Basophils are important in allergic diseases such as asthma because they produce a variety of inflammatory mediators. Activation of these cells with IgE and N-formyl-methionyl-leucyl-phenylalanine results in a variety of responses, including increased surface expression of CD203c and CD11b and release of histamine. Although considerable information is available on the effects of eicosanoids on neutrophils, eosinophils, and monocytes, less is known about their effects on basophils. In the present study, we examined the effects of various eicosanoids on the above basophil responses. Of the naturally occurring eicosanoids tested, prostaglandin D2 (PGD2; EC50, 10 nM) was by far the most potent activator of CD203c expression, with other prostanoids having little effect. This response was mediated by the DP2 receptor/chemoattractant receptor-homologous molecule expressed on Th2 cells because it was shared by the selective agonist 15R-methyl-PGD2 (EC50, 3 nM). The 5-lipoxygenase products leuko-triene B4 and 5-oxo-6,8,11,14-eicosatetraenoic acid also stimulated CD203c expression but to a lesser extent than PGD2, whereas leukotriene D4 was inactive. Neither PGD2 nor 5-oxo-6,8,11,14-eicosatetraenoic acid stimulated histamine release or CD63 expression on basophils. Both PGE2 and the DP1 receptor agonist BW245C [(4S)-(3-[(3R,S)-3-cyclohexyl-3-hydroxypropyl]-2,5-dioxo)-4-imidazolidineheptanoic acid] strongly inhibited DP2 receptor-mediated CD203c expression. The DP1 receptor antagonist BWA868C [3-[(2-cyclohexyl-2-hydroxyethyl)amino]-2,5-dioxo-1-(phenylmethyl)-4-imidazolidine-heptanoic acid] enhanced PGD2-induced CD203c expression, suggesting that interaction of PGD2 with DP1 receptors can limit activation of basophils by this prostaglandin. In conclusion, PGD2 is the most potent inducer of basophil CD203c expression among eicosanoids and may be a key mediator in asthma and other allergic diseases. The balance between DP1 and DP2 receptors may be important in determining the magnitude of basophil responses to this prostaglandin.
Basophils are similar to tissue mast cells in many respects, but unlike mast cells, do not synthesize PGD2 (Schleimer et al., 1985). They are found in the nasal mucosa and airways of allergic and asthmatic individuals and rapidly infiltrate tissues following allergen provocation (Kleinjan et al., 2000). Although mast cells are very prominent in the initial response to allergen, basophils appear to be more important in the late response (Schleimer et al., 1985). Basophils express high levels of high-affinity IgE receptors and are key cells in allergic reactions, especially late-phase reactions (Falcone et al., 2000). Following allergen stimulation, they release various inflammatory mediators, including histamine and LTC4 (Bochner, 2000), and are major sources of IL-4 and IL-13 (Devouassoux et al., 1999). The adhesion molecule CD11b (Bochner and Sterbinsky, 1991) and the basophil activation marker CD203c (Bühring et al., 1999) are also up-regulated following immunological activation of these cells.
Among hematopoietic cells, expression of CD203c is restricted to basophils, mast cells, and their CD34+ precursors, and it is the only selective marker for cells of this lineage (Bühring et al., 1999). This molecule is also known as ectonucleotide phosphodiesterase/pyrophosphatase 3 (Bühring et al., 2001) and is also highly expressed in the prostate and uterus in humans (Goding, 2000). Little is known about its role in basophils, although there is evidence that it may be involved in basophil adhesion and in the differentiation of basophil progenitor cells (Bühring et al., 2004). Although differing results have been obtained on the degree of surface expression of CD203c on unactivated basophils (Bühring et al., 1999; Boumiza et al., 2003), it is clearly highly expressed on basophils from sensitized subjects following treatment with allergen (Bühring et al., 1999; Hauswirth et al., 2002; Boumiza et al., 2003).
Eicosanoids are lipid mediators derived principally from arachidonic acid. There are two major groups of these compounds: cyclooxygenase products, including prostaglandins (PGs) and thromboxane A2, and 5-lipoxygenase products, including leukotrienes (LTs), 5-oxo-6,8,11,14-eicosatetraenoic acid (5-oxo-ETE), and lipoxins (Funk, 2001). The actions of eicosanoids are mediated by a large number of receptors that are each highly selective for a specific member of this family of lipid mediators. Prostaglandins D2, E2, F2α, and I2 and thromboxane A2 each have selective receptors, and some (PGD2 and PGE2) have multiple receptors (Narumiya and FitzGerald, 2001). There are two specific receptors for each of LTB4 and LTD4 (Funk, 2001) and another receptor for 5-oxo-ETE (Powell et al., 1993; Hosoi et al., 2002). Although PGs D2 and E2, LTs B4 and D4, and 5-oxo-ETE are all known to have effects on a variety of inflammatory cells, relatively little is know about the responses of basophils to these mediators.
