Differential developmental regulation and functional effects on pre-TCR surface expression of human pTalpha(a) and pTalpha(b) spliced isoforms

The immunopathological landscape of human pre-TCRα deficiency: From rare to common variants

We describe humans with rare biallelic loss-of-function PTCRA variants impairing pre-α T cell receptor (pre-TCRα) expression. Low circulating naive αβ T cell counts at birth persisted over time, with normal memory αβ and high γδ T cell counts. Their TCRα repertoire was biased, which suggests that noncanonical thymic differentiation pathways can rescue αβ T cell development. Only a minority of these individuals were sick, with infection, lymphoproliferation, and/or autoimmunity. We also report that 1 in 4000 individuals from the Middle East and South Asia are homozygous for a common hypomorphic PTCRA variant. They had normal circulating naive αβ T cell counts but high γδ T cell counts. Although residual pre-TCRα expression drove the differentiation of more αβ T cells, autoimmune conditions were more frequent in these patients compared with the general population.

Notch Partners in the Long Journey of T-ALL Pathogenesis

T-cell acute lymphoblastic leukemia (T-ALL) is an aggressive hematological disease that arises from the oncogenic transformation of developing T cells during T-lymphopoiesis. Although T-ALL prognosis has improved markedly in recent years, relapsing and refractory patients with dismal outcomes still represent a major clinical issue. Consequently, understanding the pathological mechanisms that lead to the appearance of this malignancy and developing novel and more effective targeted therapies is an urgent need. Since the discovery in 2004 that a major proportion of T-ALL patients carry activating mutations that turn NOTCH1 into an oncogene, great efforts have been made to decipher the mechanisms underlying constitutive NOTCH1 activation, with the aim of understanding how NOTCH1 dysregulation converts the physiological NOTCH1-dependent T-cell developmental program into a pathological T-cell transformation process. Several molecular players have so far been shown to cooperate with NOTCH1 in this oncogenic process, and different therapeutic strategies have been developed to specifically target NOTCH1-dependent T-ALLs. Here, we comprehensively analyze the molecular bases of the cross-talk between NOTCH1 and cooperating partners critically involved in the generation and/or maintenance and progression of T-ALL and discuss novel opportunities and therapeutic approaches that current knowledge may open for future treatment of T-ALL patients.

Pre-TCRα supports CD3-dependent reactivation and expansion of TCRα-deficient primary human T-cells

Chimeric antigen receptor technology offers a highly effective means for increasing the anti-tumor effects of autologous adoptive T-cell immunotherapy, and could be made widely available if adapted to the use of allogeneic T-cells. Although gene-editing technology can be used to remove the alloreactive potential of third party T-cells through destruction of either the α or β T-cell receptor (TCR) subunit genes, this approach results in the associated loss of surface expression of the CD3 complex. This is nonetheless problematic as it results in the lack of an important trophic signal normally mediated by the CD3 complex at the cell surface, potentially compromising T-cell survival in vivo, and eliminating the potential to expand TCR-knockout cells using stimulatory anti-CD3 antibodies. Here, we show that pre-TCRα, a TCRα surrogate that pairs with TCRβ chains to signal proper TCRβ folding during T-cell development, can be expressed in TCRα knockout mature T-cells to support CD3 expression at the cell surface. Cells expressing pre-TCR/CD3 complexes can be activated and expanded using standard CD3/CD28 T-cell activation protocols. Thus, heterologous expression of pre-TCRα represents a promising technology for use in the manufacturing of TCR-deficient T-cells for adoptive immunotherapy applications.

Synergistic effects of interleukin-7 and pre-T cell receptor signaling in human T cell development

The role of IL-7 in pre-T cell receptor (TCR) signaling during human T cell development is poorly understood. To study this, we engineered Molt3, a T cell progenitor T-acute lymphoblastic leukemia cell line, using lentiviral IL-7 receptor α (IL-7Rα) to serve as a model system. IL-7 promoted pre-TCR activation in IL-7Rα(hi) Molt3 as illustrated by CD25 up-regulation after anti-CD3 stimulation. Anti-CD3 treatment activated Akt and Erk1/2 signaling pathways as proven using specific inhibitors, and IL-7 further enhanced both signaling pathways. The close association of IL-7Rα with CD3ζ in the pre-TCR complex was illustrated through live imaging confocal fluorescence microscopy. These results demonstrate a direct and cooperative role of IL-7 in pre-TCR signaling.

