title: 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.
abstract: 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.
authors.affiliation: Department of Biochemistry, College of Biological Sciences, University of Minnesota, St. Paul 55108.
authors.forename: M
authors.initials: M
authors.lastname: Bellone
authors.forename: N
authors.initials: N
authors.lastname: Ostlie
authors.forename: S J
authors.initials: SJ
authors.lastname: Lei
authors.forename: X D
authors.initials: XD
authors.lastname: Wu
authors.forename: B M
authors.initials: BM
authors.lastname: Conti-Tronconi
mesh_terms: D000818:Animals; D000906:Antibodies / Q000032:analysis; D000938:Antigen-Presenting Cells / Q000502:physiology; D015704:CD4 Antigens / Q000032:analysis*; D003429:Cross Reactions; D000939:Epitopes / Q000032:analysis*; D005260:Female; D000949:Histocompatibility Antigens Class II / Q000502:physiology*; D051379:Mice; D008810:Mice, Inbred C57BL; D009154:Mutation; D009157:Myasthenia Gravis / Q000276:immunology*; D011978:Receptors, Nicotinic / Q000276:immunology*; D006377:T-Lymphocytes, Helper-Inducer / Q000276:immunology*
chemical_list: D000906:Antibodies; D015704:CD4 Antigens; D000939:Epitopes; D000949:Histocompatibility Antigens Class II; D011978:Receptors, Nicotinic