T helper cell recognition of muscle acetylcholine receptor in myasthenia gravis. Epitopes on the gamma and delta subunits

Inhibition of ROCK activity regulates the balance of Th1, Th17 and Treg cells in myasthenia gravis

Aberrant ROCK activation has been found in patients with several autoimmune diseases, but the role of ROCK in myasthenia gravis (MG) has not yet been clearly investigated. Here, we demonstrated that ROCK activity was significantly higher in peripheral blood mononuclear cells (PBMCs) from MG patients. ROCK inhibitor Fasudil down-regulated the proportions of Th1 and Th17 cells in PBMCs of MG patients in vitro. Intraperitoneal injection of Fasudil ameliorated the severity of experimental autoimmune myasthenia gravis (EAMG) rats and restored the balance of Th1/Th2/Th17/Treg subsets. Furthermore, Fasudil inhibited the proliferation of antigen-specific Th1 and Th17 cells, and inhibited CD4 + T cells differentiated into Th1 and Th17 through decreasing phosphorylated Stat1 and Stat3, but promoted Treg cell differentiation through increasing phosphorylated Stat5. We conclude that dysregulated ROCK activity may be involved in the pathogenic immune response of MG and inhibition of ROCK activity might serve as a novel treatment strategy for MG.

Predicting HLA CD4 Immunogenicity in Human Populations

Background

Prediction of T cell immunogenicity is a topic of considerable interest, both in terms of basic understanding of the mechanisms of T cells responses and in terms of practical applications. HLA binding affinity is often used to predict T cell epitopes, since HLA binding affinity is a key requisite for human T cell immunogenicity. However, immunogenicity at the population it is complicated by the high level of variability of HLA molecules, potential other factors beyond HLA as well as the frequent lack of HLA typing data. To overcome those issues, we explored an alternative approach to identify the common characteristics able to distinguish immunogenic peptides from non-recognized peptides.

Methods

Sets of dominant epitopes derived from peer-reviewed published papers were used in conjunction with negative peptides from the same experiments/donors to train neural networks and generate an “immunogenicity score.” We also compared the performance of the immunogenicity score with previously described method for immunogenicity prediction based on HLA class II binding at the population level.

Results

The immunogenicity score was validated on a series of independent datasets derived from the published literature, representing 57 independent studies where immunogenicity in human populations was assessed by testing overlapping peptides spanning different antigens. Overall, these testing datasets corresponded to over 2,000 peptides and tested in over 1,600 different human donors. The 7-allele method prediction and the immunogenicity score were associated with similar performance [average area under the ROC curve (AUC) values of 0.703 and 0.702, respectively] while the combined methods reached an average AUC of 0.725. This increase in average AUC value is significant compared with the immunogenicity score (p = 0.0135) and a strong trend toward significance is observed when compared to the 7-allele method (p = 0.0938). The new immunogenicity score method is now freely available using CD4 T cell immunogenicity prediction tool on the Immune Epitope Database website (http://tools.iedb.org/CD4episcore).

Conclusion

The new immunogenicity score predicts CD4 T cell immunogenicity at the population level starting from protein sequences and with no need for HLA typing. Its efficacy has been validated in the context of different antigen sources, ethnicities, and disparate techniques for epitope identification.

Autoreactive T Cells from Patients with Myasthenia Gravis Are Characterized by Elevated IL-17, IFN-γ, and GM-CSF and Diminished IL-10 Production

