A thymic precursor to the NK T cell lineage

NK cells in innate immunity

NK cells have an important role in innate immune responses, particularly in anti-viral immunity. Recent studies have revealed a molecular basis for NK cell recognition of virus-infected cells, implicating the activating KIR and Ly49 receptors and NKG2D in this process. Additionally, mutual cooperation between NK cells and dendritic cells suggests that these innate cells can shape the nature of an adaptive immune response. These findings, as well as advances in understanding NK cell development and homeostasis, indicate that NK cell biology is more sophisticated than previously appreciated.

The fourth way? Harnessing aggressive tendencies in the thymus

During late stages of thymic development, T cells must chose between different fates, dictated by their TCR specificity. Typically, this is thought of as a choice between three alternatives (being positive selection for useful T cells vs negative selection or neglect for harmful or useless T cells). However, there is growing evidence for a fourth alternative, in which T cells are positively selected by agonist ligands, which would normally be expected to induce T cell deletion. In this review, we will discuss where and when agonist selection is induced and whether this should be considered as a novel form of thymic selection or as an alternative differentiation state for Ag-exposed T cells.

Recent developments in the transcriptional regulation of cytolytic effector cells

Transcription factors have a profound influence on both the differentiation and effector function of cells of the immune system. T-bet controls the cytotoxicity of CD8(+) T cells and the production of interferon-gamma, and it also affects the development and function of natural killer cells and natural killer T cells. Other factors such as eomesodermin, MEF, ETS1 and members of the interferon-regulatory factor family also contribute to the effector function of immune cells. In this review, we focus on recent studies that have shed light on the transcriptional mechanisms that regulate cellular effector function in the immune system.

Restoration of NK T cell development in fyn-mutant mice by a TCR reveals a requirement for Fyn during early NK T cell ontogeny

NK T cells are a unique lymphocyte population that have developmental requirements distinct from conventional T cells. Mice lacking the tyrosine kinase Fyn have 5- to 10-fold fewer mature NK T cells. This study shows that Fyn-deficient mice have decreased numbers of NK1.1(-) NK T cell progenitors as well. 5-Bromo-2'-deoxyuridine-labeling studies indicate that the NK T cells remaining in fyn(-/-) mice exhibit a similar turnover rate as wild-type cells. The fyn(-/-) NK T cells respond to alpha-galactosylceramide, a ligand recognized by NK T cells, and produce cytokines, but have depressed proliferative capacity. Transgenic expression of the NK T cell-specific TCR alpha-chain Valpha14Jalpha18 leads to a complete restoration of NK T cell numbers in fyn(-/-) mice. Together, these results suggest that Fyn may have a role before alpha-chain rearrangement rather than for positive selection or the peripheral upkeep of cell number. NK T cells can activate other lymphoid lineages via cytokine secretion. These secondary responses are impaired in Fyn-deficient mice, but occur normally in fyn mutants expressing the Valpha14Jalpha18 transgene. Because this transgene restores NK T cell numbers, the lack of secondary lymphocyte activation in the fyn-mutant mice is due to the decreased numbers of NK T cells present in the mutant, rather than an intrinsic defect in the ability of the other fyn(-/-) lymphoid populations to respond.

The immunoregulatory role of CD1d-restricted natural killer T cells in disease

Natural killer T (NKT) cells constitute a T cell subpopulation that shares several characteristics with NK cells. NKT cells are characterized by a narrow T cell antigen receptor (TCR) repertoire, recognize glycolipid antigen in the context of the monomorphic CD1d antigen-presenting molecule, and have the unique capacity to rapidly produce large amounts of both T helper (Th) 1 and Th2 cytokines. Important roles of NKT cells have now been demonstrated in the regulation of autoimmune, allergic, antimicrobial, and antitumor immune responses. Here, we review the immunoregulatory role of NKT cells in disease and discuss NKT cell based immunotherapeutic strategies.

Increase in hepatic NKT cells in leukocyte cell-derived chemotaxin 2-deficient mice contributes to severe concanavalin A-induced hepatitis

Leukocyte cell-derived chemotaxin 2 (LECT2) was originally identified for its possible chemotactic activity against human neutrophils in vitro. It is a 16-kDa protein that is preferentially expressed in the liver. Its homologues have been widely identified in many vertebrates. Current evidence suggests that LECT2 may be a multifunctional protein like cytokines. However, the function of LECT2 in vivo remains unclear. To elucidate the role of this protein in vivo, we have generated LECT2-deficient (LECT2(-/-)) mice. We found that the proportion of NKT cells in the liver increased significantly in LECT2(-/-) mice, although those of conventional T cells, NK cells, and other cell types were comparable with those in wild-type mice. Consistent with increased hepatic NKT cell number, the production of IL-4 and IFN-gamma was augmented in LECT2(-/-) mice upon stimulation with alpha-galactosylceramide, which specifically activates Valpha14 NKT cells. In addition, NKT cell-mediated cytotoxic activity against syngeneic thymocytes increased in hepatic mononuclear cells obtained from LECT2(-/-) mice in vitro. Interestingly, the hepatic injury was exacerbated in LECT2(-/-) mice upon treatment with Con A, possibly because of the significantly higher expression of IL-4 and Fas ligand. These results suggest that LECT2 might regulate the homeostasis of NKT cells in the liver and might be involved in the pathogenesis of hepatitis.

CD1: antigen presentation and T cell function

This review summarizes the major features of CD1 genes and proteins, the patterns of intracellular trafficking of CD1 molecules, and how they sample different intracellular compartments for self- and foreign lipids. We describe how lipid antigens bind to CD1 molecules with their alkyl chains buried in hydrophobic pockets and expose their polar lipid headgroup whose fine structure is recognized by the TCR of CD1-restricted T cells. CD1-restricted T cells carry out effector, helper, and adjuvant-like functions and interact with other cell types including macrophages, dendritic cells, NK cells, T cells, and B cells, thereby contributing to both innate and adaptive immune responses. Insights gained from mice and humans now delineate the extensive range of diseases in which CD1-restricted T cells play important roles and reveal differences in the role of CD1a, CD1b, and CD1c in contrast to CD1d. Invariant TCR alpha chains, self-lipid reactivity, and rapid effector responses empower a subset of CD1d-restricted T cells (NKT cells) to have unique effector functions without counterpart among MHC-restricted T cells. This review describes the function of CD1-restricted T cells in antimicrobial responses, antitumor immunity, and in regulating the balance between tolerance and autoimmunity.

