Immune checkpoints in viral latency

The dynamics of the relationship between the immune system and latent viruses are highly complex. Latent viruses not only avoid elimination by the host's primary immune response, they also remain with the host for life in the presence of strong acquired immunity, often exhibiting periodic reactivation and recurrence from the latent state. The continual battle between reemergent infectious virus and immunological memory cells provides an essential virus-host regulatory loop in latency. In this review, we speculate on the critical importance of immune interference mechanisms by viruses contributing to the regulatory loop in viral homeostasis of latency. Central to the notion of viral homeostasis, we further invoke the concept of threshold limits in naive and memory states of immunity to account for the failure of the host to completely eradicate these intracellular parasites.

The impact of duration versus extent of TCR occupancy on T cell activation: a revision of the kinetic proofreading model

The widely accepted kinetic proofreading theory proposes that rapid TCR dissociation from a peptide/MHC ligand allows for stimulation of early but not late T cell activation events, explaining why low-affinity TCR ligands are poor agonists. We identified a low-affinity TCR ligand which stimulated late T cell responses but, contrary to predictions from kinetic proofreading, inefficiently induced early activation events. Furthermore, responses induced by this ligand were kinetically delayed compared to its high-affinity counterpart. Using peptide/MHC tetramers, we showed that activation characteristics could be dissociated from TCR occupancy by the peptide/MHC ligands. Our data argue that T cell responses are triggered by a cumulative signal which is reached at different time points for different TCR ligands.

T-cell receptor binding kinetics in T-cell development and activation

T-cell activation is of central importance to the generation of an immune response and is also required as part of the host's ability to recognise self proteins. T cells are activated to differing extents by different ligands. Agonist ligands cause the full range of T-cell activation phenotypes – from activation of signalling cascades, to cytokine secretion or target cell killing, to T-cell proliferation. Partial agonists, which can differ from the agonist by as little as a single amino acid residue, can induce some of these responses but not all. Antagonist ligands can disable the signalling of an agonist ligand. These different types of interaction between ligand and T-cell receptor (TCR) also determine the developmental fate of maturing T cells. Much recent work has focused on how the T cell distinguishes between ligands. At least part of the answer lies in the kinetics of its binding to ligand.

Hijacking and exploitation of IL-10 by intracellular pathogens

Macrophages play a central role in infections, as a target for pathogens and in activation of the immune system. Interleukin-10 (IL-10), a cytokine produced by macrophages, is a potent immunosuppressive factor. Some intracellular pathogens specifically target macrophages for infection and use IL-10 to dampen the host immune response and stall their elimination from the host. Certain viruses induce production of cellular IL-10 by macrophages, whereas other viruses encode their own viral IL-10 homologs. Additionally, specific bacteria, including several Mycobacteria spp. and Listeria monocytogenes, can survive and replicate in macrophages while inducing cellular IL-10, highlighting a potential role for IL-10 of macrophage origin in the immunosuppressive etiology of these pathogens. Thus, the exploitation of IL-10 appears to be a common mechanism of immunosuppression by a diverse group of intracellular pathogens that can infect macrophages.

T cell receptor binding kinetics and special role of Valpha in T cell development and activation

The kinetics of the interaction between T cell receptor (TCR) and major histocompatibility complex (MHC) has an important role in determining thymocyte-positive and -negative selection in the thymus, as well as in T cell activation. The alpha chain of the TCR is the major player in determining how the TCR fits onto the MHC ligand, and thus has a major role in determining whether a T cell develops as class I or class II restricted. In this article, we summarize recent data from our laboratory and others on the role of polymorphism in the Valpha combining site in determining MHC class restriction, and on kinetic parameters in thymocyte selection.

Allelic exclusion of the T cell receptor alpha-chain: developmental regulation of a post-translational event

Allelic exclusion of the alpha and beta chains of the T cell receptor is maintained by different mechanisms. Exclusion of the beta-chain is primarily by allowing the successful rearrangement of only one of the two beta-chain loci. In the case of the alpha-chain, rearrangement on both chromosomes is very common, as is expression of alpha-chain mRNA and protein encoded by both loci. For the most part, however, functional alpha-chain allelic exclusion is maintained at the cell surface after positive selection in the thymus. The mechanism by which this is accomplished is not yet known, but recent evidence indicates that it is an active process coupled to signalling through the T cell receptor.

Cutting edge: trimolecular interaction of TCR with MHC class II and bacterial superantigen shows a similar affinity to MHC:peptide ligands

Bacterial superantigens such as Staphylococcus aureus enterotoxin A (SEA) are very potent stimulators of T cells. They bind to the Vbeta region of the TCR and to MHC class II, stimulating T cells at nanomolar concentrations. Using surface plasmon resonance measurements, we find that binding between the individual components of the complex (TCR-class II, TCR-SEA, SEA-class II) is very weak, but that the stability of the trimolecular complex is considerably enhanced, reaching an affinity similar to that found for TCR interactions with MHC:peptide ligand. Thus, the potency of SEA in stimulation of T cells is not due to particularly strong affinities between the proteins, but to a cooperative effect of interactions in the TCR-SEA-MHC class II trimolecular complex that brings the kinetics into a similar range to binding of conventional Ags. This range may be the optimum for T cell activation.

