Judith T. Zelikoff

Metal pollution-induced immunomodulation in fish

Abstract The exact relationship between disease incidence in aquatic organisms and environmental pollution is not well defined. A number of mechanisms by which aquatic pollutants may act to increase disease incidence in fish have been speculated, many suggesting immunosuppression as a link in the etiology of disease among fishes in highly contaminated areas. This article will review the effects of metal pollutants on the immune responses of fish by examining in vitro and in vivo laboratory studies carried out since 1980. It will also describe how those alterations may be responsible for pollution-associated diseases in directly exposed fish. While a large number of environmental contaminants represent aquatic pollutants of concern (e.g. polycyclic aromatic hydrocarbons, chlorinated organics, and pesticides), heavy metals were selected as the pollutants for this review because of their: (a) prevalence in polluted aquatic environments; (b) immunotoxic potential in mammalian systems; (c) ability to induce tumors in exposed rodents; and (d) their overall toxicity in a variety of species. It can be concluded that a number of heavy metal pollutants shown to be immunotoxic in mammalian systems, including cadmium, chromium, copper, lead, manganese, nickel, and zic, also alter immunoregulatory functions in a variety of fish species. These alterations may ultimately lead to increased host susceptibility to infectious and malignant diseases in fish inhabiting heavy metal-contaminated waters.

Aquatic Pollution-Induced Immunotoxicity in Wildlife Species

The potential for chemicals to adversely affect human immunologic health has traditionally been evaluated in rodents, under laboratory conditions. These laboratory studies have generated valuable hazard identification and immunotoxicologic mechanism data; however, genetically diverse populations exposed in the wild may better reflect both human exposure conditions and may provide insight into potential immunotoxic effects in humans. In addition, comparative studies of species occupying reference and impacted sites provide important information on the effects of environmental pollution on the immunologic health of wildlife populations. In this symposium overview, Peter Hodson describes physiological changes in fish collected above or below the outflows of paper mills discharging effluent from the bleaching process (BKME). Effects attributable to BKME were identified, as were physiological changes attributable to other environmental factors. In this context, he discussed the problems of identifying true cause and effect relationships in field studies. Mohamed Faisal described changes in immune function of fish collected from areas with high levels of polyaromatic hydrocarbon contamination. His studies identified a contaminant-related decreases in the ability of anterior kidney leukocytes to bind to and kill tumor cell line targets, as well as changes in lymphocyte proliferation in response to mitogens. Altered proliferative responses of fish from the contaminated site were partially reversed by maintaining fish in water from the reference site. Peter Ross described studies in which harbor seals were fed herring obtained from relatively clean (Atlantic Ocean) and contaminated (Baltic Sea) waters. Decreased natural killer cell activity and lymphoproliferative responses to T and B cell mitogens, as well as depressed antibody and delayed hypersensitivity responses to injected antigens, were identified in seals fed contaminated herring. In laboratory studies, it was determined that rats fed freeze-dried Baltic Sea herring had higher virus titers after challenge with rat cytomegalovirus (RCMV) than rats fed Atlantic Ocean herring; perinatal exposure of rats to oil extracted from Baltic herring also reduced the response to challenge with RCMV. Keith Grassman reported an association between exposure to polyhalogenated aryl hydrocarbons and decreased T cell immunity in the offspring of fish-eating birds (herring gulls and Capsian terns) at highly contaminated sites in the Great Lakes. The greatest suppression of skin test responses to phytohemagglutinin injection (an indicator of T cell immunity) was consistently found at sites with the highest contaminant concentrations. Judith Zelikoff addressed the applicability of immunotoxicity studies developed in laboratory-reared fish for detecting altered immune function in wild populations. She presented data from studies done in her laboratory with environmentally relevant concentrations of metals as examples. Although the necessity of proceeding with caution when extrapolating across species was emphasized, she concluded that published data, and results presented by the other Symposium participants, demonstrate that assays similar to those developed for use in laboratory rodents may be useful for detecting immune system defects in wildlife species directly exposed to toxicants present in the environment.

Immunomodulation by Metals

A symposium entitled Immunomodulation by Metals was held at the 32nd Annual Meeting of the Society of Toxicology (SOT) in New Orleans, Louisiana. The symposium was co-sponsored by the Immunotoxicology and Metals Specialty Sections of SOT and was designed to describe the types of adverse immunological reactions which occur in response to environmental and/or occupational exposure to metals. Epidemiological evidence and underlying mechanisms responsible for the observed alterations were also discussed. The following is a summary of each of the individual presentations.

