mortality rate of over a million people per year. This pathogen's remarkable resilience and infectivity is largely due to its unique waxy cell envelope, 40% of which comprises complex lipids. Therefore, an understanding of the structure and function of the cell wall lipids is of huge indirect clinical significance. This review provides a synopsis of the cell envelope and the major lipids contained within, including structure, biosynthesis and roles in pathogenesis.

T Cell Immunity to Bacterial Pathogens: Mechanisms of Immune Control and Bacterial Evasion

The human body frequently encounters harmful bacterial pathogens and employs immune defense mechanisms designed to counteract such pathogenic assault. In the adaptive immune system, major histocompatibility complex (MHC)-restricted αβ T cells, along with unconventional αβ or γδ T cells, respond to bacterial antigens to orchestrate persisting protective immune responses and generate immunological memory. Research in the past ten years accelerated our knowledge of how T cells recognize bacterial antigens and how many bacterial species have evolved mechanisms to evade host antimicrobial immune responses. Such escape mechanisms act to corrupt the crosstalk between innate and adaptive immunity, potentially tipping the balance of host immune responses toward pathological rather than protective. This review examines the latest developments in our knowledge of how T cell immunity responds to bacterial pathogens and evaluates some of the mechanisms that pathogenic bacteria use to evade such T cell immunosurveillance, to promote virulence and survival in the host.

New Concepts in Tuberculosis Host Defense

Tuberculosis (TB) host defense depends on cellular immunity, including macrophages and adaptively acquired CD4⁺ and CD8⁺ T cells. More recently, roles for new immune components, including neutrophils, innate T cells, and B cells, have been defined, and the understanding of the function of macrophages and adaptively acquired T cells has been advanced. Moreover, the understanding of TB immunology elucidates TB infection and disease as a spectrum. Finally, determinates of TB host defense, such as age and comorbidities, affect clinical expression of TB disease. Herein, the authors comprehensively review TB immunology with an emphasis on new advances.

Immunology of Mycobacterium tuberculosis Infections

Tuberculosis (TB) is a serious global public health challenge that results in significant morbidity and mortality worldwide. TB is caused by infection with the bacilli Mycobacterium tuberculosis (M. tuberculosis), which has evolved a wide variety of strategies in order to thrive within its host. Understanding the complex interactions between M. tuberculosis and host immunity can inform the rational design of better TB vaccines and therapeutics. This chapter covers innate and adaptive immunity against M. tuberculosis infection, including insights on bacterial immune evasion and subversion garnered from animal models of infection and human studies. In addition, this chapter discusses the immunology of the TB granuloma, TB diagnostics, and TB comorbidities. Finally, this chapter provides a broad overview of the current TB vaccine pipeline.

CD1b Tetramers Broadly Detect T Cells That Correlate With Mycobacterial Exposure but Not Tuberculosis Disease State

The non-polymorphic nature of CD1 proteins creates a situation in which T cells with invariant T cell receptors (TCRs), like CD1d-specific NKT cells, are present in all humans. CD1b is an abundant protein on human dendritic cells that presents M. tuberculosis (Mtb) lipid antigens to T cells. Analysis of T cell clones suggested that semi-invariant TCRs exist in the CD1b system, but their prevalence in humans is not known. Here we used CD1b tetramers loaded with mycolic acid or glucose monomycolate to study polyclonal T cells from 150 Peruvian subjects. We found that CD1b tetramers loaded with mycolic acid or glucose monomycolate antigens stained TRAV1-2⁺ GEM T cells or TRBV4-1⁺ LDN5-like T cells in the majority of subjects tested, at rates ~10-fold lower than NKT cells. Thus, GEM T cells and LDN5-like T cells are a normal part of the human immune system. Unlike prior studies measuring MHC- or CD1b-mediated activation, this large-scale tetramer study found no significant differences in rates of CD1b tetramer-mycobacterial lipid staining of T cells among subjects with Mtb exposure, latent Mtb infection or active tuberculosis (TB) disease. In all subjects, including “uninfected” subjects, CD1b tetramer⁺ T cells expressed memory markers at high levels. However, among controls with lower mycobacterial antigen exposure in Boston, we found significantly lower frequencies of T cells staining with CD1b tetramers loaded with mycobacterial lipids. These data link CD1b-specific T cell detection to mycobacterial exposure, but not TB disease status, which potentially explains differences in outcomes among CD1-based clinical studies, which used control subjects with low Mtb exposure.

