Precursors of CD3+CD4+CD8+ cells in the human thymus are defined by expression of CD34. Delineation of early events in human thymic development

Mechanism study of ubiquitination in T cell development and autoimmune disease

T cells play critical role in multiple immune processes including antigen response, tumor immunity, inflammation, self-tolerance maintenance and autoimmune diseases et. Fetal liver or bone marrow-derived thymus-seeding progenitors (TSPs) settle in thymus and undergo T cell-lineage commitment, proliferation, T cell receptor (TCR) rearrangement, and thymic selections driven by microenvironment composed of thymic epithelial cells (TEC), dendritic cells (DC), macrophage and B cells, thus generating T cells with diverse TCR repertoire immunocompetent but not self-reactive. Additionally, some self-reactive thymocytes give rise to Treg with the help of TEC and DC, serving for immune tolerance. The sequential proliferation, cell fate decision, and selection during T cell development and self-tolerance establishment are tightly regulated to ensure the proper immune response without autoimmune reaction. There are remarkable progresses in understanding of the regulatory mechanisms regarding ubiquitination in T cell development and the establishment of self-tolerance in the past few years, which holds great potential for further therapeutic interventions in immune-related diseases.

'Off-the-Shelf' Immunotherapy: Manufacture of CD8+ T Cells Derived from Hematopoietic Stem Cells

Cellular immunotherapy is revolutionizing cancer treatment. However, autologous transplants are complex, costly, and limited by the number and quality of T cells that can be isolated from and expanded for re-infusion into each patient. This paper demonstrates a stromal support cell-free in vitro method for the differentiation of T cells from umbilical cord blood hematopoietic stem cells (HSCs). For each single HSC cell input, approximately 5 × 104 T cells were created with an initial five days of HSC expansion and subsequent T cell differentiation over 49 days. When the induced in vitro differentiated T cells were activated by cytokines and anti-CD3/CD28 beads, CD8+ T cell receptor (TCR) γδ+ T cells were preferentially generated and elicited cytotoxic function against ovarian cancer cells in vitro. This process of inducing de novo functional T cells offers a possible strategy to increase T cell yields, simplify manufacturing, and reduce costs with application potential for conversion into chimeric antigen receptor (CAR)-T cells for cancer immunotherapy and for allogeneic transplantation to restore immune competence.

SCFAs promote intestinal double-negative T cells to regulate the inflammatory response mediated by NLRP3 inflammasome

Short-chain fatty acids (SCFAs) are a product of intestinal bacteria metabolism. Our previous study has found that intestinal bacteria in patients with Alzheimer's disease (AD) can promote the activation of NLRP3 inflammasome and mediate neuroinflammation. In this study, we mainly explored the regulation of intestinal microenvironmental immunity by intestinal bacterial metabolite SCFAs and the mechanism of NLRP3 activation. First, wild-type (WT) and APP/PS1 mice were intervened with SCFAs. As a result, the proportion of double-negative T cells (CD3+CD4-CD8-, DNTs) in the intestine was increased, SCFAs could promote the expression of intestinal NLRP3 and inflammatory factors (IL-18, IL-6 and TNF-α). Moreover, SCAFs could also promote the level of inflammatory factors in the cerebrospinal fluid (CSF) of mice and aggravate the cognitive impairment in AD mice. CD3+ T cells isolated from the spleen were pre-treated with SCFAs, followed by detection of the proportion of DNTs. Consequently, SCFAs could promote the formation of DNTs, activate OX40 signal and simultaneously up-regulate the protein expression of Bcl-2, Bcl-xl and Survivin. Knockdown of OX40 could inhibit SCFAs-induced differentiation of DNTs. The co-culture of DNTs and intestinal macrophages showed that DNTs could activate Fas/FasL-TNF-α signal and induce the activation of NLRP3 inflammasome. In AD mouse models, treatment with Fas and TNFR1 inhibitors could significantly inhibit SCFAs-induced NLRP3 activation and inflammatory factors, while attenuate the inflammatory response in the brain tissue of mice and improve the cognitive ability of mice, however, without significant effect on the level of DNTs. The present study showed that SCFAs can promote the formation of DNTs through OX40. DNTs could induce the activation of NLRP3 inflammasome and the release of inflammatory factors in macrophages through Fas/FasL-TNF-α signals, thereby increasing the level of inflammatory factors in the central nervous system. When Fas and TNFR1 were inhibited by suppressing the functions of DNTs and macrophages, the activation of NLRP3 was inhibited. DNTs are affected by SCFAs, which is a new mechanism of neuroinflammation in AD.

The thymus: General concepts on embryology, anatomy, histology and immunohistochemistry

A fundamental aspect that is commonly overlook when assessing thymic tumors is the normal histology and immunohistochemical features of the normal thymus. Given the fact that most epithelial tumors occur in the adult population, it is only rarely that we are confronted with assessing normal immunohistochemistry of the thymus. However, we consider that such knowledge is of utmost importance is assessing pathological conditions including epithelial tumors or tumors of other lineages. Therefore, in this writing we have concentrated our efforts in providing an overview of the embryology and anatomy of the thymus as well as putting the normal histology and immunohistochemistry in perspective when assessing pathological conditions.

Leaving no one behind: tracing every human thymocyte by single-cell RNA-sequencing

The thymus is the primary organ for T-cell development, providing an essential microenvironment consisting of the appropriate cytokine milieu and specialized stromal cells. Thymus-seeding progenitors from circulation immigrate into the thymus and undergo the stepwise T-cell specification, commitment, and selection processes. The transcriptional factors, epigenetic regulators, and signaling pathways involved in the T-cell development have been intensively studied using mouse models. Despite our growing knowledge of T-cell development, major questions remain unanswered regarding the ontogeny and early events of T-cell development at the fetal stage, especially in humans. The recently developed single-cell RNA-sequencing technique provides an ideal tool to investigate the heterogeneity of T-cell precursors and the molecular mechanisms underlying the divergent fates of certain T-cell precursors at the single-cell level. In this review, we aim to summarize the current progress of the study on human thymus organogenesis and thymocyte and thymic epithelial cell development, which is to shed new lights on developing novel strategies for in vitro T-cell regeneration and thymus rejuvenation.

Single-Cell RNA Sequencing Resolves Spatiotemporal Development of Pre-thymic Lymphoid Progenitors and Thymus Organogenesis in Human Embryos

Generation of the first T lymphocytes in the human embryo involves the emergence, migration, and thymus seeding of lymphoid progenitors together with concomitant thymus organogenesis, which is the initial step to establish the entire adaptive immune system. However, the cellular and molecular programs regulating this process remain unclear. We constructed a single-cell transcriptional landscape of human early T lymphopoiesis by using cells from multiple hemogenic and hematopoietic sites spanning embryonic and fetal stages. Among heterogenous early thymic progenitors, one subtype shared common features with a subset of lymphoid progenitors in fetal liver that are known as thymus-seeding progenitors. Unbiased bioinformatics analysis identified a distinct type of pre-thymic lymphoid progenitors in the aorta-gonad-mesonephros (AGM) region. In parallel, we investigated thymic epithelial cell development and potential cell-cell interactions during thymus organogenesis. Together, our data provide insights into human early T lymphopoiesis that prospectively direct T lymphocyte regeneration, which might lead to development of clinical applications.

