Characteristics and Regulation of Human Eosinophil ETosis In Vitro

Cytolytic ETosis is a type of programmed cell death that is distinct from apoptosis and necrosis and plays a major role in the innate immune system and disease progression. Through the process of ETosis, cells release their chromatin with diverse antimicrobial proteins into the extracellular milieu, forming extracellular traps (ETs). Although ETosis has been reported in several leukocyte types, few studies have compared ETosis and the component proteins of ETs in leukocytes. The aim of this study was to better understand the characteristics of eosinophil ETosis (EETosis) compared with other leukocytes. We isolated human blood eosinophils, neutrophils, basophils, monocytes, and lymphocytes and stimulated them with known ETosis inducers: the protein kinase C activator PMA and calcium ionophore A23187. Both stimuli induced eosinophil cell death and ET release after 180 minutes of stimulation in a nicotinamide adenine dinucleotide phosphate oxidase-dependent manner. PMA also induced nicotinamide adenine dinucleotide phosphate oxidase-dependent ETosis in neutrophils, whereas little or no significant ETosis was observed in basophils, monocytes, or lymphocytes at 180 minutes. Mass spectrometry-based proteomic analysis of eosinophil- and neutrophil-derived ETs identified 997 and 1,415 proteins, respectively. Among the physiological stimuli tested, immobilized IgA and IgG induced EETosis. CCL11 (C-C motif chemokine ligand 11) and IL-5 were weak inducers of EETosis, but costimulation significantly induced rapid EETosis. Under high serum or albumin conditions, costimulation with CCL11 and IL-5 paradoxically prolonged cell survival by preventing spontaneous apoptosis. This study provides an in-depth characterization of EETosis and highlights the precise regulation of eosinophil survival and cell death pathways.