Genomic structure and chromosomal location of the mouse pre-T-cell receptor alpha gene

Identification of the pre-T-cell receptor alpha chain in nonmammalian vertebrates challenges the structure-function of the molecule

In humans and mice, the early development of αβ T cells is controlled by the pre-T-cell receptor α chain (pTα) that is covalently associated with the T-cell receptor β (TCRβ) chain to form the pre-T-cell receptor (pre-TCR) at the thymocyte surface. Pre-TCR functions in a ligand-independent manner through self-oligomerization mediated by pTα. Using in silico and gene synteny-based approaches, we identified the pTα gene (PTCRA) in four sauropsid (three birds and one reptile) genomes. We also identified 25 mammalian PTCRA sequences now covering all mammalian lineages. Gene synteny around PTCRA is remarkably conserved in mammals but differences upstream of PTCRA in sauropsids suggest chromosomal rearrangements. PTCRA organization is highly similar in sauropsids and mammals. However, comparative analyses of the pTα functional domains indicate that sauropsids, monotremes, marsupials, and lagomorphs display a short pTα cytoplasmic tail and lack most residues shown to be critical for human and murine pre-TCR self-oligomerization. Chicken PTCRA transcripts similar to those in mammals were detected in immature double-negative and double-positive thymocytes. These findings give clues about the evolution of this key molecule in amniotes and suggest that the ancestral function of pTα was exclusively to enable expression of the TCRβ chain at the thymocyte surface and to allow binding of pre-TCR to the CD3 complex. Together, our data provide arguments for revisiting the current model of pTα signaling.

Perturbation of T-cell development by insertional mutation of a PrP transgene

The normal cellular form of the prion protein PrP(C) is a glycosylphosphatidylinositol-linked cell-surface glycoprotein expressed primarily by cells of the nervous and immune systems. There is evidence to suggest that PrP(C) is involved in cell signalling and cellular homeostasis. We have investigated the immune composition of peripheral lymphoid tissue in PrP-/-, wild-type, tg19 and tga20 strains of mice, which express 0, 1-, 3-5- and 4-7-fold higher levels of PrP(C), respectively, relative to wild-type mice. Our data show that tga20 mice have a reduced number of spleen T-cell receptor (TCR)-alphabeta(+) T cells and an increased number of TCR-gammadelta(+) T cells compared with wild-type mice. This was not seen in tg19 mice, which also express elevated levels of PrP(C). In addition, we have found that the Prnp transgene in the tga20 genome is located centrally on chromosome 17, in or around genes involved in T-cell development. Significantly, mRNA transcripts from pre-TCR-alpha (pTalpha), a T-cell development gene located on mouse chromosome 17, are drastically reduced in tga20 mice, indicative of a perturbation in pTalpha gene regulation. We propose that the immune cell phenotype of tga20 mice may be caused by the insertional mutation of the Prnp transgene into the pTalpha gene or its regulatory elements.

MHC-restricted T cell receptor signaling is required for alpha beta TCR replacement of the pre T cell receptor

A developmental block is imposed on CD25(+)CD44(-) thymocytes at the beta-selection checkpoint in the absence of the pre T cell receptor (preTCR) alpha-chain, pTalpha. Early surface expression of a transgenic alphabeta TCR has been shown to partially circumvent this block, such that thymocytes progress to the CD4(+)CD8(+) double-positive stage. We wanted to analyze whether a restricting MHC element is required for alphabeta TCR-expressing double-negative (DN) thymocytes to overcome the developmental block in pTalpha-deficient animals. We used the HY-I knock-in model that endows thymocytes with alphabeta TCR expression in the DN compartment but has the advantage of physiological expression levels, in contrast to conventional TCR transgenes. On a pTalpha-deficient background, this HY-I TCR transgene 'rescued' CD25(+)CD44(-) thymocytes from apoptosis and enabled progression to later differentiation stages. On a non-selecting MHC background, however, pTalpha-deficient HY-I mice presented a pronounced reduction in numbers of splenocytes and thymocytes when compared to animals of selecting MHC genotype, showing that MHC restriction is necessary to drive HY-TCR-mediated rescue of pTalpha-deficient thymocytes.