Myasthenia gravis (MG) is a prototypical autoimmune disease that is among the few for which the target Ag and the pathogenic autoantibodies are clearly defined. The pathology of the disease is affected by autoantibodies directed toward the acetylcholine receptor (AChR). Mature, Ag-experienced B cells rely on the action of Th cells to produce these pathogenic Abs. The phenotype of the MG Ag-reactive T cell compartment is not well defined; thus, we sought to determine whether such cells exhibit both a proinflammatory and a pathogenic phenotype. A novel T cell library assay that affords multiparameter interrogation of rare Ag-reactive CD4(+) T cells was applied. Proliferation and cytokine production in response to both AChR and control Ags were measured from 3120 T cell libraries derived from 11 MG patients and paired healthy control subjects. The frequency of CCR6(+) memory T cells from MG patients proliferating in response to AChR-derived peptides was significantly higher than that of healthy control subjects. Production of both IFN-γ and IL-17, in response to AChR, was also restricted to the CCR6(+) memory T cell compartment in the MG cohort, indicating a proinflammatory phenotype. These T cells also included an elevated expression of GM-CSF and absence of IL-10 expression, indicating a proinflammatory and pathogenic phenotype. This component of the autoimmune response in MG is of particular importance when considering the durability of MG treatment strategies that eliminate B cells, because the autoreactive T cells could renew autoimmunity in the reconstituted B cell compartment with ensuing clinical manifestations.

Granzyme B: evidence for a role in the origin of myasthenia gravis

PURPOSE OF RESEARCH

Although the pathogenesis of myasthenia gravis (MG) as an antibody mediated disorder of acetylcholine receptors (AChRs) at neuromuscular junctions is well understood, the origin of the autoimmune response is unclear. The thymus is intimately involved in initiation of the autoimmune response; the antigen, AChR, is present in the thymus, but how the autoimmune response is triggered is not known. Granzyme B (GrB), a proteolytic enzyme present in cytolytic T cells and natural killer (NK) cells, selectively cleaves many potential autoantigens (but few non-autoantigens), generating novel fragments that trigger autoreactive responses. This protease has been strongly implicated in the pathogenesis of several autoimmune diseases including lupus, rheumatoid arthritis, dermatomyositis, and others. In the studies described in this manuscript, we examined the ability of GrB to cleave the AChR subunits, and performed biochemical, immunohistochemical and molecular studies on thymus glands from myasthenic patients and controls to assess GrB expression.

MAIN RESULTS

GrB efficiently and specifically cleaves subunits of AChR, especially the epsilon subunit. GrB is present in thymus glands from myasthenia patients, but is absent in control thymuses.

CONCLUSIONS

Our results provide evidence supporting a potential role for GrB in the process of initiation of MG, and are consistent with the concept of an immunodominant epsilon epitope.

Assessment of in vitro cytokine response in hemophilia A patients with or without factor VIII inhibitory antibody

Factor VIII (FVIII) inhibitor antibodies are produced in a proportion of hemophilia A patients. Development of anti-FVIII inhibitor antibodies is a T cell-dependent response, mediated by FVIII specific CD4(+) T cells. This study was performed to investigate the contribution of T helper (Th) cell-mediated cytokine response in inhibitor production. Peripheral blood mononuclear cells (PBMCs) were obtained from hemophilia A patients with (n = 14) or without inhibitor (n = 14) and from normal individuals (n = 14). Following stimulation of PBMCs with rFVIII and phytohemagglutinin (PHA) mitogen, the secreted cytokines, interferon-gamma (IFN-gamma), interleukin-10 (IL-10), and transforming growth factor-beta1 (TGF-beta1), in culture supernatant and the proliferative response were assessed using sandwich ELISA and (3)H-thymidine incorporation, respectively. No significant proliferative response to FVIII was observed, whereas PHA induced a strong response in all groups. No cytokine secretion was observed in response to FVIII stimulation. Although PHA induced IL-10, TGF-beta1 and IFN-gamma secretion in all groups, the level of IFN-gamma was significantly lower in hemophilia A patients than in normal individuals (p < 0.0001). The levels of TGF-beta1 and IL-10 were similarly higher in patients compared with normal subjects, but the difference was not statistically significant. Lack of FVIII-induced proliferative response and cytokine production together with reduced secretion of PHA-induced IFN-gamma in both groups of patients suggest involvement of nonspecific immunosuppression possibly due to hepatitis C virus (HCV) infection observed in the majority of patients.

