Developmentally regulated expression of CD3 components independent of clonotypic T cell antigen receptor complexes on immature thymocytes

Notch and the pre-TCR coordinate thymocyte proliferation by induction of the SCF subunits Fbxl1 and Fbxl12

Proliferation is tightly regulated during T cell development and is limited to immature CD4⁻CD8⁻ thymocytes. The major proliferative event is initiated at the ‘β-selection’ stage following successful rearrangement of Tcrβ and is triggered by and dependent on concurrent signaling by Notch and the pre-TCR; however, it is unclear how these signals cooperate to promote cell proliferation. Here we found that β-selection-associated proliferation required the combined activity of two SCF ubiquitin ligase complexes that included as substrate recognition subunits the F-box proteins Fbxl1 or Fbxl12. Both SCF complexes targeted the cyclin-dependent kinase inhibitor Cdkn1b for ubiquitinylaton and degradation. We found that Notch signals induced the transcription of Fbxl1 whereas pre-TCR signals induced the transcription of Fbxl12. Thus, concurrent Notch and pre-TCR signaling induced the expression of two genes, Fbxl1 and Fbxl12, whose products functioned identically but additively to promote degradation of Cdkn1b, cell cycle progression, and proliferation of β-selected thymocytes.

Alterations in the expression pattern of TCR zeta chain in T cells from patients with hematological diseases

The TCR zeta chain, a component of the T cell receptor (TCR)/CD3 complex, plays a significant role in the assembly of the receptor complex and in connecting antigen recognition to the intracellular signal transduction apparatus. Recently, studies have demonstrated altered expression and function of this signal transduction molecule in T cells from patients with hematological diseases. In this review, current knowledge concerning the biological feature and function of TCR zeta protein, splice variant and mutation of TCR zeta chain gene and alteration of expression pattern in hematological diseases and the related mechanism are summarized.

An orderly inactivation of intracellular retention signals controls surface expression of the T cell antigen receptor

Exit from the endoplasmic reticulum (ER) is an important checkpoint for proper assembly of multimeric plasma membrane receptors. The six subunits of the T cell receptor (TCR; TCRα, TCRβ, CD3γ, CD3δ, CD3ɛ, and CD3ζ) are each endowed with ER retention/retrieval signals, and regulation of its targeting to the plasma membrane is therefore especially intriguing. We have studied the importance of the distinct ER retention signals at different stages of TCR intracellular assembly. To this end, we have characterized first the presence of ER retention signals in CD3γ. Despite the presence of multiple ER retention signals in CD3γ, ɛγ dimers reach the cell surface when the single CD3ɛ ER retention signal is deleted. Furthermore, inclusion of this CD3ɛ mutant promoted plasma membrane expression of incomplete αβγɛ and αβδɛ complexes without CD3ζ. It therefore appears that the CD3ɛ ER retention signal is dominant and that it is only overridden upon the incorporation of CD3ζ. We propose that the stepwise assembly of the TCR complex guarantees that all assembly intermediates have at least one functional ER retention signal and that only a full signaling-competent TCR complex is expressed on the cell surface.

Cyclic adenosine 5'-monophosphate response element binding protein plays a central role in mediating proliferation and differentiation downstream of the pre-TCR complex in developing thymocytes

The roles played by specific transcription factors during the regulation of early T cell development remain largely undefined. Several key genes induced during the primary checkpoint of T cell development, beta-selection, contain cAMP response element sites within their enhancer-promoter region that are regulated by CREB activation. In this study, we show that CREB is constitutively phosphorylated in the thymus, but not the spleen. We also show that CREB is activated downstream of the pre-TCR complex, and that the induction of CREB activity is regulated by protein kinase C alpha- and ERK-MAPK-mediated signals. We addressed the importance of this activation by expressing a naturally occurring inhibitor of CREB, inducible cAMP early repressor in wild-type fetal liver-derived lymphoid progenitor cells, and assessed their developmental potential. Fetal thymic organ cultures reconstituted with cells constitutively expressing inducible cAMP early repressor displayed a delay in generating CD4(+)CD8(+) thymocytes and a decrease in cellularity compared with control fetal thymic organ cultures. Taken together, our studies establish that CREB plays a central role in relaying proliferation and differentiation signals from the pre-TCR complex into the nucleus in developing thymocytes.

Initiation of TCR signaling: regulation within CD3 dimers

The number of possible T cell activation outcomes resulting from T cell receptor (TCR) engagement suggests that the TCR is able to differentially activate a myriad of signaling pathways depending on the nature of the stimulus. The complex structural organization of the TCR itself could underlie this diversity of responses. Assembly and stoichiometric studies have helped us to shed some light on the initiation of TCR signaling. The TCR is composed of TCR and CD3 dimers. Changes in the interaction between CD3 subunits within the CD3 dimers and in the interaction of these dimers with the TCR heterodimer could be the triggering mechanism that initiates the first activation events. One of the hallmarks of these early changes in TCR conformation is the induced recruitment of the adapter protein Nck to a proline-rich sequence of the cytoplasmic tail of CD3epsilon, but there may be others. According to our most recent observations, the TCR is organized in pre-existing clusters within plasma membrane microdomains, exhibiting a complexity above and beyond that of dimer composition complexity. How the presence of TCR in clusters influences TCR avidity and propagation of TCR signals is something that has yet to be investigated.

