Recent advances in iNKT cell development

Recent advances regarding the potential roles of invariant natural killer T cells in cardiovascular diseases with immunological and inflammatory backgrounds

Invariant natural killer T (iNKT) cells, which bear αβ-type T-cell antigen-receptors (TCRs), recognize glycolipid antigens in a cluster of differentiation 1d (CD1d)-restricted manner. Regarding these cells, the unique modes of thymic selection and maturation elucidate innateness, irrespective of them also being members of the adaptive immune system as a T-cell. iNKT cells develop and differentiate into NKT1 [interferon γ (IFN-γ)-producing], NKT2 [interleukin 4 (IL-4)/IL-13-producing], or NKT17 (IL-17-producing) subsets in the thymus. After egress, NKT10 (IL-10-producing), follicular helper NKT (NKTfh; IL-21-producing), and regulatory NKT (NKTreg) subsets emerge following stimulation in the periphery. Moreover, iNKT cells have been shown to possess several physiological or pathological roles. iNKT cells exhibit dual alleviating or aggravating roles in experimentally induced immune and/or inflammatory diseases in mice. These findings indicate that the modulation of iNKT cells can be employed for therapeutic use or prevention of human diseases. In this review, we discuss the potential roles of iNKT cells in the development of immune/inflammatory diseases of the cardiovascular system, with emphasis on atherosclerosis, aortic aneurysms, and cardiac remodeling.

invariant Natural Killer T cell therapy as a novel therapeutic approach in hematological malignancies

Invariant Natural Killer T cell therapy is an emerging platform of immunotherapy for cancer treatment. This unique cell population is a promising candidate for cell therapy for cancer treatment because of its inherent cytotoxicity against CD1d positive cancers as well as its ability to induce host CD8 T cell cross priming. Substantial evidence supports that iNKT cells can modulate myelomonocytic populations in the tumor microenvironment to ameliorate immune dysregulation to antagonize tumor progression. iNKT cells can also protect from graft-versus-host disease (GVHD) through several mechanisms, including the expansion of regulatory T cells (Treg). Ultimately, iNKT cell-based therapy can retain antitumor activity while providing protection against GVHD simultaneously. Therefore, these biological properties render iNKT cells as a promising "off-the-shelf" therapy for diverse hematological malignancies and possible solid tumors. Further the introduction of a chimeric antigen recetor (CAR) can further target iNKT cells and enhance function. We foresee that improved vector design and other strategies such as combinatorial treatments with small molecules or immune checkpoint inhibitors could improve CAR iNKT in vivo persistence, functionality and leverage anti-tumor activity along with the abatement of iNKT cell dysfunction or exhaustion.

Innate-like T cell subset commitment in the murine thymus is independent of TCR characteristics and occurs during proliferation

How T-cell receptor (TCR) characteristics determine subset commitment during T-cell development is still unclear. Here, we addressed this question for innate-like T cells, mucosal-associated invariant T (MAIT) cells, and invariant natural killer T (iNKT) cells. MAIT and iNKT cells have similar developmental paths, leading in mice to two effector subsets, cytotoxic (MAIT1/iNKT1) and IL17-secreting (MAIT17/iNKT17). For iNKT1 vs iNKT17 fate choice, an instructive role for TCR affinity was proposed but recent data argue against this model. Herein, we examined TCR role in MAIT and iNKT subset commitment through scRNAseq and TCR repertoire analysis. In our dataset of thymic MAIT cells, we found pairs of T-cell clones with identical amino acid TCR sequences originating from distinct precursors, one of which committed to MAIT1 and the other to MAIT17 fates. Quantitative in silico simulations indicated that the number of such cases is best explained by lineage choice being independent of TCR characteristics. Comparison of TCR features of MAIT1 and MAIT17 clonotypes demonstrated that the subsets cannot be distinguished based on TCR sequence. To pinpoint the developmental stage associated with MAIT sublineage choice, we demonstrated that proliferation takes place both before and after MAIT fate commitment. Altogether, we propose a model of MAIT cell development in which noncommitted, intermediate-stage MAIT cells undergo a first round of proliferation, followed by TCR characteristics-independent commitment to MAIT1 or MAIT17 lineage, followed by an additional round of proliferation. Reanalyzing a published iNKT TCR dataset, we showed that this model is also relevant for iNKT cell development.

Kidins220 and Aiolos promote thymic iNKT cell development by reducing TCR signals

Development of T cells is controlled by the signal strength of the TCR. The scaffold protein kinase D-interacting substrate of 220 kilodalton (Kidins220) binds to the TCR; however, its role in T cell development was unknown. Here, we show that T cell-specific Kidins220 knockout (T-KO) mice have strongly reduced invariant natural killer T (iNKT) cell numbers and modest decreases in conventional T cells. Enhanced apoptosis due to increased TCR signaling in T-KO iNKT thymocytes of developmental stages 2 and 3 shows that Kidins220 down-regulates TCR signaling at these stages. scRNA-seq  indicated that the transcription factor Aiolos is down-regulated in Kidins220-deficient iNKT cells. Analysis of an Aiolos KO demonstrated that Aiolos is a downstream effector of Kidins220 during iNKT cell development. In the periphery, T-KO iNKT cells show reduced TCR signaling upon stimulation with α-galactosylceramide, suggesting that Kidins220 promotes TCR signaling in peripheral iNKT cells. Thus, Kidins220 reduces or promotes signaling dependent on the iNKT cell developmental stage.

