Requirement for TNF-alpha and IL-1 alpha in fetal thymocyte commitment and differentiation

Massive apoptosis of thymocytes in T-cell-deficient Id1 transgenic mice

Id1 is an inhibitor of a group of basic helix-loop-helix transcription factors, collectively called E proteins, which includes E12, E47, E2-2, and HEB. We have generated transgenic mice in which Id1 is specifically expressed in T cells. The total number of thymocytes in these mice is less than 4% of that in wild-type mice. The majority of the transgenic thymocytes are CD4 and CD8 double negative and bear the cell surface markers of multipotent progenitor cells. A small number of thymocytes, however, differentiate into CD4 or CD8 single-positive T cells, which also display different characteristics from their wild-type counterparts. More importantly, apoptotic cells constitute about 50% of the total thymocytes. These apoptotic thymocytes have rearranged their T-cell receptor genes, suggesting that they are differentiating T cells. This finding has raised the possibility that the T-cell deficiency in Id1 transgenic mice is the result of a massive apoptosis of differentiating T cells triggered by Id1 expression as opposed to a developmental block at the earliest progenitor stage. The progenitor cells accumulated in the transgenic mice might have survived because they are not susceptible to the apoptotic signals. Despite the massive cell death of the thymocytes at young ages, Id1 transgenic mice frequently develop T-cell lymphoma later in their life span, and lymphomagenesis appears to occur at different stages of T-cell development. Taken together, our data suggest that E proteins, being the targets of Id1, are essential regulators for normal T-cell differentiation and tumor suppression.

Expression of interleukin-1 receptors in the later period of foetal thymic organ culture and during suspension culture of thymocytes from aged mice

Interleukin-1 has been reported to be involved in thymocyte development by exerting a variety of effects on immature CD4-CD8- double-negative (DN) thymocytes. In contrast to the well-documented involvement of IL-1 in thymocyte development, expression of IL-1 receptors (IL-1R) on thymocytes has not been well demonstrated. In the present study, expression of IL-1R on the developing thymocytes was investigated. Although normal thymocytes barely express IL-1R, expression of IL-1R (type I) substantially increased at days 12-15 of foetal thymic organ culture (FTOC), with an increase of the DN subset. The CD4/CD8 profile of the IL-1R (type I)+ cells showed that these cells were mostly restricted to the DN and CD4+CD8+ subsets. Interestingly, in vitro culture of the thymocytes from an aged mouse, but not those from young adult or newborn mice, revealed similar results to those of FTOC. In addition, half of the IL-1R+ cells that increased in the later period of FTOC were gammadelta thymocytes. These results demonstrate IL-1R expression on thymocytes during ex vivo culture and suggest that IL-1R is expressed in a certain environment during normal thymocyte differentiation.

Transcriptional regulation of lymphocyte lineage commitment

The development of T cells and B cells from pluripotent hematopoietic precursors occurs through a stepwise narrowing of developmental potential that ends in lineage commitment. During this process, lineage-specific genes are activated asynchronously, and lineage-inappropriate genes, although initially expressed, are asynchronously turned off. These complex gene expression events are the outcome of the changes in expression of multiple transcription factors with partially overlapping roles in early lymphocyte and myeloid cell development. Key transcription factors promoting B-cell development and candidates for this role in T-cell development are discussed in terms of their possible modes of action in fate determination. We discuss how a robust, stable, cell-type-specific gene expression pattern may be established in part by the interplay between endogenous transcription factors and signals transduced by cytokine receptors, and in part by the network of effects of particular transcription factors on each other.

T Cell Development in PU.1-Deficient Mice

These studies address the role of PU.1 in T cell development through the analysis of PU.1−/− mice. We show that the majority of PU.1−/− thymocytes are blocked in differentiation prior to T cell commitment, and contain a population of thymocyte progenitors with the cell surface phenotype of CD44+, HSAbright, c-kitint, Thy-1−, CD25−, Sca-1−, CD4−, and CD8−. These cells correspond in both number and cell surface phenotype with uncommitted thymocyte progenitors found in wild-type fetal thymus. RT-PCR analysis demonstrated that PU.1 is normally expressed in this early progenitor population, but is down-regulated during T cell commitment. Rare PU.1−/− thymi, however, contained small numbers of thymocytes expressing markers of T cell commitment. Furthermore, almost 40% of PU.1−/− thymi placed in fetal thymic organ culture are capable of T cell development. Mature PU.1−/− thymocytes generated during organ culture proliferated and produced IL-2 in response to stimulation through the TCR. These data demonstrate that PU.1 is not absolutely required for T cell development, but does play a role in efficient commitment and/or early differentiation of most T progenitors.

AND-34, a Novel p130Cas-Binding Thymic Stromal Cell Protein Regulated by Adhesion and Inflammatory Cytokines

We have characterized a novel cDNA whose steady state mRNA levels rise in the thymus 2 to 6 h following the induction of CD4+CD8+ thymocyte apoptosis by in vivo cross-linking of CD3ε. This cDNA, AND-34-1, contains an open reading frame (ORF) encoding a protein with an amino-terminal Src homology 2 (SH2) domain and a carboxyl-terminal domain homologous to GDP-exchange factors (GEFs). Northern analysis demonstrates widespread expression of the AND-34 gene. Anti-CD3ε treatment induces up-regulation of the AND-34 mRNA levels in total thymic RNA but not in RNA from purified thymocytes, suggesting that this transcript is derived from a thymic stromal cell population. IL-1 and TNF increase AND-34 transcript levels in thymic cortical reticular, thymic nurse, and fibroblast cell lines. In the thymic cortical reticular cell line, IL-1 and TNF induce a protein of the predicted 93-kDa size reactive with anti-AND-34 peptide antisera. Fifteen minutes of serum stimulation of vanadate-pretreated AND-34-1-transfected NIH3T3 fibroblasts induces tyrosine phosphorylation of AND-34 as well as coprecipitating 95-, 125-, and 130-kDa proteins. One of these tyrosine phosphorylated proteins is identified as p130Cas (Crk-associated substrate), a signaling molecule previously known to bind to a GDP-exchange factor (C3G) and inducibly associate with the focal adhesion complex. Consistent with such an association, AND-34 tyrosine phosphorylation is induced following adherence of trypsinized fibroblasts to fibronectin or poly-l-lysine-coated surfaces.

Molecular and cellular aspects of induced thymus development in recombinase-deficient mice

Thymus development and microenvironment organization require stage- and site-specific cross-talk between thymocyte and stroma. In this study we have used recombinase-activating gene-deficient (RAG-2(-/-)) mice to analyze regulated gene expression both in thymocytes and stromal cells following injection of anti-CD3 monoclonal antibodies as inducer of thymus development. We show that IFN-gamma, TNF-alpha and lymphotactin are transcriptionally regulated in thymocytes, whereas cytoskeletal keratin 14, IL-1alpha and TNF-alpha are regulated in the stroma, quantitatively reproducing the variations associated with beta selection of thymocytes. In addition, RAG-2(-/-) thymus development is associated with entry of epithelial cells into the cell cycle. The histochemical evidence that expanded RAG-2(-/-) thymus becomes undistinguishable from wild-type cortex further suggests that cross-talk phenomena occurring during beta selection of thymocyte are reproduced in this system.

