Regulates Thymocyte Development Research Articles

OR19-04 Glucocorticoid Production in the Nervous and Immune Systems: Evidence for a Local HPA Axis Homolog

Glucocorticoid Production in the Nervous and Immune Systems: Evidence for a Local HPA Axis HomologThe hypothalamic-pituitary-adrenal (HPA) axis is a critical stress response system in vertebrates. The hypothalamus secretes corticotropin-releasing hormone (CRH), which binds its receptor (CRH-R1) in the anterior pituitary. The anterior pituitary then secretes adrenocorticotropic hormone (ACTH), which binds its receptor (MC2R) in the adrenal glands and stimulates secretion of glucocorticoids into the bloodstream. Glucocorticoids are critical modulators of neural and immune system development. During early development (postnatal day (PND) 2 to 12), mice show decreased adrenal glucocorticoid secretion at baseline and in response to stressors, termed the stress hyporesponsive period (SHRP) (1). Traditionally, glucocorticoids have been thought to be synthesized only in the adrenal glands. However, recent evidence demonstrates that glucocorticoids are also produced in extra-adrenal tissues, such as the brain and lymphoid organs (2). This may be of particular importance during the SHRP, as local production allows glucocorticoid modulation of specific tissues and cells, without general effects throughout the organism. Importantly, the factors that regulate local glucocorticoid production remain unknown. To study the regulation of local glucocorticoid production, we examined whether mediators of the HPA axis are locally expressed at baseline and in response to an immune stressor. We assessed systemic and local glucocorticoid levels in neonatal (PND5) C57BL/6J mice 4hr after an immune challenge with lipopolysaccharide (50µg/kg i.p.) or vehicle control. We examined blood, microdissected brain regions (prefrontal cortex, hippocampus, hypothalamus), and lymphoid organs (thymus, spleen, bone marrow). A panel of 7 steroids was measured via liquid chromatography tandem mass spectrometry (LC-MS/MS). Gene expression of Crh, Crh-R1, Pomc, and Mc2r was quantified via qPCR. Preliminary data indicate that corticosterone was 2-fold higher in tissues than in blood after an immune stressor. The thymus expressed all genes of interest, supporting the existence of a local HPA axis “homolog” in the thymus. Brain, spleen and bone marrow expressed a subset of the genes of interest. These exciting data demonstrate that all the mediators of the HPA axis are locally expressed within the thymus, likely to regulate thymocyte development and reactivity. Greater understanding of local glucocorticoid production will provide crucial insight into neural and immune development and function. Reference: (1) Sapolsky et al., Brain Res Rev. 1986 11(1):65–76. (2) Taves et al., Endocrinology. 2015 156(2):511–522.

Sumoylation of RORgt regulates TH17 differentiation and thymocyte development

Abstract RORgt controls the differentiation of TH17 cells involved in autoimmunity such as experimental autoimmune encephalomyelitis (EAE). RORgt also regulates thymocyte development and lymph node genesis. Here we show sumoylation as a novel mechanism to regulate RORgt function. Loss of Sumo3, but not Sumo1, dampens TH17 differentiation and delays the progression of thymic immature CD8+cells (ISP). RORgt is SUMO3-modified at lysine 31 (K31), and mutation of K31 to arginine (RORgtK31R) in mice to prevent sumoylation impairs RORgt-dependent function including defective TH17 differentiation, resistance to TH17-mediated EAE, accumulation of thymic ISP and lack of Peyer’s patches. Mechanically, RORgt-K31 sumoylation stabilizes the binding of SRC1 through recruiting histone acetyltransferase KAT2A. Lastly, E3 ligase PIAS4 binds and sumoylates RORgt at K31, and regulates RORgt-dependent TH17 differentiation and progression of thymic ISP. This study thus demonstrates sumoylation as a critical mechanism for regulating RORgt function, and reveal new drug targets for preventing TH17-mediated autoimmunity.

