Human CD4+ memory phenotype T cells use mitochondrial metabolism to generate sensitive IFN-γ responses

The transition of naive T lymphocytes into antigenically activated effector cells is associated with a metabolic shift from oxidative phosphorylation to aerobic glycolysis. This shift facilitates production of the key anti-tumor cytokine interferon (IFN)-γ; however, an associated loss of mitochondrial efficiency in effector T cells ultimately limits anti-tumor immunity. Memory phenotype (MP) T cells are a newly recognized subset that arises through homeostatic activation signals following hematopoietic transplantation. We show here that human CD4+ MP cell differentiation is associated with increased glycolytic and oxidative metabolic activity, but MP cells retain less compromised mitochondria compared to effector CD4+ T cells, and their IFN-γ response is less dependent on glucose and more reliant on glutamine. MP cells also produced IFN-γ more efficiently in response to weak T cell receptor (TCR) agonism than effectors and mediated stronger responses to transformed B cells. MP cells may thus be particularly well suited to carry out sustained immunosurveillance against neoplastic cells.

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

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

Analysis of Butyrophilin-Mediated Activation of γδ T Cells from Human Spleen

There is considerable interest in therapeutically engaging human γδ T cells. However, due to the unique TCRs of human γδ T cells, studies from animal models have provided limited directly applicable insights, and human γδ T cells from key immunological tissues remain poorly characterized. In this study, we investigated γδ T cells from human spleen tissue. Compared to blood, where Vδ2+Vγ9+ T cells are the dominant subset, splenic γδ T cells included a variety of TCR types, with Vδ1+ T cells typically being the most frequent. Intracellular cytokine staining revealed that IFN-γ was produced by a substantial fraction of splenic γδ T cells, IL-17A by a small fraction, and IL-4 was minimal. Primary splenic γδ T cells frequently expressed NKG2D (NK group 2 member D) and CD16, whereas expression of DNAM-1 (DNAX accessory molecule 1), CD28, PD-1, TIGIT, and CD94 varied according to subset, and there was generally little expression of natural cytotoxicity receptors, TIM-3, LAG-3, or killer Ig-like receptors. In vitro expansion was associated with marked changes in expression of these activating and inhibitory receptors. Analysis of functional responses of spleen-derived Vδ2+Vγ9+, Vδ1+Vγ9+, and Vδ1+Vγ9- T cell lines to recombinant butyrophilin BTN2A1 and BTN3A1 demonstrated that both Vδ2+Vγ9+ and Vδ1+Vγ9+ T cells were capable of responding to the extracellular domain of BTN2A1, whereas the addition of BTN3A1 only markedly enhanced the responses of Vδ2+Vγ9+ T cells. Conversely, Vδ1+Vγ9+ T cells appeared more responsive than Vδ2+Vγ9+ T cells to TCR-independent NKG2D stimulation. Thus, despite shared recognition of BTN2A1, differential effects of BTN3A1 and coreceptors may segregate target cell responses of Vδ2+Vγ9+ and Vδ1+Vγ9+ T cells.

Inflammatory CD4/CD8 double-positive human T cells arise from reactive CD8 T cells and are sufficient to mediate GVHD pathology

An important paradigm in allogeneic hematopoietic cell transplantations (allo-HCTs) is the prevention of graft-versus-host disease (GVHD) while preserving the graft-versus-leukemia (GVL) activity of donor T cells. From an observational clinical study of adult allo-HCT recipients, we identified a CD4+/CD8+ double-positive T cell (DPT) population, not present in starting grafts, whose presence was predictive of ≥ grade 2 GVHD. Using an established xenogeneic transplant model, we reveal that the DPT population develops from antigen-stimulated CD8 T cells, which become transcriptionally, metabolically, and phenotypically distinct from single-positive CD4 and CD8 T cells. Isolated DPTs were sufficient to mediate xeno-GVHD pathology when retransplanted into naïve mice but provided no survival benefit when mice were challenged with a human B-ALL cell line. Overall, this study reveals human DPTs as a T cell population directly involved with GVHD pathology.

SEC is an antiangiogenic virulence factor that promotes Staphylococcus aureus endocarditis independent of superantigen activity

The superantigen staphylococcal enterotoxin C (SEC) is critical for Staphylococcus aureus infective endocarditis (SAIE) in rabbits. Superantigenicity, its hallmark function, was proposed to be a major underlying mechanism driving SAIE but was not directly tested. With the use of S. aureus MW2 expressing SEC toxoids, we show that superantigenicity does not sufficiently account for vegetation growth, myocardial inflammation, and acute kidney injury in the rabbit model of native valve SAIE. These results highlight the critical contribution of an alternative function of superantigens to SAIE. In support of this, we provide evidence that SEC exerts antiangiogenic effects by inhibiting branching microvessel formation in an ex vivo rabbit aortic ring model and by inhibiting endothelial cell expression of one of the most potent mediators of angiogenesis, VEGF-A. SEC's ability to interfere with tissue revascularization and remodeling after injury serves as a mechanism to promote SAIE and its life-threatening systemic pathologies.

Harnessing invariant natural killer T cells to control pathological inflammation

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

iNKT cells coordinate immune pathways to enable engraftment in nonconditioned hosts

Invariant natural killer T (iNKT) cells are a conserved population of innate T lymphocytes that interact with key antigen-presenting cells to modulate adaptive T-cell responses in ways that can either promote protective immunity, or limit pathological immune activation. Understanding the immunological networks engaged by iNKT cells to mediate these opposing functions is a key pre-requisite to effectively using iNKT cells for therapeutic applications. Using a human umbilical cord blood xenotransplantation model, we show here that co-transplanted allogeneic CD4+ iNKT cells interact with monocytes and T cells in the graft to coordinate pro-hematopoietic and immunoregulatory pathways. The nexus of iNKT cells, monocytes, and cord blood T cells led to the release of cytokines (IL-3, GM-CSF) that enhance hematopoietic stem and progenitor cell activity, and concurrently induced PGE2-mediated suppression of T-cell inflammatory responses that limit hematopoietic stem and progenitor cell engraftment. This resulted in successful long-term hematopoietic engraftment without pretransplant conditioning, including multi-lineage human chimerism and colonization of the spleen by antibody-producing human B cells. These results highlight the potential for using iNKT cellular immunotherapy to improve rates of hematopoietic engraftment independently of pretransplant conditioning.

Identification and function of a CD4+/CD8αβ+ T cell population that is predictive of GVHD development in a xenogeneic transplant model

Graft-vs-host disease (GVHD) is mediated by donor reactive T cells that have a hierarchical classification based on CD4 and CD8 expression. While CD4 and CD8 lineages are thought to have fixed expression, CD4+/CD8αβ+ double positive (DP) T cells have been reported in cases of human cancers and autoimmune diseases though the lack of a suitable model system has hindered their research. In this study, we transplanted primary human graft tissue into non-conditioned immunodeficient mice and observed the development of a human DP T cell population that was not present in the starting grafts. This DP T cell population developed irrespective of graft tissue (peripheral blood, bone marrow or umbilical cord blood), accessory cells (transplantation with isolated T cells) and immunodeficient mouse strain (NSG and NBSGW). Furthermore, an increase in the percentage of DP T cells in the blood of these mice is correlated and predictive of GVHD development. We also observed that DP T cells are functionally active with significantly elevated IFNγ and TNFα secretion compared to CD4 and CD8 single positive T cells. DP T cell also display elements of the cytotoxic machinery including NKG2D and perforin/granzyme expression. Interestingly, transplantation of isolated CD4+ cells did not result in the development of DP T cells while a robust population developed after transplantation of isolated CD8+ T cells. DP T cells were also identified in primary clinical samples taken from HSCT patients with their clinical relevance to GVHD currently under investigation. In conclusion, this ongoing study has identified a novel human DP T cell that arises from the CD8+ T cell population, is functional active and is predictive of GVHD in a xenogeneic transplant model.

