CXCL12 inhibits expression of the NMDA receptor's NR2B subunit through a histone deacetylase-dependent pathway contributing to neuronal survival

Homeostatic chemokines, such as CXCL12, can affect neuronal activity by the regulation of inhibitory and excitatory neurotransmission, but the mechanisms involved are still undefined. Our previous studies have shown that CXCL12 protects cortical neurons from excitotoxicity by promoting the function of the gene-repressor protein Rb, which is involved in the recruitment of chromatin modifiers (such as histone deacetylases (HDACs)) to gene promoters. In neurons, Rb controls activity-dependent genes essential to neuronal plasticity and survival, such as the N-methyl--aspartic acid (NMDA) receptor''s subunit NR2B, the expression of which in the tetrameric ion channel largely affects calcium signaling by glutamate. In this study, we report that CXCL12 differentially modulates intracellular responses after stimulation of synaptic and extrasynaptic NMDA receptors, by a specific regulation of the NR2B gene that involves HDACs. Our results show that CXCL12 selectively inhibits NR2B expression in vitro and in vivo altering NMDA-induced calcium responses associated with neuronal death, while promoting prosurvival pathways that depend on stimulation of synaptic receptors. Along with previous studies, these findings underline the role of CXCL12/CXCR4 in the regulation of crucial components of glutamatergic transmission. These novel effects of CXCL12 may be involved in the physiological function of the chemokine in both developing and mature brains.     

CXCR3 mediates region-specific antiviral T cell trafficking within the central nervous system during West Nile virus encephalitis

Regional differences in inflammation during viral infections of the CNS suggest viruses differentially induce patterns of chemoattractant expression, depending on their cellular targets. Previous studies have shown that expression of the chemokine CXCL10 by West Nile virus (WNV)-infected neurons is essential for the recruitment of CD8 T cells for the purpose of viral clearance within the CNS. In the current study we used mice deficient for the CXCL10 receptor, CXCR3, to evaluate its role in leukocyte-mediated viral clearance of WNV infection within various CNS compartments. WNV-infected CXCR3-deficient mice exhibited significantly enhanced mortality compared with wild-type controls. Immunologic and virologic analyses revealed that CXCR3 was dispensable for control of viral infection in the periphery and in most CNS compartments but, surprisingly, was required for CD8 T cell-mediated antiviral responses specifically within the cerebellum. WNV-specific, CXCR3-expressing T cells preferentially migrated into the cerebellum, and WNV-infected cerebellar granule cell neurons expressed higher levels of CXCL10 compared with similarly infected cortical neurons. These results indicate that WNV differentially induces CXCL10 within neuronal populations and suggest a novel model for nonredundancy in chemokine-mediated inflammation among CNS compartments.     

CXCR7 Is Involved in Human Oligodendroglial Precursor Cell Maturation

Differentiation of oligodendroglial precursor cells (OPCs), a crucial prerequisite for central nervous system (CNS) remyelination in diseases such as Multiple Sclerosis (MS), is modulated by a multitude of extrinsic and intrinsic factors. In a previous study we revealed that the chemokine CXCL12 stimulates rodent OPC differentiation via activation of its receptor CXCR7. We could now demonstrate that CXCR7 is also expressed on NogoA- and Nkx2.2-positive oligodendroglial cells in human MS brains and that stimulation of cultured primary fetal human OPCs with CXCL12 promotes their differentiation as measured by surface marker expression and morphologic complexity. Pharmacological inhibition of CXCR7 effectively blocks these CXCL12-dependent effects. Our findings therefore suggest that a specific activation of CXCR7 could provide a means to promote oligodendroglial differentiation facilitating endogenous remyelination activities.     

