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Major basic protein (MBP), the predominant cationic protein of human eosinophil specific granules, is stored within crystalloid cores of these granules. Secretion of MBP contributes to the immunopathogenesis of varied diseases. Prior electron microscopy (EM) of eosinophils in sites of inflammation noted losses of granule cores in the absence of granule exocytosis and suggested that eosinophil granule proteins might be released through piecemeal degranulation (PMD), a secretory process mediated by transport vesicles. Because release of eosinophil granule-derived MBP through PMD has not been studied, we evaluated secretion of this cationic protein by human eosinophils. Intracellular localizations of MBP were studied within non-stimulated and eotaxin-stimulated human eosinophils by both immunofluorescence and a pre-embedding immunonanogold electron microscopy (EM) method which enables optimal epitope preservation and antigen access to membrane microdomains. In parallel, quantification of transport vesicles was assessed in eosinophils from a patient with hypereosinophilic syndrome (HES). Our data demonstrate vesicular trafficking of MBP within eotaxin-stimulated eosinophils. Vesicular compartments, previously implicated in transport from granules to the plasma membrane, including large vesiculotubular carriers termed eosinophil sombrero vesicles (EoSVs), were found to contain MBP. These secretory compartments were significantly increased in numbers within HES eosinophils. Moreover, in addition to granule-stored MBP, even unstimulated eosinophils contained appreciable amounts of MBP within secretory vesicles, as evidenced by immunonanogold EM and immunofluorescent co-localizations of MBP and CD63. These data suggest that eosinophil MBP, with its multiple extracellular activities, can be mobilized from granules via PMD into secretory vesicles and both granule- and secretory vesicle-stored pools of MBP are available for agonist-elicited secretion of MBP from human eosinophils. The recognition of PMD as a secretory process to release MBP is important to understand the pathological basis of allergic and other eosinophil-associated inflammatory diseases.

Intracellular granules in several types of leukocytes contain preformed proteins whose secretions contribute to immune and inflammatory functions of leukocytes, including eosinophils, cells notably associated with asthma, allergic inflammation, and helminthic infections. Cytokines and chemokines typically elicit extracellular secretion of granule proteins by engaging receptors expressed externally on the plasma membranes of cells, including eosinophils. Eosinophil granules, in addition to being intracellular organelles, are found as intact membrane-bound structures extracellularly in tissue sites of eosinophil-associated diseases. Neither the secretory capacities of cell-free eosinophil granules nor the presence of functional cytokine and chemokine receptors on membranes of leukocyte granules have been recognized. Here, we show that granules of human eosinophils express membrane receptors for a cytokine, IFN-γ, and G protein–coupled membrane receptors for a chemokine, eotaxin, and that these receptors function by activating signal-transducing pathways within granules to elicit secretion from within granules. Capacities of intracellular granule organelles to function autonomously outside of eosinophils as independent, ligand-responsive, secretion-competent structures constitute a novel postcytolytic mechanism for regulated secretion of eosinophil granule proteins that may contribute to eosinophil-mediated inflammation and immunomodulation.

Eosinophils are innate immune leukocytes implicated in the initiation and maintenance of type 2 immune responses, including asthma and allergy. The ability to store and rapidly secrete preformed cytokines distinguishes eosinophils from most lymphocytes, which must synthesize cytokine proteins prior to secretion and may be a factor in the apparent Th2 bias of eosinophils. Multiple studies confirm that human eosinophils from atopic or hypereosinophilic donors can secrete over 30 cytokines with a varying and often opposing immune-polarizing potential. However, it remains unclear whether all of these cytokines are constitutively preformed and available for rapid secretion from eosinophils in the circulation of healthy individuals or are restricted to eosinophils from atopic donors. Likewise, the relative concentrations of cytokines stored within eosinophils have not been studied. Here, we demonstrate that human blood eosinophils are not singularly outfitted with Th2-associated cytokines but rather, constitutively store a cache of cytokines with nominal Th1, Th2, and regulatory capacities, including IL-4, IL-13, IL-6, IL-10, IL-12, IFN-γ, and TNF-α. We demonstrate further rapid and differential release of each cytokine in response to specific stimuli. As agonists, strong Th1 and inflammatory cytokines elicited release of Th2-promoting IL-4 but not Th1-inducing IL-12. Moreover, a large quantity of IFN-γ was secreted in response to Th1, Th2, and inflammatory stimuli. Delineations of the multifarious nature of preformed eosinophil cytokines and the varied stimulus-dependent profiles of rapid cytokine secretion provide insights into the functions of human eosinophils in mediating inflammation and initiation of specific immunity.

