Biophysical studies of T-cell receptors and their ligands

Affinity-enhanced T-cell receptors for adoptive T-cell therapy targeting MAGE-A10: strategy for selection of an optimal candidate

Circulating T-cells that have passed thymic selection generally bear T-cell receptors (TCRs) with sub-optimal affinity for cancer-associated antigens, resulting in a limited ability to detect and eliminate tumor cells. Engineering TCRs to increase their affinity for cancer targets is a promising strategy for generating T-cells with enhanced potency for adoptive immunotherapy in cancer patients. However, this manipulation also risks generating cross-reactivity to antigens expressed by normal tissue, with potentially serious consequences. Testing in animal models might not detect such cross-reactivity due to species differences in the antigenic repertoire. To mitigate the risk of off-target toxicities in future clinical trials, we therefore developed an extensive in vitro testing strategy. This approach involved systematic substitution at each position of the antigenic peptide sequence using all natural amino acids to generate a profile of peptide specificity (“X-scan”). The likelihood of off-target reactivity was investigated by searching the human proteome for sequences matching this profile, and testing against a panel of primary cell lines. Starting from a diverse panel of parental TCRs, we engineered several affinity-enhanced TCRs specific for the cancer-testis antigen MAGE-A10. Two of these TCRs had affinities and specificities which appeared to be equally optimal when tested in conventional biochemical and cellular assays. The X-scan method, however, permitted us to select the most specific and potent candidate for further pre-clinical and clinical testing.

Immunomodulating effects of casein-derived peptides QEPVL and QEPV on lymphocytes in vitro and in vivo

Lymphocytes serve an important function in mediating specific immune responses. When the body is stimulated by internal or external antigens, activated lymphocytes proliferate to clear pathogens by secreting antibodies or cytokines. Some bioactive peptides were isolated from fermented milk in previous studies. One of the peptides, Gln-Glu-Pro-Val-Leu (QEPVL), was synthesized and used in this experiment. Results show that QEPVL can significantly activate lymphocytes both in vitro and in vivo. QEPVL can also increase the lymphocyte proliferation rate and cyclic AMP levels. This positive regulation had a dose-effect relationship within certain concentration ranges. QEPVL can also inhibit LPS-induced inflammation by regulating nitric oxide release and the production of the cytokines IL-4, IL-10, IFN-γ, and TNF-α in vivo. Digesting QEPVL in artificial gastrointestinal juice yields the digestion product Gln-Glu-Pro-Val (QEPV), which exhibits bioactivities similar to those of QEPVL in vitro. Overall, QEPVL has significant immunomodulating effects on lymphocytes and contributes to inflammation treatment through the oral route as a functional food ingredient.

Production of protein complexes via co-expression

Multi-protein complexes are involved in essentially all cellular processes. A protein's function is defined by a combination of its own properties, its interacting partners, and the stoichiometry of each. Depending on binding partners, a transcription factor can function as an activator in one instance and a repressor in another. The study of protein function or malfunction is best performed in the relevant context. While many protein complexes can be reconstituted from individual component proteins after being produced individually, many others require co-expression of their native partners in the host cells for proper folding, stability, and activity. Protein co-expression has led to the production of a variety of biological active complexes in sufficient quantities for biochemical, biophysical, structural studies, and high throughput screens. This article summarizes examples of such cases and discusses critical considerations in selecting co-expression partners, and strategies to achieve successful production of protein complexes.

Genetic engineering of T cells for adoptive immunotherapy

To be effective for the treatment of cancer and infectious diseases, T cell adoptive immunotherapy requires large numbers of cells with abundant proliferative reserves and intact effector functions. We are achieving these goals using a gene therapy strategy wherein the desired characteristics are introduced into a starting cell population, primarily by high efficiency lentiviral vector-mediated transduction. Modified cells are then expanded using ex vivo expansion protocols designed to minimally alter the desired cellular phenotype. In this article, we focus on strategies to (1) dissect the signals controlling T cell proliferation; (2) render CD4 T cells resistant to HIV-1 infection; and (3) redirect CD8 T cell antigen specificity.

A nanoscopic multivalent antigen-presenting carrier for sensitive detection and drug delivery to T cells

Both monoclonal T cell-specific antibodies and multivalent major histocompatibility complex proteins are used as diagnostic reagents for T cell-mediated diseases. However, their widespread use as vehicles for drug delivery has been hindered by the lack of versatile methods that couple the targeting potential of these reagents with drugs of clinical relevance. To address this problem, we engineered a multivalent nanoscopic drug carrier that flexibly tethers to a variety of T-cell antigens. Our carriers bound their target T cells specifically and with enhanced sensitivities as compared with free antigen. Additionally, they consistently inhibited the proliferation of the target T cells in vitro and in vivo, whereas drug-free constructs elicited strong stimulation of the target populations. As a result of the flexibility of incorporating multivalent antigen and drug, these carriers have wide potential use as sensitive T-cell detection reagents as well as promising immunostimulatory or immunosuppressive tools.