T cell recognition of the A2 domain of coagulation factor VIII in hemophilia patients and healthy subjects

Hemophilia A patients treated with coagulation factor VIII (FVIII), and also some healthy subjects, may develop anti-FVIII antibodies (Ab), whose synthesis is driven by FVIII-specific CD4+ T cells. Some Ab block the procoagulant function of FVIII (inhibitors). Many inhibitors recognize epitopes on the FVIII A2 domain. Here, we have sought to identify A2 epitopes recognized by CD4+ T cells. We tested the proliferative response of CD4+ blood lymphocytes (BL) from hemophilia patients and healthy subjects, to overlapping synthetic peptides spanning the A2 domain sequence. Many A2 peptides induced proliferative responses of CD4+ BL from one or more subjects. The peptide-induced responses were strongest in hemophilia patients with inhibitors, weakest in healthy subjects. A2 peptides comprising residues 371-400, 621-650 and 671-690 elicited frequent and strong responses in hemophilia A patients, and especially in those with inhibitors. Healthy subjects recognized frequently only the sequence 371-400. A three-dimensional model of the A2 domain suggests that these CD4+ epitope sequences have structural features typical of 'universal' CD4+ T epitopes.

Epitope repertoire of human CD4(+) T cells on the A3 domain of coagulation factor VIII

Severe hemophilia A patients treated with factor (F)VIII may develop antibodies (Ab) that block FVIII function (inhibitors). Autoimmune inhibitors may develop in subjects without congenital hemophilia, and cause acquired hemophilia. Hemophiliacs without inhibitors and healthy subjects may also have small amounts of antiFVIII Ab. FVIII-specific CD4(+) T cells induce antiFVIII Ab synthesis. Here, we have examined their epitope repertoire in hemophilia patients and healthy subjects. We used overlapping synthetic peptides, spanning the sequence of the FVIII A3 domain, to challenge blood CD4(+) T cells in proliferation assays. The epitopes recognized in hemophilia A patients with or without inhibitors, acquired hemophilia patients, or healthy subjects overlapped, yet had characteristic differences. Most members of one or more study groups recognized the sequence regions 1691-1710, 1801-1820, 1831-1850, and 1941-60. In the proposed three-dimensional structure of the A3 domain, these sequences are largely exposed to the solvent and flanked by flexible sequence loops: these are structural features characteristic of 'universal' CD4(+) T epitopes. Hemophilia A patients with inhibitors recognized prominently only the sequence 1801-1820, which overlaps a known inhibitor binding site. This is consistent with the possibility that CD4(+) T cells recognizing epitopes within residues 1801-1820 have a role in inducing inhibitor synthesis. In contrast, CD4(+) T cells sensitized to sequences 1691-1710 and 1941-60, which are recognized by healthy subjects and hemophilia A patients without inhibitors, might curb inhibitor synthesis.

Association of the gene encoding the δ-subunit of the muscle acetylcholine receptor (CHRND) with acquired autoimmune myasthenia gravis

The muscle acetylcholine receptor (AChR) is the main target self-antigen in acquired autoimmune myasthenia gravis (MG). Here, we investigated an association of MG with the CHRND gene encoding the delta-subunit of the AChR. Using a microsatellite repeat located in the second intron of the gene, we observed a preferential transmission of the allele 268 in 114 one-generation families with one myasthenic child (Pc=0.0154). This allele was also over-represented in a group of 350 unrelated nonthymoma MG patients (OR=1.78, P=0.038), but not in 84 thymoma patients, compared to 168 healthy controls. Moreover, among nonthymoma patients, those lacking serum anti-titin antibodies appeared to be best associated (OR=2.07, P=0.017). In contrast, there was no distortion in the transmission of a single-nucleotide substitution polymorphisms (SNPs) in the 3' untranslated region of CHRND nor in that of two SNPs located in the closely linked CHRNG gene, 4.5 kb telomeric to CHRND. The data warrant a detailed investigation of CHRND polymorphism in MG patients.

