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.

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.

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.

Characterization of human hybridoma clones isolated from hemophilia patients with specificity for different domains of coagulating factor VIII

Hemophilia A patients treated with human coagulating factor VIII (FVIII) may develop inhibitory antibodies (inhibitors). Characterization of the inhibitors at the clonal level may help exploring new therapeutic strategies. We have generated lymphoblastoid cell lines (LCLs) producing anti-FVIII antibodies from peripheral blood lymphocytes of hemophilia A patients with high inhibitor titers. We fused the anti-FVIII-positive LCLs with a heteromyeloma, to produce FVIII specific hybridomas. We determined the specificity, isotype, idiotypic and immunoglobulin (Ig) variable region heavy (VH) chain gene family profiles of the secreted antibodies (Ab) by ELISA, immunoblotting and RT-PCR. We established eight hybridomas which produced high titers of anti-FVIII Ab. All hybridomas secreted IgM Ab, associated with either kappa(5/8) or lambda(3/8) light chain. Analysis of the expressed VH genes by RT-PCR revealed that the hybridomas utilized only the VH1 (63%) or the VH3 (37%) gene families. Among the cross-reactive idiotypes (CRIs) we tested, only the VH1 and VK3b-associated CRIs were expressed by 3 hybridomas. Immunoblotting of thrombin-digested FVIII demonstrated distinct patterns of reactivity of the monoclonal Ab (MAb) secreted by the hybridomas, which recognized either the A2 domain of the Fvm heavy chain, or the light chain, or both. Our findings suggest that: a) the isotype of the anti-FVIII Ab secreted by LCLs and hybridoma clones (IgM) differs from that of anti-FVIII Ab in vivo, which are predominantly IgG4: this suggests a negative selection of the isotype-switched FVIII-specific B-cells in the periphery of these patients; b) the anti-FVIII Ab have a biased representation of the VH1 gene family, and c) somatic mutations in the VH genes coding for FVIII specificity occur in the anti-FVIII Ab response, as evidenced by lack of expression of the VH-associated CRI.

CD4+ T cells specific for factor VIII as a target for specific suppression of inhibitor production

The studies we reviewed here have begun to clarify the complex cellular mechanisms involved in the immune response to fVIII, and the circumstances under which fVIII inhibitors develop. Further characterization and comparison of the immune response to fVIII in both hemophilia patients and healthy subjects will help to further elucidate these mechanisms. The murine hemophilia model will hopefully provide further insights into the mechanisms of inhibitor formation, and prove to be a suitable tool for the design and testing of therapeutic strategies aimed at preventing the development of fVIII inhibitors.

Human bronchial epithelial and endothelial cells express alpha7 nicotinic acetylcholine receptors

The epithelial or endothelial cells that line the human bronchi and the aorta express nicotinic acetylcholine receptors (nAChRs) of alpha3 subtypes. We report here that human bronchial epithelial cells (BEC) and aortic endothelial cells (AEC) express also the nAChR alpha7 subunit, which forms functional nAChRs. Polymerase chain reaction and in situ hybridization experiments detected alpha7 subunit mRNA in cultured human BEC and AEC and in sections of rat trachea. The binding of radiolabeled alpha-bungarotoxin revealed a few thousand binding sites per cell in cultured human BEC and human and bovine AEC. Western blot and immunohistochemistry experiments demonstrated that cultured BEC and AEC express a protein(s) recognized by anti-alpha7 antibodies. Whole-cell patch-clamp studies of cultured human BEC demonstrated the presence of fast-desensitizing currents activated by choline and nicotine that were blocked reversibly by methyllycaconitine (1 nM) and irreversibly by alpha-bungarotoxin (100 nM), consistent with the expression of functional alpha7 nAChRs. In some cells, choline activated also slowly decaying currents, confirming previous reports that BEC express functional alpha3beta4 nAChRs. Exposure of cultured BEC to nicotine (1 microM) for 3 days up-regulated functional alpha7 and alpha3 nAChRs, as indicated by the increased number of cells responding to acetylcholine and choline, with both fast-desensitizing currents, which were blocked irreversibly by alpha-bungarotoxin, and with slowly desensitizing currents, which are alpha-bungarotoxin-insensitive currents. The presence of alpha7 nAChRs in BEC and AEC suggests that some toxic effects of tobacco smoke could be mediated through these nicotine-sensitive receptors.

Transgenic expression of IL-10 in T cells facilitates development of experimental myasthenia gravis

Ab to the acetylcholine receptor (AChR) cause experimental myasthenia gravis (EMG). Th1 cytokines facilitate EMG, whereas Th2 cytokines might be protective. IL-10 inhibits Th1 responses but facilitates B cell proliferation and Ig production. We examined the role of IL-10 in EMG by using wild-type (WT) C57BL/6 mice and transgenic (TG) C57BL/6 mice that express IL-10 under control of the IL-2 promoter. We immunized the mice with doses of AChR that cause EMG in WT mice or with low doses ineffective at causing EMG in WT mice. After low-dose AChR immunization, WT mice did not develop EMG and had very little anti-AChR serum Ab, which were mainly IgG1, whereas TG mice developed EMG and had higher levels of anti-AChR serum Ab, which were mainly IgG2, in addition to IgG1. At the higher doses, TG mice developed EMG earlier and more frequently than WT mice and had more serum anti-AChR Ab. Both strains had similar relative serum concentrations of anti-AChR IgG subclasses and IgG and complement at the muscle synapses. CD8(+)-depleted splenocytes from all AChR-immunized mice proliferated in the presence of AChR and recognized a similar epitope repertoire. CD8(+)-depleted splenocytes from AChR-immunized TG mice stimulated in vitro with AChR secreted significantly more IL-10, but less of the prototypic Th1 cytokine IFN-gamma, than those from WT mice. They secreted comparable amounts of IL-4 and slightly but not significantly reduced amounts of IL-2. This suggests that TG mice had reduced activation of anti-Torpedo AChR Th1 cells, but increased anti-AChR Ab synthesis, that likely resulted from IL-10-mediated stimulation of anti-AChR B cells. Thus, EMG development is not strictly dependent on Th1 cell activity.