Directed evolution of human T cell receptor CDR2 residues by phage display dramatically enhances affinity for cognate peptide-MHC without increasing apparent cross-reactivity

The mammalian alpha/beta T cell receptor (TCR) repertoire plays a pivotal role in adaptive immunity by recognizing short, processed, peptide antigens bound in the context of a highly diverse family of cell-surface major histocompatibility complexes (pMHCs). Despite the extensive TCR-MHC interaction surface, peptide-independent cross-reactivity of native TCRs is generally avoided through cell-mediated selection of molecules with low inherent affinity for MHC. Here we show that, contrary to expectations, the germ line-encoded complementarity determining regions (CDRs) of human TCRs, namely the CDR2s, which appear to contact only the MHC surface and not the bound peptide, can be engineered to yield soluble low nanomolar affinity ligands that retain a surprisingly high degree of specificity for the cognate pMHC target. Structural investigation of one such CDR2 mutant implicates shape complementarity of the mutant CDR2 contact interfaces as being a key determinant of the increased affinity. Our results suggest that manipulation of germ line CDR2 loops may provide a useful route to the production of high-affinity TCRs with therapeutic and diagnostic potential.

CD8 kinetically promotes ligand binding to the T-cell antigen receptor

The mechanism of CD8 cooperation with the TCR in antigen recognition was studied on live T cells. Fluorescence correlation measurements yielded evidence of the presence of two TCR and CD8 subpopulations with different lateral diffusion rate constants. Independently, evidence for two subpopulations was derived from the experimentally observed two distinct association phases of cognate peptide bound to class I MHC (pMHC) tetramers and the T cells. The fast phase rate constant ((1.7 +/- 0.2) x 10(5) M(-1) s(-1)) was independent of examined cell type or MHC-bound peptides' structure. Its value was much faster than that of the association of soluble pMHC and TCR ((7.0 +/- 0.3) x 10(3) M(-1) s(-1)), and close to that of the association of soluble pMHC with CD8 ((1-2) x 10(5) M(-1) s(-1)). The fast binding phase disappeared when CD8-pMHC interaction was blocked by a CD8-specific mAb. The latter rate constant was slowed down approximately 10-fold after cells treatment with methyl-beta-cyclodextrin. These results suggest that the most efficient pMHC-cell association route corresponds to a fast tetramer binding to a colocalized CD8-TCR subpopulation, which apparently resides within membrane rafts: the reaction starts by pMHC association with the CD8. This markedly faster step significantly increases the probability of pMHC-TCR encounters and thereby promotes pMHC association with CD8-proximal TCR. The slow binding phase is assigned to pMHC association with a noncolocalized CD8-TCR subpopulation. Taken together with results of cytotoxicity assays, our data suggest that the colocalized, raft-associated CD8-TCR subpopulation is the one capable of inducing T-cell activation.

Soluble HIV-specific T cell receptor: expression, purification and analysis of the specificity

We have produced soluble T cell receptor (TCR) derived from a human CD8(+) cytotoxic T lymphocyte (CTL) clone D3 that recognizes the immunodominant HIV Gag peptide SLYNTVATL (SL9) in association with major histocompatibility complex (MHC) class I protein HLA-A2. Drosophila Schneider cells (S2) were used to express genes coding the TCR alpha and beta chains under an inducible promoter. Both chains were labeled with two different tags: a (His)(6) was introduced at the C-terminal end of alpha chain, while beta chain was terminated by c-myc. Since an isolated alpha chain is unstable unless it is associated with a beta chain, this design permits rapid separation of alpha,beta-heterodimer from unpaired beta chain in a single step of Ni-NTA Agarose chromatography yielding 90% pure alpha,beta-TCR. Introduction of the c-myc epitope to the beta chain allows capture of soluble D3 from the culture supernatant by immobilized anti-c-myc antibody, without the need for receptor purification. The TCR specificity was then examined by analyzing the binding of peptide-HLA-A2/tetramer in an ELISA assay. Using this assay, we have also evaluated the binding of monomeric SL9-HLA-A2 complex to the immobilized D3 TCR and determined that the affinity measurement of the D3-SL9-HLA-A2 reaction is similar to that obtained by a biosensor instrument. We propose that the approach described here is generally useful for purification of other soluble TCRs and will allow rapid analysis of their specificity.

A novel single chain I-A(b) molecule can stimulate and stain antigen-specific T cells

Multimers of soluble major histocompatibility complex class I and II molecules have proven to be useful reagents in quantifying and following specific T cell populations. This study describes the design, generation, and characterization of a novel, single chain I-A(b) molecule which utilizes a unique linker derived from the murine invariant chain. A fragment of the invariant chain, residues 58-85, binds to a region proximal to the class II peptide binding groove and stabilizes occupancy of the class II invariant chain-associated peptide. We have utilized this fragment, replacing CLIP with the Ealpha peptide sequence, to lock the attached peptide into the class II binding groove. The single chain I-A(b) molecule was recognized by a panel of conformation-sensitive, I-A(b)-specific, monoclonal antibodies. Membrane-bound and soluble forms of the single chain I-A(b) stimulated an antigen-specific T cell hybridoma, and tetramers made from soluble monomers stained these cells. The unique features of this molecule may be useful in the design of recombinant T cell receptor ligands containing peptides with low affinity for MHC.

Structure and function of natural killer cell receptors: multiple molecular solutions to self, nonself discrimination

In contrast to T cell receptors, signal transducing cell surface membrane molecules involved in the regulation of responses by cells of the innate immune system employ structures that are encoded in the genome rather than generated by somatic recombination and that recognize either classical MHC-I molecules or their structural relatives (such as MICA, RAE-1, or H-60). Considerable progress has recently been made in our understanding of molecular recognition by such molecules based on the determination of their three-dimensional structure, either in isolation or in complex with their MHC-I ligands. Those best studied are the receptors that are expressed on natural killer (NK) cells, but others are found on populations of T cells and other hematopoietic cells. These molecules fall into two major structural classes, those of the immunoglobulin superfamily (KIRs and LIRs) and of the C-type lectin-like family (Ly49, NKG2D, and CD94/NKG2). Here we summarize, in a functional context, the structures of the murine and human molecules that have recently been determined, with emphasis on how they bind different regions of their MHC-I ligands, and how this allows the discrimination of tumor or virus-infected cells from normal cells of the host.