Different Human Immune Lineage Compositions Are Generated in Non-Conditioned NBSGW Mice Depending on HSPC Source

NBSGW mice are highly immunodeficient and carry a hypomorphic mutation in the c-kit gene, providing a host environment that supports robust human hematopoietic expansion without pre-conditioning. These mice thus provide a model to investigate human hematopoietic engraftment in the absence of conditioning-associated damage. We compared transplantation of human CD34⁺ HSPCs purified from three different sources: umbilical cord blood, adult bone marrow, and adult G-CSF mobilized peripheral blood. HSPCs from mobilized peripheral blood were significantly more efficient (as a function of starting HSPC dose) than either cord blood or bone marrow HSPCs at generating high levels of human chimerism in the murine blood and bone marrow by 12 weeks post-transplantation. While T cells do not develop in this model due to thymic atrophy, all three HSPC sources generated a human compartment that included B lymphocytic, myeloid, and granulocytic lineages. However, the proportions of these lineages varied significantly according to HSPC source. Mobilized blood HSPCs produced a strikingly higher proportion of granulocyte lineage cells (~35% as compared to ~5%), whereas bone marrow HSPC output was dominated by B lymphocytic cells, and cord blood HSPC output was enriched for myeloid lineages. Following transplantation, all three HSPC sources showed a shift in the CD34⁺ subset towards CD45RA⁺ progenitors along with a complete loss of the CD45RA^-CD49f⁺ long-term HSC subpopulation, suggesting this model promotes mainly short-term HSC activity. Mice transplanted with cord blood HSPCs maintained a diversified human immune compartment for at least 36 weeks after the primary transplant, although mice given adult bone marrow HSPCs had lost diversity and contained only myeloid cells by this time point. Finally, to assess the impact of non-HSPCs on transplantation outcome, we also tested mice transplanted with total or T cell-depleted adult bone marrow mononuclear cells. Total bone marrow mononuclear cell transplants produced significantly lower human chimerism compared to purified HSPCs, and T-depletion rescued B cell levels but not other lineages. Together these results reveal marked differences in engraftment efficiency and lineage commitment according to HSPC source and suggest that T cells and other non-HSPC populations affect lineage output even in the absence of conditioning-associated inflammation.

Early T Cell Activation Metrics Predict Graft-versus-Host Disease in a Humanized Mouse Model of Hematopoietic Stem Cell Transplantation

Acute graft-versus-host disease (GVHD) is a frequent complication of hematopoietic transplantation, yet patient risk stratification remains difficult, and prognostic biomarkers to guide early clinical interventions are lacking. We developed an approach to evaluate the potential of human T cells from hematopoietic grafts to produce GVHD. Nonconditioned NBSGW mice transplanted with titrated doses of human bone marrow developed GVHD that was characterized by widespread lymphocyte infiltration and organ pathology. Interestingly, GVHD was not an inevitable outcome in our system and was influenced by transplant dose, inflammatory status of the host, and type of graft. Mice that went on to develop GVHD showed signs of rapid proliferation in the human T cell population during the first 1-3 wk posttransplant and had elevated human IFN-γ in plasma that correlated negatively with the expansion of the human hematopoietic compartment. Furthermore, these early T cell activation metrics were predictive of GVHD onset 3-6 wk before phenotypic pathology. These results reveal an early window of susceptibility for pathological T cell activation following hematopoietic transplantation that is not simply determined by transient inflammation resulting from conditioning-associated damage and show that T cell parameters during this window can serve as prognostic biomarkers for risk of later GVHD development.

An EBNA3A-Mutated Epstein-Barr Virus Retains the Capacity for Lymphomagenesis in a Cord Blood-Humanized Mouse Model

Epstein-Barr virus (EBV) causes B cell lymphomas and transforms B cells in vitro The EBV protein EBNA3A collaborates with EBNA3C to repress p16 expression and is required for efficient transformation in vitro An EBNA3A deletion mutant EBV strain was recently reported to establish latency in humanized mice but not cause tumors. Here, we compare the phenotypes of an EBNA3A mutant EBV (Δ3A) and wild-type (WT) EBV in a cord blood-humanized (CBH) mouse model. The hypomorphic Δ3A mutant, in which a stop codon is inserted downstream from the first ATG and the open reading frame is disrupted by a 1-bp insertion, expresses very small amounts of EBNA3A using an alternative ATG at residue 15. Δ3A caused B cell lymphomas at rates similar to their induction by WT EBV but with delayed onset. Δ3A and WT tumors expressed equivalent levels of EBNA2 and p16, but Δ3A tumors in some cases had reduced LMP1. Like the WT EBV tumors, Δ3A lymphomas were oligoclonal/monoclonal, with typically one dominant IGHV gene being expressed. Transcriptome sequencing (RNA-seq) analysis revealed small but consistent gene expression differences involving multiple cellular genes in the WT EBV- versus Δ3A-infected tumors and increased expression of genes associated with T cells, suggesting increased T cell infiltration of tumors. Consistent with an impact of EBNA3A on immune function, we found that the expression of CLEC2D, a receptor that has previously been shown to influence responses of T and NK cells, was markedly diminished in cells infected with EBNA3A mutant virus. Together, these studies suggest that EBNA3A contributes to efficient EBV-induced lymphomagenesis in CBH mice.IMPORTANCE The EBV protein EBNA3A is expressed in latently infected B cells and is important for efficient EBV-induced transformation of B cells in vitro In this study, we used a cord blood-humanized mouse model to compare the phenotypes of an EBNA3A hypomorph mutant virus (Δ3A) and wild-type EBV. The Δ3A virus caused lymphomas with delayed onset compared to the onset of those caused by WT EBV, although the tumors occurred at a similar rate. The WT EBV and EBNA3A mutant tumors expressed similar levels of the EBV protein EBNA2 and cellular protein p16, but in some cases, Δ3A tumors had less LMP1. Our analysis suggested that Δ3A-infected tumors have elevated T cell infiltrates and decreased expression of the CLEC2D receptor, which may point to potential novel roles of EBNA3A in T cell and NK cell responses to EBV-infected tumors.

Loss of Chondroitin Sulfate Modification Causes Inflammation and Neurodegeneration in skt Mice

One major aspect of the aging process is the onset of chronic, low-grade inflammation that is highly associated with age-related diseases. The molecular mechanisms that regulate these processes have not been fully elucidated. We have identified a spontaneous mutant mouse line, small with kinky tail (skt), that exhibits accelerated aging and age-related disease phenotypes including increased inflammation in the brain and retina, enhanced age-dependent retinal abnormalities including photoreceptor cell degeneration, neurodegeneration in the hippocampus, and reduced lifespan. By positional cloning, we identified a deletion in chondroitin sulfate synthase 1 (Chsy1) that is responsible for these phenotypes in skt mice. CHSY1 is a member of the chondroitin N-acetylgalactosaminyltransferase family that plays critical roles in the biosynthesis of chondroitin sulfate, a glycosaminoglycan (GAG) that is attached to the core protein to form the chondroitin sulfate proteoglycan (CSPG). Consistent with this function, the Chsy1 mutation dramatically decreases chondroitin sulfate GAGs in the retina and hippocampus. In addition, macrophage and neutrophil populations appear significantly altered in the bone marrow and spleen of skt mice, suggesting an important role for CHSY1 in the functioning of these immune cell types. Thus, our study reveals a previously unidentified impact of CHSY1 in the retina and hippocampus. Specifically, chondroitin sulfate (CS) modification of proteins by CHSY1 appears critical for proper regulation of immune cells of the myeloid lineage and for maintaining the integrity of neuronal tissues, since a defect in this gene results in increased inflammation and abnormal phenotypes associated with age-related diseases.

