Low Levels of Acetylcholine Receptor Delta-Subunit Message and Protein in Human Thymus Suggests the Occurrence of ‘Triplet Receptors’ in Thymic Myoid Cells

Myasthenia gravis (MG) is an autoimmune disease caused by autoantibodies against the acetylcholine receptor (AChR) at the neuromuscular junction [1]. The muscular AChR has been extensively characterized [2], but whether the muscular AChR plays a role during the initiation of MG is unknown [3]. The muscular AChR is a pentameric ion channel composed of 4 different subunits [2, 4]. The fetal AChR expressed during intrauterine life and after denervation of adult muscle exhibits an α2βδγ composition, while the adult AChR expressed after birth in innervated muscle exhibits an α2βδγ composition [4]. The α-subunit contains the main epitopes recognized by MG autoantibodies [2]. The human muscle AChR α-subunit exists as two isoforms, P3A- and P3A+ [5]. The P3A+ isoform does not bind α-bungarotoxin or monoclonal antibodies against the AChR main immunogenic region and is not integrated into functional AChR [6]. Studies on the expression of the various AChR subunits in thymus and thymoma are reported by Gutsche et al [16].