Identification of clonogenic common Flt3+M-CSFR+ plasmacytoid and conventional dendritic cell progenitors in mouse bone marrow

Lymphoid tissue plasmacytoid and conventional dendritic cells (DCs) are continuously regenerated from hematopoietic stem cells. The cytokine dependence and biology of plasmacytoid and conventional DCs suggest that regeneration might proceed through common DC-restricted developmental intermediates. By selecting for cytokine receptor expression relevant to DC development, we identify here highly cycling Lin(-)c-Kit(int)Flt3(+)M-CSFR(+) cells with a distinct gene-expression profile in mouse bone marrow that, on a clonal level in vitro and as a population both in vitro and in vivo, efficiently generated plasmacytoid and conventional DCs but no other lineages, which increased in number after in vivo injection of the cytokine Flt3 ligand. These clonogenic common DC progenitors thus define a cytokine-regulated DC developmental pathway that ensures the supply of various DC populations.

Regulation of IgA production by naturally occurring TNF/iNOS-producing dendritic cells

Immunoglobulin-A has an irreplaceable role in the mucosal defence against infectious microbes. In human and mouse, IgA-producing plasma cells comprise approximately 20% of total plasma cells of peripheral lymphoid tissues, whereas more than 80% of plasma cells produce IgA in mucosa-associated lymphoid tissues (MALT). One of the most biologically important and long-standing questions in immunology is why this 'biased' IgA synthesis takes place in the MALT but not other lymphoid organs. Here we show that IgA class-switch recombination (CSR) is impaired in inducible-nitric-oxide-synthase-deficient (iNOS-/-; gene also called Nos2) mice. iNOS regulates the T-cell-dependent IgA CSR through expression of transforming growth factor-beta receptor, and the T-cell-independent IgA CSR through production of a proliferation-inducing ligand (APRIL, also called Tnfsf13) and a B-cell-activating factor of the tumour necrosis factor (TNF) family (BAFF, also called Tnfsf13b). Notably, iNOS is preferentially expressed in MALT dendritic cells in response to the recognition of commensal bacteria by toll-like receptor. Furthermore, adoptive transfer of iNOS+ dendritic cells rescues IgA production in iNOS-/- mice. Further analysis revealed that the MALT dendritic cells are a TNF-alpha/iNOS-producing dendritic-cell subset, originally identified in mice infected with Listeria monocytogenes. The presence of a naturally occurring TNF-alpha/iNOS-producing dendritic-cell subset may explain the predominance of IgA production in the MALT, critical for gut homeostasis.

The dynamics of dendritic cell: mediated innate immune regulation

After taking up pathogen-derived antigens, dendritic cells (DCs) leave peripheral organs and migrate into sentinel lymph nodes via afferent lymphatic vessels. During this process, they undergo maturation and produce proinflammatory cytokines, which leads to efficient antigen (Ag) presentation and activation of the innate and acquired immune systems. Recent evidence indicates that DC subsets cooperate to activate the innate immune system. It is becoming clear that the total DC population is composed of a network of DC subsets with distinct functions that are critical for sensing pathogens and orchestrating immune responses.

The Pten/PI3K pathway governs the homeostasis of Valpha14iNKT cells

The tumor suppressor PTEN is mutated in many human cancers. We previously used the Cre-loxP system to generate mice (LckCrePten mice) with a Pten mutation in T-lineage cells. Here we describe the phenotype of Pten-deficient Valpha14iNKT cells. A failure in the development of Valpha14iNKT cells occurs in the LckCrePten thymus between stage 2 (CD44(high)NK1.1(-)) and stage 3 (CD44(high)NK1.1(+)), resulting in decreased numbers of peripheral Valpha14iNKT cells. In vitro, Pten-deficient Valpha14iNKT cells show reduced proliferation and cytokine secretion in response to alphaGalCer stimulation but enhanced inhibitory Ly49 receptor expression. Following interaction with dendritic cells (DCs) loaded with alphaGalCer, Pten-deficient Valpha14iNKT cells demonstrate activation of PI3K. Indeed, the effects of the Pten mutation require intact function of the PI3K subunits p110gamma and p110delta. In vivo, LckCrePten mice display reduced serum IFNgamma after alphaGalCer administration. Importantly, Valpha14iNKT cell-mediated protection against the metastasis of melanoma cells to the lung was impaired in the absence of Pten. Thus, the Pten/PI3K pathway is indispensable for the homeostasis and antitumor surveillance function of Valpha14iNKT cells.

Essential roles of DC-derived IL-15 as a mediator of inflammatory responses in vivo

Interleukin (IL)-15 is expressed in a variety of inflammatory diseases. However, the contribution of dendritic cell (DC)–derived IL-15 to the development of diseases is uncertain. Using established models of Propionibacterium acnes (P. acnes)– and zymosan-induced liver inflammation, we observed granuloma formation in the livers of wild-type (WT) and RAG-2^−/− mice but not in those of IL-15^−/− mice. We demonstrate that this is likely caused by an impaired sequential induction of IL-12, IFN-γ, and chemokines necessary for monocyte migration. Likewise, lethal endotoxin shock was not induced in P. acnes– and zymosan-primed IL-15^−/− mice or in WT mice treated with a new IL-15–neutralizing antibody. In both systems, proinflammatory cytokine production was impaired. Surprisingly, neither granuloma formation, lethal endotoxin shock, nor IL-15 production was induced in mice deficient for DCs, and adoptive transfer of WT but not IL-15^−/− DCs restored the disease development in IL-15^−/− mice. Collectively, these data indicate the importance of DC-derived IL-15 as a mediator of inflammatory responses in vivo.

Interleukin 15-dependent crosstalk between conventional and plasmacytoid dendritic cells is essential for CpG-induced immune activation

The function of interleukin 15 (IL-15) in unmethylated CpG oligodeoxynucleotide (CpG)-induced immune responses remains unknown. Here, in response to CpG, both wild-type and natural killer cell-depleted mice produced IL-12 and became resistant to a lethal dose of Listeria monocytogenes. In contrast, CpG-treated IL-15-deficient mice produced little IL-12 and succumbed to L. monocytogenes. CpG-stimulated conventional dendritic cells (cDCs) were the main producers of both IL-15 and IL-12, but cDCs did not produce IL-12 in the absence of plasmacytoid DCs (pDCs). The cDC-derived IL-15 induced CD40 expression by cDCs. Interaction between CD40 on cDCs and CD40 ligand on pDCs led to IL-12 production by cDCs. Thus, IL-15-dependent crosstalk between cDCs and pDCs is essential for CpG-induced immune activation.

