Chemokines constitute a family of small cytokines that are produced in inflammation and regulate leukocyte recruitment (Baggiolini M. et al., Adv. Immunol. 55: 97-179 (1994); Springer, T. A., Annu. Rev. Physiol. 57: 827-872 (1995); and Schall, T. J. and K. B. Bacon, Curr. Opin. Immunol. 6: 865-873 (1994)). Chemokines are capable of selectively inducing chemotaxis of the formed elements of the blood (other than red blood cells), including leukocytes such as neutrophils, monocytes, macrophages, eosinophils, basophils, mast cells, and lymphocytes, such as T cells and B cells. In addition to stimulating chemotaxis, other changes can be selectively induced by chemokines in responsive cells, including changes in cell shape, transient rises in the concentration of intracellular free calcium ions ([Ca2+]i), granule exocytosis, integrin upregulation, formation of bioactive lipids (e.g., leukotrienes) and respiratory burst, associated with leukocyte activation. Thus, the chemokines are early triggers of the inflammatory response, causing inflammatory mediator release, chemotaxis and extravasation to sites of infection or inflammation.
Two subfamilies of chemokines, designated as CXC and CC chemokines, are distinguished by the arrangement of the first two of four conserved cysteine residues, which are either separated by one amino acid (as in CXC chemokines IL-8, γIP-10, Mig, PF4, ENA-78, GCP-2, GROα, GROβ, GROγ, NAP-2, NAP-4) or are adjacent residues (as in CC chemokines MIP-1α, MIP-1β, RANTES, MCP-1, MCP-2, MCP-3, I-309). Most CXC chemokines attract neutrophil leukocytes. For example, the CXC chemokines interleukin 8 (IL-8), platelet factor 4 (PF4), and neutrophil-activating peptide 2 (NAP-2) are potent chemoattractants and activators of neutrophils. The CXC chemokines designated Mig (monokine induced by gamma interferon) and IP-10 (γIP-10, interferon-gamma inducible 10 kDa protein) are particularly active in inducing chemotaxis of activated peripheral blood lymphocytes. CC chemokines are generally less selective and can attract a variety of leukocyte cell types, including monocytes, eosinophils, basophils, T lymphocytes and natural killer cells. CC chemokines such as human monocyte chemotactic proteins 1-3 (MCP-1, MCP-2 and MCP-3), RANTES (Regulated on Activation, Normal T Expressed and Secreted), and the macrophage inflammatory proteins 1α and 1β (MIP-1α and MIP-1β) have been characterized as chemoattractants and activators of monocytes or lymphocytes, but do not appear to be chemoattractants for neutrophils.
CC and CXC chemokines act through receptors which belong to a superfamily of seven transmembrane spanning G protein-coupled receptors (Murphy, P. M., Annu. Rev. Immunol., 12: 593-633 (1994); Gerard, C. and N. P. Gerard, Curr. Opin. Immunol., 6: 140-145 (1994)). This family of G-protein coupled (serpentine) receptors comprises a large group of integral membrane proteins, containing seven transmembrane-spanning regions. The receptors are coupled to G proteins, which are heterotrimeric regulatory proteins capable of binding GTP and mediating signal transduction from coupled receptors, for example, by the production of intracellular mediators.
The chemokine receptors can be divided into two groups: CC chemokine receptors 1 through 5 (CCR1-5), which bind CC chemokines, and CXC chemokine receptors 1 through 4 (CXCR1-4), which bind CXC chemokines. In general, the CC chemokine receptors occur on several types of leukocytes, and are important for the migration of monocytes, eosinophils, basophils, and T cells (Qin, S., et al., Eur. J. Immunol., 26:640-647 (1996); Carr, M. W., et al., Proc. Natl. Acad. Sci. USA, 91(9):3652-3656 (1994); Taub, D. D., et al., J. Clin. Invest., 95(3):1370-1376 (1995); Neote, K. et al., Cell, 72: 415-425 (1993); Gao, J. -L. et al., J. Exp. Med., 177: 1421-1427 (1993); Charo, I. F. et al., Proc. Natl. Acad. Sci. USA, 91: 2752-2756 (1994); Myers, S. J., et al., J. Biol. Chem., 270: 5786-5792 (1995); Combadiere, C. et al., J. Biol. Chem., 270 (27): 16491-16494 (1995); and Correction, J. Biol. Chem., 270: 30235 (1995); Ponath, P. D. et al., J. Exp. Med., 183: 2437-2448 (1996); and Daugherty, B. L. et al., J. Exp. Med., 183: 2349-2354 (1996); Power, C. A. et al., 1995, J. Biol. Chem., 270: 19495-19500 (1995); Hoogewerf, A. J. et al., Biochem. Biophys. Res. Commun., 218: 337-343 (1996); Samson, M. et al., Biochemistry, 35: 3362-3367 (1996)). In contrast, the two IL-8 receptors, CXCR1 and CXCR2, are largely restricted to neutrophils and are important for the migration of neutrophils (Baggiolini, M., et al., Adv. Immunol., 55:97-179 (1994)). The IL-8 receptors, CXCR1 (IL-8R1, interleukin-8 receptor type 1; Holmes, W. E. et al., Science, 253: 1278-1280 (1991)) and CXCR2 (IL-8R2, interleukin-8 receptor type 2; Murphy, P. M. and H. L. Tiffany, Science, 253: 1280-1283 (1991)) recognize the NH2-terminal Glu-Leu-Arg (ELR) motif, an essential binding epitope observed in CXC chemokines that induce neutrophil chemotaxis (Clark-Lewis, I. et al., J. Biol. Chem., 266: 23128-23134 (1991); Hébert, C. A. et al., J. Biol. Chem., 266: 18989-18994 (1991); and Clark-Lewis, I. et al., Proc. Natl. Acad. Sci. USA, 90: 3574-3577 (1993)).
In contrast to monocytes and granulocytes, lymphocyte responses to chemokines are not well understood. Notably, none of the receptors of known specificity appear to be restricted to lymphocytes and the chemokines that recognize these receptors cannot, therefore, account for events such as the selective recruitment of T lymphocytes that is observed in T cell-mediated inflammatory conditions. Moreover, although a number of proteins with significant sequence similarity and similar tissue and leukocyte subpopulation distribution to known chemokine receptors have been identified and cloned, the ligands for these receptors remain undefined. Thus, these proteins are referred to as orphan receptors. The characterization of the ligand(s) of a receptor, is essential to an understanding of the interaction of chemokines with their target cells, the events stimulated by this interaction, including chemotaxis and cellular activation of leukocytes, and the development of therapies based upon modulation of receptor function.