Chapter 9 The Duffy Antigen Receptor for Chemokines. Chemokines, Part B. Elsevier; 2009. pp. 191-206. [DOI: 10.1016/s0076-6879(09)05409-3]

The Involvement of Pial Microvessels in Leukocyte Invasion after Mild Traumatic Brain Injury

The pathophysiological mechanisms underlying mild traumatic brain injury (mTBI) are not well understood, but likely involve neuroinflammation. Here the controlled cortical impact model of mTBI in rats was used to test this hypothesis. Mild TBI caused a rapid (within 6 h post-mTBI) upregulation of synthesis of TNF-α and IL-1β in the cerebral cortex and hippocampus, followed by an increase in production of neutrophil (CXCL1-3) and monocyte (CCL2) chemoattractants. While astrocytes were not a significant source of CXC chemokines, they highly expressed CCL2. An increase in production of CXC chemokines coincided with the influx of neutrophils into the injured brain. At 6 h post-mTBI, we observed a robust influx of CCL2-expressing neutrophils across pial microvessels into the subarachnoid space (SAS) near the injury site. Mild TBI was not accompanied by any significant influx of neutrophils into the brain parenchyma until 24 h after injury. This was associated with an early induction of expression of intercellular adhesion molecule 1 on the endothelium of the ipsilateral pial, but not intraparenchymal, microvessels. At 6 h post-mTBI, we also observed a robust influx of neutrophils into the ipsilateral cistern of velum interpositum (CVI), a slit-shaped cerebrospinal fluid space located above the 3rd ventricle with highly vascularized pia mater. From SAS and CVI, neutrophils appeared to move along the perivascular spaces to enter the brain parenchyma. The monocyte influx was not observed until 24 h post-mTBI, and these inflammatory cells predominantly entered the ipsilateral SAS and CVI, with a limited invasion of brain parenchyma. These observations indicate that the endothelium of pial microvessels responds to injury differently than that of intraparenchymal microvessels, which may be associated with the lack of astrocytic ensheathment of cerebrovascular endothelium in pial microvessels. These findings also suggest that neuroinflammation represents the potential therapeutic target in mTBI.

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.

One-step immunopurification and lectinochemical characterization of the Duffy atypical chemokine receptor from human erythrocytes

Duffy antigen/receptor for chemokines (DARC) is a glycosylated seven-transmembrane protein acting as a blood group antigen, a chemokine binding protein and a receptor for Plasmodium vivax malaria parasite. It is present on erythrocytes and endothelial cells of postcapillary venules. The N-terminal extracellular domain of the Duffy glycoprotein carries Fy(a)/Fy(b) blood group antigens and Fy6 linear epitope recognized by monoclonal antibodies. Previously, we have shown that recombinant Duffy protein expressed in K562 cells has three N-linked oligosaccharide chains, which are mainly of complex-type. Here we report a one-step purification method of Duffy protein from human erythrocytes. DARC was extracted from erythrocyte membranes in the presence of 1% n-dodecyl-β-D-maltoside (DDM) and 0.05% cholesteryl hemisuccinate (CHS) and purified by affinity chromatography using immobilized anti-Fy6 2C3 mouse monoclonal antibody. Duffy glycoprotein was eluted from the column with synthetic DFEDVWN peptide containing epitope for 2C3 monoclonal antibody. In this single-step immunoaffinity purification method we obtained highly purified DARC, which migrates in SDS-polyacrylamide gel as a major diffuse band corresponding to a molecular mass of 40-47 kDa. In ELISA purified Duffy glycoprotein binds anti-Duffy antibodies recognizing epitopes located on distinct regions of the molecule. Results of circular dichroism measurement indicate that purified DARC has a high content of α-helical secondary structure typical for chemokine receptors. Analysis of DARC glycans performed by means of lectin blotting and glycosidase digestion suggests that native Duffy N-glycans are mostly triantennary complex-type, terminated with α2-3- and α2-6-linked sialic acid residues with bisecting GlcNAc and α1-6-linked fucose at the core.

