Surface expression of a T cell receptor beta (TCR-beta) chain in the absence of TCR-alpha, -delta, and -gamma proteins

Unprecedented effect of vitamin D3 on T-cell receptor beta subunit and alpha7 nicotinic acetylcholine receptor expression in a 3-nitropropionic acid induced mouse model of Huntington's disease

Introduction

3-NP induction in rodent models has been shown to induce selective neurodegeneration in the striatum followed by the cortex (Brouillet, 2014). However, it remains unclear whether, under such a neurotoxic condition, characterized by neuroinflammation and oxidative stress, the gene expression of the immune resident protein, T-cell receptor beta subunit (TCR-β), α7 nicotinic acetylcholine receptor (α7 nAChRs), the nuclear factor kappa B (NF-κB), inflammatory cytokines (TNF-α and IL-6), and antioxidants (Cat and GpX4) get modulated on Vitamin D3 (VD) supplementation in the central nervous system.

Methods

In the present study, real-time polymerase chain reaction (RT-PCR) was performed to study the expression of respective genes. Male C57BL/6 mice (8-12 weeks) were divided into four groups namely, Group I: Control (saline); Group II: 3-NP induction via i.p (HD); Group III: Vitamin D3 (VD) and Group IV: (HD + VD) (Manjari et al., 2022).

Results

On administration of 500IU/kg/day of VD, HD mice showed a significant reduction in the gene expression of the immune receptor, TCR-β subunit, nuclear factor kappa B (NF-κB), inflammatory cytokines, and key antioxidants, followed by a decrease in the acetylcholinesterase activity.

Conclusion

A novel neuroprotective effect of VD in HD is demonstrated by combating the immune receptor, TCR-β gene expression, antioxidant markers, and inflammatory cytokines. In addition, HD mice on VD administration for 0-15 days showed an enhancement in cholinergic signaling with restoration in α7 nAChRs mRNA and protein expression in the striatum and cortex.

Expression of T-cell receptor genes during early T-cell development

Lymphoid cell development is an ordered process that begins in the embryo in specific sites and progresses through multiple differentiative steps to production of T- and B-cells. Lymphoid cell production is marked by the rearrangement process, which gives rise to mature cells expressing antigen-specific T-cell receptors (TCR) and immunoglobulins (Ig). While most transcripts arising from TCR or Ig loci reflect fully rearranged genes, germline transcripts have been identified, but these have always been thought to have no specific purpose. Germline transcription from either unrearranged TCR or unrearranged Ig loci was commonly associated with an open chromatin configuration during VDJ recombination. Since only early T and B cells undergo rearrangement, the association of germline transcription with the rearrangement process has served as an appropriate explanation for expression of these transcripts in early T- and B-cell progenitors. However, germline TCR-V beta 8.2 transcripts have now been identified in cells from RAG(-/-) mice, in the absence of the VDJ rearrangement event and recombinase activity. Recent data now suggest that germline TCR-V beta transcription is a developmentally regulated lymphoid cell phenomenon. Germline transcripts could also encode a protein that plays a functional role during lymphoid cell development. In the least, germline transcripts serve as markers of early lymphoid progenitors.

Transcriptional regulation of the mouse IL-7 receptor alpha promoter by glucocorticoid receptor

Expression of the IL-7R alpha-chain (IL-7Ralpha) is strictly regulated during the development and maturation of lymphocytes. Glucocorticoids (GC) have pleiotypic effects on the growth and function of lymphocytes. Although GC have been reported to induce the transcription of IL-7Ralpha gene in human T cells, its molecular mechanism is largely unknown. In this study, we show that GC up-regulate the levels of IL-7Ralpha mRNA and protein in mouse T cells. This effect does not require protein synthesis de novo, because protein synthesis inhibitors do not block the process. Mouse IL-7Ralpha promoter has striking homology with human and rat, containing consensus motifs of Ikaros, PU.1, and Runx1 transcription factors. In addition, a conserved noncoding sequence (CNS) of approximately 270 bp was found 3.6-kb upstream of the promoter, which was designated as CNS-1. A GC receptor (GR) motif is present in the CNS-1 region. Importantly, we show by reporter assay that the IL-7Ralpha promoter has specific transcription activity in T cells. This activity highly depends on the PU.1 motif. Furthermore, GC treatment augments the transcriptional activity through the GR motif in the CNS-1 region. We also demonstrate that GR binds to the GR motif by EMSA. In addition, by chromatin immunoprecipitation assay, we show that GR is rapidly recruited to endogenous CNS-1 chromatin after GC stimulation. These results demonstrate that GR binds to the GR motif in the CNS-1 region after GC stimulation and then activates the transcription of the IL-7Ralpha promoter. Thus, this study identifies the IL-7Ralpha CNS-1 region as a GC-responsive element.

Use of a microgravity organ culture dish system to demonstrate the signal dampening effects of modeled microgravity during T cell development

Recently, we have shown that exposure of fetal thymus organ cultures (FTOC) to modeled microgravity (MMG) using a clinostat with a microgravity organ culture dish system (MOCDS) blocks T cell development in a manner independent of steroid stress hormones present in vivo. In this study, we describe the development of the MOCDS system, as well as its use in attempting to understand the mechanism by which T cell development is inhibited in MMG. We show that after MMG exposure FTOC exhibited a significant reduction in CD4+CD8+ double positive (DP) cell production, but those DP cells which remained expressed higher levels of the T cell receptor (TCR) associated molecule, CD3. Interestingly, CD4-CD8- double negative (DN) cells expressed lower levels of CD3 on their surface. DN, as well as immature single positive (ISP) cells, also expressed reduced levels of the IL-7 receptor alpha chain (CD127). These changes in CD3 and CD127 expression were concomitantly associated with an increased production of tumor necrosis factor (TNF)-alpha. We were also able to show that addition of an exogenous signal (anti-CD3epsilon monoclonal antibody) to these cultures effectively mitigated the MMG-induced effects, suggesting that MMG-exposure causes a signal dampening effect on developing thymocytes.