Norepinephrine regulates hepatic innate immune system in leptin-deficient mice with nonalcoholic steatohepatitis

It is not known why natural killer T (NKT) cells, which modulate liver injury by regulating local cytokine production, are reduced in leptin-deficient ob/ob mice. NKT cells express adrenoceptors. Thus, we hypothesize that the low norepinephrine (NE) activity of ob/ob mice promotes depletion of liver NKT cells, thereby sensitizing ob/ob livers to lipopolysaccharide (LPS) toxicity. To evaluate this hypothesis, hepatic NKT cells were quantified in wild-type mice before and after treatment with NE inhibitors, and in dopamine beta-hydroxylase knockout mice (which cannot synthesize NE) and ob/ob mice before and after 4 weeks of NE supplementation. Decreasing NE activity consistently reduces liver NKT cells, while increasing NE has the opposite effect. Analysis of hepatic and thymic NKT cells in mice of different ages demonstrate an age-related accumulation of hepatic NKT cells in normal mice, while liver NKT cells become depleted after birth in ob/ob mice, which have increased apoptosis of hepatic NKT cells. NE treatment inhibits apoptosis and restores hepatic NKT cells. In ob/ob mice with reduced hepatic NKT cells, hepatic T and NKT cells produce excessive T helper (Th)-1 proinflammatory cytokines and the liver is sensitized to LPS toxicity. NE treatment decreases Th-1 cytokines, increases production of Th-2 cytokines, and reduces hepatotoxicity. Studies of CD1d-deficient mice, which lack the receptor required for NKT cell development, demonstrate that they are also unusually sensitive to LPS hepatotoxicity. In conclusion, low NE activity increases hepatic NKT cell apoptosis and depletes liver NKT cells, promoting proinflammatory polarization of hepatic cytokine production that sensitizes the liver to LPS toxicity.

NF-kappa B controls cell fate specification, survival, and molecular differentiation of immunoregulatory natural T lymphocytes

Ontogenetic, homeostatic, and functional deficiencies within immunoregulatory natural T (iNKT) lymphocytes underlie various inflammatory immune disorders including autoimmunity. Signaling events that control cell fate specification and molecular differentiation of iNKT cells are only partly understood. Here we demonstrate that these processes within iNKT cells require classical NF-kappaB signaling. Inhibition of NF-kappaB signaling blocks iNKT cell ontogeny at an immature stage and reveals an apparent, novel precursor in which negative selection occurs. Most importantly, this block occurs due to a lack of survival signals, as Bcl-x(L) overexpression rescues iNKT cell ontogeny. Maturation of immature iNKT cell precursors induces Bcl-2 expression, which is defective in the absence of NF-kappaB signaling. Bcl-x(L) overexpression also rescues this maturation-induced Bcl-2 expression. Thus, antiapoptotic signals relayed by NF-kappaB critically control cell fate specification and molecular differentiation of iNKT cells and, hence, reveal a novel role for such signals within the immune system.

Parallels and distinctions between T and NKT cell development in the thymus

NKT cells are emerging as an extremely influential regulatory subset of T lymphocytes that are functionally and developmentally distinct from their mainstream counterparts. Like other T cells, NKT cells are thymus-dependent but their apparently unique pathway of differentiation is poorly characterized. Given the strong association between NKT cell deficiency and increased incidences of autoimmunity and cancer it is imperative that the mechanisms by which NKT cells are generated becomes better understood. This review examines what is known about NKT cell development in the thymus and highlights elements of the pathway that differ significantly from mainstream T-cell development. It is here that NKT cell-specific disorders may originate and may best be addressed.

T-bet regulates the terminal maturation and homeostasis of NK and Valpha14i NKT cells

Natural killer (NK) and CD1d-restricted Valpha14i natural killer T (NKT) cells play a critical early role in host defense. Here we show that mice with a targeted deletion of T-bet, a T-box transcription factor required for Th1 cell differentiation, have a profound, stem cell-intrinsic defect in their ability to generate mature NK and Valpha14i NKT cells. Both cell types fail to complete normal terminal maturation and are present in decreased numbers in peripheral lymphoid organs of T-bet(-/-) mice. T-bet expression is regulated during NK cell differentiation by NK-activating receptors and cytokines known to control NK development and effector function. Our results identify T-bet as a key factor in the terminal maturation and peripheral homeostasis of NK and Valpha14i NKT cells.

Aging of innate immunity: functional comparisons of NK/LAK cells obtained from bulk cultures of young and aged mouse spleen cells in high concentrations of interleukin-2

The technique of bulk cultivation of aged mouse spleen cells in high concentration of IL-2 was employed to obtain NK/LAK cells in sufficient number and enrichment for studies on the effects of aging on their functions. The yield and enrichment were equivalent to that of young mouse spleen cells. The aged and young mouse NK/LAK cells were equivalent also in their functional competence to proliferate, kill target cells and produce IFNgamma; i.e. they did not display age-associated defects typical of freshly-isolated NK/LAK cells. In two respects, however, the NK/LAK cells derived from aged mouse spleen were altered: (a) in the efficiency of nuclear translocation of transcription factors STAT 5A and 5B, and (b) in the deficiency in production of mRNA transcripts representing several chemokines. We recommend caution in the use of bulk cultivation in IL-2 to obtain NK/LAK cells for studies on aging. However, it does appear from this study that aging may severely affect chemokine production, at least in the case of NK/LAK cells.

Antigen-induced tolerance by intrathymic modulation of self-recognizing inhibitory receptors

CD1d-restricted invariant natural killer T cell (iNKT cells) have a limited T cell receptor (TCR) repertoire and share characteristics common to T cells and natural killer cells. While intrathymic selection facilitates the production of T cells carrying self major histocompatibility complex-restricted TCRs, natural killer cells carry an appropriate repertoire of self major histocompatibility complex-recognizing receptors to avoid self-reactivity. Here we show that chronic exposure to specific glycolipid antigen resulted in iNKT cell disappearance and thymus-dependent repopulation of iNKT cells with increased expression of inhibitory Ly-49 molecules that resulted in impaired responsiveness. Thymic selection of peripheral Ly-49-expressing iNKT cell repertoire inhibited cytokine production and other functions in vivo. These observations emphasize the acquisition of self-recognizing inhibitory receptors on NKT cells as a previously unknown mechanism of thymic tolerance after chronic antigen exposure.

