Leukocyte recruitment in inflammation and immunity is regulated by a large number of CXC and CC chemokines (1). In the past few years several new proteins of this class have been discovered and considerable information has been gained about the functions of the known ones. It has been shown in particular that the monocyte chemotactic proteins (MCP-1, MCP-2, MCP-3, and MCP-4) do not only act on monocytes, as their name suggests, but also on lymphocytes (2, 3) and basophils (4, 5), and that MCP-2, MCP-3 and MCP-4 are potent attractants for eosinophils (6–8).
Eosinophilia and tissue infiltration by eosinophils are frequently observed in allergic inflammation and parasitic diseases (9). The mechanisms by which these cells are recruited and activated is widely studied because of the pathological consequences resulting from the release of their phlogogenic and cytotoxic products. The challenge is to understand the selectivity of the chemotactic process and to develop efficient inhibitors. Several factors have been proposed as eosinophil attractants such as the anaphylatoxin C5a (10), platelet-activating factor (11), and more recently RANTES (12) and IL-16 (13). None of these stimuli is selective, however, and the discovery of eotaxin, which was originally shown to attract only eosinophils and to be specific for a single chemokine receptor, CCR3 (14–16), was greeted as a promising advance (17).
Within a large-scale sequencing and expression program for the discovery of new chemokines, we have recently identified MCP-4, a CC chemokine with powerful effects on eosinophils (8). In this paper, we describe a chemokine that is also active on eosinophils and is functionally very similar to eotaxin. It attracts and activate human eosinophils and basophils via the eotaxin receptor, CCR3, and has no activity on other leukocytes. Because of these properties, we have named the novel chemokine eotaxin-2.
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Materials and Methods
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Cloning, Expression, Purification and Analysis.
The EST representing CKβ-6 cDNA was identified in the database of Human Genome Sciences Inc. (Rockville, MD) on the basis of the CC motif and the homolgy to known CC chemokines (18). The cDNA was isolated from a library derived from activated human monocytes, and the mature protein was expressed in Sf9 insect cells (CRL 1711; American Type Culture Collection, Rockville, MD). Purification was performed by cation exchange, heparin affinity, and size exclusion chromatograpy (poros 50 HS, poros 20 HE1; Perseptive Biosystem; and Sephacryl S200 HR; Pharmacia) in the presence of protease inhibitors (20 mg/ml Pefabloc SC; Boehringer Mannheim, 1 mg/ml leupeptin, 1 mg/ml E64, and 1 mM EDTA). The purified protein was analyzed by laser desorption mass spectrometry (matrix-assisted laser desorption ionization-time of flight) and by Edman degradation after partial proteolysis with the endoprotease GluC (Boehringer Mannheim).
Chemokines.
Eotaxin, MCP-3, MCP-4, RANTES, and MIP-1
were used as standards. MCP-4 was cloned and expressed as described previously (8), and the other chemokines were chemically synthesized by Dr. I. Clark-Lewis (Biomedical Research Centre, University of British Columbia, Vancouver, Canada) (19).
CCR3-blocking Antibody.
The anti-CCR3 monoclonal antibody 7B11 which selectively blocks the eotaxin receptor (20) was kindly provided by Dr. Charles Mackay (LeukoSite Inc., Cambridge, MA).
Cells.
Monocytes (21), lymphocytes (3), and neutrophils (22) were isolated from donor blood buffy coats. The lymphocytes were cultured in the presence of IL-2 as previously described (3). Eosinophils (12) and basophils (23, 24) were purified from the venous blood of healthy volunteers. The eosinophil preparations were >95% pure and the basophil preparations consisted of 70– 80% basophils and 20–30% lymphocytes.
[Ca2+]i Changes.
Changes in the cytosolic free Ca2+ concentration ([Ca2+]i) were assayed in monocytes, eosinophils, lymphocytes, and neutrophils loaded with Fura-2 (25). Receptor desensitization was tested by monitoring [Ca2+]i changes in response to sequential stimulation with chemokines (21).
