Differential effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin, bis(tri-n-butyltin) oxide and cyclosporine on thymus histophysiology

Effects of PRRSV Infection on the Porcine Thymus

Porcine reproductive and respiratory syndrome virus (PRRSV) dramatically affects the thymus and its ability to carry out its normal functions. In particular, infection incapacitates PRRSV-susceptible CD14^pos antigen-presenting cells (APCs) in the thymus and throughout the body. PRRSV-induced autophagy in thymic epithelial cells modulates the development of T cells, and PRRSV-induced apoptosis in CD4^posCD8^pos thymocytes modulates cellular immunity against PRRSV and other pathogens. Pigs are less able to resist and/or eliminate secondary infectious agents due the effect of PRRSV on the thymus, and this susceptibility phenomenon is long recognized as a primary characteristic of PRRSV infection.

Tributyltin oxide exposure impairs mouse oocyte maturation and its possible mechanisms

Tributyltin oxide (TBTO) has been widely used as marine antifouling composition, preservative, biocide, and a stabilizer in plastic industry. Previous studies have indicated that TBTO can cause immunotoxicity as an environmental pollutant. However, little is known about its reproductive toxicity, especially on female oocyte maturation and the underlying mechanisms. In this study, mouse oocytes were cultured with different concentrations of TBTO in vitro, and several crucial events during meiotic maturation were evaluated. We found that the first polar body extrusion rate was significantly reduced, which reflected the disruption of meiotic maturation. The rate of abnormal spindle organization increased significantly, accompanied with a higher rate of chromosome misalignment. In addition, TBTO treatment increased reactive oxygen species generation markedly, which also accelerated the early-stage apoptosis. Moreover, heterogeneous mitochondrial distribution, mitochondrial dysfunction, and higher rate of aneuploidy were detected, which consequently disrupted in vitro fertilization. In conclusion, our results indicated that TBTO exposure could impair mouse oocyte maturation by affecting spindle organization, chromosome alignment, mitochondria functions, oxidative stress, and apoptosis.

The aryl hydrocarbon receptor and food allergy

The immune system is important for protection against pathogens and malignant cells. However, malfunction of the immune system can also result in detrimental auto-immune diseases, inflammatory diseases, cancers and allergies. The aryl hydrocarbon receptor (AhR), present in numerous tissues and cell subsets, including cells of the immune system, plays an important role in the functioning of the immune system. Activation of the AhR is for example associated with various effects on dendritic cells (DCs), regulatory T cells and the Th1/Th2 cell balance. These cells play a major role in the development of food allergy. Food allergy is an increasing health problem in both humans and animals. Despite the knowledge in risk factors and cellular mechanisms for food allergy, no approved treatments are available yet. Recently, it has been shown that activation of the AhR by dioxin-like compounds suppresses allergic sensitization by suppressing the absolute number of precursor and effector T cells, by preserving CD4(+)CD25(+)Foxp3(+) Treg cells and by affecting DCs and their interaction with effector T cells. Future research should elucidate whether and how AhR activation can be used to interfere in food allergic responses in humans and in animals. This may lead to new prevention strategies and therapeutic possibilities for food allergy.

Transcriptomics analysis of primary mouse thymocytes exposed to bis(tri-n-butyltin)dioxide (TBTO)

The biocide bis(tri-n-butyltin)oxide (TBTO) causes thymus atrophy in rodents and is toxic to many cell types of which thymocytes are the most sensitive. To obtain insight in the mechanisms of action of TBTO, we exposed primary mouse thymocytes in vitro for 3, 6 and 11 h to 0.1, 0.5, 1 and 2 μM TBTO. Subsequently, the cells were subjected to whole-genome gene expression profiling. Biological interpretation of the gene expression data revealed that TBTO affects a wide range of processes. Cell proliferation related genes were downregulated by all treatments except for 3 and 6 h 0.5 μM TBTO which upregulated these genes. Treatment with TBTO resulted in upregulation of genes involved in endoplasmatic reticulum (ER) stress, NFkB and TNFα pathways, and genes involved in DNA damage, p53 signaling and apoptosis. Remarkably, TBTO also increased the expression of genes that are known to be upregulated during T cell activation or during negative selection of thymocytes. The effect of TBTO on expression of genes involved in ER stress and apoptosis was confirmed by qPCR. Induction of the T cell activation response was corroborated by demonstrating that TBTO exposure resulted in translocation of NFAT to the nucleus, which is an essential event for T cell activation.