Generation of functional antigen-specific T cells in defined genetic backgrounds by retrovirus-mediated expression of TCR cDNAs in hematopoietic precursor cells

We have developed an alternative to transgenesis for producing antigen-specific T cells in vivo. In this system, clonal naive T cells with defined antigen specificity are generated by retrovirus-mediated expression of T cell antigen receptor cDNAs in RAG1-deficient murine hematopoietic precursor cells. These T cells can be stimulated to proliferate and produce cytokines by exposure to antigen in vitro, and they become activated and expand in vivo after immunization. IL-2-deficient T cells generated by this technique show decreased proliferation and cytokine production, both of which can be rescued by exogenous addition of this growth factor. Thus, retrovirus-mediated expression of T cell antigen receptor cDNAs in hematopoietic precursor cells permits the rapid and efficient analysis of the life history of antigen-specific T cells in different genetic backgrounds and may allow for the long-term production of antigen-specific T cells with different functional properties for prophylactic and therapeutic purposes.

Binding interactions between peptides and proteins of the class II major histocompatibility complex

The activation of helper T cells by peptides bound to proteins of the class II Major Histocompatibility Complex (MHC II) is pivotal to the initiation of an immune response. The primary functional requirement imposed on MHC II proteins is the ability to efficiently bind thousands of different peptides. Structurally, this is reflected in a unique architecture of binding interactions. The peptide is bound in an extended conformation within a groove on the membrane distal surface of the protein that is lined with several pockets that can accommodate peptide side-chains. Conserved MHC II protein residues also form hydrogen bonds along the length of the peptide main-chain. Here we review recent advances in the study of peptide-MHC II protein reactions that have led to an enhanced understanding of binding energetics. These results demonstrate that peptide-MHC II protein complexes achieve high affinity binding from the array of hydrogen bonds that are energetically segregated from the pocket interactions, which can then add to an intrinsic hydrogen bond-mediated affinity. Thus, MHC II proteins are unlike antibodies, which utilize cooperativity among binding interactions to achieve high affinity and specificity. The significance of these observations is discussed within the context of possible mechanisms for the HLA-DM protein that regulates peptide presentation in vivo and the design of non-peptide molecules that can bind MHC II proteins and act as vaccines or immune modulators.

CD8beta endows CD8 with efficient coreceptor function by coupling T cell receptor/CD3 to raft-associated CD8/p56(lck) complexes

The extraordinary sensitivity of CD8+ T cells to recognize antigen impinges to a large extent on the coreceptor CD8. While several studies have shown that the CD8beta chain endows CD8 with efficient coreceptor function, the molecular basis for this is enigmatic. Here we report that cell-associated CD8alphabeta, but not CD8alphaalpha or soluble CD8alphabeta, substantially increases the avidity of T cell receptor (TCR)-ligand binding. To elucidate how the cytoplasmic and transmembrane portions of CD8beta endow CD8 with efficient coreceptor function, we examined T1.4 T cell hybridomas transfected with various CD8beta constructs. T1.4 hybridomas recognize a photoreactive Plasmodium berghei circumsporozoite (PbCS) peptide derivative (PbCS (4-azidobezoic acid [ABA])) in the context of H-2K(d), and permit assessment of TCR-ligand binding by TCR photoaffinity labeling. We find that the cytoplasmic portion of CD8beta, mainly due to its palmitoylation, mediates partitioning of CD8 in lipid rafts, where it efficiently associates with p56(lck). In addition, the cytoplasmic portion of CD8beta mediates constitutive association of CD8 with TCR/CD3. The resulting TCR-CD8 adducts exhibit high affinity for major histocompatibility complex (MHC)-peptide. Importantly, because CD8alphabeta partitions in rafts, its interaction with TCR/CD3 promotes raft association of TCR/CD3. Engagement of these TCR/CD3-CD8/lck adducts by multimeric MHC-peptide induces activation of p56(lck) in rafts, which in turn phosphorylates CD3 and initiates T cell activation.

An overview of the immune system with specific reference to membrane encapsulation and islet transplantation

The concept of immunoisolation by use of a bioartificial membrane is discussed, concentrating on the immunological mechanisms that are likely to be operative in the light of recent information on the workings of the immune system. Special attention is given to the use of encapsulation for the purpose of treating autoimmune diabetes by implantation of xenogeneic islet tissue. It is argued that the term immunoisolation is misleading because the immune system is always activated by the indirect pathway of antigen presentation and that the term immunomodulation would be more appropriate.

Inhibitory effects of T-cell stimulation and co-stimulation observed at high concentrations of plate-bound antibodies

Plate-bound monoclonal antibodies (mAb) are often used as a way of stimulating lymphocytes in vitro. Our observations show that the concentrations of mAb used in functional assays in vitro must be carefully assessed before conclusions are drawn about lymphocyte activation or co-activation.

Retroviral antibody binding of the MHC class II molecule: a biochemical influence on CD4 T cell differentiation in HIV infection?