Epidermal-type fatty acid binding protein as a negative regulator of IL-12 production in dendritic cells

Fatty acids and their metabolites have recently been shown to modulate various functions of dendritic cells (DCs) including their differentiation and cytokine production, although the mechanisms underlying their cellular functions are not fully understood. In view of our previous finding that epidermal-type fatty acid binding protein (E-FABP) was exclusively expressed in splenic DCs among FABP family, we examined the phenotype of E-FABP-null mutant mice in order to elucidate the functional significance of E-FABP expression in DCs. Although E-FABP-null mutant mice showed no apparent abnormalities in the population density and subset distribution of DCs as well as the microscopic morphology in the spleen, DCs isolated from E-FABP-null spleen showed enhanced production of IL-12p70, a key cytokine for innate immune responses, in response to appropriate stimuli as compared with wild-type. In real-time PCR, the expression level of IL-12p35 mRNA after LPS stimuli was much higher in mutant DCs when compared with wild-type, while no apparent change of IL-12p40 mRNA level was detected. Phosphorylated forms of p38 mitogen-activated protein kinase (p38MAPK) and IkappaB-alpha, molecules critical for IL-12 production, were detected at higher levels in E-FABP-null-mutant DCs after LPS stimuli when compared with wild-type counterparts. Collectively, it is suggested that E-FABP may be a novel negative regulator of IL-12 production in DCs, and this regulation may be exerted via its involvement in the p38MAPK-mediated transcription of IL-12p35.

Synergistic effect of Nod1 and Nod2 agonists with toll-like receptor agonists on human dendritic cells to generate interleukin-12 and T helper type 1 cells

A synthetic Nod2 agonist, muramyldipeptide (MDP), and two Nod1 agonists, FK565 and FK156, mimic the bacterial peptidoglycan moiety and are powerful adjuvants that induce cell-mediated immunity, especially delayed-type hypersensitivity. In this study, we used human dendritic cell (DC) cultures to examine possible T helper type 1 (Th1) responses induced by MDP and FK565/156 in combination with various synthetic Toll-like receptor (TLR) agonists, including synthetic lipid A (TLR4 agonist), the synthetic triacyl lipopeptide Pam3CSSNA (TLR2 agonist), poly(I:C) (TLR3 agonist), and CpG DNA (TLR9 agonist). Immature DCs derived from human monocytes expressed mRNAs for Nod1, Nod2, TLR2, TLR3, TLR4, and TLR9. The stimulation of DCs with MDP and FK565 in combination with lipid A, poly(I:C), and CpG DNA, but not with Pam3CSSNA, synergistically induced interleukin-12 (IL-12) p70 and gamma interferon (IFN-gamma), but not IL-18, in culture supernatants and induced IL-15 on the cell surface. In correlation with the cytokine induction, an upregulation of the mRNA expression of these cytokine genes was observed. Notably, IL-12 p35 mRNA expression increased >1,000-fold upon stimulation with lipid A plus either MDP or FK565 compared with stimulation with each stimulant alone. In contrast, for the expression of CD83 and costimulatory molecules such as CD40, CD80, and CD86, no synergistic effects were observed upon stimulation with Nod plus TLR agonists. The culture supernatants of DCs stimulated with lipid A plus either MDP or FK565 activated human T cells to produce high levels of IFN-gamma, and the activity was attributable to DC-derived IL-12. These findings suggest that Nod1 and Nod2 agonists in combination with TLR3, TLR4, and TLR9 agonists synergistically induce IL-12 and IFN-gamma production in DCs to induce Th1-lineage immune responses.

Selective contribution of IFN-alpha/beta signaling to the maturation of dendritic cells induced by double-stranded RNA or viral infection

A complex mechanism may be operational for dendritic cell (DC) maturation, wherein Toll-like receptor and other signaling pathways may be coordinated differently depending on the nature of the pathogens, in order for DC maturation to be most effective to a given threat. Here, we show that IFN-alpha/beta signaling is selectively required for the maturation of DCs induced by double-stranded RNA or viral infection in vitro. Interestingly, the maturation is still observed in the absence of either of the two target genes of IFN-alpha/beta, TLR3 and PKR (double-stranded-RNA-dependent protein kinase R), indicating the complexity of the IFN-alpha/beta-induced transcriptional program in DCs. We also show that the DCs stimulated in vivo by these agents can migrate into the T cell zone of the spleen but fail to mature without the IFN signal. The immune system may have acquired the selective utilization of this cytokine system, which is essential for innate antiviral immunity, to effectively couple with the induction of adaptive immunity.

TCR binding kinetics measured with MHC class I tetramers reveal a positive selecting peptide with relatively high affinity for TCR

The interaction between TCR and peptide-MHC (pMHC) complexes is crucial for the activation of T cells as well as for positive and negative selection in the thymus. The kinetics and affinity of this interaction and the densities of TCR and pMHC complexes on the cell surface are determining factors for different outcomes during thymic selection. In general, it is thought that agonist pMHC, which cause negative selection, have higher affinities and, in particular, slower off-rates than partial or weak agonists and antagonists, which cause positive selection. In this study, we have used pMHC tetramers to investigate the kinetics of TCR-pMHC interaction for agonist, weak agonist, and antagonist ligands of the anti-lymphocytic choriomeningitis virus P14 TCR. Kinetics determined on the cell surface may be biologically more relevant than methods using soluble proteins. We can distinguish between agonists and weak agonists or antagonists based on the half-life and the avidity of tetramer-TCR interaction. Furthermore, we show that a weak agonist self-peptide that positively selects P14 TCR(+) thymocytes has a tetramer half-life and avidity only slightly weaker than strong agonists. We show that, in fact, it can act as quite a strong agonist, but that its poor ability to stabilize MHC causes it instead to have a weak agonist phenotype.

