Efficient renal recruitment of macrophages and T cells in mice lacking the duffy antigen/receptor for chemokines

The atypical chemokine receptor 2 reduces T cell expansion and tertiary lymphoid tissue but does not limit autoimmune organ injury in lupus-prone B6lpr mice

Introduction

The atypical chemokine receptor 2 (ACKR2) is a chemokine scavenger receptor, which limits inflammation and organ damage in several experimental disease models including kidney diseases. However, potential roles of ACKR2 in reducing inflammation and tissue injury in autoimmune disorders like systemic lupus erythematosus (SLE) and lupus nephritis are unknown, as well as its effects on systemic autoimmunity.

Methods

To characterize functional roles of ACKR2 in SLE, genetic Ackr2 deficiency was introduced into lupus-prone C57BL/6lpr (Ackr2-/- B6lpr) mice.

Results

Upon inflammatory stimulation in vitro, secreted chemokine levels increased in Ackr2 deficient tubulointerstitial tissue but not glomeruli. Moreover, Ackr2 expression was induced in kidneys and lungs of female C57BL/6lpr mice developing SLE. However, female Ackr2-/- B6lpr mice at 28 weeks of age showed similar renal functional parameters as wildtype (WT)-B6lpr mice. Consistently, assessment of activity and chronicity indices for lupus nephritis revealed comparable renal injury. Interestingly, Ackr2-/- B6lpr mice showed significantly increased renal infiltrates of CD3+ T and B cells, but not neutrophils, macrophages or dendritic cells, with T cells predominantly accumulating in the tubulointerstitial compartment of Ackr2-/- B6lpr mice. In addition, histology demonstrated significantly increased peribronchial lung infiltrates of CD3+ T cells in Ackr2-/- B6lpr mice. Despite this, protein levels of pro-inflammatory chemokines and mRNA expression of inflammatory mediators were not different in kidneys and lungs of WT- and Ackr2-/- B6lpr mice. This data suggests compensatory mechanisms for sufficient chemokine clearance in Ackr2-deficient B6lpr mice in vivo. Analysis of systemic autoimmune responses revealed comparable levels of circulating lupus-associated autoantibodies and glomerular immunoglobulin deposition in the two genotypes. Interestingly, similar to kidney and lung CD4+ T cell numbers and activation were significantly increased in spleens of Ackr2-deficient B6lpr mice. In lymph nodes of Ackr2-/- B6lpr mice abundance of activated dendritic cells decreased, but CD4+ T cell numbers were comparable to WT. Moreover, increased plasma levels of CCL2 were present in Ackr2-/- B6lpr mice, which may facilitate T cell mobilization into spleens and peripheral organs.

Discussion

In summary, we show that ACKR2 prevents expansion of T cells and formation of tertiary lymphoid tissue, but is not essential to limit autoimmune tissue injury in lupus-prone B6lpr mice.

Up-regulated PLA2G10 in cancer impairs T cell infiltration to dampen immunity

T cells are often absent from human cancer tissues during both spontaneously induced immunity and therapeutic immunotherapy, even in the presence of a functional T cell-recruiting chemokine system, suggesting the existence of T cell exclusion mechanisms that impair infiltration. Using a genome-wide in vitro screening platform, we identified a role for phospholipase A2 group 10 (PLA2G10) protein in T cell exclusion. PLA2G10 up-regulation is widespread in human cancers and is associated with poor T cell infiltration in tumor tissues. PLA2G10 overexpression in immunogenic mouse tumors excluded T cells from infiltration, resulting in resistance to anti-PD-1 immunotherapy. PLA2G10 can hydrolyze phospholipids into small lipid metabolites, thus inhibiting chemokine-mediated T cell mobility. Ablation of PLA2G10's enzymatic activity enhanced T cell infiltration and sensitized PLA2G10-overexpressing tumors to immunotherapies. Our study implicates a role for PLA2G10 in T cell exclusion from tumors and suggests a potential target for cancer immunotherapy.

