Calcium-dependent potentiation of the pro-inflammatory functions of human neutrophils by tigecycline in vitro

Abstract

Tigecycline is the prototype of the recently introduced, intravenously administered glycylcycline class of antibiotics, developed in response to the increasing problem of antibiotic resistance in Gram-positive bacteria, especially Staphylococcus aureus, as well as Gram-negative bacteria and anaerobes. However, relatively little is known about the immunomodulatory potential of tigecycline, specifically its interactions with human neutrophils. In the current study we investigated the effects of tigecycline at therapeutically relevant concentrations and greater (0.625-10 mg/L) on alterations in cytosolic Ca(2+) concentrations, generation of antimicrobial reactive oxygen species (ROS) and release of granule proteases [elastase, matrix metalloproteinase-8 (MMP-8) and matrix metalloproteinase-9 (MMP-9)] by human blood neutrophils activated with the chemoattractant N-formyl-L-methionyl-L-leucyl-L-phenylalanine (fMLP; 1 μM). Cytosolic Ca(2+) concentrations were measured using fura-2/AM-based spectrofluorimetry and radiometric procedures, generation of ROS by oxygen consumption and myeloperoxidase-mediated auto-iodination, and protease release by ELISA procedures. Exposure of the cells to fMLP resulted in activation of the generation of ROS, as well as release of the granule proteases, all of which were significantly increased by pre-incubation of the cells with tigecycline in a dose-dependent manner. Tigecycline-mediated enhancement of these neutrophil functions was associated with elevations in the concentrations of cytosolic Ca(2+), which appeared to result from the Ca(2+) ionophore activity of tigecycline. Tigecycline, by functioning as a Ca(2+) ionophore, and independent of antimicrobial activity, potentiates the pro-inflammatory functions of human neutrophils in vitro.