Distinct mechanisms contribute to generate and change the CD4:CD8 cell ratio during thymus development: a role for the Notch ligand, Jagged1

Preclinical assessment of thrombin-preconditioned human Wharton's jelly-derived mesenchymal stem cells for neonatal hypoxic-ischaemic brain injury

Hypoxic-ischaemic encephalopathy (HIE) is a type of brain injury affecting approximately 1 million newborn babies per year worldwide, the only treatment for which is therapeutic hypothermia. Thrombin-preconditioned mesenchymal stem cells (MSCs) exert neuroprotective effects by enriching cargo contents and boosting exosome biogenesis, thus showing promise as a new therapeutic strategy for HIE. This study was conducted to evaluate the tissue distribution and potential toxicity of thrombin-preconditioned human Wharton's jelly-derived mesenchymal stem cells (th-hWJMSCs) in animal models before the initiation of clinical trials. We investigated the biodistribution, tumorigenicity and general toxicity of th-hWJMSCs. MSCs were administered the maximum feasible dose (1 × 105 cells/10 µL/head) once, or at lower doses into the cerebral ventricle. To support the clinical use of th-hWJMSCs for treating brain injury, preclinical safety studies were conducted in newborn Sprague-Dawley rats and BALB/c nude mice. In addition, growth parameters were evaluated to assess the impact of th-hWJMSCs on the growth of newborn babies. Our results suggest that th-hWJMSCs are non-toxic and non-tumorigenic in rodent models, survive for up to 7 days in the brain and hold potential for HIE therapy.

CD4:CD8 ratio as a frontier marker for clinical outcome, immune dysfunction and viral reservoir size in virologically suppressed HIV-positive patients

Introduction

Absolute CD4 T cell count and plasma viral load have been established as predictors of HIV disease progression, and CD4 T cell count is used as an indicator for initiation of antiretroviral therapy. Following long-term therapy, patients generally present with significant CD4 T cell recovery contrasting with persistently elevated CD8 T cell counts, which leads to a partial restoration of CD4:CD8 ratio. This review focuses on the relevance of the CD4:CD8 ratio on clinical outcomes, immune dysfunction and HIV reservoir size in long-term treated patients.

Method

We conducted a comprehensive literature review of publications in English language using major electronic databases. Our search was focused on factors contributing to CD4:CD8 T cell ratio and clinical outcome in adult HIV-positive patients in the context of treated infection.

Discussion

Low CD4:CD8 ratio has been linked to ageing and acts as a predictor of mortality in the general population. This ratio may represent the combined effects of inflammation and immunological changes called “inflammaging.” Although the mechanisms underlying partial correction of the CD4:CD8 ratio and persistently elevated CD8 T cell count in long-term treated patients remain poorly understood, it has been recently indicated that patients with optimal CD4 T cell recovery and low CD4:CD8 ratio still harbour increased immune activation, an immune senescent phenotype and have a higher risk of non-AIDS morbidity and mortality. This review reconsiders CD4:CD8 ratio in the light of advances in the understanding of immune dysfunction and examines its pathophysiological features and implications on clinical outcome and HIV reservoir size in long-term treated HIV-positive adults.

Conclusion

The CD4:CD8 ratio can contribute to the immunological evaluation of treated patients in a long-term follow-up and may be applied for monitoring both immune dysfunction and viral reservoir size in immune-based clinical trials.

Staphylococcal enterotoxin B administration during pregnancy imprints the increased CD4:CD8 T-cell ratio in the peripheral blood from neonatal to adult offspring rats

Our previous study demonstrated that Staphylococcal enterotoxin B (SEB) administration during pregnancy could alter the percentage of T cells subpopulation in the thymus of the neonatal rats; however, little is known about the effect of maternal SEB administration during pregnancy on T cells subpopulation in the peripheral blood of the offspring rats. In the present study, pregnant rats at gestational day 16 were intravenously injected with 15 µg SEB. The present study found that prenatal exposure to SEB significantly decreased the percentages of CD8 T cells in the peripheral blood of both neonatal rats on the fifth day after delivery and the adult offspring rats. Furthermore, it significantly increased the percentage of CD4 T cells as well as the ratios of CD4 to CD8 T cells in both neonatal and adult offspring rats. Prenatal exposure to SEB significantly decreased the expression levels of IL-4 and IFN-γ in the plasma of neonatal and adult offspring rats. Furthermore, SEB restimulation significantly increased the percentage of CD8 T cells and significantly decreased the percentage of CD4 T cells. These data suggest the prenatal exposure to SEB can imprint the increased CD4:CD8 T cell ratio in the peripheral blood from the neonate to adulthood through the decreased CD8 T cells and the increased CD4 T cells, and altered the response characteristics of CD4 and CD8 T cells to secondary SEB administration in the peripheral blood of the adult offspring rats.