Recognition of coagulation factor VIII by CD4+ T cells of healthy humans

Hemophilia A patients treated with coagulation factor (F)VIII may develop an anti-FVIII immune response. Anti-FVIII antibodies may occur also in healthy subjects. To understand the extent to which an immune response to FVIII occurs in healthy subjects, we investigated the proliferative response of blood CD4+ T cells from 90 blood donors to FVIII and to pools of overlapping synthetic peptides spanning the sequences of individual FVIII domains (A1-A3, C1-C2). Most subjects responded to FVIII and several FVIII domains. Men had stronger responses to FVIII than women, and older subjects than younger subjects. The domain-induced responses were weaker than the FVIII-induced responses, yet their intensity in individual subjects correlated with that of the response to FVIII. We examined whether Th1 and/or Th2 cells responded to FVIII in 68 subjects, by determining the CD4+ T cells that secreted interferon-gamma (IFN-gamma) or interleukin (IL)-5 after stimulation with FVIII: 25 subjects had FVIII-specific IFN-gamma-secreting cells, and seven of them had also FVIII-specific IL-5-secreting cells. None had only IL-5-secreting cells. Thus, a CD4+ T cell response to FVIII, which first involves Th1 cells, is common among subjects with a normal procoagulant function.

T Cells and Cytokines in the Pathogenesis of Acquired Myasthenia Gravis

Although the symptoms of myasthenia gravis (MG) and experimental MG (EAMG) are caused by autoantibodies, CD4(+) T cells specific for the target antigen, the nicotinic acetylcholine receptor, and the cytokines they secrete, have an important role in these diseases. CD4(+) T cells have a pathogenic role, by permitting and facilitating the synthesis of high-affinity anti-AChR antibodies. Th1 CD4(+) cells are especially important because they drive the synthesis of anti-AChR complement-fixing IgG subclasses. Binding of those antibodies to the muscle AChR at the neuromuscular junction will trigger the complement-mediated destruction of the postsynaptic membrane. Thus, IL-12, a crucial cytokine for differentiation of Th1 cells, is necessary for development of EAMG. Th2 cells secrete different cytokines, with different effects on the pathogenesis of EAMG. Among them, IL-10, which is a potent growth and differentiation factor for B cells, facilitates the development of EAMG. In contrast, IL-4 appears to be involved in the differentiation of AChR-specific regulatory CD4(+) T cells, which can prevent the development of EAMG and its progression to a self-maintaining, chronic autoimmune disease. Studies on the AChR-specific CD4(+) cells commonly present in the blood of MG patients support a crucial role of CD4(+) T cells in the development of MG. Circumstantial evidence supports a pathogenic role of IL-10 also in human MG. On the other hand, there is no direct or circumstantial evidence yet indicating a role of IL-4 in the modulatory or immunosuppressive circuits in MG.

Human CD4+ T-cell epitope repertoire on the C2 domain of coagulation factor VIII

Approximately 25% of severe hemophilia A patients develop antibodies (Ab) that neutralize the procoagulant function of factor (F)VIII (inhibitors). Autoimmune FVIII inhibitors may develop in individuals without congenital FVIII deficiency and cause acquired hemophilia. Low titers of anti-FVIII Ab may be present in hemophilia A patients without inhibitors and in healthy blood donors. FVIII-specific CD4+ T-cells drive the synthesis of anti-FVIII Ab. We examined the epitope repertoire of CD4+ T-cells from 15 healthy subjects, 10 hemophilia A patients without inhibitors, 11 hemophilia A patients with inhibitors, and six acquired hemophilia patients. Blood CD4+ T-cells were challenged in proliferation assays with a panel 16 overlapping synthetic peptides, spanning the sequence of the FVIII C2 domain. The sequence region 2291-2330 contained the most frequently and strongly recognized peptides in each of the four subject groups. Crystallographic B factor data and the location of these peptides within the three-dimensional structure of the C2 domain confirm that this region has a high degree of solvent exposure and flexibility within the peptide backbone, which are structural features typical of immunodominant universal CD4+ epitopes. Furthermore, this sequence region overlaps inhibitor-binding sites, suggesting that CD4+ T-cells recognizing peptide sequences within this region might be involved in inhibitor synthesis. The sequence regions 2191-2210 (recognized strongly by each study group except hemophilia A patients with inhibitors) and 2241-2290 (recognized primarily by acquired hemophilia patients and healthy subjects) share the same structural features, and also overlap inhibitor-binding sites. Although similar, there appear to be important differences in the CD4+ epitope repertoires of congenital and acquired hemophilia patients.

