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

Identifying protein-protein interaction sites using covering algorithm

Identification of protein-protein interface residues is crucial for structural biology. This paper proposes a covering algorithm for predicting protein-protein interface residues with features including protein sequence profile and residue accessible area. This method adequately utilizes the characters of a covering algorithm which have simple, lower complexity and high accuracy for high dimension data. The covering algorithm can achieve a comparable performance (69.62%, Complete dataset; 60.86%, Trim dataset with overall accuracy) to a support vector machine and maximum entropy on our dataset, a correlation coefficient (CC) of 0.2893, 58.83% specificity, 56.12% sensitivity on the Complete dataset and 0.2144 (CC), 53.34% (specificity), 65.59% (sensitivity) on the Trim dataset in identifying interface residues by 5-fold cross-validation on 61 protein chains. This result indicates that the covering algorithm is a powerful and robust protein-protein interaction site prediction method that can guide biologists to make specific experiments on proteins. Examination of the predictions in the context of the 3-dimensional structures of proteins demonstrates the effectiveness of this method.

TCR-MHC docking orientation: natural selection, or thymic selection?

T cell receptors (TCR) dock on their peptide-major histocompatibility complex (pMHC) targets in a conserved orientation. Since amino acid sidechains are the foundation of specific protein-protein interactions, a simple explanation for the conserved docking orientation is that key amino acids encoded by the TCR and MHC genes have been selected and maintained through evolution in order to preserve TCR/pMHC binding. Expectations that follow from the hypothesis that TCR and MHC evolved to interact are discussed in light of the data that both support and refute them. Finally, an alternative and equally simple explanation for the driving force behind the conserved docking orientation is described.

Overview of protein folds in the immune system

The rapid advancement of X-ray crystallography and nuclear magnetic resonance techniques in recent years has resulted in the solution of macromolecular structures at an unprecedented rate. This review aims at providing a comprehensive description of structures and folds related to the function of the immune system. Focus is placed on immunologically relevant proteins such as immunoreceptors and major histocompatibility complexes. Information is also provided regarding protein structure data banks.

CD1d-restricted glycolipid antigens: presentation principles, recognition logic and functional consequences

Invariant natural killer T (iNKT) cells are innate lymphocytes whose functions are regulated by self and foreign glycolipid antigens presented by the antigen-presenting molecule CD1d. Activation of iNKT cells in vivo results in rapid release of copious amounts of effector cytokines and chemokines with which they regulate innate and adaptive immune responses to pathogens, certain types of cancers and self-antigens. The nature of CD1d-restricted antigens, the manner in which they are recognised and the unique effector functions of iNKT cells suggest an innate immunoregulatory role for this subset of T cells. Their ability to respond fast and our ability to steer iNKT cell cytokine response to altered lipid antigens make them an important target for vaccine design and immunotherapies against autoimmune diseases. This review summarises our current understanding of CD1d-restricted antigen presentation, the recognition of such antigens by an invariant T-cell receptor on iNKT cells, and the functional consequences of these interactions.

Hypothesis: TCR signal transduction--A novel tri-modular signaling system

Antigenic peptides initiate an immune response in T cells via the T cell receptor (TCR). The TCR itself is widely regarded as one of the most complex receptors in nature, as it is comprised of at least six different subunits, the antigen recognizing TCRalpha and beta chains, and the signal transmitting CD3deltavarepsilon, gammaepsilon, and zeta2 dimers. In order for a signal to be transmitted from the TCR to the cytoplasm, the CD3 chains must "sense" that an antigenic peptide has been presented to the TCRalpha and beta subunits. After accomplishing this, there are a total of 10 different immunoreceptor tyrosine activation motifs (ITAMs) present within the CD3 chains which effectively activate the T cell and hence the immune response. The importance of each CD3 chain and subsequently each ITAM has been the focus of intense research. However, the precise role(s) played by each CD3 chain has remained elusive. Using the immunomodulatory peptide termed core peptide (CP), which is proposed to inhibit TCR activation by disrupting TCR-CD3 interactions, a tri-modular signaling system for T cell activation is proposed. By contrast to the existing two distinct signaling model (zeta2, CD3epsilongamma/epsilondelta), in this model each of the three dimers, CD3gammaepsilon, deltaepsilon, and zeta2, are proposed to act as three separate and distinct signaling modules, performing both specific and redundant functions.

