T cells expressing the transcription factor PLZF regulate the development of memory-like CD8+ T cells

Unraveling the phenotypic states of human innate-like T cells: Comparative insights with conventional T cells and mouse models

The "innate-like" T cell compartment, known as Tinn, represents a diverse group of T cells that straddle the boundary between innate and adaptive immunity. We explore the transcriptional landscape of Tinn compared to conventional T cells (Tconv) in the human thymus and blood using single-cell RNA sequencing (scRNA-seq) and flow cytometry. In human blood, the majority of Tinn cells share an effector program driven by specific transcription factors, distinct from those governing Tconv cells. Conversely, only a fraction of thymic Tinn cells displays an effector phenotype, while others share transcriptional features with developing Tconv cells, indicating potential divergent developmental pathways. Unlike the mouse, human Tinn cells do not differentiate into multiple effector subsets but develop a mixed type 1/type 17 effector potential. Cross-species analysis uncovers species-specific distinctions, including the absence of type 2 Tinn cells in humans, which implies distinct immune regulatory mechanisms across species.

Direct presentation of inflammation-associated self-antigens by thymic innate-like T cells induces elimination of autoreactive CD8+ thymocytes

Upregulation of diverse self-antigens that constitute components of the inflammatory response overlaps spatially and temporally with the emergence of pathogen-derived foreign antigens. Therefore, discrimination between these inflammation-associated self-antigens and pathogen-derived molecules represents a unique challenge for the adaptive immune system. Here, we demonstrate that CD8+ T cell tolerance to T cell-derived inflammation-associated self-antigens is efficiently induced in the thymus and supported by redundancy in cell types expressing these molecules. In addition to thymic epithelial cells, this included thymic eosinophils and innate-like T cells, a population that expressed molecules characteristic for all major activated T cell subsets. We show that direct T cell-to-T cell antigen presentation by minute numbers of innate-like T cells was sufficient to eliminate autoreactive CD8+ thymocytes. Tolerance to such effector molecules was of critical importance, as its breach caused by decreased thymic abundance of a single model inflammation-associated self-antigen resulted in autoimmune elimination of an entire class of effector T cells.

Promyelocytic leukemia zinc finger controls type 2 immune responses in the lungs by regulating lineage commitment and the function of innate and adaptive immune cells

Type 2 immune responses are critical for host defense, mediate allergy and Th2-high asthma. The transcription factor, promyelocytic leukemia zinc finger (PLZF), has emerged as a significant regulator of type 2 inflammation in the lung; however, its exact mechanism remains unclear. In this review, we summarized recent findings regarding the ability of PLZF to control the development and function of innate lymphoid cells (ILCs), iNKT cells, memory T cells, basophils, and other immune cells that drive type 2 responses. We discussed the important role of PLZF in the pathogenesis of Th2-high asthma. Collectively, prior studies have revealed the critical role of PLZF in the regulation of innate and adaptive immune cells involved in type 2 inflammation in the lung. Therefore, targeting PLZF signaling represents a promising therapeutic approach to suppress Th2-high asthma.

Unraveling the Phenotypic States of Human innate-like T Cells: Comparative Insights with Conventional T Cells and Mouse Models

The "innate-like" T cell compartment, known as Tinn, represents a diverse group of T cells that straddle the boundary between innate and adaptive immunity, having the ability to mount rapid responses following activation. In mice, this ability is acquired during thymic development. We explored the transcriptional landscape of Tinn compared to conventional T cells (Tconv) in the human thymus and blood using single cell RNA sequencing and flow cytometry. We reveal that in human blood, the majority of Tinn cells, including iNKT, MAIT, and Vδ2+Vγ9+ T cells, share an effector program characterized by the expression of unique chemokine and cytokine receptors, and cytotoxic molecules. This program is driven by specific transcription factors, distinct from those governing Tconv cells. Conversely, only a fraction of thymic Tinn cells displays an effector phenotype, while others share transcriptional features with developing Tconv cells, indicating potential divergent developmental pathways. Unlike the mouse, human Tinn cells do not differentiate into multiple effector subsets but develop a mixed type I/type III effector potential. To conduct a comprehensive cross-species analysis, we constructed a murine Tinn developmental atlas and uncovered additional species-specific distinctions, including the absence of type II Tinn cells in humans, which implies distinct immune regulatory mechanisms across species. The study provides insights into the development and functionality of Tinn cells, emphasizing their role in immune responses and their potential as targets for therapeutic interventions.

IL-4/IL-4 Ab complex enhances the accumulation of both antigen-specific and bystander CD8 T cells in mouse lungs infected with influenza A virus

BACKGROUND

Unlike conventional T cells, innate and virtual-memory CD8 T cells in naïve mice acquire their memory phenotypes and functions in the absence of antigenic encounters in a cytokine-dependent manner. The relevant cytokines include interleukin-4 (IL-4), type I interferon, and interleukin-15 (IL-15). Moreover, exogenous IL-4 can also induce de novo generation and/or expansion of the virtual-memory CD8 T cell population. In this study, we investigated whether exogenous IL-4 could enhance the immune response to a viral infection.

RESULTS

In vivo administration of IL-4 and an anti-IL-4 antibody complex (IL-4C) increased CXCR3 expression in both memory and naïve phenotype CD8 T cells in the absence of antigenic stimulation, and protected mice from lethal influenza infection. Flow cytometric analysis of lung-infiltrating immune cells on day 5 after virus infection revealed higher numbers of antigen-specific and bystander CD8 T cells in IL-4C-treated mice than in control mice. In particular, the bystander CD8 T cells were a naïve or evident memory phenotypes. Crucially, an anti-CXCR3 blocking antibody abrogated this IL-4C effect, reflecting that the increased accumulation of CD8 T cells in the lungs after IL-4C treatment is dependent on CXCR3.

CONCLUSIONS

These data demonstrate that exogenous IL-4C plays a protective role by enhancing CXCR3-dependent migration of CD8 T cells into influenza-infected lungs.

