Neutrophil antimicrobial defense against Staphylococcus aureus is mediated by phagolysosomal but not extracellular trap‐associated cathelicidin

Neutrophils kill invading pathogens by AMPs, including cathelicidins, ROS, and NETs. The human pathogen Staphylococcus aureus exhibits enhanced resistance to neutrophil AMPs, including the murine cathelicidin CRAMP, in part, as a result of alanylation of teichoic acids by the dlt operon. In this study, we took advantage of the hypersusceptible phenotype of S. aureus ΔdltA against cationic AMPs to study the impact of the murine cathelicidin CRAMP on staphylococcal killing and to identify its key site of action in murine neutrophils. We demonstrate that CRAMP remained intracellular during PMN exudation from blood and was secreted upon PMA stimulation. We show first evidence that CRAMP was recruited to phagolysosomes in infected neutrophils and exhibited intracellular activity against S. aureus. Later in infection, neutrophils produced NETs, and immunofluorescence revealed association of CRAMP with S. aureus in NETs, which similarly killed S. aureus wt and ΔdltA, indicating that CRAMP activity was reduced when associated with NETs. Indeed, the presence of DNA reduced the antimicrobial activity of CRAMP, and CRAMP localization in response to S. aureus was independent of the NADPH oxidase, whereas killing was partially dependent on a functional NADPH oxidase. Our study indicates that neutrophils use CRAMP in a timed and locally coordinated manner in defense against S. aureus.