Antigen recognition and peptide-mediated immunotherapy in autoimmune disease

Peptides from common viral and bacterial pathogens can efficiently activate diabetogenic T-cells

Cross-reactivity between an autoantigen and unknown microbial epitopes has been proposed as a molecular mechanism involved in the development of insulin-dependent diabetes (type 1 diabetes). Type 1 diabetes is an autoimmune disease that occurs in humans and the nonobese diabetic (NOD) mouse. BDC2.5 is an islet-specific CD4+ T-cell clone derived from the NOD mouse whose natural target antigen is unknown. A biometrical analysis of screening data from BDC2.5 T-cells and a positional scanning synthetic combinatorial library (PS-SCL) was used to analyze and rank all peptides in public viral and bacterial protein databases and identify potential molecular mimic sequences with predicted reactivity. Selected sequences were synthesized and tested for stimulatory activity with BDC2.5 T-cells. Active peptides were identified, and some of them were also able to stimulate spontaneously activated T-cells derived from young, pre-diabetic NOD mice, indicating that the reactivity of the BDC2.5 T-cell is directed at numerous mouse peptides. Our results provide evidence for their possible role as T-cell ligands involved in the activation of diabetogenic T-cells.

The I-Ag7 MHC class II molecule linked to murine diabetes is a promiscuous peptide binder

Susceptibility to insulin-dependent diabetes mellitus is linked to MHC class II genes. The only MHC class II molecule expressed by nonobese diabetic (NOD) mice, I-Ag7, shares a common alpha-chain with I-Ad but has a peculiar beta-chain. As with most beta-chain alleles linked to diabetes susceptibility, I-Ag7 contains a nonaspartic residue at position beta57. We have produced large amounts of empty I-Ag7 molecules using a fly expression system to characterize its biochemical properties and peptide binding by phage-displayed peptide libraries. The identification of a specific binding peptide derived from glutamic acid decarboxylase (GAD65) has allowed us to crystallize and obtain the three-dimensional structure of I-Ag7. Structural information was critical in evaluating the binding studies. I-Ag7, like I-Ad, appears to be very promiscuous in terms of peptide binding. Their binding motifs are degenerate and contain small and/or small hydrophobic residues at P4 and P6 of the peptide, a motif frequently found in most globular proteins. The degree of promiscuity is increased for I-Ag7 over I-Ad as a consequence of a larger P9 pocket that can specifically accommodate negatively charged residues, as well as possibly residues with bulky side chains. So, although I-Ad and I-Ag7 are structurally closely related, stable molecules and good peptide binders, they differ functionally in their ability to bind significantly different peptide repertoires that are heavily influenced by the presence or the absence of a negatively charged residue at position 57 of the beta-chain. These characteristics link I-Ag7 with autoimmune diseases, such as insulin-dependent diabetes mellitus.

A structural framework for deciphering the link between I-Ag7 and autoimmune diabetes

Susceptibility to murine and human insulin-dependent diabetes mellitus correlates strongly with major histocompatibility complex (MHC) class II I-A or HLA-DQ alleles that lack an aspartic acid at position beta57. I-Ag7 lacks this aspartate and is the only class II allele expressed by the nonobese diabetic mouse. The crystal structure of I-Ag7 was determined at 2.6 angstrom resolution as a complex with a high-affinity peptide from the autoantigen glutamic acid decarboxylase (GAD) 65. I-Ag7 has a substantially wider peptide-binding groove around beta57, which accounts for distinct peptide preferences compared with other MHC class II alleles. Loss of Asp(beta57) leads to an oxyanion hole in I-Ag7 that can be filled by peptide carboxyl residues or, perhaps, through interaction with the T cell receptor.