Human thymuses express incomplete sets of muscle acetylcholine receptor subunit transcripts that seldom include the delta subunit

In myasthenia gravis (MG) the muscle acetylcholine receptor (AChR) is the target of an immune response that might begin in the thymus. The thymus expresses binding sites for specific ligands of muscle AChR, a complex protein composed of alpha, beta, gamma (or epsilon) and delta subunits. The thymus expresses the AChR alpha subunit, but there is controversy regarding the expression in the thymus of the gamma, epsilon and delta subunits. We investigated the presence of messenger RNA (mRNA) for the different muscle AChR subunits in thymus tissue from 20 healthy subjects and 13 myasthenic patients. We detected mRNA for the alpha and epsilon subunits in all samples, for the beta subunit in all but one sample and for the gamma subunit in most samples although at lower levels than the epsilon subunit. Myasthenic thymuses expressed levels of gamma subunit mRNA similar to control thymuses but more abundant epsilon subunit mRNA. None of the myasthenic thymuses and only two control thymuses expressed detectable delta subunit mRNA. This supports the hypothesis that human thymus may express AChR proteins that do not include the delta subunit. Such receptors, which would have different antigenic structure than the muscle AChRs, might have a role in triggering the autoimmune response that causes MG.

Mechanism of the immune response to human factor VIII in murine hemophilia A

Mice genetically deficient in factor VII (fVIII) are a model of hemophilia A. As a first step to reproduce in this mouse model what occurs over time in hemophilia A patients treated with human fVIII (hfVIII), we have investigated the time course and the characteristics of their immune response to hfVIII, after multiple intravenous injections. Anti-hfVIII antibodies appeared after four to five injections. They were IgG1 and to a lesser extent IgG2, indicating that they were induced by both Th2 and Th1 cells. Inhibitors appeared after six injections. CD4+ enriched splenocytes from hfVIII-treated mice proliferated in response to fVIII and secreted IL-10: in a few mice they secreted also IFN-gamma and in one mouse IL-4, but never IL-2. A hfVIII-specific T cell line derived from hfVIII-treated mice secreted both IL-4 and IFN-gamma, suggesting that it included both Th1 and Th2 cells. CD4+ enriched splenocytes of hfIII-treated mice recognized all hfVIII domains. Thus, hemophilic mice develop an immune response to hfVIII administered intravenously similar to that of hemophilia A patients. Their anti-hfVIII antibodies can be inhibitors and belong to IgG subclasses homologous to those of inhibitors in hemophilic patients; their anti-hfVIII CD4+ cells recognize a complex repertoire and both Th1 and Th2 cytokines, and especially IL-10, may drive the antibody synthesis.

Sensitization of CD4+ T cells to coagulation factor VIII: response in congenital and acquired hemophilia patients and in healthy subjects

Antibodies (Ab) that inhibit factor VIII (fVIII) may develop in patients with hemophilia A and rarely in individuals without congenital fVIII deficiency (acquired hemophilia). Synthesis of fVIII inhibitors requires CD4+ T cells. We investigated the proliferative response of blood CD4+ cells from 11 patients with congenital or acquired hemophilia and 12 healthy subjects, to recombinant human fVIII, and to pools of overlapping synthetic peptides spanning the sequences of individual fVIII domains. All patients had CD4+ cells that responded to fVIII. The intensity of the responses fluctuated over time: several patients had brief periods when they did not respond to fVIII. All healthy subjects had transient CD4+ responses to fVIII, that were significantly lower than those of hemophilia patients. Also, healthy subjects responded to fVIII less frequently and for shorter periods than hemophilia patients. All patients and healthy subjects recognized several fVIII domains: the A3 domain was recognized most strongly and frequently. The transient sensitization of CD4+ cells to fVIII in healthy subjects suggests that inadequate tolerization of CD4+ cells to fVIII, due to lack of endogenous fVIII, is an important factor in the development of clinically significant anti-fVIII antibodies in hemophilia A.

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.

Absence of IFN-gamma or IL-12 has different effects on experimental myasthenia gravis in C57BL/6 mice

Immunization with acetylcholine receptor (AChR) causes experimental myasthenia gravis (EMG). Th1 cells facilitate EMG development. IFN-gamma and IL-12 induce Th1 responses: we investigated whether these cytokines are necessary for EMG development. We immunized wild-type (WT) C57BL/6 mice and IFN-gamma and IL-12 knockout mutants (IFN-gamma-/-, IL-12-/-) with Torpedo AChR (TAChR). WT and IFN-gamma-/- mice developed EMG with similar frequency, IL-12-/-mice were resistant to EMG. All strains synthesized anti-AChR Ab that were not IgM or IgE. WT mice had anti-AChR IgG1, IgG2b, and IgG2c, IFN-gamma-/- mice had significantly less IgG2c, and IL-12-/- mice less IgG2b and IgG2c. All mice had IgG bound to muscle synapses, but only WT and IFN-gamma-/- mice had complement; WT mice had both IgG2b and IgG2c, IFN-gamma-/- only IgG2b, and IL-12-/- neither IgG2b nor IgG2c. CD4+ cells from all AChR-immunized mice proliferated in response to AChR and recognized similar epitopes. After stimulation with TAChR, CD4+ cells from IFN-gamma-/- mice secreted less IL-2 and similar amounts of IL-4 and IL-10 as WT mice. CD4+ cells from IL-12-/- mice secreted less IFN-gamma, but more IL-4 and IL-10 than WT mice, suggesting that they developed a stronger Th2 response to TAChR. The EMG resistance of IL-12-/- mice is likely due to both reduction of anti-TAChR Ab that bind complement and sensitization of modulatory Th2 cells. The reduced Th1 function of IFN-gamma-/- mice does not suffice to reduce all complement-fixing IgG subclasses, perhaps because as in WT mice a protective Th2 response is missing.

