Requirement of cell interactions through adhesion molecules in the early phase of T cell development

CD11a is essential for normal development of hematopoietic intermediates

The process of lymphopoiesis begins in the bone marrow (BM) and requires multiple cellular intermediates. For T cell production, lymphoid progenitors exit the BM and home to the thymus where maturation and selection ensue. These processes are dependent on a number of factors, including chemokines and adhesion molecules. Although the β2 integrin CD11a plays an important role in the migration of lymphocytes to lymph nodes, the role of CD11a in T cell development is largely undefined. Our studies now show that, in CD11a(-/-) mice, thymic cellularity was decreased and early T cell development was partially impaired. Remarkably, CD11a was critical for generation of common lymphoid progenitors (CLPs) and lymphoid-primed multipotent progenitors. However, in intact CD11a(-/-) mice, peripheral B and T cell subsets were only modestly altered, suggesting that compensatory mechanisms were operating. In contrast, competitive BM-reconstitution assays revealed an essential role for CD11a in the generation of thymocytes and mature T and B cells. This defect was linked to the requirement for CD11a in the development of CLPs. Furthermore, our results identified CLPs, and not lymphoid-primed multipotent progenitors, as the requisite CD11a-dependent precursor for lymphocyte development. Thus, these findings established a key role for CD11a in lymphopoiesis.

Developing T-cell migration: role of semaphorins and ephrins

Cell migration is a crucial event for normal T-cell development, and various ligand/receptor pairs have been implicated. Most of them, including chemokines and extracellular matrix proteins, have attractant properties on thymocytes. We discuss herein two further groups of ligand/receptor pairs, semaphorins/neuropilins and ephs/ephrins, which are constitutively expressed by thymocytes and thymic microenvironmental cells. Evidence shows that the corresponding interactions are relevant for developing T-cell migration, including the entry of bone marrow progenitor cells, migration of CD4/CD8-defined thymocyte subpopulations triggered by chemokines and/or extracellular matrix proteins, and thymocyte export. Conceptually, the data summarized here show that thymocyte migration results from a complex network of molecular interactions, which generate not only attraction, but also repulsion of migrating T-cell precursors.

Semaphorins and neuropilins: new players in the neuroendocrine control of the intrathymic T-cell migration in humans

Cell migration is a key event for proper intrathymic T-cell differentiation, and several ligand-receptor interactions contribute to the well-co ordinated movement of developing thymocytes within the thymic lobules. Herein we summarize recent data that place semaphorin 3A (Sema3A) and its receptor neuropilin 1 (NRP1) as further players in the physiological process of cell migration in the human thymus. These molecules, as well as class A plexins (necessary for the intracellular signalling transduction triggered by Sema3A-NRP1 ligation), are constitutively expressed by both developing thymocytes and components of the thymic microenvironment, including epithelial and dendritic cells. Functionally, Sema3A decreases the adhesion of human thymocytes on thymic epithelial cell monolayers and exerts per se a dose-dependent chemorepulsive effect on human thymocytes. Moreover, Sema3A inhibits chemoattractant migratory responses induced by other ligands, including fibronectin, laminin and CXCL12 (chemokine CXC motif ligand 12). These data should be placed in the context of the concept that migration of developing T cells is a multivectorial system, in which the resulting migration vector derives from a balance of several simultaneous and/or sequential ligand-receptor pair interactions. Accordingly, semaphorins and neuropilins can be considered as further players in the system.

