The implications of subunit interactions for the structure of the T cell receptor-CD3 complex

Advances in CD247

CD247, which is also known as CD3ζ, CD3H, CD3Q, CD3Z, IMD25, T3Z, and TCRZ, encodes CD3ζ protein, which is expressed primarily in natural killer (NK) and T cells. Since the discovery of the ζ peptide in 1986, it has been continuously investigated. In this paper, we review the composition, molecular mechanisms and regulatory factors of CD247 expression in T cells; and review the autoimmune diseases, tumors and inflammatory diseases associated with CD247, providing a detailed and comprehensive reference for further research on the mechanism of CD247 and related diseases.

αβ TCR-mediated recognition: relevance to tumor-antigen discovery and cancer immunotherapy

αβ T lymphocytes sense perturbations in host cellular body components induced by infectious pathogens, oncogenic transformation, or chemical or physical damage. Millions to billions of these lymphocytes are generated through T-lineage development in the thymus, each endowed with a clonally restricted surface T-cell receptor (TCR). An individual TCR has the capacity to recognize a distinct "foreign" peptide among the myriad of antigens that the mammalian host must be capable of detecting. TCRs explicitly distinguish foreign from self-peptides bound to major histocompatibility complex (MHC) molecules. This is a daunting challenge, given that the MHC-linked peptidome consists of thousands of distinct peptides with a relevant nonself target antigen often embedded at low number, among orders of magnitude higher frequency self-peptides. In this Masters of Immunology article, I review how TCR structure and attendant mechanobiology involving nonlinear responses affect sensitivity as well as specificity to meet this requirement. Assessment of human tumor-cell display using state-of-the-art mass spectrometry physical detection methods that quantify epitope copy number can help to provide information about requisite T-cell functional avidity affording protection and/or therapeutic immunity. Future rational CD8 cytotoxic T-cell-based vaccines may follow, targeting virally induced cancers, other nonviral immunogenic tumors, and potentially even nonimmunogenic tumors whose peptide display can be purposely altered by MHC-binding drugs to stimulate immune attack.

Structural Features of the αβTCR Mechanotransduction Apparatus That Promote pMHC Discrimination

The αβTCR was recently revealed to function as a mechanoreceptor. That is, it leverages mechanical energy generated during immune surveillance and at the immunological synapse to drive biochemical signaling following ligation by a specific foreign peptide-MHC complex (pMHC). Here, we review the structural features that optimize this transmembrane (TM) receptor for mechanotransduction. Specialized adaptations include (1) the CβFG loop region positioned between Vβ and Cβ domains that allosterically gates both dynamic T cell receptor (TCR)-pMHC bond formation and lifetime; (2) the rigid super β-sheet amalgams of heterodimeric CD3εγ and CD3εδ ectodomain components of the αβTCR complex; (3) the αβTCR subunit connecting peptides linking the extracellular and TM segments, particularly the oxidized CxxC motif in each CD3 heterodimeric subunit that facilitates force transfer through the TM segments and surrounding lipid, impacting cytoplasmic tail conformation; and (4) quaternary changes in the αβTCR complex that accompany pMHC ligation under load. How bioforces foster specific αβTCR-based pMHC discrimination and why dynamic bond formation is a primary basis for kinetic proofreading are discussed. We suggest that the details of the molecular rearrangements of individual αβTCR subunit components can be analyzed utilizing a combination of structural biology, single-molecule FRET, optical tweezers, and nanobiology, guided by insightful atomistic molecular dynamic studies. Finally, we review very recent data showing that the pre-TCR complex employs a similar mechanobiology to that of the αβTCR to interact with self-pMHC ligands, impacting early thymic repertoire selection prior to the CD4(+)CD8(+) double positive thymocyte stage of development.

Constitutively oxidized CXXC motifs within the CD3 heterodimeric ectodomains of the T cell receptor complex enforce the conformation of juxtaposed segments

The CD3ϵγ and CD3ϵδ heterodimers along with the CD3ζζ homodimer are the signaling components of the T cell receptor (TCR). These invariant dimers are non-covalently associated on the T cell plasma membrane with a clone-specific (i.e. clonotypic) αβ heterodimer that binds its cognate ligand, a complex between a particular antigenic peptide, and an MHC molecule (pMHC). These four TCR dimers exist in a 1:1:1:1 stoichiometry. At the junction between the extracellular and transmembrane domains of each mammalian CD3ϵ, CD3γ, and CD3δ subunit is a highly conserved CXXC motif previously found to be important for thymocyte and T cell activation. The redox state of each CXXC motif is presently unknown. Here we show using LC-MS and a biotin switch assay that these CXXC segments are constitutively oxidized on resting and activated T cells, consistent with their measured reduction potential. NMR chemical shift perturbation experiments comparing a native oxidized CD3δ CXXC-containing segment with that of a mutant SXXS-containing CD3δ segment in LPPG (1-palmitoyl-sn-glycero-3-phospho-(1'-rac-glycerol) (sodium salt)) micelles show extensive chemical shift differences in residues within the membrane-proximal motif as well as throughout the transmembrane and cytoplasmic domains as a result of the elimination of the native disulfide. Likewise, direct comparison of the native CD3δ segment in oxidizing and reducing conditions reveals numerous spectral differences. The oxidized CXXC maintains the structure within the membrane-proximal stalk region as well as that of its contiguous transmembrane and cytoplasmic domain, inclusive of the ITAM (immunoreceptor tyrosine-based activation motif) involved in signaling. These results suggest that preservation of the CD3 CXXC oxidized state may be essential for TCR mechanotransduction.