Cloning of adiponectin receptors that mediate antidiabetic metabolic effects

Adiponectin (also known as 30-kDa adipocyte complement-related protein; Acrp30) is a hormone secreted by adipocytes that acts as an antidiabetic and anti-atherogenic adipokine. Levels of adiponectin in the blood are decreased under conditions of obesity, insulin resistance and type 2 diabetes. Administration of adiponectin causes glucose-lowering effects and ameliorates insulin resistance in mice. Conversely, adiponectin-deficient mice exhibit insulin resistance and diabetes. This insulin-sensitizing effect of adiponectin seems to be mediated by an increase in fatty-acid oxidation through activation of AMP kinase and PPAR-alpha. Here we report the cloning of complementary DNAs encoding adiponectin receptors 1 and 2 (AdipoR1 and AdipoR2) by expression cloning. AdipoR1 is abundantly expressed in skeletal muscle, whereas AdipoR2 is predominantly expressed in the liver. These two adiponectin receptors are predicted to contain seven transmembrane domains, but to be structurally and functionally distinct from G-protein-coupled receptors. Expression of AdipoR1/R2 or suppression of AdipoR1/R2 expression by small-interfering RNA supports our conclusion that they serve as receptors for globular and full-length adiponectin, and that they mediate increased AMP kinase and PPAR-alpha ligand activities, as well as fatty-acid oxidation and glucose uptake by adiponectin.

Weak agonist self-peptides promote selection and tuning of virus-specific T cells

Recent progress has begun to define the interactions and signaling pathways that are triggered during positive selection. To identify and further examine self-peptides that can mediate positive selection, we searched a protein-database to find peptides that have minimal homology with the viral peptide (p33) that activates a defined P14 transgenic TCR. We identified four peptides that could bind the restriction element H-2D(b) and induce proliferation of P14 transgenic splenocytes at high concentration. Two of the four peptides (DBM and RPP) were able to positively select the virus-specific TCR in fetal thymic organ culture but were unable to induce clonal deletion. Reverse-phase HPLC and mass spectrometry demonstrated that these peptides were presented by H-2D(b) molecules on thymic epithelial cell lines. We also examined whether the selecting ligands altered T cell responsiveness in vitro. DBM-selected T cells lost their ability to respond to the positively selecting ligand DBM, whereas RPP-selected T cells only retained their ability to respond to high concentrations of RPP. These results demonstrate that self-peptides that mediate positive selection can differentially "tune" the activation threshold of T cells and alter the functional repertoire of T cells.

PI3K and Btk differentially regulate B cell antigen receptor-mediated signal transduction

Phosphoinositide-3 kinase (PI3K) is thought to activate the tyrosine kinase Btk. However, through analysis of PI3K-/- and Btk-/- mice, B cell antigen receptor (BCR)-induced activation of Btk in mouse B cells was found to be unaffected by PI3K inhibitors or by a lack of PI3K. Consistent with this observation, PI3K-/- Btk-/- double-deficient mice had more severe defects than either single-mutant mouse. NF-kappaB activation along with Bcl-xL and cyclin D2 induction were severely blocked in both PI3K-/- and Btk-/- single-deficient B cells. Transgenic expression of Bcl-xL restored the development and BCR-induced proliferation of B cells in PI3K-/- mice. Our results indicate that PI3K and Btk have unique roles in proximal BCR signaling and that they have a common target further downstream in the activation of NF-kappaB.

IFN-gamma and pro-inflammatory cytokine production by antigen-presenting cells is dictated by intracellular thiol redox status regulated by oxygen tension

Murine mature splenic DC with elevated intracellular glutathione, pretreated with IL-18, strikingly augmented the production of IFN-gamma in response to IL-12, whereas intracellular glutathione deprivation ablated this effect of IL-18. Likewise, macrophages with elevated intracellular glutathione augmented IFN-gamma production upon LPS or IL-12+IL-18 stimulation, whereas macrophages with reduced intracellular glutathione showed the reciprocal response. Under hypoxia, macrophages displayed a functional phenotype with decreased intracellular glutathione, i.e. decreased NO and IL-12, and elevated IL-10 production. However, mature DC and macrophages produced an elevated amount of IFN-gamma under hypoxia. Taken together, our results suggest that the intracellular redox status of DC and macrophages may play a pivotal role in local innate immunity, depending on local oxygen tension.

IFN-gamma and pro-inflammatory cytokine production by antigen-presenting cells is dictated by intracellular thiol redox status regulated by oxygen tension

Murine mature splenic DC with elevated intracellular glutathione, pretreated with IL-18, strikingly augmented the production of IFN-gamma in response to IL-12, whereas intracellular glutathione deprivation ablated this effect of IL-18. Likewise, macrophages with elevated intracellular glutathione augmented IFN-gamma production upon LPS or IL-12+IL-18 stimulation, whereas macrophages with reduced intracellular glutathione showed the reciprocal response. Under hypoxia, macrophages displayed a functional phenotype with decreased intracellular glutathione, i.e. decreased NO and IL-12, and elevated IL-10 production. However, mature DC and macrophages produced an elevated amount of IFN-gamma under hypoxia. Taken together, our results suggest that the intracellular redox status of DC and macrophages may play a pivotal role in local innate immunity, depending on local oxygen tension.

Critical role for IL-15 in innate immunity

Although some functional activities of interleukin (IL)-15 on NK and T cells overlap with those of IL-2, recent findings obtained from gene-targeted mice deficient in components of IL-2/IL-15 system demonstrate distinct roles of IL-15 for the activation of innate immune system. IL-15 is a pivotal cytokine for the development and survival of NK cells, NKT cells, TCRydelta+ intestinal intraepithelial lymphocytes (ilEL), and for the functional maturation of dendritic cells and macrophages. IL-15 is also important for memory T cell maintenance in vivo. In this review, I summarize recent progress of studies in the IL-15/IL-15R system.

Role of antigen-presenting cells in innate immune system

Activation of antigen-presenting cells (APC) and natural killer (NK) cells initiates the production of various proinflammatory cytokines including interleukin 12 (IL-12), interferon gamma (IFN-gamma) and nitric oxide (NO), which are important in the innate immune response for controlling infection by intracellular pathogens. In this review, we focus on these cytokines produced by APC and summarize the current understanding of how APC functions are regulated by cytokines in innate immunity.

