Modulation of CD1d-restricted NKT cell responses by CD4

Human CD4+ iNKT cell adoptive immunotherapy induces anti-tumour responses against CD1d-negative EBV-driven B lymphoma

Invariant natural killer T (iNKT) cells are a conserved population of innate T lymphocytes that are uniquely suitable as off-the-shelf cellular immunotherapies due to their lack of alloreactivity. Two major subpopulations of human iNKT cells have been delineated, a CD4- subset that has a TH1/cytolytic profile, and a CD4+ subset that appears polyfunctional and can produce both regulatory and immunostimulatory cytokines. Whether these two subsets differ in anti-tumour effects is not known. Using live cell imaging, we found that CD4- iNKT cells limited growth of CD1d+ Epstein-Barr virus (EBV)-infected B-lymphoblastoid spheroids in vitro, whereas CD4+ iNKT cells showed little or no direct anti-tumour activity. However, the effects of the two subsets were reversed when we tested them as adoptive immunotherapies in vivo using a xenograft model of EBV-driven human B cell lymphoma. We found that EBV-infected B cells down-regulated CD1d in vivo, and administering CD4- iNKT cells had no discernable impact on tumour mass. In contrast, xenotransplanted mice bearing lymphomas showed rapid reduction in tumour mass after administering CD4+ iNKT cells. Immunotherapeutic CD4+ iNKT cells trafficked to both spleen and tumour and were associated with subsequently enhanced responses of xenotransplanted human T cells against EBV. CD4+ iNKT cells also had adjuvant-like effects on monocyte-derived DCs and promoted antigen-dependent responses of human T cells in vitro. These results show that allogeneic CD4+ iNKT cellular immunotherapy leads to marked anti-tumour activity through indirect pathways that do not require tumour cell CD1d expression and that are associated with enhanced activity of antigen-specific T cells.

CD4 and CD8 co-receptors modulate functional avidity of CD1b-restricted T cells

T cells recognize mycobacterial glycolipid (mycolipid) antigens presented by CD1b molecules, but the role of CD4 and CD8 co-receptors in mycolipid recognition is unknown. Here we show CD1b-mycolipid tetramers reveal a hierarchy in which circulating T cells expressing CD4 or CD8 co-receptor stain with a higher tetramer mean fluorescence intensity than CD4-CD8- T cells. CD4+ primary T cells transduced with mycolipid-specific T cell receptors bind CD1b-mycolipid tetramer with a higher fluorescence intensity than CD8+ primary T cells. The presence of either CD4 or CD8 also decreases the threshold for interferon-γ secretion. Co-receptor expression increases surface expression of CD3ε, suggesting a mechanism for increased tetramer binding and activation. Targeted transcriptional profiling of mycolipid-specific T cells from individuals with active tuberculosis reveals canonical markers associated with cytotoxicity among CD8+ compared to CD4+ T cells. Thus, expression of co-receptors modulates T cell receptor avidity for mycobacterial lipids, leading to in vivo functional diversity during tuberculosis disease.

Harnessing invariant natural killer T cells to control pathological inflammation

Invariant natural killer T (iNKT) cells are innate T cells that are recognized for their potent immune modulatory functions. Over the last three decades, research in murine models and human observational studies have revealed that iNKT cells can act to limit inflammatory pathology in a variety of settings. Since iNKT cells are multi-functional and can promote inflammation in some contexts, understanding the mechanistic basis for their anti-inflammatory effects is critical for effectively harnessing them for clinical use. Two contrasting mechanisms have emerged to explain the anti-inflammatory activity of iNKT cells: that they drive suppressive pathways mediated by other regulatory cells, and that they may cytolytically eliminate antigen presenting cells that promote excessive inflammatory responses. How these activities are controlled and separated from their pro-inflammatory functions remains a central question. Murine iNKT cells can be divided into four functional lineages that have either pro-inflammatory (NKT1, NKT17) or anti-inflammatory (NKT2, NKT10) cytokine profiles. However, in humans these subsets are not clearly evident, and instead most iNKT cells that are CD4⁺ appear oriented towards polyfunctional (T_H0) cytokine production, while CD4^- iNKT cells appear more predisposed towards cytolytic activity. Additionally, structurally distinct antigens have been shown to induce T_H1- or T_H2-biased responses by iNKT cells in murine models, but human iNKT cells may respond to differing levels of TCR stimulation in a way that does not neatly separate T_H1 and T_H2 cytokine production. We discuss the implications of these differences for translational efforts focused on the anti-inflammatory activity of iNKT cells.

