CX3CL1 in allergic diseases: not just a chemotactic molecule

Genetic support for the causal association between 91 circulating inflammatory proteins and atopic dermatitis: A two-sample Mendelian randomization trial

BACKGROUND

Atopic dermatitis (AD) is a refractory disease that occurs in clinical practice. One of the most common inflammatory skin diseases, its occurrence and development are related to inflammation. Nevertheless, the precise nature of the relationship between circulating inflammatory proteins and AD remains uncertain.

METHODS

A two-sample MR analysis was performed to determine the causal relationship between the expression of 91 circulating inflammatory proteins and AD by using genome-wide association study (GWAS) summary statistics data from the FinnGen consortia. The robustness of the MR results was assessed by means of sensitivity analysis.

RESULTS

The causal relationship between the expression of nine specific circulating inflammatory proteins and AD was corroborated by the inverse variance weighted (IVW) method. The findings indicated that three circulating inflammatory proteins, namely, interleukin-18 receptor 1 [OR (CI) = 1.08 (1.05-1.11); p = 0.000001)], interleukin-8 [OR (CI) = 1.07 (1.00-1.14); p = 0.036244)], and tumor necrosis factor ligand superfamily member 14 [OR (CI) = 1.05 (1.00-1.10); p = 0.036842)], were positively correlated with AD. Additionally, six circulating inflammatory proteins were negatively correlated with AD: the T-cell surface glycoprotein CD5 [OR (CI) = 0.89 (0.84-0.95); p = 0.000191)], macrophage colony-stimulating factor 1 [OR (CI) = 0.93 (0.88-0.99); p = 0.031422)], fractalkine [OR (CI) = 0.91 (0.85-0.97); p = 0.003067)], interleukin-24 [OR (CI) = 0.91 (0.83-0.99); p = 0.031673)], signaling lymphocytic activation molecule [OR(CI) = 0.94 (0.89-1.00); p = 0.039818)], and urokinase-type plasminogen activator [OR(CI) = 0.95 (0.90-1.00); p = 0.037037)].

CONCLUSION

This study confirms the potential causal relationship between circulating inflammatory proteins and AD and provides guidance for the clinical diagnosis and treatment of AD.

Neutralization of CX3CL1 Attenuates TGF-β-Induced Fibroblast Differentiation Through NF-κB Activation and Mitochondrial Dysfunction in Airway Fibrosis

BACKGROUND

Severe asthma, characterized by inflammation and airway remodeling, involves fibroblast differentiation into myofibroblasts expressing α-SMA. This process leads to the production of fibronectin and connective tissue growth factor (CTGF), driven by factors such as transforming growth factor (TGF)-β. Furthermore, the persistent presence of myofibroblasts is associated with resistance to apoptosis and mitochondrial dysfunction. The chemokine (C-X3-C motif) ligand 1 (CX3CL1) plays a role in tissue fibrosis. However, it is currently unknown whether neutralization of CX3CL1 decreases TGF-β-induced fibroblast differentiation and mitochondrial dysfunction in normal human lung fibroblasts (NHLFs).

METHODS

CX3CL1/C-X3-C motif chemokine receptor 1 (CX3CR1), CX3CL1 was analyzed by immunofluorescence (IF) or immunohistochemical (IHC) staining of ovalbumin-challenged mice. CX3CL1 release was detected by ELISA. TGF-β-induced CTGF, fibronectin, and α-SMA expression were evaluated in NHLFs following neutralization of CX3CL1 (TP213) treatment for the indicated times by Western blotting or IF staining. Mitochondrion function was detected by a JC-1 assay and seahorse assay. Cell apoptosis was observed by a terminal uridine nick-end labeling (TUNEL) assay.

RESULTS

An increase in CX3CL1 expression was observed in lung tissues from mice with ovalbumin-induced asthma by IF staining. CX3CR1 was increased in the subepithelial layer of the airway by IHC staining. Moreover, CX3CR1 small interfering (si)RNA downregulated TGF-β-induced CTGF and fibronectin expression in NHLFs. CX3CL1 induced CTGF and fibronectin expression in NHLFs. TGF-β-induced CX3CL1 secretion from NHLFs. Furthermore, TP213 decreased TGF-β-induced CTGF, fibronectin, and α-SMA expression in NHLFs. Mitochondrion-related differentially expressed genes (DEGs) were examined after CX3CL1 neutralization in TGF-β-treated NHLFs. TP213 alleviated TGF-β-induced mitochondrial dysfunction and apoptosis resistance in NHLFs. CX3CL1 induced p65, IκBα, and IKKα phosphorylation in a time-dependent manner. Furthermore, CX3CL1-induced fibronectin expression and JC-1 monomer were decreased by p65 siRNA. TP213 reduced TGF-β-induced p65 and α-SMA expression in NHLFs.