Expression of 4-1BB and 4-1BBL in thymocytes during thymus regeneration

4-1BB, a member of the tumor necrosis factor receptor (TNFR) superfamily, is a major costimulatory receptor that is rapidly expressed on the surface of CD4(+) and CD8(+) T cells after antigen- or mitogen-induced activation. The interaction of 4-1BB with 4-1BBL regulates immunity and promotes the survival and expansion of activated T cells. In this study, the expression of 4-1BB and 4-1BBL was examined during regeneration of the murine thymus following acute cyclophosphamide- induced involution. Four-color flow cytometry showed that 4-1BB and 4-1BBL were present in the normal thymus and were preferentially expressed in the regenerating thymus, mainly in CD4(+)CD8(+) double-positive (DP) thymocytes. Furthermore, the CD4(lo)CD8(lo), CD4(+)CD8(lo) and CD4(lo)CD8(+) thymocyte subsets, representing stages of thymocyte differentiation intermediate between DP and single-positive (SP) thymocytes, also expressed 4-1BB and 4-1BBL during thymus regeneration but to a lesser degree. Interestingly, the 4-1BB and 4-1BBL positive cells among the CD4(+)CD8(+) DP thymocytes present during thymus regeneration were TCR(hi) and CD69(+) unlike the corresponding controls. Moreover, the 4-1BB and 4-1BBL positive cells among the intermediate subsets present during thymus regeneration also exhibited TCR(hi/int+) and CD69(+/int) phenotypes, indicating that 4-1BB and 4-1BBL are predominantly expressed by the positively selected population of the CD4(+)CD8(+) DP and the intermediate thymocytes during thymus regeneration. RT-PCR and Western blot analyses confirmed the presence and elevated levels of 4-1BB and 4-1BBL mRNA and protein in thymocytes during thymus regeneration. We also found that the interaction of 4-1BB with 4-1BBL promoted thymocyte adhesion to thymic epithelial cells. Our results suggest that 4-1BB and 4-1BBL participate in T lymphopoiesis associated with positive selection during recovery from acute thymic involution.

Laser capture microdissection technology

Deciphering the cellular and molecular interactions that drive disease within the tissue microenvironment holds promise for discovering drug targets of the future. In order to recapitulate the in vivo interactions through molecular analysis, one must be able to analyze specific cell populations within the context of their heterogeneous tissue microecology. Laser capture microdissection is a method to procure subpopulations of tissue cells under direct microscopic visualization. Laser capture microdissection technology can harvest the cells of interest directly or can isolate specific cells by cutting away unwanted cells to give histologically pure enriched cell populations. A variety of downstream applications exist: DNA genotyping and loss-of-heterozygosity analysis, RNA transcript profiling, cDNA library generation, mass spectrometry proteomics discovery and signal pathway profiling.

Laser-capture microdissection

Deciphering the cellular and molecular interactions that drive disease within the tissue microenvironment holds promise for discovering drug targets of the future. In order to recapitulate the in vivo interactions thorough molecular analysis, one must be able to analyze specific cell populations within the context of their heterogeneous tissue microecology. Laser-capture microdissection (LCM) is a method to procure subpopulations of tissue cells under direct microscopic visualization. LCM technology can harvest the cells of interest directly or can isolate specific cells by cutting away unwanted cells to give histologically pure enriched cell populations. A variety of downstream applications exist: DNA genotyping and loss-of-heterozygosity (LOH) analysis, RNA transcript profiling, cDNA library generation, proteomics discovery and signal-pathway profiling. Herein we provide a thorough description of LCM techniques, with an emphasis on tips and troubleshooting advice derived from LCM users. The total time required to carry out this protocol is typically 1-1.5 h.

Trafficking on serpentines: molecular insight on how maturating T cells find their winding paths in the thymus

Maintenance of the peripheral T-cell pool throughout the life requires uninterrupted generation of T cells. The majority of peripheral T cells are generated in the thymus. However, the thymus does not contain hematopoietic progenitors with unlimited self-renewing potential, and continuous production of T cells requires importation of such progenitors from the bone marrow into the thymus. Thymus-homing progenitors enter the thymus and subsequently migrate throughout distinct intrathymic microenvironments while differentiating into mature T cells. At each step of this scheduled journey, developing thymocytes interact intimately with the local stroma, which allow them to proceed to the next stage of their differentiation and maturation program. Undoubtedly, thymocyte/stroma interactions are instrumental for both thymocytes and stroma, because only their ongoing interplay generates and maintains a fully operational thymus, able to guarantee unimpaired T-cell supply. Therefore, proper T-cell generation intrinsically involves polarized cell migration during both adult life and embryogenesis when the thymus primordium develops into a functional thymus. The molecular mechanisms controlling cell migration during thymus development and postnatal T-cell differentiation are beginning to be defined. This review focuses on recent data regarding the role of cell migration in both colonization of the fetal thymus and T-cell development during postnatal life in mice.