Critical role of IL-15-IL-15R for antigen-presenting cell functions in the innate immune response

Activation of dendritic cells (DCs) and macrophages by infectious agents leads to secretion of interleukin 12 (IL-12), which subsequently induces interferon-gamma (IFN-gamma) production by multiple cell types that include DCs and macrophages. In turn, IFN-gamma acts on macrophages to augment IL-12 secretion and to produce nitric oxide (NO), which eradicates infected microbes. We show here that in cytokine common gamma subunit-deficient and/or IL-2 receptor beta-deficient mice, production of IL-12, IFN-gamma and NO by DCs and macrophages was severely impaired, as was up-regulation of major histocompatibility complex class II and CD40. Similar phenotypes were observed in DCs and macrophages from IL-15-deficient mice but not in those from IL-2-deficient mice. This shows that the IL-15-IL-15R interaction is critical in early activation of antigen-presenting cells and plays an important role in the innate immune system.

T cell-specific loss of Pten leads to defects in central and peripheral tolerance

PTEN, a tumor suppressor gene, is essential for embryogenesis. We used the Cre-loxP system to generate a T cell-specific deletion of the Pten gene (Pten(flox/-) mice). All Pten(flox/-) mice develop CD4+ T cell lymphomas by 17 weeks. Pten(flox/-) mice show increased thymic cellularity due in part to a defect in thymic negative selection. Pten(flox/-) mice exhibit elevated levels of B cells and CD4+ T cells in the periphery, spontaneous activation of CD4+ T cells, autoantibody production, and hypergammaglobulinemia. Pten(flox/-) T cells hyperproliferate, are autoreactive, secrete increased levels of Th1/Th2 cytokines, resist apoptosis, and show increased phosphorylation of PKB/Akt and ERK. Peripheral tolerance to SEB is also impaired in Pten(flox/-) mice. PTEN is thus an important regulator of T cell homeostasis and self-tolerance.

Overexpression of Bcl-2 Differentially Restores Development of Thymus-Derived CD4−8+ T Cells and Intestinal Intraepithelial T Cells in IFN-Regulatory Factor-1-Deficient Mice

Mice lacking IFN-regulatory factor (IRF)-1 have reduced numbers of mature CD8+ T cells within the thymus and peripheral lymphoid organs, suggesting a critical role of IRF-1 in CD8(+) T cell differentiation. Here we show that endogenous Bcl-2 expression is substantially reduced in IRF-1(-/-)CD8+ thymocytes and that introduction of a human Bcl-2 transgene driven by Emu or lck promoter in IRF-1(-/-) mice restores the CD8(+) T cell development. Restored CD8+ T cells are functionally mature in terms of allogeneic MLR and cytokine production. In contrast to thymus-derived CD8+ T cells, other lymphocyte subsets including NK, NK T, and TCR-gammadelta(+) intestinal intraepithelial lymphocytes, which are also impaired in IRF-1(-/-) mice, are not rescued by expressing human Bcl-2. Our results indicate that IRF-1 differentially regulates the development of these lymphocyte subsets and that survival signals involving Bcl-2 are critical for the development of thymus-dependent CD8+ T cells.

Negative regulation of T cell proliferation and interleukin 2 production by the serine threonine kinase GSK-3

Glycogen synthase kinase (GSK)-3 is a protein serine/threonine kinase that regulates differentiation and cell fate in a variety of organisms. This study examined the role of GSK-3 in antigen-specific T cell responses. Using resting T cells from P14 T cell receptor (TCR)-transgenic mice (specific for the lymphocytic choriomeningitis virus and H-2D(b)), we demonstrated that GSK-3beta was inactivated by serine phosphorylation after viral peptide-specific stimulation in vitro. To further investigate the role of GSK-3, we have generated a retroviral vector that expresses a constitutively active form of GSK-3beta that has an alanine substitution at the regulatory amino acid, serine 9 (GSK-3betaA9). Retroviral transduction of P14 TCR-transgenic bone marrow stem cells, followed by reconstitution, led to the expression of GSK-3betaA9 in bone marrow chimeric mice. T cells from chimeric mice demonstrate a reduction in proliferation and interleukin (IL)-2 production. In contrast, in vitro assays done in the presence of the GSK-3 inhibitor lithium led to dramatically prolonged T cell proliferation and increased IL-2 production. Furthermore, in the presence of lithium, we show that nuclear factor of activated T cells (NF-AT)c remains in the nucleus after antigen-specific stimulation of T cells. Together, these data demonstrate that GSK-3 negatively regulates the duration of T cell responses.

Identification of a cross-reactive self ligand in virus-mediated autoimmunity

Molecular mimicry has been considered to be one of the potential mechanisms underlying the induction of autoimmune diseases. Using a TCR-transgenic model specific for lymphocytic choriomeningitis virus (LCMV) we have examined the potential for cross-reactive recognition of tissue-restricted self peptides. Several peptides were identified that were able to cross-react with the TCR-transgenic virus-specific T cells in vitro. One peptide was derived from dopamine beta-mono-oxygenase, an enzyme expressed in the adrenal medulla. Interestingly, after activation of the transgenic T cells with LCMV glycoprotein peptides or viruses, infiltration of the adrenal medulla was detected in conjunction with alterations in dopamine metabolism. However, complete destruction of the adrenal medulla was not observed. This suggests that molecular mimicry may be sufficient for self recognition and infiltration, but other factors clearly contribute to chronic autoimmune disease.

Interleukin 12-dependent interferon gamma production by CD8alpha+ lymphoid dendritic cells

We investigated the role of antigen-presenting cells in early interferon (IFN)-gamma production in normal and recombinase activating gene 2-deficient (Rag-2(-/-)) mice in response to Listeria monocytogenes (LM) infection and interleukin (IL)-12 administration. Levels of serum IFN-gamma in Rag-2(-/-) mice were comparable to those of normal mice upon either LM infection or IL-12 injection. Depletion of natural killer (NK) cells by administration of anti-asialoGM1 antibodies had little effect on IFN-gamma levels in the sera of Rag-2(-/-) mice after LM infection or IL-12 injection. Incubation of splenocytes from NK cell-depleted Rag-2(-/-) mice with LM resulted in the production of IFN-gamma that was completely blocked by addition of anti-IL-12 antibodies. Both dendritic cells (DCs) and monocytes purified from splenocytes were capable of producing IFN-gamma when cultured in the presence of IL-12. Intracellular immunofluorescence analysis confirmed the IFN-gamma production from DCs. It was further shown that IFN-gamma was produced predominantly by CD8alpha+ lymphoid DCs rather than CD8alpha- myeloid DCs. Collectively, our data indicated that DCs are potent in producing IFN-gamma in response to IL-12 produced by bacterial infection and play an important role in innate immunity and subsequent T helper cell type 1 development in vivo.

