Role of peptide backbone in T cell recognition

T cells recognize self and nonself peptides presented by molecules of the MHC. Amino acid substitutions in the antigenic peptide showed that T cell specificity is highly degenerate. Recently, determination of the crystal structure of several TCR/MHC-peptide complexes suggested that the peptide backbone may significantly contribute to the interaction with the TCR. To directly investigate the role of the peptide backbone in T cell recognition, we performed a methylene-amino scan on the backbone of an antigenic peptide and measured the capacity of such pseudopeptides to bind their cognate MHC molecule, to sensitize target cells for T cell lysis, and to stimulate IL-2 secretion by two T cell hybridomas. For one of these pseudopeptides, we prepared fluorescent tetramers of MHC molecules and compared the staining of two T cell hybridomas. Our results demonstrate that the peptide backbone has an important contribution to TCR binding and suggest that some interactions between the peptide backbone and the TCR may be partially conserved. We discuss this finding in the perspective of TCR plasticity and T cell function.     

Distinct requirements for deletion versus anergy during CD8 T cell peripheral tolerance in vivo

Activation of naive T cells by quiescent APCs results in tolerance through deletion and anergy. The underlying basis for these distinct fates is unclear. Using clone 4 TCR transgenic animals as a source of naive CD8 T cells, we examined the requirements for peripheral deletion in vivo. Our results demonstrate that independent of the amount of Ag used for stimulation, a single dose was insufficient to achieve complete clonal deletion. Instead, further antigenic exposure was required to completely eliminate all of the activated T cells. Additionally, consecutive stimulations with low doses of Ag were highly effective in promoting deletion. In contrast, although stimulation with high doses of Ag initially led to the apoptosis of many of the activated T cells, it induced hyporesponsiveness in a portion of the responding cells, thereby sparing them from further activation and deletion. These data explain why some conditions promote tolerance through clonal deletion whereas others promote anergy. Furthermore, these data provide a framework to devise protocols for effective deletion of potentially autoreactive T cells.     

Stereochemical requirements for receptor recognition of the mu-opioid peptide endomorphin-1

A series of diastereoisomers of endomorphin-1 (EM1, Tyr(1)-Pro(2)-Trp(3)-Phe(4)-NH(2)) have been synthesized and their potency measured using the guinea pig ileum assay. [D-Phe(4)]EM1 possessed 1/10 the potency of EM1, while potencies of [D-Tyr(1)]EM1 and [D-Trp(3)]EM1 were 50- and 100-fold lower, respectively. Drastic loss of activity occurred in the [D-Pro(2)]EM1 peptide. The structural determinants for the inactivity and reduced potency of the diastereoisomers were investigated using NMR spectroscopy and conformational analysis. Simulations of trans-[D-Pro(2)]EM1 using NOE-derived distance constraints afforded well-defined structures in which Tyr and Trp side chains stack against the proline ring. The inactivity of [D-Pro(2)]EM1 was explained by structural comparison with EM1 (, FEBS Lett. 439:13-20). The two peptides showed an opposite orientation of the Trp(3) residue with respect to Tyr(1), thus suggesting a role of Pro(2) as a stereochemical spacer in orienting Trp(3) and Phe(4) toward regions suitable for mu-receptor interaction. The agonist activity of [D-Tyr(1)]EM1 and [D-Trp(3)]EM1 was attributed to their ability to adopt low-energy conformations that mimic those of EM1. The requirements for mu-receptor activation were examined further by comparing EM1 with the mu-peptide [D-Ala(2), MePhe(4), Gly-ol]-enkephalin (DAMGO). Conformations of DAMGO with a Tyr(1)-MePhe(4) phenyl ring separation of approximately 12 A were found to mimic Tyr(1)-Phe(4) of EM1, thus suggesting overlapping binding modes between these two peptides.     

