T cell receptor antagonists and partial agonists

CD4 and CD8: modulators of T-cell receptor recognition of antigen and of immune responses?

The response of T cells to antigen involves the participation of a number of distinct receptor-ligand engagements. The major players in the recognition of complexes of major histocompatibility complex molecules and peptide antigens are the T-cell receptors and the co-receptors CD4 and CD8. Progress in understanding the physical structures of these molecules, and how complexes between them are formed, is helping our understanding of how they participate in regulating the signals transduced to T cells.

Mechanisms of viral clearance and persistence

Using examples predominantly drawn from study of the lymphocytic choriomeningitis virus (LCMV) model system, this review describes the mechanisms involved in control of virus infections by the cell-mediated immune response, and some of the different strategies viruses have evolved to evade such immune clearance so that they can persist in their hosts. The important role played by the CD8+ cytotoxic lymphocyte (CTL) response in clearance of many systemic virus infections is discussed; and it is emphasized that although CD8+ CTL are classically thought of as lymphocytes which mediate lysis of virus-infected target cells, the principal mechanism by which CD8+ T cells effect clearance of persistent and many acute virus infections via production of antiviral cytokines such as tumour necrosis factor-alpha and interferon-gamma, not via destruction of virus-producing cells. To avoid immune-mediated clearance, viruses frequently use a combination of several different strategies. These can be grouped into mechanisms for avoiding recognition by the immune response (such as establishing latent infections, replicating in immune-privileged sites, down-regulating the expression of immune recognition signals on the surface of infected cells, or undergoing antigenic variation); and mechanisms for suppressing the immune response. The latter include generalized immune suppression mechanisms, and strategies for more precisely disabling the specific immune response such as inducing tolerance or exhaustion of virus-specific CTL. The value of understanding both immune clearance mechanisms and viral evasion strategies in the rational design of immune-based therapies to combat persistent virus infections is discussed.

Positive Selection of Thymocytes Expressing the Same TCR by Different MHC Ligands Results in the Production of Functionally Distinct Thymocytes Distinguished by Differential Expression of the Heat Stable Antigen

The issue of whether the signaling process during positive selection can affect the efficiency by which the positively selected T cells respond to antigenic stimulation has not been addressed. We approached this question by determining the consequences of positive selection of a particular transgenic TCR (2C TCR) in the H-2b and the H-2k thymus. The H-2b thymus provides a strong positive-selecting environment for the 2C TCR, whereas the H-2k thymus selects weakly for the 2C TCR. Although the positively selected CD8 thymocytes from the H-2b or H-2k thymus expressed similar levels of the CD8 coreceptor molecule, those for the H-2k thymus expressed a slightly lower level of the 2C TCR. This lower level of 2C TCR expression by H-2k CD8 thymocytes was not a result of coexpression of endogenous TCRs. Interestingly, CD8 thymocytes from H-2k mice were hyporesponsive to Ag stimulation compared with those from the H-2b mice. The functional maturity of positively selected CD8 thymocytes from the H-2b or H-2k thymus was inversely correlated with the level of heat stable Ag expressed by these cells. Furthermore, TCR-derived signals appear to be more efficiently coupled to downstream pathways leading to proliferation and cytokine production in CD8 thymocytes from H-2b 2C mice than those derived from H-2k 2C mice. These results provide the first demonstration that the intensity of the signaling process during positive selection affects the efficiency by which TCR-derived signals in positively selected thymocytes are coupled to downstream effector pathways.

Persistence of peptide-induced CD4+ T cell anergy in vitro

Clonal T cell unresponsiveness, or anergy, has been proposed as a mechanism of peripheral tolerance in vivo, and as a potential means of curbing unwanted T cell responses. In this study, anergy was induced in a T helper cell (Th) clone reactive to hemoglobin (Hb) peptide 64-76 by coculture of the T cells with live antigen-presenting cells (APCs) and 74L, a peptide analog of Hb(64-76) that contains a single amino acid substitution of leucine for glycine at position 74, or with a low concentration of the agonist ligand. The anergic state was characterized by blunted proliferation and interleukin (IL) 2 production upon restimulation with Hb(64-76), and was not the result of impaired TCR/CD3 downmodulation. The addition of exogenous IL-12 transiently restored proliferation of the anergic lines, but removal of IL-12 from culture returned the T cells to their nonproliferative state. Interestingly, persistence of the anergic phenotype was observed despite biweekly restimulation with antigen, APCs, and IL-2. Thus, T cell unresponsiveness induced by a peptide produced a stable, persistent anergic state in a Th0 clone that was not reversible by stimulation with IL-2 or -12.

Human T-cell response to myelin basic protein peptide (83-99): extensive heterogeneity in antigen recognition, function, and phenotype

Multiple sclerosis (MS) is considered a T cell-mediated autoimmune disease, and myelin proteins are the most likely candidate autoantigens. Based on experiments performed in experimental allergic encephalomyelitis (EAE), innovative immunotherapies have been developed that target either the specific trimolecular complex of encephalitogenic T cells, consisting of T-cell receptor (TCR), major histocompatibility complex (MHC; HLA in humans) class II molecule, and autoantigenic peptide, or the effector functions of these cells. To provide the basis for the transfer of these specific immunotherapies to MS, we extensively characterized the human T-cell response to one major myelin epitope, the myelin basic protein peptide (83-99). We analyzed restriction element, TCR usage and affinity, fine specificity, cytokine production, cytolytic activity, and expression of surface molecules on 41 T-cell clones (TCCs) derived from MS patients and normal controls. We demonstrate a high degree of complexity of recognition patterns as well as of functional phenotypes among T cells responding to the same epitope. In contrast to results from animal models, these findings indicate that the design of epitope-based specific immunotherapies for MS is more difficult than previously thought.

Minors held by majors: the H13 minor histocompatibility locus defined as a peptide/MHC class I complex

The products of minor histocompatibility (H) loci are serious barriers to tissue transplantation even among major histocompatibility complex (MHC) identical individuals, frequently causing chronic graft rejection and graft versus host disease. Over 50 minor H loci map to mouse autosomal chromosomes but none are known at the molecular level. By expression cloning, we identified the H13 locus, a classical minor H locus first detected 30 years ago by the trait of graft rejection. The H13a allele is located on chromosome 2 and encodes a novel protein that yields the rare naturally processed nonapeptide SSVVGVWYL (SVL9) for presentation by the Db MHC class I molecule. The SVL9 peptide binds Db MHC despite the absence of the consensus binding motif, and a conservative methyl group substitution (Valine 4 <--> Isoleucine) explains why reciprocal T cell responses are elicited in H13a and H13b congenic strains.