Selection of the T cell repertoire

Advances in gene technology have allowed the manipulation of molecular interactions that shape the T cell repertoire. Although recognized as fundamental aspects of T lymphocyte development, only recently have the mechanisms governing positive and negative selection been examined at a molecular level. Positive selection refers to the active process of rescuing MHC-restricted thymocytes from programmed cell death. Negative selection refers to the deletion or inactivation of potentially autoreactive thymocytes. This review focuses on interactions during thymocyte maturation that define the T cell repertoire, with an emphasis placed on current literature within this field.

Cutting edge: LFA-1 is required for liver NK1.1+TCR alpha beta+ cell development: evidence that liver NK1.1+TCR alpha beta+ cells originate from multiple pathways

Using mice deficient for LFA-1, CD44, and ICAM-1, we examined the role of these adhesion molecules in NK1.1+TCR alpha beta+ (NKT) cell development. Although no defect in NKT cell development was observed in CD44-/- and ICAM-1-/- mice, a dramatic reduction of liver NKT cells was observed in LFA-1-/- mice. Normal numbers of NKT cells were present in other lymphoid organs in LFA-1-/- mice. When LFA-1-/- splenocytes were injected i.v. into wild-type mice, the frequency of NKT cells among donor-derived cells in the recipient liver was normal. In contrast, when LFA-1-/- bone marrow (BM) cells were injected i.v. into irradiated wild-type mice, the frequency of liver NKT cells was significantly lower than that of mice injected with wild-type BM cells. Collectively, these data indicate that LFA-1 is required for the development of liver NKT cells, rather than the migration to and/or subsequent establishment of mature NKT cells in the liver.

Cutting Edge: LFA-1 Is Required for Liver NK1.1+TCRαβ+ Cell Development: Evidence That Liver NK1.1+TCRαβ+ Cells Originate from Multiple Pathways

Using mice deficient for LFA-1, CD44, and ICAM-1, we examined the role of these adhesion molecules in NK1.1+TCRαβ+ (NKT) cell development. Although no defect in NKT cell development was observed in CD44−/− and ICAM-1−/− mice, a dramatic reduction of liver NKT cells was observed in LFA-1−/− mice. Normal numbers of NKT cells were present in other lymphoid organs in LFA-1−/− mice. When LFA-1−/− splenocytes were injected i.v. into wild-type mice, the frequency of NKT cells among donor-derived cells in the recipient liver was normal. In contrast, when LFA-1−/− bone marrow (BM) cells were injected i.v. into irradiated wild-type mice, the frequency of liver NKT cells was significantly lower than that of mice injected with wild-type BM cells. Collectively, these data indicate that LFA-1 is required for the development of liver NKT cells, rather than the migration to and/or subsequent establishment of mature NKT cells in the liver.

Degree of TCR Internalization and Ca2+ Flux Correlates with Thymocyte Selection

Recent evidence suggests that TCR down-regulation directly reflects the number of TCRs that have engaged MHC/peptide ligand complexes. Here, we examined the influence of defined peptides on thymic selection based on their ability to induce differential TCR internalization. Our results demonstrate that there is a direct correlation: peptides that induce strong TCR down-regulation are most efficient at mediating negative selection, whereas peptides that induce suboptimal TCR internalization are more efficient at triggering positive selection. As a consequence of suboptimal TCR internalization, a proportion of TCR complexes that remain on the cell surface may be able to relay continual signals required for survival and differentiation. In addition, we show that the magnitude of Ca2+ influx set by these peptides reflects the hierarchy of TCR down-regulation and correlates with positive vs negative selection of transgenic thymocytes. Together, our data suggest that T cell selection is mediated by differing intensities of the same TCR-mediated signal, rather than by distinct signals.

Degree of TCR internalization and Ca2+ flux correlates with thymocyte selection

Recent evidence suggests that TCR down-regulation directly reflects the number of TCRs that have engaged MHC/peptide ligand complexes. Here, we examined the influence of defined peptides on thymic selection based on their ability to induce differential TCR internalization. Our results demonstrate that there is a direct correlation: peptides that induce strong TCR down-regulation are most efficient at mediating negative selection, whereas peptides that induce suboptimal TCR internalization are more efficient at triggering positive selection. As a consequence of suboptimal TCR internalization, a proportion of TCR complexes that remain on the cell surface may be able to relay continual signals required for survival and differentiation. In addition, we show that the magnitude of Ca2+ influx set by these peptides reflects the hierarchy of TCR down-regulation and correlates with positive vs negative selection of transgenic thymocytes. Together, our data suggest that T cell selection is mediated by differing intensities of the same TCR-mediated signal, rather than by distinct signals.

The transcription factor interferon regulatory factor 1 (IRF-1) is important during the maturation of natural killer 1.1+ T cell receptor-alpha/beta+ (NK1+ T) cells, natural killer cells, and intestinal intraepithelial T cells

In contrast to conventional T cells, natural killer (NK) 1.1+ T cell receptor (TCR)-alpha/beta+ (NK1+T) cells, NK cells, and intestinal intraepithelial lymphocytes (IELs) bearing CD8-alpha/alpha chains constitutively express the interleukin (IL)-2 receptor (R)beta/15Rbeta chain. Recent studies have indicated that IL-2Rbeta/15Rbeta chain is required for the development of these lymphocyte subsets, outlining the importance of IL-15. In this study, we investigated the development of these lymphocyte subsets in interferon regulatory factor 1-deficient (IRF-1-/-) mice. Surprisingly, all of these lymphocyte subsets were severely reduced in IRF-1-/- mice. Within CD8-alpha/alpha+ intestinal IEL subset, TCR-gamma/delta+ cells and TCR-alpha/beta+ cells were equally affected by IRF gene disruption. In contrast to intestinal TCR-gamma/delta+ cells, thymic TCR-gamma/delta+ cells developed normally in IRF-1-/- mice. Northern blot analysis further revealed that the induction of IL-15 messenger RNA was impaired in IRF-1-/- bone marrow cells, and the recovery of these lymphocyte subsets was observed when IRF-1-/- cells were cultured with IL-15 in vitro. These data indicate that IRF-1 regulates IL-15 gene expression, which may control the development of NK1+T cells, NK cells, and CD8-alpha/alpha+ IELs.