Single-cell transcriptomics reveal different maturation stages and sublineage commitment of human thymic invariant natural killer T cells

Invariant natural killer T cells are a rare, heterogeneous T-cell subset with cytotoxic and immunomodulatory properties. During thymic development, murine invariant natural killer T cells go through different maturation stages differentiating into distinct sublineages, namely, invariant natural killer T1, 2, and 17 cells. Recent reports indicate that invariant natural killer T2 cells display immature properties and give rise to other subsets, whereas invariant natural killer T1 cells seem to be terminally differentiated. Whether human invariant natural killer T cells follow a similar differentiation model is still unknown. To define the maturation stages and assess the sublineage commitment of human invariant natural killer T cells during thymic development, in this study, we performed single-cell RNA sequencing analysis on human Vα24+Vβ11+ invariant natural killer T cells isolated from thymocytes. We show that these invariant natural killer T cells displayed heterogeneity, and our unsupervised analysis identified 5 clusters representing different maturation stages, from an immature profile with high expression of genes important for invariant natural killer T cell development and proliferation to a mature, fully differentiated profile with high levels of cytotoxic effector molecules. Evaluation of expression of sublineage-defining gene sets revealed mainly cells with an invariant natural killer T2 signature in the most immature cluster, whereas the more differentiated ones displayed an invariant natural killer T1 signature. Combined analysis with a publicly available single-cell RNA sequencing data set of human invariant natural killer T cells from peripheral blood suggested that the 2 main subsets exist both in thymus and in the periphery, while a third more immature one was restricted to the thymus. Our data point to the existence of different maturation stages of human thymic invariant natural killer T cells and provide evidence for sublineage commitment of invariant natural killer T cells in the human thymus.

Immune cells and RBCs derived from human induced pluripotent stem cells: method, progress, prospective challenges

Blood has an important role in the healthcare system, particularly in blood transfusions and immunotherapy. However, the occurrence of outbreaks of infectious diseases worldwide and seasonal fluctuations, blood shortages are becoming a major challenge. Moreover, the narrow specificity of immune cells hinders the widespread application of immune cell therapy. To address this issue, researchers are actively developing strategies for differentiating induced pluripotent stem cells (iPSCs) into blood cells in vitro. The establishment of iPSCs from terminally differentiated cells such as fibroblasts and blood cells is a straightforward process. However, there is need for further refinement of the protocols for differentiating iPSCs into immune cells and red blood cells to ensure their clinical applicability. This review aims to provide a comprehensive overview of the strategies and challenges facing the generation of iPSC-derived immune cells and red blood cells.

Unique adipose tissue invariant natural killer T cell subpopulations control adipocyte turnover in mice

Adipose tissue invariant natural killer T (iNKT) cells are a crucial cell type for adipose tissue homeostasis in obese animals. However, heterogeneity of adipose iNKT cells and their function in adipocyte turnover are not thoroughly understood. Here, we investigate transcriptional heterogeneity in adipose iNKT cells and their hierarchy using single-cell RNA sequencing in lean and obese mice. We report that distinct subpopulations of adipose iNKT cells modulate adipose tissue homeostasis through adipocyte death and birth. We identify KLRG1+ iNKT cells as a unique iNKT cell subpopulation in adipose tissue. Adoptive transfer experiments showed that KLRG1+ iNKT cells are selectively generated within adipose tissue microenvironment and differentiate into a CX3CR1+ cytotoxic subpopulation in obese mice. In addition, CX3CR1+ iNKT cells specifically kill enlarged and inflamed adipocytes and recruit macrophages through CCL5. Furthermore, adipose iNKT17 cells have the potential to secrete AREG, and AREG is involved in stimulating adipose stem cell proliferation. Collectively, our data suggest that each adipose iNKT cell subpopulation plays key roles in the control of adipocyte turnover via interaction with adipocytes, adipose stem cells, and macrophages in adipose tissue.

RIPK1 deficiency prevents thymic NK1.1 expression and subsequent iNKT cell development

Receptor Interacting Protein Kinase 1 (RIPK1) and caspase-8 (Casp8) jointly orchestrate apoptosis, a key mechanism for eliminating developing T cells which have autoreactive or improperly arranged T cell receptors. Mutations in the scaffolding domain of Ripk1 gene have been identified in humans with autoinflammatory diseases like Cleavage Resistant RIPK1 Induced Autoinflammatory (CRIA) and Inflammatory Bowel Disease. RIPK1 protein also contributes to conventional T cell differentiation and peripheral T cell homeostasis through its scaffolding domain in a cell death independent context. Ripk1 deficient mice do not survive beyond birth, so we have studied the function of this kinase in vivo against a backdrop Ripk3 and Casp8 deficiency which allows the mice to survive to adulthood. These studies reveal a key role for RIPK1 in mediating NK1.1 expression, including on thymic iNKT cells, which is a key requirement for thymic stage 2 to stage 3 transition as well as iNKT cell precursor development. These results are consistent with RIPK1 mediating responses to TcR engagement, which influence NK1.1 expression and iNKT cell thymic development. We also used in vivo and in vitro stimulation assays to confirm a role for both Casp8 and RIPK1 in mediating iNKT cytokine effector responses. Finally, we also noted expanded and hyperactivated iNKT follicular helper (iNKTFH) cells in both DKO (Casp8-, Ripk3- deficient) and TKO mice (Ripk1-, Casp8-, Ripk3- deficient). Thus, while RIPK1 and Casp8 jointly facilitate iNKT effector function, RIPK1 uniquely influenced thymic iNKT cell development most likely by regulating molecular responses to T cell receptor engagement. iNKT developmental and functional aberrances were not evident in mice expressing a kinase-dead version of RIPK1 (RIPK1kd), indicating that the scaffolding function of this protein exerts the critical regulation of iNKT cells. Our findings suggest that small molecule inhibitors of RIPK1 could be used to regulate iNKT cell development and effector function to alleviate autoinflammatory conditions in humans.