LAPTM5 promotes lysosomal degradation of intracellular CD3ζ but not of cell surface CD3ζ

The lysosomal protein LAPTM5 has been shown to negatively regulate cell surface T cell receptor (TCR) expression and T-cell activation by promoting CD3ζ degradation in lysosomes, but the mechanism remains largely unknown. Here we show that LAPTM5 promotes lysosomal translocation of intracellular CD3ζ but not of the cell surface CD3ζ associated with the mature TCR complex. Kinetic analysis of the subcellular localization of the newly synthesized CD3ζ suggests that LAPTM5 targets CD3ζ in the Golgi apparatus and promotes its lysosomal translocation. Consistently, a Golgi-localizing mutant CD3ζ can be transported to and degraded in the lysosome by LAPTM5. A CD3ζ YF mutant in which all six tyrosine residues in the immunoreceptor tyrosine-based activation motif are mutated to phenylalanines is degraded as efficiently as is wild type CD3ζ, further suggesting that TCR signaling-triggered tyrosine phosphorylation of CD3ζ is dispensable for LAPTM5-mediated degradation. Previously, Src-like adapter protein (SLAP) and E3 ubiquitin ligase c-Cbl have been shown to mediate the ubiquitination of CD3ζ in the internalized TCR complex and its subsequent lysosomal degradation. We show that LAPTM5 and SLAP/c-Cbl function in distinct genetic pathways to negatively regulate TCR expression. Collectively, these results suggest that CD3ζ can be degraded by two pathways: SLAP/c-Cbl, which targets internalized cell surface CD3ζ dependent on TCR signaling, and LAPTM5, which targets intracellular CD3ζ independent of TCR signaling.

Gene expression profiling of blood in ruptured intracranial aneurysms: in search of biomarkers

The molecular mechanisms underlying the systemic response to subarachnoid hemorrhage (SAH) from ruptured intracranial aneurysms (RAs) are not fully understood. We investigated whether the analysis of gene expression in peripheral blood could provide clinically relevant information regarding the biologic consequences of SAH. Transcriptomics were performed using Illumina HumanHT-12v4 microarrays for 43 RA patients and 18 controls (C). Differentially expressed transcripts were analyzed for overrepresented functional groups and blood cell type-specific gene expression. The set of differentially expressed transcripts was validated using quantitative polymerase chain reaction in an independent group of subjects (15 RA patients and 14 C). There were 135 differentially expressed genes (false discovery rate 1%, absolute fold change 1.7): the abundant levels of 78 mRNAs increased and 57 mRNAs decreased. Among RA patients, transcripts specific to T lymphocyte subpopulations were downregulated, whereas those related to monocytes and neutrophils were upregulated. Expression profiles of a set of 16 genes and lymphocyte-to-monocyte-and-neutrophil gene expression ratios distinguished RA patients from C. These results indicate that SAH from RAs strongly influences the transcription profiles of blood cells. A specific pattern of these changes suggests suppression in lymphocyte response and enhancements in monocyte and neutrophil activities. This is probably related to the immunodepression observed in SAH.

Piecing together the family portrait of TCR-CD3 complexes

The pre-T-cell receptor (TCR)-, αβTCR-, and γδTCR-CD3 complexes are members of a family of modular biosensors that are responsible for driving T-cell development, activation, and effector functions. They inform essential checkpoint decisions by relaying key information from their ligand-binding modules (TCRs) to their signaling modules (CD3γε + CD3δε and CD3ζζ) and on to the intracellular signaling apparatus. Their actions shape the T-cell repertoire, as well as T-cell-mediated immunity; yet, the mechanisms that underlie their activity remain an enigma. As with any molecular machine, understanding how they function depends upon understanding how their parts fit and work together. In the 30 years since the initial biochemical and genetic characterizations of the αβTCR, the structure and function of the individual components of these family members have been extensively characterized. Cumulatively, this information has allowed us to piece together a portrait of the αβTCR-CD3 complex and outline the form of the remaining family members. Here we review the known structural and functional characteristics of the components of these TCR-CD3 complex family members. We then discuss how these data have informed our understanding of the architecture of the αβTCR-CD3 complex as well as their implications for the other family members. The intent is to provide a framework for considering: (i) how these thematically similar complexes diverge to execute their specific functions and (ii) how our knowledge of the form and function of these distinct family members can cross-inform our understanding of the other family members.

The structural basis of αβ T-lineage immune recognition: TCR docking topologies, mechanotransduction, and co-receptor function

Self versus non-self discrimination is at the core of T-lymphocyte recognition. To this end, αβ T-cell receptors (TCRs) ligate 'foreign' peptides bound to major histocompatibility complex (MHC) class I or class II molecules (pMHC) arrayed on the surface of antigen-presenting cells (APCs). Since the discovery of TCRs approximately 30 years ago, considerable structural and functional data have detailed the molecular basis of their extraordinary ligand specificity and sensitivity in mediating adaptive T-cell immunity. This review focuses on the structural biology of the Fab-like TCRαβ clonotypic heterodimer and its unique features in conjunction with those of the associated CD3εγ and CD3εδ heterodimeric molecules, which, along with CD3ζζ homodimer, comprise the TCR complex in a stoichiometry of 1:1:1:1. The basis of optimized TCRαβ docking geometry on the pMHC linked to TCR mechanotransduction and required for T-cell signaling as well as CD4 and CD8 co-receptor function is detailed. A model of the TCR ectodomain complex including its connecting peptides suggests how force generated during T-cell immune surveillance and at the immunological synapse results in dynamic TCR quaternary change involving its heterodimeric components. Potential insights from the structural biology relevant to immunity and immunosuppression are revealed.