The present study focuses on the effects of eicosanoids on the expression of CD203c and CD11b and histamine release by basophils. We were particularly interested in the effects of PGD2 on these responses because basophils express both receptors for this substance: the DP1 receptor and the DP2 receptor (also known as chemoattractant receptor-homologous molecule expressed on Th2 cells) (Hirai et al., 2001). The DP1 receptor is coupled to Gs, resulting in elevation of cAMP levels in platelets and other cell types following stimulation (Narumiya and FitzGerald, 2001). Although the role of this receptor in regulating basophil responses has not previously been investigated, it could potentially have an inhibitory effect as do other agents that act through stimulation of adenylyl cyclase (Peachell et al., 1988). Recently, our group (Monneret et al., 2001) and Hirai et al. (2001) independently discovered a second PGD2 receptor. Activation of the DP2 receptor in eosinophils elicits a variety of responses, including chemotaxis, Ca2+ mobilization, CD11b expression, actin polymerization, and shedding of l-selectin (Monneret et al., 2001). It also selectively induces the migration of eosinophils and basophils from a preparation of neutrophil-depleted leukocytes, as well as the migration of differentiated Th2 cells (Hirai et al., 2001). We report here that among eicosanoids, PGD2 is the most potent activator of CD203c expression in basophils. However, at high concentrations of PGD2, this response is tempered by the DP1 receptor, which exerts a negative effect on basophil activation.
Materials and Methods
Materials. Unless indicated otherwise, all of the eicosanoids as well as BW245C, CAY10399, and carbaprostacyclin were purchased from Cayman Chemical (Ann Arbor, MI). 5-Oxo-ETE was synthesized chemically as described previously (Khanapure et al., 1998). BWA868C was kindly provided by GlaxoSmithKline (Uxbridge, Middlesex, UK). fMLP was obtained from Sigma-Aldrich (St. Louis, MO). The following monoclonal antibodies and their respective isotype controls were used: PE-conjugated anti-CD203c (Beckman Coulter, Fullerton, CA), phycoerythrin-cyanin 5 (PC5)-conjugated anti-human CD45 (Immunotech, Marseille, France), PE-conjugated anti-human CD63 (Immunotech), polyclonal FITC-conjugated anti-human IgE (BioSource International, Camarillo, CA), PE-conjugated anti-CD11b (Beckman Coulter), and PC5-conjugated anti-CD49d (BD Biosciences PharMingen, San Diego, CA).
Preparation of Blood Cells for Flow Cytometry. Blood (4 ml), collected from healthy volunteers using citrate as the anticoagulant, was diluted with 20 ml of phosphate-buffered saline (PBS; 137 mM NaCl, 2.7 mM KCl, 1.5 mM KH2PO4, and 8.1 mM Na2HPO4 at a pH of 7.4). Following centrifugation at 200g for 15 min at 4°C, the pellet was resuspended in PBS containing CaCl2 (1 mM) and MgCl2 (1 mM) to a final volume of 4 ml. Aliquots of this suspension were used for all studies on expression of CD203c and CD11b.
Measurement of CD203c Expression. Aliquots (100 μl) of washed whole blood cells were preincubated at 37°C for 5 min in the presence or absence of various inhibitors, followed by the addition of either vehicle or agonist in 10 μl of PBS containing Ca2+, Mg2+, and 0.1% bovine serum albumin. After 10 min, the incubations were terminated by addition of ice-cold Facsflow (2 ml; BD Biosciences, San Jose, CA), and the tubes were placed in ice. After centrifugation at 700g for 6 min at 4°C, the pellets were incubated in the dark with a mixture of FITC-labeled anti-IgE (1.25 μl), PE-labeled anti-CD203c (5 μl), and PBS (3.75 μl) for 30 min at 4°C. Optilyse C (0.5 ml; Beckman Coulter) was then added, and the mixtures were kept in the dark at room temperature for 15 min. The samples were then centrifuged at 700g for 6 min at 4°C, and the pellets were resuspended in 0.4 ml of PBS containing 1% formaldehyde. The distribution of fluorescence intensities due to PE-anti-CD203c labeling was measured by flow cytometry (FACSCalibur; BD Biosciences) in basophils, which were gated on the basis of intense labeling with FITC-anti-IgE and forward scatter as shown in Fig. 1. Positive cells were then further gated based on side scatter (Fig. 1). The results are expressed as percentages of the maximal response to PGD2.