TGF-β Promotes the Postselection Thymic Development and Peripheral Function of IFN-γ-Producing Invariant NKT cells

IFN-γ-producing invariant NKT (iNKT)1 cells are lipid-reactive innate-like lymphocytes that are resident in the thymus and peripheral tissues where they protect against pathogenic infection. The thymic functions of iNKT1 cells are not fully elucidated, but subsets of thymic iNKT cells modulate CD8 T cell, dendritic cell, B cell, and thymic epithelial cell numbers or function. In this study, we show that a subset of murine thymic iNKT1 cells required TGF-β-induced signals for their postselection development, to maintain hallmark TGF-β-induced genes, and for expression of the adhesion receptors CD49a and CD103. However, the residency-associated receptor CD69 was not TGF-β signaling-dependent. Recently described CD244+ c2 thymic iNKT1 cells, which produce IFN-γ without exogenous stimulation and have NK-like characteristics, reside in this TGF-β-responsive population. Liver and spleen iNKT1 cells do not share this TGF-β gene signature, but nonetheless TGF-β impacts liver iNKT1 cell phenotype and function. Our findings provide insight into the heterogeneity of mechanisms guiding iNKT1 cell development in different tissues and suggest a close association between a subset of iNKT1 cells and TGF-β-producing cells in the thymus that support their development.

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.

Helminth Infection-Induced Increase in Virtual Memory CD8 T Cells Is Transient, Driven by IL-15, and Absent in Aged Mice

CD8 virtual memory T (TVM) cells are Ag-naive CD8 T cells that have undergone partial differentiation in response to common γ-chain cytokines, particularly IL-15 and IL-4. TVM cells from young individuals are highly proliferative in response to TCR and cytokine stimulation but, with age, they lose TCR-mediated proliferative capacity and exhibit hallmarks of senescence. Helminth infection can drive an increase in TVM cells, which is associated with improved pathogen clearance during subsequent infectious challenge in young mice. Given the cytokine-dependent profile of TVM cells and their age-associated dysfunction, we traced proliferative and functional changes in TVM cells, compared with true naive CD8 T cells, after helminth infection of young and aged C57BL/6 mice. We show that IL-15 is essential for the helminth-induced increase in TVM cells, which is driven only by proliferation of existing TVM cells, with negligible contribution from true naive cell differentiation. Additionally, TVM cells showed the greatest proliferation in response to helminth infection and IL-15 compared with other CD8 T cells. Furthermore, TVM cells from aged mice did not undergo expansion after helminth infection due to both TVM cell-intrinsic and -extrinsic changes associated with aging.

Virtual Memory CD8+ T Cells: Origin and Beyond

The presence of CD8+ T cells with a memory phenotype in nonimmunized mice has been noted for decades, but it was not until about 2 decades ago that they began to be studied in greater depth. Currently called virtual memory CD8+ T cells, they consist of a heterogeneous group of cells with memory characteristics, without any previous contact with their specific antigens. These cells were identified in mice, but a few years ago, a cell type with characteristics equivalent to the murine ones was described in healthy humans. In this review, we address the different aspects of its biology mainly developed in murine models and what is currently known about its cellular equivalent in humans.

Single-cell analysis reveals differences among iNKT cells colonizing peripheral organs and identifies Klf2 as a key gene for iNKT emigration

Invariant natural killer T cell (iNKT) subsets are differentially distributed in various immune organs. However, it remains unclear whether iNKT cells exhibit phenotypical and functional differences in different peripheral organs and how thymic iNKT cells emigrate to peripheral organs. Here, we used single-cell RNA-seq to map iNKT cells from peripheral organs. iNKT1 cells from liver, spleen, and lymph node appear to have distinct phenotypic profiles and functional capabilities. However, iNKT17 transcriptomes were comparable across peripheral organs. In addition, by integrating data with a thymic iNKT cell study, we uncovered a transient population of recent thymic emigrants, a cluster of peripheral iNKT cells with high expression of transcription factor Kruppel-like factor 2 (Klf2). Deletion of Klf2 led to a severe impairment of iNKT differentiation and migration. Our study revealed that iNKT subsets are uniquely distributed in peripheral organs with some inter-local tissue variation, especially for iNKT1 cell, and identified Klf2 as a rheostat for iNKT cell migration and differentiation.

Innate and Innate-like Effector Lymphocytes in Health and Disease

Lymphocytes can be functionally partitioned into subsets belonging to the innate or adaptive arms of the immune system. Subsets of innate and innate-like lymphocytes may or may not express Ag-specific receptors of the adaptive immune system, yet they are poised to respond with innate-like speed to pathogenic insults but lack the capacity to develop classical immunological memory. These lymphocyte subsets display a number of common properties that permit them to integrate danger and stress signals dispatched by innate sensor cells to facilitate the generation of specialized effector immune responses tailored toward specific pathogens or other insults. In this review, we discuss the functions of distinct subsets of innate and innate-like lymphocytes. A better understanding of the mechanisms by which these cells are activated in different contexts, their interactions with other immune cells, and their role in health and disease may inform the development of new or improved immunotherapies.

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.