CONCLUSIONS

These findings suggest that neutralizing CX3CL1 attenuates lung fibroblast activation and mitochondrial dysfunction. Understanding the impacts of CX3CL1 neutralization on fibroblast mitochondrial function could contribute to the development of therapeutic strategies for managing airway remodeling in severe asthma.

Weighted gene co-expression network analysis reveals key biomarkers and immune infiltration characteristics for bronchial epithelial cells from asthmatic patients

BACKGROUND

Asthma ranks among the most prevalent non-communicable diseases worldwide. Previous studies have elucidated the significant role of the immune system in its pathophysiology. Nevertheless, the immune-related mechanisms underlying asthma are complex and still inadequately understood. Thus, our objective was to investigate novel key biomarkers and immune infiltration characteristics associated with asthma by employing integrated bioinformatics tools.

METHODS

In this study, we conducted a weighted gene co-expression network analysis (WGCNA) to identify key modules and genes potentially implicated in asthma. Functional annotation of these key modules and genes was carried out through gene ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis. Additionally, we constructed a protein-protein interaction (PPI) network using the STRING database to identify 10 hub genes. Furthermore, we evaluated the relative proportion of immune cells in bronchial epithelial cell samples from 20 healthy individuals and 88 asthmatic patients using CIBERSORT. Finally, we validated the hub genes and explored their correlation with immune infiltration.

RESULTS

Furthermore, 20 gene expression modules and 10 hub genes were identified herein. Among them, complement component 3 (C3), prostaglandin I2 receptor (PTGIR), parathyroid hormone-like hormone (PTHLH), and C-X3-C motif chemokine ligand 1 (CX3CL1) were closely correlated with the infiltration of immune cells. They may be novel candidate biomarkers or therapeutic targets for asthma. Furthermore, B cells memory, and plasma cells might play an important role in immune cell infiltration after asthma.

CONCLUSIONS

C3, PTGIR, CX3CL1, and PTHLH have important clinical diagnostic values and are correlated with infiltration of multiple immune cell types in asthma. These hub genes, B cells memory, and plasma cells may become important biological targets for therapeutic asthma drug screening and drug design.

Alterations in CX3CL1 Levels and Its Role in Viral Pathogenesis

CX3CL1, also named fractalkine or neurotactin, is the only known member of the CX3C chemokine family that can chemoattract several immune cells. CX3CL1 exists in both membrane-anchored and soluble forms, with each mediating distinct biological activities. CX3CL1 signals are transmitted through its unique receptor, CX3CR1, primarily expressed in the microglia of the central nervous system (CNS). In the CNS, CX3CL1 acts as a regulator of microglia activation in response to brain disorders or inflammation. Recently, there has been a growing interest in the role of CX3CL1 in regulating cell adhesion, chemotaxis, and host immune response in viral infection. Here, we provide a comprehensive review of the changes and function of CX3CL1 in various viral infections, such as human immunodeficiency virus (HIV), SARS-CoV-2, influenza virus, and cytomegalovirus (CMV) infection, to highlight the emerging roles of CX3CL1 in viral infection and associated diseases.

Food protein-induced allergic proctocolitis in infants is associated with low serum levels of macrophage inflammatory protein-3a

BACKGROUND

Food protein-induced allergic proctocolitis (FPIAP) is a nonimmunoglobulin (IgE)-mediated food hypersensitivity and the exact mechanisms that cause FPIAP are unknown. Chemokines play crucial roles in the development of allergic diseases.

OBJECTIVE

To examine serum levels of a group of chemokines in infants with FPIAP.

