Antimicrobial Peptides Initiate IL-1β Posttranslational Processing: A Novel Role Beyond Innate Immunity

Abstract

Human monocytes stimulated with LPS produce large quantities of prointerleukin-1β, but little of this cytokine product is released extracellularly as the mature biologically active species. To demonstrate efficient proteolytic cleavage and export, cytokine-producing cells require a secondary effector stimulus. In an attempt to identify agents that may serve as initiators of IL-1β posttranslational processing in vivo, LPS-activated human monocytes were treated with several individual antimicrobial peptides. Two peptides derived from porcine neutrophils, protegrin (PTG)-1 and PTG-3, promoted rapid and efficient release of mature IL-1β. The PTG-mediated response engaged a mechanism similar to that initiated by extracellular ATP acting via the P2X7 receptor. Thus, both processes were disrupted by a caspase inhibitor, both were sensitive to ethacrynic acid and CP-424,174, two pharmacological agents that suppress posttranslational processing, and both were negated by elevation of extracellular potassium. Moreover, the PTGs, like ATP, promoted a dramatic change in monocyte morphology and a loss of membrane latency. The PTG response was concentration dependent and was influenced profoundly by components within the culture medium. In contrast, porcine neutrophil antimicrobial peptides PR-26 and PR-39 did not initiate IL-1β posttranslational processing. The human defensin HNP-1 and the frog peptide magainin 1 elicited export of 17-kDa IL-1β, but these agents were less efficient than PTGs. As a result of this ability to promote release of potent proinflammatory cytokines such as IL-1β, select antimicrobial peptides may possess important immunomodulatory functions that extend beyond innate immunity.