Copolymer 1 acts against the immunodominant epitope 82-100 of myelin basic protein by T cell receptor antagonism in addition to major histocompatibility complex blocking

The synthetic random amino acid copolymer Copolymer 1 (Cop 1, Copaxone, glatiramer acetate) suppresses experimental autoimmune encephalomyelitis, slows the progression of disability, and reduces relapse rate in multiple sclerosis (MS). Cop 1 binds to various class II major histocompatibility complex (MHC) molecules and inhibits the T cell responses to several myelin antigens. In this study we attempted to find out whether, in addition to MHC blocking, Cop 1, which is immunologically cross-reactive with myelin basic protein (MBP), inhibits the response to this autoantigen by T cell receptor (TCR) antagonism. Two experimental systems, "prepulse assay" and "split APC assay," were used to discriminate between competition for MHC molecules and TCR antagonism. The results in both systems using T cell lines/clones from mouse and human origin indicated that Cop 1 is a TCR antagonist of the 82-100 epitope of MBP. In contrast to the broad specificity of the MHC blocking induced by Cop 1, its TCR antagonistic activity was restricted to the 82-100 determinant of MBP and could not be demonstrated for proteolipid protein peptide or even for other MBP epitopes. Yet, it was shown for all the MBP 82-100-specific T cell lines/clones tested that were derived from mice as well as from an MS patient. The ability of Cop 1 to act as altered peptide and induce TCR antagonistic effect on the MBP p82-100 immunodominant determinant response elucidates further the mechanism of Cop 1 therapeutic activity in experimental autoimmune encephalomyelitis and MS.

A new anti-rheumatic drug, T-614, effectively suppresses the development of autoimmune encephalomyelitis

In the present study, we examined the therapeutic effects of T-614 (3-formylamino-7-methylsulfonylaminoxy-4H-1-benzopyran-4-one), a new anti-rheumatic drug, on a T cell-mediated autoimmune disease, experimental autoimmune encephalomyelitis (EAE). T-614 dose-dependently suppressed the development of active EAE induced in Lewis rats by immunization with myelin basic protein (MBP) when administered for 2 weeks starting on the day of immunization (day 0 to 14). Amelioration of clinical signs was also obtained by the treatment at the effector phase (day 7 to 14) of the disease. Furthermore, T-614 treatment of recipient rats that had received MBP-sensitized lymphoid cells resulted in suppression of the clinical severity of EAE. Immunohistological examination revealed that the number of TCR alpha beta-expressing T cells and the extent of MHC class II expression in the spinal cord of rats treated with T-614 was markedly reduced. In vitro study using MBP-specific T cells showed that the addition of T-614 inhibited the proliferative responses of T cells and the production of pro-inflammatory cytokines such as IFN-gamma, IL-6 and TNF produced by T and accessory cells. Taken together, these findings imply that T-614 suppresses the development of EAE by inhibiting the proliferation of autoreactive T cells and pro-inflammatory cytokine production not only by T cells but also by macrophages/microglia. This may be attributable to the result that T-614 is more effective at the effector phase rather than the induction phase. Thus, this drug has a potential value for the treatment of various T cell-mediated autoimmune diseases including multiple sclerosis (MS) as well as rheumatoid arthritis.

A peptide-binding motif for I-A(g7), the class II major histocompatibility complex (MHC) molecule of NOD and Biozzi AB/H mice

The class II major histocompatibility complex molecule I-A(g7) is strongly linked to the development of spontaneous insulin-dependent diabetes mellitus (IDDM) in non obese diabetic mice and to the induction of experimental allergic encephalomyelitis in Biozzi AB/H mice. Structurally, it resembles the HLA-DQ molecules associated with human IDDM, in having a non-Asp residue at position 57 in its beta chain. To identify the requirements for peptide binding to I-A(g7) and thereby potentially pathogenic T cell epitopes, we analyzed a known I-A(g7)-restricted T cell epitope, hen egg white lysozyme (HEL) amino acids 9-27. NH2- and COOH-terminal truncations demonstrated that the minimal epitope for activation of the T cell hybridoma 2D12.1 was M12-R21 and the minimum sequence for direct binding to purified I-A(g7) M12-Y20/K13-R21. Alanine (A) scanning revealed two primary anchors for binding at relative positions (p) 6 (L) and 9 (Y) in the HEL epitope. The critical role of both anchors was demonstrated by incorporating L and Y in poly(A) backbones at the same relative positions as in the HEL epitope. Well-tolerated, weakly tolerated, and nontolerated residues were identified by analyzing the binding of peptides containing multiple substitutions at individual positions. Optimally, p6 was a large, hydrophobic residue (L, I, V, M), whereas p9 was aromatic and hydrophobic (Y or F) or positively charged (K, R). Specific residues were not tolerated at these and some other positions. A motif for binding to I-A(g7) deduced from analysis of the model HEL epitope was present in 27/30 (90%) of peptides reported to be I-A(g7)-restricted T cell epitopes or eluted from I-A(g7). Scanning a set of overlapping peptides encompassing human proinsulin revealed the motif in 6/6 good binders (sensitivity = 100%) and 4/13 weak or non-binders (specificity = 70%). This motif should facilitate identification of autoantigenic epitopes relevant to the pathogenesis and immunotherapy of IDDM.

