Antibodies Raised Against the Second Extracellular Loop of the Human Muscarinic M3 Receptor Mimic Functional Autoantibodies in Sjögren's Syndrome

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Abstract
Functional antimuscarinic M3 receptor (M3R) autoantibodies have been shown to inhibit cholinergic neurotransmission at the postsynaptic level and appear to mediate parasympathetic dysfunction, including sicca symptoms in Sjögren's syndrome (SS). The precise epitope(s) involved in the inhibition of M3R‐mediated cholinergic neurotransmission has not been defined. In this study, an active immunization approach to raise antibodies with functional activity against the second extracellular loop of the M3R was used and their functional properties were compared with those of human autoantibodies. Peptides corresponding to the second extracellular loop of the M3R were used as immunogens in rabbits, and antisera were tested for inhibition of carbachol‐evoked colon smooth muscle contraction in parallel with immunoglobulin G from a patient with SS. Anti‐M3R antibodies were affinity purified on a peptide representing a dominant functional epitope at the COOH terminus of the second extracellular loop of the M3R and tested for concentration‐dependent inhibition. Experimentally raised anti‐M3R antibodies, like the human autoantibodies, showed concentration‐dependent and noncompetitive inhibition of carbachol‐evoked colon contractions. Inhibitory activity was detected by functional assays at concentrations as low as 3 ng/ml, which was below the threshold of detection of antibody by peptide enzyme‐linked immunosorbent assay. It is concluded that the experimentally raised anti‐M3R antibodies share the functional properties of autoantibodies in patients with SS.