Mechanisms by which the I-ABM12 mutation influences susceptibility to experimental myasthenia gravis: a study in homozygous and heterozygous mice

How clinical trials of myasthenia gravis can inform pre-clinical drug development

Pre-clinical evaluations often provide the rationale for therapeutic assessments in humans; however, in many diseases an agent found successful in animal models does not show efficacy in human subjects. Our contention is that the approach of rigorous, clinical trials can be used to inform how preclinical assessments should be performed. Clinical trials in humans are carefully designed investigations executed with consideration of critical methodological issues, such as pre-specified entrance criteria and validated, outcome measures coupled with power analysis to identify sample size. Blinding of evaluators of subjective measures and randomization of subjects are also critical aspects of trial performance. Investigative agents are also tested in subjects with active disease, rather than prior to disease induction as in some pre-clinical assessments. Application of standard procedures, including uniform reporting standards, would likely assist in reproducibility of pre-clinical experiments. Adapting methods of clinical trial performance will likely improve the success rate of therapeutics to ultimately achieve human use.

Maintenance of immune tolerance to a neo-self acetylcholine receptor antigen with aging: implications for late-onset autoimmunity

Age-related changes in immune regulation are likely to account for the age-associated increase in serum autoantibody levels and in certain autoimmune disorders, such as myasthenia gravis (MG). To demonstrate directly a loss of immune tolerance in older individuals, responses to the acetylcholine receptor, the autoantigen in MG, were assessed in transgenic mice expressing the Torpedo californica acetylcholine receptor (TAChR) alpha-chain as a neo-self Ag. T cells from young transgenic mice had been shown to be tolerant to p146-162, the TAChR alpha-chain peptide that dominated young nontransgenic T cell responses in vitro. The immunodominance of p146-162 was not lost with age; fine specificity was preserved. Moreover, T cell tolerance to p146-162, as well as to other epitopes of the TAChR alpha-chain extracellular domain, was maintained in old transgenic mice. Even multiple TAChR immunizations coupled with the MG-enhancing cytokine, IL-12, did not break tolerance. In addition, T cells exhibiting CD4 upregulation, an early activation marker, were reduced in frequency equivalently in old and young transgenic animals, suggesting that immune regulation in this model was not impacted by aging. Moreover, B cell tolerance was also maintained with age. The persistence of immune tolerance was accompanied by an increase in the proportion of T regulatory cells; it is speculated that this may compensate for deficiencies in central tolerance that occur owing to thymic involution. In summary, our study reveals, for the first time, that some immune tolerance mechanisms do survive aging; this suggests that certain late-onset autoimmune disorders may be induced by a specific insult that disrupts immune homeostasis.

Severely impaired neuromuscular synaptic transmission causes muscle weakness in theCacna1a-mutant mouserolling Nagoya

The ataxic mouse rolling Nagoya (RN) carries a missense mutation in the Cacna1a gene, encoding the pore-forming subunit of neuronal Ca(v)2.1 (P/Q-type) Ca2+ channels. Besides being the predominant type of Ca(v) channel in the cerebellum, Ca(v)2.1 channels mediate acetylcholine (ACh) release at the peripheral neuromuscular junction (NMJ). Therefore, Ca(v)2.1 dysfunction induced by the RN mutation may disturb ACh release at the NMJ. The dysfunction may resemble the situation in Lambert-Eaton myasthenic syndrome (LEMS), in which autoantibodies target Ca(v)2.1 channels at NMJs, inducing severely reduced ACh release and resulting in muscle weakness. We tested neuromuscular function of RN mice and characterized transmitter release properties at their NMJs in diaphragm, soleus and flexor digitorum brevis muscles. Clinical muscle weakness and fatigue were demonstrated using repetitive nerve-stimulation electromyography, grip strength testing and an inverted grid hanging test. Muscle contraction experiments showed a compromised safety factor of neuromuscular transmission. In ex vivo electrophysiological experiments we found severely impaired ACh release. Compared to wild-type, RN NMJs had 50-75% lower nerve stimulation-evoked transmitter release, explaining the observed muscle weakness. Surprisingly, the reduction in evoked release was accompanied by an approximately 3-fold increase in spontaneous ACh release. This synaptic phenotype suggests a complex effect of the RN mutation on different functional Ca(v)2.1 channel parameters, presumably with a positive shift in activation potential as a prevailing feature. Taken together, our studies indicate that the gait abnormality of RN mice is due to a combination of ataxia and muscle weakness and that RN models aspects of the NMJ dysfunction in LEMS.