Modeling graft-vs-host disease in humanized mice using different hematopoietic stem cells sources reveals inherent differences in Tc and myeloid cell biology

Cancers and autoimmune diseases, which affect 1.6% and 4.9% of the U.S. population respectively, are often treated with chemotherapy and immunosuppressive drugs with variable response rates. One alternative treatment option are hematopoietic stem cell (HSC) transplantations. Unfortunately, the biology behind these transplants remains unclear despite being in practice for over 60 years. Our research aims to use a humanized mouse model to understand the different molecular mechanisms that lead to acute GVHD, hematopoietic recovery/rejection and GVL. Here, we will focus on our data recapitulating clinical GVHD in different graft sources. Injecting equivalent numbers (2E7) of bone marrow (BM) mononuclear cells and G-CSF mobilized (MB) mononuclear cells into non-irradiated NSG mice results in 50% & 0% penetrance of GVHD respectively at 12 weeks. Concurrently, we have measured the Tc proliferation and IFNγ secretion in these mice over the course of 12 weeks that reveals an elevated activation status in BM Tc’s from an early stage. Interestingly, stimulating the Tc’s directly in vitro results in equivalent activation potentials suggesting a prominent role of myeloid cells in the induction of GVHD and not Tc’s. The following investigations have shown an increased expression of the B7 complex on BM myeloid cells and an increase in their IL-6 secretion. These early observations suggest that the “accessory cells” injected from the different HSC grafts are vastly important for driving clinical outcomes. Understanding the molecular cross-talk between the “accessory” cells and the underlying cellular differences attributed to each HSC source will better inform clinicians on the best treatment practices for patients.

Mucosal associated invariant T cells from human breast ducts mediate a Th17-skewed response to bacterially exposed breast carcinoma cells

BACKGROUND

Antimicrobial T cells play key roles in the disease progression of cancers arising in mucosal epithelial tissues, such as the colon. However, little is known about microbe-reactive T cells within human breast ducts and whether these impact breast carcinogenesis.

METHODS

Epithelial ducts were isolated from primary human breast tissue samples, and the associated T lymphocytes were characterized using flow cytometric analysis. Functional assays were performed to determine T-cell cytokine secretion in response to bacterially treated human breast carcinoma cells.

RESULTS

We show that human breast epithelial ducts contain mucosal associated invariant T (MAIT) cells, an innate T-cell population that recognizes specific bacterial metabolites presented by nonclassical MR1 antigen-presenting molecules. The MAIT cell population from breast ducts resembled that of peripheral blood in its innate lymphocyte phenotype (i.e., CD161, PLZF, and interleukin [IL]-18 receptor coexpression), but the breast duct MAIT cell population had a distinct T-cell receptor Vβ use profile and was markedly enriched for IL-17-producing cells compared with blood MAIT cells. Breast carcinoma cells that had been exposed to Escherichia coli activated MAIT cells in an MR1-dependent manner. However, whereas phorbol 12-myristate 13-acetate/ionomycin stimulation induced the production of both interferon-γ and IL-17 by breast duct MAIT cells, bacterially exposed breast carcinoma cells elicited a strongly IL-17-biased response. Breast carcinoma cells also showed upregulated expression of natural killer group 2 member D (NKG2D) ligands compared with primary breast epithelial cells, and the NKG2D receptor contributed to MAIT cell activation by the carcinoma cells.

CONCLUSIONS

These results demonstrate that MAIT cells from human breast ducts mediate a selective T-helper 17 cell response to human breast carcinoma cells that were exposed to E. coli. Thus, cues from the breast microbiome and the expression of stress-associated ligands by neoplastic breast duct epithelial cells may shape MAIT cell responses during breast carcinogenesis.

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

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

Modeling Human Antitumor Responses In Vivo Using Umbilical Cord Blood-Engrafted Mice

Mice engrafted with human immune cells offer powerful in vivo model systems to investigate molecular and cellular processes of tumorigenesis, as well as to test therapeutic approaches to treat the resulting cancer. The use of umbilical cord blood mononuclear cells as a source of human immune cells for engraftment is technically straightforward, and provides T lymphocytes and autologous antigen-presenting cells (including B cells, monocytes, and DCs) that bear cognate antigen presenting molecules. By using a human-specific oncogenic virus, such as Epstein-Barr virus, de novo neoplastic transformation of the human B cells can be induced in vivo in a manner that models progressive stages of tumorigenesis from nascent neoplasia to the establishment of vascularized tumor masses with an immunosuppressive environment. Moreover, since tumorigenesis occurs in the presence of autologous T cells, this type of system can be used to investigate how T cells become suppressed during tumorigenesis, and how immunotherapies counteract immunosuppression. This minireview will provide a brief overview of the use of human umbilical cord blood transplanted into immunodeficient murine hosts to model antitumor responses.

Adoptively transferred Vγ9Vδ2 T cells show potent antitumor effects in a preclinical B cell lymphomagenesis model

A central issue for adoptive cellular immunotherapy is overcoming immunosuppressive signals to achieve tumor clearance. While γδ T cells are known to be potent cytolytic effectors that can kill a variety of cancers, it is not clear whether they are inhibited by suppressive ligands expressed in tumor microenvironments. Here, we have used a powerful preclinical model where EBV infection drives the de novo generation of human B cell lymphomas in vivo, and autologous T lymphocytes are held in check by PD-1/CTLA-4-mediated inhibition. We show that a single dose of adoptively transferred Vδ2+ T cells has potent antitumor effects, even in the absence of checkpoint blockade or activating compounds. Vδ2+ T cell immunotherapy given within the first 5 days of EBV infection almost completely prevented the outgrowth of tumors. Vδ2+ T cell immunotherapy given more than 3 weeks after infection (after neoplastic transformation is evident) resulted in a dramatic reduction in tumor burden. The immunotherapeutic Vδ2+ T cells maintained low cell surface expression of PD-1 in vivo, and their recruitment to tumors was followed by a decrease in B cells expressing PD-L1 and PD-L2 inhibitory ligands. These results suggest that adoptively transferred PD-1lo Vδ2+ T cells circumvent the tumor checkpoint environment in vivo.

Human Invariant NKT Cells Induce IL-1β Secretion by Peripheral Blood Monocytes via a P2X7-Independent Pathway

The cytokine IL-1β plays a central role in inflammatory responses that are initiated by microbial challenges, as well as in those that are due to endogenous processes (often called sterile inflammation). IL-1β secretion that occurs independently of microbial stimulation is typically associated with the presence of endogenous alarmins, such as extracellular ATP (an indicator of cytopathic damage). In this study, we show that IL-2-activated human invariant NKT (iNKT) cells stimulate the secretion of IL-1β protein by human peripheral blood monocytes in a manner that requires neither the presence of microbial compounds nor signaling through the extracellular ATP receptor P2X7 Monocyte IL-1β production was specifically induced by iNKT cells, because similarly activated polyclonal autologous T cells did not have this effect. Secretion of IL-1β protein occurred rapidly (within 3-4 h) and required cell contact between the iNKT cells and monocytes. Similar to IL-1β production induced by TLR stimulation, the iNKT-induced pathway appeared to entail a two-step process involving NF-κB signaling and IL1B gene transcription, as well as assembly of the NLRP3 inflammasome and activation of caspase-1. However, in contrast to the classical inflammasome-mediated pathway of IL-1β production, activation of monocytes via P2X7 was dispensable for iNKT-induced IL-1β secretion, and potassium efflux was not required. Moreover, the iNKT-induced effect involved caspase-8 activity, yet it induced little monocyte death. These results suggest that IL-2-activated human iNKT cells induce monocytes to produce IL-1β through a distinctive pathway that does not require the presence of microbial danger signals or alarmins associated with cytopathic damage.