An architectural perspective on signaling by the pre-, alphabeta and gammadelta T cell receptors

The T cell antigen receptor (TCR) is a multimeric complex composed of an antigen-binding clonotypic heterodimer and a signal transducing complex consisting of the CD3 dimers (CD3gammaepsilon and CD3deltaepsilon) and a TCR-zeta homodimer. In all jawed vertebrates there are two T cell lineages, alphabeta and gammadelta, distinguished by the clonotypic subunits contained within their TCRs (TCR-alpha and -beta or TCR-gamma and -delta, respectively). A third receptor complex, the preTCR, is only expressed on immature T cells. The preTCR, which contains the invariant pre-Talpha (pTalpha) chain in lieu of TCR-alpha, plays a critical role in the early development of alphabeta lineage cells. The subunit composition of the signal transducing complexes of the pre-, alphabeta- and gammadeltaTCRs was previously thought to be identical. However, recent data demonstrate that there are significant differences in the signal transducing complexes of these three TCRs. For example, alphabetaTCRs contain both CD3gammaepsilon and CD3deltaepsilon dimers, whereas gammadeltaTCRs contain only CD3gammaepsilon dimers. Moreover, preTCR function appears to be unaffected in the absence of CD3delta, suggesting that CD3deltaepsilon dimers are dispensable for pre-TCR assembly. In this review, we summarize current data relating to the subunit composition of the pre-, alphabeta- and gammadeltaTCRs and discuss how these structural differences may impact receptor signaling and alphabeta/gammadelta lineage determination.

Early growth response transcription factors are required for development of CD4(-)CD8(-) thymocytes to the CD4(+)CD8(+) stage

Progression of immature CD4(-)CD8(-) thymocytes beyond the beta-selection checkpoint to the CD4(+)CD8(+) stage requires activation of the pre-TCR complex; however, few of the DNA-binding proteins that serve as molecular effectors of those pre-TCR signals have been identified. We demonstrate in this study that members of the early growth response (Egr) family of transcription factors are critical effectors of the signals that promote this developmental transition. Specifically, the induction of three Egr family members (Egr1, 2, and 3) correlates with pre-TCR activation and development of CD4(-)CD8(-) thymocytes beyond the beta-selection checkpoint. Enforced expression of each of these Egr factors is able to bypass the block in thymocyte development associated with defective pre-TCR function. However, Egr family members may play somewhat distinct roles in promoting thymocyte development, because there are differences in the genes modulated by enforced expression of particular Egr factors. Finally, interfering with Egr function using dominant-negative proteins disrupts thymocyte development from the CD4(-)CD8(-) to the CD4(+)CD8(+) stage. Taken together, these data demonstrate that the Egr proteins play an essential role in executing the differentiation program initiated by pre-TCR signaling.

Immature thymocytes that fail to express TCRbeta and/or TCRgamma delta proteins die by apoptotic cell death in the CD44(-)CD25(-) (DN4) subset

Pre-TCR/CD3 signals are essential for survival and maturation of (CD44(-)25(+)) DN3 thymocytes via the (CD44(-)25(-)) DN4 stage to CD4(+)CD8(+) (DP) cells, a process termed beta-selection. The exact developmental stages of apoptosis resulting from lack of pre-TCR/CD3 signals have so far not been determined. Here we analyzed apoptotic cell death in relation to expression of clonotypic TCR polypeptides and to cell cycle status in immature thymocyte subpopulations of wild type (wt) mice and of several strains of mice with compromised pre-TCR/CD3 signaling complexes. In wt mice or pre-TCR/CD3-deficient mice, apoptotic cells could not be detected among DN3 cells but accumulated in a subset of DN4 expressing CD69. Apoptotic CD69(+)DN4 cells were rare in wt mice and were found among DN4 cells that were negative or low for intracellular TCRbeta and negative for TCRgamma delta polypeptide chains. Apoptotic CD69(+)DN4 cells were abundant in pre-TCR/CD3 signaling-deficient mice in which most DN4 cells failed to express clonotypic TCR polypeptides. Survival of DN4 cells, but not maturation of DN3 cells to DN4, was found to depend on the expression of clonotypic TCR polypeptides in the same cell. The results suggest that thymocytes unsuccessful in alpha beta or in gamma delta lineage development die by apoptosis in the DN4 subset.