Proinflammatory IFNγ Is Produced by but Not Required for the Generation of Eomes+ Thymic Innate CD8 T Cells

Innate CD8 T cells are proinflammatory effector T cells that achieve functional maturation in the thymus prior to their export into and maturation in peripheral tissues. Innate CD8 T cells produce the Th1 cytokine IFNγ but depend on the Th2 cytokine IL-4 for their generation. Thus, innate CD8 T cells can permute the intrathymic cytokine milieu by consuming a Th2 cytokine but driving a Th1 cytokine response. The cellular source of IL-4 is the NKT2 subset of invariant NKT (iNKT) cells. Consequently, NKT2 deficiency results in the lack of innate CD8 T cells. Whether NKT2 is the only iNKT subset and whether IL-4 is the only cytokine required for innate CD8 T cell generation, however, remains unclear. Here, we employed a mouse model of NKT1 deficiency, which is achieved by overexpression of the cytokine receptor IL-2Rβ, and assessed the role of other iNKT subsets and cytokines in innate CD8 T cell differentiation. Because IL-2Rβ-transgenic mice failed to generate both NKT1 and innate CD8 T cells, we postulated an in vivo requirement for IFNγ-producing NKT1 cells for innate CD8 T cell development. In-depth analyses of IL-2Rβ-transgenic mice and IFNγ-deficient mice, however, demonstrated that neither NKT1 nor IFNγ was required to induce Eomes or to drive innate CD8 T cell generation. Instead, in vivo administration of recombinant IL-4 sufficed to restore the development of innate CD8 T cells in NKT1-deficient mice, affirming that intrathymic IL-4, and not IFNγ, is the limiting factor and key regulator of innate CD8 T cell generation in the thymus.

Invariant natural killer T cells in lung diseases

Invariant natural killer T (iNKT) cells are a subset of T cells that are characterized by a restricted T-cell receptor (TCR) repertoire and a unique ability to recognize glycolipid antigens. These cells are found in all tissues, and evidence to date suggests that they play many immunological roles in both homeostasis and inflammatory conditions. The latter include lung inflammatory diseases such as asthma and infections: the roles of lung-resident iNKT cells in these diseases have been extensively researched. Here, we provide insights into the biology of iNKT cells in health and disease, with a particular focus on the role of pulmonary iNKT cells in airway inflammation and other lung diseases.

Impaired thymic iNKT cell differentiation at early precursor stage in murine haploidentical bone marrow transplantation with GvHD

Introduction

Early recovery of donor-derived invariant natural killer T (iNKT) cells are associated with reduced risk of graft-versus-host disease (GvHD) and overall survival. Patients with severe GvHD, however, had much slower iNKT cell reconstitution relative to conventional T cells.

Methods

To characterize the delay of iNKT cell reconstitution and explore its possible causes, we used a haploidentical bone marrow transplantation (haplo-BMT) mouse model with GvHD. We found the delayed recovery of thymic and peripheral iNKT cell numbers with markedly decreased thymic NKT1 subset in GvHD mice. The defective generation of thymic iNKT precursors with egress capability contributed to the reduced peripheral iNKT cells in GvHD mice. We further identified intermediate NK1.1- NKT1 precursor subpopulations under steady-state conditions and found that the differentiation of these subpopulations was impaired in the thymi of GvHD mice. Detailed characterization of iNKT precursors and thymic microenvironment showed a close association of elevated TCR/co-stimulatory signaling provided by double positive thymocytes and macrophages with defective down-regulation of proliferation, metabolism, and NKT2 signature in iNKT precursor cells. Correspondingly, NKT2 but not NKT1 differentiation was favored in GvHD mice.

Discussion

These data underline the important roles of TCR and co-stimulatory signaling in the differentiation of thymic iNKT subsets under transplantation conditions.

Tnpo3 controls splicing of the pre-mRNA encoding the canonical TCR α chain of iNKT cells

Unconventional T cells, such as innate natural killer T cells (iNKT) cells, are an important part of vertebrate immune defences. iNKT recognise glycolipids through a T cell receptor (TCR) that is composed of a semi-invariant TCR α chain, paired with a restricted set of TCR β chains. Here, we show that splicing of the cognate Trav11-Traj18-Trac pre-mRNA encoding the characteristic Vα14Jα18 variable region of this semi-invariant TCR depends on the presence of Tnpo3. The Tnpo3 gene encodes a nuclear transporter of the β-karyopherin family whose cargo includes various splice regulators. The block of iNKT cell development in the absence of Tnpo3 can be overcome by transgenic provision of a rearranged Trav11-Traj18-Trac cDNA, indicating that Tnpo3 deficiency does not interfere with the development of iNKT cells per se. Our study thus identifies a role for Tnpo3 in regulating the splicing of the pre-mRNA encoding the cognate TCRα chain of iNKT cells.

Towards a better understanding of human iNKT cell subpopulations for improved clinical outcomes

Invariant natural killer T (iNKT) cells are a unique T lymphocyte population expressing semi-invariant T cell receptors (TCRs) that recognise lipid antigens presented by CD1d. iNKT cells exhibit potent anti-tumour activity through direct killing mechanisms and indirectly through triggering the activation of other anti-tumour immune cells. Because of their ability to induce potent anti-tumour responses, particularly when activated by the strong iNKT agonist αGalCer, they have been the subject of intense research to harness iNKT cell-targeted immunotherapies for cancer treatment. However, despite potent anti-tumour efficacy in pre-clinical models, the translation of iNKT cell immunotherapy into human cancer patients has been less successful. This review provides an overview of iNKT cell biology and why they are of interest within the context of cancer immunology. We focus on the iNKT anti-tumour response, the seminal studies that first reported iNKT cytotoxicity, their anti-tumour mechanisms, and the various described subsets within the iNKT cell repertoire. Finally, we discuss several barriers to the successful utilisation of iNKT cells in human cancer immunotherapy, what is required for a better understanding of human iNKT cells, and the future perspectives facilitating their exploitation for improved clinical outcomes.

