Elucidation of CXCR7-mediated signaling events and inhibition of CXCR4-mediated tumor cell transendothelial migration by CXCR7 ligands

CXCR7 binds chemokines CXCL11 (I-TAC) and CXCL12 (SDF-1) but does not act as a classical chemoattractant receptor. Using CCX771, a novel small molecule with high affinity and selectivity for CXCR7, we found that, although CXCR7 is dispensable for "bare filter" in vitro chemotaxis, CXCR7 plays an essential role in the CXCL12/CXCR4-mediated transendothelial migration (TEM) of CXCR4(+)CXCR7(+) human tumor cells. Importantly, although CXCL11 is unable to stimulate directly the migration of these cells, it acts as a potent antagonist of their CXCL12-induced TEM. Furthermore, even though this TEM is driven by CXCR4, the CXCR7 ligand CCX771 is substantially more potent at inhibiting it than the CXCR4 antagonist AMD3100, which is more than 100 times weaker at inhibiting TEM when compared with its ability to block bare filter chemotaxis. Far from being a "silent" receptor, we show that CXCR7 displays early hallmark events associated with intracellular signaling. Upon cognate chemokine binding, CXCR7 associates with beta-arrestin2, an interaction that can be blocked by CXCR7-specific mAbs. Remarkably, the synthetic CXCR7 ligand CCX771 also potently stimulates beta-arrestin2 recruitment to CXCR7, with greater potency and efficacy than the endogenous chemokine ligands. These results indicate that CXCR7 can regulate CXCL12-mediated migratory cues, and thus may play a critical role in driving CXCR4(+)CXCR7(+) tumor cell metastasis and tissue invasion. CXCR7 ligands, such as the chemokine CXCL11 and the newly described synthetic molecule CCX771, may represent novel therapeutic opportunities for the control of such cells.

Chemerin and the recruitment of NK cells to diseased skin

Natural killer (NK) cells play a major role in the initial control of many viral pathogens and in the rejection of tumors. Consistent with their roles as immune sentinels, NK cells are found in inflamed skin, including lichen planus, psoriasis and atopic dermatitis (AD) lesions. In oral lichen planus lesions, the recruitment as well as intradermal colocalization of NK cells and pDC (plasmacytoid dendritic cells) appear to be mediated by chemerin, a recently identified protein ligand for chemokine-like receptor 1 (CMKLR1), a chemoattractant receptor expressed by both cell types. Dendritic cells can regulate NK cell activity, and NK cells can regulate DC-mediated responses. Since chemerin was recently implicated in recruitment of pDC to psoriatic skin, in this work we determined whether chemerin facilitates interactions between NK and pDC in psoriatic plaques through controlling influx of NK cells to diseased skin. We demonstrate that circulating NK cells from normal donors as well as psoriasis and AD patients respond similarly in functional migration assays to chemerin. However, differences in the distribution of NK cells and pDC in skin lesions suggest that recruitment of both NK cells and pDC is unlikely to be controlled solely by chemerin.

Chemerin and the recruitment of NK cells to diseased skin

Natural killer (NK) cells play a major role in the initial control of many viral pathogens and in the rejection of tumors. Consistent with their roles as immune sentinels, NK cells are found in inflamed skin, including lichen planus, psoriasis and atopic dermatitis (AD) lesions. In oral lichen planus lesions, the recruitment as well as intradermal colocalization of NK cells and pDC (plasmacytoid dendritic cells) appear to be mediated by chemerin, a recently identified protein ligand for chemokine-like receptor 1 (CMKLR1), a chemoattractant receptor expressed by both cell types. Dendritic cells can regulate NK cell activity, and NK cells can regulate DC-mediated responses. Since chemerin was recently implicated in recruitment of pDC to psoriatic skin, in this work we determined whether chemerin facilitates interactions between NK and pDC in psoriatic plaques through controlling influx of NK cells to diseased skin. We demonstrate that circulating NK cells from normal donors as well as psoriasis and AD patients respond similarly in functional migration assays to chemerin. However, differences in the distribution of NK cells and pDC in skin lesions suggest that recruitment of both NK cells and pDC is unlikely to be controlled solely by chemerin.

