We recently reported the cloning of two triggering receptors expressed by myeloid cells (TREM), TREM-2a and TREM-2b, which are highly homologous to each other. These receptors associate with DAP12, and ligation of TREM-2 on the surface of macrophages leads to the release of nitric oxide. Using the immunoglobulin (Ig) domain of TREM-2 to screen a mouse EST database we have isolated a novel receptor, derived from a WEHI-3 macrophage library, which shows homology to TREM-2 (20%). The DNA sequence of this receptor has been submitted to Genbank with the name TREM-3. The predicted amino acid sequence contains a single Ig domain and a transmembrane lysine residue. We found transcripts for TREM-3 in two macrophage cell lines (RAW264.7 and MT2) but not in P388D1 macrophage cells. TREM-3 transcripts could also be detected at low levels in T cell lines, but were not detectable in NK, B cell, or mast cell lines. Furthermore, in macrophage cells, transcripts for TREM-3 were up-regulated by LPS, but were down-regulated by IFN-gamma. Like TREM-1 and TREM-2, TREM-3 signals through DAP12, and when TREM-3 is transfected into an NK cell line it mediates redirected lysis. Thus, TREM-3 functions as an activating receptor. Analysis of the mouse genome reveals that the gene for TREM-3 lies adjacent to the gene for TREM-1 and in close proximity to a number of other single Ig domain receptors, including TREM-2. Thus, TREM-3 is a novel member of a family of immunoglobulin receptors that form an innate immune gene complex on chromosome 17.
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