Redefining the Foreign Antigen and Self-Driven Memory CD4+ T-Cell Compartments via Transcriptomic, Phenotypic, and Functional Analyses

Under steady-state conditions, conventional CD4⁺ T lymphocytes are classically divided into naïve (CD44^lo CD62L^hi) and memory (CD44^hi CD62L^lo) cell compartments. While the latter population is presumed to comprise a mixture of distinct subpopulations of explicit foreign antigen (Ag)-specific “authentic” memory and foreign Ag-independent memory-phenotype (MP) cells, phenotypic markers differentially expressed in these two cell types have yet to be identified. Moreover, while MP cells themselves have been previously described as heterogeneous, it is unknown whether they consist of distinct subsets defined by marker expression. In this study, we demonstrate using combined single-cell RNA sequencing and flow cytometric approaches that self-driven MP CD4⁺ T lymphocytes are divided into CD127^hi Sca1^lo, CD127^hi Sca1^hi, CD127^lo Sca1^hi, and CD127^lo Sca1^lo subpopulations that are Bcl2^lo, while foreign Ag-specific memory cells are CD127^hi Sca1^hi Bcl2^hi. We further show that among the four MP subsets, CD127^hi Sca1^hi lymphocytes represent the most mature and cell division-experienced subpopulation derived from peripheral naïve precursors. Finally, we provide evidence arguing that this MP subpopulation exerts the highest responsiveness to Th1-differentiating cytokines and can induce colitis. Together, our findings define MP CD4⁺ T lymphocytes as a unique, self-driven population consisting of distinct subsets that differ from conventional foreign Ag-specific memory cells in marker expression and establish functional relevance for the mature subset of CD127^hi Sca1^hi MP cells.

Mucosal-Associated Invariant T Cells in T-Cell Non-Hodgkin Lymphomas: A Case Series

Simple Summary

Mucosal-associated invariant T (MAIT) cells are a subgroup of T lymphocytes whose role has recently been investigated in several types of diseases, including cancer. However, little is known about these cells in lymphomas. In this case series, we investigated the presence of MAIT cells in biopsies obtained from patients diagnosed with T-cell non-Hodgkin lymphomas, uncommon hematological malignancies with often not clearly defined etiopathology.

Abstract

Background: Mucosal-associated invariant T (MAIT) cells are a subset of unconventional T lymphocytes expressing a semi-invariant α/β T-cell receptor (TCR). The physiological functions of these cells, which are particularly abundant in normal liver and mucosal sites, have become clear only in recent years, but their role in most human diseases is still unknown. Since the cellular origin and etiopathogenesis of most T-lymphomas are still elusive, we decided to explore the presence of MAIT cells in biopsies from these neoplasms. Methods: Sixteen biopsies obtained from patients with a T-cell lymphoma diagnosis were analyzed via immunofluorescence staining using an anti-Vα7.2 antibody and the MR1-antigen tetramer. Positive cases were subjected to a polymerase chain reaction for the detection of Vα7.2–Jα33, Vα7.2–Jα20, or Vα7.2–Jα12 rearrangements, followed by sequencing of the CDR3α region. Results: CD3+/Vα7.2+ and CD3+/MR1-Ag-tetramer+ cells were found in 4 of 16 samples analyzed. The identification of specific TCR rearrangements confirmed the presence of these cells in all four samples. PCR and sequencing results documented the presence of multiple clones of MAIT cells in each positive sample. Conclusions: MAIT cells are frequently found in T-cell lymphomas. More in-depth studies and a larger number of samples are needed to better clarify the contribution of MAIT cells to this rare neoplasm.

A population of naive-like CD4+ T cells stably polarized to the TH 1 lineage

T-bet is the lineage-specifying transcription factor for CD4+ TH 1 cells. T-bet has also been found in other CD4+ T cell subsets, including TH 17 cells and Treg, where it modulates their functional characteristics. However, we lack information on when and where T-bet is expressed during T cell differentiation and how this impacts T cell differentiation and function. To address this, we traced the ontogeny of T-bet-expressing cells using a fluorescent fate-mapping mouse line. We demonstrate that T-bet is expressed in a subset of CD4+ T cells that have naïve cell surface markers and transcriptional profile and that this novel cell population is phenotypically and functionally distinct from previously described populations of naïve and memory CD4+ T cells. Naïve-like T-bet-experienced cells are polarized to the TH 1 lineage, predisposed to produce IFN-γ upon cell activation, and resist repolarization to other lineages in vitro and in vivo. These results demonstrate that lineage-specifying factors can polarize T cells in the absence of canonical markers of T cell activation and that this has an impact on the subsequent T-helper response.

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.

Memory-phenotype CD4+ T cells: a naturally arising T lymphocyte population possessing innate immune function

In conventional adaptive immune responses, upon recognition of foreign antigens, naive CD4+ T lymphocytes are activated to differentiate into effector/memory cells. In addition, emerging evidence suggests that in the steady state, naive CD4+ T cells spontaneously proliferate in response to self-antigens to acquire a memory phenotype (MP) through homeostatic proliferation. This expansion is particularly profound in lymphopenic environments but also occurs in lymphoreplete, normal conditions. The 'MP T lymphocytes' generated in this manner are maintained by rapid proliferation in the periphery and they tonically differentiate into T-bet-expressing 'MP1' cells. Such MP1 CD4+ T lymphocytes can exert innate effector function, producing IFN-γ in response to IL-12 in the absence of antigen recognition, thereby contributing to host defense. In this review, we will discuss our current understanding of how MP T lymphocytes are generated and persist in steady-state conditions, their populational heterogeneity as well as the evidence for their effector function. We will also compare these properties with those of a similar population of innate memory cells previously identified in the CD8+ T lymphocyte lineage.

Myeloid Cell-Specific IL-4 Receptor Knockout Partially Protects from Adipose Tissue Inflammation

IL-4 receptor signaling is supposed to play a major role in anti-inflammatory polarization and proliferation of adipose tissue macrophages. In this study, we examined the metabolic and inflammatory phenotype of C57BL/6J mice (IIl4ra) with LysM-dependent knockout (IIl4ra ^Δmyel) of the IL-4 receptor α-chain (IL-4Rα), the mandatory signaling component of IL-4 and IL-13, on chow and high-fat diet. Lean IIl4ra ^Δmyel mice showed decreased insulin sensitivity, no divergent adipose tissue macrophage polarization, but an increased percentage of CD8⁺ T cells in visceral adipose tissue. After 20 wk of a high-fat diet, IIl4ra ^Δmyel mice exhibited higher glucose tolerance, no changes in the lymphocyte compartment and fewer M1 macrophages in visceral adipose tissue. In vivo adipose tissue macrophage proliferation measured by BrdU incorporation was unaffected by Il4ra knockout. Interestingly, we show that IL-4Rα signaling directly augmented Itgax (Cd11c) gene expression in bone marrow-derived macrophages and increased the amount of CD11c⁺ macrophages in adipose tissue explants. Myeloid cell-specific knockout of Il4ra deteriorated insulin sensitivity in lean mice but improved parameters of glucose homeostasis and partially protected from adipose tissue inflammation in obese mice. Hence, IL-4Rα signaling probably plays a minor role in maintaining the macrophage M2 population and proliferation rates in vivo. Moreover, our data indicate that IL-4 signaling plays a proinflammatory role in adipose tissue inflammation by directly upregulating CD11c on adipose tissue macrophages.

