Triggering receptor expressed on myeloid cells 1 (TREM-1) is a recently discovered molecule that is expressed on the cell surface of monocytes and neutrophils. Engagement of TREM-1 triggers synthesis of proinflammatory cytokines in response to microbes, but the extent and mechanism by which TREM-1 modulates the inflammatory response is poorly defined. In the present study, we investigated the functional effects of blocking TREM-1 on the Toll-like receptor (TLR)4-mediated signaling pathway in macrophages. By transfecting cells with small hairpin interfering RNA molecules to TREM-1 (shRNA), we confirmed that TREM-1 mRNA and protein expression was greatly attenuated in RAW cells in response to treatment with LPS. PCR array for genes related to or activated by the TLR pathway revealed that although the expression of TLR4 itself was not significantly altered by silencing of TREM-1, expression of several genes, including MyD88, CD14, IkappaBalpha, IL-1beta, MCP-1, and IL-10 was significantly attenuated in the TREM-1 knockdown cells in response to treatment with LPS. These data indicate that expression of TREM-1 modulates the TLR signaling in macrophages by altering the expression of both adaptor and effector proteins that are critical to the endotoxin response.

Triggering receptor expressed on myeloid cells (TREM)‐1 is a recently identified immunoglobulin receptor that is expressed on neutrophils and monocytes where it amplifies the acute inflammatory response to bacteria. We examined the transcriptional regulation of TREM‐1 in macrophages. Treatment of RAW cells with Escherichia coli LPS or Pseudomonas aeruginosa led to the induction of TREM‐1 within 1 h with an expression lasting up to at least 24 h in vitro as detected by RT‐PCR. Since the promoter of TREM‐1 has multiple binding sites for NF‐κB and PU.1 (one of the members of the ets family of transcription factors), we investigated the role of these transcription factors in the induction of TREM‐1. Treatment of cells with NF‐κB inhibitors abolished the expression of message of TREM‐1 induced by LPS and P. aeruginosa. In contrast, the expression of TREM‐1 was increased after stimulation with LPS or P. aeruginosa in cells that had gene of PU.1 silenced. Additionally, over‐expression of PU.1 led to inhibition of TREM‐1 induction in response to LPS and P. aeruginosa. These data suggest that both these transcription factors are involved in the expression of TREM‐1. NF‐κB functions as a positive regulator whereas PU.1 is a negative regulator of the TREM‐1 gene.

Triggering receptor expressed on myeloid cells (TREM)-1 plays an important role in myeloid cell-activated inflammatory responses. Although TLR ligands such as LPS and lipoteichoic acid have been shown to upregulate TREM-1 expression in macrophage and neutrophils, the role of specific TLR in inducing the expression of TREM-1 remains unclear. In this study, we investigated whether the presence of TLR is necessary for the expression of TREM-1. We show that BM-derived macrophages from TLR4 and TLR2 KO mice failed to induce expression of TREM-1 message and protein in response to their specific ligands. Interestingly, the expression of TREM-1 in response to LPS is not altered in myeloid differentiation factor 88 (MyD88) KO macrophages, suggesting that downstream of TLR a MyD88-independent pathway induces the expression of TREM-1. Inhibiting toll/IL-1R domain-containing adaptor-inducing IFN-b (TRIF) expression by siRNA decreased TREM-1 expression in response to LPS, suggesting that the expression of TREM-1 in response to LPS was mediated by the TRIF signaling pathway. On the other hand, the expression of TREM-1 in response to lipoteichoic acid is dependent on MyD88 expression. These data indicate that the expression of TREM-1 in response to TLR ligands occurs secondary to downstream signaling events and that the presence of TLR is necessary for the expression of TREM-1 in response to their specific ligands. However, the downstream signaling required for the expression of TREM-1 is dependent on the stimulus and the surface receptor through which the signaling is initiated.Key words: Innate immunity . TLR . TREM-1 IntroductionTriggering receptor expressed on myeloid cells 1 (TREM-1) belongs to the TREM super-family of receptors, which is expressed on monocytes and neutrophils [1][2][3][4][5]. These receptors activate downstream signaling pathways with the help of an adaptor molecule, DAP12 [2]. Although the natural ligands for TREM-1 have not been identified the engagement of TREM-1 synergizes with the effects of the TLR ligands and amplifies the synthesis of inflammatory cytokines [2][3][4]6]. Because blocking of TREM-1 improves the survival of mice with bacterial sepsis TREM-1 has been perceived to play an essential role in acute inflammatory responses in murine models of septic shock [2,[7][8][9][10][11][12][13].In the presence of infection, inflammation is triggered through the recognition of microorganism-by the pathogen associated molecules such as the TLR, which are expressed on the surface of innate immune cells such as monocytes, macrophages, and dendritic cells [14][15][16]. To date ten members of TLR have been identified in human, and 13 in mice, and a series of genetic series have revealed their respective ligands. On engagement with 162ligands, TLR recruit specific adapter molecules that propagate downstream signaling [17]. This interaction then leads to the activation of NF-kB and to the release of pro-inflammatory TNF-a, IL-12, IL-1, IL-6, IL-8, and anti-inflammatory (IL-10 and TGF-b) cytokines [18]. TLR ...

No abstract