Positive selection by the pre-TCR yields mature CD8+ T cells

It has been of much interest whether there is functional redundancy between the constitutively signaling pre-Talpha/TCRbeta (pre-TCR) and ligated TCRalphabeta complexes, which independently operate the two distinct checkpoints during thymocyte development, i.e., the pre-TCR involved in beta-selection at the CD4(-)CD8(-) double-negative stage and the TCRalphabeta being crucial for positive/negative selection at the CD4(+)CD8(+) double-positive stage. We found that the pre-TCR expressed on double-positive cells in TCRalpha-deficient (TCRalpha(-/-)) mice produced a small number of mature CD8(+) T cells. Surprisingly, when pre-Talpha was overexpressed, resulting in augmentation of pre-TCR expression, there was a striking increase of the CD8(+) T cells. In addition, even in the absence of up-regulation of pre-TCR expression, a similar increase of CD8(+) T cells was also observed in TCRalpha(-/-) mice overexpressing Egr-1, which lowers the threshold of signal strength required for positive selection. In sharp contrast, the CD8(+) T cells drastically decreased in the absence of pre-Talpha on a TCRalpha(-/-) background. Thus, the pre-TCR appears to functionally promote positive selection of CD8(+) T cells. The biased production of CD8(+) T cells via the pre-TCR might also support the potential involvement of signal strength in CD4/CD8 lineage commitment.

Lack of evidence for aggregation-dependent enhancement of p56lck in the signal transduction upon major histocompatibility complex recognition by mature T cells

The kinase activity of lymphocyte-specific tyrosine kinase p56lck (Lck) upon physiological major histocompatibility complex (MHC) recognition by normal mature T cells was examined. Recognition of the target MHC molecules by T cells induced phosphorylation of the zeta-chain without obvious enhancement of the background Lck activity. There was no sign of enhancement of Lck through putative T-cell receptor (TCR)-independent class II MHC/CD4 interactions either. As has been reported, cross-linking of CD4 molecules by antibodies induced a marked enhancement of Lck activity. However, it did not have an immediate relevance to TCR-mediated signal transduction, as judged from the lack of detectable de novo phosphorylation of zeta-chain and the absence of functional responses of T cells. These results strongly favour the model in which TCR-mediated signal transduction does not involve aggregation-dependent enhancement of Lck, suggesting that the signal can be triggered simply by the recruitment of already active Lck with basal kinase activity through the formation of a TCR/MHC/CD4 ternary complex.

E2A and HEB activate the pre-TCR alpha promoter during immature T cell development

The pre-TCRalpha (pTalpha) is exclusively expressed in immature thymocytes and constitutes the pre-TCR complex with TCRbeta, which regulates early T cell differentiation. Despite the recent identification of the pTalpha enhancer, the contribution of the promoter region, the direct DNA-protein interaction, and the regulation of such interaction along with T cell development have not been investigated. We analyzed the pTalpha promoter region and identified the critical elements for transcription of the pTalpha gene. The pTalpha promoter was found to contain two consecutive E-box elements that are critical for pTalpha transcription. The E-box elements in the promoter region formed the specific DNA-protein complex that was exclusively observed in immature thymocytes, not in mature thymocytes and T cells. The E proteins in this complex were identified as E2A and HeLa E-box binding protein (HEB), and overexpression of E2A and HEB resulted in activation of the pTalpha promoter. The binding complex in the consecutive E-boxes in the pTalpha promoter changed along with T cell development, as a distinct DNA-binding complex was observed in mature T cells. Comparing the E-box regions in the enhancer and the promoter, those in the promoter appear to make a greater contribution to pTalpha gene transcription.

Involvement of SHP-1 tyrosine phosphatase in TCR-mediated signaling pathways in lipid rafts

To elucidate the process of TCR-mediated signaling pathways in lipid rafts, we constructed a chimeric molecule that localizes activated SHP-1 to rafts. Raft targeting of activated SHP-1 in Jurkat-derived transfectants completely inhibited the expression of CD69 and transcriptional factors after TCR cross-linking. Whereas the inducible tyrosine phosphorylation of TCR zeta and ZAP-70 and the kinase activity of Lck were intact, phosphorylated LAT was rapidly dephosphorylated by raft targeting of activated SHP-1, leading to defects in LAT activation and subsequent downstream signaling events. Intriguingly, recruitment of endogenous SHP-1 to rafts and its association with LAT were dramatically increased after TCR engagement, suggesting that SHP-1 is involved in raft-mediated T cell activation.

A pivotal role of cysteine 3 of Lck tyrosine kinase for localization to glycolipid-enriched microdomains and T cell activation

Lck, a Src family protein tyrosine kinase (PTKs), is post-translationally modified by palmitoylation, a process thought to regulate the biological function, membrane affinity and glycolipid-enriched microdomain (GEM) localization of this molecule. To examine the importance of palmitoylation sites Cys3 and Cys5 in Lck, one or both of these residues was mutated to serine to create mutants S3, S5, and S3,5, respectively. Immunofluorescence and confocal microscopy of COS-7 cells transfected with these constructs showed that while S5 and S3 localized to the plasma membrane, S3,5 was localized to the cytoplasm, suggesting that palmitoylation at at least one site is essential for membrane localization. Sucrose gradient based fractionation of these mutants expressed in COS-7 cells showed that while S5 localized to GEMs in similar fashion to the wild type, GEM localization of S3 was severely inhibited. Expression of these mutants in Lck-negative JCaM1 cells showed that although S5 reconstituted activation of nuclear factor NFAT as per the wild type, S3 expression failed to do so. These results suggest that Cys3 of Lck plays a more important role than Cys5 in GEM localization and T cell activation. Additionally, it was found that the degree of T cell function recovery is positively correlated with the degree of Lck expression in GEMs.

The cytoskeleton-associated TCR zeta chain is constitutively phosphorylated in the absence of an active p56(lck) form

The TCR recognizes peptide-MHC complexes and transmits activation signals leading to cellular responses. We have previously characterized two TCR populations expressed on the T cell surface; one is linked to the cytoskeleton via a detergent-insoluble cytoskeleton-associated zeta (cska-zeta) chain, while the other is detergent soluble and not linked to the cytoskeleton. The cska-zeta form displays unique properties: it is constitutively phosphorylated, does not undergo hyperphosphorylation upon TCR stimulation as opposed to its non-cytoskeleton-associated counterpart (non-cska-zeta) and it maintains a molecular mass of 16 kDa. It is well established that p56(lck) and possibly p59(fyn) are responsible for the generation of the 21/23-kDa phosphorylated detergent-soluble zeta form. We now demonstrate that the phosphorylation of cska-zeta does not require the activity of p56(lck). We also show that although Lck does not phosphorylate cska-zeta in vivo, it retains the capacity to phosphorylate cska-zeta in vitro. Moreover, differences in zeta-associated kinase activity were detected for non-cska-zeta and cska-zeta. Our results indicating that different kinases phosphorylate the two zeta forms are consistent with a growing consensus that each TCR form may regulate distinct cellular functions.

