C57BL/6 mice genetically deficient in IL-12/IL-23 and IFN-gamma are susceptible to experimental autoimmune myasthenia gravis, suggesting a pathogenic role of non-Th1 cells

Immunization with Torpedo acetylcholine receptor (TAChR) induces experimental autoimmune myasthenia gravis (EAMG) in C57BL/6 (B6) mice. EAMG development needs IL-12, which drives differentiation of Th1 cells. The role of IFN-gamma, an important Th1 effector, is not clear and that of IL-17, a proinflammatory cytokine produced by Th17 cells, is unknown. In this study, we examined the effect of simultaneous absence of IL-12 and IFN-gamma on EAMG susceptibility, using null mutant B6 mice for the genes of both the IL-12/IL-23 p40 subunit and IFN-gamma (dKO mice). Wild-type (WT) B6 mice served as control for EAMG induction. All mice were immunized with TAChR in Freund's adjuvant. dKO mice developed weaker anti-TAChR CD4(+)T cells and Ab responses than WT mice. Yet, they developed EAMG symptoms, anti-mouse acetylcholine receptor (AChR) Ab, and CD4(+) T cell responses against mouse AChR sequences similar to those of WT mice. dKO and WT mice had similarly reduced AChR content in their muscles, and IgG and complement at the neuromuscular junction. Naive dKO mice had significantly fewer NK, NKT, and CD4(+)CD25(+)Foxp3(+) T regulatory (Treg) cells than naive WT mice. Treg cells from TAChR-immunized dKO mice had significantly less suppressive activity in vitro than Treg cells from TAChR-immunized WT mice. In contrast, TAChR-specific CD4(+) T cells from TAChR-immunized dKO and WT mice secreted comparable amounts of IL-17 after stimulation in vitro with TAChR. The susceptibility of dKO mice to EAMG may be due to reduced Treg function, in the presence of a normal function of pathogenic Th17 cells.

CD4+ T and B cells cooperate in the immunoregulation of Experimental Autoimmune Myasthenia Gravis

C57Bl6 mice (B6 mice) immunized with Torpedo acetylcholine receptor (TAChR) in Freund's adjuvants (FA) develop Experimental Autoimmune Myasthenia Gravis (EAMG). In mouse EAMG Th2 cytokines may be protective. Aluminum hydroxide (Alum) was used to immunize B6 mice to the TAChR and prime CD4+ T and B cells secreting Th2 cytokines. Mice immunized with TAChR/Alum developed anti-AChR CD4+ T cells response, but minimal antibody levels and symptoms. TAChR/Alum treatments prior immunization with TAChR/FA protected mice from EAMG. Cell transfer experiments demonstrated that B and CD4+ T cells mediated the protective effect by causing intense reduction of complement-fixing anti-TAChR IgG subclasses.

T Cells and Cytokines in the Pathogenesis of Acquired Myasthenia Gravis

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

Absence of IL-4 facilitates the development of chronic autoimmune myasthenia gravis in C57BL/6 mice

Myasthenia gravis (MG) is a T cell-dependent, Ab-mediated autoimmune disease. Ab against muscle acetylcholine receptor (AChR) cause the muscular weakness that characterizes MG and its animal model, experimental MG (EMG). EMG is induced in C57BL6 (B6) mice by three injections of Torpedo AChR (TAChR) in adjuvant. B6 mice develop anti-TAChR Ab that cross-react with mouse muscle AChR, but their CD4+ T cells do not cross-react with mouse AChR sequences. Moreover, murine EMG is not self-maintaining as is human MG, and it has limited duration. Several studies suggest that IL-4 has a protecting function in EMG. Here we show that B6 mice genetically deficient in IL-4 (IL-4-/-) develop long-lasting muscle weakness after a single immunization with TAChR. They develop chronic self-reactive Ab, and their CD4+ T cells respond not only to the TAChR and TAChR subunit peptides, but also to several mouse AChR subunit peptides. These results suggest that in B6 mice, regulatory mechanisms that involve IL-4 contribute to preventing the development of a chronic Ab-mediated autoimmune response to the AChR.