A primer on cytokines: sources, receptors, effects, and inducers

Protection against pathogens is a prerequisite for survival of most organisms. To cope with this continuous challenge, complex defense mechanisms have evolved. The construction, adaptation, and maintenance of these mechanisms are under control of an extensive network of regulatory proteins called cytokines. A great number of cytokines have been described over the last 2 decades. This review consists of an overview of cytokines that are involved in immune responses and describes some historical and general aspects as well as prospective clinical applications. Major biological effects together with information on cytokine receptors, producers, inducers, and biochemical and molecular characteristics are listed in tables. In addition, some basic information is given on cytokine receptor signal transduction. Finally, the recent discoveries of cytokine receptors functioning as coreceptors in the pathogenesis of human immunodeficiency virus are summarized.

What can we learn about human immunodeficiency virus infection from a study of lymphocytic choriomeningitis virus?

The role of cytotoxic T lymphocytes (CTL) in human immunodeficiency virus (HIV) infection remains elusive. Since the discovery 10 years ago of high levels of specific CTL in this disease, some have argued that they play an important role in virus control, others that they drive disease progression through destruction of T helper cells, and others still that they play no obvious role at all. By contrast, the central role of CTL in murine lymphocytic choriomeningitis virus (LCMV) infection has been very clearly worked out through the use of in vivo depletion and adoptive transfer experiments, as well as knockout and transgenic mice. To interpret the possible roles for CTL in HIV, we have therefore made a comparison between what is known about CTL and their interaction with virus-infected cells in these two infections. This illustrates a potential critical role for these cells in both control of HIV replication and immune-mediated pathology, but one that is highly dependent on virus dose, distribution and dynamics.

CD4 augments the response of a T cell to agonist but not to antagonist ligands

The recognition of peptide variants by the T cell receptor (TCR) has revealed a wide range of possible responses. Here, using a series of CD4+ and CD4- variants of the same T cell hybridoma, we find that while the expression of CD4 converts weak agonists into full agonists, none of the antagonist peptides are efficiently recognized as agonists. Furthermore, in antagonist assays, little difference can be seen in the response of CD4+ and CD4- T cells. Together with previous work showing a marked difference in stability between TCR binding to agonist versus antagonist ligands, these data suggest that CD4 engagement occurs after a TCR-peptide/MHC complex has formed and that it requires a certain minimal half-life of the ternary complex to be fully engaged in signaling.

Double-positive T cell receptor(high) thymocytes are resistant to peptide/major histocompatibility complex ligand-induced negative selection

To investigate negative selection events during intrathymic ontogeny, we established T cell receptor (TCR)-transgenic mice [N15tg/RAG-2-/- (H-2b)] expressing a single TCR specific for vesicular stomatitis virus nuclear octapeptide N52-59 (VSV8) in the context of the major histocompatibility complex (MHC) class I molecule, K(b). Administration of VSV8 in vivo induced apoptosis in less than 4 h, deleting the majority of immature double-positive (DP) thymocytes by 24 h. In contrast, DP TCRhigh as well as single-positive (SP) thymocytes were refractory to this death process. Moreover, DP TCRhigh cells differentiated into SP thymocytes in vitro and in vivo, maturing into functional cytotoxic T lymphocytes upon intrathymic transfer to beta RAG 2-/- recipients. Hence, negative selection processes involving MHC-bound peptide ligands are operative only prior to the late DP thymocyte stage in this MHC class I-restricted TCR transgene system.

Topology of T cell receptor-peptide/class I MHC interaction defined by charge reversal complementation and functional analysis

The molecular interactions between the CD8 co-receptor dependent N15 and N26 T cell receptors (TCRs) and their common ligand, the vesicular stomatitis virus octapeptide (VSV8) bound to H-2Kb, were studied to define the docking orientation(s) of MHC class I restricted TCRs during immune recognition. Guided by the molecular surfaces of the crystallographically defined peptide/MHC and modeled TCRs, a series of mutations in exposed residues likely contacting the TCR ligand were analyzed for their ability to alter peptide-triggered IL-2 production in T cell transfectants. Critical residues which diminished antigen recognition by 1000 to 10,000-fold in molar terms were identified in both N15 Valpha (alphaE94A or alphaE94R, Y98A and K99) and Vbeta (betaR96A, betaW97A and betaD99A) CDR3 loops. Mutational analysis indicated that the Rp1 residue of VSV8 is critical for antigen recognition of N15 TCR, but R62 of H-2Kb is less critical. More importantly, the alphaE94R mutant could be fully complemented by a reciprocal charge reversal at Kb R62 (R62E). This result suggests a direct interaction between N15 TCR Valpha E94R and Kb R62E residues. As Rp1 of VSV8 is adjacent to R62 in the VSV8/Kb complex and essential for T cell activation, this orientation implies that the N15 Valpha CDR3 loop interacts with the N-terminal residues of VSV8 with the Valpha domain docking to the Kb alpha2 helix while the N15 Vbeta CDR3 loop interacts with the more C-terminal peptide residues and the Vbeta domain overlies the Kb alpha1 helix. An equivalent orientation is suggested for N26, a second VSV8/Kb specific TCR. Given that genetic analysis of two different class II MHC-restricted TCRs and two crystallographic studies of class I restricted TCRs offers a similar overall orientation of V domains relative to alpha-helices, these data raise the possibility of a common docking mode between TCRs and their ligands regardless of MHC restriction.

In vivo longitudinal analysis of a dominant TCR repertoire selected in human response to influenza virus

Recent studies have demonstrated biased usage of TCR V beta 17 and a high degree of diversity in J beta usage within the influenza virus matrix epitope (M.58-66)-specific CTL response. In contrast, in the course of a study on the cellular response to influenza A virus, we found preferential usage of V beta 17-J beta 2.2 rearrangement in an individual with an unexpectedly high number of CTL precursors (CTLp). We took advantage of such situation to study the longitudinal repertoire of the CD8+ T cell precursors. By limiting dilution analysis combined with the use of a clonotypic primer corresponding to the CDR3 region of this matrix-specific TCR V beta chain, the influenza-specific CTLp were shown to be stable for a period of 6 years. Overall, our results show that virus-specific CTLp can be directly monitored in vivo by molecular fingerprinting without in vitro restimulation. These findings might be extremely important for evaluation of the specific immune response to a given human pathogen.

Dynamics of HIV variants and specific cytotoxic T-cell recognition in nonprogressors and progressors

Infection with the human immunodeficiency virus (HIV) results in a disease characterized by a rapid viral replication, immunodeficiency and chronic immune activation. The vigorous polyspecific cytotoxic T-cell (CTL) response directed against multiple HIV epitopes reduces HIV-infected cell numbers, although unable to eradicate the virus. The plasticity of the specific CTL repertoire ensures adaptation to the high rate of viral variation that can be found in CTL epitopes of several HIV-1 proteins. However, viral persistence occurs despite continuous CTL recognition and although functional importance of conserved sites in the different HIV proteins may impose constraints to viral variation. In the reverse transcriptase (RT) which is a major target for antiretroviral therapy, the impact of the continuous pressure of drug therapy is more obvious than that of the CTLs. Shifts in immunodominant RT regions seem to allow the maintenance of the HIV-1 RT CTL recognition with disease progression and antiretroviral therapy. In respect to new highly active drug combinations, understanding the capacity of virus-specific CTLs to control residual viral variants seems very important and may allow development of efficient immunotherapies to prevent drug-induced viral resistance.

