TREM proteins (triggering receptors expressed on myeloid cells) are a family of cell surface receptors involved in innate immune responses. They are expressed in myeloid cells and have both positive and negative functions in regulating myeloid cell activation and differentiation. Humans have two members, TREM1 and TREM2. TREM1 is considered an amplifier of the immune response, while TREM2 is believed to be a negative regulator of inflammatory responses (Sharif & Knaap 2008). TREM proteins consist of a single extracellular V-type Ig-like domain, a transmembrane region and a short cytoplasmic tail lacking any signalling motifs (Kelker et al. 2004). Both receptors associate with DAP12 for signalling.
The ligand for TREM1 is unknown. TREM1 associates with DAP12 dimer. This interaction is mediated by aspartic acid and adjacent threonine residues in the DAP12 dimer that interface with lysine residues in the TREM1 transmembrane region. TREM1 engagement triggers the production of inflammatory chemokines and cytokines such as IL-8 and myeloperoxidase (MPO) in neutrophils and IL-8, MCP-1, and TNF in monocytes (Tessarz & Cerwenka 2008, Bouchon et al. 2000).