Small neutralizing molecules to inhibit actions of the chemokine CXCL12

The chemokine CXCL12 and the receptor CXCR4 play pivotal roles in normal vascular and neuronal development, in inflammatory responses, and in infectious diseases and cancer. For instance, CXCL12 has been shown to mediate human immunodeficiency virus-induced neurotoxicity, proliferative retinopathy and chronic inflammation, whereas its receptor CXCR4 is involved in human immunodeficiency virus infection, cancer metastasis and in the rare disease known as the warts, hypogammaglobulinemia, immunodeficiency, and myelokathexis (WHIM) syndrome. As we screened chemical libraries to find inhibitors of the interaction between CXCL12 and the receptor CXCR4, we identified synthetic compounds from the family of chalcones that reduce binding of CXCL12 to CXCR4, inhibit calcium responses mediated by the receptor, and prevent CXCR4 internalization in response to CXCL12. We found that the chemical compounds display an original mechanism of action as they bind to the chemokine but not to CXCR4. The highest affinity molecule blocked chemotaxis of human peripheral blood lymphocytes ex vivo. It was also active in vivo in a mouse model of allergic eosinophilic airway inflammation in which we detected inhibition of the inflammatory infiltrate. The compound showed selectivity for CXCL12 and not for CCL5 and CXCL8 chemokines and blocked CXCL12 binding to its second receptor, CXCR7. By analogy to the effect of neutralizing antibodies, this molecule behaves as a small organic neutralizing compound that may prove to have valuable pharmacological and therapeutic potential.     

Transgenic expression of stromal cell-derived factor-1/CXC chemokine ligand 12 enhances myeloid progenitor cell survival/antiapoptosis in vitro in response to growth factor withdrawal and enhances myelopoiesis in vivo

Hemopoiesis is regulated in part by survival/apoptosis of hemopoietic stem/progenitor cells. Exogenously added stromal cell-derived factor-1 ((SDF-1)/CXC chemokine ligand (CXCL)12) enhances survival/antiapoptosis of myeloid progenitor cells in vitro. To further evaluate SDF-1/CXCL12 effects on progenitor cell survival, transgenic mice endogenously expressing SDF-1/CXCL12 under a Rous sarcoma virus promoter were produced. Myeloid progenitors (CFU-granulocyte-macrophage, burst-forming unit-erythroid, CFU-granulocyte-erythrocyte-megakaryocyte-monocyte) from transgenic mice were studied for in vitro survival in the context of delayed addition of growth factors. SDF-1-expressing transgenic myeloid progenitors were enhanced in survival and antiapoptosis compared with their wild-type littermate counterparts. Survival-enhancing effects were due to release of low levels of SDF-1/CXCL12 and mediated through CXCR4 and G(alpha)i proteins as determined by ELISA, an antagonist to CXCR4, Abs to CXCR4 and SDF-1, and pertussis toxin. Transgenic effects of low SDF-1/CXCR4 may be due to synergy of SDF-1/CXCL12 with other cytokines; low SDF-1/CXCL12 synergizes with low concentrations of other cytokines to enhance survival of normal mouse myeloid progenitors. Consistent with in vitro results, progenitors from SDF-1/CXCL12 transgenic mice displayed enhanced marrow and splenic myelopoiesis: greatly increased progenitor cell cycling and significant increases in progenitor cell numbers. These results substantiate survival effects of SDF-1/CXCL12, now extended to progenitors engineered to endogenously produce low levels of this cytokine, and demonstrate activity in vivo for SDF-1/CXCL12 in addition to cell trafficking.     

Epigenetic changes of CXCR4 and its ligand CXCL12 as prognostic factors for sporadic breast cancer

Chemokines and their receptors are involved in the development and cancer progression. The chemokine CXCL12 interacts with its receptor, CXCR4, to promote cellular adhesion, survival, proliferation and migration. The CXCR4 gene is upregulated in several types of cancers, including skin, lung, pancreas, brain and breast tumors. In pancreatic cancer and melanoma, CXCR4 expression is regulated by DNA methylation within its promoter region. In this study we examined the role of cytosine methylation in the regulation of CXCR4 expression in breast cancer cell lines and also correlated the methylation pattern with the clinicopathological aspects of sixty-nine primary breast tumors from a cohort of Brazilian women. RT-PCR showed that the PMC-42, MCF7 and MDA-MB-436 breast tumor cell lines expressed high levels of CXCR4. Conversely, the MDA-MB-435 cell line only expressed CXCR4 after treatment with 5-Aza-CdR, which suggests that CXCR4 expression is regulated by DNA methylation. To confirm this hypothesis, a 184 bp fragment of the CXCR4 gene promoter region was cloned after sodium bisulfite DNA treatment. Sequencing data showed that cell lines that expressed CXCR4 had only 15% of methylated CpG dinucleotides, while the cell line that not have CXCR4 expression, had a high density of methylation (91%). Loss of DNA methylation in the CXCR4 promoter was detected in 67% of the breast cancer analyzed. The absence of CXCR4 methylation was associated with the tumor stage, size, histological grade, lymph node status, ESR1 methylation and CXCL12 methylation, metastasis and patient death. Kaplan-Meier curves demonstrated that patients with an unmethylated CXCR4 promoter had a poorer overall survival and disease-free survival. Furthermore, patients with both CXCL12 methylation and unmethylated CXCR4 had a shorter overall survival and disease-free survival. These findings suggest that the DNA methylation status of both CXCR4 and CXCL12 genes could be used as a biomarker for prognosis in breast cancer.     

