T-cell antigen receptor (TCR)-α/β heterodimer formation is a prerequisite for association of CD3-ζ2 into functionally competent TCR·CD3 complexes

Quantitative analysis of CD3ε in a cloned canine lymphoma cell line by selected reaction monitoring assay

We established a mass spectrometry-based quantitative method of assaying CD3ε, a component of the T-cell receptor complex. It revealed a CD3ε level of 1 mol per cell in a newly derived canine T-cell lymphoma cell line. Our results suggest that this method has sufficient sensitivity to quantify CD3ε levels in canine lymphoma cells reliably.

Depletion of cytotoxic T-cells does not protect NUP98-HOXD13 mice from myelodysplastic syndrome but reveals a modest tumor immunosurveillance effect

Myelodysplastic syndrome (MDS) and aplastic anemia (AA) patients both present with symptoms of bone marrow failure. In many AA patients, these features are thought to result from an oligoclonal expansion of cytotoxic T-cells that destroy haematopoietic stem or progenitor cells. This notion is supported by the observation that AA patients respond to immunosuppressive therapy. A fraction of MDS patients also respond well to immunosuppressive therapy suggesting a similar role for cytotoxic T-cells in the etiology of MDS, however the role of cytotoxic T-cells in MDS remains unclear. Mice that express a NUP98-HOXD13 (NHD13) transgene develop a MDS that closely mimics the human condition in terms of dysplasia, ineffective hematopoiesis, and transformation to acute myeloid leukemia (AML). We followed a cohort of NHD13 mice lacking the Rag1 protein (NHD13/Rag1KO) to determine if the absence of lymphocytes might 1) delay the onset and/or diminish the severity of the MDS, or 2) effect malignant transformation and survival of the NHD13 mice. No difference was seen in the onset or severity of MDS between the NHD13 and NHD13/Rag1KO mice. However, NHD13/Rag1KO mice had decreased survival and showed a trend toward increased incidence of transformation to AML compared to the NHD13 mice, suggesting protection from AML transformation by a modest immuno-surveillance effect. In the absence of functional Tcrb signaling in the NHD13/Rag1KO T-cell tumors, Pak7 was identified as a potential Tcrb surrogate survival signal.

A conserved CXXC motif in CD3epsilon is critical for T cell development and TCR signaling

Structural integrity of the extracellular membrane-proximal stalk region of CD3ε is required for efficient signaling by the T cell antigen receptor complex. The results in this article suggest that receptor aggregation may not be sufficient for a complete T cell receptor signal and that some type of direct allosteric signal may be involved.

Virtually all T cell development and functions depend on its antigen receptor. The T cell receptor (TCR) is a multi-protein complex, comprised of a ligand binding module and a signal transmission module. The signal transmission module includes proteins from CD3 family (CD3ε, CD3δ, CD3γ) as well as the ζ chain protein. The CD3 proteins have a short extracellular stalk connecting their Ig-like domains to their transmembrane regions. These stalks contain a highly evolutionarily conserved CXXC motif, whose function is unknown. To understand the function of these two conserved cysteines, we generated mice that lacked endogenous CD3ε but expressed a transgenic CD3ε molecule in which these cysteines were mutated to serines. Our results show that the mutated CD3ε could incorporate into the TCR complex and rescue surface TCR expression in CD3ε null mice. In the CD3ε mutant mice, all stages of T cell development and activation that are TCR-dependent were impaired, but not eliminated, including activation of mature naïve T cells with the MHCII presented superantigen, staphylococcal enterotoxin B, or with a strong TCR cross-linking antibody specific for either TCR-Cβ or CD3ε. These results argue against a simple aggregation model for TCR signaling and suggest that the stalks of the CD3 proteins may be critical in transmitting part of the activation signal directly through the membrane.