Allosteric Changes in the TCR/CD3 Structure Upon Interaction With Extra- or Intra-cellular Ligands

T lymphocytes are activated by the interaction between the T-cell antigen receptor (TCR) and peptides presented by major histocompatibility complex (MHC) molecules. The avidity of this TCR-pMHC interaction is very low. Therefore, several hypotheses have been put forward to explain how T cells become specifically activated despite this handicap: conformational change model, aggregation model, kinetic segregation model, sequential interaction model and permissive geometry model. In the present paper, we conducted experiments to distinguish between the TCR-aggregation model and the TCR-conformational change model. The results obtained using a TCR capture ELISA with Brij 98-solubilized TCR molecules from normal or activated T cells showed that the ligand-TCR interaction causes structural changes in the CD3 epsilon cytoplasmic tail as well as in the extracellular TCR beta FG loop region. Size-fractionation experiments with Brij 98-solubilized TCR/CD3/co-receptor complexes from naïve or activated CD4(+) or CD8(+) T cells demonstrated that such complexes are found as either dimers or tetramers. No monomers or multimers were detected. We propose that: (1) ligand-TCR interaction results in conformational changes in the CD3 epsilon cytoplasmic tail leading to T-cell activation; (2) CD3 epsilon cytoplasmic tail interaction with intracellular proteins may dissociate pMHC and co-receptors (CD4 or CD8) from TCR/CD3 complexes, thus leading to the arrest of T-cell activation; and (3) T-cell activation appears to occur among dimers or tetramers of TCR/CD3/co-receptor complexes interacting with self and non-self (foreign) peptide-MHC complexes.

Loss of N-terminal charged residues of mouse CD3 epsilon chains generates isoforms modulating antigen T cell receptor-mediated signals and T cell receptor-CD3 interactions

The antigen T cell receptor (TCR)-CD3 complexes present on the cell surface of CD4(+) T lymphocytes and T cell lines express CD3 epsilon chain isoforms with different isoelectric points (pI), with important structural and functional consequences. The pI values of the isoforms fit the predicted pI values of CD3 epsilon chains lacking one, two, and three negatively charged amino acid residues present in the N-terminal region. Different T cells have different ratios of CD3 epsilon chain isoforms. At a high pI, degraded CD3 epsilon isoforms can be better recognized by certain anti-CD3 monoclonal antibodies such as YCD3-1, the ability of which to bind to the TCR-CD3 complex is directly correlated with the pI of CD3 epsilon. The abundance of CD3 epsilon isoforms can be modified by treatment of T cells with the proteinase inhibitor phenanthroline. In addition, these CD3 epsilon isoforms have functional importance. This is shown, first, by the different structure of TCR-CD3 complexes in cells possessing different amounts of isoforms (as observed in surface biotinylation experiments), by their different antigen responses, and by the stronger interaction between low pI CD3 epsilon isoforms and the TCR. Second, incubation of cells with phenanthroline diminished the proportion of degraded high pI CD3 epsilon isoforms, but also the ability of the cells to deliver early TCR activation signals. Third, cells expressing mutant CD3 epsilon chains lacking N-terminal acid residues showed facilitated recognition by antibody YCD3-1 and enhanced TCR-mediated activation. Furthermore, the binding avidity of antibody YCD3-1 was different in distinct thymus populations. These results suggest that changes in CD3 epsilon N-terminal chains might help to fine-tune the response of the TCR to its ligands in distinct activation situations or in thymus selection.

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.

A natural structural variant of the mouse TCR beta-chain displays intrinsic receptor function and antigen specificity

The Cbeta0 alternate cassette exon is located between the Jbeta1 and Cbeta1 genes in the mouse TCR beta-locus. In T cells with a VDJbeta1 rearrangement, the Cbeta0 exon may be included in TCRbeta transcripts (herein called TCRbeta-Cbeta0 transcripts), potentially inserting an additional 24 aa between the V and C domains of the TCR beta-chain. These TCRbeta splice isoforms may be differentially regulated after Ag activation, because we detected TCRbeta-Cbeta0 transcripts in a high proportion (>60%) of immature and mature T cells having VDJbeta1 rearrangements but found a substantially reduced frequency (<35%) of TCRbeta-Cbeta0 expression among CD8 T cells selected by Ag in vivo. To study the potential activity of the TCRbeta-Cbeta0 splice variant, we cloned full-length TCR cDNAs by single-cell RT-PCR into retroviral expression vectors. We found that the TCRbeta-Cbeta0 splice isoform can function during an early stage of T cell development normally dependent on TCR beta-chain expression. We also demonstrate that T hybridoma-derived cells expressing a TCRbeta-Cbeta0 isoform together with the clonally associated TCR alpha-chain recognize the same cognate peptide-MHC ligand as the corresponding normal alphabetaTCR. This maintenance of receptor function and specificity upon insertion of the Cbeta0 peptide cassette signifies a remarkable adaptability for the TCR beta-chain, and our findings open the possibility that this splice isoform may function in vivo.

