Surface expression of alternative forms of the TCR/CD3 complex

Treating solid tumors with TCR-based chimeric antigen receptor targeting extra domain B-containing fibronectin

BACKGROUND

The suppression of chimeric antigen receptor (CAR) T cells by the tumor microenvironment (TME) is a crucial obstacle in the T-cell-based treatment of solid tumors. Extra domain B (EDB)-fibronectin is an oncofetal antigen expressed on the endothelium layer of the neovasculature and cancer cells. Though recognized as a T cell therapy target, engineered CAR T cells thus far have failed to demonstrate satisfactory in vivo efficacy. In this study, we report that targeting EDB-fibronectin by redirected TCR-CAR T cells (rTCR-CAR) bypasses the suppressive TME for solid tumor treatment and sufficiently suppressed tumor growth.We generated EDB-targeting CAR by fusing single-chain variable fragment to CD3ε, resulting in rTCR-CAR. Human primary T cells and Jurkat cells were used to study the EDB-targeting T cells. Differences to the traditional second-generation CAR T cell in signaling, immune synapse formation, and T cell exhaustion were characterized. Cytotoxicity of the rTCR-CAR T cells was tested in vitro, and therapeutic efficacies were demonstrated using xenograft models.

METHODS

RESULTS: In the xenograft models, the rTCR-CAR T cells demonstrated in vivo efficacies superior to that based on traditional CAR design. A significant reduction in tumor vessel density was observed alongside tumor growth inhibition, extending even to tumor models established with EDB-negative cancer cells. The rTCR-CAR bound to immobilized EDB, and the binding led to immune synapse structures superior to that formed by second-generation CARs. By a mechanism similar to that for the conventional TCR complex, EDB-fibronectin activated the rTCR-CAR, resulting in rTCR-CAR T cells with low basal activation levels and increased in vivo expansion.

CONCLUSION

Our study has demonstrated the potential of rTCR-CAR T cells targeting the EDB-fibronectin as an anticancer therapeutic. Engineered to possess antiangiogenic and cytotoxic activities, the rTCR-CAR T cells showed therapeutic efficacies not impacted by the suppressive TMEs. These combined characteristics of a single therapeutic agent point to its potential to achieve sustained control of solid tumors.

The role of the different CD3γ domains in TCR expression and signaling

The CD3 subunits of the T-cell antigen receptor (TCR) play a central role in regulation of surface TCR expression levels. Humans who lack CD3γ (γ^—) show reduced surface TCR expression levels and abolished phorbol ester (PMA)-induced TCR down-regulation. The response to PMA is mediated by a double leucine motif in the intracellular (IC) domain of CD3γ. However, the molecular cause of the reduced TCR surface expression in γ^— lymphocytes is still not known. We used retroviral vectors carrying wild type CD3γ or CD3δ or the following chimeras (EC-extracellular, TM-transmembrane and IC): δ_ECγ_TMγ_IC (δγγ for short), γγδ, γδδ and γγ-. Expression of γγγ, γγδ, γδδ or γγ- in the γ^— T cell line JGN, which lacks surface TCR, demonstrated that cell surface TCR levels in JGN were dependent on the EC domain of CD3γ and could not be replaced by the one of CD3δ. In JGN and primary γ^— patient T cells, the tested chimeras confirmed that the response to PMA maps to the IC domain of CD3γ. Since protein homology explains these results better than domain structure, we conclude that CD3γ contributes conformational cues that improve surface TCR expression, likely at the assembly or membrane transport steps. In JGN cells all chimeric TCRs were signalling competent. However, an IC domain at CD3γ was required for TCR-induced IL-2 and TNF-α production and CD69 expression, indicating that a TCR without a CD3γ IC domain has altered signalling capabilities.

CD3G or CD3D Knockdown in Mature, but Not Immature, T Lymphocytes Similarly Cripples the Human TCRαβ Complex

The human αβ T-cell receptor (TCR) is composed of a variable heterodimer (TCRαβ) and three invariant dimers (CD3γε, CD3δε, and ζζ/CD247₂). The role of each invariant chain in the stepwise interactions among TCR chains along the assembly is still not fully understood. Despite the high sequence homology between CD3γ and CD3δ, the clinical consequences of the corresponding immunodeficiencies (ID) in humans are very different (mild and severe, respectively), and mouse models do not recapitulate findings in human ID. To try to understand such disparities, we stably knocked down (KD) CD3D or CD3G expression in the human Jurkat T-cell line and analyzed comparatively their impact on TCRαβ assembly, transport, and surface expression. The results indicated that TCR ensembles were less stable and CD3ε levels were lower when CD3γ, rather than CD3δ, was scarce. However, both defective TCR ensembles were strongly retained in the ER, lacked ζζ/CD247₂, and barely reached the T-cell surface (<11% of normal controls) in any of the CD3 KD cells. This is in sharp contrast to human CD3γ ID, whose mature T cells express higher levels of surface TCR (>30% vs. normal controls). CD3 KD of human T-cell progenitors followed by mouse fetal thymus organ cultures showed high plasticity in emerging immature polyclonal T lymphocytes that allowed for the expression of significant TCR levels which may then signal for survival in CD3γ, but not in CD3δ deficiency, and explain the immunological and clinical disparities of such ID cases.

CAR- and TRuC-redirected regulatory T cells differ in capacity to control adaptive immunity to FVIII

Regulatory T cells (Tregs) control immune responses in autoimmune disease, transplantation, and enable antigen-specific tolerance induction in protein-replacement therapies. Tregs can exert a broad array of suppressive functions through their T cell receptor (TCR) in a tissue-directed and antigen-specific manner. This capacity can now be harnessed for tolerance induction by "redirecting" polyclonal Tregs to overcome low inherent precursor frequencies and simultaneously augment suppressive functions. With the use of hemophilia A as a model, we sought to engineer antigen-specific Tregs to suppress antibody formation against the soluble therapeutic protein factor (F)VIII in a major histocompatibility complex (MHC)-independent fashion. Surprisingly, high-affinity chimeric antigen receptor (CAR)-Treg engagement induced a robust effector phenotype that was distinct from the activation signature observed for endogenous thymic Tregs, which resulted in the loss of suppressive activity. Targeted mutations in the CD3ζ or CD28 signaling motifs or interleukin (IL)-10 overexpression were not sufficient to restore tolerance. In contrast, complexing TCR-based signaling with single-chain variable fragment (scFv) recognition to generate TCR fusion construct (TRuC)-Tregs delivered controlled antigen-specific signaling via engagement of the entire TCR complex, thereby directing functional suppression of the FVIII-specific antibody response. These data suggest that cellular therapies employing engineered receptor Tregs will require regulation of activation thresholds to maintain optimal suppressive function.

