CD4 and CD8 expression and T cell antigen receptor gene rearrangement in early intrathymic precursor cells

From stem cell to T cell: one route or many?

T cells developing in the adult thymus ultimately derive from haematopoietic stem cells in the bone marrow. Here, we summarize research into the identity of the haematopoietic progenitors that leave the bone marrow, migrate through the blood and settle in the thymus to generate T cells. Accumulating data indicate that various different bone-marrow progenitors are T-cell-lineage competent and might contribute to intrathymic T-cell development. Such developmental flexibility implies a mechanism of T-cell-lineage commitment that can operate on a range of T-cell-lineage-competent progenitors, and further indicates that only those T-cell-lineage-competent progenitors able to migrate to, and settle in, the thymus should be considered physiological T-cell progenitors.

Intrathymic effect of acute pathogenic SHIV infection on T-lineage cells in newborn macaques

We intrarectally infected newborn macaques with a pathogenic simian/human immunodeficiency virus (SHIV) that induced rapid and profound CD4 (+) T cell depletion, and examined the early effects of this SHIV on the thymus. After intrarectal infection, viral loads were much higher in the thymus than in other lymphoid tissues in newborns. In contrast, no clear difference was seen in the viral loads of different tissues in adults. Histological and immunohistochemical observations showed severe thymic involution. Depletion of CD4 (+) thymocytes began in the medulla at 2 weeks post infection and spread over the whole thymus. After in vivo infection, the CD2 (+) subpopulation, which represents a relatively later stage of T cell progenitors, was selectively reduced and development of thymocytes from CD3 (-) CD4 (-) CD8 (-) cells to CD4 (+) CD8 (+) cells was impaired. These results suggest that profound and irreversible loss of CD4 (+) cells that are observed in the peripheral blood of SHIV-infected monkeys are due to destruction of the thymus and impaired thymopoiesis as a result of SHIV infection in the thymus.

The lymphoid past of mouse plasmacytoid cells and thymic dendritic cells

There has been controversy over the possible lymphoid origin of certain dendritic cell (DC) subtypes. To resolve this issue, DC and plasmacytoid pre-DC isolated from normal mouse tissues were analyzed for transient (mRNA) and permanent (DNA rearrangement) markers of early stages of lymphoid development. About 27% of the DNA of CD8(+) DC from thymus, and 22-35% of the DNA of plasmacytoid pre-DC from spleen and thymus, was found to contain IgH gene D-J rearrangements, compared with 40% for T cells. However, the DC DNA did not contain IgH gene V-D-J rearrangements nor T cell Ag receptor beta gene D-J rearrangements. The same DC lineage populations containing IgH D-J rearrangements expressed mRNA for CD3 chains, and for pre-T alpha. In contrast, little of the DNA of the conventional DC derived from spleen, lymph nodes, or skin, whether CD8(+) or CD8(-), contained IgH D-J rearrangements and splenic conventional DC expressed very little CD3 epsilon or pre-T alpha mRNA. Therefore, many plasmacytoid pre-DC and thymic CD8(+) DC have shared early steps of development with the lymphoid lineages, and differ in origin from conventional peripheral DC.

T-lineage specification and commitment: a gene regulation perspective

T lymphocytes originate from pluripotent precursors and undergo lasting commitment to the T cell developmental fate during their processing in the thymus. Commitment includes both the acquisition of essential T cell characteristics and the foreclosing of other developmental options. Gain of T cell characteristics is probably mediated by separate mechanisms, at least in detail, from loss of alternative developmental potentials. Programmed shifts in survival requirements make changes irreversible. Here we review the current evidence identifying the regulatory components of this commitment pathway, and the first hints of how they work together. Roles for PU.1, GATA-3, and their target genes are highlighted.