T cell receptor recognition via cooperative conformational plasticity

Although T cell receptor cross-reactivity is a fundamental property of the immune system and is implicated in numerous autoimmune pathologies, the molecular mechanisms by which T cell receptors can recognize and respond to diverse ligands are incompletely understood. In the current study we examined the response of the human T cell lymphotropic virus-1 (HTLV-1) Tax-specific T cell receptor (TCR) A6 to a panel of structurally distinct haptens coupled to the Tax 11-19 peptide with a lysine substitution at position 5 (Tax5K, LLFG[K-hapten]PVYV). The A6 TCR could cross-reactively recognize one of these haptenated peptides, Tax-5K-4-(3-Indolyl)-butyric acid (IBA), presented by HLA-A*0201. The crystal structures of Tax5K-IBA/HLA-A2 free and in complex with A6 reveal that binding is mediated by a mechanism of cooperative conformational plasticity involving conformational changes on both sides of the protein-protein interface, including the TCR complementarity determining region (CDR) loops, Valpha/Vbeta domain orientation, and the hapten-modified peptide. Our findings illustrate the complex role that protein dynamics can play in TCR cross-reactivity and highlight that T cell receptor recognition of ligand can be achieved through diverse and complex molecular mechanisms that can occur simultaneously in the interface, not limited to molecular mimicry and CDR loop shifts.

Lipidation and glycosylation of a T cell antigen receptor (TCR) transmembrane hydrophobic peptide dramatically enhances in vitro and in vivo function

A T cell antigen receptor (TCR) transmembrane sequence derived peptide (CP) has been shown to inhibit T cell activation both in vitro and in vivo at the membrane level of the receptor signal transduction. To examine the effect of sugar or lipid conjugations on CP function, we linked CP to 1-aminoglucosesuccinate (GS), N-myristate (MYR), mono-di-tripalmitate (LP1, LP2, or LP3), and a lipoamino acid (LA) and examined the effects of these compounds on T cell activation in vitro and by using a rat model of adjuvant-induced arthritis, in vivo. In vitro, antigen presentation results demonstrated that lipid conjugation enhanced CP's ability to lower IL-2 production from 56.99%+/-15.69 S.D. observed with CP, to 12.08%+/-3.34 S.D. observed with LA. The sugar conjugate GS resulted in only a mild loss of in vitro activity compared to CP (82.95%+/-14.96 S.D.). In vivo, lipid conjugation retarded the progression of adjuvant-induced arthritis by approximately 50%, whereas the sugar conjugated CP, GS, almost completely inhibited the progression of arthritis. This study demonstrates that hydrophobic peptide activity is markedly enhanced in vitro and in vivo by conjugation to lipids or sugars. This may have practical applications in drug delivery and bioavailability of hydrophobic peptides.

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.

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.

Mycobacterium tuberculosis in the adjuvant modulates the balance of Th immune response to self-antigen of the CNS without influencing a “core” repertoire of specific T cells

In the present study, we use modified CDR3 beta-chain spectratyping (immunoscope) to dissect the effect of Mycobacterium tuberculosis (MT)-derived proteins on individual PLP139-151-specific cells in the SJL mouse strain. In this model, the immunoscope technique allows the characterization of a public TCR that involves rearrangement of Vbeta10 and Jbeta1.1 and a semi-private TCR characterized by rearrangement of Vbeta4 and Jbeta1.6. Both rearrangements are specific for PLP139-151 and sequences of the CDR3 region of the two beta-chains show a conserved motif for the public rearrangement and related but more variable sequences for the semi-private rearrangement. MT-derived proteins promote increase of IFN-gamma-secreting cells. However, we observe that the presence and amount of MT used during immunization have no effect on the frequency of usage, polarization and in vivo expansion of cells carrying the studied rearrangements. Rather, the strong Th1-promoting effect of adjuvant is possibly due to recruitment toward Th1 of a wider spectrum of TCR repertoires. Therefore, instead of having a comprehensive effect on the entire repertoire, MT modulates the immune response by affecting a subset of antigen-specific T cells whose polarization can be adapted to the environment. This step establishes the final balance between Th1 and Th2 and may be essential for the enhancement or protection of disease.

Mechanistic basis of pre–T cell receptor–mediated autonomous signaling critical for thymocyte development

The pre-T cell receptor (TCR) is crucial for early T cell development and is proposed to function in a ligand-independent way. However, the molecular mechanism underlying the autonomous signals remains elusive. Here we show that the pre-TCR complex spontaneously formed oligomers. Specific charged residues in the extracellular domain of the pre-TCR alpha-chain mediated formation of the oligomers in vitro. Alteration of these residues eliminated the ability of the pre-TCR alpha-chain to support pre-TCR signaling in vivo. Dimerization but not raft localization of CD3epsilon was sufficient to simulate pre-TCR function and promote beta-selection. These results suggest that the pre-TCR complex can deliver its signal autonomously through oligomerization of the pre-TCR alpha-chain mediated by charged residues.