The T cells of the immune system have surface receptors that detect unique features (called antigens) of foreign invaders such as viruses, bacteria and toxins. An encounter between an antigen and the T cell receptor sets off a chain of events that activates the T cell to proliferate and thus call to action the various arms of the immune response that ultimately eliminate the invader. A set of proteins, called CD3, associates with the T cell receptor, spanning the cell membrane. Their function is to deliver a signal to the inside of T cell that its receptor has encountered antigen on the outside of the cell. Two general ideas have been proposed to explain how the CD3 proteins accomplish this: That the engagement of the T cell receptor outside the cell directly causes a change in conformation in the intracellular portion of the associated CD3 proteins that is recognized by the intracellular signaling machinery; and that engagement of the T cell receptor causes clustering of multiple receptor and CD3 proteins such that interactions among the cytoplasmic portions of the many CD3 proteins now attract other proteins to start the chain of intercellular signaling. These two ideas are not mutually exclusive. We show here that mutations in a highly conserved extracellular portion of one of the CD3 proteins can impair the transmission of the activation signal without preventing receptor clustering. These results suggest that direct transmission of a conformational change across the membrane may constitute part of the CD3-mediated activation signal.

Protein quality control in the early secretory pathway

Eukaryotic cells are able to discriminate between native and non-native polypeptides, selectively transporting the former to their final destinations. Secretory proteins are scrutinized at the endoplasmic reticulum (ER)-Golgi interface. Recent findings reveal novel features of the underlying molecular mechanisms, with several chaperone networks cooperating in assisting the maturation of complex proteins and being selectively induced to match changing synthetic demands. 'Public' and 'private' chaperones, some of which enriched in specializes subregions, operate for most or selected substrates, respectively. Moreover, sequential checkpoints are distributed along the early secretory pathway, allowing efficiency and fidelity in protein secretion.

T-B+NK+ severe combined immunodeficiency caused by complete deficiency of the CD3zeta subunit of the T-cell antigen receptor complex

CD3zeta is a subunit of the T-cell antigen receptor (TCR) complex required for its assembly and surface expression that also plays an important role in TCR-mediated signal transduction. We report here a patient with T(-)B(+)NK(+) severe combined immunodeficiency (SCID) who was homozygous for a single C insertion following nucleotide 411 in exon 7 of the CD3zeta gene. The few T cells present contained no detectable CD3zeta protein, expressed low levels of cell surface CD3epsilon, and were nonfunctional. CD4(+)CD8(-)CD3epsilon(low), CD4(-)CD8(+)CD3epsilon(low), and CD4(-)CD8(-)CD3epsilon(low) cells were detected in the periphery, and the patient also exhibited an unusual population of CD56(-)CD16(+) NK cells with diminished cytolytic activity. Additional studies demonstrated that retrovirally transduced patient mutant CD3zeta cDNA failed to rescue assembly of nascent complete TCR complexes or surface TCR expression in CD3zeta-deficient MA5.8 murine T-cell hybridoma cells. Nascent transduced mutant CD3zeta protein was also not detected in metabolically labeled MA5.8 cells, suggesting that it was unstable and rapidly degraded. Taken together, these findings provide the first demonstration that complete CD3zeta deficiency in humans can cause SCID by preventing normal TCR assembly and surface expression.

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.

CD38 Signaling in T Cells Is Initiated within a Subset of Membrane Rafts Containing Lck and the CD3-ζ Subunit of the T Cell Antigen Receptor

