Immunotoxicity of repeated low level exposure to T-2 toxin, a trichothecene mycotoxin, in CD-1 mice

Consensus on the Key Characteristics of Immunotoxic Agents as a Basis for Hazard Identification

BACKGROUND

Key characteristics (KCs), properties of agents or exposures that confer potential hazard, have been developed for carcinogens and other toxicant classes. KCs have been used in the systematic assessment of hazards and to identify assay and data gaps that limit screening and risk assessment. Many of the mechanisms through which pharmaceuticals and occupational or environmental agents modulate immune function are well recognized. Thus KCs could be identified for immunoactive substances and applied to improve hazard assessment of immunodulatory agents.

OBJECTIVES

The goal was to generate a consensus-based synthesis of scientific evidence describing the KCs of agents known to cause immunotoxicity and potential applications, such as assays to measure the KCs.

METHODS

A committee of 18 experts with diverse specialties identified 10 KCs of immunotoxic agents, namely, 1) covalently binds to proteins to form novel antigens, 2) affects antigen processing and presentation, 3) alters immune cell signaling, 4) alters immune cell proliferation, 5) modifies cellular differentiation, 6) alters immune cell-cell communication, 7) alters effector function of specific cell types, 8) alters immune cell trafficking, 9) alters cell death processes, and 10) breaks down immune tolerance. The group considered how these KCs could influence immune processes and contribute to hypersensitivity, inappropriate enhancement, immunosuppression, or autoimmunity.

DISCUSSION

KCs can be used to improve efforts to identify agents that cause immunotoxicity via one or more mechanisms, to develop better testing and biomarker approaches to evaluate immunotoxicity, and to enable a more comprehensive and mechanistic understanding of adverse effects of exposures on the immune system. https://doi.org/10.1289/EHP10800.

An update on immunotoxicity and mechanisms of action of six environmental mycotoxins

Paradoxically, aflatoxin B1 (AFB1), ochratoxin A (OTA), deoxynivalenol (DON), T-2 toxin (T-2), fumonisin B1 (FB1), and zearalenone (ZEA) have both immunosuppressive and immunostimulatory effects. The immunotoxicity of six mycotoxins exhibits immune suppression or stimulation, which depends on multiple factors. Low doses of mycotoxins can induce an inflammatory response, but elevated levels of ones can induce immunosuppression; long-term instead of short-term mycotoxin exposure is immunosuppressive. These six mycotoxins play anti-inflammatory roles when the immunologic stimulants are present but pro-inflammatory roles when the immunologic stimulants are absent. Pigs are most sensitive animals to mycotoxins, followed by humans and poultry, rodent, and marine organism, and ruminants are the least susceptible. Female animals are more susceptible to mycotoxins than male ones. The immunosuppresion mechanism of mycotoxins are mainly in, oxidative stress, apoptosis and autophagy of immune cells, as well as inhibits the immunity-related signal pathways; and AFB1, OTA, DON, and T-2 induce immunostimulation via directly activating the TLRs/NF-κB pathway and other crossing pathways including cyclooxygenase-2 (COX-2) and mitogen-activated protein kinase (MAPK). This review strongly dispels the viewpoint that "immunotoxicity is equivalent to immunosuppression", clearly demonstrates the mechanistic pathway and how it contributes to immunosuppression or immunostimulation, thereby providing reliable references for future studies.