Retroviral antibody capable of binding to the major histocompatibility complex (MHC) Class II molecule has been documented in human immunodeficiency virus-1 (HIV-1)-infected patients. Interactions between the MHC Class II receptor and the T-cell receptor (TCR) are central to the immune response. Importantly, retroviral antibody possesses a much higher binding affinity for the MHC Class II receptor, when compared to the TCR. Experiments have manipulated a number of factors related to antigen-presenting cell (APC) interaction with differentiating T-cells. These studies have observed the effects of lowering antigen dose and reducing ligand density on precursor Th (T helper) cell differentiation. Studies have also examined the effect of downregulated MHC Class II receptors and co-stimulatory molecules on APC-Th cell interaction. In addition, the sequestration of antigens away from the Class II processing pathway has been studied. These investigations reveal a general trend that can determine whether a naive CD4 T-cell becomes a Th1 or Th2-like cell. If the experimental manipulation weakens the APC-Th cell interaction, a weak ligating TCR signal results. Consequently, a weak ligating TCR signal can influence precursor Th cells to become Th2-like cells. Retroviral antibody binding of MHC Class II receptors may mimic a number of experimental conditions responsible for creating a weak ligating TCR signal.

T lymphocytes promote the development of bone marrow-derived APC in the central nervous system

Certain cells within the CNS, microglial cells and perivascular macrophages, develop from hemopoietic myelomonocytic lineage progenitors in the bone marrow (BM). Such BM-derived cells function as CNS APC during the development of T cell-mediated paralytic inflammation in diseases such as experimental autoimmune encephalomyelitis and multiple sclerosis. We used a novel, interspecies, rat-into-mouse T cell and/or BM cell-transfer method to examine the development and function of BM-derived APC in the CNS. Activated rat T cells, specific for either myelin or nonmyelin Ag, entered the SCID mouse CNS within 3-5 days of cell transfer and caused an accelerated recruitment of BM-derived APC into the CNS. Rat APC in the mouse CNS developed from transferred rat BM within an 8-day period and were entirely sufficient for induction of CNS inflammation and paralysis mediated by myelin-specific rat T cells. The results demonstrate that T cells modulate the development of BM-derived CNS APC in an Ag-independent fashion. This previously unrecognized regulatory pathway, governing the presence of functional APC in the CNS, may be relevant to pathogenesis in experimental autoimmune encephalomyelitis, multiple sclerosis, and/or other CNS diseases involving myelomonocytic lineage cells.

A diverse set of oligomeric class II MHC-peptide complexes for probing T-cell receptor interactions

BACKGROUND

T-cells are activated by engagement of their clonotypic cell surface receptors with peptide complexes of major histocompatibility complex (MHC) proteins, in a poorly understood process that involves receptor clustering on the membrane surface. Few tools are available to study the molecular mechanisms responsible for initiation of activation processes in T-cells.

RESULTS

A topologically diverse set of oligomers of the human MHC protein HLA-DR1, varying in size from dimers to tetramers, was produced by varying the location of an introduced cysteine residue and the number and spacing of sulfhydryl-reactive groups carried on novel and commercially available cross-linking reagents. Fluorescent probes incorporated into the cross-linking reagents facilitated measurement of oligomer binding to the T-cell surface. Oligomeric MHC-peptide complexes, including a variety of MHC dimers, trimers and tetramers, bound to T-cells and initiated T-cell activation processes in an antigen-specific manner.

CONCLUSION

T-cell receptor dimerization on the cell surface is sufficient to initiate intracellular signaling processes, as a variety of MHC-peptide dimers differing in intramolecular spacing and orientation were each able to trigger early T-cell activation events. The relative binding affinities within a homologous series of MHC-peptide oligomers suggest that T-cell receptors may rearrange in the plane of the membrane concurrent with oligomer binding.

Modeling of the TCR-MHC-peptide complex

The methodology for generating a homology model of the T1 TCR-PbCS-K(d) class I major histocompatibility complex (MHC) class I complex is presented. The resulting model provides a qualitative explanation of the effect of over 50 different mutations in the region of the complementarity determining region (CDR) loops of the T cell receptor (TCR), the peptide and the MHC's alpha(1)/alpha(2) helices. The peptide is modified by an azido benzoic acid photoreactive group, which is part of the epitope recognized by the TCR. The construction of the model makes use of closely related homologs (the A6 TCR-Tax-HLA A2 complex, the 2C TCR, the 14.3.d TCR Vbeta chain, the 1934.4 TCR Valpha chain, and the H-2 K(b)-ovalbumine peptide), ab initio sampling of CDR loops conformations and experimental data to select from the set of possibilities. The model shows a complex arrangement of the CDR3alpha, CDR1beta, CDR2beta and CDR3beta loops that leads to the highly specific recognition of the photoreactive group. The protocol can be applied systematically to a series of related sequences, permitting the analysis at the structural level of the large TCR repertoire specific for a given peptide-MHC complex.

Recombinant T-cell receptors: an immunologic link to cancer therapy

Cytotoxic T cells can specifically kill target cells that express antigens recognized by the T-cell receptor. These are membrane-bound proteins that are not ubiquitous and thus are difficult to purify and study at the protein level. The advent of recombinant DNA technology has facilitated these objectives, thereby enabling researchers to gain valuable information about major T-cell receptor characteristics. Genetic manipulation of T-cell receptors has also been used to exploit specificity of killing by cytotoxic T lymphocytes, which represents an attractive feature for therapeutic purposes. The objective of this review was to provide an overview of the major strategies adopted to genetically manipulate T-cell receptors.