Generation of functional thymic organoids from human pluripotent stem cells

The thymus is critical for the establishment of a functional and self-tolerant adaptive immune system but involutes with age, resulting in reduced naive T cell output. Generation of a functional human thymus from human pluripotent stem cells (hPSCs) is an attractive regenerative strategy. Direct differentiation of thymic epithelial progenitors (TEPs) from hPSCs has been demonstrated in vitro, but functional thymic epithelial cells (TECs) only form months after transplantation of TEPs in vivo. We show the generation of TECs in vitro in isogenic stem cell-derived thymic organoids (sTOs) consisting of TEPs, hematopoietic progenitor cells, and mesenchymal cells, differentiated from the same hPSC line. sTOs support T cell development, express key markers of negative selection, including the autoimmune regulator (AIRE) protein, and facilitate regulatory T cell development. sTOs provide the basis for functional patient-specific thymic organoid models, allowing for the study of human thymus function, T cell development, and transplant immunity.

Uncovering the Underworld of Axial Spondyloarthritis

Axial spondyloarthritis (axial-SpA) is a multifactorial disease characterized by inflammation in sacroiliac joints and spine, bone reabsorption, and aberrant bone deposition, which may lead to ankylosis. Disease pathogenesis depends on genetic, immunological, mechanical, and bioenvironmental factors. HLA-B27 represents the most important genetic factor, although the disease may also develop in its absence. This MHC class I molecule has been deeply studied from a molecular point of view. Different theories, including the arthritogenic peptide, the unfolded protein response, and HLA-B27 homodimers formation, have been proposed to explain its role. From an immunological point of view, a complex interplay between the innate and adaptive immune system is involved in disease onset. Unlike other systemic autoimmune diseases, the innate immune system in axial-SpA has a crucial role marked by abnormal activity of innate immune cells, including γδ T cells, type 3 innate lymphoid cells, neutrophils, and mucosal-associated invariant T cells, at tissue-specific sites prone to the disease. On the other hand, a T cell adaptive response would seem involved in axial-SpA pathogenesis as emphasized by several studies focusing on TCR low clonal heterogeneity and clonal expansions as well as an interindividual sharing of CD4/8 T cell receptors. As a result of this immune dysregulation, several proinflammatory molecules are produced following the activation of tangled intracellular pathways involved in pathomechanisms of axial-SpA. This review aims to expand the current understanding of axial-SpA pathogenesis, pointing out novel molecular mechanisms leading to disease development and to further investigate potential therapeutic targets.

Cullin 3-Mediated Regulation of Intracellular Iron Homeostasis Promotes Thymic Invariant NKT Cell Maturation

The E3 ubiquitin ligase cullin 3 (Cul3) is critical for invariant NKT (iNKT) cell development, as iNKT cells lacking Cul3 accumulate in the immature developmental stages. However, the mechanisms by which Cul3 mediates iNKT cell development remain unknown. In this study, we investigated the role of Cul3 in both immature and mature thymic iNKT cells using a mouse model with a T cell-specific deletion of Cul3. We found that mature iNKT cells lacking Cul3 proliferated and died more than wild-type cells did. These cells also displayed increased glucose metabolism and autophagy. Interestingly, we found that tight regulation of iron homeostasis is critical for iNKT cell development. Without Cul3, mature iNKT cells harbored higher levels of cytosolic iron, a phenotype associated with increased cell death. Taken together, our data suggest that Cul3 promotes iNKT cell development partially through intracellular iron homeostasis.

The implications of IL-15 trans-presentation on the immune response

Interleukin-15 is a pleiotropic cytokine type I four alpha-helical bundle cytokine that along with IL-2, IL-4, IL-7, IL-9, and IL-21 shares the common cytokine receptor γ chain, γ_c. IL-15 is vital for the development, survival, and expansion of natural killer cells and for the development of CD8⁺ memory T cells. Whereas other family γ_c cytokines signal by directly binding to their target cells, IL-15 is distinctive in that it binds to IL-15Rα, a sushi domain containing binding protein that is expressed on a number of cell types, including monocytes and dendritic cells as well as T cells, and then is trans-presented to responding cells that express IL-2Rβ and γ_c. This distinctive mechanism for IL-15 relates to its role in signaling in the context of cell-cell interactions and signaling synapses. The actions of IL-15 and ways of manipulating its actions to potential therapeutic benefit are discussed.

A humanized mouse model for in vivo evaluation of invariant Natural Killer T cell responses

Invariant natural killer T (iNKT) cells mediate immune responses when stimulated by glycolipid agonists presented by CD1d. In extensive studies of synthetic analogues of α-galactosyl ceramides, we identified numerous examples of significant differences in the recognition of specific glycolipids in wild type mice versus human iNKT cell clones or PBMC samples. To predict human iNKT cell responses more accurately in a mouse model, we derived a mouse line in which compound genetic modifications were used to express a human-like iNKT cell TCR along with human CD1d in place of the endogenous mouse proteins. Detailed transcriptional and phenotypic profiling demonstrated that these partially humanized mice developed an expanded population of T cells recognizing CD1d-presented glycolipid antigens, among which a subset characterized by expression of chemokine receptor CXCR6 had features characteristic of authentic iNKT cells. Responses to iNKT cell activating glycolipids in these mice generated cytokine production in vitro and in vivo that showed a pattern of fine specificity that closely resembled that of cultured human iNKT cell clones. Anti-tumor responses to variants of α-galactosyl ceramide in VαKI mice also correlated with their potency for stimulating human iNKT cells. This genetically modified mouse line provides a practical model for human presentation and recognition of iNKT cell activators in the context of a normally functioning immune system, and may furnish valuable opportunities for preclinical evaluation of iNKT cell-based therapies.

