Kinetics of thymocyte developmental process in fetal and neonatal mice

Distinct T-cell receptor (TCR) gene segment usage and MHC-restriction between foetal and adult thymus

Here, we sequenced rearranged TCRβ and TCRα chain sequences in CD4+CD8+ double positive (DP), CD4+CD8- single positive (SP4) and CD4-CD8+ (SP8) thymocyte populations from the foetus and young adult mouse. We found that life-stage had a greater impact on TCRβ and TCRα gene segment usage than cell-type. Foetal repertoires showed bias towards 3'TRAV and 5'TRAJ rearrangements in all populations, whereas adult repertoires used more 5'TRAV gene segments, suggesting that progressive TCRα rearrangements occur less frequently in foetal DP cells. When we synchronised young adult DP thymocyte differentiation by hydrocortisone treatment the new recovering DP thymocyte population showed more foetal-like 3'TRAV and 5'TRAJ gene segment usage. In foetus we identified less influence of MHC-restriction on α-chain and β-chain combinatorial VxJ usage and CDR1xCDR2 (V region) usage in SP compared to adult, indicating weaker impact of MHC-restriction on the foetal TCR repertoire. The foetal TCRβ repertoire was less diverse, less evenly distributed, with fewer non-template insertions, and all foetal populations contained more clonotypic expansions than adult. The differences between the foetal and adult thymus TCR repertoires are consistent with the foetal thymus producing αβT-cells with properties and functions that are distinct from adult T-cells: their repertoire is less governed by MHC-restriction, with preference for particular gene segment usage, less diverse with more clonotypic expansions, and more closely encoded by genomic sequence.

MHCII+CD80+ thymic eosinophils increase in abundance during neonatal development in mice and their accumulation is microbiota dependent

Eosinophils are present in the thymus of mammals, yet their function at this site during homeostatic development is unknown. We used flow cytometry to determine the abundance and phenotype of eosinophils (here defined as SSchigh SiglecF+ CD11b+ CD45+ cells) in the thymus of mice during the neonatal period, the later postnatal period, and into adulthood. We show that both the total number of thymic eosinophils and their frequency among leukocytes increase over the first 2 wk of life and that their accumulation in the thymus is dependent on the presence of an intact bacterial microbiota. We report that thymic eosinophils express the interleukin-5 receptor (CD125), CD80, and IDO, and that subsets of thymic eosinophils express CD11c and major histocompatibility complex II (MHCII). We found that the frequency of MHCII-expressing thymic eosinophils increases over the first 2 wk of life, and that during this early-life period the highest frequency of MHCII-expressing thymic eosinophils is located in the inner medullary region. These data suggest a temporal and microbiota-dependent regulation of eosinophil abundance and functional capabilities in the thymus.

Intrauterine desensitization enables long term survival of human oligodendrocyte progenitor cells without immunosuppression

Immune rejection can be reduced using immunosuppressants which are not viable for premature infants. However, desensitization can induce immune tolerance for premature infants because of underdeveloped immune system. The fetuses of Wistar rats at 15-17 days gestation were injected via hOPCs-1 into brain, muscles, and abdomen ex utero and then returned while the fetuses of control without injection. After 6 weeks of desensitization, the brain and muscles were transplanted with hOPCs-1, hNSCs-1, and hOPCs-2. After 10 and 34 weeks of desensitization, hOPCs-1 and hNSCs-1 in desensitized groups was higher than that in the control group while hOPCs-2 were rejected. Treg, CD4CD28, CD8CD28, and CD45RC between the desensitization and the control group differed significantly. Inflammatory cells in group with hOPCs-1 and hNSCs-1 was lower than that in the control group. hOPCs-1 can differentiate into myelin in desensitized groups. Wistar rats with desensitization developed immune tolerance to desensitized and transplanted cells.