Sumoylation of ROR\u03b3t regulates TH17 differentiation and thymocyte development

RORγt controls the differentiation of TH17 cells, which are mediators of autoimmune conditions such as experimental autoimmune encephalomyelitis (EAE). RORγt also regulates thymocyte development and lymph node genesis. Here we show that the function of RORγt is regulated by its sumoylation. Loss of Sumo3, but not Sumo1, dampens TH17 differentiation and delays the progression of thymic CD8+ immature single-positive cells (ISPs). RORγt is SUMO3-modified by E3 ligase PIAS4 at lysine 31 (K31), and the mutation of K31 to arginine in mice prevents RORγt sumoylation, leading to impaired TH17 differentiation, resistance to TH17-mediated EAE, accumulation of thymic ISPs, and a lack of Peyer’s patches. Mechanistically, sumoylation of RORγt-K31 recruits histone acetyltransferase KAT2A, which stabilizes the binding of SRC1 to enhance RORγt transcription factor activity. This study thus demonstrates that sumoylation is a critical mechanism for regulating RORγt function, and reveals new drug targets for preventing TH17-mediated autoimmunity.

Enriched environment regulates thymocyte development and alleviates experimental autoimmune encephalomyelitis in mice

Environmental and social factors have profound impacts on immune homeostasis. Our work on environmental enrichment (EE) has revealed a novel anti-obesity and anticancer phenotype associated with enhanced activity of CD8+ cytotoxic T lymphocytes in secondary lymphoid tissues. Here we investigated how an EE modulated thymus and thymocyte development. EE decreased thymus mass and cellularity, decreased the double positive thymocyte population, increased the proportion of CD8+ T cells, reduced the CD4:CD8 ratio, and downregulated CD69 expression in T cells. In a model of multiple sclerosis: experimental autoimmune encephalomyelitis (EAE), EE alleviated symptoms, inhibited spinal cord inflammation through regulation of type 1 T-helper cells mediated by glucocorticoid receptor signaling, and prevented EAE-induced thymic disturbance. Our mechanistic studies demonstrated that hypothalamic BDNF activated a hypothalamic-pituitary-adrenal axis mediating the EE’s thymic effects. Our results indicate that a lifestyle intervention links the nervous, endocrine, and adaptive immune system, allowing the body to adapt to internal and external environments.

Genetic Ablation of the ABC Transporter Abcc4 Impairs Lymphoid Leukemogenesis

ABCC4 is an ABC transporter of the C sub‐family that is ubiquitously expressed and exports key endogenous compounds including cAMP and prostaglandins, and cancer chemotherapeutic drugs. cAMP and prostaglandins are signaling molecules that, among other functions, regulate thymocyte development and function. Thymocyte progenitors, IL7 receptor positive cells, migrate from the bone marrow to the thymus and give rise to mature T‐cells. The maturation of thymocytes can be followed by monitoring for the presence of surface markers CD4 and CD8. Briefly, thymocytes are initially double negative (DN) cells, with neither CD4 nor CD8; after T‐cell receptor rearrangement, CD4 and CD8 are expressed on double positive cells (DP), which following positive and negative selection, become mature, single positive (SP) cells expressing either CD4 or CD8. Abcc4 is highly expressed in DP and CD4‐positve thymocytes, while being undetectable in DN thymocytes from wild‐type (WT) mice, suggesting a developmental regulation of Abcc4. Therefore, we hypothesized that ablation of Abcc4 from mice would lead to aberrant T‐cell development and function.ABCC4 absence had no measureable impact on the number of hematopoietic progenitors. We hypothesized that lymphoid progenitors in Abcc4‐knockout(KO) mice may have an impaired leukemogenesis potential. To test this hypothesis, hematopoietic progenitors from Abcc4‐KO or WT mice were transduced with a retrovirus harboring a MYCN cDNA, a known driver of lymphoid leukemia, and subsequently transplanted into lethally irradiated congenic WT recipient mice. Mice transplanted with either WT or Abcc4‐KO MYCN‐transduced hematopoietic progenitors developed both B‐ and T‐cell leukemia, with tumors expressing Abcc4 on the plasma membrane. Kaplan‐Meier survival analysis revealed that mice transduced with Abcc4‐KO progenitors had a dramatic increase in median survival from 81.5 days in mice who received WT progenitors to 112 days in mice who received Abcc4‐KO progenitors. These studies suggested the lymphoid progenitors from Abcc4‐KO mice harbored an intrinsic defect manifested by reduced lymphoid leukemia. This was further investigated by performing a competitive bone‐marrow transplantation experiment. Briefly, hematopoietic progenitors from Abcc4‐KO and wildtype mice were isolated and mixed in equal numbers (each harboring a different surface marker to enable identification), then transplanted into congenic wildtype mice following lethal irradiation; subsequently the differentiated lymphoid progeny were identified in the peripheral blood. These studies revealed that the number of CD4+ cells from Abcc4‐KO donors was reduced, thus confirming a defect in Abcc4‐KO lymphoid progenitors. A defect in peripheral thymocyte function is also a potential mechanism, as genes in the arachidonic acid pathway regulate further differentiation and function of mature thymocytes. We discovered altered expression of genes in the arachidonic acid pathway such as Cox‐1 and Ptger4, when compared to age‐matched WT mice. These data suggest that genetic ablation of Abcc4 has an effect on arachidonic acid signaling, which may in turn affect thymocyte function.Overall, the data presented herein suggest that ABCC4 impacts lymphoid progenitors and differentiated thymocytes. One implication of these findings is that deficiency or insufficiency of ABCC4 may reduce the possibility of developing lymphoid malignancies.Support or Funding InformationThis work was funded by NIH and ALSACThis abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Retinoblastoma 1 protects T cell maturation from premature apoptosis by inhibiting E2F1.

