Benzo[a]pyrene-induced immune alteration in the Japanese medaka Oryzias latipes: Probing the underlying cellular mechanisms

Increasing public concerns regarding the use of mammals in toxicological studies have led researchers to seek “alternative models” to investigate the biological effects of xenobiotics. Due to its small size and adaptability to a laboratory environment, the Japanese medaka ( Oryzias latipes) lends itself well to such studies. Although medaka are used routinely for carcinogenicity testing, little emphasis has been placed upon this species for assessing the immunotoxic effects of chemicals. Given the known effects of benzo(a)pyrene (BaP) on the immune system of mammals, studies were conducted using medaka to examine the effects and cellular mechanisms by which exposure to BaP might alter the immune status of this laboratory fish model. Results demonstrated that IP treatment with BaP suppressed medaka lymphocyte proliferation, T-lymphocyte dependent antibody-forming cell (AFC) numbers, and phagocyte intracellular superoxide production. Thus, exposure of medaka to BaP resulted in suppression of both innate and adaptive immune parameters. Additional experiments demonstrated induction of CYP1A expression and activity within kidney tubules and mononuclear immune cells, indicating that medaka immune organs/cells are capable of metabolizing BaP into immunotoxic metabolites in situ.; Suppression of humoral immunity was not seen in medaka following exposure to the BaP congener, benzo(e)pyrene (BeP); BeP has low affinity for the aryl hydrocarbon receptor (AhR) and does not induce CYP1A in this system. Furthermore, co-exposure of medaka to BaP and compounds that inhibit CYP1A activity (i.e., α-naphthoflavone [ANF] or dehydroepiandrosterone [DHEA]) ameliorated the immunotoxicity produced by exposure to BaP alone. This suggests that BaP metabolism into reactive metabolites may be required for suppression of immune function. In vitro exposure of medaka immune cells demonstrated the ability of BaP, BaP-7,8-dihydrodiol (BD), and BaP-7,8-dihydrodiol-9,10-epoxide (BPDE) to suppress AFC numbers. Similar to that observed following in vivo BaP exposure, CYP1A inhibitors (i.e., ANF or ellipticine [ELP]) reversed BaP- and BD-induced immunotoxicity. The immunotoxicity observed following exposure to BPDE was not altered by ANF or ELP; suggesting that BPDE does not require metabolism to produce immunosuppression. Given that inhibition of CYP1A activity alleviates BaP-induced immunosuppression (both in vitro and in vivo), metabolism of BaP into its metabolites appears necessary for immunotoxicity in medaka.