Expression and selection of productively rearranged TCR beta VDJ genes are sequentially regulated by CD3 signaling in the development of NK1.1(+) alpha beta T cells

The generation of thymic NK1.1(+)alpha beta T (NKT) cells involves positive selection of cells enriched for V(alpha)14/V(beta)8 TCR by CD1d MHC class I molecules. However, it has not been determined whether positive selection is preceded by pre-TCR-dependent beta selection. Here we studied NKT cell development in CD3 signaling-deficient mice (CD3 zeta/eta(-/-) and/or p56(lck-/-)) and TCR alpha-deficient mice. In contrast to wild-type mice, NK1.1(+) thymocytes in CD3 signaling-deficient mice are approximately 10-fold reduced in number, do not exhibit V(alpha)14-J(alpha)281 rearrangements and fail to express alpha beta TCR at the cell surface. However, they exhibit TCR beta VDJ rearrangements and pre-T alpha mRNA, suggesting that they contain pre-NKT cells. Strikingly, pre-NKT cells of CD3 zeta/Lck double-deficient mice fail to express TCR beta mRNA and protein. Whereas in wild-type NKT cells TCR beta VDJ junctions are selected for productive V(beta)8 and against productive V(beta)5 rearrangements, V(beta)8 and V(beta)5 rearrangements are non-selected in pre-NKT cells of CD3 signaling-deficient mice. Thus, pre-NKT cell development in CD3 signaling-deficient mice is blocked after rearrangement of TCR beta VDJ genes but before expression of TCR beta proteins. Most NKT cells of TCR alpha-deficient mice exhibit cell surface gamma delta TCR. In contrast to pre-NKT cells of CD3 signaling-deficient mice, approximately 25% of NKT cells of TCR alpha-deficient mice exhibit intracellular TCR beta polypeptide chains. Moreover, both V(beta)8 and V(beta)5 families are selected for in-frame VDJ joints in the TCR beta(+) NKT cell subset of TCR alpha-deficient mice. The data suggest that CD3 signals regulate initial TCR beta VDJ gene expression prior to beta selection in developing pre-NKT cells.

Developmentally regulated expression of the transmembrane adaptor protein trim in fetal and adult T cells

TRIM is a recently identified transmembrane adaptor protein which is exclusively expressed in T cells and natural killer (NK) cells. In peripheral blood T cells TRIM has been reported to coprecipitate, comodulate, and cocap with the T-cell receptor (TCR), suggesting that it is an integral component of the TCR/CD3/zeta complex. Here we investigate the expression of TRIM mRNAs and proteins in developing thymocytes. Two splicing isoforms with open reading frames are observed, namely a full length (TRIM) and a truncated version (DeltaTM-TRIM). The latter lacks the extracellular and transmembrane domains as well as the first 10 cytoplasmic aminoacids and is significantly expressed only as mRNA in early fetal thymocytes. TRIM mRNA is detected in all mainstream thymocyte subsets in adult mice. TRIM protein, in contrast, first appears in the DN2 (CD44+ CD25+) subset of adult double negative (DN) cells. In fetal thymocyte development, TRIM mRNA is seen from dg 14.5 onwards whereas TRIM protein appears first on dg 16.5. In contrast to the adult, the TRIM protein was seen in a subset of fetal DN1 cells. In fetal and adult thymocytes, TRIM protein expression was highest in DN2, DN3 (CD44-25+) and in DP cells, compatible with a functional role at or around phases of thymic selection.

Identification of a novel pre-TCR isoform in which the accessibility of the TCR beta subunit is determined by occupancy of the 'missing' V domain of pre-T alpha

We have identified a novel pre-TCR isoform that is structurally distinct from conventional pre-TCR complexes and whose TCR beta chains are inaccessible to anti-TCR beta antibodies. We term this pre-TCR isoform the MB (masked beta)-pre-TCR. Pre-T alpha (pT alpha) subunits of MB-pre-TCR complexes have a larger apparent mol. wt due to extensive modification with O:-linked carbohydrates; however, preventing addition of O-glycans does not restore antibody recognition of the TCR beta subunits of MB-pre-TCR complexes. Importantly, accessibility of TCR beta chains in MB-pre-TCR complexes is restored by filling in the 'missing' variable (V) domain of pT alpha with a V domain from TCR alpha. Moreover, the proportion of pre-TCR complexes in which the TCR beta subunits are accessible to anti-TCR beta antibody varies with the cellular context, suggesting that TCR beta accessibility is controlled by a trans-acting factor. The way in which this factor might control TCR beta accessibility as well as the physiologic relevance of TCR beta masking for pre-TCR function are discussed.

A comprehensive characterization of the T-cell antigen receptor complex composition by microcapillary liquid chromatography-tandem mass spectrometry

It has become apparent that many intracellular signaling processes involve the dynamic reorganization of cellular proteins into complex signaling assemblies that have a specific subunit composition, function, and subcellular location. Since the elements of such assemblies interact physically, multiprotein signaling complexes can be isolated and analyzed. Recent technical advances in highly sensitive protein identification by electrospray-tandem mass spectrometry have dramatically increased the sensitivity with which such analyses can be performed. The T-cell antigen receptor (TCR) is an oligomeric transmembrane protein complex that is essential to T-cell recognition and function. The extracellular protein domains are responsible for ligand binding while intracellular domains generate and transduce signals in response to specific receptor-ligand interactions. We used microbore capillary chromatography-tandem mass spectrometry to investigate the composition of the TCR protein complex isolated from resting and activated cells of the murine T-cell line CD11.3. We identified all the previously known subunits of the TCR/CD3 complex as well as proteins previously not known to associate with the TCR. The catalytic activities of some of these proteins could potentially be used to interfere pharmacologically with TCR signaling.