T cell recognition of Mlsc. I. Influence of MHC gene products in Mlsc-specific T cell recognition

Monospecific T cell clones have been proven to be powerful tools for the characterization of T cell recognition in many Ag-specific as well as allo-specific T cell responses. In this report, in order to elucidate the mechanism of T cell recognition of minor stimulating locus Ag (Mlsc) determinants, Mlsc-specific cloned T cells were employed together with primary T cell responses to clarify the role of MHC-gene products in Mlsc-specific T cell recognition. The results indicated that T cells recognize Mlsc determinants in conjunction with I-region MHC gene products. Moreover, certain MHC haplotypes (e.g., H-2a and H-2k) appear to function efficiently in the "presentation" of Mlsc, whereas other haplotypes (e.g., H-2b and H-2q) function poorly if at all in presenting Mlsc. Experiments with the use of stimulators derived from F1 hybrids between the low stimulatory H-2b, Mlsc strain, C3H.SW, and a panel of Mlsb, H-2-different or intra-H-2 recombinant strains strongly suggested that expression of E alpha E beta molecules on stimulators plays a critical role for Mlsc stimulation. The functional importance of the E alpha E beta product in Mlsc recognition was further demonstrated by the ability of anti-E alpha monoclonal antibody to inhibit the response of cloned Mlsc-specific T cells. Inhibition of the same Mlsc-specific response by anti-A beta k antibody suggests that the A beta product may also play a role in T cell responses to Mlsc.     

Identification of I-A beta-chain residues critical for T cell recognition of peptide antigens

Major histocompatibility complex (MHC)-restricted recognition of antigen by T lymphocytes involves the formation of a complex composed of the T cell receptor, antigen, and restricting MHC molecule. To elucidate the interactions occurring within the antigen recognition complex, we have evaluated the ability of a panel of cell lines expressing mutated I-Ak molecules to function in the recognition by T hybridoma cells of two distinct peptide antigens. Our results indicate that while alterations along the entire length of the proposed helical structure in the carboxyterminal half of the beta 1 domain interfere with the I-Ak-restricted recognition of human fibrinopeptide B, mutations which affect recognition of hen egg lysozyme/I-Ak fall almost exclusively in the central portion of the helix. On the basis of these and previous results, we propose a "T cell receptor-mediated peptide exchange model" for formation of the antigen recognition complex.     

Molecular requirements for trinitrophenyl recognition by antihapten cytotoxic T lymphocytes

The requirement of direct covalent association of trinitrophenyl (Tnp) groups with cell surface components for functional interactions with anti-Tnp cytotoxic T lymphocytes (CTLs) was analyzed. This question was approached by comparing the ability of two methods of trinitrophenylation to render cells susceptible to lysis by anti-Tnp CTLs. As previously shown, cells modified with trinitrobenzene sulfonic acid were susceptible to H-2-restricted lysis by anti-Tnp CTLs. However, cells incubated with Sendai virus covalently associated with Tnp groups, were not rendered susceptible to lysis by anti-Tnp CTLs. These same target cells, however, were susceptible to H-2-restricted lysis by anti-Sendai virus CTLs. Direct analysis of the number of Tnp groups on cells modified by either method indicates no significant difference in the number of Tnp molecules associated with the different target cells. The results suggest that direct covalent association of Tnp groups with cell surface specific components is a minimal molecular requirement for recognition and lysis by anti-Tnp CTLs.     

Investigating T cell activation and tolerance in vivo: peptide challenge in allergic asthmatics

In the atopic allergic individual, challenge with allergen elicits manifestations of both the humoral (IgE-mediated or early-phase reaction) and the cell-mediated immune response (late-phase reaction). Detailed study of late-phase cell-mediated events is confounded by the effects of the earlier IgE-mediated response which results in mast cell and basophil activation. Thus the relative contributions to allergic inflammation of individual cell types, such as T cells and eosinophils, are difficult to define. In this review we describe experiments, largely from our own group, in which we have attempted to dissociate early and late allergic reactions. To this end, cell-mediated responses were induced in the absence of preceding IgE-mediated events, by the delivery of synthetic peptides representing T cell epitopes of the allergen. Activation of T cells resulted in airway narrowing and an increase in airway reactivity to non-specific stimuli. Furthermore, we describe the induction of antigen-specific hyporesponsiveness or "tolerance" following intradermal, but not mucosal, peptide delivery. The induction of peptide-induced hyporesponsiveness could be temporally dissociated from the initial T cell activation resulting in bronchoconstriction and likely occurred through a different mechanism. Analysis of in vitro allergen responses of peripheral blood cells revealed that hyporesponsiveness was associated with reductions in both Th1 and Th2 cytokines, together with a concomitant increase in the regulatory cytokine IL-10. We conclude that activation of T cells in vivo may result in manifestations of chronic allergic inflammation including bronchoconstriction and hyperreactivity. Additionally, when administered systemically at low dose, peptides may induce long-lasting hyporesponsiveness in the T cell compartment, through a mechanism that is associated with induction of IL-10.     