TCR Mechanobiology: Torques and Tunable Structures Linked to Early T Cell Signaling

Mechanotransduction is a basis for receptor signaling in many biological systems. Recent data based upon optical tweezer experiments suggest that the TCR is an anisotropic mechanosensor, converting mechanical energy into biochemical signals upon specific peptide-MHC complex (pMHC) ligation. Tangential force applied along the pseudo-twofold symmetry axis of the TCR complex post-ligation results in the αβ heterodimer exerting torque on the CD3 heterodimers as a consequence of molecular movement at the T cell-APC interface. Accompanying TCR quaternary change likely fosters signaling via the lipid bilayer predicated on the magnitude and direction of the TCR-pMHC force. TCR glycans may modulate quaternary change, thereby altering signaling outcome as might the redox state of the CxxC motifs located proximal to the TM segments in the heterodimeric CD3 subunits. Predicted alterations in TCR TM segments and surrounding lipid will convert ectodomain ligation into the earliest intracellular signaling events.

T cell receptors are structures capable of initiating signaling in the absence of large conformational rearrangements

Native and non-native ligands of the T cell receptor (TCR), including antibodies, have been proposed to induce signaling in T cells via intra- or intersubunit conformational rearrangements within the extracellular regions of TCR complexes. We have investigated whether any signatures can be found for such postulated structural changes during TCR triggering induced by antibodies, using crystallographic and mutagenesis-based approaches. The crystal structure of murine CD3ε complexed with the mitogenic anti-CD3ε antibody 2C11 enabled the first direct structural comparisons of antibody-liganded and unliganded forms of CD3ε from a single species, which revealed that antibody binding does not induce any substantial rearrangements within CD3ε. Saturation mutagenesis of surface-exposed CD3ε residues, coupled with assays of antibody-induced signaling by the mutated complexes, suggests a new configuration for the complex within which CD3ε is highly exposed and reveals that no large new CD3ε interfaces are required to form during antibody-induced signaling. The TCR complex therefore appears to be a structure that is capable of initiating intracellular signaling in T cells without substantial structural rearrangements within or between the component subunits. Our findings raise the possibility that signaling by native ligands might also be initiated in the absence of large structural rearrangements in the receptor.

Defective synthesis or association of T-cell receptor chains underlies loss of surface T-cell receptor-CD3 expression in enteropathy-associated T-cell lymphoma

Enteropathy-associated T-cell lymphoma, an often fatal complication of celiac disease, can result from expansion of aberrant intraepithelial lymphocytes in refractory celiac disease type II (RCD II). Aberrant intraepithelial lymphocytes and lymphoma cells are intracellularly CD3epsilon(+) but lack expression of the T-cell receptor (TCR)-CD3 complex on the cell surface. It is unknown what causes the loss of TCR-CD3 expression. We report the isolation of a cell line from an RCD II patient with the characteristic phenotype of enteropathy-associated T-cell lymphoma. We demonstrate that in this cell line the TCR-alpha and -beta chains as well as the CD3gamma, CD3delta, CD3epsilon, and zeta-chains are present intracellularly and that assembly of the CD3gammaepsilon, CD3deltaepsilon, and zetazeta-dimers is normal. However, dimerization of the TCR chains and proper assembly of the TCR-CD3 complex are defective. On introduction of exogenous TCR-beta chains, but not of TCR-alpha chains, assembly and functional cell surface expression of the TCR-CD3 complex were restored. Defective synthesis of both TCR chains was found to underlie loss of TCR expression in similar cell lines isolated from 2 additional patients. (Pre)malignant transformation in RCD II thus correlates with defective synthesis or defective association of the TCR chains, resulting in loss of surface TCR-CD3 expression.

Common themes in the assembly and architecture of activating immune receptors

Each of the many different cell types of the immune system expresses one or several activating receptors which serve a central role in the cell's surveillance function. Many of these cell-surface receptors share a distinctive modular design that consists of a ligand-binding module with no intrinsic signalling capability that is non-covalently associated with one or more dimeric signalling modules. Receptor assembly is directed by unique polar contacts within the transmembrane domains, whereas extracellular contacts can contribute to stability and specificity. This Review discusses the structural basis of receptor assembly and the implications of these findings for the mechanisms of receptor triggering.

Different composition of the human and the mouse gammadelta T cell receptor explains different phenotypes of CD3gamma and CD3delta immunodeficiencies

The γδ T cell receptor for antigen (TCR) comprises the clonotypic TCRγδ, the CD3 (CD3γε and/or CD3δε), and the ζζ dimers. γδ T cells do not develop in CD3γ-deficient mice, whereas human patients lacking CD3γ have abundant peripheral blood γδ T cells expressing high γδ TCR levels. In an attempt to identify the molecular basis for these discordant phenotypes, we determined the stoichiometries of mouse and human γδ TCRs using blue native polyacrylamide gel electrophoresis and anti-TCR–specific antibodies. The γδ TCR isolated in digitonin from primary and cultured human γδ T cells includes CD3δ, with a TCRγδCD3ε₂δγζ₂ stoichiometry. In CD3γ-deficient patients, this may allow substitution of CD3γ by the CD3δ chain and thereby support γδ T cell development. In contrast, the mouse γδ TCR does not incorporate CD3δ and has a TCRγδCD3ε₂γ₂ζ₂ stoichiometry. CD3γ-deficient mice exhibit a block in γδ T cell development. A human, but not a mouse, CD3δ transgene rescues γδ T cell development in mice lacking both mouse CD3δ and CD3γ chains. This suggests important structural and/or functional differences between human and mouse CD3δ chains during γδ T cell development. Collectively, our results indicate that the different γδ T cell phenotypes between CD3γ-deficient humans and mice can be explained by differences in their γδ TCR composition.