Altered hapten ligands antagonize trinitrophenyl-specific cytotoxic T cells and block internalization of hapten-specific receptors

Low molecular chemicals (haptens) frequently cause T cell-mediated adverse immune reactions. Our previous work provided evidence that hapten-specific T cells, in analogy to those specific for nominal peptide antigens, direct their TCR towards hapten-modified, MHC-associated peptides. We now demonstrate that trinitrophenyl (TNP)-specific, class I MHC-restricted CTL from mice may exhibit exquisite specificity for subtle structural details of these hapten determinants, surpassing even the specificity of immunoglobulins. More importantly, these CTL could be antagonized by ligands altered either in their peptide sequence or in their hapten structure. The system was employed to examine the molecular basis of T cell antagonism. Whereas agonists resulted in a dose-dependent downregulation of TCR in different mouse T cell clones, antagonistic peptides totally failed to do so despite engaging the specific TCR. Moreover, simultaneous presentation of antagonist and agonist on the same antigen presenting cell prevented TCR internalization. No signs of anergy or functional receptor inactivation were observed in CTL treated with antagonist-loaded target cells. Based on a serial triggering model of T cell activation, our data favor a model in which antagonists block T cell functions by competitively engaging the specific TCR in unproductive interactions.

Kinetics and extent of T cell activation as measured with the calcium signal

We have characterized the calcium response of a peptide-major histocompatibility complex (MHC)-specific CD4(+) T lymphocyte line at the single cell level using a variety of ligands, alone and in combination. We are able to distinguish four general patterns of intracellular calcium elevation, with only the most robust correlating with T cell proliferation. Whereas all three antagonist peptides tested reduce the calcium response to an agonist ligand, two give very different calcium release patterns and the third gives none at all, arguing that (a) antagonism does not require calcium release and (b) it involves interactions that are more T cell receptor proximal. We have also measured the time between the first T cell-antigen-presenting cell contact and the onset of the calcium signal. The duration of this delay correlates with the strength of the stimulus, with stronger stimuli giving a more rapid response. The dose dependence of this delay suggests that the rate-limiting step in triggering the calcium response is not the clustering of peptide-MHC complexes on the cell surface but more likely involves the accumulation of some intracellular molecule or complex with a half-life of a few minutes.

T cell-mediated elimination of B7.2 transgenic B cells

Transgenic mice were generated to explore the effects on lymphoid development and immune function of constitutive expression of murine B7.2 on B and T cells. The number of B lymphocytes in primary and secondary lymphoid tissues is normal in B7.2 transgenic lines expressing low levels of B7.2 on B cells, but markedly reduced in transgenic lines expressing moderate to high levels of the transgene on B cells. This reduction is not due to an intrinsic abnormality of the transgenic B cells, but is rather the consequence of an elimination by an immune mechanism requiring the engagement of CD28 on T cells. Interestingly, during cognate antigen-specific interaction with T cells in vivo, B7.2 transgenic B cells are not eliminated, but proliferate and differentiate normally. Our findings suggest that, in the absence of high affinity ligand for the TCR, the CD28-B7.2 system participates in the regulation of B cell homeostasis.

Do holes in the T-cell repertoire have a center-surround regulatory structure? A rationale for the bifurcation of the Th1 and Th2 pathways of differentiation

Regulatory strategies controlling the balance of Th1 versus Th2 T-helper cell responses have been a long-standing mystery with important consequences for immunological disease. A novel model is presented to explain the comparative differentiation of Th1 and Th2 T cells as part of a mechanism to ensure self-tolerance. This model is based on the assumption that thymic interactions of T cell antigen receptors with self major histocompatibility complex ligands may vary in efficacy. By this model, fully agonistic major histocompatibility complex ligands elicit apoptosis during thymic selection to generate 'holes' in the repertoire. Conversely, major histocompatibility complex ligands having some degree of partial efficacy (i.e. a mixed agonist/antagonist) may promote Th2 differentiation whereas fully antagonistic major histocompatibility complex ligands elicit Th0 differentiation. Differentiation of Th2 T cells may continue in the extrathymic tissues upon continued interactions with self major histocompatibility complex ligands having mixed agonist/ antagonist properties. By this mechanism, each 'hole' in the repertoire will develop an inhibitory surround comprised to Th2 T cell clones having partial reactivities to a particular self major histocompatibility complex ligand. During immune responses to self-mimicking foreign antigens, the more numerous Th2 T cells of the inhibitory surround would prevent clonal expansion and Th1 differentiation of any fully autoreactive T cell.

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.

On the mechanisms of immunodominance in cytotoxic T lymphocyte responses to minor histocompatibility antigens

Although there are numerous minor histocompatibility antigens (MiHA), T cell responses leading to graft-versus-host (GVH) and graft-versus-tumor effects involve only a small number of immunodominant MiHA. The goal of the present study was to analyze at the cellular and molecular levels the mechanisms responsible for MiHA immunodominance. Cytotoxic T lymphocytes (CTL) generated in eight combinations of H2b strains of mice were tested against syngeneic targets sensitized with HPLC-fractionated peptides eluted from immunizing cells. The number of dominant MiHA was found to range from as little as two up to ten depending on the strain combination used. The nature of dominant MiHA was influenced by both the antigen profile of the antigen-presenting cells (APC) and the repertoire of responding CTL. When C57BL/6 dominant MiHA (B6dom) and H-Y were presented on separate APC, they showed similar immunogenicity. In contrast, when they were presented on the same APC, B6dom MiHA totally dominated H-Y. B6dom MiHA did not suppress anti-H-Y responses by acting as T cell receptor antagonists for anti-H-Y CTL, nor were anti-B6dom CTL precursors more abundant than anti-H-Y CTL precursors. Dominance resulted from competition for the APC surface between anti-B6dom and anti-H-Y CTL; the crucial difference between the dominant and the dominated MiHA appears to depend on the differential avidity of their respective CTL for APC. The only B6dom epitope thus far identified is the nonapeptide AAPDNRETF presented by H2-D(b). We found that compared with other known D(b)-binding peptides, AAPDNRETF is expressed at very high levels on the cell surface, binds to the D(b) molecule with very high affinity, and dissociates very slowly from its presenting class I molecule. These data indicate that one cannot predict which MiHA will be dominant or dominated based simply on their respective immunogenicity when presented on separate APC. Indeed, the avidity of T cell/APC interactions appears to determine which antigen(s) will trigger T cell responses when numerous epitopes are presented by the same APC.

