Functional insights into autoantibodies targeting muscarinic acetylcholine receptors in Sjögren’s disease

Objectives G protein-coupled receptors (GPCRs) participate in various pathophysiological processes in Sjögren's disease (SjD); in particular, autoantibodies against muscarinic acetylcholine receptor 3 (M 3 R) inhibit the secretion of saliva and tears by exocrine glands. We aimed to identify autoantibodies targeting other muscarinic receptors in SjD and assess their potential functional relevance to signalling pathways. Methods Autoantibodies against all muscarinic acetylcholine receptor subtypes were investigated in the serum of patients with SjD (n = 347) using enzyme-linked immunosorbent assays (ELISAs). Healthy controls (HCs; n=50), patients with non-Sjögren's sicca syndrome (NSS; n=44), and patients with other autoimmune diseases (AIDs; n=67) served as controls. To analyse the functional role of muscarinic receptor autoantibodies against muscarinic acetylcholine receptors 1 to 3 and 5 (M 1 R, M 2 R, M 3 R, and M 5 R, respectively), purified immunoglobulin (Ig) G fractions from patients with SjD (n = 10) and HCs (n = 10) were examined in Chinese hamster ovary (CHO) cells transfected with the specific receptor using calcium mobilisation and cyclic adenosine monophosphate (cAMP) accumulation assays. Results IgG autoantibodies against M 1 R to M 5 R were significantly increased in patients with SjD compared with HCs ( p ≤ .05) and detected in up to 30% of patients. A combination of anti-M 2 R, anti-M 3 R, and anti-M 5 R identified up to 31% of Ro/SSA-negative SjD cases, with 96% and 75% specificity for SjD vs HCs and SjD vs NSS, respectively. Functionally, purified IgG from SjD serum reduced calcium flux by up to 30% in M 1 R-, M 3 R-, and M 5 R-expressing CHO cells, whereas its effects on cAMP accumulation in M 2 R-transfected cells were absent. Conclusions The association between autoantibodies targeting M 1 R to M 5 R in patients with SjD and their functional quantification provides strong evidence that autoantibodies against all muscarinic receptor isotypes may have pathogenic relevance in SjD.