Biased MAIT TCR Usage Poised for Limited Antigen Diversity?

Mucosal-associated invariant T (MAIT) cells are a subset of unconventional T cells that recognize the evolutionarily conserved major histocompatibility complex (MHC) class I-like antigen-presenting molecule known as MHC class I related protein 1 (MR1). Since their rise from obscurity in the early 1990s, the study of MAIT cells has grown substantially, accelerating our fundamental understanding of these cells and their possible roles in immunity. In the context of recent advances, we review here the relationship between MR1, antigen, and TCR usage among MAIT and other MR1-reactive T cells and provide a speculative discussion.

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.

LFA-1 Ligation by High-Density ICAM-1 Is Sufficient To Activate IFN-γ Release by Innate T Lymphocytes

By binding to its ligand ICAM-1, LFA-1 is known to mediate both adhesion and costimulatory signaling for T cell activation. The constitutively high LFA-1 cell surface expression of invariant NKT (iNKT) cells has been shown to be responsible for their distinctive tissue homing and residency within ICAM-rich endothelial vessels. However, the functional impact of LFA-1 on the activation of iNKT cells and other innate T lymphocyte subsets has remained largely unexplored. In particular, it is not clear whether LFA-1 contributes to innate-like pathways of T cell activation, such as IFN-γ secretion in response to IL-12. Using a recombinant ICAM-1-Fc fusion protein to stimulate human iNKT cells in the absence of APCs, we show that LFA-1 engagement enhances their IL-12-driven IFN-γ production. Surprisingly, exposure to high densities of ICAM-1 was also sufficient to activate iNKT cell cytokine secretion independently of IL-12 and associated JAK/STAT signaling. LFA-1 engagement induced elevated cytoplasmic Ca²⁺ and rapid ERK phosphorylation in iNKT cells, and the resulting IFN-γ secretion was dependent on both of these pathways. Analysis of freshly isolated human PBMC samples revealed that a fraction of lymphocytes that showed elevated LFA-1 cell surface expression produced IFN-γ in response to plate-bound ICAM-1-Fc. A majority of the responding cells were T cells, with the remainder NK cells. The responding T cells included iNKT cells, MAIT cells, and Vδ2⁺ γδ T cells. These results delineate a novel integrin-mediated pathway of IFN-γ secretion that is a shared feature of innate lymphocytes.

Immature murine NKT cells pass through a stage of developmentally programmed innate IL-4 secretion

We assessed the production of the canonical Th2 cytokine IL-4 by NKT cells directly in vivo using IL-4-substituting strains of reporter mice that provide faithful and sensitive readouts of cytokine production without the confounding effects of in vitro stimulation. Analysis in naïve animals revealed an "innate" phase of IL-4 secretion that did not need to be triggered by administration of a known NKT cell ligand. This secretion was by immature NKT cells spanning Stage 1 of the maturation process in the thymus (CD4(+) CD44(lo) NK1.1(-) cells) and Stage 2 (CD4(+) CD44(hi) NK1.1(-) cells) in the spleen. Like ligand-induced IL-4 production by mature cells, this innate activity was independent of an initial source of IL-4 protein and did not require STAT6 signaling. A more sustained level of innate IL-4 production was observed in animals on a BALB/c background compared with a C57BL/6 background, suggesting a level of genetic regulation that may contribute to the "Th2-prone" phenotype in BALB/c animals. These observations indicate a regulated pattern of IL-4 expression by maturing NKT cells, which may endow these cells with a capacity to influence the development of surrounding cells in the thymus.

Developmental dynamics of post-selection thymic DN iNKT

BACKGROUND

Invariant natural killer T (iNKT) cells develop in the thymus and branch off from the maturation pathway of conventional T cell at the DP stage. While different stages of iNKT cellular development have been defined, the actual time that iNKT cell precursors spend at each stage is still unknown.

METHODOLOGY/PRINCIPAL FINDING

Here we report on maturation dynamics of post-selection DN iNKT cells by injecting wild-type DP(dim) thymocytes into the thymus of TCRα(-/-) mice and using the Vα14-Jα18 rearrangements as a molecular marker to follow the maturation dynamics of these cells.

