T cell receptor antagonists and partial agonists

Inhibition of thymocyte negative selection by T cell receptor antagonist peptides

The T cell receptor (TCR) recognizes antigenic peptide presented by major histocompatibility complex (MHC) molecules. Analogs of antigenic peptides have been shown to inhibit antigen-specific T cell responses, a phenomenon described as TCR antagonism. We have examined the effect of a natural variant of an antigenic peptide and a synthetic peptide analog, on the responses of mature T cells and immature thymocytes from an alpha-beta TCR-transgenic mouse (F5), the TCR of which recognizes a nonamer peptide from the nucleoprotein (NP) of influenza virus in the context of the H-2Db MHC molecule. Both peptides were shown to antagonize specifically the T cells cytolytic response without being able directly to stimulate mature T cells from these transgenic mice. Furthermore, a negative selection assay in vitro was used to demonstrate for the first time that antagonistic peptides are capable of antagonizing thymocyte deletion induced by antigenic peptides. These data suggest that the final selection of a T cell could be the result of a balance between the positive and negative influences of endogenous peptide ligands.

Kinetic discrimination in T-cell activation

We propose a quantitative model for T-cell activation in which the rate of dissociation of ligand from T-cell receptors determines the agonist and antagonist properties of the ligand. The ligands are molecular complexes between antigenic peptides and proteins of the major histocompatibility complex on the surfaces of antigen-presenting cells. Binding of ligand to receptor triggers a series of biochemical reactions in the T cell. If the ligand dissociates after these reactions are complete, the T cell receives a positive activation signal. However, dissociation of ligand after completion of the first reaction but prior to generation of the final products results in partial T-cell activation, which acts to suppress a positive response. Such a negative signal is brought about by T-cell ligands containing the variants of antigenic peptides referred to as T-cell receptor antagonists. Results of recent experiments with altered peptide ligands compare favorably with T-cell responses predicted by this model.

An HLA-B35-restricted epitope modified at an anchor residue results in an antagonist peptide

Peptides associated with HLA-B35 commonly have a proline or occasionally a serine residue in the P2 anchor position of the peptide, with a tyrosine at the C terminus. Based on this motif, we identified an octamer epitope from influenza A matrix protein which is presented by HLA-B35. The requirements for MHC binding and T cell receptor contact have been analyzed using analogs of this peptide with substitutions at positions 1, 2, 4, 7 and 8. The natural epitope contains a serine residue at P2 of the peptide. Substitution of this residue with proline (the favored amino acid in this position in B35-associated peptides) considerably enhances binding to HLA-B35 in the T2-B35 cell line, but the peptide is not recognized by the majority of CTL clones and can antagonize recognition of the index peptide. This suggests that a conservative substitution at the P2 anchor position results in a conformational change in the peptide-MHC surface exposed to the T cell receptor.

New approaches to specific immunomodulation in transplantation

T cells can recognize foreign MHC antigens by two distinct routes, either directly as intact molecules, or indirectly as processed peptides. Recent evidence strongly suggests that the indirect pathway of allorecognition plays a key role in initiating and sustaining graft rejection. Theoretically, all mismatched HLA alloantigens could generate immunogenic peptides which may be recognized in the context of any of the two self HLA-DR molecules. However, indirect recognition appears to be limited to a single peptide determinant of an allogeneic HLA-DR molecule and restricted by one self HLA-DR molecule. Furthermore, T cells involved in the self-restricted allopeptide recognition express a limited array of T cell receptor variable genes. These findings suggest that selective immune interventions, such as peptide blockade of the self HLA-DR molecule involved in the presentation of the dominant allopeptide, induction of high-zone tolerance or TCR antagonism, may be devised to prevent graft rejection.