T cell receptor assembly and expression in the absence of calnexin

Most subunits of the alphabeta deltaepsilon gammaepsilon zetazeta T cell antigen receptor (TCR) complex associate with the molecular chaperone calnexin shortly after their synthesis in the endoplasmic reticulum, including clonotypic TCRalpha,beta molecules and invariant CD3gamma,delta,epsilon chains. While calnexin interaction is suggested to be important for the stability of newly synthesized TCRalpha subunits, the role of calnexin in the survival and assembly of remaining TCR components is unknown. Here we evaluated the expression of TCR proteins in CEM T cells and the calnexin-deficient CEM variant CEM.NK(R). We found that CEM and CEM.NK(R) cells constitutively synthesized all TCR subunits except for TCRalpha and that CD3gamma,delta,epsilon components and CD3-beta complexes were effectively assembled together in both cell types. The stability and folding of core CD3epsilon chains were similar in CEM and CEM.NK(R) cells. Interestingly, TCRalpha synthesis was differentially induced by phorbol myristate acetate treatment in CEM and CEM.NK(R) cells and TCRalpha proteins synthesized in CEM.NK(R) cells showed reduced survival compared to those made in CEM cells. Importantly, these data show that TCR complexes were inducibly expressed on CEM.NK(R) cells in the absence of calnexin synthesis. These results demonstrate that TCR complexes can be expressed in the absence of calnexin and suggest that the role of calnexin in the quality control of TCR assembly is primarily restricted to the stabilization of newly synthesized TCRalpha proteins.

The CD3ε Subunit of the TCR Contains Endocytosis Signals

Ligand binding to TCR induces its internalization and cell surface down-modulation. These phenomena contribute to the extinction of activation signals. Due to the multicomponent nature of the TCR-CD3 complex, its internalization may be mediated by one or several of its subunits. Although it has been reported that CD3γ and CD3δ contain endocytosis motifs involved in the internalization of the TCR-CD3 complex, other subunits could also be involved in this process. For instance, CD3ε and CDζ display amino acid sequences reminiscent of internalization motifs. To investigate whether CD3ε bears endocytosis signals, we have analyzed the internalization capacity of a panel of deletion and point mutants of CD3ε that were expressed on the cell surface independently of other TCR-CD3 subunits. Here we report that CD3ε displays endocytosis determinants. These data indicate that CD3ε could contribute to the internalization and cell surface down-regulation of TCR-CD3 complexes. Moreover, the existence of endocytosis signals in this polypeptide could serve to retrieve unassembled CD3ε subunits or partial CD3 complexes from the plasma membrane, thus restricting the expression on the cell surface to fully functional TCR-CD3 complexes.

Timing and Casting for Actors of Thymic Negative Selection

We have recently proposed a new model for the differentiation pathway of αβ TCR thymocytes, with the CD4 and CD8 coreceptors undergoing an unexpectedly complex series of expression changes. Taking into account this new insight, we reinvestigated the timing of thymic negative selection. We found that, although endogenous superantigen-driven thymic negative selection could occur at different steps during double-positive/single-positive cell transition, this event was never observed among CD4lowCD8low TCRint CD69+ thymocytes, i.e., within the first subset to be generated upon TCR-mediated activation of immature double-positive cells. We confirm a role for CD40/CD40L interaction, and the absence of involvement of CD28 costimulation, in thymic deletion in vivo. Surprisingly, we found that thymic negative selection was impaired in the absence of Fas, but not FasL, molecule expression. Finally, we show involvement in opposing directions for p59fyn and SHP-1 molecules in signaling for thymic negative selection.

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.

Specific requirement for CD3epsilon in T cell development

T cell antigen receptor (TCR) and pre-TCR complexes are composed of clonotypic heterodimers in association with dimers of signal transducing invariant subunits (CD3gamma, -delta, -epsilon, and zeta). The role of individual invariant subunits in T cell development has been investigated by generating gene-specific mutations in mice. Mutation of CD3gamma, -delta, or zeta results in an incomplete block in development, characterized by reduced numbers of mature T cells that express low levels of TCR. In contrast, mature T cells are absent from CD3epsilon-/- mice, and thymocyte development is arrested at the early CD4(-)CD8(-) stage. Although these results suggest that CD3epsilon is essential for pre-TCR and TCR expression/function, their interpretation is complicated by the fact that expression of the CD3gamma and CD3delta genes also is reduced in CD3epsilon-/- mice. Thus, it is unclear whether the phenotype of CD3epsilon-/- mice reflects the collective effects of CD3gamma, -delta, and -epsilon deficiency. By removing the selectable marker (PGK-NEO) from the targeted CD3epsilon gene via Cre/loxP-mediated recombination, we generated mice that lack CD3epsilon yet retain normal expression of the closely linked CD3gamma and CD3delta genes. These (CD3epsilonDelta/Delta) mice exhibited an early arrest in T cell development, similar to that of CD3epsilon-/- mice. Moreover, the developmental defect could be rescued by expression of a CD3epsilon transgene. These results identify an essential role for CD3epsilon in T cell development not shared by the CD3gamma, CD3delta, or zeta-family proteins and provide further evidence that PGK-NEO can influence the expression of genes in its proximity.