Pre-T cell receptor self-MHC sampling restricts thymocyte dedifferentiation

Programming T cells to distinguish self from non-self is a vital, multi-step process that occurs in the thymus1-4. Signalling through the pre-T cell receptor (preTCR), a CD3-associated heterodimer comprising an invariant pTα chain and a clone-specific β chain, is a critical early checkpoint in thymocyte development within the αβ T cell lineage5,6. PreTCRs arrayed on CD4-CD8- double-negative thymocytes ligate peptides bound to major histocompatibility complex molecules (pMHC) on thymic stroma, similar to αβ T cell receptors that appear on CD4+CD8+ double-positive thymocytes, but via a different molecular docking strategy7-10. Here we show the consequences of these distinct interactions for thymocyte progression using synchronized fetal thymic progenitor cultures that differ in the presence or absence of pMHC on support stroma, and single-cell transcriptomes at key thymocyte developmental transitions. Although major histocompatibility complex (MHC)-negative stroma fosters αβ T cell differentiation, the absence of preTCR-pMHC interactions leads to deviant thymocyte transcriptional programming associated with dedifferentiation. Highly proliferative double-negative and double-positive thymocyte subsets emerge, with antecedent characteristics of T cell lymphoblastic and myeloid malignancies. Compensatory upregulation of diverse MHC class Ib proteins in B2m/H2-Ab1 MHC-knockout mice partially safeguards in vivo thymocyte progression, although disseminated double-positive thymic tumours may develop with ageing. Thus, as well as promoting β chain repertoire broadening for subsequent αβ T cell receptor utilization, preTCR-pMHC interactions limit cellular plasticity to facilitate normal thymocyte differentiation and proliferation that, if absent, introduce developmental vulnerabilities.

Heterogeneity of type 2 innate lymphoid cells

More than a decade ago, type 2 innate lymphoid cells (ILC2s) were discovered to be members of a family of innate immune cells consisting of five subsets that form a first line of defence against infections before the recruitment of adaptive immune cells. Initially, ILC2s were implicated in the early immune response to parasitic infections, but it is now clear that ILC2s are highly diverse and have crucial roles in the regulation of tissue homeostasis and repair. ILC2s can also regulate the functions of other type 2 immune cells, including T helper 2 cells, type 2 macrophages and eosinophils. Dysregulation of ILC2s contributes to type 2-mediated pathology in a wide variety of diseases, potentially making ILC2s attractive targets for therapeutic interventions. In this Review, we focus on the spectrum of ILC2 phenotypes that have been described across different tissues and disease states with an emphasis on human ILC2s. We discuss recent insights in ILC2 biology and suggest how this knowledge might be used for novel disease treatments and improved human health.

Type 2 innate lymphoid cells (ILC2s) have diverse phenotypes across different tissues and disease states. Recent insights into ILC2 biology raise new possibilities for the improved treatment of cancer and of metabolic, infectious and chronic inflammatory diseases.

HPTE-Induced Embryonic Thymocyte Death and Alteration of Differentiation Is Not Rescued by ERα or GPER Inhibition but Is Exacerbated by Concurrent TCR Signaling

Organochlorine pesticides, such as DDT, methoxychlor, and their metabolites, have been characterized as endocrine disrupting chemicals (EDCs); suggesting that their modes of action involve interaction with or abrogation of endogenous endocrine function. This study examined whether embryonic thymocyte death and alteration of differentiation induced by the primary metabolite of methoxychlor, HPTE, rely upon estrogen receptor binding and concurrent T cell receptor signaling. Estrogen receptor inhibition of ERα or GPER did not rescue embryonic thymocyte death induced by HPTE or the model estrogen diethylstilbestrol (DES). Moreover, adverse effects induced by HPTE or DES were worsened by concurrent TCR and CD2 differentiation signaling, compared with EDC exposure post-signaling. Together, these data suggest that HPTE- and DES-induced adverse effects on embryonic thymocytes do not rely solely on ER alpha or GPER but may require both. These results also provide evidence of a potential collaborative signaling mechanism between TCR and estrogen receptors to mediate adverse effects on embryonic thymocytes, as well as highlight a window of sensitivity that modulates EDC exposure severity.