Development of thymic and splenic dendritic cell populations from different hemopoietic precursors

The antigen-presenting dendritic cells (DCs) found in mouse lymphoid tissues are heterogeneous. Several types of DCs have been identified on the basis of the expression of different surface molecules, including CD4, CD8alpha, and DEC-205. Previous studies by the authors showed that the mouse intrathymic lymphoid-restricted precursors (lin(-)c-kit(+)Thy-1(low)CD4(low)) can produce DCs in the thymus and spleen upon intravenous transfer, suggesting a lymphoid origin of these DCs. In the current study, the potential for DC production by the newly identified bone marrow (BM) common lymphoid precursors (CLPs), common myeloid precursors (CMPs), and committed granulocyte and macrophage precursors was examined. It was found that both the lymphoid and the myeloid precursors had the potential to produce DCs. All the different DC populations identified in mouse thymus and spleen could be produced by all these precursor populations. However, CLPs produced predominantly the CD4(-)CD8alpha(+) DCs, whereas CMPs produced similar numbers of CD4(-)CD8alpha(+) and CD4(+)CD8alpha(-) DCs, although at different peak times. On a per cell basis, the CLPs were more potent than the CMPs at DC production, but this may have been compensated for by an excess of CMPs over CLPs in BM. Overall, this study shows that the expression of CD8alpha does not delineate the hemopoietic precursor origin of DCs, and the nature of the early precursors may bias but does not dictate the phenotype of the DC product.

Characterization of thymus-seeding precursor cells from mouse bone marrow

The nature of the cells that seed the thymus of an irradiated recipient after intravenous (IV) transfer of bone marrow (BM) cells was investigated using 2 approaches. First, direct entry of a small number of donor BM cells into the thymus was tracked using a Ly-5 marker. Second, secondary IV transfer of the seeded thymus cells into a secondary recipient was used as an assay for precursor activity. A range of cell types was found to enter the recipient thymus initially, including B-lineage cells and myeloid cells, but T precursors were undetectable by flow cytometry over the first few days. Although all cells initially entering the thymus proliferated, no sustained thymus reconstitution was seen until day 4, when recognizable T-lineage precursors began to appear. The secondary transfer assays revealed the presence of lymphoid precursors in the recipient thymus, including T, NKT, NK, and B precursor activity, with a notable early burst of B-lineage generative capacity. There was no evidence of sustained myeloid precursor or multipotent stem cell activity, even though these were seen if BM cells were injected directly into the recipient thymus rather than introduced into the bloodstream. It is concluded that even though many cell types may initially enter an irradiated thymus, the thymus acts as a sieve, allowing lymphoid precursors, but not multipotent stem cells, to seed the environmental niches that permit selected precursor cell development and thymus reconstitution. (Blood. 2001;98:696-704)

Stepwise specification of lymphocyte developmental lineages

B and T lymphocytes differentiate from multipotent precursors through distinct specification and commitment steps. New findings on the unique role of Pax5 in B-lineage commitment, dichotomous action of Notch signaling in B versus T cell development, and the gene expression changes comprising T-lineage specification and commitment now illuminate this process.

The interleukin 7 receptor is required for T cell receptor gamma locus accessibility to the V(D)J recombinase

Defects in the interleukin (IL)-7 signal transduction pathway lead to severe immunodeficiency in humans and in mice. In IL-7 receptor-deficient (IL-7R-/-) mice, lymphoid precursors show a reduced survival rate and variable/diversity/joining region V(D)J recombination is variously affected in different loci, being arrested in the T cell receptor (TCR)-gamma locus, aberrant in the immunoglobulin heavy chain (IgH) locus, and delayed in the TCR-beta locus. Here, we analyze the recombination defect of the TCR-gamma locus. Using ligation-mediated polymerase chain reaction, we sought intermediates of the recombination process. In the absence of the IL-7 signal, no initiation of recombination of the TCR-gamma locus was observed, whereas recombination intermediates at the TCR-beta locus could be detected. Thus, the failure to rearrange the TCR-gamma locus is due to a failure to initiate cleavage rather than a failure to religate broken DNA ends. V(D)J recombination was previously thought to begin at the pro-T2 stage of T cell development after the arrest of IL-7R-/- thymocytes at the pro-T1 stage. However, here we show that both TCR-gamma and -beta recombination intermediates are readily detectable in normal T1 cells, but only TCR-beta intermediates were detected in IL-7R-/- T1 cells, supporting a mechanistic role for IL-7 in TCR-gamma locus rearrangement. Since reduced recombination activating gene (rag) expression has been reported in the absence of the IL-7 signal, we directly tested whether the TCR-gamma locus is accessible to cleavage by recombinant Rag proteins in vitro. We found a reduction in chromatin accessibility for Rag-mediated cleavage in IL-7R-/- thymocytes compared with wild-type. Thus, IL-7 controls recombination at the TCR-gamma locus by regulating locus accessibility.