In this study we present data supporting that most CD38 is pre-assembled in a subset of Brij 98-resistant raft vesicles, which were stable at 37 degrees C, and have relatively high levels of Lck and the CD3-zeta subunit of T cell antigen receptor-CD3 complex in contrast with a Brij 98-soluble pool, where CD38 is associated with CD3-zeta, and Lck is not detected. Our data further indicate that following CD38 engagement, LAT and Lck are tyrosine phosphorylated exclusively in Brij 98-resistant rafts, and some key signaling components translocate into rafts (i.e. Sos and p85-phosphatidylinositol 3-kinase). Moreover, N-Ras results activated within rafts immediately upon CD38 ligation, whereas activated Erk was mainly found in soluble fractions with delayed kinetics respective to Ras activation. Furthermore, full phosphorylation of CD3-zeta and CD3-epsilon only occurs in rafts, whereas partial CD3-zeta tyrosine phosphorylation occurs exclusively in the soluble pool, which correlated with increased levels of c-Cbl tyrosine phosphorylation in the non-raft fractions. Taken together, these results suggest that, unlike the non-raft pool, CD38 in rafts is able to initiate and propagate several activating signaling pathways, possibly by facilitating critical associations within other raft subsets, for example, LAT rafts via its capacity to interact with Lck and CD3-zeta. Overall, these findings provide the first evidence that CD38 operates in two functionally distinct microdomains of the plasma membrane.

Mutational analysis of conserved amino acids in the T cell receptor alpha-chain transmembrane region: a critical role of leucine 112 and phenylalanine 127 for assembly and surface expression

Correct assembly of all TCR complex polypeptides is essential for its cell surface expression and function. The transmembrane region of the TCRalpha chain is highly conserved and to gain insight into the structural and functional role of these residues, single amino acid substitutions were introduced and surface expression and signaling ability studied in T hybridoma cells. Introduction of acid residues within the TCRalpha chain transmembrane region were mostly tolerated, indicating that the net charge within this region of the TCR complex is not crucial to either assembly or signaling. However, mutations of leucine 112 or phenylalanine 127 to aspartic acids (L112D or F127D, respectively) resulted in dramatic loss of surface expression and, therefore, their signaling ability. Intracellular flow cytometry showed that the mutant TCRalpha polypeptides were present at levels comparable to wild-type, indicating that the reduced surface expression was not a consequence of impaired protein survival. The defect was characterized by immunoprecipitation and showed that residues L112 and F127 were involved in early interactions with the CD3 complex. A large proportion of the TCRalpha chain mutants L112D and F127D consisted of immature protein, indicative of a problem during early assembly of the TCR. Our findings provide evidence for the involvement of the conserved L112 and F127 residues of the TCRalpha chain transmembrane region in the assembly process of the TCR complex.

In vitro production and characterization of partly assembled human CD3 complexes

Pairwise assembly of human CD3 chains takes place in the endoplasmic reticulum of T cells. Subsequently, the CD3 heterodimers form complexes with Ti alpha and Tiss chains forming hexameric Ti alpha beta CD3 gamma epsilon delta epsilon complexes. Finally, association with the zeta 2 homodimer occurs in Golgi apparatus before the fully assembled T-cell receptor is transported to the cell surface. To study the structural properties of the human CD3 chains, we have developed new methods to produce and fold the extracellular domains of CD3 gamma, CD3 delta and CD3 epsilon. Proteins were expressed in Escherichia coli as denatured chains and de novo folded in vitro. CD3 gamma and CD3 epsilon folded as soluble monomers, whereas CD3 delta did not yield any soluble proteins. When folding the chains pairwise, soluble CD3 gamma epsilon and CD3 delta epsilon heterodimers could be isolated, whereas CD3 gamma delta heterodimers were not produced. Using antibodies as structural probes, we identified two different types of antigenic epitopes that were dependent on heterodimerization. Our data indicate that CD3 epsilon undergoes a conformational change after dimerization with CD3 gamma or CD3 delta. Furthermore, we demonstrated that the CD3 gamma epsilon heterodimer could be purified using immunoaffinity chromatography.