Neuronal nicotinic receptors in non-neuronal cells: new mediators of tobacco toxicity?

The nicotinic acetylcholine receptors are prototypic ionotropic receptors that mediate fast synaptic transmission. However, also non-excitable cells, and particularly the tegumental cells that line external and internal body surfaces, express acetylcholine receptors of neuronal type sensitive to nicotine. Bronchial epithelial cells, endothelial cells of blood vessels and skin keratinocytes express neuronal nicotinic receptors composed of alpha(3), alpha(5), beta(2) and beta(4) subunits, similar to those expressed in sympathetic ganglia, and neuronal nicotinic receptors composed of alpha(7) subunits. Neuronal nicotinic receptors in tegumental cells are involved in modulating cell shape and motility, and therefore in maintaining the integrity of the surfaces lined by those cells. Neuronal nicotinic receptors in non-neuronal tissues may modulate other functions, including cell proliferation and differentiation. Acetylcholine is synthesized, secreted and degraded by a variety of cells, including the tegumental cells that express neuronal nicotinic receptors. Thus, acetylcholine may function as a local "hormone" that is able to modulate cell functions that require fast adaptation to new conditions. The presence of neuronal nicotinic receptors sensitive to nicotine in tissues known to be involved in tobacco toxicity, like bronchi and blood vessels, raises the possibility that they mediate some of the toxic effects of smoking.

Universal epitopes for human CD4+ cells on tetanus and diphtheria toxins

Previous studies suggested that tetanus and diphtheria toxoids (TTD and DTD, respectively) contain "universal" epitopes for human CD4+ cells (residues 632-651 and 950-969 of TTD and 271-290, 321-350, 351-370, 411-430, and 431-450 of DTD). To investigate whether CD4+ cells of 100 randomly selected subjects recognized those sequences, the proliferation of CD4+ cell-enriched blood lymphocytes to TTD and DTD and individual synthetic universal epitopes was measured. CD4+ cells of 98 subjects recognized both toxoids, those of 1 subject only TTD, and those of 1 only DTD. The TTD peptides and DTD peptides 271-290 and 331-350 were recognized by >/=80% of the toxoid-sensitized subjects. The other DTD sequences were recognized by 63%-71% of subjects. DR-homozygous subjects recognized several universal epitopes less frequently than did DR-heterozygous subjects. The intensity of responses to the epitope peptides correlated with that to TTD or DTD, consistent with recognition of the peptides by CD4+ cells specific for the cognate toxoid.

CD4+ T cell response to factor VIII in hemophilia A, acquired hemophilia, and healthy subjects

These studies have begun to clarify the complex cellular mechanisms involved in the immune response to factor VIII. Although vigorous sensitization of CD4+ cells occurs in healthy subjects, the absence of clinically significant levels of inhibitor antibodies is likely related to the prompt down-regulation of the immune response. It may also be possible that the specific epitope repertoire recognized by CD4+ cells plays a role in the outcome of the immune response to factor VIII. Further characterization and comparison of the CD4+ repertoire in healthy subjects with that of hemophilia patients with and without inhibitors will help clarify which mechanism explains the absence of productive inhibitor synthesis in certain individuals. Also, it might identify CD4+ epitopes recognized by T helper cells that are essential for inhibitor synthesis. Additional studies to further characterize the role of Th1 and Th2 cells in the immune response to factor VIII may also be needed for the design of novel therapeutic strategies aimed at preventing inhibitor development.

Interleukin-4 deficiency facilitates development of experimental myasthenia gravis and precludes its prevention by nasal administration of CD4+ epitope sequences of the acetylcholine receptor

Immunization with acetylcholine receptor (AChR) causes experimental myasthenia gravis (EMG). We investigated EMG in interleukin (IL)-4 knock out B6 (KO) mice, that lack Th2 cells. EMG was more frequent in KO than in wild type B6 mice. KO and B6 mice developed similar amounts of anti-AChR antibodies. They were IgG2a and IgG2b in KO mice, IgG1 and IgG2b in B6 mice. CD4+ cells from KO and B6 mice recognized the same AChR epitopes. Nasal administration of synthetic AChR CD4+ epitopes reduced antibody synthesis and prevented EMG in B6, not in KO mice. Thus, Th2 cells may have protective functions in EMG.

Myasthenia in SCID mice grafted with myasthenic patient lymphocytes: role of CD4+ and CD8+ cells

OBJECTIVES

Acetylcholine receptor (AChR)-specific CD4+ cells are present in MG patients, and synthesis of the high-affinity immunoglobulin G (IgG) autoantibodies (autoAb) against the muscle AChR that causes MG symptoms requires intervention of CD4+ cells. The role of CD4+ cells in MG pathogenesis has been postulated but never proven. MG patients do not have anti-AChR cytotoxic phenomena, and it has been assumed that CD8+ cells do not have a pathogenic role in MG. However, CD8+ cells may facilitate rodent experimental MG, raising the possibility that CD8+ cells might be necessary also in MG. In this study we examined whether CD4+ and CD8+ cells play a role in the pathogenesis of MG and whether CD4+ cells specific for AChR epitope sequences recognized by most MG patients ("universal" epitopes) drive the synthesis of pathogenic antibodies.

