Human Eosinophils Secrete Preformed, Granule‐Stored Interleukin‐4 Through Distinct Vesicular Compartments

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Abstract
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.