Early exposure to environmental levels of sulfamethoxazole triggers immune and inflammatory response of healthy zebrafish larvae.

Trace levels of antibiotics are increasingly being detected in aquatic environment and their potential toxicity to aquatic organisms is concerning. Sulfamethoxazole (SMX), a veterinary sulfonamide widely used across the globe, exists ubiquitously in aquatic environment with concentrations up to micrograms per liter. This study aims to investigate the effects of environmentally relevant levels (0.1, 1, 10, 100 μg/L) of SMX on the health of zebrafish during early development. Our results show that SMX delays the hatchment of embryos and reduces the body length. A dose-response relationship of oxidative stress indicators including total-antioxidant capacity (T-AOC), inducible nitric oxide synthase (iNOS) and total nitric oxide synthase (TNOS), catalase (CAT) has been observed. Additionally, SMX up-regulates the gene expression of several key proinflammatory cytokines and their corresponding proteins including interleukin-1β (IL-1β), interferon-γ (IFN-γ) and interleukin-11 (IL-11) and the expression of genes including interleukin-6 (il-6), tumor necrosis factor-α (tnf-α). This indicates that early exposure of SMX may evoke inflammation response in healthy fish. Inhibition of lysozyme and recombination-activating genes (rags) suggests that SMX suppresses the ability of zebrafish to resist pathogen. The reduction of the expression of Toll-like receptors (TLRs) related genes and significant correlations between TLRs and other immune-related genes reveal that TLRs might be an immunoregulator of SMX for zebrafish embryos and larvae. The novelty of this study lies in that early exposure to environmental levels of SMX not only affects the growth and development of zebrafish larvae, but also triggers oxidative stress and inflammation, resulting in a reduction in host immune defense via TLRs in healthy fish.