Conservation of molecular and cellular phenotypes of invariant NKT cells between humans and non-human primates

Invariant NKT (iNKT) cells in both humans and non-human primates are activated by the glycolipid antigen, α-galactosylceramide (α-GalCer). However, the extent to which the molecular mechanisms of antigen recognition and in vivo phenotypes of iNKT cells are conserved among primate species has not been determined. Using an evolutionary genetic approach, we found a lack of diversifying selection in CD1 genes over 45 million years of evolution, which stands in stark contrast to the history of the MHC system for presenting peptide antigens to T cells. The invariant T cell receptor (TCR)-α chain was strictly conserved across all seven primate clades. Invariant NKT cells from rhesus macaques (Macaca mulatta) bind human CD1D-α-GalCer tetramer and are activated by α-GalCer-loaded human CD1D transfectants. The dominant TCR-β chain cloned from a rhesus-derived iNKT cell line is nearly identical to that found in the human iNKT TCR, and transduction of the rhesus iNKT TCR into human Jurkat cells show that it is sufficient for binding human CD1D-α-GalCer tetramer. Finally, we used a 20-color flow cytometry panel to probe tissue phenotypes of iNKT cells in a cohort of rhesus macaques. We discovered several tissue-resident iNKT populations that have not been previously described in non-human primates but are known in humans, such as TCR-γδ iNKTs. These data reveal a diversity of iNKT cell phenotypes despite convergent evolution of the genes required for lipid antigen presentation and recognition in humans and non-human primates.

Harnessing donor unrestricted T-cells for new vaccines against tuberculosis

Mycobacterium bovis bacille Calmette-Guérin (BCG) prevents extrapulmonary tuberculosis (TB) and death among infants but fails to consistently and sufficiently prevent pulmonary TB in adults. Thus, TB remains the leading infectious cause of death worldwide, and new vaccine approaches are urgently needed. T-cells are important for protective immunity to Mycobacterium tuberculosis (Mtb), but the optimal T-cell antigens to be included in new vaccines are not established. T-cells are often thought of as responding mainly to peptide antigens presented by polymorphic major histocompatibility complex (MHC) I and II molecules. Over the past two decades, the number of non-peptidic Mtb derived antigens for αβ and γδ T-cells has expanded rapidly, creating broader perspectives about the types of molecules that could be targeted by T-cell-based vaccines against TB. Many of these non-peptide responsive T-cell subsets in humans are activated in a manner that is unrestricted by classical MHC-dependent antigen-presenting systems, but instead require essentially nonpolymorphic presentation systems. These systems are Cluster of differentiation 1 (CD1), MHC related protein 1 (MR1), butyrophilin 3A1, as well as the nonclassical MHC class Ib family member HLA-E. Thus, the resulting T-cell responses can be shared among a genetically diverse population, creating the concept of donor-unrestricted T-cells (DURTs). Here, we review evidence that DURTs are an abundant component of the human immune system and recognize many antigens expressed by Mtb, including antigens that are expressed in BCG and other candidate whole cell vaccines. Further, DURTs exhibit functional diversity and demonstrate the ability to control microbial infection in small animal models. Finally, we outline specific knowledge gaps and research priorities that must be addressed to realize the full potential of DURTs as part of new TB vaccines approaches.