A bone marrow-derived stroma cell line, ST2, can support the differentiation of fetal thymocytes from the CD4+ CD8+ double negative to the CD4+ CD8+ double positive differentiation stage in vitro

T-cell precursors differentiate into mature T cells predominantly in the thymus. However, it has also been reported that T-cell precursors mature in extrathymic organs such as the liver, bone marrow, or intestines. In order to investigate the nature of the extrathymic microenvironment that supports T-cell maturation, we examined the effect of a bone marrow-derived stroma cell line, ST2, on T-cell precursors by using a reaggregate thymic organ culture (RTOC) system. We found that ST2 cells supported the differentiation of fetal thymocytes at day 14.5 of gestation from a CD4- CD8- double negative (DN) to a CD4+ CD8+ double positive (DP) differentiation stage in a manner similar to that observed in thymus. Anti-interleukin-7 receptor (IL-7R) and anti-c-kit antibodies blocked the growth of thymocytes in RTOC with ST2 cells, but did not inhibit the generation of DP thymocytes. These data indicate that a bone marrow-derived stroma cell, ST2, which supports B-cell differentiation, is also able to support T-cell development and may constitute one of the microenvironmental components for extrathymic T-cell development.

The overlooked "nonclassical" functions of major histocompatibility complex (MHC) class II antigens in immune and nonimmune cells

Besides their "classical" antigenic peptide-presenting activity, major histocompatibility complex (MHC) class II antigens can activate different cellular functions in immune and nonimmune cells. However, this "nonclassical" role and its functional consequences are still substantially overlooked. In this review, we will focus on these alternative functional properties of MHC class II antigens, to reawaken attention to their present and foreseeable immunobiologic and pathogenetic implications. The main issues that will be addressed concern 1) the role of MHC class II molecules as basic components of exchangeable oligomeric protein complexes with intracellular signaling ability; 2) the nonclassical functions of MHC class II antigens in immune cells; 3) the pathogenetic role of MHC class II antigens in inflammatory/autoimmune and infectious disease; and 4) the functional role of MHC class II antigens in solid malignancies.

Down-regulation of tumor necrosis factor alpha expression by activating transcription factor 2 increases UVC-induced apoptosis of late-stage melanoma cells

To identify mechanisms whereby activating transcription factor 2 (ATF2) alters the radiation resistance of human melanoma cells, we examined the possible role of ATF2 in UVC-induced apoptosis. Forced expression of full-length or truncated (Delta1-195 amino acids) forms of ATF2 in LU1205, a late-stage human melanoma cell line, elevated the levels of UVC-induced apoptosis. At the same time, either truncated or full-length forms of ATF2 reduced UVC-induced activation of the tumor necrosis factor-alpha (TNFalpha) promoter and decreased expression of TNFalpha. Forced expression of c-Jun in ATF2-expressing melanoma cells restored TNFalpha expression, suggesting that both forms of ATF2 sequestered transcription factors that positively regulate TNFalpha expression in response to UV irradiation. Antagonistic antibodies to Fas, but not to TNFR1, efficiently suppressed UVC-induced apoptosis, suggesting that the Fas pathway mediates the primary apoptotic signal in melanoma cells whereas the TNFR1 pathway elicits a survival signal. Indeed, treatment of melanoma cells with TNFalpha before UVC irradiation partially suppressed UVC-induced apoptosis, further supporting the protective role of TNFalpha in UVC-treated melanoma cells. Taken together, our findings suggest that ATF2 contributes to UVC-induced apoptosis through transcriptional silencing of TNFalpha, which balances Fas-mediated cell death in melanoma.

High-level replication of human immunodeficiency virus in thymocytes requires NF-kappaB activation through interaction with thymic epithelial cells

We have previously demonstrated that interaction of infected thymocytes with autologous thymic epithelial cells (TEC) is a prerequisite for a high level of human immunodeficiency virus type 1 (HIV-1) replication in thymocytes (M. Rothe, L. Chêne, M. Nugeyre, F. Barré-Sinoussi, and N. Israël, J. Virol. 72:5852-5861, 1998). We report here that this activation of HIV replication takes place at the transcriptional level through activation of the Rel/NF-kappaB transcription factors. We first demonstrate that an HIV-1 provirus (SF-2 strain) very effectively replicates in thymocytes cocultured with TEC whereas this provirus, with kappaB sites deleted, fails to replicate. We provide evidence that several NF-kappaB complexes are constitutively found in the nuclei of thymocytes either freshly isolated from the thymus or maintained in coculture with autologous or heterologous TEC. The prevalent complex is the heterodimer p50-p65. NF-kappaB activity is tightly correlated with the transcriptional activity of a long terminal repeat (LTR) of HIV-1 transfected in thymocytes. The cotransfection of this LTR with a mutated IkappaBalpha molecule formally demonstrates that LTR transactivation is regulated by members of the Rel/NF-kappaB family in thymocytes. We also showed that tumor necrosis factor (TNF) and to a lesser extent interleukin-1 (IL-1), secreted within the coculture, induce NF-kappaB activity and a correlative LTR transactivation. However IL-7, a crucial factor for thymopoiesis that is secreted mainly by TEC, is a necessary cofactor for NF-kappaB activation elicited by TNF or IL-1. Together, these data indicate that NF-kappaB activation, required for a high level of HIV replication in thymocytes, is regulated in a specific manner in the thymic microenvironment which provides the necessary cytokines: TNF, IL-1, and IL-7.

Upregulated Expression of Fibronectin Receptors Underlines the Adhesive Capability of Thymocytes to Thymic Epithelial Cells During the Early Stages of Differentiation: Lessons From Sublethally Irradiated Mice

A 250-cGy whole-body γ-radiation dose was used to induce thymus regression in mice, and to study the expression and function of extracellular matrix (ECM) receptors in distinct thymocyte subsets emerging during repopulation of the organ. The onset of regeneration was detected from day 2 to 3 postirradiation (P-Ir), when a remarkable increase in the absolute counts of CD3−CD25hiCD44+ and CD3−CD25in/hiCD44−cells occurred. Enhanced expression of L-selectin, 4, and 5 integrin chains (L-selhi 4hi5hi) was also exhibited by these cells. This pattern of expression was maintained until the CD4+CD8+ (DP) young stage was achieved. Afterward, there was a general downregulation of these ECM receptors in DP as well as in CD4+ or CD8+ single positive (SP) thymocytes (L-selin 4in5in). In some recently generated SP cells, 4 expression was downregulated before the 5 chain, and L-selectin was upregulated in half of more mature cells. The expression of the 6 integrin chain was downregulated only in maturing CD4+cells. Importantly, the increased expression of L-selectin and 4 and 5 chains in thymocytes was strongly correlated with their adhesiveness to thymic epithelial cells (TEC) in vitro. Blocking experiments with monoclonal antibody or peptides showed the following: (1) that the LDV rather than the REDV cell attachment motif in the IIIC segment of fibronectin is targeted by the 4 integrin during thymocyte/TEC adhesion; (2) that the RGD motif of the 120-kD fragment of fibronectin, a target for 5 integrin, has a secondary role in this adhesion; and (3) that the YIGSR cell attachment motif of the β1 chain of laminin/merosin recognized by a nonintegrin receptor is not used for thymocyte adherence. In conclusion, our results show that an upregulated set of receptors endows CD25+ precursors and cells up to the young DP stage with a high capability of interacting with thymic ECM components.