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

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

Analysis of Immune Cells from Human Mammary Ductal Epithelial Organoids Reveals Vδ2+ T Cells That Efficiently Target Breast Carcinoma Cells in the Presence of Bisphosphonate

Developing strategies to enhance cancer prevention is a paramount goal, particularly given recent concerns about surgical treatment of preinvasive states such as ductal carcinoma in situ. Promoting effective immunosurveillance by leukocytes that scan for nascent neoplastic transformations represents a potential means to achieve this goal. Because most breast cancers arise within the ductal epithelium, enhancing protective immunosurveillance will likely necessitate targeting one or more of the distinctive lymphocyte types found in these sites under normal conditions. Here, we have characterized the intraepithelial lymphocyte compartment of non-cancerous human breast tissue and identified a subset of T lymphocytes that can be pharmacologically targeted to enhance their responses to breast cancer cells. Specifically, Vδ2(+) γδ T cells were consistently present in preparations of mammary ductal epithelial organoids and they proliferated in response to zoledronic acid, an aminobisphosphonate drug. Vδ2(+) T cells from breast ductal organoids produced the antitumor cytokine IFNγ and efficiently killed bisphosphonate-pulsed breast carcinoma cells. These findings demonstrate the potential for exploiting the ability of Vδ2(+) γδ T cells to respond to FDA-approved bisphosphonate drugs as a novel immunotherapeutic approach to inhibit the outgrowth of breast cancers.

Transcriptional regulation of the human TNFSF11 gene in T cells via a cell type-selective set of distal enhancers

In addition to osteoblast lineage cells, the TNF-like factor receptor activator of NF-κB ligand (RANKL) is expressed in both B and T cells and may play a role in bone resorption. Rankl gene (Tnfsf11) expression in mouse T cells is mediated through multiple distal elements marked by increased transcription factor occupancy, histone tail acetylation, and RNA polymerase II recruitment. Little is known, however, of the regulation of human TNFSF11 in T cells. Accordingly, we examined the consequence of T cell activation on the expression of this factor both in Jurkat cells and in primary human T cells. We then explored the mechanism of this regulation by scanning over 400 kb of DNA surrounding the TNFSF11 locus for regulatory enhancers using ChIP-chip analysis. Histone H3/H4 acetylation enrichment identified putative regulatory regions located between -170 and -220 kb upstream of the human TNFSF11 TSS that we designated the human T cell control region (hTCCR). This region showed high sequence conservation with the mouse TCCR. Inhibition of MEK1/2 by U0126 resulted in decreased RANKL expression suggesting that stimulation through MEK1/2 was a prerequisite. ChIP-chip analysis also revealed that c-FOS was recruited to the hTCCR as well. Importantly, both the human TNFSF11 D5a/b (RLD5a/b) enhancer and segments of the hTCCR mediated robust inducible reporter activity following TCR activation. Finally, SNPs implicated in diseases characterized by dysregulated BMD co-localized to the hTCCR region. We conclude that the hTCCR region contains a cell-selective set of enhancers that plays an integral role in the transcriptional regulation of the TNFSF11 gene in human T cells.

LMP1-deficient Epstein-Barr virus mutant requires T cells for lymphomagenesis

Epstein-Barr virus (EBV) infection transforms B cells in vitro and is associated with human B cell lymphomas. The major EBV oncoprotein, latent membrane protein 1 (LMP1), mimics constitutively active CD40 and is essential for outgrowth of EBV-transformed B cells in vitro; however, EBV-positive diffuse large B cell lymphomas and Burkitt lymphomas often express little or no LMP1. Thus, EBV may contribute to the development and maintenance of human lymphomas even in the absence of LMP1. Here, we found that i.p. injection of human cord blood mononuclear cells infected with a LMP1-deficient EBV into immunodeficient mice induces B cell lymphomas. In this model, lymphoma development required the presence of CD4+ T cells in cord blood and was inhibited by CD40-blocking Abs. In contrast, LMP1-deficient EBV established persistent latency but did not induce lymphomas when directly injected into mice engrafted with human fetal CD34+ cells and human thymus. WT EBV induced lymphomas in both mouse models and did not require coinjected T cells in the cord blood model. Together, these results demonstrate that LMP1 is not essential for EBV-induced lymphomas in vivo and suggest that T cells supply signals that substitute for LMP1 in EBV-positive B cell lymphomagenesis.

Mice engrafted with human fetal thymic tissue and hematopoietic stem cells develop pathology resembling chronic graft-versus-host disease

Chronic graft-versus-host disease (cGVHD) is a significant roadblock to long-term hematopoietic stem cell (HSC) transplantation success. Effective treatments for cGVHD have been difficult to develop, in part because of a paucity of animal models that recapitulate the multiorgan pathologies observed in clinical cGVHD. Here we present an analysis of the pathology that occurs in immunodeficient mice engrafted with human fetal HSCs and implanted with fragments of human fetal thymus and liver. Starting at time points generally later than 100 days post-transplantation, the mice developed signs of illness, including multiorgan cellular infiltrates containing human T cells, B cells, and macrophages; fibrosis in sites such as lungs and liver; and thickened skin with alopecia. Experimental manipulations that delayed or reduced the efficiency of the HSC engraftment did not affect the timing or progression of disease manifestations, suggesting that pathology in this model is driven more by factors associated with the engrafted human thymic organoid. Disease progression was typically accompanied by extensive fibrosis and degradation of the thymic organoid, and there was an inverse correlation of disease severity with the frequency of FoxP3(+) thymocytes. Hence, the human thymic tissue may contribute T cells with pathogenic potential, but the generation of regulatory T cells in the thymic organoid may help to control these cells before pathology resembling cGVHD eventually develops. This model thus provides a new system to investigate disease pathophysiology relating to human thymic events and to evaluate treatment strategies to combat multiorgan fibrotic pathology produced by human immune cells.

Expression of CD1c enhances human invariant NKT cell activation by α-GalCer

Invariant natural killer T (iNKT) cells are innate T lymphocytes that specifically recognize α-linked glycosphingolipids (α-GSLs) as antigens presented by CD1d molecules. Activating iNKT cells by administering α-GSLs improves disease outcomes in murine cancer models and, thus, there is great interest in the clinical potential of these lipids for treating human cancers. However, humans possess several other CD1 isoforms that are not present in mice and it is not clear whether these CD1 molecules, which also bind lipids, affect human iNKT cell responses. We demonstrate here that CD1c, which is co-expressed with CD1d on blood dendritic cells and on a fraction of B cells, is able to present α-galactosylceramide (α-GalCer) as a weak agonist to human iNKT cells, and that the presence of CD1c synergistically enhances α-GalCerdependent activation of iNKT cells by CD1d. Primary human B cells expressing CD1c induced stronger iNKT cell responses to α-GalCer than the CD1c- subset, and an antibody against CD1c inhibited iNKT cell cytokine secretion. These results suggest that therapeutic activation of human iNKT cells by α-GSLs will be driven preferentially by CD1c+ cell types. Thus, B cell neoplasias that co-express CD1c and CD1d may be particularly susceptible to α-GSL therapy, and cancer vaccines using α-GSLs as adjuvants may be most effective when presented by CD1c+ antigen-presenting cells.