Cysteine Protease Activators of Plasma Chemoattractant Chemerin (92.4)

Chemerin, a ligand for seven-pass transmembrane, G protein-associated receptor CMKLR1 requires C-terminal proteolytic processing to unleash its chemoattractant activity. Proteolytically-processed chemerin selectively attracts specific subsets of immunoregulatory antigen presenting cells, such as immature plasmacytoid dendritic cells (pDC) and macrophages that express CMKLR1. Chemerin is predicted to belong to the structural cathelicidin/cystatin family of proteins that comprise inhibitors of cysteine proteinases (cystatins). Cysteine proteinases of the papain-like family are well characterized proteolytic enzymes, best known for their role in intracellular proteolytic pathways. Nevertheless, recent studies show that lysosomal cathepsins can also exert their proteolytic activity at extracellular sites in secreted or cell surface-bound forms. Extracellular cathepsins contribute to a variety of pathophysiological processes, including inflammation and neoplastic progression. Homology between chemerin and cystatins suggests that chemerin interacts directly with cysteine proteases. In this work, we demonstrate that chemerin does not display substantial inhibitory activity against human cysteine proteases. Instead, chemerin was found to be a new substrate for these proteases. The interaction between chemerin and the proteases resulted in the generation of potent attractant activity, eliciting a robust chemotactic response by CMKLR1 transfectants as well as human blood-derived PDC. Our data demonstrate that cathepsins may contribute to immune responses through the recruitment of pDC.

Potential role of chemerin in recruitment of plasmacytoid dendritic cells to diseased skin

Interferon alpha-producing plasmacytoid dendritic cells (pDC) are crucial contributors to pro-inflammatory or tolerogenic immune responses and are important in autoimmune diseases such as psoriasis. pDC accumulate in the lesional skin of psoriasis patients, but are rarely found in the affected skin of patients with atopic dermatitis (AD). While homeostatic chemokine CXCL12 and inducible pro-inflammatory CXCR3 chemokine ligands may regulate pDC influx to psoriatic skin, the mechanism responsible for selective pDC recruitment in psoriasis vs. AD remains unknown. Circulating pDC from normal donors express a limited number of chemoattractant receptors, including CXCR3 and CMKLR1 (chemokine-like receptor 1). In this work, we demonstrate that circulating pDC from normal donors as well as psoriasis and AD patients express similar levels of CXCR3 and responded similarly in functional migration assays to CXCL10. We next found that blood pDC from normal, AD, and psoriasis patients express functional CMKLR1. In contrast to normal skin, however, lesional skin from psoriasis patients contains the active form of the CMKLR1 ligand chemerin. Furthermore, in affected skin from psoriatic patients the level of active chemerin was generally higher than in AD skin. Taken together, these results indicate that local generation of active chemerin may contribute to pDC recruitment to psoriatic skin.

Regulation of chemerin bioactivity by plasma carboxypeptidase N, carboxypeptidase B (activated thrombin-activable fibrinolysis inhibitor), and platelets

Chemerin is a potent chemoattractant for cells expressing the serpentine receptor CMKLR1 (chemokine-like receptor 1), such as plasmacytoid dendritic cells and tissue macrophages. The bioactivity of chemerin is post-translationally regulated; the attractant circulates in blood in a relatively inactive form (prochemerin) and is activated by carboxyl-terminal proteolytic cleavage. We discovered that plasma carboxypeptidase N (CPN) and B (CPB or activated thrombin-activable fibrinolysis inhibitor, TAFIa) enhanced the bioactivity of 10-mer chemerin peptide NH(2)-YFPGQFAFSK-COOH by removing the carboxyl-terminal lysine (K). Sequential cleavages of either a prochemerin peptide (NH(2)-YFPGQFAFSKALPRS-COOH) or recombinant full-length prochemerin by plasmin and CPN/CPB substantially increased their chemotactic activities. Endogenous CPN present in circulating plasma enhanced the activity of plasmin-cleaved prochemerin. In addition, we discovered that platelets store chemerin protein and release it upon stimulation. Thus circulating CPN/CPB and platelets may potentially contribute to regulating the bioactivity of leukocyte chemoattractant chemerin, and further extend the molecular link between blood coagulation/fibrinolysis and CMKLR1-mediated immune responses.

