CX3CR1 is required for monocyte homeostasis and atherogenesis by promoting cell survival

Monocyte function and trafficking in cardiovascular disease

Monocytes are key effectors of the immune homeostasis and play a crucial role in (vascular) injury repair. Despite their role in immune defense and tissue repair mechanisms, monocytes are also involved in several pathological conditions such as autoimmune and cardiovascular diseases as well as cancer. This suggests that monocytes can be used as diagnostic and as therapeutic targets. A better understanding and characterisation of monocytes and their function in both physiological and pathological situations is thus of great interest. This review focuses on recent advances on the role of monocytes in cardiovascular diseases and describes the value of monocytes as either disease marker or therapeutic target for (cardio)vascular diseases.

CD16+ monocytes control T-cell subset development in immune thrombocytopenia

Immune thrombocytopenia (ITP) results from decreased platelet production and accelerated platelet destruction. Impaired CD4(+) regulatory T-cell (Treg) compartment and skewed Th1 and possibly Th17 responses have been described in ITP patients. The trigger for aberrant T-cell polarization remains unknown. Because monocytes have a critical role in development and polarization of T-cell subsets, we explored the contribution of monocyte subsets in control of Treg and Th development in patients with ITP. Unlike circulating classic CD14(hi)CD16(-) subpopulation, the CD16(+) monocyte subset was expanded in ITP patients with low platelet counts on thrombopoietic agents and positively correlated with T-cell CD4(+)IFN-γ(+) levels, but negatively with circulating CD4(+)CD25(hi)Foxp3(+) and IL-17(+) Th cells. Using a coculture model, we found that CD16(+) ITP monocytes promoted the expansion of IFN-γ(+)CD4(+) cells and concomitantly inhibited the proliferation of Tregs and IL-17(+) Th cells. Th-1-polarizing cytokine IL-12, secreted after direct contact of patient T-cell and CD16(+) monocytes, was responsible for the inhibitory effect on Treg and IL-17(+)CD4(+) cell proliferation. Our findings are consistent with ITP CD16(+) monocytes promoting Th1 development, which in turn negatively regulates IL-17 and Treg induction. This underscores the critical role of CD16(+) monocytes in the generation of potentially pathogenic Th responses in ITP.

Touch of chemokines

Chemoattractant cytokines or chemokines constitute a family of structurally related proteins found in vertebrates, bacteria, or viruses. So far, 48 chemokine genes have been identified in humans, which bind to around 20 chemokine receptors. These receptors belong to the seven transmembrane G-protein-coupled receptor family. Chemokines and their receptors were originally studied for their role in cellular trafficking of leukocytes during inflammation and immune surveillance. It is now known that they exert different functions under physiological conditions such as homeostasis, development, tissue repair, and angiogenesis but also under pathological disorders including tumorigenesis, cancer metastasis, inflammatory, and autoimmune diseases. Physicochemical properties of chemokines and chemokine receptors confer the ability to homo- and hetero-oligomerize. Many efforts are currently performed in establishing new therapeutically compounds able to target the chemokine/chemokine receptor system. In this review, we are interested in the role of chemokines in inflammatory disease and leukocyte trafficking with a focus on vascular inflammatory diseases, the operating synergism, and the emerging therapeutic approaches of chemokines.

Hematopoietic Interferon Regulatory Factor 8-Deficiency Accelerates Atherosclerosis in Mice

Objective—

Inflammatory leukocyte accumulation drives atherosclerosis. Although monocytes/macrophages and polymorphonuclear neutrophilic leukocytes (PMN) contribute to lesion formation, sequelae of myeloproliferative disease remain to be elucidated.

Methods and Results—

We used mice deficient in interferon regulatory factor 8 (IRF8 −/− ) in hematopoietic cells that develop a chronic myelogenous leukemia-like phenotype. Apolipoprotein E-deficient mice reconstituted with IRF8 −/− or IRF8 −/− apolipoprotein E-deficient bone marrow displayed an exacerbated atherosclerotic lesion formation compared with controls. The chronic myelogenous leukemia-like phenotype in mice with IRF8 −/− bone marrow, reflected by an expansion of PMN in the circulation, was associated with an increased lesional accumulation and apoptosis of PMN, and enlarged necrotic cores. IRF8 −/− compared with IRF8 +/+ PMN displayed unaffected reactive oxygen species formation and discharge of PMN granule components. In contrast, accumulating in equal numbers at sites of inflammation, IRF8 −/− macrophages were defective in efferocytosis, lipid uptake, and interleukin-10 cytokine production. Importantly, depletion of PMN in low-density lipoprotein receptor or apolipoprotein E-deficient mice with IRF8 −/− or IRF8 −/− apolipoprotein E-deficient bone marrow abrogated increased lesion formation.

Conclusion—

These findings indicate that a chronic myelogenous leukemia-like phenotype contributes to accelerated atherosclerosis in mice. Among proatherosclerotic effects of other cell types, this, in part, is linked to an expansion of functionally intact PMN.

Monocytes in health and disease - Minireview

Monocytes are important cell types of the innate immune system. Recent scientific evidence suggests that monocytes not only play a crucial role in our innate immune system by defending the host from intruding microbial pathogens but they also contribute to the pathogenesis and progression of diseases such as liver fibrosis, atherosclerosis, multiple sclerosis, and tumor metastasis. In addition, monocytes and monocyte-derived macrophages play a crucial beneficial role in the liver fibrosis regression, muscle regeneration, and the clearance of the β-amyloid plaques in Alzheimer's disease. Here, we summarize the origin, plasticity, and pathogenic potential of monocytes and monocyte-derived macrophages, as well as their positive role in the regression of some common diseases. Elucidating the comprehensive immunological role of monocytes will provide therapeutic advantages in either controlling disease progression or favoring the regression of the disease state.

CX3CR1 deficiency does not influence trafficking of adipose tissue macrophages in mice with diet-induced obesity

Adipose tissue macrophages (ATMs) accumulate in fat during obesity and resemble foam cells in atherosclerotic lesions, suggesting that common mechanisms underlie both inflammatory conditions. CX(3)CR1 and its ligand fractalkine/CX(3)CL1 contribute to macrophage recruitment and inflammation in atherosclerosis, but their role in obesity-induced adipose tissue inflammation is unknown. Therefore, we tested the hypothesis that CX(3)CR1 regulates ATM trafficking to epididymal fat and contributes to the development of adipose tissue inflammation during diet-induced obesity. Cx(3)cl1 and Cx(3)cr1 expression was induced specifically in epididymal fat from mice fed a high-fat diet (HFD). CX(3)CR1 was detected on multiple myeloid cells within epididymal fat from obese mice. To test the requirement of CX(3)CR1 for ATM trafficking and obesity-induced inflammation, Cx(3)cr1(+/GFP) and Cx(3)cr1(GFP/GFP) mice were fed a HFD. Ly-6c(Low) monocytes were reduced in lean Cx(3)cr1(GFP/GFP) mice; however, HFD-induced monocytosis was comparable between strains. Total ATM content, the ratio of type 1 (CD11c(+)) to type 2 (CD206(+)) ATMs, expression of inflammatory markers, and T-cell content were similar in epididymal fat from obese Cx(3)cr1(+/GFP) and Cx(3)cr1(GFP/GFP) mice. Cx(3)cr1 deficiency did not prevent the development of obesity-induced insulin resistance or hepatic steatosis. In summary, our data indicate that CX(3)CR1 is not required for the recruitment or retention of ATMs in epididymal adipose tissue of mice with HFD-induced obesity even though CX(3)CR1 promotes foam cell formation. This highlights an important point of divergence between the mechanisms regulating monocyte trafficking to fat with obesity and those that contribute to foam cell formation in atherogenesis.