T lymphocytes are key cellular components of an acquired immune system and play essential roles in cell-mediated immunity. T cell development occurs in the thymus where 95% of immature thymocytes are eliminated via apoptosis. It is known that mutation of Zeb1, one of the retinoblastoma 1 (Rb1) target genes, results in a decrease in the number of immature T cells in mice. E2F1, an RB1-interacting protein, has been shown to regulate mature T cell development by interfering with thymocyte apoptosis. However, whether Rb1 regulates thymocyte development in vivo still needs to be further investigated. Here, we use a zebrafish model to investigate the role of Rb1 in T cell development. We show that Rb1-deficient fish exhibit a significant reduction in T cell number during early development that it is attributed to the accelerated apoptosis of immature T cells in a caspase-dependent manner. We further show that E2F1 overexpression could mimic the reduced T lymphocytes phenotype of Rb1 mutants, and E2F1 knockdown could rescue the phenotype in Rb1-deficient mutants. Collectively, our data indicate that the Rb1-E2F1-caspase axis is crucial for protecting immature T cells from apoptosis during early T lymphocyte maturation.

Peroxisome Proliferator-Activated Receptor Delta (PPARδ) activity is required for normal T cell development

Abstract PPARδ is a nuclear receptor that functions as a transcriptional regulator of various processes including metabolic homeostasis, cell proliferation and differentiation; however, whether PPARδ regulates T cell development is not known. We observed that PPARδ-deficient (PPARδ−/−) mice have smaller spleens and thymi compared to wild-type (WT) mice. This reduction in thymus cellularity was apparent across all stages of thymocyte development. To further narrow in on the cellular basis of this effect, radiation bone marrow chimeras were generated where WT or PPARδ−/− mice were reconstituted with WT or PPARδ−/− bone marrow. These experiments clearly revealed that the defects in thymus and spleen mapped to the radio-resistant compartment. This was furthered confirmed by the finding that mice with T cell-restricted deficiency of PPARδ did not exhibit defective thymocyte homeostasis. Given that thymic epithelial cells (TECs) are the major radio-resistant cell population in the thymus, we next generated mice with a TEC-restricted deficiency of PPARδ (PPARδflox/floxFoxN1-Cre+). We observed that PPARδflox/floxFoxN1-Cre+ mice exhibit a trend for reduced thymus and spleen cellularity compared to PPARδflox/flox mice; however, this phenotype was not as striking as seen in PPARδ−/− mice. These results suggest that, in addition to acting within TECs, PPARδ may regulate the production of a circulating factor that in turn regulates thymocyte development. Large-scale serum profiling studies are underway to identify this factor. Altogether, this work highlights a role for PPARδ in T cell development.

T cell development involves TRAF3IP3-mediated ERK signaling in the Golgi.

Generation of T lymphocytes in the thymus is guided by signal transduction from the T cell receptor (TCR), but the underlying mechanism is incompletely understood. Here we have identified a Golgi-associated factor, TRAF3-interacting protein 3 (TRAF3IP3), as a crucial mediator of thymocyte development. TRAF3IP3 deficiency in mice attenuates the generation of mature thymocytes caused by impaired thymocyte-positive selection. TRAF3IP3 mediates TCR-stimulated activation of the mitogen-activated protein kinase (MAPK) extracellular signal-regulated kinase (ERK) and its upstream kinase mitogen/extracellular signal-regulated kinase (MEK). Interestingly, TRAF3IP3 exerts this signaling function through recruiting MEK to the Golgi and, thereby, facilitating the interaction of MEK with its activator BRAF. Transgenic expression of a constitutively active MEK rescues the T cell development block in Traf3ip3 knockout mice. These findings establish TRAF3IP3 as a novel regulator of T cell development and suggest a Golgi-specific ERK signaling mechanism that regulates thymocyte development.