Induction of central deletional T cell tolerance by gene therapy

Transgenic mice expressing an alloreactive TCR specific for the MHC class I Ag K(b) were used to examine the mechanism by which genetic engineering of bone marrow induces T cell tolerance. Reconstitution of lethally irradiated mice with bone marrow infected with retroviruses carrying the MHC class I gene H-2K(b) resulted in lifelong expression of K(b) on bone marrow-derived cells. While CD8 T cells expressing the transgenic TCR developed in control mice reconstituted with mock-transduced bone marrow, CD8 T cells expressing the transgenic TCR failed to develop in mice reconstituted with H-2K(b) transduced bone marrow. Analysis of transgene-expressing CD8 T cells in the thymus and periphery of reconstituted mice revealed that CD8 T cells expressing the transgenic TCR underwent negative selection in the thymus of mice reconstituted with K(b) transduced bone marrow. Negative selection induced by gene therapy resulted in tolerance to K(b). Thus, genetic engineering of bone marrow can be used to alter T cell education in the thymus by inducing negative selection.     

T cell receptor binding transition states and recognition of peptide/MHC

T cell receptor recognition of peptide/MHC has been described as proceeding through a "two-step" process in which the TCR first contacts the MHC molecule prior to formation of the binding transition state using the germline-encoded CDR1 and CDR2 loops. The receptor then contacts the peptide using the hypervariable CDR3 loops as the transition state decays to the bound state. The model subdivides TCR binding into peptide-independent and peptide-dependent steps, demarcated at the binding transition state. Investigating the two-step model, here we show that two TCRs that recognize the same peptide/MHC bury very similar amounts of solvent-accessible surface area in their transition states. However, 1300-1500 A2 of surface area is buried in each, a significant amount suggestive of participation of peptide and associated CDR3 surface. Consistent with this interpretation, analysis of peptide and TCR variants indicates that stabilizing contacts to the peptide are formed within both transition states. These data are incompatible with the original two-step model, as are transition state models built using the principle of minimal frustration commonly employed in the investigation of protein folding and binding transition states. These findings will be useful in further explorations of the nature of TCR binding transition states, as well as ongoing efforts to understand the mechanisms by which T cell receptors recognize the composite peptide/MHC surface.     

T cell recognition of distinct peptide:I-Au conformers in murine experimental autoimmune encephalomyelitis

We have used T cells bearing TCRs that are closely related in sequence as probes to detect conformational variants of peptide-MHC complexes in murine experimental autoimmune encephalomyelitis in H-2(u) mice. The N-terminal epitope of myelin basic protein (MBP) is immunodominant in this model. Our studies have primarily focused on T cell recognition of a position 4 analog of this peptide (MBP1-9[4Y]) complexed with I-A(u). Using site-directed mutagenesis, we have mapped the functionally important complementarity determining region residues of the 1934.4 TCR Valpha domain. One of the resulting mutants (Tyr(95) to alanine in CDR3alpha, Y95A) has interesting properties: relative to the parent wild-type TCR, this mutant poorly recognizes Ag complexes generated by pulsing professional APCs (PL-8 cells) with MBP1-9[4Y] while retaining recognition of MBP1-9[4Y]-pulsed unconventional APCs or insect cell-expressed complexes of I-A(u) containing tethered MBP1-9[4Y]. Insect cell expression of recombinant I-A(u) with covalently tethered class II-associated invariant chain peptide or other peptides which bind relatively weakly, followed by proteolytic cleavage of the peptide linker and replacement by MBP1-9[4Y] in vitro, results in complexes that resemble peptide-pulsed PL-8 cells. Therefore, the distinct conformers can be produced in recombinant form. T cells that can distinguish these two conformers can also be generated by the immunization of H-2(u) mice, indicating that differential recognition of the conformers is observed for responding T cells in vivo. These studies have relevance to understanding the molecular details of T cell recognition in murine experimental autoimmune encephalomyelitis. They are also of particular importance for the effective use of multimeric peptide-MHC complexes to characterize the properties of Ag-specific T cells.     

