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Eosinophils and basophils, when activated, become major sources of cysteinyl leukotrienes, eicosanoid mediators pertinent to allergic inflammation. We show that the C-C chemokines, eotaxin and RANTES (regulated upon activation normal T cell expressed and secreted), activate eosinophils and basophils for enhanced leukotriene C 4 (LTC 4 ) generation by distinct signaling and compartmentalization mechanisms involving the induced formation of new cytoplasmic lipid body organelles. Chemokineinduced lipid body formation and enhanced LTC 4 release were both mediated by CCR3 receptor G protein-linked downstream signaling involving activation of phosphoinositide 3-kinase, extracellular signal-regulated kinases 1 and 2, and p38 mitogen-activated protein kinases. Chemokine-elicited lipid body numbers correlated with increased calcium ionophore-stimulated LTC 4 production; and as demonstrated by intracellular immunofluorescent localization of newly formed eicosanoid, lipid bodies were the predominant sites of LTC 4 synthesis in both chemokine-stimulated eosinophils and chemokine-primed and ionophore-activated eosinophils. Eotaxin and RANTES initiated signaling via phosphoinositide 3-kinase and mitogen-activated protein kinases both elicits the formation of lipid body domains and promotes LTC 4 formation at these specific extranuclear sites.

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Lipid bodies (also known as lipid droplets) are emerging as inflammatory organelles with roles in the innate immune response to infections and inflammatory processes. In this study, we identified MCP-1 as a key endogenous mediator of lipid body biogenesis in infection-driven inflammatory disorders and we described the cellular mechanisms and signaling pathways involved in the ability of MCP-1 to regulate the biogenesis and leukotriene B4 (LTB4) synthetic function of lipid bodies. In vivo assays in MCP-1−/− mice revealed that endogenous MCP-1 produced during polymicrobial infection or LPS-driven inflammatory responses has a critical role on the activation of lipid body-assembling machinery, as well as on empowering enzymatically these newly formed lipid bodies with LTB4 synthetic function within macrophages. MCP-1 triggered directly the rapid biogenesis of distinctive LTB4-synthesizing lipid bodies via CCR2-driven ERK- and PI3K-dependent intracellular signaling in in vitro-stimulated macrophages. Disturbance of microtubule organization by microtubule-active drugs demonstrated that MCP-1-induced lipid body biogenesis also signals through a pathway dependent on microtubular dynamics. Besides biogenic process, microtubules control LTB4-synthesizing function of MCP-1-elicited lipid bodies, in part by regulating the compartmentalization of key proteins, as adipose differentiation-related protein and 5-lipoxygenase. Therefore, infection-elicited MCP-1, besides its known CCR2-driven chemotactic function, appears as a key activator of lipid body biogenic and functional machineries, signaling through a microtubule-dependent manner.

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In noninfected rats, challenge with allergen following local IgE sensitization induced a pleurisy marked by intense protein exudation that plateaued from 30 min to 4 h after challenge, reducing thereafter. Infection of rats with Angiostrongylus costaricensis induced a 5-fold increase in blood eosinophil numbers by 25 days postinfection, whereas the numbers of eosinophils in the pleural cavity ranged from normal to a weak increase. In infected rats, identically sensitized, challenge with Ag induced a much shorter duration of pleural edema with complete resolution by 4 h, but no change in the early edema response. In parallel, infection increased the number of eosinophils recovered from the pleural cavity at 4 h, but not at 30 min, following allergen challenge. Pretreatment with IL-5 (100 IU/kg, i.v.) also increased eosinophil numbers in blood and, after allergen challenge, shortened the duration of the pleural edema and increased pleural eosinophil numbers. There were increases in the levels of both PGE2 and lipoxin A4 (LXA4) in pleural exudate. Selective cyclooxygenase (COX)-2 inhibitors, NS-398, meloxicam, and SC-236, did not alter pleural eosinophilia, but reversed the curtailment of the edema in either infected or IL-5-pretreated rats. Pretreatment of noninfected animals with the PGE analogue, misoprostol, or two stable LXA4 analogues did not alter the magnitude of pleural exudation response, but clearly shortened its duration. These results indicate that the early resolution of allergic pleural edema observed during A. costaricensis infection coincided with a selective local eosinophilia and seemed to be mediated by COX-2-derived PGE2 and LXA4.