Fratricide-resistant CD1a-specific CAR T cells for the treatment of cortical T-cell acute lymphoblastic leukemia

Relapsed/refractory T-cell acute lymphoblastic leukemia (T-ALL) has a dismal outcome, and no effective targeted immunotherapies for T-ALL exist. The extension of chimeric antigen receptor (CAR) T cells (CARTs) to T-ALL remains challenging because the shared expression of target antigens between CARTs and T-ALL blasts leads to CART fratricide. CD1a is exclusively expressed in cortical T-ALL (coT-ALL), a major subset of T-ALL, and retained at relapse. This article reports that the expression of CD1a is mainly restricted to developing cortical thymocytes, and neither CD34⁺ progenitors nor T cells express CD1a during ontogeny, confining the risk of on-target/off-tumor toxicity. We thus developed and preclinically validated a CD1a-specific CAR with robust and specific cytotoxicity in vitro and antileukemic activity in vivo in xenograft models of coT-ALL, using both cell lines and coT-ALL patient-derived primary blasts. CD1a-CARTs are fratricide resistant, persist long term in vivo (retaining antileukemic activity in re-challenge experiments), and respond to viral antigens. Our data support the therapeutic and safe use of fratricide-resistant CD1a-CARTs for relapsed/refractory coT-ALL.

Construction of a functional thymic microenvironment from pluripotent stem cells for the induction of central tolerance

The thymus is required for generation of a self-tolerant, self-restricted T-cell repertoire. The capacity to manipulate or replace thymus function therapeutically would be beneficial in a variety of clinical settings, including for improving recovery following bone marrow transplantation, restoring immune system function in the elderly and promoting tolerance to transplanted organs or cells. An attractive strategy would be transplantation of thymus organoids generated from cells produced in vitro, for instance from pluripotent stem cells. Here, we review recent progress toward this goal, focusing on advances in directing differentiation of pluripotent stem cells to thymic epithelial cells, a key cell type of the thymic stroma, and related direct reprogramming strategies.

Regulation of the transcriptional program by DNA methylation during human αβ T-cell development

Thymocyte differentiation is a complex process involving well-defined sequential developmental stages that ultimately result in the generation of mature T-cells. In this study, we analyzed DNA methylation and gene expression profiles at successive human thymus developmental stages. Gain and loss of methylation occurred during thymocyte differentiation, but DNA demethylation was much more frequent than de novo methylation and more strongly correlated with gene expression. These changes took place in CpG-poor regions and were closely associated with T-cell differentiation and TCR function. Up to 88 genes that encode transcriptional regulators, some of whose functions in T-cell development are as yet unknown, were differentially methylated during differentiation. Interestingly, no reversion of accumulated DNA methylation changes was observed as differentiation progressed, except in a very small subset of key genes (RAG1, RAG2, CD8A, PTCRA, etc.), indicating that methylation changes are mostly unique and irreversible events. Our study explores the contribution of DNA methylation to T-cell lymphopoiesis and provides a fine-scale map of differentially methylated regions associated with gene expression changes. These can lay the molecular foundations for a better interpretation of the regulatory networks driving human thymopoiesis.

Tracking migration during human T cell development

Specialized microenvironments within the thymus are comprised of unique cell types with distinct roles in directing the development of a diverse, functional, and self-tolerant T cell repertoire. As they differentiate, thymocytes transit through a number of developmental intermediates that are associated with unique localization and migration patterns. For example, during one particular developmental transition, immature thymocytes more than double in speed as they become mature T cells that are among the fastest cells in the body. This transition is associated with dramatic changes in the expression of chemokine receptors and their antagonists, cell adhesion molecules, and cytoskeletal components to direct the maturing thymocyte population from the cortex to medulla. Here we discuss the dynamic changes in behavior that occur throughout thymocyte development, and provide an overview of the cell-intrinsic and extrinsic mechanisms that regulate human thymocyte migration.

Fetal immune response to chorioamnionitis

Chorioamnionitis is a frequent cause of preterm birth and is associated with an increased risk for injury responses in the lung, gastrointestinal tract, brain, and other fetal organs. Chorioamnionitis is a polymicrobial nontraditional infectious disease because the organisms causing chorioamnionitis are generally of low virulence and colonize the amniotic fluid often for extended periods, and the host (mother and the fetus) does not have typical infection-related symptoms such as fever. In this review, we discuss the effects of chorioamnionitis in experimental animal models that mimic the human disease. Our focus is on the immune changes in multiple fetal organs and the pathogenesis of chorioamnionitis-induced injury in different fetal compartments. As chorioamnionitis disproportionately affects preterm infants, we discuss the relevant developmental context for the immune system. We also provide a clinical context for the fetal responses.

A simple model system enabling human CD34(+) cells to undertake differentiation towards T cells

Background

Channelling the development of haematopoietic progenitor cells into T lymphocytes is dependent upon a series of extrinsic prompts whose temporal and spatial sequence is critical for a productive outcome. Simple models of human progenitor cells development depend in the main on the use of xenogeneic systems which may provide some limitations to development.

Methods and Findings

Here we provide evidence that a simple model system which utilises both human keratinocyte and fibroblast cell lines arrayed on a synthetic tantalum coated matrix provides a permissive environment for the development of human CD34⁺ haematopoietic cells into mature CD4⁺ or CD8⁺ T lymphocytes in the presence of Interleukin 7 (IL-7), Interleukin 15 (IL-15) and the Fms-like tyrosine kinase 3 ligand (Flt-3L). This system was used to compare the ability of CD34⁺ cells to produce mature thymocytes and showed that whilst these cells derived from cord blood were able to productively differentiate into thymocytes the system was not permissive for the development of CD34⁺ cells from adult peripheral blood.

Conclusions/Significance

Our study provides direct evidence for the capacity of human cord blood CD34⁺ cells to differentiate along the T lineage in a simple human model system. Productive commitment of the CD34⁺ cells to generate T cells was found to be dependent on a three-dimensional matrix which induced the up-regulation of the Notch delta-like ligand 4 (Dll-4) by epithelial cells.