The efficiency of CD4 recruitment to ligand-engaged TCR controls the agonist/partial agonist properties of peptide-MHC molecule ligands

One hypothesis seeking to explain the signaling and biological properties of T cell receptor for antigen (TCR) partial agonists and antagonists is the coreceptor density/kinetic model, which proposes that the pharmacologic behavior of a TCR ligand is largely determined by the relative rates of (a) dissociation ofligand from an engaged TCR and (b) recruitment oflck-linked coreceptors to this ligand-engaged receptor. Using several approaches to prevent or reduce the association of CD4 with occupied TCR, we demonstrate that consistent with this hypothesis, the biological and biochemical consequence of limiting this interaction is to convert typical agonists into partial agonist stimuli. Thus, adding anti-CD4 antibody to T cells recognizing a wild-type peptide-MHC class II ligand leads to disproportionate inhibition of interleukin-2 (IL-2) relative to IL-3 production, the same pattern seen using a TCR partial agonist/antagonist. In addition, T cells exposed to wild-type ligand in the presence of anti-CD4 antibodies show a pattern of TCR signaling resembling that seen using partial agonists, with predominant accumulation of the p21 tyrosine-phosphorylated form of TCR-zeta, reduced tyrosine phosphorylation of CD3epsilon, and no detectable phosphorylation of ZAP-70. Similar results are obtained when the wild-type ligand is presented by mutant class II MHC molecules unable to bind CD4. Likewise, antibody coligation of CD3 and CD4 results in an agonist-like phosphorylation pattern, whereas bivalent engagement of CD3 alone gives a partial agonist-like pattern. Finally, in accord with data showing that partial agonists often induce T cell anergy, CD4 blockade during antigen exposure renders cloned T cells unable to produce IL-2 upon restimulation. These results demonstrate that the biochemical and functional responses to variant TCR ligands with partial agonist properties can be largely reproduced by inhibiting recruitment of CD4 to a TCR binding a wild-type ligand, consistent with the idea that the relative rates of TCR-ligand disengagement and of association of engaged TCR with CD4 may play a key role in determining the pharmacologic properties of peptide-MHC molecule ligands. Beyond this insight into signaling through the TCR, these results have implications for models of thymocyte selection and the use of anti-coreceptor antibodies in vivo for the establishment ofimmunological tolerance.

Functional inactivation in the whole population of human V gamma 9/V delta 2 T lymphocytes induced by a nonpeptidic antagonist

Nonpeptidic compounds stimulate human T cells bearing the TCR-gamma delta in the absence of major histocompatibility complex restriction. We report that one of these ligands, 2,3-diphosphoglyceric acid (DPG), which induces expansion of V gamma 9/V delta T cells ex vivo, antagonizes the same cell population after repetitive activation. Stimulation with DPG results in partial early protein tyrosine phosphorylation and a prolonged, but reversible, state of unresponsiveness to agonist ligands in V gamma 9/V delta 2, but not in other T cells. These findings show that TCR antagonism is a general phenomenon of T cells. However, in contrast to the clonal specificity of altered peptides antagonizing alpha beta T cells, all the tested V gamma 9/V delta 2 polyclonal cell lines and clones become unresponsive, a fact that may be relevant for the regulation of their response in vivo.

Antigen-specific inhibition of CD4+ T-cell responses to beta-lactoglobulin by its single amino acid-substituted mutant form through T-cell receptor antagonism

T-cell responses can be antagonized by some single amino acid-substituted analogs of a peptide ligand for T-cell receptors (TCR), and these are called TCR antagonists. In this study, we addressed the question of whether TCR antagonism can be elicited by a whole protein antigen carrying a mutated T-cell determinant region corresponding to a TCR antagonist peptide. To clarify this, we examined the ability of a single amino acid-substituted mutant form of bovine beta-lactoglobulin (beta-Lg) to inhibit three CD4+ T-cell clones recognizing a peptide corresponding to an immunodominant determinant region 119-133 of beta-Lg (p119-133). First, we identified pD129A, an analog of p119-133 with a substitution of Ala for 129Asp, as an antagonist which can inhibit the response of two of the three T-cell clones. Then, using a yeast expression system, we prepared a mutant beta-Lg (mutD129A) with the same substitution of Ala for 129Asp as that in pD129A. This mutant protein could inhibit the proliferation of the two T-cell clones in a manner similar to the effect of pD129A. From these results we can demonstrate that TCR antagonism can be elicited by peptides naturally processed from a single-substituted mutant protein as well as by the corresponding peptides added exogenously.

Fas receptor is expressed in human lung squamous cell carcinomas, whereas bcl-2 and apoptosis are not pronounced: a preliminary report

We report a pilot study on the Fas receptor (APO-1, CD95) in vivo in 15 human squamous cell (non-small) carcinomas and ten normal bronchial specimens. The principal aim was to investigate whether the so-called death receptor, Fas, is present in these tumours. Ligation of Fas promptly induces apoptosis, particularly in T Jurkat cells in vitro, and expression of Fas on human cancer would thus theoretically be of great interest. The immunoreactivity for the anti-apoptotic protein Bcl-2 was also investigated, and the degree of apoptosis was evaluated by TdT dUTP nick end labelling (TUNEL) and conventional morphological criteria. Fas was present in all initial tumours but absent in control tissue, that is in the potential precursor cells of bronchial epithelium (P = 0.001). Fas was not detectable after radiotherapy (P = 0.03). We propose that radiotherapy induces an early selection of tumour cells rather than a down-regulation of Fas. Both Bcl-2 and apoptosis (TUNEL) were generally expressed at a modest level. In agreement with other studies, we did not find any significant correlation between Bcl-2 and prognosis, or between Bcl-2 and TUNEL. Hence, in this preliminary report, we have demonstrated Fas receptor in human squamous cell carcinomas in vivo. This is a novel finding, and the apparent absence of Fas after radiotherapy may have important therapeutic implications.

Exploration of requirements for peptidomimetic immune recognition. Antigenic and immunogenic properties of reduced peptide bond pseudopeptide analogues of a histone hexapeptide

We present a detailed analysis of the antigenic and immunogenic properties of a series of very stable peptidomimetics of a model hexapeptide corresponding to the C-terminal residues 130-135 of histone H3. Five pseudopeptide analogues of the natural sequence IRGERA were synthesized by systematically replacing, in each analogue, one peptide bond at a time by a reduced peptide bond Psi(CH2-NH). Three important features of the resulting analogues were examined. First, the analogues were tested in a biosensor system for their ability to bind monoclonal antibodies generated against the parent natural peptide, and their kinetic rate constants were measured. The results show that reduced peptide bond analogues can very efficiently mimic the parent peptide. The position of reduced bonds which were deleterious for the binding was found to depend on the antibody tested, and one monoclonal antibody recognized all five analogues. The equilibrium affinity constant toward reduced peptide bond analogues of four antibodies of IgG1 isotype induced against the parent hexapeptide was higher (up to 670 times) with certain analogues than toward the homologous peptide. Second, immunogenic properties of the five analogues were studied, and it was found that polyclonal antibodies induced against analogues in which Psi(CH2-NH) bonds were introduced between residues 130-131, 131-132, and 132-133 (R1-R2, R2-R3, and R3-R4) cross-reacted strongly with the cognate protein H3. Third, we tested the protease resistance of analogues. Altogether, the results provide a strong support for the potent applicability of reduced peptide bond pseudopeptides as components of synthetic vaccines and open a new field for the development of immunomodulatory agents.