CONCLUSION/SIGNIFICANCE

This study shows that the developmental dynamics of DN iNKT cells in DP(dim) are very rapid and that it takes less than 1 day to down-regulate CD4 and CD8 and become DN. These DN cells are precursors of peripheral DN iNKT cells and appear in the spleen in 1-2 days. Thymic DN iNKT residents are predominantly derived from cells that quickly return from the periphery. The expansion of a very small subset of DN iNKT precursors could also play a small role in this process. These data are an example of measuring T cell maturation in the thymus and show that the maturation dynamics of selected DN iNKT cells fall within the same general time frame as conventional αβ T cells.

Invariant NKT cells: regulation and function during viral infection

Natural killer T cells (NKT cells) represent a subset of T lymphocytes that express natural killer (NK) cell surface markers. A subset of NKT cells, termed invariant NKT cells (iNKT), express a highly restricted T cell receptor (TCR) and respond to CD1d-restricted lipid ligands. iNKT cells are now appreciated to play an important role in linking innate and adaptive immune responses and have been implicated in infectious disease, allergy, asthma, autoimmunity, and tumor surveillance. Advances in iNKT identification and purification have allowed for the detailed study of iNKT activity in both humans and mice during a variety of chronic and acute infections. Comparison of iNKT function between non-pathogenic simian immunodeficiency virus (SIV) infection models and chronic HIV-infected patients implies a role for iNKT activity in controlling immune activation. In vitro studies of influenza infection have revealed novel effector functions of iNKT cells including IL-22 production and modulation of myeloid-derived suppressor cells, but ex vivo characterization of human iNKT cells during influenza infection are lacking. Similarly, as recent evidence suggests iNKT involvement in dengue virus pathogenesis, iNKT cells may modulate responses to a number of emerging pathogens. This Review will summarize current knowledge of iNKT involvement in responses to viral infections in both human and mouse models and will identify critical gaps in knowledge and opportunities for future study. We will also highlight recent efforts to harness iNKT ligands as vaccine adjuvants capable of improving vaccination-induced cellular immune responses.

NKG2D performs two functions in invariant NKT cells: direct TCR-independent activation of NK-like cytolysis and co-stimulation of activation by CD1d

Invariant NKT cells are important in the activation and regulation of immune responses. They can also function as CD1d-restricted killer cells. However, the role of activating innate NK-cell receptors expressed on NKT cells in triggering cytolytic function is poorly characterized. Here, we initially confirmed that the cellular stress-ligand receptor NKG2D is expressed on CD4- NKT cells, whereas most CD4+ NKT cells lack this receptor. Interestingly, NKG2D+ NKT cells frequently expressed perforin, and both NKG2D and perforin localized at the site of contact with NKG2D ligand-expressing target cells. CD4- NKT cells degranulated in response to NKG2D engagement in a redirected activation assay independent of stimulation via their invariant TCR. NKT cells killed P815 cells coated with anti-NKG2D mAb and CD1d-negative K562 tumor target cells in an NKG2D-dependent manner. Furthermore, NKG2D engagement co-stimulated TCR-mediated NKT-cell activation in response to endogenous CD1d-presented ligands or suboptimal levels of anti-CD3 triggering. These data indicate that the CD4- subset of human NKT cells can mediate direct lysis of target cells via NKG2D engagement independent of CD1d, and that NKG2D also functions as a co-stimulatory receptor in these cells. NKG2D thus plays both a direct and a co-stimulatory role in the activation of NKT cells.

Directing traffic on the NKT-cell highway: a key role for ThPOK

CD1d-restricted NKT cells include CD4(+) and DN subsets, with an additional CD8(+) subset that is present in humans but not in mice. The molecular regulation of CD4/CD8 expression by NKT cells, and differentiation of these NKT-cell subsets, is poorly understood. The transcription factors GATA3 and ThPOK regulate lineage commitment of conventional MHC class II-restricted CD4(+) T cells; however, their role in CD4/CD8 expression by CD1d-restricted NKT cells is less clear. A new study in this issue of the European Journal of Immunology demonstrates a key role for ThPOK in differentiation of NKT-cell subsets. This study reveals that GATA3 and ThPOK are necessary for the development of CD4(+) NKT cells. Furthermore, ThPOK-deficient mice generate an unusual population of CD8(+) NKT cells, which are absent in control mice. This study sheds new light on the underlying molecular events leading to the emergence of distinct NKT-cell subsets.