Upregulated Expression of Fibronectin Receptors Underlines the Adhesive Capability of Thymocytes to Thymic Epithelial Cells During the Early Stages of Differentiation: Lessons From Sublethally Irradiated Mice

A 250-cGy whole-body γ-radiation dose was used to induce thymus regression in mice, and to study the expression and function of extracellular matrix (ECM) receptors in distinct thymocyte subsets emerging during repopulation of the organ. The onset of regeneration was detected from day 2 to 3 postirradiation (P-Ir), when a remarkable increase in the absolute counts of CD3−CD25hiCD44+ and CD3−CD25in/hiCD44−cells occurred. Enhanced expression of L-selectin, 4, and 5 integrin chains (L-selhi 4hi5hi) was also exhibited by these cells. This pattern of expression was maintained until the CD4+CD8+ (DP) young stage was achieved. Afterward, there was a general downregulation of these ECM receptors in DP as well as in CD4+ or CD8+ single positive (SP) thymocytes (L-selin 4in5in). In some recently generated SP cells, 4 expression was downregulated before the 5 chain, and L-selectin was upregulated in half of more mature cells. The expression of the 6 integrin chain was downregulated only in maturing CD4+cells. Importantly, the increased expression of L-selectin and 4 and 5 chains in thymocytes was strongly correlated with their adhesiveness to thymic epithelial cells (TEC) in vitro. Blocking experiments with monoclonal antibody or peptides showed the following: (1) that the LDV rather than the REDV cell attachment motif in the IIIC segment of fibronectin is targeted by the 4 integrin during thymocyte/TEC adhesion; (2) that the RGD motif of the 120-kD fragment of fibronectin, a target for 5 integrin, has a secondary role in this adhesion; and (3) that the YIGSR cell attachment motif of the β1 chain of laminin/merosin recognized by a nonintegrin receptor is not used for thymocyte adherence. In conclusion, our results show that an upregulated set of receptors endows CD25+ precursors and cells up to the young DP stage with a high capability of interacting with thymic ECM components.

Expression of CD10 on lymphoid cells associated with thymoma

BACKGROUND

The thymoma is a mixture, in varying proportions, of epithelial and lymphoid cells. The aim of this study was to detect immature T-lineage components of lymphoid cells infiltrating thymoma tissue.

METHODS

Tissue-infiltrating lymphoid cells (N = 10) and normal thymocytes (TC, N = 3) were retrieved from surgical specimens. The surface antigens of these lymphocytes were examined with flow cytometry.

RESULTS

CD3 was detected on 48 +/- 19% of tissue-infiltrating lymphoid cells and 66 +/- 16% of TC, while CD19 was expressed on neither. CD4 and CD8 were co-expressed on 70 +/- 21% of Tissue-infiltrating lymphoid cells and 68 +/- 28% of TC. CD10 was expressed on 22 +/- 10% of tissue-infiltrating lymphoid cells, but was not expressed on TC (5 +/- 3%, p = 0.0003). Two-color analysis showed that the CD10+ fraction was weakly stained with CD3. It was also stained with anti-CD38 and anti-CD4, but not with anti-CD34.

CONCLUSIONS

Tissue-infiltrating lymphoid cells included CD10-positive T-lineage cells. This fraction corresponds to very immature thymocyte subsets. These observations suggest that the epithelial component of a thymoma either inhibits the normal differentiation of T-lineage cells, or attracts immature T-lineage cells of bone-marrow origin.

Regulation of NK1.1 Expression During Lineage Commitment of Progenitor Thymocytes

We recently identified a stage in fetal ontogeny (NK1.1+/CD117+) that defines committed progenitors for T and NK lymphocytes. These cells are found in the fetal thymus as early as day 13 of gestation, but are absent in the fetal liver. Nonetheless, multipotent precursors derived from both the fetal thymus and fetal liver are capable of rapidly differentiating to the NK1.1+ stage upon transfer into fetal thymic organ culture (FTOC). This suggests that expression of NK1.1 marks a thymus-induced lineage commitment event. We now report that a subset of the most immature fetal thymocytes (NK1.1−/CD117+) is capable of up-regulating NK1.1 expression spontaneously upon short-term in vitro culture. Interestingly, fetal liver-derived CD117+ precursors remain NK1.1− upon similar culture. Spontaneous up-regulation of NK1.1 surface expression is minimally affected by transcriptional blockade, mitogen-induced activation, or exposure of these cells to exogenous cytokines or stromal cells. These data suggest that induction of NK1.1 expression on cultured thymocytes may be predetermined by exposure to the thymic microenvironment in vivo. Importantly, multipotent CD117+ thymocytes subdivided on the basis of NK1.1 expression after short-term in vitro culture show distinct precursor potential in lymphocyte lineage reconstitution assays. This demonstrates that even the earliest precursor thymocyte population, although phenotypically homogeneous, contains a functionally heterogeneous subset of lineage-committed progenitors. These findings characterize a thymus-induced pathway in the control of lymphocyte lineage commitment to the T and NK cell fates.

γc Gene Transfer in the Presence of Stem Cell Factor, FLT-3L, Interleukin-7 (IL-7), IL-1, and IL-15 Cytokines Restores T-Cell Differentiation From γc(−) X-Linked Severe Combined Immunodeficiency Hematopoietic Progenitor Cells in Murine Fetal Thymic Organ Cultures

X-linked severe combined immunodeficiency (SCID-Xl) is a rare human inherited disorder in which early T and natural killer (NK) lymphocyte development is blocked. The genetic disorder results from mutations in the common γc chain that participates in several cytokine receptors including the interleukin-2 (IL-2), IL-4, IL-7, IL-9, and IL-15 receptors. We have shown in a previous report that γc gene transfer into SCID-Xl bone marrow (BM) cells restores efficient NK cell differentiation. In this study, we have focused on the introduction of the γc gene into SCID-Xl hematopoietic stem cells with the goal of obtaining differentiation into mature T cells. For this purpose, we used the in vitro hybrid fetal thymic organ culture (FTOC) system in which a combination of cytokines consisting of stem cell factor (SCF), Flt-3L, IL-7, IL-1, and IL-15 is added concomitantly. In this culture system, CD34+ marrow cells from two SCID-Xl patients were able to mature into double positive CD4+ CD8+ cells and to a lesser degree into CD4+ TCRβ+ single positive cells after retroviral-mediated γc gene transfer. In addition, examination of the output cell population at the TCR DJβ1 locus exhibited multiple rearrangements. These results indicate that restoration of the γc/JAK/STAT signaling pathway during the early developmental stages of thymocytes can correct the T-cell differentiation block in SCID-Xl hematopoietic progenitor cells and therefore establishes a basis for further clinical γc gene transfer studies.