Development of innate CD4+ alpha-chain variable gene segment 24 (Valpha24) natural killer T cells in the early human fetal thymus is regulated by IL-7

Natural killer (NK) T cells are innate CD1d-restricted immune cells involved in regulation of immune tolerance, tumor immunity, and immunity to infectious pathogens. Human alpha-chain variable gene segment 24 (Valpha24) NK T cells exist in the periphery as two functionally distinct subsets: one CD4+ and one CD4- subset. However, the developmental pathway of human Valpha24 NK T cells is not well understood. Here, we show that Valpha24 NK T cells develop in the fetal thymus. The relative number of intrathymic NK T cell precursors decline in a linear manner with gestational age, and they are very rare in the neonatal thymus, indicating that these cells preferentially develop in the early fetal thymus. Their restriction element, CD1d, is expressed by a vast majority of thymocytes. A majority of intrathymic Valpha24 NK T cell progenitors are CD4+, whereas a minority are CD4/8(+/+). CD4+ Valpha24 NK T cell precursors show features of mature NK T cells, such as high levels of their semiinvariant T cell receptor and CD3 and some expression of CD161, whereas the CD4/8(+/+) precursors seem less mature. The cytokine IL-7 shows a biphasic effect on Valpha24 NK T cell progenitors in fetal thymic organ culture, with high doses driving proliferation of immature CD161-progenitors and low doses supporting survival and maturation. Thus, the data demonstrate that human Valpha24 NK T cells of the CD4+, but not the CD4-, subset develop in the early fetal thymus. Furthermore, data suggest an intrathymic pathway of CD4+ Valpha24 NK T cell development that is regulated by IL-7.

Distinct cell types control lymphoid subset development by means of IL-15 and IL-15 receptor alpha expression

IL-15 and the IL-15 receptor (IL-15R)alpha chain are essential for normal development of naive CD8 T cells, intestinal intraepithelial lymphocytes (IEL), and natural killer (NK)/NK/T cells. However, whether IL-15R alpha expression by these subsets is necessary for their production and which cell type needs to produce IL-15 to drive development are unknown. We analyzed the requirements for IL-15 and IL-15R alpha expression by bone marrow-derived or parenchymal cells for mediating lymphocyte subset development. Naive CD8 T cell development required IL-15R alpha expression by both bone marrow-derived and parenchymal cells, whereas memory-phenotype CD8 T cells required IL-15R alpha expression only by hematopoietic cells. In contrast and surprisingly, the development of IEL subsets, particularly CD8 alpha alpha Thy1(-)V gamma 5(+) T cell antigen receptor gamma delta and the CD8 alpha alpha Thy1(-) T cell antigen receptor alpha beta IEL populations, depended completely on parenchymal cell expression of IL-15R alpha and IL-15 but not IL-15R beta. In the case of NK and NK/T cell generation and maturation, expression of IL-15 and IL-15R alpha by both parenchymal and hematopoietic cells was important, although the latter played the greatest role. These results demonstrated dichotomous mechanisms by which IL-15 regulated lymphoid development, interacting with distinct cell types depending on the developmental pathway.

Lineage choices in the developing thymus: choosing the T and NKT pathways

Thymic development proceeds through several defined stages that generate not only alpha beta and gamma delta T cells but can produce dendritic cells and B cells. The earliest thymocytes exist in the CD4(-)CD8(-) double negative compartment within a heterogeneous fraction termed DN1. Recent progress has identified several candidate populations that may be the bone fide T-cell progenitor population. The potential roles of these populations, which include hematopoietic stem cells, early lymphocyte precursors, common lymphoid progenitors, and early T lineage progenitors are being elucidated. The alpha beta T-cell lineage consists of distinct subsets, one of which is NKT cells. The developmental relationship of NKT cells to conventional T cells has been controversial. Recent work has shown that these cells are probably derived from CD4(+)CD8(+) thymocytes. The discovery and application of CD1d tetramers has made it possible to more fully describe NKT-cell development.

Generation of Valpha14 NKT cells in vitro from hematopoietic precursors residing in bone marrow and peripheral blood

We previously reported the generation of Valpha14 invariant TCR+ (Valpha14i) NK1.1+ natural killer T (NKT) cells in the cytokine-activated suspension culture of murine fetal liver cells. In this study, we attempted to apply this finding to the induction of Valpha14i NKT cell differentiation in the culture of hematopoietic precursors residing in bone marrow or peripheral blood. Preferential generation of NKT cells was found in the culture of Thy-1(+)-depleted bone marrow cells in the presence of culture supernatant from Con A-stimulated spleen T cells and a combination of recombinant IL-3, IL-4, IL-7 and GM-CSF. NKT cell development from peripheral blood hematopoietic precursors was induced when they were cultured on stromal cell monolayers prepared from Thy-1(+)-depleted bone marrow or fetal liver cells, suggesting that certain environments derived from hematopoietic organs are required for the induction of NKT cells from precursors in vitro. A significant fraction of NKT cells generated in the culture were positive for staining with CD1-alpha-galactosylceramide tetramer, indicating that Valpha14i NKT cells were the major subset among the NKT cells. The present methods for obtaining NKT cells in the culture of bone marrow or peripheral blood cells are applicable to the treatment of patients suffering from diseases with numerical and functional disorders of NKT cells.

Control of NKT cell differentiation by tissue-specific microenvironments

CD1d-restricted Valpha14 NKT cells play an important role in both Th1- and Th2-type immune responses. To determine whether NKT cells develop two functionally distinct subsets that provoke different types of responses, we examined the phenotypes and cellular functions of NK1.1(+) and DX5(+) T cells. We found that both NK1.1(+) and DX5(+) T cells are CD1d-restricted Valpha14 T cells with identical Ag specificities, phenotypes, tissue locations, and functions. Similar to the NK1.1 marker, the DX5 marker (CD49b) is expressed on mature NKT cells in both NK1.1 allele-positive and allele-negative strains. However, when NK1.1(+) and DX5(+) NKT cells isolated from different tissues were compared, we found that thymic and splenic NKT cells differed not only in their cytokine profiles, but also in their phenotype and requirements for costimulatory signals. Thymic NKT cells displayed the phenotype of activated T cells and could be fully activated by TCR ligation. In contrast, splenic NKT cells displayed the phenotype of memory T cells and required a costimulatory signal for activation. Furthermore, the function and phenotype of thymic and splenic NKT cells were modulated by APCs from various tissues that expressed different levels of costimulatory molecules. Modulation of NKT cell function and differentiation may be mediated by synergic effects of costimulatory molecules on the surface of APCs. The results of the present study suggest that the costimulatory signals of tissue-specific APCs are key factors for NKT cell differentiation, and these signals cannot be replaced by anti-CD28 or anti-CD40 ligand Abs.