Gene expression profiling of Bis(tri-n-butyltin)oxide (TBTO)-induced immunotoxicity in mice and rats

Bis(tri-n-butyltin)oxide (TBTO) is one of the organotin compounds that have been used as biocides and occur as persistent environmental pollutants. Human exposure to these compounds occurs through consumption of meat and fish products in which they accumulate. The most sensitive endpoint of TBTO exposure is immunotoxicity. TBTO causes thymus atrophy and thereby interferes with T-lymphocyte-mediated immune responses. Tributyltin compounds have been found to adversely affect a wide range of cellular components and processes in many species, organ systems, and cell types. Both inhibition of proliferation and induction of apoptosis have been observed in thymocytes. We conducted microarray experiments in mice and rats in order to investigate if the immunosuppressive actions of TBTO could be detected by gene expression profiling, and if so, to elucidate the mechanisms of action. Gene expression changes that were detected in mouse thymuses after exposure to a maximum tolerable dose of TBTO correlated to previously observed effects. Most notably, reduction of expression of cell surface determinants and T-cell receptor chains, suppression of cell proliferation, and a possible involvement of nuclear receptors in interference with lipid metabolism by TBTO were observed. The TBTO-induced thymus involution may therefore primarily be caused by inhibition of thymocyte proliferation. In contrast, in rats only limited effects of a lower dose of TBTO were found at the gene expression level in the thymus, even though thymus involution was observed. Here, most gene expression regulation by TBTO was detected in the liver. These preliminary results indicate that gene expression analysis is able to reveal effects of TBTO and to gain insight into its molecular mechanism of action. It may even be a suitable tool to investigate immunotoxicology in general. However, dose and inter-species differences are apparently clearly reflected in the gene expression profiles.

In vitro immunotoxicity of bis(tri-n-butyltin)oxide (TBTO) studied by toxicogenomics

The biocide and environmental pollutant bis(tri-n-butyltin)oxide (TBTO) causes thymus atrophy in rodents. Whether the depletion of thymic lymphocytes by tributyltin compounds may be the result of inhibition of cell proliferation or induction of apoptosis is subject of debate. We examined gene expression profiles in primary rat thymocytes exposed to TBTO in vitro at dose levels of 0, 0.1, 0.3, 0.5, and 1.0microM. By measuring cell viability and apoptosis, exposure conditions were selected that would provide information on changes in gene expression preceding or accompanying functional effects of TBTO. Several processes related to TBTO-induced toxicity were detected at the transcriptome level. Effects on lipid metabolisms appeared to be the first indication of disruption of cellular function. Many transcriptional effects of TBTO at higher dose levels were related to apoptotic processes, which corresponded to present or subsequent thymocyte apoptosis observed phenotypically. The gene expression profile was, however, not unambiguous since expression of apoptosis-related genes was both increased and decreased. Stimulation of glucocorticoid receptor signaling appeared to be a relevant underlying mechanism of action. These findings suggest that TBTO exerts its toxic effects on the thymus primarily by affecting apoptotic processes, but the possibility is discussed that this may in fact represent an early effect that precedes inhibition of cell proliferation. At the highest dose level tested, TBTO additionally repressed mitochondrial function and immune cell activation. Our in vitro toxicogenomics approach thus identified several cellular and molecular targets of TBTO that may mediate the toxicity towards thymocytes and thereby its immunosuppressive effects.