KMT1E-mediated chromatin modifications at the FcγRIIb promoter regulate thymocyte development.

This work examines the role the lysine methyltransferase KMT1E (Setdb1) in thymocyte development. We have developed and described a T cell-specific conditional knockout of Setdb1. A partial block was seen at the double-positive to single-positive transition, causing reduced numbers of single-positive T cells in the thymus and periphery. Knockout thymocytes had reduced numbers of CD69(+) and T-cell receptor TCRβ(+) cells and increased numbers of apoptotic cells in the double-positive compartment, suggesting an alteration in the selection process. Transcriptional profiling of thymocytes revealed that Setdb1 deletion derepresses expression of FcγRIIb, the inhibitory Fc receptor. We demonstrate that a KMT1E-containing complex directly interacts with the FcγRIIb promoter and that histone H3 at lysine 9 tri-methylation at this promoter is dependent on Setdb1 expression. Derepression of FcγRIIb causes exacerbated signaling through the TCR complex, with specifically increased phosphorylation of ZAP70, affecting selection. This work identifies KMT1E as a novel repressor of FcγRIIb and identifies an underappreciated role of FcγRIIb in fine tuning thymocyte development.

Gene targeting RhoA reveals its essential role in coordinating mitochondrial function and thymocyte development.

Thymocyte development is regulated by complex signaling pathways. How these signaling cascades are coordinated remains elusive. RhoA of the Rho family small GTPases plays an important role in actin cytoskeleton organization, cell adhesion, migration, proliferation, and survival. Nonetheless, the physiological function of RhoA in thymocyte development is not clear. By characterizing a conditional gene targeting mouse model bearing T cell deletion of RhoA, we show that RhoA critically regulates thymocyte development by coordinating multiple developmental events. RhoA gene disruption caused a strong developmental block at the pre-TCR checkpoint and during positive selection. Ablation of RhoA led to reduced DNA synthesis in CD4(-)CD8(-), CD4(+)CD8(-), and CD4(-)CD8(+) thymocytes but not in CD4(+)CD8(+) thymocytes. Instead, RhoA-deficient CD4(+)CD8(+) thymocytes showed an impaired mitosis. Furthermore, we found that abrogation of RhoA led to an increased apoptosis in all thymocyte subpopulations. Importantly, we show that the increased apoptosis was resulted from reduced pre-TCR expression and increased production of reactive oxygen species (ROS), which may be because of an enhanced mitochondrial function, as manifested by increased oxidative phosphorylation, glycolysis, mitochondrial membrane potential, and mitochondrial biogenesis in RhoA-deficient thymocytes. Restoration of pre-TCR expression or treatment of RhoA-deficient mice with a ROS scavenger N-acetylcysteine partially restored thymocyte development. These results suggest that RhoA is required for thymocyte development and indicate, to our knowledge, for the first time that fine-tuning of ROS production by RhoA, through a delicate control of metabolic circuit, may contribute to thymopoiesis.

The SRC Kinase Family Member Lymphocyte Specific Kinase (p56Lck) Regulates Endothelial Cell Proliferation and Survival