Crystal structures of T cell receptor (beta) chains related to rheumatoid arthritis

The crystal structures of the Vbeta17+ beta chains of two human T cell receptors (TCRs), originally derived from the synovial fluid (SF4) and tissue (C5-1) of a patient with rheumatoid arthritis (RA), have been determined in native (SF4) and mutant (C5-1(F104-->Y/C187-->S)) forms, respectively. These TCR beta chains form homo-dimers in solution and in crystals. Structural comparison reveals that the main-chain conformations in the CDR regions of the C5-1 and SF4 Vbeta17 closely resemble those of a Vbeta17 JM22 in a bound form; however, the CDR3 region shows different conformations among these three Vbeta17 structures. At the side-chain level, conformational differences were observed at the CDR2 regions between our two ligand-free forms and the bound JM22 form. Other significant differences were observed at the Vbeta regions 8-12, 40-44, and 82-88 between C5-1/SF4 and JM22 Vbeta17, implying that there is considerable variability in the structures of very similar beta chains. Structural alignments also reveal a considerable variation in the Vbeta-Cbeta associations, and this may affect ligand recognition. The crystal structures also provide insights into the structure basis of T cell recognition of Mycoplasma arthritidis mitogen (MAM), a superantigen that may be implicated in the development of human RA. Structural comparisons of the Vbeta domains of known TCR structures indicate that there are significant similarities among Vbeta regions that are MAM-reactive, whereas there appear to be significant structural differences among those Vbeta regions that lack MAM-reactivity. It further reveals that CDR2 and framework region (FR) 3 are likely to account for the binding of TCR to MAM.

Structure of a human autoimmune TCR bound to a myelin basic protein self-peptide and a multiple sclerosis-associated MHC class II molecule

Multiple sclerosis is mediated by T-cell responses to central nervous system antigens such as myelin basic protein (MBP). To investigate self-peptide/major histocompatibility complex (MHC) recognition and T-cell receptor (TCR) degeneracy, we determined the crystal structure, at 2.8 A resolution, of an autoimmune TCR (3A6) bound to an MBP self-peptide and the multiple sclerosis-associated MHC class II molecule, human leukocyte antigen (HLA)-DR2a. The complex reveals that 3A6 primarily recognizes the N-terminal portion of MBP, in contrast with antimicrobial and alloreactive TCRs, which focus on the peptide center. Moreover, this binding mode, which may be frequent among autoimmune TCRs, is compatible with a wide range of orientation angles of TCR to peptide/MHC. The interface is characterized by a scarcity of hydrogen bonds between TCR and peptide, and TCR-induced conformational changes in MBP/HLA-DR2a, which likely explain the low observed affinity. Degeneracy of 3A6, manifested by recognition of superagonist peptides bearing substitutions at nearly all TCR-contacting positions, results from the few specific interactions between 3A6 and MBP, allowing optimization of interface complementarity through variations in the peptide.

Structural Studies on the αβ T-cell Receptor

Alphabeta T-cell receptor (TcR) recognition of antigenic peptides bound to the major histocompatibility complex (pMHC), is integral to the cellular immune system. Crystallographic studies over the last decade have provided significant insight into this unique trimolecular recognition event. The TcR-pMHC structural information has been paralleled by biophysical studies that have further explored the emerging binding models in an attempt to answer fundamental immunological questions regarding MHC restriction, T-cell immunodominance and TcR cross-reactivity. However, despite the important data that has been generated regarding TcR-pMHC interactions, the scope of this information is still incomplete due to the limited range of TcRs that have been studied. These limitations are primarily due to difficulties in obtaining high yields of recombinant alphabeta TcR for crystallographic and biophysical analysis; here we will discuss some of the protein engineering strategies that have been employed to expand the pool of recombinant TcRs suitable for crystallographic studies and the subsequent studies that have utilized these proteins.

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.

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

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

Distinct footprints of TCR engagement with highly homologous ligands

T cell receptor engagement promotes proliferation, differentiation, survival, or death of T lymphocytes. The affinity/avidity of the TCR ligand and the maturational stage of the T cell are thought to be principal determinants of the outcome of TCR engagement. We demonstrate in this study that the same mouse TCR preferentially uses distinct residues of homologous peptides presented by the MHC molecules to promote specific cellular responses. The preference for distinct TCR contacts depends on neither the affinity/avidity of TCR engagement (except in the most extreme ranges), nor the maturity of engaged T cells. Thus, different portions of the TCR ligand appear capable of biasing T cells toward specific biological responses. These findings explain differences in functional versatility of TCR ligands, as well as anomalies in the relationship between affinity/avidity of the TCR for the peptide/MHC and cellular responses of T cells.

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.

Structure and function of natural killer cell surface receptors

Since mid-1990, with cloning and identification of several families of natural killer (NK) receptors, research on NK cells began to receive appreciable attention. Determination of structures of NK cell surface receptors and their ligand complexes led to a fast growth in our understanding of the activation and ligand recognition by these receptors as well as their function in innate immunity. Functionally, NK cell surface receptors are divided into two groups, the inhibitory and the activating receptors. Structurally, they belong to either the immunoglobulin (Ig)-like receptor superfamily or the C-type lectin-like receptor (CTLR) superfamily. Their ligands are either members of class I major histocompatibility complexes (MHC) or homologs of class I MHC molecules. The inhibitory form of NK receptors provides the protective immunity through recognizing class I MHC molecules with self-peptides on healthy host cells. The activating, or the noninhibitory, NK receptors mediate the killing of tumor or virally infected cells through their specific ligand recognition. The structures of activating and inhibitory NK cell surface receptors and their complexes with the ligands determined to date, including killer immunoglobulin-like receptors (KIRs) and their complexes with HLA molecules, CD94, Ly49A, and its complex with H-2Dd, and NKG2D receptors and their complexes with class I MHC homologs, are reviewed here.