Murine cytomegalovirus infection down-regulates MHC class II expression on macrophages by induction of IL-10

Herpesviruses utilize many strategies for weakening the host immune response. For CMV, this includes avoidance of NK clearance and inhibition of MHC class I and class II presentation pathways. In this study, we report that mouse CMV (MCMV) specifically causes a premature and transient activation of host IL-10 very early in the course of infection, resulting in a dramatic and selective reduction in MHC class II surface expression. The expression of IL-10 is normally late in the immune response to a pathogen, serving to dampen the response by suppression of the production of inflammatory cytokines. In infection of macrophages, we show that MCMV induces the production of IL-10, leading to an early and selective reduction in the expression of MHC class II on the surface of the cells. Inhibition of MHC class II expression was not observed in the presence of neutralizing Abs to IL-10 or in macrophages from IL-10-deficient mice. Moreover, MCMV-infected IL-10-deficient mice developed an early and significantly more robust macrophage MHC class II induction than normal mice. Altogether, our results demonstrate that viral induction of an IL-10 autocrine pathway plays an essential early role in selectively reducing MHC class II expression on the surface of APC prior to stimulation by IFN-gamma.

Reciprocal expression in CD4 or CD8 subsets of different members of the V alpha 11 gene family correlates with sequence polymorphism

Previous staining studies with TCR V alpha 11-specific mAbs showed that V alpha 11.1/11.2 (AV11S1 and S2) expression was selectively favored in the CD4+ peripheral T cell population. As this phenomenon was essentially independent of the MHC haplotype, it was suggested that AV11S1 and S2 TCRs exert a preference for recognition of class II MHC molecules. The V alpha segment of the TCR alpha-chain is suggested to have a primary role in shaping the T cell repertoire due to selection for class I or II molecules acting through the complementarity determining regions (CDR) 1 alpha and CDR2 alpha residues. We have analyzed the repertoire of V alpha 11 family members expressed in C57BL/6 mice and have identified a new member of this family; AV11S8. We show that, whereas AV11S1 and S2 are more frequent in CD4+ cells, AV11S3 and S8 are more frequent in CD8+ cells. The sequences in the CDR1 alpha and CDR2 alpha correlate with differential expression in CD4+ or CD8+ cells, a phenomenon that is also observed in BALB/c mice. With no apparent restriction in TCR J alpha usage or CDR3 alpha length in C57BL/6, these findings support the idea of V alpha-dependent T cell repertoire selection through preferential recognition of MHC class I or class II molecules.

Qualitative and quantitative differences in T cell receptor binding of agonist and antagonist ligands

The kinetics of interaction between TCR and MHC-peptide show a general relationship between affinity and the biological response, but the reported kinetic differences between antigenic and antagonistic peptides are very small. Here, we show a remarkable difference in the kinetics of TCR interactions with strong agonist ligands at 37 degrees C compared to 25 degrees C. This difference is not seen with antagonist/positive selecting ligands. The interaction at 37 degrees C shows biphasic binding kinetics best described by a model of TCR dimerization. The altered kinetics greatly increase the stability of complexes with agonist ligands, accounting for the large differences in biological response compared to other ligands. Thus, there may be an allosteric, as well as a kinetic, component to the discrimination between agonists and antagonists.

Thymic skewing of the CD4/CD8 ratio maps with the T-cell receptor alpha-chain locus

The thymic preference for CD4+ T cells over CD8+ T cells is often attributed to a default pathway favouring CD4+ T cells or to homeostatic mechanisms. It is also clear, however, that T-cell receptor (TCR) preferences for major histocompatibility complex (MHC) class I versus class II binding will strongly influence an individual clone's skewing to the CD4 or CD8 subset. The variable region of each TCR alpha chain (V alpha) studied to date is found to be overrepresented in either CD4+ or CD8+ cells, suggesting that each V alpha element can interact more favourably with either MHC class I or class II molecules. Indeed, TCRs appear to have an intrinsic ability to interact with MHC molecules, and single amino acid residues present in germline-encoded complementarity determining region 1 (CDR1) and CDR2 of the V alpha element can be responsible for determining MHC specificity. Interestingly, the degree of CD4/CD8 skewing is variable among different mouse strains and in human populations. Here, we have shown that polymorphism in CD4/CD8 skewing between B6 and BALB/c mice is determined by the stem cell genotype and not by environmental effects, and that it maps in or near the TCR alpha-chain complex, Tcra. This was confirmed by comparing Tcra(b) with Tcra(a) or Tcra(c) haplotypes in congenic mice. We propose that the array of V alpha genes in various Tcra haplotypes exerts influence over the proportion of CD4 and CD8 subsets generated and may account in part for the observed thymic skewing. Thus, while it has been suggested that the TCR genes have been selected by evolution for MHC binding, our results further indicate selection for class II MHC preference.