METHODS

First we characterized a chimeric human-mouse model of MG in severe combined immunodeficiency (SCID) mice engrafted with blood lymphocytes (BL) from MG patients. We used that model to determine whether CD4+ and CD8+ cells are necessary for transfer of MG symptoms. We engrafted SCID mice intraperitoneum with BL from 19 MG patients and 5 healthy controls. We engrafted some mice with either BL, BL depleted in CD4+ or CD8+ cells from the same patient, or CD4+ depleted BL reconstituted with CD4+ T cells from the same patient, specific for "universal" AChR epitopes or for two unrelated antigens, tetanus and diphtheria toxoids. We tested the mice for myasthenic symptoms for 7 to 18 weeks.

RESULTS

Mice transplanted with BL, or CD8+ depleted BL, or CD4+-depleted BL reconstituted with anti-AChR CD4+ cells from MG patients frequently developed myasthenic weakness. The mice had human anti-AChR Ab in the serum and bound to muscle AChR. Mice transplanted with BL from controls, or CD4+-depleted BL from MG patients, or CD4+-depleted BL from an MG patient reconstituted with CD4+ cells specific for tetanus or diphtheria toxoids did not develop myasthenic weakness or anti-AChR Ab.

CONCLUSIONS

CD4+ cells are necessary for MG pathogenesis; CD8+ cells may not be. CD4+ cells specific for "universal" AChR epitopes help the synthesis of pathogenic Ab.

Subcutaneous administration of T-epitope sequences of the acetylcholine receptor prevents experimental myasthenia gravis

Immunization with acetylcholine receptor (AChR) causes experimental myasthenia gravis (EMG). The s.c. administration to C57B1/6 mice of synthetic AChR CD4+ epitopes, before and during AChR immunization, reduced the epitope-specific CD4+ responses and the anti-AChR Ab synthesis, and prevented EMG. The s.c. administration of solubilized AChR had effects similar to those of peptide treatment. Sham-tolerized mice had only Th1 anti-AChR cells, whereas peptide-treated mice had also Th2 cells, and Th2-induced anti-peptide Ab. Established EMG was not affected by s.c. peptide treatment, whereas it worsened after s.c. administration of solubilized AChR.

TCR-Vbeta usage in the thymus and blood of myasthenia gravis patients

In myasthenia gravis (MG) the muscle acetylcholine receptor (AChR) is the target of an autoimmune response. The anti-AChR response may originate in the thymus, which is abnormal in most MG patients and contains anti-AChR T and B cells. Microbial superantigens (sAg) may trigger autoimmune responses and in this study we sought clues as to whether sAg play a role in the pathogenesis of MG. We investigated the frequency of use of the different TCR Vbeta families by the thymus and blood T cells in MG patients and in control subjects, using a multi-primer PCR assay. Identical TCR-Vbeta usage was found in the thymi of MG patients and controls, except Vbeta2, which showed a small increase in MG patients' thymi. Blood T cells of MG patients used Vbeta4, Vbeta6, Vbeta15, Vbeta16 and Vbeta24 significantly more than those of the controls. Vbeta4 and Vbeta6 are the gene families most frequently used by anti-AChR CD4(+) cells in MG patients. Blood T cells from MG patients used Vbeta12, Vbeta14, Vbeta17 and Vbeta18 significantly less than controls. MG patients used Vbeta4 and Vbeta6 significantly more in the blood than in the thymus, while the opposite occurred for Vbeta7, Vbeta12 and Vbeta14. Controls used Vbeta17 more and Vbeta24 less in the blood than in the thymus. The preferential expansion of Vbeta4 and Vbeta6 in MG patients might reflect the immunodominance of certain AChR epitopes, or the action of a sAg outside the thymus. The minimal differences in the TCR-Vbeta usage in the blood and thymus of control subjects might be due to expansion of T cell clones specific for common antigens. Identical Vbeta usage in the thymi of MG patients and controls does not support an important role of the thymus as the location of anti-AChR sensitization when MG is clinically evident. The differences observed in the Vbeta usage in blood and thymi of MG patients are likely to be due to preferential Vbeta usage by the anti-AChR T cells in the blood.

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.

Human and rodent bronchial epithelial cells express functional nicotinic acetylcholine receptors

We demonstrated previously that human skin keratinocytes express acetylcholine receptors (AChRs) sensitive to acetylcholine and nicotine, which regulate cell adhesion and motility. We demonstrate here that human and rodent bronchial epithelial cells (BECs) express AChRs similar to those expressed by keratinocytes and by some neurons. Patch-clamp experiments demonstrated that the BEC AChRs are functional, and they are activated by acetylcholine and nicotine. They are blocked by kappa-bungarotoxin, a specific antagonist of the AChR isotypes expressed by neurons in ganglia. Their ion-gating properties are consistent with those of AChR isotypes expressed in ganglia, formed by alpha3, alpha5, and beta2 or beta4 subunits. Reverse transcription-polymerase chain reaction and in situ hybridization experiments demonstrated the presence in BECs of mRNA transcripts for all those AChR subunits, both in cell cultures and in tissue sections, whereas we could not detect transcripts for the alpha2, alpha4, alpha6, and beta3 AChR subunits. The expression of alpha3 and alpha5 proteins in BEC in vivo was verified by the binding of subunit-specific antibodies to sections of trachea. Mecamylamine and kappa-bungarotoxin, which are cholinergic antagonists able to block the ganglionic alpha3 AChRs, caused a reversible change of the cell shape of cultured, confluent human BECs. This resulted in a reduction of the area covered by the cell and in cell/cell detachment. The presence of AChRs sensitive to nicotine on the lining of the airways raises the possibility that the high concentrations of nicotine resulting from tobacco smoking will cause an abnormal activation, a desensitization, or both of the bronchial AChRs. This may mediate or facilitate some of the toxic effects of cigarette smoking in the respiratory system.