Discovery and Computer Aided Potency Optimization of a Novel Class of Small Molecule CXCR4 Antagonists

Amongst the chemokine signalling axes involved in cancer, chemokine CXCL12 acting on chemokine receptor CXCR4 is particularly significant since it orchestrates migration of cancer cells in a tissue-specific metastatic process. High CXCR4 tumour expression is associated with poor prognosis of lung, brain, CNS, blood and breast cancers. We have identified a new class of small molecule CXCR4 antagonists based on the use of computational modelling studies in concert with experimental determination of in vitro activity against CXCL12-induced intracellular calcium mobilisation, proliferation and chemotaxis. Molecular modelling proved to be a useful tool in rationalising our observed potencies, as well as informing the direction of the synthetic efforts aimed at producing more potent compounds.     

CXCL12 limits inflammation by localizing mononuclear infiltrates to the perivascular space during experimental autoimmune encephalomyelitis

The inflammatory response in the CNS begins with the movement of leukocytes across the blood-brain barrier in a multistep process that requires cells to pass through a perivascular space before entering the parenchyma. The molecular mechanisms that orchestrate this movement are not known. The chemokine CXCL12 is highly expressed throughout the CNS by microendothelial cells under normal conditions, suggesting it might play a role maintaining the blood-brain barrier. We tested this hypothesis in the setting of experimental autoimmune encephalomyelitis (EAE) by using AMD3100, a specific antagonist of the CXCL12 receptor CXCR4. We demonstrate that the loss of CXCR4 activation enhances the migration of infiltrating leukocytes into the CNS parenchyma. CXCL12 is expressed at the basolateral surface of CNS endothelial cells in normal spinal cord and at the onset of EAE. This polarity is lost in vessels associated with an extensive parenchymal invasion of mononuclear cells during the peak of disease. Inhibition of CXCR4 activation during the induction of EAE leads to loss of the typical intense perivascular cuffs, which are replaced with widespread white matter infiltration of mononuclear cells, worsening the clinical severity of the disease and increasing inflammation. Taken together, these data suggest a novel anti-inflammatory role for CXCL12 during EAE in that it functions to localize CXCR4-expressing mononuclear cells to the perivascular space, thereby limiting the parenchymal infiltration of autoreactive effector cells.     

JCV agnoprotein-induced reduction in CXCL5/LIX secretion by oligodendrocytes is associated with activation of apoptotic signaling in neurons

An indispensable role for oligodendrocytes in the protection of axon function and promotion of neuronal survival is strongly supported by the finding of progressive neuron/axon degeneration in human neurological diseases that affect oligodendrocytes. Imaging and pathological studies of the CNS have shown the presence of neuroaxonal injury in progressive multifocal leukoencephalopathy (PML), a demyelinating disease of the CNS, resulting from destruction of oligodendrocytes upon productive replication of the pathogenic neurotropic polyomavirus JC. Here, we examined the extracellular factors involved in communication between oligodendrocytes and neurons. Culturing cortical neurons with conditioned medium (CM) from rat CG4 oligodendrocytic cells that express the JCV agnoprotein showed that CXCL5/LIX, which is a chemokine closely related to the human CXCL5/ENA78 and CXCL6/GCP-2 chemokines, is essential for neuronal cell survival. We found that in CM from agnoprotein-producing CG-4 cells level of CXC5/LIX is decreased compared to control cells. We also demonstrated that a reduced expression of CXCL5/LIX by CG4 GFP-Agno cells triggered a cascade of signaling events in cortical neurons. Analysis of mitogen-activated protein kinases (MAPK) and glycogen synthase kinase (GSK3) pathways showed that they are involved in mechanisms of neuronal apoptosis in response to the depletion of CXCL5/LIX signaling. These data suggest that agnoprotein-induced dysregulation of chemokine production by oligodendrocytes may contribute to neuronal/axonal injury in the pathogenesis of PML lesions.     

