Sterile-filtered saliva is a strong inducer of IL-6 and IL-8 in oral fibroblasts

Saliva has been implicated to support oral wound healing, a process that requires a transient inflammatory reaction. However, definitive proof that saliva can provoke an inflammatory response remained elusive.

We investigated the ability of freshly harvested and sterile-filtered saliva to cause an inflammatory response of oral fibroblasts and epithelial cells. The expression of cytokines and chemokines was assessed by microarray, RT-PCR, immunoassays, and Luminex technology. The involvement of signaling pathways was determined by Western blot analysis and pharmacologic inhibitors.

We report that sterile-filtered whole saliva was a potent inducer of IL-6 and IL-8 in fibroblasts from the gingiva, the palate, and the periodontal ligament, but not of oral epithelial cells. This strong inflammatory response requires nuclear factor-kappa B and mitogen-activated protein kinase signaling. The pro-inflammatory capacity is heat stable and has a molecular weight of <40 kDa. Genome-wide microarrays and Luminex technology further revealed that saliva substantially increased expression of other inflammatory genes and various chemokines. To preclude that the observed pro-inflammatory activity is the result of oral bacteria, sterile-filtered parotid saliva, collected under almost aseptic conditions, was used and also increased IL-6 and IL-8 expression in gingiva fibroblasts. The inflammatory response was, furthermore, independent of MYD88, an adapter protein of the Toll-like receptor signaling pathway.

We conclude that saliva can provoke a robust inflammatory response in oral fibroblasts involving the classical nuclear factor-kappa B and mitogen-activated protein kinase signaling pathway.

Since fibroblasts but not epithelial cells show a strong inflammatory response, saliva may support the innate immunity of defect sites exposing the oral connective tissue.