CD1b presents self and Borrelia burgdorferi diacylglycerols to human T cells

Lyme disease is a common multisystem disease caused by infection with a tick‐transmitted spirochete, Borrelia burgdorferi and related Borrelia species. The monoglycosylated diacylglycerol known as B. burgdorferi glycolipid II (BbGL‐II) is a major target of antibodies in sera from infected individuals. Here, we show that CD1b presents BbGL‐II to human T cells and that the TCR mediates the recognition. However, we did not detect increased frequency of CD1b‐BbGL‐II binding T cells in the peripheral blood of Lyme disease patients compared to controls. Unexpectedly, mapping the T cell specificity for BbGL‐II‐like molecules using tetramers and activation assays revealed a concomitant response to CD1b‐expressing APCs in absence of BbGL‐II. Further, among all major classes of self‐lipid tested, BbGL‐II responsive TCRs show strong cross‐reactivity to diacylglycerol, a self‐lipid antigen with structural similarities to BbGL‐II. Extending prior work on MHC and CD1b, CD1c, and CD1d proteins, this study provides evidence for cross‐reactive CD1b‐restricted T cell responses to bacterial and self‐antigens, and identifies chemically defined targets for future discovery of self and foreign antigen cross‐reactive T cells.

Unconventional T Cell Targets for Cancer Immunotherapy

Most studies on the immunotherapeutic potential of T cells have focused on CD8 and CD4 T cells that recognize peptide antigens (Ag) presented by polymorphic major histocompatibility complex (MHC) class I and MHC class II molecules, respectively. However, unconventional T cells, which interact with MHC class Ib and MHC-I like molecules, are also implicated in tumor immunity, although their role therein is unclear. These include unconventional T cells targeting MHC class Ib molecules such as HLA-E and its murine ortholog Qa-1b, natural killer T (NKT) cells, mucosal associated invariant T (MAIT) cells, and γδ T cells. Here, we review the current understanding of the roles of these unconventional T cells in tumor immunity and discuss why further studies into the immunotherapeutic potential of these cells is warranted.

Research and development of new tuberculosis vaccines: a review

Tuberculosis kills more people worldwide than any other single infectious disease agent, a threat made more dire by the spread of drug-resistant strains of Mycobacterium tuberculosis (Mtb). Development of new vaccines capable of preventing TB disease and new Mtb infection are an essential component of the strategy to combat the TB epidemic. Accordingly, the WHO considers the development of new TB vaccines a major public health priority. In October 2017, the WHO convened a consultation with global leaders in the TB vaccine development field to emphasize the WHO commitment to this effort and to facilitate creative approaches to the discovery and development of TB vaccine candidates. This review summarizes the presentations at this consultation, updated with scientific literature references, and includes discussions of the public health need for a TB vaccine; the status of efforts to develop vaccines to replace or potentiate BCG in infants and develop new TB vaccines for adolescents and adults; strategies being employed to diversify vaccine platforms; and new animal models being developed to facilitate TB vaccine development. A perspective on the status of these efforts from the major funders and organizational contributors also is included. This presentation highlights the extraordinary progress being made to develop new TB vaccines and provided a clear picture of the exciting development pathways that are being explored.

Research and development of new tuberculosis vaccines: a review

Tuberculosis kills more people worldwide than any other single infectious disease agent, a threat made more dire by the spread of drug-resistant strains of Mycobacterium tuberculosis (Mtb). Development of new vaccines capable of preventing TB disease and new Mtb infection are an essential component of the strategy to combat the TB epidemic. Accordingly, the WHO considers the development of new TB vaccines a major public health priority. In October 2017, the WHO convened a consultation with global leaders in the TB vaccine development field to emphasize the WHO commitment to this effort and to facilitate creative approaches to the discovery and development of TB vaccine candidates. This review summarizes the presentations at this consultation, updated with scientific literature references, and includes discussions of the public health need for a TB vaccine; the status of efforts to develop vaccines to replace or potentiate BCG in infants and develop new TB vaccines for adolescents and adults; strategies being employed to diversify vaccine platforms; and new animal models being developed to facilitate TB vaccine development. A perspective on the status of these efforts from the major funders and organizational contributors also is included. This presentation highlights the extraordinary progress being made to develop new TB vaccines and provided a clear picture of the exciting development pathways that are being explored.

