Human CLEC-1 is transcribed in placenta, lung, and bone marrow (Colonna et al. 2000). Lower expression levels are found in thymus and heart, and very low levels are also detected in pancreas, kidney, bladder, prostate, testis, ovary, small intestine, and colon (Colonna et al. 2000). The wide tissue distribution of human CLEC-1 transcription is also found in the rat, with high levels of expression in spleen, kidney, lung, lymph node, and aorta; moderate expression in liver, heart, muscle, and brain; and low expression levels in bone marrow, thymus, and testis (Thebault et al. 2009; Flornes et al. 2010). At the protein level, rat CLEC-1 is strongly expressed in lung, moderately expressed in spleen and heart, and weakly expressed in thymus, consistent with the transcript expression distribution (Thebault et al. 2009).
At the cellular level, human CLEC-1 is transcribed in unstimulated DC and in DC stimulated with lipopolysaccharides (LPS), tumor necrosis factor alpha (TNF-α), or by CD40-CD40 ligand interaction (Colonna et al. 2000; Sobanov et al. 2001). High levels of human CLEC-1 transcripts are also detected in endothelial cells (unstimulated and LPS-stimulated human umbilical vein endothelial cells (HUVEC)) (Sobanov et al. 2001). CLEC-1 is weakly expressed in primary human monocytes (Colonna et al. 2000) and in the U937 human monocytic lymphoma cell line following LPS stimulation (Sobanov et al. 2001). Similarly, low levels have been detected in primary human granulocytes but not in lymphocytes, NK cells, or platelets (Sattler et al. 2012). Different levels of CLEC-1 mRNA have been detected in DC from different origins, with the highest levels in cord blood-derived interstitial-type DC, when compared with monocyte-derived DC, Langerhans cells, and peripheral blood myeloid DC (Sattler et al. 2012).
In the rat, CLEC-1 is expressed in concavalin-A stimulated blasts, DC, peritoneal macrophages, B cells, granulocytes, and LAK (IL2-stimulated NK cells or lymphokine-activated killers) (Thebault et al. 2009; Flornes et al. 2010). Low-level rat-CLEC-1 transcription is detected in CD4+ and CD8+ T cells, RNK16 (NK cell line), and an endothelial cell line (Thebault et al. 2009; Flornes et al. 2010). Rat CLEC-1 is observed at the cell surface of bone marrow-derived DC, macrophages, and endothelial cells (Thebault et al. 2009). However, in transfected COS cells (Colonna et al. 2000) and transfected HUVEC (Sobanov et al. 2001; Sattler et al. 2012), CLEC-1 is located intracellularly. Immunofluorescence studies demonstrate that CLEC-1 accumulates in perinuclear regions of the transfected cells (Sobanov et al. 2001; Sattler et al. 2012). Additionally, there is some suggestion that human CLEC-1 is a constitutive ER-associated protein, based on the observation that after subcellular fractionation of HUVEC the highest levels of CLEC-1 are found in the microsomal fraction (Sattler et al. 2012), although CLEC-1 lacks any classic ER retention motifs. These observations raise the possibility that CLEC-1 may need to associate with a second molecule to be expressed as a transmembrane protein at the cell surface. Certain C-type lectin molecules such as CLEC4E interact with adaptor proteins in the plasma membrane, and this interaction is required for efficient cell-surface expression (Yamasaki et al. 2008). It is unclear whether CLEC-1 has a similar requirement, but it does not have a charged residue in its predicted transmembrane region, and such associations are typically mediated by charge-charge interactions within the plasma membrane.
In HUVEC, recombinant CLEC-1 is expressed as an N-glycosylated homodimer, which is a common finding with other C-type lectin-like receptors, such as LOX-1 (Sattler et al. 2012).
Regulation of CLEC-1 Expression
As discussed above, CLEC-1 expression is induced in human monocytic U937 cells following LPS stimulation (Sobanov et al. 2001). However, in rat bone marrow-derived DC, CLEC-1 expression is reduced by LPS, interferon (IFN) gamma, or poly (I:C) treatment after 48 h (Thebault et al. 2009). IFN-γ and LPS treatment also significantly reduce CLEC-1 expression in human aortic endothelial cells and human monocyte–derived DC, respectively. In contrast, treatment with IL10 or TGF-beta increases CLEC-1 expression in rat bone marrow-derived DC and human monocyte-derived DC. Furthermore, IL10 and TGF-beta are able to inhibit the LPS-induced downregulation of CLEC-1 expression in human monocyte-derived DC (Thebault et al. 2009). In HUVEC, stimulation with either LPS or TNF-α has no significant effect on CLEC-1 expression, whereas treatment with TGF-β significantly upregulates CLEC-1 in these cells. There is, however, no detectable increase in cell-surface expression after treatment with TGF- β (Sattler et al. 2012).
In a model of rat allograft tolerance, CLEC-1 has been shown to be overexpressed on PECAM-1+ endothelial cells of tolerated allografts compared with allografts that are chronically rejected or syngeneic grafts (Thebault et al. 2009). In vitro and in vivo induction of CLEC-1 expression in the endothelial cells of tolerated allografts is dependent upon alloantigen-specific regulatory CD4+CD25+ T cells from tolerant recipients, involving a cell contact-dependent mechanism. CD4+CD25+ T cells from naive rats are not able to upregulate CLEC-1 in allograft endothelial cells in vitro, and neither regulatory CD4+CD25+ T cells from tolerated recipients nor T cells from naive rats can induce CLEC-1 expression on syngeneic grafts in vivo (Thebault et al. 2009). Therefore, regulatory CD4+CD25+ T cells from tolerant recipients that accumulate in allografts may be able to regulate the expression of CLEC-1 in donor-specific endothelial cells.