Hepatic iNKT cells produce type 2 cytokines and restrain antiviral T cells during acute hepacivirus infection

Chronic hepatitis C virus (HCV) infection is a curable disease, but the absence of a vaccine remains a major problem in infection prevention. The lack of small animal models and limited access to human liver tissue impede the study of hepatic antiviral immunity and the development of new vaccine strategies. We recently developed an immune-competent mouse model using an HCV-related rodent hepacivirus which shares immunological features with human viral hepatitis. In this study, we used this new model to investigate the role of invariant natural killer T (iNKT) cells during hepacivirus infection in vivo. These cells are enriched in the liver, however their role in viral hepatitis is not well defined. Using high-dimensional flow cytometry and NKT cell deficient mice we analyzed a potential role of iNKT cells in mediating viral clearance, liver pathology or immune-regulation during hepacivirus infection. In addition, we identified new immune-dominant MHC class I restricted viral epitopes and analyzed the impact of iNKT cells on virus-specific CD8+ T cells. We found that rodent hepacivirus infection induced the activation of iNKT cell subsets with a mixed NKT1/NKT2 signature and significant production of type 2 cytokines (IL-4 and IL-13) during acute infection. While iNKT cells were dispensable for viral clearance, the lack of these cells caused higher levels of liver injury during infection. In addition, the absence of iNKT cells resulted in increased effector functions of hepatic antiviral T cells. In conclusion, our study reports a regulatory role of hepatic iNKT cells during hepacivirus infection in vivo. Specifically, our data suggest that iNKT cells skewed towards type 2 immunity limit liver injury during acute infection by mechanisms that include the regulation of effector functions of virus-specific T cells.

The RAG1 Ubiquitin Ligase Domain Stimulates Recombination of TCRβ and TCRα Genes and Influences Development of αβ T Cell Lineages

RAG1/RAG2 (RAG) endonuclease-mediated assembly of diverse lymphocyte Ag receptor genes by V(D)J recombination is critical for the development and immune function of T and B cells. The RAG1 protein contains a ubiquitin ligase domain that stabilizes RAG1 and stimulates RAG endonuclease activity in vitro. We report in this study that mice with a mutation that inactivates the Rag1 ubiquitin ligase in vitro exhibit decreased rearrangements and altered repertoires of TCRβ and TCRα genes in thymocytes and impaired thymocyte developmental transitions that require the assembly and selection of functional TCRβ and/or TCRα genes. These Rag1 mutant mice present diminished positive selection and superantigen-mediated negative selection of conventional αβ T cells, decreased genesis of invariant NK T lineage αβ T cells, and mature CD4+ αβ T cells with elevated autoimmune potential. Our findings reveal that the Rag1 ubiquitin ligase domain functions in vivo to stimulate TCRβ and TCRα gene recombination and influence differentiation of αβ T lineage cells, thereby establishing replete diversity of αβ TCRs and populations of αβ T cells while restraining generation of potentially autoreactive conventional αβ T cells.

Genomic and Transcriptional Mechanisms Governing Innate-like T Lymphocyte Development

Innate-like lymphocytes are a subset of lymphoid cells that function as a first line of defense against microbial infection. These cells are activated by proinflammatory cytokines or broadly expressed receptors and are able to rapidly perform their effector functions owing to a uniquely primed chromatin state that is acquired as a part of their developmental program. These cells function in many organs to protect against disease, but they release cytokines and cytotoxic mediators that can also lead to severe tissue pathologies. Therefore, harnessing the capabilities of these cells for therapeutic interventions will require a deep understanding of how these cells develop and regulate their effector functions. In this review we discuss recent advances in the identification of the transcription factors and the genomic regions that guide the development and function of invariant NKT cells and we highlight related mechanisms in other innate-like lymphocytes.

Current insights in mouse iNKT and MAIT cell development using single cell transcriptomics data

Innate T (T_inn) cells are a collection of T cells with important regulatory functions that have a crucial role in immunity towards tumors, bacteria, viruses, and in cell-mediated autoimmunity. In mice, the two main αβ T_inn cell subsets include the invariant NKT (iNKT) cells that recognize glycolipid antigens presented by non-polymorphic CD1d molecules and the mucosal associated invariant T (MAIT) cells that recognize vitamin B metabolites presented by the non-polymorphic MR1 molecules. Due to their ability to promptly secrete large quantities of cytokines either after T cell antigen receptor (TCR) activation or upon exposure to tissue- and antigen-presenting cell-derived cytokines, T_inn cells are thought to act as a bridge between the innate and adaptive immune systems and have the ability to shape the overall immune response. Their swift response reflects the early acquisition of helper effector programs during their development in the thymus, independently of pathogen exposure and prior to taking up residence in peripheral tissues. Several studies recently profiled, in an unbiased manner, the transcriptomes of mouse thymic iNKT and MAIT cells at the single cell level. Based on these data, we re-examine in this review how T_inn cells develop in the mouse thymus and undergo effector differentiation.

Protein abundance of the cytokine receptor γc controls the thymic generation of innate-like T cells

Innate-like T (iT) cells comprise a population of immunoregulatory T cells whose effector function is imposed during their development in the thymus to provide protective immunity prior to antigen encounter. The molecular mechanism that drives the generation of iT cells remains unclear. Here, we report that the cytokine receptor γc plays a previously unappreciated role for thymic iT cells by controlling their cellular abundance, lineage commitment, and subset differentiation. As such, γc overexpression on thymocytes dramatically altered iT cell generation in the thymus, as it skewed the subset composition of invariant NKT (iNKT) cells and promoted the generation of IFNγ-producing innate CD8 T cells. Mechanistically, we found that the γc-STAT6 axis drives the differentiation of IL-4-producing iNKT cells, which in turn induced the generation of innate CD8 T cells. Collectively, these results reveal a cytokine-driven circuity of thymic iT cell differentiation that is controlled by the abundance of γc proteins.