Mast cell-expressed orphan receptor CCRL2 binds chemerin and is required for optimal induction of IgE-mediated passive cutaneous anaphylaxis

Mast cells contribute importantly to both protective and pathological IgE-dependent immune responses. We show that the mast cell–expressed orphan serpentine receptor mCCRL2 is not required for expression of IgE-mediated mast cell–dependent passive cutaneous anaphylaxis but can enhance the tissue swelling and leukocyte infiltrates associated with such reactions in mice. We further identify chemerin as a natural nonsignaling protein ligand for both human and mouse CCRL2. In contrast to other “silent” or professional chemokine interreceptors, chemerin binding does not trigger ligand internalization. Rather, CCRL2 is able to bind the chemoattractant and increase local concentrations of bioactive chemerin, thus providing a link between CCRL2 expression and inflammation via the cell-signaling chemerin receptor CMKLR1.

NMR assignment of human chemerin, a novel chemoattractant

Chemerin is a potent chemoattractant for cells expressing the GPCR CMKLR1, and is thought to play important roles in cell migration and recruitment to sites of tissue damage and inflammation. Here we report the NMR assignments of the 15.6 kDa active form of uniformly (15)N, (13)C labeled chemerin.

Chemerin, a novel adipokine that regulates adipogenesis and adipocyte metabolism

Obesity is an alarming primary health problem and is an independent risk factor for type II diabetes, cardiovascular diseases, and hypertension. Although the pathologic mechanisms linking obesity with these co-morbidities are most likely multifactorial, increasing evidence indicates that altered secretion of adipose-derived signaling molecules (adipokines; e.g. adiponectin, leptin, and tumor necrosis factor alpha) and local inflammatory responses are contributing factors. Chemerin (RARRES2 or TIG2) is a recently discovered chemoattractant protein that serves as a ligand for the G protein-coupled receptor CMKLR1 (ChemR23 or DEZ) and has a role in adaptive and innate immunity. Here we show an unexpected, high level expression of chemerin and its cognate receptor CMKLR1 in mouse and human adipocytes. Cultured 3T3-L1 adipocytes secrete chemerin protein, which triggers CMKLR1 signaling in adipocytes and other cell types and stimulates chemotaxis of CMKLR1-expressing cells. Adenoviral small hairpin RNA targeted knockdown of chemerin or CMKLR1 expression impairs differentiation of 3T3-L1 cells into adipocytes, reduces the expression of adipocyte genes involved in glucose and lipid homeostasis, and alters metabolic functions in mature adipocytes. We conclude that chemerin is a novel adipose-derived signaling molecule that regulates adipogenesis and adipocyte metabolism.

Staphylococcus aureus-Derived Staphopain B, a Potent Cysteine Protease Activator of Plasma Chemerin

Chemerin is an attractant for cells that express the serpentine receptor CMKLR1, which include immature plasmacytoid dendritic cells (pDC) and macrophages. Chemerin circulates in the blood where it exhibits low biological activity, but upon proteolytic cleavage of its C terminus, it is converted to a potent chemoattractant. Enzymes that contribute to this conversion include host serine proteases of the coagulation, fibrinolytic, and inflammatory cascades, and it has been postulated that recruitment of pDC and macrophages by chemerin may serve to balance local tissue immune and inflammatory responses. In this work, we describe a potent, pathogen-derived proteolytic activity capable of chemerin activation. This activity is mediated by staphopain B (SspB), a cysteine protease secreted by Staphylococcus aureus. Chemerin activation is triggered by growth medium of clinical isolates of SspB-positive S. aureus, but not by that of a SspB(null) mutant. C-terminal processing by SspB generates a chemerin isoform identical with the active endogenous attractant isolated from human ascites fluid. Interestingly, SspB is a potent trigger of chemerin even in the presence of plasma inhibitors. SspB may help direct the recruitment of specialized host cells, including immunoregulatory pDC and/or macrophages, contributing to the ability of S. aureus to elicit and maintain a chronic inflammatory state.