Recognition of lyso-phospholipids by human natural killer T lymphocytes

Natural killer T (NKT) cells are a subset of T lymphocytes with potent immunoregulatory properties. Recognition of self-antigens presented by CD1d molecules is an important route of NKT cell activation; however, the molecular identity of specific autoantigens that stimulate human NKT cells remains unclear. Here, we have analyzed human NKT cell recognition of CD1d cellular ligands. The most clearly antigenic species was lyso-phosphatidylcholine (LPC). Diacylated phosphatidylcholine and lyso-phosphoglycerols differing in the chemistry of the head group stimulated only weak responses from human NKT cells. However, lyso-sphingomyelin, which shares the phosphocholine head group of LPC, also activated NKT cells. Antigen-presenting cells pulsed with LPC were capable of stimulating increased cytokine responses by NKT cell clones and by freshly isolated peripheral blood lymphocytes. These results demonstrate that human NKT cells recognize cholinated lyso-phospholipids as antigens presented by CD1d. Since these lyso-phospholipids serve as lipid messengers in normal physiological processes and are present at elevated levels during inflammatory responses, these findings point to a novel link between NKT cells and cellular signaling pathways that are associated with human disease pathophysiology.

How do natural killer T cells help B cells?

CD1d-restricted natural killer (NKT) cells are important contributors to antigen-specific antibody responses. There is, therefore, considerable interest in the design and implementation of strategies to appropriately activate NKT cells and boost vaccine-induced protective antibody responses. In order to achieve these goals, investigators are examining the mechanisms by which NKT cells enhance antibody responses. Although information is limited, it is now appreciated that both cognate and noncognate interactions between CD1d-expressing B cells and NKT cells drive enhanced antibody responses. NKT cells may provide B-cell help in the form of direct receptor-mediated interactions as well as by secretion of soluble effectors, including cytokines. In this article, we review the evidence in support of these mechanisms and discuss how they likely take place in the context of interactions of NKT cells with other cell types, such as dendritic cells and helper T cells. We also discuss the evidence that NKT cells affect discrete differentiation events in the multistep process by which a naive B cell experiences antigen and develops into a memory B cell or an antibody-secreting plasma cell. Since most information on NKT cells and humoral immunity has been derived from murine studies, we discuss what is known about human NKT cells and humoral immunity. We offer thoughts on whether the findings in murine systems will translate to humans.

Invariant TCR rather than CD1d shapes the preferential activities of C-glycoside analogues against human versus murine invariant NKT cells

C-glycoside analogues of alpha-galactosylceramide were shown to activate both human and mouse invariant NKT (iNKT) cells. Among these analogues, GCK152, which has an aromatic ring in the acyl chain, exhibited a stronger stimulatory activity against human iNKT cells and a much weaker activity against murine iNKT cells than GCK127 that has an almost identical fatty acyl chain as alpha-galactosylceramide. In this study, we have found that invariant TCR (invTCR) expressed by iNKT cells, but not CD1d expressed by APCs, command the species-specific preferential activity of C-glycosides, and that their preferential activity against human vs murine iNKT cells correlate with the binding affinity of glycolipid-CD1d complex to invTCR of respective iNKT cells rather than that of glycolipid to human or murine CD1d molecules. Overall, the structural difference of invTCR appears to supersede those of CD1d molecule in shaping the strength of the biological activity of C-glycoside analogues.

Severe functional impairment and elevated PD-1 expression in CD1d-restricted NKT cells retained during chronic HIV-1 infection

Invariant CD1d-restricted NKT cells play important roles in regulating both innate and adaptive immunity. They are targeted by HIV-1 infection and severely reduced in number or even lost in many infected subjects. Here, we have investigated the characteristics of NKT cells retained by some patients despite chronic HIV-1 infection. NKT cells preserved under these circumstances displayed an impaired ability to proliferate and produce IFN-gamma in response to CD1d-restricted lipid antigen as compared with cells from uninfected control subjects. HIV-1 infection was associated with an elevated expression of the inhibitory programmed death-1 (PD-1) receptor (CD279) on the CD4(-) subset of NKT cells. However, blocking experiments indicated that the functional defects in NKT cells were largely PD-1-independent. Furthermore, the elevated PD-1 expression and the functional defects were not restored by anti-retroviral treatment, and the NKT cell numbers in blood did not recover significantly in response to treatment. The functional phenotype of NKT cells in these patients suggests an irreversible immune exhaustion due to chronic activation in vivo. The data demonstrate a severe functional impairment in the remaining NKT-cell compartment in HIV-1-infected patients, which limits the prospects to mobilize these cells in immunotherapy approaches in patients.