Expression and characterization of recombinant soluble human CD3 molecules: presentation of antigenic epitopes defined on the native TCR-CD3 complex

The TCR-CD3 complex consists of the clonotypic disulfide-linked TCRalphabeta or TCRdeltagamma heterodimers, and the invariant CD3delta, epsilon, gamma and zeta chains. We generated plasmid constructs expressing the extracellular domains of the CD3delta, epsilon or gamma subunits fused to human IgG1 Fc. Recombinant fusion proteins consisting of individual CD3delta, epsilon or gamma subunits reacted poorly with anti-CD3 mAb including G19-4, BC3, OKT3 and 64.1. Co-expression of the CD3epsilon-Ig with either the CD3delta-Ig (CD3epsilondelta-Ig) or the CD3gamma-Ig (CD3epsilongamma-Ig) resulted in fusion proteins with much increased binding to G19-4. A brief acid treatment of the purified CD3epsilondelta-Ig fusion protein substantially improved its binding to BC3, OKT3 and 64.1. Surface plasmon resonance analysis revealed that the dissociation constants for CD3epsilondelta-Ig and anti-CD3 mAb ranged from 10(-8) to 10(-9) M. Based on these results, a single-chain (sc) construct encoding the CD3delta chain linked to the CD3epsilon chain with a flexible linker followed by human IgG1 Fc was expressed. The sc CD3deltaepsilon-scIg reacted with anti-CD3 mAb without requiring acid treatment. Moreover, anti-CD3 mAb bound CD3epsilondelta-Ig at a higher affinity than CD3epsilongamma-Ig, suggesting potential structural differences between the CD3epsilondelta and CD3epsilongamma subunits. In summary, we report the expression of soluble recombinant CD3 proteins that demonstrate structural characteristics of the native CD3 complex expressed on the T cell surface. These CD3 fusion proteins can be used to further analyze the structure of the TCR-CD3 complex, and to identify molecules that can interfere with TCR-CD3-mediated signal transduction by disrupting the interaction between CD3 and TCR subunits.

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 CD3-gamma delta epsilon transducing module mediates CD38-induced protein-tyrosine kinase and mitogen-activated protein kinase activation in Jurkat T cells

We have examined the ability of the CD3-gamma delta epsilon and CD3-zeta signaling modules of the T cell receptor (TCR) to couple CD38 to intracellular signaling pathways. The results demonstrated that in TCR+ T cells that express the whole set of CD3 subunits CD38 ligation led to complete tyrosine phosphorylation of both CD3-zeta and CD3-epsilon polypeptide chains. In contrast, in TCR+ cells with a defective CD3-zeta association CD38 engagement caused tyrosine phosphorylation of CD3-epsilon but not of CD3-zeta. Despite these differences, in both cell types CD38 ligation resulted in protein-tyrosine kinase and mitogen-activated protein kinase activation. However, in cells expressing chimerical CD25-zeta or CD25-epsilon receptors or in a TCR-beta- Jurkat T cell line, CD38 ligation did not result in tyrosine phosphorylation of the chimeric receptors, or CD3 subunits, or protein-tyrosine kinase or mitogen-activated protein kinase activation. In summary, these results support a model in which CD38 transduces activating signals inside the cell by means of CD3-epsilon and CD3-zeta tyrosine phosphorylation. Moreover, these data identify the CD3-gamma delta epsilon signaling module as a necessary and sufficient component of the TCR/CD3 complex involved in T cell activation through CD38.

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.

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.

In vitro translation and assembly of a complete T cell receptor-CD3 complex

The T cell receptor for antigen (TCR) is a multisubunit complex that consists of at least seven polypeptides: the clonotypic, disulfide-linked alpha/beta heterodimer that is noncovalently associated with the invariant polypeptides of the CD3 complex (CD3-gamma, -delta, -epsilon) and zeta, a disulfide-linked homodimer. We achieved the complete assembly of the human TCR in an in vitro transcription/translation system supplemented with dog pancreas microsomes by simultaneous translation of the messenger RNAs encoding the TCR-alpha, -beta and CD3-gamma, -delta, -epsilon, and -zeta subunits. CD3-epsilon, one of the subunits that initiates the assembly of the TCR in living cells, forms misfolded, disulfide-linked homooligomers when translated alone. However, co-translation of one of its first binding partners in the course of assembly, CD3-gamma or -delta, led to the expression of mainly monomeric and correctly folded epsilon subunits, the only form we could detect as part of a properly assembled TCR complex. In the absence of these subunits, the ER-resident chaperone calnexin interacted with oligomeric, i.e. misfolded, structures of CD3-epsilon in a glycan-independent manner. A glycan-dependent interaction between CD3-epsilon and calnexin was mediated by CD3-gamma and concerned only monomeric CD3-epsilon complexed with CD3-gamma, but was dispensable for proper folding of CD3-epsilon. We suggest that in addition to its signaling function, CD3-epsilon serves as a monitor for proper subunit assembly of the TCR.

