Human gingival fibroblasts (HGFs), a predominant cell type in tooth-supporting structure, are presently recognized for their active role in the innate immune response. They produce a variety of inflammatory cytokines in response to microbial components such as LPS from the key periodontal pathogen, Porphyromonas gingivalis. In this study, we demonstrated that HGFs expressed mRNA of TLRs 1, 2, 3, 4, 5, 6, and 9, but not TLRs 7, 8, and 10. Stimulation of HGFs with highly purified TLR2 ligand (P. gingivalis LPS), TLR3 ligand (poly(I:C)), TLR4 ligand (Escherichia coli LPS), and TLR5 ligand (Salmonella typhimurium flagellin) led to expression of IL-8 and IDO. A potent TLR 9 ligand, CpG oligodeoxynucleotide 2006 had no effect, although HGFs showed a detectable TLR9 mRNA expression. No significant enhancement on IL-8 or IDO expression was observed when HGFs were stimulated with various combinations of TLR ligands. Surprisingly, the TLR9 ligand CpG oligodeoxynucleotide 2006 was able to specifically inhibit poly(I:C)-induced IL-8 and IDO expression. TNF-α enhanced TLR ligand-induced IL-8 production in HGFs, whereas IFN-γ enhanced TLR ligand-induced IDO expression. HGF production of IDO in response to P. gingivalis LPS, IFN-γ, or the two in combination inhibited T cell proliferation in MLRs. The observed T cell inhibition could be reversed by addition of either 1-methyl-dl-tryptophan or l-tryptophan. Our results suggest an important role of HGFs not only in orchestrating the innate immune response, but also in dampening potentially harmful hyperactive inflammation in periodontal tissue.

These findings indicate that LL-37 induces IL-8 expression via the P2X(7) receptor and the MEK1/2-dependent p44/42 MAP kinases in HGFs, suggesting both direct and indirect involvement of LL-37 in neutrophil recruitment into an inflammatory site within diseased periodontal tissues.

The ADAM8 levels are elevated in the GCF of patients with periodontal diseases, including gingivitis, CP, and AgP, in comparison to control participants who are healthy, and they correlate with four clinical parameters that reflect the degree of disease severity.

The significant correlations between the hCAP18/LL-37 and the CS levels were found in CP, but not in aggressive periodontitis. The presence versus absence of such correlations may be clinically applicable to help clinicians distinguish between two distinct types of periodontitis.

These findings indicate that LL-37 favorably induces IL-8 expression and secretion in HGECs, suggesting both direct and indirect involvement of LL-37 in neutrophil recruitment into an inflammatory site within diseased periodontal tissues.