The Protective Effect of Selenium on T-2-Induced Nephrotoxicity Is Related to the Inhibition of ROS-Mediated Apoptosis in Mice Kidney

T-2 toxin is produced by the Fusarium genus. Ingestion of food or feed contaminated by T-2 toxin will cause damage to kidney. Selenium (Se), an essential trace element, showed the significant protective effects against kidney and renal cell damage induced by toxic substances. To explore the protective effects and mechanisms of Se against T-2-induced renal lesions, forty-eight male Kunming mice were exposed to T-2 toxin (1.0 mg/kg) and/or Se (0.2 mg/kg) for 28 days. In this study, we found that Se alleviated T-2-induced nephrotoxicity, presenting as increasing the body weight and kidney coefficient, relieving the renal structure injury, decreasing the contents of renal function-related biomarkers, decreasing the levels of reactive oxygen species (ROS), and increasing the mitochondrial membrane potential in T-2 toxin-treated mice. In addition, inhibition of renal cell apoptosis by Se was associated with blocking the mitochondrial pathway in T-2 toxin-treated mice, presenting as decreasing the protein expression of cytochrome-c, activities of caspase-3/9, as well as regulating the protein and mRNA expressions of Bax and Bcl-2. These results documented that the alleviating effect of Se on T-2-induced nephrotoxicity is related to the inhibition of ROS-mediated renal apoptosis.

Hypoxia, oxidative stress, and immune evasion: a trinity of the trichothecenes T-2 toxin and deoxynivalenol (DON)

T-2 toxin and deoxynivalenol (DON) are type A and B trichothecenes, respectively. They widely occur as pollutants in food and crops and cause a series of toxicities, including immunotoxicity, hepatotoxicity, and neurotoxicity. Oxidative stress is the primary mechanistic basis of these toxic effects. Increasing amounts of evidence have shown that mitochondria are significant targets of apoptosis caused by T-2 toxin- and DON-induced oxidative stress via regulation of Bax/B-cell lymphoma-2 and caspase-3/caspase-9 signaling. DNA methylation and autophagy are involved in oxidative stress related to apoptosis, and hypoxia and immune evasion are related to oxidative stress in this context. Hypoxia induces oxidative stress by stimulating mitochondrial reactive oxygen species production and regulates the expression of cytokines, such as interleukin-1β and tumor necrosis factor-α. Programmed cell death-ligand 1 is upregulated by these cytokines and by hypoxia-inducible factor-1, which allows it to bind to programmed cell death-1 to enable escape of immune cell surveillance and achievement of immune evasion. This review concentrates on novel findings regarding the oxidative stress mechanisms of the trichothecenes T-2 toxin and DON. Importantly, we discuss the new evidence regarding the connection of hypoxia and immune evasion with oxidative stress in this context. Finally, the trinity of hypoxia, oxidative stress and immune evasion is highlighted. This work will be conducive to an improved understanding of the oxidative stress caused by trichothecene mycotoxins.

In vitro effects of diacetoxyscirpenol (DAS) on human and rat granulo-monocytic progenitors

Diacetoxyscirpenol (DAS) is a trichothecene mycotoxin produced by various species of fungi. Trichothecenes are known as major contaminants of cereals and cereal-containing foods. DAS has been detected in agricultural products worldwide and persists in products after processing. In human as well as in animals, DAS consumption has been shown to induce haematological disorders (neutropenia, aplastic anemia). Granulo-monocytic progenitors (CFU-GM) from human umbilical cord blood and rat bone marrow have been cultured in the presence of DAS (from 10(-8) M to 5 x 10(-10) M) for 14 days. Study of concentration and effect relationships have shown a sharp effect of DAS on rat CFU-GM between 10(-7) M and 10(-8) M, while human CFU-GM are able to grow in the presence of 10(-8) M of the toxin. IC50 values on day 14 are respectively, 7.6 x 10(-9) M for human CFU-GM and 6.2 x 10(-9) M for rat CFU-GM.

