Characterization of a 58- and a 78-kD monocytic membrane protein with affinity to the acetylcholine receptor in myasthenia gravis patients.

The autoimmune disease myasthenia gravis (MG), caused by the effect of specific antibodies, directed towards the nicotinic acetylcholine receptor, is triggered by autoantigen-specific T cells. In order to investigate cellular parts of the immune response in MG, the authors investigated the binding of the nicotinic acetylcholine receptor (AChR) to peripheral blood mononuclear cells (PBMC) from MG patients. AChR binding cells were identified by rosetting experiments using AChR-coated fluorescein beads. Applying this technique, a significant percentage of PBMC (21.2 +/- 7.65%) from MG patients formed rosettes with AChR-coated beads. Membrane preparations of nycodenz- or percoll-separated monocytes from MG patients or T-cell depleted monocytic subpopulations were applied to SDS-PAGE under reducing conditions. Ligand-blotting studies with biotinylated AChRs revealed two cell-membrane proteins with molecular weights of 58- and 78-kD. In parallel the same results were obtained by affinity chromatography of monocytic membrane proteins using AChR-sepharose. A possible interference of anti-AChR IgG was excluded. The 58- and the 78-kD proteins are detectable under reducing conditions by ligand blotting with AChR-biotin, while under non-reducing conditions only the 58-kD protein can be detected. Furthermore, in experiments using Endoglycosidase-H, the 58-kD protein appears to be non-glycosylated, while the 78-kD protein bears carbohydrates. These findings suggest that monocytes which bind the AChR via specific membrane proteins on their surface might act as antigen-presenting cells and may lead to an induction of the T-cell response, in the early phase of the disease.