Cell surface expression of calnexin, a molecular chaperone in the endoplasmic reticulum

The folding and assembly of nascent proteins in the endoplasmic reticulum are assisted by the molecular chaperone calnexin, which is itself retained within the endoplasmic reticulum. It was up to now assumed that calnexin was selectively expressed on the surface of immature thymocytes because of a particular characteristic of the protein sorting machinery in these cells. We now report that a small fraction of calnexin is normally expressed on the surface of various cells such as mastocytoma cells, murine splenocytes, fibroblast cells, and human HeLa cells. Surface biotinylation followed by chase culture of living cells revealed that calnexin is continuously delivered to the cell surface and then internalized for lysosomal degradation. These results suggest that there is continuous exocytosis and endocytosis of calnexin, and the amount of calnexin on the plasma membrane results from the balance of the rates of these two events. To study the structural requirement of calnexin for surface expression, we created deletion mutants of calnexin and found that the luminal domain, particularly the glycoprotein binding domain, is necessary. These findings suggest that the surface expression of calnexin depends on the association with glycoproteins and that calnexin may play a certain role as a chaperone on the plasma membrane as well.

Serine 6 of Lck tyrosine kinase: a critical site for Lck myristoylation, membrane localization, and function in T lymphocytes

Lck is a member of the Src family kinases expressed predominantly in T cells, and plays a pivotal role in TCR-mediated signal transduction. Myristoylation of glysine 2 in the N-terminal Src homology 4 (SH4) domain of Lck is essential for membrane localization and function. In this study, we examined a site within the SH4 domain of Lck regulating myristoylation, membrane localization, and function of Lck. A Lck mutant in which serine 6 (Ser6) was substituted by an alanine was almost completely cytosolic in COS-7 cells, and this change of localization was associated with a drastic inhibition of myristoylation in this mutant. To assess the role of Ser6 of Lck in T cell function, we established stable transfectants expressing various Lck mutants using Lck-negative JCaM1 cells. The Lck mutant of Ser6 to alanine, most of which did not target to the plasma membrane, was not able to reconstitute TCR-mediated signaling events in JCaM1 cells, as analyzed by tyrosine phosphorylation of intracellular proteins and CD69 expression. These results demonstrate that Ser6 is a critical factor for Lck myristoylation, membrane localization, and function in T cells, presumably because the residue is important for N-myristoyl transferase recognition.

Isolation and characterization of a novel HS1 SH3 domain binding protein, HS1BP3

We have isolated a novel gene, HS1BP3, which encodes an HS1 binding protein. Analysis of HS1BP3 cDNA indicates several potentially important segments, including a PX domain, a leucine zipper, immunoreceptor tyrosine-based inhibitory motif-like motifs and proline-rich regions. HS1BP3 associates with HS1 proteins in vivo as confirmed by immunoprecipitation in B and T cell lines. HS1BP3 preferentially associates with the HS1 SH3 domains rather than with other SH3 molecules, suggesting a role of HS1BP3 as an HS1 signaling mediator. Overexpression of mutant HS1BP3 protein in T cell lines results in decreased IL-2 production. Our data suggest a novel role for HS1BP3 in lymphocyte activation.

Translocation of tyrosine-phosphorylated TCRzeta chain to glycolipid-enriched membrane domains upon T cell activation

Recent studies point to glycolipid-enriched membrane (GEM) microdomains as the critical sites for TCR-mediated signal transduction. However, whether the TCR complex is localized in the GEM domain is not well-defined. In the present study, we analyzed localization of the TCR-CD3 complex in the GEM domain by isolating the GEM fraction with sucrose density gradient centrifugation. Although 10% of TCRzeta chains was localized in the GEM fraction, most of the TCR complexes were excluded from the GEM before and after T cell activation, and the amount of TCRzeta in the GEM was not increased after activation. However, the tyrosine-phosphorylated form of TCRzeta was strongly concentrated in the GEM fraction upon TCR engagement. A kinetic study revealed that tyrosine phosphorylation of TCRzeta occurred initially in the Triton X-100-soluble membrane fraction followed by the accumulation of phosphorylated TCRzeta in the GEM. Thus, these results indicate that phosphorylated TCRzeta migrates into the GEM domains on T cell activation. We speculate that the GEM microdomains may function as a reservoir of activation signals from triggered TCR.

Expression of the T cell antigen receptor zeta chain following activation is controlled at distinct checkpoints. Implications for cell surface receptor down-modulation and re-expression

The multisubunit T cell antigen receptor (TCR) is involved in antigen recognition and signal transduction, leading to T cell activation and rapid down-modulation of the cell surface expressed TCRs. Although the levels of TCR cell surface expression are pivotal to the efficiency and duration of the immune response, the molecular mechanisms controlling TCR down-modulation and re-expression upon activation, remain obscure. Here, we provide a biochemical characterization of the regulatory mechanisms governing TCR expression following long-term T cell activation. We focused primarily on the TCR zeta chain, as this is considered the limiting factor in TCR complex formation and transport to the cell surface. We found that following TCR-mediated activation zeta mRNA is up-regulated by a transcription-dependent mechanism. Concomitantly, zeta protein levels are modified according to a biphasic pattern: rapid degradation coinciding with TCR cell surface down-regulation, followed by a rebound to normal levels 24 h subsequent to T cell activation. Even though there are adequate levels of all the TCR subunits within the cell following 24 h of activation, TCR cell surface expression remained very low, provided the activating antibody is continuously present. Correlative with the latter, we detected a previously undescribed monomeric form of the zeta chain. This form could be indicative of adverse endoplasmic reticulum conditions affecting correct protein folding, dimerization, and TCR assembly, all critical for optimal receptor surface re-expression. Cumulatively, our results indicate that the levels of TCR expression following activation, are tightly controlled at several checkpoints.

Control of early thymocyte development by the pre-T cell receptor complex: A receptor without a ligand?