Disruption of extracellular interactions impairs T cell receptor-CD3 complex stability and signaling

The alphabeta T cell antigen receptor (TCR), in complex with the CD3deltavarepsilon, gammavarepsilon, and zetazeta signaling subunits, is the chief determinant for specific CD4(+) and CD8(+) T cell responses to self and foreign antigens. Although transmembrane domain charge interactions are critical for the assembly of the complex, the location of extracellular contacts between the TCR and CD3 subunits and their contributions to stability and signal transduction have not been defined. Here we used mutagenesis to demonstrate that the CD3deltavarepsilon and CD3gammavarepsilon subunits interact with the TCR via adjacent Calpha DE and Cbeta CC' loops, respectively. The TCR-CD3deltavarepsilon interactions helped stabilize CD3gammavarepsilon within the complex and were important for normal T cell and thymocyte responses to TCR engagement. These data demonstrate that extracellular TCR-CD3 subunit interactions contribute to the structural integrity and function of this multisubunit receptor.

Importance of the CD3γ Ectodomain Terminal β-Strand and Membrane Proximal Stalk in Thymic Development and Receptor Assembly

CD3epsilongamma and CD3epsilondelta are noncovalent heterodimers; each consists of Ig-like extracellular domains associated side-to-side via paired terminal beta-strands that are linked to individual subunit membrane proximal stalk segments. CD3epsilon, CD3gamma, and CD3delta stalks contain the RxCxxCxE motif. To investigate the functional importance of a CD3 stalk and terminal beta-strand, we created a CD3gamma double mutant CD3gamma(C82S/C85S) and a CD3gamma beta-strand triple mutant CD3gamma(Q76S/Y78A/Y79A) for use in retroviral transduction of lymphoid progenitors for comparison with CD3gammawt. Although both mutant CD3gamma molecules reduced association with CD3epsilon in CD3epsilongamma heterodimers, CD3gamma(Q76S/Y78A/Y79A) abrogated surface TCR expression whereas CD3gamma(C82S/C85S) did not. Furthermore, CD3gamma(C82S/C85S) rescued thymic development in CD3gamma(-/-) fetal thymic organ culture. However, the numbers of double-positive and single-positive thymocytes after CD3gamma(C82S/C85S) transduction were significantly reduced despite surface pre-TCR and TCR expression comparable to that of CD3gamma(-/-) thymocytes transduced in fetal thymic organ culture with a retrovirus harboring CD3gammawt cDNA. Furthermore, double-negative thymocyte development was perturbed with attenuated double-negative 3/double-negative 4 maturation and altered surface-expressed CD3epsilongamma, as evidenced by the loss of reactivity with CD3gamma N terminus-specific antisera. Single histidine substitution of either CD3gamma stalk cysteine failed to restore CD3epsilongamma association and conformation in transient COS-7 cell transfection studies. Thus, CD3gamma(C82) and CD3gamma(C85) residues likely are either reduced or form a tight intrachain disulfide loop rather than contribute to a metal coordination site in conjunction with CD3epsilon(C80) and CD3epsilon(C83). The implications of these results for CD3epsilongamma and TCR structure and signaling function are discussed.

Efficiency of T-cell receptor expression in dual-specific T cells is controlled by the intrinsic qualities of the TCR chains within the TCR-CD3 complex

Genetic engineering of T lymphocytes is an attractive strategy to specifically redirect T-cell immunity toward viral infections and malignancies. We previously demonstrated redirected antileukemic reactivity of cytomegalovirus (CMV)–specific T cells by transfer of minor histocompatibility antigen HA-2–specific T-cell receptors (TCRs). HA-2–TCR-transferred CMV-specific T cells were potent effectors against HA-2–expressing leukemic cells, as well as CMV-expressing cells. Functional activity of these T cells correlated with TCR cell-surface expression. In the present study we analyzed which properties of transferred and endogenous TCRs are crucial for efficient cell-surface expression. We demonstrate that expression of the introduced TCR is not a random process but is determined by characteristics of both the introduced and the endogenously expressed TCR. The efficiency of TCR cell-surface expression is controlled by the intrinsic quality of the TCR complex. In addition, we demonstrate that chimeric TCRs can be formed and that efficiency of TCR expression is independent of whether TCRs are retrovirally introduced or naturally expressed. In conclusion, introduced, endogenous, and chimeric TCRs compete for cell-surface expression in favor of the TCR-CD3 complex with best-pairing properties.

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.

Reconstitution of Allogeneic Hemopoietic Stem Cells: The Essential Role of FcRγ and the TCR β-Chain-FCp33 Complex

Transplantation of purified allogeneic hemopoietic stem cells (SC) alone is characterized by a decreased risk of graft-vs-host disease but increased incidence of engraftment failure. It has been established that the facilitating cell (FC) promotes allogeneic SC reconstitution and results in donor-specific transplantation tolerance across MHC disparities, without graft-vs-host disease. Although the requirements for this facilitating function are not well-characterized, it is known that facilitation is dependent on FC expression of a unique heterodimer consisting of the TCR beta-chain (TCRbeta) and a 33-kDa protein, FCp33. The current study confirms that CD3epsilon and TCRbeta expression are present on the FC at the time of transplantation and demonstrates that the majority of cells in the FC population express the TCR signaling molecule, FcRgamma, rather than the more conventional CD3zeta receptor. Of particular significance, we have now demonstrated that FC-mediated allogeneic SC reconstitution is critically dependent on FcRgamma expression and that FcRgamma coprecipitates with the TCRbeta-FCp33 heterodimer. The mandatory requirement of TCRbeta and FcRgamma for FC function provides the first evidence of a previously undescribed role for FcRgamma in the facilitation of allogeneic SC reconstitution and establishes that FcRgamma is part of the TCRbeta-FCp33 complex uniquely expressed on FC.