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.

Identification of CMS as a cytosolic adaptor of the human pTalpha chain involved in pre-TCR function

The T-cell receptor beta (TCRbeta)/pre-TCRalpha (pTalpha) pre-TCR complex (pre-TCR) signals the expansion and differentiation of de-veloping thymocytes. Functional pro-perties of the pre-TCR rely on its unique pTalpha chain, which suggests the participation of specific intracellular adaptors. However, pTalpha-interacting molecules remain unknown. Here, we identified a polyproline-arginine sequence in the human pTalpha cytoplasmic tail that interacted in vitro with SH3 domains of the CIN85/CMS family of adaptors, and mediated the recruitment of multiprotein complexes involving all (CMS, CIN85, and CD2BP3) members. Supporting the physiologic relevance of this interaction, we found that 1 such adaptor, CMS, interacted in vivo with human pTalpha, and its expression was selectively up-regulated during human thymopoiesis in pre-TCR-activated thymocytes. Upon activation, pre-TCR clustering was induced, and CMS and polymerized actin were simultaneously recruited to the pre-TCR activation site. CMS also associated via its C-terminal region to the actin cytoskeleton in the endocytic compartment, where it colocalized with internalized pTalpha in traffic to lysosomal degradation. Notably, deletion of the pTalpha CIN85/CMS-binding motif impaired pre-TCR-mediated Ca(2+) mobilization and NFAT transcriptional activity, and precluded activation induced by overexpression of a CMS-SH3 N-terminal mutant. These results provide the first molecular evidence for a pTalpha intracellular adaptor involved in pre-TCR function.

Cell surface expression of a peptide encoded by the unrearranged TCR-Vβ8.2 gene

Germline transcription of T-cell receptor (TCR) genes has been described in early lymphoid cells. The most common explanation for this phenomenon is that transcription of unrearranged Vbeta genes directs gene usage during the rearrangement event. Germline transcription of the TCR-Vbeta8.2 gene has been detected in a precursor T-cell line, C1-V13D, which shows no rearrangement at any of the TCR gene loci. This cell line also shows weak binding of specific anti-Vbeta8.2 antibody to the cell surface, consistent with expression of a truncated TCRbeta chain. RT-PCR has been used to confirm expression of spliced germline transcripts of TCR-Vbeta8.2 in C1-V13D initiated from both leader (L)5.1 and L8.2. Transcripts initiated from L8.2 were also detectable in unspliced form. In order to test expression and subcellular localisation of any encoded peptides, amplified germline transcripts in both spliced and unspliced form were cloned into the pEGFP-N1 fusion vector for stable transfection and overexpression in C1-V13D. Cell surface expression of a fusion protein between EGFP and a Vbeta peptide has been confirmed in C1-V13D but not in control COS-7 cells. Results presented here raise the possibility of a new pre-TCR structure specific to early lymphoid cells and based on TCR-Vbeta8.2 gene expression.

Function of alternative splicing

Alternative splicing is one of the most important mechanisms to generate a large number of mRNA and protein isoforms from the surprisingly low number of human genes. Unlike promoter activity, which primarily regulates the amount of transcripts, alternative splicing changes the structure of transcripts and their encoded proteins. Together with nonsense-mediated decay (NMD), at least 25% of all alternative exons are predicted to regulate transcript abundance. Molecular analyses during the last decade demonstrate that alternative splicing determines the binding properties, intracellular localization, enzymatic activity, protein stability and posttranslational modifications of a large number of proteins. The magnitude of the effects range from a complete loss of function or acquisition of a new function to very subtle modulations, which are observed in the majority of cases reported. Alternative splicing factors regulate multiple pre-mRNAs and recent identification of physiological targets shows that a specific splicing factor regulates pre-mRNAs with coherent biological functions. Therefore, evidence is now accumulating that alternative splicing coordinates physiologically meaningful changes in protein isoform expression and is a key mechanism to generate the complex proteome of multicellular organisms.