Macrophage differentiation and function in atherosclerosis: opportunities for therapeutic intervention?

The macrophage is exquisitely sensitive to its microenvironment, as demonstrated primarily through in vitro study. Changes in macrophage phenotype and function within the atherosclerotic plaque have profound consequences for plaque biology, including rupture and arterial thrombosis leading to clinical events such as myocardial infarction. We review the evidence for dynamic changes in macrophage numbers and macrophage differentiation within the atherosclerotic plaque microenvironment and discuss potential approaches to target macrophage differentiation for therapeutic benefit in cardiovascular disease.

Contribution of platelet CX(3)CR1 to platelet-monocyte complex formation and vascular recruitment during hyperlipidemia

OBJECTIVE

The chemokine receptor CX(3)CR1 is an inflammatory mediator in vascular diseases. On platelets, its ligation with fractalkine (CX(3)CL1) induces platelet activation followed by leukocyte recruitment to activated endothelium. Here, we evaluated the expression and role of platelet-CX(3)CR1 during hyperlipidemia and vascular injury.

METHODS AND RESULTS

The existence of CX(3)CR1 on platelets at mRNA and protein level was analyzed by RT-PCR, quantitative (q)PCR, FACS analysis, and Western blot. Elevated CX(3)CR1 expression was detected on human platelets after activation and, along with increased binding of CX(3)CL1, platelet CX(3)CR1 was also involved in the formation of platelet-monocyte complexes. Interestingly, the expression of CX(3)CR1 was elevated on platelets from hyperlipidemic mice. Accordingly, CX(3)CL1-binding and the number of circulating platelet-monocyte complexes were increased. In addition, CX(3)CR1 supported monocyte arrest on inflamed smooth muscle cells in vitro, whereas CX(3)CR1-deficient platelets showed decreased adhesion to the denuded vessel wall in vivo.

CONCLUSIONS

Platelets in hyperlipidemic mice display increased CX(3)CR1-expression and assemble with circulating monocytes. The formation of platelet-monocyte complexes and the detection of platelet-bound CX(3)CL1 on inflamed smooth muscle cells suggest a significant involvement of the CX(3)CL1-CX(3)CR1 axis in platelet accumulation and monocyte recruitment at sites of arterial injury in atherosclerosis.

Fractalkine stimulates cell growth and increases its expression via NF-κB pathway in RA-FLS

BACKGROUND

After the onset of rheumatoid arthritis (RA), fibroblast-like synoviocytes (RA-FLS) which are specialized types of fibroblasts, become tumor-like, keeping their ability to increase proliferation and invasion. The mechanism of their tumor-like growth is unclear. Fractalkine (FKN), also called CX3CL1, plays an important role in the proliferation of cells. FKN may stimulate the proliferation of RA-FLS and the by nuclear factor κB (NF-κB) pathway may be one of the steps in this process.

OBJECTIVE

To investigate whether FKN can stimulate cell growth and increase its expression in RA-FLS, and the relationship between the NF-κB pathway and the function of FKN.

METHODS

FLS were isolated from primary synovial tissue obtained from three patients with RA who had undergone total joint replacement surgery or synovectomy in the Third Hospital Affiliated to Sun Yat-sen University from February 2009 to January 2010. FKN was used in different concentrations to stimulate RA-FLS with or without NF-κB pathway blocker (PDTC), and to test the proliferation of FLS after 24 h by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay. RA-FLS was treated with 100 ng/mL FKN or 100 μM PDTC for different periods, and messenger RNA (mRNA) expression of FKN and CX3CR1 in RA-FLS was detected by reverse transcription - polymerase chain reaction. We then tested the protein expression of NF-κBp65 in the cytoplasm and nucleus, respectively by Western blotting after treating the RA-FLS with 100 ng/mL FKN for different time periods.

RESULTS

FKN stimulated cell growth in RA-FLS at the concentration of 50 or 100 ng/mL (P = 0.005 and P = 0.022, respectively). NF-κB pathway blocker inhibited FKN, promoting proliferation of RA-FLS. RA-FLS could express FKN and CX3CR1 mRNA in vitro. FKN up-regulated FKN expression after 18-h treatment (P = 0.012). PDTC disturbed the expression of FKN mRNA after 16-18 h treatment (P = 0.001 and P < 0.001, respectively). After stimulation with FKN for 1 h, the expression of NF-κBp65 in cytoplasm began to decrease (P = 0.010), and the expression of NF-κBp65 in the nucleus began to increase after 2 h (P = 0.011).

CONCLUSION

These results suggest that FKN stimulates cells growth in RA-FLS and NF-κB pathway blocker inhibits FKN, promoting proliferation of RA-FLS. FKN induced activation of NF-κB activity. FKN up-regulates FKN mRNA expression in RA-FLS via the NF-κB pathway.

Fractalkine: a survivor's guide: chemokines as antiapoptotic mediators

Chemokines are a family of low-molecular-weight proteins essential to the directed migration of cells under homeostatic and pathological conditions. Fractalkine (CX3CL1) is an unusual chemokine that can act as either a soluble or membrane-bound mediator and signals through the G protein-coupled chemokine receptor CX3CR1, expressed on monocytes, natural killer cells, T cells, and smooth muscle cells. Accumulating evidence suggests that fractalkine, in addition to its role in chemotaxis and adhesion of leukocytes, supports the survival of multiple cell types during homeostasis and inflammation. This review presents the evidence obtained from several disease models implying an antiapoptotic function for fractalkine and shows how this is relevant to the pathology of atherosclerosis and other vascular diseases. We discuss whether the key role of fractalkine, unlike other chemokines, is the promotion of cell survival and whether this has implications for vascular disease.

Atherosclerosis: current pathogenesis and therapeutic options

Coronary artery disease (CAD) arising from atherosclerosis is a leading cause of death and morbidity worldwide. The underlying pathogenesis involves an imbalanced lipid metabolism and a maladaptive immune response entailing a chronic inflammation of the arterial wall. The disturbed equilibrium of lipid accumulation, immune responses and their clearance is shaped by leukocyte trafficking and homeostasis governed by chemokines and their receptors. New pro- and anti-inflammatory pathways linking lipid and inflammation biology have been discovered, and genetic profiling studies have unveiled variations involved in human CAD. The growing understanding of the inflammatory processes and mediators has uncovered an intriguing diversity of targetable mechanisms that can be exploited to complement lipid-lowering therapies. Here we aim to systematically survey recently identified molecular mechanisms, translational developments and clinical strategies for targeting lipid-related inflammation in atherosclerosis and CAD.