DNA methylation patterns in T lymphocytes are generally stable in human pregnancies but CD3 methylation is associated with perinatal psychiatric symptoms

Objectives

To determine whether DNA methylation patterns in genes coding for selected T-lymphocyte proteins are associated with perinatal psychiatric distress or with complications of pregnancy.

Methods

T lymphocyte DNA was obtained from pregnant women across three time points in pregnancy and the postpartum period and epigenetic patterns were assessed using Illumina 450 ​K Methylation Beadchips. Seven selected genes critical for T cell function were analyzed for methylation changes during pregnancy and for associations of methylation patterns with psychiatric distress or with pregnancy complications, with particular attention paid to spatial aggregations of methyl groups, termed ‘hotspots,’ within the selected genes.

Results

In the candidate gene approach, DNA methylation density within a single cluster of 9 contiguous CpG loci within the CD3 gene was found to be strongly associated with anxiety and depression in mid- and late pregnancy, and weakly associated with the presence of complications of pregnancy. Average DNA methylation density across each of the seven genes examined, and assay-wide, was found to be relatively stable across pregnancy and postpartum, but methylation within the CD3 hotspot was more malleable and changes over time were coordinated across the nine cytosines in the hotspot. CD3 CpGs did not pass array-wide tests for significance, but CpG clusters in two other genes, DTNBP1 and OXSR1, showed array-wide significant associations with anxiety.

Conclusions

Despite the need for tolerating the fetal hemi-allograft, overall DNA methylation patterns in T lymphocytes are generally stable over the mid to late course of human pregnancies and postpartum. However, site-specific changes in DNA methylation density in CD3 appear linked to both symptoms of depression and anxiety in pregnancy and, less strongly, to adverse pregnancy outcomes.

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Associations exist between DNA methylation density in T cells and measures of stress and mental health in pregnant women.

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Global DNA methylation density is generally stable over the course of pregnancy.

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A subregion within the CD3 gene has unusually variable DNA methylation density and is associated with anxiety and depression.

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Spatial and gene specificity may be important elements of epigenetic regulation of immune function in pregnancy.

Peculiar Expression of CD3-Epsilon in Kidney of Ginbuna Crucian Carp

TCR/CD3 complex is composed of the disulfide-linked TCR-αβ heterodimer that recognizes the antigen as a peptide presented by the MHC, and non-covalently paired CD3γε- and δε-chains together with disulfide-linked ζ-chain homodimers. The CD3 chains play key roles in T cell development and T cell activation. In the present study, we found nor or extremely lower expression of CD3ε in head- and trunk-kidney lymphocytes by flow cytometric analysis, while CD3ε was expressed at the normal level in lymphocytes from thymus, spleen, intestine, gill, and peripheral blood. Furthermore, CD4-1⁺ and CD8α⁺ T cells from kidney express Zap-70, but not CD3ε, while the T cells from other tissues express both Zap-70 and CD3ε, although expression of CD3ε was low. Quantitative analysis of mRNA expression revealed that the expression level of T cell-related genes including tcrb, cd3ε, zap-70, and lck in CD4-1⁺ and CD8α⁺ T cells was not different between kidney and spleen. Western blot analysis showed that CD3ε band was detected in the cell lysates of spleen but not kidney. To be interested, CD3ε-positive cells greatly increased after 24 h in in vitro culture of kidney leukocytes. Furthermore, expression of CD3ε in both transferred kidney and spleen leukocytes was not detected or very low in kidney, while both leukocytes expressed CD3ε at normal level in spleen when kidney and spleen leukocytes were injected into the isogeneic recipient. Lower expression of CD3ε was also found in kidney T lymphocytes of goldfish and carp. These results indicate that kidney lymphocytes express no or lower level of CD3ε protein in the kidney, although the mRNA of the gene was expressed. Here, we discuss this phenomenon from the point of function of kidney as reservoir for T lymphocytes in teleost, which lacks lymph node and bone marrow.

Cutting Edge: CD3 ITAM Diversity Is Required for Optimal TCR Signaling and Thymocyte Development

For the αβ or γδTCR chains to integrate extracellular stimuli into the appropriate intracellular cellular response, they must use the 10 ITAMs found within the CD3 subunits (CD3γε, CD3δε, and ζζ) of the TCR signaling complex. However, it remains unclear whether each specific ITAM sequence of the individual subunit (γεδζ) is required for thymocyte development or whether any particular CD3 ITAM motif is sufficient. In this article, we show that mice utilizing a single ITAM sequence (γ, ε, δ, ζa, ζb, or ζc) at each of the 10 ITAM locations exhibit a substantial reduction in thymic cellularity and limited CD4^-CD8^- (double-negative) to CD4⁺CD8⁺ (double-positive) maturation because of low TCR expression and signaling. Together, the data suggest that ITAM sequence diversity is required for optimal TCR signal transduction and subsequent T cell maturation.

