Activation of immune cells has to be tightly controlled to prevent detrimental hyperactivation. In this regulatory process molecules of the C-type lectin-like family play a central role. Here we describe a new member of this family, CLEC12B. The extracellular domain of CLEC12B shows considerable homology to the activating natural killer cell receptor NKG2D, but unlike NKG2D, CLEC12B contains an immunoreceptor tyrosine-based inhibition motif in its intracellular domain. Despite the homology, CLEC12B does not appear to bind NKG2D ligands and therefore does not represent the inhibitory counterpart of NKG2D. However, CLEC12B has the ability to counteract NKG2D-mediated signaling, and we show that this function is dependent on the immunoreceptor tyrosine-based inhibition motif and the recruitment of the phosphatases SHP-1 and SHP-2. Using monoclonal anti-CLEC12B antibodies we found de novo expression of this receptor on in vitro generated human macrophages and on the human myelo-monocytic cell line U937 upon phorbol 12-myristate 13-acetate treatment, suggesting that this receptor plays a role in myeloid cell function.Activation of the immune system has to be tightly regulated to prevent the attack of self-antigens and the development of autoimmunity (1). This is in part accomplished by a sophisticated system of various receptors that create a delicate balance of activating and inhibitory signals (2, 3). These receptors are present on different immune cells but play a major role for the innate immune system.Many receptors share common signaling motifs to transmit their signal into the cell. Activating receptors usually signal via the immunoreceptor tyrosine-based activation motif containing tyrosine residues that can be phosphorylated upon receptor engagement and can recruit Syk family kinases (4). Inhibitory receptors often possess an Immunoreceptor Tyrosine-based Inhibition Motif (ITIM), 4 which can recruit phosphatases upon phosphorylation (5). These phosphatases, like SHP-1 or SHP-2, are able to de-phosphorylate and therefore inhibit intracellular factors that otherwise would promote cellular activation (6).Interestingly, many activating and inhibitory receptors come in pairs, with a strong homology of the extracellular domains and partially overlapping ligand specificity but opposite signaling capacities (2). One reason for the existence of these receptor pairs might be to avoid extreme immune reactions to minor threats. Examples for these paired receptors are killer cell Iglike receptors on natural killer (NK) cells, which comprise inhibitory and activating receptors, both recognizing the same major histocompatibility complex class I ligand (5, 7). Also in the family of lectin-like receptors can be found antithetic pairs that are specific for the same ligand but transmit opposing signals (8). For example, the NKG2A-CD94 heterodimer transmits an inhibitory signal and NKG2C-CD94 signals in an activating fashion while both receptors recognize the same ligand, HLA-E.NKG2D is a C-type lectin-like receptor and plays a...