Extratubular Polymerized Uromodulin Induces Leukocyte Recruitment and Inflammation In Vivo

Uromodulin (UMOD) is produced and secreted by tubular epithelial cells. Secreted UMOD polymerizes (pUMOD) in the tubular lumen, where it regulates salt transport and protects the kidney from bacteria and stone formation. Under various pathological conditions, pUMOD accumulates within the tubular lumen and reaches extratubular sites where it may interact with renal interstitial cells. Here, we investigated the potential of extratubular pUMOD to act as a damage associated molecular pattern (DAMP) molecule thereby creating local inflammation. We found that intrascrotal and intraperitoneal injection of pUMOD induced leukocyte recruitment in vivo and led to TNF-α secretion by F4/80 positive macrophages. Additionally, pUMOD directly affected vascular permeability and increased neutrophil extravasation independent of macrophage-released TNF-α. Interestingly, pUMOD displayed no chemotactic properties on neutrophils, did not directly activate β2 integrins and did not upregulate adhesion molecules on endothelial cells. In obstructed neonatal murine kidneys, we observed extratubular UMOD accumulation in the renal interstitium with tubular atrophy and leukocyte infiltrates. Finally, we found extratubular UMOD deposits associated with peritubular leukocyte infiltration in kidneys from patients with inflammatory kidney diseases. Taken together, we identified extratubular pUMOD as a strong inducer of leukocyte recruitment, underlining its critical role in mounting an inflammatory response in various kidneys pathologies.

C-C chemokine receptor type 2 mediates glomerular injury and interstitial fibrosis in focal segmental glomerulosclerosis

BACKGROUND

Glomerulosclerosis and tubulointerstitial fibrosis are hallmarks of chronic kidney injury leading to end-stage renal disease. Inflammatory mechanisms contribute to glomerular and interstitial scarring, including chemokine-mediated recruitment of leucocytes. In particular, accumulation of C-C chemokine receptor type 2 (CCR2)-expressing macrophages promotes renal injury and fibrotic remodelling in diseases like glomerulonephritis and diabetic nephropathy. The functional role of CCR2 in the initiation and progression of primary glomerulosclerosis induced by podocyte injury remains to be characterized.

METHODS

We analysed glomerular expression of CCR2 and its chemokine ligand C-C motif chemokine ligand 2 (CCL2) in human focal segmental glomerulosclerosis (FSGS). Additionally, CCL2 expression was determined in stimulated murine glomeruli and glomerular cells in vitro. To explore pro-inflammatory and profibrotic functions of CCR2 we induced adriamycin nephropathy, a murine model of FSGS, in BALB/c wild-type and Ccr2-deficient mice.

RESULTS

Glomerular expression of CCR2 and CCL2 significantly increased in human FSGS. In adriamycin-induced FSGS, progressive glomerular scarring and reduced glomerular nephrin expression was paralleled by induced glomerular expression of CCL2. Adriamycin exposure stimulated secretion of CCL2 and tumour necrosis factor-α (TNF) in isolated glomeruli and mesangial cells and CCL2 in parietal epithelial cells. In addition, TNF induced CCL2 expression in all glomerular cell populations, most prominently in podocytes. In vivo, Ccr2-deficient mice with adriamycin nephropathy showed reduced injury, macrophage and fibrocyte infiltration and inflammation in glomeruli and the tubulointerstitium. Importantly, glomerulosclerosis and tubulointerstitial fibrosis were significantly ameliorated.

CONCLUSIONS

Our data indicate that CCR2 is an important mediator of glomerular injury and progression of FSGS. CCR2- targeting therapies may represent a novel approach for its treatment.

Proximal Tubule Translational Profiling during Kidney Fibrosis Reveals Proinflammatory and Long Noncoding RNA Expression Patterns with Sexual Dimorphism

BACKGROUND

Proximal tubule injury can initiate CKD, with progression rates that are approximately 50% faster in males versus females. The precise transcriptional changes in this nephron segment during fibrosis and potential differences between sexes remain undefined.

METHODS

We generated mice with proximal tubule-specific expression of an L10a ribosomal subunit protein fused with enhanced green fluorescent protein. We performed unilateral ureteral obstruction surgery on four male and three female mice to induce inflammation and fibrosis, collected proximal tubule-specific and bulk cortex mRNA at day 5 or 10, and sequenced samples to a depth of 30 million reads. We applied computational methods to identify sex-biased and shared molecular responses to fibrotic injury, including up- and downregulated long noncoding RNAs (lncRNAs) and transcriptional regulators, and used in situ hybridization to validate critical genes and pathways.