Human vascular endothelial cells express functional nicotinic acetylcholine receptors

ACh receptors sensitive to nicotine (nAChR) are present in human skin keratinocytes and in bronchial epithelial cells. They are stimulated by ACh secreted by the same cells that express them, and they modulate cell motility and shape. A variety of non-neuronal tissues, including endothelial cells, synthesize ACh, which raises the possibility that they are sensitive to nicotine. We demonstrate here that endothelial cells that line blood vessels express functional nAChRs. Their structure and ion-gating properties are similar to those of the nAChRs expressed by ganglionic neurons and by skin keratinocytes and bronchial epithelial cells. In situ hybridization experiments using primary cultures of endothelial cells from human aorta demonstrated the presence in these cells of the subunits believed to contribute to ganglionic ACh receptors (AChRs) of the alpha3 subtype: alpha3, alpha5, beta2 and beta4. Binding of radiolabeled epibatidine-a high-affinity specific ligand of certain neuronal AChRs, including the alpha3 subtypes-revealed the presence of approximately 900 specific binding sites per cell. We assessed the presence of functional AChRs by patch-clamp experiments. Cultured human endothelial cells express ion channels that are opened by (+)-anatoxin-a and are blocked by dihydro-beta-erythroidine. These are specific agonist and antagonist, respectively, of neuronal AChRs of the alpha3 subtype. The ion-gating properties of the endothelial AChRs were similar to those of neuronal ganglionic AChRs. The presence of AChRs sensitive to nicotine in endothelial cells may be related to the toxic effects of nicotine on the vascular system.

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.

T-cell recognition of muscle acetylcholine receptor subunits in generalized and ocular myasthenia gravis

OBJECTIVES

Our purpose was to identify the muscle acetylcholine receptor (AChR) subunits recognized by autoimmune CD4+ T cells in myasthenia gravis (MG) and determine whether they differ in generalized (gMG) and ocular MG (oMG), and as gMG progresses.

METHODS

We tested the proliferative response of blood CD4+ cells from 25 patients with gMG and four patients with oMG to synthetic peptides spanning the sequence of each subunit of human muscle AChR. We also investigated the antisubunit response of Th1 cells (a CD4+ subset frequently involved in autoimmune phenomena) using an enzyme-linked immunospot (ELISPOT) assay of antigen-induced secretion of interferon-gamma by individual CD4 cells.

RESULTS

In gMG patients both the total CD4+ population and the Th1 subset recognized all AChR subunits to comparable extents. oMG patients recognized the AChR epsilon subunit minimally, and other subunits consistently and more strongly. gMG patients whose disease had lasted less than 5 years had lower antisubunit responses, and several of them did not recognize some AChR subunits; patients whose disease had lasted for 5 or more years had higher antisubunit responses and always responded to all AChR subunits.

CONCLUSIONS

CD4+ and Thl responses in MG involve the entire AChR molecule. This likely results from spreading of the CD4+ sensitization to increasingly larger parts of the AChR as the disease progresses. The differential recognition of AChR subunits in oMG might be related to the preferential involvement of extrinsic ocular muscles, which express AChR containing the gamma subunit.

Amino acids within residues 181-200 of the nicotinic acetylcholine receptor alpha1 subunit involved in nicotine binding

Structural determinants of L-[3H]nicotine binding to the sequence flanking Cys 192 and Cys 193 of the Torpedo acetylcholine receptor alpha1 subunit were investigated using synthetic peptides (residues 181-200) and fusion proteins (residues 166-211). Nicotine binding at a single concentration (30 nM) was compared with 71 peptides and fusion proteins in which individual amino acids at positions 181-200 were substituted. Substitution of Lys 185, Tyr 190, Cys 192, Cys 193, Thr 196, and Tyr 198 resulted in the greatest reduction in nicotine binding. Equilibrium binding of [3H]nicotine to peptide 181-200 revealed a binding component with an apparent KD of 1.2 microM. Substitution of Lys 185 (with Glu), His 186, Tyr 190, Cys 192, Cys 193, and Tyr 198 resulted in a significant reduction in affinity. Affinity was not affected significantly by substitution of Arg 182, Lys 185 (with Gly or Arg), Val 188, Tyr 189, Pro 194, Asp 195, Thr 196, and Asp 200. It is concluded that Lys 185, His 186, Tyr 190, Cys 192, Cys 193, and Tyr 198 play the greatest role in nicotine binding to residues 181-200 of the alpha1 subunit. Previous studies have implicated Tyr 190, Cys 192, Cys 193, and Tyr 198 in agonist binding to the acetylcholine receptor. These results confirm a role for these residues and also demonstrate a function for Lys 185 and His 186 in nicotine binding.

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.

