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

Atlas of the anatomical localization of atypical chemokine receptors in healthy mice.

Atypical chemokine receptors (ACKRs) scavenge chemokines and can contribute to gradient formation by binding, internalizing, and delivering chemokines for lysosomal degradation. ACKRs do not couple to G-proteins and fail to induce typical signaling induced by chemokine receptors. ACKR3, which binds and scavenges CXCL12 and CXCL11, is known to be expressed in vascular endothelium, where it has immediate access to circulating chemokines. ACKR4, which binds and scavenges CCL19, CCL20, CCL21, CCL22, and CCL25, has also been detected in lymphatic and blood vessels of secondary lymphoid organs, where it clears chemokines to facilitate cell migration. Recently, GPR182, a novel ACKR-like scavenger receptor, has been identified and partially deorphanized. Multiple studies point towards the potential coexpression of these 3 ACKRs, which all interact with homeostatic chemokines, in defined cellular microenvironments of several organs. However, an extensive map of ACKR3, ACKR4, and GPR182 expression in mice has been missing. In order to reliably detect ACKR expression and coexpression, in the absence of specific anti-ACKR antibodies, we generated fluorescent reporter mice, ACKR3GFP/+, ACKR4GFP/+, GPR182mCherry/+, and engineered fluorescently labeled ACKR-selective chimeric chemokines for in vivo uptake. Our study on young healthy mice revealed unique and common expression patterns of ACKRs in primary and secondary lymphoid organs, small intestine, colon, liver, and kidney. Furthermore, using chimeric chemokines, we were able to detect distinct zonal expression and activity of ACKR4 and GPR182 in the liver, which suggests their cooperative relationship. This study provides a broad comparative view and a solid stepping stone for future functional explorations of ACKRs based on the microanatomical localization and distinct and cooperative roles of these powerful chemokine scavengers

DARC shuttles inflammatory chemokines across the blood-brain barrier during autoimmune central nervous system inflammation

Trafficking of T cells into the CNS is a pathophysiological hallmark of multiple sclerosis. Using an invitro model of the blood-brain barrier, Minten etal. reveal that the atypical chemokine receptor DARC shuttles inflammatory chemokines across the barrier, where they contribute to immune cell trafficking into the brai

CXCR7 Functions as a Scavenger for CXCL12 and CXCL11

CXCR7 (RDC1), the recently discovered second receptor for CXCL12, is phylogenetically closely related to chemokine receptors, but fails to couple to G-proteins and to induce typical chemokine receptor mediated cellular responses. The function of CXCR7 is controversial. Some studies suggest a signaling activity in mammalian cells and zebrafish embryos, while others indicate a decoy activity in fish. Here we investigated the two propositions in human tissues. We provide evidence and mechanistic insight that CXCR7 acts as specific scavenger for CXCL12 and CXCL11 mediating effective ligand internalization and targeting of the chemokine cargo for degradation. Consistently, CXCR7 continuously cycles between the plasma membrane and intracellular compartments in the absence and presence of ligand, both in mammalian cells and in zebrafish. In accordance with the proposed activity as a scavenger receptor CXCR7-dependent chemokine degradation does not become saturated with increasing ligand concentrations. Active CXCL12 sequestration by CXCR7 is demonstrated in adult mouse heart valves and human umbilical vein endothelium. The finding that CXCR7 specifically scavenges CXCL12 suggests a critical function of the receptor in modulating the activity of the ubiquitously expressed CXCR4 in development and tumor formation. Scavenger activity of CXCR7 might also be important for the fine tuning of the mobility of hematopoietic cells in the bone marrow and lymphoid organs

Chemotactic Activity and Receptor Binding of Neutrophil Attractant/Activation Protein‐1 (NAP‐1) and Structurally Related Host Defense Cytokines: Interaction of NAP‐2 With the NAP‐1 Receptor

Neutrophil attractant/activation protein‐1 (NAP‐1) has sequence similarity to platelet factor‐4 (PF‐4) and to NAP‐2 (a truncated form of connective tissue activating protein‐Ill [CTAP‐III(des 1–15)]. We compared chemotactic activity for neutrophils of these related proteins. We also included for comparison CTAP‐III, CTAP‐III(des 1–13), the C‐terminal dodecapeptide of PF‐4 [PF‐4(59–70)], and C5a. Chemotactic potency (EC50) was highest for NAP‐1 and C5a. Although chemotactic efficacy (peak percentage of neutrophils migrating) was comparable for C5a, NAP‐1, and NAP‐2, the NAP‐2 response occurred only at concentrations 100‐fold higher than the NAP‐1 EC50 of 10‐8 M. Data for the CTAP‐III proteins confirmed that CTAP‐III is not an attractant and that chemotactic activity appears as a result of cleavage of residues at the N‐terminus to make CTAP‐III(des 1–13) or NAP‐2 [CTAP‐III(des 1–15)]. Chemotactic activity of PF‐4 was low and variable, with no significant response by neutrophils from six of nine subjects. In contrast, PF‐4(59–70) regularly induced high chemotactic responses, although the EC50 of 1.6 × 10‐5 M was 1,000‐fold greater than that of NAP‐1. The binding of fluoresceinated NAP‐1 to neutrophils was inhibited by unlabeled NAP‐1 or NAP‐2 but not by PF‐4 or PF‐4 (59–70). This suggests that NAP‐2 interacts with the neutrophil NAP‐1 receptor. Despite the low chemotactic potency of NAP‐2, it is a potential attractant at sites of injury because of the relatively large amounts of the parent CTAP‐III released from platelets, as indicated by a serum concentration of approximately 10‐6 M.Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/141697/1/jlb0258.pd

Elevated expression of the chemokine-scavenging receptor D6 is associated with impaired lesion development in psoriasis

D6 is a scavenging-receptor for inflammatory CC chemokines that are essential for resolution of inflammatory responses in mice. Here, we demonstrate that D6 plays a central role in controlling cutaneous inflammation, and that D6 deficiency is associated with development of a psoriasis-like pathology in response to varied inflammatory stimuli in mice. Examination of D6 expression in human psoriatic skin revealed markedly elevated expression in both the epidermis and lymphatic endothelium in "uninvolved" psoriatic skin (ie, skin that was more than 8 cm distant from psoriatic plaques). Notably, this increased D6 expression is associated with elevated inflammatory chemokine expression, but an absence of plaque development, in uninvolved skin. Along with our previous observations of the ability of epidermally expressed transgenic D6 to impair cutaneous inflammatory responses, our data support a role for elevated D6 levels in suppressing inflammatory chemokine action and lesion development in uninvolved psoriatic skin. D6 expression consistently dropped in perilesional and lesional skin, coincident with development of psoriatic plaques. D6 expression in uninvolved skin also was reduced after trauma, indicative of a role for trauma-mediated reduction in D6 expression in triggering lesion development. Importantly, D6 is also elevated in peripheral blood leukocytes in psoriatic patients, indicating that upregulation may be a general protective response to inflammation. Together our data demonstrate a novel role for D6 as a regulator of the transition from uninvolved to lesional skin in psoriasis