Immunotoxic effects of benzo[a]pyrene on rainbow trout (Oncorhynchus mykiss)

Abstract

Polycyclic aromatic hydrocarbons (PAHs) are a group of compounds with immunotoxic and carcinogenic potential that may pose a threat to fish populations. The mechanism underlying immune toxicity of PAHs in fishes remains unclear. Some evidence supports the requirement for metabolism to more toxic metabolites. This twopart 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. In the first part of the study, 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 d. A new flow cytometry method for absolute counts of differential leukocyte 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. The second part of the study examined whether intraperitoneal exposure of rainbow trout to BaP caused leukocyte mRNA expression changes in five cytochrome P450 (CYP) enzymes; to their transcription factor, AhR; or to an extrinsic pathway apoptosis checkpoint, p53. mRNA expression was analyzed in immunomagnetically isolated B cells and thrombocytes from blood, spleen, or head kidney in an effort to clarify the tissue and cell specific toxicity of BaP. Significant inductions above control levels were observed in CYP1A1 in liver, blood B cells, and blood thrombocytes; CYP1B1 in blood B cells, and blood thrombocytes; CYP1A3 in liver, blood and spleen B cells; and AhR in spleen thrombocytes. No significant changes were found in CYP1C1, CYP1C2, or p53. Increased mRNA expression was observed 14 d after exposure, indicating a prolonged physiological effect of a single BaP injection. Although there were differences in expression, it was concluded that variations in the presence or induction of CYP enzymes is not enough to explain the difference in toxicity observed between B cells and thrombocytes. An induction of AhR in thrombocytes is interesting, but difficult to explain given current knowledge on endogenous AhR regulation and the limited toxicity in thrombocytes. Overall, our findings provide support for a complex mechanism of toxicity for chronic PAH exposure, and suggest that CYP expression profiles do not fully explain these effects