γc Gene Transfer in the Presence of Stem Cell Factor, FLT-3L, Interleukin-7 (IL-7), IL-1, and IL-15 Cytokines Restores T-Cell Differentiation From γc(−) X-Linked Severe Combined Immunodeficiency Hematopoietic Progenitor Cells in Murine Fetal Thymic Organ Cultures

X-linked severe combined immunodeficiency (SCID-Xl) is a rare human inherited disorder in which early T and natural killer (NK) lymphocyte development is blocked. The genetic disorder results from mutations in the common γc chain that participates in several cytokine receptors including the interleukin-2 (IL-2), IL-4, IL-7, IL-9, and IL-15 receptors. We have shown in a previous report that γc gene transfer into SCID-Xl bone marrow (BM) cells restores efficient NK cell differentiation. In this study, we have focused on the introduction of the γc gene into SCID-Xl hematopoietic stem cells with the goal of obtaining differentiation into mature T cells. For this purpose, we used the in vitro hybrid fetal thymic organ culture (FTOC) system in which a combination of cytokines consisting of stem cell factor (SCF), Flt-3L, IL-7, IL-1, and IL-15 is added concomitantly. In this culture system, CD34+ marrow cells from two SCID-Xl patients were able to mature into double positive CD4+ CD8+ cells and to a lesser degree into CD4+ TCRβ+ single positive cells after retroviral-mediated γc gene transfer. In addition, examination of the output cell population at the TCR DJβ1 locus exhibited multiple rearrangements. These results indicate that restoration of the γc/JAK/STAT signaling pathway during the early developmental stages of thymocytes can correct the T-cell differentiation block in SCID-Xl hematopoietic progenitor cells and therefore establishes a basis for further clinical γc gene transfer studies.

Lineage commitment and differentiation of T and natural killer lymphocytes in the fetal mouse

T cells and natural killer (NK) cells are presumed to share a common intrathymic precursor. The development of conventional alpha beta T lymphocytes begins within the early fetal thymus, after the colonization of multipotent CD117+ precursors. Irrevocable commitment to the T lineage is marked by thymus-induced expression of CD25. However, the contribution of the fetal thymus to NK lineage commitment and differentiation remains largely unappreciated. Recently, we demonstrated that the development of functional mouse NK cells occurs first in the fetal thymus. Moreover, the appearance of mature fetal thymic NK cells (NK1.1+/CD117-) is preceded by a thymus-induced developmental stage (NK1.1+/CD117+) that marks lineage commitment of multipotent hematopoietic precursors to the T and NK-cell fates. Commitment to the T/NK bipotent stage is induced by fetal thymic stroma, but is not thymus dependent. Recent data indicate that CD90+/CD117lo fetal blood prothymocytes exhibit NK lineage potential and are phenotypically and functionally identical to fetal thymic NK1.1+/CD117+ progenitors. This finding also indicates that full commitment of circulating precursors to the T-cell lineage occurs after thymus colonization. In this review, we discuss recent insights into the cellular and molecular events involved in fetal mouse T and NK lineage commitment and differentiation to unipotent progenitors.

Requirement for the thymus in alphabeta T lymphocyte lineage commitment

We recently identified a fetal thymic developmental stage (NK1.1+/CD117(lo)) that characterizes committed T/NK progenitors. We now report the existence of phenotypically and functionally identical T/NK progenitors in mouse fetal blood and spleen but not in fetal liver. These precursors are indistinguishable from previously characterized fetal blood "prothymocytes" (CD90+/CD117(lo)), with the exception that they express NK1.1, lack markers associated with T lineage commitment, maintain a germline TCRbeta locus, and can give rise to both T and NK cells. Moreover, NK1.1+/CD90+/CD117(lo) fetal blood precursors are present in athymic nude mice. These results suggest that the T/NK lineage commitment pathway is thymus-independent. In contrast, full commitment to the alphabeta T lineage does not precede thymus colonization.

Tumor necrosis factor induces neuroendocrine differentiation in small cell lung cancer cell lines

We studied tumor necrosis factor (TNF)-alpha as a candidate cytokine to promote neuroendocrine cell differentiation in a nitrosamine-hyperoxia hamster lung injury model. Differential screening identified expression of the genes modulated by TNF-alpha preceding neuroendocrine cell differentiation. Undifferentiated small cell lung carcinoma (SCLC) cell lines NCI-H82 and NCI-H526 were treated with TNF-alpha for up to 2 wk. Both cell lines demonstrated rapid induction of gastrin-releasing peptide (GRP) mRNA; H82 cells also expressed aromatic-L-amino acid decarboxylase mRNA within 5 min after TNF-alpha was added. Nuclear translocation of nuclear factor-kappaB immunostaining occurred with TNF-alpha treatment, suggesting nuclear factor-kappaB involvement in the induction of GRP and/or aromatic-L-amino acid decarboxylase gene expression. We also demonstrated dense core neurosecretory granules and immunostaining for proGRP and neural cell adhesion molecule in H82 cells after 7-14 days of TNF-alpha treatment. We conclude that TNF-alpha can induce phenotypic features of neuroendocrine cell differentiation in SCLC cell lines. Similar effects of TNF-alpha in vivo may contribute to the neuroendocrine cell differentiation/hyperplasia associated with many chronic inflammatory pulmonary diseases.

Contact with thymic epithelial cells as a prerequisite for cytokine-enhanced human immunodeficiency virus type 1 replication in thymocytes

We report here that human immunodeficiency virus type 1 (HIV-1)-infected human thymocytes, in the absence of any exogenous stimulus but cocultivated with autologous thymic epithelial cells (TEC), obtained shortly (3 days) after thymus excision produce a high and sustained level of HIV-1 particles. The levels and kinetics of HIV-1 replication were similar for seven distinct viral strains irrespective of their phenotypes and genotypes. Contact of thymocytes with TEC is a critical requirement for optimal viral replication. Rather than an inductive signal resulting from the contact itself, soluble factors produced in the mixed culture are responsible for this effect. Specifically, the synergistic effects of tumor necrosis factor, interleukin-1 (IL-1), IL-6, and granulocyte-macrophage colony-stimulating factor may account by themselves for the high level of HIV-1 replication in thymocytes observed in mixed cultures. In conclusion, the microenvironment generated by TEC-thymocyte interaction might greatly favor optimal HIV-1 replication in the thymus.