Regulatory T cells promote alloengraftment in a model of late-gestation in utero hematopoietic cell transplantation

In utero hematopoietic cell transplantation (IUHCT) has the potential to cure congenital hematologic disorders including sickle cell disease. However, the window of opportunity for IUHCT closes with the acquisition of T-cell immunity, beginning at approximately 14 weeks gestation, posing significant technical challenges and excluding from treatment fetuses evaluated after the first trimester. Here we report that regulatory T cells can promote alloengraftment and preserve allograft tolerance after the acquisition of T-cell immunity in a mouse model of late-gestation IUHCT. We show that allografts enriched with regulatory T cells harvested from either IUHCT-tolerant or naive mice engraft at 20 days post coitum (DPC) with equal frequency to unenriched allografts transplanted at 14 DPC. Long-term, multilineage donor cell chimerism was achieved in the absence of graft-versus-host disease or mortality. Decreased alloreactivity among recipient T cells was observed consistent with donor-specific tolerance. These findings suggest that donor graft enrichment with regulatory T cells could be used to successfully perform IUHCT later in gestation.

Exposure of pregnant rats to staphylococcal enterotoxin B attenuates the response of increased Tregs to re-exposure to SEB in the thymus of adult offspring

Regulatory T cells (Tregs) play an essential role during homeostasis and tolerance of the immune system. Based on our previous study that exposure of pregnant rats to staphylococcal enterotoxin B (SEB) can alter the percentage of CD4/CD8 subsets in the thymus of the offspring, in this study, we focus on the influence of exposure of pregnant rats to SEB on number, function and response of Tregs in the thymus of the offspring. Pregnant rats at gestational day of 16 were intravenously injected with 15 μg SEB and the thymuses of the neonatal and adult offspring were harvested for this study. We found that exposure of pregnant rats to SEB could significantly increase the absolute number of Tregs and the FoxP3 expression level in the thymus of not only neonatal but also adult offspring. Re-exposure of adult offspring to SEB remarkably reduced the suppressive capacity of Tregs to CD4⁺ T cells and the expression levels of TGF-β and IL-10 in the thymus, but had no effect on production of IL-4 and IFN-γ. Furthermore, it also notedly decreased the absolute number of Tregs and the FoxP3 expression level. These data suggest that prenatal exposure of pregnant rats to SEB attenuates the response of increased Tregs to re-exposure to SEB in the thymus of adult offspring.

Bi-directional signaling by membrane-bound KitL induces proliferation and coordinates thymic endothelial cell and thymocyte expansion

The ligand for the c-Kit receptor, KitL, exists as a membrane-associated (mKitL) and a soluble form (sKitL). KitL functions outside c-Kit activation have not been identified. We show that co-culture of c-Kit– and mKitL–expressing NIH3T3 cells results in signaling through mKitL: c-Kit–bound mKitL recruits calcium-modulating cyclophilin ligand (CAML) to selectively activate Akt, leading to CREB phosphorylation, mTOR pathway activation, and increased cell proliferation. Activation of mKitL in thymic vascular endothelial cells (VECs) induces mKitL- and Akt-dependent proliferation, and genetic ablation of mKitL in thymic VECs blocks their c-Kit responsiveness and proliferation during neonatal thymic expansion. Therefore, mKitL–c-Kit form a bi-directional signaling complex that acts in the developing thymus to coordinate thymic VEC and early thymic progenitor (ETP) expansion by simultaneously promoting ETP survival and VEC proliferation. This mechanism may be relevant to both normal tissues and malignant tumors that depend on KitL–c-Kit signaling for their proliferation.

c-Kit receptor–Kit ligand complex signaling is known to activate c-Kit and is essential for tissue development. Here, Buono et al. show that membrane-bound KitL signaling induces proliferation via CAML-Akt-CREB pathway activation, establishing a role for bidirectional signaling in tissue expansion.