Thymus-Derived CD4+CD8+ Cells Reside in Mediastinal Adipose Tissue and the Aortic Arch

Double-positive CD4⁺CD8αβ⁺ (DP) cells are thought to reside as T cell progenitors exclusively within the thymus. We recently discovered an unexpected CD4⁺ and CD8αβ⁺ immune cell population in healthy and atherosclerotic mice by single-cell RNA sequencing. Transcriptomically, these cells resembled thymic DPs. Flow cytometry and three-dimensional whole-mount imaging confirmed DPs in thymus, mediastinal adipose tissue, and aortic adventitia, but nowhere else. Deep transcriptional profiling revealed differences between DP cells isolated from the three locations. All DPs were dependent on RAG2 expression and the presence of the thymus. Mediastinal adipose tissue DPs resided in close vicinity to invariant NKT cells, which they could activate in vitro. Thymus transplantation failed to reconstitute extrathymic DPs, and frequencies of extrathymic DPs were unaltered by pharmacologic inhibition of S1P1, suggesting that their migration may be locally confined. Our results define two new, transcriptionally distinct subsets of extrathymic DPs that may play a role in aortic vascular homeostasis.

Brief homogeneous TCR signals instruct common iNKT progenitors whose effector diversification is characterized by subsequent cytokine signaling

Innate-like T cell populations expressing conserved TCRs play critical roles in immunity through diverse developmentally acquired effector functions. Focusing on the prototypical lineage of invariant natural killer T (iNKT) cells, we sought to dissect the mechanisms and timing of fate decisions and functional effector differentiation. Utilizing induced expression of the semi-invariant NKT cell TCR on double positive thymocytes, an initially highly synchronous wave of iNKT cell development was triggered by brief homogeneous TCR signaling. After reaching a uniform progenitor state characterized by IL-4 production potential and proliferation, effector subsets emerged simultaneously, but then diverged toward different fates. While NKT17 specification was quickly completed, NKT1 cells slowly differentiated and expanded. NKT2 cells resembled maturing progenitors, which gradually diminished in numbers. Thus, iNKT subset diversification occurs in dividing progenitor cells without acute TCR input but utilizes multiple active cytokine signaling pathways. These data imply a two-step model of iNKT effector differentiation.

Emerging Roles of T Cells in the Pathogenesis of Nonalcoholic Steatohepatitis and Hepatocellular Carcinoma

Nonalcoholic fatty liver disease (NAFLD) has become the most common chronic liver disease worldwide. A significant proportion of patients with NAFLD develop a progressive inflammatory condition termed nonalcoholic steatohepatitis (NASH), which may eventually advance to cirrhosis and hepatocellular carcinoma (HCC). NASH is characterized by steatosis, hepatocyte ballooning, and lobular inflammation. Heightened immune cell infiltration is a hallmark of NASH, yet the mechanisms whereby hepatic inflammation occurs in NASH and how it contributes to disease initiation and progression remain incompletely understood. Emerging evidence indicates that intrahepatic T cell immune mechanisms play an integral role in the pathogenesis of NASH and its transition to HCC. In this review, we summarize the current knowledge regarding the T cell-mediated mechanisms of inflammation in NASH. We highlight recent preclinical and human studies implicating various subsets of conventional and innate-like T cells in the onset and progression of NASH and HCC. Finally, we discuss the potential therapeutic strategies targeting T cell-mediated responses for the treatment of NASH.

Endoplasmic Reticulum Quality Control in Immune Cells

The endoplasmic reticulum quality control (ERQC) system, including endoplasmic reticulum-associated degradation (ERAD), the unfolded protein response (UPR), and autophagy, presides over cellular protein secretion and maintains proteostasis in mammalian cells. As part of the immune system, a variety of proteins are synthesized and assembled correctly for the development, activation, and differentiation of immune cells, such as dendritic cells (DCs), macrophages, myeloid-derived-suppressor cells (MDSCs), B lymphocytes, T lymphocytes, and natural killer (NK) cells. In this review, we emphasize the role of the ERQC in these immune cells, and also discuss how the imbalance of ER homeostasis affects the immune response, thereby suggesting new therapeutic targets for immunotherapy.

Natural Killer T (NKT) Cells and Periodontitis: Potential Regulatory Role of NKT10 Cells

Natural killer T (NKT) cells constitute a unique subset of T lymphocytes characterized by specifically interacting with antigenic glycolipids conjugated to the CD1d receptor on antigen-presenting cells. Functionally, NKT cells are capable of performing either effector or suppressor immune responses, depending on their production of proinflammatory or anti-inflammatory cytokines, respectively. Effector NKT cells are subdivided into three subsets, termed NKT1, NKT2, and NKT17, based on the cytokines they produce and their similarity to the cytokine profile produced by Th1, Th2, and Th17 lymphocytes, respectively. Recently, a new subgroup of NKT cells termed NKT10 has been described, which cooperates and interacts with other immune cells to promote immunoregulatory responses. Although the tissue-specific functions of NKT cells have not been fully elucidated, their activity has been associated with the pathogenesis of different inflammatory diseases with immunopathogenic similarities to periodontitis, including osteolytic pathologies such as rheumatoid arthritis and osteoporosis. In the present review, we revise and discuss the pathogenic characteristics of NKT cells in these diseases and their role in the pathogenesis of periodontitis; particularly, we analyze the potential regulatory role of the IL-10-producing NKT10 cells.

