Putative prethymic T cell precursors within the early human embryonic liver: a molecular and functional analysis

A cell atlas of human thymic development defines T cell repertoire formation

The thymus provides a nurturing environment for the differentiation and selection of T cells, a process orchestrated by their interaction with multiple thymic cell types. We used single-cell RNA sequencing to create a cell census of the human thymus across the life span and to reconstruct T cell differentiation trajectories and T cell receptor (TCR) recombination kinetics. Using this approach, we identified and located in situ CD8αα⁺ T cell populations, thymic fibroblast subtypes, and activated dendritic cell states. In addition, we reveal a bias in TCR recombination and selection, which is attributed to genomic position and the kinetics of lineage commitment. Taken together, our data provide a comprehensive atlas of the human thymus across the life span with new insights into human T cell development.

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

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

Dynamics of Thymus-Colonizing Cells during Human Development

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

Phenotypic characterization of CD3-7+ cells in developing human intestine and an analysis of their ability to differentiate into T cells

We have identified a large population of CD3(-)7(+) cells in human fetal gut. Three- and four-color flow cytometry revealed a distinct surface Ag profile on this population; the majority were negative for CD4 and CD8, whereas most of the remainder expressed the CD8alphaalpha homodimer. In contrast about half of CD3(+) cells expressed CD4 and half expressed CD8alpha. A large proportion of CD3(-)7(+) cells expressed CD56, CD94, and CD161, and whereas CD3(+) T cells also expressed CD161, they only rarely expressed CD56 or CD94. Further studies were conducted to determine whether the CD3(-)7(+) cells have the potential to differentiate into CD3(+) cells. About half of CD3(-)7(+) cells contain intracellular CD3epsilon. Rearranged TCR gamma-chains were detected in highly purified CD3(-)7(+) cells as an early molecular sign of T cell commitment, and the pattern of rearrangement with V regions spliced to the most 5' Jgamma segment is reminiscent of early thymocyte differentiation. In reaggregate thymic organ cultures, CD3(-)7(+) cells also gave rise to CD3(+) T cells. Thus, we demonstrate that the CD3(-)7(+) cells present in the human fetal gut display a distinct phenotype and are able to develop into CD3(+) T cells.

Early T cell development can be traced in rat fetal liver

The degree of T cell commitment reached by cell precursors present in the fetal liver is a controversial issue. In the present work, the occurrence of fully T cell-committed progenitors among CD45+Thy-1+CD44+ 13-day-old rat fetal liver cells was demonstrated when limiting numbers of these cells in vitro reconstituted SCID mouse fetal thymic lobes providing single lineage-containing lobes for T, natural killer or dendritic cells. In addition, expression of rat pre-TCRalpha chain mRNA was detected in the CD45+ but not in the CD45- fetal liver cells and fully rearranged TCR VBeta8-Cbeta mRNA transcripts were specifically detected in the former population, demonstrating early transcription of some rearranged TCRVBeta genes in the rat fetal liver of 13 days of gestation. Finally, fetal liver organ cultures provided low numbers of TCR gamma delta T cells and CD2+CD8+NKR-P1A- intracytoplasmic CD3+ immature T cells, which intracellularly reacted with a mAb specific to the TCRalpha Beta molecule. These results prove T, NK and DC cell lineage determination at a prethymic stage in the fetal liver.

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.

Characterization of prethymic progenitors within the chicken embryo

The thymic primordium in both birds and mammals is first colonized by cells emerging from the intra-embryonic mesenchyme but the nature of these precursors is poorly understood. We demonstrate here an early embryonic day 7 prethymic population with T lymphoid potential. Our work is a phenotypic analysis of, to date, the earliest embryonic prethymic progenitors arising in the avian para-aortic area during ontogeny. The phenotype of these cells, expressing the cell surface molecules alpha2beta1 integrin, c-kit, thrombomucin/MEP21, HEMCAM and chL12, reflects functional properties required for cell adhesion, migration and growth factor responsiveness. Importantly, the presence of these antigens was found to correlate with the recolonization of the recipient thymus following intrathymic cell transfers. These intra-embryonic cells were also found to express the Ikaros transcription factor, the molecular function of which is considered to be prerequisite for embryonic lymphoid development.