CLEC-1 Is an Orphan Receptor
There are no known endogenous or exogenous ligands for CLEC-1. Unlike classical C-type lectins, which contain conserved residues that mediate the coordination of calcium involved in sugar binding, CLEC-1 is a C-type lectin-like receptor that lacks these conserved residues and so is unlikely to have a carbohydrate ligand. CLEC-1 may bind to protein ligands, as has been shown for other C-type lectin-like molecules, including CLEC-2 (Watson and O’Callaghan 2005; Suzuki-Inoue et al. 2006; Suzuki-Inoue et al. 2007; Christou et al. 2008), NKG2D (Bauer et al. 1999; Cerwenka et al. 2000; Diefenbach et al. 2000), and CD94/NKG2A/B (Braud et al. 1998). CLEC-1 belongs to the Dectin-1 family of C-type lectin-like receptors, which are encoded in the natural killer gene complex on human chromosome 12 and mouse chromosome 6 ((Sobanov et al. 2001); reviewed in (Kanazawa 2007; Huysamen and Brown 2009)), and Dectin-1 is able to bind to fungal β-glucans (Brown and Gordon 2001) in a calcium-independent manner and so has a different mode of sugar binding to the classical C-type lectins (Adachi et al. 2004; Brown et al. 2007). A carbohydrate ligand for CLEC-1 cannot be ruled out.
CLEC-1 is highly conserved; rat and mouse CLEC-1 have 95.5% identity, human and rat CLEC-1 have 71.3% identity, and human and mouse CLEC-1 have 69.4% identity (Flornes et al. 2010).
CLEC-1 Function and Regulation of Its Activity
CLEC-1 contains a tyrosine residue within the sequence YSST in its cytoplasmic domain, which may represent a novel signaling motif. Mouse and rat CLEC-1 contain an additional tyrosine residue in the intracellular domain (YxxT x 13 YxxT) (Flornes et al. 2010) (Fig. 1). The cytoplasmic domain of CLEC-1 also contains several serine and threonine residues that are putative phosphorylation sites. Putative CLEC-1 signaling and downstream effector functions have not been deciphered, but it is possible that ligand binding to CLEC-1 may lead to phosphorylation of the intracellular tyrosine(s), triggering effector functions. A knockout mouse model of CLEC-1 has not yet been generated.
Inhibition of CLEC-1 expression in rat LPS-stimulated bone marrow-derived DC using RNAi does not affect DC generation or maturation as assessed by cytokine production and expression of MHC class II, CD86, and CD80 cell-surface markers (Thebault et al. 2009). However, in a mixed leukocyte reaction, inhibition of CLEC-1 in DC increases the differentiation of allogeneic Th17 T cells and decreases the number of regulatory Foxp3+ T cells, but has no overall effect on T cell proliferation (Thebault et al. 2009). These effects could not be accounted for by levels of the cytokines IL6 and TGF-β, which were unaffected by CLEC-1 inhibition. The increase in CD4+ Th17 differentiation is not due to a direct effect on Th17 differentiation of naïve T cells or to the plasticity of regulatory CD4+CD25+ T cells. It is suggested that the Th17 differentiation may be due to an effect on the suppression mediated by regulatory CD4+CD25+ T cells (Thebault et al. 2009). Therefore, CLEC-1 expressed on DC can inhibit Th17 differentiation and increase the numbers of regulatory Foxp3+ CD4+CD25+ T cells. Inhibition of CLEC-1 expression also downregulates IL13 secretion by CD4+CD25 − T cells (Thebault et al. 2009). This suggests a role for CLEC-1 in Th2 differentiation.
In a rat model of long-term tolerated allografts, in which CLEC-1 expression on graft endothelial cells is upregulated, the level of IL17 transcript is reduced, and the level of Foxp3 transcript is increased compared to the levels in chronically rejected allografts, which do not significantly express CLEC-1 (Thebault et al. 2009). This is consistent with the idea that CLEC-1 regulates the balance between effector and regulatory T cells and, hence, modulates T-cell activation. CLEC-1 may therefore play a role in allograft tolerance.
CLEC-1 is a C-type lectin-like receptor belonging to the Dectin-1 family of immune receptors. It is predominantly expressed by DC, endothelial cells, monocytes, and macrophages. The protein has been found at high levels intracellularly and may be retained in the ER membrane. Expression of CLEC-1 is downregulated by inflammatory stimuli and increased by the immunoregulatory mediators IL10 or transforming growth factor beta (TGF-β). CLEC-1 is overexpressed on endothelial cells of tolerated allografts compared with allografts that are chronically rejected or syngeneic grafts in a rat model, and CLEC-1 is believed to play a role in allograft tolerance by regulating the balance between Th17 effector and Foxp3+ regulatory T cells. A ligand for CLEC-1 has not yet been identified. Ligand binding to CLEC-1 might lead to phosphorylation of the intracellular tyrosine(s) present in the cytoplasmic domain, and this may trigger effector functions. Future studies to identify endogenous and exogenous ligands for CLEC-1 are required to better understand CLEC-1 signaling and function.
- Sobanov Y, Bernreiter A, Derdak S, Mechtcheriakova D, Schweighofer B, Duchler M, et al. A novel cluster of lectin-like receptor genes expressed in monocytic, dendritic and endothelial cells maps close to the NK receptor genes in the human NK gene complex. Eur J Immunol. 2001;31(12):3493–503.CrossRefPubMedGoogle Scholar
- Thebault P, Lhermite N, Tilly G, Le Texier L, Quillard T, Heslan M, et al. The C-type lectin-like receptor CLEC-1, expressed by myeloid cells and endothelial cells, is up-regulated by immunoregulatory mediators and moderates T cell activation. J Immunol. 2009;183(5):3099–108.CrossRefPubMedGoogle Scholar