SRSF1 plays a critical role in invariant natural killer T cell development and function

Invariant natural killer T (iNKT) cells are highly conserved innate-like T lymphocytes that originate from CD4⁺CD8⁺ double-positive (DP) thymocytes. Here, we report that serine/arginine splicing factor 1 (SRSF1) intrinsically regulates iNKT cell development by directly targeting Myb and balancing the abundance of short and long isoforms. Conditional ablation of SRSF1 in DP cells led to a substantially diminished iNKT cell pool due to defects in proliferation, survival, and TCRα rearrangement. The transition from stage 0 to stage 1 of iNKT cells was substantially blocked, and the iNKT2 subset was notably diminished in SRSF1-deficient mice. SRSF1 deficiency resulted in aberrant expression of a series of regulators that are tightly correlated with iNKT cell development and iNKT2 differentiation, including Myb, PLZF, Gata3, ICOS, and CD5. In particular, we found that SRSF1 directly binds and regulates pre-mRNA alternative splicing of Myb and that the expression of the short isoform of Myb is substantially reduced in SRSF1-deficient DP and iNKT cells. Strikingly, ectopic expression of the Myb short isoform partially rectified the defects caused by ablation of SRSF1. Furthermore, we confirmed that the SRSF1-deficient mice exhibited resistance to acute liver injury upon α-GalCer and Con A induction. Our findings thus uncovered a previously unknown role of SRSF1 as an essential post-transcriptional regulator in iNKT cell development and functional differentiation, providing new clinical insights into iNKT-correlated disease.

Chronic Porphyromonas gingivalis lipopolysaccharide induces adverse myocardial infarction wound healing through activation of CD8+ T cells

Oral and gum health have long been associated with incidence and outcomes of cardiovascular disease. Periodontal disease increases myocardial infarction (MI) mortality by sevenfold through mechanisms that are not fully understood. The goal of this study was to evaluate whether lipopolysaccharide (LPS) from a periodontal pathogen accelerates inflammation after MI through memory T-cell activation. We compared four groups [no MI, chronic LPS, day 1 after MI, and day 1 after MI with chronic LPS (LPS + MI); n = 68 mice] using the mouse heart attack research tool 1.0 database and tissue bank coupled with new analyses and experiments. LPS + MI increased total CD8+ T cells in the left ventricle versus the other groups (P < 0.05 vs. all). Memory CD8+ T cells (CD44 + CD27+) were 10-fold greater in LPS + MI than in MI alone (P = 0.02). Interleukin (IL)-4 stimulated splenic CD8+ T cells away from an effector phenotype and toward a memory phenotype, inducing secretion of factors associated with the Wnt/β-catenin signaling that promoted monocyte migration and decreased viability. To dissect the effect of CD8+ T cells after MI, we administered a major histocompatibility complex-I-blocking antibody starting 7 days before MI, which prevented effector CD8+ T-cell activation without affecting the memory response. The reduction in effector cells diminished infarct wall thinning but had no effect on macrophage numbers or MertK expression. LPS + MI + IgG attenuated macrophages within the infarct without effecting CD8+ T cells, suggesting these two processes were independent. Overall, our data indicate that effector and memory CD8+ T cells at post-MI day 1 are amplified by chronic LPS to potentially promote infarct wall thinning.NEW & NOTEWORTHY Although there is a well-documented link between periodontal disease and heart health, the mechanisms are unclear. Our study indicates that in response to circulating periodontal endotoxins, memory CD8+ T cells are activated, resulting in an acceleration of macrophage-mediated inflammation after MI. Blocking activation of effector CD8+ T cells had no effect on the macrophage numbers or wall thinning at post-MI day 1, indicating that this response was likely due in part to memory CD8+ T cells.

Signals for antigen-independent differentiation of memory CD8+ T cells

Conventional CD8+ memory T cells develop upon stimulation with foreign antigen and provide increased protection upon re-challenge. Over the past two decades, new subsets of CD8+ T cells have been identified that acquire memory features independently of antigen exposure. These antigen-inexperienced memory T cells (TAIM) are described under several names including innate memory, virtual memory, and memory phenotype. TAIM cells exhibit characteristics of conventional or true memory cells, including antigen-specific responses. In addition, they show responsiveness to innate stimuli and have been suggested to provide additional levels of protection toward infections and cancer. Here, we discuss the current understanding of TAIM cells, focusing on extrinsic and intrinsic molecular conditions that favor their development, their molecular definitions and immunological properties, as well as their transcriptional and epigenetic regulation.

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.

Virtual memory T cells orchestrate extralymphoid responses conducive to resident memory

A primary immune response is initiated in secondary lymphoid organs. Virtual memory CD8+ T (TVM) cells are antigen-inexperienced T cells of a central memory phenotype, acquired through self-antigen–driven homeostatic proliferation. Unexpectedly, we find that TVM cells are composed of CCR2+ and CCR2− subsets that differentially elaborate a spectrum of effector- and memory-poised functions directly in the tissue. During a primary influenza infection, TVM cells rapidly infiltrate the lungs in the first day after infection and promote early viral control. TVM cells that recognize viral antigen are retained in the tissue, clonally expand independent of secondary lymphoid organs, and give rise to tissue-resident memory cells. By orchestrating an extralymphoid primary response, heterogenous TVM cells bridge innate reaction and adaptive memory directly in the infected tissue.