Measurement of CD11b Expression. CD11b expression was measured exactly as described above except that the cells were stained by addition of a mixture of FITC-labeled anti-IgE (1.25 μl), PE-labeled anti-CD11b (10 μl), PC5-labeled anti-CD49d (5 μl), and PBS (3.75 μl). The distribution of fluorescence intensities due to PE-anti-CD11b labeling was measured by flow cytometry in basophils, which were identified as described above, and eosinophils, which were identified on the basis of high expression of CD49d and high side scatter.
Measurement of CD63. Aliquots (100 μl) of whole blood, collected in heparin, were incubated for 10 min at 37°C with agonists in PBS (10 μl). The tubes were then placed on ice and incubated with FITC-labeled anti-IgE, PE-labeled anti-CD63, and PC5-labeled anti-CD45 for 30 min at 4°C in the dark. Red blood cells were then lysed using the Q-prep system (Beckman Coulter). The samples were analyzed by flow cytometry using a Coulter EPICS XL instrument with System II software (Beckman Coulter) as previously described (Boumiza et al., 2003). A region enriched in basophils that also contained lymphocytes and monocytes was first identified on the basis of side scatter and high expression of CD45. Basophils were then selected from this population based on high expression of IgE. The percentage of basophils that were positive for CD63 was then determined using a threshold that was defined by the PE fluorescence of cells treated with vehicle alone in the absence of any agonist.
Measurement of Histamine Release. Histamine was quantified using a radioimmunoassay kit from Immunotech following the procedure supplied by the manufacturer. Aliquots (100 μl) of whole blood were diluted with 300 μl of dilution buffer (provided by the manufacturer) and incubated for 30 min at 37°C with agonists dissolved in 200 μl of PBS. The tubes were then placed on ice, and, following centrifugation, histamine was measured in the supernatants. Total histamine content was determined in a control tube after a water total cell lysis. Results obtained with agonists are expressed as percentages of total cellular histamine.
Data Analysis. The results are presented as means ± S.E. The statistical significance of the differences between treatments was assessed using two-way analysis of variance with the Student-Newman-Keuls test being used for post hoc analysis. P < 0.05 was considered to be statistically significant.
PGD2 Is a Potent Stimulus of CD203c Expression in Basophils. We investigated the effects of PGD2 and other eicosanoids on the expression of CD203c in basophils in whole blood following removal of plasma. Basophils were identified on the basis of forward scatter and high expression of IgE using flow cytometry (Fig. 1A) and further selected by gating on side scatter as shown in the inset to Fig. 1A. The effect of PGD2 (100 nM) on CD203c expression by basophils is shown in Fig. 1B. Unstimulated basophils displayed very low CD203c expression (shaded histogram), whereas basophils treated with PGD2 strongly expressed this protein.
Concentration-response curves for PGD2 and other prostanoids are shown in Fig. 2A. PGD2 is a potent stimulator of CD203c expression, with an EC50 value of 10 ± 2 nM. Because there was some variability in expression levels among individual donors, the results are shown as percentages of the maximal response to PGD2, which was 4.3 ± 0.6 times the basal level of CD203c. In contrast to PGD2, PGE2 and the thromboxane A2 analog U46619 had no effect on CD203c expression. The prostacyclin analog and selective IP receptor agonist carbaprostacyclin were also without effect on CD203c expression (data not shown), whereas PGF2α had a slight effect at concentrations of 1 μM or higher. The selective DP2 receptor agonist 15R-methyl-PGD2 was the most potent compound tested, with an EC50 value (3.2 ± 0.5 nM) about 3 times lower than that of PGD2.
