Up-regulation of M3-muscarinic receptors in labial salivary gland acini in primary Sjogren's syndrome

M3-muscarinic receptors (M3R) mediate parasympathetic cholinergic neurotransmission to salivary and lacrimal glands, and autoantibodies to these receptors have been implicated in sicca symptoms and autonomic dysfunction in Sjogren's syndrome. We have investigated the expression of M3R in paraffin-embedded labial salivary glands (LSG) from seven patients with primary Sjogren's syndrome (pSS) and five healthy controls using high-resolution confocal microscopy and an affinity-purified goat polyclonal antibody raised against the COOH-terminal sequence of the human M3R. Immunolocalization of M3R was similar in control and pSS glands, with punctate staining of M3R in the basal membrane of acinar cells and in the luminal and abluminal membrane of myoepithelial cells. Bright, granular M3R staining was also detected in the cytoplasm and membranes of all intercalated and striated ducts, and infiltrating lymphocytes in pSS. All immunoreactivity was specifically blocked by the immunizing peptide. An increase in M3R expression specifically in acini in pSS was demonstrated by a 30% increase in receptor number per cluster and a 68% increase in the number of clusters in the membrane. This up-regulation is consistent with inhibition of parasympathetic neurotransmission, possibly by antagonistic autoantibodies to M3R. The up-regulation, rather than down-regulation, of M3R in acini of pSS LSG can explain the effectiveness of muscarinic agonists in treating sicca symptoms in pSS.