Endogenous memory T cells with donor-reactivity: early post-transplant mediators of acute graft injury in unsensitized recipients

The pretransplant presence of endogenous donor-reactive memory T cells is an established risk factor for acute rejection and poorer transplant outcomes. A major source of these memory T cells in unsensitized recipients is heterologously generated memory T cells expressing reactivity to donor allogeneic MHC molecules. Multiple clinical studies have shown that the pretransplant presence of high numbers of circulating endogenous donor-reactive memory T cells correlates with higher incidence of acute rejection and decreased graft function during the first-year post-transplant. These findings have spurred investigation in preclinical models to better understand mechanisms underlying endogenous donor-reactive memory T-cell-mediated allograft injury in unsensitized graft recipients. These studies have led to the identification of unique mechanisms underlying the activation of these memory T cells within allografts at early times after transplant. In particular, optimal activation to mediate acute allograft injury is dependent on the intensity of ischaemia-reperfusion injury. Therapeutic strategies directed at the recruitment and activation of endogenous donor-reactive memory T cells are effective in attenuating acute injury in allografts experiencing increased ischaemia-reperfusion injury in preclinical models and should be translatable to clinical transplantation.

NR4A3 Mediates Thymic Negative Selection

Central tolerance aims to limit the production of T lymphocytes bearing TCR with high affinity for self-peptide presented by MHC molecules. The accumulation of thymocytes with such receptors is limited by negative selection or by diversion into alternative differentiation, including T regulatory cell commitment. A role for the orphan nuclear receptor NR4A3 in negative selection has been suggested, but its function in this process has never been investigated. We find that Nr4a3 transcription is upregulated in postselection double-positive thymocytes, particularly those that have received a strong selecting signal and are destined for negative selection. Indeed, we found an accumulation of cells bearing a negative selection phenotype in NR4A3-deficient mice as compared with wild-type controls, suggesting that Nr4a3 transcriptional induction is necessary to limit accumulation of self-reactive thymocytes. This is consistent with a decrease of cleaved caspase-3⁺-signaled thymocytes and more T regulatory and CD4⁺Foxp3^-HELIOS⁺ cells in the NR4A3-deficient thymus. We further tested the role for NR4A3 in negative selection by reconstituting transgenic mice expressing the OVA Ag under the control of the insulin promoter with bone marrow cells from OT-I Nr4a3 ^+/+ or OT-I Nr4a3 ^-/- mice. Accumulation of autoreactive CD8 thymocytes and autoimmune diabetes developed only in the absence of NR4A3. Overall, our results demonstrate an important role for NR4A3 in T cell development.

Allotransplantation Is Associated With Exacerbation of CD8 T-Cell Senescence: The Particular Place of the Innate CD8 T-Cell Component

Immunosenescence is a physiological process that is associated with changes in the immune system, particularly among CD8 T-cells. Recent studies have hypothesized that senescent CD8 T-cells are produced with chronologic age by chronic stimulation, leading to the acquisition of hallmarks of innate-like T-cells. While conventional CD8 T-cells are quite well characterized, CD8 T-cells sharing features of NK cells and memory CD8 T-cells, are a newly described immune cell population. They can be distinguished from conventional CD8 T-cells by their combined expression of panKIR/NKG2A and Eomesodermin (E), a unique phenotype closely associated with IFN-γ production in response to innate stimulation. Here, we first provided new evidence in favor of the innate character of panKIR/NKG2A(+) E(+) CD8 T-cells in normal subjects, documenting their position at an intermediate level in the innateness gradient in terms of both innate IFN-γ production and diminished mitochondrial mass. We also revealed that CD8 E(+) panKIR/NKG2A(+) T-cells, hereafter referred to as Innate E(+) CD8 T-cells, exhibit increased senescent (CD27(-) CD28(-)) phenotype, compared to their conventional memory counterparts. Surprisingly, this phenomenon was not dependent on age. Given that inflammation related to chronic viral infection is known to induce NK-like marker expression and a senescence phenotype among CD8 T-cells, we hypothesized that innate E(+) CD8 T-cells will be preferentially associated with exacerbated cellular senescence in response to chronic alloantigen exposure or CMV infection. Accordingly, in a pilot cohort of stable kidney allotransplant recipients, we observed an increased frequency of the Innate E(+) CD8 T-cell subset, together with an exacerbated senescent phenotype. Importantly, this phenotype cannot be explained by age alone, in clear contrast to their conventional memory counterparts. The senescent phenotype in CD8 T-cells was further increased in cytomegalovirus (CMV) positive serology transplant recipients, suggesting that transplantation and CMV, rather than aging by itself, may promote an exacerbated senescent phenotype of innate CD8 T-cells. In conclusion, we proposed that kidney transplantation, via the setting of inflammatory stimuli of alloantigen exposure and CMV infection, may exogenously age the CD8 T-cell compartment, especially its innate component. The physiopathological consequences of this change in the immune system remain to be elucidated.

Developing the right tools for the job: Lin28 regulation of early life T-cell development and function

T cells comprise a functionally heterogeneous cell population that has important roles in the immune system. While T cells are broadly considered to be a component of the antigen-specific adaptive immune response, certain T-cell subsets display innate-like effector characteristics whereas others perform immunosuppressive functions. These functionally diverse T-cell populations preferentially arise at different stages of ontogeny and are tailored to the immunological priorities of the organism over time. Many differences in early life versus adult T-cell phenotypes can be attributed to the cell-intrinsic properties of the distinct progenitors that seed the thymus throughout development. It is becoming clear that Lin28, an evolutionarily conserved, heterochronic RNA-binding protein that is differentially expressed among early life and adult hematopoietic progenitor cells, plays a substantial role in influencing early T-cell development and function. Here, we discuss the mechanisms by which Lin28 shapes the T-cell landscape to protect the developing fetus and newborn. Manipulation of the Lin28 gene regulatory network is being considered as one means of improving hematopoietic stem cell transplant outcomes; as such, understanding the impact of Lin28 on T-cell function is of clinical relevance.