Prostaglandin (PG)D2 is a key mediator of allergic inflammatory diseases that is mainly synthesized by mast cells, which constitutively express high levels of the terminal enzyme involved in PGD2 synthesis, the hematopoietic PGD synthase (H-PGDS). Here, we investigated whether eosinophils are also able to synthesize, and therefore, supply biologically active PGD2. PGD2 synthesis was evaluated within human blood eosinophils, in vitro-differentiated mouse eosinophils, and eosinophils infiltrating inflammatory site of mouse allergic reaction. Biological function of eosinophil-derived PGD2 was studied by employing inhibitors of synthesis and activity. Constitutive expression of H-PGDS was found within non-stimulated human circulating eosinophils. Acute stimulation of human eosinophils with A23187 (0.1 – 5 μM) evoked PGD2 synthesis, which was located at the nuclear envelope and was inhibited by pre-treatment with HQL-79 (10 μM), a specific H-PGDS inhibitor. Pre-stimulation of human eosinophils with arachidonic acid (AA; 10 μM) or human eotaxin (6 nM) also enhanced HQL-79-sensitive PGD2 synthesis, which, by acting on membrane-expressed specific receptors (DP1 and DP2), displayed an autocrine/paracrine ability to trigger leukotriene (LT)C4 synthesis and lipid body biogenesis, hallmark events of eosinophil activation. In vitro-differentiated mouse eosinophils also synthesized paracrine/autocrine active PGD2 in response to AA stimulation. In vivo, at late time point of the allergic reaction, infiltrating eosinophils found at the inflammatory site appeared as an auxiliary PGD2-synthesizing cell population. Our findings reveal that eosinophils are indeed able to synthesize and secrete PGD2, hence representing during allergic inflammation an extra cell source of PGD2, which functions as an autocrine signal for eosinophil activation.

We investigated whether cysteinyl leukotrienes (cysLT) are intracrine signal transducers that regulate human eosinophil degranulation mechanisms. Interleukin (IL)-16, eotaxin, and RANTES stimulate vesicular transport–mediated release of preformed, granule-derived IL-4 and RANTES from eosinophils and the synthesis at intracellular lipid bodies of LTC4, the dominant 5-lipoxygenase–derived eicosanoid in eosinophils. 5-Lipoxygenase inhibitors blocked IL-16–, eotaxin-, and RANTES-induced IL-4 release; but neither exogenous LTC4, LTD4, nor LTE4 elicited IL-4 release. Only after membrane permeabilization enabled cysLTs to enter eosinophils did LTC4 and LTD4 stimulate IL-4, but not RANTES, release. LTC4-elicited IL-4 release was pertussis toxin inhibitable, but inhibitors of the two known G protein–coupled cysLT receptors (cysLTRs) (CysLT1 and CysLT2) did not block LTC4-elicited IL-4 release. LTC4 was 10-fold more potent than LTD4 and at low concentrations (0.3–3 nM) elicited, and at higher concentrations (>3 nM) inhibited, IL-4 release from permeabilized eosinophils. Likewise with intact eosinophils, LTC4 export inhibitors, which increased intracellular LTC4, inhibited eotaxin-elicited IL-4 release. Thus, LTC4 acts, via an intracellular cysLTR distinct from CysLT1 or CysLT2, as a signal transducer to selectively regulate IL-4 release. These results demonstrate that LTC4, well recognized as a paracrine mediator, may also dynamically govern inflammatory and immune responses as an intracrine mediator of eosinophil cytokine secretion.