T Cells Development Is Different between Thymus from Normal and Intrauterine Growth Restricted Pig Fetus at Different Gestational Stage

This experiment was conducted to evaluate the development of T cells in intrauterine growth retarded (IUGR) piglets at different gestational stages, and tentatively explore the relationship between T cells development and the Notch signaling pathway. A total of 18 crossbred (Landrace×Large white) primiparous sows were mated at similar weights and estruses and euthanized at d 60, 90 and 110 of gestation with six replicates for each time point. One IUGR and one normal fetus were picked from each litter. The T-cell subsets, mRNA expression of Delta-like1, Delta-like4, Jagged1, and Notch2 genes in the thymus were investigated. Compared to normal piglets, CD3⁺CD4⁻CD8⁺ cells in IUGR fetuses at d 90 was 0.13% lower (p<0.05). At d 110 of gestation CD8⁺ T cells in IUGR fetuses was 0.19% lower (p<0.05). The percentage of CD8⁺ T cells was 3.14% lower (p<0.05) of the total T cells in IUGR pigs at d 60. The abundance of Notch2 and Delta-like4 mRNA at d 110 was 20.93% higher and 0.77% (p<0.05) lower, and Delta-like1 mRNA at d 90 was 0.19% (p<0.05) higher compared to normal pigs. These results suggested that normal fetuses had a greater proportion of T-cell subsets at earlier gestation periods, and the Notch signaling pathway was likely partially responsible for these differences to some degree.

Active Wnt/beta-catenin signaling is required for embryonic thymic epithelial development and functionality ex vivo

The Wnt/beta-catenin signaling pathway plays an important role in the commitment and development of thymic epithelial precursors. Here we document similarities of thymic epithelial development during embryogenesis in human and mouse. We stained for thymic epithelial surface markers (EpCAM1, Ly51, K8) and ligand/receptor pair (Wnt4, Fz4). Our results confirm the relevance of using murine test systems to model human embryonic thymic epithelial cell development. We have efficiently transduced murine embryonic epithelial cells using mock (GFP) and Wnt/beta-catenin-inhibiting (ICAT-encoding) recombinant adenoviral vectors. The effect of Wnt4 was assayed in the form of Wnt4-containing supernatant. Gene expressional changes were assessed by Q-PCR and also morphology using conventional and confocal fluorescent microscopy. Functional aberration caused by ICAT was assessed through evaluation of thymocyte maturation. Our results demonstrate that ICAT and Wnt4 have reciprocal effects during embryonic thymic epithelial cell development. While Wnt4 is capable of increasing the expression level of characteristic intracellular (FoxN1), surface (MHCII) and secreted (IL7) molecules, Wnt/beta-catenin inhibition through ICAT can moderately decrease their expression. Morphological changes induced by ICAT resulted in the development of hollow, inflated thymic lobes with reduced epithelial cell numbers. The ICAT-treated thymic lobes also showed significant impairment in supporting thymocyte development and maturation.

Reshaping of T-lymphocyte compartment in adult prepubertaly ovariectomised rats: a putative role for progesterone deficiency

This study explores the role of ovarian hormones in the phenotypic shaping of peripheral T-cell pool over the reproductive lifespan of rats. For this purpose, 2-month-old prepubertally ovariectomised (Ox) rats, showing oestrogen and progesterone deficiency, and 11-month-old Ox rats, exhibiting only progesterone deficiency, were examined for thymus output, and cellularity and composition of major TCRαβ+ peripheral blood lymphocyte (PBL) and splenocyte subsets. Although ovariectomy increased thymic output in both 2- and 11-month-old rats, the count of both CD4+ and CD8+ PBLs and splenocytes increased only in the former. In the blood and spleen of 11-month-old Ox rats only the count of CD8+ cells increased. Although ovariectomy affected the total CD4+ count in none of the examined compartments from the 11-month-old rats, it increased CD4+FoxP3+ PBL and splenocyte relative proportions over those in the age-matched controls. The age-related differences in the cellularity and the major subset composition in Ox rats were linked to the differences in the ovarian steroid hormone levels registered in 2- and 11-month-old rats. The administration of progesterone to Ox rats during the seven days before the sacrificing confirmed contribution of this hormone deficiency to the ovariectomy-induced changes in the TCRαβ+ PBL and splenocyte pool from 11-month-old rats. The expansion of the CD8+ splenocyte subset in the 11-month-old Ox rats reflected increases in cellularity of memory and, particularly, naïve cells. This was due to greater thymic output of CD8+ cells and homeostatic proliferation than apoptosis in 11-month-old Ox rats when compared with age-matched sham-Ox control rats. The homeostatic changes within CD8+ splenocyte pool from 11-month-old Ox rats, most likely, reflected the enhanced splenic IL-7 and TGF-β mRNA expression. Overall, in adult female rats, circulating oestrogen and progesterone provide maintenance of T-cell counts, a diversity of T-cell repertoire, and the main T-cell subset composition in the periphery. Progesterone deficiency affects mainly the CD8+ lymphocyte compartment through increasing thymic CD8+ cell export and upsetting homeostatic regulation within the CD8+ splenocyte pool. These alterations were reversible through progesterone supplementation.