The role of subclass switching in the pathogenesis of endemic pemphigus foliaceus

Endemic pemphigus foliaceus, like the sporadic form seen in the developed world, is mediated by IgG antibodies to desmoglein-1. We studied an endemic focus in Limao Verde, Brazil, where disease prevalence is 3.4%. We previously detected IgG antibodies to desmoglein-1 in 97% of patients, but also in 55% of normal subjects in the endemic focus, with progressively lower levels in normal subjects in surrounding areas. An environmental trigger is hypothesized to explain these and other findings. In this study we sought to determine if patients and enzyme-linked-immunosorbent-assay-positive normal subjects in Limao Verde differ in IgG subclass response to desmoglein-1. We developed a sensitive and specific subclass enzyme-linked immunosorbent assay using recombinant desmoglein-1 and standardized the assay to enable comparability between the four subclasses. We found that normal subjects have an IgG1 and IgG4 response, whereas patients have similar levels of IgG1 but a mean 19.3-fold higher IgG4 response. Patients in remission have a weak IgG4 response, and a 74.3-fold higher IgG4 response is associated with active disease. Finally, in five patients in whom we had blood samples from both before and after the onset of clinical disease, a mean 103.08-fold rise in IgG4 was associated with onset of clinical disease, but only a mean 3.45-fold rise in IgG1. These results suggest that the early antibody response in normal subjects living in the endemic area and in patients before the onset of clinical disease is mainly IgG1. Acquisition of an IgG4 response is a key step in the development of clinical disease.

Markedly enhanced susceptibility to experimental autoimmune myasthenia gravis in the absence of decay-accelerating factor protection

Myasthenia gravis (MG) is an autoimmune neuromuscular transmission disorder characterized by loss of acetylcholine receptors (AChR's) due primarily to the production of anti-AChR autoantibodies. In this study we investigated whether the presence of decay-accelerating factor (DAF or CD55), an intrinsic complement regulator, protects against the development of disease. Experimental autoimmune MG was induced in Daf1(-/-) mice (devoid of neuromuscular DAF protein) and their Daf1(+/+) littermates by injection of rat anti-AChR mAb McAb-3. After twenty-four hours, grip strength assessment revealed that Daf1(-/-) mice exhibited hold times of less than 30 seconds, compared with more than 8 minutes for the Daf1(+/+) controls. The weakness was reversed by edrophonium, consistent with a myasthenic disorder. Immunohistochemistry revealed greatly augmented C3b deposition localized at postsynaptic junctions, and radioimmunoassays showed more profound reductions in AChR levels. Electron microscopy demonstrated markedly greater junctional damage in the Daf1(-/-) mice compared with the Daf1(+/+) littermates. Control studies showed equivalent levels of other cell surface regulators, i.e., Crry and CD59. The results demonstrate that mice that lack DAF are markedly more susceptible to anti-AChR-induced MG, which simulates the primary mechanism in the human disease, and strongly suggest that in disease flares complement inhibitors might have therapeutic value.

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.

