Bacterial Siderophores Hijack Neutrophil Functions

Abstract

Abstract Neutrophils are primary immune cells that respond to inflammation and eliminate microbial transgression. Yet, E. coli have been reported to bloom during gut inflammation despite the heightened neutrophil activity. The survivability of E. coli in inflamed gut are not completely understood, although their production of enterobactin (Ent; a siderophore) to circumvent inflammation-induced iron scarcity may be one of the potential mechanisms. Herein, we report that Ent is not only an iron chelator, but is an immune-regulator that counter an array of neutrophil functions. We showed that Ent inhibited PMA and LPS induced generation of reactive oxygen species (ROS) and neutrophil extracellular traps (NETs) in both mouse and human neutrophils. Ent also impaired the degranulation of primary granules, inhibited phagocytosis and bactericidal activity of neutrophils, but without affecting their migration and chemotaxis. Molecular analysis revealed that Ent can chelate intracellular labile iron that are required for neutrophil oxidative responses. Other siderophores (pyoverdine, ferrichrome, deferoxamine) likewise inhibited ROS and NETs in neutrophils, thus indicating that the chelation of iron may, in part, explain their inhibitory effects. To counter iron theft by Ent, neutrophils rely on the siderophore-binding protein lipocalin 2 (Lcn2) in a ‘tug-of-war’ for iron. The inhibition of neutrophil ROS and NETs by Ent was augmented in Lcn2-deficient than WT neutrophils, but rescued by the exogenous addition of recombinant Lcn2. Taken together, our findings illustrate the novel concept that microbial siderophore’s iron scavenging property may serve as an antiradical defense system, that neutralize immune functions of neutrophils.