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.

Targeting SLP76:ITK interaction separates GVHD from GVL in allo-HSCT

Allogeneic hematopoietic stem cell transplantation (allo-HSCT) is a curative therapy for hematological malignancies, due to graft-versus-leukemia (GVL) activity mediated by alloreactive donor T cells. However, graft-versus-host disease (GVHD) is also mediated by these cells. Here, we assessed the effect of attenuating TCR-mediated SLP76:ITK interaction in GVL vs. GVHD effects after allo-HSCT. CD8+ and CD4+ donor T cells from mice expressing a Y145F mutation in SLP-76 did not cause GVHD but preserved GVL effects against B-ALL cells. SLP76Y145FKI CD8+ and CD4+ donor T cells also showed less inflammatory cytokine production and migration to GVHD target organs. We developed a novel peptide to specifically inhibit SLP76:ITK interactions, resulting in decreased phosphorylation of PLCγ1 and ERK, decreased cytokine production in human T cells, and separation of GVHD from GVL effects. Altogether, our data suggest that inhibiting SLP76:ITK interaction could be a therapeutic strategy to separate GVHD from GVL effects after allo-HSCT treatment.

Phenotypic and Clonal Stability of Antigen-Inexperienced Memory-like T Cells across the Genetic Background, Hygienic Status, and Aging

Ag-inexperienced memory-like T (AIMT) cells are functionally unique T cells, representing one of the two largest subsets of murine CD8⁺ T cells. However, differences between laboratory inbred strains, insufficient data from germ-free mice, a complete lack of data from feral mice, and an unclear relationship between AIMT cells formation during aging represent major barriers for better understanding of their biology. We performed a thorough characterization of AIMT cells from mice of different genetic background, age, and hygienic status by flow cytometry and multiomics approaches, including analyses of gene expression, TCR repertoire, and microbial colonization. Our data showed that AIMT cells are steadily present in mice, independent of their genetic background and hygienic status. Despite differences in their gene expression profiles, young and aged AIMT cells originate from identical clones. We identified that CD122 discriminates two major subsets of AIMT cells in a strain-independent manner. Whereas thymic CD122^LOW AIMT cells (innate memory) prevail only in young animals with high thymic IL-4 production, peripheral CD122^HIGH AIMT cells (virtual memory) dominate in aged mice. Cohousing with feral mice changed the bacterial colonization of laboratory strains but had only minimal effects on the CD8⁺ T cell compartment, including AIMT cells.

Protocol for isolation of spermatids from mouse testes

Post-meiotic spermatids become spermatozoa through developmental stages during spermiogenesis. Isolation of spermatid fractions is required to examine the change of protein expression during spermiogenesis. Here, we present a simple method to isolate spermatid fractions from mouse testes using unit gravity sedimentation in a BSA density gradient. Isolation of spermatid fractions can be used to analyze changes of transcript or protein during spermiogenesis.

For complete details on the use and execution of this protocol, please refer to Kim et al. (2020).

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A protocol for isolation of post-meiotic spermatids from mouse testes

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Preparation of single-cell suspension through serial digestion of testes

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Analysis of cell fractions from unit gravity sedimentation in BSA density gradient

The thymus medulla and its control of αβT cell development

αβT cells are an essential component of effective immune responses. The heterogeneity that lies within them includes subsets that express diverse self-MHC-restricted αβT cell receptors, which can be further subdivided into CD4⁺ helper, CD8⁺ cytotoxic, and Foxp3⁺ regulatory T cells. In addition, αβT cells also include invariant natural killer T cells that are very limited in αβT cell receptor repertoire diversity and recognise non-polymorphic CD1d molecules that present lipid antigens. Importantly, all αβT cell sublineages are dependent upon the thymus as a shared site of their development. Ongoing research has examined how the thymus balances the intrathymic production of multiple αβT cell subsets to ensure correct formation and functioning of the peripheral immune system. Experiments in both wild-type and genetically modified mice have been essential in revealing complex cellular and molecular mechanisms that regulate thymus function. In particular, studies have demonstrated the diverse and critical role that the thymus medulla plays in shaping the peripheral T cell pool. In this review, we summarise current knowledge on functional properties of the thymus medulla that enable the thymus to support the production of diverse αβT cell types.

Targeting Interleukin-2-Inducible T-Cell Kinase (ITK) Differentiates GVL and GVHD in Allo-HSCT

Allogeneic hematopoietic stem cell transplantation is a potentially curative procedure for many malignant diseases. Donor T cells prevent disease recurrence via graft-versus-leukemia (GVL) effect. Donor T cells also contribute to graft-versus-host disease (GVHD), a debilitating and potentially fatal complication. Novel treatment strategies are needed which allow preservation of GVL effects without causing GVHD. Using murine models, we show that targeting IL-2-inducible T cell kinase (ITK) in donor T cells reduces GVHD while preserving GVL effects. Both CD8⁺ and CD4⁺ donor T cells from Itk^-/- mice produce less inflammatory cytokines and show decrease migration to GVHD target organs such as the liver and small intestine, while maintaining GVL efficacy against primary B-cell acute lymphoblastic leukemia (B-ALL). Itk^-/- T cells exhibit reduced expression of IRF4 and decreased JAK/STAT signaling activity but upregulating expression of Eomesodermin (Eomes) and preserve cytotoxicity, necessary for GVL effect. Transcriptome analysis indicates that ITK signaling controls chemokine receptor expression during alloactivation, which in turn affects the ability of donor T cells to migrate to GVHD target organs. Our data suggest that inhibiting ITK could be a therapeutic strategy to reduce GVHD while preserving the beneficial GVL effects following allo-HSCT treatment.

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.