Chemokine-like receptor 1 expression by macrophages in vivo: regulation by TGF-beta and TLR ligands

OBJECTIVE

Chemokine-like receptor 1 (CMKLR1) is expressed by human antigen presenting cells and binds to chemerin, a proteolytically activatable chemoattractant. Here we assessed the expression of mCMKLR1 on mouse leukocytes, focusing on ex vivo dendritic cells (DC) and macrophages. mCMKLR1-expressing cells were evaluated for functional responses to chemerin. We examined the regulation of mCMKLR1 expression by exposure to toll-like receptor (TLR) ligands and cytokines. Finally, we evaluated ex vivo human ascites macrophages for huCMKLR1 expression and chemerin responsiveness.

METHODS

A novel anti-mCMKLR1 monoclonal antibody was generated to assess mCMKLR1 expression by mouse leukocytes using flow cytometry. Mouse bone marrow-derived DC precursors, mouse peritoneal macrophages, and human ascites leukocytes were examined in functional assays (in vitro chemotaxis and intracellular calcium mobilization).

RESULTS

During DC differentiation from bone marrow, mCMKLR1 is upregulated early and then diminishes with time in culture. Most DC in vivo do not detectably express the receptor. In contrast, freshly isolated F4/80+CD11b+ mouse serosal macrophages express mCMKLR1, bind a fluorescently labeled chemerin peptide, and display calcium signaling and migration to the active ligand. Interestingly, macrophage mCMKLR1 is suppressed by proinflammatory cytokines and TLR ligands, whereas treatment with TGF-beta upregulates the receptor. A small population of blood-borne F4/80+CD11b+ macrophages also expresses mCMKLR1. Freshly isolated macrophages from human ascites fluid express CMKLR1 and are chemerin responsive, as well.

CONCLUSION

The conserved expression of CMKLR1 by macrophages in mouse and man, coupled with the stimuli-specific regulation of CMKLR1, may reflect a critical role for CMKLR1:chemerin in shaping the nature (either proinflammatory or suppressive) in macrophage-mediated immune responses.

Chemoattractants, extracellular proteases, and the integrated host defense response

The host response to tissue injury and/or infection is dependent on the action of numerous extracellular proteases. Proteolytic cascades trigger blood clotting, fibrinolysis, and complement activation, while proteases released upon leukocyte degranulation are integral to the processes of inflammation and immunity. Modulation of effector protein activity by proteases provides a critical layer of posttranslational control that enables rapid enzymatic regulation of target proteins. This report reviews the emerging literature describing a novel class of proteolytic targets, leukocyte chemoattractants, and, in particular, chemerin, a dendritic cell and macrophage chemoattractant activated by serine proteases of the coagulation, fibrinolytic, and inflammatory cascades. As chemoattractants are critical for both systemic leukocyte positioning by triggering integrin activation and subsequent recruitment from circulation, and local intratissue leukocyte positioning via chemotaxis, modulation of attractant activities by proteases may have profound effects on the immune response.