METHODS

In 67 infants with FPIAP and 65 healthy infants, we measured serum levels of mucosa-associated epithelial chemokine (MEC/CCL28), thymus-expressed chemokine (TECK/CCL25), CX3CL1 and macrophage inflammatory protein (MIP)-3a/CCL20.

RESULTS

Infants with FPIAP had a lower median value of MIP3a/CCL20 than healthy infants [0.7 (0-222) vs. 4 (0-249) pg/mL, respectively] (p < 0.001). Infants with MIP3a/CCL20 levels ≤0.95 pg/mL have 13.93 times more risk of developing FPIAP than infants with MIP3a/CCL20 levels >0.95 pg/mL. Serum MEC/CCL28, TECK/CCL25, and CX3CL1 levels were similar between the infants with FPIAP and the control group.

CONCLUSION

MIP3a/CCL20 serum levels were reduced in infants with FPIAP compared with healthy controls. Whether this finding has a role in pathogenesis remains to be determined.

FARSB serves as a novel hypomethylated and immune cell infiltration related prognostic biomarker in hepatocellular carcinoma

PURPOSE

Hepatocellular carcinoma (HCC) is a prevalent tumor with high morbidity, and an unfavourable prognosis. FARSB is an aminoacyl tRNA synthase, and plays a key role in protein synthesis in cells. Furthermore, previous reports have indicated that FARSB is overexpressed in gastric tumor tissues and is associated with a poor prognosis and tumorigenesis. However, the function of FARSB in HCC has not been studied.

RESULTS

The results showed that FARSB mRNA and protein levels were upregulated in HCC and were closely related to many clinicopathological characteristics. Besides, according to multivariate Cox analysis, high FARSB expression was linked with a shorter survival time in HCC and may be an independent prognostic factor. In addition, the FARSB promoter methylation level was negatively associated with the expression of FARSB. Furthermore, enrichment analysis showed that FARSB was related to the cell cycle. And TIMER analysis revealed that the FARSB expression was closely linked to tumor purity and immune cell infiltration. The TCGA and ICGC data analysis suggested that FARSB expression is greatly related to m6A modifier related genes. Potential FARSB-related ceRNA regulatory networks were also constructed. What's more, based on the FARSB-protein interaction network, molecular docking models of FARSB and RPLP1 were constructed. Finally, drug susceptibility testing revealed that FARSB was susceptible to 38 different drugs or small molecules.

CONCLUSIONS

FARSB can serve as a prognostic biomarker for HCC and provide clues about immune infiltration, and m6A modification.

Immune Mechanisms of Pulmonary Fibrosis with Bleomycin

Fibrosis and structural remodeling of the lung tissue can significantly impair lung function, often with fatal consequences. The etiology of pulmonary fibrosis (PF) is diverse and includes different triggers such as allergens, chemicals, radiation, and environmental particles. However, the cause of idiopathic PF (IPF), one of the most common forms of PF, remains unknown. Experimental models have been developed to study the mechanisms of PF, and the murine bleomycin (BLM) model has received the most attention. Epithelial injury, inflammation, epithelial-mesenchymal transition (EMT), myofibroblast activation, and repeated tissue injury are important initiators of fibrosis. In this review, we examined the common mechanisms of lung wound-healing responses after BLM-induced lung injury as well as the pathogenesis of the most common PF. A three-stage model of wound repair involving injury, inflammation, and repair is outlined. Dysregulation of one or more of these three phases has been reported in many cases of PF. We reviewed the literature investigating PF pathogenesis, and the role of cytokines, chemokines, growth factors, and matrix feeding in an animal model of BLM-induced PF.

CX3CR1 deficiency exacerbates immune-mediated hepatitis by increasing NF-κB-mediated cytokine production in macrophage and T cell