The CD8 beta polypeptide is required for the recognition of an altered peptide ligand as an agonist

T cell activation is triggered by the specific recognition of cognate peptides presented by MHC molecules. Altered peptide ligands are analogs of cognate peptides which have a high affinity for MHC molecules. Some of them induce complete T cell responses, i.e. they act as agonists, whereas others behave as partial agonists or even as antagonists. Here, we analyzed both early (intracellular Ca2+ mobilization), and late (interleukin-2 production) signal transduction events induced by a cognate peptide or a corresponding altered peptide ligand using T cell hybridomas expressing or not the CD8 alpha and beta chains. With a video imaging system, we showed that the intracellular Ca2+ response to an altered peptide ligand induces the appearance of a characteristic sustained intracellular Ca2+ concentration gradient which can be detected shortly after T cell interaction with antigen-presenting cells. We also provide evidence that the same altered peptide ligand can be seen either as an agonist or a partial agonist, depending on the presence of CD8beta in the CD8 co-receptor dimers expressed at the T cell surface.

Antagonist HIV-1 Gag peptides induce structural changes in HLA B8

In the cellular immune response, recognition by CTL-TCRs of viral antigens presented as peptides by HLA class I molecules, triggers destruction of the virally infected cell (Townsend, A.R.M., J. Rothbard, F.M. Gotch, G. Bahadur, D. Wraith, and A.J. McMichael. 1986. Cell. 44:959-968). Altered peptide ligands (APLs) which antagonise CTL recognition of infected cells have been reported (Jameson, S.C., F.R. Carbone, and M.J. Bevan. 1993. J. Exp. Med. 177:1541-1550). In one example, lysis of antigen presenting cells by CTLs in response to recognition of an HLA B8-restricted HIV-1 P17 (aa 24-31) epitope can be inhibited by naturally occurring variants of this peptide, which act as TCR antagonists (Klenerman, P., S. Rowland Jones, S. McAdam, J. Edwards, S. Daenke, D. Lalloo, B. Koppe, W. Rosenberg, D. Boyd, A. Edwards, P. Giangrande, R.E. Phillips, and A. McMichael. 1994. Nature (Lond.). 369:403-407). We have characterised two CTL clones and a CTL line whose interactions with these variants of P17 (aa 24-31) exhibit a variety of responses. We have examined the high resolution crystal structures of four of these APLs in complex with HLA B8 to determine alterations in the shape, chemistry, and local flexibility of the TCR binding surface. The variant peptides cause changes in the recognition surface by three mechanisms: changes contributed directly by the peptide, effects transmitted to the exposed peptide surface, and induced effects on the exposed framework of the peptide binding groove. While the first two mechanisms frequently lead to antagonism, the third has more profound effects on TCR recognition.

A motif within the T cell receptor alpha chain constant region connecting peptide domain controls antigen responsiveness

Mutant alphabeta TCRs were generated by replacing domains of the alpha and beta chain constant regions with homologous domains from TCR delta and gamma chains, respectively. Chimeric TCRs in which the alpha chain contains TCR delta chain sequences within the connecting peptide domain are unresponsive to alloantigens and superantigens, and have defective interactions with the CD3/zeta complex. Although these antigen-unresponsive TCRs undergo zeta chain phosphorylation upon stimulation with superantigen, they do not generate a full signal capable of producing IL-2. Mutant TCRs acquire signaling activity with a combination of superantigen and calcium ionophore, indicating a defect in calcium-mediated signaling. Finally, a conserved motif, FETDxNLN, present in the alpha chain connecting peptide domain, is disrupted in all signaling-defective TCRs. This conserved alpha chain connecting peptide motif might mediate the transfer of signals from the alphabeta heterodimer to the CD3/zeta complex.

Amino acid substitutions at position 97 in HLA-A2 segregate cytolysis from cytokine release in MART-1/Melan-A peptide AAGIGILTV-specific cytotoxic T lymphocytes

CD8+ T lymphocytes recognize antigenic peptides presented by major histocompatibility complex (MHC) class I molecules. Individual peptide termini appear to be fixed at the C- and N-terminal ends. In contrast, central peptide side chains residues may point in different directions and exhibit limited flexibility, dependent on the MHC class I structural variation. For instance, position 97 in HLA-A201 has been shown to shift individual peptide species into different coordinations, one oriented towards the peptide N terminus, or more towards the C-terminal end. The conformational shape of such non-anchor peptide residues may affect the affinity of MHC/peptide/TCR interaction, resulting in quantitative, or qualitative different T cell effector functions. To characterize the impact of different amino acid residues occupying position 97 in HLA-A2 on peptide binding and presentation to CTL, we generated a panel of mutated HLA-A2 molecules containing either M, K, T, V, G, Q, W, P or H at position 97. The HLA-A0201 presented melanoma-associated MART-1/Melan-A derived peptide AAGIGILTV was employed to assess the impact of such position-97 mutations on HLA-A2 in peptide binding measured in an HLA-A2 reconstitution assay and presentation to AAGIGILTV-specific polyclonal or clonal T lymphocytes as measured by cytotoxicity, or interferon (IFN)-gamma and granulocyte/ macrophage colony-stimulating factor (GM-CSF) secretion. The high-affinity AAGIGILTV peptide bound to all position-97 mutants, albeit with differential efficiencies, and elicited specific release of IFN-gamma and GM-CSF by CTL. CTL responses were triggered only by the HLA-A2 wild type, by HLA-A2-H97 (histidine position 97 mutant), and HLA-A2-W97. The HLA-A2-M97 presenting molecule elicited enhanced cytokine release and CTL effector functions by polyclonal and by clonal effector T cells. These results indicate that MHC class I-bound peptides can trigger specific cytokine release by effector T cells independently of their ability to induce cytolysis. We conclude that relatively minor changes in the MHC class I peptide binding groove, including substitutions at position 97, can affect recognition by antigen-specific T cells. Mutant MHC class I molecules, such as those described here, may act as partial peptide antagonists and could be useful for inducing T lymphocytes with qualitatively different effector functions.