Chemokines: established and novel targets in atherosclerosis

In their role as small chemotactic cytokines, chemokines are crucial mediators and regulators of leukocyte trafficking during immune surveillance and inflammation. Their involvement in the development and progression of inflammatory diseases has been subject of intense investigation. Concordantly, the chemokine system has been explored in search for therapeutic targets to prevent or treat inflammatory disorders, such as atherosclerosis. Targeting the chemokine system offers various entry points for a causative treatment of this widespread and chronic illness. Although this approach has encountered some setbacks, several innovative compounds are currently in an advanced stage of development. In this review, the current standing of this dynamic field is highlighted and the potential advantages and drawbacks of particular strategies are discussed.

CCL17-expressing dendritic cells drive atherosclerosis by restraining regulatory T cell homeostasis in mice

Immune mechanisms are known to control the pathogenesis of atherosclerosis. However, the exact role of DCs, which are essential for priming of immune responses, remains elusive. We have shown here that the DC-derived chemokine CCL17 is present in advanced human and mouse atherosclerosis and that CCL17+ DCs accumulate in atherosclerotic lesions. In atherosclerosis-prone mice, Ccl17 deficiency entailed a reduction of atherosclerosis, which was dependent on Tregs. Expression of CCL17 by DCs limited the expansion of Tregs by restricting their maintenance and precipitated atherosclerosis in a mechanism conferred by T cells. Conversely, a blocking antibody specific for CCL17 expanded Tregs and reduced atheroprogression. Our data identify DC-derived CCL17 as a central regulator of Treg homeostasis, implicate DCs and their effector functions in atherogenesis, and suggest that CCL17 might be a target for vascular therapy.

Monocyte recruitment during infection and inflammation

Monocytes originate from progenitors in the bone marrow and traffic via the bloodstream to peripheral tissues. During both homeostasis and inflammation, circulating monocytes leave the bloodstream and migrate into tissues where, following conditioning by local growth factors, pro-inflammatory cytokines and microbial products, they differentiate into macrophage or dendritic cell populations. Recruitment of monocytes is essential for effective control and clearance of viral, bacterial, fungal and protozoal infections, but recruited monocytes also contribute to the pathogenesis of inflammatory and degenerative diseases. The mechanisms that control monocyte trafficking under homeostatic, infectious and inflammatory conditions are being unravelled and are the focus of this Review.

In vivo structure/function and expression analysis of the CX3C chemokine fractalkine

The CX3C chemokine family is composed of only one member, CX3CL1, also known as fractalkine, which in mice is the sole ligand of the G protein-coupled, 7-transmembrane receptor CX3CR1. Unlike classic small peptide chemokines, CX3CL1 is synthesized as a membrane-anchored protein that can promote integrin-independent adhesion. Subsequent cleavage by metalloproteases, either constitutive or induced, can generate shed CX3CL1 entities that potentially have chemoattractive activity. To study the CX3C interface in tissues of live animals, we generated transgenic mice (CX3CL1cherry:CX3CR1gfp), which express red and green fluorescent reporter genes under the respective control of the CX3CL1 and CX3CR1 promoters. Furthermore, we performed a structure/function analysis to differentiate the in vivo functions of membrane-tethered versus shed CX3CL1 moieties by comparing their respective ability to correct established defects in macrophage function and leukocyte survival in CX3CL1-deficient mice. Specifically, expression of CX3CL1(105Δ), an obligatory soluble CX3CL1 isoform, reconstituted the formation of transepithelial dendrites by intestinal macrophages but did not rescue circulating Ly6Clo CX3CR1hi blood monocytes in CX3CR1gfp/gfp mice. Instead, monocyte survival required the full-length membrane-anchored CX3CL1, suggesting differential activities of tethered and shed CX3CL1 entities.

Deficiency of CX3CR1 delays burn wound healing and is associated with reduced myeloid cell recruitment and decreased sub-dermal angiogenesis

The development of a good blood supply is a key step in burn wound healing and appears to be regulated in part by myeloid cells. CX3CR1 positive cells have recently been identified as myeloid cells with a potential role in angiogenesis. The role of functional CX3CR1 system in burn wound healing is not previously investigated. A 2% contact burn was induced in CX3CR1(+/gfp) and CX3CR1(gfp/gfp) mice. These transgenic mice facilitate the tracking of CX3CR1 cells (CX3CR1(+/gfp)) and allow evaluation of the consequence of CX3CR1 functional knockout (CX3CR1(gfp/gfp)) on burn wound healing. The progression of wound healing was monitored before tissue was harvested and analyzed at day 6 and day 12 for migration of CX3CR1 cells into burn wound. Deficiency of a functional CX3CR1 system resulted in decreased recruitment of CX3CR1 positive cells into the burn wound associated with decreased myeloid cell recruitment (p<0.001) and reduced maintenance of new vessels (p<0.001). Burn wound healing was prolonged (p<0.05). Our study is the first to establish a role for CX3CR1 in burn wound healing which is associated with sub-dermal angiogenesis. This chemokine receptor pathway may be attractive for therapeutic manipulation as it could increase sub dermal angiogenesis and thereby improve time to healing.

Differential role of monocyte subsets in atherosclerosis

Endothelial dysfunction and inflammation of the arterial wall continuously drive the development of atherosclerosis. Details regarding the sequential involvement of different monocyte subsets in the pathology of this disease have recently emerged. This review concentrates on major monocyte subpopulations in mouse and men and specifically addresses their phenotype, function and recruitment during primary atherosclerosis as well as their contribution to angiogenesis and tissue regeneration secondary to plaque rupture.

Toll-like receptors and macrophage activation in atherosclerosis

Atherosclerosis is a multi-factorial inflammatory disease and is the primary initiator of coronary artery and cerebrovascular disease. Initially believed to be exclusively lipid-driven, recent evidence demonstrates that inflammation is a significant driving force of the disease. Cellular components of innate immunity, for example monocytes and macrophages, play a predominant role in atherosclerosis. Toll-like receptors (TLRs) are the most characterised innate immune receptors and recent evidence demonstrates an important role in atherogenesis. Engagement of TLRs results in the transcription of pro-inflammatory cytokines, foam cell formation and activation of adaptive immunity. Recently they have also been implicated in protection from vascular disease. In this review, we detail the role of the innate immune system, specifically macrophages and TLR signalling, in atherosclerosis and acute cardiovascular complications, and thereby identify the potential of TLRs to act as therapeutic targets.

How dendritic cells shape atherosclerosis

Atherosclerosis is an inflammatory disease of the arteries, which results in major morbidity and mortality. Immune cells initiate and sustain local inflammation. Here, we focus on how dendritic cell (DC)-mediated processes might be relevant to atherosclerosis. Although only small numbers of DCs are detected in healthy arteries, these numbers dramatically increase during atherosclerosis development. In the earliest fatty streaks, DCs are found next to the vascular endothelium. During plaque growth, new DCs are actively recruited, and their egress from the vessel wall is dampened. In the adventitia next to mature atherosclerotic lesions, tertiary lymphoid organs develop, which also contain DCs. Thus, DCs probably participate in all stages of atherosclerosis from fatty streaks to mature lesions.