Secretion of interleukin-4 (IL-4) by leukocytes is important for varied immune responses including allergic inflammation. Within eosinophils, unlike lymphocytes, IL-4 is stored in granules (termed specific granules) and can be rapidly released by brefeldin A (BFA)-inhibitable mechanisms upon stimulation with eotaxin, a chemokine that activates eosinophils. In studying eotaxin-elicited IL-4 secretion, we identified at the ultrastructural level distinct vesicular IL-4 transport mechanisms. Interleukin-4 traffics from granules via two vesicular compartments, large vesiculotubular carriers, which we term eosinophil sombrero vesicles (EoSV), and small classical spherical vesicles. These two vesicles may represent alternative pathways for transport to the plasma membrane. Loci of both secreted IL-4 and IL-4-loaded vesicles were imaged at the plasma membranes by a novel EliCell assay using a fluoronanogold probe. Three dimensional electron tomographic reconstructions revealed EoSVs to be folded, flattened and elongated tubules with substantial membrane surfaces. As documented with quantitative electron microscopy, eotaxin-induced significant formation of EoSVs while BFA pretreatment suppressed eotaxin-elicited EoSVs. Electron tomography showed that both EoSVs and small vesicles interact with and arise from granules in response to stimulation. Thus, this intracellular vesicular system mediates the rapid mobilization and secretion of preformed IL-4 by activated eosinophils. These findings, highlighting the participation of large tubular carriers, provide new insights into vesicular trafficking of cytokines.

Eosinophils, leukocytes involved in allergic, inflammatory and immunoregulatory responses, have a distinct capacity to rapidly secrete preformed granule-stored proteins through piecemeal degranulation (PMD), a secretion process based on vesicular transport of proteins from within granules for extracellular release. Eosinophil-specific granules contain cytokines and cationic proteins, such as major basic protein (MBP). We evaluated structural mechanisms responsible for mobilizing proteins from within eosinophil granules. Human eosinophils stimulated for 30–60 min with eotaxin, regulated on activation, normal, T-cell expressed and secreted (RANTES) or platelet activating factor exhibited ultrastructural features of PMD (e.g. losses of granule contents) and extensive vesiculotubular networks within emptying granules. Brefeldin A inhibited granule emptying and collapsed intragranular vesiculotubular networks. By immunonanogold ultrastructural labelings, CD63, a tetraspanin membrane protein, was localized within granules and on vesicles outside of granules, and mobilization of MBP into vesicles within and extending from granules was demonstrated. Electron tomography with three dimension reconstructions revealed granule internal membranes to constitute an elaborate tubular network able to sequester and relocate granule products upon stimulation. We provide new insights into PMD and identify eosinophil specific granules as organelles whose internal tubulovesicular networks are important for the capacity of eosinophils to secrete, by vesicular transport, their content of preformed and granule-stored cytokines and cationic proteins.

Eosinophil granules are notable for their content of preformed proteins whose secretion underlies many of the contributions eosinophils make in allergic and anthelminthic responses. We have recently shown that cell-free eosinophil granules respond to eotaxin and IFN-γ via cognate signaling-coupled granule membrane-expressed receptors, triggering phosphorylation of intragranular signaling molecules and eliciting specific secretion of granule-derived proteins. Thus, fully intact eosinophil granules deposited extracellularly within diseased tissues maintain a capacity to function as secretory competent organelles, demanding a fresh look at our understanding of the secretion of eosinophil granule-derived proteins, both intracellularly and extracellularly. Here, we review a series of our recent work revealing the unique ultrastructure of human eosinophil granules and mechanisms of intragranule sorting and mobilization of specific cytokine products from intact human eosinophils. Implications of these findings t...

Although leukocytes of the innate immune system, including eosinophils, contain within their granules preformed stores of cytokines available for selective and rapid release, little is known about the mechanisms governing the mobilization and secretion of these cytokines. Here we show that a cytokine receptor, the IL-4 receptor α chain, mediates eotaxin-stimulated mobilization of preformed IL-4 from eosinophil granules into secretory vesicles. Eosinophils contain substantial intracellular quantities of several granule- and vesicle-associated cytokine receptors, including IL-4, IL-6, and IL-13 receptors as well as CCR3. Both IL-4 and IL-4 receptor α chain colocalized in eosinophil granules; and after eotaxin-stimulation, IL-4 receptor α chain, bearing bound IL-4, was mobilized into secretory vesicles. These findings indicate that intracellular cytokine receptors within secretory vesicles transport their cognate cytokines requisite for the secretion of cytokines preformed in innate immune leukocytes.

Summary A distinguishing feature of eosinophils is their ability to rapidly release preformed cytokines from intracellular pools. Cytokines are delivered to the cell surface from granule stores by transport vesicles and are released in small packets at discrete locations along the cell surface through a process termed “piecemeal” degranulation. The study of this process has been hindered by lack of an assay sensitive enough to register minute protein concentrations and the inability to visualize morphology of cytokine secreting cells. These hindrances have necessitated our development of the EliCell assay, an agarose-based dual cytokine capture and detection system through which cytokine secretion and cellular morphology may be analyzed in concert. Cells are embedded within capture antibodycontaining agarose and stimulated under conditions of interest. Extracellularly released cytokine is captured within the matrix at the point of release from the cell and can be labeled with a fluorochromeconjugated antibody. Cytokine release and cellular morphology are visualized in parallel by phase contrast and fluorescence microscopy, respectively.