The effects of benzo[a]pyrene on leucocyte distribution and antibody response in rainbow trout (Oncorhynchus mykiss)

• Injection of benzo[a]pyrene results in sustained excretion for up to 42 days.
• Differential effects between cell types are distinguishable using monoclonal antibodies.
• Decreases in certain leukocytes occurred most strongly 21 days after exposure.
• Pre-exposure followed by antigen challenge caused a significant reduction in antibody.
• Different time profiles of effects suggest a complex mechanism of action.

Polycyclic aromatic hydrocarbons (PAHs) are a group of compounds with immunotoxic and carcinogenic potential that may pose a threat to fish populations. This study aims to utilize a newly developed fish immunotoxicology model to determine the immune tissue/cell population level effects of PAHs on rainbow trout, using benzo[a]pyrene (BaP) as a representative immunotoxic PAH. Intraperitoneal injection of 25 or 100 mg/kg BaP resulted in sustained exposure as indicated by biliary fluorescence at BaP wavelengths for up to 42 days. A new flow cytometry method for absolute counts of differential leucocyte distributions in spleen, blood, and head kidney was developed by combining absolute quantitative counts of total leukocytes in the tissue (3,3′-dihexyloxacarbocyanine iodide (DiOC6) dye) with relative differential counts using monoclonal antibodies for B cells, T cells, myeloid cells, and thrombocytes. Experiments indicated dose- and time-dependent decreases in the absolute number of B cells, myeloid cells, or T cells in blood, spleen, or head kidney after 7, 14 or 21 d of exposure. There was no change in the absolute numbers of erythrocytes or thrombocytes in any tissue. When rainbow trout were exposed to inactivated Aeromonas salmonicida after a 21 d exposure to 100 mg/kg BaP, circulating antibody concentrations were decreased by 56%. It was concluded that BaP has a cell lineage-specific toxic effect on some immune cells of rainbow trout, and causes a decrease in circulating antibody levels.