A Single-Cell Transcriptomic Atlas of Thymus Organogenesis Resolves Cell Types and Developmental Maturation

Thymus development is critical to the adaptive immune system, yet a comprehensive transcriptional framework capturing thymus organogenesis at single-cell resolution is still needed. We applied single-cell RNA sequencing (RNA-seq) to capture 8 days of thymus development, perturbations of T cell receptor rearrangement, and in vitro organ cultures, producing profiles of 24,279 cells. We resolved transcriptional heterogeneity of developing lymphocytes, and genetic perturbation confirmed T cell identity of conventional and non-conventional lymphocytes. We characterized maturation dynamics of thymic epithelial cells in vivo, classified cell maturation state in a thymic organ culture, and revealed the intrinsic capacity of thymic epithelium to preserve transcriptional regularity despite exposure to exogenous retinoic acid. Finally, by integrating the cell atlas with human genome-wide association study (GWAS) data and autoimmune-disease-related genes, we implicated embryonic thymus-resident cells as possible participants in autoimmune disease etiologies. This resource provides a single-cell transcriptional framework for biological discovery and molecular analysis of thymus organogenesis.

Apoptotic Diminution of Immature Single and Double Positive Thymocyte Subpopulations Contributes to Thymus Involution During Murine Polymicrobial Sepsis

To generate and maintain functional T-cell receptor diversity, thymocyte development is tightly organized. Errors in this process may have dramatic consequences, provoking, for example, autoimmune diseases. Probably for this reason, the thymus reacts to septic stress with involution, decreasing the numbers of thymocytes. Because it is still unclear which thymocyte subpopulation contributes to thymus involution and whether thymocyte emigration is altered, we were interested to clarify this question in detail. Here, we show, using the cecal ligation and puncture (CLP) mouse model of polymicrobial sepsis, that predominantly immature thymocytes are reduced. The number of immature single positive thymocytes was most marked diminished (CLP: 6.54 × 10 ± 3.79 × 10 vs. sham: 4.54 × 10 ± 7.66 × 10 cells/thymus [24 h], CLP: 2.60 × 10 ± 2.14 × 10 vs. sham: 2.17 × 10 ± 1.90 × 10 cells/thymus [48 h]), and was consequently associated with the highest rate of apoptosis (8.4 [CLP] vs. 2.2% [sham]), the reduction in double positive thymocytes being associated with a smaller apoptotic response (number, CLP: 2.33 × 10 ± 1.38 × 10 vs. sham: 1.07 × 10 ± 2.72 × 10 cells/thymus [24 h], CLP: 2.34 × 10 ± 9.08 × 10 vs. sham: 3.5 × 10 ± 9.62 × 10 cells/thymus [48 h]; apoptosis: 2.5% [CLP] vs. 0.7% [sham]). Analysis of T-cell receptor excision circles revealed that the emigration of mature thymocytes was not inhibited. Real-time qPCR analysis revealed upregulation of pro-apoptotic Bim expression and suggested interference between Notch receptor expression on thymocytes and the respective ligands on thymic stromal cells during CLP-dependent sepsis, which might be responsible for the altered thymocyte viability in CLP-dependent sepsis.

Epithelial LTβR signaling controls the population size of the progenitors of medullary thymic epithelial cells in neonatal mice

The establishment of T cell central tolerance critically relies on the development and maintenance of the medullary thymic epithelial cells (mTECs). Disrupted signaling of lymphotoxin beta receptor (LTβR) results in dramatically reduced mTEC population. However, whether LTβR directly or indirectly control mTECs remains undetermined; how LTβR controls this process also remain unclear. In this study, by utilizing K14-Cre × Ltbr^fl/fl conditional knockout (cKO) mice, we show that epithelial intrinsic LTβR was essential for the mTEC development postnatally. Mechanistically, LTβR did not directly impact the proliferation or survival of mTECs; the maturation of mTECs from MHC-II^lo to MHC-II^hi stage was also unaltered in the absence of LTβR; interestingly, the number of mTEC progenitors (Cld3,4^hiSSEA-1⁺) was found significantly reduced in LTβR cKO mice at the neonatal stage, but not at E18.5. Consequently, epithelial deficiency of LTβR resulted in significant defect of thymic negative selection as demonstrated using OT-I and RIP-OVA transgenic mouse system. In summary, our study clarifies the epithelial intrinsic role of LTβR on mTEC development and function; more importantly, it reveals a previously unrecognized function of LTβR on the control of the size of mTEC progenitor population.