The transcription factor NR4A1 (Nur77) controls bone marrow differentiation and the survival of Ly6C- monocytes

The transcription factors that regulate differentiation into the monocyte subset in bone marrow have not yet been identified. Here we found that the orphan nuclear receptor NR4A1 controlled the differentiation of Ly6C- monocytes. Ly6C- monocytes, which function in a surveillance role in circulation, were absent from Nr4a1-/- mice. Normal numbers of myeloid progenitor cells were present in Nr4a1-/- mice, which indicated that the defect occurred during later stages of monocyte development. The defect was cell intrinsic, as wild-type mice that received bone marrow from Nr4a1-/- mice developed fewer patrolling monocytes than did recipients of wild-type bone marrow. The Ly6C- monocytes remaining in the bone marrow of Nr4a1-/- mice were arrested in S phase of the cell cycle and underwent apoptosis. Thus, NR4A1 functions as a master regulator of the differentiation and survival of 'patrolling' Ly6C- monocytes.

Monocyte and macrophage dynamics during atherogenesis

Vascular inflammation is associated with and in large part driven by changes in the leukocyte compartment of the vessel wall. Here, we focus on monocyte influx during atherosclerosis, the most common form of vascular inflammation. Although the arterial wall contains a large number of resident macrophages and some resident dendritic cells, atherosclerosis drives a rapid influx of inflammatory monocytes (Ly-6C(+) in mice) and other monocytes (Ly-6C(-) in mice, also known as patrolling monocytes). Once in the vessel wall, Ly-6C(+) monocytes differentiate to a phenotype consistent with inflammatory macrophages and inflammatory dendritic cells. The phenotype of these cells is modulated by lipid uptake, Toll-like receptor ligands, hematopoietic growth factors, cytokines, and chemokines. In addition to newly recruited macrophages, it is likely that resident macrophages also change their phenotype. Monocyte-derived inflammatory macrophages have a short half-life. After undergoing apoptosis, they may be taken up by surrounding macrophages or, if the phagocytic capacity is overwhelmed, can undergo secondary necrosis, a key event in forming the necrotic core of atherosclerotic lesions. In this review, we discuss these and other processes associated with monocytic cell dynamics in the vascular wall and their role in the initiation and progression of atherosclerosis.

Monocyte trafficking in acute and chronic inflammation

Environmental signals at the site of inflammation mediate rapid monocyte mobilization and dictate differentiation programs whereby these cells give rise to macrophages or dendritic cells. Monocytes participate in tissue healing, clearance of pathogens and dead cells, and initiation of adaptive immunity. However, recruited monocytes can also contribute to the pathogenesis of infection and chronic inflammatory disease, such as atherosclerosis. Here, we explore monocyte trafficking in the context of acute inflammation, relying predominantly on data from microbial infection models. These mechanisms will be compared to monocyte trafficking during chronic inflammation in experimental models of atherosclerosis. Recent developments suggest that monocyte trafficking shares common themes in diverse inflammatory diseases; however, important differences exist between monocyte migratory pathways in acute and chronic inflammation.

Fractalkine is a novel human adipochemokine associated with type 2 diabetes

OBJECTIVE

Leukocyte infiltration of adipose is a critical determinant of obesity-related metabolic diseases. Fractalkine (CX3CL1) and its receptor (CX3CR1) comprise a chemokine system involved in leukocyte recruitment and adhesion in atherosclerosis, but its role in adipose inflammation and type 2 diabetes is unknown.

RESEARCH DESIGN AND METHODS

CX3CL1 mRNA and protein were quantified in subcutaneous adipose and blood during experimental human endotoxemia and in lean and obese human adipose. CX3CL1 cellular source was probed in human adipocytes, monocytes, and macrophages, and CX3CL1-blocking antibodies were used to assess its role in monocyte-adipocyte adhesion. The association of genetic variation in CX3CR1 with metabolic traits was determined in a community-based sample. Finally, plasma CX3CL1 levels were measured in a case-control study of type 2 diabetes.

RESULTS

Endotoxemia induced adipose CX3CL1 mRNA (32.7-fold, P < 1 × 10(-5)) and protein (43-fold, P = 0.006). Obese subjects had higher CX3CL1 levels in subcutaneous adipose compared with lean (0.420 ± 0.387 vs. 0.228 ± 0.187 ng/mL, P = 0.04). CX3CL1 was expressed and secreted by human adipocytes and stromal vascular cells. Inflammatory cytokine induction of CX3CL1 in human adipocytes (27.5-fold mRNA and threefold protein) was completely attenuated by pretreatment with a peroxisome proliferator-activated receptor-γ agonist. A putative functional nonsynonymous single nucleotide polymorphism (rs3732378) in CX3CR1 was associated with adipose and metabolic traits, and plasma CX3CL1 levels were increased in patients with type 2 diabetes vs. nondiabetics (0.506 ± 0.262 vs. 0.422 ± 0.210 ng/mL, P < 0.0001).

CONCLUSIONS

CX3CL1-CX3CR1 is a novel inflammatory adipose chemokine system that modulates monocyte adhesion to adipocytes and is associated with obesity, insulin resistance, and type 2 diabetes. These data provide support for CX3CL1 as a diagnostic and therapeutic target in cardiometabolic disease.

Suppressed monocyte recruitment drives macrophage removal from atherosclerotic plaques of Apoe-/- mice during disease regression

Experimental models of atherosclerosis suggest that recruitment of monocytes into plaques drives the progression of this chronic inflammatory condition. Cholesterol-lowering therapy leads to plaque stabilization or regression in human atherosclerosis, characterized by reduced macrophage content, but the mechanisms that underlie this reduction are incompletely understood. Mice lacking the gene Apoe (Apoe-/- mice) have high levels of cholesterol and spontaneously develop atherosclerotic lesions. Here, we treated Apoe-/- mice with apoE-encoding adenoviral vectors that induce plaque regression, and investigated whether macrophage removal from plaques during this regression resulted from quantitative alterations in the ability of monocytes to either enter or exit plaques. Within 2 days after apoE complementation, plasma cholesterol was normalized to wild-type levels, and HDL levels were increased 4-fold. Oil red O staining and quantitative mass spectroscopy revealed that esterified cholesterol content was markedly reduced. Plaque macrophage content decreased gradually and was 72% lower than baseline 4 weeks after apoE complementation. Importantly, this reduction in macrophages did not involve migratory egress from plaques or CCR7, a mediator of leukocyte emigration. Instead, marked suppression of monocyte recruitment coupled with a stable rate of apoptosis accounted for loss of plaque macrophages. These data suggest that therapies to inhibit monocyte recruitment to plaques may constitute a more viable strategy to reduce plaque macrophage burden than attempts to promote migratory egress.

Fractalkine/CX3CR1 and atherosclerosis

Fractalkine is a unique chemokine which has both adhesive and chemoattractant functions. With the increasing emphasis on the importance of inflammation in atherosclerosis, more attention has been focused on the role of chemokines in atherosclerosis. It has been shown that fractalkine/CX3CR1 participates in the atherosclerotic pathological process through mediating the recruitment of leukocytes and the interaction of vascular cells and leukocytes. Some signal pathways are simultaneously activated through fractalkine/CX3CR1 coupling to promote the inflammatory response in atherosclerotic vessels. Additionally, fractalkine has cytotoxic effects on endothelium as well as anti-apoptosis and proliferative effects on vascular cells which consequently changes plaque components and stability in plaque. Several studies have showed that fractalkine or CX3CR1 deficiency in atherosclerotic mice would ameliorate the severity of plaque. Population studies on CX3CR1 polymorphism have confirmed that 280M-containing haplotype is associated with reduced risk of atherosclerotic disease. Despite the apparent association with atherosclerosis, further studies on fractalkine/CX3CR1 chemokine pair are clearly warranted to more fully elucidate this relationship.