Alteration of major cytokines produced by mitogen-activated peritoneal macrophages and splenocytes in T-2 toxin-treated male CD-1 mice

Fusarium T-2 toxin has immunotoxic properties that may be related to the modulation of cytokine expression by cells of the immune system. Male CD-1 mice were used to study the effect of in vivo exposure to T-2 toxin on the alteration of interleukin (IL)-1α, tumor necrosis factor α (TNF), and IL-6 in lipopolysaccharide-stimulated peritoneal macrophages, and IL-2, IL-3, and interferon γ (IFNγ) in concanavalin A (Con A)-stimulated splenocytes. Mice were orally dosed with 0, 0.1, 0.5, and 2.5 mg T-2 toxin/kg body weight for 2 weeks on alternate days. Northern blot analysis of IL-1α, TNF, and IL-6 mRNA from activated peritoneal macrophages showed no significant differences between control and treated groups. Measurements of secreted protein by immunoassay demonstrated suppression of these cytokines in all treated groups, suggesting that T-2 toxin affects the translational or post-translational regulation of these cytokines from peritoneal macrophages. Levels of IL-2, IL-3, and IFNγ mRNA from Con A-activated splenocytes were higher in all treated groups. The increases were significant for IL-2 and IFNγ in the groups receiving low (0.1 mg/kg) and high (2.5 mg/kg) doses of T-2 toxin, and for IL-3 in the group receiving a medium (0.5 mg/kg) dose of this toxin (P ≤ 0.05). Results indicated that T-2 toxin given orally at low or medium doses induces transcription or increases mRNA stability of IL-2, IFNγ, and IL-3. Protein levels of all three cytokines were also increased, indicating that T-2 toxin also increases translational/post-translational efficiency of IFNγ, IL-2, and IL-3. Possible mechanisms in the immunosuppressive effects of T-2 toxin may involve endotoxemia resulting after the toxin administration, alteration of the stability of mRNA, or previously described effects of T-2 toxin on protein synthesis.

Comparison of toxicity induced by T-2 toxin on human and rat granulo-monocytic progenitors with an in vitro model

T-2 toxin is a trichothecene mycotoxin produced by various species of fungi. Trichothecenes are known as major contaminants of cereals and their derivatives. In man as well as in animals, T-2 toxin has been shown to induce alimentary intoxication and, among others, haematological symptoms. Granulo-monocytic progenitors from human umbilical cord blood on the one hand and granulo-monocytic progenitors from rat bone marrow on the other, were cultured in the presence of T-2 toxin (from 10(-7) to 10(-10) M) for 14 days. A study of concentration and effect relationships showed a strong and rapid effect of T-2 toxin on rat colony forming unit-granulocyte and macrophage (CFU-GM) between 5 x 10(-9) M and 10(-9) M. On the other hand, human CFU-GM were able to grow in the presence of the same T-2 toxin concentrations. IC50 were determined on day 7, 10 and 14. They were, respectively, 1.6 x 10(-9) M; 3.6 x 10(-9) M; 1.4 x 10(-9) M for human cells, and 2.2 x 10(-9) M; 3.3 x 10(-9) M; 2.6 x 10(-9) M for rat cells. The present study was prompted by the need to define precisely the cytotoxic and inhibitory T-2 toxin concentrations for rat and human CFU-GM. It is particularly relevant for the investigation of cellular T-2 toxin targets and in order to elucidate the mechanism of trichothecene haematotoxicity.

Effects of aflatoxin B1 and T-2 toxin on the granulocyte-macrophage progenitor cells in mouse bone marrow cultures