Human invariant natural killer T cells acquire transient innate responsiveness via histone H4 acetylation induced by weak TCR stimulation

Weak TCR stimulation of iNKT cells, such as that resulting from self-antigen recognition, induces histone modifications at the IFNG locus that allow the iNKT cells to subsequently produce IFN-γ in response to proinflammatory cytokines alone.

Invariant NKT cells (iNKT cells) are innate T lymphocytes that are thought to play an important role in producing an early burst of IFN-γ that promotes successful tumor immunosurveillance and antimicrobial immunity. The cellular activation processes underlying innate IFN-γ production remain poorly understood. We show here that weak T cell receptor (TCR) stimulation that does not directly activate iNKT cell IFN-γ messenger RNA transcription nevertheless induces histone H4 acetylation at specific regions near the IFNG gene locus. This renders the iNKT cells able to produce IFN-γ in an innate manner (i.e., not requiring concurrent TCR stimulation) upon exposure to IL-12 and IL-18. The iNKT cells retain the capacity for innate activation for hours to days after the initial weak TCR stimulation, although their innate responsiveness gradually declines as a function of histone deacetylation. These results explain how iNKT cells are able to mediate rapid innate IFN-γ secretion in a manner that does not require them to undergo permanent T_H1 differentiation. Moreover, our results also indicate that iNKT cell motility is maintained during activation by IL-12 and IL-18. Therefore, iNKT cells activated through this pathway can continue to migrate and may thus disseminate the IFN-γ that they produce, which may amplify its impact.

A live imaging cell motility screen identifies prostaglandin E2 as a T cell stop signal antagonist

The T cell migration stop signal is a central step in T cell activation and inflammation; however, its regulatory mechanisms remain largely unknown. Using a live-cell, imaging-based, high-throughput screen, we identified the PG, PGE(2), as a T cell stop signal antagonist. Src kinase inhibitors, microtubule inhibitors, and PGE(2) prevented the T cell stop signal, and impaired T cell-APC conjugation and T cell proliferation induced by primary human allogeneic dendritic cells. However, Src inhibition, but not PGE(2) or microtubule inhibition, impaired TCR-induced ZAP-70 signaling, demonstrating that T cell stop signal antagonists can function either upstream or downstream of proximal TCR signaling. Moreover, we found that PGE(2) abrogated TCR-induced activation of the small GTPase Rap1, suggesting that PGE(2) may modulate T cell adhesion and stopping through Rap1. These results identify a novel role for PGs in preventing T cell stop signals and limiting T cell activation induced by dendritic cells.

Analysis of the CD1 antigen presenting system in humanized SCID mice

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

The 2.5 Å structure of CD1c in complex with a mycobacterial lipid reveals an open groove ideally suited for diverse antigen presentation

CD1 molecules function to present lipid-based antigens to T cells. Here we present the crystal structure of CD1c at 2.5 Å resolution, in complex with the pathogenic Mycobacterium tuberculosis antigen mannosyl-β1-phosphomycoketide (MPM). CD1c accommodated MPM's methylated alkyl chain exclusively in the A' pocket, aided by a unique exit portal underneath the α1 helix. Most striking was an open F' pocket architecture lacking the closed cavity structure of other CD1 molecules, reminiscent of peptide binding grooves of classical major histocompatibility complex molecules. This feature, combined with tryptophan-fluorescence quenching during loading of a dodecameric lipopeptide antigen, provides a compelling model by which both the lipid and peptide moieties of the lipopeptide are involved in CD1c presentation of lipopeptides.

Human NKT cells direct the differentiation of myeloid APCs that regulate T cell responses via expression of programmed cell death ligands

NKT cells are innate lymphocytes that can recognize self or foreign lipids presented by CD1d molecules. NKT cells have been shown to inhibit the development of autoimmunity in murine model systems, however, the pathways by which they foster immune tolerance remain poorly understood. Here we show that autoreactive human NKT cells stimulate monocytes to differentiate into myeloid APCs that have a regulatory phenotype characterized by poor conjugate formation with T cells. The NKT cell instructed myeloid APCs show elevated expression of the inhibitory ligand PD-L2, and blocking PD-L1 and PD-L2 during interactions of the APCs with T cells results in improved cluster formation and significantly increased T cell proliferative responses. The elevated expression of PD-L molecules on NKT-instructed APCs appears to result from exposure to extracellular ATP that is produced during NKT-monocyte interactions, and blocking purinergic signaling during monocyte differentiation results in APCs that form clusters with T cells and stimulate their proliferation. Finally, we show that human monocytes and NKT cells that are injected into immunodeficient mice co-localize together in spleen and liver, and after 3 days in vivo in the presence of NKT cells a fraction of the myeloid cells have upregulated markers associated with differentiation into professional APCs. These results suggest that autoreactive human NKT cells may promote tolerance by inducing the differentiation of regulatory myeloid APCs that limit T cell proliferation through expression of PD-L molecules.

Natural killer T cells: innate lymphocytes positioned as a bridge between acute and chronic inflammation?

Natural killer T cells are an innate population of T lymphocytes that recognize antigens derived from host lipids and glycolipids. In this review, we focus on how these unique T cells are positioned to influence both acute and chronic inflammatory processes through their early recruitment to sites of inflammation, interactions with myeloid antigen presenting cells, and recognition of lipids associated with inflammation.

Primary deficiency of microsomal triglyceride transfer protein in human abetalipoproteinemia is associated with loss of CD1 function

Abetalipoproteinemia (ABL) is a rare Mendelian disorder of lipid metabolism due to genetic deficiency in microsomal triglyceride transfer protein (MTP). It is associated with defects in MTP-mediated lipid transfer onto apolipoprotein B (APOB) and impaired secretion of APOB-containing lipoproteins. Recently, MTP was shown to regulate the CD1 family of lipid antigen-presenting molecules, but little is known about immune function in ABL patients. Here, we have shown that ABL is characterized by immune defects affecting presentation of self and microbial lipid antigens by group 1 (CD1a, CD1b, CD1c) and group 2 (CD1d) CD1 molecules. In dendritic cells isolated from ABL patients, MTP deficiency was associated with increased proteasomal degradation of group 1 CD1 molecules. Although CD1d escaped degradation, it was unable to load antigens and exhibited functional defects similar to those affecting the group 1 CD1 molecules. The reduction in CD1 function resulted in impaired activation of CD1-restricted T and invariant natural killer T (iNKT) cells and reduced numbers and phenotypic alterations of iNKT cells consistent with central and peripheral CD1 defects in vivo. These data highlight MTP as a unique regulator of human metabolic and immune pathways and reveal that ABL is not only a disorder of lipid metabolism but also an immune disease involving CD1.