Emerging concepts in myeloid cell biology after spinal cord injury

Traumatic spinal cord injury (SCI) affects the activation, migration, and function of microglia, neutrophils and monocyte/macrophages. Because these myeloid cells can positively and negatively affect survival of neurons and glia, they are among the most commonly studied immune cells. However, the mechanisms that regulate myeloid cell activation and recruitment after SCI have not been adequately defined. In general, the dynamics and composition of myeloid cell recruitment to the injured spinal cord are consistent between mammalian species; only the onset, duration, and magnitude of the response vary. Emerging data, mostly from rat and mouse SCI models, indicate that resident and recruited myeloid cells are derived from multiple sources, including the yolk sac during development and the bone marrow and spleen in adulthood. After SCI, a complex array of chemokines and cytokines regulate myelopoiesis and intraspinal trafficking of myeloid cells. As these cells accumulate in the injured spinal cord, the collective actions of diverse cues in the lesion environment help to create an inflammatory response marked by tremendous phenotypic and functional heterogeneity. Indeed, it is difficult to attribute specific reparative or injurious functions to one or more myeloid cells because of convergence of cell function and difficulties in using specific molecular markers to distinguish between subsets of myeloid cell populations. Here we review each of these concepts and include a discussion of future challenges that will need to be overcome to develop newer and improved immune modulatory therapies for the injured brain or spinal cord.

Analyses of phenotypic and functional characteristics of CX3CR1-expressing natural killer cells

We previously demonstrated a correlation between the frequency of CX3CR1-expressing human natural killer (NK) cells and disease activity in multiple sclerosis and showed that CX3CR1(high) NK cells were more cytotoxic than their CX3CR1(neg/low) counterparts. Here we aimed to determine whether human NK cell fractions defined by CX3CR1 represent distinct subtypes. Phenotypic and functional NK cell analyses revealed that, distinct from CX3CR1(high), CX3CR1(neg/low) NK cells expressed high amounts of type 2 cytokines, proliferated robustly in response to interleukin-2 and promoted a strong up-regulation of the key co-stimulatory molecule CD40 on monocytes. Co-expression analyses of CX3CR1 and CD56 demonstrated the existence of different NK cell fractions based on the surface expression of these two surface markers, the CX3CR1(neg) CD56(bright), CX3CR1(neg) CD56(dim) and CX3CR1(high) CD56(dim) fractions. Additional investigations on the expression of NK cell receptors (KIR, NKG2A, NKp30 and NKp46) and the maturation markers CD27, CD62L and CD57 indicated that CX3CR1 expression of CD56(dim) discriminated between an intermediary CX3CR1(neg)  CD56(dim) and fully mature CX3CR1(high) CD56(dim) NK cell fractions. Hence, CX3CR1 emerges as an additional differentiation marker that may link NK cell maturation with the ability to migrate to different organs including the central nervous system.

Analyses of phenotypic and functional characteristics of CX3CR1-expressing natural killer cells

We previously demonstrated a correlation between the frequency of CX3CR1-expressing human natural killer (NK) cells and disease activity in multiple sclerosis and showed that CX3CR1(high) NK cells were more cytotoxic than their CX3CR1(neg/low) counterparts. Here we aimed to determine whether human NK cell fractions defined by CX3CR1 represent distinct subtypes. Phenotypic and functional NK cell analyses revealed that, distinct from CX3CR1(high), CX3CR1(neg/low) NK cells expressed high amounts of type 2 cytokines, proliferated robustly in response to interleukin-2 and promoted a strong up-regulation of the key co-stimulatory molecule CD40 on monocytes. Co-expression analyses of CX3CR1 and CD56 demonstrated the existence of different NK cell fractions based on the surface expression of these two surface markers, the CX3CR1(neg) CD56(bright), CX3CR1(neg) CD56(dim) and CX3CR1(high) CD56(dim) fractions. Additional investigations on the expression of NK cell receptors (KIR, NKG2A, NKp30 and NKp46) and the maturation markers CD27, CD62L and CD57 indicated that CX3CR1 expression of CD56(dim) discriminated between an intermediary CX3CR1(neg)  CD56(dim) and fully mature CX3CR1(high) CD56(dim) NK cell fractions. Hence, CX3CR1 emerges as an additional differentiation marker that may link NK cell maturation with the ability to migrate to different organs including the central nervous system.

The CD14(+/low)CD16(+) monocyte subset is more susceptible to spontaneous and oxidant-induced apoptosis than the CD14(+)CD16(-) subset

Human monocytes can be classified into two subsets with distinctive characteristics. In this study, we report a difference in apoptotic potential between these two subsets with CD14^+/lowCD16⁺ monocytes being more susceptible than CD14⁺CD16⁻ monocytes to undergo spontaneous apoptosis and apoptosis induced by reactive oxygen species (ROS). By global transcriptomic and proteomic approaches, we observed that CD14^+/lowCD16⁺ monocytes expressed higher levels of pro-apoptotic genes and proteins such as TNFα, caspase 3, Bax and cytochrome c and showed more caspases 3 and 7 activities. They also exhibited greater aerobic respiration resulting in a higher production of ROS from the mitochondria. CD14⁺CD16⁻ monocytes, in contrast, showed higher expression of glutathione (GSH)-metabolizing genes such as GSH peroxidase and microsomal GSH S-transferase and were more resistant to oxidative stress than CD14^+/lowCD16⁺ monocytes. The apoptosis of CD14^+/lowCD16⁺ monocytes was ROS dependent as reducing ROS levels significantly reduced cell death. This is the first report of a differential apoptotic propensity of human monocyte subsets, and gaining a better understanding of this process may help to provide a better understanding of the roles of these subsets during homeostasis and under pathological conditions, particularly in situations in which high levels of oxidants are present.

Hyperlipidemia-Triggered Neutrophilia Promotes Early Atherosclerosis

Background—

Inflammation and activation of immune cells are key mechanisms in the development of atherosclerosis. Previous data indicate important roles for monocytes and T lymphocytes in lesion formation, whereas the contribution of neutrophils remains to be firmly established. Here, we investigate the effect of hypercholesterolemia on peripheral neutrophil counts, neutrophil recruitment to atherosclerotic lesions, and the importance of neutrophils in atherosclerotic lesion formation in Apoe −/− mice.

Methods and Results—

Hypercholesterolemia induces neutrophilia, which was attributable to enhanced granulopoiesis and enhanced mobilization from the bone marrow. The degree of hypercholesterolemia-induced neutrophilia was positively correlated with the extent of early atherosclerotic lesion formation. In turn, neutropenic mice display reduced plaque sizes at early but not late stages of atherosclerotic lesion formation. Flow cytometry of enzymatically digested aortas further shows altered cellular plaque composition in neutropenic mice with reduced numbers of inflammatory monocytes and macrophages. Aortic neutrophil infiltration peaks 4 weeks after the start of a high-fat diet and decreases afterward. The recruitment of neutrophils to large arteries was found to depend on CCR1, CCR2, CCR5, and CXCR2, which contrasts to peripheral venous recruitment, which requires CCR2 and CXCR2 only. The involvement of CCR1 and CCR5 corresponded to the endothelial deposition of the platelet-derived chemokine CCL5 in arteries but not in veins.