Transcriptomic analysis supports similar functional roles for the two thymuses of the tammar wallaby

Background

The thymus plays a critical role in the development and maturation of T-cells. Humans have a single thoracic thymus and presence of a second thymus is considered an anomaly. However, many vertebrates have multiple thymuses. The tammar wallaby has two thymuses: a thoracic thymus (typically found in all mammals) and a dominant cervical thymus. Researchers have known about the presence of the two wallaby thymuses since the 1800s, but no genome-wide research has been carried out into possible functional differences between the two thymic tissues. Here, we used pyrosequencing to compare the transcriptomes of a cervical and thoracic thymus from a single 178 day old tammar wallaby.

Results

We show that both the tammar thoracic and the cervical thymuses displayed gene expression profiles consistent with roles in T-cell development. Both thymuses expressed genes that mediate distinct phases of T-cells differentiation, including the initial commitment of blood stem cells to the T-lineage, the generation of T-cell receptor diversity and development of thymic epithelial cells. Crucial immune genes, such as chemokines were also present. Comparable patterns of expression of non-coding RNAs were seen. 67 genes differentially expressed between the two thymuses were detected, and the possible significance of these results are discussed.

Conclusion

This is the first study comparing the transcriptomes of two thymuses from a single individual. Our finding supports that both thymuses are functionally equivalent and drive T-cell development. These results are an important first step in the understanding of the genetic processes that govern marsupial immunity, and also allow us to begin to trace the evolution of the mammalian immune system.

Notch ligand delta-like 4 regulates development and pathogenesis of allergic airway responses by modulating IL-2 production and Th2 immunity

Activation of the canonical Notch pathways has been implicated in Th cell differentiation, but the role of specific Notch ligands in Th2-mediated allergic airway responses has not been completely elucidated. In this study, we show that delta-like ligand 4 (Dll4) was upregulated on dendritic cells in response to cockroach allergen. Blocking Dll4 in vivo during either the primary or secondary response enhanced allergen-induced pathogenic consequences including airway hyperresponsiveness and mucus production via increased Th2 cytokines. In vitro assays demonstrated that Dll4 regulates IL-2 in T cells from established Th2 responses as well as during primary stimulation. Notably, Dll4 blockade during the primary, but not the secondary, response increased IL-2 levels in lung and lymph node of allergic mice. The in vivo neutralization of Dll4 was associated with increased expansion and decreased apoptosis during the primary allergen sensitization. Moreover, Dll4-mediated Notch activation of T cells during primary stimulation in vitro increased apoptosis during the contraction/resting phase of the response, which could be rescued by exogenous IL-2. Consistent with the role for Dll4-mediated IL-2 regulation in overall T cell function, the frequency of IL-4-producing cells was also significantly altered by Dll4 both in vivo and in vitro. These data demonstrate a regulatory role of Dll4 both in initial Th2 differentiation and in Th2 cytokine production in established allergic responses.

