Muscarinic acetylcholine type-3 receptor desensitization due to chronic exposure to Sjögren's syndrome-associated autoantibodies

OBJECTIVE

Anti-muscarinic acetylcholine type-3 receptor (anti-M3R) autoantibodies have been shown to inhibit M3R-mediated responses by both in vitro contractility and fura-2 microfluorimetry analyses of intracellular calcium mobilization, suggesting an important role for anti-M3R autoantibody in secretory dysfunction. We investigated whether chronic stimulation of M3R by anti-M3R autoantibodies and/or the agonist pilocarpine results in a general M3R desensitization.

METHODS

Carbachol-evoked responses of mouse-bladder smooth muscle strips were measured following exposure to anti-M3R-positive and/or anti-M3R-negative sera from either NOD/Lt mice, a model of SS-like disease, or human patients with primary SS.

RESULTS

Bladder smooth muscle strips isolated from NOD/Lt mice with circulating anti-M3R autoantibodies exhibited lower carbachol-evoked responses than smooth muscle strips from anti-M3R autoantibody-negative NOD/Lt mice and control C57BL/6 mice. Repeated pilocarpine injections of NOD mice for 6 days also revealed M3R desensitization in the agonist-evoked contractile assay, whereas age and sex matched C57BL/6 mice injected with pilocarpine for the same period showed a 2-fold higher response. Incubation of smooth muscle strips with sera obtained from patients with primary SS resulted in both stimulated and inhibited responses.

CONCLUSION

These results support the hypothesis that chronic stimulation of membrane-bound M3R can result in receptor desensitization, and raise questions about repeated use of pilocarpine by patients positive for anti-M3R autoantibodies.

[The influence of aminostigmine on the parameters of nonspecific resistance and immune system of the organism upon acute poisoning of benzyl 3-quinuclidylate]

The results of experiments on outbred rats weighing 180 -240 g showed that the acute poisoning with benzyl 3-quinuclidylate decreases the parameters of nonspecific resistance of the organism, reduces the antibody production mainly to T-dependent antigens (sheep red blood cells), decreases the activity of natural killers and the antibody-dependent cell-mediated cytotoxicity, and suppresses the formation of delayed-type hypersensitivity. Aminostigmine partly inhibits the immunotoxicity benzyl 3-quinuclidylate.

An alternative perspective to the immune response in autoimmune exocrinopathy: induction of functional quiescence rather than destructive autoaggression

Sjögren's syndrome is characterized by dryness of the eyes and the mouth due to mononuclear cell infiltration of the lacrimal and salivary glands. The aetiology is unknown but autoimmunity is considered to play a significant role in the pathogenesis. Recent studies have focused on the fact that tear and salivary flow involves an entire functional system that includes the mucosal surfaces with adnexes (the site of inflammation), efferent nerve signals sent to the midbrain (lacrimal and salivary response region), and afferent neural signals from the brain to the acinar/ductal epithelial structures in the gland. Mononuclear cell infiltration in exocrine glands can lead to glandular destruction, suggested to be mediated through apoptosis. However, the functional impairment of exocrine glands could be regulated by cytokines and/or antibodies against the muscarinic M3 receptor by inhibiting the neural stimulation of the residual glands. This review discusses the possibility that the pathogenesis of Sjögren's syndrome comprises aberrant immune-mediated neuro-hormonal events.

Circulating antibodies against rat parotid gland M3 muscarinic receptors in primary Sjögren's syndrome

In this study we demonstrate that IgG present in the sera of patients with primary Sjögren's syndrome (PSS) could bind and activate muscarinic acetylcholine receptors (mAChRs) of rat parotid gland. These antibodies were able to inhibit in a non-competitive manner the binding of 3H-quinuclidinyl benzilate (QNB) to mAChRs of purified rat parotid gland membranes. Moreover, IgG from PSS could modify biological effects mediated by mAChR activation; i.e. decrease cAMP, increase phosphoinositide turnover without affecting cGMP. Atropine and 4-DAMP blocked all of these effects, and carbachol mimicked them, confirming the M3 subtype mAChRs mediated PSS IgG action. Neither binding nor biological effect were obtained with IgG from sera of normal women. The prevalence of cholinergic antibody was 100% in PSS, and was independent of Ro/SS-A and La/SS-B antibodies. It could be concluded that antibody against mAChRs may be another serum factor to be considered in the pathophysiology of the development of PSS.