Three-dimensional structural location and molecular functional effects of missense SNPs in the T cell receptor V? domain

The mechanisms by which human single nucleotide polymorphisms (SNPs) influence susceptibility to disease are not yet well understood. In a previous study, we developed a structure-based model that may be used to identify which missense SNPs are neutral and which are deleterious to protein function and so potentially involved in disease (Wang and Moult, Hum Mutat 2001;263-270). The model has now been applied to a set of 54 missense cSNPs in the 46 functional T-cell receptor Vbeta-genes. Most of these missense cSNPs are found to be neutral, but 10 are identified as likely deleterious to protein function. Only one was previously associated with disease. We suggest that the others may be disease related but that redundancy in the T-cell response prevents any simple, monogenic effect. Therefore, these SNPs are the most likely contributors to complex, polygenic disease traits. It has been noted that there is a surprisingly high (74%) fraction of nonsynonymous SNPs in these genes. Contrary to expectation, the analysis shows that these are not associated with an unusually high fraction of deleterious SNPs, nor do they significantly contribute to a larger range of antigen recognition or a reduced superantigen-binding repertoire.

Another View of T Cell Antigen Recognition: Cooperative Engagement of Glycolipid Antigens by Va14Ja18 Natural TCR

Va14Ja18 natural T (iNKT) cells rapidly elicit a robust effector response to different glycolipid Ags, with distinct functional outcomes. Biochemical parameters controlling iNKT cell function are partly defined. However, the impact of iNKT cell receptor beta-chain repertoire and how alpha-galactosylceramide (alpha-GalCer) analogues induce distinct functional responses have remained elusive. Using altered glycolipid ligands, we discovered that the Vb repertoire of iNKT cells impacts recognition and Ag avidity, and that stimulation with suboptimal avidity Ag results in preferential expansion of high-affinity iNKT cells. iNKT cell proliferation and cytokine secretion, which correlate with iNKT cell receptor down-regulation, are induced within narrow biochemical thresholds. Multimers of CD1d1-alphaGalCer- and alphaGalCer analogue-loaded complexes demonstrate cooperative engagement of the Va14Ja18 iNKT cell receptor whose structure and/or organization appear distinct from conventional alphabeta TCR. Our findings demonstrate that iNKT cell functions are controlled by affinity thresholds for glycolipid Ags and reveal a novel property of their Ag receptor apparatus that may have an important role in iNKT cell activation.

TCR binding kinetics measured with MHC class I tetramers reveal a positive selecting peptide with relatively high affinity for TCR

The interaction between TCR and peptide-MHC (pMHC) complexes is crucial for the activation of T cells as well as for positive and negative selection in the thymus. The kinetics and affinity of this interaction and the densities of TCR and pMHC complexes on the cell surface are determining factors for different outcomes during thymic selection. In general, it is thought that agonist pMHC, which cause negative selection, have higher affinities and, in particular, slower off-rates than partial or weak agonists and antagonists, which cause positive selection. In this study, we have used pMHC tetramers to investigate the kinetics of TCR-pMHC interaction for agonist, weak agonist, and antagonist ligands of the anti-lymphocytic choriomeningitis virus P14 TCR. Kinetics determined on the cell surface may be biologically more relevant than methods using soluble proteins. We can distinguish between agonists and weak agonists or antagonists based on the half-life and the avidity of tetramer-TCR interaction. Furthermore, we show that a weak agonist self-peptide that positively selects P14 TCR(+) thymocytes has a tetramer half-life and avidity only slightly weaker than strong agonists. We show that, in fact, it can act as quite a strong agonist, but that its poor ability to stabilize MHC causes it instead to have a weak agonist phenotype.

Production, characterization, and immunogenicity of a soluble rat single chain T cell receptor specific for an encephalitogenic peptide

The encephalitogenic rat T cell clone C14 recognizes the myelin basic protein 69-89 peptide in the context of the RT1B major histocompatibility complex (MHC) class II molecule. Modeling of the C14 TCR molecule indicated that previously identified CDR3 motifs are likely to be central to interaction with MHC class II-presented peptide. Here we report the cloning and expression of C14-derived single chain TCR (scTCR) molecules in an Escherichia coli expression system. The recombinant molecule consists of the Valpha2 domain connected to the Vbeta8.2 domain via a 15-residue linker. Soluble C14 scTCR was purified using conventional chromatography techniques and refolded by a rapid dilution procedure. C14 scTCR was able to bind soluble rat MHC class II molecules bearing covalently coupled Gp-BP-(69-89) peptide, as analyzed using surface plasmon resonance. Immune recognition of the C14 scTCR protein as an antigen revealed that limited regions of the TCR may be more likely to induce responsiveness.