Potential advantages of CD1-restricted T cell immunotherapy in cancer

Adoptive cell therapy (ACT) using tumor-specific "conventional" MHC-restricted T cells obtained from tumor-infiltrating lymphocytes, or derived ex vivo by either antigen-specific expansion or genetic engineering of polyclonal T cell populations, shows great promise for cancer treatment. However, the wide applicability of this therapy finds limits in the high polymorphism of MHC molecules that restricts the use in the autologous context. CD1 antigen presenting molecules are nonpolymorphic and specialized for lipid antigen presentation to T cells. They are often expressed on malignant cells and, therefore, may represent an attractive target for ACT. We provide a brief overview of the CD1-resticted T cell response in tumor immunity and we discuss the pros and cons of ACT approaches based on unconventional CD1-restricted T cells.

Validation of a CD1b tetramer assay for studies of human mycobacterial infection or vaccination

CD1 tetramers loaded with lipid antigens facilitate the identification of rare lipid-antigen specific T cells present in human blood and tissue. Because CD1 proteins are structurally non-polymorphic, these tetramers can be applied to genetically diverse human populations, unlike MHC-I and MHC-II tetramers. However, there are no standardized assays to quantify and characterize lipid antigen-specific T cells present within clinical samples. We incorporated CD1b tetramers loaded with the mycobacterial lipid glucose monomycolate (GMM) into a multi-parameter flow cytometry assay. Using a GMM-specific T-cell line, we demonstrate that the assay is linear, reproducible, repeatable, precise, accurate, and has a limit of detection of approximately 0.007%. Having formally validated this assay, we performed a cross-sectional study of healthy U.S. controls and South African adolescents with and without latent tuberculosis infection (LTBI). We show that GMM-specific T cells are specifically detected in South African subjects with LTBI and not in U.S. healthy controls. This assay can be expanded to include additional tetramers or phenotypic markers to characterize GMM-specific T cells in studies of mycobacterial infection, disease, or vaccination.

The Conventional Nature of Non-MHC-Restricted T Cells

The definition “unconventional T cells” identifies T lymphocytes that recognize non-peptide antigens presented by monomorphic antigen-presenting molecules. Two cell populations recognize lipid antigens and small metabolites presented by CD1 and MR1 molecules, respectively. A third cell population expressing the TCR Vγ9Vδ2 is stimulated by small phosphorylated metabolites. In the recent past, we have learnt a lot about the selection, tissue distribution, gene transcription programs, mode of expansion after antigen recognition, and persistence of these cells. These studies depict their functions in immune homeostasis and diseases. Current investigations are revealing that unconventional T cells include distinct sub-populations, which display unexpected similarities to classical MHC-restricted T cells in terms of TCR repertoire diversity, antigen specificity variety, functional heterogeneity, and naïve-to-memory differentiation dynamic. This review discusses the latest findings with a particular emphasis on these T cells, which appear to be more conventional than previously appreciated, and with the perspective of using CD1 and MR1-restricted T cells in vaccination and immunotherapy.

Molecular recognition of microbial lipid-based antigens by T cells

The immune system has evolved to protect hosts from pathogens. T cells represent a critical component of the immune system by their engagement in host defence mechanisms against microbial infections. Our knowledge of the molecular recognition by T cells of pathogen-derived peptidic antigens that are presented by the major histocompatibility complex glycoproteins is now well established. However, lipids represent an additional, distinct chemical class of molecules that when presented by the family of CD1 antigen-presenting molecules can serve as antigens, and be recognized by specialized subsets of T cells leading to antigen-specific activation. Over the past decades, numerous CD1-presented self- and bacterial lipid-based antigens have been isolated and characterized. However, our understanding at the molecular level of T cell immunity to CD1 molecules presenting microbial lipid-based antigens is still largely unexplored. Here, we review the insights and the molecular basis underpinning the recognition of microbial lipid-based antigens by T cells.

