Page 520 - The Toxicology of Fishes

500 The Toxicology of Fishes

Bacillus cereus (Robohm, 1986). Given that the immunomodulatory effects of Cd in mammals as well
as fish depends on dose, mode of Cd exposure, and time of exposure in relation to immunization in both
fish (Albergoni and Viola, 1995a,b), contradictory results are not surprising.
The effects of Cd on innate immune function represent the best studied area of Cd-induced immuno-
toxicity in fish (Bennani et al., 1996; Lemaire-Gony et al., 1995; Sanchez-Daron et al., 1999; Voccia et
al., 1996; Zelikoff et al., 1994, 1995, 1996b). Studies in this laboratory (Zelikoff et al., 1995) have
demonstrated that in the absence of overt toxicity (i.e., changes in total body weight, lysozyme activity,
or cell viability) waterborne exposure of mature Aeromonas salmonicidae-injected rainbow trout (Onco-
rhynchus mykiss) to Cd at 2 µg/L significantly reduced phorbol myristate acetate (PMA)-stimulated free-
–
radical production (i.e., superoxide [·O ] and hydrogen peroxide [H O ] production) after 30 days of
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exposure. In the same study, phagocytosis was enhanced by Cd exposure after 8 days but returned to
control levels after longer exposure durations (17 and 30 days). Carp (Cyprinus carpio) exposed to Cd
under similar conditions also demonstrated enhanced phagocytic activity (Witeska, 1998); however,
unlike that seen in adults, Cd exposure of juvenile trout for 30 days depressed phagocytosis (Sanchez-
Dardon et al., 1999; Voccia et al., 1996).
Studies by Zelikoff et al. (1995), Voccia et al. (1996), and Sanchez-Daron et al. (1999) examined the
effects of low-dose waterborne Cd exposure on rainbow trout (Oncorhynchus mykiss) and demonstrated
suppressive effects on PMA-stimulated H O production. Intraperitoneal injection of Cd also reduced
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reactive oxygen intermediate (ROI) production (Bennani et al., 1996). In this study, a single i.p. injection
of Cd dose-dependently inhibited bacterially stimulated ROI production by sea bass (Dicentrarchus
labrax) pronephros phagocytes. On the other hand, in vitro treatment of bass phagocytes with Cd had
a dose-dependent opposite effect to that observed in vivo. It was suggested that discrepancies between
the in vivo and in vitro studies may have been due to differences in the differentiation state of the MØs.
Alternatively, effects observed in vivo may have been mediated by serum levels of corticosteroids and
catecholamines not operative in vitro. Inasmuch as Cd acts by inhibiting cellular respiration and uncou-
pling oxidative phosphorylation in mammalian systems, suppressive effects of Cd on phagocyte ROI
production were not unexpected. In contrast to the enhanced effects produced in rainbow trout (Sanchez-
Daron et al., 1999; Voccia et al., 1996; Zelikoff et al., 1995) and carp (Bennani et al., 1996), waterborne
exposure of Japanese medaka (Oryzias latipes) to 6 µg Cd/L for 5 days increased H O production
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compared to control fish (Zelikoff et al., 1996b). Overall, results of the aforementioned in vivo and in
vitro studies demonstrate the sensitivity of fish phagocytes to the immunomodulating effects of Cd. In
addition, the findings suggest that ROI production may be the most sensitive indicator of immunotoxic
effects associated with exposure to low, environmentally relevant doses of Cd. In fact, its applicability
as a biomarker in fish to predict the toxicological impact of contaminated aquatic environments has been
suggested (Zelikoff et al., 1994, 1995, 1996b).
In a study examining the effects of metal mixtures on rainbow trout (Oncorhynchus mykiss) immunity
(Sanchez-Dardon et al., 1999), waterborne exposure for 5 weeks to zinc (30 or 50 µg/L) significantly
reduced Cd-induced suppressive effects on MØ phagocytosis and ROI production. Co-exposure to zinc,
however, failed to protect against Cd-induced effects on antibody production or B- and T-lymphocyte
proliferation. Results of these studies suggest that zinc may act to protect innate immune functions from
Cd-induced toxicity in fish by mechanisms similar to those operative in mammals (e.g., induction of
metallothionein, prevention of Cd entry into the cell, competition with Cd for intracellular binding sites).
The heterogeneity of the results emphasizes the complexity of Cd and the need for further studies to
fully understand the mechanisms underlying Cd toxicity in aquatic life.
Although the majority of studies examining the effects of Cd on fish innate immune functions have
focused upon MØs, other cells important in maintaining nonspecific host defense also appear to be
sensitive targets for Cd-induced immunotoxicity. Studies by Viola et al. (1996), for example, demon-
strated that in vitro exposure of catfish (Ictalurus melas) NCCs to a concentration of soluble Cd as low
as 5 µM inhibited their ability to kill human tumor cells; in the presence of 10 µM Cd, cytotoxic activity
was completely ameliorated. In vitro studies with mammalian NK cells demonstrating the inhibitory
effects of Cd on NK cytotoxicity support these findings (Cifone et al., 1990).
Although only a limited number of studies in fish have examined the effects of Cd on cell-mediated
immunity, strong evidence exists demonstrating altered T-lymphocyte proliferative responses (Albergoni