Saliva initiates the formation of pro-inflammatory macrophages in vitro

Saliva can support oral wound healing, a process that requires a temporary inflammatory reaction. We have reported previously that saliva provokes a strong inflammatory response in oral fibroblasts. Bone marrow cells also give rise to macrophages, a heterogeneous subset of cell population involved in wound healing. Lipopolysaccharide (LPS) and interleukin 4 (IL-4) induce activation of pro-(M1), and anti-(M2) inflammatory macrophages, respectively. Yet, the impact of saliva on programming bone marrow cells into either M1 or M2 macrophages remains unclear .

Herein, we examined whether sterile saliva affects the in vitro process of macrophage polarization based on murine bone marrow cultures and RAW264.7 mouse macrophages.

We report that sterile saliva, similar to lipopolysaccharides, provoked a robust activation of the M1 phenotype which is characterized by a strong increase of the respective genes IL-12 and IL-6, based on a real-time gene expression analysis, and for IL-6 with immunoassay. Arginase-1 and Ym1, both genes characteristic for the M2 phenotype, were not considerably modulated by saliva. Inhibition of TLR4 signaling with TAK-242, blocking NFκB signaling with Bay 11-7085, but also autoclaving saliva greatly reduced the development of the M1 phenotype.

These data suggest that saliva activates the TLR4 dependent polarization into pro-inflammatory M1 macrophages in vitro.