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 TCR C beta FG loop regulates alpha beta T cell development

The TCRbeta chain constant domain contains an unusually elongated, solvent-exposed FG loop. This structural element forms one component of an alphabeta TCR cavity against which CD3epsilongamma may abut to facilitate Ag-specific signaling. Consistent with this notion, in the present study we show that N15alphabeta TCR transfectants expressing a FG loop-deleted chain (betaDeltaFG) stimulate less tyrosine protein phosphorylation than those bearing a wild-type beta-chain (betawt) upon TCR cross-linking. Furthermore, coimmunoprecipitation studies suggest a weakened association between the CD3epsilongamma heterodimer and the beta-chain in TCR complexes containing the betaDeltaFG variant. To further investigate the biologic role of the Cbeta FG loop in development, we competitively reconstituted the thymus of Ly5 congenic or RAG-2-/- mice using bone marrow cells from betawt or betaDeltaFG transgenic C57BL/6 (B6) mice. Both betawt and betaDeltaFG precursor cells generate thymocytes representative of all maturational stages. However, betaDeltaFG-expressing thymocytes dominate during subsequent development, resulting in an excess of betaDeltaFG-expressing peripheral T cells with reduced proliferative and cytokine production abilities upon TCR stimulation. Collectively, our results show that the unique Cbeta FG loop appendage primarily controls alphabeta T cell development through selection processes.

A conformation- and avidity-based proofreading mechanism for the TCR–CD3 complex

During antigen recognition, T cells show high sensitivity and specificity, and a wide dynamic range. Paradoxically, these characteristics are based on low-affinity receptor-ligand interactions [between the T-cell antigen receptor (TCR-CD3) complex and the antigen peptide bound to MHC]. Recent evidence indicates that the TCR-CD3 is expressed as multivalent complexes in the membrane of non-stimulated T cells and that conformational changes in the TCR-CD3 can be induced by strong but not weak agonists. Here, we propose a thermodynamic model whereby the specificity of the TCR-CD3-pMHC interaction is explained by its multivalent nature. We also propose that the free energy barriers involved in the change in conformation of the receptor impose a response threshold and determine the kinetic properties of recognition. Finally, we suggest that multivalent TCR-CD3s can amplify signals by spreading them from pMHC-engaged TCR-CD3s to unengaged complexes as a consequence of the cooperativity in the system.

Deconstructing the form and function of the TCR/CD3 complex

When T cells encounter antigens via the T cell antigen receptor (TCR), information about the quantity and quality of antigen engagement is relayed to the intracellular signal transduction machinery. This process is poorly understood. The TCR itself lacks a significant intracellular domain. Instead, it is associated with CD3 molecules that contain intracellular signaling domains that couple the TCR/CD3 complex to the downstream signaling machinery. The earliest events in TCR signaling must involve the transfer of information from the antigen binding TCR subunit to the CD3 signaling subunits of the TCR/CD3 complex. Elucidating the structural organization of the TCR with the associated CD3 signaling molecules is necessary for understanding the mechanism by which TCR engagement is coupled to activation. Here, we review the current state of our understanding of the structure and organization of the TCR/CD3 complex.

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.

Structural and Mutational Analyses of a CD8αβ Heterodimer and Comparison with the CD8αα Homodimer

The crystal structure of a recombinant mouse single chain CD8alphabeta ectodomains at 2.4 A resolution reveals paired immunoglobulin variable region-like domains with a striking resemblance to CD8alphaalpha in size, shape, and surface electrostatic potential of complementarity-determining regions (CDR), despite <20% sequence identity between the CD8alpha and CD8beta subunits. Unlike the CD8alpha subunit(s) in the heterodimer or homodimer, the CDR1 loop of CD8beta tilts away from its corresponding CDR2 and CDR3 loops. Consistent with this observation, independent mutational studies reveal that alanine substitutions of residues in the CDR1 loop of CD8beta have no effect on CD8alphabeta coreceptor function, whereas mutations in CD8beta CDR2 and CDR3 loops abolish CD8alphabeta coreceptor activity. The implications of these findings and additional CD8alpha mutational studies for CD8alphabeta- versus CD8alphaalpha-MHCI binding are discussed.

