To investigate the relationship between pattern recognition receptor triggering receptors expressed on myeloid cells-1 (TREM-1) and M1/M2 polarization in macrophages stimulated by Porphyromonas gingivalis lipopolysaccharide (Pg-LPS), so as to explore the mechanism of TREM-1 in periodontitis.

Human monocytic cell line THP-1 were induced to differentiate into macrophages by phorbol-12-myristate-13-acetate and stimulated by 0 (blank control group) and 1 μg/ml Pg-LPS (LPS group), respectively. LP17, the TREM-1 inhibitor (LPS+LP17 group) and its control peptide (LPS+control peptide group) with final concentration of 0.1 μg/ml were added at the same time. After 24 hours stimulation, the expression of TREM-1, M1 markers and related cytokines [CD86, tumor necrosis factor-α (TNF-α), interleukin (IL)-1β], M2 markers and related cytokines (CD206,IL-10) mRNA were detected by real-time quantitative-PCR (RT-qPCR), the level of TREM-1, CD86 and CD206 proteins were detected by Western blotting, and TNF-α, IL-1β and IL-10 in the macrophage culture supernatant were detected by enzyme linked immunosorbent assay.

After 24 h of cell culture, the relative expressions of TREM-1 mRNA (1.40±0.14) and protein (3.85±0.24) in macrophages in the LPS group increased compared with the blank control group (1.01±0.18 and 1.00±0.05, respectively) (P<0.05). Meanwhile, the expression levels of M1 markers CD86 mRNA and protein [LPS group vs blank control group were (1.42±0.01 vs 1.00±0.09) and (1.55±0.07 vs 1.00±0.10), respectively] were up-regulated (P<0.01), and the expressions of mRNA and protein of M1 related cytokines TNF-α and IL-1 increased (P<0.05). After the addition of TREM-1 blocker LP17, the levels of mRNA and protein of TREM-1 showed no significant changes (P>0.05), while the levels of CD86 mRNA (0.96±0.00) and protein (1.36±0.02) decreased (P<0.05), and the levels of TNF-α and IL-1 further decreased (P<0.05). For M2 marker CD206 and related cytokine IL-10, CD206 mRNA (0.56±0.05) and protein (0.25±0.04) were significantly down-regulated (P<0.01) compared with the blank control group (1.02±0.25 and 1.00±0.10, respectively), and IL-10 mRNA was up-regulated compared with the blank control group (P<0.05), with no significant change in protein (P>0.05). After the addition of LP17, the expressions of CD206 and IL-10 mRNA in the LPS+LP17 group were further down-regulated compared with the LPS group (P<0.05), and there was no statistically significant difference between the two groups in protein level (P>0.05).

TREM-1 and its downstream signaling pathway might be involved in M1 polarization of Pg-LPS-mediated macrophages, thus playing a pro-inflammatory role in the development of periodontitis. There is no obvious evidence that TREM-1 is involved in regulating M2 polarization of Pg-LPS-mediated macrophages.

Periodontitis; Porphyromonas gingivalis; Triggering receptors expressed by myeloid cells-1; Macrophages; Polarization