How TCRs bind MHCs, peptides, and coreceptors

Since the first crystal structure determinations of alphabeta T cell receptors (TCRs) bound to class I MHC-peptide (pMHC) antigens in 1996, a sizable database of 24 class I and class II TCR/pMHC complexes has been accumulated that now defines a substantial degree of structural variability in TCR/pMHC recognition. Recent determination of free and bound gammadelta TCR structures has enabled comparisons of the modes of antigen recognition by alphabeta and gammadelta T cells and antibodies. Crystal structures of TCR accessory (CD4, CD8) and coreceptor molecules (CD3epsilondelta, CD3epsilongamma) have further advanced our structural understanding of most of the components that constitute the TCR signaling complex. Despite all these efforts, the structural basis for MHC restriction and signaling remains elusive as no structural features that define a common binding mode or signaling mechanism have yet been gleaned from the current set of TCR/pMHC complexes. Notwithstanding, the impressive array of self, foreign (microbial), and autoimmune TCR complexes have uncovered the diverse ways in which antigens can be specifically recognized by TCRs.

Genes, pathways and checkpoints in lymphocyte development and homeostasis

A plethora of genes involved in murine B and T cell development have been identified, and developmental pathways within the primary lymphoid tissues have been well delineated. The generation of a functional, but non-self reacting lymphocyte repertoire results from the completion of several checkpoints during lymphocyte development and competition for survival factors in the periphery. Improved knowledge of these developmental checkpoints and homeostatic mechanisms is critical for understanding human immunodeficiency, leukaemia/lymphoma and autoimmunity, which are conditions where checkpoints and homeostasis are likely to be deregulated.

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.

Masking of the CD3 gamma di-leucine-based motif by zeta is required for efficient T-cell receptor expression

The T-cell receptor (TCR) is a multimeric receptor composed of the Ti alpha beta heterodimer and the noncovalently associated CD3 gamma delta epsilon and zeta(2) chains. All of the TCR chains are required for efficient cell surface expression of the TCR. Previous studies on chimeric molecules containing the di-leucine-based endocytosis motif of the TCR subunit CD3 gamma have indicated that the zeta chain can mask this motif. In this study, we show that successive truncations of the cytoplasmic tail of zeta led to reduced surface expression levels of completely assembled TCR complexes. The reduced TCR expression levels were caused by an increase in the TCR endocytic rate constant in combination with an unaffected exocytic rate constant. Furthermore, the TCR degradation rate constant was increased in cells with truncated zeta. Introduction of a CD3 gamma chain with a disrupted di-leucine-based endocytosis motif partially restored TCR expression in cells with truncated zeta chains, indicating that the zeta chain masks the endocytosis motif in CD3 gamma and thereby stabilizes TCR cell surface expression.

Solution structure of the CD3epsilondelta ectodomain and comparison with CD3epsilongamma as a basis for modeling T cell receptor topology and signaling

Invariant CD3 subunit dimers (CD3epsilongamma, CD3epsilondelta, and CD3zetazeta) are the signaling components of the alphabeta T cell receptor (TCR). The recently solved structure of murine CD3epsilongamma revealed a unique side-to-side interface and central beta-sheets conjoined between the two C2-set Ig-like ectodomains, with the pairing of the parallel G strands implying a potential concerted piston-type movement for signal transduction. Although CD3gamma and CD3delta each dimerize with CD3epsilon, there are differential CD3 subunit requirements for receptor assembly and signaling among T lineage subpopulations, presumably mandated by structural differences. Here we present the solution structure of the heterodimeric CD3epsilondelta complex. Whereas the CD3epsilon subunit conformation is virtually identical to that in CD3epsilongamma, the CD3delta ectodomain adopts a C1-set Ig fold, with a narrower GFC front face beta-sheet that is more parallel to the ABED back face than those beta-sheets in CD3epsilon and CD3gamma. The dimer interface between CD3delta and CD3epsilon is highly conserved among species and of similar character to that in CD3epsilongamma. Glycosylation sites in CD3delta are arranged such that the glycans may point away from the membrane, consistent with a model of TCR assembly that allows the CD3delta chain to be in close contact with the TCR alpha-chain. This and many other structural and biological features provide a basis for modeling putative TCR/CD3 extracellular domain associations. The fact that the two clusters of transmembrane helices, namely, the three CD3epsilon-CD3gamma-TCRbeta segments and the five CD3epsilon-CD3delta-TCRalpha-CD3zeta-CD3zeta segments, are presumably centered beneath the G strand-paired CD3 heterodimers has important implications for TCR signaling.

Crystal structure of a human CD3-epsilon/delta dimer in complex with a UCHT1 single-chain antibody fragment

The alpha/beta T cell receptor complex transmits signals from MHC/peptide antigens through a set of constitutively associated signaling molecules, including CD3-epsilon/gamma and CD3-epsilon/delta. We report the crystal structure at 1.9-A resolution of a complex between a human CD3-epsilon/delta ectodomain heterodimer and a single-chain fragment of the UCHT1 antibody. CD3-epsilon/delta and CD3-epsilon/gamma share a conserved interface between the Ig-fold ectodomains, with parallel packing of the two G strands. CD3-delta has a more electronegative surface and a more compact Ig fold than CD3-gamma; thus, the two CD3 heterodimers have distinctly different molecular surfaces. The UCHT1 antibody binds near an acidic region of CD3-epsilon opposite the dimer interface, occluding this region from direct interaction with the TCR. This immunodominant epitope may be a uniquely accessible surface in the TCR/CD3 complex, because there is overlap between the binding site of the UCHT1 and OKT3 antibodies. Determination of the CD3-epsilon/delta structure completes the set of TCR/CD3 globular ectodomains and contributes information about exposed CD3 surfaces.