Modeling the development of the post-natal mouse thymus in the absence of bone marrow progenitors

Many mathematical models have been published with the purpose of explaining aspects of T-cell development in the thymus. In this manuscript we adapted a four-compartment model of the thymus and used a range of mathematical approaches with the aim of explaining the dynamics of the four main thymocyte populations in the mouse thymus, from the emergence of the first fetal thymocyte until the death of the animal. At various pre-natal and post-natal stages we investigated experimentally the number and composition of thymocytes populations, their apoptosis and proliferation, along with data from literature, to create and validate the model. In our model the proliferation processes are characterized by decreasing proliferation rates, which allows us to model the natural involution of the thymus. The best results were obtained when different sets of parameters were used for the fetal and post-natal periods, suggesting that birth may induce a discontinuity in the modeled processes. Our model is able to model the development of both pre-natal and post-natal thymocyte populations. Also, our findings showed that the post-natal thymus is able to develop in the absence of the daily input of bone marrow progenitors, providing more evidence to support the autonomous development of the post-natal thymus.

The expansion of thymopoiesis in neonatal mice is dependent on expression of high mobility group a 2 protein (Hmga2)

Cell number in the mouse thymus increases steadily during the first two weeks after birth. It then plateaus and begins to decline by seven weeks after birth. The factors governing these dramatic changes in cell production are not well understood. The data herein correlate levels of High mobility group A 2 protein (Hmga2) expression with these temporal changes in thymopoiesis. Hmga2 is expressed at high levels in murine fetal and neonatal early T cell progenitors (ETP), which are the most immature intrathymic precursors, and becomes almost undetectable in these progenitors after 5 weeks of age. Hmga2 expression is critical for the initial, exponential expansion of thymopoiesis, as Hmga2 deficient mice have a deficit of ETPs within days after birth, and total thymocyte number is repressed compared to wild type littermates. Finally, our data raise the possibility that similar events occur in humans, because Hmga2 expression is high in human fetal thymic progenitors and falls in these cells during early infancy.

Genetic interaction between mutations in c-Myb and the KIX domains of CBP and p300 affects multiple blood cell lineages and influences both gene activation and repression

Adult blood cell production or definitive hematopoiesis requires the transcription factor c-Myb. The closely related KAT3 histone acetyltransferases CBP (CREBBP) and p300 (EP300) bind c-Myb through their KIX domains and mice homozygous for a p300 KIX domain mutation exhibit multiple blood defects. Perplexingly, mice homozygous for the same KIX domain mutation in CBP have normal blood. Here we test the hypothesis that the CBP KIX domain contributes subordinately to hematopoiesis via a genetic interaction with c-Myb. We assessed hematopoiesis in mice bearing compound mutations of c-Myb and/or the KIX domains of CBP and p300, and measured the effect of KIX domain mutations on c-Myb-dependent gene expression. We found that in the context of a p300 KIX mutation, the CBP KIX domain mutation affects platelets, B cells, T cells, and red cells. Gene interaction (epistasis) analysis provides mechanistic evidence that blood defects in KIX mutant mice are consistent with reduced c-Myb and KIX interaction. Lastly, we demonstrated that the CBP and p300 KIX domains contribute to both c-Myb-dependent gene activation and repression. Together these results suggest that the KIX domains of CBP, and especially p300, are principal mediators of c-Myb-dependent gene activation and repression that is required for definitive hematopoiesis.