MHC-dependent and -independent modulation of endogenous Ly49 receptors on NK1.1+ T lymphocytes directed by T-cell receptor type

Natural killer (NK) T lymphocytes are thought to act as regulatory cells directing early events during immune responses. Murine NKT cells express inhibitory receptors of the Ly49 family. These receptors have a well-established and crucial role in modulating NK cell activities, but their physiological role in regulating NKT cells is not well understood, nor is the influence of major histocompatibility (MHC) ligands on endogenous Ly49 expression. We have further investigated how the expression of inhibitory NK receptors is regulated on NKT cells, and demonstrate a non-random expression of ligated Ly49 molecules on CD1d-restricted NKT cells. The nature of the T-cell receptor on the NKT cell crucially determines the profile of expressed Ly49 isoforms. Further, we show that MHC class I ligands efficiently modulate the expression levels of the inhibitory receptors, and the frequencies of cells positive for the Ly49 members. In addition, we find a several-fold increase in Ly49C/I-expressing NKT cells in adult thymus, apparently independent of MHC class I molecules. Abundant expression of Ly49 receptors on NKT cells, and the striking differences found in Ly49 isoform patterns on NKT-cell subsets differing in T-cell receptor expression, suggest that the pattern of Ly49 expression is tuned to fit the T-cell receptor and to emphasize further a role for these receptors in NKT immunity.

Activation of natural killer T cells in NZB/W mice induces Th1-type immune responses exacerbating lupus

In vivo treatment of mice with the natural killer T (NKT) cell ligand, alpha-galactosylceramide (alphaGalCer), ameliorates autoimmune diabetes and experimental autoimmune encephalomyelitis (EAE) by shifting pathogenic Th1-type immune responses to nonpathogenic Th2-type responses. In the current study, in vivo activation of NKT cells in adult NZB/W mice by multiple injections of alphaGalCer induced an abnormal Th1-type immune response as compared with the Th2-type response observed in nonautoimmune C57BL/6 mice. This resulted in decreased serum levels of IgE, increased levels of IgG2a and IgG2a anti-double-stranded DNA (anti-dsDNA) Ab's, and exacerbated lupus. Conversely, treatment of NZB/W mice with blocking anti-CD1d mAb augmented Th2-type responses, increased serum levels of IgE, decreased levels of IgG2a and IgG2a anti-dsDNA Ab's, and ameliorated lupus. While total CD4+ T cells markedly augmented in vitro IgM anti-dsDNA Ab secretion by splenic B cells, the non-CD1d-reactive (CD1d-alphaGalCer tetramer-negative) CD4+ T cells (accounting for 95% of all CD4+ T cells) failed to augment Ab secretion. The CD1d-reactive tetramer-positive CD4+ T cells augmented anti-dsDNA Ab secretion about tenfold. In conclusion, activation of NKT cells augments Th1-type immune responses and autoantibody secretion that contribute to lupus development in adult NZB/W mice, and anti-CD1d mAb might be useful for treating lupus.

Constitutive cytokine mRNAs mark natural killer (NK) and NK T cells poised for rapid effector function

Natural killer (NK) and NK T cells are tissue lymphocytes that secrete cytokines rapidly upon stimulation. Here, we show that these cells maintain distinct patterns of constitutive cytokine mRNAs. Unlike conventional T cells, NK T cells activate interleukin (IL)-4 and interferon (IFN)-γ transcription during thymic development and populate the periphery with both cytokine loci previously modified by histone acetylation. Similarly, NK cells transcribe and modify the IFN-γ gene, but not IL-4, during developmental maturation in the bone marrow. Lineage-specific patterns of cytokine transcripts predate infection and suggest evolutionary selection for invariant but distinct types of effector responses among the earliest responding lymphocytes.

Innate immunity in the liver

PURPOSE OF REVIEW

The liver is constantly exposed to large varieties of antigens that are derived from the gastrointestinal tract, including dietary antigens, pathogens, and toxins. Its function as a major immune organ is now being appreciated. The liver lymphocyte population is enriched in macrophages (ie, Kupffer cells), natural killer and natural killer T cells, which constitute the innate immune system. This review will focus on recent advances in the understanding of mechanisms that regulate the hepatic innate immune system because the innate immune system may mediate many chronic liver diseases, including nonalcoholic fatty liver disease, and nonalcoholic steatohepatitis.

RECENT FINDINGS

Hepatic natural killer T cells modulate liver injury by balancing local production of proinflammatory (Th-1) and antiinflammatory (Th-2) cytokines. Hepatic natural killer T cell depletion leads to Th-1 polarization of hepatic cytokine production, increasing tumor necrosis factor alpha and interferon gamma. This potentates lipopolysaccharide-induced hepatotoxicity. The hepatic natural killer T cells themselves are regulated by Kupffer-cell-produced cytokines, dietary factors, and certain neurotransmitters, such as norepinephrine. In leptin-deficient ob/ob mice, an animal model for nonalcoholic fatty liver disease and nonalcoholic steatohepatitis, insufficient norepinephrine increases hepatic natural killer T cell apoptosis, depleting hepatic natural killer T cells and inducing proinflammatory cytokine polarization. This contributes to chronic inflammation, increased lipopolysaccharide-induced hepatotoxicity, and insulin resistance in ob/ob mice.

SUMMARY

Assuming that defects in the hepatic innate immune system that promote Th-1 cytokine polarization are common pathogenic mechanisms for hepatic insulin resistance and nonalcoholic steatohepatitis, therapies that inhibit inflammatory activity may be beneficial for these disorders.

Repeated α-Galactosylceramide Administration Results in Expansion of NK T Cells and Alleviates Inflammatory Dermatitis in MRL-lpr/lpr Mice

NK T (NKT) cells expressing the invariant Valpha14-Jalpha18 TCR alpha-chain recognize glycolipid Ags such as alpha-galactosylceramide (alpha-GalCer) presented by the MHC class I-like molecule CD1d. Upon activation by alpha-GalCer, invariant NKT cells secrete multiple cytokines and confer protection in certain immune-mediated disorders. Here we have investigated the role of NKT cells in the development of inflammatory dermatitis in MRL-lpr/lpr mice, which shares features with lupus in humans. Our results show that the numbers Sand functions of NKT (TCRbeta(+)CD1d/alpha-GalCer tetramer(+)) cells, particularly of the NK1.1(-) subset, are reduced in MRL-lpr/lpr mice compared with MRL-fas/fas and/or nonautoimmune C3H/Hej and BALB/c mice. Repeated treatments with alpha-GalCer result in the expansion of NKT cells and alleviate dermatitis in MRL-lpr/lpr mice. Our results indicate that NKT cell deficiency can be corrected by repeated alpha-GalCer treatment and that NKT cells may play a protective role in inflammatory dermatitis of lupus-prone mice.