The Stalk Domain of NKp30 Contributes to Ligand Binding and Signaling of a Preassembled NKp30-CD3ζ Complex

The natural cytotoxicity receptor (NCR) NKp30 (CD337) is a key player for NK cell immunosurveillance of infections and cancer. The molecular details of ligand recognition and its connection to CD3ζ signaling remain unsolved. Here, we show that the stalk domain (¹²⁹KEHPQLGAGTVLLLR¹⁴³) of NKp30 is very sensitive to sequence alterations, as mutations lead to impaired ligand binding and/or signaling capacity. Surprisingly, the stalk domains of NKp30 and NKp46, another NCR employing CD3ζ for signaling, were not exchangeable without drastic deficiencies in folding, plasma membrane targeting, and/or ligand-induced receptor signaling. Further mutational studies, N-glycosylation mapping, and plasma membrane targeting studies in the absence and presence of CD3ζ suggest two interconvertible types of NCR-CD3ζ assemblies: 1) a signaling incompetent structural NKp30-CD3ζ complex and 2) a ligand-induced signaling competent NKp30-CD3ζ complex. Moreover, we propose that ligand binding triggers translocation of Arg-143 from the membrane interface into the membrane to enable alignment with oppositely charged aspartate residues within CD3ζ and activation of CD3ζ-signaling.

Porcine CD3(+)NKp46(+) Lymphocytes Have NK-Cell Characteristics and Are Present in Increased Frequencies in the Lungs of Influenza-Infected Animals

The CD3⁻NKp46⁺ phenotype is frequently used for the identification of natural killer (NK) cells in various mammalian species. Recently, NKp46 expression was analyzed in more detail in swine. It could be shown that besides CD3⁻NKp46⁺ lymphocytes, a small but distinct population of CD3⁺NKp46⁺ cells exists. In this study, we report low frequencies of CD3⁺NKp46⁺ lymphocytes in blood, lymph nodes, and spleen, but increased frequencies in non-lymphatic organs, like liver and lung. Phenotypic analyses showed that the majority of CD3⁺NKp46⁺ cells coexpressed the CD8αβ heterodimer, while a minor subset expressed the TCR-γδ, which was associated with a CD8αα⁺ phenotype. Despite these T-cell associated receptors, the majority of CD3⁺NKp46⁺ lymphocytes displayed a NK-related phenotype (CD2⁺CD5⁻CD6⁻CD16⁺perforin⁺) and expressed mRNA of NKp30, NKp44, and NKG2D at similar levels as NK cells. Functional tests showed that CD3⁺NKp46⁺ lymphocytes produced IFN-γ and proliferated upon cytokine stimulation to a similar extent as NK cells, but did not respond to the T-cell mitogen, ConA. Likewise, CD3⁺NKp46⁺ cells killed K562 cells with an efficiency comparable to NK cells. Cross-linking of NKp46 and CD3 led to degranulation of CD3⁺NKp46⁺ cells, indicating functional signaling pathways for both receptors. Additionally, influenza A(H1N1)pdm09-infected pigs had reduced frequencies of CD3⁺NKp46⁺ lymphocytes in blood, but increased frequencies in the lung in the early phase of infection. Thus, CD3⁺NKp46⁺ cells appear to be involved in the early phase of influenza infections. In summary, we describe a lymphocyte population in swine with a mixed phenotype of NK and T cells, with results so far indicating that this cell population functionally resembles NK cells.

Human congenital T-cell receptor disorders

Immunodeficiencies of most T-cell receptor (TCR) components (TCRID) have been reported in almost 40 patients worldwide who have also, at times, shown signs of autoimmunity. We updated their clinical, immunological, and molecular features with an emphasis on practical diagnosis, as the range of the disorder grows in complexity with new partial defects. Cellular and animal models are also reviewed and in some cases reveal their limitations for predicting TCRID immunopathology.

Differential Biological Role of CD3 Chains Revealed by Human Immunodeficiencies

The biological role in vivo of the homologous CD3gamma and delta invariant chains within the human TCR/CD3 complex is a matter of debate, as murine models do not recapitulate human immunodeficiencies. We have characterized, in a Turkish family, two new patients with complete CD3gamma deficiency and SCID symptoms and compared them with three CD3gamma-deficient individuals belonging to two families from Turkey and Spain. All tested patients shared similar immunological features such as a partial TCR/CD3 expression defect, mild alphabeta and gammadelta T lymphocytopenia, poor in vitro proliferative responses to Ags and mitogens at diagnosis, and very low TCR rearrangement excision circles and CD45RA(+) alphabeta T cells. However, intrafamilial and interfamilial clinical variability was observed in patients carrying the same CD3G mutations. Two reached the second or third decade in healthy conditions, whereas the other three showed lethal SCID features with enteropathy early in life. In contrast, all reported human complete CD3delta (or CD3epsilon) deficiencies are in infants with life-threatening SCID and very severe alphabeta and gammadelta T lymphocytopenia. Thus, the peripheral T lymphocyte pool was comparatively well preserved in human CD3gamma deficiencies despite poor thymus output or clinical outcome. We propose a CD3delta >> CD3gamma hierarchy for the relative impact of their absence on the signaling for T cell production in humans.

The Impact of Single Amino Acid Substitutions in CD3γ on the CD3ϵγ Interaction and T-Cell Receptor–CD3 Complex Formation

The human T-cell receptor-CD3 complex consists of at least eight polypeptide chains; CD3gamma- and delta-dimers associate with the disulfide linked alphabeta- and zetazeta-dimers to form a functional receptor complex. The exact structure of this complex is still unknown. We now have examined the interaction between CD3gamma and CD3 in human T-cells. For this purpose, we have generated site-directed mutants of CD3gamma that were introduced in human T-cells defective in CD3gamma expression. Cell-surface and intracellular expression of the introduced CD3gamma chains was determined, as was the association with CD3delta, CD3, and the T-cell receptor. Although the introduction of wild type CD3gamma and CD3gamma (78Y-F) fully restored T-cell receptor assembly and expression, the introduction of CD3gamma (82C-S), CD3gamma (85C-S), and CD3gamma (76Q-E) all resulted in an impaired association between CD3gamma and CD3 and a lack of cell-surface expressed CD3gamma. Finally, the introduction of CD3gamma (76Q-L) and CD3gamma (78Y-A) restored the expression of TCR-CD3deltagammazeta2 complexes, although the association between CD3gamma and CD3 was impaired. These results indicate that several amino acids in CD3gamma are essential for an optimal association between CD3gamma and CD3 and the assembly of a cell-surface expressed TCR-CD3deltagammazeta2 complex.