Cross-lineage expression of Ig-beta (B29) in thymocytes: positive and negative gene regulation to establish T cell identity

Developmental commitment involves activation of lineage-specific genes, stabilization of a lineage-specific gene expression program, and permanent inhibition of inappropriate characteristics. To determine how these processes are coordinated in early T cell development, the expression of T and B lineage-specific genes was assessed in staged subsets of immature thymocytes. T lineage characteristics are acquired sequentially, with germ-line T cell antigen receptor-beta transcripts detected very early, followed by CD3epsilon and terminal deoxynucleotidyl transferase, then pTalpha, and finally RAG1. Only RAG1 expression coincides with commitment. Thus, much T lineage gene expression precedes commitment and does not depend on it. Early in the course of commitment to the T lineage, thymocytes lose the ability to develop into B cells. To understand how this occurs, we also examined expression of well defined B lineage-specific genes. Although lambda5 and Ig-alpha are not expressed, the mu 0 and I mu transcripts from the unrearranged IgH locus are expressed early, in distinct patterns, then repressed just before RAG1 expression. By contrast, RNA encoding the B cell receptor component Ig-beta was found to be transcribed in all immature thymocyte subpopulations and throughout most thymocyte differentiation. Ig-beta expression is down-regulated only during positive selection of CD4(+)CD8(-) cells. Thus several key participants in the B cell developmental program are expressed in non-B lineage-committed cells, and one is maintained even through commitment to an alternative lineage, and repressed only after extensive T lineage differentiation. The results show that transcriptional activation of "lymphocyte-specific" genes can occur in uncommitted precursors, and that T lineage commitment is a composite of distinct positive and negative regulatory events.

Transcriptional regulation of interleukin-1beta gene by interleukin-1beta itself is mediated in part by Oct-1 in thymic stromal cells

Interleukin (IL)-1 is involved in many processes, including thymic development. However, control of IL-1 expression in thymic-derived stromal cells (TSC) has not been reported. We found that IL-1beta increased steady-state mRNA levels for IL-1alpha and IL-1beta in TSC-936 and TSC-2C4 cells; stability was not a major determinant of this effect. To study transcriptional regulation of IL-1beta, we functionally characterized 4 kilobase pairs of the 5'-flanking region and first intron of the bovine IL-1beta gene. The -470/+14 fragment was sufficient to confer maximal responsiveness to IL-1beta upon transfection into these cell lines. Progressive 5' deletions identified several IL-1beta-responsive regions, including -308 to -226, which we further characterized. Electrophoretic mobility shift and supershift analyses showed that IL-1beta induced the ability to form multiple protein complexes with -261/-226 and that one of these contained nuclear factor Oct-1. A competitor containing a mutated Oct consensus site failed to compete not only for this complex but others as well, suggesting that this sequence regulates binding of other proteins to this region. Functional analysis confirmed that this element was essential for maximal induction of transcription. These findings document a heretofore undescribed mechanism utilized by TSC for regulation of IL-1beta transcription by IL-1beta itself.

A linkage between dendritic cell and T-cell development in the mouse thymus: the capacity of sequential T-cell precursors to form dendritic cells in culture

The earliest T precursor population in the adult mouse thymus (CD4lo8-3-44+25-c-kit+) was previously shown to be lymphoid-restricted (T, B, NK) but to have a capacity to form dendritic cells (DC). This led to the concept of a lineage of lymphoid-derived DC. DC could be generated with high efficiency in culture from this low CD4 precursor, using a complex mix of cytokines, a mix that notably did not include GM-CSF, the cytokine normally used for development in culture of myeloid-derived DC. Using this new culture system we now show that the capacity to form DC extends to the pro-T precursor population (CD4-8-3-44+25+c-kit+) but is lost by the pre-T precursor stage (CD4-8-3-44-25+c-kit+), the point of T-cell antigen-receptor beta-gene rearrangement. The DC generated in the cultures resemble mature thymic DC by most markers, but differ in their lack of expression of BP-1 and CD8 alpha.

Nuclear Factor κB Is Required for Peptide Antigen-Induced Differentiation of a CD4+CD8+ Thymocyte Line

NF-κB transcription factors are known to regulate the expression of a number of genes involved in T cell activation and function. Some evidence has suggested that they also play a role in T cell development. However, the role of NF-κB in Ag-induced thymocyte differentiation has not been directly addressed to date. Here we critically examine this role by employing DPK, a CD4+CD8+ thymocyte line that undergoes differentiation upon TCR engagement in a process that closely mimics positive selection. Expression of a degradation-resistant form of IκBα in DPK cells results in constitutive inhibition of NF-κB activity. We find that in the absence of NF-κB activity, MHC-peptide-induced differentiation of DPK is blocked. Furthermore, differentiation induced by a nonphysiologic stimulus, anti-TCR Ab, is greatly reduced. Altogether, our data indicate a requirement for NF-κB in the developmental changes associated with positive selection.

Fas/Fas Ligand Signaling During Gestational T Cell Development

Most thymocytes express high levels of Fas Ag (Apo-1/CD95); however, the role of Fas/Fas ligand-mediated apoptosis in thymocyte development remains unclear. During gestational development of thymocytes in C57BL/6(B6) +/+ mice, the highest levels of Fas ligand mRNA and Fas ligand protein expression were detected at gestational day (GD) 15, and there was a ninefold decrease in Fas ligand mRNA expression between GD 15 and 17 accompanied by a sixfold increase in Fas mRNA. Apoptotic thymocytes were first detected in the medulla at GD 15, and increasing numbers of cortical clusters and scattered, single apoptotic cells were present on GD 16 and 17. Thus, early apoptosis correlated with high expression of Fas ligand. High levels of Fas ligand mRNA were maintained throughout gestational development in thymocytes of Fas-deficient B6-lpr/lpr mice, but cortical clusters and scattered apoptotic cells were decreased relative to B6 +/+ mice before GD 17. Kinetic analysis of fetal thymic organ cultures treated with anti-Fas Ab demonstrated that thymocytes become sensitive to Fas-mediated apoptosis during the transition from the CD4−CD8− to the CD4+CD8+ phenotype. More mature CD4+CD8+ thymocytes and CD4+ and CD8+ thymocytes became resistant to Fas-mediated apoptosis after GD 17, despite high expression of Fas. However, low avidity engagement of the TCR on Fas-sensitive CD4+CD8+ thymocytes before GD 17 induced resistance to Fas-mediated apoptosis. The present results indicate that Fas plays a critical role in mediating apoptosis during early gestational thymocyte development and that thymocytes that receive a survival signal through TCR/CD3 become resistant to Fas-mediated apoptosis.