Molecular recognition in antibody-antigen complexes

With the numerous detailed molecular descriptions of antibody-antigen interfaces, the structual study of these molecular interactions has evolved from an attempt to understand to immunological function to their use as model systems for protein-protein interactions. In this chapter, we describe the structual aspects common to antibody-antigen interfaces and discuss the roles they may play in antibody cross-rectivity and molecular mimicry. More detailed analysis of these interfaces has required the marriage of structural studies with extensive mutagenesis and thermodynamic analysis efforts. Here, we discuss the thermodynamic mapping of interfaces for two model antibody-antigen complexes, including the identification of thermodynamic hot spots in binding and the various mechanism used to accommodate interface mutations. We also discuss the functional roles for protein plasticity in antigen recognition, including the entropic control of antibody affinity maturation and the use of induced fit mechanism of different types and to varying degrees by mature antibodies in binding their specific antigens.

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.

Binding free energy differences in a TCR-peptide-MHC complex induced by a peptide mutation: a simulation analysis

Recognition by the T-cell receptor (TCR) of immunogenic peptides presented by class I major histocompatibility complexes (MHCs) is the determining event in the specific cellular immune response against virus-infected cells or tumor cells. It is of great interest, therefore, to elucidate the molecular principles upon which the selectivity of a TCR is based. These principles can in turn be used to design therapeutic approaches, such as peptide-based immunotherapies of cancer. In this study, free energy simulation methods are used to analyze the binding free energy difference of a particular TCR (A6) for a wild-type peptide (Tax) and a mutant peptide (Tax P6A), both presented in HLA A2. The computed free energy difference is 2.9 kcal/mol, in good agreement with the experimental value. This makes possible the use of the simulation results for obtaining an understanding of the origin of the free energy difference which was not available from the experimental results. A free energy component analysis makes possible the decomposition of the free energy difference between the binding of the wild-type and mutant peptide into its components. Of particular interest is the fact that better solvation of the mutant peptide when bound to the MHC molecule is an important contribution to the greater affinity of the TCR for the latter. The results make possible identification of the residues of the TCR which are important for the selectivity. This provides an understanding of the molecular principles that govern the recognition. The possibility of using free energy simulations in designing peptide derivatives for cancer immunotherapy is briefly discussed.

The 1.5 A crystal structure of a highly selected antiviral T cell receptor provides evidence for a structural basis of immunodominance

Despite a potential repertoire of >10(15) alphabeta T cell receptors (TcR), the HLA B8-restricted cytolytic T cell response to a latent antigen of Epstein-Barr virus (EBV) is strikingly limited in the TcR sequences that are selected. Even in unrelated individuals this response is dominated by a single highly restricted TcR clonotype that selects identical combinations of hypervariable Valpha, Vbeta, D, J, and N region genes. We have determined the 1.5 A crystal structure of this "public" TcR, revealing that five of the six hypervariable loops adopt novel conformations providing a unique combining site that contains a deep pocket predicted to overlay the HLA B8-peptide complex. The findings suggest a structural basis for the immunodominance of this clonotype in the immune response to EBV.

Probing T cell membrane organization using dimeric MHC-Ig complexes

In this report, we review a novel method for probing the membrane organization of T cells using dimeric major histocompatibility complexes (MHC), MHC-Ig. MHC-Ig complexes are useful reagents for quantitative analysis of binding data since their valency is controlled. These complexes can be easily labeled and loaded with a variety of peptides. A binding assay using these dimers and quantitative analysis of the MHC-Ig dimer-T cell binding curves is described in detail. Using this approach, we show that the organization of TCR on activated T cells is different from TCR organization on nai;ve T cells. The implications of these findings are discussed with regards to current models of T cell recognition. This analysis offers insight into how T cell controls their biological range of responsiveness. Specifically, these findings reveal the biophysical basis of the ability of activated T cells to recognize low amounts of antigen independent of costimulation.

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.

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.

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.

CD4 T cells selected by antigen under Th2 polarizing conditions favor an elongated TCR alpha chain complementarity-determining region 3