RabGEF1 regulates stem cell factor/c-Kit-mediated signaling events and biological responses in mast cells

We recently reported that RabGEF1 is a negative regulator of high-affinity Fc receptor for IgE (Fc epsilonRI)-dependent mast cell activation and that mice lacking RabGEF1 develop severe skin inflammation and increased numbers of dermal mast cells. To better understand how RabGEF1 can regulate signaling events and biological responses in mast cells, we examined the responses of bone marrow-derived cultured mast cells (BMCMCs) from wild-type (+/+) and Rabgef1 knockout (-/-) mice after stimulation with the c-Kit ligand, stem cell factor (SCF), an important regulator of mast cell development, survival, proliferation, and activation. We found that RabGEF1-deficient mast cells exhibited enhanced and prolonged activation of Ras and extracellular regulated kinase, and significantly elevated IL-6 secretion, after stimulation with SCF. SCF-induced activation of c-Jun N-terminal kinase was increased in Rabgef1-/- BMCMCs, but without corresponding significant increases in SCF-induced migration or adhesion. SCF-mediated activation of the survival-enhancing kinase, Akt, also was increased in Rabgef1-/- BMCMCs, and these cells had a survival advantage over their +/+ counterparts in vitro. Despite enhanced Ras activation in the absence of RabGEF1, SCF-induced proliferation was lower in Rabgef1-/- BMCMCs compared with their +/+ counterparts. Finally, we found that c-Kit internalization was delayed in the absence of RabGEF1, probably reflecting a positive role for RabGEF1 in the regulation of endocytic events, and that infection of Rabgef1-/- BMCMCs with a wild-type RabGEF1 lentiviral construct normalized c-Kit internalization to the levels seen in +/+ BMCMCs. Thus, RabGEF1 plays a critical role in the regulation of SCF/c-Kit-mediated signaling events and biological responses in mast cells.

Chemerin activation by serine proteases of the coagulation, fibrinolytic, and inflammatory cascades

Proteases function at every level in host defense, from regulating vascular hemostasis and inflammation to mobilizing the "rapid responder" leukocytes of the immune system by regulating the activities of various chemoattractants. Recent studies implicate proteolysis in the activation of a ubiquitous plasma chemoattractant, chemerin, a ligand for the G-protein-coupled receptor CMKLR1 present on plasmacytoid dendritic cells and macrophages. To define the pathophysiologic triggers of chemerin activity, we evaluated the ability of serum- and inflammation-associated proteases to cleave chemerin and stimulate CMKLR1-mediated chemotaxis. We showed that serine proteases factor XIIa and plasmin of the coagulation and fibrinolytic cascades, elastase and cathepsin G released from activated neutrophil granules and mast cell tryptase are all potent activators of chemerin. Activation results from cleavage of the labile carboxyl terminus of the chemoattractant at any of several different sites. Activation of chemerin by the serine protease cascades that trigger rapid defenses in the body may direct CMKLR1-positive plasmacytoid dendritic cell and tissue macrophage recruitment to sterile sites of tissue damage, as well as trafficking to sites of infectious and allergic inflammation.

Chemokine-like receptor 1 expression and chemerin-directed chemotaxis distinguish plasmacytoid from myeloid dendritic cells in human blood

Plasmacytoid dendritic cells (pDCs) are versatile cells of the immune response, secreting type I IFNs and differentiating into potent immunogenic or tolerogenic APCs. pDCs can express adhesion and chemokine receptors for lymphoid tissues, but are also recruited by unknown mechanisms during tissue inflammation. We use a novel mAb specific for serpentine chemokine-like receptor 1 (CMKLR1) to evaluate its expression by circulating leukocytes in humans. We show that CMKLR1 is expressed by circulating pDCs in human blood, whereas myeloid DCs (mDCs) as well as lymphocytes, monocytes, neutrophils, and eosinophils are negative. We identify a major serum agonist activity for CMKLR1 as chemerin, a proteolytically activated attractant and the sole known ligand for CMKLR1, and we show that chemerin is activated during blood coagulation and attracts pDC but not mDC in ex vivo chemotaxis assays. We conclude that CMKLR1 expression and chemerin-mediated chemotaxis distinguish circulating pDCs from mDCs, providing a potential mechanism for their differential contribution to or regulation of immune responses at sites of bleeding or inflammatory protease activity.