Mapping myasthenia gravis-associated T cell epitopes on human acetylcholine receptors in HLA transgenic mice

Susceptibility to myasthenia gravis (MG) is positively linked to expression of HLA-DQ8 and DR3 molecules and negatively linked to expression of the DQ6 molecule. To elucidate the molecular basis of this association, we have induced experimental autoimmune MG (EAMG) in mice transgenic for HLA-DQ8, DQ6, and DR3, and in DQ8xDQ6 and DQ8xDR3 F(1) transgenic mice, by immunization with human acetylcholine receptor (H-AChR) in CFA. Mice expressing transgenes for one or both of the HLA class II molecules positively associated with MG (DQ8 and DR3) developed EAMG. T cells from DQ8 transgenic mice responded well to three cytoplasmic peptide sequences of H-AChR (alpha320-337, alpha304-322, and alpha419-437), of which the response to alpha320-337 was the most intense. DR3 transgenic mice also responded to this sequence very strongly. H-AChR- and alpha320-337 peptide-specific lymphocyte responses were restricted by HLA class II molecules. Disease resistance in DQ6 transgenic mice was associated with reduced synthesis of anti-AChR IgG, IgG(2b), and IgG(2c) Ab's and reduced IL-2 and IFN-gamma secretion by H-AChR- and peptide alpha320-337-specific lymphocytes. Finally, we show that DQ8 imparts susceptibility to EAMG and responsiveness to an epitope within the sequence alpha320-337 as a dominant trait.

T cell recognition of muscle acetylcholine receptor in ocular myasthenia gravis

We examined the proliferative response of blood CD4(+) cells to muscle acetylcholine receptor (AChR) subunits and the epitope repertoire of the epsilon and gamma subunits, in ocular myasthenia gravis (oMG) patients and healthy subjects. oMG patients seldom recognized all subunits. The frequency and intensity of recognition was the same for all subunits, irrespective of the disease duration. The responses in oMG were lower than in generalized myasthenia gravis. Healthy subjects had frequent, low responses to one or more subunits. oMG patients recognized several epitopes on the gamma and epsilon subunits, that partially overlapped those recognized in gMG. The subunits and epitopes recognized by individual oMG patients changed over time. Thus, oMG patients have minimal and unstable sensitization of anti-AChR CD4(+) cells, in agreement with their low and inconsistent synthesis of anti-AChR antibody.

CD4+ T cell repertoire on the epsilon subunit of muscle acetylcholine receptor in myasthenia gravis

We have identified sequence regions of the human muscle acetylcholine receptor epsilon subunit recognized by CD4+ T cells from myasthenia gravis patients. We tested the proliferative response in vitro of blood CD4+ cells from 18 myasthenic patients and 5 controls, to individual overlapping synthetic peptides spanning the epsilon subunit sequence. All patients recognized a complex epitope repertoire. The peptides recognized by the CD4+ cells included sequence regions of the epsilon subunit that were diverged as compared to the homologous sequences of the other receptor subunits. Recognition of epitopes formed by sequence regions unique to the epsilon subunit suggests a direct role of this subunit in sensitizing the CD4+ cells. Several epsilon subunit peptides were recognized by many patients. Thus the epsilon subunit, like other acetylcholine receptor subunits, forms 'universal' CD4+ epitopes. The healthy subjects recognized some epsilon subunit peptides sporadically and at a low level.