RESULTS

We identified >17,000 genes in each proximal tubule group, including 145 G-protein-coupled receptors. More than 700 transcripts were differentially expressed in the proximal tubule of males versus females. The >4000 genes displaying altered expression during fibrosis were enriched for proinflammatory and profibrotic pathways. Our identification of nearly 150 differentially expressed proximal tubule lncRNAs during fibrosis suggests they may have unanticipated regulatory roles. Network analysis prioritized proinflammatory and profibrotic transcription factors such as Irf1, Nfkb1, and Stat3 as drivers of fibrosis progression.

CONCLUSIONS

This comprehensive transcriptomic map of the proximal tubule revealed sexually dimorphic gene expression that may reflect sex-related disparities in CKD, proinflammatory gene modules, and previously unappreciated proximal tubule-specific bidirectional lncRNA regulation.

Sox7 is involved in antibody-dependent endothelial cell activation and renal allograft injury via the Jagged1-Notch1 pathway

BACKGROUND

Antibody-mediated rejection (AMR) can cause graft loss and reduces long-term graft survival after kidney transplantation. Human leukocyte antigen (HLA) and/or non-HLA antibodies play a key role in the pathogenesis of AMR by targeting the allograft epithelium via complement activation and complement-independent mechanisms. However, the precise mechanisms of AMR remain unclear and treatment is still limited.

METHODS

In this study, we investigated the role of the endothelial-associated transcription factor Sox7 in AMR, using the anti-HLA antibody W6/32, shRNA-mediated Sox7 knockdown, and by manipulating the Notch pathway. We used an in vitro human kidney glomerular endothelial cells (HKGECs) model and an in vivo MHC-mismatched kidney transplantation model.

RESULTS

Sox7 expression was upregulated and the Jagged1-Notch1 pathway was activated in HKGECs after W6/32 activation. W6/32 antibodies increased the expression of adhesion molecules (VCAM-1, ICAM-1), inflammatory cytokines (IL-6, TNF-α), and chemokines (CXCL8, CXCL10), and Sox7 knockdown and inhibition of the Notch pathway by DAPT significantly reduced these effects. Jagged1 overexpression rescued the inhibitory effects of Sox7 knockdown. In addition, Sox7 knockdown attenuated acute allograft kidney injury in mice and reduced the expression of adhesion molecules and Jagged1-Notch1 signaling after transplantation.

CONCLUSIONS

Our findings suggest that Sox7 plays an important role in mediating HLA I antibody-dependent endothelial cell activation and acute kidney allograft rejection via the Jagged1-Notch1 pathway. Manipulating Sox7 in donor organs may represent a useful treatment for AMR in solid organ transplantation.

The Atypical Chemokine Receptor 2 Limits Progressive Fibrosis after Acute Ischemic Kidney Injury

Following renal ischemia-reperfusion injury (IRI), resolution of inflammation allows tubular regeneration, whereas ongoing inflammatory injury mediated by infiltrating leukocytes leads to nephron loss and renal fibrosis, typical hallmarks of chronic kidney disease. Atypical chemokine receptor 2 (ACKR2) is a chemokine decoy receptor that binds and scavenges inflammatory CC chemokines and reduces local leukocyte accumulation. We hypothesized that ACKR2 limits leukocyte infiltration, inflammation, and fibrotic tissue remodeling after renal IRI, thus preventing progression to chronic kidney disease. Compared with wild type, Ackr2 deficiency increases CC chemokine ligand 2 levels in tumor necrosis factor-stimulated tubulointerstitial tissue in vitro. In Ackr2-deficient mice with early IRI 1 or 5 days after transient renal pedicle clamping, tubular injury was similar to wild type, although accumulation of mononuclear phagocytes increased in postischemic Ackr2^-/- kidneys. Regarding long-term outcomes, Ackr2^-/- kidneys displayed more tubular injury 5 weeks after IRI, which was associated with persistently increased renal infiltrates of mononuclear phagocytes, T cells, Ly6C^high inflammatory macrophages, and inflammation. Moreover, Ackr2 deficiency caused substantially aggravated renal fibrosis in Ackr2^-/- kidneys 5 weeks after IRI, shown by increased expression of matrix molecules, renal accumulation of α-smooth muscle actin-positive myofibroblasts, and bone marrow-derived fibrocytes. ACKR2 is important in limiting persistent inflammation, tubular loss, and renal fibrosis after ischemic acute kidney injury and, thus, can prevent progression to chronic renal disease.

