An investigation of interactions between the immune system and stimulus-secretion coupling in mouse submandibular acinar cells. A possible mechanism to account for reduced salivary flow rates associated with the onset of Sjögren's syndrome

Update on Pathogenesis of Sjogren's Syndrome

Sjogren's syndrome is a common autoimmune disease that presents with sicca symptoms and extraglandular features. Sjogren's syndrome is presumably as common as RA; yet it is poorly understood, underdiagnosed and undertreated. From the usual identity as an autoimmune exocrinopathy to its most recent designate as an autoimmune epithelitis - the journey of SS is complex. We herein review some of the most important milestones that have shed light on different aspects of pathogenesis of this enigmatic disease. This includes role of salivary gland epithelial cells, and their interaction with cells of the innate and adaptive immune system. Non-immune factors acting in concert or in parallel with immune factors may also be important. The risk genes identified so far have only weak association, nevertheless advances in genetics have enhanced understanding of disease mechanisms. Role of epigenetic and environmental role factors is also being explored. SS has also some unique features such as congenital heart block and high incidence of lymphoma; disease mechanisms accounting for these manifestations are also reviewed.

An autoimmunized mouse model recapitulates key features in the pathogenesis of Sjögren's syndrome

The pathogenesis of Sjögren's syndrome (SS) is poorly understood. To evaluate an autoimmunization-induced experimental SS model, we firstly observed the phenotype of lymphocyte infiltration in the enlarged submandibular gland (SG). Furthermore, significant activation of caspase-3 and a high ratio of Bax-to-Bcl-2 were detected, indicating the inflammatory apoptosis associated with developmental foci. Meanwhile, the dysregulated cytokines, such as tumor necrosis factor α, IL-1β and IL-6 mRNA expression, were found to be over-expressed. A progressive decrease of aquaporin 5 and its subcellular translocation from apical to basal membrane in SG was found to be associated with the abnormally expressed M3 muscarinic acetylcholine receptor. This pattern was found to be similar to that seen in human SS and possibly contributed to the saliva secretion deficiency. Thus, this autoimmunization-induced model recapitulates the key features of human SS and may have potential for studying the pathogenesis of human SS.

Effects of hydroxychloroquine on salivary flow rates and oral complaints of Sjögren patients: a prospective sample study

OBJECTIVE

The objective of this study was to evaluate whether hydroxychloroquine (HCQ) therapy effects subjective and/or objective complaints and salivary flow rates of patients with primary Sjögren's syndrome (PSS).

STUDY DESIGN

Thirty women recently diagnosed with PSS, scheduled for HCQ treatment (400 mg daily), participated and were clinically examined before initiation of 30 weeks of HCQ treatment. During baseline evaluation, both the objective and/or subjective oral findings were recorded. Unstimulated (uSFR) and stimulated salivary flow rates (sSFR) were determined. After initiation of HCQ treatment, study parameters were assessed at 6, 12, 18, 24, and 30 weeks. Each patient served as her own control; measurements of the baseline and control times were analyzed by ANOVA.

RESULTS

uSFR values increased significantly with HCQ treatment, but sSFR values, objective and/or subjective complaints did not change considerably.

CONCLUSION

A positive impact of 30 weeks of HCQ treatment only on uSFRs of SS patients was revealed.

Antimuscarinic antibodies in primary Sjögren's syndrome reversibly inhibit the mechanism of fluid secretion by human submandibular salivary acinar cells

OBJECTIVE

Sjögren's syndrome (SS) is an autoimmune condition affecting salivary glands, for which a clearly defined pathogenic autoantibody has yet to be identified. Autoantibodies that bind to the muscarinic M3 receptors (M3R), which regulate fluid secretion in salivary glands, have been proposed in this context. However, there are no previous data that directly show antisecretory activity. This study was undertaken to investigate and characterize the antisecretory activity of anti-M3R.

METHODS

Microfluorimetric Ca2+ imaging and patch clamp electrophysiologic techniques were used to measure the secretagogue-evoked increase in [Ca2+]i and consequent activation of Ca2+-dependent ion channels in individual mouse and human submandibular acinar cells. Together, these techniques form a sensitive bioassay that was used to determine whether IgG isolated from patients with primary SS and from control subjects has antisecretory activity.

RESULTS

IgG (2 mg/ml) from patients with primary SS reduced the carbachol-evoked increase in [Ca2+]i in both mouse and human acinar cells by approximately 50%. IgG from control subjects had no effect on the Ca2+ signal. Furthermore, the inhibitory action of primary SS patient IgG on the Ca2+ signal was acutely reversible. We repeated our observations using rabbit serum containing antibodies raised against the second extracellular loop of M3R and found an identical pattern of acutely reversible inhibition. Anti-M3R-positive serum had no effect on Ca2+-dependent ion channel activation evoked by the direct intracellular infusion of inositol 1,4,5-triphosphate.

