Ex-vivo analysis of human natural killer T cells demonstrates heterogeneity between tissues and within established CD4(+) and CD4(-) subsets

Our understanding of human type 1 natural killer T (NKT) cells has been heavily dependent on studies of cells from peripheral blood. These have identified two functionally distinct subsets defined by expression of CD4, although it is widely believed that this underestimates the true number of subsets. Two recent studies supporting this view have provided more detail about diversity of the human NKT cells, but relied on analysis of NKT cells from human blood that had been expanded in vitro prior to analysis. In this study we extend those findings by assessing the heterogeneity of CD4(+) and CD4(-) human NKT cell subsets from peripheral blood, cord blood, thymus and spleen without prior expansion ex vivo, and identifying for the first time cytokines expressed by human NKT cells from spleen and thymus. Our comparative analysis reveals highly heterogeneous expression of surface antigens by CD4(+) and CD4(-) NKT cell subsets and identifies several antigens whose differential expression correlates with the cytokine response. Collectively, our findings reveal that the common classification of NKT cells into CD4(+) and CD4(-) subsets fails to reflect the diversity of this lineage, and that more studies are needed to establish the functional significance of the antigen expression patterns and tissue residency of human NKT cells.

The BH3-only proteins Bim and Puma cooperate to impose deletional tolerance of organ-specific antigens

Although the proapoptotic BH3-only protein, Bim, is required for deletion of autoreactive thymocytes, Bim-deficient mice do not succumb to extensive organ-specific autoimmune disease. To determine whether other BH3-only proteins safeguard tolerance in the absence of Bim, we screened mice lacking Bim as well as other BH3-only proteins. Most strains showed no additional defects; however, mice deficient for both Puma and Bim spontaneously developed autoimmunity in multiple organs, and their T cells could transfer organ-specific autoimmunity. Puma- and Bim-double-deficient mice had a striking accumulation of mature, single-positive thymocytes, suggesting an additional defect in thymic deletion was the basis for disease. Transgenic mouse models of thymocyte deletion by peripheral neoantigens confirmed that the loss of Bim and Puma allowed increased numbers of autoreactive thymocytes to escape deletion. Our data show that Puma cooperates with Bim to impose a thymic-deletion checkpoint to peripheral self-antigens and cement the notion that defects in apoptosis alone are sufficient to cause autoimmune disease.

Presumed guilty: natural killer T cell defects and human disease

Natural killer T (NKT) cells are important regulatory lymphocytes that have been shown in mouse studies, to have a crucial role in promoting immunity to tumours, bacteria and viruses, and in suppressing cell-mediated autoimmunity. Many clinical studies have indicated that NKT cell deficiencies and functional defects might also contribute to similar human diseases, although there is no real consensus about the nature of the NKT cell defects or whether NKT cells could be important for the diagnosis and/or treatment of these conditions. In this Review, we describe the approaches that have been used to analyse the NKT cell populations of various patient groups, suggest new strategies to determine how (or indeed, if) NKT cell defects contribute to human disease, and discuss the prospects for using NKT cells for therapeutic benefit.

An NZW-derived interval on chromosome 7 moderates sialadenitis, but not insulitis in congenic nonobese diabetic mice

Autoimmune lymphocytic infiltration of the salivary glands, termed sialadenitis, is a pathologic feature of Sjögren's syndrome (SjS) that is also prominent in nonobese diabetic (NOD) mice. Genetic factors regulate sialadenitis, and a previous (NOD x NZW)F2 study detected linkage to murine chromosome (Chr) 7. The locus, subsequently annotated as Ssial3, maps to the distal end of Chr7 and overlaps a region associated with type 1 diabetes susceptibility in NOD mice. To examine whether Ssial3 could contribute to both diseases, or was specific for SjS, we generated a congenic mouse strain that harbored an NZW-derived Chr7 interval on the NOD genetic background. This congenic strain exhibited reduced sialadenitis compared with NOD mice and confirmed Ssial3. This reduction, however, did not ameliorate saliva abnormalities associated with SjS-like disease in NOD mice, nor were congenic mice protected against insulitis (lymphocytic infiltration of the pancreatic islets) or diabetes onset. Thus, the Ssial3 locus appears to have a tissue-specific effect for which the NZW allele is unable to prevent other autoimmune traits in the NOD mouse. Anomalous increases for antinuclear Ab production and frequency of marginal-zone B cells were also identified in congenic mice, indicating that the NZW-derived Chr7 interval has a complex effect on the NOD immune system.