Differential expression of notch signaling-related transcripts accompanies Pro-thymocyte proliferation and phenotype transition induced by epidermal growth factor plus insulin in fetal thymus organ cultures

Thymus regression upon stressing stimuli, such as infectious diseases, is followed by organ reconstitution, paralleling its development in ontogeny. A narrow window of thymus development was here studied, encompassing the pro-T lymphoid precursor expansion during specification stages, by the use of epidermal growth factor plus insulin (INS) in murine fetal thymus organ cultures. Aiming to disclose signaling pathways related to these stages, cultured thymus lobes had their RNA extracted, for the search of transcripts differentially expressed using RNAse protection assays and reverse transcriptase-polymerase chain reactions. We found no difference that could explain INS-driven thymocyte growth, in the pattern of transcripts for death/proliferation mediators, or for a series of growth factor receptors and transcriptional regulators known as essential for thymus development. Thymocyte suspensions from cultured lobes, stained for phenotype analysis by fluorescence activated cell sorting, showed a decreased staining for Notch1 protein at cell surfaces upon INS addition. We analyzed the expression of Notch-related elements, and observed the recruitment of a specific set of transcripts simultaneous and compatible with INS-driven thymocyte growth, namely, transcripts for Notch3, for its ligand Jagged2, and for Deltex1, a mediator of a poorly characterized alternative pathway downstream of the Notch receptor.

T lymphoid differentiation in human bone marrow

The unique role of the thymus in the development of T cells was established >4 decades ago. To elucidate how uncommitted lymphoid progenitor cells are instructed to migrate from bone marrow to the thymus to undergo T lymphoid differentiation, we generated and analyzed a genome-wide gene expression profile of CD7+ CD10+ human bone marrow T cell lineage precursors (TLPs) by using the serial analysis of gene expression technique. Unexpectedly, the serial analysis of gene expression profile identified a high number of (pre-) T cell receptor antigen (TCR)-related transcripts in bone marrow TLPs. To determine the configuration of the TCRbeta locus in these cells at a quantitative level, we sorted and analyzed bone marrow TLPs from five donors by single-cell PCR. Similar proportions of TLPs harbored TCRbeta germ-line alleles, D-J, or V-DJ gene rearrangements. Thus, bone marrow TLPs are heterogenous with respect to TCRbeta rearrangement status, suggesting an active recombination machinery that is consistent with the expression of RAG1, RAG2, and TdT in this population. As a hallmark of ongoing TCRbeta V-DJ rearrangement, we could amplify broken-ended recombination-signal sequence DNA intermediates from bone marrow TLPs, but not from mature T cells by ligation-mediated PCR. Approximately half of the TCRbeta rearrangements were compatible with the expression of a functional pre-TCR, which is in agreement with surface expression of pre-Talpha on bone marrow TLPs as shown by confocal laser microscopy and flow cytometry. At a frequency <0.5% of mononucleated cells in human bone marrow, this population is rare, yet it exemplifies T lymphoid differentiation in the human already before immigration into the thymus.

A role for the cytoplasmic tail of the pre-T cell receptor (TCR) alpha chain in promoting constitutive internalization and degradation of the pre-TCR