Dynamics of thymus organogenesis and colonization in early human development

The thymus is the central site of T-cell development and thus is of fundamental importance to the immune system, but little information exists regarding molecular regulation of thymus development in humans. Here we demonstrate, via spatial and temporal expression analyses, that the genetic mechanisms known to regulate mouse thymus organogenesis are conserved in humans. In addition, we provide molecular evidence that the human thymic epithelium derives solely from the third pharyngeal pouch, as in the mouse, in contrast to previous suggestions. Finally, we define the timing of onset of hematopoietic cell colonization and epithelial cell differentiation in the human thymic primordium, showing, unexpectedly, that the first colonizing hematopoietic cells are CD45(+)CD34(int/-). Collectively, our data provide essential information for translation of principles established in the mouse to the human, and are of particular relevance to development of improved strategies for enhancing immune reconstitution in patients.

Characterization of CD34⁺ thymocytes in newborn dogs

Using two-color flow cytometry, we characterized CD34(+) cells in the newborn canine thymus. CD34(+) thymic cells comprised approximately 5% of cells recovered by thymus tissue teasing and both large and small thymocytes have been present in this population, the former being 7-12 times more frequent. All CD34(+) cells expressed the pan-leukocyte antigen CD45. The expression of CD44 profile on the large and small CD34(+) thymocytes differed: almost all large CD34(+) cells were CD44(+), while only 75% of small CD34(+) thymocytes co-expressed the CD44 antigen. We have previously described that CD172α is present on the surface of CD34(+) bone marrow cells in dogs. In the thymus, CD172α was expressed on 5-10% and less than 5% of large and small CD34(+) cells, respectively. Some CD34(+) thymocytes also co-expressed T-lineage-specific markers like CD3, CD4, CD8, TCR1 and TCR2. Their expression increased during the large-to-small thymocyte transition. Based on our findings we suggest that thymocyte progenitors enter their primary differentiation center as large CD34(+), CD44(+), CD45(+) and CD172α(+) cells. T-cell specific markers appear on their surface at early stages of differentiation. As the size of progenitors decreases with terminal primary differentiation, the CD34, CD44, and CD172α surface markers are down-regulated.

Notch1 and IL-7 receptor signalling in early T-cell development and leukaemia

Notch receptors are master regulators of many aspects of development and tissue renewal in metazoans. Notch1 activation is essential for T-cell specification of bone marrow-derived multipotent progenitors that seed the thymus, and for proliferation and further progression of early thymocytes along the T-cell lineage. Deregulated activation of Notch1 significantly contributes to the generation of T-cell acute lymphoblastic leukaemia (T-ALL). In addition to Notch1 signals, survival and proliferation signals provided by the IL-7 receptor (IL-7R) are also required during thymopoiesis. Our understanding of the molecular mechanisms controlling stage-specific survival and proliferation signals provided by Notch1 and IL-7R has recently been improved by the discovery that the IL-7R is a transcriptional target of Notch1. Thus, Notch1 controls T-cell development, in part by regulating the stage- and lineage-specific expression of IL-7R. The finding that induction of IL-7R expression downstream of Notch1 also occurs in T-ALL highlights the important contribution that deregulated IL-7R expression and function may have in this pathology. Confirming this notion, oncogenic IL7R gain-of-function mutations have recently been identified in childhood T-ALL. Here we discuss the fundamental role of Notch1 and IL-7R signalling pathways in physiological and pathological T-cell development in mice and men, highlighting their close molecular underpinnings.

HIV-1 infection inhibits cytokine production in human thymic macrophages

OBJECTIVE

The thymus serves as a critical site of T-lymphocyte ontogeny and selection. Thymic infection by HIV-1 is known to disrupt thymocyte maturation by both direct and indirect means; however, the mechanism behind these effects remains poorly defined. Macrophages represent one of the most important peripheral targets of HIV-1 infection, are resident in the thymic stroma, and play a central role in thymocyte maturation.

MATERIALS AND METHODS

Studies presented here define three primary features and outcomes of thymic macrophages (TM) and HIV-1 infection: (1) The distinctive TM phenotype (surface markers and cytokine production measured by immunofluorescence, fluorescence-activated cell sorting, and reverse transcriptase polymerase chain reaction) relative to macrophages from other sources (blood [monocyte-derived macrophages] and bone marrow); (2) infection of TM by different HIV-1 subtypes (X4, R5, and X4/R5) measured by enzyme-linked immunosorbent assay and polymerase chain reaction; and (3) consequences of HIV-1 infection on cytokine production by TM measured by reverse transcriptase polymerase chain reaction.

RESULTS

The results demonstrate that TM display a distinctive phenotype of HIV-1 receptors (CD4(lo), CXCR4(lo), CCR5(med), CCR3(hi)), chemokine production (macrophage inflammatory protein-1α(+); regulated on activation, normal T expressed and secreted(+); macrophage inflammatory protein-1b(-); stromal cell-derived factor -1(-)); and cytokine production (tumor necrosis factor-α(+), interleukin-8(+), macrophage colony-stimulating factor(+), interleukin-6(-)) relative to either monocyte-derived macrophages or bone marrow. TM were infected in vitro with R5 and X4/R5-tropic HIV-1 subtypes, and developed syncytia formation during long-term X4/R5 culture. In contrast, TM supported only transient replication of X4-tropic HIV-1. Lastly, infection of TM with HIV-1 abolished the production of all cytokines tested in long-term in vitro cultures.

CONCLUSIONS

Taken together, these results indicate that TM are a potential direct target of in situ HIV-1 infection, and that this infection may result in the disruption of macrophage functions that govern normal thymocyte maturation.

Differential IL-7 responses in developing human thymocytes

Interleukin (IL)-7 is a factor essential for mouse and human thymopoiesis. Mouse thymocytes have altered sensitivities to IL-7 at different developmental stages. CD4/CD8 double positive (DP) mouse thymocytes are shielded from the influence of IL-7 because of loss of CD127 (IL-7Ralpha). In this study, we assessed IL-7 receptor expression and IL-7 signaling in human thymocytes. We found human DP cells to be severely limited in their ability to phosphorylate STAT-5 in response to IL-7. The relative expression levels of the IL-7-inducible proteins Bcl-2 and Mcl-1 were also lower in human DP cells, consistent with a stage-specific decrease in IL-7 responsiveness. IL-7 responses were restored in a subset of cells that matured past the DP stage. Unlike the regulation of IL-7 signaling in mouse thymocytes, loss of IL-7 signaling in human DP cells was not due to absence of CD127, but instead correlated with downregulation of CD132 (common gamma chain).