Testing the NKT cell hypothesis in lenalidomide-treated myelodysplastic syndrome patients

Myelodysplastic syndrome (MDS) comprises a group of clonal bone marrow disorders characterized by ineffective hematopoiesis and increased predisposition to acute myeloid leukemia. The causes of MDS remain poorly defined, but several studies have reported the NKT cell compartment of patients with MDS is deficient in number and functionally defective. In support of a central role for NKT cells, a pilot clinical study reported that lenalidomide (an approved treatment for MDS) increased NKT cell numbers in patients with MDS, and several in vitro studies showed lenalidomide specifically promoted NKT cell proliferation and cytokine production. We tested this in a much larger study and confirm a moderate in vitro augmentation of some NKT cell functions by lenalidomide, but find no impact on the NKT cell compartment of patients treated with lenalidomide, despite a consistently positive clinical response. We further show that the frequency and cytokine production of NKT cells is normal in patients with MDS before treatment and remains stable throughout 10 months of lenalidomide therapy. Collectively, our data challenge the concept that NKT cell defects contribute to the development of MDS, and show that a clinical response to lenalidomide is not dependent on modulation of NKT cell frequency or function.

Immune characterization of an individual with an exceptionally high natural killer T cell frequency and her immediate family

Natural killer T cells (NKT) are a regulatory subset of T lymphocytes whose frequency in peripheral blood is highly variable within the human population. Lower than normal NKT frequencies are associated with increased predisposition to a number of diseases, including type 1 diabetes and some forms of cancer, raising the possibility that an increased frequency may be protective. However, there is little or no understanding of how high NKT frequencies arise or, most importantly, whether the potential exists to boost and maintain NKT levels for therapeutic advantage. Here, we provide a detailed functional and phenotypic characterization of the NKT compartment of a human donor with NKT levels approximately 50 times greater than normal, including an analysis of NKT in her immediate family members. The study focuses upon the characteristics of this donor and her family, but demonstrates more broadly that the size and flexibility of the NKT niche is far greater than envisioned previously. This has important implications for understanding how the human NKT compartment is regulated, and supports the concept that the human NKT compartment might be expanded successfully for therapeutic benefit.

NKT cell development in the absence of the autoimmune regulator gene (Aire)

Autoimmune regulator gene (Aire)-deficient mice develop an array of autoimmune lesions that reflect failures of immune tolerance. Negative selection is clearly compromised in these mice, but there is evidence to suggest that other mechanisms of tolerance might also be affected, including a possible impairment of regulatory T cell (Treg) development. Studies to date have failed to demonstrate any significant impact on the development or function of the FOXP3+ Treg compartment, but NKT cells represent a distinct regulatory cell lineage that also develop in the thymus and which are known to influence self-tolerance. Aire-related defects coincide with NKT cell deficiencies in a number of animal models, but the direct consequence of Aire-deficiency on NKT cell development has not been established. In this study, we demonstrate that the frequency, distribution and cytokine production of NKT cells and their subsets is principally normal in Aire-deficient mice. We conclude that Aire has little or no effect on regulatory T cell development in general and NKT cells in particular.