Posttranslational regulation of TCR Valpha allelic exclusion during T cell differentiation

We have previously shown that phenotypic allelic exclusion of TCR alpha-chain is functional only in mature thymocytes. A significant proportion of immature thymocytes (TCRlow) express more than one cell surface alpha-chain, but mature thymocytes (TCRhigh) show phenotypic allelic exclusion and express only a single alpha-chain. We have analyzed thymocytes for both surface and intracellular alpha-chain expression and find that the majority of mature thymocytes express a second alpha-chain intracellularly. This result is predicted by a model in which the developmentally regulated allelic exclusion of the TCR alpha-chain is caused by competition between alpha-chains for the beta-chain rather than by models in which one alpha-chain is down-regulated or in which selection favors cells with only a single alpha-chain species. Changes in the relative amounts of alpha- and beta-chains available for pairing may therefore allow competition between the two alpha-chains for the beta-chain. Peripheral T cells also frequently express second alpha-chains in the cytoplasm (18-27%), despite a rather low frequency of dual alpha-chain expression on the cell surface (2-4%). The frequency of nonsurface expressed alpha-chains is reduced somewhat compared with thymocytes, indicating that an additional level of control of allelic exclusion operates during the maturation of peripheral T cells.

Polymorphism within a TCRAV family influences the repertoire through class I/II restriction

Antibody-staining experiments have shown that closely related members of the TCRAV3 family are reciprocally selected into the CD4 or CD8 peripheral T cell subsets. This has been attributed to the individual AV3 members interacting preferentially with either MHC class I or MHC class II molecules. Single amino acid residues present in the complementarity-determining regions (CDR) CDR1alpha and CDR2alpha are important in determining MHC class specificity. We have now extended these observations to survey the expressed repertoire of the AV3 family in C57BL/6 mice. Three of the four expressed AV3 members are preferentially selected into the CD4+ subset of T cells. These share the same amino acid residue in both CDR1alpha and CDR2alpha that differ from the only CD8-skewed member. Preferential expression of an individual AV3 is not caused by other endogenous alpha- or beta-chains, by any conserved CDR3 sequence, or by the usage of TCRAJ regions. This study shows that residues in the CDR1 and CDR2 regions are primary determinants for MHC class discrimination and suggests that polymorphism found within a TCRAV family has an important effect on the overall shaping of the T cell repertoire.

V alpha 3.2 selection in MHC class I mutant mice: evidence for an alternate orientation of TCR-MHC class I interaction

TCR V alpha elements are expressed preferentially in CD4 or CD8 subsets in a manner that is largely independent of MHC haplotype. It is likely that the V alphas interact preferentially with conserved regions of class I or class II molecules. To investigate the topology of binding of TCR to MHC-peptide complexes, we screened a panel of H-2Kbm mutants for differential V alpha expression. One strain, bm23, showed a consistent alteration in V alpha expression, with increased V alpha 3.2 expression in CD8 peripheral T cells. This overselection is manifest in CD8 single-positive thymocytes and appears to be due to enhanced positive selection on Kbm23. There is no apparent effect of V beta elements. The Kbm23 molecule is unique compared with Kb and the other Kbm molecules at residue 75 on the helix of the alpha 1 domain, suggesting an interaction between V alpha 3.2 and the alpha 1 helix at this point. Such an interaction is inconsistent with the orientation of TCR and MHC defined in two crystal structures, but is consistent with an orientation where the TCR is rotated by 180 degrees relative to MHC.

Natural killer cells: influence of the home environment

Recent results show that the repertoire of anti-MHC class 1-specific inhibitory receptors expressed by natural killer cells is influenced by self class 1 molecules in such a way as to minimize the number of cells with multiple self-reactive inhibitory receptors.

V alpha 3.2 selection in MHC class I mutant mice: evidence for an alternate orientation of TCR-MHC class I interaction

TCR V alpha elements are expressed preferentially in CD4 or CD8 subsets in a manner that is largely independent of MHC haplotype. It is likely that the V alphas interact preferentially with conserved regions of class I or class II molecules. To investigate the topology of binding of TCR to MHC-peptide complexes, we screened a panel of H-2Kbm mutants for differential V alpha expression. One strain, bm23, showed a consistent alteration in V alpha expression, with increased V alpha 3.2 expression in CD8 peripheral T cells. This overselection is manifest in CD8 single-positive thymocytes and appears to be due to enhanced positive selection on Kbm23. There is no apparent effect of V beta elements. The Kbm23 molecule is unique compared with Kb and the other Kbm molecules at residue 75 on the helix of the alpha 1 domain, suggesting an interaction between V alpha 3.2 and the alpha 1 helix at this point. Such an interaction is inconsistent with the orientation of TCR and MHC defined in two crystal structures, but is consistent with an orientation where the TCR is rotated by 180 degrees relative to MHC.

Selection of phage-displayed superantigen by binding to cell-surface MHC class II

We have expressed the superantigen staphylococcal enterotoxin A (SEA) on the surface of bacteriophage as a fusion with the gene VIII protein (gVIIIp). This phage-displayed superantigen retains the properties inherent in the natural protein. It binds to MHC class II and activates T-cells bearing appropriate V beta regions. A flexible 5-amino acid linker sequence between the SEA molecule and the phage coat protein improved the production of functional phage-displayed SEA. Binding to MHC class II-expressing cells effectively selected SEA-phage from non-SEA-phage background. This indicates that this will be an effective method for selecting new specificities of superantigen from libraries of SEA mutants and for cloning of novel superantigens.