Immunologic effects of perinatal exposure to dioxins, PCBs and organochlorine pesticides in Japanese infants

Effects of perinatal exposure to dioxins, PCBs and organochlorine pesticides on lymphocyte subsets were investigated in the peripheral blood from 101 Japanese infants with approximately 10 months of age. Perinatal exposure to these organochlorine compounds were estimated by their contamination levels in the breast milk of the mothers. Lymphocyte subsets such as CD16+, HLA-DR+, CD4+, CD4+8+, CD8+, CD3+ and CD20+ cells in peripheral venous blood were assessed in a subgroup of 92 infants. Greater exposures to HCE, chlordane and dioxins were significantly associated with the increase in the percentages of CD8+ and CD3+ T lymphocytes and CD4+/CD8+ T cell ratios, respectively. In addition, higher HCH exposure was also associated with a decrease in the percentage of HLA-DR+ T lymphocytes. Furthermore effects of dioxins, DDT and PCBs on the percentage of CD16+ T lymphocyte were more pronounced by the combined exposure of dioxins and PCBs or by the combined exposure of DDT and PCBs. Effects of HCE on the percentages of CD8+ T lymphocyte were also more pronounced by the combined exposure of HCE and chlordane. In conclusion, our study suggests that greater exposures to dioxins, PCBs and organochlorine pesticides determined in this study (except dieldrin) influence the immune system of Japanese infant, although the clinical significance of these changes is uncertain.

Histopathology of the thymus

The thymus is a primary lymphoid organ that manifests dynamic physiological changes as animals age in addition to being exquisitely sensitive to stress and toxic insult. It is typically the first lymphoid tissue to respond to immunotoxic xenobiotics, with the first change being loss of cortical lymphocytes by apoptosis. This is followed by removal of the apoptotic cellular debris and, in the absence of recovery, may lead to loss of the cortico-medullary demarcation and organ atrophy. Nonneoplastic proliferative changes include focal lymphoid hyperplasia and proliferation of medullary epithelial cells, often with formation of ribbons, cords, or tubules. Thymomas are relatively rare tumors that exhibit a wide spectrum of morphologic types but do not metastasize. Thymic lymphomas are common in some mouse strains and can become leukemic with hematogenous spread throughout the body.

Normal structure, function and histology of the thymus

The thymus, a primary lymphoid organ and the initial site for development of T cell immunological function, is morphologically similar across species. It is actually an epithelial organ in which its epithelial cells provide a framework containing T cells as well as smaller numbers of other lymphoid cells. A symbiotic interaction exists between the thymic microenvironment and developing T cells, and the specificity of T cell release into the systemic circulation is under thymic control. The thymic cortex in a young animal is heavily populated by developing T cells along with a smaller proportion of associated epithelial cells. Larger, more mature T cells are found in the medulla where epithelial and other cell types are more abundant. Understanding normal morphological features of the thymus and their perturbations provides a cornerstone to assessing immune system function.

Extended histopathology in immunotoxicity testing: interlaboratory validation studies

There has been considerable interest in the use of expanded histopathology as a primary screen for immunotoxicity assessment. To determine the utility of a semiquantitative histopathology approach for examining specific structural and architectural changes in lymphoid tissues, a validation effort was initiated. This study addresses the interlaboratory reproducibility of extended histopathology, using tissues from studies of ten test chemicals and both negative and positive controls from the National Toxicology Program's immunotoxicology testing program. We examined the consistency between experienced toxicologic pathologists, who had varied expertise in immunohistopathology in identifying lesions in immune tissues, and in the sensitivity of the individual and combined histopathological endpoints to detect chemical effects and dose response. Factor analysis was used to estimate the association of each pathologist with a so-called "common factor" and analysis-of-variance methods were used to evaluate biases. Agreement between pathologists was highest in the thymus, in particular, when evaluating cortical cellularity of the thymus; good in spleen follicular cellularity and in spleen and lymph node-germinal center development; and poorest in spleen red-pulp changes. In addition, the ability to identify histopathological change in lymphoid tissues was dependent upon the experience/training that the individual pathologist possessed in examining lymphoid tissue and the apparent severity of the specific lesion.