Conclusions—

Our data provide evidence that hypercholesterolemia-induced neutrophilia is multifactorial and that neutrophils infiltrate arteries primarily during early stages of atherosclerosis. Collectively, these data suggest an important role of neutrophils in the initiation of atherosclerosis.

The fractalkine receptor CX₃CR1 protects against liver fibrosis by controlling differentiation and survival of infiltrating hepatic monocytes

Chemokines modulate inflammatory responses that are prerequisites for organ fibrosis upon liver injury. Monocyte-derived hepatic macrophages are critical for the development, maintenance, and resolution of hepatic fibrosis. The specific role of monocyte-associated chemokine (C-X3-C motif) receptor 1 (CX₃CR1) and its cognate ligand fractalkine [chemokine (C-X3-C motif) ligand 1)] in liver inflammation and fibrosis is currently unknown. We examined 169 patients with chronic liver diseases and 84 healthy controls; we found that CX₃CL1 is significantly up-regulated in the circulation upon disease progression, whereas CX₃CR1 is down-regulated intrahepatically in patients with advanced liver fibrosis or cirrhosis. To analyze the functional relevance of this pathway, two models of experimental liver fibrosis were applied to wild-type (WT) and CX₃CR1-deficient mice. Fractalkine expression was induced upon liver injury in mice, primarily in hepatocytes and hepatic stellate cells. CX₃CR1(-/-) animals developed greater hepatic fibrosis than WT animals with carbon tetrachloride-induced and bile duct ligation-induced fibrosis. CX₃CR1(-/-) mice displayed significantly increased numbers of monocyte-derived macrophages within the injured liver. Chimeric animals that underwent bone marrow transplantation revealed that CX₃CR1 restricts hepatic fibrosis progression and monocyte accumulation through mechanisms exerted by infiltrating immune cells. In the absence of CX₃CR1, intrahepatic monocytes develop preferentially into proinflammatory tumor necrosis factor-producing and inducible nitric oxide synthase-producing macrophages. CX₃CR1 represents an essential survival signal for hepatic monocyte-derived macrophages by activating antiapoptotic bcl2 expression. Monocytes/macrophages lacking CX₃CR1 undergo increased cell death after liver injury, which then perpetuates inflammation, promotes prolonged inflammatory monocyte infiltration into the liver, and results in enhanced liver fibrosis.

CONCLUSION

CX₃CR1 limits liver fibrosis in vivo by controlling the differentiation and survival of intrahepatic monocytes. The opposing regulation of CX₃CR1 and fractalkine in patients suggests that pharmacological augmentation of this pathway may represent a possible therapeutic antifibrotic strategy.

CX3CR1 is required for airway inflammation by promoting T helper cell survival and maintenance in inflamed lung

Allergic asthma is a T helper type 2 (T(H)2)-dominated disease of the lung. In people with asthma, a fraction of CD4(+) T cells express the CX3CL1 receptor, CX3CR1, and CX3CL1 expression is increased in airway smooth muscle, lung endothelium and epithelium upon allergen challenge. Here we found that untreated CX3CR1-deficient mice or wild-type (WT) mice treated with CX3CR1-blocking reagents show reduced lung disease upon allergen sensitization and challenge. Transfer of WT CD4(+) T cells into CX3CR1-deficient mice restored the cardinal features of asthma, and CX3CR1-blocking reagents prevented airway inflammation in CX3CR1-deficient recipients injected with WT T(H)2 cells. We found that CX3CR1 signaling promoted T(H)2 survival in the inflamed lungs, and injection of B cell leukemia/lymphoma-2 protein (BCl-2)-transduced CX3CR1-deficient T(H)2 cells into CX3CR1-deficient mice restored asthma. CX3CR1-induced survival was also observed for T(H)1 cells upon airway inflammation but not under homeostatic conditions or upon peripheral inflammation. Therefore, CX3CR1 and CX3CL1 may represent attractive therapeutic targets in asthma.

Cell death in the neighbourhood: direct microenvironmental effects of apoptosis in normal and neoplastic tissues

Here we consider the impact of the physiological cell-death programme on normal tissue homeostasis and on disease pathogenesis, with particular reference to evolution and progression of neoplasia. We seek to describe the direct contributions played by apoptosis in creating the microenvironments of normal and malignant tissues and to discuss the molecular mechanisms underlying the elements of the '3Rs' that define the meaning of apoptosis: recognition, response, and removal. Apoptotic cells elicit responses in other cell types-both phagocytic and non-phagocytic-through short- and long-range signalling modes that range from direct contact to intercellular communication via membrane-bound microparticles. Such cellular responses include migration, proliferation, and differentiation, as well as production of immunomodulatory and anti-inflammatory mediators together with, in the case of phagocytes, engulfment, and breakdown of apoptotic cells. In normal tissues, the removal of apoptotic cells is rapid and typically non-phlogistic. We discuss the importance of this clearance process in tissue homeostasis and the consequences of its failure in disease pathogenesis. Using the typical cell culture environment in vitro as an illustrative example in which apoptosis occurs commonly in the absence of the removal mechanisms, we also discuss the inhibitory effects of persistent apoptotic cells on their otherwise viable neighbours. Since apoptosis is a common and sustained event in high-grade malignancies, we hypothesize on its purposeful role in conditioning the tumour microenvironment. We propose that apoptosis subserves several pro-tumour functions-trophic, anti-inflammatory, and immunomodulatory-and we identify strategies targeting host responses to apoptotic cells as promising modes of future therapies that could be applied to multiple cancer types in which tumour-cell apoptosis is active.

Novel characterization of monocyte-derived cell populations in the meninges and choroid plexus and their rates of replenishment in bone marrow chimeric mice

The mouse dura mater, pia mater, and choroid plexus contain resident macrophages and dendritic cells (DCs). These cells participate in immune surveillance, phagocytosis of cellular debris, uptake of antigens from the surrounding cerebrospinal fluid and immune regulation in many pathologic processes. We used Cx3cr1 knock-in, CD11c-eYFP transgenic and bone marrow chimeric mice to characterize the phenotype, density and replenishment rate of monocyte-derived cells in the meninges and choroid plexus and to assess the role of the chemokine receptor CX3CR1 on their number and tissue distribution. Iba-1 major histocompatibility complex (MHC) Class II CD169 CD68 macrophages and CD11c putative DCs were identified in meningeal and choroid plexus whole mounts. Comparison of homozygous and heterozygous Cx3cr1 mice did not reveal CX3CR1-dependancy on density, distribution or phenotype of monocyte-derived cells. In turnover studies, wild type lethally irradiated mice were reconstituted with Cx3cr1/-positive bone marrow and were analyzed at 3 days, 1, 2, 4 and 8 weeks after transplantation. There was a rapid replenishment of CX3CR1-positive cells in the dura mater (at 4 weeks) and the choroid plexus was fully reconstituted by 8 weeks. These data provide the foundation for future studies on the role of resident macrophages and DCs in conditions such as meningitis, autoimmune inflammatory disease and in therapies involving irradiation and hematopoietic or stem cell transplantation.