Adoptive protection from experimental myasthenia gravis with T cells from mice treated nasally with acetylcholine receptor epitopes

Nasal administration of synthetic CD4(+) epitopes of the acetylcholine receptor (AChR) prevents experimental myasthenia gravis (EMG) in C57Bl/6 mice, but not in IL4-deficient C57Bl/6 (IL4(-/-)) mice. Here we verify that nasal tolerance requires IL4, by showing that CD4(+) cells from C57Bl/6 mice treated nasally with a pool of AChR CD4(+) epitopes protected IL4(-/-) mice from EMG and caused a reduced production of anti-AChR antibody. CD4(+) cells from C57Bl/6 mice treated with unrelated peptides or sham-treated did not induce protection. CD4(+) cells from C57Bl/6 mice treated with just one AChR peptide protected IL4(-/-) mice from EMG without affecting antibody synthesis.

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

The I-Abm12 mutation in C57B1/6 (B6) mice yields the B6.C-H-2bm12 (bm12) strain, which is resistant to Experimental Myasthenia Gravis (EMG) induced by immunization with Torpedo acetylcholine receptor (TAChR), while the parental B6 strain is highly susceptible to EMG. CD4+ cells from bm12 mice immunized with TAChR do not recognize three sequence regions of the TAChR alpha subunit which dominate the CD4+ cell sensitization in B6 mice. We immunized with TAChR bm12, B6 and (bm12 x B6)F1 mice. B6 and F1 mice developed EMG with comparable frequency. Their CD4+ cells recognized the same TAChR alpha subunit peptide sequences (T alpha 150-169, T alpha 181-200 and T alpha 360-378). CD4+ cells from TAChR-sensitized F1 mice were challenged with TAChR and alpha subunit epitope peptides, using F1, B6 or bm12 APC. B6 and F1 APC presented all these Ag efficiently, while bm12 APC presented TAChR and peptide T alpha 150-169 poorly and erratically. Anti-TAChR and anti-alpha subunit epitope CD4+ lines propagated from F1 and B6 mice had similar TcR V beta usage. All lines but those specific for the sequence T alpha 150-169 had unrestricted V beta usage. Anti-T alpha 150-169 lines from both B6 and F1 mice had a strong preferential usage of V beta 6. Anti-T alpha 150-169 lines from F1 mice had also a slightly higher V beta 14 usage. B6, bm12 and F1 mice developed similar anti-TAChR Ab titres, and had Ab bound to muscle AChR in comparable amounts. Therefore EMG resistance of bm12 mice must be due to a subtle shift in the anti-AChR Ab repertoire, and absence of special Ab able to cause destruction and/or dysfunction of muscle AChR. This is probably related to the absence of CD4+ cells sensitized to epitopes within the sequence T alpha 150-160, consequent to the inability of the I-Abm12 molecule to present this sequence.

Clustering of B and T epitopes within short sequence regions of the nicotinic acetylcholine receptor

The epitope repertoire of B cells, due to their selective ability to process their specific antigen and the potential bias imposed on the resulting peptides by the surface immunoglobulins bound to the antigen, may influence the T-helper repertoire. Immunization of C57B1/6 mice with Torpedo acetylcholine receptor (TAChR) causes experimental autoimmune myasthenia gravis (EAMG). Anti-TAChR CD4+ cells recognize epitopes within three sequence regions of the TAChR alpha subunit ('dominant epitopes'). Immunization of mice with denatured or synthetic TAChR antigens sensitizes CD4+ cells to other TAChR sequence regions ('cryptic epitopes'). We investigated here whether clustering of B and T epitopes within the same short sequence segments occurs during the anti-TAChR response, as previously described for the response to hexogenous antigens unrelated to homologous self proteins. Twelve 19-20 residue synthetic sequences of the TAChR alpha, gamma and delta subunits, containing dominant or cryptic CD4+ epitopes for C57B1/6 mice, were tested for ability to induce anti-peptide antibody production. C57B1/6 mice were immunized with the individual peptides. Ten peptides stimulated antibody production. Therefore > 80% of these short TAChR sequences also contain B epitopes. Therefore also in the anti-TAChR response leading to EAMG T and B cell epitopes frequently reside within the same short sequence segment.

