Kinetics of TREM-1 expression on canine neutrophils after in vitro and in vivo stimulation with microbial products

The triggering receptor expressed on myeloid cells-1 (TREM-1) is a recently discovered cell surface molecule expressed on neutrophils, mature monocytes, and macrophages [1]. Activation of TREM-1 synergistically enhances proinflammatory cytokine production induced by toll-like receptor (TLR) stimulation [2]. A soluble form of TREM-1 has shown promise as a sensitive and specific biomarker for sepsis in humans [3–5]. Expression and function of TREM-1 in the dog has yet to be characterized. We hypothesize that the expression of canine TREM-1 will be upregulated after stimulation with TLR agonists. We assessed TREM-1 expression on canine neutrophils after exposure to TLR agonists in vitro and in vivo after i.v. LPS administration. In vitro, expression of TREM-1 on neutrophils is significantly upregulated by stimulation with microbe-derived agonists against TLR2/6 (Pam2CSK4), TLR1/2 (Pam3CSK4), and TLR4/MD2 (ultrapure LPS and wildtype LPS) (paired t test, P < 0.05). The TLR5 agonist flagellin did not significantly upregulate TREM-1 expression at any time point. See Figure 1. In contrast, i.v. administration of LPS to dogs resulted in a significant decrease in both TREM-1 expression and the percentage of TREM-1-positive neutrophils from 6 hours through 12 hours post LPS administration. See Figure 2. The disparity between in vitro and in vivo effects of LPS suggest other factors, such as systemic and local cytokine production and neutrophil turnover, may influence expression and shedding of TREM-1 on canine neutrophils. We suggest that naturally occurring sepsis in the dog represents the ideal model for defining diagnostic biomarkers and discovering efficacious therapeutics for use in human sepsis.

Figure 1

Kinetics of TREM-1 expression after TLR agonist stimulation in vitro.

Figure 2

Kinetics of TREM-1 expression post LPS administration in vivo.