Early human T cell development: analysis of the human thymus at the time of initial entry of hematopoietic stem cells into the fetal thymic microenvironment

Identification of a human recent thymic emigrant phenotype

The ability to measure human thymic output would be an invaluable tool for the study of the development of the naive T cell repertoire, as well as naive T cell regeneration after intensive cytotoxic chemotherapy or effective antiretroviral therapy of progressive HIV infection. We and others have demonstrated previously that quantification of T cell receptor rearrangement excision circles (TREC) within peripheral T cell populations provides insight into the frequency of recent thymic emigrants (RTE) and, therefore, into thymic function. However, measurement of RTE by this approach is complicated by the fact that TREC levels also are determined by turnover within the naive T cell compartment. Here, we report a phenotypic approach to RTE measurement. We demonstrate that alphaE integrin (CD103) expression is up-regulated very late in thymic development on a subset of CD8(+)/CD4(-) thymocytes and also defines a distinct subset of naive CD8(+) T cells in the periphery. The latter subset is differentiated from circulating CD103(+) mucosa-associated memory T cells by its naive T cell phenotype (CD45RO(-), CD62L(bright), CD27(bright), CD11a(dim), CD95(dim)) and its high concentration of TREC. Indeed, sorted CD103(+) naive CD8(+) cells display higher levels of TREC than their CD103(-) naive counterparts, and these cells demonstrate an age-related decline in frequency that is enhanced significantly by thymectomy. The thymic dependence of this subset and the cells' relatively evanescent presence in the periphery suggest that these cells are a population of RTE and that quantification of their frequency in peripheral blood provides an estimate of the level of ongoing thymopoiesis.

Evidence of alloreactive T lymphocytes in fetal liver: implications for fetal hematopoietic stem cell transplantation

The use of hematopoietic stem cells for in utero transplantation to create permanent hematochimerism represents a new concept in fetal therapy, although this approach has provided heterogeneous results. In this paper we have undertaken molecular, phenotypic and functional studies aimed at identifying the presence of fully competent T lymphocytes in samples of fetal livers and cord blood. We found mature VDJ TCR beta chain transcripts in fetal liver cells taken from 7 to 16 weeks of gestation and a similar pattern was detected in cord blood cells sampled from 13.5 to 20.5 weeks of gestation. A Vbeta8 gene sequence comparable to that detected in adult PBMC was found in fetal liver samples at 9 or 17 weeks gestation. PreTalpha message was detected in all samples and its expression decreased in fetal blood samples with increasing gestational age while Calpha message appeared at 9.4 weeks and its expression increased during gestational age. T cell clones obtained from fetal liver cells showed a mature TCR alphabeta+, CD8+ phenotype and displayed strong alloreactivity against allo-MHC class I molecules. The presence of alloreactive T lymphocytes may explain the failure to engraft in fetuses older than 13 to 16 weeks and may provide insights into fetal liver transplantation. Bone Marrow Transplantation (2000) 25, 135-141.

Resistance of CD7-deficient mice to lipopolysaccharide-induced shock syndromes

CD7 is an immunoglobulin superfamily molecule involved in T and natural killer (NK) cell activation and cytokine production. CD7-deficient animals develop normally but have antigen-specific defects in interferon (IFN)-gamma production and CD8(+) CTL generation. To determine the in vivo role of CD7 in systems dependent on IFN-gamma, the response of CD7-deficient mice to lipopolysaccharide (LPS)-induced shock syndromes was studied. In the high-dose LPS-induced shock model, 67% of CD7-deficient mice survived LPS injection, whereas 19% of control C57BL/6 mice survived LPS challenge (P < 0.001). CD7-deficient or C57BL/6 control mice were next injected with low-dose LPS (1 microgram plus 8 mg D-galactosamine [D-gal] per mouse) and monitored for survival. All CD7-deficient mice were alive 72 h after injection of LPS compared with 20% of C57BL/6 control mice (P < 0.001). After injection of LPS and D-gal, CD7-deficient mice had decreased serum IFN-gamma and tumor necrosis factor (TNF)-alpha levels compared with control C57BL/6 mice (P < 0.001). Steady-state mRNA levels for IFN-gamma and TNF-alpha in liver tissue were also significantly decreased in CD7-deficient mice compared with controls (P < 0.05). In contrast, CD7-deficient animals had normal liver interleukin (IL)-12, IL-18, and interleukin 1 converting enzyme (ICE) mRNA levels, and CD7-deficient splenocytes had normal IFN-gamma responses when stimulated with IL-12 and IL-18 in vitro. NK1.1(+)/ CD3(+) T cells are known to be key effector cells in the pathogenesis of toxic shock. Phenotypic analysis of liver mononuclear cells revealed that CD7-deficient mice had fewer numbers of liver NK1.1(+)/CD3(+) T cells (1.5 +/- 0.3 x 10(5)) versus C57BL/6 control mice (3.7 +/- 0.8 x 10(5); P < 0.05), whereas numbers of liver NK1.1(+)/CD3(-) NK cells were not different from controls. Thus, targeted disruption of CD7 leads to a selective deficiency of liver NK1.1(+)/ CD3(+) T cells, and is associated with resistance to LPS shock. These data suggest that CD7 is a key molecule in the inflammatory response leading to LPS-induced shock.