Constitutive expression of the pre-TCR enables development of mature T cells

Expression and signalling through the pre-TCR and the TCRalphabeta resemble two critical checkpoints during T cell development. We investigated to which extent a pre-TCR can functionally replace mature TCRalpha chains during T cell development. For this purpose, transgenic mice were generated expressing the pre-TCRalpha (pTalpha) under the transcriptional control of TCRbeta regulatory elements. We report here on the interesting finding that constitutive pTalpha expression allows complete T cell maturation. The pre-TCR complex permits a subset of beta-selected thymocytes to mature in the absence of TCRalpha into peripheral T cells (betaT cells) comprising up to 10% of all lymphocytes. Lymphopenia-driven proliferation of these betaT cells is similar to that of conventional alphabetaT cells. Furthermore, betaT cells proliferated and acquired effector function upon stimulation with allogeneic MHC.

Genes, pathways and checkpoints in lymphocyte development and homeostasis

A plethora of genes involved in murine B and T cell development have been identified, and developmental pathways within the primary lymphoid tissues have been well delineated. The generation of a functional, but non-self reacting lymphocyte repertoire results from the completion of several checkpoints during lymphocyte development and competition for survival factors in the periphery. Improved knowledge of these developmental checkpoints and homeostatic mechanisms is critical for understanding human immunodeficiency, leukaemia/lymphoma and autoimmunity, which are conditions where checkpoints and homeostasis are likely to be deregulated.

Unique features of the pre-T-cell receptor α-chain: not just a surrogate

The pre-T-cell receptor (pre-TCR) has a crucial role in the normal development of alphabeta T cells. Different views have emerged concerning the structure and function of the pre-TCR. This molecular complex can be viewed as a variant of the alphabeta-TCR in which the pre-TCR alpha-chain that is covalently associated with the TCR beta-chain is a 'surrogate' TCR alpha-chain. Alternatively, the unique structure of the pre-TCR might be associated with a unique function, owing to evolutionary selection of a pre-TCR alpha-chain that has different capabilities from the TCR alpha-chain. As described here, I consider that experimental evidence favours the latter view.

Transposable B2 SINE elements can provide mobile RNA polymerase II promoters

Short interspersed elements (SINEs) are highly abundant components of mammalian genomes that are propagated by retrotransposition. SINEs are recognized as a causal agent of human disease and must also have had a profound influence in shaping eukaryotic genomes. The B2 SINE family constitutes approximately 0.7% of total mouse genomic DNA (ref. 2) and is also found at low abundance in humans. It resembles the Alu family in several respects, such as its mechanism of propagation. B2 SINEs are derived from tRNA and are transcribed by RNA polymerase (pol) III to generate short transcripts that are not translated. We find here, however, that one B2 SINE also carries an active pol II promoter located outside the tRNA region. Indeed, a B2 element is responsible for the production of a mouse Lama3 transcript. The B2 pol II promoters can be bound and stimulated by the transcription factor USF (for upstream stimulatory factor), as shown by transient transfection experiments. Moreover, this pol II activity does not preclude the pol III transcription necessary for retrotransposition. Dispersal of B2 SINEs by retrotransposition may therefore have provided numerous opportunities for creating regulated pol II transcription at novel genomic sites. This mechanism may have allowed the evolution of new transcription units and new genes.

Expression of the mouse pre-T cell receptor alpha gene is controlled by an upstream region containing a transcriptional enhancer

The pre-T cell receptor alpha (pTalpha) protein is a critical component of the pre-T cell receptor complex in early thymocytes. The expression of the pTalpha gene is one of the earliest markers of the T cell lineage and occurs exclusively in pre-T cells. To investigate the molecular basis of thymocyte-specific gene expression, we searched for the genomic elements regulating transcription of the mouse pTalpha gene. We now report that expression of the pTalpha gene is primarily controlled by an upstream genomic region, which can drive thymocyte-specific expression of a marker gene in transgenic mice. Within this region, we have identified two specific DNase-hypersensitive sites corresponding to a proximal promoter and an upstream transcriptional enhancer. The pTalpha enhancer appears to function preferentially in pre-T cell lines and binds multiple nuclear factors, including YY1. The enhancer also contains two G-rich stretches homologous to a critical region of the thymocyte-specific lck proximal promoter. Here we show that these sites bind a common nuclear factor and identify it as the zinc finger protein ZBP-89. Our data establish a novel experimental model for thymocyte-specific gene expression and suggest an important role for ZBP-89 in T cell development.

Pleiotropic changes controlled by the pre-T-cell receptor

The construction of various gene-deficient mice has facilitated the understanding of the role of various receptors and signaling pathways that control the generation of alphabeta lineage cells. A predominant role is occupied by the pre-TCR, which not only generates large numbers of alphabeta lineage cells but also controls TCRbeta allelic exclusion as well as commitment to the gammadelta lineage versus the alphabeta lineage.