The effects of PGD2 are compared with those of 5-lipoxygenase products and fMLP in Fig. 2B. In contrast to PGD2, the response to fMLP was highly variable among subjects, as can be seen by the much larger standard errors. Low concentrations (≤10 nM) of these two substances induced similar increases in CD203c expression, whereas at higher concentrations (≥100 nM), fMLP was usually more effective. In contrast, all of the 5-lipoxygenase products tested were less active than PGD2 in stimulating CD203c expression. LTB4 (EC50, 12 ± 2 nM) was nearly as potent at PGD2 but only 43 ± 5% as efficacious. 5-Oxo-ETE (EC50, 37 ± 12 nM) was less potent and had a maximal response that was 27 ± 7% that of PGD2. LTD4 had virtually no effect on CD203c expression by basophils.
PGD2 Stimulates Expression of CD11b on Basophils. PGD2 is a potent stimulator of CD11b expression on basophils with an EC50 value of 11 ± 2 nM (Fig. 3A) and a maximal response of 140 ± 28% above control. In contrast, 5-oxo-ETE only weakly stimulates CD11b expression on these cells, with an EC50 of 95 ± 33 nM and a maximal response about 33 ± 13% that of PGD2. To ensure that 5-oxo-ETE was still fully active in preparations of whole blood lacking only plasma, we tested its effects on eosinophils in the same samples as those used to measure basophil responses (Fig. 3B). 5-Oxo-ETE (EC50, 23 ± 2 nM) was somewhat less potent than PGD2 in stimulating CD11b expression by eosinophils but induced a greater maximal response (47 ± 7% above that of PGD2). The slightly lower potency of 5-oxo-ETE may have been due to nonspecific binding to red blood cells in the samples because we have consistently observed a small shift to the right in the concentration-response curve for 5-oxo-ETE-induced responses in whole blood compared with isolated leukocytes. As was the case for CD203c expression, LTD4 failed to stimulate CD11b expression in basophils (data not shown).
Activation of DP1 Receptors Inhibits Expression of CD203c by Basophils. Although PGD2 clearly stimulates expression of CD203c by basophils via DP2 receptors, it is theoretically possible that this response could be tempered by activation of DP1 receptors on these cells. Therefore, we investigated the effects of the selective DP1 receptor agonist BW245C on the response of basophils to the selective DP2 receptor agonist 15R-methyl-PGD2. BW245C (1 μM) reduced the maximal response to 15R-methyl-PGD2 by 50 ± 7% (Fig. 4A). The concentration-response curve for the inhibitory effect of BW245C on the response to a near-maximal concentration of 15R-methyl-PGD2 (10 nM) is shown in Fig. 4B. BW245C was a potent inhibitor of CD203c expression (IC50, 8 ± 3 nM) with a maximal inhibitory effect of 77 ± 2%. BW245C also inhibited CD203c expression induced by fMLP (10 nM), but the maximal response (44 ± 3%) was less than that for inhibition of the response to 15R-methyl-PGD2.
The DP1 Receptor Antagonist BWA868C Enhances PGD2-Elicited CD203c Expression. Although the predominant response of basophils to PGD2 is clearly stimulation, this prostaglandin could also activate the inhibitory Gs-coupled DP1 receptor, which could potentially limit PGD2-elicited CD203c expression. To examine this possibility, blood cells were preincubated with the selective DP1 receptor antagonist BWA868C (1 μM) for 5 min prior to addition of PGD2. BWA868C had no effect on CD203c expression on its own and, if anything, tended to reduce expression induced by low concentrations of PGD2 (not significant; Fig. 5A). However, at concentrations of PGD2 at or above 1 μM, BWA868C significantly enhanced CD203c expression (P < 0.01), suggesting that interaction of PGD2 with the DP1 receptor can reduce the response of basophils to high concentrations of this prostaglandin. To further test this hypothesis, we examined the effect of BWA868C on the response of basophils to 15R-methyl-PGD2, which is devoid of DP1 receptor agonist activity. In contrast to its stimulatory effect on PGD2-induced CD203c expression, BWA868C slightly diminished CD203c expression in response to 15R-methyl-PGD2 (P < 0.02) (Fig. 5B).