Beta-selection refers to a developmental checkpoint linking thymocyte survival to the outcome of antigen receptor gene rearrangement. Immature thymocytes that productively rear-range the gene segments of the TCRbeta locus undergo proliferative expansion and mature to the CD4(+)CD8(+)stage; those failing to do so die by apoptosis. How are these precursor cells alerted that TCRbeta rearrangement has been productive? While it is clear that this process involves signals transduced by a surrogate form of the TCR termed the pre-TCR, it remains unclear how pre-TCR signals are triggered. In this review, we will discuss the implications of recent experimental attempts to address this issue, as well as how pre-TCR activation is linked to the changes in gene expression that underlie thymocyte development.

CAST, a novel CD3epsilon-binding protein transducing activation signal for interleukin-2 production in T cells

Antigen recognition through T cell receptor (TCR)-CD3 complex transduces signals into T cells, which regulate activation, function, and differentiation of T cells. The TCR-CD3 complex is composed of two signaling modules represented by CD3zeta and CD3epsilon. Signaling through CD3zeta has been extensively analyzed, but that via CD3epsilon, which is also crucial in immature thymocyte development, is still not clearly understood. We isolated cDNA encoding a novel CD3epsilon-binding protein CAST. CAST specifically interacts in vivo and in vitro with CD3epsilon but not with CD3zeta or FcRgamma via a unique membrane-proximal region of CD3epsilon. CAST is composed of 512 amino acids including a single tyrosine and undergoes tyrosine phosphorylation upon TCR stimulation. Overexpression of two dominant-negative types of CAST, a minimum CD3epsilon-binding domain and a tyrosine-mutant, strongly suppressed NFAT activation and interleukin-2 production. These results demonstrate that CAST serves as a component of preformed TCR complex and transduces activation signals upon TCR stimulation and represents a new signaling pathway via the CD3epsilon-containing TCR signaling module.

T Lymphocyte Development in the Absence of CD3ε or CD3γδεζ

CD3γ, δ, ε, and ζ proteins together with the pre-TCR α-chain (pTα) and a rearranged TCR β-chain assemble to form the pre-TCR that controls the double negative (DN) to double positive (DP) stages of thymopoiesis. The CD3 proteins are expressed before pTα and TCR β-chains in prothymocytes and are expressed intracellularly in precursor NK cells, suggesting that the CD3 complex may function independent of pTα and TCRβ. In this report, both the role of CD3ε exclusively, and the role of CD3 proteins collectively, in thymocyte and NK cell development were examined. In a mouse strain termed εΔP, a neomycin cassette inserted within the CD3ε promoter abolishes CD3ε and δ expression and also abolishes CD3γ expression in all but a small minority (≤1%) of prothymocytes. These prothymocytes became deficient in CD3ε alone upon reconstitution of CD3δ expression and were severely, but not completely, arrested at the DN stage, as small numbers of double positive thymocytes were detected. In de facto CD3γδεζnull mice generated by crossing the εΔP mice with CD3ζ−/− mice, thymopoiesis were arrested at the CD44−CD25+ DN stage as observed in RAG−/− mice, DJ and VDJ recombination at the TCRβ locus was functional, and normal numbers of NK cells were detected. Together, the findings demonstrate that during thymocyte development, the CD3 complex collectively is not essential until the critical CD44−CD25+ DN stage in which pre-TCR begins to function, whereas CD3ε is critical for the assembly of pre-TCR. Moreover, CD3 proteins are dispensable for NK cell development.

Molecular cloning of pTAC12 an alternative splicing product of the CD3gamma chain as a component of the pre-T cell antigen-receptor complex

We have reported that a 12-kDa molecule (pTAC12 as a pre-T cell receptor (TCR)-associated chain) was associated as a dimer with the pre-TCR complex as well as the clonotype-independent CD3 complex on the cell surface of immature thymocytes. We now report by protein sequencing and molecular cloning that pTAC12 is an alternatively spliced product of the CD3gamma chain lacking exon 4 containing the transmembrane region. The transcript of pTAC12 is expressed in most T cell lineages and parallels the expression of CD3gamma. However, the pTAC12 protein is expressed on the cell surface of immature thymocytes but not mature T cells, despite the fact that mature T cells express a low level of pTAC12 in association with the TCR complex within the cells. These results indicate that pTAC12 may play a special role for the transport/expression and assembly of the pre-TCR.CD3 complex as well as the clonotype-independent CD3 complex in immature thymocytes.

Essential and partially overlapping role of CD3gamma and CD3delta for development of alphabeta and gammadelta T lymphocytes

CD3gamma and CD3delta are two highly related components of the T cell receptor (TCR)-CD3 complex which is essential for the assembly and signal transduction of the T cell receptor on mature T cells. In gene knockout mice deficient in either CD3delta or CD3gamma, early thymic development mediated by pre-TCR was either undisturbed or severely blocked, respectively, and small numbers of TCR-alphabeta+ T cells were detected in the periphery of both mice. gammadelta T cell development was either normal in CD3delta-/- mice or partially blocked in CD3gamma-/- mice. To examine the collective role of CD3gamma and CD3delta in the assembly and function of pre-TCR and in the development of gammadelta T cells, we generated a mouse strain with a disruption in both CD3gamma and CD3delta genes (CD3gammadelta-/-). In contrast to mice deficient in either CD3gamma or CD3delta chains, early thymic development mediated by pre-TCR is completely blocked, and TCR-alphabeta+ or TCR-gammadelta+ T cells were absent in the CD3gammadelta-/- mice. Taken together, these studies demonstrated that CD3gamma and CD3delta play an essential, yet partially overlapping, role in the development of both alphabeta and gammadelta T cell lineages.

Growth Factor Receptor-Bound Protein 2 (Grb2) Association with Hemopoietic Specific Protein 1: Linkage Between Lck and Grb2

To analyze the growth factor receptor-bound protein 2 (Grb2) signaling pathway in lymphoid cells, we used expression cloning to isolate the genes encoding proteins that associate with Grb2. We find that the Src homology 3 domains of Grb2 directly associate, in vitro and in vivo, with murine hemopoietic specific protein 1 (HS1), a protein identical to Lck-binding protein 1. Because HS1 associates with the p56lck and p59lyn tyrosine kinases in vitro and in vivo, and becomes tyrosine phosphorylated upon various receptor stimulations, our present data suggest that HS1 mediates linkage between Lck or Lyn and Grb2 in lymphoid lineage cells.