Hiding in Plain Sight: Virtually Unrecognizable Memory Phenotype CD8+ T cells

Virtual memory T (T_VM) cells are a recently described population of conventional CD8⁺ T cells that, in spite of their antigen inexperience, express markers of T cell activation. T_VM cells exhibit rapid responsiveness to both antigen-specific and innate stimuli in youth but acquire intrinsic antigen-specific response defects in the elderly. In this article, we review how the identification of T_VM cells necessitates a re-evaluation of accepted paradigms for conventional memory T (T_MEM) cells, the potential for heterogeneity within the T_VM population, and the defining characteristics of T_VM cells. Further, we highlight recent literature documenting the development of T_VM cells as a distinct CD8⁺ T cell lineage as well their biological significance in the context of disease.

Quantitative Difference in PLZF Protein Expression Determines iNKT Lineage Fate and Controls Innate CD8 T Cell Generation

Zbtb16 encodes the zinc-finger protein PLZF, which is often used as a lineage marker for innate-like T cells and is specifically required for the generation of invariant natural killer T (iNKT) cells in the thymus. Here, we report that not only PLZF expression itself but also the relative abundance of PLZF proteins plays critical roles in iNKT cell development. Utilizing a Zbtb16 hypomorphic allele, PLZF^GFPCre, which produces PLZF proteins at only half of the level of the wild-type allele, we show that decreased PLZF expression results in a significant decrease in iNKT cell numbers, which is further associated with profound alterations in iNKT lineage choices and subset composition. These results document that there is a quantitative aspect of PLZF expression in iNKT cells, demonstrating that the availability of PLZF protein is a critical factor for both effective iNKT cell generation and subset differentiation.

YY1lo NKT cells are dedicated IL-10 producers

Co-expression of Yin Yang 1 (YY1) is required for the full function of the transcription factor, PLZF, which is essential for the development of natural killer T cell (NKT cell) effector functions. Discordant expression of YY1 and PLZF, therefore, might define NKT cell subsets with distinct effector functions. A subset of NKT cells was identified that expressed low levels of YY1. YY1lo NKT cells were found in all tissues, had a mature phenotype and, distinct from other NKT cells, expressed almost no ThPOK or Tbet. When activated, YY1lo NKT cells produced little IL-4 or IFN-γ. YY1lo NKT cells were found to constitutively transcribe IL-10 mRNA and, accordingly, produced IL-10 upon primary activation. Finally, we find that tumor infiltrating NKT cells are highly enriched for the YY1lo subset. Low YY1 expression, therefore, defines a previously unrecognized NKT cell subset that is committed to producing IL-10.

Innate T-αβ lymphocytes as new immunological components of anti-tumoral "off-target" effects of the tyrosine kinase inhibitor dasatinib

Kinase inhibitors hold great potential as targeted therapy against malignant cells. Among them, the tyrosine kinase inhibitor dasatinib is known for a number of clinically relevant off-target actions, attributed in part to effects on components of the immune system, especially conventional T-cells and natural killer (NK)-cells. Here, we have hypothesized that dasatinib also influences non-conventional T-αβ cell subsets known for their potential anti-tumoral properties, namely iNKT cells and the distinct new innate CD8 T-cell subset. In mice, where the two subsets were originally characterized, an activated state of iNKT cells associated with a shift toward an iNKT cell Th1-phenotype was observed after dasatinib treatment in vivo. Despite decreased frequency of the total memory CD8 T-cell compartment, the proportion of innate-memory CD8 T-cells and their IFNγ expression in response to an innate-like stimulation increased in response to dasatinib. Lastly, in patients administered with dasatinib for the treatment of BCR-ABL-positive leukemias, we provided the proof of concept that the kinase inhibitor also influences the two innate T-cell subsets in humans, as attested by their increased frequency in the peripheral blood. These data highlight the potential immunostimulatory capacity of dasatinib on innate T-αβ cells, thereby opening new opportunities for chemoimmunotherapy.

Recent advances in iNKT cell development

Recent studies suggest that murine invariant natural killer T (iNKT) cell development culminates in three terminally differentiated iNKT cell subsets denoted as NKT1, 2, and 17 cells. Although these studies corroborate the significance of the subset division model, less is known about the factors driving subset commitment in iNKT cell progenitors. In this review, we discuss the latest findings in iNKT cell development, focusing in particular on how T-cell receptor signal strength steers iNKT cell progenitors toward specific subsets and how early progenitor cells can be identified. In addition, we will discuss the essential factors for their sustenance and functionality. A picture is emerging wherein the majority of thymic iNKT cells are mature effector cells retained in the organ rather than developing precursors.

Constitutive CD8 expression drives innate CD8+ T-cell differentiation via induction of iNKT2 cells

Temporal down-regulation of the CD8 co-receptor after receiving positive-selection signals has been proposed to serve as an important determinant to segregate helper versus cytotoxic lineages by generating differences in the duration of TCR signaling between MHC-I and MHC-II selected thymocytes. By contrast, little is known about whether CD8 also modulates TCR signaling engaged by the non-classical MHC-I-like molecule, CD1d, during development of invariant natural killer T (iNKT) cells. Here, we show that constitutive transgenic CD8 expression resulted in enhanced differentiation of innate memory-like CD8⁺ thymocytes in both a cell-intrinsic and cell-extrinsic manner, the latter being accomplished by an increase in the IL-4-producing iNKT2 subset. Skewed iNKT2 differentiation requires cysteine residues in the intracellular domain of CD8α that are essential for transmitting cellular signaling. Collectively, these findings shed a new light on the relevance of CD8 down-regulation in shaping the balance of iNKT-cell subsets by modulating TCR signaling.