CONCLUSION

These observations show for the first time that IgG from patients with primary SS contains autoantibodies capable of damaging saliva production and contributing to xerostomia. The unusual but not unprecedented acute reversibility of the effects of anti-M3 autoantibodies is the subject of further research.

Sjogren's syndrome

Sjögren's syndrome (SS) is a chronic autoimmune disease affecting the exocrine glands, primarily the salivary and lacrimal glands. It has been suggested that exogenous agents may trigger SS in genetically predisposed individuals. However, at present, the etiology of SS is far from being understood, and no direct evidence for any of these triggers has been presented. The salivary and lacrimal glands from patients with SS harbor unique and highly selected T- and B-cell populations. Disturbance in glandular cell apoptosis may be one possible explanation for the sicca symptoms in SS. However, discrepancies between glandular destruction and salivary flow give rise to processes causing glandular dysfunction preceding or triggering glandular cell destruction. Recent reports suggested autoantibodies inhibiting neuronal innervation of acinar cells and defective water transport to be implicated in salivary secretion deficiency observed in SS. Several types of autoantibodies have been suggested to contribute to the pathogenesis of SS. However, how the tolerance to these structures is broken down is unknown at present. Studies on B-cell activating factor indicated that diminished apoptosis and disturbed B-cell maturation could be responsible for the occurrence of autoreactive B-cells and B-cell hyperreactivity. B-cell activation may also provide a basis for lymphoma development observed in up to 5% of the patients with SS.

Selective use of a reserved mechanism for inducing calcium oscillations

Concentration-dependent transformation of hormone- and neurotransmitter-induced calcium oscillation is a common phenomenon in diverse types of cells especially of the secretory type. The rodent submandibular acinar cells are an exception to this rule, which show elevated plateau increase in intracellular calcium under all stimulatory concentrations of both norepinephrine and acetylcholine. However, under depolarized state this cell type could also show a variation of periodic calcium changes. This reserved mechanism of calcium oscillation is jump-started by depolarization only with muscarinic cholinergic stimulation, but not with adrenergic stimulation. This latter effect is attributable to alpha receptor activation, not due to simultaneous activation of alpha and beta receptors, with beta receptor activation only serving to enhance the magnitude. These data suggest that this reserved mechanism for inducing calcium oscillation can be selectively used only by specific receptor-signaling pathways, and may therefore partly explain the long-known differences between secretion induced by sympathetic and parasympathetic stimulation in the submandibular gland.

Inhibitory effects of autoantibodies on the muscarinic receptors in Sjögren's syndrome

Sjogren's syndrome (SS) is a systemic autoimmune disease that involves reduced salivary secretions. Recently, circulating autoantibodies from SS patients against the type 3 muscarinic cholinergic receptor (M3R) has been reported in the sera of SS patients. However, the role of these autoantibodies in the development of SS has not been elucidated. In this study, purified IgG was obtained from the sera of 11 SS patients, and its inhibitory effect on the M3R of the salivary glands was evaluated using RT-PCR, microspectrofluorimetry, immunohistochemistry, and Western blot analysis. Stimulation with carbachol (CCh) evoked a [Ca2+]i transient in the fura-2 loaded HSG cells. However, pretreatment of the cells with SS IgG (0.5 mg/ml) for 12 or 24 h significantly reduced the magnitude of the CCh-induced [Ca2+]i transient (CICT). We found that the magnitude of CICT was decreased by 62-45% when cells were pretreated with the SS IgG. However, the [Ca2+]i response to ATP was not altered by the pretreatment of SS IgG. The effect of SS IgG on CICT was abrogated by the inclusion of excessive competitive peptides that encode the amino-acid sequence of M3R, which was not recapitulated by nonspecific peptides. The inhibitory effect of SS IgG on the aquaporin (AQP)-5 expression was also examined. After confirming the apical localization of AQP-5 along with its increase by pilocarpine (10(-5) M), we examined whether SS IgG had an effect on pilocarpine-induced AQP-5 trafficking to the apical membrane (APM) using rat parotid acinar cells. After incubating the cells with SS IgG for 12 h, the amount of pilocarpine-induced AQP-5 significantly decreased compared to the control groups. In conclusion, autoantibodies from the SS patients inhibit the function of the human M3R that is mediated by Ca2+ mobilization and AQP-5 trafficking. Our results could partly explain the underlying mechanisms of glandular dysfunction and associated features of impaired autonomic function in SS patients.