Cyclical mobilization and gated importation of thymocyte progenitors in the adult mouse: evidence for a thymus-bone marrow feedback loop

It has recently been observed, as in the fetal thymus, that the importation of hematogenous thymocyte progenitors by the adult thymus is a gated phenomenon, whereby saturating numbers of progenitors periodically enter the thymus and occupy a finite number of intrathymic niches. In addition, the mobilization of thymocyte progenitors from the bone marrow appears to be a cyclical process that coincides temporally with the periods of thymic receptivity (open gate). It is proposed that these events are coordinated by a thymus-bone marrow feedback loop in which a wave of developing triple negative (CD3- CD4- CD8-) thymocytes interacts with stromal cells in the stratified regions of the thymus cortex to sequentially induce the release of diffusible cytokines that regulate the production, mobilization, and recruitment of thymocyte progenitors. The likely components of this feedback loop are described here, as are the properties of the intrathymic vascular gates and niches for thymocyte progenitors. The cyclical production and release of thymocyte progenitors from the bone marrow is placed in the context of a general phenomenon of oscillatory feedback regulation involving all lymphohemopoietic cell lineages. Lastly, the question of whether the gated (as opposed to the continuous) entry of thymocyte progenitors is essential for normal thymocytopoiesis in adult life is discussed.

Trafficking from the bone marrow to the thymus: a prerequisite for thymopoiesis

T-cell development in the thymus requires periodic importation of hematopoietic progenitors from the bone marrow. Such thymus settling progenitors arise from hematopoietic stem cells (HSCs) that are retained in a specific bone marrow microenvironmental niche. Vacation of this niche is required for HSC proliferation and differentiation into downstream progenitors. In order to reach the thymus, progenitors must then be mobilized from bone marrow to blood. Finally, progenitors in blood must settle in the thymus. Here we review signals and molecular interactions that are likely to play a role in trafficking from the bone marrow to the thymus, focusing on how these interactions may regulate which progenitors physiologically contribute to thymopoiesis.

Signaling via the AHR leads to enhanced usage of CD44v10 by murine fetal thymic emigrants: possible role for CD44 in emigration

Signaling via the endogenous arylhydrocarbon receptor (AHR) affects proliferation, differentiation, function and gene expression of thymocytes. In the present study, we show that treatment of mouse fetal thymus lobes in organ culture (FTOC) with AHR ligands results in (a) a drastic decrease in the emigration of thymocytes in terms of numbers and types of cells, and (b) preferential emigration of CD4-CD8- (DN) cells expressing CD44v7- and CD44v10-containing isoforms on the cell surface. Moreover, a higher level of transcripts of various other CD44 variant isoforms (CD44v) could be detected by RT-PCR in emigrants from fetal thymi exposed to either AHR-agonist during culture. Expression of CD44v9-10-containing isoforms could be exclusively detected in DN thymic emigrants. Thus, signaling via AHR by ligands alters CD44v expression patterns in a thymocyte subpopulation. Furthermore, emigration could be decreased by the addition of anti-panCD44 antibodies to TCDD-treated FTOCs, suggesting a role for CD44 in emigration.

Identification of the earliest prethymic T-cell progenitors in murine fetal blood

During murine fetal development, hemato-poietic progenitors start to colonize the thymic anlage at day 11 of gestation via blood stream. The present study aims at identifying the earliest prethymic progenitors in circulation. Here, we show that the interleukin-7 receptor-positive (IL-7R+) cells in Lin- c-kit+ population are circulating exclusively between days 11 and 14 of fetal age. Clonal analysis revealed that these IL-7R+ cells mostly contain T-cell lineage-restricted progenitors (p-Ts). The proportion of circulating p-Ts reaches 30% of the total p-Ts during these fetal ages, whereas virtually all B-cell lineage-restricted progenitors stay in the fetal liver, suggesting that the p-Ts are selectively released to the circulation. The circulating p-Ts retain the potential to generate natural killer cells and dendritic cells and exhibit extensive proliferation before the occurrence of T-cell receptor beta (TCRbeta) chain gene rearrangement. We propose that the wave of p-Ts in fetal blood disclosed by this study represents the ontogenically earliest thymic immigrants.