Vivax malaria in Mauritania includes infection of a Duffy-negative individual

Background

Duffy blood group polymorphisms are important in areas where Plasmodium vivax is present because this surface antigen is thought to act as a key receptor for this parasite. In the present study, Duffy blood group genotyping was performed in febrile uninfected and P. vivax-infected patients living in the city of Nouakchott, Mauritania.

Methods

Plasmodium vivax was identified by real-time PCR. The Duffy blood group genotypes were determined by standard PCR followed by sequencing of the promoter region and exon 2 of the Duffy gene in 277 febrile individuals. Fisher's exact test was performed in order to assess the significance of variables.

Results

In the Moorish population, a high frequency of the FYB^ES/FYB^ES genotype was observed in uninfected individuals (27.8%), whereas no P. vivax-infected patient had this genotype. This was followed by a high level of FYA/FYB, FYB/FYB, FYB/FYB^ES and FYA/FYB^ES genotype frequencies, both in the P. vivax-infected and uninfected patients. In other ethnic groups (Poular, Soninke, Wolof), only the FYB^ES/FYB^ES genotype was found in uninfected patients, whereas the FYA/FYB^ES genotype was observed in two P. vivax-infected patients. In addition, one patient belonging to the Wolof ethnic group presented the FYB^ES/FYB^ES genotype and was infected by P. vivax.

Conclusions

This study presents the Duffy blood group polymorphisms in Nouakchott City and demonstrates that in Mauritania, P. vivax is able to infect Duffy-negative patients. Further studies are necessary to identify the process that enables this Duffy-independent P. vivax invasion of human red blood cells.

Mendelian randomization: potential use of genetics to enable causal inferences regarding HIV-associated biomarkers and outcomes

PURPOSE OF REVIEW

It is unknown whether biomarkers simply correlate with or are causal for HIV-associated outcomes. Mendelian randomization is a genetic epidemiologic approach used to disentangle causation from association. Here, we discuss the potential use of Mendelian randomization for differentiating whether biomarkers are correlating with or causal for HIV-associated outcomes.

RECENT FINDINGS

Mendelian randomization refers to the random allocation of alleles at the time of gamete formation. In observational epidemiology, this refers to the use of genetic variants to estimate a causal effect between a modifiable risk factor and an outcome of interest. A formal Mendelian randomization study using a genetic marker as a proxy for the biomarker has not been conducted in the HIV field. However, in the postgenomic era, this approach is being used increasingly. Examples are evidence for the causal role of BMI in blood pressure and noncausal role of C-reactive protein in coronary heart disease. We discuss the conceptual framework, uses, and limitations of Mendelian randomization in the context of HIV infection as well as specific biomarkers (IL-6, C-reactive protein) and genetic determinants (e.g., in CCR5, chemokine, and DARC genes) that associate with HIV-related outcomes.

SUMMARY

Making the distinction between correlation and causality has particular relevance when a biomarker (e.g., IL-6) is potentially modifiable, in which case a biomarker-guided targeted treatment strategy may be feasible. Although the tenets of Mendelian randomization rest on strong assumptions, and conducting a Mendelian randomization study in HIV infection presents many challenges, it may offer the potential to identify causal biomarkers for HIV-associated outcomes.

A recombinant dromedary antibody fragment (VHH or nanobody) directed against human Duffy antigen receptor for chemokines

Fy blood group antigens are carried by the Duffy antigen receptor for chemokines (DARC), a red cells receptor for Plasmodium vivax broadly implicated in human health and diseases. Recombinant VHHs, or nanobodies, the smallest intact antigen binding fragment derivative from the heavy chain-only antibodies present in camelids, were prepared from a dromedary immunized against DARC N-terminal extracellular domain and selected for DARC binding. A described VHH, CA52, does recognize native DARC on cells. It inhibits P. vivax invasion of erythrocytes and displaces interleukin-8 bound to DARC. The targeted epitope overlaps the well-defined DARC Fy6 epitope. K (D) of CA52-DARC equilibrium is sub-nanomolar, hence ideal to develop diagnostic or therapeutic compounds. Immunocapture by immobilized CA52 yielded highly purified DARC from engineered K562 cells. This first report on a VHH with specificity for a red blood cell protein exemplifies VHHs' potentialities to target, to purify, and to modulate the function of cellular markers.