Interleukin-12 enhances clinical experimental autoimmune myasthenia gravis in susceptible but not resistant mice

Experimental autoimmune myasthenia gravis (EAMG) is induced by antibodies against the nicotinic acetylcholine receptor (AChR). Studies indicate a role for interferon-gamma (IFN-gamma) in EAMG. We examined the effect of IL-12, a major inducer of IFN-gamma production, on EAMG in C57BL/6 mice. Five doses of IL-12 accelerated and enhanced clinical disease in AChR-immunized mice. Control B6 mice, IFN-gamma gene-knockout mice, and EAMG-resistant bm12 mice showed no enhancement of disease. Shifting to a Th1-type antibody isotype distribution was insufficient to cause disease. Other factors, such as direct effects of Th1 cytokines on muscle tissue, may be involved in EAMG susceptibility.

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.

Animal models of myasthenia gravis

Myasthenia gravis (MG) is an antibody-mediated, autoimmune neuromuscular disease. Animal models of experimental autoimmune myasthenia gravis (EAMG) can be induced in vertebrates by immunization with Torpedo californica acetylcholine receptors (AChR) in complete Freund's adjuvant. The MHC class II genes influence the cellular and humoral immune response to AChR and are involved in the development of clinical EAMG in mice. A dominant epitope within the AChR alpha146-162 region activates MHC class II-restricted CD4 cells and is involved in the production of pathogenic anti-AChR antibodies by B cells. Neonatal or adult tolerance to this T-cell epitope could prevent EAMG. During an immune response to AChR in vivo, multiple TCR genes are used. The CD28-B7 and CD40L-CD40 interaction is required during the primary immune response to AChR. However, CTLA-4 blockade augmented T- and B-cell immune response to AChR and disease. Cytokines IFN-gamma and IL-12 upregulate, while IFN-alpha downregulates, EAMG pathogenesis. However, the Th2 cytokine IL-4 fails to play a significant role in the development of antibody-mediated EAMG. Systemic or mucosal tolerance to AChR or its dominant peptide(s) has prevented EAMG in an antigen-specific manner. Antigen-specific tolerance and downregulation of pathogenic cytokines could achieve effective therapy of EAMG and probably MG.

Myasthenia gravis and its animal model: T cell receptor expression in an antibody mediated autoimmune disease

Myasthenia gravis (MG) is a prototypic antibody-mediated autoimmune disease. Since the primary target antigen of the autoimmune response is known and a well-characterized animal model is available, MG is often considered an excellent situation for the application of novel specific immunotherapies, many of which are directed at T lymphocytes. CD4+ helper T cells are required for the development of the animal model, experimental autoimmune MG (EAMG). Even though the target antigen, acetylcholine receptor (AChR) is immunologically complex, the T cell response to AChR in mice is dominated by recognition of a single peptide by about 50% of the T cells. These T cells, in turn, utilize a restricted set of TCR gene elements and conserved CDR3 regions. While specific therapy directed at the immunodominant T cells is capable of reducing the magnitude of the anti-AChR response, considerable flexibility is apparent and reveals the ability of additional T cells to provide the requisite B cell help. In human MG patients, AChR-specific T cells have been identified but in many studies the frequencies were surprisingly low. In a very few cases, AChR-specific T cells have been cloned from MG patients. Analysis reveals heterogeneity in epitope recognition and MHC restriction. Little information on TCR structure is available. Our own studies using antigen-specific as well as non-specific methods for examining clonal T cell expansions in MG have led to an alternative hypothesis concerning T-B collaboration in MG.

Experimental autoimmune encephalomyelitis induced in B6.C-H-2bm12 mice by myelin oligodendrocyte glycoprotein: effect of MHC class II mutation on immunodominant epitope selection and fine epitope specificity of encephalitogenic T cells

The effect of the bm12 mutation on susceptibility to MOG-induced EAE, TCR repertoire and fine epitope specificity of the encephalitogenic T-cells, was assessed. prMOG35-55 was encephalitogenic for H-2bm12 and H-2b mice. Despite only minor differences in TCRVbeta expression and fine epitope specificity, H-2bm12/ and H-2b/prMOG35-55-specific T-cells failed to recognize Ab/prMOG35-55 and Abm12/prMOG35-55, respectively. rhMOG-induced EAE was milder in H-2bm12 mice, possibly as a result of co-dominant responses to prMOG35-55 and to the non-encephalitogenic pMOG94-116, rather than a single dominant response to prMOG35-55 in H-2b mice.

