Background Loss of apico-basal polarity in acinar cells is a characteristic of labial salivary glands (LSG) of Sjögren's syndrome (SS) patients (1). In these glands, the ectopic presence of salivary mucins in the extracellular matrix has been described (1). The structure of mucins consists of a protein backbone with large number of O-linked oligosaccharides covalently attached. In SS-patients, ectopic mucin oligosaccharides might be recognized as not-self antigens by innate immunity receptors, inducing or increasing a pro-inflammatory response.
Objectives To determine whether exogenous salivary mucins or mucin oligosaccharides induce the expression of pro-inflammatory cytokines, to evaluate if the Toll-like receptor-4 (TLR4) is involved in this response and if pro-inflammatory cytokines are able to induce the expression of salivary mucins.
Methods Human submandibular gland (HSG) cells were stimulated with mucin from bovine submaxillary glands (BSM), Sulfo-Lewis (SO3-3Galβ1-3GlcNAc) oligosaccharide and human recombinant TNF-α or IFN-γ at different concentrations and for different periods of time. For TLR4 signaling assays, HSG cells were pre-treated with the TIRAP inhibitory peptide TBX2 or with an anti-TLR4 blocking antibody and then stimulated with BSM. The mRNA expression of mucins and cytokines was determined by real time PCR.
Results Mucins and Sulfo-Lewis induced a significant increase in CXCL8, TNF-α, IFN-α, IFN-β, IL-6 and IL-1β mRNA levels. The induction of cytokines was mediated by TLR4 as shown by reversion of the effect using TBX2 or the anti-TLR4 antibody. TNF-α and IFN-γ induced in vitro the expression of salivary mucins such as MUC1Y/ and MUC1/SEC, which are overexpressed in LSG of SS-patients.
Conclusions Salivary mucins and mucin oligosaccharides were recognized by TLR4 in epithelial cells initiating a pro-inflammatory response that could attract inflammatory cells. The overexpression and aberrant localization of mucins in LSG of SS-patients and their induction upon stimulation with pro-inflammatory cytokines support a self-perpetuating mucin-cytokine signaling loop that may facilitate the maintenance of the inflammatory environment leading to disruption of salivary gland homeostasis in SS-patients.
Barrera MJ, Bahamondes V, Sepúlveda D, Quest AFG, Castro I, Cortés J, Aguilera S, Urzúa U, Molina C, Pérez P, Ewert P, Alliende C, Hermoso MA, González S, Leyton C and González MJ. Sjögren's Syndrome: J Autoimmun. 2013 May;42:7-18.
Acknowledgements Fondecyt 1120062 (MJG, SA, CM, SG) and PhD fellowship Conicyt-Chile (MJB and JC).
Disclosure of Interest None declared