T cell autoreactivity directed toward CD1c itself rather than toward carried self lipids

The hallmark function of αβ T cell antigen receptors (TCRs) involves the highly specific co-recognition of a major histocompatibility complex molecule and its carried peptide. However, the molecular basis of the interactions of TCRs with the lipid antigen-presenting molecule CD1c is unknown. We identified frequent staining of human T cells with CD1c tetramers across numerous subjects. Whereas TCRs typically show high specificity for antigen, both tetramer binding and autoreactivity occurred with CD1c in complex with numerous, chemically diverse self lipids. Such extreme polyspecificity was attributable to binding of the TCR over the closed surface of CD1c, with the TCR covering the portal where lipids normally protrude. The TCR essentially failed to contact lipids because they were fully seated within CD1c. These data demonstrate the sequestration of lipids within CD1c as a mechanism of autoreactivity and point to small lipid size as a determinant of autoreactive T cell responses.

The versatility of the CD1 lipid antigen presentation pathway

The family of non-classical major histocompatibility complex (MHC) class-I like CD1 molecules has an emerging role in human disease. Group 1 CD1 includes CD1a, CD1b and CD1c, which function to display lipids on the cell surface of antigen-presenting cells for direct recognition by T-cells. The recent advent of CD1 tetramers and the identification of novel lipid ligands has contributed towards the increasing number of CD1-restricted T-cell clones captured. These advances have helped to identify novel donor unrestricted and semi-invariant T-cell populations in humans and new mechanisms of T-cell recognition. However, although there is an opportunity to design broadly acting lipids and harness the therapeutic potential of conserved T-cells, knowledge of their role in health and disease is lacking. We briefly summarize the current evidence implicating group 1 CD1 molecules in infection, cancer and autoimmunity and show that although CD1 are not as diverse as MHC, recent discoveries highlight their versatility as they exhibit intricate mechanisms of antigen presentation.

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.

Molecular features of lipid-based antigen presentation by group 1 CD1 molecules

Lipids are now widely considered to play a variety of important roles in T-cell mediated immunity, including serving as antigens. Lipid-based antigens are presented by a specialised group of glycoproteins termed CD1. In humans, three classes of CD1 molecules exist: group 1 (CD1a, CD1b, CD1c), group 2 (CD1d), and group 3 (CD1e). While CD1d-mediated T-cell immunity has been extensively investigated, we have only recently gained insights into the structure and function of group 1 CD1 molecules. Structural studies have revealed how lipid-based antigens are presented by group 1 CD1 molecules, as well as shedding light on the molecular requirements for T-cell recognition. Here, we provide an overview of our current understanding of lipid presentation by group 1 CD1 molecules in humans and their recognition by T-cells, as well as examining the potential differences in lipid presentation that may occur across different species.

CD1: From Molecules to Diseases

The human cluster of differentiation (CD)1 system for antigen display is comprised of four types of antigen-presenting molecules, each with a distinct functional niche: CD1a, CD1b, CD1c, and CD1d. Whereas CD1 proteins were thought solely to influence T-cell responses through display of amphipathic lipids, recent studies emphasize the role of direct contacts between the T-cell receptor and CD1 itself. Moving from molecules to diseases, new research approaches emphasize human CD1-transgenic mouse models and the study of human polyclonal T cells in vivo or ex vivo in disease states. Whereas the high genetic diversity of major histocompatibility complex (MHC)-encoded antigen-presenting molecules provides a major hurdle for designing antigens that activate T cells in all humans, the simple population genetics of the CD1 system offers the prospect of discovering or designing broadly acting immunomodulatory agents.