Characterization in vitro and engraftment potential in vivo of human progenitor T cells generated from hematopoietic stem cells

T-cell development follows a defined set of stage-specific differentiation steps. However, molecular and cellular events occurring at early stages of human T-cell development remain to be fully elucidated. To address this, human umbilical cord blood (UCB) hematopoietic stem cells (HSCs) were induced to differentiate to the T lineage in OP9-DL1 cocultures. A developmental program involving a sequential and temporally discrete expression of key differentiation markers was revealed. Quantitative clonal analyses demonstrated that CD34(+)CD38(-) and CD34(+)CD38(lo) subsets of UCB contain a similarly high T-lineage progenitor frequency, whereas the frequency in CD34(+)CD38(+/hi) cells was 5-fold lower. Delta-like/Notch-induced signals increased the T-cell progenitor frequency of CD34(+)CD38(-/lo) cells differentiated on OP9-DL1, and 2 distinct progenitor subsets, CD34(+)CD45RA(+)CD7(++)CD5(-)CD1a(-) (proT1) and CD34(+)CD45RA(+)CD7(++)CD5(+)CD1a(-) (proT2), were identified and their thymus engrafting capacity was examined, with proT2 cells showing a 3-fold enhanced reconstituting capacity compared with the proT1 subset. Furthermore, in vitro-generated CD34(+)CD7(++) progenitors effectively engrafted the thymus of immunodeficient mice, which was enhanced by the addition of an IL-7/IL-7 antibody complex. Taken together, the identification of T-progenitor subsets readily generated in vitro may offer important avenues to improve cellular-based immune-reconstitution approaches.

Restoration of adenosine deaminase-deficient human thymocyte development in vitro by inhibition of deoxynucleoside kinases

Mutations in the gene encoding adenosine deaminase (ADA), a purine salvage enzyme, lead to immunodeficiency in humans. Although ADA deficiency has been analyzed in cell culture and murine models, information is lacking concerning its impact on the development of human thymocytes. We have used chimeric human/mouse fetal thymic organ culture to study ADA-deficient human thymocyte development in an "in vivo-like" environment where toxic metabolites accumulate in situ. Inhibition of ADA during human thymocyte development resulted in a severe reduction in cellular expansion as well as impaired differentiation, largely affecting mature thymocyte populations. Thymocyte differentiation was not blocked at a discrete stage; rather, the paucity of mature thymocytes was due to the induction of apoptosis as evidenced by activation of caspases and was accompanied by the accumulation of intracellular dATP. Inhibition of adenosine kinase and deoxycytidine kinase prevented the accumulation of dATP and restored thymocyte differentiation and proliferation. Our work reveals that multiple deoxynucleoside kinases are involved in the phosphorylation of deoxyadenosine when ADA is absent, and suggests an alternate therapeutic strategy for treatment of ADA-deficient patients.

IL-7 activates the phosphatidylinositol 3-kinase/AKT pathway in normal human thymocytes but not normal human B cell precursors

IL-7 signaling culminates in different biological outcomes in distinct lymphoid populations, but knowledge of the biochemical signaling pathways in normal lymphoid populations is incomplete. We analyzed CD127/IL-7Ralpha expression and function in normal (nontransformed) human thymocytes, and human CD19(+) B-lineage cells purified from xenogeneic cord blood stem cell/MS-5 murine stromal cell cultures, to further clarify the role of IL-7 in human B cell development. IL-7 stimulation of CD34(+) immature thymocytes led to phosphorylation (p-) of STAT5, ERK1/2, AKT, and glycogen synthase kinase-3 beta, and increased AKT enzymatic activity. In contrast, IL-7 stimulation of CD34(-) thymocytes (that included CD4(+)/CD8(+) double-positive, and CD4(+) and CD8(+) single-positive cells) only induced p-STAT5. IL-7 stimulation of CD19(+) cells led to robust induction of p-STAT5, but minimal induction of p-ERK1/2 and p-glycogen synthase kinase-3 beta. However, CD19(+) cells expressed endogenous p-ERK1/2, and when rested for several hours following removal from MS-5 underwent de-phosphorylation of ERK1/2. IL-7 stimulation of rested CD19(+) cells resulted in robust induction of p-ERK1/2, but no induction of AKT enzymatic activity. The use of a specific JAK3 antagonist demonstrated that all IL-7 signaling pathways in CD34(+) thymocytes and CD19(+) B-lineage cells were JAK3-dependent. We conclude that human CD34(+) thymocytes and CD19(+) B-lineage cells exhibit similarities in activation of STAT5 and ERK1/2, but differences in activation of the PI3K/AKT pathway. The different induction of PI3K/AKT may at least partially explain the different requirements for IL-7 during human T and B cell development.

Mouse models with human immunity and their application in biomedical research

Biomedical research in human beings is largely restricted to in vitro studies that lack complexity of a living organism. To overcome this limitation, humanized mouse models are developed based on immunodeficient characteristics of severe combined immunodeficiency (SCID) or recombination activating gene (Rag)(null) mice, which can accept xenografts. Peripheral constitution of human immunity in SCID or Rag(null) mice has been achieved by transplantation of mature human immune cells, foetal human thymus, bone marrow, liver tissues, lymph nodes or a combination of these, although efficiency needs to be improved. These mouse models with constituted human immunity (defined as humanized mice in the present text) have been widely used to investigate the basic principles of human immunobiology as well as complex pathomechanisms and potential therapies of human diseases. Here, elements of an ideal humanized mouse model are highlighted including genetic and non-genetic modification of recipient mice, transplantation strategies and proposals to improve engraftments. The applications of the humanized mice to study the development and response of human immune cells, human autoimmune diseases, virus infections, transplantation biology and tumour biology are reviewed as well.

A human postnatal lymphoid progenitor capable of circulating and seeding the thymus

Identification of a thymus-seeding progenitor originating from human bone marrow (BM) constitutes a key milestone in understanding the mechanisms of T cell development and provides new potential for correcting T cell deficiencies. We report the characterization of a novel lymphoid-restricted subset, which is part of the lineage-negative CD34(+)CD10(+) progenitor population and which is distinct from B cell-committed precursors (in view of the absence of CD24 expression). We demonstrate that these Lin(-)CD34(+)CD10(+)CD24(-) progenitors have a very low myeloid potential but can generate B, T, and natural killer lymphocytes and coexpress recombination activating gene 1, terminal deoxynucleotide transferase, PAX5, interleukin 7 receptor alpha, and CD3epsilon. These progenitors are present in the cord blood and in the BM but can also be found in the blood throughout life. Moreover, they belong to the most immature thymocyte population. Collectively, these findings unravel the existence of a postnatal lymphoid-polarized population that is capable of migrating from the BM to the thymus.