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

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

Recognition of lyso-phospholipids by human natural killer T lymphocytes

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

Human NKT cells promote monocyte differentiation into suppressive myeloid antigen-presenting cells

NKT cells have been shown to promote peripheral tolerance in a number of model systems, yet the processes by which they exert their regulatory effects remain poorly understood. Here, we show that soluble factors secreted by human NKT cells instruct human peripheral blood monocytes to differentiate into myeloid APCs that have suppressive properties. NKT instructed monocytes acquired a cell surface phenotype resembling myeloid DCs. However, whereas control DCs that were generated by culturing monocytes with recombinant GM-CSF and IL-4 had a proinflammatory phenotype characterized by the production of IL-12 with little IL-10, NKT-instructed APCs showed the opposite cytokine production profile of high IL-10 with little or no IL-12. The control DCs efficiently stimulated peripheral blood T cell IFN-gamma secretion and proliferation, whereas NKT-instructed APCs silenced these T cell responses. Exposure to NKT cell factors had a dominant effect on the functional properties of the DCs, since DCs differentiated by recombinant GM-CSF and IL-4 in the presence of NKT cell factors inhibited T cell responses. To confirm their noninflammatory effects, NKT-instructed APCs were tested in an in vivo assay that depends on the activation of antigen-specific human T cells. Control DCs promoted substantial tissue inflammation; however, despite a marked neutrophilic infiltrate, there was little edema in the presence of NKT-instructed APCs, suggesting the inflammatory cascade was held in check. These results point to a novel pathway initiated by NKT cells that can contribute to the regulation of human antigen-specific Th1 responses.

An in vivo model of the human CD1 antigen presenting system (42.24)

CD1 molecules are a family of antigen presenting molecules that present lipids and glycolipids for recognition by specialized T cells. The CD1 system has been implicated in microbial infections, autoimmune and allergic disorders, and immunological control of tumors. However, our understanding of its role in human disease remains limited because animal models that resemble the human CD1 system are lacking. Here we report that a severely immune deficient mouse engrafted with human fetal tissues and hematopoietic stem cells develops an intact human CD1 antigen presenting system. Human CD1a, b, c, and d isoforms are highly expressed in the thymic organoid that develops in the engrafted mice, and histological analysis indicates the presence of CD1a+ APCs. Moreover, CD1+ APCs are expressed in peripheral tissues including the spleen and liver, and are capable of presenting bacterial glycolipids to human T cells specific for CD1a, b, or c. Staining with CD1d tetramers detected comparable percentages of human CD1d-restricted T cells in engrafted mice and human blood, and CD1d-restricted T cells isolated from the mice expressed a characteristic Vα24+/Vβ11+ TCR and recognized the glycolipid antigen α-GalCer. These results demonstrate the functional reconstitution of the human CD1 system in a mouse model.

Natural killer T-cell autoreactivity leads to a specialized activation state

Natural killer T (NKT) cells are innate-like T cells that recognize specific microbial antigens and also display autoreactivity to self-antigens. The nature of NKT-cell autoreactive activation remains poorly understood. We show here that the mitogen-activated protein kinase (MAPK) pathway is operative during human NKT-cell autoreactive activation, but calcium signaling is severely impaired. This results in a response that is biased toward granulocyte macrophage colony-stimulating factor (GM-CSF) secretion because this cytokine requires extracellular signal-regulated kinase (ERK) signaling but is not highly calcium dependent, whereas interferon-gamma (IFN-gamma), interleukin (IL)-4, and IL-2 production are minimal. Autoreactive activation was associated with reduced migration velocity but did not induce arrest; thus, NKT cells retained the ability to survey antigen presenting cells (APCs). IL-12 and IL-18 stimulated autoreactively activated NKT cells to secrete IFN-gamma, and this was mediated by Janus kinase-signal transducers and activators of transcription (JAK-STAT)-dependent signaling without induction of calcium flux. This pathway did not require concurrent contact with CD1d(+) APCs but was strictly dependent on preceding autoreactive stimulation that induced ERK activation. In contrast, NKT-cell responses to the glycolipid antigen alpha-galactosyl ceramide (alpha-GalCer) were dampened by prior autoreactive activation. These results show that NKT-cell autoreactivity induces restricted cytokine secretion and leads to altered basal activation that potentiates innate responsiveness to costimulatory cytokines while modulating sensitivity to foreign antigens.

Selective activation, expansion, and monitoring of human iNKT cells with a monoclonal antibody specific for the TCR alpha-chain CDR3 loop

A significant fraction of CD1d-restricted T cells express an invariant T cell receptor (TCR) alpha-chain. These highly conserved invariant NKT (iNKT) populations are important regulators of a wide spectrum of immune responses. The ability to directly identify and manipulate iNKT cells is essential to understanding their function and to exploit their therapeutic potential. To this end, we sought monoclonal and polyclonal antibodies specific for iNKT cells by immunizing CD1d KO mice, which lack iNKT cells, with a cyclic peptide modeled after the TCRalpha CDR3 loop. One mAb (6B11) was specific for cloned and primary human but not rodent iNKT cells and the human invariant TCRalpha, as shown by transfection and reactivity with human invariant TCRalpha transgenic T cells ex vivo and in situ. 6B11 was utilized to identify, purify, and expand iNKT cells from an otherwise minor component of human peripheral blood lymphocytes and to specifically identify human iNKT cells in tissue. Thus, we report a novel and general strategy for the generation of mAb specific for the CDR3 loop encoded by the TCR of interest. Specifically, an anti-Valpha24Jalpha18 CDR3 loop clonotypic TCR mAb is available for the enumeration and therapeutic manipulation of human and non-human primate iNKT populations.

Serum lipids regulate dendritic cell CD1 expression and function

Dendritic cells (DCs) are highly potent antigen-presenting cells (APCs) and play a vital role in stimulating naïve T cells. Treatment of human blood monocytes with the cytokines granulocyte-macrophage colony-stimulating factor (GM-CSF) and interleukin (IL)-4 stimulates them to develop into immature dendritic cells (iDCs) in vitro. DCs generated by this pathway have a high capacity to prime and activate resting T cells and prominently express CD1 antigen-presenting molecules on the cell surface. The presence of human serum during the differentiation of iDCs from monocytes inhibits the expression of CD1a, CD1b and CD1c, but not CD1d. Correspondingly, T cells that are restricted by CD1c showed poor responses to DCs that were generated in the presence of human serum, while the responses of CD1d-restricted T cells were enhanced. We chemically fractionated human serum to isolate the bioactive factors that modulate surface expression of CD1 proteins during monocyte to DC differentiation. The human serum components that affected CD1 expression partitioned with polar organic soluble fractions. Lysophosphatidic acid and cardiolipin were identified as lipids present in normal human serum that potently modulate CD1 expression. Control of CD1 expression was mediated at the level of gene transcription and correlated with activation of the peroxisome proliferator-activated receptor (PPAR) nuclear hormone receptors. These findings indicate that the ability of human DCs to present lipid antigens to T cells through expression of CD1 molecules is sensitively regulated by lysophosphatidic acid and cardiolipin in serum, which are ligands that can activate PPAR transcription factors.

Modulation of CD1d-restricted NKT cell responses by CD4

CD4+ and CD4- NKT cell populations have been shown to be functionally distinct, but the role of CD4 molecules in NKT cell activation is not clear. Here, we have used human CD1d-restricted NKT cell clones to investigate the contribution of CD4 to NKT cell functional responses. Coligation of CD4 with the TCR/CD3 complex resulted in enhanced cytokine secretion and increased calcium flux by CD4+ NKT cell clones, indicating that CD4 is functionally active in these cells. CD4 blockade specifically inhibited cytokine secretion and proliferation of CD4+ NKT cell clones in response to CD1d+ APCs but did not affect cytotoxicity, suggesting that CD4 preferentially modulates some NKT cell functional responses and not others. Anti-CD4 mAb treatment inhibited NKT cell responses to both MHC class II(+) and MHC class II(-) APCs, indicating that its effect was not due to blocking CD4 binding to MHC class II molecules on APCs. The inhibitory effect of the anti-CD4 mAb also did not require recognition of CD1d by the NKT cell, since calcium flux was reduced in response to anti-CD3 mAb stimulation. Western blot analysis revealed that anti-CD4 treatment resulted in increased phosphorylation of an inhibitory site of p56(lck) (tyrosine 505). Thus, CD4 blockade interferes with the course of CD3-mediated signaling events in NKT cells. These results indicate that CD4 can contribute to NKT cell activation independently of the presence of a CD4-ligand on APCs and suggest that it preferentially modulates cytokine and proliferative responses.

