Novel anti-bacterial activities of β-defensin 1 in human platelets: Suppression of pathogen growth and signaling of neutrophil extracellular trap formation

Human β-defensins (hbd) are antimicrobial peptides that curb microbial activity. although hbd's are primarily expressed by epithelial cells,we show that human platelets express hbd-1 that has both predicted and novel antibacterial activities. we observed that activated platelets surround staphylococcus aureus (s. aureus),forcing the pathogens into clusters that have a reduced growth rate compared to s. aureus alone. given the microbicidal activity of β-defensins,we determined whether hbd family members were present in platelets and found mrna and protein for hbd-1. we also established that hbd-1 protein resided in extragranular cytoplasmic compartments of platelets. consistent with this localization pattern,agonists that elicit granular secretion by platelets did not readily induce hbd-1 release. nevertheless,platelets released hbd-1 when they were stimulated by α-toxin,a s. aureus product that permeabilizes target cells. platelet-derived hbd-1 significantly impaired the growth of clinical strains of s. aureus. hbd-1 also induced robust neutrophil extracellular trap (net) formation by target polymorphonuclear leukocytes (pmns),which is a novel antimicrobial function of β-defensins that was not previously identified. taken together,these data demonstrate that hbd-1 is a previously-unrecognized component of platelets that displays classic antimicrobial activity and,in addition,signals pmns to extrude dna lattices that capture and kill bacteria. © 2011 kraemer et al.