Engagement of the alpha beta T cell receptor (TCR) by its ligand results in the down-modulation of TCR cell surface expression, which is thought to be a central event in T cell activation. On the other hand, pre-TCR signaling is a key process in alpha beta T cell development, which appears to proceed in a constitutive and ligand-independent manner. Here, comparative analyses on the dynamics of pre-TCR and TCR cell surface expression show that unligated pre-TCR complexes expressed on human pre-T cells behave as engaged TCR complexes, i.e. they are rapidly internalized and degraded in lysosomes and proteasomes but do not recycle back to the cell surface. Thus, pre-TCR down-regulation takes place constitutively without the need for extracellular ligation. By using TCR alpha/p Tau alpha chain chimeras, we demonstrate that prevention of recycling and induction of degradation are unique pre-TCR properties conferred by the cytoplasmic domain of the pT alpha chain. Finally, we show that pre-TCR internalization is a protein kinase C-independent process that involves the combination of src kinase-dependent and -independent pathways. These data suggest that constitutive pre-TCR down-modulation regulates pre-TCR surface expression levels and hence the extent of ligand-independent signaling through the pre-TCR.

Development of alphabeta T cells in the human thymus

The thymus is the main producer of alphabeta T cells and is, therefore, crucial for a normal immune system. The intrathymic developmental pathway of human alphabeta T cells has now been delineated. The production of new T cells by the thymus decreases with age, and the thymus was thought to be redundant in adults once the peripheral T-cell pool has been formed early in life. However, recent work has shown that the thymus can function even at an advanced age. Research into the production of T cells in clinical settings that are associated with loss of T cells in the periphery has sparked renewed interest in the function of the human thymus.

Regulation of surface expression of the human pre-T cell receptor complex

Considerable progress has recently been made in defining the role that pre-antigen receptor complexes, namely the pre-T and pre-B cell receptors, play in lymphocyte development. It is now established that these receptors direct, in a similar way, the survival, expansion, clonality and further differentiation of pre-T and pre-B lymphocytes, respectively. However, less is known about the mechanisms which ensure that only minute amounts of pre-TCR and pre-BCR reach the plasma membrane of developing lymphocytes. In this review, we discuss the implications of recent experimental approaches which address the developmental regulation of human pre-TCR expression and the molecular mechanisms that control surface pre-TCR expression levels.

On the brink of becoming a T cell

Recent studies provide fresh insight into the mechanisms by which precursor cells are committed to and develop within the T-lymphocyte lineage. Precursor/product studies have identified developmental stages between that of the pluripotent hematopoietic stem cell and thymocytes committed to the T lineage. Specific ligands and signaling pathways interacting with the Notch-1 receptor and its ability to influence commitment within the lymphoid lineage have been described. Although the structural features or putative ligands endowing the pre-TCR with constitutive signaling capacity remain elusive, numerous distal mediators of pre-TCR signaling have been identified. It remains for the future to determine what roles they may have in survival, proliferation, lineage commitment and allelic exclusion of TCR genes. Receptor editing and lineage commitment of alphabeta T cells still represent controversial topics that need further study.

Combined expression of pTalpha and Notch3 in T cell leukemia identifies the requirement of preTCR for leukemogenesis

Notch receptors are conserved regulators of cell fate and have been implicated in the regulation of T cell differentiation and lymphomagenesis. However, neither the generality of Notch involvement in leukemia, nor the molecules with which Notch may interact have been clarified. Recently, we showed that transgenic mice expressing the constitutively active intracellular domain of Notch3 in thymocytes and T cells developed early and aggressive T cell neoplasias. Although primarily splenic, the tumors sustained features of immature thymocytes, including expression of pTalpha, a defining component of the pre T cell receptor, known to be a potent signaling complex provoking thymocyte survival, proliferation, and activation. Thus, enforced expression of Notch3, which is ordinarily down-regulated as thymocytes mature, may sustain pre T cell receptor expression, causing dysregulated hyperplasia. This hypothesis has been successfully tested in this article by the observation that deletion of pTalpha in Notch3 transgenic mice abrogates tumor development, indicating a crucial role for pTalpha in T cell leukemogenesis. Parallel observations were made in humans, in that all T cell acute lymphoblastic leukemias examined showed expression of Notch3 and of the Notch target gene HES-1, as well as of pTalpha a and b transcripts, whereas the expression of all these genes was dramatically reduced or absent in remission. Together, these results suggest that the combined expression of Notch3 and pTalpha sustains T cell leukemogenesis and may represent pathognomonic molecular features of human T-ALL.