T Cell Development in Mice Expressing CD1d Directed by a Classical MHC Class II Promoter

CD1d and nonclassical MHC molecules differ markedly from classical MHC ligands in their ability to promote the selection and differentiation of developing T cells. Whereas classical MHC-restricted T cells have a predominantly naive phenotype and a broad TCR repertoire, most other T cells have a memory and/or NKT phenotype with a restricted repertoire. Because the nonclassical ligands selecting these memory-type cells are expressed by bone marrow-derived cells, it has been suggested that the development of large repertoires of naive-type cells was dependent on the classical MHC expression pattern in the thymus cortex, high on epithelial cells and low on cortical thymocytes. We redirected CD1d expression using the classical MHC II Ealpha promoter. pEalpha-CD1d mice lacked memory-type NKT cells, but, surprisingly, they did not acquire the reciprocal ability to select a diverse population of naive CD1d-restricted cells. These findings suggest that, whereas the development of NKT cells is dependent on the pattern of CD1d expression, the absence of a broad, naive CD1d-restricted T cell repertoire may reflect intrinsic limitations of the pool of TCR genes or lipid Ags.

Glycolipid Antigen Drives Rapid Expansion and Sustained Cytokine Production by NK T Cells

NKT cells are enigmatic lymphocytes that respond to glycolipid Ags presented by CD1d. Although they are key immunoregulatory cells, with a critical role in immunity to cancer, infection, and autoimmune diseases, little is known about how they respond to antigenic challenge. Current theories suggest that NKT cells die within hours of stimulation, implying that their direct impact on the immune system derives from the initial cytokine burst released before their death. Here we show that NKT cell disappearance results from TCR down-regulation rather than apoptosis, and that they expand to many times their normal number in peripheral tissues within 2-3 days of stimulation, before contracting to normal numbers over subsequent days. This expansion is associated with ongoing cytokine production, biased toward a Th1 (IFN-gamma(+) IL-4(-)) phenotype, in contrast to their initial Th0 (IFN-gamma(+)IL-4(+)) phenotype. This study provides critical new insight into how NKT cells can have such a major impact on immune responses, lasting many days beyond the initial stimulation of these cells.

The response of natural killer T cells to glycolipid antigens is characterized by surface receptor down-modulation and expansion

CD1d-restricted natural killer T (NKT) cells are a subset of regulatory T cells that react with glycolipid antigens. Although preclinical studies have effectively targeted NKT cells for immunotherapy, little is known regarding the early in vivo response of these cells to antigenic stimulation. We have analyzed the early response of NKT cells to glycolipid antigens and bacterial infection by using specific reagents for tracking these cells. Our results demonstrate dramatic in vivo expansion and surface phenotype alterations after NKT cell activation with alpha-galactosylceramide. In addition, we show significant NK1.1 down-modulation on NKT cells in the setting of oral Salmonella infection. Our results indicate that in vivo activation of NKT cells leads to a dynamic response characterized by surface receptor down-modulation and expansion. These findings alter current understanding of NKT cell biology and should aid in the rational design of NKT cell-based immunotherapies.

Differential chemokine responses and homing patterns of murine TCR alpha beta NKT cell subsets

NKT cells play important roles in the regulation of diverse immune responses. Therefore, chemokine receptor expression and chemotactic responses of murine TCRalphabeta NKT cells were examined to define their homing potential. Most NKT cells stained for the chemokine receptor CXCR3, while >90% of Valpha14i-positive and approximately 50% of Valpha14i-negative NKT cells expressed CXCR6 via an enhanced green fluorescent protein reporter construct. CXCR4 expression was higher on Valpha14i-negative than Valpha14i-positive NKT cells. In spleen only, subsets of Valpha14i-positive and -negative NKT cells also expressed CXCR5. NKT cell subsets migrated in response to ligands for the inflammatory chemokine receptors CXCR3 (monokine induced by IFN-gamma/CXC ligand (CXCL)9) and CXCR6 (CXCL16), and regulatory chemokine receptors CCR7 (secondary lymphoid-tissue chemokine (SLC)/CC ligand (CCL)21), CXCR4 (stromal cell-derived factor-1/CXCL12), and CXCR5 (B cell-attracting chemokine-1/CXCL13); but not to ligands for other chemokine receptors. Two NKT cell subsets migrated in response to the lymphoid homing chemokine SLC/CCL21: CD4(-) Valpha14i-negative NKT cells that were L-selectin(high) and enriched for expression of Ly49G2 (consistent with the phenotype of most NKT cells found in peripheral lymph nodes); and immature Valpha14i-positive cells lacking NK1.1 and L-selectin. Mature NK1.1(+) Valpha14i-positive NKT cells did not migrate to SLC/CCL21. BCA-1/CXCL13, which mediates homing to B cell zones, elicited migration of Valpha14i-positive and -negative NKT cells in the spleen. These cells were primarily CD4(+) or CD4(-)CD8(-) and were enriched for Ly49C/I, but not Ly49G2. Low levels of chemotaxis to CXCL16 were only detected in Valpha14i-positive NKT cell subsets. Our results identify subsets of NKT cells with distinct homing and localization patterns, suggesting that these populations play specialized roles in immunological processes in vivo.

