Immune response is modulated by different substances that are present in the environment. Nevertheless, some of these may cause an immunotoxic effect. In this paper, the effect of organophosphorus pesticides (frequent substances spilled in aquatic ecosystems) on the immune system of fishes and in immunotoxicology is reviewed. Furthermore, some cellular and molecular mechanisms that might be involved in immunoregulation mechanisms of organophosphorus pesticides are discussed.
Organophosphorus pesticides (OPs) are a group of insecticides derived from the phosphoric or phosphorothioic acid; its use has increased in the recent years for the improvement of agriculture production, in the industry and prevention of human health through control and/or eradication of unwanted insects, plants, animals, and disease vectors [
Even though OPs have limited persistence in the environment, they are highly toxic for humans and are responsible for most of accidental intoxications [
After its application on agricultural crops, residual OPs enter water bodies as result of spray drift, soil leaching, and running off soils dedicated to agriculture, provoking adverse effects on the target species but also on a wide range of nontarget organisms, especially those that inhabit aquatic ecosystems such as invertebrates, birds, and fishes [
Among the nontarget species exposed to OPs, it is important to mention fishes, since these organisms are transcendental due to their status as top consumer species in the food chain, besides of playing an important role in the maintenance of the balance of aquatic ecosystems. From an evolutionary point of view, fishes are important organisms because they appeared over 560 million years ago; they are a group of vertebrates phylogenetically antique; there are over 25,000 species; therefore their great diversity stands out in comparison to other vertebrates [
Fishes are the first group of organisms that present an innate and adaptive immunity system; therefore the study of these organisms is of great relevance due to the information it gives about evolution of the immune system in vertebrates [
The innate immune system is of paramount importance in fishes [
On the other hand, adaptive immunity mechanisms in fishes play a vital role in the protection against recurrent infections, response that is mediated by T- and B-lymphocytes and antibodies. Fishes are the first vertebrates where clonal selection and genetic rearrangement in receptors of lymphocytes are present. Likewise, leucocytes with T cell activity have been reported, similar to the cooperative and cytotoxic T cells of mammals (CD4+-like, CD8+-like). Apart from that, based on the profile of cytokines, there have been reports of T cells subpopulations similar to the ones reported in mammals [
In recent years, an immunotoxic effect of OPs has been reported in diverse organisms, including fishes. Immune system is the first defense line against pathogenic organisms; however, it is a very sensitive system to be altered by stressing factors present in the environment (biotic and abiotic) [
Toxic effects of OPs in humoral and cellular immune parameters in fishes.
Parameters | OP | Effects | Tissue/cell line | Exposure time | Concentration | Species | References |
---|---|---|---|---|---|---|---|
