The effects of the alkyl polycyclic aromatic hydrocarbon retene on rainbow trout (Oncorhynchus mykiss) immune response

The objective of this study was to examine the immune toxicity of the alkyl polycyclic aromatic hydrocarbon (PAH) retene (7-isopropyl-1-methyl phenanthrene) in rainbow trout. Retene is a common alkyl PAH associated with combustion of terrestrial plants or industrial effluents. Rainbow trout were injected intraperitoneally with a single dose of 0, 1, or 10 mg/kg of retene and sampled at 21 d. Within each retene treatment, co-injection of formalin-killed Aeromonas salmonicida or a phosphate buffered saline sham was used to stimulate the trout immune system. At the highest retene dose (10 mg/kg) there was an increase in total blood leukocyte counts but only in the A. salmonicida-injected group. This result was paralleled by an increase in leukocytes in differential blood cell counts. There was an overall increase in A. salmonicida-specific antibody titre due to the antigen injection and there was also a significant stimulation of antibody production response at the 10 mg/kg retene dose. At the tissue level, immunohistochemistry revealed a greater density of B-lymphocytes at the highest retene dose in spleen and head kidney. A number of immune specific transcripts including the Th2 marker CD4, TCR, MHCII TNFα, and the Th1 marker CD8, INFγ were examined by quantitative RT-PCR in spleen and head kidney. There was no influence of the A. salmonicida antigen on the expression of Th1 related genes in either tissue but increased production of Th2 related CD4 co-receptor transcripts was observed in spleen at both retene concentrations. Retene appeared to be mildly immunostimulatory, either on its own, or in combination with an inactivated A. salmonicida immune challenge and these data suggest that exposures of fishes to retene may not pose significant risk of eliciting immunotoxic effects.