Progression of intracranial glioma disrupts thymic homeostasis and induces T-cell apoptosis in vivo

The thymus is the site where all T-cell precursors develop, mature, and subsequently leave as mature T-cells. Since the mechanisms that mediate and regulate thymic apoptosis are not fully understood, we utilized a syngenic GL261 murine glioma model to further elucidate the fate of T-cells in tumor bearing C57BL/6 mice. First, we found a dramatic reduction in the size of the thymus accompanied by a decrease in thymic cellularity in response to glioma growth in the brains of affected mice. There was a marked reduction of double positive subset and an increase in the frequency of CD4(+) and CD8(+) single positive T-cell subsets. Analysis of double negative thymocytes showed an increase in the accumulation of CD44(+) cells. In contrast, there was a marked loss of CD44 and CD122 expression in CD4(+) and CD8(+) subsets. The growth of intracranial tumors was also associated with decreased levels of HO-1, a mediator of anti-apoptotic function, and increased levels of Notch-1 and its ligand, Jagged-1. To determine whether thymic atrophy could be due to the effect of Notch and its ligand expression by glioma in vivo, we performed a bone marrow transplant experiment. Our results suggest that Notch-1 and its ligand Jagged-1 can induce apoptosis of thymocytes, thereby influencing thymic development, immune system homeostasis, and function of the immune cells in a model of experimental glioma.

Histochemical and molecular overview of the thymus as site for T-cells development

The thymus represents the primary site for T cell lymphopoiesis, providing a coordinated set for critical factors to induce and support lineage commitment, differentiation and survival of thymus-seeding cells. One irrefutable fact is that the presence of non-lymphoid cells through the thymic parenchyma serves to provide coordinated migration and differentiation of T lymphocytes. Moreover, the link between foetal development and normal anatomy has been stressed in this review. Regarding thymic embryology, its epithelium is derived from the embryonic endodermal layer, with possible contributions from the ectoderm. A series of differentiating steps is essential, each of which must be completed in order to provide the optimum environment for thymic development and function. The second part of this article is focused on thymic T-cell development and differentiation, which is a stepwise process, mediated by a variety of stromal cells in different regions of the organ. It depends strongly on the thymic microenvironment, a cellular network formed by epithelial cells, macrophages, dendritic cells and fibroblasts, that provide the combination of cellular interactions, cytokines and chemokines to induce thymocyte precursors for the generation of functional T cells. The mediators of this process are not well defined but it has been demonstrated that some interactions are under neuroendocrine control. Moreover, some studies pointed out that reciprocal signals from developing T cells also are essential for establishment and maintenance of the thymic microenvironment. Finally, we have also highlighted the heterogeneity of the lymphoid, non-lymphoid components and the multi-phasic steps of thymic differentiation. In conclusion, this review contributes to an understanding of the complex mechanisms in which the foetal and postnatal thymus is involved. This could be a prerequisite for developing new therapies specifically aimed to overcome immunological defects, linked or not-linked to aging.

Transition from enhanced T cell infiltration to inflammation in the myelin-degenerative central nervous system

Myelin degeneration in the central nervous system (CNS) is often associated with elevated numbers of T cells in brain and spinal cord (SC). In some degenerative diseases, this T cell immigration has no clinical relevance, in others, it may precede severe inflammation and tissue damage. We studied T cells in the myelin-degenerative SC of transgenic (tg) Lewis rats overexpressing the proteolipid protein (PLP). These lymphocytes are T(H)1/T(C)1 cells and represent different T cell clones unique to individual animals. The SC-infiltrating CD8(+) T cell pool is more restricted than its CD4(+) counterpart, possibly due to constrictions in the peripheral CD8(+) T cell repertoire. Some SC-infiltrating T cells are highly motile and cover large distances within their target tissue, others are tethered to MHC class II(+) microglia cells. The activation of the tethered cells may trigger the formation of inflammatory foci and could pave the way for inflammation in degenerative CNS disease.

Zoned out: functional mapping of stromal signaling microenvironments in the thymus

All hematopoietic cells, including T lymphocytes, originate from stem cells that reside in the bone marrow. Most hematopoietic lineages also mature in the bone marrow, but in this respect, T lymphocytes differ. Under normal circumstances, most T lymphocytes are produced in the thymus from marrow-derived progenitors that circulate in the blood. Cells that home to the thymus from the marrow possess the potential to generate multiple T and non-T lineages. However, there is little evidence to suggest that, once inside the thymus, they give rise to anything other than T cells. Thus, signals unique to the thymic microenvironment compel multipotent progenitors to commit to the T lineage, at the expense of other potential lineages. Summarizing what is known about the signals the thymus delivers to uncommitted progenitors, or to immature T-committed progenitors, to produce functional T cells is the focus of this review.