Residues within the alpha subunit sequence 304-322 of muscle acetylcholine receptor forming autoimmune CD4+ epitopes in BALB/c mice

BALB/c mice develop myasthenic symptoms after immunization with rodent acetylcholine receptor (AChR). After immunization with Torpedo AChR (TAChR), their CD4+ cells become strongly sensitized against a conserved region of the TAChR alpha subunit sequence (residues alpha 304-322), and cross-react vigorously with the homologous sequences of mouse and human AChR, which are almost identical. Therefore AChR-specific potentially autoreactive CD4+ cells exist in this strain. We immunized BALB/c mice with the synthetic TAChR sequence alpha 304-322. The CD4+ cells thus sensitized responded to TAChR, indicating that they recognize an epitope(s) produced upon TAChR processing. They recognized peptide alpha 304-322 in association with the I-Ad molecule. Anti-alpha 304-322 CD4+ cells cross-reacted well with the corresponding murine and human synthetic sequences. To identify residues involved in formation of an autoimmune epitope(s), CD4+ cells from mice immunized with peptide alpha 304-322 were challenged in vitro with single residue glycine-substituted analogues of this sequence. Substitution of residue W311, and of any residue within the sequence alpha 313-319 (RKVFIDT), consistently and, in some cases, strongly affected the CD4+ cells response. Substitution of residues in the region alpha 311-319 had variable effects in different experiments, and in general affected moderately the CD4+ response. These results suggest that anti-alpha 304-322 CD4+ cells comprise several clones, recognizing overlapping epitopes which share residues alpha 311-319. The importance of the sequence region alpha 311-319 for formation of CD4+ cell epitope(s) was verified by testing CD4+ cells sensitized to T alpha 304-322 with analogues of this sequence, carrying non-conservative substitutions at positions Q310, K314 and D318. Substitution of Q310 had minimal or no effects, while those of K314 or D318 strongly affected the CD4+ cell response.

Preferential pairing of T and B cells for production of antibodies without covalent association of T and B epitopes

T cell from H-2b mice recognize at least 12 sequence regions on the Torpedo acetylcholine receptor (TAChR) alpha, gamma and delta subunits. Immunization of C57BL/6 mice with individual synthetic TAChR sequences known to contain CD4+ epitopes resulted in most cases (10 out of 12 peptides) in anti-peptide antibody (Ab) production, indicating that short TAChR sequences contain both CD4+ and B epitopes. Immunization of C57BL/6 mice with a mixture of a CD4+ epitope peptide, from the TAChR or from an unrelated protein, plus another TAChR sequence forming a "pure" B epitope (T alpha 63-80), induced in most cases anti-peptide Ab and CD4+ cell sensitization only against the peptide containing the CD4+ epitope. However, when the T epitope peptide T alpha 360-378 was co-injected with the B epitope, Ab were also produced against the B epitope peptide. Injection of the individual peptides T alpha 360-378 and T alpha 63-80 at different and distant sites along the back of mice elicited sensitization of CD4+ cells and Ab production only against peptide T alpha 360-378. Therefore, when optimal cooperation between T and B cells occurs, spatial proximity but not covalent association of the B and the CD4+ epitope is necessary for production of Ab against the B epitope.

Cryptic epitopes on the nicotinic acetylcholine receptor are recognized by autoreactive CD4+ cells

Experimental autoimmune myasthenia gravis is induced in C57BL/6 mice by injection of Torpedo nicotinic acetylcholine receptor (TAChR). We investigated here the presence of cryptic CD4+ epitopes on the TAChR molecule, and their relationship with potentially autoreactive CD4+ cells, which survived clonal deletion. CD4+ cells from C57BL/6 mice immunized with native or denatured TAChR were challenged in vitro with overlapping synthetic peptides, 20-residue long, screening the sequences of TAChR alpha, gamma, and delta subunits. Only three epitopes on the alpha subunit were recognized consistently. Mice immunized with large doses (nanomoles) of TAChR clearly recognized only the immunodominant sequence T alpha 150-169. Anti-TAChR CD4+ cells did not cross-react with murine alpha subunit sequences, or with any synthetic sequence of human gamma and delta subunits, which are very similar to the corresponding murine subunits. To facilitate recognition of cryptic epitopes, we injected mice with pools of synthetic peptides corresponding to the sequences of TAChR alpha, gamma, and delta subunits. In addition to the three immunodominant alpha subunit epitopes, other epitopes were recognized by CD4+ cells within the sequences T alpha 304-322, T gamma 105-124, T gamma 120-139, T gamma 401-420, T gamma 357-376, T delta 16-35, T delta 61-80, T delta 121-140, and T delta 301-320. CD4+ cells thus sensitized cross-reacted with the mammalian sequences alpha 304-322, gamma 105-124, gamma 120-139, and delta 301-320. Mice were immunized with large doses (approximately 40 nmol) of individual TAChR synthetic cryptic epitopes. CD4+ cells sensitized to five cryptic epitopes (the ones listed above plus delta 121-140) cross-reacted with autologous sequences. We determined the dose dependence of the sensitization of CD4+ cells in vivo to the strongly immunodominant epitope peptide T alpha 150-169 and to the cryptic epitope peptides T gamma 120-139 and T delta 301-320 by immunizing mice with increasing doses of peptide (approximately 1.2 to approximately 20 nmol), and testing the in vitro anti-peptide response of the CD4+ cells. No difference was found for the epitopes tested. Doses of 3 to 10 micrograms induced a strong CD4+ sensitization, and the dose dependence of the in vitro response of the sensitized cells to the relevant peptide was comparable. Production of cryptic epitopes upon in vitro TAChR processing was investigated by testing peptide-sensitized CD4+ cells with native TAChR: only two cryptic epitopes were produced.