Cytotoxic T-lymphocyte clones specific for an immunodominant epitope display discerning antagonistic response to naturally occurring Epstein-Barr virus variants

In the present study we have identified Epstein-Barr virus isolates which encode variant sequences within an HLA B35-restricted immunodominant cytotoxic T-lymphocyte (CTL) epitope that act as natural antagonists and can inhibit CTL activity on the wild-type epitope. This effect can be demonstrated if the wild-type epitope is presented as a synthetic peptide or when processed from a full-length Epstein-Barr virus protein expressed by recombinant vaccinia constructs. However, this antagonistic effect was only selectively seen with some CTL clones, while a strong agonistic effect was evident for other clones in the presence of the same variant peptide. The data presented in this study strongly suggest that it is unlikely that the variant viruses can completely antagonize a virus-specific CTL response by this mechanism since the host immune response is capable of generating CTLs expressing a diverse array of T-cell receptors. Moreover, many of these CTLs can recognize the variant sequences as efficiently as wild-type epitope.

Selective activation of the calcium signaling pathway by altered peptide ligands

We previously demonstrated that altered peptide ligands (APL) can partially activate T cells, resulting in multiple distinct functional phenotypes, including the induction of anergy. Such APL stimulate a unique pattern of T cell receptor (TCR) phospho-zeta species, and lack associated ZAP-70 kinase activity. While these data suggested that selective signaling pathways downstream of the TCR/CD3 molecules are activated upon APL stimulation, they did not directly demonstrate this. Thus, we pursued intracellular signaling events successfully stimulated by APL. Because our previous studies showed that cyclosporin A (CsA) completely inhibited anergy induction, we assessed whether TCR ligation by APL cause a rise in cytosolic calcium (Ca+2). Our results show that these ligands can induce Ca+2 transients, in contrast to data generated using analogue peptides in other antigen systems. These opposing results may reflect differences in the intracellular signaling pathways utilized by different APL, or may be due to the exquisite sensitivity of the assay used here. Importantly, the APL-stimulated Ca+2 induction is both initiated and sustained at lower levels than that stimulated by a strong agonist signal, but resembles that stimulated by a weaker agonist stimulus. Alone, the less than optimal Ca+2 induction does not cause anergy, because ionomycin treatment together with the APL does not result in a proliferative signal. Instead, we propose that a combination of this and other signaling pathways induces T cell anergy. Overall, these data support the concept of differential signaling in T cells, as a direct consequence of the phosphotyrosine status of the TCR/CD3 molecules.

Strategy for isolating and sequencing biologically derived MHC class I peptides

The presentation of MHC class I peptides at cell surfaces and the subsequent cytolytic T-lymphocyte response are critical components of the mammalian immune response. However, the identification and sequencing of such peptides present a considerable analytical challenge since > 10,000 peptides at 10(-15)-10(-18) M concentrations are often present in the mixture. We describe a two-dimensional chromatography approach in conjunction with tandem mass spectrometry to sequence and identify such peptides. After immunoaffinity concentration, and subsequent acetic acid release of MHC class I peptides from MHC protein complex, the peptides are subjected to reversed phase HPLC, where they are separated based on their hydrophilic-hydrophobic character. These coarse fractions are then loaded onto a specially designed membrane preconcentration-capillary electrophoresis cartridge (mPC-CE) and subsequently subjected to on-line mPC-CE-MS analysis. The second dimension of chromatography by CE separation affords resolution of peptides based on their charge/mass (to a first approximation) ratio. Ultimately peptides are sequenced using mPC-CE-tandem mass spectrometry (mPC-CE-MS-MS). We describe the strategy for sequencing < 60 femtomoles of a peptide obtained from 3.10(9) Kb-derived EL-4 cells.

Interleukin 2 production, not the pattern of early T-cell antigen receptor-dependent tyrosine phosphorylation, controls anergy induction by both agonists and partial agonists

Full activation of T cells requires signaling through the T-cell antigen receptor (TCR) and additional surface molecules interacting with ligands on the antigen-presenting cell. TCR recognition of agonist ligands in the absence of accessory signals frequently results in the induction of a state of unresponsiveness termed anergy. However, even in the presence of costimulation, anergy can be induced by TCR partial agonists. The unique pattern of early receptor-induced tyrosine phosphorylation events induced by partial agonists has led to the hypothesis that altered TCR signaling is directly responsible for the development of anergy. Here we show that anergy induction is neither correlated with nor irreversibly determined by the pattern of early TCR-induced phosphorylation. Rather, it appears to result from the absence of downstream events related to interleukin 2 receptor occupancy and/or cell division. This implies that the anergic state can be manipulated independently of the precise pattern of early biochemical changes following TCR occupancy, a finding with implications for understanding the induction of self-tolerance and the use of partial agonist ligands in the treatment of autoimmune diseases.

CD28: a signalling perspective

CD28 and the related molecule cytotoxic T lymphocyte-associated molecule-4 (CTLA-4), together with their natural ligands B7.1 and B7.2, have been implicated in the differential regulation of several immune responses. CD28 provides signals during T cell activation which are required for the production of interleukin 2 and other cytokines and chemokines, and it has also been implicated in the regulation of T cell anergy and programmed T cell death. The biochemical signals provided by CD28 are cyclosporin A-resistant and complement those provided by the T cell antigen receptor to allow full activation of T cells. Multiple signalling cascades which may be independent of, or dependent on, protein tyrosine kinase activation have been demonstrated to be activated by CD28, including activation of phospholipase C, p21ran, phosphoinositide 3-kinase, sphingomyelinase/ceramide and 5-lipoxygenase. The relative contributions of these cascades to overall CD28 signalling are still unknown, but probably depend on the state of activation of the T cell and the level of CD28 activation. The importance of these signalling cascades (in particular the phosphoinositide 3-kinase-mediated cascade) to functional indications of CD28 activation, such as interleukin 2 gene regulation, has been investigated using pharmacological and genetic manipulations. These approaches have demonstrated that CD28-activated signalling cascades regulate several transcription factors involved in interleukin 2 transcriptional activation. This review describes in detail the structure and expression of the CD28 and B7 families, the functional outcomes of CD28 ligation and the signalling events that are thought to mediate these functions.

Altered T cell receptor ligands trigger a subset of early T cell signals

TCR ligands are complexes of peptides and MHC proteins on the surfaces of APCs. Some of these ligands cause T cell proliferation (agonists), while others block it (antagonists). We compared the acid release, calcium flux, and proliferation response of helper T cells to a variety of ligands. We found that all agonist ligands but not most antagonist ligands trigger acid release, a general indicator of early cellular activation. Only a subset of ligands triggering acid release cause sustained calcium flux, and only a subset of these ligands cause T cell proliferation. Antagonist ligands and anti-CD4 antibodies both effectively block T cell proliferation. However, significantly greater antagonist ligand or antibody concentrations are required to block acid release and initial calcium influx. These data demonstrate a hierarchy of early T cell signaling steps and show that altered TCR ligands can initiate some steps while blocking the completion of others.

Evolution and plasticity of CTL responses against HIV

Exceptionally potent cytotoxic T lymphocyte responses are generated after HIV invasion and probably control the primary infection as well as the asymptomatic phase of HIV infection. The chronic phase appears as a quasi-equilibrium between waves of new HIV variants and variant-specific CTLs, thus sustaining continuous CTL activation which eventually fails to eradicate HIV disease progression and the reascension of viral replication. Meanwhile, both the host and the virus develop various strategies either to stop or to evade this potentially deleterious permanent CTL activity. The transient effectiveness of CTLs opens perspectives for understanding disease progression generally as well as for immune therapeutic strategies.