The human thymus. A chimeric organ comprised of central and peripheral lymphoid components

The human thymus is a lymphoepithelial organ in which T cells develop during fetal life. After maturation and selection in the fetal thymic microenvironment, T cells emigrate to peripheral lymphoid tissues such as the spleen, gut, and lymph nodes, and establish the peripheral T cell repertoire. Although the thymus has enormous regenerative capacity during fetal development, the regenerative capacity of the human postnatal thymus decreases over time. With the advent of intensive chemotherapy regimens for a variety of cancer syndromes, and the discovery that infection with the Human Immunodeficiency Virus (HIV) leads to severe loss of CD4+ T cells, has come the need to understand the role of the human thymus in reconstitution of the immune system in adults. During a recent study of the thymus in HIV infection, we observed many CD8+ T cells in AIDS thymuses that had markers consistent with those of mature effector cytotoxic T cells usually found in peripheral immune tissues, and noted these CD8+ effector T cells were predominantly located in a thymic zone termed the thymic perivascular space. This article reviews our own work on the thymus in HIV-1 infection, and discusses the work of others that, taken together, suggest that the thymus contains peripheral immune cell components not only in the setting of HIV infection, but also in myasthenia gravis, as well as throughout normal life during the process of thymus involution. Thus, the human thymus can be thought of as a chimeric organ comprised of both central and peripheral lymphoid tissues. These observations have led us to postulate that the thymic epithelial atrophy and decrease in thymopoiesis that occurs in myasthenia gravis, HIV-1 infection, and thymic involution may in part derive from cytokines or other factors produced by peripheral immune cells within the thymic perivascular space.

HCA, an Immunoglobulin-Like Adhesion Molecule Present on the Earliest Human Hematopoietic Precursor Cells, Is Also Expressed by Stromal Cells in Blood-Forming Tissues

We have previously shown that the HCA/ALCAM (CD166) glycoprotein, a member of the immunoglobulin family that mediates both homophilic and heterophilic cell-cell adhesion, via the CD6 ligand, is expressed at the surface of all of the most primitive CD38−/lo, Thy-1+, rho123lo, CD34+hematopoietic cells in human fetal liver and fetal and adult bone marrow. In the present report we show that HCA is also expressed by subsets of stromal cells in the primary hematopoietic sites that sequentially develop in the human embryo and fetus, ie, the paraaortic mesoderm, liver, thymus, and bone marrow. Adult bone marrow stromal cells established in vitro, including those derived from Stro-1+ progenitors and cells from immortalized cell lines, express HCA. In contrast, no HCA expression could be detected in peripheral lymphoid tissues, fetal spleen, and lymph nodes. HCA membrane molecules purified from marrow stromal cells interact with intact marrow stromal cells, CD34+ CD38−hematopoietic precursors, and CD3+ CD6+peripheral blood lymphocytes. Finally, low but significant levels of CD6 are here for the first time detected at the surface of CD34+ rho123med/lo progenitors in the bone marrow and in mobilized blood from healthy individuals. Altogether, these results indicate that the HCA/ALCAM surface molecule is involved in homophilic or heterophilic (with CD6) adhesive interactions between early hematopoietic progenitors and associated stromal cells in primary blood-forming organs.