Peripheral NK1.1 NKT cells are mature and functionally distinct from their thymic counterparts

One interesting aspect of NKT cell development is that although they are thymus dependent, the pivotal transition from NK1.1(-) to NK1.1(+) can often take place after immature NK1.1(-) NKT cells are exported to the periphery. NK1.1(-) NKT cells in general are regarded as immature precursors of NK1.1(+) NKT cells, meaning that peripheral NK1.1(-) NKT cells are regarded as a transient, semimature population of recent thymic emigrant NKT cells. In this study, we report the unexpected finding that most NK1.1(-) NKT cells in the periphery of naive mice are actually part of a stable, mature and functionally distinct NKT cell population. Using adult thymectomy, we show that the size of the peripheral NK1.1(-) NKT cell pool is maintained independently of thymic export and is not the result of NK1.1 down-regulation by mature cells. We also demonstrate that most peripheral NK1.1(-) NKT cells are functionally distinct from their immature thymic counterparts, and from NK1.1(+) NKT cells in the periphery. We conclude that the vast majority of peripheral NK1.1(-) NKT cells are part of a previously unrecognized, mature NKT cell subset.

Control points in NKT-cell development

CD1d-dependent natural killer T (NKT) cells are a unique T-cell subset with the ability to regulate the immune system in response to a broad range of diseases. That low NKT-cell numbers are associated with many different disease states in mice and humans, combined with the fact that NKT-cell numbers vary widely between individuals, makes it crucial to understand how these cells develop and how their numbers are maintained. Here, we review the current state of knowledge of NKT-cell development and attempt to highlight the most important questions in this field.

IL-21 Is Produced by NKT Cells and Modulates NKT Cell Activation and Cytokine Production

The common gamma-chain cytokine, IL-21, is produced by CD4(+) T cells and mediates potent effects on a variety of immune cells including NK, T, and B cells. NKT cells express the receptor for IL-21; however, the effect of this cytokine on NKT cell function has not been studied. We show that IL-21 on its own enhances survival of NKT cells in vitro, and IL-21 increases the proliferation of NKT cells in combination with IL-2 or IL-15, and particularly with the CD1d-restricted glycosphingolipid Ag alpha-galactosylceramide. Similar to its effects on NK cells, IL-21 enhances NKT cell granular morphology, including granzyme B expression, and some inhibitory NK receptors, including Ly49C/I and CD94. IL-21 also enhanced NKT cell cytokine production in response to anti-CD3/CD28 in vitro. Furthermore, NKT cells may be subject to autocrine IL-21-mediated stimulation because they are potent producers of this cytokine following in vitro stimulation via CD3 and CD28, particularly in conjunction with IL-12 or following in vivo stimulation with alpha-galactosylceramide. Indeed, NKT cells produced much higher levels of IL-21 than conventional CD4 T cells in this assay. This study demonstrates that NKT cells are potentially a major source of IL-21, and that IL-21 may be an important factor in NKT cell-mediated immune regulation, both in its effects on NK, T, and B cells, as well as direct effects on NKT cells themselves. The influence of IL-21 in NKT cell-dependent models of tumor rejection, microbial clearance, autoimmunity, and allergy should be the subject of future investigations.

Antigen challenge inhibits thymic emigration

T cell development in the thymus involves a series of TCR-mediated control points including TCR-beta selection and positive and negative selection. Approximately half of the thymic sojourn is spent in the medulla, where thymocytes undergo final maturation before emigrating to the periphery. Although it is acknowledged that thymic emigration is an active process, relatively little is known about how this is regulated, why it takes so long, and whether TCR-mediated signaling can influence this step. Using wild-type and TCR transgenic mice, we found that Ag injected i.v. or intrathymically led to a striking reduction in the number of recent thymic emigrants (RTE) in the periphery. This was caused by inhibition of T cell export rather than peripheral deletion, because a cohort of RTE that was already released before in vivo Ag challenge was not depleted, and similar results were observed in Bim-deficient mice, which have impaired T cell deletion. Within the thymus, the loss of RTE was associated with retention of medullary thymocytes rather than increased negative selection. In addition to Ag-specific inhibition of export, some TCR-independent suppression of emigration was also observed that appeared to be partly the result of the inflammatory cytokine TNF. Thus, in addition to its accepted role in intrathymic selection events, TCR signaling can also play an important role in the regulation of thymic emigration.