Binding of viral antigens to major histocompatibility complex class I H-2Db molecules is controlled by dominant negative elements at peptide non-anchor residues. Implications for peptide selection and presentation

Binding of viral antigens to major histocompatibility complex (MHC) class I molecules is a critical step in the activation process of CD8(+) cytotoxic T lymphocytes. In this study, we investigated the impact of structural factors at non-anchor residues in peptide-MHC interaction using the model of lymphocytic choriomeningitis virus (LCMV) infection of its natural host, the mouse. Altering viral genes by making reassortants, recombinants, and using synthetic peptides, CD8(+) cytotoxic T lymphocytes were shown to recognize only three H-2Db-restricted epitopes, GP amino acids 33-41/43, GP 276-286, and NP 396-404. However, LCMV NP and GP proteins contain 31 other peptides bearing the H-2Db motif. These 34 LCMV peptides and 11 other known H2-Db-restricted peptides were synthesized and examined for MHC binding properties. Despite the presence of the H-2Db binding motif, the majority of LCMV peptides showed weak or no affinity for H-2Db. We observed that dominant negative structural elements located at non-anchor positions played a crucial role in peptide-MHC interaction. By comparative sequence analysis of strong versus non-binders and using molecular modeling, we delineated these negative elements and evaluated their impact on peptide-MHC interaction. Our findings were validated by showing that a single mutation of a favorable non-anchor residue in the sequence of known viral epitopes for a negative element resulted in dramatic reduction of antigen presentation properties, while conversely, substitution of one negative for a positive element in the sequence of a non-binder conferred to the peptide an ability to now bind to MHC molecules.

Partial signaling by CD8+ T cells in response to antagonist ligands

Structural variants of an agonist peptide-major histocompatibility complex (MHC) molecule ligand can show partial agonist and/or antagonist properties. A number of such altered ligands appear to act as pure antagonists. They lack any detectable ability to induce T cell effector function and have been described as unable to induce calcium transients and turnover of inositol phosphates. This has been interpreted as an inability of these ligands to initiate any T cell receptor (TCR)-dependent signal transduction, with their antagonist properties ascribed to competition with offered agonist for TCR occupancy. Yet antagonists for mature CD8+ T cells can induce positive selection of thymocytes, implying active induction of T cell differentiation events, and partial agonists or agonist/antagonist combinations elicit a distinctive pattern of early TCR-associated tyrosine phosphorylation events in CD4+ T cells. We have therefore directly examined proximal TCR signaling in a CD8+ T cell line in response to various related ligands. TCR engagement with natural peptide-MHC class I agonist resulted in the same pattern of early TCR-associated tyrosine phosphorylation events as seen with CD4+ cells, including accumulation of both the p21 and p23 forms of phosphorylated zeta, phosphorylation of CD3 epsilon, and association of phosphorylated ZAP-70 with the TCR. Two antagonists that lacked the ability to induce any detectable CTL effector response (cytolysis, esterase release, gamma interferon secretion, interleukin-2 receptor alpha upregulation) were nevertheless found to also induce TCR-dependent phosphorylation events. In these cases, there was preferential accumulation of the p21 form of phospho-zeta without net phosphorylation of CD3 epsilon, as well as the association of nonphosphorylated ZAP-70 kinase with the receptor. These data show that variant ligands induce similar TCR-dependent phosphorylation events in CD8+ T cells as first observed in CD4+ cells. More importantly, they demonstrate that some putatively pure antagonists are actually a subset of partial agonists able to induce intracellular biochemical changes through the TCR. This delivery of a partial signal by antagonists raises the possibility that antagonism in some cases may result from active interference with stimulation of effector activity by agonist in mature T cells, while the same variant signal could selectively trigger intracellular events that allow positive without negative selection in thymocytes.

A TCR binds to antagonist ligands with lower affinities and faster dissociation rates than to agonists

T lymphocyte activation is mediated by the interaction of specific TCR with antigenic peptides bound to MHC molecules. Single amino acid substitutions are often capable of changing the effect of a peptide from stimulatory to antagonistic. Using surface plasmon resonance, we have analyzed the interaction between a complex consisting of variants of the MCC peptide bound to a mouse class II MHC (Ek) and a specific TCR. Using both an improved direct binding method as well as a novel inhibition assay, we show that the affinities of three different antagonist peptide-Ek complexes are approximately 10-50 times lower than that of the wildtype MCC-Ek complex for the TCR, largely due to an increased off-rate. These results suggest that the biological effects of peptide antagonists and partial agonists may be largely based on kinetic parameters.

Regulation of colony-stimulating factor 1 receptor signaling by the SH2 domain-containing tyrosine phosphatase SHPTP1

SHPTP1 (PTP1C, HCP, SHP) is an SH2 domain-containing tyrosine phosphatase expressed predominantly in hematopoietic cells. A frameshift mutation in the SHPTP1 gene causes the motheaten (me/me) mouse. These mice are essentially SHPTP1 null and display multiple hematopoietic abnormalities, most prominently hyperproliferation and inappropriate activation of granulocytes and macrophages. The me/me phenotype suggests that SHPTP1 negatively regulates macrophage proliferative pathways. Using primary bone marrow-derived macrophages from me/me mice and normal littermates, we examined the role of SHPTP1 in regulating signaling by the major macrophage mitogen colony-stimulating factor 1 (CSF-1) (also known as macrophage colony-stimulating factor). Macrophages from me/me mice hyperproliferate in response to CSF-1. In the absence of SHPTP1, the CSF-1 receptor (CSF-1R) is hyperphosphorylated upon CSF-1 stimulation, suggesting that SHPTP1 dephosphorylates the CSF-1R. At least some CSF-1R-associated proteins also are hyperactivated. SHPTP1 is associated constitutively, via its SH2 domains, with an unidentified 130-kDa phosphotyrosyl protein (P130). P130 and SHPTP1 are further tyrosyl phosphorylated upon CSF-1 stimulation. Tyrosyl-phosphorylated SHPTP1 binds to Grb2 via the Grb2 SH2 domain. Moreover, in me/me macrophages, Grb2 is associated, via its SH3 domains, with several tyrosyl phosphoproteins. These proteins are hyperphosphorylated on tyrosyl residues in me/me macrophages, suggesting that Grb2 may recruit substrates for SHPTP1. Our results indicate that SHPTP1 is a critical negative regulator of CSF-1 signaling in vivo and suggest a potential new function for Grb2.