Diacylglycerol metabolism attenuates T-cell receptor signaling and alters thymocyte differentiation

Diacylglycerol (DAG) metabolism has a critical function in Ras-regulated functions in mature T cells, but causal data linking defects in DAG-based signals with altered thymus development are missing. To study the effect of increased DAG metabolism in T-cell development, we engineered a membrane-targeted constitutive active version of DAG kinase-α (DGKα). We show that transgenic expression of constitutive active DGK leads to developmental defects in T cells, with a marked accumulation of immature CD8 thymocytes and a reduction in positive selected populations. These alterations are reflected in the periphery by a CD4/CD8 cell imbalance and general T-cell lymphopenia. The results link DAG metabolism to T-cell homeostasis, and show that correctly controlled generation and consumption of this lipid at the plasma membrane ensure T-cell passage through quality-control checkpoints during differentiation.

Impaired lymphoid organ development in mice lacking the heparan sulfate modifying enzyme glucuronyl C5-epimerase

The development of lymphoid organs depends on cross talk between hematopoietic cells and mesenchymal stromal cells and on vascularization of the lymphoid primordia. These processes are orchestrated by cytokines, chemokines, and angiogenic factors that require tight spatiotemporal regulation. Heparan sulfate (HS) proteoglycans are molecules designed to specifically bind and regulate the bioactivity of soluble protein ligands. Their binding capacity and specificity are controlled by modification of the HS side chain by HS-modifying enzymes. Although HS proteoglycans have been implicated in the morphogenesis of several organ systems, their role in controlling lymphoid organ development has thus far remained unexplored. In this study, we report that modification of HS by the HS-modifying enzyme glucuronyl C5-epimerase (Glce), which controls HS chain flexibility, is required for proper lymphoid organ development. Glce(-/-) mice show a strongly reduced size of the fetal spleen as well as a spectrum of defects in thymus and lymph node development, ranging from dislocation to complete absence of the organ anlage. Once established, however, the Glce(-/-) primordia recruited lymphocytes and developed normal architectural features. Furthermore, Glce(-/-) lymph node anlagen transplanted into wild-type recipient mice allowed undisturbed lymphocyte maturation. Our results indicate that modification of HS by Glce is required for controlling the activity of molecules that are instructive for early lymphoid tissue morphogenesis but may be dispensable at later developmental stages and for lymphocyte maturation and differentiation.

Immunization of newborn and adult mice with low numbers of BCG leads to Th1 responses, Th1 imprints and enhanced protection upon BCG challenge

Neonatal bacille Calmette–Guerin (BCG) vaccination is widely employed to protect against tuberculosis. Predominant Th1 but not mixed Th1/Th2 responses are thought to be protective. If so, effective vaccination must cause Th1 imprints. The immune system of infants differs from that of adults and such differences could critically affect neonatal vaccination. We demonstrate that BCG infection of infant and adult mice produces similar responses. Infection with low and high numbers of BCG, respectively, leads to sustained Th1 and mixed Th1/Th2 responses. Low-dose but not high-dose infection also results in Th1 imprints, guaranteeing a Th1 response upon high-dose challenge, and resulting in optimal bacterial clearance. Our observations on low-dose Th1 imprinting are intriguing in the context of the well-known madras trial. In this trial, the highest dose of BCG, which had insignificant side effects, was administered to over 250,000 human subjects. This high-dose vaccination resulted in insignificant protection against tuberculosis.

Prenatal cadmium exposure dysregulates sonic hedgehog and Wnt/beta-catenin signaling in the thymus resulting in altered thymocyte development