Molecular mechanisms for the assembly of the T cell receptor-CD3 complex

The T cell receptor (TCR)-CD3 complex represents on of the most intricate membrane receptor structures since it is built from six distinct chains. This complexity led to a number of different proposals for the arrangement of the receptor subunits, its stoichiometry and the mechanisms responsible for receptor triggering. Early work had demonstrated that basic and acidic transmembrane (TM) residues were involved in the assembly but the molecular arrangement could not be deduced due to the complexity of the receptor. Using a novel method for the isolation of intact radiolabeled protein complexes, we demonstrated that the complex assembled in the ER contains only a single TCRalphabeta heterodimer and one copy of each of the CD3deltaepsilon, CD3gammaepsilon and zeta-zeta signaling dimers. Surprisingly, assembly of each of the three signaling dimers with TCR was dependent on one of the three basic TCR TM residues as well as both acidic residues located in the TM domains of the interacting signaling dimer. Each assembly step thus results in the formation of a three-helix interface in the membrane that involves one basic and two acidic TM residues, and this arrangement effectively shields these ionizable residues at protein-protein interfaces from the lipid. Since proteins whose TM domains have exposed ionizable residues are not stably integrated into the lipid bilayer, assembly based on shielding of ionizable residues permits full equilibration of the receptor into the lipid bilayer and prevents degradation. Assembly, export of intact receptor complexes and degradation of unassembled components thus rely on the same organizing principle.

TCR comodulation of nonengaged TCR takes place by a protein kinase C and CD3 gamma di-leucine-based motif-dependent mechanism

One of the earliest events following TCR triggering is TCR down-regulation. However, the mechanisms behind TCR down-regulation are still not fully known. Some studies have suggested that only directly triggered TCR are internalized, whereas others studies have indicated that, in addition to triggered receptors, nonengaged TCR are also internalized (comodulated). In this study, we used transfected T cells expressing two different TCR to analyze whether comodulation took place. We show that TCR triggering by anti-TCR mAb and peptide-MHC complexes clearly induced internalization of nonengaged TCR. By using a panel of mAb against the Ti beta chain, we demonstrate that the comodulation kinetics depended on the affinity of the ligand. Thus, high-affinity mAb (K(D) = 2.3 nM) induced a rapid but reversible comodulation, whereas low-affinity mAb (K(D) = 6200 nM) induced a slower but more permanent type of comodulation. Like internalization of engaged TCR, comodulation was dependent on protein tyrosine kinase activity. Finally, we found that in contrast to internalization of engaged TCR, comodulation was highly dependent on protein kinase C activity and the CD3 gamma di-leucine-based motif. Based on these observations, a physiological role of comodulation is proposed and the plausibility of the TCR serial triggering model is discussed.

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.

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.

Structural and thermodynamic correlates of T cell signaling

The first crystal structures of intact T cell receptors (TCRs) bound to class I peptide-MHC (pMHCs) antigens were determined in 1996. Since then, further structures of class I TCR/pMHC complexes have explored the degree of structural variability in the TCR-pMHC system and the structural basis for positive and negative selection. The recent determination of class II and allogeneic class I TCR/pMHC structures, as well as those of accessory molecules (e.g., CD3), has pushed our knowledge of TCR/pMHC interactions into new realms, shedding light on clinical pathologies, such as graft rejection and graft-versus-host disease. Furthermore, the determination of coreceptor structures lays the foundation for a more comprehensive structural description of the supramolecular TCR signaling events and those assemblies that arise in the immunological synapse. While these telling photodocumentaries of the TCR/pMHC interaction are composed mainly from static crystal structures, a full description of the biological snapshots in T cell signaling requires additional analytical methods that record the dynamics of the process. To this end, surface plasmon resonance (SPR), isothermal titration calorimetry (ITC), and ultracentrifugation (UC) have furnished both affinities and kinetics of the TCR/pMHC association. In the past year, structural, biochemical, and molecular biological data describing TCR/pMHC interactions have sublimely coalesced into a burgeoning well of understanding that promises to deliver further insights into T cell recognition. The coming years will, through a more intimate union of structural and kinetic data, allow many pressing questions to be addressed, such as how TCR/pMHC ligation is affected by coreceptor binding and what is the mechanism of TCR signaling in both early and late stages of T cell engagement with antigen-presenting cells.

Defective expression and tyrosine phosphorylation of the T cell receptor zeta chain in peripheral blood T cells from systemic lupus erythematosus patients

We have reported that tyrosine phosphorylation and expression of the T cell receptor zeta chain (TCR zeta) was decreased in two systemic lupus erythematosus (SLE) patients with an abnormal TCR zeta lacking exon-7. To examine further the TCR zeta defect and any possible relationship with specific clinical features, we studied the expression of TCR zeta in peripheral blood T cells from 44 patients with SLE, 53 with other rheumatic diseases (30 rheumatoid arthritis (RA), 11 systemic sclerosis (SSc) and 12 primary Sjögren's syndrome(SjS)) and 39 healthy individuals. Flow cytometric analysis demonstrated a significant decrease in the expression of TCR zeta in SLE (P < 0.001), but not in the other rheumatic diseases. Immunoprecipitation experiments confirmed that the expression of TCR zeta in SLE T cells was decreased dramatically (normal: 111.4 +/- 22.6%, SLE: 51.6 +/- 37.4%, P < 0.0001). The decrease in TCR zeta did not correlate with disease activity, or with the dose of prednisolone (PSL). There were, however, three SLE patients in whom the level of TCR zeta expression normalized after treatment, suggesting that mechanisms responsible for the TCR zeta defect appear to be heterogeneous. These results confirm the defective expression and altered tyrosine phosphorylation of TCR zeta in a large proportion of SLE patients, suggesting that it may play an important role in T cell dysfunction in SLE.

