Chronic exposure to environmental concentrations of enrofloxacin impairs neutrophil function in zebrafish via HPT axis disruption and hematopoietic dysregulation.

The widespread detection of fluoroquinolone antibiotics like enrofloxacin (ENR) in aquatic ecosystems raises significant concerns about ecological and immunological impacts. While acute ENR toxicity is documented, the immunotoxic effects of chronic, environmentally relevant exposure remain poorly understood. This study investigates the mechanisms by which long-term ENR exposure (28 days at 10-100 μg/L,concentrations reflecting common pollution levels in freshwater systems near aquaculture facilities and extreme contamination scenarios due to wastewater treatment failure) compromises zebrafish immunity, focusing on neutrophil dysfunction. Our results demonstrate that ENR reduces neutrophil counts by 12.2-55.1 % in hematopoietic organs (blood, kidney, spleen) and impairs neutrophil extracellular trap (NET) formation by suppressing reactive oxygen species (ROS) production. RNA-seq analysis reveals ENR disrupts hematopoietic differentiation in the kidney, downregulating gata2 (3.5-fold) and upregulating apoptosis-related genes (bax, 3.3-fold). Crucially, ENR indirectly impairs neutrophil function by dysregulating the hypothalamic-pituitary-thyroid (HPT) axis, reducing thyroxine (T3) levels by 29.9 % and thyrotropin-releasing hormone (TRH) by 13.5 %. These endocrine disruptions correlate with diminished ROS generation capacity (49.8 % reduction vs. controls) and compromised defenses against pathogenic infection, evidenced by 28 % higher mortality after Aeromonas hydrophila infection and 17.6 % higher mortality after SVCV infection. Notably, in vitro ENR exposure showed no direct neutrophil toxicity, highlighting the centrality of systemic endocrine-immune crosstalk. This work provides the first evidence linking chronic ENR exposure to HPT axis-mediated neutrophil suppression, offering critical insights into the ecological risks of antibiotic persistence in aquatic environments. SYNOPSIS: Limited studies have explored enrofloxacin's impact on immunity in aquatic environments. This study demonstrates that enrofloxacin disrupts zebrafish immune defenses by impairing thyroid signaling and the renal microenvironment, thereby posing a potential threat to aquatic ecosystems.