In Vitro Chemotaxis.
Chemotaxis was assessed in 48-well chambers (Neuro Probe, Cabin John, MD) using polyvinylpyrrolidone-free polycarbonate membranes (Nucleopore, Neuro Probe, Cabin John, MD) with 5-µm pores for eosinophils, basophils, neutrophils and monocytes, and 3-µm pores for lymphocytes (3, 6). Cell suspensions and chemokine dilutions were made in RPMI 1640 supplemented with 20 mM Hepes, pH 7.4, and 1% pasteurized plasma protein solution (Central Laboratory of the Swiss Red Cross). Migration was allowed to proceed for 60 min at 37°C in 5% CO2. The membrane was then removed, washed on the upper side with PBS, fixed, and stained. All assays were done in triplicate, and the migrated cells were counted in five randomly selected fields at 1,000-fold magnification. Spontaneous migration was determined in the absence of chemoattractant.
Histamine and Leukotriene C4 (LTC4) Release.
Basophils (0.1– 0.3 x 106 cells/ml) in 20 mM Hepes, pH 7.4, containing 125 mM NaCl, 5 mM KCl, 1 mM MgCl2, 1 mM CaCl2, 0.5 mM glucose and 0.025% BSA were warmed to 37°C, primed with 10 ng/ml IL-3 for 5 min, and then challenged with a chemokine. The reaction was stopped on ice after 20 min and histamine and LTC4 were measured in the supernatants (26). Histamine release was expressed as percent of the total cellular content (determined after cell lysis). LTC4 generation was expressed in ng per 106 basophils.
Enzyme Release.
N-acetyl-β-D-glucosaminidase release was assayed in monocytes (21) and elastase release in neutrophils (22) exactly as described previously.
In Vivo Activity.
A male rhesus monkey of 7.5 kg was anesthesized by injection of 10 mg/kg Ketamine (Ketalar, Parke Davis) i.m. and 15 mg/kg Na Thiopental (Pentotal, Abbott) i.v. The chemokines (100 pmol eotaxin, 100 and 1,000 pmol eotaxin-2 in 100 µl pyrogen-free isotonic saline) were then administered intradermally on the back, and full skin thickness punch biopsies of 8-mm diameter were taken from the injection sites after 4 h. The biopsies were fixed in formalin, embedded in paraffin, and 5-µm sections were prepared. The sections were stained with Giemsa solution or hematoxylin and eosin, and the eosinophil infiltrates were evaluated by two independent observers. In each section, eosinophils were counted at a magnification of 630x in five randomly selected fields including a postcapillary venule, using a grid of 0.19 x 0.19 mm, and the number of eosinophils per mm2 was calculated. Photographs were taken at a magnification of 1,000x with a Nikon Microphot microscope (Nikon AG, Switzerland).
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Results and Discussion
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Eotaxin-2 Structure.
The purified material contained a protein of
8.5 kD, as judged on SDS-PAGE. The material was treated with GluC under conditions ensuring cleavage COOH-terminally of glutamic acid, and three fragments were obtained. The first two were homogeneous and extended from residue 1 to 18 and 19 to 54, respectively, while the third showed some COOH-terminal heterogeneity. Sequencing and mass spectrometry revealed tree COOH-terminally truncated forms consisting of residues 55–73, 55–75, and 55–76 in addition to the main form extending from residue 55 to 78 (Fig. 1). The mass of the 78-residue form of eotaxin-2 is 8,778.3 and the amino acid identities to reference chemokines were 43% for MCP-4, 42% for MIP-1
, 39% for MCP-3 and eotaxin, and 32% for RANTES. Since the four COOH-terminal variants could not be separated all tests were performed with the mixture. The sequence shown in Fig. 1 corresponds to the cDNA-deduced sequence of the chemokine MPIF-2 that was shown to inhibit the proliferation of myeloid progenitors (18), except for two substitutions, Ala35 for Gly and Ser47 for Phe.
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