Murine pregnancy leads to reduced proliferation of maternal thymocytes and decreased thymic emigration

During mammalian pregnancy the maternal thymus undergoes significant involution, and then recovers in size after birth. The mechanism behind this involution is not known, but it has been suggested that elevated levels of hormones during pregnancy induce the involution. We have recently shown that injection of 17beta-oestradiol into mice causes loss of early thymocyte precursors and inhibits proliferation of developing thymocytes. This suggests that elevated oestrogen in pregnancy may contribute to thymic involution. We have investigated this idea by examining the fate of thymocytes during mouse pregnancy in much greater detail than has been previously reported. Looking over a broad time-course, we find that pregnancy does not affect thymocyte precursor populations in the bone marrow, but induces a profound loss of early thymic progenitors in the thymus as early as day 12 x 5 of pregnancy. This loss is accompanied by decreased thymocyte proliferation, which returns to normal 2-4 days postpartum. No enhancement of apoptosis is detectable at any stage of pregnancy. We also find that there is a reduction in recent thymic emigrants after oestrogen treatment and at day 17 x 5 of pregnancy, suggesting that thymic involution during pregnancy influences the peripheral T-cell repertoire. The similarities between oestrogen-mediated involution and pregnancy-mediated involution suggest that oestrogen is a significant contributor to loss of thymocyte cellularity during pregnancy, and probably functions primarily by reducing thymocyte proliferation.

Lymphoid organ development in rabbits: major lymphocyte subsets

Although rabbits represent an important animal model, little is known about the lymphoid organ development in this species. In the present study, lymphocyte subsets in peripheral blood, spleen, mesenteric and popliteal lymph nodes in newborn and 2-, 4-, 6- and 8-week old and adult were characterized. Lymphocyte subsets were detected using flow cytometry and monoclonal antibodies against rabbit CD4, CD8, T-cell-specific antigen and cross-reactive antibody against B-cell antigen CD79alpha. In neonates, lower numbers of T cells were detected in both peripheral blood and spleen than in mesenteric lymph nodes. In comparison with other compartments, CD79alpha(+) cells prevailed in the spleen. Post-natal development was characterized by a decreased CD4(+)/CD8(+) ratio due to increasing frequency of CD8(+) lymphocytes in all organs but mesenteric lymph nodes, where it was due to decreased numbers of CD4(+) lymphocytes. Another significant feature was the increase of B cells in peripheral blood and mesenteric lymph nodes.

Establishment and functioning of intrathymic microenvironments

The thymus supports the production of self-tolerant T cells from immature precursors. Studying the mechanisms regulating the establishment and maintenance of stromal microenvironments within the thymus therefore is essential to our understanding of T-cell production and ultimately immune system functioning. Despite our ability to phenotypically define stromal cell compartments of the thymus, the mechanisms regulating their development and the ways by which they influence T-cell precursors are still unclear. Here, we review recent findings and highlight unresolved issues relating to the development and functioning of thymic stromal cells.

Expression of JAGGED1 in T-lymphocytes results in thymic involution by inducing apoptosis of thymic stromal epithelial cells

Proper development of the thymus and differentiation of T-lymphocytes requires cell-cell interactions between the developing T-lymphocytes and the thymic epithelia. The Delta/Serrate/Lag-2 (DSL)/Notch signal-transduction pathway is known to govern cell fate decisions required for proper development through direct cell-cell interactions. The functional consequences of specific DSL/Notch interactions during the development of a complex organ, such as the thymus, have not been thoroughly elucidated, however. In order to examine the role of DSL proteins during thymus development and T-lymphocyte differentiation, we targeted expression of JAGGED1 in T-lymphocyte progenitors via the control of the proximal lck promoter. Here, we report that expression of JAGGED1 in T cells causes premature involution of the thymus by directing thymic epithelial cells to undergo an apoptotic program. Adoptive transfer of JAGGED1 transgenic bone marrow into non-transgenic mice revealed that JAGGED1 expression on T cells does not alter T-cell differentiation, but is directly responsible for involution of the thymus. We propose that the phenotype of the lck-JAGGED1 transgenic mice is a direct result of specific DSL/Notch interactions and improper cell-to-cell signaling.

Complementary contribution of CD4 and CD8 T lymphocytes to T-cell infiltration of the intact and the degenerative spinal cord

The central role of T cells in inflammatory reactions of the central nervous system (CNS) is well documented. However, there is little information about the few T cells found within the noninflamed CNS. In particular, the contribution of CD4+ and CD8+ T cells to the lymphocyte pool infiltrating the intact CNS, the location of these cells in CNS white and gray matter, and changes in the cellular composition of T-cell infiltrates coinciding with degeneration are primarily undefined. To address these points, we studied T cells in the intact and degenerative rat spinal cord. In the intact spinal cord, T cells were preferentially located within the gray matter. CD8+ T cells were more numerous than CD4+ lymphocytes. In cases of neuroaxonal degeneration or myelin degeneration/oligodendrocyte death, T cells were predominantly seen in areas of degeneration and were present in increased numbers. These effects were more pronounced for the CD4+ than for the CD8+ T-cell subset. Collectively, these data provide evidence for a clear cellular and compartmental bias in T-cell infiltration of the intact and degenerative spinal cord. This could indicate that CD4+ and CD8+ T cells might fulfill complementary roles in the intact and the diseased organ.