AlphaIIb integrin expression during development of the murine hemopoietic system

Integrin alphaIIb is a cell adhesion molecule expressed in association with beta3 by cells of the megakaryocytic lineage, from committed progenitors to platelets. While it is clear that lymphohemopoietic cells differentiating along other lineages do not express this molecule, it has been questioned whether mammalian hemopoietic stem cells (HSC) and various progenitor cells express it. In this study, we detected alphaIIb expression in midgestation embryo in sites of HSC generation, such as the yolk sac blood islands and the hemopoietic clusters lining the walls of the major arteries, and in sites of HSC migration, such as the fetal liver. Since c-Kit, which plays an essential role in the early stages of hemopoiesis, is expressed by HSC, we studied the expression of the alphaIIb antigen in the c-Kit-positive population from fetal liver and adult bone marrow differentiating in vitro and in vivo into erythromyeloid and lymphocyte lineages. Erythroid and myeloid progenitor activities were found in vitro in the c-Kit(+)alphaIIb(+) cell populations from both origins. On the other hand, a T cell developmental potential has never been considered for c-Kit(+)alphaIIb(+) progenitors, except in the avian model. Using organ cultures of embryonic thymus followed by grafting into athymic nude recipients, we demonstrate herein that populations from murine fetal liver and adult bone marrow contain T lymphocyte progenitors. Migration and maturation of T cells occurred, as shown by the development of both CD4(+)CD8- and CD4-CD8(+) peripheral T cells. Multilineage differentiation, including the B lymphoid lineage, of c-Kit(+)alphaIIb(+) progenitor cells was also shown in vivo in an assay using lethally irradiated congenic recipients. Taken together, these data demonstrate that murine c-Kit(+)alphaIIb(+) progenitor cells have several lineage potentialities since erythroid, myeloid, and lymphoid lineages can be generated.

Functional demonstration of intrathymic binding sites and microvascular gates for prothymocytes in irradiated mice

Quantitative intrathymic (i.t.) and i.v. adoptive transfer assays for prothymocytes show strict log dose saturation kinetics, consistent with a finite number of i.t. binding sites (microenvironmental niches). This inference is supported here by demonstration of competitive antagonism obeying one-on-one receptor occupancy kinetics during the establishment of thymic chimerism in irradiated adult mice. The results of primary and secondary transfer experiments suggested that hematogenous precursors (i) enter specific i.t. niches between 4 and 24 h after injection, (ii) compete reversibly with subsequently introduced precursors, (iii) establish insurmountable competition within 5-7 days, (iv) mature through the initial stages of thymocytopoiesis preceding proliferative expansion, and (v) vacate the niches between 7 and 14 days after entry. The results also suggested that, as in non-irradiated mice, prothymocyte importation in irradiated mice is a gated phenomenon. Gate closure was indicated by the inability of i.v.-, but not i.t.-, injected bone marrow (BM) cells to induce thymic chimerism when administered 7--14 days after a primary injection and gate opening by the ability of i.v.-injected BM cells to induce thymic chimerism in competition with circulating host prothymocytes. Gate closing was log dose-responsive and could be induced in individual thymic lobes by unilateral i.t. injection, whereas gate opening, which occurs bilaterally, was not initiated until most of the niches for prothymocytes had been vacated. We therefore posit the existence of a series of associated microvascular gates and microenvironmental niches that act in concert to regulate prothymocyte importation and early thymocyte differentiation.

Variant isoforms of CD44 are required in early thymocyte development

The earliest T cells homing to the thymus (CD3-CD4loCD8-) express CD117 (c-kit), CD43 (leukosialin), and the integrins CD11a (alphaL), CD11b (alphaM), CD29 (beta1), CD49f (alpha6), and CD44. Using reagents specific for CD44 variant isoforms (CD44v), we demonstrated that CD44v were expressed on virtually all early thymocytes,whereas cells carrying only the standard molecule (CD44s, not containing any variant domains), which is ubiquitously found on mature lymphocytes later, are very sparse. The expression of CD44v was closely correlated with CD43 and CD117 and was restricted to the CD3-CD4loCD8- stage. CD44v were detected on lymphocyte progenitor populations in the fetal blood, liver, thymus and spleen, as well as in the adult bone marrow. Functional studies demonstrated that only cells expressing CD44v from fetal liver and adult bone marrow could efficiently populate fetal thymic stroma and develop into mature T cells. In fetal thymic organ cultures anti-CD44v antibodies specifically blocked thymocyte development. We also present evidence that CD44v were required for the initial interaction of hematopoietic progenitor cells with the thymic stroma. Our data imply that CD44v are not only a useful marker for hematopoietic progenitors, but also play a functional role in the initiation of thymocyte development.