The initiation of antigen-induced B cell antigen receptor signaling viewed in living cells by fluorescence resonance energy transfer

Binding of antigen to the B cell antigen receptor (BCR) triggers signaling that ultimately leads to B cell activation. Using quantitative fluorescence resonance energy transfer imaging, we provide evidence here that the BCR is a monomer on the surface of resting cells. Binding of multivalent antigen clustered the BCR, resulting in the simultaneous phosphorylation of and a conformational change in the BCR cytoplasmic domains from a closed to an open form. Notably, the open conformation required immunoreceptor tyrosine-activation motif and continuous Src family kinase activity but not binding of the kinase Syk. Thus, the initiation of BCR signaling is a very dynamic process accompanied by reversible conformational changes induced by Src family kinase activity.

The T cell receptor: critical role of the membrane environment in receptor assembly and function

Recent studies have demonstrated that cell membranes provide a unique environment for protein-protein and protein-lipid interactions that are critical for the assembly and function of the T cell receptor (TCR)-CD3 complex. Highly specific polar interactions among transmembrane (TM) domains that are uniquely favorable in the lipid environment organize the association of the three signaling dimers with the TCR. Each of these three assembly steps depends on the formation of a three-helix interface between one basic and two acidic residues in the membrane environment. The same polar TM residues that drive assembly also play a central role in quality control and export by directing the retention and degradation of free subunits and partial complexes, while membrane proximal cytoplasmic signals control recycling and degradation of surface receptors. Recent studies also suggest that interactions between the membrane and the cytoplasmic domains of CD3 proteins may be important for receptor triggering.

Twisting tails exposed: the evidence for TCR conformational change

The mechanism by which the ligand occupancy state of the T cell receptor complex is converted into intracellular signaling information has been a controversial topic. Although the majority of structural studies argue against a conformational change, recent studies support the possibility for such a change within the CD3 components of the TCR complex. In this commentary, the evidence for TCR conformational change is reviewed and potential mechanisms for its initiation are explored.

How T cells 'see' antigen

T lymphocytes bearing alphabeta T cell receptors are pivotal in the immune response of most vertebrates. For example, helper T cells orchestrate antibody production by B cells as well as stimulating other cells, whereas cytotoxic T cells kill virally infected or abnormal cells. Regulatory T cells act to dampen responsiveness, and natural killer-like T cells monitor lipid metabolism. The specificity of these cells is governed by the alphabeta T cell receptors - antibody-like heterodimeric receptors that detect antigenic fragments (peptides) or lipids bound to histocompatibility molecules. Intriguing clues as to how these peculiar ligands are recognized have gradually emerged over the years and tell a remarkable story of biochemical and cellular novelty. Here we summarize some of the more recent work on alphabeta T cell receptor recognition and discuss the implications for activation.

Agonist/endogenous peptide-MHC heterodimers drive T cell activation and sensitivity

Alphabeta T lymphocytes are able to detect even a single peptide-major histocompatibility complex (MHC) on the surface of an antigen-presenting cell. This is despite clear evidence, at least with CD4+ T cells, that monomeric ligands are not stimulatory. In an effort to understand how this remarkable sensitivity is achieved, we constructed soluble peptide-MHC heterodimers in which one peptide is an agonist and the other is one of the large number of endogenous peptide-MHCs displayed by presenting cells. We found that some specific combinations of these heterodimers can stimulate specific T cells in a CD4-dependent manner. This activation is severely impaired if the CD4-binding site on the agonist ligand is ablated, but the same mutation on an endogenous ligand has no effect. These data correlate well with analyses of lipid bilayers and cells presenting these ligands, and indicate that the basic unit of helper T cell activation is a heterodimer of agonist peptide- and endogenous peptide-MHC complexes, stabilized by CD4.

Severe combined immunodeficiency caused by deficiency in either the delta or the epsilon subunit of CD3

We investigated the molecular mechanism underlying a severe combined immunodeficiency characterized by the selective and complete absence of T cells. The condition was found in 5 patients and 2 fetuses from 3 consanguineous families. Linkage analysis performed on the 3 families revealed that the patients were carrying homozygous haplotypes within the 11q23 region, in which the genes encoding the gamma, delta, and epsilon subunits of CD3 are located. Patients and affected fetuses from 2 families were homozygous for a mutation in the CD3D gene, and patients from the third family were homozygous for a mutation in the CD3E gene. The thymus from a CD3delta-deficient fetus was analyzed and revealed that T cell differentiation was blocked at entry into the double positive (CD4+CD8+) stage with the accumulation of intermediate CD4-single positive cells. This indicates that CD3delta plays an essential role in promoting progression of early thymocytes toward double-positive stage. Altogether, these findings extend the known molecular mechanisms underlying severe combined immunodeficiency to a new deficiency, i.e., CD3epsilon deficiency, and emphasize the essential roles played by the CD3epsilon and CD3delta subunits in human thymocyte development, since these subunits associate with both the pre-TCR and the TCR.