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.

TCRαβ Chains Associate with the Plasma Membrane Independently of CD3 and TCRζ Chains in Murine Primary T Cells

The TCR is a multisubunit complex composed of the clonotypic α/β disulfide-linked heterodimer and noncovalently linked invariant CD3γε and CD3δε and TCRζ chains. Recent studies demonstrate that the surface expression of CD3 components can occur independently of the clonotypic TCR complexes in both thymocytes and splenic T cells. In this study, we report that free noncovalently associated TCRαβ heterodimers that exist independently of CD3 and TCRζ chains are expressed on the cell surface of immature thymocytes and peripheral T cells, but not of T cell lines and T cell hybridomas. This suggests that the regulation of surface expression of TCRαβ heterodimers differs between primary T cells and T cell lines or T cell hybridomas. The isolation and biochemical characterization of surface clonotype-independent CD3 complexes and free membrane-associated TCRαβ complexes may provide a structural basis for the quantitative difference in amount of T cell proliferation stimulated by anti-CD3ε and anti-TCRβ.

Calnexin associates with monomeric and oligomeric (disulfide-linked) CD3delta proteins in murine T lymphocytes

The antigen-binding receptor expressed on most T lymphocytes consists of disulfide-linked clonotypic alphabeta heterodimers noncovalently associated with monomeric CD3gamma,delta,epsilon proteins and disulfide-linked zeta zeta homodimers, collectively referred to as the T cell antigen receptor (TCR) complex. Here, we examined and compared the disulfide linkage status of newly synthesized TCR proteins in murine CD4(+)CD8(+) thymocytes and splenic T cells. These studies demonstrate that CD3delta proteins exist as both monomeric and oligomeric (disulfide-linked) species that differentially assemble with CD3epsilon subunits in CD4(+)CD8(+) thymocytes and splenic T cells. Interestingly, unlike previous results on glucose trimming and TCR assembly of CD3delta proteins in splenic T cells (Van Leeuwen, J. E. M., and K. P. Kearse (1996) J. Biol. Chem. 271, 9660-9665), we found that glucose residues were not invariably removed from CD3delta glycoproteins prior to their assembly with CD3epsilon subunits in CD4(+)CD8(+) thymocytes. Finally, these studies show that calnexin associates with both monomeric and disulfide-linked CD3delta proteins in murine T cells. The data in the current report demonstrate that CD3delta proteins exist as both monomeric and disulfide-linked molecules in murine T cells that differentially associate with partner TCR chains in CD4(+)CD8(+) thymocytes and splenic T cells. These results are consistent with the concept that folding and assembly of CD3delta proteins is a function of their oxidation state.

Characterization of the region involved in CD3 pairwise interactions within the T cell receptor complex

Assembly of the six-chain T cell antigen receptor-CD3 complex takes place by pairwise interactions. Thus, CD3-epsilon interacts with either CD3-gamma or CD3-delta, and these dimers then associate with the TCR heterodimer (alpha.beta or gamma.delta) and the CD3-zeta homodimer to constitute a full complex. We have now mapped the site in CD3-epsilon responsible for the interaction with CD3-gamma and CD3-delta by analysis of a series of deletional mutants encompassing the most conserved regions. We found that the highly conserved juxtamembrane domain is mainly responsible for the interaction. Thus, deletion of this 16-amino acid extracellular sequence resulted in the inhibition of up to 95% of the CD3-epsilon/gamma interaction. A highly conserved sequence is also present in both CD3-gamma and CD3-delta, suggesting that the domain in these two chains may reciprocally be involved in the interaction with CD3-epsilon. Indeed, an immobilized synthetic peptide corresponding to the CD3-gamma sequence specifically associated to a bacterially expressed CD3-epsilon protein, suggesting the 16-amino acid domain is sufficient to promote CD3-epsilon/CD3-gamma assembly. The conservation of the motif in the CD3 chains suggest that, in addition to CD3-epsilon/CD3-gamma and CD3-epsilon/CD3-delta interactions, it may also mediate homotypic interactions. Indeed, it is shown that it mediates the formation of disulfide-linked homodimers and that the formation of homo- and heterodimers are mutually excluded. Finally, this domain contains a Cys-X-X-Cys sequence that resembles that of p56(lck), which is responsible for the interaction with the cytoplasmic tails of CD4 and CD8. Since the replacement of the two cysteines (Cys97 and Cys100) in CD3-epsilon by alanines strongly inhibited pair formation, the existence of a Cys-X-X-Cys motif involved in protein-protein interactions is suggested.