Control of MHC restriction by TCR Valpha CDR1 and CDR2

Individual T cell receptor (TCR) Valpha elements are expressed preferentially in CD4 or CD8 peripheral T cell subsets. The closely related Valpha3.1 and Valpha3.2 elements show reciprocal selection into CD4 and CD8 subsets, respectively. Transgenic mice expressing site-directed mutants of a Valpha3.1 gene were used to show that individual residues in either the complementarity-determining region 1 (CDR1) or CDR2 were sufficient to change selection from the CD4 subset to the CD8 subset. Thus, the germline-encoded Valpha elements are a major influence on major histocompatibility class complex (MHC) restriction, most likely by a preferential interaction with one or the other class of MHC molecule.

T-cell-receptor affinity and thymocyte positive selection

Development of thymocytes involves two distinct outcomes resulting from superficially similar events. Recognition by thymocytes of major histocompatibility complex (MHC) proteins plus peptides leads to their rescue from apoptosis (positive selection), and recognition of antigenic peptide induces cell death (negative selection). Antigen analogues, and sometimes low concentrations of antigenic peptide, induce positive selection; such analogues are often antagonists of mature T-cell clones. Various models seek to explain how recognition of different peptide/MHC complexes leads to such different outcomes: quantitative models relate response to the affinity, avidity or kinetics of T-cell-antigen receptor (TCR) binding, whereas qualitative models require conformational or spatial changes in the TCR or associated molecules to modulate signal transduction. We have used surface plasmon resonance to measure the kinetics of TCR interactions with positively and negatively selecting ligands to distinguish between these models, and find that affinity correlates to the outcome of selection. A 'window' of affinity resulting in positive selection extends over a 1-log range starting threefold below the affinity for negative selection.

Allelic exclusion of mouse T cell receptor alpha chains occurs at the time of thymocyte TCR up-regulation

We report a detailed analysis of TCR V alpha and V beta chain expression on immature versus mature thymocytes of normal, TCR beta-transgenic, and TCR alpha-hemizygous mice. Chain pairing between TCR V alpha and V beta chains is random on immature thymocytes, but individual chain pairs are selected in mature thymocytes. This indicates that V alpha-V beta chain pairing preferences are determined during thymic selection, and not as a result of structural constraints. Dual V alpha chain expression is found frequently on immature, but not mature thymocytes. It is not found in TCR alpha-hemizygous mice, showing that cell surface expression of dual alpha chains is caused by lack of allelic exclusion in immature thymocytes. Down-regulation of one of the alpha chains occurs concurrently with differentiation from TCRlo, CD69- to TCRhi, CD69+ phenotype, suggesting that it is associated with positive selection of the functional TCR.

Selection of TCR V alpha by MHC class II predicts superantigen reactivity

Recognition of superantigens by T cells predominantly involves the TCR V beta region. The contribution of reactivity from the non-V beta portion of the TCR remains less clear. We have investigated the V alpha repertoire of T cells bearing one V beta element responding to different superantigen or polyclonal stimuli. The data indicate that the V alpha chain is not directly involved in superantigen recognition, since unrelated superantigens do not stimulate cells bearing different V alpha elements. Instead, analysis of V alpha elements used preferentially in the CD4+ or CD8+ cells, shows that the V alpha-regions predominating in the superantigen response are the same V alphas that are positively selected into the CD4+ lineage. Thus the ability of a V alpha-region to either aid or hinder the interaction with class II predicts its frequency in a superantigen-responding population of T cells.

Monoclonal antibody-superantigen fusion proteins: tumor-specific agents for T-cell-based tumor therapy

The bacterial superantigen staphylococcal enterotoxin A (SEA) is an extremely potent activator of T lymphocytes when presented on major histocompatibility complex (MHC) class II molecules. To develop a tumor-specific superantigen for cancer therapy, we have made a recombinant fusion protein of SEA and the Fab region of the C215 monoclonal antibody specific for human colon carcinoma cells. SEA as part of a fusion protein showed a > 10-fold reduction in MHC class II binding compared to native SEA, and accordingly, the affinity of the FabC215-SEA fusion protein for the C215 tumor antigen was approximately 100-fold stronger than to MHC class II molecules. The FabC215-SEA fusion protein efficiently targeted T cells to lyse C215+ MHC class II- human colon carcinoma cells, which demonstrates functional substitution of the MHC class II-dependent presentation of SEA with tumor specificity. Treatment of mice carrying B16 melanoma cells expressing a transfected C215 antigen resulted in 85-99% inhibition of tumor growth and allowed long-term survival of animals. The therapeutic effect was dependent on antigen-specific targeting of the FabC215-SEA fusion protein, since native SEA and an antigen-irrelevant FabC242-SEA fusion protein did not influence tumor growth. The results suggest that Fab-SEA fusion proteins convey superantigenicity on tumor cells, which evokes T cells to suppress tumor growth.

T-cell activation by superantigens

Superantigens stimulate powerful T-cell responses that can have marked effects in vivo, sometimes leading to shock or even death. The demonstration that strong T-cell responses to superantigens in vivo can be followed by tolerance, reflecting either clonal elimination or anergy, has provided important insights into how mature T cells can be regulated. Further progress in understanding the factors that control these responses relies heavily on defining the specific interactions between T-cell receptors, superantigens and major histocompatibility complex molecules which lead to T-cell activation as well as on the characterization of the specific signal transduction events and molecules involved in this activation. Significant progress has been made, during the past year, in the first area and these findings are summarized below; though less information is available in the latter area, recent observations relevant to this issue are discussed.