Involvement of the TCR Cbeta FG loop in thymic selection and T cell function

The asymmetric disposition of T cell receptor (TCR) Cbeta and Calpha ectodomains creates a cavity with a side-wall formed by the rigid Cbeta FG loop. To investigate the significance of this conserved structure, we generated loop deletion (betaDeltaFG) and betawt transgenic (tg) mice using the TCR beta subunit of the N15 CTL. N15betawt and N15betaDeltaFG H-2(b) animals have comparable numbers of thymocytes in S phase and manifest developmental progression through the CD4(-)CD8(-) double-negative (DN) compartment. N15betaDeltaFG facilitates transition from DN to CD4(+)8(+) double-positive (DP) thymocytes in recombinase activating gene (RAG)-2(-/-) mice, showing that pre-TCR function remains. N15betaDeltaFG animals possess approximately twofold more CD8(+) single-positive (SP) thymocytes and lymph node T cells, consistent with enhanced positive selection. As an altered Valpha repertoire observed in N15betaDeltaFG mice may confound the deletion's effect, we crossed N15alphabeta TCR tg RAG-2(-/-) with N15betaDeltaFG tg RAG-2(-/-) H-2(b) mice to generate N15alphabeta RAG-2(-/-) and N15alphabeta.betaDeltaFG RAG-2(-/-) littermates. N15alphabeta.betaDeltaFG RAG-2(-/-) mice show an 8-10-fold increase in DP thymocytes due to reduced negative selection, as evidenced by diminished constitutive and cognate peptide-induced apoptosis. Compared with N15alphabeta, N15alphabeta.betaDeltaFG T cells respond poorly to cognate antigens and weak agonists. Thus, the Cbeta FG loop facilitates negative selection of thymocytes and activation of T cells.

Structural basis of T cell recognition of peptides bound to MHC molecules

Helper T lymphocytes play a critical role in immune system activation following recognition of MHC class II-bound peptide ligands (pMHCII). These CD4 T cells stimulate B cell antibody production and cytolytic T cell generation. Until recently, the structural basis of coordinate T cell receptor (TCR) and CD4 co-receptor interaction with a given pMHCII was unknown. Here we review current structural data on specific pMHCII recognition by T cells and compare TCR and co-receptor docking to pMHCI versus pMHCII ligands. The implications of these findings for thymic selection, helper versus cytolytic T cell recognition and alloreactivity are discussed.

Expression of the signal transduction molecule zeta in peripheral and tumour-associated lymphocytes in Hodgkin's disease in relation to the Epstein-Barr virus status of the tumour cells

We investigated whether the described immune evasion of Epstein-Barr virus (EBV)-infected malignant Hodgkin and Reed-Sternberg (HRS) cells in Hodgkin's disease (HD) is paralleled by a disturbed expression of the signal transduction molecule zeta associated with CD3 and CD16 in tumour-associated T lymphocytes (TAL). Flow cytometric analysis revealed a significantly lower zeta expression in CD3+/4+, CD3+/8+ and CD16+ patient peripheral blood lymphocytes (PBL; n = 10) compared with normal donor PBLs (n = 11). When patient PBLs were compared with the corresponding TAL, the latter showed a significantly higher (CD3+/4+) or equal (CD3+/8+) zeta expression. The EBV status of the tumours did not correlate with zeta expression in the TAL. Immunohistochemical staining revealed zeta-positive lymphocytes among the adjacent bystander cells of the HRS cells in all analysed tumours (n = 8), irrespective of tumour EBV status. In conclusion, these results do not support downregulation of zeta in TAL as a critical mechanism contributing specifically to the immune escape of EBV+ HRS cells.

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.

Early TCR αβ Expression Promotes Maturation of T Cells Expressing FcεRIγ Containing TCR/CD3 Complexes

In a previous study we presented data indicating that the expanded population of CD4(-)CD8(-) (DN) alphabeta T cells in TCRalpha-chain-transgenic mice was partially if not entirely derived from gammadelta T cell lineage cells. The development of both gammadelta T cells and DN alphabeta T cells is poorly understood; therefore, we thought it would be important to identify the immediate precursors of the transgene-induced DN alphabeta T cells. We have in this report studied the early T cell development in these mice and we show that the transgenic TCRalpha-chain is expressed by precursor thymocytes already at the CD3(-)CD4(-)CD8(-) (triple negative, TN) CD44(+)CD25(-) stage of development. Both by using purified precursor populations in reconstitution experiments and by analyzing fetal thymocyte development, we demonstrated that early TN precursors expressing endogenous TCRbeta-chains matured into DN alphabeta T cells at several stages of development. The genes encoding the gamma-chain of the high affinity receptor for IgE (FcepsilonRIgamma) and the CD3zeta protein were found to be reciprocally expressed in TN thymocytes such that during development the FcepsilonRIgamma expression decreased whereas CD3zeta expression increased. Furthermore, in a fraction of the transgene-induced DN alphabeta T cells the FcepsilonRIgamma protein colocalized with the TCR/CD3 complex. These data suggest that similarly to gammadelta T cells and NKT cells, precursors expressing the TCR early in the common alphabetagammadelta developmental pathway may use the FcepsilonRIgamma protein as a signaling component of the TCR/CD3 complex.

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.

Human dendritic cells shed a functional, soluble form of the mannose receptor

Human monocyte-derived dendritic cells (DC) use mannose receptor (MR)-mediated endocytosis for efficient antigen capture and targeting to the endosomal/lysosomal compartment. Active biosynthesis of the MR takes place in such cells. We now report that a substantial percentage (up to 20%) of these newly synthesized MR are secreted into the culture medium. The secretion of the soluble MR (sMR) was found to be proportional to the rate of synthesis. The addition of the inflammatory mediator lipopolysaccharide (LPS) to DC, known to induce maturation, strongly reduced MR synthesis, expression and shedding of the MR. The sMR is approximately 10 kDa smaller than the membrane-bound form, but contains an intact N-terminus, indicating the lack of the cytoplasmic and transmembrane region. The sMR appeared to be directly generated from the cell-bound form, indicative of proteolytic cleavage. Importantly, the sMR has maintained its mannose-binding properties since it was capable of binding a mannosylated ligand. The high amount of sMR released by DC and its ability to bind mannosylated ligand might indicate that this molecule plays a role in the transport of mannosylated proteins from the site of inflammation to other parts of the body. Whether that contributes to the generation of immune responses remains to be determined.