The Generation of Human γδ T Cell Repertoires During Fetal Development

The nature of how human γδ T cells are normally generated is not clear. We have used an RT-PCR assay and DNA sequencing to identify and compare δ-encoded TCRs (TCRDs) that are generated de novo in the fetal gut, liver, and thymus and to determine when, where, and how the TCRD repertoire is established during normal embryonic development. Rearranged TCRDV genes are first expressed outside of the thymus in the liver and primitive gut between 6 and 9 wk gestation. Although DV1Rs and/or DV2Rs predominated, differences in the pattern of TCRDV gene rearrangement and transcription in each tissue during ontogeny were identified. Specific, DV2-encoded TCRs are highly conserved throughout ontogeny in the tissues from the same and between genetically distinct donors. Although the thymic and intestinal γδ T cell repertoires partially overlap early in development, they diverge and become nonoverlapping during the second trimester, and the generation of the intestinal γδ T cell repertoire is characterized by differences in the processing of DV1Rs and DV2Rs. Whereas the structural diversity of DV1Rs progressively increases during gut development up to birth, DV2Rs have limited structural diversity throughout ontogeny. Together, our findings provide evidence for the ability of different fetal tissues to support the development of γδ T cells.

Human thymic stroma supports human natural killer (NK) cell development from immature progenitors

NK cells are lymphocytes which exhibit spontaneous cytotoxicity against a variety of target cells, including cancer cells. Mature NK and T cells may derive from a common precursor which differentiates into T or NK cells depending on the microenvironment. We evaluated the effect of human fetal thymic stroma on human CD34+Lin- progenitors. The culture medium was supplemented with human AB serum with or without interleukin-2 (IL2; 1000 U/ml) and interleukin-7 (IL7; 1000 U/ml). After 3 weeks of culture, CD45/56 cells were detected by flow cytometry and their activity was tested against K562 targets. In cultures with IL2 the percentage of CD56-positive cells was much higher in the Transwell cultures (60.8 +/- 12.5% from CD34+Lin-DR+ and 51% from CD34+Lin-progenitors) than in adherent cultures (25 +/- 21.9% from CD34+Lin-DR+ and 25.3 +/- 9.5% from CD34+Lin-progenitors) or suspension cultures (23 +/- 21.4% from CD34+Lin-DR+ progenitors and 43.1 +/- 14.2% from CD34+Lin-progenitors). Cytolytic activity as measured by K562 lysis was also higher in Transwell cultures with IL2. NK cells were also obtained in cultures without factors or supplemented with IL7, but in smaller numbers. These data indicate that NK cells can be obtained in vitro by coculture of immature hematopoietic progenitors with thymic stromal cells and that IL2 appears to strongly favor their development in the absence of stromal contact. This would indicate a direct inhibitory effect of the thymic stroma on NK progenitors.

FLT-3 Ligand and Marrow Stroma-Derived Factors Promote CD3γ, CD3δ, CD3ζ, and RAG-2 Gene Expression in Primary Human CD34+LIN−DR− Marrow Progenitors

We hypothesize that early lymphoid commitment from primitive hematopoietic marrow progenitors is governed by signals from the marrow microenvironment leading to sequential induction of lineage-specific genes. Using expression of lymphoid genes as markers of differentiation, we characterize a highly purified population (>99.8% by double sorting) of primary human CD34+Lin−DR− progenitors. This population was then used to evaluate the effects of supplemental cytokines (interleukin-2 [IL-2], IL-3, IL-7, c-kit ligand), FLT-3 ligand (FL), and stroma-derived factors on lymphoid differentiation in vitro. CD3, RAG-1, Ikaros, CD10, and TdT transcripts were detected in the starting CD34+Lin−DR− population. By contrast, CD3γ, CD3δ, CD3ζ, and RAG-2 transcripts were not present in any samples tested. The presence of supplemental cytokines alone at culture initiation permitted stimulation of the expression of CD3ζ, but not of CD3γ or CD3δ. However, when FL and stroma-derived factors were added to cytokines, CD3 gene expression was induced in all samples. The predominant CD3 transcripts induced by optimal culture conditions were alternatively spliced isoforms lacking transmembrane sequences (CD3δ and CD3γ) and portions of the intracellular and extracellular domains (CD3γ). The combination of cytokines, FL, and stromal factors also provided a potent stimulus for RAG-2 gene expression. These findings show that FL in combination with stroma-derived factors provide important signals to promote early events required for lymphoid differentiation.