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.

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.

Determinant spreading and immune responses to acetylcholine receptors in myasthenia gravis

In myasthenia gravis (MG), antibodies to the muscle acetylcholine receptor (AChR) cause muscle weakness. Experimental autoimmune myasthenia gravis (EAMG) can be induced by immunisation against purified AChR; the main immunogenic region (MIR) is a conformation-dependent site that includes alpha 67-76. EAMG can also occur after immunisation against extracellular AChR sequences, but this probably involves intramolecular determinant spreading. In MG patients, thymic hyperplasia and germinal centres are found in about 50%, and thymoma in 10-15%. The heterogeneous, high affinity, IgG anti-AChR antibodies appear to be end-products of germinal centre responses, and react mainly with the MIR or a site on fetal AChR; the latter contains a gamma subunit and is mainly expressed on myoid cells in the thymic medulla. T cells cloned against recombinant AChR subunits recognise principally two naturally processed epitopes: epsilon 201-219 derived from adult AChR which is expressed in muscle, and sometimes in thymic epithelium, and alpha 146-160, common to fetal and adult AChR. Since AChR is not normally co-expressed with class II, it is unclear how CD4+ responses to AChR alpha and epsilon subunits are initiated, and how and where these spread to induce antibodies against fetal AChR. Various possibilities, including upregulation of class II on muscle/myoid cells and involvement of CD8+ responses to AChR and other muscle antigens, are discussed.

T cell responses in EAMG-susceptible and non-susceptible mouse strains after immunization with overlapping peptides encompassing the extracellular part of Torpedo californica acetylcholine receptor alpha chain. Implication to role in myasthenia gravis of autoimmune T-cell responses against receptor degradation products

To study the role in myasthenia gravis (MG) of peptides resulting from acetylcholine receptor (AChR) degradation, we examined the ability of AChR peptides to induce T cell responses that are capable of cross-reacting with intact AChR. The studies were carried out in an experimental autoimmune MG (EAMG)-susceptible mouse strain [C57BL/6 (B6)] as well as in two non-susceptible strains [B6.C-H-2bm12 (bm12) and C3H/He]. A set of overlapping peptides encompassing the extracellular part (residues 1-210) of the alpha-chain of Torpedo californica (t) AChR were used, individually or in equimolar mixtures, as immunogens. In B6, immunization with peptides alpha45-60, alpha111-126, alpha146-162 and alpha182-198 gave T cells that responded in vitro to the correlate immunizing peptide. Only the T cells against the latter three peptides cross-reacted with tAChR. Peptide alpha146-162 exhibited the highest in vitro reaction with the immunizing peptide and cross-reaction with tAChR. T cells obtained by immunization of B6 with an equimolar mixture of the peptides responded in vitro to peptides alpha111-126, alpha146-162 and alpha182-198 and cross-reacted very strongly with tAChR. In bm12 and C3H/He, a number of peptides evoked, when used individually as immunogens, strong or moderate T cell responses that recognized in vitro the correlate immunizing peptide but cross-reacted poorly with tAChR. Immunization with the mixture of the peptides gave T cells that recognized several peptides in each strain butdid not cross-react with alpha146-162 or tAChR. The results indicate that the ability to recognize alpha146-162 or AChR by T cells against peptides resulting from receptor degradation can account for the susceptibility to, and aggravation of, MG in B6.

