Exogenous reactive oxygen species alter antioxidative state and immunological responses of rainbow trout

Authors: LIU, DIBO - Leibniz Institute Of Freshwater Ecology And Inland Fisheries; LAZADO, CARLO - Technical University Of Denmark; PEDERSEN, LARS-FLEMMING - Technical University Of Denmark; Straus, David - Dave; MEINELT, THOMAS - Leibniz Institute Of Freshwater Ecology And Inland Fisheries

Submitted to: Fish and Shellfish Immunology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/12/2020
Publication Date: 1/13/2020
Citation: Liu, D., Lazado, C.C., Pedersen, L., Straus, D.L., Meinelt, T. 2020. Exogenous reactive oxygen species alter antioxidative state and immunological responses of rainbow trout. Fish and Shellfish Immunology. https://doi.org/10.1016/j.aquaculture.2020.734956.
DOI: https://doi.org/10.1016/j.aquaculture.2020.734956

Interpretive Summary: In this study, we exposed rainbow trout to the water disinfectant peracetic acid at concentrations that are typically used in aquaculture in Europe. We were looking for impacts on oxidative balance, immunity and surface tissue. Our results indicate an increase of free radicals at the gill and in the blood, but not in the liver when we measured total antioxidative capacity. This increase was responded to with greater antioxidative protection from the cells. As a result, immunity and surface tissue were barely affected.

Technical Abstract: Endogenously produced reactive oxygen species (ROS) and reactive nitrogen species (RNS) play important roles in the vertebrate innate immune response. Besides their direct inhibitory activity against pathogens, ROS provide a chemotactic signal for the recruitment of circulating leukocytes. However, little is known about how exogenous ROS can influence internal ROS production and the host innate immune response. In the present study, rainbow trout (Oncorhynchus mykiss), were exposed to low, therapeutic concentrations of peracetic acid (PAA), which contains hydrogen peroxide (H2O2) when in equilibrium and also as a degradation product, as a source of exogenous ROS for the 6-week study. The treatments were achieved with periodic or continuous exposure method. For the periodic exposure, a single dose of 1 mg L-1 PAA (containing 1.4 mg L-1 H2O2) was applied biweekly and this dose spontaneously degraded within hours. For continuous exposure, a constant drip was applied which aimed to maintain 0.2 mg L-1 PAA (containing 0.28 mg L-1 H2O2) in the inflow water but was counteracted by escalating degradation by the second week. The exogenous PAA/H2O2 altered the level of endogenous total free radicals, either ROS or RNS. This was realized probably through diffusion at the body surface, as well as stress-activated endogenous generation (by periodic exposure only at a higher exogenous ROS concentrations). In response, the total antioxidative capacity (TAC) in gill and serum was significantly elevated. Liver showed no significant changes in the levels of ROS/RNS and TAC. Epidermal mucous cell density was significantly lower in response to constant low concentrations of exogenous ROS of the continuous exposure. Such a sensitive response was absent in the periodic exposure, probably due to rapid epidermal recovery. In contrast, the branchial structural adjustment of trout in response to exogenous ROS was less sensitive. Minimal hyperplasia of lamellae was present only in the periodic exposure with a higher exogenous ROS concentration. Enzymes responsible for innate cutaneous and humoral immunity, including lysozyme, alkaline phosphatase, myeloperoxidase and esterase, were not influenced by either exposure treatment. However, there was a ferroxidase enzyme that was highly sensitive to exogenous ROS: the ceruloplasmin. It was significantly higher in skin, independent of the exogenous ROS concentration and exposure method. In serum, ceruloplasmin reduction was positively correlated to the reduction of antiprotease activity. In conclusion, defense mechanisms in rainbow trout responded to exogenous ROS in a dose- and duration-dependent manner. Ceruloplasmin may have a key role in the host antioxidative and anti-proteolytic adaptation.