Cell migration and the anatomic control of thymocyte precursor differentiation

The thymus performs several essential functions during the steady-state production of T lymphocytes in adults, including expansion of the precursor pool, differentiation into multiple lineages and screening for TCRs with restricted specificities. Other than those functions attributed to the TCR, most of the factors that control these processes remain undefined. One potential mechanism for such control may be related to the movement of precursor cells between distinct anatomical compartments in the thymus. Histological studies show that the majority of CD4- CD8- cells are found in the subcapsular region. However; vascular tissues that support the migration of precursor cells into the thymus (postcapillary venules) are located deep in the tissue, near the cortico-medullary junction. This implies that blood-borne cells entering the thymus must transit outward across the cortex in order to accumulate in the SCR. Differentiation of DN cells into the CD4+ 8+ stage correlates with a reversal in polarity and migration inward, while mature cells ultimately transit the CMJ in the opposite direction of cells first entering the organ. Here we review evidence for a model in which differentiation is induced and proliferation is controlled by this progressive translocation of immature precursors through discrete stromal compartments. In addition, we attempt to summarize what is known about the molecular mechanisms that may support polarized migration of early CD4- 8- thymocytes in the adult, as well as how and where the relevant differentiative and/or proliferative signals may be compartmentalized.

Interaction of merosin (laminin 2) with very late activation antigen-6 is necessary for the survival of CD4+ CD8+ immature thymocytes

The laminin alpha2-chain is a component of merosin, a member of the laminin family molecules, which is mainly expressed in the basement membranes of striated muscle. It is known that laminin alpha2 gene (lama2) null mutant mice (dy3k/dy3k) exhibit congenital muscular dystrophy (CMD). Because the laminin alpha2-chain is also expressed in the thymus, the role of merosin in the thymus was examined. In association with the onset of muscular dystrophy, CD4+ CD8+ double-positive (DP) thymocytes disappear by apoptotic cell death, while CD4+ CD8- or CD4- CD8+ thymocytes remain. In order to study the mechanisms leading to the selective death of DP cells in the absence of merosin, the role of the interaction between very late activation antigen-6 (VLA-6), a candidate merosin ligand in the thymus, and merosin was examined. The in vitro survival of thymocytes from normal mice was maintained by the addition of either anti-VLA-6 monoclonal antibodies (mAbs) or merosin. Furthermore, when the normal thymocytes were cultured on thymic epithelial cell lines, viable DP cell recoveries on wild-type epithelial cells were better than on cells from null mutant mice. The results suggest that DP cells are more sensitive to an uncharacterized apoptotic death signal, and that survival is supported by the interaction between VLA-6 and merosin.

Mobilization of hematopoietic primitive and committed progenitor cells into blood in mice by anti-vascular adhesion molecule-1 antibody alone or in combination with granulocyte colony-stimulating factor

OBJECTIVE

One of the mechanisms for mobilization of hematopoietic stem cells and progenitor cells is alternation of adhesion molecules. We investigated the mobilization of hematopoietic progenitor cells in blood by administration of anti-vascular cell adhesion molecule (VCAM)-1 antibody (Ab) in mice.

MATERIALS AND METHODS

Twelve- to 14-week old C57BL/6J mice were injected intravenously with anti-VCAM-1 Ab and anti-very late antigen (VLA)-4 Ab at a dose of 5 mg/kg for 2 days.

RESULTS

The number of colony-forming cells (CFCs) in blood was increased 11.4-fold after anti-VCAM-1 Ab treatment, but the number of CFCs was not increased after treatment with anti-VLA-4 Ab. The number of colony-forming unit spleen (CFU-S) also was increased 21.6-fold in the peripheral blood by administration of anti-VCAM-1 Ab. The number of CFCs and CFU-S in the bone marrow of mice treated with anti-VCAM-1 Ab was decreased and that in the spleen also was decreased. On administration of recombinant human granulocyte colony-stimulating factor (125 microg/kg twice daily) with anti-VCAM-1 Ab, the numbers of CFCs and CFU-S were increased 141.8-fold and 439-fold, respectively.

