SummaryA human receptor that is selective for the CXC chemokines IP10 and Mig was cloned and characterized. The receptor cDNA has an open reading frame of 1104-bp encoding a protein of 368 amino acids with a molecular mass of 40,659 dalton. The sequence includes seven putative transmembrane segments characteristic of G-protein coupled receptors. It shares 40.9 and 40.3% identical amino acids with the two IL-8 receptors, and 34.2-36.9% identity with the five known CC chemokine receptors. The IPl0/Mig receptor is highly expressed in IL-2-activated T lymphocytes, but is not detectable in resting T lymphocytes, B lymphocytes, monocytes and granulocytes. It mediates Ca 2+ mobilization and chernotaxis in response to IP10 and Mig, but does not recognize the CXC-chemokines IL-8, GROom, NAP-2, GCP-2, ENA78, PF4, the CC-chemokines MCP-1, MCP-2, MCP-3, MCP-4, MIP-lot, MIP-I[~, RANTES, I309, eotaxin, nor lymphotactin. The exclusive expression in activated T-lymphocytes is of high interest since the receptors for chemokines which have been shown so far to attract lymphocytes, e.g., MCP-1, MCP-2, MCP-3, MIP-lot, MIP-I[3, and R_ANTES, are also found in monocytes and granulocytes. The present observations suggest that the IP10/Mig receptor is involved in the selective recruitment of effector T cells.

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Drug-induced hypersensitivity reactions have been explained by the hapten concept, according to which a small chemical compound is too small to be recognized by the immune system. Only after covalently binding to an endogenous protein the immune system reacts to this so called hapten-carrier complex, as the larger molecule (protein) is modified, and thus immunogenic for B and T cells. Consequently, a B and T cell immune response might develop to the drug with very heterogeneous clinical manifestations. In recent years, however, evidence has become stronger that not all drugs need to bind covalently to the MHC-peptide complex in order to trigger an immune response. Rather, some drugs may bind directly and reversibly to immune receptors like the major histocompatibility complex (MHC) or the T cell receptor (TCR), thereby stimulating the cells similar to a pharmacological activation of other receptors. This concept has been termed pharmacological interaction with immune receptors the (p-i) concept. While the exact mechanism is still a matter of debate, non-covalent drug presentation clearly leads to the activation of drug-specific T cells as documented for various drugs (lidocaine, sulfamethoxazole (SMX), lamotrigine, carbamazepine, p-phenylendiamine, etc.). In some patients with drug hypersensitivity, such a response may occur within hours even upon the first exposure to the drug. Thus, the reaction to the drug may not be due to a classical, primary response, but rather be mediated by stimulating existing, pre-activated, peptide-specific T cells that are cross specific for the drug. In this way, certain drugs may circumvent the checkpoints for immune activation imposed by the classical antigen processing and presentation mechanisms, which may help to explain the peculiar nature of many drug hypersensitivity reactions.

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Serpentine receptors serve as ligand-activated molecular switches, relaying signals from extracellular ligands to heterotrimeric (␣␤␥) G proteins on the cytoplasmic face of the plasma membrane. These receptors catalyze ligand-dependent exchange of guanosine triphosphate (GTP) for guanosine diphosphate (GDP) on the ␣ subunit of the heterotrimer, causing dissociation of ␣⅐GTP from the ␤␥ dimer; ␣⅐GTP and free ␤␥ subsequently activate effector enzymes and ion channels (1, 2). More than 1,000 serpentine receptors of mammals share with their counterparts in yeast and plants a conserved three-dimensional architecture, comprising seven ␣-helices in a transmembrane bundle (3-6). The switch mechanism is also conserved, as indicated by the abilities of mammalian receptors to activate G protein trimers in yeast (7-9). The switch clearly resides in the seven-helix bundle: swapping of extra-or intracellular loops preserves the ability of ligands to activate G proteins while transferring specificity of ligand binding or G protein activation, respectively, from one receptor to another (10 -13).A static model of the three-dimensional structure of the helix bundle is beginning to take shape. A low resolution (6 Å) electron cryomicroscopic structure of rhodopsin (14), the retinal light receptor, reveals relative positions and tilts of seven transmembrane helices in the plane of the membrane. Based on many mutations, the rhodopsin structure, and an analysis of the primary structures of more than 500 rhodopsin-like serpentine receptors, Baldwin and co-workers (15) constructed an ␣-carbon template of the helix bundle, hereafter termed the Baldwin model. In this model, transmembrane helices I-VII bundle together in clockwise order as viewed from the cytoplasm. The probable arrangement of helices and the positions of specific amino acids in the model are inferred from patterns of conserved hydrophobic and hydrophilic residues in many receptors. The Baldwin model specifies which helix corresponds to which density in the electron projection map, approximate orientations of cognate amino acids around the helical axes, and the cytoplasmic and extracellular limits of each transmembrane sequence (16).How does the switch work? It is difficult to infer a conserved switch mechanism from the functional effects of site-directed mutations reported in a large number of different receptors, because relatively few positions have been mutated in any one receptor (17,18). Accordingly, we undertook a systematic genetic analysis of a single serpentine receptor, with the goal of identifying functionally important residues and sites of helixhelix interactions that relay the ligand signal to G protein activation. We selected functional receptors after random saturation mutagenesis of the four transmembrane helices (III, V, VI, and VII) most consistently implicated in ligand binding or G protein activation by various serpentine receptors (3-6, 17, 18). This comprehensive approach determines the relative importance of side chains in each ␣-helix by identify...