Physical and functional association between thymic shared antigen-1/stem cell antigen-2 and the T cell receptor complex

Thymic shared antigen-1 (TSA-1)/stem cell Ag-2 (Sca-2) is a glycosylphosphatidylinositol (GPI)-anchored antigen expressed on lymphocytes. We have previously demonstrated that a signal via TSA-1/Sca-2 inhibits T cell receptor (TCR)-mediated T cell activation and apoptosis. To elucidate a molecular mechanism for TSA-1-mediated modulation of the TCR-signaling pathway, we examined whether TSA-1 is physically coupled to the TCR in the present study. TSA-1 was clearly associated with CD3zeta chains in T cell hybridomas, activated T cells, and COS-7 cells transfected with TSA-1 and CD3zeta cDNA. The physical association was confirmed on the surface of T cells in immunoprecipitation and confocal microscopy. The analysis using stable and transient transfectants expressing a transmembrane form of TSA-1 revealed that the association of CD3zeta did not require the GPI anchor of TSA-1. Finally, tyrosine phosphorylation of CD3zeta chains was induced after stimulation with anti-TSA-1, suggesting that a functional association between these two molecules also exists. These results imply that the physical association to CD3zeta underlies a regulatory role of TSA-1/Sca-2 in the TCR-signaling pathway.

The transmembrane region of CD2-associated signal-transducing proteins is crucial for the outcome of CD2-mediated T-cell activation

Signalling through the CD2 molecule was shown previously to employ similar signalling molecules as the T-cell receptor (TCR). Here, we show that CD2-mediated signalling is strongly influenced by the expressed transmembrane region of the employed signal-transducing molecule. We used TCR-negative cells expressing chimeric fusion proteins that consist of human interleukin-2 (IL-2) receptor alpha-chain-derived sequences (hCD25) fused to mouse-specific zeta-chain segments (hCD25-zeta). One set of TCR-negative cell lines expressed the hCD25-derived extracellular part fused to mouse-specific transmembrane and cytoplasmic zeta-protein sequences ('TZZ'). The second type of cell lines expressed the hCD25-derived extracellular and transmembrane portions fused to the mouse-specific zeta-chain cytoplasmic segment ('TTZ'). After cross-linking the hCD25-zeta molecules with specific monoclonal antibodies (mAb), all TCR-negative cell lines produced similar amounts of IL-2. Cross-linking with stimulating pairs of CD2-specific mAb, however, led to IL-2 production only in cell lines expressing the zeta-chain-specific transmembrane segment. Co-cross-linking of CD25 and CD2 molecules resulted in an effective stimulation of both TZZ- and TTZ-expressing cell lines. Moreover, TTZ- and TZZ-expressing cell lines differed in their pattern of tyrosine-phosphorylated proteins after stimulation with hCD25-specific mAb. Thus, although CD2 and TCR molecules share signalling components and pathways, the fine tuning of CD2 co-receptor function appears to be regulated in part by transmembrane regions of signal-transducing molecules like the TCR-associated zeta-chain.

A novel 90-kDa tyrosine-phosphorylated protein associated with TCR complex in thymocytes

Ligation of the TCR-CD3 complex initiates a cascade of tyrosine phosphorylation that results in T cell activation. Initial activation of tyrosine kinases depends on the phosphorylation of activation motifs on CD3 chains. We previously found that a 90-kDa protein was tyrosine phosphorylated upon TCR cross-linking and the induction of the phosphorylation was dependent on the structure of the CD3 complex. In this study, we further characterized p90 phosphorylation. Phosphorylation of p90 was induced only by stimulation through the TCR-CD3 complex but not by other kinds of stimulation including CD28- or hydrogen peroxide-mediated activation and was dynamically regulated. Phosphorylated p90 was associated with the TCR-CD3 complex upon T cell activation. In a normal T cell population, thymocytes but not splenic T cells induced the tyrosine phosphorylation of p90 upon TCR cross-linking. These results suggest that p90 is a novel phosphoprotein associated with the TCR-CD3 complex and may play a role in TCR signaling during thymocyte differentiation.

Subunit composition of the pre-T-cell receptor complex analysed by monoclonal antibody against the pre-T-cell receptor alpha chain

The pre-T-cell receptor (TCR) complex, which consists of a heterodimer of the TCR beta-chain and the pre-TCR alpha-chain, is known to regulate early thymocyte development. The pre-TCR complex contains CD3 subunits as a signal-transducing molecule, but the exact subunit composition of the fully assembled pre-TCR complex remains to be elucidated. In particular, the association of the CD3 zeta-chain with the pre-TCR is controversial. In the present study, we have generated a monoclonal antibody against the cytoplasmic portion of the pre-TCR alpha-chain, and analysed a subunit composition of the pre-TCR complex. We demonstrated that the CD3 zeta-chain is physically associated with the pre-TCR in immature T cells. Thus, the result strongly supports the previous findings that CD3 zeta contributes to signalling mediated through the pre-TCR complex.

TCR zeta chain in T cell development and selection

Current data suggest that an important function of the multimeric structure of the TCR is to enable the assembly of structurally and functionally different forms of the TCR, the pre-TCR and alphabetaTCR complexes, at different stages in development. Four distinct TCR subunits (the CD3gamma, delta, and epsilon chains and the zeta chain) contain signal transducing motifs; however, the zeta chain is notable for containing three of these elements. These motifs, especially those within the zeta chain, function to amplify signals generated by the TCR, and this property is especially critical during thymocyte selection. The results of several recent experiments argue that positive and negative selection of thymocytes may involve activation of distinct downstream signaling pathways. The outcome of thymocyte selection can also be influenced, however, by quantitative effects such as changes in ligand concentration or direct alteration of the TCR signaling potential. Recent studies pertaining to the kinetics of TCR-ligand interactions may provide insight into how signaling through the TCR can be regulated either quantitatively or qualitatively.

CD3 delta deficiency arrests development of the alpha beta but not the gamma delta T cell lineage

The CD3 complex found associated with the T cell receptor (TCR) is essential for signal transduction following TCR engagement. During T cell development, TCR-mediated signalling promotes the transition from one developmental stage to the next and controls whether a thymocyte undergoes positive or negative selection. The roles of particular CD3 components in these events remain unclear. Indeed, it is unknown whether they have specialized or overlapping roles. However, the multiplicity of CD3 components and their evolutionary conservation suggest that they serve distinct functions. Here the developmental requirement for the CD3 delta chain is analyzed by generating a mouse line specifically lacking this component (delta-/- mice). Strikingly, CD3 delta is shown to be differentially required during development. In particular, CD3 delta is not needed for steps in development mediated by pre-TCR or gamma delta TCR, but is required for further development of thymocytes expressing alpha beta TCR. Absence of CD3 delta specifically blocks the thymic selection processes that mediate the transition from the double-positive to single-positive stages of development.