The atypical chemokine receptor 2 limits renal inflammation and fibrosis in murine progressive immune complex glomerulonephritis

The atypical chemokine receptor 2 (ACKR2), also named D6, regulates local levels of inflammatory chemokines by internalization and degradation. To explore potential anti-inflammatory functions of ACKR2 in glomerulonephritis, we induced autologous nephrotoxic nephritis in C57/BL6 wild-type and Ackr2-deficient mice. Renal ACKR2 expression increased and localized to interstitial lymphatic endothelium during nephritis. At two weeks Ackr2^-/-mice developed increased albuminuria and urea levels compared to wild-type mice. Histological analysis revealed increased structural damage in the glomerular and tubulointerstitial compartments within Ackr2^-/- kidneys. This correlated with excessive renal leukocyte infiltration of CD4⁺ T cells and mononuclear phagocytes with increased numbers in the tubulointerstitium but not glomeruli in knockout mice. Expression of inflammatory mediators and especially markers of fibrotic tissue remodeling were increased along with higher levels of ACKR2 inflammatory chemokine ligands like CCL2 in nephritic Ackr2^-/- kidneys. In vitro, Ackr2 deficiency in TNF-stimulated tubulointerstitial tissue but not glomeruli increased chemokine levels. These results are in line with ACKR2 expression in interstitial lymphatic endothelial cells, which also assures efflux of activated leukocytes into regional lymph nodes. Consistently, nephritic Ackr2^-/- mice showed reduced adaptive cellular immune responses indicated by decreased regional T-cell activation. However, this did not prevent aggravated injury in the kidneys of Ackr2^-/- mice with nephrotoxic nephritis due to simultaneously increased tubulointerstitial chemokine levels, leukocyte infiltration and fibrosis. Thus, ACKR2 is important in limiting renal inflammation and fibrotic remodeling in progressive nephrotoxic nephritis. Hence, ACKR2 may be a potential target for therapeutic interventions in immune complex glomerulonephritis.

Reliable and High Efficiency Extraction of Kidney Immune Cells

Immune system activation occurs in multiple kidney diseases and pathophysiological processes. The immune system consists of both adaptive and innate components and multiple cell types. Sometimes, the cell type of interest is present in very low numbers among the large numbers of total cells isolated from the kidney. Hence, reliable and efficient isolation of kidney mononuclear cell populations is important in order to study the immunological problems associated with kidney diseases. Traditionally, tissue isolation of kidney mononuclear cells have been performed via enzymatic digestions using different varieties and strengths of collagenases/DNAses yielding varying numbers of viable immune cells. Recently, with the development of the mechanical tissue disruptors for single cell isolation, the collagenase digestion step is avoided and replaced by a simple mechanical disruption of the kidneys after extraction from the mouse. Herein, we demonstrate a simple yet efficient method for the isolation of kidney mononuclear cells for every day immune cell extractions. We further demonstrate an example of subset analysis of immune cells in the kidney. Importantly, this technique can be adapted to other soft and non-fibrous tissues such as the liver and brain.

S-Nitrosoglutathione ameliorates acute renal dysfunction in a rat model of lipopolysaccharide-induced sepsis

OBJECTIVE

Sepsis induces an inflammatory response that results in acute renal failure (ARF). The current study is to evaluate the role of S-Nitrosoglutathione (GSNO) in renoprotection from lipopolysaccharide (LPS)-induced sepsis.

METHODS

Rats were divided to three groups. First group received LPS (5 mg/kg body weight), second group was treated with LPS + GSNO (50 μg/kg body weight), and third group was administered with vehicle (saline). They were sacrificed on day 1 and 3 post-LPS injection. Serum levels of nitric oxide (NO), creatinine and blood urea nitrogen (BUN) were analysed. Tissue morphology, T lymphocyte infiltrations, and the expression of inflammatory (TNF-α, iNOS) and anti-inflammatory (IL-10) mediators as well as glutathione (GSH) levels were determined.

KEY FINDING

Lipopolysaccharide significantly decreased body weight and increased cellular T lymphocyte infiltration, caspase-3 and iNOS and decreased PPAR-γ in renal tissue. NO, creatinine and BUN were significantly elevated after LPS challenge, and they significantly decreased after GSNO treatment. TNF-α level was found significantly increased in LPS-treated serum and kidney. GSNO treatment of LPS-challenged rats decreased caspase-3, iNOS, TNF-α, T lymphocyte infiltration and remarkably increased levels of IL-10, PPAR-γ and GSH.