CONCLUSIONS

These observations demonstrated that administration of anti-VCAM-1 Ab induced mobilization of hematopoietic progenitor cells into blood from bone marrow and spleen and that granulocyte colony-stimulating factor has synergistic effects on anti-VCAM-1 Ab-induced mobilization.

Roles of Integrins and CD44 on the Adhesion and Migration of Fetal Liver Cells to the Fetal Thymus

Adhesion and migration of mouse fetal liver (FL) cells to the thymus were investigated using cells from green fluorescent protein transgenic (GFP+) mice. FL cells from GFP+ embryos at 12 gestational days (E12) of mice were incubated with 2′-deoxyguanosine-treated fetal thymus lobe (from E14) by thymic repopulation (hanging drop) culture methods. GFP+ cells were observed in the thymus lobe at the end of the repopulation culture period. A large part of the infiltrated cells expressed CD44 until day 2 of culture on a permeable membrane, then lost the expression. CD25 expression was observed from day 1 to day 4. Around day 8, GFP+ cells became both CD4+ and CD8+. The results support the early observation of the sequential expression of CD44, CD25, and CD4/8 during the early stages of thymocyte development. When anti-CD44 mAb was added at the beginning of the repopulation culture period, GFP+ FL cells adhered to the surface of the thymus lobe but did not migrate into the thymus. Pretreatment of the thymus with hyaluronidase or hyaluronate produced results similar to the results of anti-CD44 treatment. On the other hand, the addition of anti-integrin α4 mAb inhibited adhesion to the thymus, and almost no GFP+ cells were seen on the surface of the thymus lobe. The data suggest that integrin α4 and CD44 play different roles, i.e., integrin α4 is required for the adhesion of FL cells to the thymus lobe and CD44 is required for the migration of the cells into the thymus.

Enhancement of activation-induced cell death by fibronectin in murine CD4+ CD8+ thymocytes

Development of T cells in the thymus is achieved through the interactions of thymocytes with their microenvironments. This study focused on the function of fibronectin (FN), a major extracellular matrix molecule in the thymus, in the cell death induced by activation via the T-cell antigen receptor. FN alone did not increase cell death in murine thymocytes above the baseline level, but it significantly enhanced the cell death induced by fixed anti-CD3 monoclonal antibody (mAb), especially when a high concentration of anti-CD3 mAb was used. DNA fragmentation increased in parallel with cell death, indicating that cell death was a result of the apoptosis. Fluorescence-activated cell sorter (FACS) analysis revealed that the activation-induced cell death (AICD) caused by anti-CD3 mAb alone, or by a combination of anti-CD3 mAb and FN, occurred selectively in CD4+ CD8+ thymocytes. Very late activation antigen (VLA)-4 and VLA-5 are two major ligands to FN on thymocytes. The expression of both ligands was investigated at different stages of thymocyte development. VLA-4 was predominantly expressed at the CD4- CD8- stage, and thereafter the expression was reduced, whereas VLA-5 was constantly expressed during maturation. Furthermore, the enhancing effect by FN was inhibited in the presence of the Gly-Arg-Gly-Asp-Ser-Pro (GRGDSP) peptide but not in the presence of the connecting segment-1 (CS-1) peptide, suggesting that enhancement of AICD observed in CD4+ CD8+ thymocytes is mediated through VLA-5.

Isolation of the most immature population of murine fetal thymocytes that includes progenitors capable of generating T, B, and myeloid cells

Thymus cells of murine fetuses at day 12 of gestation are exclusively of the CD3-CD4-CD8-CD44+CD25- phenotype, which is known as a hallmark of the most immature subset of thymus cells. In the present study, we show that day 12 fetal thymus (FT) cells express Fc gamma RII/ III (FcR) at a broad range of levels on their surface. The FcR+ FT cells seem to represent T lineage cells, because a large majority of them express the T lineage specific transcription factors TCF-1 and GATA-3 as well as CD3 epsilon in the cytoplasm. Also shown is that the FcR- population contains progenitors capable of developing into not only T cells but also B and myeloid cells, whereas FcR+ progenitors are mostly committed to the T lineage. These findings indicate that thymic T lineage cells express FcR on their surface at the earliest stage of differentiation, and thus FcR is a useful marker in isolating the most immature population of murine FT cells.

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.