Background and Objective: The Src family consists of Src, Lck (p56Lck), Fyn, Hck, Blk, Lyn, Fgr, Yes, and Yrk protein tyrosine kinases, which display cell and tissue specific expression. These non-receptor protein tyrosine kinases regulate many cellular processes including cell growth, differentiation, shape, migration, and survival. p56Lck is primarily expressed in T lymphocytes where it stimulates T cell receptor-mediated signaling to regulate thymocyte development. Similar to other SRC family members, p56Lck activity is negatively regulated by phosphorylation of Tyr505 and undergoes autophosphorylation on Tyr394 in the active state. Our previous studies showed that p56Lck activation occurred in proliferating endothelial cells exposed to cleaved high molecular weight kininogen (HKa), which induced endothelial cell apoptosis.The purpose of this study is to define the role of Lck in regulation of endothelial cell survival and angiogenesis.Methods: Primary cultures of endothelial cells were treated with Lck siRNA or with mammalian expression vectors or lentiviral constructs containing Lck cDNA. The proliferative capacity, viability, and tube-forming ability of Lck and control endothelial cells were determined.Results: Transfection of endothelial cells with Lck siRNA markedly changed their morphology and significantly enhanced their ability to proliferate in response to bFGF. In contrast, endothelial cells transfected with a Lck expression vector or transduced with a Lck-lentivirus not only failed to proliferate in response to growth factors, but also underwent apoptosis on several different extracellular matrix proteins. Finally, overexpression of Lck in endothelial cells led to impaired tube formation in a matrigel-based, in vitro angiogenesis assay; apoptosis of endothelial cells in this system was suggested by increased staining with Annexin V.Conclusion: Taken together, these results suggest that regulation of Lck activation provides a key node in determination of endothelial cell proliferation and survival that is likely to be relevant to the regulation of angiogenesis. Moreover, they suggest that specific endothelial cell Src family kinase members may play contrasting roles in regulation of these processes. Activation of Lck may represent a potential mechanism for controlling aberrant angiogenesis in pathological conditions. [Display omitted] DisclosuresNo relevant conflicts of interest to declare.

The catalytic activity of the mitogen-activated protein kinase extracellular signal-regulated kinase 3 is required to sustain CD4+ CD8+ thymocyte survival.

Extracellular signal-regulated kinase 3 (ERK3) is an atypical member of the mitogen-activated protein kinase (MAPK) family whose function is largely unknown. Given the central role of MAPKs in T cell development, we hypothesized that ERK3 may regulate thymocyte development. Here we have shown that ERK3 deficiency leads to a 50% reduction in CD4(+) CD8(+) (DP) thymocyte number. Analysis of hematopoietic chimeras revealed that the reduction in DP thymocytes is intrinsic to hematopoietic cells. We found that early thymic progenitors seed the Erk3(-/-) thymus and can properly differentiate and proliferate to generate DP thymocytes. However, ERK3 deficiency results in a decrease in the DP thymocyte half-life, associated with a higher level of apoptosis. As a consequence, ERK3-deficient DP thymocytes are impaired in their ability to make successful secondary T cell receptor alpha (TCRα) gene rearrangement. Introduction of an already rearranged TCR transgene restores thymic cell number. We further show that knock-in of a catalytically inactive allele of Erk3 fails to rescue the loss of DP thymocytes. Our results uncover a unique role for ERK3, dependent on its kinase activity, during T cell development and show that this atypical MAPK is essential to sustain DP survival during RAG-mediated rearrangements.

FADD regulates thymocyte development at the β-selection checkpoint by modulating Notch signaling.

Non-apoptotic functions of Fas-associated protein with death domain (FADD) have been implicated in T lineage lymphocytes, but the nature of FADD-dependent non-apoptotic mechanism in early T-cell development has not been completely elucidated. In this study, we show that tissue-specific deletion of FADD in immature (CD44–CD25+) thymocytes results in severe perturbation of αβ lineage development. Meanwhile, loss of FADD signaling at a later (CD44–CD25–) developmental stage does not affect subsequent T-cell development. Collectively, our work presents that FADD deficiency induces failed survival in double-negative 4 (DN4) cells, while pre-T-cell receptor (TCR) signal remains intact. In addition, Notch signaling is positive regulated on DN4 and double-positive thymocytes in T-cell-specific FADD-knockout mice, which express higher levels of a subset of Notch-target genes, including Hes1, Deltex1 and CD25. Moreover, a transcriptional repressor of Notch1, NKAP is downregulated coupled with the loss of FADD in thymocytes and is found to associate with FADD. These data suggest that as a death receptor, FADD is also required for cell survival in β-selection as a regulator of Notch1 expression.

Peroxisome proliferator-activated receptor delta (PPARδ) limits Th1 inflammation through effects on interferon (IFN)-γ transcription and lymphocyte homeostasis (IRM7P.480)