CX3CL1-CX3CR1 interaction prevents carbon tetrachloride-induced liver inflammation and fibrosis in mice

Chronic liver disease is associated with hepatocyte injury, inflammation, and fibrosis. Chemokines and chemokine receptors are key factors for the migration of inflammatory cells such as macrophages and noninflammatory cells such as hepatic stellate cells (HSCs). The expression of CX3CR1 and its ligand, CX3CL1, is up-regulated in chronic liver diseases such as chronic hepatitis C. However, the precise role of CX3CR1 in the liver is still unclear. Here we investigated the role of the CX3CL1-CX3CR1 interaction in a carbon tetrachloride (CCl(4))-induced liver inflammation and fibrosis model. CX3CR1 was dominantly expressed in Kupffer cells in the liver. In contrast, the main source of CX3CL1 was HSCs. Mice deficient in CX3CR1 showed significant increases in inflammatory cell recruitment and cytokine production [including tumor necrosis factor α (TNF-α); monocyte chemoattractant protein 1; macrophage inflammatory protein 1β; and regulated upon activation, normal T cell expressed, and secreted (RANTES)] after CCl(4) treatment versus wild-type (WT) mice. This suggested that CX3CR1 signaling prevented liver inflammation. Kupffer cells in CX3CR1-deficient mice after CCl(4) treatment showed increased expression of TNF-α and transforming growth factor β and reduced expression of the anti-inflammatory markers interleukin-10 (IL-10) and arginase-1. Coculture experiments showed that HSCs experienced significantly greater activation by Kupffer cells from CCl(4)-treated CX3CR1-deficient mice versus WT mice. Indeed, augmented fibrosis was observed in CX3CR1-deficient mice versus WT mice after CCl(4) treatment. Finally, CX3CL1 treatment induced the expression of IL-10 and arginase-1 in WT cultured Kupffer cells through CX3CR1, which in turn suppressed HSC activation.

CONCLUSION

The CX3CL1-CX3CR1 interaction inhibits inflammatory properties in Kupffer cells/macrophages and results in decreased liver inflammation and fibrosis.

Resident intimal dendritic cells and the initiation of atherosclerosis

PURPOSE OF REVIEW

To highlight the fact that regional differences in the normal arterial intima are critical to atherosclerotic lesion formation driven by systemic risk factors.

RECENT FINDINGS

At arterial curvatures, bifurcations and branches unique hemodynamics influence endothelial cell signaling and gene expression patterns, which create a proinflammatory environment, with low-grade recruitment of monocytes and accumulation of cells with dendritic features in the intima. Upon induction of hypercholesterolemia, these resident intimal dendritic cells initiate atherosclerosis by rapidly engulfing lipid and becoming the first foam cells in nascent lesions. This step precedes endothelial cell activation and increased monocyte recruitment.

SUMMARY

The unique features of the arterial intima at atherosclerosis-susceptible sites do not lead to disease under normal physiological conditions, but this intimal environment promotes the initiation of atherogenesis upon induction of systemic risk factors such as hypercholesterolemia.

Therapeutic implications of chemokine-mediated pathways in atherosclerosis: realistic perspectives and utopias

Current perspectives on the pathogenesis of atherosclerosis strongly support the involvement of inflammatory mediators in the establishment and progression of atherosclerostic lesions. Chemokine-mediated mechanisms are potent regulators of such processes by orchestrating the interactions of inflammatory cellular components of the peripheral blood with cellular components of the arterial wall. The increasing evidence supporting the role of chemokine pathways in atherosclerosis renders chemokine ligands and their receptors potential therapeutic targets. In the following review, we aim to highlight the special structural and functional features of chemokines and their receptors in respect to their roles in atherosclerosis, and examine to what extent available data can be applied in disease management practices.

The multiple roles of monocyte subsets in steady state and inflammation

Monocytes participate importantly in immunity. Produced in the bone marrow and released into the blood, they circulate in blood or reside in a spleen reservoir before entering tissue and giving rise to macrophages or dendritic cells. Monocytes are more than transitional cells that adapt to a particular tissue environment indiscriminately. Accumulating evidence now indicates that monocytes are heterogeneous in several species and are themselves predetermined for particular function in the steady state and inflammation. Future therapeutics may harness this heterogeneity to target harmful functions while sparing those that are beneficial. Here, we review recent advances on the ontogeny and function of monocytes and their subsets in humans and mice.

Bone marrow chimeric mice reveal a role for CX₃CR1 in maintenance of the monocyte-derived cell population in the olfactory neuroepithelium

Macrophages in the olfactory neuroepithelium are thought to play major roles in tissue homeostasis and repair. However, little information is available at present about possible heterogeneity of these monocyte-derived cells, their turnover rates, and the role of chemokine receptors in this process. To start addressing these issues, this study used Cx₃cr1(gfp) mice, in which the gene sequence for eGFP was knocked into the CX₃CR1 gene locus in the mutant allele. Using neuroepithelial whole-mounts from Cx₃cr1(gfp/+) mice, we show that eGFP(+) cells of monocytic origin are distributed in a loose network throughout this tissue and can be subdivided further into two immunophenotypically distinct subsets based on MHC-II glycoprotein expression. BM chimeric mice were created using Cx₃cr1(gfp/+) donors to investigate turnover of macrophages (and other monocyte-derived cells) in the olfactory neuroepithelium. Our data indicate that the monocyte-derived cell population in the olfactory neuroepithelium is actively replenished by circulating monocytes and under the experimental conditions, completely turned over within 6 months. Transplantation of Cx₃cr1(gfp/gfp) (i.e., CX₃CR1-deficient) BM partially impaired the replenishment process and resulted in an overall decline of the total monocyte-derived cell number in the olfactory epithelium. Interestingly, replenishment of the CD68(low)MHC-II(+) subset appeared minimally affected by CX₃CR1 deficiency. Taken together, the established baseline data about heterogeneity of monocyte-derived cells, their replenishment rates, and the role of CX₃CR1 provide a solid basis to further examine the importance of different monocyte subsets for neuroregeneration at this unique frontier with the external environment.

Age-dependent alterations of monocyte subsets and monocyte-related chemokine pathways in healthy adults

Background

Recent experimental approaches have unraveled essential migratory and functional differences of monocyte subpopulations in mice. In order to possibly translate these findings into human physiology and pathophysiology, human monocyte subsets need to be carefully revisited in health and disease. In analogy to murine studies, we hypothesized that human monocyte subsets dynamically change during ageing, potentially influencing their functionality and contributing to immunosenescence.

Results

Circulating monocyte subsets, surface marker and chemokine receptor expression were analyzed in 181 healthy volunteers (median age 42, range 18-88). Unlike the unaffected total leukocyte or total monocyte counts, non-classical CD14⁺CD16⁺ monocytes significantly increased with age, but displayed reduced HLA-DR and CX₃CR1 surface expression in the elderly. Classical CD14⁺⁺CD16^- monocyte counts did not vary dependent on age. Serum MCP-1 (CCL2), but not MIP1α (CCL3), MIP1β (CCL4) or fractalkine (CX₃CL1) concentrations increased with age. Monocyte-derived macrophages from old or young individuals did not differ with respect to cytokine release in vitro at steady state or upon LPS stimulation.