Citrobacter rodentium espB is necessary for signal transduction and for infection of laboratory mice

Citrobacter rodentium is the causative agent of transmissible murine colonic hyperplasia and contains a locus of enterocyte effacement (LEE) similar to that found in enteropathogenic Escherichia coli (EPEC). EPEC espB is necessary for intimate attachment and signal transduction between EPEC and cultured cell monolayers. Mice challenged with wild-type C. rodentium develop a mucosal immunoglobulin A response to EspB. In this study, C. rodentium espB has been cloned and its nucleotide sequence has been determined. C. rodentium espB was found to have 90% identity to EPEC espB. A nonpolar insertion mutation in C. rodentium espB was constructed and used to replace the chromosomal wild-type allele. The C. rodentium espB mutant exhibited reduced cell association and had no detectable fluorescent actin staining activity on cultured cell monolayers. The C. rodentium espB mutant also failed to colonize laboratory mice following experimental inoculation. The espB mutation could be complemented with a plasmid-encoded copy of the gene, which restored both cell association and fluorescent actin staining activity, as well as the ability to colonize laboratory mice. These studies indicate that espB is necessary for signal transduction and for colonization of laboratory mice by C. rodentium.

Human G protein-coupled receptor GPR-9-6/CC chemokine receptor 9 is selectively expressed on intestinal homing T lymphocytes, mucosal lymphocytes, and thymocytes and is required for thymus-expressed chemokine-mediated chemotaxis

TECK (thymus-expressed chemokine), a recently described CC chemokine expressed in thymus and small intestine, was found to mediate chemotaxis of human G protein-coupled receptor GPR-9-6/L1.2 transfectants. This activity was blocked by anti-GPR-9-6 monoclonal antibody (mAb) 3C3. GPR-9-6 is expressed on a subset of memory alpha4beta7(high) intestinal trafficking CD4 and CD8 lymphocytes. In addition, all intestinal lamina propria and intraepithelial lymphocytes express GPR-9-6. In contrast, GPR-9-6 is not displayed on cutaneous lymphocyte antigen-positive (CLA(+)) memory CD4 and CD8 lymphocytes, which traffic to skin inflammatory sites, or on other systemic alpha4beta7(-)CLA(-) memory CD4/CD8 lymphocytes. The majority of thymocytes also express GPR-9-6, but natural killer cells, monocytes, eosinophils, basophils, and neutrophils are GPR-9-6 negative. Transcripts of GPR-9-6 and TECK are present in both small intestine and thymus. Importantly, the expression profile of GPR-9-6 correlates with migration to TECK of blood T lymphocytes and thymocytes. As migration of these cells is blocked by anti-GPR-9-6 mAb 3C3, we conclude that GPR-9-6 is the principal chemokine receptor for TECK. In agreement with the nomenclature rules for chemokine receptors, we propose the designation CCR-9 for GPR-9-6. The selective expression of TECK and GPR-9-6 in thymus and small intestine implies a dual role for GPR-9-6/CCR-9, both in T cell development and the mucosal immune response.

Genetic and biochemical characterization of Citrobacter rodentium sp. nov

An unusual bacterial pathogen of laboratory mice has been previously classified as an atypical biotype of Citrobacter freundii. Designated C. freundii biotype 4280, this bacterium is the etiologic agent of transmissible murine clonic hyperplasia. An eaeA gene has been shown to be present in this organism and to be necessary for virulence in laboratory mice. However, other biotypes of C. freundii lack DNA homology with the eaeA gene. Because of the recent reclassification in which five named species and three unnamed species, all previously considered C. freundii, were described, we determined the taxonomic status of C. freundii biotype 4280. With a battery of biochemical tests and DNA relatedness studies, three isolates of C. freundii biotype 4280 were shown to be members of an unnamed Citrobacter species, designated species 9. In total, six isolates of Citrobacter species 9, but none of the type strains of the other eight named species or of the two remaining unnamed species of Citrobacter, were shown to possess DNA homology with both the eaeA and the eaeB genes. Species 9 was named Citrobacter rodentium sp. nov.