Cadmium (Cd) is both an environmental pollutant and a component of cigarette smoke. Although evidence demonstrates that adult exposure to Cd causes changes in the immune system, there are limited reports in the literature of immunomodulatory effects of prenatal exposure to Cd. The sonic hedgehog (Shh) and Wnt/beta-catenin pathways are required for thymocyte maturation. Several studies have demonstrated that Cd exposure affects these pathways in different organ systems. This study was designed to investigate the effect of prenatal Cd exposure on thymocyte development, and to determine if these effects were linked to dysregulation of Shh and Wnt/beta-catenin pathways. Pregnant C57Bl/6 mice were exposed to an environmentally relevant dose (10 ppm) of Cd throughout pregnancy and effects on the thymus were assessed on the day of birth. Thymocyte phenotype was determined by flow cytometry. A Gli:luciferase reporter cell line was used to measure Shh signaling. Transcription of target genes and translation of key components of both signaling pathways were assessed using real-time RT-PCR and western blot, respectively. Prenatal Cd exposure increased the number of CD4(+) cells and a subpopulation of double-negative cells (DN; CD4(-)CD8(-)), DN4 (CD44(-)CD25(-)). Shh and Wnt/beta-catenin signaling were both decreased in the thymus. Target genes of Shh (Patched1 and Gli1) and Wnt/beta-catenin (c-fos, and c-myc) were affected differentially among thymocyte subpopulations. These findings suggest that prenatal exposure to Cd dysregulates two signaling pathways in the thymus, resulting in altered thymocyte development.

Lymphocyte differentiation in sea bass thymus: CD4 and CD8-alpha gene expression studies

Different developmental stages (from eggs to 1-year-old juveniles) of the teleost fish Dicentrarchus labrax (L.) were assayed for CD4 gene expression. RT-PCR revealed the appearance of CD4 transcripts in post-larvae from 51 days post-hatching (dph). This finding overlaps the first detection of CD8-alpha mRNA. Real-time PCR with specific primers quantified CD4, CD8-alpha and TCR-beta transcripts in larvae and post-larvae (25, 51, 75 and 92 dph) and 1-year-old thymus. At 92 dph, TcR-beta and CD8-alpha transcripts were significantly higher (P < 0.001) than in previous stages, as CD4 transcripts compared with 51 dph (P < 0.01). High levels of TCR-beta and CD8-alpha transcripts were found in the thymus, while CD4 transcripts were lower (P < 0.05 vs. TCR-beta). In situ hybridization identified CD4 mRNAs at 51 dph, localized in thymocytes of the outer and lateral zones of the thymic glands. From 75 dph on the signal was mainly detected in the outer region, drawing a cortex-medulla demarcation. Developmental expression of CD4 and CD8-alpha almost coincided. In each adult thymic lobe CD4(+) and CD8-alpha(+) thymocytes filled the cortex. The expression patterns of CD4 and CD8-alpha largely overlap, except in the medulla, where CD4(+) thymocytes were isolated, while CD8-alpha(+) ones mainly arranged in cords. These results provide new information about the thymic compartmentalization and lymphocyte differentiation pathways in a teleost, almost demonstrating that double negative thymocytes fill the cortex giving rise to further selection in the medulla.

Cell surface glycosylation diversity of embryonic thymic tissues

In the thymus, glycosylation status of many cell surface molecules changes during the thymocyte maturation and selection processes. In this study, we evaluated the glycosylation changes and possible relationships with programmed cell death in the thymic tissues from mouse embryos at the days 14 (E14), 15 (E15), 16 (E16), 17 (E17) and 18 (E18) of embryonic development. In order to determine glycosylation changes we used three different plant lectins: peanut agglutinin (PNA), Maackia amurensis leucoagglutinin (MAL or MAAI) and Sambucus nigra agglutinin (SNA), which recognize core disaccharide galactose (1-3) N-acetylgalactosamine [Galbeta(1-->3)GalNAc], sialic acid linked (2-->3) to galactose [SAalpha(2-->3)Gal] and sialic acid linked to galactose [SAalpha(2-->6)Gal] structures, respectively. Our lectin histochemistry and lectin blotting studies indicated that glycosylation pattern was modified in thymocytes at the embryonic developmental stages analyzed. The immature cortical thymocytes were labeled by PNA, whereas medullary thymocytes were positive for MAL and SNA binding. Many medullary thymocytes exhibited alpha(2-->6)-linked sialic acid on their surface and this increased throughout the gestational stages. In the lectin blotting studies, different protein bands of various molecular weights were identified in thymocytes. Two of them were putatively identified as CD43 and CD45 glycoproteins. In addition, TUNEL (deoxynucleotdyltransferase-mediated dUDP nick end labeling) indicated that only PNA-positive cortical thymocytes were deleted in all embryonic stages. These results indicate that the glycosylation pattern was modified in thymocytes at all embryonic developmental stages, and these modifications can affect the T cell deletion, probably via the galectin-1 molecule in the embryonic thymus.