Cypermethrin-induced alteration of thymocyte distribution and functions in prenatally-exposed rats

The synthetic pyrethroid insecticide, cypermethrin (50 mg/kg) was given during gestation to pregnant rats by gavage in corn oil. Prenatal cypermethrin exposure induced a significant decrease in the absolute number of all thymocyte subsets during the first 30 days after birth, being the double negative CD4-CD8-, single positive CD4 and CD8 T cells preferentially affected. Later on day 60 and 90 double positive CD4+CD8+ and single positive thymocytes gradually recovered, while the total number of CD4 CD8 cells was increased. Moreover, thymocytes from rats prenatally exposed to cypermethrin showed an impaired ability to proliferate in response to different doses of Concanavalin A (ConA) and human recombinant interleukin-2 (hrIL-2) and to produce and/or release IL-2. Overall, our results indicate that cypermethrin administered during prenatal period can affect multiple steps in thymocyte differentiation pathways resulting in an altered cell subset distribution and an impairment of thymocyte functions.

Natural Killer Cell Development and Function Precede αβ T Cell Differentiation in Mouse Fetal Thymic Ontogeny

Natural killer (NK) cells mediate MHC-unrestricted cytolysis of virus-infected cells and tumor cells. In the adult mouse, NK cells are bone marrow-derived lymphocytes that mature predominantly in extrathymic locations but have also been suggested to share a common intrathymic progenitor with T lymphocytes. However, mature NK cells are thought to be absent in mouse fetal ontogeny. We report the existence of thymocytes with a mature NK cell phenotype (NK1.1+/CD117−) as early as day 13 of gestation, approximately 3 days before the appearance of CD4+/CD8+ cells in T lymphocyte development. These mature fetal thymic NK cells express genes associated with NK cell effector function and, when freshly isolated, display MHC-unrestricted cytolytic activity in vitro. Moreover, the capacity of fetal thymic NK cells for sustained growth both in vitro and in vivo, in addition to their close phenotypic resemblance to early precursor thymocytes, confounds previous assessments of NK lineage precursor function. Thus, mature NK cells may have been inadvertently included in previous attempts to identify multipotent and bipotent precursor thymocytes. These results provide the first evidence of functional NK lymphocytes in mouse fetal ontogeny and demonstrate that NK cell maturation precedes αβ T cell development in the fetal thymus.

Tumor necrosis factor (TNF) receptor type 2 mediates thymocyte proliferation independently of TNF receptor type 1

Tumor necrosis factor (TNF) mediates its biological effects by binding to two distinct but homologous receptor molecules. The type 1 receptor (TNF-R1) has been shown to be essential and sufficient for most cellular responses to soluble TNF. In contrast, only limited data exist concerning the role of the type 2 receptor (TNF-R2) in TNF responses, both in vitro and in vivo. Here, we demonstrate by the use of thymocytes from TNF-R-deficient mice that the TNF-R2-dependent enhancement of proliferation and secretion of granulocyte-macrophage colony-stimulating factor is in fact mediated by TNF-R2 on its own, independent of co-expression and/or stimulation of TNF-R1.

Development of Natural Killer Cells, B Lymphocytes, Macrophages, and Mast Cells From Single Hematopoietic Progenitors in Culture of Murine Fetal Liver Cells

We have recently established a clonal culture system that supports the growth of immature natural killer (NK) cells from murine fetal thymocytes. We now describe a culture system for mixed NK cell colony formation from single lymphohematopoietic progenitors. When Sca-1+c-kit+ fetal liver cells were cultured in methylcellulose media with interleukin (IL)-2, IL-7, IL-11, and steel factor (SF ), we found mixed colonies consisting of diffuse small round cells characteristic of immature NK cells and other types of cells. The single cell origin of the mixed colonies was established by micromanipulation. Individual mixed colonies derived from single cells were characterized by flow cytometric analysis and May-Grünwald Giemsa staining. All mixed colonies contained Thy-1+B220− cells, which can differentiate to mature NK cells in fetal thymus organ culture. Most of the colonies contained B220+ B-lineage cells and macrophages, and some contained mast cells. IL-1α and IL-3, which have previously been shown to inhibit the T- and B-cell potentials of blast colonies, suppressed the formation of mixed NK cell colonies. The clonal culture assay presented here may be useful in analysis of the developmental pathway and commitment of NK cells from multipotential progenitors.

An indirect effect of Stat5a in IL-2-induced proliferation: a critical role for Stat5a in IL-2-mediated IL-2 receptor alpha chain induction

Stat5a was identified as a prolactin-induced transcription factor but also is activated by other cytokines, including interleukin-2 (IL-2) and IL-7. We have now analyzed the immune system of Stat5a-deficient mice. Stat5a-/- splenocytes exhibited defective IL-2-induced expression of the IL-2 receptor alpha chain (IL-2R alpha), a protein that together with IL-2R beta and the common cytokine receptor gamma chain (gamma(c)) mediates high-affinity IL-2 binding. Correspondingly, Stat5a-/- splenocytes exhibited markedly decreased proliferation to IL-2, although maximal proliferation was still achieved at IL-2 concentrations high enough to titrate intermediate-affinity IL-2R beta/gamma(c) receptors. Thus, defective Stat5a expression results in diminished proliferation by an indirect mechanism, resulting from defective receptor expression. Correspondingly, we show that Stat5a is essential for maximal responsiveness to antigenic stimuli in vivo, underscoring the physiological importance of IL-2-induced IL-2R alpha expression.

Cell-autonomous defects in dendritic cell populations of Ikaros mutant mice point to a developmental relationship with the lymphoid lineage

The transcription factor Ikaros is a major determinant of lymphocyte differentiation. Mice homozygous for an Ikaros dominant-negative (DN-/-) mutation lack all cells of lymphoid origin, including T, B, and natural killer (NK) cells. Mice homozygous for an Ikaros null allele lack B and NK cells but display specific defects in T lymphocytes. Nonetheless, both Ikaros mutant lines make an excess of monocytes and macrophages. Here we report that the production of subsets of antigen-presenting dendritic cells (DCs) is also defective. By constructing bone marrow chimeras, we demonstrate that the Ikaros-mediated defect in lymphocytes and DCs is intrinsic to their precursors and is not environment dependent. The selective defects in DCs manifested with the Ikaros null mutation suggest a tight linkage between development of T cells and CD8alpha+ DCs. The complete lack of DCs in the lymphoid organs of Ikaros DN-/- micke points to an essential role for the Ikaros gene family in the development of all DCs.

E2A deficiency leads to abnormalities in alphabeta T-cell development and to rapid development of T-cell lymphomas

The E2A gene products, E12 and E47, are critical for proper early B-cell development and commitment to the B-cell lineage. Here we reveal a new role for E2A in T-lymphocyte development. Loss of E2A activity results in a partial block at the earliest stage of T-lineage development. This early T-cell phenotype precedes the development of a T-cell lymphoma which occurs between 3 and 9 months of age. The thymomas are monoclonal and highly malignant and display a cell surface phenotype similar to that of immature thymocytes. In addition, the thymomas generally express high levels of c-myc. As assayed by comparative genomic hybridization, each of the tumor populations analyzed showed a nonrandom gain of chromosome 15, which contains the c-myc gene. Taken together, the data suggest that the E2A gene products play a role early in thymocyte development that is similar to their function in B-lineage determination. Furthermore, the lack of E2A results in development of T-cell malignancies, and we propose that E2A inactivation is a common feature of a wide variety of human T-cell proliferative disorders, including those involving the E2A heterodimeric partners tal-1 and lyl-1.

