Immunodominant regions for T helper-cell sensitization on the human nicotinic receptor alpha subunit in myasthenia gravis.

Abstract

In myasthenia gravis an autoimmune response against the nicotinic acetyicholine receptor (AChR) occurs. The a subunit of the AChR contains both the epitope(s) that dominates the antibody response (main immunogenic region) and epitopes involved in T helper cell sensitization. In this study, overlapping synthetic peptides corresponding to the complete AChR a-subunit sequence were used to propagate polyclonal AChR-specific T helper cell lines from four myasthenic patients of different HLA types. Response ofthe T helper lines to the individual peptides was studied. Four immunodominant sequence segments were identified-i.e., residues 48-67, 101-120, 304-322, and 419-437. These regions did not include residues known to form the main immunogenic region or the cholinergic binding site, and they frequently contained sequence motifs that have been proposed to be related to Tepitope formation.

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

“…It is conceivable that, even in the absence of a CD4+ response to epitopes on a given AChR subunit, the anti-AChR antibodies of that patient may still recognize epitopes formed by residues within that subunit: this is particularly likely for the a subunit, whose corresponding a pool was not recognized by patient 14, since the a subunit contains the MIR, the set of largely overlapping epitopes which dominates the antibody response in MG (21)(22)(23). However, given the conformation dependence of most (1,3,4,6,8,11,12,14,16,17,18, and 20) as described in reference 78 or by RFLP and oligonucleotide hybridization (7,10,13,15,19,21, and 22) as described in reference 79. t Classified as described in reference 80. § The anti-AChR antibody titers, measured as described in reference 81, were usually evaluated on serum collected the day of the experiment.…”

“…However, long-term Th lines are not ideal to study the Th epitope repertoire in MG, since during their propagation in vitro selective clonal loss or enrichment may occur, resulting in a biased repertoire of the anti-AChR Th lines. Long-term Th lines specific for the AChR a subunit recognize only epitopes within four immunodominant sequence regions (14), while direct testing of unselected CD4+ cells from MG patients' blood reveals several other epitopes (41 ). Furthermore, propagation of Th lines is time consuming and labor intensive, and very few patients can be studied by this approach ( 14,15,30).…”

“…AChR-specific long-term Th cell lines can be propagated from the blood of MG patients by stimulation with pools of synthetic peptides corresponding to the sequences of the human AChR a, f3, -y, and 5 subunits (13)(14)(15)30, and Moiola, L., and B. M. Conti-Tronconi, unpublished observations). However, long-term Th lines are not ideal to study the Th epitope repertoire in MG, since during their propagation in vitro selective clonal loss or enrichment may occur, resulting in a biased repertoire of the anti-AChR Th lines.…”

“…The AChR a subunit contains the socalled main immunogenic region (MIR), i.e., the set oflargely overlapping epitopes that dominates the antibody response in MG (21)(22)(23), and important constituent elements of binding sites for cholinergic ligands (reviewed in reference 24). Most studies on the sequence regions forming T cell epitopes have focused on the a subunit (10)(11)(12)(13)(14). However, since T and B cells tend to recognize different parts of the same protein antigen ("complementarity" of the T and B repertoire [25,26]), other AChR subunits may form epitopes important for sensitization of pathogenetic T cells.…”

Current Treatment, Emerging Translational Therapies, and New Therapeutic Targets for Autoimmune Myasthenia Gravis

“…Although anti-AChR antibodies directly contribute to the degradation of AChR at the neuromuscular junction, autoreactive T cells provide help to B cells that synthesize anti-AChR antibodies [11,12]. CD4+ T helper (Th) and T regulatory (Treg) cells recognize AChR epitopes in the context of major histocompatibility complex class II and exert a helper function on B cells to proliferate and differentiate into plasma cells.…”