Immunomodulatory peptides with high binding affinity for class II MHC molecules for the prevention of graft-versus-host disease

Graft-versus-host disease (GVHD) represents the major barrier to successful allogeneic bone marrow transplantation. Positive and negative selection studies have unequivocally demonstrated that donor T cells are responsible for the induction phase of GVHD. Inhibition of the early steps of T cell antigen recognition leading to graft-versus-host disease has become an area of intense investigation. Peptides with high binding affinity for class II MHC molecules have been shown to compete for the single class II binding site and to inhibit T cell proliferative responses in vitro. Recent work has extended this approach to the prevention of murine GVHD in vivo.

HLA-DPB1 glutamate 69: a genetic marker of beryllium disease

Chronic beryllium disease (CBD) is a lung disorder related to beryllium exposure and is characterized by the accumulation in the lung of beryllium-specific CD4+ major histocompatibility complex (MHC) class II-restricted T lymphocytes. Evaluation of MHC class II genes in 33 CBD cases and 44 controls has shown a negative association with HLA-DPB1*0401 (P < 0.001) and a positive association with HLA-DPB1*0201 (P < 0.05) alleles, which differ at residues 36, 55 to 56, and 69 of the beta 1 chain. Among CBD cases, 97 percent expressed the HLA-DPB1*0201-associated glutamic acid (unaffected population, 30 percent; P < 0.001) at residue 69, a position involved in susceptibility to autoimmune disorders. This suggests that HLA-DP has a role in conferring susceptibility and that residue 69 of HLA-DPB1 could be used in risk assessment for CBD.

T-cell receptor genes in autoimmunity

T cells are primary participants in the pathogenesis of the MHC-dependent autoimmune diseases, and therefore, evidence for association of TCR V-gene repertoires with such disorders has been actively sought. With very few exceptions, no clear-cut evidence for correlation of particular RFLP-defined V-C-region genomic polymorphisms with autoimmune disease predisposition has thus far been demonstrated. With regard to TCR V-gene repertoires engaged in responses to autoantigens, restricted use of certain V beta and V alpha genes in response to myelin basic protein has been documented in animal models. In many spontaneous and experimentally induced animal and human autoimmune diseases, however, the picture is far from clear. Although dominance of certain TCR V genes has been noted, the clonal restrictions are not absolute; they differ from one study to another and from one patient to another. Such variations may be caused by MHC allele-dependent determinant selection mechanisms, secondary T-cell infiltrates in inflammatory sites, different patient populations and stages of disease, or the involvement of different pathogens that, nevertheless, lead to the same clinical entity. Overall, the results indicate that efforts to intervene therapeutically in autoimmune diseases by vaccination with modified T-cell clones, V region-synthetic peptides, or TCR blocking analogues may not be easily applicable. Further studies on the characterization of the specific antigens involved in autoimmune disease pathogenesis is required in order to accurately address the issue of TCR utilization in autoimmune diseases.

Impact of herpesvirus infections in the future

Numerous diverse, often opposing, trends will determine the frequency (and severity) of herpesvirus infections in the future. A major factor is the growing population of iatrogenically immunocompromised patients, accruing especially from large increases in organ and autologous bone marrow transplantation. Of even greater importance is the immense number of human immunodeficiency virus (HIV)-infected patients for whom herpesviruses may: (1) have been a cofactor in acquisition of HIV infection; (2) contribute to in vivo activation of HIV; (3) cause severe infections. Another factor influencing severe herpesvirus infections is the aging of populations in developed countries; this will be associated with a greater prevalence, and more morbid manifestations, of herpes zoster. Sociologic changes will also be important. The consequences of these, such as attempts to influence sexual practices, will be difficult to predict. Other changes, such as more frequent use of early child care facilities, will predictably lead to early acquisition of most herpesviruses, thereby decreasing the incidence of severe disease in adulthood. Factors that will reduce the incidence and/or severity of herpesvirus infections include vaccines (varicella, herpes simplex, cytomegalovirus); prophylactic strategies for immunocompromised hosts (passive immunization, antiviral drugs, blood product selection); more rapid and sensitive diagnostic methods; and suppressive or early antiviral therapy for common infections, such as genital herpes simplex and varicella.(ABSTRACT TRUNCATED AT 250 WORDS)