Identification of a novel developmental stage marking lineage commitment of progenitor thymocytes

Bipotent progenitors for T and natural killer (NK) lymphocytes are thought to exist among early precursor thymocytes. The identification and functional properties of such a progenitor population remain undefined. We report the identification of a novel developmental stage during fetal thymic ontogeny that delineates a population of T/NK-committed progenitors (NK1. 1(+)/CD117(+)/CD44(+)/CD25(-)). Thymocytes at this stage in development are phenotypically and functionally distinguishable from the pool of multipotent lymphoid-restricted (B, T, and NK) precursor thymocytes. Exposure of multipotent precursor thymocytes or fetal liver- derived hematopoietic progenitors to thymic stroma induces differentiation to the bipotent developmental stage. Continued exposure to a thymic microenvironment results in predominant commitment to the T cell lineage, whereas coculture with a bone marrow-derived stromal cell line results in the generation of mature NK cells. Thus, the restriction point to T and NK lymphocyte destinies from a multipotent progenitor stage is marked by a thymus-induced differentiation step.

Targeted complementation of MHC class II deficiency by intrathymic delivery of recombinant adenoviruses

De novo differentiation of CD4+ T cells was provoked in mice lacking major histocompatibility complex (MHC) class II molecules by intrathymic injection of adenovirus vectors carrying class II genes. This permits a new approach to questions concerning the dynamics of CD4+ T cell compartments in the thymus and peripheral lymphoid organs. Here two issues are explored. First, we show that mature CD4+ CD8- cells reside in the thymus for a protracted period before emigrating to the periphery, highlighting the potential importance of, and our ignorance of, the postselection maturation period. Second, we demonstrate that the survival of CD4+ cells in peripheral lymphoid organs is markedly curtailed when class II molecules are absent and is not further reduced in the absence of both class II and class I molecules, raising the possibility that MHC-mediated selection may continue in the periphery.

Clonal Proliferation and Cytokine Requirement of Murine Progenitors for Natural Killer Cells

We have established a clonal cell culture system that supports the proliferation of committed natural killer (NK) cell progenitors of mice to investigate the pathway and cytokine regulation of NK cell development. Day 14 fetal thymocytes cultured in methylcellulose with interleukin-7 (IL-7), IL-15, and steel factor (SF ) formed diffuse colonies that could not be classified to known colony types. Single-cell origin of the colonies was established by micromanipulation of the colony-forming cells. Cells in the colonies are very blastic, showing no cytoplasmic differentiation, and express Ly5, Thy-1, and CD25 but not myeloid, B, mature T, or NK cell markers. The cells lack T, B, and myeloid potentials but can differentiate to mature NK cells in fetal thymus organ culture, suggesting that the colonies consist of NK committed progenitors. Examination of the minimal cytokine requirement for the NK colony formation showed that IL-7 and SF are indispensable for the formation of immature NK cell colonies. Both IL-2 and IL-15 increased the frequency of colonies. In contrast to IL-2, IL-7, and IL-15, IL-4 strongly inhibited the formation of the colonies. This quantitative clonal culture will provide a useful means to examine the mechanism of NK cell development.

From stem cells to lymphocytes: biology and transplantation

We review the development of the hematopoietic system, focusing on the transition from hematopoietic stem cells (HSCs) to T cells. This includes the isolation of HSCs, and recent progress in understanding their ontogeny, homing properties, and differentiation. HSC transplantation is reviewed, including the kinetics of reconstitution, engraftment across histocompatibility barriers, the facilitation of allogeneic engraftment, and the mechanisms of graft rejection. We describe progress in understanding T-cell development in the bone marrow and thymus as well as the establishment of lymph nodes. Finally, the role of bcl-2 in regulating homeostasis in the hematopoietic system is discussed.

Regulation of interleukin (IL)-1alpha, IL-1beta, and IL-6 expression by growth hormone and prolactin in bovine thymic stromal cells

Growth hormone (GH) and prolactin (PRL) have been implicated in T-cell development, but relatively little is known about the mechanism(s) of their actions on the multiple cell types in this complex tissue. Here, we investigated the effects of GH and PRL on the expression of interleukin (IL)-1alpha, IL-1beta and IL-6 in thymic stromal cells (TSC). These cytokine mRNAs were increased by GH, PRL and placental lactogen (PL) in primary cultures prepared from mid-gestational fetuses in a dose-dependent manner. IL-1 receptor antagonist (IL-1ra) abolished the hormone-induced IL-6 expression, suggesting that the induction of IL-6 was secondary to IL-1 activity. To examine the effects of these hormones on an individual cell type and develop a system in which signalling mechanisms can be studied, we generated immortalized cell lines using a strategy of conditional transformation. In the cell line, TSC-936, which displayed vimentin-positive staining and morphological characteristics of mesenchymal cells, both GH and PRL increased levels of steady-state mRNAs for IL-1alpha and IL-1beta. Nuclear run-on analysis revealed that the transcription rate of the IL-1beta gene was significantly increased by GH and PRL at 30 and 60 min, respectively, but that for IL-1alpha was not significantly changed, suggesting the possibility of an alternative mechanism mediating this response. These data suggest that modulation of cytokine gene expression is one mechanism by which GH and PRL facilitate thymic development and T-cell maturation.

Early alpha beta T cell development in the thymus of normal and genetically altered mice

The vast majority of T lymphocytes, with the exception of gut-associated, intraepithelial lymphocytes, differentiate and mature inside the thymus. Early T cell development is characterized by expansion and differentiation of thymocytes which do not yet express mature TCRs on their cell surface. Important events in early thymocyte development are controlled by a pre-TCR complex consisting of a conventional TCR beta chain and a novel transmembrane protein termed pre-TCR alpha (p T alpha chain) which are noncovalently associated with components of CD3. Recent studies of pre-TCR function have led to a better understanding of the molecular events in early thymocyte development.