CD4-CD8-C.B-17 SCID thymocytes enter the CD4+CD8+ stage in the presence of neonatally grafted T cells

The aim of this work was to study the selection of donor T cells and their influence on thymic development in C.B-17 scid/scid (severe combined immunodeficient; SCID) mice during chronic graft-versus-host disease (GVHD). Recipient SCID mice (H-2d), neonatally grafted with allogeneic peripheral T cells from CBA/J strain (H-2k) of mice, only developed a mild acute GVHD, and were, at the chronic stage, devoid of pathological symptoms. Thymic cell numbers of injected mice differed from 10(5) to 1.2 x 10(7) at 2-3 weeks post-injection (p.i.), and from 4 x 10(5) to 8.5 x 10(7) at 2 months p.i. In these mice, the thymus size was correlated to the CD4-CD8- (double negative; DN) to CD4+CD8+ (double positive; DP) cell ratio, where at 2 months p.i., 8 out of 16 treated SCID mice contained 5 x 10(6) cells or more and also possessed the highest frequencies of endogenous DP cells (25-95%). In contrast to previous findings, peripheral donor T cells from allogeneic and syngeneic mice, infiltrating the host thymus, had a positive effect on the development of endogenous DP thymocytes. Furthermore, these thymocytes were developmentally blocked at the DP stage, occasionally in combination with the expression of CD25, CD44 and CD117 but in the absence of T-cell receptor (TCR) expression. Also, at this time-point, the CBA/J donor TCR Vbeta repertoire was equal to that of normal CBA/J mice, but purified responding donor cells were proliferatively inhibited against H-2d stimulators in ex vivo mixed lymphocyte cultures. In contrast, the same responders showed a pronounced proliferation against syngeneic H-2Kk stimulators, suggesting either a reversion from anergy of autoreactive CBA/J T cells or a vast expansion of multiple self-reactive T-cell clones, when parked in a milieu with a lower concentration of self-antigens.

The CD3gamma chain is essential for development of both the TCRalphabeta and TCRgammadelta lineages

CD3gamma and CD3delta are the most closely related CD3 components, both of which participate in the TCRalphabeta-CD3 complex expressed on mature T cells. Interestingly, however, CD3delta does not appear to participate functionally in the pre-T-cell receptor (TCR) complex that is expressed on immature T cells: disruption of CD3delta gene expression has no effect on the developmental steps controlled by the pre-TCR. Here we report that in contrast with CD3delta, CD3gamma is an essential component of the pre-TCR. We generated mice selectively lacking expression of CD3gamma, in which expression of CD3delta, CD3epsilon, CD3zeta, pTalpha and TCRbeta remained undisturbed. Thus, all components for composing a pre-TCR are available, with the exception of CD3gamma. Nevertheless, T-cell development is severely inhibited in CD3gamma-deficient mice. The number of cells in the thymus is reduced to <1% of that in normal mice, and the large majority of thymocytes lack CD4 and CD8 and are arrested at the CD44-CD25+ double negative (DN) stage of development. Peripheral lymphoid organs are also practically devoid of T cells, with absolute numbers of peripheral T cells reduced to only 2-5% of those in normal mice. Both TCRalphabeta and TCRgammadelta lineages fail to develop effectively in CD3gamma-deficient mice, although absence of CD3gamma has no effect on gene rearrangements of the TCRbeta, delta and gamma loci. Furthermore, absence of CD3gamma results in a severe reduction in the level of TCR and CD3epsilon expression at the cell surface of thymocytes and peripheral T cells. The defect in the DN to double positive transition in mice lacking CD3gamma can be overcome by anti-CD3epsilon-mediated cross-linking. CD3gamma is thus essential for pre-TCR function.

Sequestration of CD4-Associated Lck from the TCR Complex May Elicit T Cell Hyporesponsiveness in Nonobese Diabetic Mice

The Lck protein tyrosine kinase associates noncovalently with the cytoplasmic domain of CD4. Upon ligand engagement of the TCR, CD4-associated Lck is rapidly activated and recruited to the TCR complex. Coupling of this complex to an intracellular signaling pathway may result in T cell proliferation. Previously, we reported that thymocytes from nonobese diabetic (NOD) mice (≥6 wk of age) exhibit a proliferative hyporesponsiveness after TCR stimulation, which is associated with defective TCR-mediated signaling along the protein kinase C/Ras/mitogen-activated protein kinase pathway of T cell activation. Here, we investigated whether differential association of Lck with TCR or CD4 mediates the control of NOD thymocyte hyporesponsiveness. We demonstrate that less CD4-associated Lck is recruited to the TCR in activated NOD thymocytes than in control thymocytes. This CD4-mediated sequestration of Lck from the TCR correlates with the increased binding of CD4-associated Lck through its Src homology 2 domain to free TCRζ and CD3γε chains on the plasma membrane. Sequestration of Lck by CD4 does not occur in activated thymocytes from 3-wk-old NOD mice and is only apparent in thymocytes from NOD mice &gt;5 to 6 wk of age. This diminished recruitment of CD4-associated Lck to the TCR is not mediated by an increase in the amount of CD8-associated Lck. Thus, impaired recruitment of CD4-associated Lck to the TCR complex may represent an early event that results in deficient coupling of the TCR complex to downstream signaling events and gives rise to NOD thymocyte hyporesponsiveness.