Genomic structure of the human pre-T cell receptor alpha chain and expression of two mRNA isoforms

The pre-TCR, which is minimally composed of the TCRbeta chain, the pre-Talpha chain, and the CD3 complex, regulates early T cell development. The pre-Talpha chain is a 33-kDa type I transmembrane glycoprotein with an extracellular part similar to the constant domain of the immunoglobulin supergene family. We have sequenced (11 kb) the human pTalpha gene, which like the murine pTalpha gene consists of four exons: exon 1 encodes the 5' untranslated region, the leader peptide and the first three amino acids of the mature protein, exon 2 the extracellular immunoglobulin (Ig)-like domain, exon 3 a 15-amino acid peptide including a cysteine required for heterodimerization with TCRbeta, exon 4 the transmembrane region, the cytoplasmic tail and the 3' untranslated sequence. The human pTalpha gene is located on chromosome 6p21.3, close to the HLA-A locus. Reverse transcription-PCR studies with human thymus and leukemic cells showed that alternative splicing produces a shorter pTalpha isoform, which lacks the Ig-like domain but contains the transmembrane elements and the extracytoplasmic cystein and which could thus permit pairing with TCRbeta chain and association with CD3 molecules. The conserved splice sites suggest a yet ill-defined biological function of the short pTalpha protein.

Thymocyte selection in Vav and IRF-1 gene-deficient mice

T cells undergo a defined program of phenotypic and genetic changes during differentiation within the thymus. These changes define commitment of T-cell receptor (TCR) gamma delta and TCR alpha beta cells and lineage differentiation into CD4+ T helper and CD8+ cytotoxic T cells. T-cell differentiation and selection in the thymus constitute a tightly co-ordinated multistep journey through a network that can be envisaged as a three-dimensional informational highway made up of stromal cells and extracellular matrix molecules. This intrathymic journey is controlled by information exchange, with thymocytes depending on two-way cellular interactions with thymic stromal cells in order to receive essential signals for maturation and selection. Genetic inactivation of surface receptors, signal transduction molecules, and transcription factors using homologous recombination has provided novel insight into the signaling cascades that relay surface receptor engagement to gene transcription and subsequent progression of the developmental program. In this review we discuss molecular mechanisms of T lymphocyte development in mice that harbour genetic mutations in the guanine nucleotide exchange factor Vav and the interferon regulatory transcription factor 1 (IRF-1). We also propose a novel model of T-cell selection based on TCR alpha chain-directed signals for allelic exclusion and TCR alpha-based selection for single receptor usage.

Crucial function of the pre-T-cell receptor (TCR) in TCR beta selection, TCR beta allelic exclusion and alpha beta versus gamma delta lineage commitment

The analysis of T-cell receptor (TCR) beta selection, TCR beta allelic exclusion and TCR beta rearrangement in gamma delta T cells from normal and pre-TCR-deficient mice has shown that the pre-TCR has a crucial role in T-lymphocyte development: The pre-TCR is by far the most effective receptor that generates large numbers of CD4+8+ T cells with productive TCR beta rearrangements. In the absence of the pre-TCR, TCR beta rearrangement proceeds in developing cells irrespective of whether they already contain a productive TCR beta gene. The pre-TCR directs developing T cells to the alpha beta lineage because gamma delta T cells from pT alpha-/- mice proceed much further in TCR beta rearrangement than gamma delta T cells from wild-type mice. It is argued that the pre-TCR commits developing T cells to the alpha beta lineage by an instructive mechanism, which has largely replaced an evolutionarily more ancient mechanism that involves stochastic alpha beta lineage commitment.

Cutting Edge: The Expression In Vivo of a Second Isoform of pTα: Implications for the Mechanism of pTα Action

A second isoform of pTα, “pTαb,” is derived from the pTα locus by tissue-specific, alternative splicing. pTαb is coexpressed in the thymus with the previously characterized form of pTα (which we term pTαa) and is also expressed in peripheral cells without pTαa. While pTαa acts to retain most TCR β-chains intracellularly, pTαb permits higher levels of cell surface TCRβ expression and facilitates signaling from a CD3-TCRβ complex.

