T cell immunoglobulin-domain and mucin-domain (TIM) proteins constitute a receptor family that was identified first on kidney and liver cells; recently it was also shown to be expressed on T cells. TIM-1 and -3 receptors denote different subsets of T cells and have distinct regulatory effects on T cell function. Ferritin is a spherical protein complex that is formed by 24 subunits of H- and L-ferritin. Ferritin stores iron atoms intracellularly, but it also circulates. H-ferritin, but not L-ferritin, shows saturable binding to subsets of human T and B cells, and its expression is increased in response to inflammation. We demonstrate that mouse TIM-2 is expressed on all splenic B cells, with increased levels on germinal center B cells. TIM-2 also is expressed in the liver, especially in bile duct epithelial cells, and in renal tubule cells. We further demonstrate that TIM-2 is a receptor for H-ferritin, but not for L-ferritin, and expression of TIM-2 permits the cellular uptake of H-ferritin into endosomes. This is the first identification of a receptor for ferritin and reveals a new role for TIM-2.

By homology to triggering receptor expressed by myeloid cells-2, we screened the mouse expressed sequence tag database and isolated a new single Ig domain receptor, which we have expressed and characterized. The receptor is most similar in sequence to the human CMRF-35 receptor, and thus we have named it CMRF-35-like molecule (CLM)-1. By screening the mouse genome, we determined that CLM-1 was part of a multigene family located on a small segment of mouse chromosome 11. Each contains a single Ig domain, and they are expressed mainly in cells of the myeloid lineage. CLM-1 contains multiple cytoplasmic tyrosine residues, including two that lie in consensus immunoreceptor tyrosine-based inhibitory motifs, and we demonstrate that CLM-1 can associate with Src-homology 2 containing phosphatase-1. Expression of CLM-1 mRNA is down-regulated by treatment with receptor activator of NF-κB ligand (RANKL), a cytokine that drives osteoclast formation. Furthermore, expression of CLM-1 in the osteoclastogenic cell line RAW (RAW.CLM-1) prevents osteoclastogenesis induced by RANKL and TGF-β. RAW.CLM-1 cells fail to multinucleate and do not up-regulate calcitonin receptor, but they express tartrate-resistant acid phosphatase, cathepsin K, and β3 integrin, suggesting that osteoclastogenesis is blocked at a late-intermediate stage. Thus, we define a new family of myeloid receptors, and demonstrate that the first member of this family, CLM-1, is an inhibitory receptor, able to block osteoclastogenesis.

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‐γ. 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.