Definition of a T-cell receptor beta gene core enhancer of V(D)J recombination by transgenic mapping

V(D)J recombination in differentiating lymphocytes is a highly regulated process in terms of both cell lineage and the stage of cell development. Transgenic and knockout mouse studies have demonstrated that transcriptional enhancers from antigen receptor genes play an important role in this regulation by activating cis-recombination events. A striking example is the T-cell receptor beta-chain (TCRbeta) gene enhancer (Ebeta), which in the mouse consists of at least seven nuclear factor binding motifs (betaE1 to betaE7). Here, using a well-characterized transgenic recombination substrate approach, we define the sequences within Ebeta required for recombination enhancer activity. The Ebeta core is comprised of a limited set of motifs (betaE3 and betaE4) and an additional previously uncharacterized 20-bp sequence 3' of the betaE4 motif. This core element confers cell lineage- and stage-specific recombination within the transgenic substrates, although it cannot bypass the suppressive effects resulting from transgene integration in heterochromatic centromeres. Strikingly, the core enhancer is heavily occupied by nuclear factors in immature thymocytes, as shown by in vivo footprinting analyses. A larger enhancer fragment including the betaE1 through betaE4 motifs but not the 3' sequences, although active in inducing germ line transcription within the transgenic array, did not retain the Ebeta recombinational activity. Our results emphasize the multifunctionality of the TCRbeta enhancer and shed some light on the molecular mechanisms by which transcriptional enhancers and associated nuclear factors may impact on cis recombination, gene expression, and lymphoid cell differentiation.

RAG2 is regulated differentially in B and T cells by elements 5' of the promoter

To study RAG2 gene regulation in vivo, we developed a blastocyst complementation method in which RAG2-deficient embryonic stem cells were transfected with genomic clones containing RAG2 and then assessed for their ability to generate lymphocytes. A RAG2 genomic clone that contained only the RAG2 promoter sequences rescued V(D)J recombination in RAG2-deficient pro-B cell lines, but did not rescue development of RAG2-deficient lymphocytes in vivo. However, inclusion of varying lengths of sequences 5' of the RAG2 promoter generated constructs capable of rescuing only in vivo B cell development, as well as other constructs that rescued both B and T cell development. In particular, the 2-kb 5' region starting just upstream of the RAG2 promoter, as well as the region from 2-7 kb 5', could independently drive B cell development, but not efficient T cell development. Deletion of the 2-kb 5' region from the murine germ line demonstrated that this region was not required for RAG expression sufficient to generate normal B or T cell numbers, implying redundancy among 5' elements. We conclude that RAG2 expression in vivo requires elements beyond the core promoter, that such elements contribute to differential regulation in the B vs. T lineages, and that sequences sufficient to direct B cell expression are located in the promoter-proximal 5' region.