HCA, an Immunoglobulin-Like Adhesion Molecule Present on the Earliest Human Hematopoietic Precursor Cells, Is Also Expressed by Stromal Cells in Blood-Forming Tissues

We have previously shown that the HCA/ALCAM (CD166) glycoprotein, a member of the immunoglobulin family that mediates both homophilic and heterophilic cell-cell adhesion, via the CD6 ligand, is expressed at the surface of all of the most primitive CD38−/lo, Thy-1+, rho123lo, CD34+hematopoietic cells in human fetal liver and fetal and adult bone marrow. In the present report we show that HCA is also expressed by subsets of stromal cells in the primary hematopoietic sites that sequentially develop in the human embryo and fetus, ie, the paraaortic mesoderm, liver, thymus, and bone marrow. Adult bone marrow stromal cells established in vitro, including those derived from Stro-1+ progenitors and cells from immortalized cell lines, express HCA. In contrast, no HCA expression could be detected in peripheral lymphoid tissues, fetal spleen, and lymph nodes. HCA membrane molecules purified from marrow stromal cells interact with intact marrow stromal cells, CD34+ CD38−hematopoietic precursors, and CD3+ CD6+peripheral blood lymphocytes. Finally, low but significant levels of CD6 are here for the first time detected at the surface of CD34+ rho123med/lo progenitors in the bone marrow and in mobilized blood from healthy individuals. Altogether, these results indicate that the HCA/ALCAM surface molecule is involved in homophilic or heterophilic (with CD6) adhesive interactions between early hematopoietic progenitors and associated stromal cells in primary blood-forming organs.

In Vitro Intrathymic Differentiation Kinetics of Human Fetal Liver CD34+CD38− Progenitors Reveals a Phenotypically Defined Dendritic/T-NK Precursor Split

Human CD34+CD38− hematopoietic precursor cells from fetal liver are able to develop into T, NK, and dendritic cells in a hybrid human/mouse fetal thymic organ culture (FTOC). In this report, we pay particular attention to the early events in differentiation of these precursor cells. We show that the CD34+CD38− precursor cells, which are CD4−CD7−cyCD3−HLA-DR−/++ (cy, cytoplasmatic), differentiate into a CD4+ population that remained CD7−cyCD3−HLA-DR++ and a CD4− population that expressed CD7 and cyCD3. The CD4+CD7−cyCD3− cells differentiate into phenotypically and functionally mature dendritic cells, but do not differentiate into T or NK cells. The CD4−CD7+cyCD3+ population later differentiates into a CD4+CD7+cyCD3+HLA-DR− population, which has no potential to differentiate into dendritic cells but is able to differentiate into NK cells and γδ and αβ T lymphocytes. These findings support the notion that the T/NK split occurs downstream of the NK/dendritic split.

Pulmonary inflammation associated with repeated, prenatal exposure to an E1, E3-deleted adenoviral vector in sheep

Fetal gene therapy may prove useful in treating diseases that manifest in the perinatal or early postnatal period. Adenoviruses effectively transfer gene expression to a variety of tissues but also stimulate inflammatory and immune responses. The purpose of this study was to test the hypothesis that exposure of fetal sheep to a first generation adenovirus vector encoding bacterial beta-galactosidase, Av1nBg, before the development of the immune system, is safe, minimizes inflammatory and immune responses and induces tolerance. A total of 22 fetal sheep was studied; of these, two were born with respiratory distress, seven were electively killed and 13 died in utero. The incidence of mortality was higher than the < or = 10% we have experienced with other fetal sheep studies and was not likely related to complications arising from surgical or anesthetic procedures. Inflammatory and fibrotic responses were observed in the lungs and may represent untoward long-term consequences of in utero adenoviral gene therapy. Tolerance to Av1nBg was not established, and repeated exposure to Av1nBg before birth was associated with significant pathology and mortality.

The human thymus. A chimeric organ comprised of central and peripheral lymphoid components

The human thymus is a lymphoepithelial organ in which T cells develop during fetal life. After maturation and selection in the fetal thymic microenvironment, T cells emigrate to peripheral lymphoid tissues such as the spleen, gut, and lymph nodes, and establish the peripheral T cell repertoire. Although the thymus has enormous regenerative capacity during fetal development, the regenerative capacity of the human postnatal thymus decreases over time. With the advent of intensive chemotherapy regimens for a variety of cancer syndromes, and the discovery that infection with the Human Immunodeficiency Virus (HIV) leads to severe loss of CD4+ T cells, has come the need to understand the role of the human thymus in reconstitution of the immune system in adults. During a recent study of the thymus in HIV infection, we observed many CD8+ T cells in AIDS thymuses that had markers consistent with those of mature effector cytotoxic T cells usually found in peripheral immune tissues, and noted these CD8+ effector T cells were predominately located in a thymic zone termed the thymic perivascular space. This article reviews our own work on the thymus in HIV-1 infection, and discusses the work of others that, taken together, suggest that the thymus contains peripheral immune cell components not only in the setting of HIV infection, but also in myasthenia gravis, as well as throughout normal life during the process of thymus involution. Thus, the human thymus can be thought of as a chimeric organ comprised of both central and peripheral lymphoid tissues. These observations have led us to postulate that the thymic epithelial atrophy and decrease in thymopoiesis that occurs in myasthenia gravis, HIV-1 infection, and thymic involution may in part derive from cytokines or other factors produced by peripheral immune cells within the thymic perivascular space.