Allelic exclusion in pTalpha-deficient mice: no evidence for cell surface expression of two T cell receptor (TCR)-beta chains, but less efficient inhibition of endogeneous Vbeta--> (D)Jbeta rearrangements in the presence of a functional TCR-beta transgene

Although individual T lymphocytes have the potential to generate two distinct T cell receptor (TCR)-beta chains, they usually express only one allele, a phenomenon termed allelic exclusion. Expression of a functional TCR-beta chain during early T cell development leads to the formation of a pre-T cell receptor (pre-TCR) complex and, at the same developmental stage, arrest of further TCR-beta rearrangements, suggesting a role of the pre-TCR in mediating allelic exclusion. To investigate the potential link between pre-TCR formation and inhibition of further TCR-beta rearrangements, we have studied the efficiency of allelic exclusion in mice lacking the pre-TCR-alpha (pTalpha) chain, a core component of the pre-TCR. Staining of CD3+ thymocytes and lymph node cells with antibodies specific for Vbeta6 or Vbeta8 and a pool of antibodies specific for most other Vbeta elements, did not reveal any violation of allelic exclusion at the level of cell surface expression. This was also true for pTalpha-deficient mice expressing a functionally rearranged TCR-beta transgene. Interestingly, although the transgenic TCR-beta chain significantly influenced thymocyte development even in the absence of pTalpha, it was not able to inhibit fully endogeneous TCR-beta rearrangements either in total thymocytes or in sorted CD25+ pre-T cells of pTalpha-/- mice, clearly indicating an involvement of the pre-TCR in allelic exclusion.

Restoration of thymopoiesis in pT alpha-/- mice by anti-CD3epsilon antibody treatment or with transgenes encoding activated Lck or tailless pT alpha

Mice deficient for the pre-TCR alpha (pT alpha) chain cannot form a pre-T cell receptor (TCR) and exhibit a severe defect in early T cell development, characterized by lack of "beta selection" and impaired generation of double-positive (DP) thymocytes. Here, we demonstrate that intraperitoneal injection of CD3epsilon-specific antibodies into pT alpha-/- x RAG-/- mice or introduction of an activated p56(lck) transgene in pT alpha-/- mice fully restores the number of DP thymocytes, and that expression of a transgenic pT alpha chain lacking its cytoplasmic portion can overcome all developmental defects associated with pT alpha deficiency. These results allow a better definition of the role of pT alpha in pre-TCR signal transduction and provide conclusive evidence that the cytoplasmic tail of pT alpha is not essential for pre-TCR signaling.

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.

Structure and function of the pre-T cell receptor

The pre-T cell receptor (pre-TCR) that minimally consists of the TCR beta chain and the disulfide-linked pre-T cell receptor alpha (pT alpha) chain in association with signal-transducing CD3 molecules rescues from programmed cell death cells with productive TCR beta rearrangements. The pre-TCR induces expansion and differentiation of these cells such that they become TCR alpha beta bearing CD4+8+ thymocytes, which express only a single TCR beta chain and then either die of neglect or--upon TCR-ligand interaction--undergo either positive or negative selection. The newly discovered pT alpha gene encodes a transmembrane protein that belongs to the Ig superfamily and contains a cytoplasmic tail that, however, has no essential function in signal transduction, which is mediated by CD3 molecules and most likely p56lck. Experiments in pT alpha gene-deficient mice show that the pre-TCR has a crucial role in maturation as well as allelic exclusion of alpha beta T cells but is not required for the development of gamma delta-expressing cells. The function of the pre-TCR cannot be fully assumed by an alpha beta TCR that is expressed abnormally early in T cell development.

Functions of TCR and pre-TCR subunits: lessons from gene ablation

Recent gene-targeting experiments have highlighted the existence of checkpoints that ensure that alpha beta T cells do not complete intrathymic differentiation if they have not attained certain landmark events. These 'proofreading' mechanisms operate by way of the pre-TCR and TCR complexes, which are sequentially expressed during T-cell development. These complexes are likely to signal via their associated CD3 subunits. By activating intracellular effectors, the CD3 subunits probably modulate gene expression profiles and drive the maturing alpha beta T cells through a precise developmental sequence.

Cloning and comparative analysis of the human pre-T-cell receptor alpha-chain gene

In immature T cells the T-cell receptor (TCR) beta-chain gene is rearranged and expressed before the TCR alpha-chain gene. At this stage TCR beta chain can form disulfide-linked heterodimers with the pre-T-cell receptor alpha chain (pTalpha). Using the recently isolated murine pTalpha cDNA as a probe, we have isolated the human pTalpha cDNA. The complete nucleotide sequence predicts a mature protein of 282 aa consisting of an extracellular immunoglobulin-like domain, a connecting peptide, a transmembrane region, and a long cytoplasmic tail. Amino acid sequence comparison of human pTalpha with the mouse pTalpha molecule reveals high sequence homology in the extracellular as well as the transmembrane region. In contrast, the cytoplasmic region differs in amino acid composition and in length from the murine homologue. The human pTalpha gene is expressed in immature but not mature T cells and is located at the p21.2-p12 region of the short arm of chromosome 6.