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.

Isolation of adult murine thymic stromal cells that naturally express Notch ligands

This communication summarizes the procedures that enabled isolation of adult murine thymic stromal cell lines that naturally express Notch ligands Jagged-1 or Delta-1. Histochemical characterization of these cell lines, in terms of ligand and cell type, revealed epithelial cells that bear an antigen characteristic of the thymic medulla and express either Jagged-1 or Delta-1. FACS sorting of stromal cells that naturally express these and other ligands is thus feasible, and such cells can be used to investigate the activity of each ligand in Notch-mediated commitment to the T-lymphocyte pathway.

Notch ligands Delta 1 and Jagged1 transmit distinct signals to T-cell precursors

Signaling through the Notch pathway plays an essential role in inducing T-lineage commitment and promoting the maturation of immature thymocytes. Using an in vitro culture system, we show that 2 different classes of Notch ligands, Jagged1 or Delta1, transmit distinct signals to T-cell progenitors. OP9 stromal cells expressing either Jagged1 or Delta1 inhibit the differentiation of DN1 thymocytes into the B-cell lineage, but only the Delta1-expressing stromal cells promote the proliferation and maturation of T-cell progenitors through the early double-negative (DN) stages of thymocyte development. Whereas the majority of bone marrow-derived stem cells do not respond to Jagged1 signals, T-cell progenitors respond to Jagged1 signals during a brief window of their development between the DN1 and DN3 stages of thymic development. During these stages, Jagged1 signals can influence the differentiation of immature thymocytes along the natural killer (NK) and gamma delta T-cell lineages.

Thymocyte-intrinsic genetic factors influence CD8 T cell lineage commitment and affect selection of a tumor-reactive TCR

Selection of immature CD4CD8 double-positive (DP) thymocytes for CD4 or CD8-lineage commitment is controlled by the interaction of the TCR with stromal cell-expressed peptide/MHC. We show that thymocyte-intrinsic genes influence the pattern of expression of a MHC class I-restricted transgenic (tg) TCR so that in DBA/2 mice, DP thymocytes with a characteristically high expression of tg TCR, infrequently transit to CD8 single-positive thymocytes. In contrast, in B10.D2 mice, the same tg TCR is expressed at lower levels on a subpopulation of DP thymocytes that more frequently transit to CD8 single-positive thymocytes. These characteristics were not influenced by thymic stromal components that control positive selection. Radiation chimeras reconstituted with a mixture of BM from tg TCR mice of the two genetic backgrounds revealed that the relative frequency of transit to the CD8 lineage remained thymocyte-intrinsic. Identifying the gene products whose polymorphism controls CD8 T cell development may shed new light on the mechanisms controlling T cell commitment/selection in mice other than the most studied "C57BL/6"-based strains.

Thymic generation and regeneration

The thymus is a complex epithelial organ in which thymocyte development is dependent upon the sequential contribution of morphologically and phenotypically distinct stromal cell compartments. It is these microenvironments that provide the unique combination of cellular interactions, cytokines, and chemokines to induce thymocyte precursors to undergo a differentiation program that leads to the generation of functional T cells. Despite the indispensable role of thymic epithelium in the generation of T cells, the mediators of this process and the differentiation pathway undertaken by the primordial thymic epithelial cells are not well defined. There is a lack of lineage-specific cell-surface-associated markers, which are needed to characterize putative thymic epithelial stem cell populations. This review explores the role of thymic stromal cells in T-cell development and thymic organogenesis, as well as the molecular signals that contribute to the growth and expansion of primordial thymic epithelial cells. It highlights recent advances in these areas, which have allowed for a lineage relationship amongst thymic epithelial cell subsets to be proposed. While many fundamental questions remain to be addressed, collectively these works have broadened our understanding of how the thymic epithelium becomes specialized in the ability to support thymocyte differentiation. They should also facilitate the development of novel, rationally based therapeutic strategies for the regeneration and manipulation of thymic function in the treatment of many clinical conditions in which defective T cells have an important etiological role.