Comparative study of the role of professional versus semiprofessional or nonprofessional antigen presenting cells in the rejection of vascularized organ allografts

The immune systems of transplant recipients are progressively challenged with exposure to the multiple lineages of donor cells that comprise the vascularized organ allograft. Each lineage of such donor tissue constitutively expresses or can be induced to express varying densities of MHC antigens ranging from no expression of MHC to MHC class I only to both MHC class I and class II. In addition, the cell surface expression of a diverse assortment of costimulatory and cell adhesion molecules also varies in density in a tissue specific fashion within the allograft. The MHC class I/II molecules displayed on the donor cells contain within their clefts a constellation of processed protein antigens in the form of peptides derived from intracellular and to some extent extracellular sources. Therefore, the potential for each cell lineage to induce alloactivation and serve as a target for allospecific immune responses is dependent on the diversity and density of peptide-bearing MHC molecules, costimulatory molecules, and cell adhesion molecules. In addition, the T cell receptor repertoire of the recipient also contributes to the magnitude of the allogeneic response. Consequently, the variety of clinical outcomes following organ transplantation even with the institution of potent immunosuppressive (drug) therapies is not surprising, as it appears reasonable for such therapies to influence the allogeneic response against distinct lineages differentially. Our failure to prevent chronic human allograft rejection may therefore be due to our limited appreciation of the full spectrum of alloactivating experiences encountered by host T cells as they interact with donor cells of diverse tissue lineages. Investigations by our laboratory of the immunopathogenesis of chronic cardiac allograft rejection have revealed an intrinsic inability of human cardiac myocytes to process and present antigens, not only for primary but also for secondary alloimmune responses. One obvious explanation for this phenomenon is the fact that cardiac myocytes do not constitutively express MHC class II molecules and express only low levels of class I molecules. However, this immunological unresponsiveness is maintained even after the induction of MHC class II and upregulation of MHC class I on these cells by interferon-gamma (IFN-gamma). Similar results have also been reported for cells of different tissue lineages (e.g. chondrocytes, keratinocytes, neural cells). Until now, cells have been defined as professional or nonprofessional for the purposes of defining their potential for antigen presentation to T cells. Professional antigen presenting cells have been identified as cells that are of haematopoietic origin, that constitutively express MHC class I and class II molecules as well as potent costimulatory molecules, and that are able to induce both primary and secondary immune responses, whereas nonprofessional antigen presenting cells are not bone marrow derived, do not constitutively express MHC class II, but may in some cases initiate primary and secondary immune responses after induction of MHC class II antigen by proinflammatory cytokines (e.g. IFN-gamma). The findings of our laboratory and others suggest that cells of certain lineages be considered in the separate class of 'nonantigen presenting cells'. Indeed, nonprofessional antigen presenting cells can be reclassified into three categories: semiprofessional-, nonprofessional-, or nonantigen presenting cells that are able to present antigen to and activate naive T cells, activated T cells, or no T Cells, respectively. The aim of this review is to identify and (re)examine the antigen presentation characteristics of cells of different tissue lineages in terms of their ability to activate different subsets of T cells. This approach is taken in an attempt to synthesize these concepts into a unified picture of T cell activation in the context of antigen processing and presentation by different cell types.

Preferential usage of the T-cell receptor by influenza virus hemagglutinin-specific human CD4+ T lymphocytes: in vitro life span of clonotypic T cells

Human helper T-cell (Th) responses to influenza A virus were studied by analyzing T-cell receptor V beta gene diversity in hemagglutinin-specific Th lymphocytes. The T-lymphocyte population from peripheral blood became quickly oligoclonal when stimulated in vitro with the HA306-329 peptide, and T-cell receptor V beta 3 genes were mainly expanded. Moreover, specific junctional region oligonucleotide probes corresponding to hemagglutinin-specific clones were used to estimate temporal diversity during antigenic stimulations.

Survival of mouse pancreatic islet allografts in recipients treated with allogeneic small lymphocytes and antibody to CD40 ligand

Combined treatment with allogeneic small lymphocytes or T-depleted small lymphocytes plus a blocking antibody to CD40 ligand (CD40L) permitted indefinite pancreatic islet allograft survival in 37 of 40 recipients that differed from islet donors at major and minor histocompatibility loci. The effect of the allogeneic small lymphocytes was donor antigen-specific. Neither treatment alone was as effective as combined treatment, although anti-CD40L by itself allowed indefinite islet allograft survival in 40% of recipients. Our interpretation is that small lymphocytes expressing donor antigens in the absence of appropriate costimulatory signals are tolerogenic for alloreactive host cells. Anti-CD40L antibody may prevent host T cells from inducing costimulatory signals in donor lymphocytes or islet grafts.