Human intrathymic lineage commitment is marked by differential CD7 expression: identification of CD7- lympho-myeloid thymic progenitors

The identity and lineage potential of the cells that initiate thymopoiesis remain controversial. The goal of these studies was to determine, at a clonal level, the immunophenotype and differentiation pathways of the earliest progenitors in human thymus. Although the majority of human CD34(+)lin(-) thymocytes express high levels of CD7, closer analysis reveals that a continuum of CD7 expression exists, and 1% to 2% of progenitors are CD7(-). CD34(+)lin(-) thymocytes were fractionated by CD7 expression and tested for lineage potential in B-lymphoid, T-lymphoid, and myeloid-erythroid conditions. Progressive restriction in lineage potential correlated with CD7 expression, that is, the CD7(hi) fraction produced T and NK cells but lacked B and myelo-erythroid potential, the CD7(int) (CD10(+)) fraction produced B, T, and NK cells, but lacked myelo-erythroid potential. The CD7(-) fraction produced all lymphoid and myelo-erythroid lineages and expressed HSC-associated genes. However, CD34(+)lin(-)CD7(-) thymocytes also expressed early T lymphoid genes Tdt, pTalpha, and IL-7Ralpha and lacked engraftment capacity, suggesting the signals that direct lymphoid commitment and corresponding loss of HSC function are rapidly initiated on arrival of HSC in the human thymus. Thus, differential levels of CD7 identify the progressive stages of lineage commitment in human thymus, initiated from a primitive CD7(-) lympho-myeloid thymic progenitor.

Characterization of a progenitor cell population in childhood T-cell acute lymphoblastic leukemia

A significant proportion of children with T-cell acute lymphoblastic leukemia (T-ALL) continue to fail therapy. Consequently, characterization of the cells that proliferate to maintain the disease should provide valuable information on the most relevant therapeutic targets. We have used in vitro suspension culture (SC) and nonobese diabetic-severe combined immune deficient (NOD/SCID) mouse assays to phenotypically characterize and purify T-ALL progenitor cells. Cells from 13 pediatric cases were maintained in vitro for at least 4 weeks and expanded in 8 cases. To characterize the progenitors, cells were sorted for expression of CD34 and CD4 or CD7 and the subfractions were evaluated in vitro and in vivo. The majority of cells capable of long-term proliferation in vitro were derived from the CD34+/CD4- and CD34+/CD7- subfractions. Moreover, the CD34+/CD4- or CD7- cells were the only subfractions capable of NOD/SCID engraftment. These T-ALL cells successfully repopulated secondary and tertiary recipients with equivalent levels of engraftment, demonstrating self-renewal ability. The immunophenotype and genotype of the original leukemia cells were preserved with serial passage in the NOD/SCID mice. These data demonstrate the long-term repopulating ability of the CD34+/CD4- and CD34+/CD7- subfractions in T-ALL and suggest that a cell with a more primitive phenotype was the target for leukemic transformation in these cases.

Chronic lymphoproliferative disorder with regulatory T-cell phenotype

We report a case of T-cell chronic lymphoproliferative disorder (CLPD) that shows neither features of T-cell prolymphocytic leukemia nor disease progression for more than 34 months. Flow cytometric analyses of the lymphocytes revealed high expression of CD4 and CD25. Up-regulation of Foxp3, a master regulatory gene for developmental differentiation of regulatory T cells (Treg), was confirmed at mRNA and protein levels. To our knowledge, this is the first case of extremely indolent CLPD with Treg phenotype.

Development of human lymphoid cells

The lymphocytes, T, B, and NK cells, and a proportion of dendritic cells (DCs) have a common developmental origin. Lymphocytes develop from hematopoietic stem cells via common lymphocyte and various lineage-restricted precursors. This review discusses the current knowledge of human lymphocyte development and the phenotypes and functions of the rare intermediate populations that together form the pathways of development into T, B, and NK cells and DCs. Clearly, development of hematopoietic cells is supported by cytokines. The studies of patients with genetic deficiencies in cytokine receptors that are discussed here have illuminated the importance of cytokines in lymphoid development. Lineage decisions are under control of transcription factors, and studies performed in the past decade have provided insight into transcriptional control of human lymphoid development, the results of which are summarized and discussed in this review.

Human alpha beta and gamma delta thymocyte development: TCR gene rearrangements, intracellular TCR beta expression, and gamma delta developmental potential--differences between men and mice

To evaluate the role of the TCR in the alphabeta/gammadelta lineage choice during human thymocyte development, molecular analyses of the TCRbeta locus in gammadelta cells and the TCRgamma and delta loci in alphabeta cells were undertaken. TCRbeta variable gene segments remained largely in germline configuration in gammadelta cells, indicating that commitment to the gammadelta lineage occurred before complete TCRbeta rearrangements in most cases. The few TCRbeta rearrangements detected were primarily out-of-frame, suggesting that productive TCRbeta rearrangements diverted cells away from the gammadelta lineage. In contrast, in alphabeta cells, the TCRgamma locus was almost completely rearranged with a random productivity profile; the TCRdelta locus contained primarily nonproductive rearrangements. Productive gamma rearrangements were, however, depleted compared with preselected cells. Productive TCRgamma and delta rearrangements rarely occurred in the same cell, suggesting that alphabeta cells developed from cells unable to produce a functional gammadelta TCR. Intracellular TCRbeta expression correlated with the up-regulation of CD4 and concomitant down-regulation of CD34, and plateaued at the early double positive stage. Surprisingly, however, some early double positive thymocytes retained gammadelta potential in culture. We present a model for human thymopoiesis which includes gammadelta development as a default pathway, an instructional role for the TCR in the alphabeta/gammadelta lineage choice, and a prolonged developmental window for beta selection and gammadelta lineage commitment. Aspects that differ from the mouse are the status of TCR gene rearrangements at the nonexpressed loci, the timing of beta selection, and maintenance of gammadelta potential through the early double positive stage of development.

Dynamics of Thymus-Colonizing Cells during Human Development

Here, we identify fetal bone marrow (BM)-derived CD34hiCD45RAhiCD7+ hematopoietic progenitors as thymus-colonizing cells. This population, virtually absent from the fetal liver (FL), emerges in the BM by development weeks 8-9, where it accumulates throughout the second trimester, to finally decline around birth. Based on phenotypic, molecular, and functional criteria, we demonstrate that CD34hiCD45RAhiCD7+ cells represent the direct precursors of the most immature CD34hiCD1a- fetal thymocytes that follow a similar dynamics pattern during fetal and early postnatal development. Histological analysis of fetal thymuses further reveals that early immigrants predominantly localize in the perivascular areas of the cortex, where they form a lymphostromal complex with thymic epithelial cells (TECs) driving their rapid specification toward the T lineage. Finally, using an ex vivo xenogeneic thymus-colonization assay, we show that BM-derived CD34hiCD45RAhiCD7+ progenitors are selectively recruited into the thymus parenchyma in the absence of exogenous cytokines, where they adopt a definitive T cell fate.