CONCLUSION

GSNO can be used as a renoprotective agent for the treatment of sepsis-induced acute kidney injury.

Compartment-specific flow cytometry for the analysis of TNF-mediated recruitment and activation of glomerular leukocytes in murine kidneys

Cytokines of the TNF superfamily, particularly TNF itself, are important mediators of inflammatory leukocyte recruitment and activation in parenchymal organs. In inflammatory kidney diseases, leukocytes accumulate in glomeruli and the tubulointerstitium, leading to glomerulonephritis and tubulointerstitial nephritis, respectively. In particular, glomeruli can be the target of organ-threatening leukocyte-mediated inflammation. As microvasculatures of the glomerulus and the tubulointerstitium differ markedly in their structural and functional properties, recruitment and subsequent activation of leukocytes to these sites occur via distinct mechanisms. To understand the pathways and mediators of leukocyte-driven inflammation in the kidney it is therefore essential to analyze glomerular and tubulointerstitial leukocyte recruitment in a compartment-specific way. The protocol presented here describes an easy and rapid technique that allows compartment-specific quantitation and qualitative analysis of leukocytes present in glomeruli and tubulointerstitial tissue by flow cytometry after separation of these tissue compartments.

Role of chemokines in proteinuric kidney disorders

Experimental and human studies have shown that proteinuria contributes to the progression of renal disease. Overexposure to filtered proteins promotes the expression and release of chemokines by tubular epithelial cells, thus leading to inflammatory cell recruitment and renal impairment. This review focuses on recent progress in cellular and molecular understanding of the role of chemokines in the pathogenesis of proteinuria-induced renal injury, as well as their clinical implications and therapeutic potential.

Immune regulation by atypical chemokine receptors

Chemokines have fundamental roles in regulating immune and inflammatory responses, primarily through their control of leukocyte migration and localization. The biological functions of chemokines are typically mediated by signalling through G protein-coupled chemokine receptors, but chemokines are also bound by a small family of atypical chemokine receptors (ACKRs), the members of which are unified by their inability to initiate classical signalling pathways after ligand binding. These ACKRs are emerging as crucial regulatory components of chemokine networks in a wide range of developmental, physiological and pathological contexts. In this Review, we discuss the biochemical and immunological properties of ACKRs and the potential unifying themes in this family, and we highlight recent studies that identify novel roles for these molecules in development , homeostasis, inflammatory disease, infection and cancer.

Duffy antigen receptor for chemokines regulates post-fracture inflammation

There is now considerable experimental data to suggest that inflammatory cells collaborate in the healing of skeletal fractures. In terms of mechanisms that contribute to the recruitment of inflammatory cells to the fracture site, chemokines and their receptors have received considerable attention. Our previous findings have shown that Duffy antigen receptor for chemokines (Darc), the non-classical chemokine receptor that does not signal, but rather acts as a scavenger of chemokines that regulate cell migration, is a negative regulator of peak bone density in mice. Furthermore, because Darc is expressed by inflammatory and endothelial cells, we hypothesized that disruption of Darc action will affect post-fracture inflammation and consequently will affect fracture healing. To test this hypothesis, we evaluated fracture healing in mice with targeted disruption of Darc and corresponding wild type (WT) control mice. We found that fracture callus cartilage formation was significantly greater (33%) at 7 days post-surgery in Darc-KO compared to WT mice. The increased cartilage was associated with greater Collagen (Col) II expression at 3 days post-fracture and Col-X at 7 days post-fracture compared to WT mice, suggesting that Darc deficiency led to early fracture cartilage formation and differentiation. We then compared the expression of cytokine and chemokine genes known to be induced during inflammation. Interleukin (Il)-1β, Il-6, and monocyte chemotactic protein 1 were all down regulated in the fractures derived from Darc-KO mice at one day post-fracture, consistent with an altered inflammatory response. Furthermore, the number of macrophages was significantly reduced around the fractures in Darc-KO compared to WT mice. Based on these data, we concluded that Darc plays a role in modulating the early inflammatory response to bone fracture and subsequent cartilage formation. However, the early cartilage formation was not translated with an early bone formation at the fracture site in Darc-KO compared to WT mice.