The peculiarities of the SDF-1/CXCL12 system: in some cells,CXCR4 and CXCR7 sing solos,in others,they sing duets

The chemokine SDF-1/CXCL12 induces and modulates major steps of ontogenesis, regeneration and tumorigenesis. Depending on the organ or tissue, CXCL12 serves as a proliferation or cell survival factor, influences differentiation, induces adhesion and/or regulates cell migration. These functions are mediated by the two chemokine receptors, CXCR4 and CXCR7. Whereas CXCR4 is still viewed as the sole G-protein-activating and, hence, signaling receptor for CXCL12, CXCR7 is regarded as a non-classic scavenging or decoy receptor that modulates the function of CXCR4. However, this view might be too limited, since evidence has accumulated favoring a cell-type-specific mode of CXCL12 signaling. In addition to the “classic” CXCL12 signaling mode via CXCR4, CXCR4 and CXCR7 have to form a receptor unit for successful CXCL12 signaling in some cells. Moreover, examples exist whereby CXCL12 receptors split functions or switch roles, such that CXCR7 (instead of CXCR4) mediates signal transduction. The obvious lack of a universal mode of CXCL12 signaling urges a re-evaluation of the role of this chemokine in development, health and disease. This review depicts the exceptional characteristics of CXCL12-induced signal transduction in various cells and organs, points out remaining controversies and mentions consequences for therapeutic interventions.     

CXCL12: role in neuroinflammation

CXCL12, also known as SDF-1 (stromal cell derived factor-1) is a small protein that belongs to the chemokine family, whose members have a crucial role in directing cell migration. CXCL12 has an essential role in neural and vascular development, hematopoiesis and in immunity. It acts through two receptors, CXCR4 and CXCR7. While the former is a classic G protein-coupled transmembrane chemokine receptor, the latter primarily function as a scavenger of CXCL12. CXCL12 has been considered as a standard pro-inflammatory molecule for a long time, as it attracts leukocytes to inflammatory sites and contributes to their activation. However, recent findings indicate that this chemokine has the opposite role in neuroinflammation. In this review, basic data about molecular and functional properties of CXCL12 are presented, while its role in CNS autoimmunity is addressed in details.     

Leukocyte infiltration,but not neurodegeneration,in the CNS of transgenic mice with astrocyte production of the CXC chemokine ligand 10

The CXC chemokine ligand (CXCL)10 is induced locally in the CNS in diverse pathologic states. The impact of CXCL10 production in the CNS was examined in transgenic mice with astrocyte-directed production of this chemokine. These glial fibrillary acidic protein (GF)-CXCL10 transgenic mice spontaneously developed transgene dose- and age-related leukocyte infiltrates in perivascular, meningeal, and ventricular regions of the brain that were composed of, surprisingly, mainly neutrophils and, to a lesser extent, T cells. No other overt pathologic or physical changes were evident. In addition, the cerebral expression of a number of inflammation-related genes (e.g., cytokines) was not significantly altered in the transgenic mice. The extent of leukocyte recruitment to the brain could be enhanced markedly by peripheral immunization of GF-CXCL10 mice with CFA and pertussis toxin. This was paralleled by a modest, transient increase in the expression of some cytokine and chemokine genes. Analysis of the expression of the CXCL10 receptor, CXCR3, by the brain-infiltrating leukocytes from immunized GF-CXCL10 transgenic mice revealed a significant enrichment for CXCR3-positive cells in the CNS compared with spleen. The majority of cells positive for CXCR3 coexpressed CD3, whereas Gr1-positive granulocytes were negative for CXCR3 expression. Thus, while astrocyte production of CXCL10 can promote spontaneous and potentiate immune-induced recruitment of leukocytes to the CNS, this is not associated with activation of a degenerative immune pathology. Finally, the accumulation of neutrophils in the brain of GF-CXCL10 transgenic mice is apparently independent of CXCR3 and involves an unknown mechanism.     