FLT-3 Ligand and Marrow Stroma-Derived Factors Promote CD3γ, CD3δ, CD3ζ, and RAG-2 Gene Expression in Primary Human CD34+LIN−DR− Marrow Progenitors

We hypothesize that early lymphoid commitment from primitive hematopoietic marrow progenitors is governed by signals from the marrow microenvironment leading to sequential induction of lineage-specific genes. Using expression of lymphoid genes as markers of differentiation, we characterize a highly purified population (>99.8% by double sorting) of primary human CD34+Lin−DR− progenitors. This population was then used to evaluate the effects of supplemental cytokines (interleukin-2 [IL-2], IL-3, IL-7, c-kit ligand), FLT-3 ligand (FL), and stroma-derived factors on lymphoid differentiation in vitro. CD3, RAG-1, Ikaros, CD10, and TdT transcripts were detected in the starting CD34+Lin−DR− population. By contrast, CD3γ, CD3δ, CD3ζ, and RAG-2 transcripts were not present in any samples tested. The presence of supplemental cytokines alone at culture initiation permitted stimulation of the expression of CD3ζ, but not of CD3γ or CD3δ. However, when FL and stroma-derived factors were added to cytokines, CD3 gene expression was induced in all samples. The predominant CD3 transcripts induced by optimal culture conditions were alternatively spliced isoforms lacking transmembrane sequences (CD3δ and CD3γ) and portions of the intracellular and extracellular domains (CD3γ). The combination of cytokines, FL, and stromal factors also provided a potent stimulus for RAG-2 gene expression. These findings show that FL in combination with stroma-derived factors provide important signals to promote early events required for lymphoid differentiation.

The molecular basis of natural killer (NK) cell recognition and function

Natural Killer cells are likely to play an important role in the host defenses because they kill virally infected or tumor cells but spare normal self-cells. The molecular mechanism that explains why NK cells do not kill indiscriminately has recently been elucidated. It is due to several specialized receptors that recognize major histocompatibility complex (MHC) class I molecules expressed on normal cells. The lack of expression of one or more HLA class I alleles leads to NK-mediated target cell lysis. Different types of receptors specific for groups of HLA-C, HLA-B, and, very recently, HLA-A alleles have been identified. While in most instances, they function as inhibitory receptors, an activatory form of the HLA-C-specific receptors has been identified in some donors. Molecular cloning of HLA-C-, HLA-B- or HLA-A-specific receptors has revealed new members of the immunoglobulin superfamily with two or three Ig-like domains, respectively, in their extracellular portion. While the inhibitory form is characterized by a long cytoplasmic tail associated with a non-polar transmembrane portion, the activatory one has a short tail associated with a Lys-containing transmembrane portion. Thus, these human NK receptors are different from the murine Ly49, that is a type II transmembrane protein characterized by a C-type lectin domain. A subset of activated T lymphocytes expresses NK-type class I-specific receptors. These receptors exert an inhibiting activity on T cell receptor-mediated functions and may provide an important mechanism of downregulation of T cell responses.

Regulation of pre-T cell receptor (pT alpha-TCR beta) gene expression during human thymic development

In murine T cell development, early thymocytes that productively rearrange the T cell receptor (TCR) beta locus are selected to continue maturation, before TCR alpha expression, by means of a pre-TCR alpha- (pT alpha-) TCR beta heterodimer (pre-TCR). The aim of this study was to identify equivalent stages in human thymocyte development. We show here that variable-diversity-joining region TCR beta rearrangement and the expression of full-length TCR beta transcripts have been initiated in some immature thymocytes at the TCR alpha/beta- CD4+CD8- stage, and become common in a downstream subset of TCR alpha/beta- CD4+CD8+ thymocytes that is highly enriched in large cycling cells. TCR beta chain expression was hardly detected in TCR alpha/beta- CD4+CD8- thymocytes, whereas cytoplasmic TCR beta chain was found in virtually all TCR alpha/beta- CD4+CD8+ blasts. In addition, a TCR beta complex distinct from the mature TCR alpha/beta heterodimer was immunoprecipitated only from the latter subset. cDNA derived from TCR alpha/beta- CD4+CD8+ blasts allowed us to identify and clone the gene encoding the human pT alpha chain, and to examine its expression at different stages of thymocyte development. Our results show that high pT alpha transcription occurs only in CD4+CD8- and CD4+CD8+ TCR alpha/beta- thymocytes, whereas it is weaker in earlier and later stages of development. Based on these results, we propose that the transition from TCR alpha/beta- CD4+CD8- to TCR alpha/beta- CD4+CD8+ thymocytes represents a critical developmental stage at which the successful expression of TCR beta promotes the clonal expansion and further maturation of human thymocytes, independent of TCR alpha.