TCRzeta is transported to and retained in the Golgi apparatus independently of other TCR chains: implications for TCR assembly

It is generally assumed that TCR assembly occurs in the endoplasmic reticulum (ER), and ER retention/degradation signals have been identified in several of the TCR chains. These signals are probably responsible for retention of incompletely assembled TCR complexes and free TCR chains in the ER. This study focused on the intracellular localization and transport of partially assembled TCR complexes as determined by confocal microscopy analyses. We found that none of the TCR chains except for TCRzeta were allowed to exit the ER in T cell variants in which the hexameric CD3gammaepsilonTi alphabetaCD3 deltaepsilon complex was not formed. Interestingly, TCRzeta was exported from the ER independently of other TCR chains and was predominantly located in a compartment identified as the Golgi apparatus. Furthermore, in the TCRzeta-negative cell line MA5.8, the hexameric CD3gammaepsilonTi alphabetaCD3 deltaepsilon complex was allowed to exit the ER and was also predominantly located in the Golgi apparatus. However, neither hexameric TCR complexes nor TCRzeta chains were efficiently expressed at the cell surface without the other. The observations that TCRzeta and hexameric TCR complexes are transported from the ER to the Golgi apparatus independently of each other and that these partial TCR complexes are unable to be efficiently expressed at the cell surface suggest that final TCR assembly occurs in the Golgi apparatus.

T Lymphocyte Development in the Absence of CD3ε or CD3γδεζ

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

Role of transmembrane domains in the functions of B- and T-cell receptors

The antigen receptors on the surface of B- and T-lymphocytes are complexes of several integral membrane proteins, essential for their proper expression and function. Recent studies demonstrated that transmembrane (TM) domains of the components of these receptors play a critical role in their association and function. It was specifically demonstrated that in many cases point mutations in the TM domains can partially or completely disrupt the receptor surface expression and function. Here we review studies of the TM domains of B- and T-cell receptors. Furthermore, we use a novel method, PHDtopology, to provide estimates of the exact locations and lengths of the TM domains of the subunit components of these receptors. Most previous studies used single residue hydrophobicity as a criterion for determining the position and length of the TM domains. In contrast, PHDtopology utilizes a system of neural networks and the evolutionary information contained in multiple alignments of related sequences to predict the location, length, and orientation of transmembrane helices. Present results significantly differ from most published estimates of the TM domains of the B- and T-cell receptor components, primarily in the length of the TM domains. These results may lead to modification of putative TM motifs and re-interpretation of the results of studies using mutated TM domains. The availability of PHDtopology on the Internet would make it a valuable tool in the future studies of the TM domains of integral membrane proteins.

Prolactin receptor signaling: shared components with the T-cell antigen receptor in Nb2 lymphoma cells

Previously, we reported that activation of the human prolactin receptor (PRLR) produced a protein phosphorylation pattern strikingly similar to that provoked by Concanavalin A (Con A), an activator of the T-cell antigen receptor (TCR). These results suggested that certain signaling components of the TCR may be shared by the activated PRLR. Additional studies here assessed the levels of TCR expression following PRLR stimulation and the effect of TCR activation on PRL-stimulated proliferation in lactogen-dependent pre-T Nb2-11 lymphoma cells. The results indicated that the TCR was expressed on the surface of approx 4% of exponentially proliferating and prolactin- (PRL) treated cells. In contrast, approx 45% of quiescent cells, cultured in the absence of PRL for 24 h, expressed the TCR at the cell surface, suggesting that lactogen withdrawal may up-regulate TCR cell-surface expression. Moreover, TCR activation with anti-CD3 antibodies attenuated PRL-stimulated Nb2-11 cell proliferation in a concentration-dependent manner. In other experiments, immunoprecipitation and immunoblotting of Nb2-11 lysates revealed that activation of the PRLR resulted in rapid tyrosyl phosphorylation of ZAP-70, a critical TCR-associated tyrosine kinase. In addition, ZAP-70 was found to associate transiently with the putative guanine nucleotide exchange factor and substrate, Vav, in PRL-treated cells. ZAP-70 was also found to associate constitutively with the PRLR; PRL stimulation provoked the transient recruitment of Vav to the complex. These observations suggest that PRL signaling reflects the transient formation of a PRLR-ZAP-70-Vav complex and its immunomodulatory actions involve diverse interactions that affect TCR expression and signaling mechanisms.

The phosphorylation state of CD3gamma influences T cell responsiveness and controls T cell receptor cycling

The T cell receptor (TCR) is internalized following activation of protein kinase C (PKC) via a leucine (Leu)-based motif in CD3gamma. Some studies have indicated that the TCR is recycled back to the cell surface following PKC-mediated internalization. The functional state of recycled TCR and the mechanisms involved in the sorting events following PKC-induced internalization are not known. In this study, we demonstrated that following PKC-induced internalization, the TCR is recycled back to the cell surface in a functional state. TCR recycling was dependent on dephosphorylation of CD3gamma, probably mediated by the serine/threonine protein phosphatase-2A, but independent on microtubules or actin polymerization. Furthermore, in contrast to ligand-mediated TCR sorting, recycling of the TCR was independent of the tyrosine phosphatase CD45 and the Src tyrosine kinases p56(Lck) and p59(Fyn). Studies of mutated TCR and chimeric CD4-CD3gamma molecules demonstrated that CD3gamma did not contain a recycling signal in itself. In contrast, the only sorting information in CD3gamma was the Leu-based motif that mediated lysosomal sorting of chimeric CD4-CD3gamma molecules. Finally, we found a correlation between the phosphorylation state of CD3gamma and T cell responsiveness. Based on these observations a physiological role of CD3gamma and TCR cycling is proposed.

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.