Co-receptor-independent signal transduction in a mismatched CD8+ major histocompatibility complex class II-specific allogeneic cytotoxic T lymphocyte

The contribution of co-receptors in signal transduction upon T cell receptor (TCR)-mediated recognition of major histocompatibility complex (MHC) class II antigen by mature T lymphocytes expressing TCR derived from the apparently co-receptor-independent, I-Ak-specific allogeneic CD8+ CTL clone QM11 has been examined. Mature double-negative, CD8+ and CD4+ bulk T cell lines and clones expressing TCR(QM11) were developed from TCR(QM11) transgenic mice. All these T cells, irrespective of co-receptor expression, showed specific lytic activity on cells expressing I-Ak. Furthermore, co-receptorless mutants were obtained from a CD4+ and CD8+ clone. The responses of these co-receptorless mutants upon specific recognition of the alloantigen, as judged by cytolytic activity, granule exocytosis, lymphokine production, proliferation, and tyrosine phosphorylation of the zeta chain, were comparable to those of the original clones. Thus, the results proved the co-receptor independence of the recognition of I-Ak by TCR(QM11) and further indicated there is no indispensable unique signal transduced by co-receptors. However, when the amount of the available antigen was limited by anti-I-Ak antibody, the CD4+ T cell clone showed a remarkable resistance to the inhibition whereas the mismatched CD8+ clone was readily inhibitable. The anti-I-Ak-resistant component of the CD4+ clone showed dependency on the CD4 molecule. Taken collectively, the results indicate that the role played by a co-receptor molecule in mature T cells is purely quantitative amplification of the signal through the formation of a TCR/MHC/co-receptor ternary complex, and also indicate that the role of co-receptor molecules as TCR-independent adhesion molecules is at best minimal.

Domains of the TCR beta-chain required for early thymocyte development

The T cell receptor beta (TCR beta) chain controls the developmental transition from CD4-CD8- to CD4+8+thymocytes. We show that the extracellular constant region and the transmembrane region, but not the variable domain or cytoplasmic tail of the TCR beta chain are required for this differentiation step. TCR beta mutant chains lacking the cytoplasmic tail can be found at the cell surface both in functional TCR/CD3 complexes and in a GPI-anchored monomeric form indicating that the cytoplasmic tail of the TCR beta chain functions as an ER retention signal. The concordance between cell surface expression of the mutant chains as TCR/CD3 complexes and their capacity to mediate thymocyte differentiation supports the CD3 mediated feedback model in which preTCR/CD3 complexes control the developmental transition from CD4-CD8- to CD4+CD8+thymocytes.

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.

CD3-induced apoptosis of CD4+CD8+ thymocytes in the absence of clonotypic T cell antigen receptor

Clonal selection of T cells mediated through the T cell antigen receptor (TCR) mostly occurs at the CD4+CD8+ double positive thymocyte stage. Immature CD4+CD8+ thymocytes expressing self-reactive TCR are induced to die upon clonotypic engagement of TCR by self antigens. CD3 engagement by antibody of the surface TCR-CD3 complex is known to induce apoptosis of CD4+CD8+ thymocytes, a process that is generally thought to represent antigen-induced negative selection in the thymus. The present study shows that the CD3-induced apoptosis of CD4+CD8+ thymocytes can occur even in TCR alpha- mutant mice which do not express the TCR alpha beta/CD3 antigen receptor. Anti-CD3 antibody induces death of CD4+CD8+ thymocytes in TCR alpha- mice either in cell cultures or upon administration in vivo. Interestingly, most surface CD3 chains expressed on CD4+CD8+ thymocytes from TCR alpha- mice are not associated with clonotypic TCR chains, including TCR beta. Thus, apoptosis of CD4+CD8+ thymocytes appear to be induced through the CD3 complex even in the absence of clonotypic antigen receptor chains. These results shed light on previously unknown functions of the clonotype-independent CD3 complex expressed on CD4+CD8+ thymocytes, and suggest its function as an apoptotic receptor inducing elimination of developing thymocytes.

Composition and function of T-cell receptor and B-cell receptor complexes on precursor lymphocytes

The precursor T-cell receptors (TCRs) and B-cell receptors (BCRs) direct lymphocyte development to the mature T-cell and B-cell stage, respectively. Recent genetic and biochemical experiments reveal the striking parallel in structure and function of these receptors. They consist of TCR beta and BCR mu chains paired with surrogate TCR alpha and BCR light chains. Both receptors employ a two-component signal transduction unit: CD3 gamma epsilon for the pre-TCR, and CD79ab for the pre-BCR. Plasma membrane levels of pre-TCR/BCR complexes are kept extremely low, most probably by a mechanism involving specific retention in the endoplasmic reticulum. This mechanism may control the signalling activity of pre-TCR/BCR and therewith the lymphocyte differentiation process.

Insights into T cell development and signal transduction provided by TCR-zeta chain deficient mice

The T cell antigen receptor (TCR) transduces signals that mediate different responses depending on the stage of development of the T cell and the nature of the ligand it engages. The presence of multiple signal transducing subunits (CD3-gamma-delta,-epsilon and zeta chain) suggests the potential to control these responses by altering the subunit composition of the TCR. zeta chain represents an especially important signalling molecule as it contains multiple signalling motifs within its cytoplasmic tail. The generation and analysis of zeta deficient (zeta-/-) and zeta-transgenic mice has provided insight into the role of zeta as well as the CD3 subunits in TCR surface expression, T cell activation and thymocyte development. Herein, we discuss the results from such experiments which suggest distinct roles for zeta chain and the CD3 components at different stages of T cell development.