Alpha 6 integrins participate in pro-T cell homing to the thymus

During embryogenesis, colonization of the thymic rudiment by hemopoietic progenitor cells depends on the adhesion of these cells to the jugular endothelium. Previously, we showed that progenitor T cells (pro-T cells) interact with alpha 6 integrins present on vascular endothelium. Here, we demonstrate that anti-alpha 6 integrin antibodies reduced the number of thymocytes up to 80% in a congenic mouse model for thymus colonization by pro-T cells. In organotypic thymus cultures, the anti-alpha 6 integrin antibodies did not influence T cell development and proliferation. From this, we conclude that alpha 6 integrin participates in thymus homing. During mouse thymus ontogeny, alpha 6 integrin mRNA and protein expression was found as early as day 10 of development; at day 11, perithymic endothelial cells were alpha 6 integrin positive. Two alpha 6 integrin mRNA exist which are produced by alternative exon usage. The longer form, alpha 6 integrin, predominates during early embryonic stages, while the shorter alpha 6A form was present later during development. Although alpha 6 integrins can be displayed by immature thymocytes, strongest expression was found on intra- and perithymic vascular endothelium. These data suggest that alpha 6 integrins are involved in the homing of pro-T cells to the developing thymus by mediating adhesion of pro-T cells to the vascular endothelium.

In-vitro models of stroma-dependent lymphopoiesis

The differentiation of hematopoietic stem cells into lymphocytes can be replicated ex vivo under the inductive influence of the stromal cells that frame the bone marrow and thymus. We summarize hereafter the development of culture systems where lymphopoiesis-supporting cell compartments are maintained in either their normal three-dimensional arrangement, in organotypic culture, or as culture dish-adherent monolayers and review the recent and current uses of those in-vitro models to investigate T- and B-cell differentiation in mouse and man.

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

To determine events that transpire during the earliest stages of human T cell development, we have studied fetal tissues before (7 wk), during (8.2 wk), and after (9.5 wk to birth) colonization of the fetal thymic rudiment with hematopoietic stem cells. Calculation of the approximate volumes of the 7- and 8.2-wk thymuses revealed a 35-fold increase in thymic volumes during this time, with 7-wk thymus height of 160 microM and volume of 0.008 mm3, and 8.2-wk thymus height of 1044 microM and volume of 0.296 mm3. Human thymocytes in the 8.2-wk thymus were CD4+ CD8 alpha+ and cytoplasmic CD3 epsilon+ cCD3 delta+ CD8 beta- and CD3 zetta-. Only 5% of 8-wk thymocytes were T cell receptor (TCR)-beta+, < 0.1% were TCR-gamma+, and none reacted with monoclonal antibodies against TCR-delta. During the first 16 wk of gestation, we observed developmentally regulated expression of CD2 and CD8 beta (appearing at 9.5 wk), CD1a,b, and c molecules (CD1b, then CD1c, then CD1a), TCR molecules (TCR-beta, then TCR-delta), CD45RA and CD45RO isoforms, CD28 (10 wk), CD3 zeta (12-13 wk), and CD6 (12,75 wk). Whereas CD2 was not expressed at the time of initiation of thymic lymphopoiesis, a second CD58 ligand, CD48, was expressed at 8.2 wk, suggesting a role for CD48 early in thymic development. Taken together, these data define sequential phenotypic and morphologic changes that occur in human thymus coincident with thymus colonization by hematopoietic stem cells and provide insight into the molecules that are involved in the earliest stages of human T cell development.