Distinct Endosomal Trafficking Requirements for Presentation of Autoantigens and Exogenous Lipids by Human CD1d Molecules

CD1d molecules present both self Ags and microbial lipids to NKT cells. Previous studies have established that CD1d lysosomal trafficking is required for presentation of autoantigens to murine invariant NKT cells. We show in this study that this is not necessary for autoantigen presentation by human CD1d, but significantly affects the presentation of exogenous Ags. Wild-type and tail-deleted CD1d molecules stimulated similar autoreactive responses by human NKT clones, whereas presentation of exogenous lipids by tail-deleted CD1d was highly inefficient. Chloroquine treatment markedly inhibited exogenous Ag presentation by wild-type CD1d transfectants, but did not affect NKT autoreactive responses. Conversely, APC expression of HLA-DRalphabeta and the invariant chain (Ii) was associated with faster internalization of CD1d into the endocytic system and enhanced CD1d-mediated presentation of exogenous Ags, but did not appear to augment NKT autoreactivity. Knockdown of the Ii by small interfering RNA resulted in reduced CD1d surface expression and slower internalization in HLA-DR+ APCs, but not HLA-DR- APCs, demonstrating a direct effect of MHC/Ii complexes on CD1d trafficking. CD1d-mediated presentation of exogenous Ags was much more efficient in immature dendritic cells, which actively recycle MHC class II molecules through the endocytic system, than in mature dendritic cells that have stabilized MHC class II expression at the cell surface, suggesting a physiological role for MHC/Ii complexes in modulating CD1d function. These results indicate that autoantigens and exogenous lipids are acquired by human CD1d at distinct cellular locations, and that Ii trafficking selectively regulates CD1d-mediated presentation of extracellular Ags.

Self antigens are partial agonists for NKT cells (B221)

NKT cells are regulatory T cells that recognize glycolipid antigens presented by CD1d. The antigens recognized by NKT cells include self and foreign lipids, but it is unclear whether these elicit different functional responses. Here we show that presentation of self antigens to human NKT cell clones stimulates quantitatively and qualitatively different responses than the potent exogenous antigen α-GalCer. Self antigen recognition only efficiently activated secretion of GM-CSF and IL-13, whereas α-GalCer stimulated full cytokine production (GM-CSF, IL-13, IL-4, IFNγ, and IL-2) as well as cytotoxicity and proliferation. Recognition of self antigens resulted in TCR down-regulation, CD69 up-regulation, and ERK phosphorylation, indicating that TCR-mediated signaling had occurred. However, no NKT cell Ca++ flux was detected in response to self antigen, although α-GalCer stimulated robust Ca++ signaling. Recognition of self antigens in the presence of ionomycin, which artificially induces Ca++ flux, resulted in full functional responses, including production of IL-2 and proliferation. Conversely, stimulation by α-GalCer in the presence of cyclosporin A to block calcium signaling, preferentially inhibited IL-2 secretion. Hence, NKT cell recognition of self antigens is associated with a defect in calcium signaling, and results in a partial activation phenotype.

NKT cells direct monocytes into a DC differentiation pathway

Monocytes can differentiate into macrophages or dendritic cells (DCs). The processes that promote their differentiation along one pathway rather than the other remain unknown. NKT cells are regulatory T cells that respond functionally to self and foreign antigens presented by CD1d molecules. Hence, in addition to contributing to antimicrobial responses, they may carry out autoreactively activated functions when there is no infectious challenge. However, the immunological consequences of NKT cell autoreactivity remain poorly understood. We show here that human NKT cells direct monocytes to differentiate into immature DCs. The ability to induce monocyte differentiation was CD1d-dependent and appeared specific to NKT cells. Addition of exogenous antigens or costimulation from IL-2 was not required but could enhance the effect. DC differentiation was a result of NKT cell secretion of GM-CSF and IL-13, cytokines that were produced by the NKT cells upon autoreactive activation by monocytes. NKT cells within PBMC samples produced GM-CSF and IL-13 upon exposure to autologous monocytes directly ex vivo, providing evidence that such NKT cell-autoreactive responses can occur in vivo. These results show that when NKT cells are activated by autologous monocytes, they are capable of providing factors that specifically direct monocyte differentiation into immature DCs. Thus, autoreactively activated NKT cells may contribute to the maintenance of the immature DC population, and microbial infection or inflammatory conditions that activate NKT cells further could stimulate them to promote an increased rate of DC differentiation.

The ins and outs of CD1 molecules: bringing lipids under immunological surveillance

An emerging area of investigation is the role of lipids as immunological antigens. CD1 glycoproteins comprise a family of molecules that are specialized for presenting lipids, glycolipids and lipopeptides to T lymphocytes. Variations in the cytoplasmic tail sequences of CD1 isoforms lead to differential association with adaptor proteins and consequently divergent routes of intracellular trafficking, resulting in surveillance of distinct cellular sites for binding lipid antigens. CD1 molecules efficiently gain access to lipids from intracellular microbial pathogens in endosomal compartments, and the trafficking and lipid-binding specialization of CD1 isoforms may correlate with the endosomal segregation of structurally distinct lipids. Endosomal trafficking is also critical for CD1d molecules to load antigenic self-lipids that are presented to autoreactive CD1d-restricted natural killer (NK)T cells and is required for the positive selection of these unique T cells. Recent studies reveal a key role for accessory proteins that facilitate the uptake of lipid antigens by CD1 molecules. These include lysosomal lipid-transfer proteins, such as the saposins, and apolipoprotein E, the major serum factor that binds and delivers extracellular lipids to antigen-presenting cells. These advances in understanding the CD1 lipid antigen presentation system raise new considerations about the role of the immune response in lipid-related diseases.

Conserved and Heterogeneous Lipid Antigen Specificities of CD1d-Restricted NKT Cell Receptors

CD1d-restricted NKT cells use structurally conserved TCRs and recognize both self and foreign glycolipids, but the TCR features that determine these Ag specificities remain unclear. We investigated the TCR structures and lipid Ag recognition properties of five novel Valpha24-negative and 13 canonical Valpha24-positive/Vbeta11-positive human NKT cell clones generated using alpha-galactosylceramide (alpha-GalCer)-loaded CD1d tetramers. The Valpha24-negative clones expressed Vbeta11 paired with Valpha10, Valpha2, or Valpha3. Strikingly, their Valpha-chains had highly conserved rearrangements to Jalpha18, resulting in CDR3alpha loop sequences that are nearly identical to those of canonical TCRs. Valpha24-positive and Valpha24-negative clones responded similarly to alpha-GalCer and a closely related bacterial analog, suggesting that conservation of the CDR3alpha loop is sufficient for recognition of alpha-GalCer despite CDR1alpha and CDR2alpha sequence variation. Unlike Valpha24-positive clones, the Valpha24-negative clones responded poorly to a glucose-linked glycolipid (alpha-glucosylceramide), which correlated with their lack of a conserved CDR1alpha amino acid motif, suggesting that fine specificity for alpha-linked glycosphingolipids is influenced by Valpha-encoded TCR regions. Valpha24-negative clones showed no response to isoglobotrihexosylceramide, indicating that recognition of this mammalian lipid is not required for selection of Jalpha18-positive TCRs that can recognize alpha-GalCer. One alpha-GalCer-reactive, Valpha24-positive clone differed from the others in responding specifically to mammalian phospholipids, demonstrating that semi-invariant NKT TCRs have a capacity for private Ag specificities that are likely conferred by individual TCR beta-chain rearrangements. These results highlight the variation in Ag recognition among CD1d-restricted TCRs and suggest that TCR alpha-chain elements contribute to alpha-linked glycosphingolipid specificity, whereas TCR beta-chains can confer heterogeneous additional reactivities.