Unconventional or Preset αβ T Cells: Evolutionarily Conserved Tissue-Resident T Cells Recognizing Nonpeptidic Ligands

A majority of T cells bearing the αβ T cell receptor (TCR) are specific for peptides bound to polymorphic classical major histocompatibility complex (MHC) molecules. Smaller subsets of T cells are reactive toward various nonpeptidic ligands associated with nonpolymorphic MHC class-Ib (MHC-Ib) molecules. These cells have been termed unconventional for decades, even though only the composite antigen is different from the one seen by classical T cells. Herein, we discuss the identity of these particular T cells in light of the coevolution of their TCR and MHC-Ib restricting elements. We examine their original thymic development: selection on hematopoietic cells leading to the acquisition of an original differentiation program. Most of these cells acquire memory cell features during thymic maturation and exhibit unique patterns of migration into peripheral nonlymphoid tissues to become tissue resident. Thus, these cells are termed preset T cells, as they also display a variety of effector functions. They may act as microbial or danger sentinels, fight microbes, or regulate tissue homeostasis.

Four pathways of CD1 antigen presentation to T cells

CD1a, CD1b, CD1c and CD1d proteins migrate through distinct subcellular compartments of antigen presenting cells and so can be considered to take four separate pathways leading to display of lipid antigens to T cell receptors. This review discusses the intersection of CD1 trafficking and lipid antigen loading mechanisms in cells, highlighting key controversies relating to CD1 gene expression, size mismatches between antigens and CD1 binding clefts and unexpected mechanisms of T cell receptor-based recognition.

Inhibition of endocytic lipid antigen presentation by common lipophilic environmental pollutants

Environmental pollutants as non-heritable factors are now recognized as triggers for multiple human inflammatory diseases involving T cells. We postulated that lipid antigen presentation mediated by cluster of differentiation 1 (CD1) proteins for T cell activation is susceptible to lipophilic environmental pollutants. To test this notion, we determined whether the common lipophilic pollutants benzo[a]pyrene and diesel exhaust particles impact on the activation of lipid-specific T cells. Our results demonstrated that the expression of CD1a and CD1d proteins, and the activation of CD1a- and CD1d-restricted T cells were sensitively inhibited by benzo[a]pyrene even at the low concentrations detectable in exposed human populations. Similarly, diesel exhaust particles showed a marginal inhibitory effect. Using transcriptomic profiling, we discovered that the gene expression for regulating endocytic and lipid metabolic pathways was perturbed by benzo[a]pyrene. Imaging flow cytometry also showed that CD1a and CD1d proteins were retained in early and late endosomal compartments, respectively, supporting an impaired endocytic lipid antigen presentation for T cell activation upon benzo[a]pyrene exposure. This work conceptually demonstrates that lipid antigen presentation for T cell activation is inhibited by lipophilic pollutants through profound interference with gene expression and endocytic function, likely further disrupting regulatory cytokine secretion and ultimately exacerbating inflammatory diseases.

Targeting Innate-Like T Cells in Tuberculosis

Peptide-specific conventional T cells have been major targets for designing most antimycobacterial vaccines. Immune responses mediated by conventional T cells exhibit a delayed onset upon primary infection and are highly variable in different human populations. In contrast, innate-like T cells quickly respond to pathogens and display effector functions without undergoing extensive clonal expansion. Specifically, the activation of innate-like T cells depends on the promiscuous interaction of highly conserved antigen-presenting molecules, non-peptidic antigens, and likely semi-invariant T cell receptors. In antimicrobial immune responses, mucosal-associated invariant T cells are activated by riboflavin precursor metabolites presented by major histocompatibility complex-related protein I, while lipid-specific T cells including natural killer T cells are activated by lipid metabolites presented by CD1 proteins. Multiple innate-like T cell subsets have been shown to be protective or responsive in mycobacterial infections. Through rapid cytokine secretion, innate-like T cells function in early defense and memory response, offering novel advantages over conventional T cells in the design of anti-tuberculosis strategies.