Production of soluble alphabeta T-cell receptor heterodimers suitable for biophysical analysis of ligand binding

A method to produce alphabeta T-cell receptors (TCRs) in a soluble form suitable for biophysical analysis was devised involving in vitro refolding of a TCR fusion protein. Polypeptides corresponding to the variable and constant domains of each chain of a human and a murine receptor, fused to a coiled coil heterodimerization motif from either c-Jun (alpha) or v-Fos (beta), were overexpressed separately in Escherichia coli. Following recovery from inclusion bodies, the two chains of each receptor were denatured, and then refolded together in the presence of denaturants. For the human receptor, which is specific for the immunodominant influenza A HLA-A2-restricted matrix epitope (M58-66), a heterodimeric protein was purified in milligram yields and found to be homogeneous, monomeric, antibody-reactive, and stable at concentrations lower than 1 microM. Using similar procedures, analogous results were obtained with a murine receptor specific for an influenza nucleoprotein epitope (366-374) restricted by H2-Db. Production of these receptors has facilitated a detailed analysis of viral peptide-Major Histocompatibility Complex (peptide-MHC) engagement by the TCR using both surface plasmon resonance (SPR) and, in the case of the human TCR, isothermal titration calorimetry (ITC) (Willcox et al., 1999). The recombinant methods described should enable a wide range of TCR-peptide-MHC interactions to be studied and may also have implications for the production of other heterodimeric receptor molecules.

Interaction of the NK cell inhibitory receptor Ly49A with H-2Dd: identification of a site distinct from the TCR site

Natural killer cell function is controlled by interaction of NK receptors with MHC I molecules expressed on target cells. We describe the binding of bacterially expressed Ly49A, the prototype murine NK inhibitory receptor, to similarly engineered H-2Dd. Despite its homology to C-type lectins, Ly49A binds independently of carbohydrate and Ca2+ and shows specificity for MHC I but not bound peptide. The affinity of the Ly49A/H-2Dd interaction as determined by surface plasmon resonance is from 6 to 26 microM at 25 degrees C and is greater by ultracentrifugation at 4 degrees C. Biotinylated Ly49A stains H-2Dd-expressing cells. Competition experiments indicate that the Ly49A and T cell receptor (TCR) binding sites on MHC I are distinct, suggesting complex regulation of cells that bear both TCR and NK cell receptors.

An inverse relationship between T cell receptor affinity and antigen dose during CD4(+) T cell responses in vivo and in vitro

Multimeric peptide/class II MHC staining reagents were synthesized and shown to bind with appropriate specificity to T cell hybridomas. A small, expanded population of T cells detected with one of these reagents in peptide-immunized C57BL/10 mice persisted for several months. This population expanded further on secondary immunization. Equating the extent of binding of this reagent to T cell receptor affinity, we saw little correlation of immunizing peptide dose to T cell receptor affinity at the peak of the primary response. However, there was an inverse relation between peptide dose and the apparent receptor affinity of the T cells that were present several months after a primary response or after a secondary stimulation either in vivo or in vitro.

The X-ray crystal structure of a Valpha2.6Jalpha38 mouse T cell receptor domain at 2.5 A resolution: alternate modes of dimerization and crystal packing

We describe here the structure of a murine T cell receptor (TCR) Valpha2.6Jalpha38 (TCRAV2S6J38) domain, derived from a T cell hybridoma with specificity for the H-2Ddmajor histocompatibility complex class I molecule bound to a decamer peptide, P18-I10, from the HIV envelope glycoprotein gp120, determined by X-ray crystallography at 2.5 A resolution. Unlike other TCR Valpha domains that have been studied in isolation, this one does not dimerize in solution at concentrations below 1 mM, and the crystal fails to show dimer contacts that are likely to be physiological. In comparison to other Valpha domains, this Valpha2.6 shows great similarity in the packing of its core residues, and exhibits the same immunoglobulin-like fold characteristic of other TCR Valpha domains. There is good electron density in all three complementarity-determining regions (CDRs), where the differences between this Valpha domain and others are most pronounced, in particular in CDR3. Examination of crystal contacts reveals an association of Valpha domains distinct from those previously seen. Comparison with other Valpha domain structures reveals variability in all loop regions, as well as in the first beta strand where placement and configuration of a proline residue at position 6, 7, 8, or 9 affects the backbone structure. The great variation in CDR3 conformations among TCR structures is consistent with an evolving view that CDR3 of TCR plays a plastic role in the interaction of the TCR with the MHC/peptide complex as well as with CDR3 of the paired TCR chain.

Construction and binding analysis of recombinant single-chain TCR derived from tumor-infiltrating lymphocytes and a cytotoxic T lymphocyte clone directed against MAGE-1