Humoral immune response | |||||||
|
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Lysozyme activity | Diazinon | ↑ |
Spleen | 1, 7 d |
1.5 mg/L | Great sturgeon or beluga ( |
[ |
↓ | Plasma | 35, 42, 56, 63 d | |||||
↑ | Kidney | 63 d | |||||
↑ |
Spleen |
7 d |
2.0, 4.0 mg/L | Grass carp ( |
[ | ||
↓ | Plasma | 15, 30 d | 0.1, 0.2 mg/L | Rainbow trout ( |
[ | ||
Chlorpyrifos | ↑ |
Spleen | 3 d |
15 |
Common carp ( |
[ | |
↑ |
Plasma | 3 d |
15 | ||||
↑ |
Kidney | 3 d |
15 | ||||
↑ | Plasma | 4 d | 0.102, 0.255 mg/mL | Nile tilapia ( |
[ | ||
Phosalone | ↓ | Plasma | 7, 14 d | 0.15, 0.30, 0.60 mg/L | Common carp ( |
[ | |
|
|||||||
C-reactive protein | Metrifonate | ↑ |
Plasma | 3 d |
0.4 ppm | Rainbow trout ( |
[ |
|
|||||||
Globulin | Diazinon | ↓ | Plasma | 7, 14, 28 d | 0.1, 0.2 mg/L | Rainbow trout ( |
[ |
↓ | 7, 15, 30 d | [ | |||||
Phosalone | ↓ | Plasma | 14 d | 0.15, 0.30, 0.60 mg/L | Common carp ( |
[ | |
|
|||||||
IgM | Diazinon | → | Plasma | 4 d | 3.915, 7.830 ppm | Nile tilapia ( |
[ |
↑ | Plasma | 4 d | 1.96 mg/L | Nile tilapia ( |
[ | ||
Chlorpyrifos | ↓ |
Spleen | 1, 3, 5, 7 d |
15 |
Common carp ( |
[ | |
↑ | Kidney | 1, 3, 5 d | 15, 75 | ||||
↓ | Plasma | 1 d | 75 | ||||
↓ | Plasma | 4 d | 0.051 mg/mL | Nile tilapia ( |
[ | ||
|
|||||||
Contents of complement C3 | Chlorpyrifos | — | Spleen | 1, 3,5, 7 d | 15, 75 |
Common carp ( |
[ |
↑ | Plasma | 1 d | 75 | ||||
↑ |
Kidney | 1 d |
15 | ||||
|
|||||||
Complement C3 expression at mRNA level | Chlorpyrifos | ↑ | Spleen | 1 d | 75 |
Common carp ( |
[ |
↓ | 7 d |
15, 75 | |||||
↑ |
Kidney | 3 d |
15 | ||||
↓ | 7 d |
15, 75 | |||||
|
|||||||
IL-1 |
↑ | Spleen | 1.16, 11.6, 116 |
Common carp ( |
[ | ||
↓ | Kidney | ||||||
IL-1R relative mRNA level | ↑ | Spleen | 11.6, 116 | ||||
Chlorpyrifos | ↓ |
Kidney | 40 d | 1.16 | |||
IFN- |
↑ | Spleen | 11.6 | ||||
↓ |
Kidney | 1.16, 11.6 | |||||
|
|||||||
Cellular immune response | |||||||
|
|||||||
Cell proliferation | Diazinon | ↓ | Lymphocytes | 4 d | 7.83, 3.91, 1.95 mg/L | Nile tilapia ( |
[ |
Chlorpyrifos | → | Lymphocytes | 4 d | 0.051, 0.102, 0.255 mg/L | Nile tilapia ( |
[ | |
|
|||||||
WBC |
Diazinon | ↓ |
Blood | 10, 20, 30 d | |
Common carp, ( |
[ |
|
|||||||
WBC | Malathion | ↓ | Blood | 1, 4, 28, 42 d | 0.023, 0.46 mg/L | Nile tilapia ( |
[ |
Chlorpyrifos | ↑ | Blood | 14 d | 0.040, 0.080 mg/L | Common carp ( |
[ | |
|
|||||||
WBC |
Diazinon | ↓ |
Blood | 7, 15, 30 d | 0.1, 0.2 mg/mL | Rainbow trout ( |
[ |
|
|||||||
WBC |
Phosalone | ↓ |
Blood | 7, 14 d |
0.15, 0.30, 0.60 mg/L |
Common carp ( |
[ |
|
|||||||
WBC |
Diazinon | ↑ |
Blood | 7 d | 0.5, 1 ppm | Iridescent shark ( |
[ |
|
|||||||
Respiratory burst | Diazinon | ↑ | Splenocytes | 4 d | 1.96 mg/L | Nile tilapia ( |
[ |
|
|||||||
Phagocytic index | Diazinon | ↓ | Blood | 4 d | 7.83, 3.91 mg/L | Nile tilapia ( |
[ |
|
|||||||
Phagocytic function | |||||||
Gran | Chlorpyrifos | → | Kidney cells | 0.1, 1, 10 mg/L | Rainbow fish ( |
[ | |
Lyn | Silver perch ( | ||||||
Gran | Golden perch ( | ||||||
Lyn | ↓ | 10 mg/L | Murray cod ( |
↑: increase/activation (induction); ↓: inhibition/decrease; →: no effect; —: not detectable; WBC: white blood cell; Lym: lymphocytes; Mon: monocyte; Gra: granulocytes; Eos: eosinophil; Bas: basophil; Neu: neutrophil; d: days.