Notch and lymphopoiesis: a view from the microenvironment

The differentiation of B- and T-cells in primary lymphoid organs depends on, or is strongly influenced by, signals provided by stromal cells, extracellular matrix components as well as by direct contacts between differentiating lymphocytes and distinct environmental cells. Notch receptors and their ligands mediate intercellular contacts and are crucially important for the development of T- and B-cell lineages. Here we start by reviewing current knowledge on the expression patterns of Notch receptors and their ligands in primary lymphoid organs and the effects induced by their functional interactions. Then we shall attempt to discuss how those interactions may regulate not only lymphopoiesis per se but also morphogenesis and the functional compartmentalization of lymphopoietic organs during development.

Genetic dissection of systemic lupus erythematosus pathogenesis: evidence for functional expression of Sle3/5 by non-T cells

On the non-autoimmune C57BL/6 (B6) background, the chromosome 7-derived lupus susceptibility loci Sle3 and Sle5 have been shown to mediate an elevated CD4:CD8 ratio with an increase in activated CD4(+) T cells, decreased susceptibility to apoptosis, and a break in humoral tolerance. Development of subcongenic strains has subsequently shown that the elevated CD4:CD8 ratio is due to Sle3 but that both loci contribute to the development of autoantibodies. To elucidate the functional expression patterns of these loci, adoptive transfer experiments were conducted. All possible combinations of bone marrow reconstitution, including syngenic, were conducted between the congenic B6 and B6.Sle3/5 strains. It was found that the Sle3/5 locus was functionally expressed by bone marrow-derived cells, but not by host cells, and that the elevated CD4:CD8 phenotype could be reconstituted in radiation chimeras. Using Ly5-marked congenic strains and B6 host mice, additional experiments surprisingly demonstrated that the elevated CD4:CD8 ratio was neither an intrinsic property of the T cells nor of single positive thymocytes. Allotype-marked chimeras indicated that autoantibody production by B cells was also an extrinsic property, as shown by the fact that B cells without the Sle3/5 interval contributed to autoantibody production. These experiments strongly suggest that a gene within the B6.Sle3/5 interval was expressed by a bone marrow-derived, nonlymphocyte population in the thymus and periphery and was affecting T cell selection and/or survival.

Inducible gene knockout of transcription factor recombination signal binding protein-J reveals its essential role in T versus B lineage decision

The transcription factor recombination signal binding protein-J (RBP-J) functions immediately downstream of the cell surface receptor Notch and mediates transcriptional activation by the intracellular domain of all four kinds of Notch receptors. To investigate the function of RBP-J, we introduced loxP sites on both sides of the RBP-J exons encoding its DNA binding domain. Mice bearing the loxP-flanked RBP-J alleles, RBP-J(f/f), were mated with Mx-Cre transgenic mice and deletional mutation of the RBP-J gene in adult mice was induced by injection of the IFN-alpha inducer poly(I)-poly(C). Here we show that inactivation of RBP-J in bone marrow resulted in a block of T cell development at the earliest stage and increase of B cell development in the thymus. Lymphoid progenitors deficient in RBP-J differentiate into B but not T cells when cultured in 2'-deoxyguanosine-treated fetal thymic lobes by hanging-drop fetal thymus organ culture. Competitive repopulation assay also revealed cell autonomous deficiency of T cell development from bone marrow of RBP-J knockout mouse. Myeloid and B lineage differentiation appears normal in the bone marrow of RBP-J-inactivated mice. These results suggest that RBP-J, probably by mediating Notch signaling, controls T versus B cell fate decision in lymphoid progenitors.

Rat peripheral CD4+CD8+ T lymphocytes are partially immunocompetent thymus-derived cells that undergo post-thymic maturation to become functionally mature CD4+ T lymphocytes