The TCR is responsible for the specificity of cytotoxic T lymphocytes (CTL) by recognizing peptides presented in the context of MHC. By producing recombinant soluble TCR, it is possible to study this interaction at the molecular level. We generated single-chain TCR (scTCR) from tumor infiltrating lymphocytes (TIL) and one CTL clone directed against melanoma-associated antigen (MAGE)-1. Sixty-eight day anti-MAGE-1 TIL and one cloned anti-MAGE-1 CTL were analyzed by PCR for their Valpha and Vbeta gene usage. The TIL population showed a restriction in Valpha and Vbeta usage with only Valpha4 and Valpha9 and Vbeta2 and Vbeta7 expressed. The anti-MAGE-1 CTL clone demonstrated absolute restriction with only Valpha12 and Vbeta1 expressed. DNA sequence analysis was performed on all V regions. For the TIL, each possible Valpha-Vbeta combination (i.e. Valpha4-Vbeta2, Valpha9-Vbeta2, Valpha4-Vbeta7 and Valpha9-Vbeta7) was constructed as a distinct scTCR and the recombinant proteins expressed in bacteria. From the anti-MAGE-1 TIL, Valpha4-Vbeta2 scTCR demonstrated binding activity to HLA-A1(+) cells pulsed with MAGE-1 peptide. Results obtained from screening a panel of our scTCR constructs on HLA-A1(+) cells pulsed with MAGE-1 peptide or irrelevant peptide demonstrated that Vbeta2 plays a significant role in binding to the MAGE-1 peptide. Amino acid alignment analysis showed that each Vbeta sequence is distinctly different from the others. These findings demonstrate that soluble TCR in single-chain format have binding activity. Furthermore, the results indicate that in TCR, like antibodies, one chain may contribute a dominant portion of the binding activity.

Soluble Class I MHC with β2-Microglobulin Covalently Linked Peptides: Specific Binding to a T Cell Hybridoma

Soluble forms of the mouse MHC class I molecule, Dd, were produced in which the peptide binding groove was uniformly occupied by peptides attached via a covalent flexible peptide linker to the N terminus of the associated β2-microglobulin. The MHC heavy chain and β2-microglobulin were firmly associated, and the molecules displayed an Ab epitope requiring proper occupancy of the peptide binding groove. Soluble Dd containing a covalent version of a well-characterized Dd-binding peptide from HIV stimulated a T cell hybridoma specific for this combination. Furthermore, a tetravalent version of this molecule bound specifically with apparent high avidity to this hybridoma.

Antigens varying in affinity for the B cell receptor induce differential B lymphocyte responses

The B cell receptor (BCR) triggers a variety of biological responses that differ depending upon the properties of the antigen. A panel of M13 phage-displayed peptide ligands with varying affinity for the 3-83 antibody was generated to explore the role of antigen-BCR affinity in cell activation studies using primary 3-83 transgenic mouse B cells. Multiple parameters of activation were measured. T cell-independent B cell proliferation, antibody secretion, induction of germline immunoglobulin gamma1 transcripts, and B cell production of interleukin (IL) 2 and interferon gamma responses were better correlated with antigen-BCR affinity than with receptor occupancy. In contrast, other responses, such as upregulation of major histocompatibility complex class II and B7.2 (CD86), secretion of IL-6, and B cell proliferation in the context of CD40 signaling were only weakly dependent on antigen affinity. Biochemical analysis revealed that at saturating ligand concentrations the ability of phage to stimulate some early signaling responses, such as Ca++ mobilization and tyrosine phosphorylation of syk or Igalpha, was highly affinity dependent, whereas the ability to stimulate Lyn phosphorylation was less so. These data suggest that the BCR is capable of differential signaling. The possibility that differential BCR signaling by antigen determines whether an antibody response will be T independent or dependent is discussed.

Detection of antigen-specific T cells with multivalent soluble class II MHC covalent peptide complexes

Multimeric soluble MHC class II molecules stably occupied with covalently attached peptides bind with appropriate specificity to T cell hybridomas and T cells from T cell receptor transgenic mice. There is a direct correlation between soluble T cell receptor affinity for monomeric MHC/peptide and level of binding of multimeric MHC/peptide to T cells. While binding of the multimeric MHC/peptide complex is proportional to T cell receptor affinity and expression level, there is little influence of T cell CD4.

Presentation of antigenic peptides by products of the major histocompatibility complex

Molecules encoded by the major histocompatibility complex (MHC) are polymorphic integral membrane proteins adapted to the presentation of peptide fragments of foreign antigens to antigen-specific T-cells. The diversity of infectious agents to which an immune response must be mounted poses a unique problem for receptor-ligand interactions; how can proteins whose polymorphism is necessarily limited bind an array of peptides almost infinite in its complexity? Both MHC class I and class II determinants have achieved this goal by harnessing a limited number of peptide side chains to anchor the epitope in place while exploiting conserved features of peptide structure, independent of their primary sequence. While class I molecules interact predominantly with the N- and C-termini of peptides, class II determinants form an extensive hydrogen bonding network along the length of the peptide backbone. Such a strategy ensures high-affinity binding, while selectively exposing the unique features of each ligand for recognition by the T-cell receptor.

Superantigens of gram-positive bacteria: structure-function analyses and their implications for biological activity

Just as we thought that we know everything about superantigens, new molecular and structural studies indicate that we have only just begun to unravel the secrets of these fascinating molecules. Recent structure-function analysis of superantigens from Gram-positive bacteria, with emphasis on their interaction with major histocompatibility complex molecules, could help us decipher the role of superantigens in disease, identify host factors that potentiate their effects and design drugs that specifically block their activity.

