Salivary mucins induce a Toll-like receptor 4-mediated pro-inflammatory response in human submandibular salivary cells: are mucins involved in Sjögren's syndrome?

Objectives: A hallmark characteristic of SS patients is the ectopic presence of the mucins MUC5B and MUC7 in the extracellular matrix of salivary glands that have lost apical-basolateral acinar-cell polarity. This study aims to determine whether exogenous salivary mucins induce gene expression of pro-inflammatory cytokines, as well as to evaluate whether the Toll-like receptor-4 (TLR4) pathway is involved in this response.

Methods: Differentiated human submandibular gland (HSG) cells were stimulated with mucins or oligosaccharide residues at different concentrations and for different periods of time. The expression of pro-inflammatory cytokines and their receptors was determined by semi-quantitative real time PCR (sqPCR). TLR4-mediated responses induced by mucin were evaluated with the Toll-IL-1 receptor domain containing adaptor protein (TIRAP) inhibitory peptide or using anti-hTLR4 blocking antibody. TLR4-receptor expression was also determined in SS patients, controls and HSG cells.

Results: Mucins induced a significant increase in CXCL8, TNF-α, IFN-α, IFN-β, IL-6 and IL-1β, but not B cell activating factor (BAFF). Cytokine induction was mediated by TLR4, as shown using TIRAP or using anti-hTLR4 antibody. Sugar residues present in MUC5B, such as sulpho-Lewis (SO3-3Galβ1-3GlcNAc), also induced cytokines. Unexpectedly, mucins induced MUC5B, but not MUC7 expression.

Conclusion: Salivary mucins were recognized by TLR4 in epithelial cells initiating a pro-inflammatory response that could attract inflammatory cells to amplify and perpetuate inflammation and thereby contribute to the development of a chronic state characteristic of SS. The ectopic localization of MUC5B and MUC7 in the salivary gland extracellular matrix from SS patients and the current results reveal the importance of salivary epithelial cells in innate immunity, as well as in SS pathogenesis.