Pre-T cell receptor (TCR) and TCR-controlled checkpoints in T cell differentiation are set by Ikaros

T cell differentiation relies on pre-T cell receptor (TCR) and TCR signaling events that take place at successive steps of the pathway. Here, we show that two of these T cell differentiation checkpoints are regulated by Ikaros. In the absence of Ikaros, double negative thymocytes can differentiate to the double positive stage without expression of a pre-TCR complex. Subsequent events in T cell development mediated by TCR involving transition from the double positive to the single positive stage are also regulated by Ikaros. Nonetheless, in Ikaros-deficient thymocytes, the requirement of pre-TCR expression for expansion of immature thymocytes as they progress to the double positive stage is still maintained, and the T cell malignancies that invariably arise in the thymus of Ikaros-deficient mice are dependent on either pre-TCR or TCR signaling. We conclude that Ikaros regulates T cell differentiation, selection, and homeostasis by providing signaling thresholds for pre-TCR and TCR.

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.

RAG2:GFP knockin mice reveal novel aspects of RAG2 expression in primary and peripheral lymphoid tissues

We generated mice in which a functional RAG2:GFP fusion gene is knocked in to the endogenous RAG2 locus. In bone marrow and thymus, RAG2:GFP expression occurs in appropriate stages of developing B and T cells as well as in immature bone marrow IgM+ B cells. RAG2:GFP also is expressed in IgD+ B cells following cross-linking of IgM on immature IgM+ IgD+ B cells generated in vitro. RAG2:GFP expression is undetectable in most immature splenic B cells; however, in young RAG2:GFP mice, there are substantial numbers of splenic RAG2:GFP+ cells that mostly resemble pre-B cells. The latter population decreases in size with age but reappears following immunization of older RAG2:GFP mice. We discuss the implications of these findings for current models of receptor assembly and diversification.

The linkage between T-cell and dendritic cell development in the mouse thymus

Thymic dendritic cells (DC) mediate negative selection at a relatively late stage of the T-cell developmental pathway. We present evidence that the development of thymic DC and of T-lineage cells is linked via a common precursor at an early stage of thymocyte development. T-lineage precursor populations from the adult mouse thymus, prior to T-cell receptor gene rearrangement, display a capacity to produce DC as well as T cells in the thymus, and are very efficient precursors of DC in culture. These lymphoid/DC precursors have little capacity to form myeloid cells, indicating that thymic DC are a lymphoid-related rather than myeloid-related lineage. In contrast to myeloid-related DC, granulocyte-macrophage colony-stimulating factor is not required for the development of these lymphoid-related DC in vivo or in vitro. DC can develop in mutant mice lacking mature T cells, provided the common precursors are present. However, in mutant mice lacking functional Ikaros transcription factors, there are deficiencies in lymphoid precursor cells, in mature lymphoid cells and in DC.

Induction of insulitis by glutamic acid decarboxylase peptide-specific and HLA-DQ8-restricted CD4(+) T cells from human DQ transgenic mice

Insulin-dependent diabetes mellitus in humans is linked with specific HLA class II genes, e.g., HLA-DQA1*0301/ DQB1*0302 (DQ8). To investigate the roles of HLA-DQ8 molecules and glutamic acid decarboxylase (GAD) in disease development, we generated DQ8(+)/I-Abo transgenic mice expressing functional HLA-DQ8 molecules and devoid of endogenous mouse class II. DQ8(+)/I-Abo mice produced antigen-specific antibodies and formed germinal centers after immunization with GAD65 peptides. Two GAD peptide-specific (247-266 and 509-528), DQ8 restricted Th1 CD4(+) T cell lines, were generated from immunized DQ8(+)/I-Abo mice. They induced severe insulitis after adoptive transfer into transgene positive (but not negative) mice who were treated with a very low dose of streptozotocin that alone caused no apparent islet pathology. In addition to CD4, islet mRNA from these mice also showed expression of CD8, IFNgamma, TNFalpha, Fas, and Fas ligand. Our data suggest that a mild islet insult in the presence of HLA-DQ8 bearing antigen-presenting cells promotes infiltration of GAD peptide reactive T cells into the islet.

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.