Conformational integrity and ligand binding properties of a single chain T-cell receptor expressed in Escherichia coli

We recently showed that a soluble, heterodimeric murine D10 T-cell receptor (TCR) (Valpha2Calpha, Vbeta8.2Cbeta) expressed in insect cells binds both Vbeta8.2-specific bacterial superantigen staphylococcal enterotoxin C2 (SEC2) and a soluble, heterodimeric major histocompatibility complex class II I-Ak.conalbumin peptide complex with a low micromolar affinity. To define further the structural requirements for the TCR/ligand interactions, we have produced in Escherichia coli a soluble, functional D10 single chain (sc) TCR molecule in which the Valpha and Vbeta domains are connected by a flexible peptide linker. Purified and refolded D10 scTCR bound to SEC2 and murine major histocompatibility complex class II I-Ak.conalbumin peptide complex with thermodynamic and kinetic binding constants similar to those measured for the baculovirus-derived heterodimeric D10 TCR suggesting that neither the TCR constant domains nor potential N- or O-linked carbohydrate moieties are necessary for ligand recognition and for expression and proper folding of the D10 scTCR. Purified D10 scTCR remained soluble at concentrations up to 1 mM. Circular dichroism and NMR spectroscopy indicated that D10 scTCR is stabilized predominantly by beta-sheet secondary structure, consistent with its native-like conformation. Because of its limited size, high solubility, and structural integrity, purified D10 scTCR appears to be suitable for structural studies by multidimensional NMR spectroscopy.

Analysis of the expression of peptide-major histocompatibility complexes using high affinity soluble divalent T cell receptors

Understanding the regulation of cell surface expression of specific peptide-major histocompatibility complex (MHC) complexes is hindered by the lack of direct quantitative analyses of specific peptide-MHC complexes. We have developed a direct quantitative biochemical approach by engineering soluble divalent T cell receptor analogues (TCR-Ig) that have high affinity for their cognate peptide-MHC ligands. The generality of this approach was demonstrated by specific staining of peptide-pulsed cells with two different TCR-Ig complexes: one specific for the murine alloantigen 2C, and one specific for a viral peptide from human T lymphocyte virus-1 presented by human histocompatibility leukocyte antigens-A2. Further, using 2C TCR- Ig, a more detailed analysis of the interaction with cognate peptide-MHC complexes revealed several interesting findings. Soluble divalent 2C TCR-Ig detected significant changes in the level of specific antigenic-peptide MHC cell surface expression in cells treated with gamma-interferon (gamma-IFN). Interestingly, the effects of gamma-IFN on expression of specific peptide-MHC complexes recognized by 2C TCR-Ig were distinct from its effects on total H-2 Ld expression; thus, lower doses of gamma-IFN were required to increase expression of cell surface class I MHC complexes than were required for upregulation of expression of specific peptide-MHC complexes. Analysis of the binding of 2C TCR-Ig for specific peptide-MHC ligands unexpectedly revealed that the affinity of the 2C TCR-Ig for the naturally occurring alloreactive, putatively, negatively selecting, complex, dEV-8-H-2 Kbm3, is very low, weaker than 71 microM. The affinity of the 2C TCR for the other naturally occurring, negatively selecting, alloreactive complex, p2Ca-H-2 Ld, is approximately 1000-fold higher. Thus, negatively selecting peptide-MHC complexes do not necessarily have intrinsically high affinity for cognate TCR. These results, uniquely revealed by this analysis, indicate the importance of using high affinity biologically relevant cognates, such as soluble divalent TCR, in furthering our understanding of immune responses.

High-level production of a secreted, heterodimeric alpha beta murine T-cell receptor in Escherichia coli

For structural studies, high-level production of properly folded, disulfide-linked, unglycosylated protein in E. coli is an attractive alternative to production in eukaryotic systems. We describe here the production of heterodimeric, murine D10 T-cell receptor (sD10TCR) in E. coli as a secreted leucine zipper (LZ) fusion protein. Two genes, one (alpha-acid) encoding the alpha-chain variable and constant domains (V alpha and C alpha) of D10 TCR fused to an LZ 'acid' encoding sequence and the other (beta-base) encoding the beta-chain variable and constant domains (V beta and C beta) fused to an LZ 'base' encoding sequence, were co-expressed from a bacteriophage T7 promoter as a dicistronic message. Secreted alpha-acid and beta-base proteins formed proper inter- and intra-chain disulfide bonds in the periplasm, bypassing the need for in vitro protein refolding. Complementary LZ sequences facilitated the formation of alpha beta heterodimers. sD10TCR-LZ was purified by affinity chromotography using a D10 TCR clonotype-specific monoclonal antibody (mAb 3D3). Typical yields of purified protein were 4-5 mg/l of culture. Purified sD10TCR-LZ was reactive with a panel of conformationally sensitive TCR-specific monoclonal antibodies, consistent with its conformational integrity and appeared to be suitable for structural studies by X-ray crystallography or NMR spectroscopy.

Influence of the NH2-terminal amino acid of the T cell receptor alpha chain on major histocompatibility complex (MHC) class II + peptide recognition

The alpha/beta T cell receptor (TCR) recognizes peptide fragments bound in the groove of major histocompatibility complex (MHC) molecules. We modified the TCR alpha chain from a mouse T cell hybridoma and tested its ability to reconstitute TCR expression and function in an alpha chain-deficient variant of the hybridoma. The modified alpha chain differed from wild type only in its leader peptide and mature NH2-terminal amino acid. Reconstituted cell surface TCR complexes reacted normally with anti-TCR and anti-CD3 antibodies. Although cross-linking of this TCR with an antibody to the TCR idiotype elicited vigorous T cell hybridoma activation, stimulation with its natural MHC + peptide ligand did not. We demonstrated that this phenotype could be reproduced simply by substituting the glutamic acid (E) at the mature NH2 terminus of the wild type TCR alpha chain with aspartic acid (D). The substitution also dramatically reduced the affinity of soluble alpha/beta-TCR heterodimers for soluble MHC + peptide molecules in a cell-free system, suggesting that it did not exert its effect simply by disrupting TCR interactions with accessory molecules on the hybridoma. These results demonstrate for the first time that amino acids which are not in the canonical TCR complementarity determining regions can be critical in determining how the TCR engages MHC + peptide.