A strategy for the synthesis and screening of thiol-modified peptide variants recognized by T cells

In this study we present a strategy for the identification of novel peptide conjugates which may be used to understand the molecular details of the recognition process or to potentially regulate T cell-mediated responses. The approach involves the incorporation of cysteine into a known peptide at a position of interest and subsequent chemical conjugation using thiol-specific agents. Conjugates derived from the nonapeptide QL9 that is recognized by CTL 2C had either enhanced or reduced activity compared to the original cys-peptides. Different classes of thiol-reactive agents (alkyl halides, alkylthiolsulfonates, and disulfides) were tested with increases in activity of over 100-fold. As with standard peptide analogs, the activity depended on the position of the cysteine within the peptide and the nature of the chemically linked functional group. Use of this approach in a cysteine 'scan' of all positions of the original peptide is cost effective and with the availability of many different thiol-specific functional groups will allow the screening of considerably larger libraries of chemically modified peptides than have been used to date. Additionally, these findings may provide insight into the pathogenesis of thiol agents involved in contact sensitivity reactions.

Partially modified retro-inverso pseudopeptides as non-natural ligands for the human class I histocompatibility molecule HLA-A2

Syntheses of a series of partially modified retro-inverso analogues of the antigenic peptide M58-66 derived from the influenza virus matrix protein are reported. The retro-inverso modification phi(NH-CO) was obtained by replacement of two successive amino acid residues with a 2-substituted malonate derivative and gem-diaminoalkyl residue. The resulting compounds 1-8 were tested for their binding to the human histocompatibility class I molecule HLA-A2 in an assembly assay using lysates of peptide transporter-deficient cells T2. Specific peptide-dependent HLA-A2 assembly was revealed by an enzyme-linked immunosorbent assay. Significant HLA-A2 assembly was detected in the presence of analogues [gGly58-(S)mLeu59]-M58-66 (1a), [gGly61-(R,S)mPhe62]M58-66 (4), [gVal63-(R,S)mPhe64]M58-66 (6), and [gPhe64-(R,S)mAla65]M58-66 (7). The introduction of the retro-inverso modification between P2-P3, P3-P4, P5-P6, and P8-P9 (compounds 2, 3, 5, and 8, respectively) however led to a dramatic reduction in peptide binding to HLA-A2. Interestingly, compound 1a which contains modification between P1-P2 was found to be the most potent analogue, being able to retain the original HLA-A2 binding profile of the parent peptide M58-66. Taken together, these results and recent binding data obtained in the context of murine MHC class I molecule H-2Kd suggest that the incorporation of peptide bond surrogates in MHC class I-restricted epitopes is a useful approach to design molecules having both increased stability and high MHC-binding capacity. Depending on their agonist or antagonist effects at the T-cell receptor, such non-natural MHC ligands are likely to find many applications in the development of peptide-based vaccines or as potential therapeutic agents in the treatment of allergies and autoimmune diseases.

Essential flexibility in the T-cell recognition of antigen

alpha beta T cells specifically recognize a ligand composed of a peptide bound to a self-major-histocompatibility-complex molecule, but the recognition of slightly altered ligands by T cells can lead to a partial activation. This flexibility is crucial for T-cell development and can have both beneficial and harmful effects on peripheral T cells.

Permissive recognition during positive selection

In the periphery alpha beta T lymphocytes recognize antigens in conjunction with major histocompatibility complex (MHC) molecules. In the thymus immature T cells are positively selected on MHC molecules in the apparent absence of cognate peptides. Thus, at different developmental stages a T cell responds to different epitopes, yet uses the identical alpha beta T cell antigen receptor (TcR). To explain this paradox it has been hypothesized that during positive selection immature T cells see peptides/ligands unique to the thymus, are selected by specific antagonists related to their cognate peptides, or are driven by lowered affinity thresholds of their TcR. Though different in detail, these theories rely on defined peptides uniquely matched to select certain TcR. However, we find that in a TcR-transgenic (TcR(trans +)) mouse severely limiting the diversity of peptides does not impair positive selection. We show that many unrelated peptides, including some naturally occurring on the cell surface, induce maturation of CD4-CD8+TcR(high) thymocytes. The same peptides when presented in conjunction with the selecting MHC molecule, are not recognized by peripheral T cells expressing the same TcR(trans). Therefore, these findings point to a promiscuous rather than discriminate recognition mode used by immature T cells.

The specificity and orientation of a TCR to its peptide-MHC class II ligands

A T cell-mediated immune response is mainly determined by the 3-5 aa residues that protrude upwards from a peptide bound to an MHC molecule. Alterations of these peptide residues can diminish, eliminate or radically alter the signal that the T cell receives through its T cell receptor (TCR). We have used peptide immunizations of normal mice and mice carrying alpha or beta chain TCR transgenes to identify three distinct peptide contact points. One, near the carboxyl terminus of the peptide, involves the beta chain CDR3 region; the second was centrally located and interacted with both the alpha and beta chain CDR3 loops; the third was near the amino terminus of the peptide, and affected V alpha gene usage, but not the structure of CDR3 of either TCR chain. Based on these results, we propose an orientation for the TCR of this cloned line and argue for its generality.

Mechanisms underlying T-cell tolerance

To understand how the immune system manages to disarm potentially harmful T cells with reactivity against self-antigens, many immunologists have turned to model systems in which transgenic expression of TCRs, foreign antigens, or both, allows visualization of otherwise undetectable mechanisms. Recent data have offered several new insights into the variables that dictate the mechanism for tolerance employed by peripheral T cells. Some of these mechanisms can also dampen ongoing immune responses against foreign antigens and provide a strikingly powerful supplement to the primary mechanism of tolerance, thymic clonal deletion.

Altered peptide ligand-induced partial T cell activation: molecular mechanisms and role in T cell biology

The elucidation of the phenomena of T cell antagonism and partial activation by altered peptide ligands has necessitated a revision in the traditional concepts of TCR recognition of antigen and subsequent signal transduction. Whereas previous models supported a single ligand specificity for any particular T cell, many studies using analogs of immunogenic peptides have now demonstrated a flexibility in this recognition. Moreover, interaction with such altered peptide ligands can result in dramatically different phenotypes of the T cells, ranging from inducing selective stimulatory functions to completely turning off their functional capacity. Investigations of the biochemical basis leading to these phenotypes have shown that altered peptide ligands can induce a qualitatively different pattern of signal transduction events than does any concentration of the native ligand. Such observations imply that several signaling modules are directly linked to the TCR/CD3 complex and that they can be dissociated from each other as a direct result of the nature of the ligand bound. Interestingly, many in vivo models of T cell activation are compatible with a selective signaling model, and several studies have shown that peptide analogs can play a role in various T cell biologic phenomena. These data strongly suggest that naturally occurring altered peptide ligands for any TCR exist in the repertoire of self-peptides or, in nature, derived from pathogens, and recent reports provide compelling evidence that this is indeed the case. The concept of altered peptide ligands, their effects on T cell signaling, the hypothesized mechanisms by which they exert their effects, and their possible roles in shaping the T cell immune response are the scope of this review.