Long-Term Retention of Mature NK1.1+ NKT Cells in the Thymus

The NKT cell pool in the thymus contains immature (NK1.1(-)) and mature (NK1.1(+)) subsets that represent distinct linear stages of a linear developmental pathway. An unexplained paradox is why immature NK1.1(-) NKT cells are mainly exported to the periphery instead of the more mature and more abundant NK1.1(+) NKT cells. In this study we have determined that mature NK1.1(+) NKT cells are retained by the thymus to form an extremely long-lived resident population capable of rapid and prolonged production of IFN-gamma and IL-4. The retention of mature NKT cells provides an explanation for why the periphery is mainly seeded by immature NK1.1(-) cells despite mature NK1.1(+) NKT cells being more abundant in the thymus. This is the first study to identify a mature T cell subset retained within the thymus and is additional evidence of the distinct developmental pathways of mainstream T cells and NKT cells.

Differential antitumor immunity mediated by NKT cell subsets in vivo

We showed previously that NKT cell–deficient TCR Jα18^−/− mice are more susceptible to methylcholanthrene (MCA)-induced sarcomas, and that normal tumor surveillance can be restored by adoptive transfer of WT liver-derived NKT cells. Liver-derived NKT cells were used in these studies because of their relative abundance in this organ, and it was assumed that they were representative of NKT cells from other sites. We compared NKT cells from liver, thymus, and spleen for their ability to mediate rejection of the sarcoma cell line (MCA-1) in vivo, and found that this was a specialized function of liver-derived NKT cells. Furthermore, when CD4⁺ and CD4⁻ liver-derived NKT cells were administered separately, MCA-1 rejection was mediated primarily by the CD4⁻ fraction. Very similar results were achieved using the B16F10 melanoma metastasis model, which requires NKT cell stimulation with α-galactosylceramide. The impaired ability of thymus-derived NKT cells was due, in part, to their production of IL-4, because tumor immunity was clearly enhanced after transfer of IL-4–deficient thymus-derived NKT cells. This is the first study to demonstrate the existence of functionally distinct NKT cell subsets in vivo and may shed light on the long-appreciated paradox that NKT cells function as immunosuppressive cells in some disease models, whereas they promote cell-mediated immunity in others.

Localization of Idd11 is not associated with thymus and nkt cell abnormalities in NOD mice

Congenic mouse strains provide a unique resource for genetic dissection and biological characterization of chromosomal regions associated with diabetes progression in the nonobese diabetic (NOD) mouse. Idd11, a mouse diabetes susceptibility locus, was previously localized to a region on chromosome 4. Comparison of a panel of subcongenic NOD mouse strains with different intervals derived from the nondiabetic C57BL/6 (B6) strain now maps Idd11 to an approximately 8-Mb interval. B6-derived intervals protected congenic NOD mice from diabetes onset, even though lymphocytic infiltration of pancreatic islets was similar to that found in NOD mice. In addition, neither thymic structural irregularities nor NKT cell deficiencies were ameliorated in diabetes-resistant congenic NOD mice, indicating that Idd11 does not contribute to these abnormalities, which do not need to be corrected to prevent disease.

The influence of CD1d in postselection NKT cell maturation and homeostasis

After being positively selected on CD1d-expressing thymocytes, NKT cells undergo a series of developmental changes that can take place inside or outside the thymus. We asked whether CD1d continues to play a role in late-stage NKT cell development and, in particular, during the functionally significant acquisition of NK1.1 that is indicative of NKT cell maturity. We report that CD1d is indeed crucial for this step, because immature NK1.1(-) NKT cells fail to fully mature when transferred to a CD1d-deficient environment. Surprisingly, however, the lack of CD1d did not greatly affect the long-term survival of NKT cells, and they continued to express CD69 and slowly proliferate. This directly contradicts the currently held view that these phenomena are caused by autoreactivity directed against CD1d/TCR-restricted self-Ags. Our findings demonstrate an ongoing role for TCR-mediated signaling throughout NKT cell development, but the characteristic semiactivated basal state of NKT cells is controlled by CD1d-independent factors or is intrinsic to the cells themselves.