Interplay between superantigens and the immune system

Superantigens interact with the immune system by binding to major histocompatibility complex (MHC) class II proteins and activating T cells through the variable region of the T cell receptor beta-chain. Through this means they can cause massive proliferation and then death of a large proportion of T cells. Superantigens are produced by bacteria, mycoplasmas, retroviruses, and probably by other organisms. In some cases, the superantigen is crucial to the organism's life cycle. Mouse mammary tumor virus disseminates by activating T cells which stimulate the proliferation of B cells harboring the virus. In other cases, the superantigen may be responsible for the pathogenesis of the infection, such as in the case of Toxic Shock Syndrome. In this article, we review information on the diseases in which superantigens are involved, and the mechanisms by which the superantigens interact with T cell receptor and class II molecules.

T-cell receptor beta-chain binding to enterotoxin superantigens

The last few years have seen an enormous jump in our knowledge and understanding of T-cell activation by superantigens. Clearly, a great number of infectious and parasitic organisms utilize superantigens as part of a strategy to evade the immune response of their host. The ability to modulate superantigen effects will give us new means to fight infections, and the knowledge of T-cell activation that we have gained from study of superantigens will, in turn, allow us to modulate the immune system in new ways.

Interaction of the T cell receptor with bacterial superantigens

Bacterial toxin superantigens bind to MHC class II molecules and activate a large proportion of T cells through a direct interaction with the T cell receptor (TCR). The toxin: TCR interaction involves specific recognition between the beta-chain variable region and the toxin. Although a complete alpha beta T cell receptor is required for activation of T cells, studies using purified soluble T cell receptor beta-chain have shown that it alone is sufficient for binding the toxin: class II complex. The regions of V beta and enterotoxin involved in the recognition have been determined.

Enterotoxin residues determining T-cell receptor V beta binding specificity

Superantigens such as the staphylococcal enterotoxins bind to major histocompatibility complex (MHC) class II molecules and activate T cells through a specific interaction between the V beta region of the T-cell antigen receptor (TCR) and the toxin. The TCR beta-chain alone is sufficient to produce the interaction with the enterotoxin-class II complex. Identification of the regions of enterotoxins that interact with TCR has so far proved equivocal because of difficulties in distinguishing between direct effects on T-cell recognition and indirect effects resulting from alteration of binding to class II. For example, amino-terminal truncations of SEB abrogated T-cell stimulation whereas carboxy-terminal truncation of SEA stopped its mitogenic activity. The most comprehensive study to date, accounting for both enterotoxin binding to class II and enterotoxin interactions with the TCR, identified two functionally important regions for SEB binding to TCR. Although the amino-acid sequences of staphylococcal enterotoxins A and E are 82% identical, they activate T cells bearing different V beta elements. We have assayed the binding of cells coated with these enterotoxins to soluble secreted TCR beta-chain protein and find that V beta 3 binds enterotoxin A but not E, whereas V beta 11 binds enterotoxin but not A. To map the amino-acid residues responsible for these different binding specificities, we prepared a series of hybrids between the two staphylococcal enterotoxins. We report that just two amino-acid residues near the carboxy terminus of the enterotoxins are responsible for the discrimination between these molecules by V beta 3 and V beta 11.(ABSTRACT TRUNCATED AT 250 WORDS)

T-cell antigen receptor binding sites for the microbial superantigen staphylococcal enterotoxin A

We have examined the interaction of the microbial superantigen staphylococcal enterotoxin A (SEA) with peptides corresponding to overlapping regions of the T-cell antigen receptor beta chain variable region V beta 3. SEA is known to stimulate murine T cells bearing certain V beta elements, among them V beta 3. Five peptides were synthesized representing amino acids 1-24, 20-44, 39-60, 57-77, and 74-95 of V beta 3. We demonstrate here that soluble V beta 3-bearing beta chains can bind to a complex of SEA and major histocompatibility complex class II and that the synthetic peptide V beta 3-(57-77) blocked this interaction. The peptide V beta 3-(57-77) also inhibited SEA-induced interferon-gamma production and SEA-induced proliferation of B10.BR spleen cells. Conversely, the peptide corresponding to amino acids 57-77 of V beta 8.2, a V beta element that is not recognized by SEA, decreased staphylococcal enterotoxin C-2-induced proliferation but did not affect SEA-induced proliferation. The peptide inhibition of SEA-induced function was due at least in part to inhibition of V beta 3-bearing T-cell activity, since the percentage of T cells reactive with an anti-V beta 3 monoclonal antibody was significantly reduced by V beta 3-(57-77). These data suggest that the region of V beta 3 encompassing amino acids 57-77 is an area that displays the appropriate sequence and conformation for binding of the SEA molecule and blocking of the resultant interaction with the T-cell antigen receptor.