Development of a human thymic organ culture model for the study of HIV pathogenesis

The development of effective therapies for the treatment of AIDS would be facilitated by a better understanding of HIV pathogenesis in vivo. While some aspects of pathogenesis may be assessed by standard tissue culture assays, in vivo animal models may provide clues to other aspects of HIV-mediated progression toward AIDS. Current animal models include primate models for the study of simian immunodeficiency virus (SIV) and HIV, SCID-hu and hu-PBL SCID mouse models for the study of HIV, and feline models for the study of feline immunodeficiency virus (FIV). In general these models are costly and labor intensive. We have developed a simple human fetal thymic organ culture (TOC) system that is permissive for HIV infection and that exhibits pathology similar to that observed in vivo. A key feature of this system is the time-dependent destruction of thymocytes typified by the preferential loss of CD4-expressing cells. HIV-mediated thymocyte destruction occurs by a process involving programmed cell death. We have infected TOC with a panel of HIV isolates and found that the resulting viral replicative and pathogenic profiles are similar to those seen in the SCID-hu Thy/Liv mouse, yet different from profiles observed in standard PHA-blast tissue culture assays. In addition, we find that TOC may be used to assess efficacy of antiviral agents such as AZT (3'-azido-3'-deoxythymidine) and ddI (2',3'-dideoxyinosine) in blocking both viral replication and virus-induced pathology. These results indicate that this model is amenable to the systematic manipulation, analysis, and characterization of a variety of HIV virus isolates and antiviral therapies.

Involvement of both major histocompatibility complex class II alpha and beta chains in CD4 function indicates a role for ordered oligomerization in T cell activation

CD4 is a membrane glycoprotein on T lymphocytes that binds to the same peptide:major histocompatibility complex (MHC) class II molecule recognized by the antigen-specific receptor (TCR), thereby stabilizing interactions between the TCR and peptide;MHC class II complexes and promoting the localization of the src family tyrosine kinase p56lck into the receptor complex. Previous studies identified a solvent-exposed loop on the class II beta 2 domain necessary for binding to CD4 and for eliciting CD4 coreceptor activity. Here, we demonstrate that a second surface-exposed segment of class II is also critical for CD4 function. This site is in the alpha 2 domain, positioned in single class II heterodimers in such a way that it cannot simultaneously interact with the same CD4 molecule as the beta 2 site. The ability of mutations at either site to diminish CD4 function therefore indicates that specifically organized CD4 and/or MHC class II oligomers play a critical role in coreceptor-dependent T cell activation.

A role for CD5 in TCR-mediated signal transduction and thymocyte selection

CD5 is a transmembrane protein that is expressed on the surface of T cells and a subset of B cells. The absence of CD5 rendered thymocytes hyperresponsive to stimulation through the T cell antigen receptor (TCR) in vitro. Selection of T cells expressing three distinct transgenic TCRs was also abnormal in CD5-deficient mice. These observations indicate that CD5 can influence the fate of developing thymocytes by acting as a negative regulator of TCR-mediated signal transduction.

A tricyclic ring system replaces the variable regions of peptides presented by three alleles of human MHC class I molecules

BACKGROUND

Cytotoxic T-lymphocytes (CTLs) recognize complexes of short peptides with major histocompatibility complex (MHC) class I molecules. MHC molecules are polymorphic, and the products of different MHC alleles bind to different subsets of peptides. This is due to differences in the shape of the peptide-binding groove on the surface of the MHC protein, especially the 'pockets' into which anchor residues at each end of the peptide fit. Non-peptidic ligands for class I molecules may be useful clinically.

RESULTS

By applying computer-aided design methods guided by X-ray structures, we designed and synthesized several MHC class I ligands, based on known peptide ligands, in which the tricyclic, aromatic compound phenanthridine replaced the central amino acids of the peptides. These semi-peptidic fluorescent ligands bound with high affinity and with allelic specificity to the peptide-binding groove of different MHC class I molecules, forming crystallizable complexes.

CONCLUSIONS

Specificity for binding to different MHC class I molecules can be imparted to the common phenanthridine element by judicious choice of terminal peptidic elements from either nonamer or decamer peptides. The phenanthridine-based ligands have a long bound half-life, as do antigenic peptides.