Prevention of experimental myasthenia gravis by nasal administration of synthetic acetylcholine receptor T epitope sequences

T cell tolerization prevents and improves T cell-mediated experimental autoimmune diseases. We investigated here whether similar approaches could be used for antibody (Ab)-mediated autoimmune diseases. Myasthenia gravis, caused by IgG Ab against muscle acetylcholine receptor (AChR), is perhaps the best characterized of them. We used an animal model, experimental myasthenia gravis induced in C57Bl/6 mice by immunization with Torpedo acetylcholine receptor (TAChR), to demonstrate that nasal administration of synthetic sequences of the TAChR alpha-subunit- forming epitopes recognized by anti-TAChR CD4+ T helper cells (residues alpha150-169, alpha181-200, and alpha360-378), given before and during immunization with TAChR, causes decreased CD4+ responsiveness to those epitopes and to TAChR, reduced synthesis of anti-TAChR Ab, and prevented experimental myasthenia gravis. These effects were not induced by nasal administration of synthetic epitopes of diphtheria toxin. Secretion of IL-2, IL-4, and IL-10 by spleen T cells from TAChR immunized mice, in response to challenge with TAChR in vitro, indicated that in sham-tolerized mice only Th1 cells responded to TAChR, while peptide-treated mice had also an AChR-specific Th2 response. The TAChR peptide treatment induced also in vitro anergy to the TAChR of the spleen T cells, which was reversed by IL-2.

TCR vbeta usage of TSH receptor-specific CD4+ T cells in Graves' disease patients and healthy humans

Healthy humans have CD4+ T cells specific for self-components. Since autoreactive T cells in autoimmune patients may use a limited number of TCR V-region genes, we investigated here whether this also occurs for the potentially autoreactive CD4+ cells present in healthy persons. We studied CD4+ cells specific for human TSH receptor (TSHr) sequences, that are present with high frequency in healthy subjects and, as expected, in Graves' disease (GD) patients. We used short-term CD4+ cell lines propagated from four GD patients and five healthy subjects by cycles of stimulation with a pool of overlapping synthetic peptides corresponding to the putative extracellular parts of the TSHr sequence. The lines recognized the pool of TSHr peptides specifically and vigorously. Their epitope repertoire had been characterized previously: each line recognized one or a few TSHr peptides, different for each subject. We determined their TCR Vbeta usage by a semi-quantitative reverse transcriptase PCR assay, using primers specific for each known human Vbeta region family, in conjunction with a constant region primer. Six lines preferentially used one Vbeta family (42-94%), different for each line. In all lines, three or less Vbeta families accounted for approximately 60% or more of the Vbeta usage. Different Vbeta regions were used by each subject. There was no obvious difference between the Vbeta usage of the lines from GD patients and healthy controls. These results suggest that a limited pool of potentially autoreactive T cells survives clonal deletion. The pathogenic CD4+ cells involved in autoimmune diseases are likely recruited from that pool, since they have similar characteristics of epitope and TCR repertoire as the CD4+ cells specific for the same autoantigen in healthy subjects.

Th1 epitope repertoire on the alpha subunit of human muscle acetylcholine receptor in myasthenia gravis

In myasthenia gravis (MG), CD4+ T helper cells recognize the muscle acetylcholine receptor (AChR) alpha subunit. We investigated the epitope repertoire of anti-AChR blood CD4+ Th1 cells from 13 myasthenic patients and three healthy controls, using overlapping synthetic peptides screening the alpha subunit sequence and an enzyme-linked immunospot (ELISPOT) assay that detects antigen-induced interferon-gamma secretion of individual Th1 cells. All patients recognized a pool of the alpha subunit peptides. All but one patient recognized numerous peptides. Each patient had an individual pattern of peptide recognition, but most or all patients recognized four sequences (residues 48-67, 101-137, 304-322, and 403-437) that stimulated relatively large numbers of Th1 cells. They include previously identified "immunodominant" sequences recognized by AChR-specific CD4+ T cell lines from myasthenic patients. Peptide 1-14 was also recognized frequently. The controls recognized, with a low precursor frequency, the peptide pool and a few peptides that frequently included the immunodominant sequences described above. The present results demonstrate that Th1 cells are involved in the anti-AChR response in MG and that their epitope repertoire is very complex. This indicates that when MG is clinically evident, the AChR itself is the sensitizing antigen and the target of the autoimmune Th1 cells, although it does not exclude that molecular mimicry between one AChR epitope and a microbial structure may have triggered this autoimmune response. Although the complexity of the Th1 repertoire suggests that development of specific immunosuppressive treatments targeted on epitopes recognized by autoimmune T cells will be difficult, the existence of immunodominant T epitope sequences might facilitate that task.

Expression of the alpha 7 subunit of the nicotinic acetylcholine receptor in normal and myasthenic human thymuses

The nicotinic acetylcholine receptor (AChR) is a transmembrane glycoprotein composed of five homologous subunits. Different isoforms of the AChR alpha subunit exist (alpha 1 to alpha 9). Of them, alpha 1 is expressed in muscle, alpha 2 to alpha 9 in neuronal cells. Muscle AChR is the target autoantigen in the autoimmune disease myasthenia gravis (MG). The thymus is implicated in MG pathogenesis, and the anti-AChR autoimmune response may start in this tissue, that expresses the muscle-type alpha 1 subunit as well as other muscle AChR subunits. The thymus also expresses the "neuronal" alpha 3 and alpha 5 subunits. By using polymerase chain reaction and other molecular techniques, we demonstrate here expression of the AChR alpha 7 subunit transcript in thymuses from both myasthenic patients and normal subjects. The alpha 7 subunit can form homo-oligomeric functional AChR complexes that, like muscle AChR, bind alpha-bungarotoxin. The demonstration of expression of the alpha 7 subunit in the thymus suggests that alpha-bungarotoxin binding, functional AChRs of the neuronal type are normally present in the thymus.