Immune-mediated hepatitis is marked by liver inflammation characterized by immune cell infiltration, chemokine/cytokine production, and hepatocyte injury. C-X3C motif receptor 1 (CX3CR1), as the receptor of chemokine C-X3C motif ligand 1 (CX3CL1)/fractalkine, is mainly expressed on immune cells including monocytes and T cells. Previous studies have shown that CX3CR1 protects against liver fibrosis, but the exact role of CX3CL1/CX3CR1 in acute immune-mediated hepatitis remains unknown. Here, we investigate the role of the CX3CL1/CX3CR1 axis in immune-mediated hepatitis using concanavalin A (ConA)-induced liver injury model in CX3CR1-deficient (Cx3cr1^-/-) mice. We observed that Cx3cr1^-/- mice had severe liver injury and increased pro-inflammatory cytokines (tumor necrosis factor-alpha [TNF-α], interferon-gamma [IFN-γ], interleukin-1 beta [IL-1β], and IL-6) in serum and liver compared to wild-type (Cx3cr1^+/+) mice after ConA injection. The deficiency of CX3CR1 did not affect ConA-induced immune cell infiltration in liver but led to elevated production of TNF-α in macrophages as well as IFN-γ in T cells after ConA treatment. On the contrary, exogenous CX3CL1 attenuated ConA-induced cytokine production in wild type, but not CX3CR1-deficient macrophages and T cells. Furthermore, in vitro results showed that CX3CR1 deficiency promoted the pro-inflammatory cytokine expression by increasing the phosphorylation of nuclear factor kappa B (NF-κB) p65 (p-NF-κB p65). Finally, pre-treatment of p-NF-κB p65 inhibitor, resveratrol, attenuated ConA-induced liver injury and inflammatory responses, especially in Cx3cr1^-/- mice. In conclusion, our data show that the deficiency of CX3CR1 promotes pro-inflammatory cytokine production in macrophages and T cells by enhancing the phosphorylation of NF-κB p65, which exacerbates liver injury in ConA-induced hepatitis.

Role of Chemokines and Inflammatory Cells in Respiratory Allergy

The idea of "one airway, one disease" has been gaining importance in the last decade. In the upper and lower airways, allergic mechanisms interact with each other. In the initial stage of respiratory allergic inflammation, allergens contact the respiratory epithelium, which produces chemokines and inflammatory factors, which cause allergic reactions by binding to the corresponding receptors and chemotactic various inflammatory cells to reach the epithelium and tissues. It also drives inflammatory cells to activate and produce more inflammatory factors, thus producing a cascade amplification effect. Inflammatory cell aggregation and activation are very complex and interact with each other in a lattice structure. By blocking the action of various chemokines, inflammatory cell aggregation is reduced, and ultimately the symptoms of respiratory allergy are alleviated. Chemokines can serve as cues for coordinated recruitment of immune cells into and out of tissues, as well as directing the spatial organization of immune cells within tissues and cellular interactions. Chemokines are critical in directing immune cell migration and thus have an important role in the direction of respiratory allergy: however, chemokines are also involved in the production and recruitment of immune cells that contribute to respiratory allergy. In this article, linking the upper and lower respiratory tracts. We review the role of the chemokine system in the respiratory immune response and discuss how respiratory disease modulates overall chemokines to shape the type and outcome of the immune response to the treatment of respiratory allergic disease so that we can further deepen our knowledge of chemokines in the direction of respiratory allergy. In the future, we can do drug research and development based on this network structure and explore new research directions.

Suppression of CX3CL1 by miR-497-5p inhibits cell growth and invasion through inactivating the ERK/AKT pathway in NSCLC cells

Non-small cell lung cancer (NSCLC) is the most common lung cancer with a highest mortality rate. MiR-497-5p has been reported as tumor suppressor in many cancers, but the role and mechanism of miR-497-5p in regulating NSCLC progression are still largely unknown in vitro and in vivo. Here, miR-497-5p was significantly downregulated in human NSCLC tissues and cell lines, compared with matched adjacent tissues and normal lung epithelial cell line. Then, miR-497-5p mimic and inhibitor were, respectively, transfected into human NSCLC cells A549 and H460, CCK-8 assay, transwell assay, and flow cytometry were used to detect the capacities of cell proliferation, invasion and apoptosis. MiR-497-5p negatively regulated proliferation and invasion of NSCLC cancer cells. MiR-497-5p was demonstrated to directly bound to 3'-UTR of CX3CL1 mRNA and post-transcriptionally suppressed its expression thus inactivating its downstream oncogenic pathway ERK/AKT. Moreover, transfection with short hairpin RNA (shRNA) against CX3CL1 decreased capacity of cell proliferation and invasion and promoted cell apoptosis in NSCLC cells. In addition, ERK inhibitor U0126 attenuated the promotion effect of miR-497-5p inhibitor on activation of ERK/AKT and cell proliferation and migration. Finally, overexpression of miR-497-5p substantially suppressed activation of the ERK/AKT pathway and tumor growth in tumor-bearing mice in vivo. Taken together, our findings showed that miR-497-5p is downregulated in human NSCLC tissues and cell lines, and it inhibited tumor growth and cell invasion by targeting CX3CL1 gene to inactivate the ERK/AKT pathway in NSCLC cells.