The affinity of the MHC/peptide/TCR interaction is thought to be one factor determining the differentiation of CD4+ T cells into Th1 or Th2 phenotypes. To study whether CD4+ cells generated under conditions favoring Th1 or Th2 responses select structurally different TCRs, Th1 and Th2 clones and lines were generated from nonobese diabetic and nonobese diabetic H2-E transgenic mice against the peptides proteolipoprotein 56-70, glutamic acid decarboxylase(65) 524-543, and heat shock protein-60 peptides 168-186 and 248-264. Th1/Th2 polarization allowed the generation of clones and lines with fixed peptide specificity and class II restriction but differing in Th1/Th2 phenotype in which the impact on TCR selection and structure could be studied. The Th2 clones tended to use longer TCR complementarity-determining region (CDR)3alpha loops than their Th1 counterparts. This trend was confirmed by analyzing TCRalpha transcripts from Th1 and Th2 polarized, bulk populations. Molecular modeling of Th1- and Th2-derived TCRs demonstrated that Th2 CDR3alpha comprised larger side chain residues than Th1 TCRs. The elongated, bulky Th2 CDR3alpha loops may be accommodated at the expense of less optimal interactions between the MHC class II/peptide and other CDR loops of the TCR. We propose that CD4+ T cells selected from the available repertoire under Th2 polarizing conditions tend to have elongated TCR CDR3alpha loops predicted to alter TCR binding, reducing contact at other interfaces and potentially leading to impeded TCR triggering.

Some hints concerning the shape of T-cell receptor structures

Several models are proposed for T-cell antigen receptor (TCR) assembly and structure. However, there is little experimental data favouring directly either one or the other(s). The minimal complex appears to be composed of a TCRalphabeta/CD3deltaepsilon,gammaepsilon/zeta2 structure but at the cell membrane, multimers of this minimal structure may be formed. Quantitative cytofluometry has suggested three CD3epsilon chains for two TCRbeta (or TCRdelta) chains/complex. Such data should be repeated with monoclonal antibodies (MoAb) against extracellular (EC) parts of CD3delta or CD3gamma chains. In the present review, we have compared the TCR/CD3 assembly of pre-TCR, TCRgammadelta and TCRalphabeta containing complexes, and analysed the reactivity of antibodies (Abs) against the EC part of CD3delta chains. Our data suggest an alternative assembly pathway and structure of TCR/CD3 complexes.

The effect of IgM-enriched human Ig and rabbit antithymocyte globulin on the stimulation of mononuclear cells

Whether IgM-enriched intravenous Ig (pentaglobin) is a useful adjunct treatment for graft versus host disease (GvHD) prophylaxis in allogeneic stem-cell transplantation is unclear. Clinical data with the use of a five-agent GvHD prevention regimen, including pentaglobin and antithymocyte globulin (ATG), are encouraging. In vitro both have been reported to modulate alloreactive T cells. We compared their inhibitory effect on the phytohemagglutinin-induced lymphocyte proliferation. ATG blocked the proliferation of lymphocytes at lower doses and much stronger than pentaglobin. The combination of both was not different from ATG alone. In pentaglobin, glucose used as stabiliser, caused the effect. Starting at a concentration of 40 mg/dL glucose, glucose alone showed a dose-dependent inhibition of phytohemaglutinin (PHA)-induced proliferation. For the in vivo application of pentaglobin, the results suggest that pentaglobin does not inhibit the proliferation of T cells.

Evolution of T cell receptor (TCR) alpha beta heterodimer assembly with the CD3 complex

T cell antigen receptors (TCR) are composed of an antigen-recognizing unit, the TCRalpha beta heterodimer, and a signal transduction ensemble, the CD3 complex. Whereas mammals possess three CD3 dimers (delta epsilon, gamma epsilon, and zeta2), birds and amphibians have only two (delta/gamma-epsilon and zeta2). To understand evolutionary changes in TCR/CD3 assembly,a phylogenetic approach was employed to dissect the interaction of TCRalpha beta heterodimers with the CD3 components. While sheep and mouse TCRalpha and TCRbeta chains could replace the corresponding human chains in mutant human T cells to restore surface TCR/CD3 expression and function, chicken TCRalpha, TCRbeta and CD3delta/gamma chains were unable to replace the corresponding human chains in forming a chimeric TCR/CD3 complex. The inability of chicken TCR/CD3 components to replace the human molecules in T cells was found to result from the lack of interaction between chicken TCRalpha beta heterodimers and the human CD3 complex. In contrast, if no CD3 molecules are present (non-T cells), TCRalpha -TCRbeta chain pairing can take place in an apparently non-controlled way. Thus, the TCR-CD3 interactions have changed with the evolutionary divergence of two mammalian CD3gamma and CD3delta genes from a single prototypic chicken delta/gamma gene. Our data suggest that the structures in mammalian TCR.C regions, which distinguish between CD3delta and CD3gamma chains, have evolved with the appearance of two separate CD3delta and CD3gamma functions.

Degenerate interfaces in antigen-antibody complexes

In most of the work dealing with the analysis of protein-protein interfaces, a single X-ray structure is available or selected, and implicitly it is assumed that this structure corresponds to the optimal complex for this pair of proteins. However, we have found a degenerate interface in a high-affinity antibody-antigen complex: the two independent complexes of the camel variable domain antibody fragment cAb-Lys3 and its antigen hen egg white lysozyme present in the asymmetric unit of our crystals show a difference in relative orientation between antibody and antigen, leading to important differences at the protein-protein interface. A third cAb-Lys3-hen lysozyme complex in a different crystal form adopts yet another relative orientation. Our results show that protein-protein interface characteristics can vary significantly between different specimens of the same high-affinity antibody-protein antigen complex. Consideration should be given to this type of observation when trying to establish general protein-protein interface characteristics.