Cryptic epitopes on the nicotinic acetylcholine receptor are recognized by autoreactive CD4+ cells

Experimental autoimmune myasthenia gravis is induced in C57BL/6 mice by injection of Torpedo nicotinic acetylcholine receptor (TAChR). We investigated here the presence of cryptic CD4+ epitopes on the TAChR molecule, and their relationship with potentially autoreactive CD4+ cells, which survived clonal deletion. CD4+ cells from C57BL/6 mice immunized with native or denatured TAChR were challenged in vitro with overlapping synthetic peptides, 20-residue long, screening the sequences of TAChR alpha, gamma, and delta subunits. Only three epitopes on the alpha subunit were recognized consistently. Mice immunized with large doses (nanomoles) of TAChR clearly recognized only the immunodominant sequence T alpha 150-169. Anti-TAChR CD4+ cells did not cross-react with murine alpha subunit sequences, or with any synthetic sequence of human gamma and delta subunits, which are very similar to the corresponding murine subunits. To facilitate recognition of cryptic epitopes, we injected mice with pools of synthetic peptides corresponding to the sequences of TAChR alpha, gamma, and delta subunits. In addition to the three immunodominant alpha subunit epitopes, other epitopes were recognized by CD4+ cells within the sequences T alpha 304-322, T gamma 105-124, T gamma 120-139, T gamma 401-420, T gamma 357-376, T delta 16-35, T delta 61-80, T delta 121-140, and T delta 301-320. CD4+ cells thus sensitized cross-reacted with the mammalian sequences alpha 304-322, gamma 105-124, gamma 120-139, and delta 301-320. Mice were immunized with large doses (approximately 40 nmol) of individual TAChR synthetic cryptic epitopes. CD4+ cells sensitized to five cryptic epitopes (the ones listed above plus delta 121-140) cross-reacted with autologous sequences. We determined the dose dependence of the sensitization of CD4+ cells in vivo to the strongly immunodominant epitope peptide T alpha 150-169 and to the cryptic epitope peptides T gamma 120-139 and T delta 301-320 by immunizing mice with increasing doses of peptide (approximately 1.2 to approximately 20 nmol), and testing the in vitro anti-peptide response of the CD4+ cells. No difference was found for the epitopes tested. Doses of 3 to 10 micrograms induced a strong CD4+ sensitization, and the dose dependence of the in vitro response of the sensitized cells to the relevant peptide was comparable. Production of cryptic epitopes upon in vitro TAChR processing was investigated by testing peptide-sensitized CD4+ cells with native TAChR: only two cryptic epitopes were produced.