Myelotoxic effects of aflatoxin B1 (AFB1) and T-2 toxin on the proliferation of the granulocyte-macrophage progenitor cells to granulocyte, macrophage and granulocyte-macrophage (GM) colonies were investigated in male CD-1 mice by a semisolid in vitro culture technique. Mice received 0, 0.03, 0.145 and 0.7 mg/kg or 0, 0.1, 0.5, and 2.5 mg/kg body weight of AFB1 and T-2 toxin, respectively, for 2 weeks on alternate days. Granulocyte, macrophage, and GM-colonies were suppressed in the group that received the highest dose of AFB1 (0.7 mg/kg body weight). Treatment with T-2 toxin activated granulocyte colonies at 0.5 mg/kg and macrophage colonies at 0.1 and 2.5 mg/kg but suppressed GM-colonies at 0.5 and 2.5 mg/kg body weight. Bone marrow cells from normal CD-1 mice were cultured with different concentrations of AFB1 (1-50 microM) or T-2 toxin (1-10 nM) in vitro. Significant activation of granulocyte colonies with 1 microM AFB1 and suppression of all three types of colonies with the highest concentration of AFB1 (50 microM) were observed. The suppression of granulocyte and GM-colonies associated with T-2 toxin was concentration-dependent. The greatest suppression occurred in macrophage colonies with three highest concentrations of T-2 toxin (3, 6, and 10 nM). Results suggest that AFB1 is myelotoxic both in vivo and in vitro, whereas T-2 toxin is more toxic in vitro.

Immunotoxicity of mycotoxins

The immune system is primarily responsible for defense against invading organisms. The effects of several mycotoxins on the immune responses have been investigated; however, most data concern laboratory animals. In some instances, farm animals and cells derived from livestock species have been employed to evaluate the immunotoxicity of mycotoxins. Immune responses are highly variable, and cost considerations usually preclude the use of dairy cattle as experimental models. Immunosuppression caused by aflatoxin B1 has been demonstrated in various livestock species (e.g., turkeys, chickens, and pigs) and also in laboratory animals (mice, guinea pigs, and rabbits). The response of bovine lymphocytes to aflatoxin in vitro is similar to that of other laboratory animals. Trichothecenes are potent immunosuppressive agents that directly affect immune cells and also modify immune responses as a consequence of tissue damage elsewhere. Sheep and calves treated with fusarium T-2 toxin develop leukopenia and decreased functioning of peripheral lymphocytes. Immunosuppressive effects of ochratoxin A, rubratoxin B, and patulin have been reported. Citrinin produced lymphopenia but stimulated responses against antigens. Antibodies against mycotoxins conjugated to proteins have been produced and are useful for analytical purposes.

Modulation of resistance to mastitis pathogens by pretreatment of mice with T-2 toxin

T-2 toxin, a secondary metabolite of Fusarium species, is a mycotoxin with immunomodulatory activity. In the present investigation the effects of T-2 toxin on host resistance was studied. The virulence of Escherichia coli and Staphylococcus aureus in the mammary glands of mice treated with T-2 toxin was compared with their virulence in control mice. Virulence was estimated from the ability to induce various types of lesions and bacterial growth in the mammary gland. Pretreatment of mice with a single dose (3 mg/kg body weight) of T-2 toxin by gavage reduced the virulence of both E. coli (P less than 0.05) and S. aureus (P less than 0.01). Microscopic lesions in the infected glands varied in character, from consistently non-reactive necrosis of the entire mammary gland to limited inflammatory reactions. The former were more abundant in control mice than in mice treated with T-2 toxin. Although treatment by gavage with T-2 toxin (0.75 mg/kg body weight/day) for 14 days prior to inoculation had no significant effect on the course of the mastitis infection, virulence was slightly lower in the T-2 toxin treated mice. Both single-dose and successive treatment with T-2 toxin enhanced the respiratory burst activity of macrophages. Pre-inoculation treatment with T-2 toxin also caused a significant increase in the number of peritoneal cells, T-2 toxin did not show bacterial effects on the E. coli or S. aureus strains used for the inoculations. The data indicate that T-2 toxin has modulatory effects on the cell-mediated immune system, and that enhancement of resistance to common mastitis pathogens in mice pretreated with a single dose of T-2 toxin is associated with migration and activation of macrophages.