Immunological and physiological effects of chronic exposure of Peromyscus leucopus to Aroclor 1254 at a concentration similar to that found at contaminated sites

Polychlorinated biphenyls (PCBs) are environmental contaminants known to cause adverse health effects to biological systems. Limited data are available on their effects on the immune system of wildlife species. Previously, we found that 4 and 6-week-old white-footed mice (Peromyscus leucopus) born from dams injected with a single dose (300 mg/kg) of Aroclor 1254, had altered immunological, hematological, and biochemical responses. Here, we examined the effect of transplacental, lactational and postnatal exposure to Aroclor 1254, at a concentration similar to that found at contaminated sites, on various physiological parameters of 22-week-old white-footed mice. Liver weight and liver somatic index of PCB treated animals were significantly higher, the combined weights of the adrenal glands were significantly lower and EROD and BROD enzyme activity was significantly higher compared to control values. The number of thymocytes of the treated mice was significantly lower than that of the controls; however, thymocytes of treated mice had a higher proliferative response to the mitogen Con A. These alterations were correlated with the PCBs body burdens. Some toxic effects of chronic exposure to PCBs, at levels comparable to exposure found in contaminated sites in the USA, are still evident in adult P. leucopus.

Effect of cyclosporin A in Lewis rats in vivo and HeLa cells in vitro

The aim of this study was to compare the effect of cyclosporin A (CsA) in inbred Lewis rats with published assessment of immunotoxicity in 'classical' outbred Wistar rats. A second purpose was to consider the contribution of a panel of in vitro assays in cell cultures when added to an immunotoxicity study in vivo. The in vivo effect of CsA was investigated in a 28-day subacute immunotoxicity study in male Lewis rats at three different concentrations: 1.25, 5 and 20 mg kg(-1). The highest dose of CsA exceeded the maximum tolerated dose. A drop in body, spleen and popliteal lymph node weight of exposed animals displayed symptoms of toxicity. At a high toxic dose, haematological changes showed a decrease in the leucocyte count and in the percentage of lymphocytes, and an increase in the percentage of polymorphonuclear leucocytes. The haematocrit was significantly dose-dependently suppressed in all rats exposed to CsA. A similar dose-dependent depression of the mean cell volume of erythrocytes was found in rats given high and middle doses of CsA. The phagocytic activity of polymorphonuclear leucocytes and monocytes also was significantly dose-dependently suppressed. No significant changes in primary antibody response to sheep erythrocytes or in vitro proliferative response of spleen lymphocytes to mitogens were found in those rats.A battery of in vitro cytotoxicity methods was selected for the evaluation of metabolic and functional activity of subcellular organelles (mitochondria, lysosomes) and for the detection of drug-induced superoxide-mediated damage in HeLa cells. This cell line was chosen because it has a lower activity of superoxide dismutase (SOD) than normal cells and is sufficiently sensitive for the detection of the induction of oxygen radicals. The in vitro results indicated a direct relationship between CsA cytotoxicity and a change in the mitochondrial enzyme activity, as well as an induction of superoxide production. The results of the study indicated that a combination of selected in vivo and in vitro methods is an inexpensive way to obtain more complex information on cell status affected by xenobiotics.

Triphenyltin acetate-induced cytotoxicity and CD4(+) and CD8(+) depletion in mouse thymocyte primary cultures

Organotin compounds (OTs) find application worldwide as catalysts, stabilizers and biocides. Triphenyltin derivatives (TPs), including the fungicide triphenyltin acetate (TPTA), are OTs mostly used in our country. Some OTs were proved to be immunotoxic and in this paper the cytotoxicity, the possible selective activity upon definite lymphocyte subsets as well as the antiproliferative effect of TPTA was investigated in vitro by using primary cultures of mouse thymocytes. TPTA (5, 10 and 25 microM) was cytotoxic to these cells, as demonstrated by the significant (P<0.05) reduction of the cell viability percentage (trypan blue dye exclusion test), the neutral red uptake and the reduction of tetrazolium salts to formazan products (MTT assay). These overt effects were already noticed after 4 h of exposure to TPTA. The fungicide otherwise significantly reduced, after 24 h of incubation, the percentage of mature single positive thymocytes, particularly the CD4(+)/CD8(-) one. Finally, a significative dose-dependent inhibition of the T-cell mitogen-induced cell proliferation was observed in thymocytes exposed to 1 and 8 microM TPTA. These results are indicative of the TPTA immunotoxic properties, according to previous published reports concerning the in vitro and in vivo toxicity of some di- and triorganotin compounds.