CXCR7 Is Highly Expressed in Acute Lymphoblastic Leukemia and Potentiates CXCR4 Response to CXCL12

Recently, a novel CXCL12-binding receptor, has been identified. This CXCL12-binding receptor commonly known as CXCR7 (CXC chemokine receptor 7), has lately, based on a novel nomenclature, has received the name ACKR3 (atypical chemokine receptor 3). In this study, we aimed to investigate the expression of CXCR7 in leukemic cells, as well as its participation in CXCL12 response. Interesting, we clearly demonstrated that CXCR7 is highly expressed in acute lymphoid leukemic cells compared with myeloid or normal hematopoietic cells and that CXCR7 contributed to T-acute lymphoid leukemic cell migration induced by CXCL12. Moreover, we showed that the cellular location of CXCR7 varied among T-lymphoid cells and this finding may be related to their migration capacity. Finally, we hypothesized that CXCR7 potentiates CXCR4 response and may contribute to the maintenance of leukemia by initiating cell recruitment to bone marrow niches that were once occupied by normal hematopoietic stem cells.     

Cutting edge: profile of chemokine receptor expression on human plasma cells accounts for their efficient recruitment to target tissues

We systematically examined the repertoire of chemokine receptors expressed by human plasma cells. Fresh bone marrow plasma cells and myeloma cells consistently expressed CXCR4, CXCR6, CCR10, and CCR3. Accordingly, plasma cells responded to their respective ligands in chemotaxis and very late Ag-4-dependent cell adhesion to fibronectin. Immobilized CXC chemokine ligand (CXCL)16, a novel transmembrane-type chemokine and CXCR6 ligand, also directly induced adhesion of plasma cells without requiring G(alpha i) signaling or divalent cations. Furthermore, we revealed consistent expression of CXCL12 (CXCR4 ligand), CXCL16 (CXCR6 ligand), and CC chemokine ligand 28 (CCR10 and CCR3 ligand) in tissues enriched with plasma cells including bone marrow, and constitutive expression of CXCL12, CXCL16, and CC chemokine ligand 28 by cultured human bone marrow stromal cells. Collectively, plasma cells are likely to be recruited to bone marrow and other target tissues via CXCR4, CXCR6, CCR10, and CCR3. CXCR6 may also contribute to tissue localization of plasma cells through its direct binding to membrane-anchored CXCL16.     

CXCL12 Induces Connective Tissue Growth Factor Expression in Human Lung Fibroblasts through the Rac1/ERK,JNK, and AP-1 Pathways

CXCL12 (stromal cell-derived factor-1, SDF-1) is a potent chemokine for homing of CXCR4⁺ fibrocytes to injury sites of lung tissue, which contributes to pulmonary fibrosis. Overexpression of connective tissue growth factor (CTGF) plays a critical role in pulmonary fibrosis. In this study, we investigated the roles of Rac1, extracellular signal-regulated kinase (ERK), c-Jun N-terminal kinase (JNK), and activator protein-1 (AP-1) in CXCL12-induced CTGF expression in human lung fibroblasts. CXCL12 caused concentration- and time-dependent increases in CTGF expression and CTGF-luciferase activity. CXCL12-induced CTGF expression was inhibited by a CXCR4 antagonist (AMD3100), small interfering RNA of CXCR4 (CXCR4 siRNA), a dominant negative mutant of Rac1 (RacN17), a mitogen-activated protein kinase (MAPK) kinase (MEK) inhibitor (PD98059), a JNK inhibitor (SP600125), a p21-activated kinase inhibitor (PAK18), c-Jun siRNA, and an AP-1 inhibitor (curcumin). Treatment of cells with CXCL12 caused activations of Rac1, Rho, ERK, and c-Jun. The CXCL12-induced increase in ERK phosphorylation was inhibited by RacN17. Treatment of cells with PD98059 and SP600125 both inhibited CXCL12-induced c-Jun phosphorylation. CXCL12 caused the recruitment of c-Jun and c-Fos binding to the CTGF promoter. Furthermore, CXCL12 induced an increase in α-smooth muscle actin (α-SMA) expression, a myofibroblastic phenotype, and actin stress fiber formation. CXCL12-induced actin stress fiber formation and α-SMA expression were respectively inhibited by AMD3100 and CTGF siRNA. Taken together, our results suggest that CXCL12, acting through CXCR4, activates the Rac/ERK and JNK signaling pathways, which in turn initiates c-Jun phosphorylation, and recruits c-Jun and c-Fos to the CTGF promoter and ultimately induces CTGF expression in human lung fibroblasts. Moreover, overexpression of CTGF mediates CXCL12-induced α-SMA expression.     