Although agonists are thought to occupy binding pockets within the seven-helix core of serpentine receptors, the topography of these binding pockets and the conformational changes responsible for receptor activation are poorly understood. To identify the ligand binding pocket in the receptor for complement factor 5a (C5aR), we assessed binding affinities of hexapeptide ligands, each mutated at a single position, for seven mutant C5aRs, each mutated at a single position in the putative ligand binding site. In ChaW (an antagonist) and W5Cha (an agonist), the side chains at position 5 are tryptophan and cyclohexylalanine, respectively. Comparisons of binding affinities indicated that the hexapeptide residue at this position interacts with two C5aR residues, Ile-116 (helix III) and Val-286 (helix VII); in a C5aR model these two side chains point toward one another. Both the I116A and the V286A mutations markedly increased binding affinity of W5Cha but not that of ChaW. Moreover, ChaW, the antagonist hexapeptide, acted as a full agonist on the I116A mutant. These results argue that C5aR residues Ile-116 and Val-286 interact with the side chain at position 5 of the hexapeptide ligand to form an activation switch. Based on this and previous work, we present a docking model for the hexapeptide within the C5aR binding pocket. We propose that agonists induce a small change in the relative orientations of helices III and VII and that these helices work together to allow movement of helix VI away from the receptor core, thereby triggering G protein activation.Serpentine receptors transmit a diverse array of extracellular stimuli to heterotrimeric G proteins located on the cytoplasmic face of the plasma membrane. These receptors promote exchange of GTP for GDP bound to the ␣ subunit of the heterotrimer, allowing the ␣ and ␤␥ subunits to disengage from one another and activate intracellular effectors (1). Of several serpentine receptor families (2), the rhodopsin-like family is the largest (3). Low resolution models of the three-dimensional structure of the seven-helix bundle in the serpentine receptor core were based on patterns of conserved primary structure, biochemical observations with many receptors, and a low resolution electron cryomicroscopy structure of rhodopsin (4). Three such models, constructed independently (3, 5, 6), predict three-dimensional structures of the transmembrane helices that are remarkably similar to one another and to a recent three-dimensional crystal structure of rhodopsin at atomic resolution (7). These similarities make the crystal structure a promising platform for designing and interpreting experiments aimed at elucidating structure and molecular mechanisms of other members of the rhodopsin-like family of serpentine receptors.

The migration of leukocytes in immune surveillance and inflammation is largely determined by their response to chemokines. While the chemokine specificities and expression patterns of chemokine receptors are well defined, it is still a matter of debate how leukocytes integrate the messages provided by different chemokines that are concomitantly produced in physiologic or pathologic situations in vivo. We present evidence for a novel regulatory mechanism of leukocyte trafficking. Our data are consistent with a mode of action where CC-chemokine receptor 7 (CCR7) agonists and unrelated, nonagonist chemokines first form a heteromeric complex, in the presence of which the triggering of CCR7 can occur at a much lower agonist concentration. The increase is synergistic and can be evoked by many but not all chemokines. Chemokine-induced synergism might provide an amplification system in "chemokine-rich" tissues, rendering leukocytes more competent to respond to migratory cues.