Apoptosis but not other activation events is inhibited by a mutation in the transmembrane domain of T cell receptor beta that impairs CD3zeta association

The transmembrane domain of T cell receptor (TCR) beta contains a conserved immunoreceptor tyrosine-based activation-like motif consisting of a duplicated YXXL sequence. The motif is also present in TCRgamma, the equivalent chain to TCRbeta in gammadelta T lymphocytes but is absent in TCRalpha and TCRdelta. To determine the putative role of this sequence in TCR.CD3 complex assembly and signal transduction, a TCRbeta chain cDNA was mutated in the C-terminal tyrosine of the motif, cloned in an expression vector, and transfected into TCRbeta-negative Jurkat cells. Transfectants of the mutated chain (MUT) expressed, on average, much less TCR.CD3 complex on the membrane than wild type TCRbeta transfectants. Radiolabeling experiments suggested that the mutation caused a loose association of the CD3zeta chain resulting in a defective assembly. However, stimulation of high TCR.CD3 expressing wild type and MUT clones with monoclonal antibodies and Staphylococcus aureus enterotoxin B resulted in similar levels of CD25 and CD69 expression, interleukin-2 secretion, and TCR.CD3 complex down-regulation. By contrast, MUT cells were clearly resistant to activation-induced cell death, and they did not express CD95-ligand upon activation. These results suggest a differentiated intracellular signaling pathway leading to apoptosis in which Tyr-TM11 of the immunoreceptor tyrosine-based activation motif-like motif and CD3zeta appear to be involved.

Altered T lymphocyte signaling in rheumatoid arthritis

Synovial and peripheral blood T cells from patients with rheumatoid arthritis are functionally deficient. This may be secondary to their reduced cytokine (e.g. interleukin-2) synthesis. We have investigated the possibility of an alteration in pathways common to interleukin-2 production and proliferation in peripheral blood T cells from patients with active rheumatoid arthritis. Intracellular calcium levels ([Ca2+]i) were analyzed by flow cytometric methods in Indo1-loaded T cells. These were purified by negative selection from patients or age/sex-matched controls, and stimulated with phytohemagglutinin-P or anti-CD3. Rheumatoid [Ca2+]i responses to both stimuli were reduced (p < 0.005). Patient cell samples included a larger proportion of non-responding cells, but even in the responsive population the magnitude of the response in rheumatoid cells was impaired compared with those in normal cell samples (p < 0.0001) for both stimuli. Proliferation responses were also impaired (p < 0.005), and there was a positive correlation between the paired [Ca2+]i elevation and proliferative responses for both stimuli. CD2 and CD3 expression were normal, and the proportions of CD4, CD8 and CD45RO and CD45RA subsets were also unaffected by disease. Thus a signaling defect downstream of CD2 or CD3 surface molecules may contribute to functional deficiencies in rheumatoid T lymphocytes. This effect is not due to non-steroidal anti-inflammatory drugs which some patients were taking. We have demonstrated similar alterations in [Ca2+]i responses and proliferation in a smaller study of patients with inflammatory bowel disease, indicating that such changes might be present in other chronic inflammatory states.