Intrathymic education of alpha beta and gamma delta T cells is accompanied by cell surface expression of RNA/DNA helicase [corrected]

Despite ubiquitous expression of the gene, RNA/DNA helicase protein was found to be expressed specifically in all cells of the T cell lineage. Interestingly, immature thymocytes that are rearranging T cell receptor (TCR) genes express the helicase strongly on the cell surface and the surface expression is terminated upon engagement of functional TCR by positively selecting ligands. This provides the first evidence that a protein that binds nucleic acids can directly contact the extracellular environment in a developmentally controlled manner. Our discovery of a novel molecular link between the cell surface and nuclear events specific for thymocytes suggests that thymic education is supervised by a previously unknown molecular mechanism, which can now be experimentally explored.

Exit of the pre-TCR from the ER/cis-Golgi is necessary for signaling differentiation, proliferation, and allelic exclusion in immature thymocytes

A major issue is whether surface expression of the pre-TCR is necessary for signaling the development of immature thymocytes. To address this question, we generated transgenic mice expressing a TCRbeta chain that had a strong endoplasmic reticulum (ER) retrieval signal (TCRbetaER) and that was expressed intracellularly but failed to reach the cell surface. In TCRbetaER transgenic mice, there was a failure of allelic exclusion. Also, the transgene failed to rescue the developmental defects observed in TCRbeta-null mice. In contrast, TCRbeta transgenes with a mutant ER retrieval sequence or lacking this sequence signaled efficient allelic exclusion and suppressed the TCRbeta-/- defect. These data show that exit of the pre-TCR from the ER/cis-Golgi is required for progression through the double-negative thymocyte checkpoint.

Prethymic expression of a transgenic TCR beta chain on a precursor of T-cells

Mice carrying a rearranged TCR Vbeta 8.2 transgene express the Vbeta protein on the vast majority of peripheral T-cells. The bone marrow and peripheral blood, as well as other lymphoid organs of both untreated animals and animals depleted of T-cells by neonatal thymectomy and/or injection from birth of monoclonal anti-TCR antibodies, contain a small population of cells that express low levels of the Vbeta transgene product, but no T-cell or other detectable lineage-specific phenotypic markers. When such TG-bearing BM cells are purified and injected directly into the non-TG thymus, they show the phenotypic maturation sequences of intrathymic T-cell development and, subsequently, mature TG-bearing peripheral T-cells. However, this population failed to support long-term recovery from lethal irradiation. Both Vbeta 8.2 TG and CD3delta mRNA transcripts are strongly expressed in the cell population, but no CD3gamma, CD3epsilon, CD3zeta, CD4, CD8beta, pre-Talpha, or RAG-1 transcript was detected. The transgene-encoded TCR component is not bound to the cell membrane exclusively by a phosphatidylinositol linkage. The data show that the fully rearranged TCR transgene and transcripts for at least one of the associated CD3 components, CD3delta, can be expressed on a subpopulation of BM and PBL cells that has not passed through the thymus. The phenotypic characteristics of this cell population resemble those described for the earliest thymocyte described by others. The TG protein molecule in this model may provide a specific developmental marker for a prothymocyte lineage subset that lacks pluripotential properties.

The Ig alpha/Igbeta heterodimer on mu-negative proB cells is competent for transducing signals to induce early B cell differentiation

The immunoglobulin alpha (Ig alpha)/Ig beta heterodimer was detected on the surface of mu-negative proB cell lines in association with calnexin. The cross-linking of Ig beta on proB cells freshly isolated from bone marrow of recombination activating gene (RAG)-2-deficient mice induced a rapid and transient tyrosine-phosphorylation of Ig alpha as well as an array of intracellular proteins including Syk, PI3-kinase, Vav, and SLP-76. It also elicited the phosphorylation and activation of a MAP kinase ERK but not JNK/SAPK or p38. When RAG-2-deficient mice were treated with anti-Ig beta monoclonal antibody, developmentally arrested proB cells were induced to differentiate to the small preB cell stage as observed when the mu transgene was expressed in RAG-2-deficient mice. Thus, the cross-linking of Ig beta on proB cells appears to elicit differentiation signals analogous to those delivered by the preB cell receptor in normal B cell development.

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.

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.