Conclusions

Our study demonstrates dynamic changes of circulating monocytes during ageing in humans. The expansion of the non-classical CD14⁺CD16⁺ subtype, alterations of surface protein and chemokine receptor expression as well as circulating monocyte-related chemokines possibly contribute to the preserved functionality of the monocyte pool throughout adulthood.

Distinct role of the intracellular C-terminus for subcellular expression, shedding and function of the murine transmembrane chemokine CX3CL1

The transmembrane chemokine CX3CL1 is expressed on the endothelial surface and promotes leukocyte adhesion and transmigration by receptor interaction via its extracellular chemokine domain. Since little is known about its intracellular C-terminus, we examined the consequences of C-terminal truncation on cellular distribution, proteolytic shedding and function of murine CX3CL1. Full length murine CX3CL1 was expressed and shed by the metalloproteinase ADAM10 as described for human CX3CL1. Truncation of murine CX3CL1 led to reduced maturation and impaired trafficking to the surface. Truncation of CX3CL1 also abrogated localization to early endosomal vesicles, but increased shedding from the surface by ADAM10. Once truncated CX3CL1 was expressed on the surface, it mediated cell contact and induced leukocyte transmigration similar as full length CX3CL1. These data suggest that the C-terminus of CX3CL1 carries important determinants for cellular trafficking but not for function of the chemokine during leukocyte recruitment.

Monocytes: subsets, origins, fates and functions

PURPOSE OF REVIEW

The term mono-cyte suggests this population of cells consists of a single homogenous fraction. However, evidence from a number of laboratories indicates that monocytes are composed of several subsets, which differ in phenotype, size, nuclear morphology, granularity and gene profiles. Most importantly, recent data suggest that monocyte subsets are also functionally distinct. Here we summarize the recent advances in our understanding of monocyte subsets and their origins, fates and functions.

RECENT FINDINGS

The recent past has seen major progress in our understanding of myeloid differentiation. Specifically, the published literature now suggests a dichotomy that starts at the stage of a novel clonotypic bone marrow resident precursor, the macrophage dendritic cell progenitor (MDP). Insights into differential origins of macrophages and dendritic cells, linked with functional specifications, are likely to significantly change our current view of the mononuclear phagocyte system.

SUMMARY

Contemporary studies have demonstrated that two subsets of monocytes reside in the peripheral circulation. These subsets are surprisingly distinct; with regard to their functions and fates, for example, one subset might be dedicated to generate macrophages upon extravasation from the peripheral circulation, whereas, the other subset under inflammatory conditions may differentiate into inflammatory dendritic cells. The tissue response during pathogenesis seems to differentially mobilize these cells, thereby manipulating the local mononuclear phagocyte composition according to acute needs.

Resident Intimal Dendritic Cells Accumulate Lipid and Contribute to the Initiation of Atherosclerosis

Rationale : Atherosclerosis is an inflammatory disease in which leukocytes and oxidatively modified lipids accumulate in the arterial intima. Previously, we showed that dendritic cells (DCs) accumulate preferentially in regions predisposed to atherosclerosis in the normal murine aortic intima. The function of these cells in atherogenesis is unknown.

Objective : Our goal was to determine the role of resident intimal DCs in the initiation of atherosclerosis.

Methods and Results : En face immunostaining of nascent atherosclerotic lesions in low-density lipoprotein receptor–deficient ( Ldlr −/− ) mice fed a cholesterol-rich diet for 5 or 10 days demonstrated that foam cells expressed DC markers CD11c, 33D1, and major histocompatibility complex class II. Transmission electron microscopy revealed that the majority of intimal lipid was intracellular. The role of resident intimal DCs in lesion formation was verified by their conditional depletion using transgenic mice expressing the simian diphtheria toxin receptor in CD11c + cells. A single injection of diphtheria toxin depleted intimal CD11c + DCs by >98% within 24 hours, with 25% and 75% recovery at 1 and 3 weeks, respectively. When bred onto the Ldlr −/− background, intimal DC depletion with diphtheria toxin during 5 days of lesion formation reduced the intimal lipid area by 55% relative to undepleted controls. Transmission electron microscopy revealed few foam cells in DC-depleted mice and abundant accumulation of subendothelial extracellular lipid.

Conclusions : Induction of hypercholesterolemia in mice triggers rapid ingestion of lipid by resident intimal DCs, which initiate nascent foam cell lesion formation.

Enteric flora expands gut lamina propria CX3CR1+ dendritic cells supporting inflammatory immune responses under normal and inflammatory conditions

CD103 or CX(3)CR1 surface expression defines distinct dendritic cells (DCs) and macrophages in the murine lamina propria of the colon (cLP). We investigated the surface marker and functional phenotype of CD103(+) and CX(3)CR1(+) cLP DCs and their role in transfer colitis. cLP CD11c(+) cells were isolated from specific pathogen-free or germ-free mice to elucidate the role of the commensal flora in their development. The cLP CD11c(+) cells are a heterogeneous cell population that includes 16% CX(3)CR1(+), 34% CD103(+), 30% CD103(-)CX(3)CR1(-) DCs, and 17% CD68(+/)F4/80(+)CX(3)CR1(+)CD11c(+) macrophages. All DCs expressed high levels of MHC II but low levels of costimulatory (CD40, CD86, and CD80) and coinhibitory (programmed death ligand-1) molecules. Ex vivo confocal microscopy demonstrated that CX(3)CR1(+)CD11c(+) cells, but not CD103(+) DCs, were reduced in the cLP of germ-free (CX(3)CR1-GFP) mice. The absence of the enteric flora prevents the formation of transepithelial processes by the CX(3)CR1(+) DCs. CX(3)CR1(+) DCs preferentially supported Th1/Th17 CD4 T cell differentiation. CD103(+) DCs preferentially induced the differentiation of Foxp3-expressing regulatory T cells. The stimulation of cLP DCs with fractalkine/CX(3)CL1 increased the release of IL-6 and TNF-alpha. In the absence of CX(3)CR1, the CD45RB(high) CD4 transfer colitis was suppressed and associated with reduced numbers of DCs in the mesenteric lymph nodes and a reduction in serum IFN-gamma and IL-17. The local bacteria-driven accumulation of CX(3)CR1(+) DCs seems to support inflammatory immune responses.

Monocytes in atherosclerosis: subsets and functions

Chronic inflammation drives atherosclerosis, the leading cause of cardiovascular disease. Over the past two decades, data have emerged showing that immune cells are involved in the pathogenesis of atherosclerotic plaques. The accumulation and continued recruitment of leukocytes are associated with the development of 'vulnerable' plaques. These plaques are prone to rupture, leading to thrombosis, myocardial infarction or stroke, all of which are frequent causes of death. Plaque macrophages account for the majority of leukocytes in plaques, and are believed to differentiate from monocytes recruited from circulating blood. However, monocytes represent a heterogenous circulating population of cells. Experiments are needed to address whether monocyte recruitment to plaques and effector functions, such as the formation of foam cells, the production of nitric oxide and reactive oxygen species, and proteolysis are critical for the development and rupture of plaques, and thus for the pathophysiology of atherosclerosis, as well as elucidate the precise mechanisms involved.