CD3 delta immunodeficiency

PURPOSE OF REVIEW

The review describes advances in understanding the role of the CD3 delta subunit in human T-cell development as deduced from a recently described human immunodeficiency. The review also compares CD3 delta deficiency with other human CD3 subunit deficiencies and with corresponding animal models.

RECENT FINDINGS

In describing CD3 delta deficiency in humans this review shows that patients with profound T-cell depletion, who present at 2-3 months with severe viral infection, lack CD3 delta as a result of a mutation in the extracellular domain of this gene. The genetic aberration was discovered by comparing patients' and normal thymocytes, using mass gene screening with the microarray technique. In humans the absence of CD3 delta results in a complete arrest in thymocyte development at the stage of double negative to double positive transition and the development of gamma delta T-cell receptor-positive T cells is also impaired.

SUMMARY

Unlike patients with CD3 gamma or CD3 epsilon deficiency who have a milder condition, patients with CD3 delta deficiency present with severe lethal susceptibility to infections during early infancy. As expected, this profound immunodeficiency was cured with an allogenic bone marrow transplantation. In contrast to murine CD3 (-/) delta, which retains a normal gamma delta T-cell receptor-positive T-cell population and only partly affects the developmental transition of double positive to single positive thymocytes, CD3 delta in humans appears to be more critically required for the development of both alpha beta and gamma delta T-cell receptor-positive T-cell lineages. The studies also show for the first time that comparing relevant patients' with normal tissue using microarray technology can aid in the discovery of the genetic basis of inherited disorders.

Studies of patients' thymi aid in the discovery and characterization of immunodeficiency in humans

Studying the molecular and genetic bases of primary immunodeficiency is valuable at several levels. First, such information directly benefits patients in both short- and long-term management. Sophisticated diagnostic tools based on these studies can be used early and lead to appropriate treatment before potentially fatal infections and complications arise. Genotyping is also critical for future development and implementation of gene therapy. Secondly, investigating primary immunodeficiency helps understand the normal immune system in humans. As described in this report, the roles of zeta-associated protein of 70 kDa (Zap-70), CD25, and CD3delta are substantially different in humans when compared with the roles of homologous molecules in other species. Last, information obtained from these studies can be applied to other fields of investigation. Prominent examples for such applications include the intensive effort to design and produce specific inhibitors of Zap-70 and Janus kinase 3 as specific immunosuppressive agents. Most types of primary immunodeficiency in general and severe combined immunodeficiency in particular are rare and therefore cannot be easily studied by using traditional genetic methodology. Instead, biochemical methods were used to explore for candidate genes as was the case in the discovery of Zap-70 deficiency. Critical to the success of these discoveries was the careful analysis of patients' thymus glands. Detection of abnormalities in the thymus in these patients, which preceded identification of the genetic defect, aided in the assessment of the severity and nature of the immune disorder (primary versus secondary). Such assessment is critical before high-risk bone marrow transplantation. Equally important was the contribution of studies of the thymus to the description of novel phenotype of immunodeficiency as clearly demonstrated in defining CD8 lymphocytopenia, Zap-70 deficiency, and CD25 deficiency. Indeed, analysis of the thymus directly pointed to CD25 as candidate gene. Recently, the study of thymocyte-derived transcripts using DNA microarrays was key to discovering CD3delta deficiency. Finally, immunohistochemical analysis of the thymus was critical in pinpointing the roles of Zap-70, CD25, and CD3delta in the development of human T cells.

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.

Biochemical evidence for the presence of a single CD3delta and CD3gamma chain in the surface T cell receptor/CD3 complex

The T cell antigen receptor (TCR) consists of an alphabeta heterodimer and associated invariant CD3gamma, delta, epsilon, and zeta chains (TCR/CD3 complex). The general stoichiometry of the receptor complex, which is believed to be one molecule each of TCRalpha, TCRbeta, CD3gamma, and CD3delta and two molecules each of CD3epsilon and CD3zeta, is not clearly understood. Although it has been shown that there are two chains of CD3epsilon and CD3zeta, the stoichiometry of CD3gamma or CD3delta chains in the surface antigen receptor complex has not been determined. In the present study, transgenic mice expressing an altered form of mouse CD3delta and CD3gamma were employed to show that the surface TCR complexes contain one molecule each of CD3delta and CD3gamma. Thymocytes from wild type and CD3 chain transgenic mice on the appropriate knockout background were surface-biotinylated and immunoprecipitated using a specific antibody. The immunoprecipitates were resolved in two dimensions under nonreducing/reducing conditions to determine the stoichiometry of CD3delta and CD3gamma in the surface antigen receptor complex. Our data clearly show the presence of one molecule each of CD3delta and CD3gamma in the surface TCR/CD3 complex.

Conversion of CTLA-4 from inhibitor to activator of T cells with a bispecific tandem single-chain Fv ligand

Abs or their recombinant fragments against surface receptors of the Ig superfamily can induce or block the receptors' native function depending on whether they induce or prevent the assembly of signalosomes on their cytoplasmic tails. In this study, we introduce a novel paradigm based on the observation that a bispecific tandem single-chain variable region fragment ligand of CTLA-4 by itself converts this inhibitory receptor into an activating receptor for primary human T lymphocytes. This reversal of function results from increased recruitment of the serine/threonine phosphatase 2A to the cytoplasmic tail of CTLA-4, consistent with a role of this phosphatase in the regulation of CTLA-4 function, and assembly of a distinct signalosome that activates an lck-dependent signaling cascade and induces IL-2 production. Our data demonstrate that the cytoplasmic domain of CTLA-4 has an inherent plasticity for signaling that can be exploited therapeutically with recombinant ligands for this receptor.

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.

Mechanisms contributing to T cell receptor signaling and assembly revealed by the solution structure of an ectodomain fragment of the CD3 epsilon gamma heterodimer

The T cell receptor (TCR) consists of genetically diverse disulfide-linked alpha and beta chains in noncovalent association with the invariant CD3 subunits. CD3 epsilon and CD3 gamma are integral components of both the TCR and pre-TCR. Here, we present the solution structure of a heterodimeric CD3 epsilon gamma ectodomain complex. A unique side-to-side hydrophobic interface between the two C2-set immunoglobulin-like domains and parallel pairing of their respective C-terminal beta strands are revealed. Mutational analysis confirms the importance of the distinctive linkage as well as the membrane proximal stalk motif (RxCxxCxE) for domain-domain association. These biochemical and structural analyses offer insights into the modular pairwise association of CD3 invariant chains. More importantly, the findings suggest how the rigidified CD3 elements participate in TCR-based signal transduction.