Chromosome 14 in B10.A(18R) mice is recombinant and includes Tcra-V a alleles

Analysis of mouse Tcr genes has previously defined at least five different Tcra-V haplotypes among inbred strains of mice. For mice of the Tcra-Vb haplotype, including C57BL/10 (B10), T-cell expression of the Tcra-V11 gene subfamily can be detected with a monoclonal antibody, 1.F2. In the course of further characterizing the specificity of 1.F2, we found that it fails to recognize Tcra-V11-expressing T-cell hybrids derived from the B10 congenic strain, B10.A(18R)/SgIcr. Moreover, staining analysis indicated that the Va11 epitope recognized by 1.F2 is not expressed by peripheral T cells from several different B10.A(18R) colonies with the exception of that at the Research Institute of Scripps Clinic. Nucleotide sequences were determined for cDNA representing rearranged Tcra-V11 genes from two independent, B10.A(18R)/SgIcr derived T-cell hybrids. The two Tcra-V11 gene segments were identical and the predicted amino acid sequence differed by at least five residues from Tcra-V11 sequences previously obtained from B10.A mice. Southern blot analysis of restriction fragment length polymorphisms (RFLP) associated with Tcra-V11, as well as Tcra-V1, subfamily genes revealed that the B10.A(18R) mouse has inherited Tcra-Va alleles rather than the expected Tcra-Vb alleles from the B10 strain. RFLP analysis of the Rib-1 locus, located in close proximity to the Tcra locus on chromosome 14, showed that B10.A(18R) carries the Rib-1b allele from B10. These results indicate that the B10.A(18R) mouse has inherited a recombinant chromosome 14 with a recombination event having occurred between the Rib-1 locus and the Tcra-V gene subfamilies examined. Inheritance of Tcra-Va alleles in B10.A(18R) probably originated from strain 129/J which breeding records show was used in the first cross with B10.A in the production of B10.A(18R) and which we found exhibits Tcra-V11a RFLPs.

Profound alteration in an alpha beta T-cell antigen receptor repertoire due to polymorphism in the first complementarity-determining region of the beta chain

Amino acid residues that are critical in maintaining the framework structure of immunoglobulin heavy- and light-chain variable (V) regions are strongly conserved in the V alpha and V beta proteins of the alpha beta T-cell antigen receptor (TCR alpha beta). Consequently, it has been proposed that TCR alpha beta has a conformation similar to that of an immunoglobulin Fab fragment and that the regions of the TCR homologous to the three immunoglobulin complementarity-determining regions (CDRs 1, 2, and 3) bind to the peptide antigen-major histocompatibility complex (MHC) molecule ligand. A single amino acid substitution in the predicted CDR1 of the V beta 3 protein of certain mouse strains dramatically altered TCR alpha beta usage in an antigen-specific MHC-restricted immune response but did not abrogate V beta 3 specificity for the superantigens minor lymphocyte stimulatory locus (Mls)c and staphylococcal enterotoxin A (SEA). The results confirm the importance of the V beta CDR1 in antigen-MHC molecule recognition, supporting the Fab-like structural model of TCR alpha beta, and provide further evidence that conventional antigen-MHC recognition and superantigen recognition are mediated by distinct regions of the TCR beta chain. They also suggest that allelic polymorphism may be a significant source of diversity in the TCR repertoire.

The T cell receptor V alpha 11 gene family. Analysis of allelic sequence polymorphism and demonstration of J alpha region-dependent recognition by allele-specific antibodies

Allelic polymorphism in TCR loci may play an important role in shaping the T cell repertoire and in disease susceptibility. We have used a combination of antibody and sequence analysis to investigate polymorphism in the murine V alpha 11 family. Two different antibodies have been analyzed that recognize particular V alpha 11 family members of the V alpha b and V alpha d haplotypes. One antibody shows J alpha dependency, suggesting a conformational element to the epitope. Investigation of the anti-V alpha 11 staining pattern on different mouse strains indicates that there is a marked influence of MHC haplotype on V alpha 11 selection and that V alpha 11 is preferentially expressed on CD4+ cells. Sequence analysis of V alpha 11 genes from the V alpha a, V alpha b, and V alpha d haplotypes shows two potential regions for the haplotype-specific epitopes. The relatedness of the different V alpha 11 family members from different haplotypes suggests that the V alpha 11.1/11.2 gene duplication is relatively recent, but that V alpha 11.3 separated much earlier. Differences between V alpha 11.3 and V alpha 11.1/11.2 are concentrated in the putative complementarity determining regions (CDR), whereas differences between alleles are not clearly clustered. However, the V alpha 11.1a and V alpha 11.1d alleles differ from V alpha 11.1b and V alpha 11.2b in CDR1. A V alpha 11.2-expressing anti-cytochrome c T cell has the same V-J junction as a V alpha 11.1-bearing cell with a similar fine specificity, indicating that V alpha 11.1b and V alpha 11.2b do not contribute different Ag specificities.