Immunobiological analysis of TCR single-chain transgenic mice reveals new possibilities for interaction between CDR3alpha and an antigenic peptide bound to MHC class I

The interaction between TCRs and peptides presented by MHC molecules determines the specificity of the T cell-mediated immune response. To elucidate the biologically important structural features of this interaction, we generated TCR beta-chain transgenic mice using a TCR derived from a T cell clone specific for the immunodominant peptide of vesicular stomatitis virus (RGYVYQGL, VSV8) presented by H-2K(b). We immunized these mice with VSV8 or analogs substituted at TCR contact residues (positions 1, 4, and 6) and analyzed the CDR3alpha sequences of the elicited T cells. In VSV8-specific CTLs, we observed a highly conserved residue at position 93 of CDR3alpha and preferred Jalpha usage, indicating that multiple residues of CDR3alpha are critical for recognition of the peptide. Certain substitutions at peptide position 4 induced changes at position 93 and in Jalpha usage, suggesting a potential interaction between CDR3alpha and position 4. Cross-reactivity data revealed the foremost importance of the Jalpha region in determining Ag specificity. Surprisingly, substitution at position 6 of VSV8 to a negatively charged residue induced a change at position 93 of CDR3alpha to a positively charged residue, suggesting that CDR3alpha may interact with position 6 in certain circumstances. Analogous interactions between the TCR alpha-chain and residues in the C-terminal half of the peptide have not yet been revealed by the limited number of TCR/peptide-MHC crystal structures reported to date. The transgenic mouse approach allows hundreds of TCR/peptide-MHC interactions to be examined comparatively easily, thus permitting a wide-ranging analysis of the possibilities for Ag recognition in vivo.

On the role of CD3delta chains in TCRgammadelta/CD3 complexes during assembly and membrane expression

The present study was performed in order to analyze whether T-cell receptor (TCR)/CD3 assembly, intracellular transport and surface expression are carried in a similar way in alphabeta-and gammadelta-T cells. By means of optimal immunoprecipitation conditions with 35S-methionine/cysteine- or biotin-labelled TCR/CD3 proteins from alphabeta- or gammadelta-T-lymphoma-cell lines, as well as TCRgammadelta cDNA transfectants, it was found that CD3delta chains associate less strongly with TCRgammadelta heterodimers compared to TCRalphabeta heterodimers. This preferential reactivity of CD3delta chains appears to be structural and not owing to differences in gammadelta- versus alphabeta-T-cell intracellular environments. Our results are in accordance firstly, with data from CD3delta-deficient mice, which have gammadelta-T cells but no alphabeta-T cells, secondly with the suggested role of CD3delta chains in the positive selection of alphabeta-T cells, a process apparently not followed by gammadelta-T cells, and lastly with the differential roles of CD3delta chains versus CD3gamma chains, explaining the maintenance of two CD3delta and CD3gamma genes after the duplication from a CD3delta/gamma gene present in avians. The impaired reactivity of CD3delta chains with TCRgammadelta heterodimers seems to be owing to a less efficient association with TCRgamma chains. In contrast, CD3delta chains interact as strongly with TCRdelta chains as do CD3gamma chains with both TCRgamma and TCRdelta chains. These data may explain, at the molecular levels, why surface TCR/CD3 expression levels are impaired in gammadelta-T cells from CD3gamma-deficient mice but not from CD3delta-deficient mice.

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.

Structure of a human gammadelta T-cell antigen receptor

T-cell antigen receptors composed of gamma and delta polypeptide chains (gammadelta TCRs) can directly recognize antigens in the form of intact proteins or non-peptide compounds, unlike alphabeta TCRs, which recognize antigens bound to major histocompatibility complex molecules (MHC). About 5% of peripheral blood T cells bear gammadelta TCRs, most of which recognize non-peptide phosphorylated antigens. Here we describe the 3.1 A resolution structure of a human gammadelta TCR from a T-cell clone that is phosphoantigen-reactive. The orientation of the variable (V) and constant (C) regions of the gammadelta TCR is unique when compared with alphabeta TCRs or antibodies, and results from an unusually small angle between the Vgamma and Cgamma domains. The complementarity-determining regions (CDRs) of the V domains exhibit a chemically reasonable binding site for phosphorylated antigens, providing a possible explanation for the canonical usage of the Vgamma9 and Vdelta2 gene segments by phosphoantigen-reactive receptors. Although the gammadelta TCR V domains are similar in overall structure to those of alphabeta TCRs, gammadelta TCR C domains are markedly different. Structural differences in Cgamma and Cdelta, and in the location of the disulphide bond between them, may enable gammadelta TCRs to form different recognition/signalling complexes than alphabeta TCRs.