T-cell cytokines may control the balance of functionally distinct macrophage populations

As monocytes differentiate into mature macrophages, subsets emerge that exhibit stimulatory, suppressive or phagocytic potential. These functionally distinct subsets can be discriminated using monoclonal antibodies RFD1 and RFD7. As examples of all these subsets have been repeatedly identified within the macrophage pool in a variety of organs the overall functional capacity of this pool will depend on the relative balance of these subpopulations. This study investigates whether this balance present in mature macrophage populations can be regulated by the local influence of T-cell-derived cytokines. The dose-dependent effect of cytokines interferon-gamma (IFN-gamma), interleukins (IL) IL-2, IL-4 and IL-10 on the phenotype and function of monocyte-derived macrophages was determined. Subsets of mature cells were quantified by identifying RFD1- RFD7- stimulatory cells (D1+); RFD1- RFD7+ phagocytes (D7+) and RFD1+ RFD7+ suppressive cells (D1 D7+). IFN-gamma and IL-4 increased the relative proportions of D1+ stimulatory cells and upregulated HLA-DR expression. IFN-gamma also increased the capacity of the mature macrophage pool to stimulate T-cell proliferation. IL-10 reduced the proportions of D1+ stimulatory cells while promoting the differentiation of D7+ phagocytes and D1/D7+ suppressive cells. IL-10 induced changes also reduced the functional capacity of the mature populations to stimulate T cells in auto and allogenic mixed lymphocyte reactions (MLR). IL-2 had no effect on differentiation of monocytes. Thus IL-4 and IFN-gamma are seen to induce the development of stimulatory macrophages while IL-10 promotes differentiation of monocytes to mature phagocytes and suppressive macrophages. It is concluded that mature macrophage phenotype is 'plastic' and under the control of T-cell-derived mediators. Furthermore, within the differentiating monocytes, phenotypic change appears to carry with it functional change, thus retaining the relationship between antigen expression and activity in the mature macrophage populations.

Induction of TCR gene rearrangements in uncommitted stem cells by a subset of IL-7 producing, MHC class-II-expressing thymic stromal cells

The embryonic thymic microenvironment provides the necessary elements for T cell lineage commitment, but the precise role of individual stromal cell components remains to be determined. Here we address the question of which stromal cell types are required for initiation of V-DJ rearrangements of the TCR-beta and TCR-delta locus in CD117+CD45+ uncommitted fetal liver progenitors. We show that fetal thymic stroma alone is necessary and sufficient for induction of TCR-beta and TCR-delta rearrangements. Furthermore, the ability to induce this T cell commitment step is confined to a subset of MHC class II-positive epithelial cells. Thymic stroma derived from mice with a targeted deletion in the IL-7 gene, however, lacks this ability. These findings set the stage for a further definition of the nature of the thymic stromal cell support in the regulation of T cell commitment.

Thymocyte development in vitro: implications for studies of ageing and thymic involution

Functional defects that accumulate in the T cell compartment are thought to be responsible for the pronounced immunodeficiency that develops during ageing, and reduced production of T cells by the thymus as it undergoes involution has been suggested to contribute to this phenomenon. Understanding the mechanisms responsible for thymic involution requires a thorough knowledge of how thymopoiesis is regulated. Obtaining such information is dependent upon the availability of defined experimental systems that permit analysis of thymopoiesis at the cellular and molecular levels. Recent advances have been made in the development of such human and murine in vitro systems, and their analysis has the potential to identify thymic microenvironmental signals that regulate T cell production. This information should, in turn, provide a basis for understanding changes in thymopoiesis that occur during ageing. The features of these culture systems are reviewed in this article, and their potential application to the study of T cell production during ageing is discussed.

Dendritic cells and T lymphocytes: developmental and functional interactions

Dendritic cells (DCs) are specialized for presentation of antigen to T cells and are essential for primary T cell activation. Although DCs are generally considered to be myeloid derived, we now have evidence that a subgroup are of lymphoid origin. In particular, the DCs of the adult mouse thymus appear to be derived from the same early, lymphoid-restricted precursor cells that generate T lymphocytes. Purified early thymic T precursors have the capacity to produce T cells, B cells, NK cells and DCs, but not myeloid cells, on transfer to irradiated recipients. They also produce thymic DCs on culture with a mix of cytokines; this mix does not include GM-CSF, needed to generate myeloid-derived DCs. A subgroup of DCs in other lymphoid organs, which like thymic DCs express CD8 as an alpha alpha homodimer, may likewise be of lymphoid origin. These CD8+ DCs in mouse spleen differ functionally from the conventional CD8+ DCs. CD8+ DCs efficiently activate CD4+ T cells but then kill them via Fas ligand on the DC surface. CD8+ DCs efficiently recruit CD8+ T cells into the cell cycle, but their proliferation is then restricted by an inadequate production of interleukin 2. This subgroup of CD8+ DCs therefore appears to have a regulatory role.

Involvement of transcription factors TCF-1 and GATA-3 in the initiation of the earliest step of T cell development in the thymus

Flow cytometric and immunocytochemical analyses of murine fetal thymus (FT) cells with antibodies to various surface markers and transcription factors reveal that the synthesis of TCF-1 and GATA-3 protein begins simultaneously in a fraction of the most immature population of FT cells, which have the phenotype of CD4-CD8-CD44+CD25-. No TCF-1-producing cells is found in the fetal liver (FL). In CD44+CD25- FT cells, the production of TCF-1 is immediately followed by intracellular expression of CD3 epsilon. It is also found that the T cell development from FL, but not FT, progenitors in the FT organ culture system is severely inhibited by the addition of antisense oligonucleotides for either TCF-1 or GATA-3. These results strongly suggest that TCF-1 and GATA-3 play essential roles in the initiation of the earliest steps of T cell development in the thymus.

Thymic dendritic cell precursors: relationship to the T lymphocyte lineage and phenotype of the dendritic cell progeny

Successive T-precursors isolated from adult mouse thymus were examined for their developmental potential, by transfer to irradiated Ly 5-disparate recipients. The earliest, "low CD4" precursors formed T, B, and dendritic cells (DC), but not myeloid cells, in accordance with earlier studies. Surprisingly, the next downstream CD4-8-3 44+25+ precursor population still formed DC as well as T cells although it no longer formed B or myeloid cells. Further down-stream, the CD4-8 3-44-25+ population formed only T cells. The thymic and splenic DC progeny of the early thymic precursors all expressed high levels of CD8 alpha, in contrast with normal splenic DC and the splenic DC progeny of bone marrow stem cells, which consisted of both CD8 and CD8+ DC. A common precursor of T cells and of a subclass of DC is proposed, with CD8 alpha as a marker of the lymphoid-related DC lineage.

Murine natural killer cell differentiation: past, present, and future

Natural killer cells are bone marrow-derived lymphocytes capable of lysing a variety of target cells without prior exposure. While the biological activities and function of mature NK cells have been extensively investigated, the differentiation of NK cells from primitive hematopoietic stem cells is poorly understood. Recently, we have reported on the identification of a highly enriched bone marrow population capable of repopulating recipient mice with mature NK cells. In this review, we will summarize our findings and those of others in an attempt to clarify the current status of murine natural killer cell differentiation.

Regulation of thymocyte development from immature progenitors

T lymphocytes differentiate from hematopoietic stem cells that settle in the microenvironment of the thymus. The earliest stages of mouse alpha/beta T-cell differentiation occurring before surface expression of the TCR include three important events: proliferation, commitment to the T lineage, and rearrangement and expression of the TCR loci. Recent evidence suggests that the survival as well as differentiation of early thymocytes depends critically on molecular signals such as those generated by the recently described pre-TCR complex.