Abstract PPARδ is a nuclear receptor that functions as a transcriptional regulator of various processes including metabolic homeostasis, cell proliferation and differentiation. We previously reported that mice deficient in PPARδ (KO) exhibit more severe experimental autoimmune encephalomyelitis than wild type (WT) mice. This more severe inflammation correlated with an increased expansion of myelin-reactive IFNγ-producing cells in the spinal cord, suggesting that PPARδ may act to limit Th1 inflammatory responses. However, the mechanism of how PPARδ accomplishes this is not clear. Characterization of the spleen and thymus compartments revealed that KO mice exhibit reduced spleen and thymus cellularity as compared to WT mice. The resultant lymphopenia in KO mice associated with a higher frequency of memory CD4+ T cells in the periphery. Radiation bone marrow chimeras were generated and the defect in thymocyte development was found to map to the radio-resistant compartment, suggesting that PPARδ may operate in stromal cells to regulate thymocyte development. This was furthered confirmed by the finding that mice with T cell-restricted deficiency of PPARδ did not exhibit defective thymocyte homeostasis. Furthermore, we observed that naïve CD4+ T cell from both KO and T cell-restricted PPARδ-deficient mice had a higher propensity to produce IFNγ upon stimulation. Thus PPARδ appears to limit Th1 inflammation both through the control of lymphocyte homeostasis and IFNγ production.

Adenosine maintains the numbers

Adenosine regulates thymocyte development and maintains peripheral naive T cells through the regulation of IL-7Rα expression.

Efnb1 and Efnb2 Proteins Regulate Thymocyte Development, Peripheral T Cell Differentiation, and Antiviral Immune Responses and Are Essential for Interleukin-6 (IL-6) Signaling

Erythropoietin-producing hepatocellular kinases (Eph kinases) constitute the largest family of cell membrane receptor tyrosine kinases, and their ligand ephrins are also cell surface molecules. Because of promiscuous interaction between Ephs and ephrins, there is considerable redundancy in this system, reflecting the essential roles of these molecules in the biological system through evolution. In this study, both Efnb1 and Efnb2 were null-mutated in the T cell compartment of mice through loxP-mediated gene deletion. Mice with this double conditional mutation (double KO mice) showed reduced thymus and spleen size and cellularity. There was a significant decrease in the DN4, double positive, and single positive thymocyte subpopulations and mature CD4 and CD8 cells in the periphery. dKO thymocytes and peripheral T cells failed to compete with their WT counterparts in irradiated recipients, and the T cells showed compromised ability of homeostatic expansion. dKO naive T cells were inferior in differentiating into Th1 and Th17 effectors in vitro. The dKO mice showed diminished immune response against LCMV infection. Mechanistic studies revealed that IL-6 signaling in dKO T cells was compromised, in terms of abated induction of STAT3 phosphorylation upon IL-6 stimulation. This defect likely contributed to the observed in vitro and in vivo phenotype in dKO mice. This study revealed novel roles of Efnb1 and Efnb2 in T cell development and function.

Indian hedgehog (Ihh) both promotes and restricts thymocyte differentiation

AbstractWe show that Indian Hedgehog (Ihh) regulates T-cell development and homeostasis in both fetal and adult thymus, controlling thymocyte number. Fetal Ihh−/− thymi had reduced differentiation to double-positive (DP) cell and reduced cell numbers compared with wild-type littermates. Surprisingly, fetal Ihh+/− thymi had increased thymocyte numbers and proportion of DP cells relative to wild type, indicating that Ihh also negatively regulates thymocyte development. In vitro treatment of thymus explants with exogenous recombinant Hedgehog protein promoted thymocyte development in Ihh−/− thymi but inhibited thymocyte development in Ihh+/−, confirming both positive and negative regulatory functions of Ihh. Analysis of Rag−/−Ihh+/− thymi showed that Ihh promotes T-cell development before pre–T-cell receptor (pre-TCR) signaling, but negatively regulates T-cell development only after pre-TCR signaling has taken place. We show that Ihh is most highly expressed by the DP population and that Ihh produced by DP cells feeds back to negatively regulate the differentiation and proliferation of their double-negative progenitors. Thus, differentiation from double-negative to DP cell, and hence the size of the DP population, is dependent on the concentration of Ihh in the thymus. Analysis of Ihh conditional knockout and heterozygote adult mice showed that Ihh also influences thymocyte number in the adult.

Function of PI3Kgamma in thymocyte development, T cell activation, and neutrophil migration.

Phosphoinositide 3-kinases (PI3Ks) regulate fundamental cellular responses such as proliferation, apoptosis, cell motility, and adhesion. Viable gene-targeted mice lacking the p110 catalytic subunit of PI3Kgamma were generated. We show that PI3Kgamma controls thymocyte survival and activation of mature T cells but has no role in the development or function of B cells. PI3Kgamma-deficient neutrophils exhibited severe defects in migration and respiratory burst in response to heterotrimeric GTP-binding protein (G protein)-coupled receptor (GPCR) agonists and chemotactic agents. PI3Kgamma links GPCR stimulation to the formation of phosphatidylinositol 3,4,5-triphosphate and the activation of protein kinase B, ribosomal protein S6 kinase, and extracellular signal-regulated kinases 1 and 2. Thus, PI3Kgamma regulates thymocyte development, T cell activation, neutrophil migration, and the oxidative burst.