Innate Immunity to Spiral Ganglion Neuron Loss: A Neuroprotective Role of Fractalkine Signaling in Injured Cochlea

Immune system dysregulation is increasingly being attributed to the development of a multitude of neurodegenerative diseases. This, in large part, is due to the delicate relationship that exists between neurons in the central nervous system (CNS) and peripheral nervous system (PNS), and the resident immune cells that aid in homeostasis and immune surveillance within a tissue. Classically, the inner ear was thought to be immune privileged due to the presence of a blood-labyrinth barrier. However, it is now well-established that both vestibular and auditory end organs in the inner ear contain a resident (local) population of macrophages which are the phagocytic cells of the innate-immune system. Upon cochlear sterile injury or infection, there is robust activation of these resident macrophages and a predominant increase in the numbers of macrophages as well as other types of leukocytes. Despite this, the source, nature, fate, and functions of these immune cells during cochlear physiology and pathology remains unclear. Migration of local macrophages and infiltration of bone-marrow-derived peripheral blood macrophages into the damaged cochlea occur through various signaling cascades, mediated by the release of specific chemical signals from damaged sensory and non-sensory cells of the cochlea. One such signaling pathway is CX₃CL1-CX₃CR1, or fractalkine (FKN) signaling, a direct line of communication between macrophages and sensory inner hair cells (IHCs) and spiral ganglion neurons (SGNs) of the cochlea. Despite the known importance of this neuron-immune axis in CNS function and pathology, until recently it was not clear whether this signaling axis played a role in macrophage chemotaxis and SGN survival following cochlear injury. In this review, we will explore the importance of innate immunity in neurodegenerative disease development, specifically focusing on the regulation of the CX₃CL1-CX₃CR1 axis, and present evidence for a role of FKN signaling in cochlear neuroprotection.

Inhibition of CX3C receptor 1-mediated autophagy in macrophages alleviates pulmonary fibrosis in hyperoxic lung injury

AIMS

To investigate the role of CX3CR1 in hyperoxic lung injury induced pulmonary fibrosis.

MATERIALS AND METHODS

Hyperoxic lung injured mice were used as the disease model. Pulmonary fibrosis was determined by H&E and Masson's staining. Autophagy was investigated by western blot, immunofluorescence staining, and transmission electron microscopy.

KEY FINDINGS

We observed that increased CX3CR1 expression corresponded with increased pulmonary fibrosis. Additionally, silencing of CX3CR1 significantly alleviated the fibrosis when compared to the control. We observed that exposure of mouse to hyperoxic environment increased macrophage levels along with an increased CD11b expression in the lung tissues. Subsequently, we also observed an increased expression of LC3-II and decreased p62 expression in hyperoxic mice models, suggesting the potential role of hyperoxia induced autophagy. CD11b and LC3/CX3CR1 were expressed and co-localized in a manner indicating CX3CR1 indeed does regulate macrophage autophagy in the hyperoxic lung injury model. We observed a decrease in hyperoxia-associated fibrosis, along with a decrease in autophagy when we used 3-MA (autophagy inhibitor) in our hyperoxic lung injury model. To elucidate the pathway through which CX3CR1 regulated autophagy, we further analyzed the Akt1 pathway. Our experimental results indicated that the Akt1 inhibitor (A-674563) did significantly decrease macrophage autophagy and fibrosis in hyperoxic mice models.

SIGNIFICANCE

Thus, our data indicates a novel role of CX3CR1 in regulation of macrophage autophagy and promotion of pulmonary fibrosis in hyperoxic lung injured mice.