Distinct PLZF+CD8αα+ Unconventional T Cells Enriched in Liver Use a Cytotoxic Mechanism to Limit Autoimmunity

Hepatic immune system is uniquely challenged to mount a controlled effector response to pathogens while maintaining tolerance to diet and microbial Ags. We have identified a novel population of innate-like, unconventional CD8αα+TCRαβ+ T cells in naive mice and in human peripheral blood, called CD8αα Tunc, capable of controlling effector T cell responses. They are NK1.1+ (CD161+ in human), express NK-inhibitory receptors, and express the promyelocytic leukemia zinc finger (PLZF) transcription factor that distinguishes them from conventional CD8+ T cells. These cells display a cytotoxic phenotype and use a perforin-dependent mechanism to control Ag-induced or T cell-mediated autoimmune diseases. CD8αα Tunc are dependent upon IL-15/IL-2Rβ signaling and PLZF for their development and/or survival. They are Foxp3-negative and their regulatory activity is associated with a functionally distinct Qa-1b-dependent population coexpressing CD11c and CD244. A polyclonal TCR repertoire, an activated/memory phenotype, and the presence of CD8αα Tunc in NKT- and in MAIT-deficient as well as in germ-free mice indicates that these cells recognize diverse self-protein Ags. Our studies reveal a distinct population of unconventional CD8+ T cells within the natural immune repertoire capable of controlling autoimmunity and also providing a new target for therapeutic intervention.

Myeloid cells activate iNKT cells to produce IL-4 in the thymic medulla

Interleukin-4 (IL-4) is produced by a unique subset of invariant natural killer T (iNKT) cells (NKT2) in the thymus in the steady state, where it conditions CD8⁺ T cells to become "memory-like" among other effects. However, the signals that cause NKT2 cells to constitutively produce IL-4 remain poorly defined. Using histocytometry, we observed IL-4-producing NKT2 cells localized to the thymic medulla, suggesting that medullary signals might instruct NKT2 cells to produce IL-4. Moreover, NKT2 cells receive and require T cell receptor (TCR) stimulation for continuous IL-4 production in the steady state, since NKT2 cells lost IL-4 production when intrathymically transferred into CD1d-deficient recipients. In bone marrow chimeric recipients, only hematopoietic, not stromal, antigen-presenting cells (APCs), provided such stimulation. Furthermore, using different Cre-recombinase transgenic mouse strains to specifically target CD1d deficiency to various APCs, together with the use of diphtheria toxin receptor (DTR) transgenic mouse strains to deplete various APCs, we found that macrophages were the predominant cell to stimulate NKT2 IL-4 production. Thus, NKT2 cells appear to encounter and require different activating ligands for selection in the cortex and activation in the medulla.

Tissue-specific functions of invariant natural killer T cells

Invariant natural killer T cells (iNKT cells) are an innate-like T cell subset that expresses an invariant T cell receptor (TCR) α-chain and recognizes lipids presented on CD1d. They secrete diverse cytokines and can influence many types of immune responses. Despite having highly similar TCR specificities, iNKT cells differentiate in the thymus into distinct subsets that are analogous to T helper 1 (T_H1), T_H2 and T_H17 cell subsets. Additional iNKT cell subsets that may require peripheral activation have also been described, including one that produces IL-10. In general, iNKT cells are non-circulating, tissue-resident lymphocytes, but the prevalence of different iNKT cell subsets differs markedly between tissues. Here, we summarize the functions of iNKT cells in four tissues in which they are prevalent, namely, the liver, the lungs, adipose tissue and the intestine. Importantly, we explain how local iNKT cell responses at each site contribute to tissue homeostasis and protection from infection but can also contribute to tissue inflammation and damage.

EOMES interacts with RUNX3 and BRG1 to promote innate memory cell formation through epigenetic reprogramming

Memory CD8⁺ T cells have the ability to provide lifelong immunity against pathogens. Although memory features generally arise after challenge with a foreign antigen, naïve CD8 single positive (SP) thymocytes may acquire phenotypic and functional characteristics of memory cells in response to cytokines such as interleukin-4. This process is associated with the induction of the T-box transcription factor Eomesodermin (EOMES). However, the underlying molecular mechanisms remain ill-defined. Using epigenomic profiling, we show that these innate memory CD8SP cells acquire only a portion of the active enhancer repertoire of conventional memory cells. This reprograming is secondary to EOMES recruitment, mostly to RUNX3-bound enhancers. Furthermore, EOMES is found within chromatin-associated complexes containing BRG1 and promotes the recruitment of this chromatin remodelling factor. Also, the in vivo acquisition of EOMES-dependent program is BRG1-dependent. In conclusion, our results support a strong epigenetic basis for the EOMES-driven establishment of CD8⁺ T cell innate memory program.

TRAF3IP3 at the trans-Golgi network regulates NKT2 maturation via the MEK/ERK signaling pathway

Thymic natural killer T (NKT)2 cells are a subset of invariant NKT cells with PLZF^hiGATA3^hiIL-4⁺. The differentiation of NKT2 cells is not fully understood. In the present study, we report an important role of TRAF3-interacting protein 3 (TRAF3IP3) in the functional maturation and expansion of committed NKT2s in thymic medulla. Mice with T-cell-specific deletion of TRAF3IP3 had decreased thymic NKT2 cells, decreased IL-4-producing peripheral iNKTs, and defects in response to α-galactosylceramide. Positive selection and high PLZF expression in CD24⁺CD44^- and CCR7⁺CD44^- immature iNKTs were not affected. Only CD44^hiNK1.1^- iNKTs in Traf3ip3^-/- mice showed reduced expression of Egr2, PLZF, and IL-17RB, decreased proliferation, and reduced IL-4 production upon stimulation. This Egr2 and IL-4 expression was augmented by MEK1/ERK activation in iNKTs, and TRAF3IP3 at the trans-Golgi network recruited MEK1 and facilitated ERK phosphorylation and nuclear translocation. LTβR-regulated bone marrow-derived nonlymphoid cells in the medullary thymic microenvironment were required for MEK/ERK activation and NKT2 maturation. These data demonstrate an important functional maturation process in NKT2 differentiation that is regulated by MEK/ERK signaling at the trans-Golgi network.