TCR V beta usage by acetylcholine receptor-specific CD4+ T cells in myasthenia gravis

In myasthenia gravis the muscle acetylcholine receptor (AChR) is the target of an autoimmune response. AChR epitopes recognized by CD4+ T cells in myasthenic patients have been identified. AChR-specific CD4+ cell lines can be propagated by stimulation of blood lymphocytes with synthetic or biosynthetic AChR sequences. We analysed, using a semi-quantitative PCR assay, the T cell receptor (TCR) V beta usage of 16 anti-AChR polyclonal CD4+ T cell lines of known epitope specificity, propagated from myasthenic patients using pools of overlapping peptides corresponding to the sequence of an AChR subunit, or individual synthetic AChR sequences. Twelve lines had been propagated for less than 2 months, four lines for 3.5-5 months. Most lines had limited V beta usage, but in most cases different V beta regions were used for different epitopes in the same patient, and for the same epitope in different patients. In a few patients, the same V beta regions were used for recognition of different epitopes. The V beta 4 and V beta 6 regions were used most frequently. These findings suggest that the potentially autoimmune T cells that survive clonal deletion have a limited TCR repertoire. Although the present data do allow conclusions on the role of a superantigen in triggering the anti-AChR autoimmune response, the finding that different V beta regions were used in different patients does not support an important role of a superantigen in the maintenance of the CD4+ response in myasthenia gravis.

Epitope repertoire of human CD4+ T cells on tetanus toxin: identification of immunodominant sequence segments

Sequence regions of tetanus toxin-forming CD4+ cell epitopes in 8 HLA-disparate subjects were identified. Overlapping synthetic peptides corresponding to the complete tetanus toxin sequence were used to test, in a proliferation assay, unselected blood CD4+ cells or CD4+ cell lines propagated by stimulation with tetanus toxoid. The CD4+ cell lines recognized most peptides recognized by the blood CD4+ cells and they recognized additional peptides. Their responses were stronger than those of unselected blood CD4+ cells. Two peptides were recognized by all subjects: one largely overlapped a tetanus toxin sequence region previously identified as a "universal" T cell epitope. Thirteen other peptides elicited a CD4+ cell response in 6 or 7 of the 8 subjects, and another 10 elicited responses in 5 subjects.

Binding of native kappa-neurotoxins and site-directed mutants to nicotinic acetylcholine receptors

The kappa-neurotoxins are useful ligands for the pharmacological characterization of nicotinic acetylcholine receptors because they are potent antagonists at only a subgroup of these receptors containing either alpha 3- or alpha 4-subunits (IC50 < or = 100 nM). Four of these highly homologous, 66 amino acid peptides have been purified from the venom of Bungarus multicinctus (kappa-bungarotoxin (kappa-Bgt), kappa 2-Bgt, kappa 3-Bgt] and Bungarus flaviceps [kappa-Fvt)]. Two approaches were taken to examine the binding of these toxins to nicotinic receptors. First, venom-derived kappa-Fvt and kappa-Bgt were radioiodinated and the specific binding was measured of these toxins to overlapping synthetic peptides (16-20 amino acids in length) prepared based on the known sequence of the nicotinic receptor alpha 3-subunit. At least two main regions of interaction between the toxins and the receptor subunit were identified, both lying in the N-terminal region of the subunit that is exposed to the extracellular space. The second approach examined the importance of several sequence position in kappa-Bgt for binding to alpha 3-containing receptors in autonomic ganglia and alpha 1-containing muscle receptors. This was done using site-directed mutants of kappa-Bgt produced by an Escherichia coli expression system. Arg-34 and position 36 were important for binding to both receptor subtypes, while replacing Gln-26 with Trp-26 (an invariant in alpha-neurotoxins) increased affinity for the muscle receptor by 8-fold. The results confirm that kappa-neurotoxins bind potently to the alpha 3-subunit and bind with considerably reduced affinity (Kd approximately 10 microM) to muscle receptors. Site-directed mutagenesis of recombinant kappa-Bgt is thus an important approach for the study of structure-function relationships between kappa-Bgt and nicotinic receptors.

Antibodies as tools to study the structure of membrane proteins: the case of the nicotinic acetylcholine receptor

The nicotinic acetylcholine receptor is the prototype of the ionotropic receptor superfamily of proteins, which includes the closely related gamma- aminobutyric acid type A and glycine receptors, and more distantly related serotonin type-3 and glutamate receptors. Several models of the transmembrane topology of the nicotinic acetylcholine receptor subunits were originally proposed based on hydropathy analysis of their deduced amino acid sequences. Antibodies specific to different epitopes of the nicotinic acetylcholine receptor have proven to be valuable probes for examining the validity of those models. Despite important caveats, a viable model for the transmembrane structure and functional topology of the nicotinic acetylcholine receptor subunits has been obtained from the antibody mapping studies. This model, and the associated methodological shortcomings and obstacles that were overcome in the process of its formulation, can legitimately be extended to other members of the ionotropic receptor superfamily and to other membrane proteins as well.