Most IL-4-producing gamma delta thymocytes of adult mice originate from fetal precursors

Thy-1(dull) gammadelta T cells constitute a distinct adult gammadelta T cell subset characterized by the expression of a TCR composed of Vgamma1Cgamma4 and Vdelta6Cdelta chains with limited junctional sequence diversity. However, several features of the expressed Thy-1(dull) TCR-gammadelta genes, in particular the absence or minimal presence of N region diversity and the almost invariable Ddelta2-Jdelta1 junction, are typical of rearrangements often found in the fetal thymus. In this study, we have investigated the origin of these cells. Few Thy-1(dull) gammadelta thymocytes developed in syngeneic radiation adult chimeras, regardless of whether the recipient mice were given adult bone marrow or fetal liver cells as a source of hemopoietic precursors. In contrast, normal numbers of Thy-1(dull) gammadelta T cells developed in fetal thymi grafted into adult syngeneic recipients. Interestingly, the majority of Thy-1(dull) gammadelta thymocytes present in the grafts were of graft origin, even when most conventional gammadelta and alphabeta thymocytes in the grafted thymi originated from T cell precursors of recipient origin. Single-cell PCR analyses of the nonselected TCR-gamma rearrangements present in adult Thy-1(dull) gammadelta thymocytes revealed that more than one-half of these cells represent the progenies of a limited number of clones that greatly expanded possibly during the first weeks of life. Finally, the second TCR-delta allele of a large number of Thy-1(dull) gammadelta T cells contained incomplete TCR-delta rearrangements, thus providing an explanation for the adult-type rearrangements previously found among nonfunctional V(D)J rearrangements present in Thy-1(dull) gammadelta thymocytes.

Positive and negative selection of T cells

A functional immune system requires the selection of T lymphocytes expressing receptors that are major histocompatibility complex restricted but tolerant to self-antigens. This selection occurs predominantly in the thymus, where lymphocyte precursors first assemble a surface receptor. In this review we summarize the current state of the field regarding the natural ligands and molecular factors required for positive and negative selection and discuss a model for how these disparate outcomes can be signaled via the same receptor. We also discuss emerging data on the selection of regulatory T cells. Such cells require a high-affinity interaction with self-antigens, yet differentiate into regulatory cells instead of being eliminated.

The regulatory role of Valpha14 NKT cells in innate and acquired immune response

A novel lymphocyte lineage, Valpha14 natural killer T (NKT) cells, is now well established as distinct from conventional alphabeta T cells. Valpha14 NKT cells express a single invariant Valpha14 antigen receptor that is essential for their development. Successful identification of a specific ligand, alpha-galactosylceramide(alpha-GalCer), and the establishment of gene-manipulated mice with selective loss of Valpha14 NKT cells helped elucidate the remarkable functional diversity of Valpha14 NKT cells in various immune responses such as host defense by mediating anti-nonself innate immune reaction, homeostatic regulation of anti-self responses, and antitumor immunity.

An anti-inflammatory role for V alpha 14 NK T cells in Mycobacterium bovis bacillus Calmette-Guérin-infected mice

The possible contribution of NKT cells to resistance to Mycobacterium tuberculosis infection remains unclear. In this paper we characterized the Valpha14 NKT cell population following infection with Mycobacterium bovis bacillus Calmette-Guérin (BCG). BCG infection determined an early expansion of Valpha14 NKT cells in liver, lungs, and spleen, which peaked on day 8 and was sustained until day 30. However, an NK1.1(+) Valpha14 NKT population preferentially producing IFN-gamma predominated at an early stage (day 8), which was substituted by an NK1.1(-) population preferentially producing IL-4 at later stages (day 30). Despite the fact that Valpha14 NKT cell-deficient mice eliminated BCG as did control mice, they had significantly higher numbers of granulomas in liver and lungs. Additionally, while control mice developed organized small granulomas, those in Valpha14 NKT-deficient mice had signs of caseation, large cellular infiltrates, and some multinucleated macrophages, suggesting that Valpha14 NKT cells may actually work as anti-inflammatory cells by limiting excessive lymphocyte influx and tissue pathology. In agreement, we found an increased spontaneous production and mRNA expression of TNF-alpha in liver and lungs of Valpha14 NKT-deficient mice, whose neutralization in vivo by anti-TNF-alpha mAbs consistently reduced the number of granulomas in liver and lungs. Together, our results support a regulatory role for Valpha14 NKT cells in the course of BCG infection through their ability to limit the extent of inflammatory response and point to an important role for this cell subset as a regulator of the balance between protective responses and immunopathology.

Differential requirement for Rel/nuclear factor kappa B family members in natural killer T cell development

Natural killer T (NKT) cells have been implicated in diverse immune responses ranging from suppression of autoimmunity to tumor rejection. Thymus-dependent NKT cells are positively selected by the major histocompatibility complex class I-like molecule CD1d, but the molecular events downstream of CD1d are still poorly understood. Here, we show that distinct members of the Rel/nuclear factor (NF)-kappa B family of transcription factors were required in both hematopoietic and nonhematopoietic cells for normal development of thymic NKT cells. Activation of NF-kappa B via the classical I kappa B alpha-regulated pathway was required in a cell autonomous manner for the transition of NK-1.1-negative precursors that express the TCR V alpha 14-J alpha 18 chain to mature NK-1.1-positive NKT cells. The Rel/NF-kappa B family member RelB, on the other hand, had to be expressed in radiation resistant thymic stromal cells for the generation of early NK-1.1-negative NKT precursors. Moreover, NF-kappa B-inducing kinase (NIK) was required for both constitutive thymic DNA binding of RelB and the specific induction of RelB complexes in vitro. Thus, distinct Rel/NF-kappa B family members in hematopoietic and nonhematopoietic cells regulate NKT cell development with a unique requirement for NIK-mediated activation of RelB in thymic stroma.

NIK-dependent RelB activation defines a unique signaling pathway for the development of V alpha 14i NKT cells

A defect in RelB, a member of the Rel/nuclear factor (NF)-kappa B family of transcription factors, affects antigen presenting cells and the formation of lymphoid organs, but its role in T lymphocyte differentiation is not well characterized. Here, we show that RelB deficiency in mice leads to a selective decrease of NKT cells. RelB must be expressed in an irradiation-resistant host cell that can be CD1d negative, indicating that the RelB expressing cell does not contribute directly to the positive selection of CD1d-dependent NKT cells. Like RelB-deficient mice, aly/aly mice with a mutation for the NF-kappa B-inducing kinase (NIK), have reduced NKT cell numbers. An analysis of NK1.1 and CD44 expression on NKT cells in the thymus of aly/aly mice reveals a late block in development. In vitro, we show that NIK is necessary for RelB activation upon triggering of surface receptors. This link between NIK and RelB was further demonstrated in vivo by analyzing RelB+/- x aly/+ compound heterozygous mice. After stimulation with alpha-GalCer, an antigen recognized by NKT cells, these compound heterozygotes had reduced responses compared with either RelB+/- or aly/+ mice. These data illustrate the complex interplay between hemopoietic and nonhemopoietic cell types for the development of NKT cells, and they demonstrate the unique requirement of NKT cells for a signaling pathway mediated by NIK activation of RelB in a thymic stromal cell.

