Sombrero vesicles: from overlooked structures to primary mediators of eosinophil secretion

Eosinophils, cells of the innate immune system, have a unique architecture, which includes morphologically complex secretory granules and a prominent, activation-responsive vesiculotubular system. In human eosinophils, this system is collectively termed eosinophil sombrero vesicles (EoSVs) because of its peculiar morphology feasibly identifiable by transmission electron microscopy. Considered as "microgranules" or "small granules" in the past, EoSVs are currently defined as active components of the eosinophil secretory pathway, sprouting from specific granules and trafficking granule-derived products in the eosinophil cytoplasm. Eosinophil secretory proteins associated with EoSVs include cationic proteins, cytokines, cytokine receptors, membrane fusion complexes, and tetraspanins. Beyond the intracellular landscape, emerging evidence has revealed a more complex functional repertoire for EoSVs. This review offers an overview of EoSVs as key participants in the mechanisms of eosinophil secretion during immune responses, incorporating the recent understanding of these transport carriers as extracellular vesicles, potentially implicated in the traffic of eosinophil products in the extracellular medium and intercellular communication.