CXCL5 regulates chemokine scavenging and pulmonary host defense to bacterial infection

The chemokine sink hypothesis pertaining to erythrocyte Duffy Antigen Receptor for Chemokines (DARC) during inflammation has received considerable attention, but lacks direct in vivo evidence. Here we demonstrate, using mice with a targeted deletion in CXCL5, that CXCL5 bound erythrocyte DARC and impaired its chemokine scavenging in blood. CXCL5 increased the plasma concentrations of CXCL1 and CXCL2 in part through inhibiting chemokine scavenging, impairing chemokine gradients and desensitizing CXCR2, which led to decreased neutrophil influx to the lung, increased lung bacterial burden and mortality in an Escherichia coli pneumonia model. In contrast, CXCL5 exerted a predominant role in mediating neutrophil influx to the lung during inflammation after LPS inhalation. Platelets and lung resident cells were the sources of homeostatic CXCL5 in blood and inflammatory CXCL5 in the lung respectively. This study presents a paradigm whereby platelets and red cells alter chemokine scavenging and neutrophil-chemokine interaction during inflammation.

Plasmodium vivax and the Duffy antigen: a paradigm revisited

The Duffy blood group antigen is the portal of entry of the Plasmodiumvivax malaria parasite into human red blood cells and the receptor for a number of CXC and CC chemokines. We review here epidemiological data and evidence derived from therapeutic or experimental human infections associating P. vivax and the Duffy glycoprotein and laboratory studies indicating that P. vivax uses the Duffy antigen as a receptor to invade the red cell. We then review recent field observations indicating that the conclusion of the absolute dependence on the presence of Duffy on the red cell for P. vivax infection and development into the red cell no longer holds true and that in some parts of the world, P. vivax infects and causes disease in Duffy-negative people.

Chemokine homeostasis vs. chemokine presentation during severe acute lung injury: the other side of the Duffy antigen receptor for chemokines

Acute lung injury (ALI) still poses a major challenge in critical care medicine. Neutrophils, platelets, and chemokines are all considered key components in the development of ALI. The Duffy antigen receptor for chemokines (DARC) is thought to be involved in scavenging, transendothelial transport, and presentation of neutrophil-specific chemokines. DARC is expressed on endothelial cells and erythrocytes but not on leukocytes. Here, we show that DARC is crucial for chemokine-mediated leukocyte recruitment in vivo. However, we also demonstrate that changes in chemokine and chemokine receptor homeostasis, associated with Darc gene deficiency, exert strong anti-inflammatory effects. Neutrophils from Darc gene-deficient (Darc(-/-)) mice display a more prolonged downregulation of CXCR2 during severe inflammation than neutrophils from wild-type mice. In a CXCR2-dependent model of acid-induced ALI, Darc gene deficiency prevents ALI. Darc(-/-) mice demonstrate fully preserved oxygenation, only a small increase in vascular permeability, and a complete lack of pulmonary neutrophil recruitment. Further analysis reveals that only neutrophils but neither endothelial cells nor erythrocytes from Darc(-/-) mice confer protection from ALI. The protection appears to be due to abolished pulmonary recruitment of neutrophils from Darc(-/-) mice. The generation of neutrophil-platelet aggregates, a key mechanism in both pulmonary neutrophil recruitment and thrombus formation, is also affected by altered CXCR2 homeostasis in Darc(-/-) mice. CXCR2 blockade enhances the formation of platelet-neutrophil aggregates and thereby corrects a formerly unknown bleeding defect in Darc(-/-) mice. In summary, our study suggests that chemokine/chemokine receptor homeostasis plays a previously unrecognized and crucial role in severe ALI.