T-cell effector mechanisms: gammadelta and CD1d-restricted subsets

Gammadelta T lymphocytes and CD1d-restricted natural killer T cells are classified as innate T lymphocytes, which perform effector functions that protect from malignancy and maintain tissue integrity. Innate T cells also play important regulatory roles in autoimmunity, inflammation and infection. Recent advances have established innate T cells as both effectors and regulators of disease in biological models.

Innate self recognition by an invariant, rearranged T-cell receptor and its immune consequences

This review attempts to illuminate the glycolipid antigen presentation properties of CD1d, how CD1d controls the function of natural T (iNKT) cells and how CD1d and iNKT cells interact to jump-start the immune system. It is postulated that the CD1d-iNKT cell system functions as a sensor, sensing alterations in cellular lipid content by virtue of its affinity for such ligands. The presentation of a neo-self glycolipid, presumably by infectious assault of antigen-presenting cells, activates iNKT cells, which promptly release pro-inflammatory and anti-inflammatory cytokines and jump-start the immune system.

Generation of CD1 tetramers as a tool to monitor glycolipid-specific T cells

CD1 molecules are beta(2)m-associated HLA class-I-like glycoproteins which have the unique ability to present glycolipid and phospholipid antigens to specific T lymphocytes. To study the biology of CD1 and its role in human disease we developed novel techniques for generation of recombinant CD1/lipid complexes by in vitro refolding. Fluorescent tetrameric complexes made from soluble recombinant CD1d/alpha-galactosylceramide complexes allowed highly sensitive and specific ex vivo and in vitro detection and functional characterization of novel human T-lymphocyte populations. Furthermore, protein crystals were obtained from soluble recombinant CD1b/beta(2)m-proteins loaded either with phosphatidylinositol or ganglioside GM2, which led to the first atomic structure determination of a CD1/lipid complex. The analysis of these crystal structures clarified how CD1b molecules can bind lipid ligands of different size, and revealed a broader spectrum of potential CD1b ligands than previously predicted.

What does it take to make a natural killer?

We know how B and T cells develop, what they 'see' and the receptors they 'see with'. By contrast, and despite an unprecedented increase in the number of receptors and ligands known to regulate the activity of natural killer (NK) cells, we still have many questions regarding how these cells develop. Nevertheless, we are beginning to understand the transcriptional programmes of NK-cell maturation and the role of the effector functions of NK cells in the regulation of immune responses. An improved knowledge of NK-cell development in mice and humans might be useful to harness the power of these natural killers in the clinic to fight autoimmune diseases, infection and cancer.

Regulatory T cells in the control of transplantation tolerance and autoimmunity

A role for immunoregulatory T cells in the maintenance of self-tolerance and in transplantation tolerance has long been suggested, but the identification of such cells has not been achieved until recently. With the characterisation of spontaneously occurring CD4+CD25+ and NK1.1+ T subpopulations of T cells as regulatory cells in rodents and in humans, together with several in vitro generated regulatory T-cell populations, it seems possible that 'customised' regulatory cells possessing antidonor specificity may become therapeutic tools in clinical transplantation tolerance.

Ligand-dependent inhibition of CD1d-restricted NKT cell development in mice transgenic for the activating receptor Ly49D

In addition to their CD1d-restricted T cell receptor (TCR), natural killer T (NKT) cells express various receptors normally associated with NK cells thought to act, in part, as modulators of TCR signaling. Immunoreceptor-tyrosine activation (ITAM) and inhibition (ITIM) motifs associated with NK receptors may augment or attenuate perceived TCR signals respectively, potentially influencing NKT cell development and function. ITIM-containing Ly49 family receptors expressed by NKT cells are proposed to play a role in their development and function. We have produced mice transgenic for the ITAM-associated Ly49D and ITIM-containing Ly49A receptors and their common ligand H2-Dd to determine the importance of these signaling interplays in NKT cell development. Ly49D/H2-Dd transgenic mice had selectively and severely reduced numbers of thymic and peripheral NKT cells, whereas both ligand and Ly49D transgenics had normal numbers of NKT cells. CD1d tetramer staining revealed a blockade of NKT cell development at an early precursor stage. Coexpression of a Ly49A transgene partially rescued NKT cell development in Ly49D/H2-Dd transgenics, presumably due to attenuation of ITAM signaling. Thus, Ly49D-induced ITAM signaling is incompatible with the early development of cells expressing semi-invariant CD1d-restricted TCRs and appropriately harmonized ITIM-ITAM signaling is likely to play an important role in the developmental program of NKT cells.

CD1d-expressing dendritic cells but not thymic epithelial cells can mediate negative selection of NKT cells

Natural killer T (NKT) cells are a unique immunoregulatory T cell population that is positively selected by CD1d-expressing thymocytes. Previous studies have shown that NKT cells exhibit autoreactivity, which raises the question of whether they are subject to negative selection. Here, we report that the addition of agonist glycolipid alpha-galactosylceramide (alpha-GalCer) to a fetal thymic organ culture (FTOC) induces a dose-dependent disappearance of NKT cells, suggesting that NKT cells are susceptible to negative selection. Overexpression of CD1d in transgenic (Tg) mice results in reduced numbers of NKT cells, and the residual NKT cells in CD1d-Tg mice exhibit both an altered Vbeta usage and a reduced sensitivity to antigen. Furthermore, bone marrow (BM) chimeras between Tg and WT mice reveal that CD1d-expressing BM-derived dendritic cells, but not thymic epithelial cells, mediate the efficient negative selection of NKT cells. Thus, our data suggest that NKT cells developmentally undergo negative selection when engaged by high-avidity antigen or abundant self-antigen.

Human NKT cells express granulysin and exhibit antimycobacterial activity

Human NKT cells are a unique subset of T cells that express an invariant V alpha 24 TCR that recognizes the nonclassical Ag-presenting molecule CD1d. Activation of NKT cells is greatly augmented by the marine sponge-derived glycolipid alpha-galactosylceramide (alpha GalCer). Because human monocyte-derived cells express CD1d and can harbor the intracellular pathogen Mycobacterium tuberculosis, we asked whether the addition of alpha GalCer could be used to induce effector functions of NKT cells against infected monocytes, macrophages, and monocyte-derived dendritic cells. NKT cells secreted IFN-gamma, proliferated, and exerted lytic activity in response to alpha GalCer-pulsed monocyte-derived cells. Importantly, alpha GalCer-activated NKT cells restricted the growth of intracellular M. tuberculosis in a CD1d-dependent manner. NKT cells that exhibited antimycobacterial activity also expressed granulysin, an antimicrobial peptide shown to mediate an antimycobacterial activity through perturbation of the mycobacterial surface. Degranulation of NKT cells resulted in depletion of granulysin and abrogation of antimycobacterial activity. The detection of CD1d in granulomas of tuberculosis patients supports the potential interaction of NKT cells with CD1d-expressing cells at the site of disease activity. These studies provide evidence that alpha Gal Cer-activated CD1d-restricted T cells can participate in human host defense against M. tuberculosis infection.