The transcription factor NF-ATc1 regulates lymphocyte proliferation and Th2 cytokine production

NF-ATc1 is a member of a family of genes that encodes the cytoplasmic component of the nuclear factor of activated T cells (NF-AT). In activated T cells, nuclear NF-AT binds to the promoter regions of multiple cytokine genes and induces their transcription. The role of NF-ATc1 was investigated in recombination activating gene-1 (RAG-1)-deficient blastocyst complementation assays using homozygous NF-ATc1-/- mutant ES cell lines. NF-ATc1-/-/RAG-1-/- chimeric mice showed reduced numbers of thymocytes and impaired proliferation of peripheral lymphocytes, but normal production of IL-2. Induction in vitro of Th2 responses, as demonstrated by a decrease in IL-4 and IL-6 production, was impaired in mutant T cells. These data indicate that NF-ATc1 plays roles in the development of T lymphocytes and in the differentiation of the Th2 response.

Role for IL-15/IL-15 receptor beta-chain in natural killer 1.1+ T cell receptor-alpha beta+ cell development

Mouse TCR alphabeta+ cells expressing NKR-P1 (NK1+ T cells in the C57BL/6 strain) are classified as a unique subset that require beta2m-associated, MHC class I-like molecules for development and preferentially express particular V beta and V alphaJ domains of the TCR. We show here that NK1+ T cells express the IL-2R beta/IL-15R beta that is normally present on conventional NK cells, but not T cells. In mice lacking IL-2R beta/IL-15R beta chain, the number of NK1+ T cells is dramatically reduced. However, in IL-2-deficient mice, NK1+ T cells develop normally. Moreover, NK1+ T cells proliferate upon stimulation by IL-15, and the proliferation is blocked by the addition of anti-IL-2R beta/IL-15R beta Abs. Collectively, our data indicate that NK1+ T cells require IL-2R beta/IL-15R beta chain in an IL-2-independent manner and demonstrate that IL-15 plays a crucial role during development.

Role for IL-15/IL-15 receptor beta-chain in natural killer 1.1+ T cell receptor-alpha beta+ cell development

Mouse TCR alphabeta+ cells expressing NKR-P1 (NK1+ T cells in the C57BL/6 strain) are classified as a unique subset that require beta2m-associated, MHC class I-like molecules for development and preferentially express particular V beta and V alphaJ domains of the TCR. We show here that NK1+ T cells express the IL-2R beta/IL-15R beta that is normally present on conventional NK cells, but not T cells. In mice lacking IL-2R beta/IL-15R beta chain, the number of NK1+ T cells is dramatically reduced. However, in IL-2-deficient mice, NK1+ T cells develop normally. Moreover, NK1+ T cells proliferate upon stimulation by IL-15, and the proliferation is blocked by the addition of anti-IL-2R beta/IL-15R beta Abs. Collectively, our data indicate that NK1+ T cells require IL-2R beta/IL-15R beta chain in an IL-2-independent manner and demonstrate that IL-15 plays a crucial role during development.

Altered peptide ligands trigger perforin- rather than Fas-dependent cell lysis

CTLs lyse Fas-expressing target cells by the concomitant action of a perforin- and a Fas-dependent mechanism. This study analyzed whether target cells pulsed with T cell antagonists and other altered peptide ligands (APLs) were susceptible selectively to only one of these two mechanisms. In vivo and in vitro activated T cells from transgenic mice expressing a TCR specific for lymphocytic choriomeningitis virus were used as effector cells. To distinguish between perforin- and Fas-dependent cytotoxicity, T cells from normal or perforin-deficient mice were used to lyse peptide-pulsed Fas-positive or Fas-negative target cells. In contrast to previous reports that have shown that APLs selectively induce the Fas-dependent pathway of cytotoxicity, our results demonstrate that target cells pulsed with T cell antagonists and other APLs are lysed predominantly by the perforin-dependent pathway. The contribution of Fas-mediated cytotoxicity was similar for the full agonist and the APLs. Thus, full agonists, partial agonists, and antagonists trigger similar and not distinct pathways of cytotoxicity.

Altered peptide ligands trigger perforin- rather than Fas-dependent cell lysis

CTLs lyse Fas-expressing target cells by the concomitant action of a perforin- and a Fas-dependent mechanism. This study analyzed whether target cells pulsed with T cell antagonists and other altered peptide ligands (APLs) were susceptible selectively to only one of these two mechanisms. In vivo and in vitro activated T cells from transgenic mice expressing a TCR specific for lymphocytic choriomeningitis virus were used as effector cells. To distinguish between perforin- and Fas-dependent cytotoxicity, T cells from normal or perforin-deficient mice were used to lyse peptide-pulsed Fas-positive or Fas-negative target cells. In contrast to previous reports that have shown that APLs selectively induce the Fas-dependent pathway of cytotoxicity, our results demonstrate that target cells pulsed with T cell antagonists and other APLs are lysed predominantly by the perforin-dependent pathway. The contribution of Fas-mediated cytotoxicity was similar for the full agonist and the APLs. Thus, full agonists, partial agonists, and antagonists trigger similar and not distinct pathways of cytotoxicity.

Lack of directed V alpha 14-J alpha 281 rearrangements in NK1+ T cells

NK1.1+ T cells are an unusual subset of TCR alpha beta cells distinguished by their highly restricted V beta repertoire and predominant usage of an invariant V alpha 14-J alpha 281 chain. To assess whether a directed rearrangement mechanism could be responsible for this invariant alpha chain, we have analyzed V alpha 14 rearrangements by polymerase chain reaction and Southern blot in a panel of cloned T-T hybrids derived from thymic NK1.1+ T cells. As expected a high proportion (17/20) of the hybrids had rearranged V alpha 14 to J alpha 281. However, V alpha 14-J alpha 281 rearrangements always occurred on only one chromosome and were accompanied by other V alpha-J alpha rearrangements (not involving V alpha 14) on the homologous chromosome. These data argue that rigorous ligand selection rather than directed rearrangement is responsible for the high frequency of V alpha 14-J alpha 281 rearrangements in NK1.1+ T cells.

