To determine whether Fusobacterium nucleatum's ability to invade cells allows the bacteria to activate pro-inflammatory response through cytosolic pattern recognition receptors, independent of surface Toll-like receptors (TLRs).Methodology
HEK293T cells, which lack endogenous TLRs, and overexpressing dominant negative myeloid differentiation primary response gene 88 (MyD88DN) protein, were infected with F. nucleatum and the production of interleukin-8 (IL-8) was determined. The necessity for intracellular invasion of the bacteria for cytokine production was also investigated by blocking bacterial invasion with cytochalasin D. The roles of NFκB and p38 mitogen-activated protein kinase (MAPK) and nucleotide-binding oligomerization domain-1 (NOD-1) signalling pathways in F. nucleatum-induced IL-8 secretion were determined.Results
Fusobacterium nucleatum-infected HEK293T cells produced IL-8 independent of the MYD88 signalling. This response was inhibited by preventing F. nucleatum invasion into HEK293T cells. p38 MAPK but not the NFκB signalling pathway was required for F. nucleatum-mediated IL-8 production. HEK293T cells expressed NOD-1 but not NOD-2. Yet, inhibition of NOD-1 signalling did not affect F. nucleatum-induced IL-8 secretion.Conclusions
Fusobacterium nucleatum invasion led to cytokine production, which is mediated by the p38 MAPK signalling but independent of TLRs, NOD-1, NOD-2 and NFκB signalling.