Effect of in vivo administration of T-2 toxin on peritoneal murine macrophages

We investigated the effect of in vivo administration of T-2 toxin, a 12,13-epoxytrichothecene produced by several Fusarium species, on murine macrophage metabolism. Cytoplasmic and lysosomal enzyme levels, generation and release of superoxide anion, phagocytosis and intracellular killing of Salmonella typhi and murine P815 tumour cell lysis were measured under different experimental conditions. When T-2 toxin was administered to mice at sublethal doses (0.50-1.00 mg/kg/24 hr), the levels of lysosomal and cytoplasmic enzyme activity and the generation of superoxide anion were significantly enhanced as compared to controls. This correlated with increased phagocytosis and intracellular killing of S. typhi. Cytotoxic activity against murine P815 mastocytoma cells exhibited by macrophages isolated from mice treated with T-2 toxin was inhibited in a dose-dependent manner. In vivo administration of T-2 toxin may result in the activation of specific metabolic pathways of peritoneal macrophages, while inhibiting other paths.

Relationship of the hypothalamic-pituitary-adrenal axis with chemically induced immunomodulation. I. Stress-like response after exposure to T-2 toxin

T-2 toxin (T-2), a trichothecene mycotoxin, produced by several members of the genus Fusarium is a cytotoxic feed contaminant and has been shown to by immunomodulatory. It is suspected that T-2 associated immunomodulation is mediated partly through the hypothalamic-pituitary-adrenal (HPA) axis. T-2, prepared in 4% ethanol/corn oil, was administered orally to male CD-1 mice. Endotoxemia was evident 24 h after a single, oral exposure to T-2. Blood levels of corticosterone, indicative of the stress-response, increased 24 h after T-2 exposure. Hypothalamic norepinephrine and serum corticosterone levels increased in a dose-related manner after 2 weeks of T-2 exposure. Endotoxin, detected in the serum of animals exposed to 2.5 mg/kg T-2 for 1 week, was not associated with bacteremia. Neither endotoxin nor bacteremia were detectable after 2 or 4 weeks of T-2 exposure. Exposure to 2.5 mg/kg T-2 also affected several organs. The forestomach was ulcerated, with lymphocytic infiltration, epithelial proliferation, and hyperkeratinization. Increased spleen weight was associated with a proliferative red pulp. No histological changes were observed in the enlarged liver. Gastritis has been associated with increased corticosteroid production; cortical depletion and reduced mass of the thymus are phenomena attributable to increased corticosteroid levels. An increased corticosteroid level has been associated with thymic involution leading potentially to decreased T-dependent antibody response, a known effect of T-2.

Antibody producing ability of mouse spleen cells after subacute dietary exposure to T-2 toxin

The effects of T-2 toxin on the antibody producing ability of CD-1 mice after dietary exposure to 0, 2.5, 5 and 15 ppm of T-2 toxin for 29 days was studied. The antibody response against sheep red blood cells, a T-lymphocyte and macrophage-dependent response was suppressed at 2.5, 5 and 15 ppm of T-2 toxin exposure. In contrast, the antibody responses against dinitrophenyl-aminoethylcarbamylmethyl - Ficoll (DNP - Ficoll), a T-lymphocyte independent macrophage-dependent response, and Escherichia coli 0127 (LPS), a T-lymphocyte and macrophage-independent response, were not affected. These results suggest that the inhibitory effects of T-2 toxin on antibody-producing ability after subacute dietary exposure appear to be a direct affect on T-lymphocyte function, possibly involving the T-helper lymphocytes.