Identification of a pathogenic epitope involved in initiation of Heymann nephritis

Heymann nephritis is an experimental autoimmune disease model for human membranous nephropathy. We have recently identified a pathogenic epitope, clone 14 (C14), responsible for formation and deposition of glomerular immune complexes that is contained within the small subunit of the Heymann nephritis antigenic complex (HNAC). HNAC is a heterodimer composed of a large subunit designated gp330 and a smaller (44 kDa) subunit, which is immunologically identical to the receptor-associated protein. In this study, we prepared antibodies to fusion proteins with C-terminal deletions in the C14 sequence and assessed their ability to promote formation of immune deposits (IDs). When IgG specific for the shortest truncated fusion protein (C14/delta 3; 86 amino acids) was injected into rats, small IDs developed. In contrast, when IgG raised against the full-length C14 sequence was depleted of its reactivity toward the C14/delta 3 fusion protein (C14/delta 3-fp), no IDs could be detected. These data indicate that at least one pathogenic epitope is contained within the N-terminal 86 amino acids of C14. Since the IDs induced with the C14/delta 3-fp-specific IgG are smaller than those induced with the poly-epitope-specific anti-gp330 antibodies, it is likely that other epitopes in addition to those expressed by the C14/delta 3-fp are required for formation and growth of immune complexes.

Selective immunosuppression by administration of major histocompatibility complex (MHC) class II-binding peptides. I. Evidence for in vivo MHC blockade preventing T cell activation

Draining lymph node cells (LNC) from mice immunized with hen egg white lysozyme (HEL) display at their surface antigen-MHC complexes able to stimulate, in the absence of any further antigen addition, HEL peptide-specific, class II-restricted T cell hybridomas. Chloroquine addition to these LNC cultures fails to inhibit antigen presentation, indicating that antigenic complexes of class II molecules and HEL peptides are formed in vivo. MHC class II restriction of antigen presentation by LNC from HEL-primed mice was verified by the use of anti-class II monoclonal antibodies. Coinjection of HEL and the I-Ak-binding peptide HEL 112-129 in mice of H-2k haplotype inhibits the ability of LNC to stimulate I-Ak-restricted, HEL 46-61-specific T cell hybridomas. Similar results are obtained in mice coinjected with the HEL peptides 46-61 and 112-129. Inhibition of T hybridoma activation can also be observed using as antigen-presenting cells irradiated, T cell-depleted LNC from mice coinjected with HEL 46-61 and HEL 112-129, ruling out the possible role of either specific or nonspecific suppressor T cells. Inhibition of T cell proliferation is associated with MHC-specific inhibition of antigen presentation and with occupancy by the competitor of class II binding sites, as measured by activation of peptide-specific T cell hybridomas. These results demonstrate that administration of MHC class II binding peptide competitors selectively inhibits antigen presentation to class II-restricted T cells, indicating competitive blockade of class II molecules in vivo.

Antigen analog-major histocompatibility complexes act as antagonists of the T cell receptor

A novel mechanism for inhibition of T cell responses is described. Using the recognition of the influenza hemagglutinin (HA) 307-319 peptide in the context of DR1 class II major histocompatibility complex molecules, we have found that nonstimulatory analogs of the HA peptide preferentially inhibit HA-specific T cells in inhibition of antigen presentation assays. This antigen-specific effect could be generalized to another DR1-restricted peptide, Tetanus toxoid 830-843. Direct binding and cellular experiments indicated that the mechanism responsible was distinct from competition for binding to DR1 molecules. Likewise, negative signaling and induction of T cell tolerance could also be excluded as effector mechanisms. Thus, the most likely mechanism for this effect is engagement of antigen-specific T cell receptors by DR1-peptide analog complexes, which results in antigen-specific competitive blocking of T cell responses by virtue of their capacity to compete with DR1-antigen complexes for binding to the T cell receptor.