Factors that determine the severity of experimental myasthenia gravis

Based on our current information, the robust differences in responses of B6 and bm12 mice after immunization with AChR are as follows: (1) The AChR-specific T cell repertoires are strikingly different. The epitope specificities, as well as the rearranged TCR alpha and beta chains and their CDR3 domains, are virtually nonoverlapping in the two strains of mice. (2) The AChR antibody responses are quantitatively different, both to Torpedo AChR and to the autoantigen--mouse AChR. (3) The isotype distribution of AChR antibodies favors IgG2b in B6 mice, but not in bm12 mice. (4) The clinical manifestations of EAMG are qualitatively and quantitatively different in the two strains. These considerations have led to the following scheme, illustrated diagrammatically in FIGURE 2, to explain the differences in EAMG in B6 and bm12 mice: (1) The MHC Class II of B6 mice binds the alpha 146-162 peptide of Torpedo AChR with high affinity, while the genetically altered MHC Class II of bm12 mice does not, as previously suggested (see FIGURE 2). (2) The alpha 146-162/MHC Class II complex occurs only in B6 mice and interacts with T cells having appropriate TCRs, resulting in their stimulation and expansion. Although T cells of appropriate specificity are also available in the bm12 strain, the relevant peptide/MHC Class II complex is not present. Therefore, very few T cells with specificity for alpha 146-162 are stimulated, and those that are stimulated have different TCRs. T cells with specificity for other AChR peptides are also present and expanded in both strains of mice, but they have less influence on the outcome of the immune response. (3) The alpha 146-162-specific T cells of B6 mice, in turn, interact strongly with AChR-specific B cells of B6 mice. These B cells present the same epitope/MHC Class II complex as the APCs and therefore interact well with the alpha 146-162-specific T cells (FIGURE 2). Thus, T cells of this specificity appear to provide more efficient help for AChR antibody production than T cells with specificity for other Torpedo AChR epitopes. This results in production of greater amounts of AChR antibodies, including a critical subset that cross-reacts with autologous mouse AChR. The higher autoantibody levels contribute to the greater susceptibility to EAMG and to the greater severity of manifestations in the B6 strain compared with the bm12 strain. (4) There is a bias in B6 mice toward the production of AChR antibodies of IgG2b isotype. We suggest that T cells specific for alpha 146-162 may contribute to this isotype bias. The IgG2b antibodies appear to have particularly potent "myasthenogenic" effects in rats and mice. (5) Finally, it should be emphasized that these differences in immunological and clinical aspects of EAMG in B6 and bm12 mice are relative rather than absolute. T cells that respond to AChR epitopes other than alpha 146-162 can also provide help for AChR antibody production, albeit less potent. In a sense, this model represents a special case of molecular mimicry. In this case, the source of the foreign antigenic molecule is injection rather than the more usual route of infection. The antigen (Torpedo AChR) is one that these mice would never naturally encounter, and the critical amino acid (lysine 155) of the key epitope (alpha 146-162) is present only in the AChR of electric organs of electric fish and not in the AChR of mice, chickens, cows, or humans. The important point is that a detail of the structure of the foreign antigen--that is, a particular peptide of Torpedo AChR--can determine the severity of an antibody-mediated autoimmune disease, depending on how it interacts with a detail of the structure of the MHC Class II molecule and, in turn, on how the peptide/MHC Class II complex interacts with the available T cell repertoire. (ABSTRACT TRUNCATED)

Polymorphism at the HLA-DQ Locus Determines Susceptibility to Experimental Autoimmune Myasthenia Gravis

Studies in myasthenia gravis (MG) patients demonstrate that polymorphism at the HLA-DQ locus influences the development of MG. Several studies using the mouse models also demonstrate the influence of class II molecules, especially the H2-A, which is the mouse homologue of HLA-DQ, in experimental autoimmune myasthenia gravis (EAMG). We used transgenic mice expressing two different DQ molecules, DQ8 (DQA1*0301/B1*0302) and DQ6 (DQA1*0103/B1*0601), to evaluate the role of HLA-DQ genes in MG. These mice do not express endogenous mouse class II molecules since they contain the mutant H2-Aβ0 gene. The mice were immunized with Torpedo acetylcholine receptor, and EAMG was assessed by clinical evaluation and was confirmed by electrophysiology. Clinical scores for EAMG were highest in HLA-DQ8 transgenic mice, whereas the scores of HLA-DQ6 mice rarely exceeded grade 1. There was no incidence of EAMG in class II-deficient (H2-Aβ0) mice. These results demonstrate that polymorphism at the HLA-DQ locus affects the incidence and the severity of EAMG. The manifestation of susceptibility to EAMG in the context of human class II molecules underscores the important roles of these molecules in the initiation and perpetuation of EAMG.