Epitope repertoire of human CD4+ lines propagated with tetanus toxoid or with synthetic tetanus toxin sequences

The use of synthetic antigen sequences allows propagation in vitro of T cell lines and clones specific for rare antigens, or for individual epitopes. In the present study we investigated the extent of similarity of the epitope repertoire of CD4+ T cell line specific for the antigen tetanus toxin (TTX), propagated with the complete molecule of tetanus toxoid (TTD), and with the synthetic TTX peptides. We propagated from two healthy subjects CD4+ T cell lines specific for TTD, by cycles of stimulation in vitro with TTD or with pools of overlapping synthetic peptides, 20 residues long and overlapping by five residues, corresponding to all or part of the tetanus toxin (TTX) sequence. One pool corresponded to the complete TTX sequence (peptide pool). Two other pools corresponded to residues 1-305 of the TTX light chain and 476-780 of the TTX heavy chain (peptide minipools). The peptide pool-propagated lines recognized TTD vigorously, at levels comparable with those of the TTD-propagated lines. They recognized several peptides, most of which were also recognized by the TTD-propagated line from the same subject. They also recognized to a low extent a few peptides not recognized by the corresponding TTD-propagated line, which might contain cryptic epitopes. The TTD-propagated lines recognized also several peptides that did not elicit a detectable response by the lines propagated with the complete peptide pool. The peptide minipool propagated lines recognized most of the peptides recognized by the TTD-propagated lines. They also recognized several peptides that did not elicit a measurable response of the TTD-propagated line from the same subject, which might contain cryptic epitopes. Very few peptides recognized by the TTD-propagated line did not evoke a response from the peptide minipool propagated lines. To verify that the response to the TTD molecule of the lines propagated with the peptide pools reflected the response of clones recognizing different epitopes produced upon in vitro processing of the TTD molecule, we propagated from each of the two subjects CD4+ T cell lines by stimulation with individual peptide recognized by the TTD-specific lines of that subject (13 peptide-specific lines from subject #1, and 15 from subject #2). All lines responded to the presence of TTD to an extent comparable to the response induced by the same concentration of the relevant peptide, demonstrating that propagation by synthetic epitope sequences allows expansion of T cell clones specific for epitopes which result from processing of the complete TTD molecule. Therefore, whereas the use of very large pools of synthetic antigen peptides for propagation of antigen specific human CD4+ cell lines might lead to loss of clones recognizing less immunogenic sequence regions, peptide pools comprising a relatively limited number of synthetic sequences allow propagation of the majority of the antigen specific T cell clones. The use of peptide pools, and especially of limited peptide pools, results in propagation of polyclonal T cell lines having a more diverse repertoire than the lines propagated by stimulation with the complete antigen molecule. The T clones propagated by the use of the short peptide sequences, which are not expanded when the complete antigen molecule is used, may recognize poorly processed, cryptic epitopes. This approach may be adopted to propagate and detect minor clonal populations, recognizing less immunogenic parts of the antigen.

MHC class II presentation of human acetylcholine receptor in Myasthenia gravis: binding of synthetic gamma subunit sequences to DR molecules

Embryonic muscle acetylcholine receptor (AChR) is composed of four subunits-alpha, beta, gamma, and delta. Upon innervation, the gamma subunit is substituted by a homologous epsilon subunit. Embryonic muscle AChR might be the original autoantigen in Myasthenia gravis (MG) because it is expressed in the thymus, where the anti-AChR response might start, as well as in adult extrinsic ocular muscles, which are a preferential target in MG. MG patients have antibodies specific for embryonic AChR and CD4+ T cells, which recognize epitopes on the gamma subunit, in association with DR molecules. In the present study, we investigated the binding to several purified DR molecules (DRB1*0101, DRB1*0201, DRB1*0401 and DRB1*0701) of overlapping synthetic peptides, screening the human gamma subunit sequence. Binding of the peptides to the DR molecules were determined from their ability to compete with radiolabelled peptide probes for binding to purified DR molecules. All peptides which were recognized by CD4+ cells bound to the relevant DR molecule. On the other hand, some AChR peptides not recognized by CD4+ cells of MG patients bound well to one or more DR molecules. In terms of relative ability to bind to the DR molecules tested, the gamma subunit behaved like an exogenous antigen: some AChR peptide sequences uniquely bound one DR molecule, others bound several DR alleles, while others did not bind any of the DR molecules tested.

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.

A nicotinic acetylcholine receptor regulating cell adhesion and motility is expressed in human keratinocytes

Acetylcholine is synthesized and released by human epidermal keratinocytes and modulates the adhesion and motility of these cells. To understand the molecular basis of the effects of acetylcholine on keratinocytes, we investigated the presence, pharmacology, structure, and function of nicotinic acetylcholine receptors in human epidermal keratinocytes. Patch-clamp studies indicated that keratinocytes express acetylcholine receptors with ion gating and pharmacologic properties similar to those observed so far only in neurons, and containing the alpha 3 subunit. Specific binding of the receptor-specific ligand 125I-kappa-bungarotoxin revealed approximately 5500 binding sites per cell on undifferentiated keratinocytes in cell cultures and approximately 35,400 binding sites per cell on mature keratinocytes freshly isolated from human neonatal foreskins. Antibody binding and polymerase chain reaction experiments demonstrated the presence of alpha 3, beta 2, and beta 4 nicotinic receptor subunits. Binding of subunit-specific antibodies indicated that nicotinic receptors were associated with the suprabasal keratinocytes in epidermis and localized to the cell membranes of differentiated keratinocytes in cell cultures. Acetylcholine and the nicotinic agonist nicotine increased cell-substrate and cell-cell adherence of cultured keratinocytes and stimulated their lateral migration. The specific antagonists kappa-bungarotoxin and mecamylamine caused cell detachment and abolished migration. Thus, a nicotinic receptor expressed in keratinocytes may mediate acetylcholine control of keratinocyte adhesion and motility.