CXCR7 Functions as a Scavenger for CXCL12 and CXCL11

Background

CXCR7 (RDC1), the recently discovered second receptor for CXCL12, is phylogenetically closely related to chemokine receptors, but fails to couple to G-proteins and to induce typical chemokine receptor mediated cellular responses. The function of CXCR7 is controversial. Some studies suggest a signaling activity in mammalian cells and zebrafish embryos, while others indicate a decoy activity in fish. Here we investigated the two propositions in human tissues.

Methodology/Principal Findings

We provide evidence and mechanistic insight that CXCR7 acts as specific scavenger for CXCL12 and CXCL11 mediating effective ligand internalization and targeting of the chemokine cargo for degradation. Consistently, CXCR7 continuously cycles between the plasma membrane and intracellular compartments in the absence and presence of ligand, both in mammalian cells and in zebrafish. In accordance with the proposed activity as a scavenger receptor CXCR7-dependent chemokine degradation does not become saturated with increasing ligand concentrations. Active CXCL12 sequestration by CXCR7 is demonstrated in adult mouse heart valves and human umbilical vein endothelium.

Conclusions/Significance

The finding that CXCR7 specifically scavenges CXCL12 suggests a critical function of the receptor in modulating the activity of the ubiquitously expressed CXCR4 in development and tumor formation. Scavenger activity of CXCR7 might also be important for the fine tuning of the mobility of hematopoietic cells in the bone marrow and lymphoid organs.     

CXCR4 chemokine receptor engagement modifies integrin dependent adhesion of renal carcinoma cells

The mechanisms leading to renal cell carcinoma (RCC) metastasis are incompletely understood. Although evidence shows that the chemokine receptor CXCR4 and its ligand CXCL12 may regulate tumor dissemination, their role in RCC is not clearly defined. We examined CXCR4 expression and functionality on RCC cell lines, and explored CXCL12-triggered tumor adhesion to human endothelium (HUVEC) or extracellular matrix proteins. Functional CXCR4 was expressed on A498 tumor cells, enabling them to migrate towards a CXCL12 gradient. CXCR4 engagement by CXCL12 induced elevated cell adhesion to HUVEC, to immobilized fibronectin, laminin or collagen. Anti-CXCR4 antibodies or CXCR4 knock down by siRNA applied prior to CXCL12 stimulation impaired CXCL12-triggered tumor adhesion. However, blocking CXCR4 subsequent to CXCL12 stimulation did not. This pointed to an indirect control of tumor cell adhesion by CXCR4. In fact, CXCR4 engagement by CXCL12 also induced alterations of receptors of the integrin family, notably alpha3, alpha5, beta1 and beta3 subunits, and blocking beta1 integrins with a function-blocking antibody prevented CXCL12-induced A498 adhesion. Focal adhesion kinase (total and activated) and integrin-linked kinase significantly increased in CXCL12-treated A498 cells, accompanied by a distinct up-regulation of ERK1/2, JNK and p38 phosphorylation. Therefore, CXCR4 may be crucial in controlling adhesion of A498 cells via cross talking with integrin receptors. These data show that CXCR4 receptors contribute to RCC dissemination and may provide a novel link between CXCR4 chemokine receptor expression and integrin triggered RCC adhesion to the vascular wall and subendothelial matrix components.     

CXCL14 is no direct modulator of CXCR4

C-X-C motif chemokine 12/C-X-C chemokine receptor type 4 (CXCL12/CXCR4) signaling is involved in ontogenesis, hematopoiesis, immune function and cancer. Recently, the orphan chemokine CXCL14 was reported to inhibit CXCL12-induced chemotaxis – probably by allosteric modulation of CXCR4. We thus examined the effects of CXCL14 on CXCR4 regulation and function using CXCR4-transfected human embryonic kidney (HEK293) cells and Jurkat T cells. CXCL14 did not affect dose–response profiles of CXCL12-induced CXCR4 phosphorylation, G protein-mediated calcium mobilization, dynamic mass redistribution, kinetics of extracellular signal-regulated kinase 1 (ERK1) and ERK2 phosphorylation or CXCR4 internalization. Hence, essential CXCL12-operated functions of CXCR4 are insensitive to CXCL14, suggesting that interactions of CXCL12 and CXCL14 pathways depend on a yet to be identified CXCL14 receptor.