Human thymus contains multipotent progenitors with T/B lymphoid, myeloid, and erythroid lineage potential

It is a longstanding question which bone marrow-derived cell seeds the thymus and to what level this cell is committed to the T-cell lineage. We sought to elucidate this issue by examining gene expression, lineage potential, and self-renewal capacity of the 2 most immature subsets in the human thymus, namely CD34+ CD1a- and CD34+ CD1a+ thymocytes. DNA microarrays revealed the presence of several myeloid and erythroid transcripts in CD34+ CD1a- thymocytes but not in CD34+ CD1a+ thymocytes. Lineage potential of both subpopulations was assessed using in vitro colony assays, bone marrow stroma cultures, and in vivo transplantation into nonobese diabetic/severe combined immunodeficient (NOD/SCID) mice. The CD34+ CD1a- subset contained progenitors with lymphoid (both T and B), myeloid, and erythroid lineage potential. Remarkably, development of CD34+ CD1a- thymocytes toward the T-cell lineage, as shown by T-cell receptor delta gene rearrangements, could be reversed into a myeloid-cell fate. In contrast, the CD34+ CD1a+ cells yielded only T-cell progenitors, demonstrating their irreversible commitment to the T-cell lineage. Both CD34+ CD1a- and CD34+ CD1a+ thymocytes failed to repopulate NOD/SCID mice. We conclude that the human thymus is seeded by multipotent progenitors with a much broader lineage potential than previously assumed. These cells resemble hematopoietic stem cells but, by analogy with murine thymocytes, apparently lack sufficient self-renewal capacity.

Early virological events in various tissues of newborn monkeys after intrarectal infection with pathogenic simian human immunodeficiency virus

Children infected with human immunodeficiency virus type 1 often have higher viral loads and progress to acquired immunodeficiency syndrome more rapidly than adults. In our previous study of simian-human immunodeficiency virus (SHIV)-infected adult monkeys, immature CD4CD8 double-positive T cells in the thymus and jejunum decreased faster than mature CD4 single-positive T cells. Here, we examined the effect of virus replication on immature T cells from the same SHIV-inoculated newborn monkeys having more immature T cells than adults. The infectious viruses were more abundantly detected in the thymus than in other tissues at both 13 and 26 days post-infection (dpi). However, mature CD4(+) T cells in the thymus declined after 13 dpi and immature CD3(-) CD4 single-positive T cells remained at 26 dpi. These results suggested that many immature CD4(+) T cells in the thymus of newborns support the production of infectious viruses even after the depletion of mature CD4(+) T cells.

Induction of T-cell development from human cord blood hematopoietic stem cells by Delta-like 1 in vitro

The Notch signaling pathway plays a key role at several stages of T-lymphocyte differentiation. However, it remained unclear whether signals induced by the Notch ligand Delta-like 1 could support full T-cell differentiation from a defined source of human hematopoietic stem cells (HSCs) in vitro. Here, we show that human cord blood–derived HSCs cultured on Delta-like 1–expressing OP9 stromal cells undergo efficient T-cell lineage commitment and sustained T-cell differentiation. A normal stage-specific program of T-cell development was observed, including the generation of CD4 and CD8 αβ–T-cell receptor (TCR)–bearing cells. Induction of T-cell differentiation was dependent on the expression of Delta-like 1 by the OP9 cells. Stimulation of the in vitro–differentiated T cells by TCR engagement induced the expression of T-cell activation markers and costimulatory receptors. These results establish an efficient in vitro coculture system for the generation of T cells from human HSCs, providing a new avenue for the study of early T-cell differentiation and function.

Sonic hedgehog regulates early human thymocyte differentiation by counteracting the IL-7-induced development of CD34+ precursor cells

The Hedgehog (Hh) family of signaling molecules normally functions in the development of numerous tissues by regulating cellular differentiation and proliferation. Recent results have demonstrated that the different components of the Hh signaling pathway are expressed in the human thymus. In this study, we investigate the potential role of Sonic hedgehog (Shh) in human intrathymic T cell maturation. Results show that the expression of the two components of the Hh receptor, Patched and Smoothened, is mostly restricted to CD34+ precursor cells that are committing to the T cell lineage. Shh significantly increased the viability of CD34+ T cell precursors modulating bcl-2 and bax protein expression, and also inhibited their proliferation. The treatment of chimeric human-mouse fetal thymus organ cultures with Shh resulted in an arrested thymocyte differentiation and an accumulation of CD34+ progenitor cells. This effect was mainly attributed to the ability of Shh to counteract the IL-7-induced proliferation and differentiation of CD34+ cells. Shh down-regulated in the precursor cell population the expression of IL-7R as well as stromal-derived factor-1 chemokine receptor, CXCR4, and inhibited IL-7-dependent STAT5 phosphorylation. Therefore, Shh may function as a maintenance factor for intrathymic CD34+ precursor cells.

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

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

HIV-1 replication and pathogenesis in the human thymus

How HIV replicates and causes destruction of the thymus, and how to restore thymic function, are among the most important questions of HIV-1 pathogenesis and therapy in adult as well as pediatric patients. The thymus appears to function, albeit at reduced levels, throughout the life of adults, to respond to T cell depletion induced by HIV and to be suppressed by HIV. In this review, we summarize recent findings concerning HIV replication and pathogenesis in the human thymus, focusing on mechanistic insights gleaned from studies in the SCID-hu Thy/Liv mouse and human fetal-thymus organ culture (HF-TOC) models. First, we discuss HIV viral determinants and host factors involved in the replication of HIV in the thymus. Second, we consider evidence that both viral factors and host factors contribute to HIV-induced thymocyte depletion. We thus propose that multiple mechanisms, including depletion and suppression of progenitor cells, paracrine and direct lytic depletion of thymocytes, and altered thymocyte selection are involved in HIV-induced pathology in the thymus. With the SCID-hu Thy/Liv mouse and HF-TOC models, it will be important in the coming years to further clarify the virological, cell biological, and immunological mechanisms of HIV replication and pathogenesis in human thymus, and to correlate their significance in HIV disease progression.

Gene expression signatures in MLL-rearranged T-lineage and B-precursor acute leukemias: dominance of HOX dysregulation

Rearrangements of the MLL locus, located on human chromosome 11q23, are frequent in both infant and therapy-related leukemias. Gene expression analysis of MLL-rearranged B-precursor acute lymphoblastic leukemias (MLL B-ALLs) has identified these cases as a unique subtype of leukemia, characterized by the expression of genes associated with both lymphoid and myeloid hematopoietic lineages. Here we show that MLL fusions also generate a distinct genetic subtype of T-lineage ALL (MLL T-ALL), in which leukemic cells are characterized by an early arrest in thymocyte differentiation, with suggestive evidence of commitment to the gammadelta lineage. Interestingly, multiple genes linked to cell proliferation (eg, PCNA, MYC, CDK2, and POLA) were down-regulated in MLL-fusion samples, relative to those transformed by other T-ALL oncogenes (P <.000 001, Fisher exact test). Overall, MLL T-ALL cases consistently demonstrated increased levels of expression of a subset of major HOX genes--HOXA9, HOXA10, and HOXC6--and the MEIS1 HOX coregulator (P <.008, one-sided Wilcoxon test), a pattern of gene expression that was reiterated in MLL B-ALLs. However, expression of myeloid lineage genes, previously reported in MLL B-ALLs, was not identified in T-lineage cases with this abnormality, suggesting that myeloid gene dysregulation is dispensable in leukemic transformation mediated by MLL fusion proteins. Our findings implicate dysregulation of HOX gene family members as a dominant mechanism of leukemic transformation induced by chimeric MLL oncogenes.

