Solution structure of the CXC chemokine Interleukin-8

Chemokines and Atherogenesis Chemoattractant cytokines or chemokines are a family of more than 40 cell signalling proteins. Chemokines mediate chemotaxis and activation of a wide range of cell types including monocytes, macrophages, T cells and platelets. Because chemokines can mediate monocyte migration and macrophage differentiation we are analysing the chemokines that are expressed in human atherosclerotic lesions. We are particularly interested in analysing the expression of the linked chromosome 16q13 chemokines, MDC, fractalkine and TARC, which we have shown to be upregulated by the Th2 type cytokines IL-4 and IL-13.

Introduction to Chemokines (top, next, home)

Chemokines are small disulphide-linked polypeptides of typically 60 -70 amino acids that act as potent chemoattractants for many cell types including monocytes, macrophages, neutrophils and T-cells. The chemokine superfamily, unrecognised 10 years ago, now has over 40 different members. Chemokines have been classified into different subfamilies on the basis of conserved structural features. Most chemokines so far described contain four cysteine residues which are crosslinked Cys1-> Cys3 and Cys2->Cys4. The backbone of the chemokine molecule consists of beta strands while the N and C termini of the protein appear to have a less ordered structure.

Chemokines in which the C1 and C2 cysteine residues are separated by a single amino acid are the CXC chemokines, examples include IL-8, PF4, IP-10, I-TAC and SDF-1. Originally thought of as chemoattractants for neutrophils, CXC chemokines have been shown to be important mediators of T- and B- lymphocyte chemotaxis.

In CC chemokines the C1 and C2 cysteine residues are adjacent, examples include RANTES, MCP-1, MDC, TARC and eotaxin. Many CC chemokines exert their effects on monocytes and macrophages but CC chemokines have been shown to be important for dendritic cell chemotaxis and CC chemokines such as MDC and TARC appear to act preferentially on Th2-type T cells via the CCR4 chemokine receptor.

The T cell chemoattractant chemokine lymphotactin is the sole C chemokine with only one cysteine residue in the N terminal half of the protein. The only described CX3C chemokine is fractalkine, which has three amino acids separating C1 and C2. Fractalkine is also unusual in that it exists both as a membrane bound protein with the CX3C chemokine domain sitting on top of a mucin stalk and as a cleaved soluble molecule.

Chemokine Receptors (top, next, home)

Chemokines mediate their effects on cells via G protein coupled receptors that contain seven transmembrane spanning regions. Upon binding their cognate chemokine ligand chemokine receptors initiate cellular signalling through changes in the intracellular concentrations of calcium and cAMP. Many cellular chemokine receptors can bind more than one chemokine with similar affinities. Chemokine receptors bind either CC chemokines (CCR1-CCR9) or CXC chemokines (CXCR1-CXCR5). The chemokine receptors for which chemokine ligands have been identified are listed below. The predominant cell types known to express specific chemokine receptors are indicated.

Human Chemokine Receptors

Receptor	Chemokine ligand(s)	Cell type
CXCR1	IL-8	Neutrophils
CXCR2	IL-8, GROalpha/beta/gamma, ENA-78, NAP-2	Neutrophils
CXCR3	IP10, Mig,, I-TAC	T-cells
CXCR4	SDF-1	T-cells
CXCR5	BCA-1/BLC	B-cells
CCR1	RANTES, MIP-1alpha, MIP-1beta	Monocytes, eosinophils
CCR2	MCP-1, MCP-2, MCP-3, MCP-4	Monocytes, basophils
CCR3	Eotaxin, RANTES, MCP-2, MCP-4	Eosinophils, basophils, T-cells
CCR4	MDC, TARC	Monocytes, T-cells
CCR5	RANTES, MIP-1alpha, MIP-1beta	Monocytes, dendritic cells
CCR6	MIP-3alpha	Dendritic cells, T-cells
CCR7	SLC, MIP-3beta	Dendritic cells, T-cells
CCR8	I-309	Thymocytes, monocytes
CCR9	TECK	T-cells
CX3CR1	Fractalkine	T-cells, monocytes, NK cells
XCR1	Lymphotactin	T-cells

Chemokines and Atherogenesis (top, next, home)

Atherogenesis involves the recruitment of monocytes and T cells as well as smooth muscle cell migration. A role for chemokines in this process has been inferred from chemokine expression in human atherosclerotic lesions and expression of chemokine receptors on inflammatory cells within lesions. Direct experimental evidence for the role of the potent monocyte chemoattractant MCP-1 in atherogenesis has come from the analysis of MCP-1 and CCR2 knockout mice. ApoE-/- mice that lack either MCP-1 or CCR2 expression show markedly reduced atherosclerotic lesion development when fed a high fat diet (1,2). Experimental evidence for the role of the CXC chemokine receptor CXCR2 in atherogenesis was provided by reconstituting lethally irradiated LDL Receptor knockout mice with bone marrow from CXCR2 knockout animals (3).