Histopathologic approaches to detect changes indicative of immunotoxicity

Toxicologic pathology is crucial in the identification and characterization of health effects following exposure to xenobiotics, mainly in toxicity experiments in rodents. Regarding regulatory toxicology, histopathology of lymphoid organs and tissues is a cornerstone in the identification of immunotoxic compounds. A 2-tier testing system is usually employed in which the first tier is a general screen for (immuno)toxicity and the second tier consists of specific immune function studies, including host resistance tests or mechanistic studies. The role attributed to histopathology of lymphoid organs in the updated Organisation for Economic Cooperation and Development and Food and Drug Administration guidelines requires improvement and standardization of the histopathology procedures. Optimalization and standardization was started in an international collaborative immunotoxicity study (ICICIS). However, several problems were left unaddressed, mostly because of the few compounds tested in this study. Based on the results of the ICICIS study and the morphologic changes induced by immunotoxic/immunomodulatory compounds observed in other investigations, suggestions are given to further improve the identification and (semi)quantification of histopathologic changes in lymphoid organs and tissues.

Contaminant-induced immunosuppression and mass mortalities among harbor seals

Virus-associated mass mortalities among seals inhabiting northwestern Europe have generated an interest in immunotoxicology in this species. A morbillivirus has been isolated from victims, but a contribution of immunotoxic contaminants to the severity of the outbreaks could not be ruled out. Fish-eating seals occupy high trophic levels in the aquatic food chain, and accumulate high levels of contaminants including polychlorinated biphenyls (PCBs), polychlorinated dibenzo-p-dioxins (PCDDs), and polychlorinated dibenzofurans (PCDFs). Such chemicals have been found to be immunotoxic at low doses in studies of laboratory animals. We carried out an immunotoxicological study, in which captive harbor seals (Phoca vitulina) were fed herring from either relatively uncontaminated sites of the Atlantic Ocean, or from the highly contaminated Baltic Sea. In this report we summarize the contaminant-related immunosuppression observed in the captive group of seals fed herring from the Baltic Sea. In addition, we describe two parallel studies, in which laboratory rats are exposed as adults or perinatally to the contaminants in the Baltic Sea herring, exhibiting immunotoxicity. On the basis of these studies we conclude that complex mixtures of environmental contaminants including PCBs, PCDFs, and PCDDs may represent a real immunotoxic risk to free-ranging seals.

In vitro exposure effects of cyclosporin A and bis(tri-n-butyltin)oxide on lymphocyte proliferation, cytokine (receptor) mRNA expression, and cell surface marker expression in rat thymocytes and splenocytes

Rat thymocytes and splenocytes were exposed in vitro to the model compounds Cyclosporin A (CsA), an immunosuppressive drug, and bis(tri-n-butyltin)oxide (TBTO), an immunotoxic environmental contaminant. The lymphocyte transformation test (LTT), cytokine (receptor) mRNA expression (RT-PCR and dot blot hybridisation), and flow cytometry were evaluated as assays for in vitro immunotoxicity, at dose levels that did not show effects on viability, this being the aim of the study. LTT and RT-PCR proved useful assays. Lymphocyte transformation was suppressed by both compounds, while IL-2 mRNA expression was suppressed by CsA but not by TBTO, and both compounds suppressed IL-2R mRNA expression in splenocytes but not in thymocytes. Furthermore, the data obtained suggest that antiproliferative effects may be more relevant than apoptosis induction for TBTO induced thymus atrophy.