CD4+CD8+ double-positive (DP) T cells represent a minor subpopulation of T lymphocytes found in the periphery of adult rats. In this study, we show that peripheral DP T cells appear among the first T cells that colonize the peripheral lymphoid organs during fetal life, and represent approximately 40% of peripheral T cells during the perinatal period. Later their proportion decreases to reach the low values seen in adulthood. Most DP T cells are small size lymphocytes that do not exhibit an activated phenotype, and their proliferative rate is similar to that of the other peripheral T cell subpopulations. Only 30-40% of DP T cells expresses CD8beta chain, the remaining cells expressing CD8alphaalpha homodimers. However, both DP T cell subsets have an intrathymic origin since they appear in the recent thymic emigrant population after injection of FITC intrathymically. Functionally, although DP T cells are resistant to undergo apoptosis in response to glucocorticoids, they show poor proliferative responses upon CD3/TCR stimulation due to their inability to produce IL-2. A fraction of DP T cells are not actively synthesizing the CD8 coreceptor, and they gradually differentiate to the CD4 cell lineage in reaggregation cultures. Transfer of DP T lymphocytes into thymectomized SCID mice demonstrates that these cells undergo post-thymic maturation in the peripheral lymphoid organs and that their CD4 cell progeny is fully immunocompetent, as judged by its ability to survive and expand in peripheral lymphoid organs, to proliferate in response to CD3 ligation, and to produce IL-2 upon stimulation.

T-cell development and the CD4-CD8 lineage decision

Cell-fate decisions are controlled typically by conserved receptors that interact with co-evolved ligands. Therefore, the lineage-specific differentiation of immature CD4+ CD8+ T cells into CD4+ or CD8+ mature T cells is unusual in that it is regulated by clonally expressed, somatically generated T-cell receptors (TCRs) of unpredictable fine specificity. Yet, each mature T cell generally retains expression of the co-receptor molecule (CD4 or CD8) that has an MHC-binding property that matches that of its TCR. Two models were proposed initially to explain this remarkable outcome--'instruction' of lineage choice by initial signalling events or 'selection' after a stochastic fate decision that limits further development to cells with coordinated TCR and co-receptor specificities. Aspects of both models now appear to be correct; mistake-prone instruction of lineage choice precedes a subsequent selection step that filters out most incorrect decisions.

Notch ligand-bearing thymic epithelial cells initiate and sustain Notch signaling in thymocytes independently of T cell receptor signaling

Thymic epithelial cells are specialized to play essential roles at multiple stages of T cell development in the thymus, yet the molecular basis of this specialization is largely unknown. Recently, the Notch family of transmembrane proteins has been implicated in thymocyte development. Such proteins interact with cell surface proteins of the Delta-like and Jagged families. It is known that Notch ligands are expressed intrathymically, and that Notch signaling is regulated by Notch ligands expressed on either the same or third-party cells. However, functional analysis of Notch ligand expression, and elucidation of the mechanism of Notch ligand signaling in thymocyte development, are unclear. Here, we find that Notch ligand expression in the thymus is compartmentalized, with MHC class II(+) thymic epithelium, but not thymocytes nor dendritic cells, expressing Jagged-1, Jagged-2 and Delta-like-1. We also provide evidence that contact with Notch ligands on thymic epithelium is necessary to activate and sustain Notch signaling in thymocytes, and that this can occur independently of positive selection induction. Our data suggest that Notch ligand expression by thymic epithelium may partly explain the specialization of these cells in supporting thymocyte development, by regulating Notch activation via an inductive signaling mechanism independently of signaling leading to positive selection.

Differential effects of Notch ligands Delta-1 and Jagged-1 in human lymphoid differentiation

Notch signaling is known to differentially affect the development of lymphoid B and T cell lineages, but it remains unclear whether such effects are specifically dependent on distinct Notch ligands. Using a cell coculture assay we observed that the Notch ligand Delta-1 completely inhibits the differentiation of human hematopoietic progenitors into the B cell lineage while promoting the emergence of cells with a phenotype of T cell/natural killer (NK) precursors. In contrast, Jagged-1 did not disturb either B or T cell/NK development. Furthermore, cells cultured in the presence of either Delta-1 or Jagged-1 can acquire a phenotype of NK cells, and Delta-1, but not Jagged-1, permits the emergence of a de novo cell population coexpressing CD4 and CD8. Our results thus indicate that distinct Notch ligands can mediate differential effects of Notch signaling and provide a useful system to further address cell-fate decision processes in lymphopoiesis.

Lymphostromal interactions in thymic development and function

The generation of a peripheral T-cell pool is essential for normal immune system function. CD4+ and CD8+ T cells are produced most efficiently in the thymus, which provides a complexity of discrete cellular microenvironments. Specialized stromal cells, that make up such microenvironments, influence each stage in the maturation programme of immature T-cell precursors. Progress has recently been made in elucidating events that regulate the development of intrathymic microenvironments, as well as mechanisms of thymocyte differentiation. It is becoming increasingly clear that the generation and maintenance of thymic environments that are capable of supporting efficient T-cell development, requires complex interplay between lymphoid and stromal compartments of the thymus.