Molecular modelling and endoplasmic reticulum retention of mutated TCR/CD3 complexes

T cell receptor (TCR)/CD3 complex assembly takes place in the endoplasmic reticulum (ER). Normal TCR/CD3 complexes egress from the ER to the cis-Golgi, where the interaction with zeta2 homodimers occurs. This interaction leads to further uncontrolled transport of TCR/CD3/zeta molecules to the cell surface. The purpose of the present experiments was to determine firstly the basis for the impact of the Phe195/216 --> Val mutations on TCR/CD3 expression in Jurkat cells, and secondly why mutated J79-cell TCRalphabeta/CD3 hexamers are prevented from interacting with zeta2 homodimers. We found that Phe --> Val mutations cause serious perturbations in a so far undefined hydrophobic area formed by the two Phe195/216 on beta-strand F and aromatic/large hydrophobic amino acids on neighboring beta-strands B and A in Calpha and Cbeta domains, respectively. In addition, TCR/CD3 hexamers and zeta2 homodimers colocalize in normal Jurkat T cells, in revertant J79r58 cells, and in J79 cells transfected with wild-type TCRalpha cDNA but not in J79 mutant cells (confocal microscopy). Furthermore, mutated TCR/CD3 complexes seem to be actively retained in the ER in J79 cells but not in revertant J79r58 cells by a nondominant mechanism. We propose that a hitherto undefined ER-retention molecule controls both the protein structure and egress of TCR/CD3 complexes from the ER of alphabeta and gammadelta T cells.

Heterodimeric CD3epsilongamma extracellular domain fragments: production, purification and structural analysis

The CD3 polypeptides (epsilon, gamma, and delta) are non-covalently associated signaling subunits of the T cell receptor which form non-disulfide linked epsilongamma and epsilondelta heterodimers. With the goal of investigating their structure, Escherichia coli expression was utilized to produce CD3 ectodomain fragments including the murine CD3epsilon subunit N-terminal Ig-like extracellular domain alone or as a single chain construct with that of CD3gamma. The latter links the CD3gamma segment to the C terminus of the CD3epsilon segment via a 26 amino acid peptide (scCD3epsilongamma26). Although CD3epsilon could be produced at high yield when directed to inclusion bodies, the refolded monomeric CD3epsilon was not native as judged by monoclonal antibody binding using surface plasmon resonance and was largely unstructured by (15)N-(1)H two-dimensional NMR analysis. In contrast, scCD3epsilongamma26 could be refolded readily into a native state as shown by CD, NMR and mAb reactivity. The linker length between CD3epsilon and CD3gamma is critical since scCD3epsilongamma16 containing a 16 residue connector failed to generate a stable heterodimer. Collectively, the results demonstrate that: (i) soluble heterodimeric fragments of CD3 can be produced; (ii) cotranslation of CD3 chains insures proper folding even in the absence of the conserved ectodomain stalk region (CxxCxE); and (iii) CD3epsilon has a more stable tertiary protein fold than CD3gamma.

Dissection of the role of CD3gamma chains in profound but reversible T-cell receptor down-regulation

T-lymphocyte activity in the immune system is regulated by the quantity of surface membrane T-cell antigen receptors (TCR). The amount of surface-bound TCR is dependent on the rate of [1] biosynthesis, assembly and intracellular transport of the individual chains composing the TCR/CD3 complex and [2] the internalization and recycling of the receptors. The TCR-ligand interaction augments receptor internalization. In the present paper, we have studied short- and long-term down-regulation of TCR/CD3 complexes with monoclonal anti-TCR/CD3 antibodies, and attempted to determine which component(s) of the TCR/CD3 complex are responsible for these two phenomena. Our data indicate that short- and long-term down-regulation is mediated by different mechanisms, and that the extracellular and/or transmembrane regions of CD3gamma molecules appear to play an important role in chronic TCR/CD3 down-regulation and subsequent deficient re-expression. These results may have important implications for the understanding of induction of T-cell tolerance or anergy.

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.

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.

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.

Signaling Through a CD3γ-Deficient TCR/CD3 Complex in Immortalized Mature CD4+ and CD8+ T Lymphocytes

The biologic role of each CD3 chain and their relative contribution to the signals transduced through the TCR/CD3 complex and to downstream activation events are still controversial: they may be specialized or redundant. We have immortalized peripheral blood CD4+ and CD8+ T lymphocytes from a human selective CD3γ deficiency using Herpesvirus saimiri. The accessibility of the mutant TCR/CD3 complex to different Abs was consistently lower in immortalized CD8+ cells when compared with CD4+ cells, relative to their corresponding CD3γ-sufficient controls. Several TCR/CD3-induced downstream activation events, immediate (calcium flux), early (cytotoxicity and induction of surface CD69 or CD40L activation markers or intracellular TNF-α) and late (proliferation and secretion of TNF-α), were normal in γ-deficient cells, despite the fact that their TCR/CD3 complexes were significantly less accessible than those of controls. In contrast, the accumulation of intracellular IL-2 or its secretion after CD3 triggering was severely impaired in γ-deficient cells. The defect was upstream of protein kinase C activation because addition of transmembrane stimuli (PMA plus calcium ionophore) completely restored IL-2 secretion in γ-deficient cells. These results suggest that the propagation of signals initiated at the TCR itself can result in a modified downstream signaling cascade with distinct functional consequences when γ is absent. They also provide evidence for the specific participation of the CD3γ chain in the induction of certain cytokine genes in both CD4+ and CD8+ human mature T cells. These immortalized mutant cells may prove to be useful in isolating cytosolic signaling pathways emanating from the TCR/CD3 complex.