Apolipoprotein-mediated pathways of lipid antigen presentation

Peptide antigens are presented to T cells by major histocompatibility complex (MHC) molecules, with endogenous peptides presented by MHC class I and exogenous peptides presented by MHC class II. In contrast to the MHC system, CD1 molecules bind lipid antigens that are presented at the antigen-presenting cell (APC) surface to lipid antigen-reactive T cells. Because CD1 molecules survey endocytic compartments, it is self-evident that they encounter antigens from extracellular sources. However, the mechanisms of exogenous lipid antigen delivery to CD1-antigen-loading compartments are not known. Serum apolipoproteins are mediators of extracellular lipid transport for metabolic needs. Here we define the pathways mediating markedly efficient exogenous lipid antigen delivery by apolipoproteins to achieve T-cell activation. Apolipoprotein E binds lipid antigens and delivers them by receptor-mediated uptake into endosomal compartments containing CD1 in APCs. Apolipoprotein E mediates the presentation of serum-borne lipid antigens and can be secreted by APCs as a mechanism to survey the local environment to capture antigens or to transfer microbial lipids from infected cells to bystander APCs. Thus, the immune system has co-opted a component of lipid metabolism to develop immunological responses to lipid antigens.

Regulation of CD1d expression and function by a herpesvirus infection

Little is known about the role of CD1d-restricted T cells in antiviral immune responses. Here we show that the lytic replication cycle of the Kaposi sarcoma-associated herpesvirus (KSHV) promotes downregulation of cell-surface CD1d. This is caused by expression of the 2 modulator of immune recognition (MIR) proteins of the virus, each of which promotes the loss of surface CD1d expression following transfection into uninfected cells. Inhibition of CD1d surface expression is due to ubiquitination of the CD1d alpha-chain on a unique lysine residue in its cytoplasmic tail, which triggers endocytosis. Unlike MIR-mediated MHC class I downregulation, however, CD1d downregulation does not appear to include accelerated lysosomal degradation. MIR2-induced downregulation of CD1d results in reduced activation of CD1d-restricted T cells in vitro. KSHV modulation of CD1d expression represents a strategy for viral evasion of innate host immune responses and implicates CD1d-restricted T cells as regulators of this viral infection.

Antigen specificity of semi‐invariant CD1d‐restricted T cell receptors: The best of both worlds?

T lymphocytes are characterized by the use of structurally diverse TCR. The discovery of subsets of canonical T cells that have structurally homogeneous TCR presents an enigma: What antigens do these T cells recognize, and how does their antigen specificity relate to their functions? One subset of canonical T cells is restricted by CD1d, a non-classical antigen presenting molecule that presents lipids and glycolipids. Canonical CD1d-restricted T cells have semi-invariant TCR consisting of an invariantly rearranged TCR alpha chain, paired with diversely rearranged TCR beta chains. Most respond strongly to the unusual glycolipid alpha-galactosylceramide (alpha-GalCer), and can also respond to cellular antigens presented by CD1d. Mounting evidence indicates that alpha-GalCer responsive T cells are heterogeneous in their reactivities to cellular antigens, suggesting that an individual semi-invariant TCR may be capable of recognizing more than one ligand. Recent crystal structures of CD1b molecules with three different bound lipids indicate that the antigenic features of lipids may be localized over a smaller area than those of peptides, and that the positioning of the polar head group can vary substantially. A model that explains how CD1d-restricted T cells could possess both conserved and heterogeneous antigen specificities, is that different lipid antigens may interact with distinct areas of a TCR due to differences in the positioning of the polar head group. Hence, canonical CD1d-restricted TCR could recognize conserved antigens via the invariant TCR alpha chain, and have diverse antigen specificities that are conferred by their individual TCR beta chains.

CD1d-restricted "NKT" cells and myeloid IL-12 production: an immunological crossroads leading to promotion or suppression of effective anti-tumor immune responses?

CD1d-restricted T cells are remarkable for their unusual ability to respond to self-antigens and to contribute to both immunostimulatory and immunosuppressive responses. Their effects in different cancer models have appeared contradictory; in some cases, they are linked to the generation of effective tumor clearance, and in others, they seem to contribute to suppression of anti-tumor responses. Recent results suggest CD1d-restricted T cells are involved in critical interactions with myeloid dendritic cells (DCs) that can affect the subsequent course of the immune response, and that factors such as the strength of the antigenic signal and the presence or absence of proinflammatory cytokines may determine the outcome of these interactions. In the presence of a strong antigenic signal, CD1d-restricted T cells induced myeloid DCs to secrete interleukin (IL)-12, and these DCs in turn activated naive T cells to secrete Th1 cytokines. When exposed to the weak antigenic stimulus of self-antigens, CD1d-restricted T cells induced DCs to secrete IL-10 but not IL-12, and these DCs failed to stimulate Th1 cytokine production by naive T cells. In contrast, CD1d-restricted T cells that were stimulated by self-antigens in the presence of IL-12 potently secreted interferon-gamma (IFN-gamma) and were among the first lymphocytes to become activated in vivo. Hence, CD1d-restricted T cells may promote or prevent effective anti-tumor responses that are mediated by other lymphocytic effector cells by influencing IL-12 production by myeloid DCs and by their own production of early IFN-gamma in response to IL-12.

Lysosomal localization of murine CD1d mediated by AP-3 is necessary for NK T cell development

The presentation of lipid and glycolipid Ags to T cells is mediated through CD1 molecules. In the mouse and rat only a single isoform, CD1d, performs these functions, while humans and all other mammals studied have members of both group I (CD1a, -b, and -c) and group II (CD1d) isoforms. Murine CD1d contains a cytoplasmic tyrosine-based sorting motif that is similar to motifs recognized by adaptor protein complexes that sort transmembrane proteins. Here we show that the adaptor protein complex, AP-3, directly interacts with murine CD1d and controls its targeting to lysosomes. AP-3 deficiency results in a redistribution of CD1d from lysosomes to the cell surface of thymocytes, B cell-depleted splenocytes, and dendritic cells. The altered trafficking of CD1d in AP-3-deficient mice results in a significant reduction of NK1.1(+)TCR-beta(+) and CD1d tetramer-positive cells, consistent with a defect in CD1d self-Ag presentation and thymocyte-positive selection. The AP-3 complex has recently been shown to associate with the human CD1b isoform, which has an intracellular distribution pattern similar to that of murine CD1d. We propose that lysosomal sampling may be so critical for efficient host defense that mice have evolved mechanisms to target their single CD1 isoform to lysosomes for sampling lipid Ags. Here we show the dominant mechanism for this trafficking is mediated by AP-3.