Alteration of humoral and cellular immunity in manganese chloride-treated mice

Immunological effects of manganese chloride (MnCl2) were determined in male CD-1 mice injected (ip) daily with MnCl2 (0, 1, 3, or 10 mg/kg) for 4 wk. Liver and spleen weights increased in the 10-mg/kg MnCl2 treatment group. The weights of thymus, kidney, and adrenal glands were not affected by MnCl2 treatment. No significant differences in peripheral erythrocyte or leukocyte counts were observed; however, packed cell volumes decreased in the medium- and high-dose groups. Manganese treatment significantly increased the uptake of [3H]thymidine (3H-TdR) by cultured splenic cells. The lymphoproliferative responses to phytohemagglutinin (PHA) and concanavalin A (Con A) increased at all levels of MnCl2 exposure. No differences in the responses to lipopolysaccharide (LPS) were observed. Mixed lymphocyte responses increased significantly with exposure to 10 mg MnCl2/kg. Another immunological alteration induced by MnCl2 was a dose-dependent immunosuppressive effect on the development of antibody-forming cells. The production of anti-sheep red blood cell antibody (alpha-SRBC) nearly ceased following exposure to 10 mg MnCl2/kg. This effect was apparently reversible, as the number of plaque-forming cells in the 10-mg/kg treatment group increased after MnCl2 treatment had been halted for 2 wk. The alpha-SRBC titer also decreased significantly in the 10-mg/kg treatment group, corresponding to the reduction of antibody producing cells. MnCl2 treatment was immunomodulatory to the reduction of antibody producing cells. MnCl2 treatment was immunomodulatory in male CD-1 mice, as indicated by the increase in mitogen and mixed lymphocyte responses and decrease in antibody production.

Immunological responsiveness of mouse spleen cells after in vivo or in vitro exposure to 3-acetyldeoxynivalenol

The effects of 3-acetyldeoxynivalenol (3-AcDON) on mitogen-induced lymphocyte proliferation and antibody production were studied in male CD-1 mice exposed to 0, 2.5, 5 or 10 ppm 3-AcDON in the diet for 35 days. Mitogen-induced lymphocyte proliferation and T-cell-independent antibody responses to dinitrophenyl-ficoll or Escherichia coli were not altered by dietary exposure to 3-AcDON. The T-cell-dependent antibody response to sheep red blood cells was increased in the group fed 10 ppm 3-AcDON. In vitro, 3-AcDON inhibited lymphocyte proliferation in a dose-dependent manner. Inhibition was observed when the toxin was present during the first 8 hr in phytohaemagglutinin-stimulated cultures and during the first 24 hr in lipopolysaccharide-stimulated cultures. This suggests that 3-AcDON blocks an early step in lymphocyte activation. This inhibition was not restored by thiol reducing agents (dithiothreitol, L-cysteine or 2-mercaptoethanol). Similarly, the addition of lymphokines, including interleukin-1 or interleukin-2, did not alter the inhibitory effects of 3-AcDON. These results suggest that the in vitro effects of 3-AcDON may not reflect its in vivo immunotoxicity. However, 3-AcDON may serve as a chemical probe for examining the activation process of lymphocyte proliferation.

Dose and time related effects of T-2 toxin on mitogenic response of murine splenic cells in vitro

The effects of T-2 toxin in vitro on murine splenic cells were investigated. Cells from syngeneic male NFS/N mice were cultured with three concentrations of either lipopolysaccharide (LPS), pokeweed mitogen (PWM), phytohaemagglutinin (PHA), or concanavalin A (Con A), for 48 h. T-2 toxin (10(-12)-10(-8)M) was added at the beginning or 24 h after initiation of the cultures. Exposure to T-2 toxin (10(-11)-10(-10)M) after 24 h increased 3H-thymidine uptake by splenic cells. Stimulation by PWM increased dramatically while the response to LPS was increased to a lesser extent in this system. Conversely, exposure to T-2 toxin decreased T-cell responses to both PHA and Con A. Exposing cells to T-2 toxin at the initiation of culture increased Con A stimulation; LPS and PWM responses decreased following 48 h exposure. When splenic cells were pre-treated for 60 or 90 min at 0 degrees C with 10(-9)M T-2 toxin the responses to both PWM and LPS increased. Increased 3H-thymidine uptake was also observed following 90 min exposure to T-2 toxin alone at either 0 or 37 degrees C. The increased PWM response and the mitogenic phenomenon observed when cells were exposed only to T-2 toxin were consistent with previously observed effects of in vivo T-2 toxin exposure.