Zileuton, 5-lipoxygenase blocker, modulates acute inflammatory response in Nile tilapia

Pharmacological control of leukotriene biosynthesis is well established in mammals, but its translational value for teleost inflammation remains unclear. We evaluated oral 5-lipoxygenase (5-LOX) inhibition with zileuton in Nile tilapia challenged with a bacterin of A. hydrophila to induce acute aerocystitis. Fish received vehicle or zileuton (2.25 or 4.50 mg/kg) before challenge. Outcomes were assessed at 6, 24, and 48 h post-inoculation (hpi) to investigate cellular profiles in exudates and blood, reactive oxygen species (ROS), acute-phase proteins (APPs), and the formation of splenic melano-macrophage centers (MMCs). Zileuton significantly reduced exudate granulocytes and thrombocytes at 6 hpi and macrophages at 24 hpi. Systemically, treatment attenuated transient peaks in neutrophils and lymphocytes (6 hpi) and thrombocytes (24-48 hpi). We observed correlations between lower ROS production and reduced blood neutrophil counts and macrophages in the exudate, suggesting a restrained oxidative burst. Zileuton down-modulated positive APPs (ceruloplasmin, complement C3, α₂-macroglobulin, haptoglobin) in time- and dose-dependent patterns, while negative APPs (albumin, transferrin) and apolipoprotein A1 showed no consistent change. Histology revealed MMCs enlargement/remodeling with bacterin that was tempered by zileuton. The anti-inflammatory response was dose-dependent, since fish treated with 4.50 mg/kg were more effective than 2.25 mg/kg. Tilapia receiving 4.50 mg/kg of zileuton exhibited a pronounced decrease in inflammatory cell accumulation in the exudate (granulocytes, lymphocytes, and macrophages), reduced ROS production, stronger suppression of positive acute-phase proteins (notably ceruloplasmin, complement C3, and haptoglobin), and more evident remodeling of splenic MMCs. No clinical adverse effects were observed. To our knowledge, this is the first demonstration in fish that oral 5-LOX inhibition by zileuton mitigates acute inflammatory reaction, and underscores leukotriene pathway as a promising lever for improving fish health outcomes.