TLR7 triggers IFN-1a in HEK293 cells exposed to Porphyromonas gingivalis

Objectives: Toll-like receptor(TLR)2 and TLR4 have been identified as important receptors for a range of microbial molecules and are specifically implicated in sensing Porphyromonas gingivalis lipopolysaccharide (LPS). A high throughput screen of gene expression in human gingival epithelial cells challenged with P. gingivalis indicated significant fold differences for TLR7 mRNA, which we accepted as a priori evidence that the internally located TLR7 can sense P. gingivalis. Thus, we investigated the role of TLR7 in detecting P. gingivalis.

Methods: HEK293(null) cells were transiently transfected with a plasmid specific for TLR7. We then challenged the TLR7 transfected cells with heat-killed P. gingivalis, live P. gingivalis at various multiplicites of infection (MOI), P. gingivalis LPS, major fimbrial protein, fimbrillin (FimA), and imiquimod (R837), a synthetic TLR7 agonist. The cells were incubated at 4h or at 24h. The HEK cellular response was analyzed by measuring cytokines including TNF-a and IP-10 by ELISA. Gene expression at the mRNA level was analyzed using Reverse Transcriptase Polymerase chain Reaction (RT-PCR) and quantitative PCR for human TLR7, NOD1, NOD2, and IFN-a detection.

Results: Although heat-killed P. gingivalis did not display any significant TLR7 gene expression at the mRNA level, live P. gingivalis showed a statistically significant 4-fold increase in TLR7 expression and a significant 3-fold increase in IFN-1a expression.

Conclusions: Our results suggest that live P. gingivalis has the ability to trigger TLR7 and is a potent inducer of IFN-1a.

Supported by grant DE017384 from NIDCR