Fine specificity of T cell recognition of the same peptide in association with different I-A molecules

The fine specificity of the response of T cell clones derived from B10.BR and B10.S congenic mouse strains restricted by I-Ak and I-As molecules, respectively, and which recognize the same 17 amino acid sequence (102-118) of myoglobin, has been investigated and compared with that of T cell clones specific for the same peptide with I-A.d The critical amino acid residues within the 102-118 region of myoglobin required for stimulation of I-Ak-and I-As-restricted T cell clones specific for this determinant were compared using a panel of synthetic peptide analogs. Residues 109, 113, and 116 were critical for stimulation of clones from both haplotypes, although the precise fine specificity varied, even among clones using the same restriction molecule. Residues 109 and 116 are also critical for stimulation of myoglobin-specific I-Ad-restricted clones (Berkower, I., L. A. Matis, G.K. Buckenmeyer, F.R. N. Gurd, D. L. Longo, and J. A. Berzofsky. J. Immunol. 132:1370, 1984). There was also considerable overlap in the size of the minimal determinant necessary for full activity: 106-118 for B10.BR and B10.D2 (Cease, K. B., I. Berkower, J. York-Jolley, and J. A. Berzofsky. J. Exp. Med. 164:1779, 1986) clones and 102-117 for B10.S clones. Despite this similarity in fine specificity, T cell clones were genetically restricted and could not be stimulated with the 102-118 peptide presented by Ia molecules of other haplotypes that could also present this epitope to syngeneic clones. These results suggest that binding of an immunogenic peptide to class II molecules is not sufficient to ensure recognition by a given T cell antigen receptor specific for the peptide, but do not indicate whether the major histocompatibility complex molecules interact directly with the T cell antigen receptor or induce a different recognizable conformation of the peptide.     

T lymphocyte recognition of insolubilized peptide antigen

To study the role of antigen-presenting cells (APC) in T lymphocyte responses, the stimulation requirements of a murine T cell hybridoma specific for the peptide antigen human fibrinopeptide B (hFPB)/I-Ak was examined. The fine specificity of T cell recognition of this peptide was determined by using several hFPB homologs and analogs, which indicated that the intact 14-amino acid peptide must remain intact to preserve the antigenic determinant, and that the carboxyl terminal Arg14 was important for T cell responses. Of particular interest was the finding that APC-associated hFPB failed to stimulate the T cells, and that activation was only observed with soluble peptide or by brief hFPB treatment of the T cells and APC mixed together. In addition, hFPB covalently bound to agarose beads was able to cause T cell activation, provided that I-Ak+ APC were also present in the culture. A number of control experiments were performed that showed that hFPB was not released from the bead and that the antigenic peptide involved in T cell responses remained bound to the beads. These results indicate that the form of the hFPB peptide antigen recognized by this T cell can be provided separately from APC.     

D-amino acid-substituted atrial natriuretic peptide analogs reveal novel receptor recognition requirements

The introduction of D-amino acid residues into peptide hormones has been traditionally utilized in structure-activity studies to probe the conformational requirements of ligand-receptor interactions. A study was undertaken to examine the effect of D-amino acid substitutions into the atrial natriuretic peptide molecule on interactions with distinct subpopulations of specific membrane-associated receptors of bovine aortic smooth muscle cells. Competitive binding analysis revealed that each of 15 synthetic D-amino acid-substituted analogs showed comparable affinities for C-ANP receptors, a class of specific receptors which have been proposed to mediate the sequestration and metabolic clearance of ANP. The relative affinities of all 15 analogs did not differ more than 10-fold. In contrast, the interaction of the ANP analogs with a second receptor pool (B-ANP receptors), which is coupled to the stimulation of particulate guanylate cyclase, varied over a 1000-fold range of potency consistent with expectations for a receptor that displays rigorous conformational specificity. The indiscriminant selectivity of C-ANP receptors for D-amino acid-substituted ANP analogs is unprecedented for hormone receptors involved in biological signal transduction. These results, when coupled with the inability to correlate any direct in vitro biological effect associated with C-ANP receptor occupancy supports the hypothesis that the C-ANP receptor protein is a novel transport protein involved in the metabolic clearance of ANP.     