Glycolipid ligands of invariant natural killer T cells as vaccine adjuvants

Invariant natural killer T (iNKT) cells are a unique subset of T lymphocytes that recognize glycolipid antigens in the context of the antigen-presenting molecule CD1d. Upon glycolipid antigen stimulation, iNKT cells rapidly produce copious amounts of immunomodulatory cytokines, leading to potent activation of a variety of innate and adaptive immune cells. These immune-potentiating properties of iNKT cells hold great promise for the development of vaccine adjuvants. This review aims to summarize the immunomodulatory activities of iNKT cell ligands and to discuss prospects for developing iNKT cell-based vaccine adjuvants.

Peripheral NKT cells in simian immunodeficiency virus-infected macaques

NKT cells are a specialized population of T lymphocytes that have an increasingly recognized role in immunoregulation, including controlling the response to viral infections. The characteristics of NKT cells in the peripheral blood of macaques during simian immunodeficiency virus (SIV) or chimeric simian/human immunodeficiency virus (HIV) (SHIV) infection were assessed. NKT cells comprised a mean of 0.19% of peripheral blood lymphocytes across the 64 uninfected macaques studied. Although the range in the percentages of NKT cells was large (0 to 2.2%), levels were stable over time within individual macaques without SIV/SHIV infection. The majority of NKT cells in macaques were CD4(+) (on average 67%) with smaller populations being CD8(+) (21%) and CD4/CD8 double positive (13%). A precipitous decline in CD4(+) NKT cells occurred in all six macaques infected with CXCR4-tropic SHIV(mn229) early after infection, with a concomitant rise in CD8(+) NKT cells in some animals. The depletion of CD4(+) NKT cells was tightly correlated with the depletion of total CD4(+) T cells. R5-tropic SIV(mac251) infection of macaques resulted in a slower and more variable decline in CD4(+) NKT cells, with animals that were able to control SIV virus levels maintaining higher levels of CD4(+) NKT cells. An inverse correlation between the depletion of total and CD4(+) NKT cells and SIV viral load during chronic infection was observed. Our results demonstrate the infection-driven depletion of peripheral CD4(+) NKT cells during both SHIV and SIV infection of macaques. Further studies of the implications of the loss of NKT cell subsets in the pathogenesis of HIV disease are needed.

MHC class II-expressing thymocytes suppress invariant NKT cell development

Natural killer T (NKT) cells are positively selected on cortical thymocytes expressing the non-classical major histocompatibility complex (MHC) class I CD1d molecules. However, it is less clear how NKT cells are negatively selected in the thymus. In this study, we investigated the role of MHC class II expression in NKT cell development. Transgenic mice expressing MHC class II on thymocytes and peripheral T cells had a marked reduction in invariant NKT (iNKT) cells. Reduced numbers of iNKT cells correlated with the absence of in vivo production of cytokines in response to the iNKT cell agonist alpha-galactosylceramide. Using mixed bone marrow chimeras, we found that MHC class II-expressing thymocytes suppressed the development of iNKT cells in trans in a CD4-dependent manner. Our observations have significant implications for human iNKT cell development as human thymocytes express MHC class II, which can lead to an inefficient selection of iNKT cells.

CD1d-restricted iNKT cells, the 'Swiss-Army knife' of the immune system

Natural Killer T cells are a distinct lymphocyte lineage that regulates a broad range of immune responses. NKT cells recognize glycolipids presented by the non-classical MHC molecule CD1d. Structural insight into the TCR/glycolipid/CD1d tri-complex has revealed an unusual and unexpected mode of recognition. Recent studies have also identified some of the signaling events during NKT cell development that give NKT cells their innate phenotype. Pathogen-derived glycolipid antigens continue to be found, and new mechanisms of NKT cell activation have been described. Finally, NKT cells have been shown to be remarkably versatile in function during various immune responses. Whether these extensive functional capacities can be attributed to a single population sensitive to environmental cues or if functionally distinct NKT cell subpopulations exist remains unresolved.

Exploiting dendritic cells and natural killer T cells in immunotherapy against malignancies

A primary focus of tumor immunotherapy research is to change the immune system so that it becomes immunized and not tolerized to the presentation of antigens by or from tumor cells. Dendritic cells (DCs) are the logical target for the development of immunotherapies because DCs instruct the ensuing immune response. Upon activation, invariant natural killer T (iNKT) cells have direct antitumor effects and also induce in situ DC maturation, linking the innate and adaptive arms of the immune system in an immunogenic form. The characterization and manipulation of DC function in tumor-bearing hosts will provide new insights into mechanisms of tumor immunology and lead to the development of successful clinical strategies.