PGD2 and 5-Oxo-ETE Do Not Stimulate Histamine Release or CD63 Expression by Basophils. The effects of PGD2 and selective DP1 and DP2 receptor agonists on histamine release and expression of the degranulation marker CD63 were also examined. In contrast to its potent stimulatory effects on CD203c and CD11b expression, PGD2 had little or no effect on histamine release from basophils in whole blood (Fig. 6A). BW245C (DP1 agonist), 13,14-dihydro-15-keto-PGD2 (DP2 agonist), and 5-oxo-ETE were also without effect on histamine release. As a positive control, we used fMLP, a potent stimulator of basophil degranulation, which stimulated the release of about 45% of the total histamine content in whole blood at a concentration of 100 nM. PGD2, BW245C, 13,14-dihydro-15-keto-PGD2, and 5-oxo-ETE were also without effect on the expression of CD63 by basophils, in contrast to fMLP, which induced 45% of basophils to express this marker (Fig. 6B).
PGE2 and the IP Receptor Agonist Carbaprostacyclin Inhibit CD203c Expression by Basophils. We also investigated the potential inhibitory effects on CD203c expression of other prostanoids that act through Gs-coupled receptors, including PGE2 and the stable PGI2 analog, carbaprostacyclin. PGE2 inhibited CD203c expression in response to 15R-methyl-PGD2 with an EC50 of 53 ± 39 nM, about 5 times higher than that of BW245C (Fig. 7). However, PGE2 was more efficacious, achieving virtually complete inhibition at a concentration of 10 μM. The response to PGE2 appeared to be biphasic, with an effect being observed at concentrations as low as 0.1 nM. The selective EP2 receptor agonist CAY10399 (Tani et al., 2001) also nearly completely inhibited the response to 15R-methyl-PGD2 at the highest concentration tested (10 μM), but unlike PGE2, had little effect at concentrations at or below 10 nM. The IP receptor agonist carbaprostacyclin had a similar effect but was somewhat less efficacious.
The effects of eicosanoids on the surface expression of CD203c and CD11b by basophils have been examined by flow cytometry in unfractionated human blood cells. Basophils can readily be selected from these cells on the basis of high expression of IgE and light scattering (Fig. 1). The identity of these cells as basophils was confirmed by their high expression of the selective basophil marker CD203c following stimulation. This approach has the advantage that there is little manipulation of the cells prior to treatment with agonists, thus minimizing nonspecific activation that often accompanies lengthy purification procedures.
The results show for the first time that PGD2 is a highly potent activator of CD203c expression on basophils. PGD2 was recently reported to have a modest stimulatory effect on CD11b expression by basophils (Yoshimura-Uchiyama et al., 2004), but the maximal response (∼15% above control) was considerably less that that observed in the present study (∼140% above control), possibly because the basophils had been activated during centrifugation over Percoll. The stimulatory effect of PGD2 on CD11b expression may contribute to basophil infiltration into tissues, following its allergen-induced release from mast cells. PGD2 is considerably more active than any of the other eicosanoids tested, including agonists of all of the known prostanoid receptors, leukotriene receptors, and 5-oxo-ETE, which has its own distinct receptor. This is in contrast to most other leukocytes, which are activated to a greater extent by products of the 5-lipoxygenase pathway. For example, LTB4 (Ford-Hutchinson et al., 1980) and 5-oxo-ETE (Powell et al., 1993) strongly activate neutrophils, whereas PGD2 is inactive (Monneret et al., 2001). LTB4 is also a potent stimulator of monocyte activation, whereas 5-oxo-ETE has only a modest effect, and PGD2 is ineffective. Although both PGD2 and 5-oxo-ETE are potent activators of eosinophils, 5-oxo-ETE induces a stronger chemoattractant response at higher concentrations (Monneret et al., 2001). The DP2 receptor is also highly expressed on Th2 cells, and PGD2 is a potent chemoattractant for these cells (Hirai et al., 2001).
Among the compounds tested, 15R-methyl-PGD2 is the most potent inducer of CD203c expression by basophils. This compound, which is the most potent known agonist at the DP2 receptor, also elicits a variety of responses in eosinophils, including cell migration, CD11b expression, calcium mobilization, and actin polymerization (Monneret et al., 2003). The potency of 15R-methyl-PGD2, in which the configuration of the 15-hydroxyl group is opposite to that of naturally-occurring prostaglandins, illustrates the different selectivity pattern of the DP2 receptor compared with other prostaglandin receptors, for which the 15S configuration is required for optimal activity. Unlike other prostaglandin receptors, the DP2 receptor is also selectively activated by indomethacin (Hirai et al., 2002) and the thromboxane B2 metabolite 11-dehydro-thromboxane B2 (Bohm et al., 2004).