An alternate pathway for T cell development supported by the bone marrow microenvironment: recapitulation of thymic maturation

In the principal pathway of alpha/beta T cell maturation, T cell precursors from the bone marrow migrate to the thymus and proceed through several well-characterized developmental stages into mature CD4+ and CD8+ T cells. This study demonstrates an alternative pathway in which the bone marrow microenvironment also supports the differentiation of T cell precursors into CD4+ and CD8+ T cells. The marrow pathway recapitulates developmental stages of thymic maturation including a CD4+CD8+ intermediary cell and positive and negative selection, and is strongly inhibited by the presence of mature T cells. The contribution of the marrow pathway in vivo requires further study in mice with normal and deficient thymic or immune function.

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.

Early Intrathymic Precursor Cells Acquire a CD4low Phenotype

CD4low cells are a population of lymphoid lineage-restricted progenitor cells representing the earliest precursors present in the adult thymus. Paradoxically, thymic progenitors with a similar phenotype in fetal mice and adult RAG-2-deficient (RAG-2−/−) mice lack this characteristic low-level expression of CD4. We now show that radiation-induced differentiation of CD4+CD8+ double positive thymocytes in RAG-2−/− mice results in the appearance of low levels of CD4 on thymocytes that are phenotypically identical to CD4low progenitor cells present in the normal adult thymus. This suggests that CD4 surface expression can be passively transferred from double positive cells to early progenitor thymocytes. Analysis of mixed bone marrow chimeras, reconstituted with hematopoietic stem cells from both CD4−/− (CD45.2) and CD4wt (CD45.1) congenic mice, revealed a CD4low phenotype on cells derived from CD4−/− bone marrow cells. Furthermore, these CD4−/−-derived “CD4low” progenitors were capable of reconstituting lymphocyte-depleted fetal thymi, with all thymocytes displaying a CD4−/− phenotype. This directly demonstrates that genetically CD4-deficient thymic progenitor cells can passively acquire a CD4low phenotype. Moreover, CD4 expression on CD4low progenitor thymocytes is sensitive to mild acid treatment, indicating that CD4 may not exist as an integral cell surface molecule on this thymocyte population. Our findings demonstrate that low-level CD4 surface expression can be passively acquired by intrathymic progenitor cells from the surrounding thymic microenvironment, suggesting that other cell surface molecules expressed at low levels may also result from an acquired phenotype.

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.

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.

Dendritic cell development in culture from thymic precursor cells in the absence of granulocyte/macrophage colony-stimulating factor

The earliest lymphoid precursor population in the adult mouse thymus had previously been shown to produce not only T cells, but also dendritic cell (DC) progeny on transfer to irradiated recipients. In this study, culture of these isolated thymic precursors with a mixture of cytokines induced them to proliferate and to differentiate to DC, but not to T lineage cells. At least 70% of the individual precursors had the capacity to form DC. The resultant DC were as effective as normal thymic DC in the functional test of T cell stimulation in mixed leukocyte cultures. The cultured DC also expressed high levels of class I and class II major histocompatibility complex, together with CD11c, DEC-205, CD80, and CD86, markers characteristic of mature DC in general. However, they did not express CD8 alpha or BP-1, markers characteristic of normal thymic DC. The optimized mixture of five to seven cytokines required for DC development from these thymic precursors did not include granulocyte/macrophage colony stimulating factor (GM-CSF), usually required for DC development in culture. The addition of anti-GM-CSF antibody or the use of precursors from GM-CSF-deficient mice did not prevent DC development. Addition of GM-CSF was without effect on DC yield when interleukin (IL) 3 and IL-7 were present, although some stimulation by GM-CSF was noted in their absence. In contrast, DC development was enhanced by addition of the Flt3/Flk2 ligand, in line with the effects of the administration of this cytokine in vivo. The results indicate that the development of a particular lineage of DC, probably those of lymphoid precursor origin, may be independent of the myeloid hormone GM-CSF.

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.