The effects of adenosine A2A receptor knockout on renal interstitial fibrosis in a mouse model of unilateral ureteral obstruction

Adenosine A2A receptor (A2AR) plays an important regulatory role in the processes of inflammation and fibrosis. However, it is unknown whether A2AR can mediate renal interstitial fibrosis (RIF). To evaluate the effect of genetic A2AR knockout (KO) on the pathological progress of RIF, we applied a unilateral ureteral obstruction (UUO) model of RIF on A2AR KO mice and their wild-type (WT) littermates. Renal pathological assessment was performed at different post-UUO stages using hematoxylin and eosin (H&E) and Masson's trichrome staining as well as quantitative morphological analysis. Our data demonstrated that: (i) the extent of RIF was determined by the development of UUO in a time-dependent manner; (ii) A2AR KO exacerbated the pathological progress of RIF in mice at the early post-UUO stage, i.e. day 3 and day 7; (iii) the profibrotic effect of A2AR KO was prominent until the late post-UUO stage, i.e. day 14, at which RIF reached a similar severity level in A2AR KO and WT mice. Our findings revealed that A2AR KO significantly exacerbated the progression of UUO-induced RIF in mice, prominently at the initial stage.

Epithelial cell TGFβ signaling induces acute tubular injury and interstitial inflammation

TGFβ signaling plays a central role in the development of acute and chronic kidney diseases. Previous in vivo studies involved systemic alteration of TGFβ signaling, however, limiting conclusions about the direct role of TGFβ in tubular cell injury. Here, we generated a double transgenic mouse that inducibly expresses a ligand-independent constitutively active TGFβ receptor type 1 (TβR1) kinase specifically in tubular epithelial cells, with expression restricted by the Pax8 promoter. In this model, activation of TGFβ signaling in the tubular epithelium alone was sufficient to cause AKI characterized by marked tubular cell apoptosis and necrosis, oxidative stress, dedifferentiation and regenerative cell proliferation, reduced renal function, and interstitial accumulation of inflammatory cells. This tubular injury was associated with mitochondrial-derived generation of reactive oxygen species (ROS), but cell damage and apoptosis were partially independent of mitochondrial-derived ROS. TβR1 signaling-induced tubular injury also associated with significant leukocyte infiltration consisting of F4/80(+) macrophages, CD11c(+) F4/80(+) dendritic cells, CD11c(+) F4/80(-) Ly6C(high) dendritic cells/monocytes, and T cells. Inhibition of mitochondrial-derived ROS significantly reduced accumulation of CD11c(+) F4/80(+) dendritic cells and T cells, suggesting a role for ROS in the activation and recruitment of the adaptive immune response to tubular injury. Taken together, these results suggest that TGFβ signaling in the tubular epithelium alone is sufficient to cause acute tubular injury and inflammation; therefore, TGFβ may be a mechanistic link between acute injury and chronic progression of kidney disease.

Targeting chemokines in proteinuria-induced renal disease

INTRODUCTION

Proteinuria is a common finding in glomerular diseases that contributes to the progression of chronic kidney injury. Tubular cells reabsorb the excess of albumin and other plasma proteins from the tubular lumen, triggering several pathophysiologic responses, such as overexpression of fibrogenic mediators and inflammatory chemokines. Chemokines are implicated both in the recruitment of inflammatory infiltrate and in a number of physiological and pathological processes related to protein overload.

AREAS COVERED

In recent years, the specific chemokines and their receptors and the intracellular signaling pathways involved in proteinuria-induced renal damage have been identified. This review provides an overview of the role of chemokines and their receptors in proteinuria-related renal disease and summarizes novel therapeutic approaches to restrain the progression of renal damage.

EXPERT OPINION

Inhibition of chemokine-induced biological activities is a promising therapeutic strategy in proteinuric disorders. Neutralizing antibodies and small organic molecules targeting chemokines and chemokine receptors have been proven to prevent inflammation and renal damage in experimental models of protein overload. Some of these compounds are currently being tested in human clinical trials.

The biochemistry and biology of the atypical chemokine receptors

A subset of chemokine receptors, initially called "silent" on the basis of their apparent failure to activate conventional signalling events, has recently attracted growing interest due to their ability to internalize, degrade, or transport ligands and thus modify gradients and create functional chemokine patterns in tissues. These receptors recognize distinct and complementary sets of ligands with high affinity, are strategically expressed in different cellular contexts, and lack structural determinants supporting Gα(i) activation, a key signalling event in cell migration. This is in keeping with the hypothesis that they have evolved to fulfil fundamentally different functions to the classical signalling chemokine receptors. Based on these considerations, these receptors (D6, Duffy antigen receptor for chemokines (DARC), CCX-CKR1 and CXCR7) are now collectively considered as an emerging class of 'atypical' chemokine receptors. In this article, we review the biochemistry and biology of this emerging chemokine receptor subfamily.