Molecular analyses of HLA class II-associated susceptibility to subtypes of autoimmune diseases unique to Asians

It is well known that individuals positive for particular HLA-class II alleles show high risks for the development of Takayasu arteritis and other diseases caused by immunological disorders such as autoimmune diseases and allergies. HLA class II molecules present antigenic peptides to CD4+ T cells. Their extensive polymorphism affects the structures of peptides bound to HLA class II molecules to create individual differences in immune responses to antigenic peptides. To better understand the mechanisms for association between HLA class II alleles and susceptibility to autoimmune diseases, it is important to identify self-peptides presented by disease-susceptible HLA class II molecules and triggering disease-causative T cells. Many autoimmune diseases are observed in all ethnic groups, whereas the incidences of diseases, clinical manifestations and disease-susceptible HLA class II alleles are different among various ethnic groups for some autoimmune diseases. These phenomena suggest that differences in autoimmune self-peptide(s) in the context of disease-susceptible HLA class II molecules may cause these differences. Therefore, comparisons among disease-susceptible HLA class II alleles, autoimmune self-peptides and clinical manifestations of autoimmune diseases in different ethnic groups would be helpful in determining the pathogenesis of the diseases. In this paper, we describe our recent findings on: (1) the uniqueness of both clinical manifestations and HLA-linked genetic background of Asian-type (optico-spinal form) multiple sclerosis; (2) the structural characteristics of peptides bound to HLA-DQ molecules susceptible to insulin-dependent diabetes mellitus; (3) the identification of a disease-related autoantigenic peptide presented by disease-susceptible HLA-DQ molecules in Asians-specific infant onset myasthenia gravis; and (4) a manipulation of human T cell response by altered peptide ligands, as a possible candidate for new and antigen-specific immuno-suppressive therapy against autoimmune diseases.

Response of CD4+ T cells from myasthenic patients and healthy subjects of biosynthetic and synthetic sequences of the nicotinic acetylcholine receptor

We investigated the suitability of pools of overlapping synthetic peptides spanning the complete alpha 1 subunit sequence of the human muscle acetylcholine receptor (AChR) (alpha 1 pool) or the extracellular domain (residues 1-218, alpha 11-218 pool), and of biosynthetic alpha 1 constructs from E. coli, as stimulants of human CD4+ cells from myasthenia gravis (MG) patients and healthy subjects. A construct corresponding to residues alpha 11-209 was obtained as solubilized inclusion bodies (ib alpha 11-209), or purified by SDS gel electrophoresis (pur alpha 11-209). A second construct included the extracellular, cytoplasmic and carboxylterminal domains plus histidine residues, and was obtained as inclusion bodies (ib alpha 1NoTrans) or purified by gel permeation and histidine tag affinity chromatography (pur alpha 1NoTrans). A biosynthetic extracellular domain of the neuronal AChR alpha 7 subunit (ib alpha 71-206) isolated from E. coli as inclusion bodies served as control for bacterial contaminants. We used ib alpha 11-209, pur alpha 11-209 and peptide pools to propagate CD4+ lines from two MG patients. The lines obtained using pur alpha 11-209 and the peptide pools recognized the peptide pools and alpha 1 constructs tested well, but ib alpha 71-206 poorly or not at all. These lines recognized peptides known to form CD4+ epitopes in these patients. The ib alpha 11-209 lines recognized ib alpha 11-209 and ib alpha 71-206 strongly, but recognized poorly pur alpha 11-209 and the alpha 11-218 pool. We propagated T-cell lines from a healthy subject using pur alpha 11-209 and ib alpha 11-209. The pur alpha 11-209 line recognized pur alpha 11-209 and the alpha 11-218 pool, but not ib alpha 11-209 or ib alpha 71-206. The ib alpha 11-209 line recognized ib alpha 11-209 and ib alpha 71-206, but not pur alpha 11-209 or the alpha 11-218 pool. We tested blood CD4+ cells from six MG patients and eight healthy subjects with ib alpha 11-209, pur alpha 11-209, the alpha 11-218 pool and--in the healthy subjects--also ib alpha 71-206, ib alpha 1NoTrans and pur alpha 1NoTrans. In both populations, the alpha 11-218 pool elicited low and sporadic responses, while the constructs elicited clear responses that were frequently higher for ib alpha 11-209 than pur alpha 11-209. The responses to ib alpha 71-206 were strong and comparable to those to ib alpha 11-209, ib alpha 1NoTrans, and pur alpha 1NoTrans. These results indicate that even purified constructs from E. coli contain bacterial contaminants recognized by CD4+ cells. They should not be used to test unselected blood CD4+ cells, because they may evoke strong CD4+ responses to the bacterial antigens. Purified recombinant sequences may be suitable for propagation of CD4+ cell lines, if the specificity of the lines can be verified using different antigen preparations. Short synthetic peptide sequences can be safely used for propagation of specific CD4+ cells. Although they are poor stimulants for unselected blood CD4+ cells, the low responses they elicit are probably due to these cells.