Thymus-independent generation of NK1+ T cells in vitro from fetal liver precursors

NK1.1+TCR alpha beta+ (NK1+) T cells are an unusual subset of mouse TCR alpha beta+ cells found primarily in adult thymus and liver. In contrast to conventional TCR alpha beta+ cells, NK1+ T cells have a TCR repertoire that is highly skewed to V alpha14 and to Vbeta8, -7, and -2. The developmental origin and ligand specificity of NK1+ T cells are controversial. We show here that NK1+ T cells with a typically biased V alpha and V beta repertoire develop in cytokine-supplemented suspension cultures of fetal liver established from either normal or athymic mice. Furthermore, NK1+ T cell development in fetal liver cultures is abrogated in beta2m-deficient mice (which lack MHC class I and other related molecules) and can be partially inhibited by the presence of anti-CD1 mAbs. Moreover, mixing experiments indicate that recombination-deficient SCID fetal liver cells can reconstitute NK1+ T cell development in beta2m-deficient fetal liver cultures. Collectively, our data demonstrate that NK1+ T cells can develop extrathymically from fetal liver precursors and that a beta2m-associated ligand (putatively CD1) present on nonlymphoid cells is essential for their positive selection and/or expansion.

Thymus-independent generation of NK1+ T cells in vitro from fetal liver precursors

NK1.1+TCR alpha beta+ (NK1+) T cells are an unusual subset of mouse TCR alpha beta+ cells found primarily in adult thymus and liver. In contrast to conventional TCR alpha beta+ cells, NK1+ T cells have a TCR repertoire that is highly skewed to V alpha14 and to Vbeta8, -7, and -2. The developmental origin and ligand specificity of NK1+ T cells are controversial. We show here that NK1+ T cells with a typically biased V alpha and V beta repertoire develop in cytokine-supplemented suspension cultures of fetal liver established from either normal or athymic mice. Furthermore, NK1+ T cell development in fetal liver cultures is abrogated in beta2m-deficient mice (which lack MHC class I and other related molecules) and can be partially inhibited by the presence of anti-CD1 mAbs. Moreover, mixing experiments indicate that recombination-deficient SCID fetal liver cells can reconstitute NK1+ T cell development in beta2m-deficient fetal liver cultures. Collectively, our data demonstrate that NK1+ T cells can develop extrathymically from fetal liver precursors and that a beta2m-associated ligand (putatively CD1) present on nonlymphoid cells is essential for their positive selection and/or expansion.

Natural killer-like T cells develop in SJL mice despite genetically distinct defects in NK1.1 expression and in inducible interleukin-4 production

An unusual subset of mature T cells expresses natural killer (NK) cell-related surface markers such as interleukin-2 receptor beta (IL-2R beta; CD122) and the polymorphic antigen NK1.1. These "NK-like" T cells are distinguished by their highly skewed V alpha and V beta repertoire and by their ability to rapidly produce large amounts of IL-4 upon T cell receptor (TCR) engagement. The inbred mouse strain SJL (which expresses NK1.1 on its NK cells) has recently been reported to lack NK1.1+ T cells and consequently to be deficient in IL-4 production upon TCR stimulation. We show here, however, that SJL mice have normal numbers of IL-2R beta+ T cells with a skewed V beta repertoire characteristic of "NK-like" T cells. Furthermore lack of NK1.1 expression on IL-2R beta+ T cells in SJL mice was found by backcross analysis to be controlled by a single recessive gene closely linked to the NKR-P1 complex on chromosome 6 (which encodes the NK1.1 antigen). Analysis of a panel of inbred mouse strains further demonstrated that lack of NK1.1 expression on IL-2R beta+ T cells segregated with NKR-P1 genotype (as assessed by restriction fragment length polymorphism) and thus was not restricted to the SJL strain. In contrast, defective TCR induced IL-4 production (which appeared to be a unique property of SJL mice) seems to be controlled by two recessive genes unlinked to NKR-P1. Collectively, our data indicate that "NK-like" T cells develop normally in SJL mice despite genetically distinct defects in NK1.1 expression and inducible IL-4 production.

Tumor necrosis factor receptor p55 mediates deletion of peripheral cytotoxic T lymphocytes in vivo

Cellular death of activated lymphocytes down-regulates immune responses and is involved in maintaining self tolerance. Signals associated with ligation of the membrane molecule Fas lead to lymphocyte apoptosis, but additional, Fas-independent mechanisms have been postulated. Here, we show a marked expansion and prolonged persistence of functional activated cytotoxic T cells in mice lacking the tumor necrosis factor (TNF) receptor p55. In the absence of this receptor, peripheral lymphocyte apoptosis was significantly reduced in vivo. The prolonged thymocyte survival was associated with functional anergy, since the T cells no longer proliferated in vitro when stimulated with peptide antigen. However, specific cytotoxic effector function was easily detected in vitro. We conclude that the TNF receptor p55 is involved in peripheral T cell deletion in vivo.

Stringent V beta requirement for the development of NK1.1+ T cell receptor-alpha/beta+ cells in mouse liver

The liver of C57BL/6 mice contains a major subset of CD4+8- and CD4-8- T cell receptor (TCR)-alpha/beta+ cells expressing the polymorphic natural killer NK1.1 surface marker. Liver NK1.1+TCR-alpha/beta+ (NK1+ T) cells require interaction with beta2-microglobulin-associated, major histocompatibility complex I-like molecules on hematopoietic cells for their development and have a TCR repertoire that is highly skewed to Vbeta8.2, Vbeta7, and Vbeta2. We show here that congenic C57BL/6.Vbeta(a) mice, which lack Vbeta8- expressing T cells owing to a genomic deletion at the Vbeta locus, maintain normal levels of liver NK1+ T cells owing to a dramatic increase in the proportion of cells expressing Vbeta7 and Vbeta2 (but not other Vbetas). Moreover, in C57BL/6 congenic TCR-V Vbeta3 and -Vbeta8.1 transgenic mice (which in theory should not express other Vbeta, owing to allelic exclusion at the TCR-beta locus), endogenous TCR-Vbeta8.2, Vbeta7, and Vbeta2 (but not other Vbetas) are frequently expressed on liver NK1+T cells but absent on lymph node T cells. Finally, when endogenous V beta expression is prevented in TCR-Vbeta3 and Vbeta8.1 transgenic mice (by introduction of a null allele at the C beta locus), the development of liver NK1+T cells is totally abrogated. Collectively, our data indicate that liver NK1+T cells have a stringent requirement for expression of TCR-Vbeta8.2, Vbeta7, or Vbeta2 for their development.