CD1-Restricted T Cells at the Crossroad of Innate and Adaptive Immunity

Lipid-specific T cells comprise a group of T cells that recognize lipids bound to the MHC class I-like CD1 molecules. There are four isoforms of CD1 that are expressed at the surface of antigen presenting cells and therefore capable of presenting lipid antigens: CD1a, CD1b, CD1c, and CD1d. Each one of these isoforms has distinct structural features and cellular localizations, which promotes binding to a broad range of different types of lipids. Lipid antigens originate from either self-tissues or foreign sources, such as bacteria, fungus, or plants and their recognition by CD1-restricted T cells has important implications in infection but also in cancer and autoimmunity. In this review, we describe the characteristics of CD1 molecules and CD1-restricted lipid-specific T cells, highlighting the innate-like and adaptive-like features of different CD1-restricted T cell subtypes.

Cellular endocytic compartment localization of expressed canine CD1 molecules

CD1 molecules are glycoproteins present primarily on dendritic cells (DCs), which recognize and present a variety of foreign- and self-lipid antigens to T-cells. Humans have five different CD1 isoforms that survey distinct cellular compartments allowing for recognition of a large repertoire of lipids. The canine CD1 family consists of seven functional CD1 molecules (canine CD1a2, CD1a6, CD1a8, CD1a9, CD1b, CD1c and CD1e) and one presumed non-functional isoform (canine CD1d) due to a disrupted gene structure. The aim of this study was to describe in vitro steady-state localization ptterns of canine CD1 isoforms and their correlation with endocytic organelles. GFP-fused canine CD1 293T cell transfectants were stained with markers for early endocytic compartments (EEA-1) and late endocytic/lysosomal compartments (LAMP-1), respectively, and analyzed by confocal microscopy. Canine CD1a molecules localized to the plasma membrane and partially to the early endocytic compartment, but not to late endosomes or lysosomes. In contrast, canine CD1b was highly associated with late endosomal/lysosomal compartments and showed a predominant intracellular expression pattern. Canine CD1c protein expression localized more promiscuously to both the early endosomal compartments and the late endosomal/lysosomal compartments. The canine CD1e molecule showed a strictly intracellular expression with a partial overlap with late endosomal/lysosomal compartments. Lastly, canine CD1d was expressed abnormally showing only a diminished GFP expression. In conclusion, canine CD1 transfectants show distinct localization patterns that are similar to human CD1 proteins with the exception of the canine CD1d isoform, which most likely is non-functional. These findings imply that canine CD1 localization overall resembles human CD1 trafficking patterns. This knowledge is important for the understanding of lipid antigen-receptor immunity in the dog.

T cell receptor recognition of CD1b presenting a mycobacterial glycolipid

CD1 proteins present microbial lipids to T cells. Germline-encoded mycolyl lipid-reactive (GEM) T cells with conserved αβ T cell receptors (TCRs) recognize CD1b presenting mycobacterial mycolates. As the molecular basis underpinning TCR recognition of CD1b remains unknown, here we determine the structure of a GEM TCR bound to CD1b presenting glucose-6-O-monomycolate (GMM). The GEM TCR docks centrally above CD1b, whereby the conserved TCR α-chain extensively contacts CD1b and GMM. Through mutagenesis and study of T cells from tuberculosis patients, we identify a consensus CD1b footprint of TCRs present among GEM T cells. Using both the TCR α- and β-chains as tweezers to surround and grip the glucose moiety of GMM, GEM TCRs create a highly specific mechanism for recognizing this mycobacterial glycolipid.

Germline-encoded mycolyl lipid-reactive (GEM) T cells recognize CD1b proteins presenting mycobacterial mycolates via their T-cell receptors (TCRs). Here, the authors present the structure of this interaction and provide a molecular basis for the co-recognition of CD1b and a mycobacterial glycolipid.

