Abstract
The impact of anthropogenic contaminants on the immune system of fishes is an issue of growing concern. An important xenobiotic receptor that mediates effects of chemicals, such as halogenated aromatic hydrocarbons (HAHs) and polyaromatic hydrocarbons (PAHs), is the aryl hydrocarbon receptor (AhR). Fish toxicological research has focused on the role of this receptor in xenobiotic biotransformation as well as in causing developmental, cardiac, and reproductive toxicity. However, biomedical research has unraveled an important physiological role of the AhR in the immune system, what suggests that this receptor could be involved in immunotoxic effects of environmental contaminants. The aims of the present review are to critically discuss the available knowledge on (i) the expression and possible function of the AhR in the immune systems of teleost fishes; and (ii) the impact of AhR-activating xenobiotics on the immune systems of fish at the levels of immune gene expression, immune cell proliferation and immune cell function, immune pathology, and resistance to infectious disease. The existing information indicates that the AhR is expressed in the fish immune system, but currently, we have little understanding of its physiological role. Exposure to AhR-activating contaminants results in the modulation of numerous immune structural and functional parameters of fish. Despite the diversity of fish species studied and the experimental conditions investigated, the published findings rather uniformly point to immunosuppressive actions of xenobiotic AhR ligands in fish. These effects are often associated with increased disease susceptibility. The fact that fish populations from HAH- and PAH-contaminated environments suffer immune disturbances and elevated disease susceptibility highlights that the immunotoxic effects of AhR-activating xenobiotics bear environmental relevance.
Highlights
Immunity is a biological trait that critically relates to organism fitness [1,2]
We focus on the possible role of the aryl hydrocarbon receptor (AhR) in mediating immunotoxic effects of aquatic contaminants in fish
Evidence that the AhR pathway is functional comes from the fact that a key target gene of the AhR, CYP1A, can be upregulated in fish immune cells treated with AhR ligands
Summary
Immunity is a biological trait that critically relates to organism fitness [1,2]. The immune system is commonly understood as protection against non-self, such as infectious pathogens, but more broadly, it is concerned with the prevention of damage to the organism [3]. The various immune system-based receptors could be a molecular entry site through which environmental contaminants may interfere with fish immunity An example for such a mechanism is provided by the so-called endocrine disruptors, i.e., contaminants that bind as agonists or antagonists to steroid hormone receptors in cells of the neuroendocrine and reproductive systems and thereby can disrupt fish reproduction and sexual development [15,16,17]. Steroid hormone receptors are present in immune cells of fish [18] and estrogenor androgen-active chemicals are able to impact the reproductive and the immune system of fish [13,19]. We focus on the possible role of the aryl hydrocarbon receptor (AhR) in mediating immunotoxic effects of aquatic contaminants in fish. Sci. 2021, 22, 9460 the possible ecotoxicological consequences of contaminant-induced activation of the AhR in the fish immune system
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