Page 523 - The Toxicology of Fishes

The Immune System of Fish: A Target Organ of Toxicity 503

tyrosine phosphorylation of cell proteins (Low and Sin, 1998; MacDougal et al., 1996). Head kidney
MØs recovered from rainbow trout (Oncorhynchus mykiss) exposed for several weeks to 0.5 ppb Hg
exhibited diminished phagocytosis, respiratory burst activity, and Ig levels (Sanchez-Dardon et al., 1999).
Japanese medaka (Oryzias latipes) exposed to a tenfold higher Hg concentration for 5 days failed to
exhibit similar responses (Zelikoff et al., 1998).
Similar to that seen in trout, exposure to divalent Hg also alters the immune system of blue gourami
(Trichogaster trichopterus) (Low and Sin, 1998). Exposure of fish for 2 weeks to 0.09 ppm Hg signif-
icantly increased gourami kidney lysozyme activity. Interestingly, enzyme activity decreased to control
2–
levels following co-exposure to an equimolar concentration of selenium (as SeO ). Other studies by the
3
same authors demonstrated similar findings in tilapia (Oreochromis). In this study, fish exposed to 0.01
mg Hg per L had significantly reduced plasma agglutinating antibody titers 7 and 8 weeks following
immunization with formalin-killed Aeromonas hydrophila; waterborne exposure to 0.09 mg Hg per L
reduced agglutinating titers for 5 weeks after infection. Tilapia, co-exposed to Hg plus 0.5 mg SeO 3 2–
per L, demonstrated significantly decreased agglutinating antibody titers in the first 3 weeks following
bacterial injection. Overall, effects of Hg on fish agglutination titers were inconsistent and difficult to
interpret. In vitro studies have also been carried out to determine Hg-induced effects on lymphocyte
proliferation. In the presence of either 0.09 or 0.18 mg Hg per L, mitosis of Con A-stimulated T-lym-
phocytes was significantly inhibited, while incubation with a lower Hg dose (0.04 mg Hg per L)
significantly enhanced the mitotic rate. Taken together, the aforementioned studies support the hypothesis
that immunotoxic doses of Hg inappropriately alter fish immunity, thus modifying the ability to regulate
the magnitude and specificity of a competent immune response.

Tin
Although some inorganic tin (Sn) compounds have demonstrated toxicity, it is the synthesized orga-
notins (with the potential for persistence in aquatic environments) that pose the greatest threat to
teleost and mammalian species (O’Halloran et al., 1998; Zelikoff, 1993; Zelikoff an Cohen, 1997).
Organotins are used in a variety of products, including as stabilizers in polyvinyl chloride, catalysts
in polyurethane and silicone elastomers, and pesticides (Grinwis et al., 1998). The best studied of the
organotins, tribuyltin (TBT) is most commonly used in antifouling paints for small ships. Although
banned in many countries (including the United States), TBT can still be found in concentrations as
high as 5 ppb in some Canadian freshwaters (Maguire et al., 1986), 1.5 ppb in French marine waters
(Alzieu et al., 1989), and 7 ppb in some Netherlands harbors (Ritsema and Laane, 1991). Moreover,
fish have been shown to bioaccumulate organotins by two to three orders of magnitude. In addition
to their known hepatotoxicity and effects on the endocrine system (Snoeij et al., 1985), organotins
are well-known mammalian immunotoxicants. Exposure to organotin compounds has been shown in
mammals to reduce cellularity of the thymus and spleen, circulating and splenic lymphocyte numbers,
T-lymphocyte-dependent immunity, host resistance against infectious agents, and tumoricidal activity
(Zelikoff and Cohen, 1997).
Prior to 1994, most studies assessing the immunotoxicity of organotin compounds in fish were
performed in vitro (Rice and Weeks, 1989; Wishovsky et al., 1989). In one study, exposure to TBT
reduced phagocyte-mediated immune functions in a time- and dose-dependent manner (Rice and Weeks,
1989). In a more recent in vitro investigation, O’Halloran et al. (1998) examined the effects of TBT and
its metabolite dibutyltin (DBT) on immune cells isolated from the spleen and head kidney of juvenile
rainbow trout (Oncorhynchus mykiss). Immune function was unaltered by exposure to the lowest dose
of either organotin compound (i.e., 2.5 µg/L), but incubation of lymphocytes with 50 µg DBT/L
significantly depressed mitogen-stimulated proliferation. Mitogenesis of LPS-stimulated splenic and
kidney B-lymphocytes was also reduced by the same DBT concentration. Changes in the cell population
profile appear to be responsible for the DBT-induced inhibition of lymphocyte proliferation. In contrast,
NCC activity was unaffected by in vitro exposure to either organotin compound. It was suggested that
organotins might exhibit both functional and tissue-specific effects on the fish immune system (i.e.,
spleen > head kidney, and, in general, LPS responsivity > Con A-responsive leukocytes). Based on the
observed results, the authors questioned whether TBT did, indeed, represent the most toxic form of
organotin.