Induction of TNF-α by Filifactor alocis in THP-1 macrophagic cells

Filifactor alocis is an emerging periodontal pathogen, and macrophage-produced tumor necrosis factor-α (TNF-α) plays important roles in periodontal pathogenesis. In this study, we investigated F. alocis-stimulated TNF-α production in THP-1 macrophagic cells.

Phorbol 12-myristate 13-acetate-differentiated THP-1 macrophagic cells were challenged with F. alocis ATCC 35896 for various durations. TNF-α mRNA expression and protein secretion were determined using RT-PCR and ELISA, respectively. Activation of protein kinases and transcription factor proteins was evaluated by Western blot analysis.

Live F. alocis stimulated THP-1 cells to produce TNF-α in a dose-dependent manner. However, glutaraldehyde-killed or heat-killed F. alocis showed no effectiveness for TNF-α induction. In contrast, both live and killed Porphyromonas gingivalis robustly increased TNF-α expression. Furthermore, F. alocis was unable to stimulate TNF-α expression in Toll-like receptor 2 (TLR2) knockout THP-1 cells. F. alocis activated all three mitogen-activated protein kinases: extracellular signal-regulated kinase (ERK), p38, and c-Jun N-terminal kinase (JNK). Pharmacological inhibition of ERK and JNK, but not p38, significantly reduced F. alocis-induced TNF-α production. Finally, increased levels of phospho-c-Jun were detected in F. alocis-stimulated THP-1 cells.

These results suggest that F. alocis induces TNF-α production in THP-1 macrophagic cells primarily by activating the TLR2, JNK, and c-Jun pathways.