Working with NKT cells--pitfalls and practicalities

Our understanding of NKT cells has been rapidly advancing over recent years, with many research groups studying how these cells behave and how they can be manipulated to prevent disease. Although good progress has been made, a difficulty is the lack of a clear consensus about how to assay, or even identify, NKT cells. The different approaches have been an ongoing source of uncertainty about the biological behaviour and function of NKT cells and have complicated efforts to define their role in immunity. An important step towards reaching agreement on the behaviour of NKT cells is to have a clear appreciation of the advantages and disadvantages of the different approaches that are employed in this field of study. This should help determine the most appropriate ways to investigate NKT cell function, thus bringing us closer to successfully exploiting these cells in the treatment of human diseases.

Activation of Thymic Regeneration in Mice and Humans following Androgen Blockade

The thymus undergoes age-related atrophy, coincident with increased circulating sex steroids from puberty. The impact of thymic atrophy is most profound in clinical conditions that cause a severe loss in peripheral T cells with the ability to regenerate adequate numbers of naive CD4+ T cells indirectly correlating with patient age. The present study demonstrates that androgen ablation results in the complete regeneration of the aged male mouse thymus, restoration of peripheral T cell phenotype and function and enhanced thymus regeneration following bone marrow transplantation. Importantly, this technique is also applicable to humans, with analysis of elderly males undergoing sex steroid ablation therapy for prostatic carcinoma, demonstrating an increase in circulating T cell numbers, particularly naive (TREC+) T cells. Collectively these studies represent a fundamentally new approach to treating immunodeficiency states in humans.

The cellular mechanism of Aire control of T cell tolerance

Aire promotes the tolerization of thymocytes by inducing the expression of a battery of peripheral-tissue antigens in thymic medullary epithelial cells. We demonstrate that the cellular mechanism by which Aire exerts its tolerance-promoting function is not primarily positive selection of regulatory T cells, but rather negative selection of T effector cells. Surprisingly, supplementing its influence on the transcription of genes encoding peripheral-tissue antigens, Aire somehow enhances the antigen-presentation capability of medullary epithelial cells. Thus, this transcriptional control element promotes central tolerance both by furnishing a specific thymic stromal cell type with a repertoire of self antigens and by better arming such cells to present these antigens to differentiating thymocytes. In Aire's absence, autoimmunity and ultimately overt autoimmune disease develops.

Limited correlation between human thymus and blood NKT cell content revealed by an ontogeny study of paired tissue samples

NKT cells are a CD1d-restricted T cell subset with strong immunoregulatory properties. Human NKT deficiencies are associated with autoimmune diseases such as type 1 diabetes and several types of cancer, yet there is little understanding of how the human NKT cell pool develops or is maintained. In this study, we present the first detailed analysis of human NKT cells from donor-matched postnatal thymus and blood samples. In mice, NKT cells are a thymus-dependent population that migrates to the periphery at an immature stage. Our data show that human NKT cells also undergo early stages of development in the thymus, forming a CD4(+)CD161(-/low) population that predominates neonatal thymic and blood NKT cell pools. CD4(-) and CD161(+) NKT cells accumulate with age in the blood, but not thymus, to the point that they dominate the NKT cell compartment in adult blood. This is consistent with the post-thymic maturation of NKT cells exported from the thymus at the putatively immature CD4(+)CD161(-/low) stage. Interestingly, while thymus and peripheral NKT cell frequencies vary widely between patients and are relatively stable between age groups, there is no clear relationship between the NKT cell frequency in thymus and blood.