CD138 expression is a molecular signature but not a developmental requirement for RORγt+ NKT17 cells

Invariant NKT (iNKT) cells are potent immunomodulatory cells that acquire effector function during their development in the thymus. IL-17-producing iNKT cells are commonly referred to as NKT17 cells, and they are unique among iNKT cells to express the heparan sulfate proteoglycan CD138 and the transcription factor RORγt. Whether and how CD138 and RORγt contribute to NKT17 cell differentiation, and whether there is an interplay between RORγt and CD138 expression to control iNKT lineage fate, remain mostly unknown. Here, we showed that CD138 expression was only associated with and not required for the differentiation and IL-17 production of NKT17 cells. Consequently, CD138-deficient mice still generated robust numbers of IL-17-producing RORγt+ NKT17 cells. Moreover, forced expression of RORγt significantly promoted the generation of thymic NKT17 cells, but did not induce CD138 expression on non-NKT17 cells. These results indicated that NKT17 cell generation and IL-17 production were driven by RORγt, employing mechanisms that were independent of CD138. Therefore, our study effectively dissociated CD138 expression from the RORγt-driven molecular pathway of NKT17 cell differentiation.

NKT and NKT-like Cells in Autoimmune Neuroinflammatory Diseases-Multiple Sclerosis, Myasthenia Gravis and Guillain-Barre Syndrome

NKT cells comprise three subsets—type I (invariant, iNKT), type II, and NKT-like cells, of which iNKT cells are the most studied subset. They are capable of rapid cytokine production after the initial stimulus, thus they may be important for polarisation of Th cells. Due to this, they may be an important cell subset in autoimmune diseases. In the current review, we are summarising results of NKT-oriented studies in major neurological autoimmune diseases—multiple sclerosis, myasthenia gravis, and Guillain-Barre syndrome and their corresponding animal models.

microRNA dynamic expression regulates invariant NKT cells

Invariant natural killer T cells (iNKT) are a prevalent population of innate-like T cells in mice, but quite rare in humans that are critical for regulation of the innate and adaptive immune responses during antimicrobial immunity, tumor rejection, and inflammatory diseases. Multiple transcription factors and signaling molecules that contribute to iNKT cell selection and functional differentiation have been identified. However, the full molecular network responsible for regulating and maintaining iNKT populations remains unclear. MicroRNAs (miRNAs) are an abundant class of evolutionarily conserved, small, non-coding RNAs that regulate gene expression post-transcriptionally. Previous reports uncovered the important roles of miRNAs in iNKT cell development and function using Dicer mutant mice. In this review, we discuss the emerging roles of individual miRNAs in iNKT cells reported by our group and other groups, including miR-150, miR-155, miR-181, let-7, miR-17 ~ 92 cluster, and miR-183-96-182 cluster. It is likely that iNKT cell development, differentiation, homeostasis, and functions are orchestrated through a multilayered network comprising interactions among master transcription factors, signaling molecules, and dynamically expressed miRNAs. We provide a comprehensive view of the molecular mechanisms underlying iNKT cell differentiation and function controlled by dynamically expressed miRNAs.

Harnessing NKT cells for vaccination

Natural killer T (NKT) cells are innate-like T cells capable of enhancing both innate and adaptive immune responses. When NKT cells are stimulated in close temporal association with co-administered antigens, strong antigen-specific immune responses can be induced, prompting the study of NKT cell agonists as novel immune adjuvants. This activity has been attributed to the capacity of activated NKT cells to act as universal helper cells, with the ability to provide molecular signals to dendritic cells and B cells that facilitate T cell and antibody responses, respectively. These signals can override the requirement for conventional CD4⁺ T cell help, so that vaccines can be designed without need to consider CD4⁺ T cell repertoire and major histocompatibility complex Class II diversity. Animal studies have highlighted some drawbacks of the approach, namely, concerns around induction of NKT cell hyporesponsiveness, which may limit vaccine boosting, and potential for toxicity. Here we highlight studies that suggest these obstacles can be overcome by targeted delivery in vivo. We also feature new studies that suggest activating NKT cells can help encourage differentiation of T cells into tissue-resident memory cells that play an important role in prophylaxis against infection, and may be required in cancer therapy.

The Timing and Abundance of IL-2Rβ (CD122) Expression Control Thymic iNKT Cell Generation and NKT1 Subset Differentiation

Invariant NKT (iNKT) cells are thymus-generated innate-like T cells, comprised of three distinct subsets with divergent effector functions. The molecular mechanism that drives the lineage trifurcation of immature iNKT cells into the NKT1, NKT2, and NKT17 subsets remains a controversial issue that remains to be resolved. Because cytokine receptor signaling is necessary for iNKT cell generation, cytokines are proposed to contribute to iNKT subset differentiation also. However, the precise roles and requirements of cytokines in these processes are not fully understood. Here, we show that IL-2Rβ, a nonredundant component of the IL-15 receptor complex, plays a critical role in both the development and differentiation of thymic iNKT cells. While the induction of IL-2Rβ expression on postselection thymocytes is necessary to drive the generation of iNKT cells, surprisingly, premature IL-2Rβ expression on immature iNKT cells was detrimental to their development. Moreover, while IL-2Rβ is necessary for NKT1 generation, paradoxically, we found that the increased abundance of IL-2Rβ suppressed NKT1 generation without affecting NKT2 and NKT17 cell differentiation. Thus, the timing and abundance of IL-2Rβ expression control iNKT lineage fate and development, thereby establishing cytokine receptor expression as a critical regulator of thymic iNKT cell differentiation.

Classical MHC expression by DP thymocytes impairs the selection of non-classical MHC restricted innate-like T cells

Conventional T cells are selected by peptide-MHC expressed by cortical epithelial cells in the thymus, and not by cortical thymocytes themselves that do not express MHC I or MHC II. Instead, cortical thymocytes express non-peptide presenting MHC molecules like CD1d and MR1, and promote the selection of PLZF⁺ iNKT and MAIT cells, respectively. Here, we report an inducible class-I transactivator mouse that enables the expression of peptide presenting MHC I molecules in different cell types. We show that MHC I expression in DP thymocytes leads to expansion of peptide specific PLZF⁺ innate-like (PIL) T cells. Akin to iNKT cells, PIL T cells differentiate into three functional effector subsets in the thymus, and are dependent on SAP signaling. We demonstrate that PIL and NKT cells compete for a narrow niche, suggesting that the absence of peptide-MHC on DP thymocytes facilitates selection of non-peptide specific lymphocytes.