One of the CD3epsilon subunits within a T cell receptor complex lies in close proximity to the Cbeta FG loop

A recent crystal structure of the N15 alpha/beta-T cell receptor (TCR) in complex with an Fab derived from the H57 Cbeta-specific monoclonal antibody (mAb) shows the mAb fragment interacting with the elongated FG loop of the Cbeta domain. This loop creates one side wall of a cavity within the TCR Ti-alpha/beta constant region module (CalphaCbeta) while the CD and EF loops of the Calpha domain form another wall. The cavity size is sufficient to accommodate a single nonglycosylated Ig domain such as the CD3epsilon ectodomain. By using specific mAbs to mouse TCR-beta (H57) and CD3epsilon (2C11) subunits, we herein provide evidence that only one of the two CD3epsilon chains within the TCR complex is located in close proximity to the TCR Cbeta FG loop, in support of the above notion. Moreover, analysis of T cells isolated from transgenic mice expressing both human and mouse CD3epsilon genes shows that the heterologous human CD3epsilon component can replace the mouse CD3epsilon at this site. The location of one CD3epsilon subunit within the rigid constant domain module has implications for the mechanism of signal transduction throughout T cell development.

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.

Atomic structure of an alphabeta T cell receptor (TCR) heterodimer in complex with an anti-TCR fab fragment derived from a mitogenic antibody

Each T cell receptor (TCR) recognizes a peptide antigen bound to a major histocompatibility complex (MHC) molecule via a clonotypic alphabeta heterodimeric structure (Ti) non-covalently associated with the monomorphic CD3 signaling components. A crystal structure of an alphabeta TCR-anti-TCR Fab complex shows an Fab fragment derived from the H57 monoclonal antibody (mAb), interacting with the elongated FG loop of the Cbeta domain, situated beneath the Vbeta domain. This loop, along with the partially exposed ABED beta sheet of Cbeta, and glycans attached to both Cbeta and Calpha domains, forms a cavity of sufficient size to accommodate a single non-glycosylated Ig domain such as the CD3epsilon ectodomain. That this asymmetrically localized site is embedded within the rigid constant domain module has implications for the mechanism of signal transduction in both TCR and pre-TCR complexes. Furthermore, quaternary structures of TCRs vary significantly even when they bind the same MHC molecule, as manifested by a unique twisting of the V module relative to the C module.

Assembly of the TCR/CD3 complex: CD3 epsilon/delta and CD3 epsilon/gamma dimers associate indistinctly with both TCR alpha and TCR beta chains. Evidence for a double TCR heterodimer model

The TCR/CD3 complex is composed of six subunits which are expressed on the cell surface in a coordinate fashion after assembly in the endoplasmic reticulum (ER). The TCR/CD3 complex is assembled after a series of pairwise interactions involving the formation of dimers of CD3 epsilon with either CD3 gamma or CD3 delta. These dimers assemble with TCR alpha and TCR beta chains, and finally, the CD3 zeta homodimer is added to allow export of the full complex from the ER. A model has been proposed suggesting that during assembly the CD3 epsilon/CD3 gamma dimer interacts exclusively with TCR beta and the CD3 epsilon/CD3 delta dimer with TCR alpha to form a complex with a single TCR alpha/beta heterodimer. We show in this study, by immunoprecipitation and two-dimensional gel electrophoresis, that in the human T cell line Jurkat as well as in total human thymocytes, this preferential interaction does not occur and instead, the CD3 epsilon/CD3 gamma and CD3 epsilon/CD3 delta dimers associate with both TCR chains simultaneously and indistinctly. These data are confirmed by the analysis of the TCR alpha-negative T cell line MOLT-4 in which TCR beta is found separately associated with CD3 epsilon/CD3 gamma and with CD3 epsilon/CD3 delta dimers. Indirectly, our results support a model of stoichiometry in which two TCR alpha/beta heterodimers are present in a TCR/CD3 complex. Furthermore, immunoprecipitation with anti-CD3 gamma and anti-CD3 delta antibodies from 1% NP40 and 1% Brij96 cell lysates showed that these subunits form independent partial complexes which are cross-linked through the CD3 zeta homodimer. This suggests that CD3 zeta mediates the interaction between both TCR alpha/beta heterodimers contained in the double TCR complex. Further proof for this hypothesis is obtained after analysis of a Jurkat cell transfectant containing a point mutation in the transmembrane domain of TCR beta that impairs the association of CD3 zeta. In this mutant cell line, unlike a control line with wild-type TCR beta, the CD3 gamma- and CD3 delta-containing complexes were found completely independent. Altogether, these results support a model of TCR/CD3 assembly and stoichiometry in which two TCR-alpha/beta heterodimers form two hemicomplexes containing either CD3 epsilon/gamma or CD3 epsilon/delta dimers which become associated via the CD3 zeta homodimer.

An amphibian CD3 homologue of the mammalian CD3 gamma and delta genes

T cell receptor (TCR) genes have been identified in representatives of both cartilaginous and bony vertebrates. The CD3 chains that serve as signal transducing elements of the TCR complex in mammals have been defined to a limited extent in birds. In these studies a CD3 homologue was identified in an amphibian representative, Xenopus laevis, using degenerate oligomer primers designed from conserved regions of avian and mammalian CD3 gamma/delta subunits. The reverse transcriptase polymerase chain reaction amplified product of Xenopus splenocyte RNA was then used to isolate full-length cDNA clones from a splenic library. When employed as probes, the cDNA clones hybridized with a 1-kb mRNA transcript in Xenopus T cells, but not in other cell types. Comparison of the deduced amino acid sequence indicated a similar degree of homology with mammalian and avian CD3 gamma and delta chains. Genomic analysis indicated that the Xenopus CD3 molecule is encoded by five exons, a structure resembling the mammalian CD3 delta gene rather than the seven exon CD3 gamma gene. Southern blot analysis and sequencing of the 5' flanking region failed to yield evidence of a related Xenopus gene. This amphibian CD3 gene thus appears to represent an ancestral form of the mammalian CD3 gamma and delta genes.

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.

