Role of immune response to M3 muscarinic acethylcholine receptor in the pathogenesis of Sjögren's syndrome

[Analysis of epitopes and function of anti-M3 muscarinic acetylcholine receptor antibodies in patients with Sjögren's syndrome]

Sjögren's syndrome (SS) is an autoimmune disease that affects exocrine glands including salivary and lacrimal glands. It is characterized by lymphocytic infiltration into exocrine glands, leading to dry mouth and eyes. A number of auto-antibodies, such as anti-SS-A and SS-B antibodies, are detected in patients with SS. However, no SS-specific pathologic auto-antibodies have yet been found in this condition. M3 muscarinic acetylcholine receptor (M3R) plays a crucial role in the secretion of saliva from salivary glands. It is reported that some patients with SS carried inhibitory auto-antibodies against M3R. To clarify the epitopes and function of anti-M3R antibodies in SS, we examined antibodies to the extracellular domains (N terminal region, the first, second, and third extracellular loop) of M3R by ELISA using synthesized peptide antigens encoding these domains in 42 SS and 42 healthy controls (HC). Titers and positivity of anti-M3R antibodies to every extracellular domain of M3R were significantly higher in SS than in HC. For functional analysis, human salivary gland (HSG) cells were pre-cultured with IgG from anti-M3R antibodies positive SS, negative SS, and HC. HSG cells were stimulated with cevimeline hydrochloride and intracellular calcium concentration ([Ca(2+)](i)) was measured. IgG from anti-M3R antibodies to the second loop positive SS inhibited the increase of [Ca(2+)](i), but IgG from antibodies to the N terminal or the first loop positive SS enhanced it, while IgG from antibodies to the third loop positive SS showed no effect on [Ca(2+)](i) as well as IgG from anti-M3R antibodies negative SS and HC. These findings indicated the presence of several B cell epitopes on M3R in SS and effect of anti-M3R antibodies on the salivary secretion might differ with these epitopes.