Distinct roles of VEGFR-1 and VEGFR-2 in the aberrant hematopoiesis associated with elevated levels of VEGF

Vascular endothelial growth factor (VEGF), a major factor in tumor-host interactions, plays a critical role in the aberrant hematopoiesis observed in cancer-bearing hosts. To dissect the roles of VEGF receptor (VEGFR)-1 and VEGFR-2 in cancer-associated hematopoiesis in vivo, we selectively stimulated VEGFR-1 and VEGFR-2 by continuous infusion of receptor-specific ligands or selective blockade with VEGF receptor-specific antibodies in mice infused with recombinant VEGF at levels observed in tumor-bearing animals. We found that the effect of VEGF on the accumulation of Gr1(+)CD11b(+) cells is mediated by VEGFR-2, but that the 2 receptors have opposite effects on lymphocyte development. Pathophysiologic levels of VEGF strongly inhibit T-cell development via VEGFR-2, whereas VEGFR-1 signaling decreases this inhibition. VEGFR-1, and not VEGFR-2, signaling is responsible for the observed increase of splenic B cells. Both receptors are capable of inhibiting dendritic cell function. These data suggest that most of observed aberrant hematopoiesis caused by excess tumor-derived VEGF is mediated by VEGFR-2, and VEGFR-1 alone has very limited independent effects but clearly both positively and negatively modulates the effects of VEGFR-2. Our findings suggest that selective blockade of VEGFR-2 rather than of both receptors may optimally overcome the adverse hematologic consequences of elevated VEGF levels found in malignancy.

Neuropilin 1 and CD25 co-regulation during early murine thymic differentiation

Neuropilin 1 (NP1) is a receptor for both semaphorin and vascular endothelial growth factor expressed by subpopulations of neuronal and endothelial cells. In the immune system, NP1 is present on dendritic and regulatory T cells. Here, we show that NP1 is expressed in the murine thymus, starting on day 12.5 of gestation. In the adult, NP1 is mainly expressed by CD4(-)CD8(-) double negative cells, CD4+CD8+ double positive cells, and CD4+CD25+ regulatory T cells but barely detected in single CD4+ and CD8+ positive thymocytes. Within the CD4(-)CD8(-)CD3(-) (triple-negative, TN) immature cells, NP1 expression starts in TN3 (CD44(-)CD25+) and increases in TN4 (CD44(-)CD25(-)) cells. In order to study the role of NP1 in thymocyte differentiation, we generated mice in which the np1 gene is selectively disrupted in the T-cell lineage. The mutant mice display normal thymocyte, peripheral, conventional and CD4+CD25+Foxp3+ regulatory T-cell populations. However, we observe a down-regulation of the CD25 expression between the TN3 and TN4 stages that is (i) correlated to increased expression of NP1 in control mice and (ii) altered in mutant mice, suggesting that NP1 is co-regulated with CD25 during early immature thymocyte differentiation.

A Defect in Lineage Fate Decision during Fetal Thymic Invariant NKT Cell Development May Regulate Susceptibility to Type 1 Diabetes

A numerical and functional deficiency in invariant NKT (iNKT) cells detectable by 3 wk of age in the thymus and spleen mediates the pathogenesis of type 1 diabetes in NOD mice, but the stage of T cell development at which this deficiency first occurs is unknown. We report in this study that this deficiency develops after the CD4(+)CD8(+) double-positive stage of thymic T cell development and is due to a lineage-specific depletion of CD4(-)CD8(-) double-negative alphabeta T cells and iNKT cells from the thymus between embryonic day 18 and day 1 after birth. Thus, an inheritable defect in a lineage fate decision that elicits a deficiency in fetal thymic iNKT cell development may predispose to susceptibility to type 1 diabetes.