Thymocyte Selection Is Regulated by the Helix-Loop-Helix Inhibitor Protein, Id3

E2A, HEB, E2–2, and daughterless are basic helix-loop-helix (bHLH) proteins that play key roles in multiple developmental pathways. The DNA binding activity of E2A, HEB, and E2-2 is regulated by a distinct class of inhibitor HLH proteins, the Id gene products. Here, we show that Id3 is required for major histocompatability (MHC) class I– and class II–restricted thymocyte positive selection. Additionally, H-Y TCR–mediated negative selection is severely perturbed in Id3 null mutant mice. Finally, we show that E2A and Id3 interact genetically to regulate thymocyte development. These observations identify the HLH inhibitory protein Id3 as an essential component required for proper thymocyte maturation.

Regulatory T cells maintain peripheral tolerance to islet allografts induced by intrathymic injection of MHC class I allopeptides.

Although transplantation remains the treatment of choice for diabetes mellitus, immunological rejection of allografts continues to be a major problem. The search for strategies to prevent graft rejection led us to examine if the fate of developing T cells may be influenced by the presence of allo MHC class I peptides in the thymus because T cell receptor-MHC class I/self-peptide interaction regulates thymocyte development. We studied the effects of intrathymic (IT) injection of a short segment of a synthetic immunogenic MHC class I peptide (peptide 2, residues 67-85) of the hypervariable domain of RT1.A derived from WAG rat (RT1U) on islet graft survival in the WF(RT1U)-to-ACI combination. Adult diabetic male recipients were treated with IT injection of a single WAG-derived MHC class I peptide 7 days before intraportal islet transplantation. Long-term unresponsive islet recipients were examined for the development of alloantigen (Ag)-specific regulatory cells. The results showed that while IT injection of 150 microg peptide 2 on day -7 did not prolong graft survival in naive recipients [median survival time (MST) of 14.0 days vs. 9.6 in controls], IT injection of 300 or 600 microg peptide 2 led to normoglycemia and permanent islet survival (> 200 days) in 4/6 and 3/5 STZ-induced diabetic ACI recipients, respectively. IT injection of 150, 300, or 600 microg peptide 2 combined with 0.5 antilymphocyte serum (ALS) immunosuppression on day -7 led to 100% permanent islet allograft survival (> 200 days) compared to MST of 15.0 +/- 2.3 days in ALS alone-treated controls. Similarly prepared animals rejected third-party Brown Norway (BN) islets in an acute fashion, thus demonstrating donor specificity. Intravenous injection of 300 microg peptide 2 combined with 0.5 ml ALS did not prolong islet allograft survival. The long-term unresponsive islet allograft recipients challenged with second set grafts accepted permanently 100% donor-type cardiac allografts while rejecting third-party (BN) hearts without rejecting the primary Wistar Furth (WF) islets. In analyzing the underlying mechanisms of acquired systemic tolerance, we found no suppressor/regulatory cells in adoptive transfer studies in tolerant animals at 30 days after IT injection of allopeptides. In contrast, adoptive transfer of 5 x 10(7) unseparated spleen cells from tolerant animals at 60 and 100 days after islet transplantation into lightly irradiated [200 rad total body irradiation (TBI)] ACI recipients led to donor-specific permanent islet graft survival in 2/3 and 4/5 secondary recipients, respectively, compared to an MST of 13.8 days in lightly irradiated ACI given unmodified syngeneic spleen cells. In addition, adoptive transfer of 2 x 10(7) purified T cells obtained from long-term functioning islet recipients led to permanent donor-specific islet survival in secondary recipients. The finding that IT injection of a short segment of a synthetic immunodominant MHC class I peptide derived from WAG that shares the RT1.A(U) domain with the graft donor is capable of inducing acquired systemic tolerance to WF islets suggests that linked recognition or epitope suppression may be involved in the induction of unresponsiveness. Generation of peripheral Ag-specific regulatory cells that suppress Ag-specific alloreactive T cells is, in part, responsible for the maintenance of tolerance in this model.