Tracing the expression of CD7 and other antigens during T- and myeloid-cell differentiation in the human fetal liver and thymus

During the last decade, the function/s of the cell membrane CD7 antigen have been investigated in human mature T and NK cells, showing the direct involvement of this molecule in multiple effector functions related with activation, proliferation, production of cytokines and modification of adhesion properties. The CD7 glycoprotein is not only expressed by mature lymphoid cells, but also by early hematopoietic progenitors and several types of leukemias, suggesting a role of CD7 during hematopoiesis. However, the function of CD7 in the early stages of hematopoietic development has not yet been elucidated. CD7 has been classically considered the earliest T-cell specific marker. This assumption was based on data indicating the presence of CD45+CD7+CD3-CD4-CD8- cells in the human embryonic/fetal liver at the gestational age at which the thymic rudiment is colonized by T-cell progenitors. In the present article, we review recent results obtained by several groups concerning the expression of CD7 and various other cell surface antigens by T-, B- and myeloid-cell progenitors generated in the adult bone marrow and fetal liver. In addition, we present an hypothetical model of hematopoiesis in the fetal liver and thymus.

The development of T and non-T cell lineages from CD34+ human thymic precursors can be traced by the differential expression of CD44

In addition to T-lineage cells, a small proportion of hematopoietic non-T cells are present in the human postnatal thymus. However, the origin of this minor non-T cell thymic compartment is presently unknown. In this study we have analyzed the developmental potential of the earliest human intrathymic precursors, characterized as CD34+ cells expressing intermediate levels of CD44. We show that these CD34+CD44int thymocytes cultured with interleukin 7 were able to develop simultaneously into both T- and non-T (monocytes and dendritic cells) -lineage cells. Both developmental pathways progress through a CD1+CD4+ intermediate stage, currently believed to be the immediate precursor of double positive thymocytes. However, separate progenitors for either T or non-T cells could be characterized within CD1+CD4+ thymocytes by their opposite expression of CD44. Downregulated levels of CD44 identified CD1+CD4+ T-lineage precursors, whereas CD44 upregulation occurred on CD1+CD4+ intermediates that later differentiated into non-T cells. Therefore, commitment of human early intrathymic precursors to either T or non-T cell lineages can be traced by the differential expression of the CD44 receptor.

Expression of a wide T cell receptor V beta repertoire in human T lymphocytes derived in vitro from embryonic liver cell precursors

As shown recently, CD3+/TcR+ functional T lymphocytes can be derived in culture from embryonic liver cell precursors at a gestational age (6-8 weeks) preceding the colonization of the epithelial thymus. In this report, we analyzed the V beta repertoire of T lymphocytes derived from embryonic liver by applying a quantitative reverse transcriptase-polymerase chain reaction technique. To this end, oligonucleotide primers for C alpha or the various human V beta have been used to study both freshly derived embryonic liver cell suspensions and CD3+/TcR+ populations derived after approximately 6 weeks upon stimulation with 1% phytohemagglutinin and culture in 100 units/ml recombinant interleukin-2. In order to exclude possible contaminations with mother-derived T lymphocytes, only T cells displaying both X and Y chromosomal sequences (i.e. derived from male embryos) were further analyzed. While neither C alpha nor the various V beta could be detected in fresh liver cells, C alpha and the large majority of V beta were detected in in vitro cultured populations. The levels of the various V beta expressed by embryo-derived T cells was similar to that detected in adult peripheral blood-derived T lymphocytes. These experiments indicate that the immature liver precursors can potentially give rise in vitro to T cells which express a wide V beta repertoire and may provide a suitable in vitro system for the analysis of the selection processes mediated by either major histocompatibility complex antigen or superantigens.

Early stages in human and mouse T-cell development

One important question in lymphopoiesis is where stem cells commit to T-, B- and natural killer (NK)-cell lineages. Recent findings in human and mouse systems suggest that the thymus is seeded by a yet uncommitted progenitor cell. The earliest murine thymic progenitor cells have the capacity to develop into B, T and NK cells when introduced into the appropriate microenvironment. The mechanisms underlying T-cell commitment are unknown, but cytokines might be involved. The gamma-chain of the interleukin (IL)-2 receptor seems to play a role in development of T and NK cells, but the current data argue against a critical role for IL-2 in T- and NK-cell development. This suggests that the IL-2 receptor gamma-chain is part of a receptor for another cytokine, important for T- and NK-cell development. IL-7 might be involved in regulating T-cell receptor rearrangements and in proliferation of cells within the thymus.