The T cell receptor associated CD3-epsilon protein is phosphorylated upon T cell activation in the two tyrosine residues of a conserved signal transduction motif

Signal transduction through the T cell receptor for antigen, the TcR/CD3 complex, involves phosphorylation of tyrosine residues in the CD3-zeta chain. Since both CD3-epsilon and the zeta chain contain a tyrosine-based signaling motif, we examine phosphorylation of CD3-epsilon in human T cells. Engagement of the TcR/CD3 complex induced tyrosine phosphorylation of CD3-epsilon in vivo. Induction of CD3-epsilon phosphorylation followed similar kinetics to that of the zeta chain phosphorylation. In contrast to zeta, CD3-epsilon phosphorylation was strictly dependent upon cell surface expression of this member of the TcR/CD3 complex. Chemical and proteolytic cleavage combined with peptide-specific Western blotting established that CD3-epsilon phosphorylation occurred in the two tyrosine residues located in the signal transduction motif in the C-terminal portion of the molecule. Taken together, these data indicated that phosphorylation of CD3-epsilon by tyrosine protein kinases may serve to couple the TcR/CD3 complex to other effector molecules in the signaling cascade.

Covalent binding of guanine nucleotides to the CD3-gamma chain of the T cell receptor/CD3 complex

In a search for proteins involved in signal transduction through the T cell receptor (TcR/CD3 complex), a recently developed highly efficient method for labeling of nucleotide binding proteins in permeabilized cells was applied. Here, we report that human CD3-gamma could be labeled by periodate-oxidized [alpha-32P] GTP (GTPoxi). In contrast to GTPoxi labeling of CD3-zeta (Peter, M. E., Hall, C., Rühlmann, A., Sancho, J. and Terhorst, C., EMBO J. 1992. 11:933), GTP-specific labeling of CD3-gamma reached a maximum when nucleotides were added 60 min prior to the cross-linking reaction. As CD3-gamma did not contain a known consensus sequence for nucleotide binding and since labeling kinetics of CD3-gamma coincided with those of cytosolic GTP-binding proteins, labeling may have been caused by a GTP-binding protein. This putative protein was not T cell specific because labeling of CD3-gamma could also be achieved when expressed in the endoplasmic reticulum of Chinese hamster ovary (CHO) cells. In CHO cells, labeling by GTPoxi took place only when CD3-gamma was associated with CD3-epsilon, whereas labeling could not be established upon association of CD3-gamma with CD3-delta or TcR alpha. The observation that CD3-gamma was labeled without leaving the endoplasmic reticulum led to the hypothesis that the association of CD3-gamma with a GTP-binding protein might be involved in an early step of the TcR/CD3 complex formation or transport.

Characterization of T cell receptor assembly and expression in a Ti gamma delta-positive cell line

T cell antigen receptor (TcR) heterodimers of both the Ti-alpha beta and Ti-gamma delta types are expressed at the surface of T cells noncovalently associated with the CD3 complex composed of the monomorphic chains gamma, delta, epsilon and zeta. The structural relationship and assembly of the various components of this multimeric protein complex is still not fully understood. In this report, the human leukemic T cell line Lyon which expresses a Ti-gamma delta/CD3 complex, was characterized and compared to another human leukemic T cell line Jurkat (Ti-alpha beta/CD3). Membrane TCR-/CD3- variants of the T cell Lyon were induced and found to produce all of the Ti/CD3 components, with the exception of Ti-delta. Biochemical analysis indicated that: (1) Ti-gamma/CD3 gamma, delta, epsilon complexes were formed in the endoplasmic reticulum in the absence of Ti-delta; (2) the CD3-zeta chain did not associate with the Ti-gamma/CD3 gamma delta epsilon complex and (3) the Ti-delta chain was required for cell surface expression of the Ti-gamma delta/CD3 complex. Introduction of Jurkat wild-type Ti-alpha cDNA into Lyon T cells resulted in Ti-alpha beta/CD3 expression and abrogated Ti-gamma delta/CD3 expression. In contrast, the expression of the Ti-gamma delta/CD3 complex was not affected by transfection of a mutated Ti-alpha cDNA into Lyon cells. The mutated Ti-alpha chain formed complexes with Ti-beta and CD3 gamma delta epsilon, but the CD3-zeta chain did not associate with these complexes. Taken together analysis of Lyon cells transfected with either wild-type or mutated Ti-alpha suggested that the CD3-zeta chain may have higher affinity for Ti-alpha beta/CD3 complexes than for Ti-gamma delta/CD3 complexes.