Analysis of TCR, pT alpha, and RAG-1 in T-acute lymphoblastic leukemias improves understanding of early human T-lymphoid lineage commitment

T-acute lymphoblastic leukemias (T-ALLs) derive from human T-lymphoid precursors arrested at various early stages of development. Correlation of phenotype and T-cell receptor (TCR) status with RAG-1 and pT alpha transcription in 114 T-ALLs demonstrated that they largely reflect physiologic T-lymphoid development. Half the TCR alpha beta lineage T-ALLs expressed a pre-TCR, as evidenced by RAG-1, pT alpha, and cTCR beta expression, absence of TCR delta deletion, and a sCD3(-), CD1a(+), CD4/8 double-positive (DP) phenotype, in keeping with a population undergoing beta selection. Most TCR gamma delta T-ALLs were pT alpha, terminal deoxynucleotidyl transferase (TdT), and RAG-1(lo/neg), double-negative/single-positive (DN/SP), and demonstrated only TCR beta DJ rearrangement, whereas 40% were pT alpha, TdT, and RAG-1 positive, DP, and demonstrated TCR beta V(D)J rearrangement, with cTCR beta expression in proportion. As such they may correspond to TCR alpha beta lineage precursors selected by TCR gamma delta expression, to early gamma delta cells recently derived from a pT alpha(+) common alpha beta/gamma delta precursor, or to a lineage-deregulated alpha beta/gamma delta intermediate. Approximately 30% of T-ALLs were sCD3/cTCR beta(-) and corresponded to nonrestricted thymic precursors because they expressed non-T-restricted markers such as CD34, CD13, CD33, and CD56 and were predominantly DN, CD1a, pT alpha, and RAG-1 low/negative, despite immature TCR delta and TCR gamma rearrangements. TCR gene configuration identified progressive T-lymphoid restriction. T-ALLs, therefore, provide homogeneous expansions of minor human lymphoid precursor populations that can aid in the understanding of healthy human T-cell development.

Development of alphabeta T cells in the human thymus

The thymus is the main producer of alphabeta T cells and is, therefore, crucial for a normal immune system. The intrathymic developmental pathway of human alphabeta T cells has now been delineated. The production of new T cells by the thymus decreases with age, and the thymus was thought to be redundant in adults once the peripheral T-cell pool has been formed early in life. However, recent work has shown that the thymus can function even at an advanced age. Research into the production of T cells in clinical settings that are associated with loss of T cells in the periphery has sparked renewed interest in the function of the human thymus.

Regulation of surface expression of the human pre-T cell receptor complex

Considerable progress has recently been made in defining the role that pre-antigen receptor complexes, namely the pre-T and pre-B cell receptors, play in lymphocyte development. It is now established that these receptors direct, in a similar way, the survival, expansion, clonality and further differentiation of pre-T and pre-B lymphocytes, respectively. However, less is known about the mechanisms which ensure that only minute amounts of pre-TCR and pre-BCR reach the plasma membrane of developing lymphocytes. In this review, we discuss the implications of recent experimental approaches which address the developmental regulation of human pre-TCR expression and the molecular mechanisms that control surface pre-TCR expression levels.

Impact of cytokines on replication in the thymus of primary human immunodeficiency virus type 1 isolates from infants

Early infection of the thymus with the human immunodeficiency virus (HIV) may explain the more rapid disease progression among children infected in utero than in children infected intrapartum. Therefore, we analyzed infection of thymocytes in vitro by HIV type 1 primary isolates, obtained at or near birth, from 10 children with different disease outcomes. HIV isolates able to replicate in the thymus and impact thymopoiesis were present in all infants, regardless of the timing of viral transmission and the rate of disease progression. Isolates from newborns utilized CCR5, CXCR4, or both chemokine receptors to enter thymocytes. Viral expression was observed in discrete thymocyte subsets postinfection with HIV isolates using CXCR4 (X4) and isolates using CCR5 (R5), despite the wider distribution of CXCR4 in the thymus. In contrast to previous findings, the X4 primary isolates were not more cytopathic for thymocytes than were the R5 isolates. The cytokines interleukin-2 (IL-2), IL-4, and IL-7 increased HIV replication in the thymus by inducing differentiation and expansion of mature CD27(+) thymocytes expressing CXCR4 or CCR5. IL-2 and IL-4 together increased expression of CXCR4 and CCR5 in this population, whereas IL-4 and IL-7 increased CXCR4 but not CCR5 expression. IL-2 plus IL-4 increased the viral production of all pediatric isolates, but IL-4 and IL-7 had a significantly higher impact on the replication of X4 isolates compared to R5 isolates. Our studies suggest that coreceptor use by HIV primary isolates is important but is not the sole determinant of HIV pathogenesis in the thymus.

Gene expression signatures define novel oncogenic pathways in T cell acute lymphoblastic leukemia

Human T cell leukemias can arise from oncogenes activated by specific chromosomal translocations involving the T cell receptor genes. Here we show that five different T cell oncogenes (HOX11, TAL1, LYL1, LMO1, and LMO2) are often aberrantly expressed in the absence of chromosomal abnormalities. Using oligonucleotide microarrays, we identified several gene expression signatures that were indicative of leukemic arrest at specific stages of normal thymocyte development: LYL1+ signature (pro-T), HOX11+ (early cortical thymocyte), and TAL1+ (late cortical thymocyte). Hierarchical clustering analysis of gene expression signatures grouped samples according to their shared oncogenic pathways and identified HOX11L2 activation as a novel event in T cell leukemogenesis. These findings have clinical importance, since HOX11 activation is significantly associated with a favorable prognosis, while expression of TAL1, LYL1, or, surprisingly, HOX11L2 confers a much worse response to treatment. Our results illustrate the power of gene expression profiles to elucidate transformation pathways relevant to human leukemia.