Hierarchy of T cell antigen receptor assembly

The formation of partial complexes in an African green monkey kidney cell line (COS cells) has been used to examine assembly of newly synthesized T cell antigen receptor (TCR) chains. The identification of assembly interactions between multiple subunits and the formation of higher order subcomplexes has led to the development of a model for the hierarchy of subunit assembly leading to a complete TCR. These assembly interactions suggest likely nearest neighbour relationships in the assembling structure and probably reflect the quarternary structure of the TCR complex. This may be important when examining the association of other molecules with TCR proteins or when trying to discern structural components involved with signal transduction.

Normal development and function of natural killer cells in CD3 epsilon delta 5/delta 5 mutant mice

The CD3 epsilon polypeptide contributes to the cell surface display as well as to the signal transduction properties of the T-cell antigen receptor complex. Intriguingly, the distribution of CD3 epsilon is not restricted to T cells, since activated mouse, human, and avian natural killer (NK) cells do express intracytoplasmic CD3 epsilon polypeptides. CD3 epsilon is also present in the cytoplasm of fetal thymic T/NK bipotential progenitor cells, suggesting that it constitutes a component of the NK differentiation program. We report here that the genetic disruption of CD3 epsilon exon 5 alters neither NK cell development nor in vitro and in vivo NK functions, although it profoundly blocked T-cell development. These results support the notion that CD3 epsilon is dispensable for mouse NK cell ontogeny and function and further suggest that the common NK/T-cell progenitor cell utilizes CD3 epsilon as a mandatory component only when differentiating toward the T-cell lineage.

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.

Immunotherapeutic strategies targeting rheumatoid synovial T-cell receptors by DNA inoculation

Immunotherapy against autoreactive T-cell receptors (TCRs) has been reported to have promise in several animal models of autoimmune diseases. Facilitated DNA inoculation has many potential advantages as a modality for development of specific immune responses. Specifically, this technology is able to deliver exogenous antigens for processing via both the endogenous pathway, with subsequent presentation by class-I major histocompatibility (MHC) antigens, and the exogenous pathway, with subsequent presentation by class-II MHC antigens. This allows for induction of both arms of the cellular immune system. These cellular immune responses may be particularly important in targeting and controlling pathogenic cell populations. The application of this technology to the treatment of human autoimmune diseases depends on the availability of readily manipulated systems for the evaluation of specific interventions. Here we report the full length cloning and expression of TCRs from rheumatoid arthritis synovial tissue. These were developed by recombinant polymerase chain reaction, cloning and retroviral transduction into a TCR-alpha/beta-negative murine T-cell hybridoma. Reconstitution of CD3 expression was confirmed by flow cytometry. Similar constructs have been developed for TCR-based immunotherapy by facilitated inoculation of DNA intramuscularly. Preliminary analysis of immune responses in mice indicates that these constructs elicit anti-TCR responses. These studies indicate the ability to reconstitute expression of potentially autoreactive human TCRs in a model system wherein specific immune responses elicited against these TCRs by various immunogens can be evaluated.

Structure of the T cell receptor in a Ti alpha V beta 2, alpha V beta 8-positive T cell line

The T cell receptor (TcR) is composed of at least six different polypeptide chains consisting of the clonotypic Ti heterodimer (Ti alpha beta or Ti gamma delta) and the noncovalently associated CD3 chains (CD3 gamma delta epsilon zeta). The exact number of subunits constituting the TcR is still not known; however, it has been suggested that each TcR contains two Ti dimers. To gain insight into the structure of the TcR we constructed a Ti alpha V beta 2, alpha V beta 8-positive T cell line which expressed the endogenous human TiV beta 8 and the transfected mouse TiV beta 2 both in association with the endogenous Ti alpha and CD3 chains at the cell surface. Preclearing experiments with radioiodinated cell lysate prepared with digitonin lysis buffer demonstrated that depleting the lysate of Ti alpha V beta 8 by immunoprecipitation with anti V beta 8 monoclonal antibody (mAb) did not reduce the amount of Ti alpha V beta 2 in the lysate, and likewise, depleting the lysate of Ti alpha V beta with anti-V beta 2 mAb did not reduce the amount of Ti alpha V beta 8. Comodulation experiments showed that V beta 8 and V beta 2 did not comodulate with each other. Furthermore, functional tests demonstrated that TcR containing V beta 8 and TcR containing V beta 2 mediated transmembrane activation signals independently of each other. These data demonstrate that mouse V beta 2 and human V beta 8 were not expressed in the same TcR in agreement with a TcR model where each TcR contains only one Ti dimer.

The T cell antigen receptor alpha and beta chains interact via distinct regions with CD3 chains

Selective pairwise interactions between CD3 chains and the clonotypic T cell antigen receptor (TCR)-alpha, -beta chains has recently been established. In this study, the region of interaction between clonotypic and CD3 chains involved with assembly was examined. To determine the site of protein interaction a variety of genetically altered TCR chains were constructed. These included: truncated proteins, lacking transmembrane and or cytosolic domains; chimeric proteins, in which extracellular, transmembrane or cytosolic domains were replaced with similar domains derived from either the Tac antigen or CD4; and point mutagenized TCR chains. COS-1 cells were transfected with cDNA, metabolically labeled, and immunoprecipitates analyzed using non-equilibrium pH gel electrophoresis (NEPHGE)-SDS/PAGE. The results demonstrated that assembly between TCR-alpha and TCR-beta chains occurred at the extracellular level. Assembly of the TCR-alpha chain with CD3-delta, and CD3-epsilon was localized to an eight-amino acid motif within the transmembrane domain of TCR-alpha. Site-specific mutations of the TCR-alpha charged residues within this motif (arginine, lysine) to leucine and similar point mutations of the transmembrane CD3-epsilon and CD3-delta charge groups resulted in the abrogation of assembly. In contrast, TCR-beta and CD3-epsilon binary complexes interacted via their extracellular domain. Analogous to TCR-alpha, the site of TCR-beta and CD3-delta assembly was at the transmembrane region. Despite multiple genetic manipulations on CD3-gamma and zeta these proteins failed to assemble with TCR-alpha. Similarly, there was no interaction between TCR-beta and zeta. These findings when coupled with the information on pairwise interactions and formation of higher order subcomplexes extend our model for the structure of the TCR complex.