Usage of TCRAV and TCRBV gene families in human fetal and adult TCR rearrangements

We have investigated fetal and adult T-cell receptor (TCR) A and B V-gene repertoires both by fluorescence-activated cell sorter (FACS) analysis with the available TCR V region-specific mAbs and by the polymerase chain reaction (PCR) with TCR V gene family-specific oligonucleotides. Among the low number of CD3+ T cells, most of the TCR V regions tested for could be detected by FACS analysis in liver, bone marrow, and spleen derived from a 14-week-old fetus and two 15-week-old fetuses. Similarly, the PCR analysis showed that the majority of the TCRAV and TCRBV families were expressed in the peripheral organs of the 13-week-old fetus, although an apparent absence of particular TCR V families was found in liver and bone marrow. This was most probably the consequence of the low number of CD3+ T cells in these organs. In 17-week-old fetal thymi the level of expression of some TCRAV and TCRBV gene families, in particular those that contain a single member, was lower compared to post-partum thymi and adult peripheral blood mononuclear cells. The combined data of FACS and PCR analysis demonstrate that TCR V genes belonging to the majority of TCR V gene families can be used in TCR alpha and beta chain rearrangements during early human fetal life. Our data also suggest that the expression levels of some of the single member TCR V gene families may be influenced by the developmental stage.

T cell receptor gamma cDNA in human fetal liver and thymus: variable regions of gamma chains are restricted to V gamma I or V9, due to the absence of splicing of the V10 and V11 leader intron

Although complete in-frame transcripts of the human T cell receptor gamma V10 and V11 genes have been described, the corresponding gamma chains have never been found in gamma delta T cell receptors. In this study, we show that the leader intron of all V10 and V11 cDNA isolated from fetal thymus, fetal liver and adult peripheral blood lymphocytes are unspliced. We demonstrate that, due to the absence of splicing, V10 and V11 are pseudogenes and cannot be expressed in gamma chains. They are the first pseudogenes of this type described in a rearranging T cell receptor/immunoglobulin locus. Therefore the gamma repertoire at the protein level is limited to subgroup V gamma I and to V9. By analysis of the gamma polymerase chain reaction products from total cDNA, we find that the gamma locus is active in early ontogeny (8 weeks), as shown by the presence of rearranged V9 and V10 gene transcripts in the liver. At 13 weeks, the V gamma I genes as well as V9 and V10 have undergone productive rearrangements in the liver, and in the thymus. Most rearrangements, if not all, involve the T cell receptor gamma C1 region (JP1, JP, J1 segments) in both tissues, confirming the accessibility of the C1 region in early stages of development.

Extrathymic differentiation of T lymphocytes and natural killer cells from human embryonic liver precursors

Liver cells were isolated on Ficoll/Hypaque gradients from embryos or fetuses at 6-10 weeks of gestation; 2-20% of the cells expressed CD45 or HLA class I surface antigens and 2-6% expressed CD7. Other T- or natural-killer (NK)-cell-lineage-specific markers were undetectable. Liver-cell suspensions cultured in the presence of phytohemagglutinin and recombinant interleukin 2 gave rise to large proportions of CD3+ lymphocytes expressing either alpha/beta or gamma/delta T-cell receptors. This occurred not only in bulk cultures but also when cells were cloned under limiting dilution conditions. Importantly, these figures were obtained also in embryos at 6-8 weeks of gestation, which is before colonization of the thymic rudiment by T-cell precursors. When the same liver-cell suspensions were cultured in the presence of irradiated H9 cells and recombinant interleukin 2 (either in bulk cultures or under cloning conditions), large proportions of cells (or clones) expressed surface CD16 and CD56 antigens and displayed a strong cytolytic activity against both NK-sensitive (K562) and NK-resistant (M14) target cells. In addition, liver-derived T or NK cells expressed functional receptor molecules since they could be activated via either CD3/T-cell receptor or CD16 surface antigens, respectively. Further fractionation of liver cells on the basis of CD45 antigen expression indicated that only CD45+ cells could give rise to T or NK cells in culture. Thus, CD45 can be used as a marker for identification of an early liver-cell population containing T- and NK-cell precursors. That T or NK cells were derived from male embryos and not from the mother was shown by PCR amplification of X and Y chromosomal sequences. Our present data may offer an in vitro model for extrathymic embryonic T-cell maturation that can be used to examine fundamental aspects of human T-cell development and function.