In previous studies, our laboratory has shown that the salivary gland epithelial cells (SGEC) of patients with primary Sjögren's syndrome (SS) display an intrinsically activated phenotype, as characterised by high expression of adhesion, costimulatory and antigen-presenting molecules. Stimulation of Toll-like receptors (TLRs) expressed by epithelia by native or synthetic ligands of TLR2, TLR3 and TLR4 may be critically involved in the regulation of immune responses through the induction of various immunoactive molecules, such as the intercellular adhesion molecule-1 (ICAM-1/CD54).
Materials and methods
Cultured, non-neoplastic SGEC lines derived from minor salivary gland biopsies of patients with SS (SS-SGEC, n = 4) and controls (n = 4) were studied. The expression of TLR2, TLR3 and TLR4 mRNA was examined by PCR. SGEC were stimulated by S. aureus peptidoglycan (100 μg/ml, TLR2-ligand), the synthetic analogue of dsRNA poly-inosinic:cytidylic acid (polyI:C) (5 μg/ml, TLR3-ligand) and E. coli lipopolysaccharide (100 ng/ml, TLR4-ligand), for 24 and 48 hours. In certain experiments, cells were also primed with interferon alpha (500 IU/ml) for 24 hours, prior to the stimulation with polyI:C. The induction of ICAM-1 expression was analysed by flow cytometry on resting and stimulated cells.
SGEC lines were found to express TLR2, TLR3, and TLR4 mRNA. In agreement with their intrinsic activation status and previously published reports, SS-SGEC lines displayed high constitutive ICAM-1 expression, compared with control-SGEC lines. Triggering of TLR3 with polyI:C had resulted in significantly increased ICAM-1 expression (SS-SGEC: sixfold, controls: sevenfold) that reached maximum expression after 24-hour stimulation. The response to polyI:C was further enhanced after priming of the cells with interferon alpha. Signaling via TLR2 by peptidoglycan was found to upregulate moderately ICAM-1 expression (SS-SGEC: onefold, controls: twofold) after 24-hour stimulation. Despite expression of TLR4 mRNA (by PCR), as well as of surface CD14 protein (by flow cytometry), lipopolysaccharide stimulation failed to upregulate ICAM-1 expression.
Our results indicate the SGEC lines are able to respond to synthetic microbial analogues that stimulate TLR2 and TLR3 by upregulating the expression of the adhesion molecule ICAM-1/CD54.