The T cell receptor V alpha 11 gene family. Analysis of allelic sequence polymorphism and demonstration of J alpha region-dependent recognition by allele-specific antibodies

Allelic polymorphism in TCR loci may play an important role in shaping the T cell repertoire and in disease susceptibility. We have used a combination of antibody and sequence analysis to investigate polymorphism in the murine V alpha 11 family. Two different antibodies have been analyzed that recognize particular V alpha 11 family members of the V alpha b and V alpha d haplotypes. One antibody shows J alpha dependency, suggesting a conformational element to the epitope. Investigation of the anti-V alpha 11 staining pattern on different mouse strains indicates that there is a marked influence of MHC haplotype on V alpha 11 selection and that V alpha 11 is preferentially expressed on CD4+ cells. Sequence analysis of V alpha 11 genes from the V alpha a, V alpha b, and V alpha d haplotypes shows two potential regions for the haplotype-specific epitopes. The relatedness of the different V alpha 11 family members from different haplotypes suggests that the V alpha 11.1/11.2 gene duplication is relatively recent, but that V alpha 11.3 separated much earlier. Differences between V alpha 11.3 and V alpha 11.1/11.2 are concentrated in the putative complementarity determining regions (CDR), whereas differences between alleles are not clearly clustered. However, the V alpha 11.1a and V alpha 11.1d alleles differ from V alpha 11.1b and V alpha 11.2b in CDR1. A V alpha 11.2-expressing anti-cytochrome c T cell has the same V-J junction as a V alpha 11.1-bearing cell with a similar fine specificity, indicating that V alpha 11.1b and V alpha 11.2b do not contribute different Ag specificities.

Direct binding of secreted T-cell receptor beta chain to superantigen associated with class II major histocompatibility complex protein

The interaction of the T-cell receptor (TCR) with peptide antigen plus major histocompatibility complex (MHC) protein requires both alpha and beta chains of the TCR. The "superantigens" are a group of molecules that are recognized in association with MHC class II but that do not appear to conform to this pattern. Superantigens are defined as such because they cause the activation or thymic deletion of many or all T cells bearing specific TCR beta-chain variable region (V beta) elements. The strong association of particular V beta S with T-cell responses to superantigens suggests that their interaction with the TCR is fundamentally different from that of most antigens. We have directly investigated the involvement of the beta chain in recognition of a superantigen by using a secreted, truncated TCR beta chain and the bacterial superantigen staphylococcal enterotoxin A complexed to cell-surface MHC class II. We demonstrate that this interaction is specific for the enterotoxin and is dependent on MHC class II expression by the cell. The reaction can be inhibited by antibodies against the three components of the reaction: V beta, enterotoxin, and class II. This shows that the TCR beta chain is sufficient to mediate the interaction with a superantigen-class II complex. The TCR alpha chain and co-receptors such as CD4 are not required.

A T cell receptor V alpha region selectively expressed in CD4+ cells

The peripheral TCR V beta repertoire is strongly influenced by the processes of negative selection (deletion) and positive selection in the thymus. In order to investigate whether such selection events influence the V alpha repertoire, we have produced an anti-V alpha 11 mAb. This antibody was made by immunization with a chimeric TCR:Ig protein containing V alpha 11 in place of the VH of an IgG2a, lambda Ig. This scheme optimizes the specificity of immunization and facilitates the screening procedure. The antibody recognizes a panel of V alpha 11-expressing T cell clones. Analysis of mouse strains indicates that the antibody recognizes V alpha 11 only in mice of the C57 background. The expression of the epitope on peripheral T cells is strongly biased to the CD4+ subset, suggesting positive selection of V alpha 11 on class II MHC molecules. In some strain comparisons, the percentage of V alpha 11-expressing T cells in the CD4+ subset was elevated in I-E+ relative to I-E- strains. These data suggest that V alpha 11 can differentially influence the selection of T cells into the CD4+/CD8+ subsets.

A T cell receptor V alpha region selectively expressed in CD4+ cells

The peripheral TCR V beta repertoire is strongly influenced by the processes of negative selection (deletion) and positive selection in the thymus. In order to investigate whether such selection events influence the V alpha repertoire, we have produced an anti-V alpha 11 mAb. This antibody was made by immunization with a chimeric TCR:Ig protein containing V alpha 11 in place of the VH of an IgG2a, lambda Ig. This scheme optimizes the specificity of immunization and facilitates the screening procedure. The antibody recognizes a panel of V alpha 11-expressing T cell clones. Analysis of mouse strains indicates that the antibody recognizes V alpha 11 only in mice of the C57 background. The expression of the epitope on peripheral T cells is strongly biased to the CD4+ subset, suggesting positive selection of V alpha 11 on class II MHC molecules. In some strain comparisons, the percentage of V alpha 11-expressing T cells in the CD4+ subset was elevated in I-E+ relative to I-E- strains. These data suggest that V alpha 11 can differentially influence the selection of T cells into the CD4+/CD8+ subsets.

Transport and secretion of truncated T cell receptor beta-chain occurs in the absence of association with CD3

The T cell receptor (TCR) beta-chain is produced in the endoplasmic reticulum where it associates with the TCR alpha-chain and the members of the CD3 complex to form the complete receptor. When the other chains of the complex are not available, the beta-chain is rapidly degraded within the endoplasmic reticulum. When incomplete TCR.CD3 complexes are formed, they are transported through the Golgi apparatus and degraded in lysosomes. In this study, a truncated form of the TCR beta-chain has been made by removal of the transmembrane and cytoplasmic segments. Unlike the normal beta-chain, the truncated molecule is stable and is transported through the Golgi apparatus and secreted. This process occurs at a similar rate in both T and B cells, indicating that it is not affected by the presence or absence of CD3 components. These data suggest that an element in the transmembrane or cytoplasmic region of the beta-chain confers sensitivity to the degradative control mechanisms that regulate TCR expression.