IL-15 availability conditions homeostasis of peripheral natural killer T cells

Steady-state numbers of peripheral lymphocyte are tightly controlled. For conventional T cells, signals delivered through the interaction of the T cell receptor (TCR) with antigen-loaded MHC molecules are required for the peripheral survival of naive T cells and for their homeostatic expansion in lymphopenic hosts. Cytokines, including IL-7, are also essential for survival of peripheral naive T cells. CD1d-restricted, V alpha 14(+) natural killer (NK)-T cells are a specialized autoreactive T subset with immunoregulatory activity. The relative roles of TCR engagement and cytokine signaling in the peripheral homeostasis of V alpha 14(+) NK-T cells were investigated. After adoptive transfer, the survival and expansion of peripheral V alpha 14(+) NK-T cells was independent of CD1d expression in the host. In contrast, IL-15 (but not IL-7) was required for maintenance of peripheral CD1d-reactive V alpha 14(+) T cells. Comparison of V alpha 14(+) T cell transfers into NK-proficient vs. deficient hosts suggests that NK-T cells and NK cells compete for peripheral resources. Our results indicate that IL-15 maintains the homeostasis of peripheral V alpha 14(+) NK-T cells. In contrast, TCR "tickling" of NK-T cells, if it occurs under steady-state conditions, does not by itself provide a sufficient signal for their peripheral survival.

BATF transgenic mice reveal a role for activator protein-1 in NKT cell development

The importance of regulated AP-1 activity during T cell development was assessed using transgenic mice overexpressing BATF, a basic leucine zipper transcription factor and an AP-1 inhibitor. BATF transgenic animals possess normal thymic cellularity and all major T cell subsets, but show impaired thymocyte proliferation in vitro and no induction of IL-2, IL-4, IL-5, IL-10, and IL-13 expression. Since NKT cells are largely responsible for cytokine production in the thymus, this population was examined by detection of the V alpha 14-J alpha 281 TCR, flow cytometry of NK1.1(+) TCR beta(+) cells, and analysis of cytokine production by heat-stable Ag(low) thymocytes and peripheral NKT cells stimulated in vivo. Results show a severe under-representation of NKT cells in BATF transgenic animals, providing the first evidence that the precise control of AP-1-mediated transcription is critical for the proper emergence of thymus-derived NKT cells in the mouse.

Human invariant V alpha 24-J alpha Q TCR supports the development of CD1d-dependent NK1.1+ and NK1.1- T cells in transgenic mice

A sizable fraction of T cells expressing the NK cell marker NK1.1 (NKT cells) bear a very conserved TCR, characterized by homologous invariant (inv.) TCR V alpha 24-J alpha Q and V alpha 14-J alpha 18 rearrangements in humans and mice, respectively, and are thus defined as inv. NKT cells. Because human inv. NKT cells recognize mouse CD1d in vitro, we wondered whether a human inv. V alpha 24 TCR could be selected in vivo by mouse ligands presented by CD1d, thereby supporting the development of inv. NKT cells in mice. Therefore, we generated transgenic (Tg) mice expressing the human inv. V alpha 24-J alpha Q TCR chain in all T cells. The expression of the human inv. V alpha 24 TCR in TCR C alpha(-/-) mice indeed rescues the development of inv. NKT cells, which home preferentially to the liver and respond to the CD1d-restricted ligand alpha-galactosylceramide (alpha-GalCer). However, unlike inv. NKT cells from non-Tg mice, the majority of NKT cells in V alpha 24 Tg mice display a double-negative phenotype, as well as a significant increase in TCR V beta 7 and a corresponding decrease in TCR V beta 8.2 use. Despite the forced expression of the human CD1d-restricted TCR in C alpha(-/-) mice, staining with mCD1d-alpha-GalCer tetramers reveals that the absolute numbers of peripheral CD1d-dependent T lymphocytes increase at most by 2-fold. This increase is accounted for mainly by an increased fraction of NK1.1(-) T cells that bind CD1d-alpha-GalCer tetramers. These findings indicate that human inv. V alpha 24 TCR supports the development of CD1d-dependent lymphocytes in mice, and argue for a tight homeostatic control on the total number of inv. NKT cells. Thus, human inv. V alpha 24 TCR-expressing mice are a valuable model to study different aspects of the inv. NKT cell subset.

Extrathymic T cell lymphopoiesis: ontogeny and contribution to gut intraepithelial lymphocytes in athymic and euthymic mice

In the absence of thymopoiesis, T lymphocytes are nevertheless present, mainly in the gut epithelium. Ontogeny of the extrathymic pathway and the extent of its involvement in euthymic mice are controversial. These questions have been addressed by assessing the expression of recombinase activating gene (RAG) through the use of green fluorescent protein RAG2 transgenic mouse models. In athymic mice, T lymphopoiesis occurs mainly in the mesenteric lymph node and less in the Peyer's patches. Ontogenic steps of this lymphopoiesis resemble those of thymopoiesis, but with an apparent bias toward gamma delta T cell production and with a paucity of oligoclonal alpha beta T cells possibly resulting from a deficit in positive selection. Whether in athymic or euthymic mice, neither T intraepithelial lymphocytes (IEL) nor cryptopatch cells (reported to contain precursors of IEL) displayed fluorescence indicating recent RAG protein synthesis. Newly made T cells migrate from the mesenteric node into the thoracic duct lymph to reach the gut mucosa. In euthymic mice, this extrathymic pathway is totally repressed, except in conditions of severe lymphocytic depletion. Thus, in normal animals, all gut T IEL, including CD8 alpha alpha(+) cells, are of thymic origin, CD8 alpha alpha(+) TCR alpha beta(+) IEL being the likely progeny of double negative NK1-1(-) thymocytes, which show polyclonal V alpha and V beta repertoires.