CX3CL1 homo-oligomerization drives cell-to-cell adherence

During inflammatory response, blood leukocytes adhere to the endothelium. This process involves numerous adhesion molecules, including a transmembrane chemokine, CX3CL1, which behaves as a molecular cluster. How this cluster assembles and whether this association has a functional role remain unknown. The analysis of CX3CL1 clusters using native electrophoresis and single molecule fluorescence kinetics shows that CX3CL1 is a homo-oligomer of 3 to 7 monomers. Fluorescence recovery after photobleaching assays reveal that the CX3CL1-transmembrane domain peptide self-associates in both cellular and acellular lipid environments, while its random counterpart (i.e. peptide with the same residues in a different order) does not. This strongly indicates that CX3CL1 oligomerization is driven by its intrinsic properties. According to the molecular modeling, CX3CL1 does not associate in compact bundles but rather with monomers linearly assembled side by side. Finally, the CX3CL1 transmembrane peptide inhibits both the CX3CL1 oligomerization and the adhesive function, while its random counterpart does not. This demonstrates that CX3CL1 oligomerization is mandatory for its adhesive potency. Our results provide a new direction to control CX3CL1-dependent cellular adherence in key immune processes.

Prevention of lipopolysaccharide-induced preterm labor by the lack of CX3CL1-CX3CR1 interaction in mice

Preterm labor (PTL) is the most common cause of neonatal death and long-term adverse outcome. The pharmacological agents for PTL prevention are palliative and frequently fail to prevent PTL and improve neonatal outcome. It is essential to fully understand the molecular mechanisms of PTL in order to develop novel therapeutic methods against PTL. Several lines of evidence indicate some chemokines are expressed in gestational tissues during labor or PTL. To reveal the pathophysiological roles of the CX3CL1-CX3CR1 axis in PTL, we performed present study using LPS-induced PTL mice model in CX3CR1-deficient (Cx3cr1^-/-) mice. We indicated that PTL was suppressed in Cx3cr1^-/- mice and immunoneutralization of CX3CL1 in WT mice. From immunohistochemical and the gene expression analyses, the CX3CL1-CX3CR1 axis has detrimental roles in PTL through intrauterine recruitment of macrophages and the enhancement of macrophage-derived inflammatory mediators. Thus, the CX3CL1-CX3CR1 axis may be a good molecular target for preventing PTL.

Essential involvement of the CX3CL1-CX3CR1 axis in bleomycin-induced pulmonary fibrosis via regulation of fibrocyte and M2 macrophage migration

The potential role of macrophages in pulmonary fibrosis (PF) prompted us to evaluate the roles of CX3CR1, a chemokine receptor abundantly expressed in macrophages during bleomycin (BLM)-induced PF. Intratracheal BLM injection induced infiltration of leukocytes such as macrophages into the lungs, which eventually resulted in fibrosis. CX3CR1 expression was mainly detected in the majority of macrophages and in a small portion of α-smooth muscle actin-positive cells in the lungs, while CX3CL1 was expressed in macrophages. BLM-induced fibrotic changes in the lungs were reduced without any changes in the number of leukocytes in Cx3cr1^−/− mice, as compared with those in the wild-type (WT) mice. However, intrapulmonary CX3CR1⁺ macrophages displayed pro-fibrotic M2 phenotypes; lack of CX3CR1 skewed their phenotypes toward M1 in BLM-challenged lungs. Moreover, fibrocytes expressed CX3CR1, and were increased in BLM-challenged WT lungs. The number of intrapulmonary fibrocytes was decreased in Cx3cr1^−/− mice. Thus, locally-produced CX3CL1 can promote PF development primarily by attracting CX3CR1-expressing M2 macrophages and fibrocytes into the lungs.

CX3CR1 modulates the anti-inflammatory activity of hepatic dendritic cells in response to acute liver injury