Developmental regulation of lymphocyte-specific protein 1 (LSP1) expression in thymus during human T-cell maturation

The lymphocyte specific protein 1 (LSP1) phosphoprotein is an F-actin binding molecule restricted to cells of hematopoietic origin in mice and humans. LSP1 is localized to the internal surface of the plasma membrane, the cytoplasm, and NP-40-insoluble actin filaments and is thought to mediate cytoskeleton-driven responses in activated leukocytes that involve receptor capping, cell-cell interactions and cell motility. Here, we generated two monoclonal antibodies (MAbs), 5E3 and 14G8, that are specific for human LSP1 to define the expression of LSP1 throughout human T-cell development. Both MAbs reacted with a 52-kDa protein in BW5147 cells transfected with human LSP1 cDNA in pcDNA3, but not in cells transfected with cDNA in an antisense orientation, indicating the specificity of 5E3 and 14G8 for human LSP1. In developing T cells, LSP1 was expressed on human fetal liver CD7+ NK and T-cell precursors, the CD7+, CD3-, CD4-, CD8- human stem cell line DU-528, and on CD4-, CD8- double-negative (DN) thymocytes. Immunohistochemistry and three-color flow cytometry analysis of fetal or postnatal thymocytes revealed that LSP1 was increasingly expressed during intrathymic human T-cell maturation. While immature CD4+CD8+ double-positive (DP) thymocytes expressed low to undetectable levels of LSP1, mature CD4+CD8- and CD4-CD8+ single-positive (SP) thymocytes expressed high levels of LSP1. Thus, LSP1 is developmentally regulated during T-cell maturation within the human thymus and may play a functional role in the motility of DN and SP thymocytes.

Early stages in the development of human T, natural killer and thymic dendritic cells

T-cell development is initiated when CD34+ pluripotent stem cells or their immediate progeny leave the bone marrow to migrate to the thymus. Upon arrival in the thymus the stem cell progeny is not yet committed to the T-cell lineage as it has the capability to develop into T, natural killer (NK) and dendritic cells (DC). Primitive hematopoietic progenitor cells in the human thymus express CD34 and lack CD1a. When these progenitor cells develop into T cells they traverse a number of checkpoints. One early checkpoint is the induction of T-cell commitment, which correlates with appearance of CD1a and involves the loss of capacity to develop into NK cells and DC and the initiation of T-cell receptor (TCR) gene rearrangements. Basic helix-loop-helix transcription factors play a role in induction of T-cell commitment. CD1a+CD34+ cells develop into CD4+CD8 alpha+ beta+ cells by upregulating first CD4, followed by CD8 alpha and then CD8 beta. Selection for productive TCR beta gene rearrangements (beta selection) likely occurs in the CD4+CD8 alpha+ beta- and CD4+CD8 alpha+ beta+ populations. Although the T and NK-cell lineages are closely related to each other, NK cells can develop independently of the thymus. The fetal thymus is most likely one site of NK-cell development.

Antigenic phenotype of early intra-embryonic lymphoid progenitors in the chicken

The stem cells for the definitive haematopoiesis are derived from intra-embryonic sources originally described in an avian model and later also in mammals. However, the molecular make-up of the early embryonic haematopoietic progenitors is not yet clearly defined. We have recently characterized the phenotype of prethymic intra-embryonic progenitors capable of thymic colonization. Here we studied the ontogeny of cell-surface antigens HEMCAM, alpha2beta1 integrin, thrombomucin, chL12 and c-kit and their co-expression on prethymic T-cell progenitors. The early intra-embryonic expression of avian B-cell antigen chB6 was also demonstrated on cells derived from the intra-embryonic areas. We suggest that in the chicken, embryonic B-cell progenitors segregate earlier than T-cell progenitors in the differentiation of multipotent haematopoietic stem cells to committed progenitors.

