Doxycycline inhibits TREM-1 induction by Porphyromonas gingivalis

The triggering receptor expressed on myeloid cells 1 (TREM-1) is a cell surface receptor of the immunoglobulin superfamily, with the capacity to amplify pro-inflammatory cytokine production. Porphyromonas gingivalis is a Gram-negative anaerobic species highly implicated in inflammatory periodontal disease, with potential involvement in systemic inflammation. Porphyromonas gingivalis positively regulates TREM-1 expression and production in monocytic cells. Subantimicrobial doses of doxycycline (SDD) are used as an adjunct treatment in periodontal therapy, because of their anti-inflammatory properties. The aim of this study was to investigate the effect of SDD on P. gingivalis-induced TREM-1 expression and secretion by the myelomonocytic cell line MonoMac-6. After 24 h of challenge, P. gingivalis enhanced TREM-1 gene expression by the cells, with a concomitant increase in soluble TREM-1 release. Nevertheless, SDD concentrations between 2 and 10 mu g mL-1 abolished TREM-1 expression and release, already after 4 h of administration. Moreover, SDD reduced P. gingivalis-induced interleukin-8 secretion, confirming its anti-inflammatory effects. In conclusion, SDD inhibits bacterially induced TREM-1, and this effect may partly account for its generalized anti-inflammatory properties. This could partly explain the clinical efficacy of SDD as an adjunctive treatment for periodontal disease, but may also indicate that SDD could serve as a suitable modulator of systemic inflammatory responses.