Murine models of atherosclerosis share many features of early human atherosclerotic lesions such as the accumulation of macrophage-derived foam cells (4). Advanced atherosclerotic plaques in human arteries are, however, more heterogenous in morphology with significant numbers of recruited T lymphocytes and evidence of macrophage differentiation within atherosclerotic plaques (5). Immunohistochemical analysis has shown that macrophages within human atherosclerotic lesions express a number of T-cell chemoattractant CC chemokines including PARC (pulmonary and activation-regulated chemokine) and MIP-3beta (6). It was recently reported that the CC chemokine eotaxin is expressed by smooth muscle cells treated with the inflammatory cytokine TNFalpha (7). Eotaxin expression in human atherosclerotic lesions could be demonstrated by immunostaining and T cells positive for the eotaxin receptor CCR3 could be detected in the same lesions (7). We have observed expression of the linked chromosome 16q13 CC chemokines MDC, fractalkine and TARC by macrophages and endothelial cells in advanced but not early human atherosclerotic lesions (DRG, unpublished results).

A number of CXC chemokines have been shown to be expressed in human atherosclerotic lesions. Mach et al reported that the CXC chemokines IP-10, Mig and I-TAC are expressed in human atherosclerotic lesions (8). All three CXC chemokines are induced by interferon-gamma and hence their expression might be considered a surrogate marker of Th1 cytokine expression in human atherosclerotic lesions. The CXC chemokine SDF-1 has been reported to be expressed by endothelial cells, smooth muscle cells and macrophages within human atherosclerotic lesions (9). Interestingly, SDF-1 was shown to mediate platelet activation suggesting a potential role for SDF-1 in thrombosis associated with atherosclerotic plaque rupture (9).

Summary (top, home)

Experimental evidence for the role of MCP-1 and CCR2 in atherogenesis has been obtained using animal models of atherosclerosis.

Human atherosclerotic lesions exhibit greater heterogeneity in lesion morphology than that seen in animal models of atherogenesis. A number of chemokines have been shown to be expressed by macrophages, smooth muscle cells and emdothelial cells in human athersclerotic plaques.

Some chemokines expressed in human atherosclerotic plaques (IP-10, I-TAC and Mig) can mediate selective recruitment of Th1-type T cells (via CXCR3) while other chemokines (eotaxin, MDC and TARC) can mediate selective recruitment of Th2-type T cells (via CCR3 and CCR4).

References (top, home)

1. Gosling J, Slaymaker S, Gu L, Tseng S, Zlot CH, Young SG, Rollins BJ, Charo IF. MCP-1 deficiency reduces susceptibility to atherosclerosis in mice that overexpress human apolipoprotein B. J. Clin. Invest. 103 (1999) 773-778. (back)

2. Boring L, Gosling J, Cleary M, Charo IF. Decreased lesion formation in CCR2-/- mice reveals a role for chemokines in the initiation of atherosclerosis. Nature 394 (1998) 894-897. (back)

3. Boisvert WA, Santago R, Curtiss LK, Terkeltaub RA. A leukocyte homologue of the IL-8 receptor CXCR2 mediates the accumulation of macrophages in atherosclerotic lesions of LDL receptor-deficient mice. J. Clin. Invest. 101 (1998) 353-363. (back)

4. Knowles JW Maeda N. Genetic modifiers of atherosclerosis in mice. Arterioscler Thromb Vasc Biol. 2000; 20:2336-2345.(back)

5. Ross R, Atherosclerosis - an inflammatory disease. N. Engl. J. Med., 340 (1999) 115-126. (back)

6. Reape TJ, Rayner K, Manning CD, Gee AN, Barnette MS, Burnand KG, Groot PH. Expression and cellular localization of the CC chemokines PARC and ELC in human atherosclerotic plaques. Am. J. Pathol. 154 (1999) 365-374. (back)

7. Haley KJ, Lilly CM, Yang JH, Feng Y, Kennedy SP, Turi TG, Thompson JF, Sukhova GH, Libby P, Lee RT. Overexpression of eotaxin and the CCR3 receptor in human atherosclerosis: using genomic technology to identify a potential novel pathway of vascular inflammation. Circulation. 2000;102:2185-189. (back)

8. Mach F, Sauty A, Iarossi AS, Sukhova GK, Neote K, Libby P, Luster AD. Differential expression of three T lymphocyte-activating CXC chemokines by human atheroma-associated cells. J Clin Invest. 1999;104:1041-1050. (back)

9. Abi-Younes S, Sauty A, Mach F, Sukhova GK, Libby P, Luster AD. The stromal cell-derived factor-1 chemokine is a potent platelet agonist highly expressed in atherosclerotic plaques. Circ Res. 2000;86:131-138. (back)

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Last updated 3 January 2001

Copyright David R. Greaves 2001