Incomplete endoplasmic reticulum (ER) retention in immature thymocytes as revealed by surface expression of "ER-resident" molecular chaperones

The folding and assembly of nascent proteins in the endoplasmic reticulum (ER) is assisted by molecular chaperones that are themselves retained within the ER. We now report that a number of different ER proteins, including molecular chaperones, are selectively expressed on the surface of immature thymocytes, but their surface expression is extinguished upon further differentiation. Escape from the ER is only possible for newly synthesized ER proteins before they become permanently retained. Thus, the cellular process of ER retention is incomplete in immature thymocytes and provides an explanation for surface expression of partial receptor complexes that transduce differentiative signals during thymic 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.

The related molecular chaperones calnexin and calreticulin differentially associate with nascent T cell antigen receptor proteins within the endoplasmic reticulum

Assembly of the multisubunit T cell antigen receptor (TCR) complex is an intricate process requiring coordinated regulation of at least six different gene products (alpha, beta, gamma, delta, epsilon, and zeta) and the ordered pairing of partner chains within the endoplasmic reticulum (ER). To date, two proteins have been implicated as functioning as molecular chaperones in the assembly of nascent TCR proteins: calnexin, a resident ER transmembrane protein, which associates with all TCR components except zeta, and T cell receptor-associated protein, which selectively associates with CD3gammaepsilon pairs. In this study, we examined the association of calreticulin, a soluble protein with significant sequence homology to calnexin, with newly synthesized TCR proteins. Analogous to calnexin, processing of glycan chains by glucosidase enzymes was required for initial association of TCRalpha and -beta proteins with calreticulin; however, several major differences were noted regarding interaction of calnexin and calreticulin chaperones with TCR proteins. First, TCRalpha and -beta proteins showed prolonged association with calnexin molecules compared with calreticulin; interaction of TCRalpha proteins with calreticulin was particularly transient, with most calreticulin-TCRalpha protein complexes dissociating within 15 min of their initial assembly. Second, we found that, unlike calnexin, which associated with clonotypic TCRalpha and -beta proteins and invariant CD3delta and -epsilon polypeptides, calreticulin associated specifically with clonotypic TCRalpha and -beta proteins. These studies identify calreticulin as a molecular chaperone for nascent clonotypic TCRalpha and -beta proteins and demonstrate that calreticulin and calnexin differentially associate with newly synthesized TCR proteins within the ER.

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.

TCR activation of ZAP70 is impaired in CD4+CD8+ thymocytes as a consequence of intrathymic interactions that diminish available p56lck

The fate of developing CD4+CD8+ thymocytes is determined by signals transduced through surface TCR complexes. Here, we report that cross-linking of TCR on CD4+ CD8+ thymocytes fails to activate ZAP70 protein tyrosine kinase and fails to initiate downstream signaling events, unless the TCR are coaggregated with surface coreceptor molecules. TCR signaling in CD4+CD8+ thymocytes is impaired because the number of available p56lck molecules is diminished by intrathymic CD4-Ia interactions that initially activate p56lck molecules, which are subsequently degraded. As a consequence of intrathymic CD4-Ia interactions, TCR zeta chains are initially phosphorylated to recruit ZAP70 molecules, but the recruited ZAP70 molecules are not subsequently phosphorylated, resulting in TCR complexes that are stably associated with inactive ZAP70 molecules. Thus, intrathymic interactions that diminish p56lck regulate TCR signaling thresholds and affect TCR structure in developing CD4+CD8+ thymocytes.

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.

Calnexin associates exclusively with individual CD3 delta and T cell antigen receptor (TCR) alpha proteins containing incompletely trimmed glycans that are not assembled into multisubunit TCR complexes

Most T lymphocytes express on their surfaces an oligomeric protein complex consisting of clonotypic alpha beta polypeptides associated with invariant CD3-gamma delta epsilon and zeta chains, designated the T cell antigen receptor (TCR) complex. Assembly and intracellular transport of nascent TCR proteins is believed to be assisted by their interaction with the molecular chaperone calnexin, which for certain molecules functions as a lectin for monoglucosylated glycans. However, as most of our knowledge about calnexin-TCR protein associations has been obtained under conditions of limited TCR assembly, the role of calnexin in the formation of nascent TCR complexes is unclear. Here, we studied the role of glucose (Glc) trimming and calnexin association in the oligomerization of TCR alpha and CD3 delta glycoproteins in murine splenic T lymphocytes, a model cell type for efficient assembly of complete TCR complexes. We show that removal of Glc residues from both CD3 delta proteins and TCR alpha proteins occurred prior to their association with any other TCR components and that calnexin specifically interacted with unassembled TCR alpha and CD3 delta proteins containing incompletely trimmed oligosaccharides. Interestingly, we found that removal of Glc residues from glycan chains was necessary for efficient association of calnexin with TCR alpha glycoproteins but not with CD3 delta glycoproteins. These studies define Glc trimming and calnexin association as initial molecular events in the translation of CD3 delta and TCR alpha proteins occurring coincident with or immediately after their translocation into the endoplasmic reticulum and preceding the ordered pairing of TCR chains. In addition, these data document that calnexin assembly with CD3 delta and TCR alpha glycoproteins involves both glycan-dependent and glycan-independent mechanisms.

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.

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.