Leukocyte trafficking at the endothelium requires both cellular adhesion molecules and chemotactic factors. Fractalkine, a novel transmembrane molecule with a CX3C-motif chemokine domain atop a mucin stalk, induces both adhesion and migration of leukocytes. Here we identify a seven-transmembrane high-affinity receptor for fractalkine and show that it mediates both the adhesive and migratory functions of fractalkine. The receptor, now termed CX3CR1, requires pertussis toxin-sensitive G protein signaling to induce migration but not to support adhesion, which also occurs without other adhesion molecules but requires the architecture of a chemokine domain atop the mucin stalk. Natural killer cells predominantly express CX3CR1 and respond to fractalkine in both migration and adhesion. Thus, fractalkine and CX3CR1 represent new types of leukocyte trafficking regulators, performing both adhesive and chemotactic functions.

Prostaglandin (PG)D2, which has long been implicated in allergic diseases, is currently considered to elicit its biological actions through the DP receptor (DP). Involvement of DP in the formation of allergic asthma was recently demonstrated with DP-deficient mice. However, proinflammatory functions of PGD2 cannot be explained by DP alone. We show here that a seven-transmembrane receptor, CRTH2, which is preferentially expressed in T helper type 2 (Th2) cells, eosinophils, and basophils in humans, serves as the novel receptor for PGD2. In response to PGD2, CRTH2 induces intracellular Ca2+ mobilization and chemotaxis in Th2 cells in a Gαi-dependent manner. In addition, CRTH2, but not DP, mediates PGD2-dependent cell migration of blood eosinophils and basophils. Thus, PGD2 is likely involved in multiple aspects of allergic inflammation through its dual receptor systems, DP and CRTH2.

The chemokine superfamily consists of a large number of ligands and receptors. At first glance, this family appears redundant and their ligand-receptor relationships promiscuous, making its study challenging. However, analyzing this family from the evolutionary perspective greatly simplifies understanding both the organization and function of this apparently complex system. In particular, the functions of a subgroup of chemokines (designated homeostatic chemokines) have played pivotal roles in advancing our understanding of the organization and function of the cellular networks that shape the immune system. Here, we update the full scope of the human and mouse chemokine superfamilies, their relationships, and summarize several important roles that homeostatic chemokines play in the immune system.

Association of major histocompatibility complex (MHC) class II molecules with peptides occurs in a series of endocytic vacuoles, termed MHC class II-enriched compartments (MIICs). Morphological criteria have defined several types of MIICs, including multivesicularMIICs, which are composed of 50 -60-nm vesicles surrounded by a limiting membrane. Multivesicular MIICs can fuse with the plasma membrane, thereby releasing their internal vesicles into the extracellular space. The externalized vesicles, termed exosomes, carry MHC class II and can stimulate T-cells in vitro. In this study, we show that exosomes are enriched in the co-stimulatory molecule CD86 and in several tetraspan proteins, including CD37, CD53, CD63, CD81, and CD82. Interestingly, subcellular localization of these molecules revealed that they were concentrated on the internal membranes of multivesicular MIICs. In contrast to the tetraspans, other membrane proteins of MIICs, such as HLA-DM, Lamp-1, and Lamp-2, were mainly localized to the limiting membrane and were hardly detectable on the internal membranes of MIICs nor on exosomes. Because internal vesicles of multivesicular MIICs are thought to originate from inward budding of the limiting membrane, the differential distribution of membrane proteins on the internal and limiting membranes of MIICs has to be driven by active protein sorting. Major histocompatibility complex (MHC)1 class II molecules present peptide determinants of exogenous protein antigens to CD4 ϩ T-cells (reviewed in Refs. 1-3). MHC class II molecules are composed of an ␣ and ␤ subunit, which associate with the invariant chain (Ii) in the endoplasmic reticulum (4, 5). The ␣␤Ii complexes are transported to the trans-Golgi network, from which they are diverged from the secretory pathway and targeted to the endocytic pathway (Refs. 6 and 7; for reviews, see Refs. 2 and 8). After proteolytic processing of Ii in endocytic compartments, the Ii-degradation product class II-associated invariant chain peptide remains temporarily associated with the peptide binding groove of MHC class II but is ultimately displaced by antigenic peptides, a process that is catalyzed by HLA-DM (9 -12).Immunoelectron microscopy (IEM) studies revealed that in a variety of antigen-presenting cells, the majority of intracellular MHC class II molecules localize to a heterogeneous set of endocytic compartments, collectively termed MIICs (MHC class II-enriched compartments) (Refs. 13 and 14; reviewed in Refs. 15 and 16). MIICs are endocytic vacuoles with internal membrane vesicles and sheets, reminiscent of late endosomes and lysosomes in non-antigen-presenting cells (17). The internal vesicles of MIICs probably originate from inward vesiculation of its limiting membrane (18), analogous to the formation of multivesicular bodies in other cell types (19). MIICs are thought to represent the subcellular site at which MHC class II molecules bind peptides (20 -23). Once formed in MIICs, the ␣␤-peptide complexes are transported to the plasma membrane via largely unidentif...

