Role of muscarinic-3 receptor antibody in systemic sclerosis: correlation with disease duration and effects of IVIG

Abstract

Gastrointestinal dysmotility in systemic sclerosis (SSc) is associated with autoantibodies against muscarinic-3 receptor (M3-R). We investigated the temporal course of the site of action of these autoantibodies at the myenteric neurons (MN) vs. the smooth muscle (SM) M3-R in relation to disease duration, and determined the role of intravenous immunoglobulin (IVIG) in reversing these changes. Immunoglobulins purified from SSc patients (SScIgG) were used to assess their differential binding to MN and SM (from rat colon) employing immunohistochemistry (IHC). Effect of SScIgG on neural and direct muscle contraction was determined by cholinergic nerve stimulation and bethanechol-induced SM contraction. Effects of IVIG and its antigen-binding fragment F(ab')2 on SScIgG binding were studied by enzyme-linked immunosorbent assay (ELISA) of rat colonic longitudinal SM myenteric plexus (LSMMP) lysate and to second extracellular loop peptide of M3-R (M3-RL2). SScIgG from all patients demonstrated significantly higher binding to MN than to SM. With progression of SSc duration, binding at MN and SM increased in a linear fashion with a correlation coefficient of 0.696 and 0.726, respectively (P < 0.05). SScIgG-mediated attenuation of neural and direct SM contraction also increased with disease duration. ELISA analysis revealed that IVIG and F(ab')2 significantly reduced SScIgG binding to LSMMP lysate and M3-RL2. Dysmotility in SSc occurs sequentially, beginning with SScIgG-induced blockage of cholinergic neurotransmission (neuropathy), which progresses to inhibition of acetylcholine action at the SM cell (myopathy). IVIG reverses this cholinergic dysfunction at the neural and myogenic receptors by anti-idiotypic neutralization of SScIgG.

Review article: pathogenesis and clinical manifestations of gastrointestinal involvement in systemic sclerosis

“…The earliest evidence for neuropathy was obtained in a study that showed contractile response of the oesophageal smooth muscle of SSc patients to the direct acting muscarinic agonist methacholine but not to edrophonium, which acts indirectly . These and numerous subsequent studies suggested that in the early stages of SSc GIT involvement circulating M 3 ‐R autoantibodies block cholinergic neuro‐transmission by inhibition of acetylcholine release at the myenteric cholinergic nerves (neuropathic damage), and that progression of the disease leads to myopathy via inhibition of acetylcholine action at the GI smooth muscle cell . Some of these studies further suggested a temporal increase in binding of SScIgGs to the neural and myogenic M 3 ‐R with disease duration, observations that may account for the progressive nature of GIT involvement in SSc.…”

“…As the disease duration increases, there is progressive increase in binding at both the smooth muscle and myenteric neurons. Of substantial therapeutic significance were the observations that the neural and myogenic effects of the SScIgGs were abrogated by the administration of pooled human intravenous immunoglobulin (IVIG) and its antigen binding fragment F(ab′) 2 …”

“…We recently proposed that GIT dysfunction in SSc is a staged process beginning with neuropathy and progressing to myopathy, with eventual fibrosis . The initial neuropathic damage leads to disrupted contraction in response to neural stimulation, which is followed by a substantial reduction in amplitude of contractions signifying the development of myopathy .…”

“…We recently proposed that GIT dysfunction in SSc is a staged process beginning with neuropathy and progressing to myopathy, with eventual fibrosis . The initial neuropathic damage leads to disrupted contraction in response to neural stimulation, which is followed by a substantial reduction in amplitude of contractions signifying the development of myopathy . The earliest evidence for neuropathy was obtained in a study that showed contractile response of the oesophageal smooth muscle of SSc patients to the direct acting muscarinic agonist methacholine but not to edrophonium, which acts indirectly .…”

Detecting gastrointestinal manifestations in patients with systemic sclerosis using anti-gAChR antibodies

“…Anti-RNA polymerase III is also an important biomarker associated with severe dcSSc and a 25% risk of renal crisis [29]. Earlier studies demonstrated that GI dysmotility in SSc was associated with circulating Abs against the muscarinic AChRs and myenteric neurons [19,[29][30][31][32][33][34][35]. Recently, McMahan and colleagues reported patients with SSc and anti-RNPC3 antibodies had moderate-to-severe GI disease and interstitial lung disease [36].…”