Binding of single substituted promiscuous and designer peptides to purified DRB1*0101

MHC class II molecules present antigenic peptides to T cells. The sequence characteristics of peptides associated with various class II alleles have been examined by analysis of peptide mixtures extracted from purified class II molecules as well by direct binding assays with substituted synthetic peptides and purified class II molecules. Here, in vitro binding assays with purified DRB1*0101 and glycine substituted analogues of H gamma 321-340 and alanine substituted analogues of TT948-967, universal CD4+ epitopes of the gamma subunit of the human nicotinic acetylcholine receptor and tetanus toxin, respectively, were able to compete for binding to an extent similar to that of the unsubstituted peptides. Testing whether this is a property of promiscuous, but not allele-specific peptide epitopes, a designer peptide containing the proposed anchor residues for binding DRB1*0101 was used in similar binding assays. As expected, the binding capacity of this designer peptide was much higher than that of the universal epitope peptides. However, substitution of the anchor residues for alanine in the context of this designer peptide did not abrogate binding to DRB1*0101 and, in fact, enhanced it. Therefore, it appears that for both class II allele-specific and universal epitope peptides, binding is a result of the combinatory effects of a few residues. The individual replacement of these residues with the sterically and electrostatically neutral residue alanine does not negatively affect binding in the continued presence of other anchor residues.

Immuno-suppressive peptides for a human T cell clone autoreactive to a unique acetylcholine receptor alpha subunit peptide presented by the disease-susceptible HLA-DQ6 in infant-onset myasthenia gravis

Infant-onset myasthenia gravis, an autoimmune disease specific to Asians predominantly affects neuromuscular junctions in ocular muscles. An AChR alpha peptide (p71-91) specific autoreactive CD4+ alpha beta T cell clone was established by stimulating PBMC from a patient heterozygous for two disease-susceptible HLA-DR9-DQ9 and DR13-DQ6 haplotypes with a mixture of overlapping peptides covering AChR alpha. The T cell clone recognized the AChR alpha peptide in the context of the HLA-DQ6 molecule and produced a large amount of IFN-gamma and a trace amount of IL-4. A part (p75-83) of the core epitope of the autoantigenic peptide (p75-87) is encoded for by an exon P3A of the AChR alpha gene which can be alternatively spliced. The T cell clone responded to the recombinant AChR alpha protein with a P3A exon product, but not without a P3A exon product. We investigated responses of the T cell clone to 114 analogue peptides carrying single residue substitutions of the core AChR alpha peptide. The majority of analogues substituted at residues Phe-77, Leu-80 and Asn-82 stimulated proliferation of the T cell clone. Conversely, the majority of analogue peptides substituted at either Gln-81 or Glu-83 did not stimulate proliferative responses, and all exhibited strong or intermediate inhibitory effects on proliferative responses of the T cell clone to the wild type peptide, possibly by TCR antagonism. Thus, an HLA class II allele specific to Asians may directly control susceptibility to the Asian-specific type of myasthenia gravis. Analogues of the auto-antigenic AChR alpha peptide may prove effective for new immunosuppressive therapy.

