Toll-Like Receptors and Transforming Growth Factor-β1 Modulate Odontoblast Bacterial Responses

Objectives: Little is known about the immune functions of human odontoblasts (ODs), which are traditionally thought to form and maintain dentin. We investigate OD responses to common oral bacteria and compare their responses to those from human monocytes (MCs), cells of the immune system. We also determine the immunologic roles of toll-like receptors (TLRs) and transforming growth factor-β1 (TGF-β1), commonly released from the dentin in carious lesions.

Methods: We established human OD clones and characterized them both in vitro and in vivo. Expression of interleukin-8 (IL-8), and tumor necrosis factor-α(TNF-α) was examined in the human OD and MC cell lines, stimulated with Gram-positive oral bacteria (Streptococcus mutans, Sm; Enterococcus faecalis, Ef; and Lactobacillus casei, Lc) or Gram-negative bacteria (Porphylomonas gingivalis, Pg; Prevotella intermedia, Pi; and Fusobacterium nucleatum, Fn) with and without TGF-β1 pretreatment.

Results: In ODs, IL-8 and TNF-α increased in response to Gram-negative Pi and Fn but not to Pg, or Gram-positive Sm, Ef, or Lc. TLR4 RNA silencing attenuated these responses. In contrast, all bacteria increased expression of these cytokines in MCs. Luciferase assays for the activation of TLRs showed TLR2 stimulation without TLR4 activity from all Gram-positive bacteria. TGF-β1 inhibited IL-8 stimulation in human ODs but not in MCs whereas TNF-α inhibition occurred in both cell types. TGF-β1 decreased TLR2 and TLR4 in both cell types.

Conclusion: TLR4 signaling induced by Gram-negative oral bacteria regulates OD inflammatory responses which are diminished by TGF-β1. In contrast to MCs, ODs could differentially recognize and respond to Gram-negative and Gram-positive oral bacteria. These results establish the role of TLR4 in dental infection and suggest the potential use of TGF-β1 for the treatment of pulpal inflammation. Funded by NIH/NIDCR R01 DE-13573.