The effect of doxycycline on canine neutrophil functions.

Doxycycline is a broad-spectrum tetracycline-class antibiotic that is frequently used to treat bacterial infections. Its use has also been described in immune-mediated diseases due to its immunomodulatory properties. The aim of this study was to evaluate the immunomodulatory effect of doxycycline on canine neutrophil functions. Therefore, the release of reactive oxygen species (ROS) and the formation of neutrophil extracellular traps (NETs) were determined after incubation of canine PMNs with doxycycline in three different concentrations (4 µg/mL, 20 µg/mL and 200 µg/mL) for one and three hours, respectively. Additionally, a neutrophil killing assay with a doxycycline-resistant Staphylococcus aureus was performed to determine the bactericidal effect of doxycycline treated PMNs in presence of plasma. Doxycycline significantly diminished the production of ROS. However, doxycycline concentrations of 4 µg/mL and 20 µg/mL significantly induced NETs. A synergistic bacteriostatic effect of PMNs and doxycycline on a doxycycline-resistant Staphylococcus aureus isolate was detectable. However, already PMNs and especially doxycycline alone inhibited the growth. In summary, doxycycline showed a concentration-dependent immunomodulatory property in canine PMNs with a reduced ROS production and increased NET-induction. This immunomodulatory effect resulted in a slightly increased elimination of a doxycycline-resistant Staphylococcus aureus by the doxycycline plasma concentrations achieved in dogs.