Interactions of TCRs with MHC-peptide complexes: a quantitative basis for mechanistic models

The activation of T lymphocytes is initiated by the binding of MHC-peptide complexes on antigen-presenting cells to MHC-restricted, peptide specific TCRs. Significant progress has recently been made in understanding the structure of the TCR and in the direct quantitative examination of the primary binding interactions between MHC-peptide complexes and the TCR. Attempts to develop quantitative models for the differential activation of T cells by MHC-peptide ligands that differ subtly in their structure have largely been based on either the affinity of the MHC-peptide complexes for the TCR in question or on the dissociation kinetics of the MHC-peptide complex from the T cell.

Affinity and kinetics of the interactions between an alphabeta T-cell receptor and its superantigen and class II-MHC/peptide ligands

Immune activation is mediated by a specific interaction between the T-cell receptor (TCR) and an antigenic peptide bound to the major histocompatibility complex (MHC). T-cell activation can also be stimulated by superantigens which bind to germline-encoded variable domain sequences of certain TCR beta-chains. We have used a surface plasmon resonance biosensor to characterize the molecular interactions between a class II-restricted alphabeta TCR and its superantigen and MHC/peptide ligands. The extracellular domains of the murine D10 TCR (Valpha2, Vbeta8.2) were expressed in insect cells and secreted as a disulfide-linked heterodimer. In the absence of MHC class II, purified soluble D10 TCR bound to Staphylococcus aureus enterotoxin C2 with an association rate of 1.69+/-0.12 x 10(4)M(-1) sec(-1) and a dissociation rate of 1.9+/-0.47 x 10(-2) sec(-1), giving a dissociation constant of 1.1 microM. Binding of the TCR to S. aureus enterotoxin B was barely detectable and could not be measured accurately due to the rapid dissociation rate. Soluble D10 TCR also bound to a soluble murine MHC class II I-A(k) molecule containing a fused antigenic conalbumin peptide and complementary leucine zipper sequences to facilitate efficient chain pairing. The purified I A(k) chimera specifically stimulated proliferation of the D10 T-cell clone, and bound to immobilized soluble D10 TCR with an association rate of 1.07+/-0.19 x 10(4)M(-1)sec(-1) and a dissociation rate of 2.2+/-0.65 x 10(-2) sec(-1), giving a dissociation constant of 2.1 microM.

CD80 (B7-1) binds both CD28 and CTLA-4 with a low affinity and very fast kinetics

The structurally related T cell surface molecules CD28 and CTLA-4 interact with cell surface ligands CD80 (B7-1) and CD86 (B7-2) on antigen-presenting cells (APC) and modulate T cell antigen recognition. Preliminary reports have suggested that CD80 binds CTLA-4 and CD28 with affinities (Kd values approximately 12 and approximately 200 nM, respectively) that are high when compared with other molecular interactions that contribute to T cell-APC recognition. In the present study, we use surface plasmon resonance to measure the affinity and kinetics of CD80 binding to CD28 and CTLA-4. At 37 degrees C, soluble recombinant CD80 bound to CTLA-4 and CD28 with Kd values of 0.42 and 4 microM, respectively. Kinetic analysis indicated that these low affinities were the result of very fast dissociation rate constants (k(off)); sCD80 dissociated from CD28 and CTLA-4 with k(off) values of > or = 1.6 and > or = 0.43 s-1, respectively. Such rapid binding kinetics have also been reported for the T cell adhesion molecule CD2 and may be necessary to accommodate-dynamic T cell-APC contacts and to facilitate scanning of APC for antigen.

B cells are exquisitely sensitive to central tolerance and receptor editing induced by ultralow affinity, membrane-bound antigen

To assess the sensitivity of B cell tolerance with respect to receptor/autoantigen affinity, we identified low affinity ligands to the 3-83 (anti-major histocompatibility complex class I) antibody and tested the ability of these ligands to induce central and peripheral tolerance in 3-83 transgenic mice. Several class I protein alloforms, including Kbm3 and Dk, showed remarkably low, but detectable, affinity to 3-83. The 3-83 antibody bound Kb with K lambda approximately 2 x 10(5) M-1 and bound 10-fold more weakly to the Kbm3 (K lambda approximately 2 x 10(4) M-1) and Dk antigens. Breeding 3-83 immunoglobulin transgenic mice with mice expressing these ultralow affinity Kbm3 and Dk ligands resulted in virtually complete deletion of the autoreactive B cells from the peripheral lymphoid tissues. These low affinity antigens also induced receptor editing, as measured by elevated RAG mRNA levels in the bone marrow and excess levels of id- variant B cells bearing lambda light chains in the spleen. Reactive class I antigens were also able to mediate deletion of mature B cells when injected into the peritoneal cavity of 3-83 transgenic mice. Although the highest affinity ligand, Kk, was consistently able to induce elimination of the 3-83 peritoneal B cells, the lower affinity ligands were only partially effective. These results demonstrate the remarkable sensitivity of the deletion and receptor-editing mechanisms in immature B cells, and may suggest a higher affinity threshold for deletion of peripheral, mature B cells.