Toxicity of 3,3',4,4'-tetrachloroazoxybenzene in rats and mice

The toxicity of 3,3',4,4'-tetrachloroazoxybenzene (TCAOB) was evaluated in 13-week gavage studies in male and female F344/N rats and B6C3F1 mice. In addition to histopathology, evaluations included clinical chemistry, hematology, thyroid hormone analyses, and effects on sperm morphology and estrous cycle length. Groups of 10 rats and 10 mice of each sex were exposed to TCAOB at dose levels of 0, 0.1, 1, 3, 10, or 30 mg/kg 5 days a week for 13 weeks. In the rat studies, the major effects included death in the 30 mg TCAOB/kg dose group; at lower exposure levels, a decrease in body weight gain, a decrease in thymus weight, an increase in liver weight, an increase in hematopoietic cell proliferation in the spleen and liver, a responsive anemia, a decrease in platelet counts, a chronic active inflammation of the vasculature in the lung, an increase in cardiomyopathy, hyperplasia of the forestomach, and a marked decrease in circulating thyroxine concentrations were observed. In male rats a decrease in sperm motility in the epididymides was observed. In addition, in female rats an increase in lung, spleen, kidney, and heart weights and nephropathy was observed. Furthermore, the estrous cycle length was increased. In the mouse studies, the major effects for males and females included a decrease in thymus weights, an increase in liver and kidney weights, centrilobular hypertrophy in the liver, hematopoietic cell proliferation, hyperplasia of the forestomach, and dilatation of hair follicles. The spectrum of effects in both rats and mice after exposure to TCAOB indicates that dioxin-like effects occur in addition to effects that have not been observed with dioxin-like compounds. No no-observed-adverse-effect level was reached in male or female rats or mice.

Toxicity of 3,3',4,4'-tetrachloroazobenzene in rats and mice

The toxicity of 3,3',4,4'-tetrachloroazobenzene (TCAB) was evaluated in 13-week gavage studies in male and female F344/N rats and B6C3F1 mice. In addition to histopathology, evaluations included clinical chemistry, hematology, thyroid hormone analyses, and reproductive parameters. Groups of 10 rats and 10 mice of each sex were exposed to TCAB at dose levels of 0, 0.1, 1, 3, 10, or 30 mg/kg for 5 days a week for 13 weeks. In the rat studies, the major effects for both males and females included a 10% decrease in terminal body weight at 30 mg/kg/day, an increase in hematopoietic cell proliferation in the spleen at 10 and 30 mg/kg/day, and a responsive anemia at 10 and 30 mg/kg/day. A 15 to 30% decrease in platelet counts and a 20 to 40% decrease in thymus weights was observed at 10 and 30 mg/kg/day. An increase in liver weight up to 15% was found at 3 mg/kg/day and higher doses in males and at 10 and 30 mg/kg/day in females, respectively. An increase in spleen weights up to 15% was observed at 10 and 30 mg/kg/day in males and at 30 mg/kg/day in females. A marked decrease in circulating total thyroxine (TT4) was found in both males and females at all dose levels tested. TT4 could hardly be detected at 10 and 30 mg TCAB/kg/day. In addition, hyperplasia of the forestomach was increased at 3 mg/kg/day and higher doses in males and at 30 mg/kg/day in females. In the mouse studies, an increase in liver and spleen weight was observed up to approximately 25% in both males and females at 10 and 30 mg/kg/day. Hyperplasia of the forestomach was observed at 1 mg/kg/day and higher doses in both males and females. In males, a 30% decrease in thymus weights at 30 mg/kg/day and a 60% decrease in epididymal sperm density at 3 and 30 mg/kg/day was observed. Also in males, centrilobular hypertrophy of hepatocytes and an increase in hematopoietic cell proliferation in the spleen was observed at 3 mg/kg/day and higher doses. Based on the current study and information in the literature, TCAB has dioxin-like properties. Comparison of the effects of TCAB in the present study and in the literature to those with 2,3,7, 8-tetrachlorodibenzo-p-dioxin (TCDD) indicates that TCAB is from two to six orders of magnitude less potent than TCDD depending on the end point.