Myasthenia gravis: effect on antibody binding of conservative substitutions of amino acid residues forming the main immunogenic region of the nicotinic acetylcholine receptor

In Myasthenia Gravis most anti-acetylcholine receptor (AChR) antibodies are against a highly conserved area of the AChR alpha-subunit called the Main Immunogenic Region (MIR). Amino acid residues critical for MIR formation have been located within the sequence alpha 67-76. In the present study, binding of anti-AChR monoclonal antibodies (mAbs) to synthetic peptide analogues of the sequence alpha 67-76 of human and Torpedo AChRs containing conservative single-residue substitutions identified the amino acid residues most important to the antigenicity of the MIR sequence, and offered clues to its tridimensional structure. Conservative substitutions of residues Asn68 and Asp71 greatly diminished mAb binding, identifying them as critical contact residues for anti-MIR mAbs. Substitutions at Asp70 and Tyr72 moderately affected binding. Cross-reactive mAbs originally raised against Electrophorus AChR bound single residue-substituted synthetic peptides in a manner consistent with the possibility that Electrophorus AChR may have a glutamic acid residue at position alpha 70 or alpha 71. Substitutions at residues Asp/Ala70 and Val/Ile70 between human and Torpedo alpha-subunits may be size-compensating, suggesting these amino acids in the native AChR may be in closer proximity than proposed in previous models of the MIR.

T helper function of CD4+ cells specific for defined epitopes on the acetylcholine receptor in congenic mouse strains

We previously identified sequence segments of Torpedo acetylcholine receptor (TAChR) alpha subunit recognized by CD4+ cells of congenic mouse strains of different H-2 haplotypes, susceptible to experimental autoimmune myasthenia gravis. CD4+ cells from BALB/c and CB17 mice (H-2d) recognized the peptide sequences alpha 1-20 and alpha 304-322, while C57BL/6 and BALB/b mice (H-2b) recognized alpha 150-169 and alpha 360-378. C57BL/6 mice recognized to a lesser extent also peptide alpha 181-200. In the present study we demonstrate that CD4+ cells which recognize these epitopes have T-helper function. CD4+ cells from TAChR immunized mice, stimulated in vitro with synthetic epitope peptides, induced proliferation in vitro of B cells via soluble factors which were not strain specific, and induced secretion in vitro of anti-AChR antibodies. Upon in vitro stimulation with T-epitope peptides, they secreted interleukin-2. Immunization of mice with synthetic T-epitope peptides caused sensitization of CD4+ cells, which responded in vitro both to the immunizing peptides and to TAChR, and appearance of anti-AChR antibodies in vivo, further identifying the epitope-specific CD4+ cells as AChR-specific T-helper cells.

Experimental myasthenia gravis in congenic mice. Sequence mapping and H-2 restriction of T helper epitopes on the alpha subunits of Torpedo californica and murine acetylcholine receptors

Immunization of mice with nicotinic acetylcholine receptor from Torpedo electric organ (TAChR) causes a disease similar to human myasthenia gravis (experimental autoimmune myasthenia gravis, EAMG). Susceptibility to EAMG correlates with the H-2 haplotype. In this study we used overlapping synthetic peptide corresponding to the complete sequences of the alpha subunits from TAChR and murine muscle AChR (MAChR) to map T helper epitopes in congenic murine strains of different H-2 haplotype. C57BL/6 and BALB/B mice (highly susceptible to EAMG) and BALB/c and CB17 mice (less susceptible to EAMG), immunized with TAChR, developed similar anti-TAChR antibody titers and L3T4+ (T helper) cell sensitization. Different sequence segments of the TAChR alpha subunit were recognized by L3T4+ cells from strains of H-2b and H-2d haplotype. The sequence segments recognized by the H-2d strains have the highest predicted propensity to form amphipatic alpha helices, while those recognized by the H-2b strains do not. We investigated whether in EAMG T helper cells cross-react with autologous AChR sequences, and a true break of the tolerance occurs. Overlapping synthetic peptides, corresponding to the complete sequence of MAChR alpha subunit, were used to test L3T4+ cell from mice immunized with TAChR. L3T4+ cell strains did not cross-react with any murine peptide sequence, while L3T4+ cells from H-2d mice were strongly stimulated by the peptide sequence Ma alpha 304-322, which is very similar to the homologous Torpedo peptide.