The chemokine fractalkine (CX₃CL1) and its receptor CX₃CR1 are known to mediate leukocyte chemotaxis, adhesion and survival. In the liver, CX₃CR1 is expressed on multiple cell types including monocytes and dendritic cells. However, the function of CX₃CR1 on hepatic dendritic cells (HDCs) is still poorly understood. In this study, we investigated the role of CX₃CR1 on mouse HDCs during homeostasis and following acute liver injury. At homeostasis, CX₃CR1-expression was detected among CD11b⁺/CD103^- type 2 myeloid HDCs (mHDCs) and these cells were characterized by the production of IL-10.   Mice treatment with the hepatotoxic agent CCl₄ up-regulated liver IL-10 expression and stimulated the expansion of CX₃CR1⁺ mHDCs which also showed a more mature phenotype. The absence of CX₃CR1 in naïve CX₃CR1^gfp/gfp mice specifically reduced the CD11b⁺/IL-10⁺ mHDCs as compared to CX₃CR1-proficient animals (CX₃CR1^+/gfp).  Following CCl₄ poisoning, the liver recruitment and maturation of CD11b⁺ mHDCs was significantly attenuated in CX₃CR1^gfp/gfp mice. Furthermore, these mice suffered more severe hepatic injury and inflammation than CX₃CR1^+/gfp mice and showed a delated recovery from liver damage. Such a worsening of liver injury in CX₃CR1^gfp/gfp mice was associated with an impaired up-regulation of hepatic IL-10 expression and a lower number of IL-10 producing CD11b⁺ mHDCs. Consistently, IL-10 inactivation enhanced hepatic injury and inflammation in CX3CR1^+/gfp mice receiving CCl₄ Altogether, these data indicate a novel role of the CX₃CL1/CX₃CR1 axis in liver type 2 mHDC functions, pointing out the importance of CX₃CR1 in promoting IL-10-mediated anti-inflammatory actions of HDCs.

Mutating the CX3C Motif in the G Protein Should Make a Live Respiratory Syncytial Virus Vaccine Safer and More Effective

Respiratory syncytial virus (RSV) belongs to the family Paramyxoviridae and is the single most important cause of serious lower respiratory tract infections in young children, yet no highly effective treatment or vaccine is available. Through a CX3C chemokine motif (182CWAIC186) in the G protein, RSV binds to the corresponding chemokine receptor, CX3CR1. Since RSV binding to CX3CR1 contributes to disease pathogenesis, we investigated whether a mutation in the CX3C motif by insertion of an alanine, A186, within the CX3C motif, mutating it to CX4C (182CWAIAC187), which is known to block binding to CX3CR1, might decrease disease. We studied the effect of the CX4C mutation in two strains of RSV (A2 and r19F) in a mouse challenge model. We included RSV r19F because it induces mucus production and airway resistance, two manifestations of RSV infection in humans, in mice. Compared to wild-type (wt) virus, mice infected with CX4C had a 0.7 to 1.2 log10-fold lower virus titer in the lung at 5 days postinfection (p.i.) and had markedly reduced weight loss, pulmonary inflammatory cell infiltration, mucus production, and airway resistance after challenge. This decrease in disease was not dependent on decrease in virus replication but did correspond to a decrease in pulmonary Th2 and inflammatory cytokines. Mice infected with CX4C viruses also had higher antibody titers and a Th1-biased T cell memory response at 75 days p.i. These results suggest that the CX4C mutation in the G protein could improve the safety and efficacy of a live attenuated RSV vaccine.IMPORTANCE RSV binds to the corresponding chemokine receptor, CX3CR1, through a CX3C chemokine motif (182CWAIC186) in the G protein. RSV binding to CX3CR1 contributes to disease pathogenesis; therefore, we investigated whether a mutation in the CX3C motif by insertion of an alanine, A186, within the CX3C motif, mutating it to CX4C (182CWAIAC187), known to block binding to CX3CR1, might decrease disease. The effect of this mutation and treatment with the F(ab')2 form of the anti-RSV G 131-2G monoclonal antibody (MAb) show that mutating the CX3C motif to CX4C blocks much of the disease and immune modulation associated with the G protein and should improve the safety and efficacy of a live attenuated RSV vaccine.

Cellular crosstalk between airway epithelial and endothelial cells regulates barrier functions during exposure to double-stranded RNA

INTRODUCTION

The epithelial and endothelial barriers of the airway mucosa are critical for regulation of tissue homeostasis and protection against pathogens or other tissue damaging agents. In response to a viral infection, epithelial cells must signal to the endothelium to initiate immune cell recruitment. This is a highly temporal regulated process; however, the mechanisms of this cross-talk are not fully understood.

METHODS

In a close-contact co-culture model of human airway epithelial and endothelial cells, cellular crosstalk was analyzed using transepithelial electrical resistance (TER) measurements, immunofluorescence, electron microscopy, and ELISA. Viral infections were simulated by exposing airway epithelial cells apically to double-stranded RNA (Poly(I:C)). Using a microfluidic culture system, the temporal release of mediators was analyzed in the co-culture model.