Immunologic Characterization of CD7-Deficient Mice

Human CD7 is an Ig superfamily molecule that is expressed on mature T and NK lymphocytes. Although in vitro studies have suggested a role for CD7 in lymphoid development and function, the exact function of CD7 in vivo has remained elusive. One patient has been reported with SCID syndrome attributed to CD7 deficiency. To study in vivo functions of CD7, we have generated CD7-deficient mice and assessed their lymphoid development and function. CD7-deficient mice were viable, had normal peripheral blood and spleen lymphocyte numbers, and had normal specific Ab responses with Ag-driven Ig isotype switching. Thymocyte numbers were normal in 4-wk-old, 6-mo-old, and 1-yr-old CD7-deficient mice, but in 3-mo-old CD7-deficient mice, total thymocyte numbers were significantly increased by 60% (p &lt; 0.02) compared with normal age-matched +/+ littermates. CD7-deficient splenocytes proliferated normally in response to various mitogens, including PHA, anti-CD3, Con A, and LPS. While NK cell numbers and cytolytic activity to YAC targets were normal, CD7-deficient mice had lower Ag-induced MHC class I-restricted CTL activity against OVA-transfected target cells than did +/+ control mice. Thus, CD7-deficient mice did not have a SCID syndrome, but rather had transient increases in thymocyte numbers at age 3 mo and altered splenocyte Ag-specific CTL effecter cell activity. These data suggest a role for CD7 in regulating intrathymic T cell development and in mediating CTL effecter function.

Histological and immunohistological study of the developing and involuting superficial cervical thymus in the koala (Phascolarctos cinereus)

The thymuses of 44 koalas, ranging from less than 30 d to more than 14 y of age, were examined histologically and immunohistologically. The thymuses from 17 of these koalas dying acutely through trauma were regarded as not being significantly affected by disease and formed the basis for study of the normal thymus. Most other koalas had chronic illness and, consequently, disease affected (involuted) thymuses. Histologically, thymuses showed obvious corticomedullary differentiation with small Hassall's corpuscles visible in koalas more than 8 mo of age. Most cortical and medullary lymphocytes stained for CD3 and CD5 (T lymphocyte markers) while some cells (predominantly medullary) stained for CD79b (B lymphocytes and plasma cells), IgG (plasma cells) or MHC class II (reticular epithelium, macrophages and possibly lymphocytes). Adults of up to 5-6 y of age which had died through trauma had little evidence of involution and had prominent Hassall's corpuscles and medullary epithelial thymocytes. Thymic eosinopoiesis was an inconsistent finding. In traumatised animals over this age, involution was obvious with fibrous replacement of lobules, loss of Hassall's corpuscles and the development of dilated ducts lined by nonciliated epithelium. However, loss of lymphocytes was gradual and pockets of lymphocytes, centrally located in lobules, were still present in the oldest koala examined. In these involuted thymuses, remaining lymphocytes stained for CD3 and lesser numbers of CD5 and CD79b. Plasma cells were common and often stained both for IgG and MHC class II. Thymuses of chronically diseased koalas showed accelerated involution when age matched with thymuses from traumatised koalas. Chronically ill koalas as young as 18-24 mo showed advanced involution, but the morphological and immunohistological characteristics of involuted thymus from diseased koalas could not be distinguished from those of involuted thymuses derived from traumatised koalas. It was concluded that development of the koala thymus is completed at 8 mo of age and that for normal koalas involution is a gradual process which starts not at but after sexual maturity. Immunohistological characterisation of the thymus was comparable to that reported for a variety of eutherian mammals.

Abnormal in vitro thymocyte differentiation in a patient with severe combined immunodeficiency-Nezelof's syndrome

An in vitro coculture model system of CD34+ stem cells and allogenic cultured thymic epithelia fragments was used to evaluate thymocyte differentiation in a 9-month-old child of Amish descent with Nezelof syndrome. Though the patient's stem cells differentiate to acquire normal expression of CD2 and CD7, later steps of maturation were abnormal. There was detectable but reduced expression of CD3 and CD4 phenotypes. CD44+ expression, however, was markedly reduced. CD44 is an adhesion molecule, interacting with the matrix ligands hyaluronan and fibronectin, and is expressed early in thymocyte differentiation and subsequently in mature T cells. It is hypothesized that abnormal expression of CD44 in a variant of severe combined immunodeficiency, Nezelof's syndrome, interferes with normal thymocyte and thymic epithelial interaction, which leads to abnormal thymocyte differentiation.