Defective signal transduction via T-cell receptor-CD3 structure in T cells from rheumatoid arthritis patients

T cells from patients with active RA are known to produce low levels of IL-2 and proliferate poorly in response to various mitogenic stimuli. The present work shows that cross-linking of CD3 antigen on patients' T-cell surface induces two- to threefold lower Ca2+ response than in T cells from age-matched controls. Immunofluorescence studies indicate that the attenuated signal may be due to the suppressed expression of CD3 and/or CD45 molecules on patients' T cells. In the majority of the patients, the level of CD45 expression is reduced by 60%-70% as compared with that in the control T cells. Therefore, the poor mitogenic response of patient cells is apparently due to a defect in early stages of signal transduction through the T-cell receptor (TCR-CD3).

Sequence requirements for induction of cytolysis by the T cell antigen/Fc receptor zeta chain

The zeta chain of the T cell antigen receptor is a dimeric transmembrane protein with a very short extracellular domain and an extended cytoplasmic tail that triggers T cell effector function when aggregated by extracellular stimuli. We have reduced the active site of zeta to an 18 residue motif that can be appended to the intracellular domain of other transmembrane proteins to endow them with receptor-like activity. The compact size of the motif appears to eliminate zeta mechanisms based on enzymatic activity and suggests that one or at most a few cellular proteins interact with the zeta intracellular domain to initiate signal transduction. Analysis of individual amino acids within the 18 residue element reveals two phylogenetically conserved tyrosines that are absolutely required for activity and other residues that are less essential but contribute to the efficacy of receptor-directed cytolysis.

Genetic reconstitution of the T cell receptor (TcR) alpha/beta heterodimer restores the association of CD3 zeta 2 with the TcR/CD3 complex

The cell surface expression of the T cell receptor (TcR)/CD3 complex and, consequently, the functional competence of the cell is partly dependent on CD3 zeta. In its absence, a pentameric complex (TcR alpha/beta/CD3 gamma delta epsilon) is formed which is inefficiently transported to the cell surface. Reconstitution of CD3 zeta by transfection, in turn, restores the cell surface expression and function of the complex. Through the use of transfection experiments, we here provide direct evidence that the association of CD3 zeta 2 with the TcR/CD3 complex is dependent on the presence of both the TcR alpha and beta polypeptide chains. Despite wild-type levels of the CD3 zeta protein in a TcR alpha-negative mutant human T cell line, a complex was formed intracellularly which lacked CD3 zeta 2 and consisted of beta gamma delta epsilon and beta 2 gamma delta epsilon. Upon transfection of the mutant with a TcR alpha cDNA, a TcR/CD3 complex which contained CD3 zeta 2 was observed intracellularly. In contrast to the partial subcomplex on the cell surface of the untransfected cell line, the TcR/CD3 complex on the transfectant was functional as demonstrated by its ability to mobilize intracellular calcium after stimulation with a mitogenic CD3 epsilon-specific monoclonal antibody. Transient transfection studies performed in COS cell fibroblasts indicated that CD3 zeta 2 was not interacting with the TcR alpha protein alone, implying that a conformation provided by either the TcR alpha/beta heterodimer or the TcR alpha/beta/CD3 gamma delta epsilon complex was necessary for the association of CD3 zeta 2. Transfection studies performed in a TcR alpha/beta-negative murine T-T hybridoma confirmed the requirement of both the TcR alpha and beta proteins in CD3 zeta 2 binding. We conclude that the TcR alpha and beta chains harbor polypeptide sequences essential for the association of CD3 zeta 2 with the TcR/CD3 complex.