Natural Killer T Lymphocytes Integrate Innate Sensory Information and Relay Context to Effector Immune Responses

It is now appreciated that a group of lymphoid lineage cells, collectively called innate-like effector lymphocytes, have evolved to integrate information relayed by the innate sensory immune system about the state of the local tissue environment and to pass on this context to downstream effector innate and adaptive immune responses. Thereby, innate functions engrained into such innate-like lymphoid lineage cells during development can control the quality and magnitude of an immune response to a tissue-altering pathogen and facilitate the formation of memory engrams within the immune system. These goals are accomplished by the innate lymphoid cells that lack antigen-specific receptors, γδ T cell receptor (TCR)-expressing T cells, and several αβ TCR-expressing T cell subsets-such as natural killer T cells, mucosal-associated invariant T cells, et cetera. Whilst we briefly consider the commonalities in the origins and functions of these diverse lymphoid subsets to provide context, the primary topic of this review is to discuss how the semi-invariant natural killer T cells got this way in evolution through lineage commitment and onward ontogeny. What emerges from this discourse is the question: Has a "limbic immune system" emerged (screaming quietly in plain sight!) out of what has been dubbed "in-betweeners"?

Thymic iNKT single cell analyses unmask the common developmental program of mouse innate T cells

Most T lymphocytes leave the thymus as naïve cells with limited functionality. However, unique populations of innate-like T cells differentiate into functionally distinct effector subsets during their development in the thymus. Here, we profiled >10,000 differentiating thymic invariant natural killer T (iNKT) cells using single-cell RNA sequencing to produce a comprehensive transcriptional landscape that highlights their maturation, function, and fate decisions at homeostasis. Our results reveal transcriptional profiles that are broadly shared between iNKT and mucosal-associated invariant T (MAIT) cells, illustrating a common core developmental program. We further unmask a mutual requirement for Hivep3, a zinc finger transcription factor and adapter protein. Hivep3 is expressed in early precursors and regulates the post-selection proliferative burst, differentiation and functions of iNKT cells. Altogether, our results highlight the common requirements for the development of innate-like T cells with a focus on how Hivep3 impacts the maturation of these lymphocytes.

Adipose Tissue Immunomodulation: A Novel Therapeutic Approach in Cardiovascular and Metabolic Diseases

Adipose tissue is a critical regulator of systemic metabolism and bodily homeostasis as it secretes a myriad of adipokines, including inflammatory and anti-inflammatory cytokines. As the main storage pool of lipids, subcutaneous and visceral adipose tissues undergo marked hypertrophy and hyperplasia in response to nutritional excess leading to hypoxia, adipokine dysregulation, and subsequent low-grade inflammation that is characterized by increased infiltration and activation of innate and adaptive immune cells. The specific localization, physiology, susceptibility to inflammation and the heterogeneity of the inflammatory cell population of each adipose depot are unique and thus dictate the possible complications of adipose tissue chronic inflammation. Several lines of evidence link visceral and particularly perivascular, pericardial, and perirenal adipose tissue inflammation to the development of metabolic syndrome, insulin resistance, type 2 diabetes and cardiovascular diseases. In addition to the implication of the immune system in the regulation of adipose tissue function, adipose tissue immune components are pivotal in detrimental or otherwise favorable adipose tissue remodeling and thermogenesis. Adipose tissue resident and infiltrating immune cells undergo metabolic and morphological adaptation based on the systemic energy status and thus a better comprehension of the metabolic regulation of immune cells in adipose tissues is pivotal to address complications of chronic adipose tissue inflammation. In this review, we discuss the role of adipose innate and adaptive immune cells across various physiological and pathophysiological states that pertain to the development or progression of cardiovascular diseases associated with metabolic disorders. Understanding such mechanisms allows for the exploitation of the adipose tissue-immune system crosstalk, exploring how the adipose immune system might be targeted as a strategy to treat cardiovascular derangements associated with metabolic dysfunctions.

Impact of Aging on the Phenotype of Invariant Natural Killer T Cells in Mouse Thymus

Invariant natural killer T (iNKT) cells represent a subclass of T cells possessing a restricted repertoire of T cell receptors enabling them to recognize lipid derived ligands. iNKT cells are continuously generated in thymus and differentiate into three main subpopulations: iNKT1, iNKT2, and iNKT17 cells. We investigated the transcriptomes of these subsets comparing cells isolated from young adult (6–10 weeks old) and aged BALB/c mice (25–30 weeks of age) in order to identify genes subject to an age-related regulation of expression. These time points were selected to take into consideration the consequences of thymic involution that radically alter the existing micro-milieu. Significant differences were detected in the expression of histone genes affecting all iNKT subsets. Also the proliferative capacity of iNKT cells decreased substantially upon aging. Several genes were identified as possible candidates causing significant age-dependent changes in iNKT cell generation and/or function such as genes coding for granzyme A, ZO-1, EZH2, SOX4, IGF1 receptor, FLT4, and CD25. Moreover, we provide evidence that IL2 differentially affects homeostasis of iNKT subsets with iNKT17 cells engaging a unique mechanism to respond to IL2 by initiating a slow rate of proliferation.

Survivre et vivre: When iNKT cells met a Hippo

Invariant natural killer T (iNKT) cells are innate-like lymphocytes with unique signaling requirements for their development and differentiation. In this issue of JEM, Raynor et al. (https://doi.org/10.1084/jem.20191157) report that the Hippo signaling pathway controls the maturation and effector differentiation of iNKT cells by modulating cellular metabolism.