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.

Stoichiometry of the T-cell receptor-CD3 complex and key intermediates assembled in the endoplasmic reticulum

The T-cell receptor (TCR)-CD3 complex is critical for T-cell development and function, and represents one of the most complex transmembrane receptors. Models of different stoichiometry and valency have been proposed based on cellular experiments and these have important implications for the mechanisms of receptor triggering. Since determination of receptor stoichiometry in T-cells is not possible due to the presence of previously synthesized, unlabeled receptor components with different half-lives, we examined the stoichiometry of the receptor assembled in endoplasmic reticulum (ER) microsomes of B-cell origin. The stoichiometric relationship among all subunits was directly determined using intact radiolabeled TCR-CD3 complexes that were isolated with a sequential, non-denaturing immunoprecipitation method, and identical results were obtained with two detergents belonging to different structural classes. The results firmly establish that the alphabeta TCR-CD3 complex assembled in the ER is monovalent and composed of one copy of the TCRalphabeta, CD3deltaepsilon, CD3gammaepsilon and zeta-zeta dimers.

Crystal structure of the human T cell receptor CD3 epsilon gamma heterodimer complexed to the therapeutic mAb OKT3

The CD3 epsilon gamma heterodimer is essential for expression and function of the T cell receptor. The crystal structure of the human CD3 epsilon gamma heterodimer is described to 2.1-A resolution complexed with OKT3, a therapeutic mAb that not only activates and tolerizes mature T cells but also induces regulatory T cells. The mode of CD3 epsilon gamma dimerization provides a general structural basis for CD3 assembly and maps candidate T cell antigen receptor docking sites, including a duplicated linear region rich in acidic residues that is unique to human CD3 epsilon. OKT3 binds to an atypically small area of CD3 epsilon and has a low affinity for the isolated CD3 epsilon gamma heterodimer. The structure of the OKT3/CD3 epsilon gamma complex has implications for T cell signaling and therapeutic design.

Evidence that structural rearrangements and/or flexibility during TCR binding can contribute to T cell activation

While in many cases the half-life of T cell receptor (TCR) binding to a particular ligand is a good predictor of activation potential, numerous exceptions suggest that other physical parameter(s) must also play a role. Accordingly, we analyzed the thermodynamics of TCR binding to a series of peptide-MHC ligands, three of which are more stimulatory than their stability of binding would predict. Strikingly, we find that during TCR binding these outliers show anomalously large changes in heat capacity, an indicator of conformational change or flexibility in a binding interaction. By combining the values for heat capacity (DeltaCp) and the half-life of TCR binding (t(1/2)), we find that we can accurately predict the degree of T cell stimulation. Structural analysis shows significant changes in the central TCR contact residue of the peptide-MHC, indicating that structural rearrangements within the TCR-peptide-MHC interface can contribute to T cell activation.

Molecular interactions mediating T cell antigen recognition

Over the past decade, key protein interactions contributing to T cell antigen recognition have been characterized in molecular detail. These have included interactions involving the T cell antigen receptor (TCR) itself, its coreceptors CD4 and CD8, the accessory molecule CD2, and the costimulatory receptors CD28 and CTLA-4. A clear view is emerging of how these molecules interact with their ligands at the cell-cell interface. Structural and binding studies have confirmed that the proteins span small but comparable distances and that, overall, they interact very weakly. However, there have been important surprises as well: that TCR interactions with peptide-MHC are topologically constrained and characterized by considerable conformational flexibility at the binding interface; that coreceptors engage peptide-MHC with extraordinarily fast kinetics and at angles apparently precluding direct interactions with the TCR bound to the same peptide-MHC; that the structural mechanisms allowing recognition by costimulatory and accessory molecules to be weak and yet specific are very heterogeneous; and that because of differences in both binding affinity and stoichiometry, there is enormous variation in the stability of the various costimulatory receptor/ligand complexes. These studies provide the necessary framework for exploring how these molecular interactions initiate T cell activation.