Donor-unrestricted T cells in the human CD1 system

The CD1 and MHC systems are specialized for lipid and peptide display, respectively. Here, we review evidence showing how cellular CD1a, CD1b, CD1c, and CD1d proteins capture and display many cellular lipids to T cell receptors (TCRs). Increasing evidence shows that CD1-reactive T cells operate outside two classical immunogenetic concepts derived from the MHC paradigm. First, because CD1 proteins are non-polymorphic in human populations, T cell responses are not restricted to the donor's genetic background. Second, the simplified population genetics of CD1 antigen-presenting molecules can lead to simplified patterns of TCR usage. As contrasted with donor-restricted patterns of MHC-TCR interaction, the donor-unrestricted nature of CD1-TCR interactions raises the prospect that lipid agonists and antagonists of T cells could be developed.

The CD1 family: serving lipid antigens to T cells since the Mesozoic era

Class I-like CD1 molecules are in a family of antigen-presenting molecules that bind lipids and lipopeptides, rather than peptides for immune surveillance by T cells. Since CD1 lacks the high degree of polymorphism found in their major histocompatibility complex (MHC) class I molecules, different species express different numbers of CD1 isotypes, likely to be able to present structurally diverse classes of lipid antigens. In this review, we will present a historical overview of the structures of the different human CD1 isotypes and also discuss species-specific adaptations of the lipid-binding groove. We will discuss how single amino acid changes alter the shape and volume of the CD1 binding groove, how these minor changes can give rise to different numbers of binding pockets, and how these pockets affect the lipid repertoire that can be presented by any given CD1 protein. We will compare the structures of various lipid antigens and finally, we will discuss recognition of CD1-presented lipid antigens by antigen receptors on T cells (TCRs).

MiR-381-3p Regulates the Antigen-Presenting Capability of Dendritic Cells and Represses Antituberculosis Cellular Immune Responses by Targeting CD1c

Tuberculosis is still the widest spread infectious disease in the world, and more in-depth studies are needed on the interaction between the pathogen and the host. Due to the highest lipid components in Mycobacterium tuberculosis, the CD1 family that specifically presents antigenic lipids plays important roles in the antituberculosis immunity, especially CD1c, which functions as the intracellular Ag inspector at the full intracellular range. However, downregulation of the CD1c mRNA level has been observed in M. tuberculosis-infected cells, which is consistent with the regulatory mechanism of miRNA on gene expression. In this study, through combinatory analysis of previous miRNA transcriptomic assays and bioinformatic predictions by web-based algorithms, miR-381-3p was predicted to bind the 3'-untranslated region of CD1c gene. In vivo expression of miR-381-3p in dendritic cells (DCs) of TB patients is higher than in DCs of healthy individuals, inversely related to CD1c. Suppression of CD1c expression in bacillus Calmette-Guérin (BCG)-infected DCs was accompanied with upregulation of miR-381-3p, whereas inhibition of miR-381-3p could reverse suppression of CD1c expression and promote T cell responses against BCG infection. Further study indicated that miR-381-3p is also one of the mediators of the immune suppressor IL-10. Collectively, these results demonstrated the mechanism that suppression of CD1c by BCG infection is mediated by miR-381-3p. This finding may provide a novel approach to boost immune responses to M. tuberculosis.

The burgeoning family of unconventional T cells

While most studies of T lymphocytes have focused on T cells reactive to complexes of peptide and major histocompatibility complex (MHC) proteins, many other types of T cells do not fit this paradigm. These include CD1-restricted T cells, MR1-restricted mucosal associated invariant T cells (MAIT cells), MHC class Ib-reactive T cells, and γδ T cells. Collectively, these T cells are considered 'unconventional', in part because they can recognize lipids, small-molecule metabolites and specially modified peptides. Unlike MHC-reactive T cells, these apparently disparate T cell types generally show simplified patterns of T cell antigen receptor (TCR) expression, rapid effector responses and 'public' antigen specificities. Here we review evidence showing that unconventional T cells are an abundant component of the human immune system and discuss the immunotherapeutic potential of these cells and their antigenic targets.