CD40, CD154, Bax and Bcl-2 expression in Sjögren's syndrome salivary glands: a putative anti-apoptotic role during its effector phases

Sjögren's syndrome (SS) is an autoimmune rheumatic disorder characterized by chronic lymphocytic infiltration and decreased secretion in the salivary glands (SGs). For some time, apoptosis has been suggested to constitute the major mechanism for acinar epithelial destruction during the effector phases; however, this is still controversial. We have recently demonstrated that despite the expression of Fas and FasL, the incidence of apoptosis is not increased in SS patients compared with control individuals. Our aim was therefore to further evaluate the expression of the pro- and anti-apoptotic Bax and Bcl-2 proteins. CD40 and CD154 expression was also investigated, as CD40 ligation has been suggested to protect cells from Fas-mediated apoptosis. Immunohistochemical staining was performed on SG tissue from primary and secondary SS patients, a group of patients with non-SS-related degenerative changes as well as on healthy control individuals. The frequency of stained cells in the foci of infiltrating mononuclear cells (IMCs) and in the acinar and ductal epithelium was determined. We found the expression of Bcl-2 but rarely Bax in SS SG IMCs. Bcl-2 in epithelial cells was sparse, while Bax expression occurred frequently and with no significant difference between the patient groups. CD40 and CD154 expression was high among SS IMCs, while CD40 levels were slightly decreased in SS epithelium compared with controls. Elevated CD154 expression was found in SS epithelium, being significantly increased in the ducts. In conclusion, our study further supports the hypothesis about apoptosis resistance among SS focal IMCs and suggests a putative protective role of CD40 ligation in SS SG epithelium.

Autoantibodies in primary Sjögren's syndrome: new insights into mechanisms of autoantibody diversification and disease pathogenesis

Characterisation of autoantibodies and their target autoantigens in primary Sjögren's syndrome (SS) is an important entry point for studying this common systemic autoimmune disease. Diversification of anti-Ro/La responses is believed to occur by a process of determinant spreading following initiation of an autoimmune response to one component, possibly 52-kD Ro (Ro52). Recent evidence supports the ER-resident chaperone Grp78 as a potential candidate in the initiation of an autoimmune response against Ro52, by binding to a Grp78 binding motif in the COOH-terminal region of Ro52. The subsequent diversification of the anti-Ro/La response is influenced by distinct HLA class II alleles. Anti-salivary duct autoantibodies have been revisited and shown to be mimicked by cross-reactive isoantibodies to AB blood group antigens. Identification of autoantibodies that act as antagonists at M3-muscarinic receptors represents an important advance. As well as contributing to the sicca symptoms, the functional effects of these autoantibodies may explain associated features of autonomic dysfunction in patients with SS. Anti-M3 receptor autoantibodies occur in both primary and secondary SS and allow Sjögren's syndrome to be viewed as a disorder of anti-receptor autoimmunity.

Up-regulation of M3-muscarinic receptors in labial salivary gland acini in primary Sjögren'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 Sjögren's syndrome. We have investigated the expression of M3R in paraffin-embedded labial salivary glands (LSG) from seven patients with primary Sjögren'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.

Acetylcholine-evoked calcium mobilization and ion channel activation in human labial gland acinar cells from patients with primary Sjögren's syndrome

Recent evidence has indicated that the salivary gland dysfunction associated with Sjögren's syndrome (SjS) is not necessarily due to immune-mediated destruction of acinar tissue. SjS sufferers may possess substantial reserves of acinar tissue but nevertheless be incapable of maintaining salivary flow rates in the normal range. We have investigated the ability of isolated labial gland acinar cells from SjS patients to fluid secrete by measuring agonist-evoked changes in intracellular Ca(2+) ([Ca(2+)](i)) using fura-2 microfluorimetry and activation of K(+) and Cl(-) channels using the patch-clamp whole cell technique. We can confirm that stimulation with a super-maximal dose of acetylcholine (ACh) increased [Ca(2+)]i equally in both control acinar cells and those derived from SjS patients. However, at submaximal concentrations, the dose-response curve for ACh was shifted to the right by approximately one order of magnitude in acinar cells from SjS patients compared to control acinar cells. Patch-clamp measurements consistent with the presence of Ca(2+)-activated K(+) and Cl(-) conductances were obtained from both control acinar cells and those obtained from SjS patients. Dose-dependent activation of the ion channels by acetylcholine was also right-shifted in acinar cells from SjS patients compared to control cells. Our data show that labial gland acinar cells from SjS patients were capable of responding to agonist stimulation by mobilizing [Ca(2+)](i) and activating K(+) and Cl(-) channels consistent with the requirements of fluid secretion. However, the persistent loss of sensitivity to ACh observed in from SjS patients may account for the lack of saliva production observed in these patients in vivo.