Accessory cell requirements for the generation of cytolytic T lymphocytes.

A system is presented that may simplify the study of accessory cell requirements for CTL generation. Cortisone resistant (CR) thymocytes containing alloreactive CTL precursors do not respond to allogeneic tumor cells unless non-T accessory cells are added to culture. In addition, splenic T cells do not respond to allogeneic tumor cells in the absence of non-T accessory cells. These accessory cells share several properties of macrophages.     

Interferon-gamma enhances cytotoxic T lymphocyte recognition of endogenous peptide in keratinocytes without lowering the requirement for surface peptide

Keratinocytes expressing the human papillomavirus (HPV) type 16 E7 protein, as a transgene driven by the K14 promoter, form a murine model of HPV-mediated epithelial cancers in humans. Our previous studies have shown that K14E7 transgenic skin grafts onto syngeneic mice are not susceptible to immune destruction despite the demonstrated presence of a strong, systemic CTL response directed against the E7 protein. Consistent with this finding, we now show that cultured, E7 transgenic keratinocytes (KC) express comparable endogenous levels of E7 protein to a range of CTL-sensitive E7-expressing cell lines but are not susceptible to CTL-mediated lysis in vitro. E7 transgenic and non-transgenic KC are susceptible to conventional mechanisms of CTL-mediated lysis, including perforin and Fas/FasL interaction when an excess of exogenous peptide is provided. The concentration of exogenous peptide required to render a cell susceptible to lysis was similar between KC and other conventional CTL targets (e.g. EL-4), despite large differences in H-2Db expression at the cell surface. Furthermore, exposure of KC to IFN-gamma increased H-2Db expression, but did not substantially alter the exogenous peptide concentration required to sensitize cells for half maximal lysis. In contrast, the lytic sensitivity of transgenic KC expressing endogenous E7 is modestly improved by exposure to IFN-gamma. Thus, failure of CTL to eliminate KC expressing endogenous E7, and by inference squamous tumours expressing E7, may reflect the need for a sustained, local inflammatory environment during the immune effector phase.     

Altered recognition of antigen is a mechanism of CD8+ T cell tolerance in cancer

Antigen-specific CD8+ T-cell tolerance, induced by myeloid-derived suppressor cells (MDSCs), is one of the main mechanisms of tumor escape. Using in vivo models, we show here that MDSCs directly disrupt the binding of specific peptide-major histocompatibility complex (pMHC) dimers to CD8-expressing T cells through nitration of tyrosines in a T-cell receptor (TCR)-CD8 complex. This process makes CD8-expressing T cells unable to bind pMHC and to respond to the specific peptide, although they retain their ability to respond to nonspecific stimulation. Nitration of TCR-CD8 is induced by MDSCs through hyperproduction of reactive oxygen species and peroxynitrite during direct cell-cell contact. Molecular modeling suggests specific sites of nitration that might affect the conformational flexibility of TCR-CD8 and its interaction with pMHC. These data identify a previously unknown mechanism of T-cell tolerance in cancer that is also pertinent to many pathological conditions associated with accumulation of MDSCs.     

T cell tolerance by clonal elimination in the thymus

The monoclonal antibody KJ23a reacts with T cell receptors utilizing the V beta segment V beta 17a. T cells bearing V beta 17a+ receptors react with very high frequency with the MHC class II protein, IE. In this paper we show that T cells expressing V beta 17a are selectively eliminated from the peripheral T cell and mature thymocyte pool of mice expressing IE, but are present in expected numbers in the immature thymocyte population of such animals. These results show that in normal animals tolerance to self-MHC is due to clonal elimination rather than suppression. In addition, they indicate that tolerance induction may occur in the thymus at the time immature thymocytes are selected to move into the mature thymocyte pool.