ABSTRACT

The Duffy antigen receptor for chemokines (DARC) belongs to a family of ‘silent’ heptahelical chemokine receptors that do not couple to G proteins and fail to transmit measurable intracellular signals. DARC binds most inflammatory chemokines and is prominently expressed on venular endothelial cells, where its function has remained contentious. Here we show that DARC, like other silent receptors, internalized chemokines but did not effectively scavenge them. Instead, DARC mediated chemokine transcytosis, which led to apical retention of intact chemokines and more leukocyte migration across monolayers expressing DARC. Mice overexpressing DARC on blood vessel endothelium had enhanced chemokine-induced leukocyte extravasation and contact-hypersensitivity reactions. Thus, interactions of chemokines with DARC support their activity on apposing leukocytes in vitro and in vivo.

At sites of inflammation and in normal immune surveillance, chemokines direct leukocyte migration across the endothelium. Many cell types that are extravascular can produce chemokines, and for these mediators to directly elicit leukocyte migration from the blood, they would need to reach the luminal surface of the endothelium. This article reviews the evidence that endothelial cells are active in transcytosing chemokines to their luminal surfaces, where they are presented to leukocytes. The endothelial binding sites that transport and present chemokines include glycosaminoglycans (GAGs) and possibly the Duffy antigen/receptor for chemokines (DARC). The binding residues on chemokines that interact with GAGs are discussed, as are the carbohydrate structures on GAGs that bind these cytokines. The expression of particular GAG structures by endothelial cells may lend selectivity to the type of chemokine presented in a given tissue, thereby contributing to selective leukocyte recruitment. At the luminal surface of the endothelium, chemokines are preferentially presented to blood leukocytes on the tips of microvillous processes. Similarly, certain adhesion molecules and chemokine receptors are also preferentially distributed on leukocyte and endothelial microvilli, and evidence suggests an important role for these structures in creating the necessary surface topography for leukocyte migration. IntroductionA central feature of inflammatory diseases is the migration of leukocytes from the circulation, across the endothelium and the basement membrane, and into the affected tissue. This mechanism of extravasation is induced by chemokines (chemoattractant cytokines), which are a family of proinflammatory mediators produced at the inflammatory site. 1,2 As part of the migration process, circulating leukocytes must first adhere to the luminal surface of the endothelium. According to the current paradigm, this interaction involves the sequential engagement of leukocyte and endothelial adhesion molecules. First, selectins and their carbohydrate counterligands mediate leukocyte tethering and rolling. Then, leukocyte integrins and their ligands, including immunoglobulinlike intercellular adhesion molecules, mediate firm leukocyte adhesion. 3 Chemokines play a role in firm adhesion by activating integrins on the leukocyte cell surface. 4,5 The leukocytes are directed by chemoattractant gradients to migrate across the endothelium, and through the extracellular matrix into the tissue.The intent of this review is to focus on the endothelium and its role in transcytosing and presenting chemokines to blood leukocytes, resulting in leukocyte extravasation. The molecular nature of the endothelial binding sites that are proposed to transport and present chemokines are discussed. The mechanisms of chemokine transcytosis and presentation by endothelial cells are then fitted into the current model of how leukocytes emigrate into tissues at sites of inflammation. Chemokine-binding sites on the endotheliumIt has been traditionally held that c...