Characterization of immature thymocyte lines derived from T-cell receptor or recombination activating gene 1 and p53 double mutant mice

The T-cell receptor (TCR) beta chain is instrumental in the progression of thymocyte differentiation from the CD4-CD8- to the CD4+CD8+ stage. This differentiation step may involve cell surface expression of novel CD3-TCR complexes. To facilitate biochemical characterization of these complexes, we established cell lines from thymic lymphomas originating from mice carrying a mutation in the p53 gene on the one hand and a mutation in TCR-alpha, TCR-beta, or the recombination activating gene 1 (RAG-1) on the other hand. The cell lines were CD4+CD8+ and appeared to be monoclonal. A cell line derived from a RAG-1 x p53 double mutant thymic lymphoma expressed low levels of CD3-epsilon, -gamma, and -delta on the surface. TCR-alpha x p53 double mutant cell lines were found to express complexes consisting of TCR-beta chains associated with CD3-epsilon, -gamma, and -delta chains and CD3-zeta zeta dimers. These lines will be useful tools to study the molecular structure and signal transducing properties of partial CD3-TCR complexes expressed on the surface of immature thymocytes.

Regulation of thymocyte development through CD3: functional dissociation between p56lck and CD3 sigma in early thymic selection

We studied the extent of functional linkage between CD3 sigma and p56lck in pre-TCR-dependent thymocyte development. Differentiation of DN to DP cells was examined by treatment of RAG2/CD3 sigma and RAG1/p56lck double-deficient mice with anti-CD3 epsilon antibodies. The results suggest that CD3 sigma has no specific role in this maturation step, but may be important for amplification of signaling through the pre-TCR. In contrast, p56lck is the main protein tyrosine kinase associated with signaling through the pre-TCR-CD3 complex. In DP thymocytes, the Ca2+ response to anti-CD3 epsilon was totally abolished in CD3 sigma-I-but only reduced in p56lck-I-mice, and in vivo responses to anti-CD3 epsilon differed from one another. Thus, CD3 sigma and p56lck are functionally not tightly associated and their deficiencies cause distinct developmental defects.

Distinct domains of the CD3-gamma chain are involved in surface expression and function of the T cell antigen receptor

The T cell antigen receptor (TcR) is a multisubunit complex that consists of at least six different polypeptides. We have recently demonstrated that the CD3-delta subunit cannot substitute for the CD3-gamma subunit in TcR cell surface expression, in spite of significant amino acid homology between these two subunits. To identify CD3-gamma-specific domains that are required for assembly of the complete TcR and for surface expression and function of the TcR, chimeric CD3-gamma/CD3-delta molecules were constructed and expressed in T cells devoid of endogenous CD3-gamma. Substitution of the extracellular domain of CD3-gamma with that of CD3-delta did not allow cell surface expression of the TcR. In contrast, substitution of the transmembrane and/or the intracellular domains of CD3-gamma with those of CD3-delta did allow TcR cell surface expression. These results conclusively demonstrate that the extracellular domain of CD3-gamma plays a unique role in TcR assembly. Functional analyses of the transfectants demonstrated that the intracellular domain of CD3-gamma is required for protein kinase C-mediated down-regulation of TcR but is dispensable for the pattern of tyrosine phosphorylation observed following activation through TcR.

Development of CD4+CD8+ thymocytes in RAG-deficient mice through a T cell receptor beta chain-independent pathway

Antigen-binding diversity is generated by site-specific V(D)J recombination of the T cell receptor (TCR) and immunoglobulin loci in lymphocyte precursors. Coordinate expression of two structurally distinct recombinase activating genes, RAG-1 and RAG-2, is necessary for activation of site-specific V(D)J recombination. In mice bearing targeted disruptions of either the RAG-1 or RAG-2 genes, T and B lymphocyte development is arrested at the CD4-8- double negative (DN) thymocyte or B220+/CD43+ pro-B cell stage. Development of CD4+CD8+ double positive (DP) thymocytes is restored by expression of a functionally rearranged TCR beta transgene, suggesting that TCR beta expression is critical for this developmental transition. We have found that treatment of adult or newborn RAG-deficient mice with a single sublethal dose of gamma-irradiation rescues the DN to DP transition in early thymocytes, and this is accompanied by a dramatic increase in thymus cellularity. In contrast to the observed induction of thymocyte maturation, there was no phenotypic or functional evidence of coincident B lymphocyte development in irradiated RAG-deficient mice. Interestingly, maturation of DP thymocytes occurred without expression of TCR beta protein in the cytoplasm or on the cell surface. These results suggest an in vivo pathway for DP thymocyte development which is TCR beta chain independent.

A signal strength hypothesis of thymic selection: preliminary considerations

During their differentiation, thymocytes are subjected to two rounds of selection. First, CD4-8- double-negative (DN) thymocytes with a functional TCR-beta chain express a alpha-beta+ CD3 complex on their surface and, as a consequence, are selected to mature to the CD4+8+ double-positive (DP) stage. This round ends after the initial proliferation of young DP thymocytes and is termed beta-chain selection. Second, DP thymocytes are selected on the basis of their alpha+beta+ CD3 complex. This is termed repertoire selection and the cells are given three choices: death by neglect selection, death by positive selection, or deletion by negative selection. Using anti-CD3 epsilon mAb as invariant ligand, signals for beta-chain selection of DN cells including proliferation of DP cells do not require a Ca2+ response, are independent of CD3 zeta, and are only slightly impaired in the absence of p56lck (lck). Signals that induce positive selection of DP thymocytes require a partial Ca2+ response and CD3 zeta but are independent of lck. Deletion of DP thymocytes requires a full-blown Ca2+ response and both, CD3 zeta and lck. Thymic selection thus appears to be governed by a gradient of signal intensities.

A highly conserved phenylalanine in the alpha, beta-T cell receptor (TCR) constant region determines the integrity of TCR/CD3 complexes

In the present study, we have investigated the importance of a phenylalanine (phe195) in the Tcr-C alpha region on Tcr-alpha,beta/CD3 membrane expression. An exchange of phe195 with a tyrosine residue does not affect Tcr/CD3 membrane expression; however, exchange with aspartic acid, histidine or valine prohibit completely Tcr/CD3 membrane expression. This seems to be due to a lack of interaction between mutated Tcr-alpha,beta/CD3-gamma epsilon,delta epsilon complexes and zeta 2 homodimers. The Tcr-C alpha region around phe195 seems together with the same region in the Tcr-C beta region to constitute an interaction site for zeta 2 homodimers. The presence of phe195 on both Tcr-C alpha and Tcr-C beta causes high avidity interaction with zeta 2 homodimers, whereas his195 in both Tcr-C gamma and Tcr-C delta results in an apparently lower avidity interaction with zeta 2 homodimers. It is suggested that the phe195 region (on beta-strand F) and eventually adjacent aromatic amino acid residues on beta-strand B region may play an important role in Tcr-alpha,beta/CD3 membrane expression, in Tcr-alpha,beta/CD3 competition with Tcr-gamma,delta/CD3 complexes for zeta 2 homodimers and in the control of formation of 'mixed' Tcr heterodimers.