In fishes, molecules that are responsible of the innate and adaptative humoral response can be altered by OPs, like chlorpyrifos, diazinon, and phosalone, among others [
Thus, lysozyme is an important molecule defense of the innate immune system of fishes that is frequently altered by OPs. A study showed that the lysozyme activity increased significantly in liver and spleen of beluga (
Another important molecule of the innate immune system of fishes is the protein C3 of the complement, which is also altered by the exposition to OPs. A deregulation at concentration and mRNA expression of this molecule has been reported in anterior kidney, spleen, and plasma of common carp (
Reactive C protein (RCP) is another molecule of the innate immune system of fishes affected by exposure to this type of pesticides. In this context, it has been reported that acute exposure to metrifonate (0.4 ppm) in rainbow trout (
Other proteins that are also altered by the exposure to OPs are the globulins. Some studies have reported that, in plasma of rainbow trout (
Regarding the effect of OPs on the cytokines, it has been reported that the exposure to chlorpyrifos during 24 h (1.16, 11.6 and 116
The innate and adaptative cellular response of fishes can be deregulated by the exposure to diverse OPs. Studies show that exposure of rainbow trout and common carp to diazinon provokes a diminishment in the white blood cell (WBC) in these species. The differential account of these cells showed a diminishment in the percentage of lymphocytes, monocytes, and basophils; however, the percentage of neutrophils and eosinophils increased significantly after exposure to the pesticide [
On the other hand, it was also reported that the OPs not only induce alteration in the number of cells, but also in the morphology and functionality of them. Hence, it was reported that diazinon (15, 30, 45, 60, and 75
The effects mentioned above show that OPs alter the function of certain elements of the immune system, even though the mechanisms of immunotoxicity of the OPs are not clear. Such mechanism of OPs is not direct but it works through indirect mechanisms, topics that will be discussed in this section, based on evidence shown in different animal models (Figure
Mechanisms of immunotoxicity of OPs. The potential immunotoxicity mechanisms could involve effect directly on immune cells or through neuroimmune communication disturbance. AcCoA: acetyl-coenzyme A; ACh: acetylcholine; AChE: acetylcholinesterase; ChAT: acetylcholine transferase; CYP450: Cytochrome P450; DAPS: dialkylphosphates; mAChR: muscarinic acetylcholine receptor; nAChR: nicotinic acetylcholine receptor; OPs: organophosphorus pesticides; oxon: oxidized metabolite of OPs; PON: paraoxonase.
As previously mentioned, OPs are substances that have as target molecule the enzyme AChE, blocking its activity through the irreversible bound to the active site, which provokes an increase in the levels of the neurotransmitter ACh in the nervous system. In this context, in mammals, the influence of the nervous system on the regulation of the immune system has been demonstrated years ago [
In this way, the existence of an extraneuronal cholinergic system in lymphocytes makes them susceptible to perturbation by OPs. It has been suggested that OPs can modulate lymphocytes through cholinergic receptors, evoking an immediate intracellular signalization of diverse molecules, among them c-Fos, modulating therefore the levels of second messengers. Activation of cholinergic receptors can act upstream in the transduction of signals, causing the interruption of cellular homeostasis, decaying into apoptosis [
Besides inhibiting the enzyme AChE, OPs are capable of inhibiting serine hydrolases enzymes, such as molecules of the complement and thrombin system, which will influence directly the functionality of the immune system. In addition, the damage in the lymphoid tissue is the result of the phosphorylation, oxidative damage, and/or altered neuronal function, induced by OPs [
Alterations of the components and immune functions have also been related to the sequence and intensity of phosphorylation and dephosphorylating of protein kinases, essential process to modulate the immune response. A key molecule in this process is the protein suppressor of cytokine signaling 3 (SOCS3), which regulates protein STAT. SOCS3 mediates inhibition of phosphorylation of STAT5, which has been related with the diminishment of cellular proliferation [
Some studies have suggested the implication of OPs in apoptotic processes. It is known that the initiation of apoptosis is regulated by external and internal signals, such as the activation of dead receptors, damage to DNA, and perturbation of the mitochondrial membrane. These mechanisms carry the caspases activation and subsequently the destruction of the cell in a programmed way [
Fishes are the first vertebrates with innate and adaptative immune mechanisms, similar to mammals. Thus, fishes can be used as a model in biomedical research, allowing data in the immunotoxicology field in evolutionary terms. Besides, due to fishes being the most abundant vertebrate in the planet, a lot of them with commercial importance, data generated could have economic and ecological importance.
There are evidences that the immune response can be altered by OPs exposure. Although, the immunotoxicity mechanisms are not completely clarified, evidence suggests that OPs can target several molecules related to the immune system and execute the immunotoxic effect through the alteration of the neuroimmune communication, particularly the cholinergic neuronal and immune system. Nevertheless, further research is needed in order to understand the mechanisms of immunoregulation of this type of pesticides widely used in household and agricultural activities.
There is no conflict of interests, and the authors declare that they have no direct relationship with the previously mentioned commercial entities or any other related one.
This work was funded by a grant from the financial resources of SEP-CONACyT, Mexico, for Basic Research (Project no. 2012-179508) to M. I. Girón Pérez. K. J. G. Díaz-Resendiz and G. A. Toledo-Ibarra are students of “Posgrado en Ciencias Biológico-Agropecuarias y Pesqueras (CBAP)” of Universidad Autónoma de Nayarit (Mexico). K. J. G. Díaz-Resendiz received a grant from CONACyT, Mexico (no. 265138). The authors gratefully acknowledge Lic. Gabriela González de Pablos for her reading of the paper, correcting English language and style.