Selectively impaired development of intestinal T cell receptor gamma delta+ cells and liver CD4+ NK1+ T cell receptor alpha beta+ cells in T cell factor-1-deficient mice

T cell factor-1 (Tcf-1) is a transcription factor that binds to a sequence motif present in several T cell-specific enhancer elements. In Tcf-1-deficient (Tcf-1-/-) mice, thymocyte development is partially blocked at the transition from the CD4-8+ immature single-positive stage to the CD4+8+ double-positive stage, resulting in a marked decrease of mature peripheral T cells in lymph node and spleen. We report here that the development of most intestinal TCR gamma delta+ cells and liver CD4+ NK1.1+TCR alpha beta+ (NK1+T) cells, which are believed to be of extrathymic origin, is selectively impaired in Tcf-1-/- mice. In contrast, thymic and thymus-derived (splenic) TCR gamma delta+ cells are present in normal numbers in Tcf-1-/- mice, as are other T cell subsets in intestine and liver. Collectively, our data suggest that Tcf-1 is differentially required for the development of some extrathymic T cell subsets, including intestinal TCR gamma delta+ cells and liver CD4+ NK1+T cells.

In susceptible mice, Leishmania major induce very rapid interleukin-4 production by CD4+ T cells which are NK1.1-

Susceptibility of BALB/c mice to infection with Leishmania major is associated with a T helper type 2 (Th2) response. Since interleukin-4 (IL-4) is critically required early for Th2 cell development, the kinetics of IL-4 mRNA expression was compared in susceptible and resistant mice during the first days of infection. In contrast to resistant mice, susceptible mice exhibited a peak of IL-4 mRNA in their spleens 90 min after i.v. injection of parasites and in lymph nodes 16 h after s.c. injection. IL-12 and interferon-gamma (IFN-gamma) down-regulated this early peak of IL-4 mRNA; the effect of IL-12 was IFN-gamma dependent. Treatment of resistant C57BL/6 mice with anti-IFN-gamma allowed the expression of this early IL-4 response to L. major. The increased IL-4 mRNA expression occurred in V beta 8, 7, 2- CD4+ cells in BALB/c mice and NK1.1- CD4+ cells in anti-IFN-gamma treated C57BL/6 mice. These results show that the NK1.1+ CD4+ cells, responsible for the rapid burst of IL-4 production after i.v. injection of anti-CD3, do not contribute to the early IL-4 response to L. major.

Distinct phenotypes of antigen-selected CD8 T cells emerge at different stages of an in vivo immune response

We have previously described a unique system for identifying Ag-selected CD8 T cells during an in vivo response in normal mice. In this system, lymphocytes isolated from DBA/2 mice injected i.p. with HLA-CW3 transfected syngeneic (H-2d) P815 cells show a remarkable expansion of CD8 cells that utilize TCR expressing the V beta 10 gene segment and additional structural features characteristic of Kd-restricted CW3-specific CTL clones. We have now taken advantage of this system to characterize the surface phenotype of CD8 cells selected by Ag in vivo. We observed several distinct phenotypes at different stages of the response. At the peak of the response, Ag-selected cells were low in CD62L and CD45RB expression but displayed high levels of CD44. In addition, there was a partial down-regulation of CD8 and TCR. Cells of this phenotype were present in lymphoid tissues for several mo after immunization. Much later in the response, Ag-selected cells expressed higher levels of CD8 and TCR. Moreover, a distinct subset of these long-term immune cells emerged that now expressed CD62L and CD45RB. Analysis of CD8 cells from different tissues also revealed certain differences, particularly in TCR and co-receptor levels from liver-derived cells compared with circulating cells at the peak of the response. Our findings suggest that the function of Ag-selected CD8 cells may be regulated over time and according to location by subtle changes in cell-surface phenotype.

Distinct phenotypes of antigen-selected CD8 T cells emerge at different stages of an in vivo immune response

We have previously described a unique system for identifying Ag-selected CD8 T cells during an in vivo response in normal mice. In this system, lymphocytes isolated from DBA/2 mice injected i.p. with HLA-CW3 transfected syngeneic (H-2d) P815 cells show a remarkable expansion of CD8 cells that utilize TCR expressing the V beta 10 gene segment and additional structural features characteristic of Kd-restricted CW3-specific CTL clones. We have now taken advantage of this system to characterize the surface phenotype of CD8 cells selected by Ag in vivo. We observed several distinct phenotypes at different stages of the response. At the peak of the response, Ag-selected cells were low in CD62L and CD45RB expression but displayed high levels of CD44. In addition, there was a partial down-regulation of CD8 and TCR. Cells of this phenotype were present in lymphoid tissues for several mo after immunization. Much later in the response, Ag-selected cells expressed higher levels of CD8 and TCR. Moreover, a distinct subset of these long-term immune cells emerged that now expressed CD62L and CD45RB. Analysis of CD8 cells from different tissues also revealed certain differences, particularly in TCR and co-receptor levels from liver-derived cells compared with circulating cells at the peak of the response. Our findings suggest that the function of Ag-selected CD8 cells may be regulated over time and according to location by subtle changes in cell-surface phenotype.

Reduced development of CD4-8-B220+ T cells but normal autoantibody production in lpr/lpr mice lacking major histocompatibility complex class I molecules

The lpr gene has recently been shown to encode a functional mutation in the Fas receptor, a molecule involved in transducing apoptotic signals. Mice homozygous for the lpr gene develop an autoimmune syndrome accompanied by massive accumulation of double-negative (DN) CD4-8-B220+ T cell receptor-alpha/beta+ cells. In order to investigate the origin of these DN T cells, we derived lpr/lpr mice lacking major histocompatibility complex (MHC) class I molecules by intercrossing them with beta 2-microglobulin (beta 2m)-deficient mice. Interestingly, these lpr beta 2m-/- mice develop 13-fold fewer DNT cells in lymph nodes as compared to lpr/lpr wild-type (lprWT) mice. Analysis of anti-DNA antibodies and rheumatoid factor in serum demonstrates that lpr beta 2m-/- mice produce comparable levels of autoantibodies to lprWT mice. Collectively our data indicate that MHC class I molecules control the development of DN T cells but not autoantibody production in lpr/lpr mice and support the hypothesis that the majority of DN T cells may be derived from cells of the CD8 lineage.