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

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

A mouse with a loss-of-function mutation in the c-Cbl TKB domain shows perturbed thymocyte signaling without enhancing the activity of the ZAP-70 tyrosine kinase

The unique tyrosine kinase binding (TKB) domain of Cbl targets phosphorylated tyrosines on activated protein tyrosine kinases (PTKs); this targeting is considered essential for Cbl proteins to negatively regulate PTKs. Here, a loss-of-function mutation (G304E) in the c-Cbl TKB domain, first identified in Caenorhabditis elegans, was introduced into a mouse and its effects in thymocytes and T cells were studied. In marked contrast to the c-Cbl knockout mouse, we found no evidence of enhanced activity of the ZAP-70 PTK in thymocytes from the TKB domain mutant mouse. This finding contradicts the accepted mechanism of c-Cbl-mediated negative regulation, which requires TKB domain targeting of phosphotyrosine 292 in ZAP-70. However, the TKB domain mutant mouse does show aspects of enhanced signaling that parallel those of the c-Cbl knockout mouse, but these involve the constitutive activation of Rac and not enhanced PTK activity. Furthermore, the enhanced signaling in CD4(+)CD8(+) double positive thymocytes appears to be compensated by the selective down-regulation of CD3 on mature thymocytes and peripheral T cells from both strains of mutant c-Cbl mice.

Initiation of TCR signaling: regulation within CD3 dimers

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

An evolutionary and structural perspective on T cell antigen receptor function

After a brief overview of the themes and variations that occur in the family of receptors containing immunoreceptor tyrosine-based activation motifs (ITAMs), and of recent structural data on the ligand-binding subunits of these receptors, we use these data to revisit how information on the state and quality of occupancy of the binding site of the T cell antigen receptor (TCR) is conveyed to the proximal components of the TCR transduction cassette.

CD3 delta establishes a functional link between the T cell receptor and CD8

T cells expressing T cell receptor (TCR) complexes that lack CD3 delta, either due to deletion of the CD3 delta gene, or by replacement of the connecting peptide of the TCR alpha chain, exhibit severely impaired positive selection and TCR-mediated activation of CD8 single-positive T cells. Because the same defects have been observed in mice expressing no CD8 beta or tailless CD8 beta, we examined whether CD3 delta serves to couple TCR.CD3 with CD8. To this end we used T cell hybridomas and transgenic mice expressing the T1 TCR, which recognizes a photoreactive derivative of the PbCS 252-260 peptide in the context of H-2K(d). We report that, in thymocytes and hybridomas expressing the T1 TCR.CD3 complex, CD8 alpha beta associates with the TCR. This association was not observed on T1 hybridomas expressing only CD8 alpha alpha or a CD3 delta(-) variant of the T1 TCR. CD3 delta was selectively co-immunoprecipitated with anti-CD8 antibodies, indicating an avid association of CD8 with CD3 delta. Because CD8 alpha beta is a raft constituent, due to this association a fraction of TCR.CD3 is raft-associated. Cross-linking of these TCR-CD8 adducts results in extensive TCR aggregate formation and intracellular calcium mobilization. Thus, CD3 delta couples TCR.CD3 with raft-associated CD8, which is required for effective activation and positive selection of CD8(+) T cells.

A structural basis for the selection of dominant alphabeta T cell receptors in antiviral immunity

We have examined the basis for immunodominant or "public" TCR usage in an antiviral CTL response. Residues encoded by each of the highly selected genetic elements of an immunodominant clonotype recognizing Epstein-Barr virus were critical to the antigen specificity of the receptor. Upon recognizing antigen, the immunodominant TCR undergoes extensive conformational changes in the complementarity determining regions (CDRs), including the disruption of the canonical structures of the germline-encoded CDR1alpha and CDR2alpha loops to produce an enhanced fit with the HLA-peptide complex. TCR ligation induces conformational changes in the TCRalpha constant domain thought to form part of the docking site for CD3epsilon. These findings indicate that TCR immunodominance is associated with structural properties conferring receptor specificity and suggest a novel structural link between TCR ligation and intracellular signaling.

Not just any T cell receptor will do

Although our structural understanding of T cell recognition has rapidly evolved due to recent crystallographic results, the reality is that detailed answers to many of the most fundamental questions still remain elusive. In this issue, high-resolution insight into the phenomenon of TCR chain bias takes down another brick from the wall.

The organizing principle in the formation of the T cell receptor-CD3 complex

The T cell receptor (TCR) serves a critical function in the immune system and represents one of the most complex receptor structures. A striking feature is the presence of nine highly conserved, potentially charged residues in the transmembrane helices. Previous models have attempted to explain assembly based on pairwise interactions of these residues. Using a novel method for the isolation of intact radiolabeled protein complexes, we demonstrate that one basic and two acidic transmembrane residues are required for the assembly of each of the three signaling dimers with the TCR. This remarkable three-helix arrangement applies to all three assembly steps and represents the organizing principle for the formation of this intricate receptor structure.

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.