An activated lck transgene promotes thymocyte development in RAG-1 mutant mice

Expression of the T cell receptor beta (TCR beta) chain is necessary for the transition from the CD4CD8- stage in the major alpha beta thymocyte lineage. The protein tyrosine kinase p56lck has been implicated in the regulation of early thymocyte differentiation and of allelic exclusion at the TCR beta locus. Using mice overexpressing an activated lck transgene and mice with a disruption of the lck gene, we demonstrate that p56lck participates in a pathway that regulates the expansion of the pool of CD4+CD8+ thymocytes to wild-type levels. In addition, p56lck may be involved in the down-regulation of the putative pre-TCR on CD4+CD8+ thymocytes.

CD3 components at the surface of pro-T cells can mediate pre-T cell development in vivo

Developmentally arrested pro-T cells (CD4-8-, IL-2R+, HSA++) of RAG-1-deficient mice appear to express low levels of CD3 molecules in the absence of T cell receptor (TcR) chains at their surface, while developmentally arrested pre-T cells of TcR alpha-deficient mice express low levels of a disulfide-linked TcR beta chain in association with CD3 molecules. Cross-linking of the CD3 modules on pro-T cells of RAG-1-/- mice in vivo, with either of two different CD3 epsilon-specific monoclonal antibodies, induces differentiation of these pro-T cells into pre-T cells (CD4+8+, IL-2R-, HSA+), concomitant with a rapid expansion of the thymic T cell compartment, up to 175-fold within 12 days. The same effects can be produced by introduction of a mutant TcR beta transgene lacking most of the variable domain (delta V-TcR beta) into the RAG-1-/- background. These experiments suggest that cross-linking of the CD3 modules on pro-T cells mimics the signaling function expected of the pre-TcR complex, which is found at the surface of pre-T cells prior to functional TcR alpha gene rearrangement. The variable domain of the TcR beta chain is apparently not essential for inducing these aspects of T cell development.

Restoration of early thymocyte differentiation in T-cell receptor beta-chain-deficient mutant mice by transmembrane signaling through CD3 epsilon

Thymic repertoire selection requires the expression of the alpha beta CD3 T-cell receptor (TCR) together with the coreceptors CD4 and CD8. The appearance of CD4 and CD8 on thymocytes is the hallmark of a complex maturation step, accompanied by downregulation of the interleukin 2 receptor (IL-2R) alpha chain, arrest of rearrangement (i.e., allelic exclusion) of the TCR beta-chain locus, a burst of cell divisions, and reduction in cell size. This maturation step is inhibited in TCR beta-chain-deficient mouse strains and may depend on surface expression of an immature TCR complex containing CD3 and TCR beta chains but no TCR alpha chain. Here we show that the CD4+8+ double-positive (DP) stage can be induced by treatment of fetal thymic organ cultures with anti-CD3 epsilon monoclonal antibodies in several TCR beta-chain-deficient mouse strains: severe combined immunodeficient (scid) mice, mice carrying a mutation in the recombination activating gene 1 (Rag-1), or mice carrying a deletion in the TCR beta-chain locus itself. These findings suggest that CD3 epsilon is expressed on the thymocyte surface independent of and prior to the TCR beta chain. The data are consistent with the notion that in wild-type mice the DP stage is induced by transmembrane signaling through an immature CD3-TCR beta-chain complex, which can be bypassed by crosslinking of CD3 epsilon alone.

Regulation of thymocyte development through CD3. II. Expression of T cell receptor beta CD3 epsilon and maturation to the CD4+8+ stage are highly correlated in individual thymocytes

Recent studies have shown that maturation of CD4-8- double negative (DN) thymocytes to the CD4+8+ double positive (DP) stage is dependent on expression of the T cell receptor (TCR)-beta polypeptide. The exact mechanism by which the TCR-beta chain regulates this maturation step remains unknown. Previous experiments had suggested that in the presence of some TCR+ thymocytes, additional DN thymocytes not expressing a TCR-beta chain may be recruited to mature to the DP stage. The recent demonstration of an immature TCR-beta-CD3 complex on early thymocytes lead to the alternative hypothesis that signal transduction through an immature TCR-CD3 complex may induce maturation to the DP stage. In the latter case, maturation to the DP stage would depend on the expression of TCR-beta-CD3 in the same cell. We examined these two hypotheses by studying the expression of the intra- and extracellular CD3 epsilon, CD3 zeta, and TCR-beta polypeptides in intrathymic subpopulations during embryogenesis. CD3 epsilon and CD3 zeta were expressed intracellularly 2 and 1 d, respectively, before intracellular expression of the TCR-beta chain, potentially allowing immediate surface expression of an immature TCR-beta-CD3 complex as soon as functional rearrangement of a TCR-beta gene locus has been accomplished. Calcium mobilization could be induced by stimulation with anti-CD3 epsilon mAb as soon as intracellular TCR-beta was detectable, suggesting that a functional TCR-beta-CD3 complex is indeed expressed on the surface of early thymocytes. From day 17 on, most cells were in the DP stage, and over 95% of the DP cells expressed on the TCR-beta chain intracellularly. At day 19 of gestation, extremely low concentrations of TCR-beta chain and CD3 epsilon were detectable on the cell surface of nearly all thymocytes previously thought to be TCR-CD3 negative. These findings strongly support the hypothesis that maturation to the DP stage depends on surface expression of and subsequent signal transduction through an immature TCR-beta-CD3 complex and suggest that maturation to the DP stage by recruitment, if it occurs at all, is of minor relevance.

A role for a pre-T-cell receptor in T-cell development

Expression of a productive T-cell receptor (TCR) beta gene has profound consequences for T-cell development, preceding an increase in thymocyte number, appearance of CD4 and CD8 coreceptors and suppression of further TCR beta-gene rearrangement. Here Marcus Groettrup and Harald von Boehmer discuss the data obtained in various experimental models and describe a novel signal transducing receptor complex on pre-T cells, which may regulate the TCR beta-induced developmental changes.