Selective development of CD4+ T cells in transgenic mice expressing a class II MHC-restricted antigen receptor

T lymphocytes are predisposed to recognition of foreign protein fragments bound to cell-surface molecules encoded by the major histocompatibility complex (MHC). There is now compelling evidence that this specificity is a consequence of a selection process operating on developing T lymphocytes in the thymus. As a result of this positive selection, thymocytes that express antigen receptors with a threshold affinity for self MHC-encoded glycoproteins preferentially emigrate from the thymus and seed peripheral lymphoid organs. The specificity for both foreign antigen and MHC molecules is imparted by the alpha and beta chains of the T-cell antigen receptor (TCR). Two other T-cell surface proteins, CD4 and CD8, which bind non-polymorphic regions of class II and class I MHC molecules respectively, are also involved in these recognition events and play an integral role in thymic selection. In order to elucidate the developmental pathways of class II MHC-restricted T cells in relation to these essential accessory molecules, we have produced TCR-transgenic mice expressing a receptor specific for a fragment of pigeon cytochrome c and the Ek (class II MHC) molecule. The transgenic TCR is expressed on virtually all T cells in mice expressing Ek. The thymuses of these mice contain an abnormally high percentage of mature CD4+CD8- cells. In addition, the peripheral T-cell population is almost exclusively CD4+, demonstrating that the MHC specificity of the TCR determines the phenotype of T cells during selection in the thymus.

Secretion of a chimeric T-cell receptor-immunoglobulin protein

To produce sufficient quantities of soluble T-cell receptor protein for detailed biochemical and biophysical analyses we have explored the use of immunoglobulin--T-cell receptor gene fusions. In this report we describe a chimeric gene construct containing a T-cell receptor alpha-chain variable (V) domain and the constant (C) region coding sequences of an immunoglobulin gamma 2a molecule. Cells transfected with the chimeric gene synthesize a stable protein product that expresses immunoglobulin and T-cell receptor antigenic determinants as well as protein A binding sites. We show that the determinant recognized by the anticlonotypic antibody A2B4.2 resides on the V alpha domain of the T-cell receptor. The chimeric protein associates with a normal lambda light chain to form an apparently normal tetrameric (H2L2, where H = heavy and L = light) immunoglobulin molecule that is secreted. Also of potential significance is the fact that a T-cell receptor V beta gene in the same construct is neither assembled nor secreted with the lambda light chain, and when expressed with a C kappa region it does not assemble with the chimeric V alpha C gamma 2a protein mentioned above. This indicates that not all T-cell receptor V regions are similar enough to immunoglobulin V regions for them to be completely interchangeable.

Expression of T cell receptor genes in an antigen-specific hybridoma and radiation-induced variants

We have analyzed a series of mutants derived from a KLH-specific, I-E-restricted T hybridoma (FN1-18) which have lost antigen-reactivity while retaining both T cell receptor idiotypic determinants and the ability to respond to Con A. The variants have not gained any detectable alloreactivity, nor is there an obvious lesion in the mutants' beta chain DNA containing the utilized beta chain genes. This loss of antigen reactivity is due to a failure of stable production of the specific V beta-containing mRNA. Our results indicate that in FN1-18, the T cell receptor antigenic determinants are most likely carried by the alpha chain alone or by a complementation product of the V alpha FN1-18 with the V beta of BW5147. V beta FN1-18 represents a previously undescribed T cell receptor V region.

T-cell receptor gene structure and function

Recent progress in the serology, biochemistry, and now the molecular genetics of T-cell receptor molecules has brought within reach the prospect of solving some of the most basic questions about the nature of T-cell recognition. These include the exact nature of the receptor-major histocompatibility complex (MHC)-antigen recognition event and the sequential expression of T-cell receptor molecules in the thymus.

Variability and repertoire size of T-cell receptor V alpha gene segments

The immune system of higher organisms is composed largely of two distinct cell types, B lymphocytes and T lymphocytes, each of which is independently capable of recognizing an enormous number of distinct entities through their antigen receptors; surface immunoglobulin in the case of the former, and the T-cell receptor (TCR) in the case of the latter. In both cell types, the genes encoding the antigen receptors consist of multiple gene segments which recombine during maturation to produce many possible peptides. One striking difference between B- and T-cell recognition that has not yet been resolved by the structural data is the fact that T cells generally require a major histocompatibility determinant together with an antigen whereas, in most cases, antibodies recognize antigen alone. Recently, we and others have found that a series of TCR V beta gene sequences show conservation of many of the same residues that are conserved between heavy- and light-chain immunoglobulin V regions, and these V beta sequences are predicted to have an immunoglobulin-like secondary structure. To extend these studies, we have isolated and sequenced eight additional alpha-chain complementary cDNA clones and compared them with published sequences. Analyses of these sequences, reported here, indicate that V alpha regions have many of the characteristics of V beta gene segments but differ in that they almost always occur as cross-hybridizing gene families. We conclude that there may be very different selective pressures operating on V alpha and V beta sequences and that the V alpha repertoire may be considerably larger than that of V beta.