CD22 associates with the human surface IgM-B-cell antigen receptor complex

The B-cell surface molecule CD22, when cross-linked, modulates signaling through the surface IgM (sIgM)-B-cell receptor (BCR) complex. Here we analyzed the basis of this interaction between CD22 and the human sIgM complex. After lysis of B cells or B-cell lines in digitonin, CD22 coimmunoprecipitated a kinase activity that in vitro-phosphorylated two polypeptides of 150 and 130 kDa on tyrosine residues. By immunoblot analysis with a rabbit anti-serum specific for a synthetic peptide of CD22, we found these proteins to be CD22 itself. Furthermore, the phosphorylated 150-kDa CD22 was found in the sIgM-BCR complex maintained by digitonin, along with Ig alpha/mb-1, Ig beta/B29, and a 75-kDa polypeptide precipitated by an antiserum specific to protein-tyrosine kinase PTK72. CD22 is likely to be an important signaling partner in the sIgM-BCR complex since it is very rapidly and strikingly phosphorylated after sIgM is cross-linked and since it contains the antigen recognition homology I (ARHI) motif, present in other antigen receptor molecules.

Independent mutations of the human CD3-epsilon gene resulting in a T cell receptor/CD3 complex immunodeficiency

The T-cell receptor (TCR) is composed of two glycoproteins (alpha and beta or gamma and delta) associated with four invariant polypeptides (CD3-gamma, delta, epsilon and zeta). The majority of TCR/CD3 complexes contain six polypeptide chains, and although there is some flexibility in the complex subunit stoichiometry the CD3-epsilon chain is central to CD3 core assembly and full complex formation. We have described previously defective expression of the TCR/CD3 complex in an immunodeficient child. We now report that two independent CD3-epsilon gene mutations present in the parents have segregated in the patient, leading to defective CD3-epsilon chain synthesis and preventing normal association and membrane expression of the TCR/CD3 complex.

CD3-gamma, -delta, -epsilon, -zeta, T-cell receptor-alpha and -beta transcripts are independently regulated during thymocyte ontogeny and T-cell activation

CD3 proteins transduce signals delivered from the T-cell receptor (TcR) for antigen. The genes that encode the individual CD3 subunits are asynchronously regulated in tumour cell lines in vitro. In this report, we examined the expression of individual CD3 and TcR genes during normal murine thymocyte ontogeny in vivo. We show that CD3-gamma, -delta, -epsilon, and zeta transcripts are all expressed on Day 14 post-coitum (p.c.), along with IL-2R alpha and Thy-1 mRNA, and prior to the expression of functional TcR-alpha, -beta, CD4 and CD8 transcripts. Individual CD3 subunits display unique patterns of increased gene expression as ontogeny proceeds. Unique regulation of these T-cell transcripts is also observed in splenic cells activated with the T-cell mitogen concanavalin A (Con A). CD3-delta, -zeta, TcR-alpha and -beta mRNA expression increases, whilst CD3-gamma and -epsilon mRNA levels decrease after mitogenic activation. The potential implications of this regulation on the composition and expression of the TcR/CD3 complex during T-cell ontogeny and activation is discussed.

An endoplasmic reticulum retention signal in the CD3 epsilon chain of the T-cell receptor

Isolated polypeptide chains of the T-cell antigen receptor complex are degraded or retained in the endoplasmic reticulum (ER). Assembly of the multisubunit complex allows the individual chains to escape retention in the ER and to be expressed on the cell surface. We engineered a series of deletions in the CD3 epsilon subunit of the human T-cell receptor in order to find the sequences responsible for its retention in the ER. Deletion of amino acids 171 to 180 in the cytosolic tail resulted in the cell-surface expression of the isolated chain. This sequence also promotes retention when it is appended to CD4, a plasma membrane protein. Mutagenesis of the 10-amino-acid CD3 epsilon sequence established that the tyrosine and serine residues are important for ER retention. This and other ER retention signals must be hidden when a complete T-cell receptor complex is assembled in order to allow its expression on the cell surface.

The development of functionally responsive T cells

The work reviewed in this article separates T cell development into four phases. First is an expansion phase prior to TCR rearrangement, which appears to be correlated with programming of at least some response genes for inducibility. This phase can occur to some extent outside of the thymus. However, the profound T cell deficit of nude mice indicates that the thymus is by far the most potent site for inducing the expansion per se, even if other sites can induce some response acquisition. Second is a controlled phase of TCR gene rearrangement. The details of the regulatory mechanism that selects particular loci for rearrangement are still not known. It seems that the rearrangement of the TCR gamma loci in the gamma delta lineage may not always take place at a developmental stage strictly equivalent to the rearrangement of TCR beta in the alpha beta lineage, and it is not clear just how early the two lineages diverge. In the TCR alpha beta lineage, however, the final gene rearrangement events are accompanied by rapid proliferation and an interruption in cellular response gene inducibility. The loss of conventional responsiveness is probably caused by alterations at the level of signaling, and may be a manifestation of the physiological state that is a precondition for selection. Third is the complex process of selection. Whereas peripheral T cells can undergo forms of positive selection (by antigen-driven clonal expansion) and negative selection (by abortive stimulation leading to anergy or death), neither is exactly the same phenomenon that occurs in the thymic cortex. Negative selection in the cortex appears to be a suicidal inversion of antigen responsiveness: instead of turning on IL-2 expression, the activated cell destroys its own chromatin. The genes that need to be induced for this response are not yet identified, but it is unquestionably a form of activation. It is interesting that in humans and rats, cortical thymocytes undergoing negative selection can still induce IL-2R alpha expression and even be rescued in vitro, if exogenous IL-2 is provided. Perhaps murine thymocytes are denied this form of rescue because they shut off IL-2R beta chain expression at an earlier stage or because they may be uncommonly Bcl-2 deficient (cf. Sentman et al., 1991; Strasser et al., 1991). Even so, medullary thymocytes remain at least partially susceptible to negative selection even as they continue to mature.(ABSTRACT TRUNCATED AT 400 WORDS)