The response of basophils to 5-oxo-ETE is clearly different from that of eosinophils, for which this substance is a potent agonist (Powell et al., 1995). Because previous studies with 5-oxo-ETE were conducted with purified or partially purified eosinophils, it is theoretically possible that, because of its hydrophobicity, it could be nonspecifically adsorbed by red cells present in the preparations used in the current study, thus reducing its potency. However, when we examined eosinophil CD11b expression in the same preparation, we found that 5-oxo-ETE induced a stronger response than PGD2, despite its much weaker effect on basophils. Thus, unlike eotaxin, which stimulates both types of cells (Uguccioni et al., 1997; Yamada et al., 1997), 5-oxo-ETE is more selective for eosinophils.
The present study is the first to demonstrate a rapid up-regulation of CD203c expression in response to an endogenous inflammatory mediator. CD203c was first shown to be up-regulated on basophils following cross-linking of the IgE receptor (Bühring et al., 1999), and we have recently demonstrated that its expression is increased by the bacterial peptide fMLP (Boumiza et al., 2003). In contrast, CD203c levels were unaffected by a large panel of cytokines (Bühring et al., 1999), although more recent studies suggest that its expression is increased after treatment with IL-3 for 90 min (Bühring et al., 2004) All previous studies have shown increased CD203c expression to occur in association with basophil degranulation. However, the current results demonstrate that these two responses can be dissociated because PGD2 is a potent activator of CD203c expression but has no effect on either histamine release or CD63 expression. This is consistent with other studies in which it was found that PGD2 or 13,14-dihydro-15-keto-PGD2 on their own do not affect histamine release (Virgolini et al., 1992; Yoshimura-Uchiyama et al., 2004), although they can enhance the response of basophils to other stimuli, including antigen, phorbol 12-myristate 13-acetate, and A23187 (Peters et al., 1984; Yoshimura-Uchiyama et al., 2004). The lack of a direct degranulatory response to PGD2 cannot be explained by its interaction with the inhibitory DP1 receptor because the selective DP2 receptor agonist 13,14-dihydro-15-keto-PGD2 also failed to induce both histamine release and CD63 expression (Fig. 6).
A dissociation between degranulation and other basophil responses has also been noted for certain chemokines. Eotaxin, like PGD2 (Hirai et al., 2001), is a potent basophil chemoattractant but has only a modest (Uguccioni et al., 1997) or no (Yamada et al., 1997) effect on histamine release. In contrast, MCP-1 has only a modest effect on basophil migration but is a potent inducer of histamine release (Uguccioni et al., 1997; Yamada et al., 1997). It is also possible that PGD2 could elicit other responses in basophils such as LTC4 release, as it does in eosinophils (Raible et al., 1992). The cyclooxygenase inhibitor indomethacin, which activates the DP2 receptor (Hirai et al., 2002), has been reported to induce a shape change in basophils (Stubbs et al., 2002), which might be consistent with stimulation of actin polymerization via this receptor.
Because PGD2 could interact with both stimulatory DP2 receptors and inhibitory DP1 receptors on basophils, the final response could be determined by the relative contributions of these two pathways. The present study shows for the first time that selective activation of the DP1 receptor with BW245C (IC50, 8 nM) results in inhibition of 15R-methyl-PGD2-induced basophil activation. This effect is presumably mediated by stimulation of adenylyl cyclase because elevation of cAMP levels by phosphodiesterase inhibitors inhibits basophil activation (Weston et al., 1997). The stimulatory effect of the selective DP1 receptor antagonist BWA868C on CD203c expression in response to higher concentrations of PGD2 suggests a regulatory effect of the DP1 receptor on PGD2-induced basophil activation (Fig. 8). In contrast, BWA868C failed to enhance the responsiveness of basophils to 15R-methyl-PGD2, which is devoid of DP1 receptor activity, and instead had a slight inhibitory effect on this response, possibly due to weak antagonist activity at the DP2 receptor. The failure of BWA868C to enhance the response of basophils to lower concentrations of PGD2 may be because these concentrations of PGD2 were insufficient to achieve inhibitory levels of cAMP through activation of the DP1 receptor. This could be related to differences in the concentration-response relationships for the DP1 and DP2 receptors, as well as the greater expression levels of DP2 receptors compared with DP1 receptors (Yoshimura-Uchiyama et al., 2004). Furthermore, it is possible that BWA868C may have modest antagonist activity toward the DP2 receptor, which could possibly counterbalance its DP1-mediated effect at low concentrations of PGD2. Overall, these results suggest that the DP1 receptor could serve to limit the response of basophils to high levels of PGD2, as might occur following mast cell activation.