Duffy antigen receptor for chemokines and its involvement in patterning and control of inflammatory chemokines

Leukocyte functions are linked to their migratory responses, which, in turn, are largely determined by the expression profile of classical chemokine receptors. Upon binding their cognate chemokines, these G-protein-coupled receptors (GPCRs) initiate signaling cascades and downstream molecular and cellular responses, including integrin activation and cell locomotion. Chemokines also bind to an alternative subset of chemokine receptors, which have serpentine structure characteristic for GPCRs but lack DRYLAIV consensus motive required for coupling to G-proteins. Duffy antigen receptor for chemokines (DARC) is a member of this atypical receptor subfamily. DARC binds a broad range of inflammatory CXC and CC chemokines and is expressed by erythrocytes, venular endothelial cells, and cerebellar neurons. Erythrocyte DARC serves as blood reservoir of cognate chemokines but also as a chemokine sink, buffering potential surges in plasma chemokine levels. Endothelial cell DARC internalizes chemokines on the basolateral cell surface resulting in subsequent transcytosis of chemokines and their immobilization on the tips of apical microvilli. These DARC-mediated endothelial cell interactions allow chemokines produced in the extravascular tissues to optimally function as arrest chemokines on the luminal endothelial cell surface.

Atypical chemokine receptors

Atypical chemokine receptors (ACRs) are cell surface receptors with seven transmembrane domains structurally homologous to chemokine G-protein coupled receptors (GPCRs). However, upon ligation by cognate chemokines, ACRs fail to induce classical signaling and downstream cellular responses characteristic for GPCRs. Despite this, by affecting chemokine availability and function, ACRs impact on a multitude of pathophysiological events and have emerged as important molecular players in health and disease. This review discusses individual characteristics of the currently known ACRs, highlights their similarities and differences and attempts to establish their group identity. It summarizes the progress made in mapping ACR expression, understanding their diverse in vitro and in vivo functions of ACRs and uncovering their contributions to disease pathogeneses.

DARC and D6: silent partners in chemokine regulation?

Chemokine receptors adorn the surface of leukocytes and other cell types ready to translate the extracellular chemokine environment into functional cellular outcomes. However, there are several molecules that, in many respects, look like chemokine receptors, but which do not have the ability to confer chemotactic potential to cell lines. This apparent silence spurred the search for signalling-independent functions and led to the development of new paradigms of chemokine regulation. In this review, we summarise the experimental basis for these ideas focussing on DARC and D6, the most studied members of this group of molecules. We discuss data generated using in vitro systems and genetically deficient mice, include results from observational human studies, and summarise the key findings of recent research. We take a critical look at current models of in vivo function highlighting important gaps in our knowledge and demonstrating that there is still much to find out about these enigmatic molecules.

Targeting the recruitment of monocytes and macrophages in renal disease

Macrophages convert proinflammatory or anti-inflammatory signals of tissue microenvironments into response mechanisms. These response mechanisms largely derive from evolutionary conserved defense programs of innate host defense, wound healing, and tissue homeostasis. Hence, in many settings these programs lead to renal inflammation and tissue remodeling (ie, glomerulonephritis and sclerosis or interstitial nephritis and fibrosis). There is abundant experimental evidence that blocking macrophage recruitment or macrophage activation can ameliorate renal inflammation and fibrosis. In this review we discuss experimental tools to target renal macrophage recruitment by using antagonists against selectins, chemokines, integrins, or other important cytokines that mediate renal injury via macrophage recruitment, some of these already having been used in clinical trials.

Chemokine decoy receptors: structure-function and biological properties

Chemokines induce cell migration through the activation of a distinct family of structurally related heterotrimeric G protein-coupled receptors (GPCR). Over the last few years, several receptors in this family that recognize chemokines but do not induce cell migration have been identified. These "atypical" chemokine receptors are unable to activate transduction events that lead directly to cell migration, but appear nonetheless to play a nonredundant role in the control of leukocyte recruitment at inflammatory sites and in tumors by shaping the chemoattractant gradient, either by removing, transporting, or concentrating their cognate ligands.