Stromal cell-derived factor 1/CXCR4 signaling is critical for early human T-cell development

The present study investigated the potential role of stromal cell-derived factor 1 (SDF-1) in human intrathymic T-cell differentiation. Results show that SDF-1 is produced by human thymic epithelial cells from the subcapsular and medullary areas, and its receptor, CXCR4, is up-regulated on CD34(+) precursor cells committed to the T-cell lineage. Chimeric human-mouse fetal thymus organ culture (FTOC) seeded with purified CD34(+) thymic progenitors and treated with neutralizing antibodies against SDF-1 or CXCR4 showed a significant reduction of the number of human thymocytes and an arrested thymocyte differentiation in the transition between CD34(+) precursor cells and CD4(+) immature thymocytes. SDF-1-treated FTOC showed an increase of human thymocyte numbers, mainly affecting the most immature subpopulations. Moreover, these results suggest that CXCR4/SDF-1 signaling is not critical for the CD34(+) cell precursor recruitment to the thymus. On the other hand, SDF-1 significantly increased the viability of CD34(+) T-cell precursors modulating the expression of BCL-2 and BAX genes, and stimulated the proliferation of CD34(+) thymic precursor cells, particularly in synergy with interleukin 7 (IL-7), but not with other cytokines, such as stem cell factor or flt3-ligand. Accordingly, only IL-7 was able to up-regulate CXCR4 expression on CD34(+) thymic progenitors. In addition, deprivation of SDF-1 partially inhibited human thymocyte expansion induced by IL-7 in human-mouse FTOC. This study indicates that SDF-1/CXCR4 signaling is required for the survival, expansion, and subsequent differentiation of human early thymocytes and identifies a new mechanism by which IL-7 mediates its effects on human thymopoiesis.

Human thymic epithelial cells inhibit IL-15- and IL-2-driven differentiation of NK cells from the early human thymic progenitors

T/NK progenitors are present in the thymus; however, the thymus predominantly promotes T cell development. In this study, we demonstrated that human thymic epithelial cells (TEC) inhibit NK cell development. Most ex vivo human thymocytes express CD1a, indicating that thymic progenitors are predominantly committed to the T cell lineage. In contrast, the CD1a(-)CD3(-)CD56(+) NK population comprises only 0.2% (n = 7) of thymocytes. However, we observed increases in the percentage (20- to 25-fold) and absolute number (13- to 71-fold) of NK cells when thymocytes were cultured with mixtures of either IL-2, IL-7, and stem cell factor or IL-15, IL-7, and stem cell factor. TEC, when present in the cultures, inhibited the increases in the percentage (3- to 10-fold) and absolute number (3- to 25-fold) of NK cells. Furthermore, we show that TEC-derived soluble factors inhibit generation of NK-CFU and inhibit IL15- or IL2-driven NK cell differentiation from thymic CD34(+) triple-negative thymocytes. The inhibitory activity was found to be associated with a 8,000- to 30,000 Da fraction. Thus, our data demonstrate that TEC inhibit NK cell development from T/NK CD34(+) triple negative progenitors via soluble factor(s), suggesting that the human thymic microenvironment not only actively promotes T cell maturation but also controls the development of non-T lineage cells such as the NK lineage.

Effect of cytokines on HIV-induced depletion of thymocytes in vivo

BACKGROUND

Cytokines play an important role in the differentiation of thymocytes into mature T cells; consequently, certain cytokines could be useful for immune reconstitution after HIV infection without increasing viral load.

OBJECTIVE

To investigate whether cytokines affect immune depletion caused by HIV infection with a CXCR4-tropic strain in SCID-hu mice implanted with human fetal thymus and liver (thy/liv) tissue.

METHODS

The thy/liv implants were either mock infected or infected with HIV-1 NL4-3, a CXCR4-tropic molecular clone. Interleukin (IL)-2, IL-4, IL-7, interferon-gamma (IFN-gamma) or diluent was administered to the mice during the second and third week postinfection. Viral load and immunophenotype were determined in thymocytes.

RESULTS

Thymocyte subset distributions at 3 weeks postinfection were significantly influenced by treatment with certain cytokines. In particular, IL-2 caused the infected mice to retain a thymocyte profile that was more similar to that in mock-infected mice than that in diluent-treated infected mice, in that the percentages of immature CD4+CD8+ and CD5+CD1+ cells were slightly higher and much less variable than in diluent-treated infected mice. The effect of IFN-gamma treatment was similar to IL-2 but did not reach statistical significance. However, after IFN-gamma treatment, normal percentages of mature CD3+CD69+ cells were maintained whereas this population was relatively increased in diluent-treated infected mice. Although treatment with IL-4 and IL-7 delayed depletion of immature thymocytes, these cytokines increased viral load.

CONCLUSIONS

Cytokines such as IL-2 and IFN-gamma maintain immature thymocytes without increasing viral load and may be useful as adjuncts to improve immune reconstitution after HIV infection.

Efficient generation of human T cells from a tissue-engineered thymic organoid

Biocompatible inorganic matrices have been used to enhance bone repair by integrating with endogenous bone architecture. Hypothesizing that a three-dimensional framework might support reconstruction of other tissues as well, we assessed the capacity of a tantalum-coated carbon matrix to support reconstitution of functioning thymic tissue. We engineered a thymic organoid by seeding matrices with murine thymic stroma. Co-culture of human bone marrow-derived hematopoietic progenitor cells within this xenogeneic environment generated mature functional T cells within 14 days. The proportionate T-cell yield from this system was highly reproducible, generating over 70% CD3+ T cells from either AC133+ or CD34+ progenitor cells. Cultured T cells expressed a high level of T-cell receptor excision circles (TREC), demonstrating de novo T lymphopoiesis, and function of fully mature T cells. This system not only facilitates analysis of the T-lymphopoietic potential of progenitor cell populations; it also permits ex vivo genesis of T cells for possible applications in treatment of immunodeficiency.

Analysis of the human neonatal thymus: evidence for a transient thymic involution

The neonatal period is marked by the impairment of the major components of both innate and adaptive immunity. We report a severe depletion of cortical CD4+CD8+ double-positive thymocytes in the human neonatal thymus. This drastic reduction in immature double-positive cells, largely provoked by an increased rate of cell death, could be observed as early as 1 day after birth, delaying the recovery of the normal proportion of this thymocyte subset until the end of the first month of postnatal life. Serum cortisol levels were not increased in newborn donors, indicating that the neonatal thymic involution is a physiological rather than a stress-associated pathological event occurring in the perinatal period. Newborn thymuses also showed increased proportions of both primitive CD34+CD1- precursor cells and mature TCRalphabetahighCD69-CD1-CD45RO+/RAdull and CD45ROdull/RA+ cells, which presumably correspond to recirculating T lymphocytes into the thymus. A notable reinforcement of the subcapsular epithelial cell layer as well as an increase in the intralobular extracellular matrix network accompanied modifications in the thymocyte population. Additionally neonatal thymic dendritic cells were found to be more effective than dendritic cells isolated from children's thymuses at stimulating proliferative responses in allogeneic T cells. All these findings can account for several alterations affecting the peripheral pool of T lymphocytes in the perinatal period.