The I-Abm12 mutation, which confers resistance to experimental myasthenia gravis, drastically affects the epitope repertoire of murine CD4+ cells sensitized to nicotinic acetylcholine receptor

Susceptibility to experimental autoimmune myasthenia gravis (EAMG), which is induced in mice by injection of purified Torpedo nicotinic acetylcholine receptor (TAChR), is influenced by the I-A locus products, which restrict presentation of AChR Th epitopes. The bm12 mutation of the I-Ab molecule in the C57BL/6 strain, which is highly susceptible to EAMG, yields the EAMG resistant mutant B6.C-H-2bm12 (bm12). We investigated here the consequences of the bm 12 mutation on the CD4+ response to the TAChR alpha subunit. Upon immunization with TAChR, CD4+ cells became sensitized to TAChR and anti-AChR antibodies were produced in both bm12 and C57BL/6 strains. Overlapping synthetic peptides, corresponding to the complete sequence of TAChR alpha subunit, were used to identify Th epitopes. CD4+ cells from C57BL/6 mice recognized peptides T alpha 150-169, T alpha 181-200, and T alpha 360-378. CD4+ cells from bm12 mice did not respond to any synthetic sequence. Upon injection of the three C57BL/6 Th epitope peptides, either individually or as a pool, CD4+ cells from C57BL/6 mice recognized each peptide and TAChR. Therefore they recognized epitopes similar or identical to those originated from TAChR processing. CD4+ cells from bm12 mice injected with the same peptides responded to T alpha 360-378 strongly, to a lesser extent to T alpha 181-200, never to peptide T alpha 150-169. Only CD4+ cells sensitized against the T epitope peptide T alpha 181-200 responded to TAChR. We tested if lack of response to T alpha 150-169, and the low response to T alpha 181-200, was due to inability of the I-Abm12 molecule to present the T epitope peptides. bm12 and C57BL/6 APC were used to present the T epitope peptides to specifically sensitized CD4+ cells from C57BL/6 mice. All T epitope peptides were presented by bm12 APC, although T alpha 150-169 was presented less efficiently than by C57BL/6 APC. Resistance to EAMG induced by the bm12 mutation may be due to the change in the epitope repertoire of AChR-specific Th cells, and lack of recognition of otherwise immunodominant Th epitopes. For at least one epitope this might be due to absence of potentially reactive, specific CD4+ clones.

The I-Abm12 mutation, which confers resistance to experimental myasthenia gravis, drastically affects the epitope repertoire of murine CD4+ cells sensitized to nicotinic acetylcholine receptor

Susceptibility to experimental autoimmune myasthenia gravis (EAMG), which is induced in mice by injection of purified Torpedo nicotinic acetylcholine receptor (TAChR), is influenced by the I-A locus products, which restrict presentation of AChR Th epitopes. The bm12 mutation of the I-Ab molecule in the C57BL/6 strain, which is highly susceptible to EAMG, yields the EAMG resistant mutant B6.C-H-2bm12 (bm12). We investigated here the consequences of the bm 12 mutation on the CD4+ response to the TAChR alpha subunit. Upon immunization with TAChR, CD4+ cells became sensitized to TAChR and anti-AChR antibodies were produced in both bm12 and C57BL/6 strains. Overlapping synthetic peptides, corresponding to the complete sequence of TAChR alpha subunit, were used to identify Th epitopes. CD4+ cells from C57BL/6 mice recognized peptides T alpha 150-169, T alpha 181-200, and T alpha 360-378. CD4+ cells from bm12 mice did not respond to any synthetic sequence. Upon injection of the three C57BL/6 Th epitope peptides, either individually or as a pool, CD4+ cells from C57BL/6 mice recognized each peptide and TAChR. Therefore they recognized epitopes similar or identical to those originated from TAChR processing. CD4+ cells from bm12 mice injected with the same peptides responded to T alpha 360-378 strongly, to a lesser extent to T alpha 181-200, never to peptide T alpha 150-169. Only CD4+ cells sensitized against the T epitope peptide T alpha 181-200 responded to TAChR. We tested if lack of response to T alpha 150-169, and the low response to T alpha 181-200, was due to inability of the I-Abm12 molecule to present the T epitope peptides. bm12 and C57BL/6 APC were used to present the T epitope peptides to specifically sensitized CD4+ cells from C57BL/6 mice. All T epitope peptides were presented by bm12 APC, although T alpha 150-169 was presented less efficiently than by C57BL/6 APC. Resistance to EAMG induced by the bm12 mutation may be due to the change in the epitope repertoire of AChR-specific Th cells, and lack of recognition of otherwise immunodominant Th epitopes. For at least one epitope this might be due to absence of potentially reactive, specific CD4+ clones.