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have been divided into the two major subfamilies on the Osaka-Sayama 589-8511 basis of the arrangement of the two N-terminal cysteine Japan residues, CXC and CC, depending on whether the first two cysteine residues have an amino acid between them Chemokines are a group of small ‫41-8ف(‬ kDa), mostly (CXC) or are adjacent (CC). The genes for these families basic, structurally related molecules that regulate cell are currently designated SCY (small secreted cytokine) trafficking of various types of leukocytes through interwith SCYa corresponding to the CC subfamily and SCYb actions with a subset of seven-transmembrane, G proto the CXC subfamily. Two other classes of chemokines tein-coupled receptors. About 40 chemokines have now have been described: lymphotactin (C or SCYc) and been identified in humans. They mainly act on neutrofractalkine (CX3C or SCYd). The former one lacks cystephils, monocytes, lymphocytes, and eosinophils and ines one and three of the typical chemokine structure play a pivotal role in host defense mechanisms. The (Kelner et al., 1994), while the latter one exhibits three study of chemokines has recently overlapped more with amino acids between the first two cysteines and is also other fields of immunology. It has now become evident the only membrane-bound chemokine through a mucinthat chemokines play fundamental roles in the developlike stalk (Bazan et al., 1997). The proposed chemokine ment, homeostasis, and function of the immune system. nomenclature is based on the chemokine receptor no-The rapid increase in the number of chemokines along menclature currently in use, which uses CC, CXC, XC, with other complex issues described below have led to or CX3C followed by R (for receptor) and then a number. a situation where a newcomer attempting to understand Thus, we have CCR1-9, CXCR1-5, XCR1 (the lymphothis field faces a daunting task. In this review, our goal tactin receptor), and CX3CR1 (the fractalkine receptor). is to present a concise overview of the chemokine super-Basically, the new nomenclature replaces R with L (lifamily. The chemokines have a wide range of effects in gand instead of receptor) to designate the ligands and many different cell types beyond the immune system, uses CC for the SCYa family, CXC for SCYb, XC for including, for example, various cells of the central ner-SCYc, and CX3C for SCYd. The numbering system is vous system (Ma et al., 1998) or endothelial cells, where the one already in use to designate the genes encoding they result in either angiogenic or angiostatic effects each chemokine. Thus, a given gene will have the same (Strieter et al., 1995). However, the scope of this review number as its protein ligand (for example, ScyA 27 is makes it difficult to provide a comprehensive view of the gene encoding CCL27), a correlation that should such a complex and expanding field. For this reason, further simplify matters. Besides eliminating ambiguwe will focus on several areas of chemokine biology of ities, the new nomenclature directly indicates the class particular in...

Leukocyte migration into sites of inflammation involves multiple molecular interactions between leukocytes and vascular endothelial cells, mediating sequential leukocyte capture, rolling, and firm adhesion. In this study, we tested the role of molecular interactions between fractalkine (FKN), a transmembrane mucin-chemokine hybrid molecule expressed on activated endothelium, and its receptor (CX3CR1) in leukocyte capture, firm adhesion, and activation under physiologic flow conditions. Immobilized FKN fusion proteins captured resting peripheral blood mononuclear cells at physiologic wall shear stresses and induced firm adhesion of resting monocytes, resting and interleukin (IL)-2–activated CD8+ T lymphocytes and IL-2–activated NK cells. FKN also induced cell shape change in firmly adherent monocytes and IL-2–activated lymphocytes. CX3CR1-transfected K562 cells, but not control K562 cells, firmly adhered to FKN-expressing ECV-304 cells (ECV-FKN) and tumor necrosis factor α–activated human umbilical vein endothelial cells. This firm adhesion was not inhibited by pertussis toxin, EDTA/EGTA, or antiintegrin antibodies, indicating that the firm adhesion was integrin independent. In summary, FKN mediated the rapid capture, integrin-independent firm adhesion, and activation of circulating leukocytes under flow. Thus, FKN and CX3CR1 mediate a novel pathway for leukocyte trafficking.

Helper T cells are classified into Th1 and Th2 subsets based on their profiles of cytokine production. Th1 cells are involved in cell-mediated immunity, whereas Th2 cells induce humoral responses. Selective recruitment of these two subsets depends on specific adhesion molecules and specific chemoattractants. Here, we demonstrate that the T cell-directed CC chemokine thymus and activation-regulated chemokine (TARC) was abundantly produced by monocytes treated with granulocyte macrophage colony stimulating factor (GM-CSF) or IL-3, especially in the presence of IL-4 and by dendritic cells derived from monocytes cultured with GM-CSF + IL-4. The receptor for TARC and another macrophage/dendritic cell-derived CC chemokine macrophage-derived chemokine (MDC) is CCR4, a G protein-coupled receptor. CCR4 was found to be expressed on approximately 20% of adult peripheral blood effector/memory CD4+ T cells. T cells attracted by TARC and MDC generated cell lines predominantly producing Th2-type cytokines, IL-4 and IL-5. Fractionated CCR4+ cells but not CCR4- cells also selectively gave rise to Th2-type cell lines. When naive CD4+ T cells from adult peripheral blood were polarized in vitro, Th2-type cells selectively expressed CCR4 and vigorously migrated toward TARC and MDC. Taken together, CCR4 is selectively expressed on Th2-type T cells and antigen-presenting cells may recruit Th2 cells expressing CCR4 by producing TARC and MDC in Th2-dominant conditions.