Immunobiological studies on experimental visceral Leishmaniasis. V. The I-A(Bm12) mutation specifies resistance to infection

The I-A(Bm12) mutation of the I-A beta subunit converted Leishmania donovani-susceptible C57BL/6 (B6) mice into the relatively resistant B6C-H-2 Bm12 (Bm12) strain. The relative resistance was reflected not only in the reduced splenic and hepatic parasite burden in Bm12 (compared with B6) but also by the ability of Bm12 mice to mount a T-cell proliferative response to parasite antigens. Assay of antileishmanial antibody (immunoglobulin G (IgG)2a and IgG1) in the sera of infected mice showed that in Bm12 mice the predominant isotype was IgG2a, rather than IgG1, whereas a similar level of both isotypes were found in B6 mice. From the serum immunoglobulin isotype titre it appeared that the antileishmanial T-cell response was biased towards a T helper (Th) 1 response in Bm12 mice whereas it was a mixed Th1 and Th2 response in B6 mice. These observations provide credence to the notion that polymorphism in class II major histocompatibility complex (MHC) molecules is responsible for the difference in the disease phenotype.

How subtle differences in MHC class II affect the severity of experimental myasthenia gravis

Myasthenia gravis is an autoimmune disorder characterized by muscle weakness, due to an antibody-mediated deficit of acetylcholine receptors (AChRs) at neuromuscular junctions. We analyzed the factors that determine the severity of experimental myasthenia gravis (EAMG) induced by immunization with Torpedo AChR, in two congenic strains of mice--B6 mice, which are highly susceptible to EAMG; and bm12 mice, which are relatively resistant, and differ only in a change of three amino acids in MHC Class II. We prepared large numbers of AChR-specific T cell hybridomas from each strain and characterized their epitope specificities and T cell receptor (TCR) gene usage: Half the B6 hybridomas responded to a single AChR peptide (alpha 146-162), and their TCR genes encoded restricted V alpha and V beta chains and CDR3 motifs. bm12 hybridomas had different epitope specificities and different, less restricted TCR genes. APCs were able to present AChR or AChR-derived peptides virtually exclusively to hybridomas of their own strain. Levels of antibodies to Torpedo and autoantibodies to mouse AChR were higher in B6 mice, and were biased toward the IgG2b isotype. We conclude that the "better fit" of MHC II, peptide, and TCR in the B6 mice enhanced cognate interactions of APCs with T cells, and T cells with B cells, resulting in a more abundant and pathogenic AChR antibody response, and thus more severe EAMG.

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.

Restricted T cell receptor repertoire for acetylcholine receptor in murine myasthenia gravis

Immunization of C57BL/6 mice with AChR provokes symptoms similar to those seen in the disease myasthenia gravis. To elucidate the structural requirements for T cell recognition of AChR and to identify TcR features which might provide targets for immunotherapy, a panel of T cell hybridomas was generated after immunization of mice with the immunodominant peptide of the AChR alpha chain. The TcR genes expressed by these hybridomas were sequenced. TcR-V beta 6 was preferentially employed, but other V beta genes were also observed. A conserved acidic residue was present in all CDR3 regions, regardless of the V beta. The TcR-V alpha repertoire was somewhat skewed with three V alpha families accounting for 82% of the sequences. The utilization of multiple T cell receptor V beta genes may contribute to the inability to inhibit EAMG by elimination of V beta 6+ T cells.