Epitopes for human CD4+ cells on diphtheria toxin: structural features of sequence segments forming epitopes recognized by most subjects

The sequence regions of diphtheria toxin (DTX) recognized by CD4+ T cells of seven healthy humans of different major histocompatibility complex haplotypes were identified. Overlapping synthetic peptides, screening the DTX sequence, were used to test in proliferation assays unselected blood CD4+ cells, or DTX-specific CD4+ lines propagated by stimulation with DTX of blood mononuclear cells. Blood CD4+ cells and DTX-specific CD4+ lines gave consistent results. Although each subject had an individual pattern of peptide recognition, six peptide sequences (residues 271-290, 321-340, 331-350, 351-370, 411-430 and 431-450) were recognized by all subjects. In the native DTX molecule, these sequence regions are flanked by sequence loops exposed on the DTX surface. They overlap uncharged segments of the DTX sequence. These structural properties may be general requirements for immunodominance in CD4+ cell sensitization in humans.

Cellular immunity to a determinant common to glutamate decarboxylase and coxsackie virus in insulin-dependent diabetes

Insulin-dependent diabetes (IDD) results from the autoimmune destruction of the insulin-producing pancreatic beta cells. Autoreactive T-lymphocytes are thought to play a pivotal role in the pathogenesis of IDD; however, the target antigens of these cells, as well as the inductive events in the disease, are unclear. PBMC in persons with or at increased risk for IDD show elevated reactivity to the beta cell enzyme glutamate decarboxylase (GAD). To identify the T-lymphocyte-reactive determinants of GAD, an overlapping set of synthetic peptides was used to stimulate the PBMC from these individuals, PBMC responsiveness to GAD peptides was not restricted to those with IDD, and a number of peptides elicited responses in PBMC. However, the major determinant of GAD recognized by persons at increased risk for IDD was amino acids 247-279, a region which has significant sequence similarity to the P2-C protein of Coxsackie B virus (47% of 15 increased risk [islet cell autoantibody-positive relatives]; 25% of 16 newly diagnosed IDD patients; and 0% of 13 healthy control subjects). Responses to tetanus and insulin antigens were not different between the study groups. In addition, PBMC from individuals responding to GAD peptides within 247-279 also responded to a Coxsackie viral peptide (i.e., P2-C amino acids 32-47), an observation supporting potential molecular mimicry in this immune response. Although the role of environmental agents in the pathogenesis of the disease remains unclear, these cellular immunological findings support the epidemiological evidence suggesting an inductive role for enteroviruses like Coxsackie B in the autoimmunity underlying IDD.

Epitopes on the beta subunit of human muscle acetylcholine receptor recognized by CD4+ cells of myasthenia gravis patients and healthy subjects

We investigated the sequence regions of the human muscle acetylcholine receptor (AChR) beta subunit forming epitopes recognized by T helper cells in myasthenia gravis (MG), using overlapping synthetic peptides, 20 residues long, which screened the sequence of the AChR beta subunit. Since CD4+ lymphocytes from MG patients' blood did not respond to the peptides, we attempted propagation of beta subunit-specific T lines from six MG patients and seven healthy controls by cycles of stimulation of blood lymphocytes with the pooled peptides corresponding to the beta subunit sequence. CD4+ T lines were obtained from four patients and three controls. They secreted IL-2, not IL-4, suggesting that they comprised T helper type 1 cells. The T lines from MG patients could be propagated for several months. Three lines were tested with purified bovine muscle AChR and cross-reacted well with this antigen. All T lines were tested with the individual synthetic peptides present in the pool corresponding to the beta subunit sequence. Several beta subunit peptide sequences were recognized. Each line had an individual pattern of peptides recognition, but three sequence regions (peptides beta 181-200, beta 271-290, and the overlapping peptides beta 316-335 and beta 331-350) were recognized by most MG lines. The beta subunit-specific T lines from controls could be propagated for < 5 wk. Each line recognized several peptides, which frequently included the immunodominant regions listed above.

The nicotinic acetylcholine receptor: structure and autoimmune pathology

The nicotinic acetylcholine receptors (AChR) are presently the best-characterized neurotransmitter receptors. They are pentamers of homologous or identical subunits, symmetrically arranged to form a transmembrane cation channel. The AChR subunits form a family of homologous proteins, derived from a common ancestor. An autoimmune response to muscle AChR causes the disease myasthenia gravis. This review summarizes recent developments in the understanding of the AChR structure and its molecular recognition by the immune system in myasthenia.