RESULTS

Within 4 h of challenge, double-stranded RNA induced the release of TNF-α by epithelial cells. This activated endothelial cells by triggering the release of the chemoattractant CX3CL1 (fractalkine) by 8 h post-challenge and expression of adhesion molecules E-selectin and ICAM-1. These responses were significantly reduced by neutralising TNF-α.

CONCLUSION

By facilitating kinetic profiling, the microfluidic co-culture system has enabled identification of a key signaling mechanism between the epithelial and endothelial barriers. Better understanding of cell-cell cross-talk and its regulatory mechanisms has the potential to identify new therapeutic strategies to control airway inflammation.

CX₃CL1 (fractalkine) and its receptor CX₃CR1 regulate atopic dermatitis by controlling effector T cell retention in inflamed skin

Fractalkine interactions with its receptor, CX₃CR1, regulate CD4⁺ T cell retention in atopic dermatitis and offer a potential therapeutic target in allergic disease.

Atopic dermatitis (AD) is a chronic allergic dermatosis characterized by epidermal thickening and dermal inflammatory infiltrates with a dominant Th2 profile during the acute phase, whereas a Th1 profile is characteristic of the chronic stage. Among chemokines and chemokine receptors associated with inflammation, increased levels of CX₃CL1 (fractalkine) and its unique receptor, CX₃CR1, have been observed in human AD. We have thus investigated their role and mechanism of action in experimental models of AD and psoriasis. AD pathology and immune responses, but not psoriasis, were profoundly decreased in CX₃CR1-deficient mice and upon blocking CX₃CL1–CX₃CR1 interactions in wild-type mice. CX₃CR1 deficiency affected neither antigen presentation nor T cell proliferation in vivo upon skin sensitization, but CX₃CR1 expression by both Th2 and Th1 cells was required to induce AD. Surprisingly, unlike in allergic asthma, where CX₃CL1 and CX₃CR1 regulate the pathology by controlling effector CD4⁺ T cell survival within inflamed tissues, adoptive transfer experiments established CX₃CR1 as a key regulator of CD4⁺ T cell retention in inflamed skin, indicating a new function for this chemokine receptor. Therefore, although CX₃CR1 and CX₃CL1 act through distinct mechanisms in different pathologies, our results further indicate their interest as promising therapeutic targets in allergic diseases.

Fractalkine signaling in regulation of insulin secretion

Fractalkine is a chemokine, which has been shown to play important roles in metabolic disease in both animal models and humans. Fractalkine is a key player in the accumulation of atherosclerotic plaques, and fractalkine receptor (CX3CR1) mutations have been implicated in obesity. Serum fractalkine levels have been found to be elevated in type 2 diabetic patients, but the role of fractalkine signaling on the pancreatic β cell was unclear. Recently published findings in April 2013 issue of the journal Cell by Lee and Olefsky et al. have implicated fractalkine in β-cell insulin secretion. They demonstrate that Cx3cr1 knockout mice have impaired glucose tolerance resulting from decreased insulin secretion. In addition, fractalkine administration improved glucose tolerance and induced insulin secretion. This modulation of insulin secretion was proposed to result from an increase in intracellular calcium and potentiation of insulin secretion, which occurs in a Gαi and MEK-dependent manner. They also found that Cx3cr1 knockout animals had transcriptional repression of genes important for β-cell function, specifically NeuroD, via induction of ICER-1. One important issue that remains unresolved is how CX3CR1 signaling regulates the potentiation of calcium influx and the distal events in insulin exocytosis. Finally, testing the effects of fractalkine treatment on proliferation and survival in vivo during regenerative conditions would be critical to determine the potential use of this chemokine in diabetes. While these exciting results open the possibility for new therapeutics, there are some concerns about a potential risk for exacerbation of atherosclerosis.

Development and characterization of an aptamer binding ligand of fractalkine using domain targeted SELEX

Fractalkine (FKN) is a unique cell surface protein with potential as a therapeutic target because of its role in inflammatory diseases and cancer. We developed an aptamer, named FKN-S2, with a dissociation constant of 3.4 ± 0.7 nM that is specific to the chemokine domain of fractalkine.