A new trigger for T cells

Understanding the early events in T cell activation and signaling is an active area of research. A recent study has described a new trigger for T cell activation, involving a TCR-ligand-induced conformational change in CD3epsilon that permits binding of the adaptor protein Nck.

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.

Recruitment of Nck by CD3 epsilon reveals a ligand-induced conformational change essential for T cell receptor signaling and synapse formation

How membrane receptors initiate signal transduction upon ligand binding is a matter of intense scrutiny. The T cell receptor complex (TCR-CD3) is composed of TCR alpha/beta ligand binding subunits bound to the CD3 subunits responsible for signal transduction. Although it has long been speculated that TCR-CD3 may undergo a conformational change, confirmation is still lacking. We present strong evidence that ligand engagement of TCR-CD3 induces a conformational change that exposes a proline-rich sequence in CD3 epsilon and results in recruitment of the adaptor protein Nck. This occurs earlier than and independently of tyrosine kinase activation. Finally, by interfering with Nck-CD3 epsilon association in vivo, we demonstrate that TCR-CD3 recruitment of Nck is critical for maturation of the immune synapse and for T cell activation.

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.

The T-cell receptor signalosome: a dynamic structure with expanding complexity

Signal transduction in T cells is a dynamic process involving a large number of membrane and cytosolic proteins. The TCR macromolecular complex (signalosome) is initiated by receptor occupancy and becomes more elaborate over time. This review describes how 'vertical' displacement mechanisms and lateral coalescence of lipid-raft-associated scaffold proteins combine to form distinct signalosomes, which control signal specificity.

Structure of gammadelta T cell receptors and their recognition of non-peptide antigens

The gammadelta T cell receptors (TCRs) and alphabeta TCRs are similar in both sequence and structure; however, gammadelta+ and alphabeta+ T cells are not merely similar lymphocytes with subtly different receptors. These cell types differ in several ways, including the types of antigens recognized, the mechanism of antigen presentation and recognition and the mechanism and kinetics of downstream signaling events. gammadelta TCRs can directly recognize antigens in the form of intact proteins or non-peptidic compounds, unlike alphabeta TCRs which recognize peptide antigens bound to major histocompatibility complex molecules (MHC). One of the major classes of human gammadelta+ T cells expresses Vgamma9Vdelta2 TCRs which recognize pyrophosphomonoester, alkylamine and aminobisphosphonate antigens. This review focuses on the recently determined structure of a Vgamma9Vdelta2 TCR, with emphasis on antigen recognition and receptor signaling.

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.

Structural insights into the interactions between human IgE and its high affinity receptor FcepsilonRI

The interaction of IgE antibodies with the high affinity IgE receptor, FcepsilonRI, is a key step in the initiation of anti-parasitic immunity and allergic reactions. Recent structural studies of the receptor, the IgE-Fc and the IgE-Fc:FcepsilonRI complex have revealed how these two proteins interact to prime mast cell responses to antigen. The structures have revealed a novel arrangement for the FcepsilonRI ectodomains that is also observed in homologous members of this antibody receptor family. The crystal structure of the IgE-Fc:FcepsilonRI complex clarified how a 1:1 complex between the antibody and receptor is formed, with the receptor binding each chain of the antibody Fc dimer. The IgE-Fc structure in the absence of the receptor revealed the potential for large conformational rearrangements within the IgE that may affect receptor binding. These studies provide the basis for further investigation of the specificity of antibody:receptor binding and for the development of new treatments for allergic hypersensitivities.

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

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

The specificity of TCR/pMHC interaction

Crystal structures of 11 complexes of TCRs with peptide/MHC (pMHC), that represent 6 independent TCRs, constitute the current structural database for deriving general insights into how alphabeta TCRs recognise peptide-bound MHC class I or class II. The TCRs adopt a roughly diagonal orientation on top of the pMHCs, but the identification of a set of conserved interactions that dictate this orientation is not apparent. Furthermore, the specific interaction of each TCR with its cognate pMHC partner is quite variable and also involves bound water molecules at the TCR/pMHC interface. In two of the systems, the structural basis for binding of altered peptide ligands has illustrated that the only significant conformational changes occur in the TCR/pMHC interface, but their small magnitude is inconsistent with the enormous variation in signalling outcomes. The TCRs adjust to different agonist, partial agonist and antagonist peptides by subtle conformational changes in their complementarity-determining regions, as previously observed in induced-fit mechanisms of antibody/antigen recognition. Alloreactive-complex structures determined or modelled so far indicate increased interactions of the TCR beta-chain with the pMHC compared with their syngeneic counterparts.