Identification of CLEC12B, an Inhibitory Receptor on Myeloid Cells

Abstract

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...

The fungal pattern recognition receptor, Dectin-1, and the associated cluster of C-type lectin-like receptors

“…This nonfunctional isoform appears to be preferentially expressed in certain tissues, including the lung, mammary gland and ovary. Although the receptor was not detected on any peripheral blood leukocyte, its expression was induced upon the differentiation of monocytes into macrophages (Hoffmann et al , 2007). CLEC12B possesses an ITIM sequence (VxYxxL) within its cytoplasmic tail, which can recruit the inhibitory phosphatases, SHP-1 and SHP-2, and, using a model system, was shown to be able to inhibit certain cellular functions.…”

Surface CD107a/LAMP-1 protects natural killer cells from degranulation-associated damage

“…26 Mouse and human NK cells were isolated and cultured also as previously described. 27,28 Cell lines used in this study were HeLa and 721.221 cells transduced with sCD107a or CLEC12B 29 as a control.…”

Evolution of the C-Type Lectin-Like Receptor Genes of the DECTIN-1 Cluster in the NK Gene Complex

“…CLEC12B was identified in human macrophages [20], and correspondingly mRNA is expressed in monocytic cell lines and primary myeloid cells [21]. Sequences are available for CLEC12b in mouse (protein homology to human sequence: 72%) and predicted in dog (81%) and cow (80%).…”

“…Interestingly, a putative ortholog of human CLEC12B has been identified in platypus [34], which shares 26% identity with human CLEC12B (Table 2). CLEC12B is unusual as it is the only known receptor within the DECTIN-1 cluster, which acts as an inhibitory receptor via recruitment of Src homology phosphatase (SHP)-1 and SHP-2 [20]. We have demonstrated that human CLEC12B has several splice variants, and many of them are likely to render the corresponding protein nonfunctional.…”