Like PGD2, PGE2 has the potential to both activate basophils through EP1 or EP3 receptors and to inhibit activation through the Gs-coupled EP2 or EP4 receptors. However, we found no evidence for a stimulatory effect of PGE2 on these cells (Fig. 2A) but instead found that it is a potent inhibitor of agonist induced CD203c expression. This response appeared to be biphasic, with concentrations of PGE2 as low as 100 pM resulting in about 20% inhibition of the response to 15R-methyl-PGD2, but with complete inhibition occurring only at the highest concentration tested (10 μM). The effect of low concentrations of PGE2 may be due to the activation of EP4 receptors, as it was not observed with the highly selective EP2 receptor agonist CAY10399, whereas the effect of higher concentrations may be due to activation of EP2 receptors, as it occurred with both PGE2 and CAY10399. This would be consistent with the higher affinity of EP4 receptors for PGE2 (Abramovitz et al., 2000). PGE2 has previously been reported to stimulate adenylyl cyclase in basophils (Peachell et al., 1988) but to have relatively little effect (Peachell et al., 1988) or a modest inhibitory effect on histamine release (Peters et al., 1984; Virgolini et al., 1992). The PGE1 analog misoprostol, which activates EP1, EP3, and EP4 receptors, was reported to inhibit histamine release from basophils (Babakhin et al., 2000). Thus PGE2 or selective EP2 or EP4 receptor agonists may be useful in the treatment of allergic diseases such as asthma. Indeed, PGE2 has been shown to be a potent inhibitor of airway responses and inflammation in both humans (Gauvreau et al., 1999) and animal models (Martin et al., 2002).
There is considerable evidence that PGD2 may be an important mediator in asthma and other allergic diseases. Large amounts of PGD2 are released into the airways following allergen challenge of human asthmatic subjects (Murray et al., 1986). Transgenic mice overexpressing lipocalin-type PGD synthase exhibit enhanced pulmonary inflammation following antigen challenge (Fujitani et al., 2002). PGD2 induces migration of eosinophils, basophils, and Th2 cells through the DP2 receptor (Hirai et al., 2001; Monneret et al., 2001). The present results are consistent with a role for PGD2 in allergic diseases and suggest that it is a key endogenous mediator of basophil activation. Because the DP1 receptor can serve to attenuate DP2 receptor-mediated basophil activation, the relative numbers of these two receptors could determine the degree of response of these cells to PGD2. It will be interesting to determine whether the relative expression of these receptors is altered in allergic diseases such as asthma.
We thank Christel Tabet (Beckman Coulter) for providing PE-labeled anti-CD203c.
- Received July 23, 2004.
- Accepted September 23, 2004.
This work was supported by the Canadian Institutes of Health Research (Grant MOP-6254 to W.S.P.), the J.T. Costello Memorial Research Fund, the National Institutes of Health (Grants DK44730 and HL69835 to J.R.), and the National Science Foundation (Grant CHE-90-13145 to J.R. for an AMX-360 NMR instrument).
Article, publication date, and citation information can be found at http://jpet.aspetjournals.org.
ABBREVIATIONS: IL, interleukin; PG, prostaglandin; LT, leukotriene; 5-oxo-ETE, 5-oxo-6,8,11,14-eicosatetraenoic acid; BW245C, (4S)-(3-[(3R,S)-3-cyclohexyl-3-hydroxypropyl]-2,5-dioxo)-4-imidazolidineheptanoic acid; CAY10399, 9-oxo-11α,16S-dihydroxy-17-cyclobutylprosta-5Z,13E-dien-1-oic acid; BWA868C, 3-[(2-cyclohexyl-2-hydroxyethyl)amino]-2,5-dioxo-1-(phenylmethyl)-4-imidazolidine-heptanoic acid; fMLP, N-formyl-methionyl-leucyl-phenylalanine; PC5, phycoerythrin-cyanin 5; FITC, fluorescein isothiocyanate; PBS, phosphate-buffered saline; U46619, 9,11-dideoxy-9α,11α-methanoepoxy PGF2α.
- The American Society for Pharmacology and Experimental Therapeutics