Development of fish peritoneal macrophages as a model for higher vertebrates in immunotoxicological studies: I. Characterization of trout macrophage morphological, functional, and biochemical properties

The immune defense mechanisms of fish are not as well characterized as those of mammals but seem to be related and similarly competent. Because of this, there is an increased interest in the immune responses of fish as models for higher vertebrates in immunotoxicological studies. Prior to such studies, baseline criteria for specific components of the immune response needed to be established. For this study, we have examined trout macrophage morphology using light and scanning electron microscopy, phagocytic activity, random and stimulus-directed migration, and superoxide anion radical (O2-) production for resident and lipopolysacharide (LPS) or Aeromonas salmonicidae-elicited rainbow trout (Oncorhynchus mykiss) peritoneal macrophages (M phi). Following peritoneal lavage, greater than 89% of the cells were M phi as determined by differential counts and nonspecific esterase staining. Immunization with LPS and A. salmonicidae increased M phi number approximately 5 and 13-fold, respectively, and overall size. Trout M phi were phagocytically active engulfing serum opsonized latex particles and were mobile, migrating both randomly and in a directed fashion towards formyl-methionine-L-leucine-L-phenylalanine (FMLP) and trout serum-derived complement fragment C5a. Concentrations of FMLP (100 nM) and C5a (0.01-1%) effective for attracting trout M phi are the same as those used to attract rabbit M phi. Resident trout M phi produced negligable quantities of .O2- following stimulation with 1 micrograms/ml phorbol myristate acetate; Aeromonas-elicited M phi produced .O2- in a time-dependent manner which peaked after 60 min at 2.9 nmol per 2 x 10(5) cells and then declined. The results of this study provide a data base for future toxicological studies with trout peritoneal M phi and indicate the usefulness of this system for immunotoxicological studies.

Health Risks Associated with Prenatal Metal Exposure

A symposium entitled Health Risks Associated with Prenatal Metal Exposure was held at the 33rd Annual Meeting of the Society of Toxicology (SOT) in Dallas, Texas. The symposium was cosponsored by the Metals and Reproductive and Developmental Specialty Sections of SOT and was designed to elaborate the health risks associated with in utero exposure to metals commonly found in the workplace and/or ambient environment on the mother and developing offspring. Epidemiological and toxicological evidence that demonstrates the health effects and underlying mechanisms associated with exposure to arsenic (As), lead (Pb), and methyl mercury (MeHg) were discussed, as well as the legal ramifications and personal implications associated with prenatal metal exposure. The following is a summary of each of the individual presentations.

Biomarker selection for restoration monitoring of fishery resources

Monitoring fishery resources affected by contaminant discharges can include two distinct components: (1) monitoring contaminant exposure (e.g., residues in fish tissues), and (2) monitoring biological effects. Although exposure monitoring may be appropriate for evaluating the efficacy of ecological restoration programs, effects monitoring is an equally important and often overlooked aspect of monitoring programs. Advantages of monitoring effects indices include (1) biotic integration of diverse exposure pathways and temporal variability; (2) ability to integrate responses across multiple stressors; and (3) cost effectiveness relative to extensive chemical analyses. The objective of our work was to develop and review biomarker selection criteria including: (1) sensitivity (response time, permanence of response, degree of responsiveness); (2) specificity (specific to contaminant exposure); (3) applicability (cost-effectiveness, scientific acceptance); and (4) reproducibility (biological, methodological). Emphasis is placed on selection criteria for biomarkers associated with organochlorine, petroleum hydrocarbon, or metal exposure and effects.

Sulfur and Nitrogen Oxides

Sulfur oxides comprise both gaseous and particulate chemical species. There are four of the former, namely sulfur monoxide, sulfur dioxide, sulfur trioxide and disulfur monoxide. The particulate phase sulfur oxides consist of strongly-to-weakly acidic sulfates, namely sulfuric acid (H2SO4) and its products of neutralization with ammonia: letovicite [(NH4)3H(SO4)2], ammonium bisulfate (NH4HSO4), and ammonium sulfate [(NH4)2SO4]. Most of the toxicologic database for sulfur oxides involves sulfur dioxide (SO2) and H2SO4.