Page 517 - The Toxicology of Fishes

The Immune System of Fish: A Target Organ of Toxicity 497

Until recently, teleost antibody was thought to be of only the IgM isotype that exists as a tetramer
comprised of the classical two heavy-chain and two light-chain (H L ) subunit structure (Kaattari and
2 2
Piganelli, 1996); however, recent investigations in two teleost species have described a second isotype
similar to mammalian IgD (Miller et al., 1998). Although it appears that fish possess limited isotypic
diversity, several studies have indicated some heterogeneity of IgM heavy chains (Lobb and Olson, 1988)
and isotypic variants of light chains (Lobb et al., 1984; Sanchez and Dominguez, 1991; Whittington,
1993). Several investigators have also shown that subpopulations of teleost IgM exist that differ in
covalent structure (Lobb, 1986; Lobb and Clem, 1981, 1983; Warr, 1983; Whittington, 1993). The
tetrameric molecules appear to contain various configurations in which the presence of both inter-heavy
and inter-subunit disulfide bonds varies. This produces the appearance of an array of protein bands from
90 to 750 kDa under nonreducing, denaturing SDS-PAGE electrophoresis (Kaattari and Piganelli, 1996).
The gene arrangement of the teleost heavy chain is similar to that described for mammals, with variable
(V ), diversifying (D), and joining (J ) regions upstream of the constant segments (Miller et al., 1998).
H
H
The light-chain gene organization is similar to that of elasmobranchs in that V , J , and constant (C )
L
L
L
segments appear in multiple clusters (Miller et al., 1998). In addition, teleost B-cells demonstrate Ig
heavy-chain germline rearrangements and allelic exclusion (Miller et al., 1994).
Cell-Mediated Immunity
Until just a few years ago, characterization of specific cell-mediated cytotoxicity in teleost species had
been hampered due to a lack of defined cellular and molecular markers (Stuge et al., 2000). Early studies
of cell-mediated immunity in fish demonstrated that acute (less than 14-day) allograft rejections of skin
and scales occur in many teleost species (Manning and Nakanishi, 1996). The cell-mediated phenomenon
of graft-vs.-host reaction has also been described in studies that employed triploid and tetraploid
ginbuna–goldfish (Carassius auratus) hybrids (Nakanishi and Ototake, 1999). Finally, delayed hyper-
sensitivity responses (DHRs) have been recorded in response to bacterial and parasitic antigens in various
fish species (Manning and Nakanishi, 1996). Recently, allospecific cytotoxic responses of fish leukocytes
have been demonstrated in vitro (Fisher et al., 1998). In addition, intricate studies utilizing TNP-modified
allogeneic and autologous cells have indicated that genetically restricted target cell recognition is involved
in the specific CTL response of fish following both in vitro and in vivo priming (Hasegawa et al., 1999;
Verlhac et al., 1990). The discovery of TCR-α and -β chain sequences in fish (Zhou et al., 1997) has
allowed for the demonstration of at least four subpopulations of cells involved in the CTL response of
channel catfish (Ictalurus punctatus) (Stuge et al., 2000). One population was identified as TCRαβ +
+
allospecific cytotoxic cells (i.e., equivalent to mammalian CTL); other groups included TCRαβ non-
specific cytotoxic cells, TCRαβ allospecific cytotoxic cells, and TCRαβ nonspecific cytotoxic cells.
–
–
+
In addition, a TCRαβ cell subpopulation that proliferated in response to allogeneic stimulation, but
possessed no cytotoxicity toward target cells, was also observed. It is thought that the TCRαβ subpop-
+
ulation may represent a T-helper lymphocyte population (Stuge et al., 2000).
Immunomodulation by Chemicals

Immunotoxicology, a specific subspecialty within the field of toxicology, has gained increasing interest
in the last decade due primarily to the increased knowledge of immunology and the importance of the
immune response in maintaining the integrity of the organism. Moreover, considerable attention has
been focused on the applicability and predictability of laboratory-animal-based assays for immunotox-
icity in safety assessment studies (Thomas, 1995). Research in the area of immunotoxicology has
established various assays for identifying immunotoxicants, a large array of chemicals known to produce
immune effects by either inhibiting or enhancing responses, and specific mechanisms by which some
chemicals may produce immune dysfunction. Not all compounds within a given chemical class can act
to alter the immune response; however, certain metals, organometals, halogenated hydrocarbons, het-
erocyclic compounds, organophosphate/chlorine pesticides, and radionuclides can act to alter immune
responsiveness of a variety of animal species (including humans) at exposure levels below which other